Advanced Composites and Hybrid Materials (2022) 5:2651–2674
https://doi.org/10.1007/s42114-021-00412-z
REVIEW
Composite polymer electrolytes: progress, challenges, and future
outlook for sodium‑ion batteries
Dheeraj K. Maurya1 · Ragupathy Dhanusuraman2 · Zhanhu Guo3 · Subramania Angaiah1
Received: 26 October 2021 / Revised: 17 December 2021 / Accepted: 26 December 2021 / Published online: 28 February 2022
© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2022
Abstract
Sodium-ion battery (SIB) arises as propitious energy sources complementing the energy supply demands amidst of proliferating
energy crises and environmental trauma due to fossil fuel consumption. Higher earth abundance, similar electrochemistry
as lithium, and cost-effectiveness have driven the research focused on building better SIBs. Solid inorganic and polymer
electrolytes (PEs) are prevailing electrolyte candidates for SIBs. The bottleneck of both the electrolytes, such as low ionic
conductivity, poor mechanical and thermal stability, and high interfacial charge resistance, has retarded the rate of their commercial acceptance for futuristic energy devices. To tackle these burning issues, strategies to couple inorganic and polymer
electrolytes as composite polymer electrolytes (CPEs) are drawing immense interest in academia and industry. The present
review discusses the state-of-the-art composite polymer electrolytes for SIBs. It comprises three parts. The first part
briefs about the introduction and performance index of CPEs to assess the importance of CPEs over existing electrolytes. In
the second part, various synthesis methods for CPEs preparations are encapsulated. The third part is focused on the role of extrinsic
fillers (active and passive) and the corresponding mechanism involved in ionic transport in CPEs by recently reported works. The
role of filler engineering in addressing the remedies of CPEs is also intensely scrutinized. Finally, this review is concluded
with the perspective of CPEs toward the future of SIB development. This review is aiming to understand the insight of fillers
within CPEs and their impact on the performance of SIBs.
Keywords Sodium-ion batteries (SIBs) · Inorganic ceramic electrolyte (ICE) · Composite polymer electrolyte (CPE) ·
Fillers · Ion transport
Abbreviations
SIBSodium-ion battery
LIBLithium-ion battery
ICEInorganic ceramic electrolyte
PEGPolyethylene glycol
PEGDMEPoly(ethylene glycol) dimethyl ether
SNSuccinonitrile
PEOPolyethylene oxide
* Subramania Angaiah
a.subramania@gmail.com
1
Electro‑Materials Research Laboratory, Centre
for Nanoscience and Technology, Pondicherry University,
Puducherry 605014, India
2
Nano‑Electrochemistry Lab (NEL), Department
of Chemistry, National Institute of Technology Puducherry,
Karaikal 609‑609, India
3
Integrated Composites Laboratory (ICL), Department
of Chemical and Biomolecular Engineering, University
of Tennessee, Knoxville, TN 37996, USA
PVDFPoly(vinylidene fluoride)
PVDF-HFPPoly(vinylidene
fluoride-co-hexafluoropropylene)
PVPPolyvinylpyrrolidone
PMMAPoly(methyl methacrylate)
NaClO4Sodium perchlorate
NaTFSISodium bis(trifluoromethylsulfonyl)imide
NaFSISodium (I) bis(fluorosulfonyl)imide
QDQuantum dot
SiO2Silicon dioxide
LCLiquid crystal
ZrO2Zirconium dioxide
Al2O3Aluminum oxide
CaOCalcium oxide
Zn2Fe2O4Zinc ferrite
Si3N4Silicon nitride
NaPO3Sodium triphosphate
NaIO4Sodium periodate
NaTFSodium triflate
NaCF3SO3Sodium trifluoromethanesulfonate
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NaPF6Sodium hexafluorophosphate
TEGDMETetraethylene glycol dimethyl ether
AFMAtomic force microscopy
PEPolymer electrolyte
CPEComposite polymer electrolyte
DMFDimethylformamide
DMAcDimethylacetamide
1 Introduction
Renewable energy-driven energy storage devices have
been recognized as a powerful solution to minimize the
depletion of fossil fuels, solely responsible for severe environmental threats such as global warming and air pollution
[1–5]. Electrochemical energy research attracts enormous
attention as a component of practical applications like
batteries, fuel cells, and supercapacitors, utilizing conversion and storage of renewable energy into electrochemical
energy [5–11]. The rapid evolution of electric vehicles,
grid energy storage, and portable electronics has raised
soaring demands to develop more energy-efficient energy
storage devices. Over the past three decades, rechargeable
lithium-ion batteries (LIBs), owing to the merits such as
high voltage, high energy density, low self-discharge rate,
and long cycle life, have been deemed as a potential alternative meeting intermittent demands of portable energy
storage [12–17]. Nowadays, the colossal production of
LIBs is suffering from the economic sustainability crisis of lithium sources. Consequently, the limited lithium
Fig. 1 Comparison of performance of current LIB and
SIB technologies. (Ref. [24]
reproduced with the permission
of Elsevier)
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reserves and increased price of lithium salts have motivated researchers to develop alternatives for LIB technologies based on abundant materials.
In recent years, SIBs have been paid immense attention
due to their higher natural earth abundance than lithium
in the earth’s crust. SIBs have been considered to replace
LIBs due to their similar electrochemical charge storage
mechanism as LIBs. A good number of companies currently adapted SIB technologies (e.g., HiNa battery (China),
Faradion, and Tiamat) and conducting research and development for advanced SIBs [2, 18, 19]. The electrolyte is
the critical component of high-performance rechargeable
SIBs, which mainly transport ­Na+ and block electrons’ diffusion. In the present situation, major conventional SIBs are
employing organic liquid electrolytes comprising flammable
carbonates. Operating SIBs with the Na metal anode and
liquid electrolyte cause leakage and fire risks during
overcharge or abused operations in electrical vehicles
applications.
Additionally, SIBs encounter inevitable dendrite growth
due to the side reactions between highly reactive liquid
electrolytes with the Na anode causing capacity fade and
low cycling ability in SIBs [20–23]. Rejuvenating highperformance electrolyte materials is focused on safety
assurance and long-term cycling improvement SIBs
(Fig. 1). In the same way, as LIBs, the development of
all-solid-state batteries aims to address the safety concerns
of SIBs. The strategy of replacing liquid electrolytes with
solid-state electrolytes has significantly been pursued in
the current energy storage scenario.
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Fig. 2 Schematic of different
electrolytes for current SIBs
All-solid-state electrolytes are generally classified into
­ a+ conductive solid polymer electrolytes (SPEs), inorganic
N
ceramic electrolytes (ICEs), and solid ceramic-polymer
composite electrolytes (CPEs) (Fig. 2). ICEs and SPEs are
the prevailing solid electrolytes that have been exploited for
the last two decades for all-solid-state SIBs [25–30]. SPEs
mainly comprise the polymer matrix solvated in a sodium
salt to form a macromolecular architecture that inherits good
flexibility, easy processability, low flammability, and better
compatibility with the electrodes. These superior characteristics of SPEs stabilize them as an appealing electrolyte
material for flexible and wearable energy storage devices
[17, 31, 32]. However, the practical applicability of SPEs is
hampered due to severe drawbacks such as low ionic conductivity, narrow electrochemical stability windows, and
poor thermal stability.
On the other hand, ICEs offer a great zeal toward their
use in all-solid-state SIBs owing to their advantages such
as high ionic conductivity, high electrochemical stability,
superior thermal stability, and robust mechanical properties
[14, 21, 29, 33–39]. Their intrinsic rigidity and inevitable
poor interfacial compatibility with the electrodes led to a
tremendous challenge for more efficient high-density allsolid-state SIB. In the past decade, the researchers introduced
numerous solutions and promising approaches to eradicate
the disadvantages of SPEs and ICEs. Various strategies like
cross-linking and blending of polymers, addition of liquid
plasticizers in the polymer matrix, and developing polymerionic liquid electrolytes have been practiced for SIB development. As prepared electrolyes succeed in uplifting the ionic
conductivity but on the expensive cost in the form of reduced
long-term cyclic stability and poor mechanical strength [40,
41]. A new class of solid polymer-ceramic composite utilizes
the advantages of both ICEs and SPEs and overcomes the
disadvantages to develop high energy-efficient SIBs. Synergistically coupling the ICEs with an organic polymer have
demonstrated smart stratagem technology to harness the
favorable properties of existing inorganic–organic entities
[35, 42–46]. This particular class of electrolytes is extremely
successful in fulfilling the aim of achieving high ionic conductivity, robust mechanical strength, wide electrochemical
potential stability window, improved lithium-ion transference
number, low interfacial resistance, and dendrite suppression
capability (Fig. 3). Tremendous research has been done to
Fig. 3 Performance characteristics of inorganic ceramic
electrolytes (ICE), solid
polymer electrolytes (SPE), and
composite polymer electrolytes
(CPE)
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develop CPE for LIBs and a good number of reviews regarding composite electrolytes for LIBs are reported [4, 14, 24,
35, 42–44, 47–55]. But there is no review emphasizing the
development of composite polymer electrolytes for SIBs.
The present review discusses the state-of-the-art of
composite polymer electrolytes for SIBs. This article does
not intend to review solid and gel polymer electrolytes but
focuses on the composite polymer-ceramic-based composite polymer electrolytes (CPEs). A basic understanding and
performance index to assess the importance of CPEs over
existing classes of electrolytes is discussed. Various synthesis methods for CPEs fabrications are introduced and
a detailed overview of ongoing research on CPEs regarding SIBs is also intensely scrutinized. The role of extrinsic
fillers (active and passive) and the mechanism involved in
ionic transport in CPEs following recently reported works
are also discussed in correlation with the filler engineering
in addressing the remedies of CPEs. Finally, this review is
concluded with the perspective of CPEs toward the future of
SIB development. This review aims to understand the insight
of fillers within CPEs and their impact on the performance
of SIBs.
2 Requirement for composite polymer
electrolyte
Solid polymer electrolytes generally contain a polymer
matrix with sodium salt without any involvement of liquid solvent in it. Host polymer plays a vital role in shaping
the electrolyte with better flexibility, cost-effective process
ability, high compatibility toward electrodes, and high electrochemical performance. Unfortunately, the acceptance of
solid polymer electrolytes victimizes owing to the low ionic
conductivity and poor electrochemical stability window.
Fig. 4 Physical properties of
ingredients for CPEs in SIBs
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Quest of achieving more superiority develops promising strategies for solid polymer electrolyte revival such
as dropping liquid plasticizer [32, 46], the introduction of
ionic liquids, enthusiastic solvents [16, 56], and finally adding inorganic fillers [17, 44, 45]. Physical characteristics
of ingredients for existing all classes of composite polymer electrolytes are shown in Fig. 4. These practices have
revived solid polymer electrolytes with an inevitable sacrifice of mechanical strength, thermal, and chemical instability. But the addition of fillers led the researchers in achieving their standards of SIBs in a commercial application.
Polymer electrolytes with salt in a plasticized solvent are
termed as solid polymer electrolytes, polymer electrolytes
with salt and inheriting ionic liquids are categorized as gel
polymer electrolytes, and polymer electrolytes incorporating inorganic ceramics are commonly named as composite
polymer electrolytes. The best negotiable solution for overcoming the challenges of ICEs and SPEs is to couple both
types of electrolytes to develop CPEs. Synergistic coupling
of inorganic–organic moieties is a fruitful approach among
the practices to harness the maximum advantage of current
electrolytes. Here, we are focusing only on ceramic fillers
embedded composite polymer electrolytes.
3 Synthesis methods
3.1 In situ polymerization
In situ polymerization is a widely accepted technique for the
preparation of nanocomposites demanding uniform dispersion and direct integration of filler nanoparticles in a bulk
polymer matrix. This process simply involves the filler dispersion in the precursor monomer solution and further proceeds through the polymerization of monomer via standard
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
polymerization reactions to give nanocomposite. To avoid the
agglomeration of nanofillers, the grafting technique allows
the direct covalent linking of fillers over the polymer matrix.
Regarding SIB applications, Chen et al. prepared a
3D-CPE via in situ polymerization of PEGMA monomer
and anion-trapping boron containing crosslinker (B-HEMA)
inside a mechanically supporting PVDF-HFP polymer
matrix containing NaTF and TEGDME as plasticizer [57].
Initially, mechanical supporting PVDF-HFP matrix was prepared using phase transfer method with mass ratio of 1:10
for PVDF-HFP-acetone and water (6% mass fraction). Subsequently, boron-containing CPE was prepare by soaking
the porous PVDF-HFP polymer in the monomer solution
containing mixture of PEGMA (2.5 g), B-HEMA (0.2 g),
NaTF (0.48 g), TEGDME (1.05 mL), and AIBN (0.01 g)
solution for 0.5 h in ambient conditions. As-prepared
3D-CPE was subjected to heat treatment of 70 °C for 12 h
in vacuum. Presence of numerous polar C–C–O– groups
on host polymer chains causes rapid dissociation of NaTF.
Additionally, [–B–O–] crosslinking bonds in CPE act as a
trapping center for anions due to the Lewis acid nature of
boron moieties. Since, boron accepts electronegative anions
from NaTF salt due to its ­sp2 hybridization with an unoccupied orbital. The synthesized electrolyte shows an appreciable ionic conductivity, high transference number, and wide
electrochemical window of 0.26 mS ­cm−1 @40 °C, 0.66, and
5.1 V, respectively. Improved electrochemical performance
is accredited to the structural versatility and availability of
boron moieties in 3D-CPE for SIBs. In another reported
work, poly(methacrylate) (PMA)/poly(ethylene glycol)
(weight ratio of 65:35) based hybrid polymer electrolyte
using in situ polymerization with 5 wt.% nano-α-Al2O3 as
filler and ­NaClO4 salt was reported for all solid-state SIBs.
As-prepared CPE exhibited a high ionic conductivity of
1.46 × ­10−4 S c­ m−1 at 70 °C with a wide electrochemical
stability window (4.5 V vs. N
­ a+/Na) [58]. Similarly, a polyvinyl carbonate (PVC)-based CPE comprising PVDF-HFP
mechanically supported matrix coupled with the organic
plasticizing agent was reported using in situ polymerization
technique. As-prepared PVC-based CPE exhibited appreciable room temperature ionic conductivity of 0.12 mS c­ m−1
with electrochemical window of 5.3 V [59]. CPE also exhibited a maximum transference number of 0.60 which is due
to the high polarity of PVC and TEGDME that restrains
the migration of T
­ f− anions and thus facilitated high N
­ a+
transference value. Yi et al. adopted a versatile approach
by intertwining electrospinning technique with the in situ
polymerization technique. They reported a PMMA polymer-based CPE in which PMMA polymer was filled into
electrospinning derived ­Na3Zr2SiPO12-PVDF-HFP composite membrane using in situ polymerization. Initially,
­Na3Zr2SiPO12-PVDF-HFP composite was prepared using
electrospinning technique and then a CPE was obtained
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using mixture of PMMA, dibenzoyl peroxide, and 1 M of
liquid electrolyte (1 M of N
­ aPF6 in EC/PC, 1:1) via in situ
polymerization. As-developed CPE exhibited an excellent
ionic conductivity, wide electrochemical window, and high
transference number of 2.78 × ­10−3 S ­cm−1, ⁓4.9 V, and
⁓0.63, respectively [60].
3.2 Doctor blade/solution casting
Solution casting is a conventional technique to prepare polymer electrolyte membranes having a thickness ranging from
50 to 300 μm. A wide range of CPEs is developed using this
method with various concentrations of fillers, sodium salt,
and plasticizers. The steps involved during this synthesis of
CPEs are as follows:
➢ Preparation of homogenous and transport solution of
polymer in the desired solvent.
➢ Subsequent addition of salts in a polymer solution.
➢ Addition of filler nanoparticles.
➢ A properly dispersed solution with desired viscosity is
cast on a glass/Teflon plate using doctor blade.
➢ Slow evaporation of solvent for few days.
Very recently, Devi et al. reported a PEO/PVP-based CPE
employing InAs nanowires as fillers using solution casting
technique. Initially, a homogeneous solution of PEO (0.4 g),
PVP (0.1 g), and ­NaPF6 salt with 8:1 molar ratios of O/
Na in methanol was prepared. Further, the filler was added
in stoichiometric proportion (0.5–3.0 wt.%) to the solution.
Solution was then casted into polypropylene petri dishes and
further dried in vacuum for the complete removal of solvent
[61]. Similarly, Yu et al. reported PEO/carbon quantum dots
(CQDs)-based CPE using solution casting technique. PEO
and ­NaClO4 were dissolved in acetonitrile and kept stirring
for 12 h to acquire homogeneity. Further, the solution was
casted on polytetrafluoroethylene (PTFE) plate and dried for
24 h at 70 °C in vacuum atmosphere to get CPE [62].
Kim et al. prepared a NASICON/PVDF-HFP-based
CPE via solution casting using a doctor blade (Fig. 5a)
[48]. Initially, as-prepared NASICON filler was mixed with
PVDF-HFP polymer using planetary ball mill in solvent
(DMAc:acetone, 1:2) for 30 h. Post milling obtained viscous
solution was casted using doctor blade on a Teflon mold
substrate. As-casted membrane was immersed in water for
10 min followed by drying in an oven at 60 °C/12 h and
70 °C/6 h in a vacuum to get a solid polymer membrane.
These membranes were hot-pressed and further soaked in a
standard liquid electrolyte to get CPE. The CPE was again
pressed under 6.3 kPa pressure to remove excess liquid electrolyte (Fig. 5b). Several research works involving the solution casting technique have been reported to synthesize the
flexible quasi-solid state CPEs for SIBs [42, 63–67].
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Fig. 5 a Composite solid electrolyte prepared via doctor blade
method employing filler NASICON powder, host PVDF-HFP polymer and sodium salt (1 M N
­ aCF3SO3/TEGDME). Reproduced with
permission from ref. [48]. Copyright 2014 RSC. b Schematic of flexible all-solid-state sodium battery employing quasi solid electrolyte.
3.3 Phase inversion method
The phase inversion method is the commonly used technique to prepare polymer electrolyte membranes with different functionalities. It is a demixing process in which a
homogeneous polymer solution is transformed from a liquid
state to a solid state in a controlled manner. The structure,
properties, and chemical interaction of CPEs prepared by the
phase inversion method solely rely on the choice of polymer
and additives added in the casting solution. The steps for the
phase inversion methods are as follows:
➢ Initially, polymer solution in a suitable solvent
is casted on a glass sheet or Teflon substrate with the
required thickness.
➢ The casted membrane is immersed in a non-solvent
precipitation bath (e.g., water).
➢ An asymmetric surface structure appears on the
immersed polymer membrane which precipitates rapidly
due to the exchange of solvent and nonsolvent across the
interface.
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Reproduced with permission from ref. [70]. Copyright 2018 Wiley–
VCH. c Schematic diagram of PEO-NASICON composite polymer electrolytes by filling PEO (NaTFSI) solution inside the porous
NASICON-NZSP frameworks. Reproduced with permission from ref.
[69]. Copyright 2016 American Chemical Society
➢ Numerous phase transition occurs during the exchange
between solvent (polymer membrane) and nonsolvent
(water) which depends on the choice of casting solution
and additives.
Verma et al. prepared CPEs based on PVDF-HFP/xTiO2
(x = 0, 0.25, 0.50, 0.75, 1.0, 1.25, and 1.50 wt.%) via phase
inversion technique [68]. Initially, the transparent solution of
PVDF-HFP + DMF + ­TiO2 was prepared via the addition of
PVDF-HFP in the homogeneous suspension of DMF-TiO2.
The obtained solution was poured into a glass petri dish and
kept in a steam chamber at 100 °C. A thick, whitish, highly
porous PVDF-HFP/TiO2 CPE membrane has been obtained
after 2-h exposure. The high porosity of CPE is credited to
the two simultaneous processes during phase inversion, one
the evaporation of DMF at an elevated temperature and the
other one is the diffusion of steam into the pores of PVDFHFP created due to evaporation of DMF from PVDF-HFP
polymer membrane. Further soaking the membrane into the
standard liquid electrolyte (0.5 M ­NaPF6 in EC: PC, 1:1 wt./
wt. ratio) produces a transparent CPE.
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Porous NASICON-Na3Zr2Si2PO12 ceramic frameworks
with controlled porosity synthesized via sol–gel method
were reported by Wang et al. [69]. PEO polymer along with
NaTFSI sodium salt was then filled into the continuous pores
of NASICON-Na3Zr2Si2PO12 ceramic frameworks (10 wt.%,
25 wt.%, and 32 wt.%). In this way, they developed a threedimensional interconnected ceramic-polymer interface with
the CPEs (Fig. 5c).
3.4 Hot pressing method
The hot press technique is a facile, rapid, economically
cheaper, and solvent-free synthesis method to fabricate
polymer electrolyte membranes with good uniformity and
density. Initially, precursor materials such as polymer, salt,
plasticizers, and fillers are finely grounded for a certain duration. As-obtained mixture is heated to a temperature close
to the melting temperature of the host polymer to produce a
homogeneous and dense slurry. This slurry is further sandwiched between two stainless steels and subjected to high
pressure in a controlled manner to give CPEs.
Serra Moreno et al. developed a PEO:NaTFSI-SiO2-based
CPE using solvent-free hot pressing technique [71]. Starting materials like PEO, NaTFSI, and ­SiO2 nanoparticles
were initially dried under vacuum for the respective duration. Ramp-wise pressure was applied to the finely grounded
mixtures obtained after ball milling (17.7 kg ­cm−2/15 min,
141.5 kg ­cm−2/45 min at 90.2 °C) to get various PEO-based
CPEs. In other work, Dalvi et al. reported the PEO-NASICON-based CPE (63PEO-37[PEO1−yNaIy], y = 0.03–0.013)
using hot press method. NASICON nanoparticle synthesized
by solid state reaction with particle size ⁓30 nm was dispersed into homogeneous solution containing PEO blended
with NaI to obtain a viscous slurry. This slurry was further
vacuum dried and subjected to hot press with pressure of
2 tons ­cm−2. As-prepared CPE exhibited a homogeneous
distribution of NASICON nanoparticles in PEO matrix free
from any phase segregation (PEO-NZSP) [72]. Chandra et al.
reported a CPE containing host polymer (PEO), sodium salt
­(NaHCO3), and nanofiller ­(SiO2) for SIB using isostatic hot
pressing method. Fully dried polymer blended with sodium
salt (70PEO:30NaHCO3) was homogeneously mixed with
various concentration of S
­ iO2 nanofillers, with heating treatment at ∼70 °C to obtain slurry. This slurry was further hotpressed between stainless steel blocks to form a mechanically stable membrane [73]. Chandra et al. have also reported
a PEO-based CPE [(1 − x)(75 PEO:25 N
­ aPO 3):xSiO 2]
(x = 0–15 wt.%) via hot pressing technique [74].
3.5 Electrospinning
Electrospinning is a facile and cost-effective technique to produce one-dimensional nanostructures and three-dimensional
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interconnected nanofibrous networks in the form of nonwoven membranes. It is a versatile technique to synthesize
one-dimensional nanostructures, three-dimensional polymer
membranes, inorganic and inorganic–organic hybrid materials. Unlike conventional nanomaterials, uni-dimensional
nanofibers have admirable length, impressive mechanical
strength, and efficient electron migration. These hierarchical nanostructures in the form of nanofibers and nanofibrous
membranes with high surface area, high porosity, tunable
morphology, multi composition, good isotropy, and rich
active sites serve them as a prevalent candidate for electrode
and electrolyte materials for electrochemical energy storage
devices such as electrochemical supercapacitors [6, 75–80].
The performance of SIBs relies on the compatibility of the
electrolyte material. An extensive chemical interaction is
demanded between the electrolyte and the electrode materials to boost electrochemical performance. Owing to this reason, the electrospinning technique is widely used to prepare
the polymer membrane. Electrospun polymer membranes
have a large porosity and high electrolyte uptake, which
increases the viability of the electrolyte material. Electrospun
nanofiber-based separators are promising materials for SIB
applications due to their exceptional chemical stability, high
porosity, appreciable affinity toward organic electrolyte, and
superior electrochemical performance [78–80].
4 Performance index and characterization
of composite polymer electrolytes
The concept of CPE is introduced to overcome the demerits
of existing polymer electrolytes. Nevertheless, an ideal CPE
must have the following characteristics for SIBs (Fig. 6). The
performance index of an electrolyte can be assessed based
on the following traits:
➢ CPEs should exhibit a high ionic conductivity (up to
order ­10−3 S ­cm−1) in temperatures ranging from ambient
to high to execute high cycle and rate performance for SIBs.
➢ CPEs must have a wide electrochemical potential stability window (0 − 5 V) to prevent sluggish reactions with
the electrodes in Na-insertion and de-insertion during the
charge–discharge process of SIBs.
➢ CPEs should inherit superior thermal stability (−40 to
150 °C) and high dimensional stability to avoid deformation while operating at an elevated temperature. In general, the thermal stability of polymer electrolytes causes
severe hazards like short circuits and thermal runway due
to the deformation of polymer electrolytes in SIBs.
➢ CPEs should have a high N
­ a+ transference number
(≈ 1) for preventing ionic concentration polarization and
contributing to high power performance.
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Fig. 6 Performance index of composite polymer electrolytes
➢ CPEs should also have a high mechanical strength to
suppress the intrusion of sodium dendrites. The mechanical stiffness of a CPE is crucial for its application in wearable and flexible energy storage devices.
➢ CPEs should be cost-effective for a wide range of commercial applications in SIBs.
➢ CPEs should be low toxicity and composed of earthabundant elements to procure long-term availability of
energy-driven SIBs.
4.1 Characterization of composite polymer
electrolytes
CPEs are considered as one of the promising electrolyte
alternatives to the rapidly growing SIB market. The electrochemical performance and physical characteristics of
an electrolyte solely rely on the quality of CPEs membranes. The way separator cum electrolytes will perform
under the electrochemical process parameters like cyclic
stability, rate capability, and voltage profiles and delivered ionic conductivity is dependent on the structural and
functional properties of CPEs [14, 22, 66]. Assessment
of structural features of CPEs includes microcrystalline
nature, thickness, wettability, porosity, and electrolyte
uptake. The evaluation of chemical properties of CPEs
includes Raman, FTIR, and NMR spectroscopy (Fig. 7).
13
The functional properties crucial for electrochemical performance include the ionic conductivity, electrochemical stability window, and transport number measurement.
An ideal CPE must have to characterize in terms of its
structural, chemical, and functional characteristics to
bridge electrolyte characteristics with the performance of
SIBs [40, 81]. Essential characterization techniques for the
primary evaluation of CPEs for SIB application are summarized in Table 1. This table does not include the ex-situ
and in-situ studies for post-mortem analysis of SIBs.
5 Composite polymer electrolytes
with passive fillers
Passive fillers refer to the N
­ a + free ceramics which are
implicitly capable in contributing the ­Na + through the
electrolytes. The addition of passive fillers is a promising approach to strengthen the polymer electrolytes in
terms of their physical and electrochemical performance
for SIBs [31, 43, 44]. In SIBs, passive fillers have been
extensively harvested to alter the chemical physics of the
polymer electrolytes at the cost of their reduced crystallinity and glass transition temperature. In this section, we
have summarized the ongoing developments on composite
electrolytes for SIBs based on the passive fillers (Table 2).
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Fig. 7 Summary for research
methods of electrolytes (liquid, polymer, and solid) and
interfaces for SIBs. Adapted
with permission from ref. [22].
Copyright© 2019 Wiley–VCH
The mechanism for the ion transport of such electrolytes
is discussed separately. Different exotic materials such as
boron [57], ­TiO2 [68], ­SiO2 [71, 82–84], E8 liquid crystal [66], ­ZnFe2O3 [85], ­Si3N4 [86], ­Al2O3 [70], CaO [87],
and carbon quantum dot [88] are used as fillers for SIBs.
Implementation of these conventional fillers is gaining
attention for the CPE development in practical SIBs.
Villaluenga et al. developed a novel PEO-SiO2 nanohybrid electrolyte with a high N
­ a+ transference number [82].
The hurdle of ambipolar conductivity in CPEs is jumped by
covalently grafting ­SiO2 nanoparticles with sodium salt and
oligo-PEG chains. ­SiO2-anion and ­SiO2-PEG-anion were
dissolved into a binary mixture of PEO and PEGDME with
a weight ratio 1:1. EO/Na ratio was varied to different values
(40, 20, 10, and 6.5) for further investigation. ­SiO2-anion
(EO/Na ≈ 10) and S
­ iO2-PEG-anion (EO/Na ≈ 20) based
ion-conducting electrolytes exhibit similar ionic conductivity 2 × ­10−5 S ­cm−1. This is due to the reason that S
­ iO2
grafted PEG chains do not contribute to solvation owing to
conformal constraints. ­SiO2-anion (EO/Na ≈ 10) electrolyte
Table 1 Different characterization techniques and their capabilities in the assessment of CPEs
Parameter
Technique
Capability
Morphology
Field emission scanning electron microscopy
(FESEM)
X-ray diffraction (XRD)
•Surface morphology, nanofiber diameter determination
•Average crystal structure
•Crystallinity variation of polymer
•High spatial resolution down to atomic scale in
structure and morphology
•Variation of weight loss with the temperature
•Determination of crystallinity
•Electrolyte uptake capability
•Mechanical stability testing
•Determination of ion association and dissociation,
free anion, ion pair, and ion aggregates in the CPEs
•Bonding information
•Local structure, electronic structure, and ion-dynamics information
•Cation number information
•Ionic conductivity, bulk resistance, grain boundary, activation energy, and interfacial resistance
information
•Electrochemical voltage stability window determination of CPEs
Crystallinity
Transmission electron microscopy (TEM)
Thermal stability
Wettability
Mechanical stability
Functional group
Thermo-gravimetric analysis (TGA)
Differential scanning calorimetry (DSC)
Contact angle measurement
Stress–strain measurement
Fourier transform infrared spectroscopy (FTIR)
Ion pathway
Raman
Nuclear magnetic resonance (NMR)
Transference number
Ionic conductivity
Chronoamperometry
Complex impedance spectrometry
Electrochemical stability window Linear sweep voltammogram (LSV)
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Table 2 Recent reported work on CPEs with passive fillers
Polymer
Sodium salt
Synthesis method
Ionic conductivity
(S ­cm−1)
ESW
Ref
PVDF-HFP-boron (0.5 wt.%)
PVDF-HFP-TiO2 (0.5 wt.%)
PVDF-HFP-Al2O3
PEO-SiO2 (5 wt.%)
PEO-E8 LC
PEO-SiO2-anion
PEO-SiO2-PEG-anion
PVDF-SiO2
PMMA-SiO2 (4 wt.%)
PEO-ZnFe2O4 (0.5 wt.%)
PVDF-Si3N4
PEO-CaO (5 wt.%)
PEO-carbon QD
PVP-ZrO2 (0.5 wt.%)
PVDF-HFP-glass fiber
PVDF-HFP-glass fiber-dopamine
NaTF
NaPF6
NaPF6
NaTFSI
NaIO4
RSO2N(−)SO3CF3
RSO2N(−)SO3CF3
NaCF3SO3
NaClO4
NaCF3SO3
NaClO4
NaClO4
NaClO4
NaPO3
NaClO4
NaClO4
In situ polymerization
Phase inversion
Doctor blade
Solvent-free hot press
Solution casting
Solvent drying
Solvent drying
Solution casting
Solvent drying
Solution casting
Electrospinning
Solution casting
Solution casting
Solution casting
Coating and drying
Coating and drying
0.257 at 40 °C
1.3
1.3 at 25 °C
1.1 at 80 °C (EO/Na ≈ 20)
–
˂10−5 (EO/Na ≈ 10)
˂10−5 (EO/Na ≈ 10)
0.06 at 25 °C
3.5 at 25 °C
0.06 at 25 °C
4.1 at 25 °C
0.049 at 25 °C
0.0717
1.02 at 25 °C
4.6 at 80 °C
5.4 at 80 °C
5.1
3.5
4.3
4.9
–
4.4
3.8
4.1
3.3
4
5.4
5
4.5
2.0
4.8
4.8
[57]
[68]
[70]
[71]
[66]
[82]
[82]
[83]
[84]
[85]
[86]
[87]
[88]
[92]
[93]
[93]
shows an electrochemical stability window of 4.4 V wider
than the S
­ iO2-PEG-anion (EO/Na ≈ 20) having a value of
3.8 V owing to higher sodium concentration.
Ma et al. attempted to resolve the problematic aggregation of nanoparticles used as fillers in the polymer matrices by introducing the well-dispersed carbon QDs of size
2–3 nm diameter in the PEO matrix [88]. The strategy of
using nanoscale fillers of a smaller size such as QDs successfully overcame the hindrance in ionic conductivity. Meanwhile, carbon QDs offer various Lewis sites to increase the
dissociation degree of sodium salts. PEO-CQD-based CPE
exhibits an improved ionic conductivity of 7.71 × ­10−5 S
­cm−1.
Liquid crystals (LC) are preferred over conventional
nanofillers due to their intrinsic features like anisotropy,
self-healing, spontaneous alignment, and good process
ability. Koduru et al. used a nematic liquid crystal as a
filler to fabricate novel polymer/liquid crystal composite
electrolytes [66]. In the nematic phase of the LC system,
linear isotropy of liquid crystal favors the formation of a
high density of ion-conductive paths. PEO/E8 LC system
with 10 wt.% ­NaIO4 salt exhibits the minimum crystallinity and highest ionic conductivity of 1.05 × ­10−7 S ­cm−1.
This enhanced ionic conductivity was accounted based
on the dynamic percolation model following the Lewis
acid–base mechanism. Dielectric measurement studies of
the samples witnessed that the strong interactions among
the sodium salt ions (­ Na+ and I­ O4−), ether oxygen of host
polymer PEO, and functional groups associated with E8
LC (cyanobiphenyl and cyanoterphenyl) increased the
amorphosity of PEO/E8 LC nanocomposite electrolyte.
13
Chen et al. developed an innovative composite electrolyte comprising PVDF-HFP polymer modified with anion
trapping boron moiety (B-CPE) via in situ polymerization
method [57]. As-synthesized electrolyte exhibits an ionic
conductivity of 2.57 × ­10−4 S ­cm−1 with a high transference number (0.66) and improved interfacial stability with
the sodium metal anode. Boron atom inherits Lewis acid
character and accepts the electronegative ions obtained
from the dissociation of sodium salts (NaTF), thus
increased the N
­ a+ transference number of composite electrolyte. To smoothen the ion transport at the electrolyte/
electrode interface as well as inside electrodes, a strategy
of preparing NFM cathode via in situ solidification of polymer electrolyte inside NFM through heat polymerization
reaction was adopted. Consequently, c-NFM/B-CPE/Na
cell exhibits a high discharge capacity of 113.8 mA h ­g−1
with 80.1% capacity retention at 0.2 C (Fig. 8a–f).
Wang et al. reported a full cell comprising N
­ a3V2(PO4)3
(NVP) electrode and PVDF-based composite solid electrolyte in which ­SiO2 nanoparticles were added as fillers
along with N
­ aCF3SO3 salt via conventional solution casting
technique (Fig. 8g–k). The highest ionic conductivity for
as-developed CPE was calculated as 6.0 × ­10−5 S ­cm−1 at
25 °C. Full cell assembled using CPE has demonstrated a
specific capacity and specific energy density of 76 mA h ­g−1
and 126 W h ­kg−1 at 0.5 C rate, respectively. After 100
charge–discharge cycles, the Na cell still retained 70% of its
initial specific capacitance [69].
Xu et al. intruded CaO filler derived from waste egg
shell to develop a PEO/CaO CPE [87]. The strong alkaline
character of CaO enhanced the ionic conductivity 4.5 times
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
2661
Fig. 8 a SEM images of original porous supporting matrix (P-support)
B and b B-PC with an inset image of P-support and B-CPE. c Magnified SEM image of B-PE. d Cross-sectional SEM image of B-CPE.
e Original NFM cathode deposited on Al foil current collector and f
Cross-section image of c-NFM/B-CPE/Na full cell. a–f are reproduced
with permission from ref. [57]. Copyright 2014 Elsevier. g, h FESEM
image of PVDF-SiO2-based CPE surface. i Digital image of circular
CPE film. j, l SEM image of the cross-sectional view of CPE and k
photograph of as-prepared CPE. g–k are reproduced with permission
from ref. [83]. Copyright 2014 Elsevier. l SEM and m cross-sectional
SEM image of P/SN@PBNi electrolyte. l and m are reproduced with
permission from ref. [86]. Copyright 2014 Elsevier
higher than the pristine polymer electrolyte. Additionally,
this filler contributes high mechanical stability and a wide
electrochemical stability window up to 5 V. As-fabricated
Na/PEO-CaO/Na3V2(PO4)3 full cell battery gives an appreciable reversible capacity of 101.2 mA h ­g−1 at 0.5 C after
100 cycles.
Ma et al. perform the electrochemical and SIB characteristics by twining the separator and the electrode [86].
Considering the aspect that PVDF is used as a binder for
the electrode, it is possible to couple the PVDF-based electrode and the separator (Fig. 8l and m). Inspiring from this,
­Si3N4 embedded PVDF (P/SN) electrospun separator was
fabricated with superior characteristics like high-temperature
resistance, good chemical stability, and high dielectric properties. As-developed separator is deposited on the Prussian
blue ­Na2Ni[Fe(CN)6] (PBNi) electrode using electrospinning
technique to get a P/SN@PBNi separator-electrode combination. Crosslinking of –CH2–CF2– with the β-PVDF chains
significantly improves the thermal stability, tensile strength,
and ionic conductivity and decreases interfacial impedance.
P/SN@PBNi-based battery exhibits high initial Coulomb
efficiency (96.8%), appreciable cyclic stability (92.2% after
503 cycles), and superior rate performance (78 mA h ­g−1 at
5 C).
Passive fillers or N
­ a + free ceramics are effectively
enhanced the physical and electrochemical performance.
Ion transport is the major characteristic of CPEs for their
applications in SIBs. Passive fillers are mainly metal oxides
whereas some other nanostructures such as carbon QDs,
­Si3N4, boron, and E8 liquid crystal also lie in this category.
The mechanism for the ionic conduction in such fillers is the
same as the prominent metal oxides. The addition of passive fillers influences the host polymer-sodium salt system
in two ways.
➢ The segmental motion of host polymer chains
The inclusion of fillers in the polymer matrix suppresses
the recrystallization process of the polymer molecular
chains and increases the amorphous regions in polymer electrolytes. These amorphous regions are solely
responsible for the ionic conduction in solid polymer
electrolytes [77, 89–91]. The ionic conductivity of CPEs
increases with a rise in temperature with gradual inhibition of crystallinity of the polymer.
➢ Polymer-passive filler interface
Ion transport of CPEs with passive fillers is also encouraged via Lewis acid-base interactions. Host polymer chains
act as ion transport medium via dipole-dipole interaction.
Passive filters act as a Lewis acid whereas anions of added
sodium salt act a Lewis base. These two entities compete
with each other to form a complex with the host polymer.
In this way, fillers act as a crosslinking center for the poly-
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Aggregation of filler beyond a certain threshold concentration is a severe problem observed in CPEs. To overcome
this, carbon QDs were used as a filler in PEO-NaClO 4
polymer electrolyte. As-prepared PEO-NaClO4-CQD (3
wt.%) based CPE exhibited a substantial enhancement in
ionic conductivity and higher ­Na+ transference number
(0.42) as compared to pristine PEO-NaClO4 polymer electrolyte. Dissociation of sodium salt in PEO was investigated using FTIR analysis. The dissociation ratio for PEONaClO4-CQD (3 wt.%) is found to be 97.9% higher than
the PEO-NaClO4 polymer electrolyte (86.1%). The working
mechanism of prepared CPEs is schematically shown
(Fig. 9b). It is obvious from figure that PEO-NaClO4 polymer electrolyte consists of a good crystallinity with minimal
amorphous regions which provides flexural and lengthy ion
transport pathway for N
­ a+ conduction causing low ionic
conductivity. In contrast, PEO-NaClO 4-CQD (3 wt.%)
based CPE is having a strong Lewis acid–base interaction
among CQD fillers, sodium salt, and host polymer. These
interactions felicitated the segmental motion and large free
volume for ionic transport and also enhanced the dissociation of sodium salt results into high ionic conductivity.
Pristine PEO-NaClO4 system was synergistically coupled with CaO filler derived from waste egg shell [87].
CaO is a ceramic filler material that is more alkaline and
sodiophillic as compared to other remaining metal oxides
reported so far. Successive addition of CaO filler (0.25
wt.%, 0.5 wt.%, 1 wt.%, 3 wt.%, 0.25 wt.%, 5 wt.%, and 7
wt.%) significantly raises the ionic conductivity which was
found to be higher than the commercially purchased CaO
filler. Further increasing the filler concentration beyond 5
wt.% reduces the ionic conductivity values due to the segregation of fillers. The addition of CaO fillers influences
Fig. 9 a Schematic illustration of the mechanism of PVDF-HFPAl2O3-based CPE exhibiting synergistic Lewis acid–base intermolecular bonding host polymer and filler nanoparticles. a is adapted
with permission from ref. [70]. ­Copyright© 2019 Wiley–VCH. b
Schematic diagram of ion transport mechanism in pristine PEO SPE
and PEO/CQDs CPE. b is adapted with permission from ref. [88].
­Copyright© 2018 Wiley–VCH. c Schematic of the mechanism for
ionic transport in PEO-CaO-based CPE. Reproduced with permission
from ref. [87]. ­Copyright© 2020 Royal Society of Chemistry and the
Chinese Chemical Society. d Schematic illustration of ion transport
in B-CPE and C-CPE during charge–discharge process. Reproduced
with permission from ref. [57]. ­Copyright© 2020 Elsevier
mer and the anion. At the filler-polymer interface, the crystallinity of the polymer gets perturbed due to the formation
of a hydrogen bond between the filler and host polymer
(Fig. 9a). This led to the appearance of free volume for ion
conduction. In addition to this, surface groups of the passive fillers act as Lewis acid-base interaction centers with
the alkali salt (Lewis base) and host polymer (O atoms as
Lewis base) felicitating more salt dissociation which synergistically increased the concentration of N
­ a+ and hence
ionic conductivity [35, 43, 44, 46].
13
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
the PEO/NaClO4 polymer electrolyte in two ways. Firstly,
the crystallinity of PEO disrupted upon the intrusion of
fillers facilitating the segmental motion of polymer chains.
Secondly, adsorption/desorption of ­Na+ filler surface promotes rapid ion transport (Fig. 9c).
Chen et al. developed an innovative composite electrolyte comprising PVDF-HFP polymer modified with anion
trapping boron moiety (B-CPE) via the in situ polymerization method [57]. The two full cells with the configuration
of c-NFM/B-CPE/Na and c-NFM/C-CPE/Na were tested
in terms of their electrochemical performance in SIBs.
As tested, c-NFM/B-CPE/Na cell exhibited a high first
discharge capacity of 113.8 mA h ­g−1 with capacity retention of 80.4% of its initial capacity even after 700 cycles
at a current rate of 0.2 C compared to the c-NFM/C-CPE/
Na cell which exhibits a first discharge capacity of only
103.2 mA h ­g −1 with capacity retention of 43.3%. This
improved rate capability is a credit to the existence of low
electrode polarization due to high ionic conductivity and
high ­Na+ transference number. The proposed mechanism
for the improved electrochemical performance is shown
in Fig. 9d. The c-NFM/C-CPE/Na owing lower N
­ a+ ion
transference, resulting in concentration gradients of the
salt and cell polarization during the cycling process, ended
with the deterioration of rate capability. In contrast, the
c-NFM/B-CPE/Na inherits a single N
­ a+ conducting nature.
The presence of anion trapping boron hinders the migration of anions (­ Tf−) and stabilizes the mobility of cations
­(Na+) during the insertion/de-insertion process. This, in
turn, alleviates concentration polarization of electrolytes
during cycling, thus remarkably enhances ­Na+ transport,
Table 3 Recently reported work
on CPE with active fillers
Polymer
2663
leading to the improvement of rate capability, cycling stability, and discharge capacities of SIBs.
6 Composite polymer electrolytes
with active fillers
Unlike passive fillers, active fillers refer to the N
­ a+-containing
ceramics which explicitly supplies the N
­ a+ through the electrolytes. These active filers are having superior characteristics
like high ionic conductivity, good chemical stability, wide
electrochemical stability window, and high rigidity [42, 94,
95]. Active fillers have low activation energies due to the
presence of defects and ionic conductivity is facilitated by
mutual hopping of ions from one site to another site during
­Na+ transport [2, 10, 18]. Importantly, these fillers are capable to supply N
­ a+ for ion transport in SIBs. In LIBs, active
fillers have been adopted extensively exploited to strengthen
the viability of polymer electrolytes. Regarding SIBs, prevailing ionic conductors such as NASICONs are used as a
filler in various polymers to fabricate CPEs [47, 49, 63, 72,
96]. In this section, we have given an overview of current
strategies on CPEs for SIBs based on the active fillers. The
mechanism for the ion transport of such electrolytes is discussed separately. ­Na3Zr2Si2PO12 [62], ­Na3Zr1.8Mg0.2Si2PO12
[63], and ­Na2Zn2TeO6 [65] have been tried as an active fillers
for the CPE development (Table 3). This strategy is gaining
enormous attention due to the improved physical and electrochemical performance of pristine polymer electrolytes.
Kim et al. developed a high-performance CPE inheriting high ionic conductivity, wide electrochemical
Sodium salt
PEO-Na3Zr2Si2PO12 (40 wt.%)
NaFSI
(EO/Na ≈ 20)
PEO-Na3Zr1.8Mg0.2Si2PO12 (40 wt.%) NaFSI
(EO/Na ≈ 20)
Epoxy-Na3Zr2Si2PO12 (30 wt.%)
–
NaTF
PVDF-HFP-Na3Zr2Si2PO12
NaClO4
PVDF-HFP-Na3Zr2Si2PO12
NaTFSI
Na3Zr2Si2PO12-PEO (10 wt.%)
PEO-Na3Zr1.8Mg0.2Si2PO12 (50 wt.%) NaTFSI
(EO/Na ≈ 14)
PEO-PAN-Na3Zr2Si2PO12
NaClO4
NaTFSA
Polyether-Na3Zr2Si2PO12
PEO-Na3Zr2Si2PO12 (25 wt.%)
PMMA-PVDF-HFP-Na3Zr2Si2PO12
NaClO4
NaPF6
Synthesis method Ionic conductivity
(S ­cm−1)
ESW Ref
Solution casting
0.022 at 25 °C
–
[42]
Solution casting
0.044 at 25 °C
4.4
[42]
Polymer filling
Solution casting
Solution casting
Polymer filling
Solution casting
0.145 at 25 °C
0.12
2.25 at 25 °C
0.14 at 25 °C
2.8 at 80 °C
7
6
–
–
4.3
[47]
[48]
[49]
[69]
[63]
Solution casting
Solution casting
with UV irradiation
Solvent drying
Electrospinning
and in situ
polymerization
0.136 at 25 °C
0.010 at 25 °C
4.8
–
[62]
[67]
0.021 at 30 °C
2.78 at 30 °C
–
4.9
[64]
[60]
13
2664
stability window, and exceptional thermal stability [48].
­Na 3Zr 2Si 2PO 12 nanoparticles were uniformly dispersed
in PVDF-HFP polymer by using a binary mixture of solvents, i.e., DMAc:acetone, 1:2. This solution was casted
as a membrane by the doctor blade method and further
immersed in water for 10 min. As-developed CPE was
further activated by the addition of 1 M of NaTF/TEGDME liquid electrolyte. The final weight proportion of
the CPE is found at 70:15:15 for NASICON filler:PVDFHFP:ether-based electrolyte. Electrochemical stability
of ether-based electrolyte uplifts to 5 V from 3 V upon
addition of NASICON fillers. NASICON/PVDF-HFP CPE
shows lower bulk resistance than the NASICON/PVDFHFP composite film with activation with liquid electrolyte.
Small inclusion of liquid improves the ceramic-ceramic,
ceramic-polymer, and electrolyte–electrode interfacial
stabilities. A full cell of hard carbon/NASICON/PVDFHFP/NaFePO4 exhibits 98% Coulombic efficiency with a
discharge capacity of 120 mA h ­g −1 at 0.2 C rate. The
enhancement in flexibility and bendability of the prepared
CPEs has been demonstrated by illuminating LED under
normal static and bent conditions.
Zhang et al. achieved a high ionic conductivity of
2.4 × ­10−3 S ­cm−1 at 80 °C and low interfacial resistance
by embedding NASICON fillers (­Na 3 Zr 2 Si 2 PO 12 and
­Na3Zr1.8Mg0.2Si2PO12 (40 wt.%)) into PEO matrix [42].
The temperature dependence of ionic conductivity values
for both the electrolytes reaches its apex at 40–50 °C owing
to the crystalline amorphous transition of the host polymer. ­Na3Zr1.8Mg0.2Si2PO12 (40 wt.%)-PEO-NaFSI shows
much higher ionic conductivity than pristine PEO-NaFSI.
­Na3V2(PO4)3/PEO-Na 3Zr1.8Mg0.2Si2PO12-NaFSI/Na cell
delivers the highest reversible capacity of 106.1 mA h ­g−1
with an initial Coulombic efficiency of 94%.
Yi et al. prepared a PMMA polymer-filled ­Na3Zr2Si2PO12
(40 wt.%)-PVDF-HFP CPE via in situ polymerization [60].
As-prepared electrolyte exhibits a high ionic conductivity of
2.78 × ­10−3 S c­ m−1, an electrochemical stability window of
4.9 V, and a high cationic transference number of 0.63. Asprepared electrolyte exhibits maximum stress of 6.2 MPa.
The addition of ­Na3Zr2Si2PO12 powder into the PVDF-HFP
nanofibers not only enhanced the mechanical strength but
also reduced the strain of the prepared CPE.
Yu et al. introduced a CPE based on PEO/NaClO4/Na3Zr2Si2PO12 via solution casting technique [64]. As-prepared
solid-state CPE showed an excellent ionic conductivity of
1.45 × ­10−4 S c­ m−1 at 60 °C. Cyclic stability of symmetrical
cell-based Na/PEO-NaClO44-Na3Zr2Si2PO12/Na improves
due to the inclusion of active filler capable of suppressing sodium dendrites. Full cell Na/PEO-NaClO4-Na3Zr2Si2PO12/N0a2MnFe(CN)6 exhibits a discharge capacity of
124 mA h ­g−1 at a current rate of C/10. As-fabricated SIB
shows a stable cycling performance with cyclic stability of
13
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
83% and Coulombic efficiency of 97–99% after being cycled
300 times (Fig. 10h–k).
Wang et al. developed a NASICON/PEO-based ceramicpolymer composite in which the semi-crystalline PEO polymer was used as a filler in the NASICON framework [69].
To avoid the segregation of microsized fillers and to increase
the availability of ceramic-polymer filler interfaces, ceramic
framework-polymer filler strategies have been adopted
instead of the conventional ceramic filler polymer matrix.
Sol–gel derived ­Na3Zr2Si2PO12 framework was synergistically couple with PEO:NaTFSI filler (10 wt.%, 25 wt.%, and
32 wt.%) to develop CPE. Among these, 10 wt.% polymer
embedded CPE shows a thin coating of polymer whereas the
higher concentration of fillers develops the thick coating on
NASICON framework. As-developed CPE with 10 wt.% of
polymer filler exhibited an ionic conductivity of 1.4 × ­10−4
S ­cm−1 at room temperature. Scanning probe microscopy
(SPM) was used to investigate the ­Na+ diffusion at the interface into two modes. The first one is bimodal-AFM that
measures sample hardness and the second is ESM which
detects the surface deformation caused by ion transportation.
Cheng et al. reported a ceramic in polymer hybrid solid
electrolyte (HSE) comprising synergistically coupled polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP)
polymer with ­Na3Zr2Si2PO12 (NASICON) ceramic fillers.
As-synthesized HSE exhibits good flexibility, remarkable
high ionic conductivity of 2.25 × ­10−3 S c­ m−1 at ambient
temperature, and good interfacial stability. N
­ a3V2(PO4)3/C/
CPE/Na-based full cell exhibits an interfacial resistance of
152 Ω [49]. As-fabricated ­Na3V2(PO4)3/C/CPE/Na-based
battery shows a reversible capacity of 98 mA h ­g−1 with a
capacity retention of 85% even after 175 cycles.
Till now, most of the researchers have developed the CPE
with low content of N
­ a+ filler in the host polymer electrolytes. In this regard, Hiraoka et al. synthesized a new polyether/Na3Zr2Si2PO12 CPE with filler content varying from 0
to 300 wt.% into the host polymer matrix [67]. The sole aim
of this study is to harvest the utility of host polyether and
­Na3Zr2Si2PO12 filler to its maximum extent as an electrolyte.
CPE with 30 wt.% of filler concentration in polyether/NaTFSA
exhibited the highest ionic conductivity of 1.03 × ­10−5 S ­cm−1
at room temperature. This is due to the facilitated segmental
motion of polymer chains and promotion of dissociation of
sodium salt upon addition of lower concentration of active
fillers. The positive influence of higher content of filler concentration has a profound effect in interfacial characteristics.
Polyether/NaTFSA-Na3Zr2Si2PO12 (200 wt.%) exhibited the
lowest interfacial resistance and activation energy among prepared CPEs. This superiority is accredited to the presence
of ­Na3Zr2Si2PO12 accelerating ­Na+ transport at the electrolyte–electrode (Na metal) interface. In order to ensure the
safety, Na-ion and Na-sulfur with the respected cells Na/polyether/NaTFSA-(200 wt.%Na3Zr2Si2PO12)/NaCoO2 and Na/
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
2665
Fig. 10 a Interfacial morphology between NMTO ceramic electrode
and all-solid-state CE after 200 galvanostatic cycles, with a low and b
high magnification. c Galvanostatic cycling of Na|electrolyte|Na symmetric cells using PEO (NaTFSI) and NASICON/PEO CE as electrolytes, under constant current of 1 μA cm. a–c are reproduced with
permission from ref. [69]. ­Copyright© 2020 Elsevier. d Illustration
of the NASICON structure of the N
­ a3Zr2Si2PO12 material. e Scanning electron microscopy (SEM) image of the ­Na3Zr2Si2PO12 powder. f SEM image of the PEO-NaClO4-Na3Zr2Si2PO12 membrane. g
X-ray diffraction (XRD) patterns of the ­Na3Zr2Si2PO12 powder, PEO
membrane, and PEO-NaClO4-Na3Zr2Si2PO12 composite membrane,
as well as the reference pattern of the N
­ a3Zr2Si2PO12 compound. d–g
were reproduced with permission from ref. [64]. ­Copyright© 2019
American Chemical Society. h–j Photographs of composite polymer electrolytes with 0 wt.%, 30 wt.%, and 300 wt.% of NZSP fillers. k Variation of ionic conductivities with the temperature exhibited
by composite polymer electrolytes (0–300 wt.% of NZSP). h–k are
reproduced with permission from ref. [67]. ­Copyright© 2020 American Chemical Society. l XRD pattern of filler N
­ a2Zn2Te2O6 nanoparticles. m SEM image of NZTO powder. n Photograph of composite
polymer membrane. o Ionic conductivity of PEO-NaTFSI-Na2Zn2Te2O6 (0–60 wt.%) composite polymer electrolytes. l–o were reproduced with permission from ref. [65]. ­Copyright© 2019 Elsevier
polyether/NaTFSA-(200 wt.% N
­ a3Zr2Si2PO12)/sulfur modified polyacrylonitrile (SPAN) cells were operated at 333 K
(Fig. 10d–g).
Wu et al. developed a CPE based on PEO embedded
with novel sodium-ion-conducting filler, i.e., Ga-doped
­Na2Zn2TeO6, via solution casting technique [65]. CPEs with
different concentration of filler (0%, 10%, 20%, 30%, 40%,
50%, and 60%) have been prepared. The PEO-NaTFSI/NZTO
(50 wt.%) CPE exhibits a remarkable ionic conductivity of
4 × ­10−5 S ­cm−1 at an ambient temperature which was further elevated to 1 × ­10−3 S ­cm−1 at 80 °C with an electrochemical stability window of 4 V (Fig. 10l–o). The tensile
strength is uplifted from 0.15 to 0.54 MPa (PEO/NaTFSI)
whereas the tensile moduli raised from 1.3 to 14.2 MPa
for CPE. N
­ a2V3(PO4)3-based cathode was fabricated using
PEO:NaTFSI as a binder and ion-conducting network formed
at the electrolyte–electrode interface. Three batteries were
fabricated using three different types of cathodes with CPE
13
2666
against sodium metal anode (1) nanostructured ­Na2V3(PO4)3
with CPE, (2) microcrystalline N
­ a2V3(PO4)3 with CPE, and
(3) PEO:NaTFSI solid electrolyte with microcrystalline
­Na2V3(PO4)3. Among the three prototypes, the first battery
shows a minimal internal resistance of 250 Ω while the third
one exhibits the highest value of internal resistance, i.e., 550
Ω. The discharge capacities of the batteries were found to be
105, 95, and 72 mA h ­g−1, respectively.
Depending on concentration of active fillers in host polymer, CPEs may be “inorganic rich” (˂50 vol%) or “polymer
rich” (inorganic content > 50 vol%) [14]. In case of inorganicrich CPEs, ion conduction is expected through the 3D percolation network that critically relies on particle size and
dispersion. The overall ­Na+ conductivity is dependent on
their volume fraction and possible ion-conduction pathways.
In case of polymer-rich CPEs, the maximum contribution in
ionic transport is expected from polymer network; the probability of migrating through inorganic fillers is less. Overall the
ionic conductivity, the ionic conduction could be facilitated
through (a) combined polymer electrolyte–ceramic particles
pathways, (b) interface region between polymer and inorganic
particles, (c) the polymer electrolyte phase (polymer-rich
CPEs), and (d) inorganic electrolyte bulk phase (inorganicrich CPEs) [27, 28, 46].
Lim et al. suggested a substantial solution for facilitating ceramic contribution through the induction of ion transport channels in the oxide-based solid electrolyte using a
top-down method [47]. They filled the epoxy-resin polymer within the internal pores of oxide-based NASICON
electrolyte resulting in the improvement in physical properties and electrochemical performance of the electrolyte.
Using this method, the physical properties of the CPE is
expected to increase due to use of epoxy-resin, while the
ionic conduction of NASICON is well due to formation of
separate ion transport channel. Bare NASICON pellet was
filled with epoxy-resin polymer material followed by 5-min
curing under UV illumination. Consequently, the high ionic
conductivity of 1.45 × ­10−4 S ­cm−1 was achieved with exceptional electrochemical stability of 7 V and thermal stability
up to 400 °C. As-fabricated NVP/NASICON/Na exhibited
a specific capacity of 120 mA h ­g−1 at 0.1 C rate.
Yu et al. adopted a dual-layer approach to prepare the
CPE for ambient temperature SIBs [62]. The dual-layer
strategy took the benefit of anode compatible layer from
one side whereas the other side is cathode friendly. A novel
laminated electrolyte was developed using PAN-SN-Na3Zr2Si2PO12/PAN-Na3Zr2Si2PO12-NaClO4 that exhibited an
excellent ionic conductivity of 1.36 × ­10−4 S ­cm−1 with an
electrochemical stability window of 0–4.8 V. A full cell was
fabricated using Na-metal anode and N
­ a2MnFe(CN)6 cathode which exhibited high capacity and appreciable cyclic
stability.
13
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
6.1 Ion transport mechanism
Active fillers refer to the ­Na+-containing ceramics having
the potential to strengthen the physical and electrochemical performance of the host polymer. Highly targeted solidstate sodium-ion conducting ICEs are actively employed
as an active filler for the CPE preparation. Pure and alkali
metal/alkaline earth metal doped ­Na 3Zr 2Si 2PO 12- and
­Na2Zn2TeO6-based ICEs are exploited as active fillers owing
to their high ionic conductivity, high rigidity, and wide electrochemical stability window. Intrinsic bulk ionic conductivity is due to the presence of continuous defects causing
low activation energy for multiple ions hopping rather than
single ion movement. The role of these fillers is more enthusiastic than the passive filler as they provide more pathways
for the ions to migrate and facilitate conductivity in CPEs.
Usually, in active filler-based CPEs, the ­Na+ conduction is
through the polymer-ceramic interface and the segmental
motion of polymer chains but in contrast, active filler-based
CPEs provide a new intrinsic path through their filler’s surface. In this way, they contributed implicitly to the enhancement in ionic conductivity of CPEs.
Kim et al. in 2015 reported the active filler-based CPEs
for the first time in SIBs. They reported a N
­ a3Zr2Si2PO12
embedded PVDF-HFP electrolyte containing ether-based
liquid electrolyte [48]. ­Na+-containing CPE exhibited a
maximum ionic conductivity of 1.4 × ­10−3 S ­cm−1 at an
elevated temperature of 90 °C. This ionic conductivity
value is larger than the comprising liquid, inorganic filler,
and host polymer-based electrolytes. They recommended
three possible migration paths for N
­ a+ conduction (1) hopping of ions through the ceramic surface, (2) plasticizer
transport of liquid electrolytes, and (3) cross ion hopping
through a polymer-ceramic and ceramic-liquid interface.
Doped NASICON filler-based CPEs are reported using the
same strategy and with different solvents and polymers.
Wu et al. used a Ga-doped N
­ a2Zn2TeO6 filler filled PEO
CPEs for SIB development. Different weight percentages
of filler (0%, 10%, 20%, 30%, 40%, 50%, and 60%) were
dispersed in PEO:NaTFSI-based solid polymer electrolyte
system [65]. CPE prepared with 50 wt.% ­Na2Zn2TeO6-PEO/
NaTFSI exhibited maximum ionic conductivity 3 × ­10−3 S
­cm−1 at a temperature of 80 °C which is highest among all
the reported PEO-based CPEs for SIBs. This ionic conductivity for the higher concentration of NZTO (40 wt.%) CPEs
is due to the above melting temperature of PEO, the crystallinity of host polymer vanishes, and the ion transport due to
connected NZTO fillers is dominant. On the other hand, the
raised ionic conductivity for lower concentration is due to
suppression in crystallization of PEO polymer.
Lim et al. reported an interesting work using a top-down
approach to prepare a polymer in ceramic electrolyte rather
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
2667
than the trending ceramic in polymer-based CPE where the
contribution of ceramic fillers is negligible and maximum
ionic conductivity is due to host polymer chains [47]. Initially, NASICON solid electrolyte was sintered to introduce
ion transport channels and further as-sintered electrolyte was
filled with epoxy polymer to develop epoxy-NASICON CPE.
The physical strength of the CPE was doubled; however,
the thermal and electrochemical performances have been
retained (Fig. 11a).
Cheng et al. in 2020 reported the reason for ionic conductivity enhancement in ceramic in polymer hybrid solid
electrolyte comprising NASICON nanoparticles embedded
PVDF-HFP polymer matrix [49]. As-prepared hybrid solid
composite electrolyte facilitated a migration pathway for
sodium-ion conduction during its charging and discharging.
They suggested four possible pathways for the sodium-ion
conduction particularly termed as intrachain hopping of
sodium ions, interchain hopping of sodium ions, and through
Fig. 11 a Comparison of the ionic conductivity effect in different
ceramic ratios in a ceramic-polymer composite state. Reproduced with
permission from ref. [47]. ­Copyright© 2020 from the Royal Society
of Chemistry. b and c Schematic diagram of ionic transport in host
PVDF-HFP polymer and CPE. Reproduced with permission from ref.
[49]. ­Copyright© 2020 Springer Nature
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Advanced Composites and Hybrid Materials (2022) 5:2651–2674
polymer and polymer-NASICON interface, respectively
(Fig. 11b–c). The ionic-conductivity enhancement in the
reported electrolyte is accredited to the following four aspects:
➢ NASICON fillers in the electrolyte perturbed the crystallinity of the host PVDF-HFP polymer which in turn
facilitates the segmental motion of the polymer. This provides the more expanded region for characteristic sodiumion conduction through the electrolyte.
➢ NASICON-PVDF-HFP interface causes the permeation effect which creates an interconnected transmission
channel on the surface of the fillers, playing a vital role
in sodium-ion conduction.
The difference in sodium-ion concentration between
NASICON fillers and host PVDF-HFP polymer induces
a concentration gradient in an electrolyte which enthusiastically contributes polymer electrolytes to absorb more
sodium ions. Owing to this, there is an increase in Na-ion
vacancy promoting ionic transport.
➢ The presence of highly Na-ion conductive NASICON
implicitly contributes to the ionic conductivity improvement.
7 Electrolyte–electrode interfacial
characteristics
The major obstacle in building high-performance CPEbased SIBs is the poor interaction between CPEs with the
electrodes which increases the interfacial resistance. A high
mechanical, chemical, thermal, and electrochemical stability
is crucial to suppress sluggish charge transfer reaction that
occurs at the electrode–electrolyte interface [14, 20, 22, 23].
The fundamental concept of formation of solid electrolyte
interface is deliberately shown in Fig. 12. Accordingly, the
solid electrolyte interface is formed when the redox potential of the electrode materials exceeds the electrochemical
potential window of the electrolyte. Constructing excellent
interfaces between ceramic fillers and polymer chains in
SCEs could effectively solve the intrinsic problems of SCE
to achieve high-performance all-solid-state batteries. The
stability of the electrolyte depends upon the LUMO (lowest
unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) gaps of the electrolyte and Fermi
level of the electrodes. [14]. The electrolyte would be stable when the lowest unoccupied molecular orbital (LUMO)
of the electrolyte is higher than that of the Fermi energy
of anode. Similarly, when the highest occupied molecular
orbital (HOMO) of the electrolyte is lower than that of Fermi
energy level of cathode [15, 26, 34, 97].
Particularly, in the case of CPEs which contain both inorganic solids and polymer electrolytes, the rigid nature of
inorganic ceramic electrolytes is having poor compatibility
13
Fig. 12 Schematic open circuit energy diagram of an electrolyte. The
term μA and μC are the redox potential and ϕA and ϕC are the work
function of anode and cathode, respectively. ­Eg is the electrochemical
potential stability window of the electrolyte
with the electrodes and suffers from a large interfacial resistance. On the other hand, polymer electrolytes are more
adaptable toward interfacial stability. During operations of
SIBs, they formed a stable solid electrolyte interface which
is steadily conductive to ­Na+ but exhibits insulating behavior
for electron migration [18, 20, 33, 36, 96, 98]. This capability of polymer electrolytes plays a vital role in improving cycle life, capacity loss, rate capability, and safety concerns among SIBs [31, 40, 99, 100]. A tailored solution for
minimizing the challenge of high interfacial resistance is
to couple both the electrolytes rationally in terms of forming electrode–electrolyte interface layers [14, 26, 34, 49].
Tremendous research work has been reported for improving
electrode–electrolyte interfacial characteristics in SIBs.
Xie et al. reported a SIB comprising a quasi-solid-state
electrolyte and graphite along with graphite and Sn foil as
cathode and anode, respectively (Fig. 13a and b). The use of
Sn foil as both anode and current collector replaced the anode
active materials different strategy to enhance the energy density of as-developed SIB compared to conventional SIBs [70].
Regarding CPEs in SIBs, a strategy of introducing thin
film coating on the CPE is a promising approach for extending improved electrochemical performance. Gao et al.
enhanced the mechanical strength of PVDF-HFP polymer
electrolyte by providing commercial glass fiber as mechanical support [93]. But, the hydrophobic nature of PVDF-HFP
impedes the flow of liquid electrolytes causing poor battery
performance at high charge–discharge densities. This problem was overcome by a thin coating of hydrophilic polydopamine over PVDF-HFP electrolyte. The retention of discharge capacity for glass fiber/PVDF-HFP/polydopamine
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
2669
separator is found to be 89.4% higher than that of glass fiber/
PVDF-HFP separator which shows 84.1% discharge capacity after 100 cycles. Glass fiber/PVDF-HFP/polydopamine
separators show a stable Coulombic efficiency of 99.7% after
100 cycles compared to glass fiber/PVDF-HFP separators
(99.4%). This might be due to strong adhesion between electrode and electrolyte. The presence of catechol moieties on
the polydopamine backbone provides strong adhesion to the
substrates. Additionally, this mussel-powered adhesive is
capable of buffering the volume changes of electrode materials during repeated charge–discharge processes.
Yi et al. used a Na/C cloth as an anode material instead of
Na metal. They developed a PMMA polymer-filled NZSP/
PVDF-HFP electrospun nanocomposite membrane via in situ
polymerization [60]. Full cell ­Na0.67Ni0.23Mg0.1Mn0.67O2/
PMMA-NZSP/PVDF-HFP/Na/C exhibited remarkable rate
capability with an initial discharge capacity of 96 mA h ­g−1
and excellent cyclability of 95% after 600 cycles. The internal resistance of Na/C symmetrical cell exhibited a value of
130 Ω, 160 Ω, and 270 Ω after 1st, 20th, and 40th cycles,
respectively, which is lower than the corresponding Nabased symmetrical cell exhibiting an internal resistance of
120 Ω, 520 Ω, and 800 Ω after 1st, 20th, and 40th cycles,
respectively. A higher value of internal resistance after discharge is due to activation of Na/C electrode after the first
discharge process. To smoothen the ion transport at the electrolyte/electrode interface as well as inside the electrodes, a
strategy of preparing a composite NFM cathode via in situ
solidification of PE inside NFM through heat polymerization
reaction was adopted.
A higher concentration of polymer in ceramic also contributes to improving electrolyte–electrode interfacial characteristics. Hiraoka et al. found that polyether/NZSP (˂200
wt.%) based CPE exhibited low interfacial resistance with
Fig. 13 a Schematic illustration of the configuration and working
mechanism SIB during its charging process. a is adapted with permission from ref. [70]. ­Copyright© 2019 Wiley–VCH. b Impedance
spectra of SIBs with glass fiber, HSE, and NASICON ceramic elec-
trolyte. c Schematic diagram of ionic transport in ceramic electrolyte. Reproduced with permission from ref. [49]. ­Copyright© 2020
Springer Nature
13
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Advanced Composites and Hybrid Materials (2022) 5:2651–2674
Fig. 14 Future enhancements of
oxide ceramic-based composite electrolytes. Reproduced
with permission from ref. [47].
­Copyright© 2020 from the
Royal Society of Chemistry
Na metal compared to the NZSP free system due to high N
­ a+
transference number [67]. In a low concentration of NZSP
(> 100 wt.%), the increase in N
­ a+ transference number is
influenced by dissociation of NaTFSA salt upon addition
of NZSP. At higher concentrations, the CPE accumulates
a large number of NZSP particles and causes aggregation
of NZSP near CPE/Na interface. Charge transfer energy is
lower in high concentration CPE (˂200 wt.%) that is lower
than the desolvation energy of the solvated ether oxygen of
the polymer. This would enable higher concentration CPEs
in SIBs with high charge–discharge rates owing to their
rapid ion migration at CPE/Na interface.
Cheng et al. reported a PVDF-HFP/NZSP CPE activated
by a liquid electrolyte. N
­ a3V2(PO4)3/C/CPE/Na-based full
cell exhibits an interfacial resistance of 152 Ω very much
than smaller that of NASICON-based battery with interfacial
resistance of 792 Ω [49]. As-fabricated N
­ a3V2(PO4)3/C/CPE/
Na-based battery shows a reversible capacity of 98 mA h ­g−1
with a capacity retention of 85% even after 175 cycles
(Fig. 13b–c). In general, extraction of ­Na+ from the cathode
is accomplished at the cost of unavoidable volume changes
in the electrodes resulting in the formation of voids. These
vicinities are hardly full-filled by NASICON nanoparticles
causing high electrode–electrolyte interface resistance. Asfabricated CPEs provide compatibility and mechanical stiffness at the interface in the form of interconnected migration
paths during battery operation.
8 Conclusion and future outlook
In conclusion, we have reviewed the basic understanding
and recent developments of composite polymer electrolytes
(CPEs) for SIBs. Recent work undergoing electrolyte development for sodium-based energy storage is elucidated in this
13
review. The necessary aspect regarding synthesis, assessment, and mechanism of CPEs in SIB is describes correlating their performance indices. Still, scientists are facing the
practical ability of CPEs in SIBs and new paradigms and
efforts are still required to bridging the gap between experimental and commercial fulfillment. Major challenges still
need to be confronted are as follows:
(1) Low ionic conductivity is still barricading in meeting
the performance comparable to the highly conductive liquid electrolytes. Novel polymer materials and
respective fillers with high ionic conductivity should
be explored. Regarding SIBs, the choice of fillers is
restricted up to two or three prevailing inorganic electrolytes although a large number of ICEs for SIBs are
available.
(2) Host polymer for CPEs should be tuned with rational
engineering techniques aiming to create more amorphous entities to improve the ionic conductivity. Functionalization recipes such as copolymerization, crosslinking, and grafting should be practiced with modified
synthesis.
(3) Diagnosing the interfacial compatibility to maximize
electrochemical performance within CPEs as well as
across CPEs and electrodes (cathodes/anodes) by introducing additives in electrolytes, laminated structure
development, unifying electrode–electrolyte materials
and novel interfacial architectures (Fig. 14).
(4) Minimization of cost for the SIB development is a crucial demand for SIBs to be manufactured at a larger
scale. Implementation of low-cost materials, bioinspired polymers, and sustainable precursors should
be accelerated to reduce prices.
(5) Design of novel electrolyte system for building flexible,
transparent, and electrochromic SIBs.
Advanced Composites and Hybrid Materials (2022) 5:2651–2674
Funding Dr. AS gratefully acknowledges the UGC, New Delhi, for
their financial supports under the BSR Mid-Career award scheme (No.
F.19–214/2018).
Declarations
2671
16.
17.
Conflict of interest The authors declare no competing interests.
18.
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Publisher's Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
Mr. Dheeraj K. Maurya is working
as a research scholar at Centre for
Nanoscience & Technology,
Pondicherry University, Puducherry, India. He has received his
M.Sc. (Applied Physics) and
M.Phil. (Physics) in 2014 and
2016, respectively from BBA University, Lucknow. His research
interest includes development of
polymer nanocomposites for
energy storage and sensors
applications.
D. Ragupathy received his master
degree in 2005 from Bharathiar
University, Coimbatore, India and
Ph.D. degree from Kyungpook
National University, Daegu, South
Korea in 2010. He did postdoctoral work at Kyungpook National
University and Pusan National
University, South Korea. He is
currently working as Assistant
Professor in the Department of
Chemistry, National Institute of
Technology Puducherry, Karaikal,
India. He is recently focusing his
13
research attention on interdisciplinary topics covering synthesis ofconducting polymers, nanostructuring of materials, electrochemistry and
deviceapplications.
Zhanhu Guo earned his Ph.D.
degree in Chemical Engineeringfrom Louisiana State University in
2005. He received his three-yearpostdoctoral training in mechanical and aerospace engineering at
the University of California Los
Angeles. He is an Associate Professor in Chemical and Biomolecular Engineering and directs the
Integrated Composites Laboratory
at The University of Tennessee,
Knoxville. His current research
interests are inthe areas of optical,
optoelectronic, electric, magnetic,
and dielectric materials for energy conversion/storage, electromagnetic
interference (EMI) shielding, catalysis, sensing, and electronics.
Subramania Angaiah is currently
working as a Professor at the Centre
for Nanoscience and Technology,
Pondicherry University, India. Heobtained his Ph.D. degree from Alagappa University & CSIR-CECRI
(2001) and his postdoctoral training
from the Korean Institute of Science
and Technology(KIST), Seoul,
South Korea. His current research
interests include the development of
nanostructured materials for metalion batteries, super capacitors, quantum dot sensitized solar cells, dyesensitized solar cells and perovskite solar cells.