Chemical Engineering Journal 367 (2019) 230–238
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Hybridizing poly(vinylidene fluoride-co-hexafluoropropylene) with
Li6.5La3Zr1.5Ta0.5O12 as a lithium-ion electrolyte for solid state lithium metal
batteries
T
⁎
Juan Lua,1, Yanchen Liua,1, Penghui Yaoa,1, Zhiyu Dinga, Qiming Tanga, Junwei Wua, , Ziran Yeb,
⁎
Kevin Huangc, , Xingjun Liua
a
b
c
Shenzhen Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China
Department of Applied Physics, Zhejiang University of Technology, Hangzhou 310014, China
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
LLZTO electrolyte was compo• Garnet
site with polymer PVDF-HFP electrolyte.
solid electrolyte exhibited
• Hybrid
good electrochemical performance.
The Li/LiFePO cell showed superior
• cycling
performance with virtually no
4
capacity loss.
A R T I C LE I N FO
A B S T R A C T
Keywords:
Solid electrolyte
Polymer
Ceramic
Ionic conductivity
Capacity
Polymer/ceramic composite solid electrolyte is an appealing solution for the exploitation of flexible solid-state
lithium-metal batteries. Here we report a solid-state Li-ion electrolyte composing of poly(vinylidene fluoride-cohexafluoro propylene) (PVDF-HFP) polymer, ceramic powder Li6.5La3Zr1.5Ta0.5O12 (LLZTO) and lithium salt
LiTFSI. The composite electrolyte exhibits a high ionic conductivity of 8.80 × 10−5 S·cm−1 at room temperature. A coin battery with LiFePO4 cathode is cycled under 0.5 C at room temperature for long cycles, achieving a
Coulombic efficiency of 99.6% without virtually capacity loss (1st: 101.4 mAh·g−1 and 500th: 110.9 mAh·g−1).
Such excellent performance can be ascribed to the formation of high ionic conductivity by LLZTO active garnet
reducing polymer crystallinity. These results show that the developed polymer/ceramic composite has potential
to be a high-performance electrolyte for solid-state lithium-metal batteries.
1. Introduction
Attributed to the high voltage and long cycle life, rechargeable lithium-ion batteries (LIBs) have become a primary and indispensable
power source for our daily life [1–3]. However, the relatively low energy density, high cost and concern on safety of commercial organic
liquid electrolyte based LIBs are major barriers for the modern LIB
technology to expand from thinner electronic products such as mobile
⁎
Corresponding authors.
E-mail addresses: junwei.wu@hit.edu.cn (J. Wu), huang46@cec.sc.edu (K. Huang).
1
These authors are contributed equally to this work.
https://doi.org/10.1016/j.cej.2019.02.148
Received 13 November 2018; Received in revised form 23 January 2019; Accepted 20 February 2019
Available online 21 February 2019
1385-8947/ © 2019 Published by Elsevier B.V.
Chemical Engineering Journal 367 (2019) 230–238
J. Lu, et al.
ceramic powders into a polymer matrix. The resultant hybrid electrolyte exhibited an ionic conductivity of 5 × 10−4 S·cm−1 at room temperature, High mechanical strength and excellent thermal stability. In a
LiCoO2/Li cell composed with this composite electrolyte, the initial
discharge specific capacity reached 150 mAh·g−1 and with a high capacity retention rate of 98% after 120 cycles at 0.4 C at 25 °C. Zhang
et al. [18] also fabricated a hybrid polymer/ceramic LICEs by incorporating ∼40 nm LLZTO particles into a PEO/LiTFSI matrix. And
the ionic conductivity was almost 100 times higher than those with the
micron-sized LLZTO, reaching a ionic conductivity of 2.1 × 10−4
S·cm−1·cm−1 at 30 °C and 5.6 × 10−4 S·cm−1 at 60 °C. With LiFePO4
(LFP) and LiFe0.15Mn0.85PO4 (LFMP)-based cathodes, the Li-metal as
the anode, and the PEO/LLZTO hybrid electrolyte membranes were
assembled into pouch cells. The results showed that both cells could be
cycled for more than 200 cycles with 90% capacity retention at 0.1 C
and 60 °C. Liang et al. [23] reported a composite gel polymer electrolyte
(GPE) of Li7La3Zr2O12 (LLZO) particles which was hybridized with
PVDF-HFP by a simple mixing method. The hybrid electrolyte achieved
a high ionic conductivity of 3.71 × 10−4 S·cm−1 at 25 °C. In a Li/GPE/
LiFePO4 cell, the initial discharge capacity reached 163.1 mAh·g−1 and
with capacity retention of 83% after 200 cycles at 0.2 C. However, most
of solid-state batteries with hybrid electrolyte developed so far show
poor cycle life, typically less than 300 cycles even at lower current
densities.
Here we demonstrate a hybrid solid electrolyte (HSE) based on
PVDF-HFP and LLZTO composite, prepared by a simple solution casting
of PVDF-HFP polymer filled with submicron-sized active LLZTO. A
solid-state lithium metal cell based on LFP cathode, PVDF-HFP/LiTFSI/
LLZTO and Li metal anode shows a high and stable capacity at 0.5 C for
500 cycles.
phones to new energy electric vehicles, and ultimately renewable energy storage. Overcoming these obstacles in the coming years with
existing material systems and engineering designs will be very challenging. Invention of new functional materials, recognition of novel
storage chemistry and advancement in computational materials science
are considered three viable solutions to develop the next-generation
rechargeable batteries for scaled-up energy storage applications [4,5].
LIBs using solid-state electrolytes are widely considered a promising
next-generation advanced battery technology to overcome the barriers
of low energy density and insecure safety [6,7]. Therefore, there have
been significant research activities worldwide in recent years. A key to
the success of solid-state Li-metal batteries (LMBs) is the development
of solid-state Li-ion conducting electrolytes (LICEs). Currently, LICEs
are being developed in inorganic sulfides [8,9], ceramic oxides [10,11],
glasses and organic polymers [12,13]. Solid polymer electrolytes (SPEs)
have attracted attention due to their high flexibility, good viscoelasticity and processing friendly mechanical properties. However, their Liion conductivity (σ) and transport number (t) at room temperature are
generally low (σ < 10−6 S·cm−1 and t ≈ 0.2–0.4, respectively) [14].
Among SPEs, poly(vinylidene fluoride) (PVDF) is attracting a lot of
attention due to its nice electrochemical stability, high solubility in
dispersing media, and high thermal stability [15]. The introduction of
amorphous phase hexafluoropropylene (HFP) into PVDF, i.e. PVDFHFP, can increase the fluorine content with lower degree of crystallinity, leading to enhanced hydrophobicity and solubility in various
solvents [16,17]. In the early studies, PVDF-HFP was mainly used for
gel polymer electrolytes since its porous structure can absorb and immobilize liquid electrolytes [18]. However, the low mechanical
strength of gel electrolytes presents a challenge to membrane processing. On the other hand, the ion transport is known to mostly occur in
the amorphous region [17]. Therefore, polymers with high crystallinity
would not favor ionic transport. To increase ionic conductivity, the
crystallinity of a polymer must be reduced. The simplest and most effective method to minimize the crystallization of polymers is to introduce inorganic particles into the polymer matrix so that disorder
regions can be created with minimal crystallinity. Inorganic fillers (e. g.
Al2O3, ZrO2, SiO2 and TiO2) [19–22] have been previously reported to
be effective in reducing polymeric crystallization, thus retaining the
ionic conductivity of amorphous polymers [23]. However, inorganic
fillers are non-electrochemical active. Too much filler could in fact
decrease the conductivity and increase the mass of the battery.
Among inorganic LICEs, garnet-structured oxides such as the tantalum doped lithium lanthanum zirconium oxide (Li6.5La3Zr1.5Ta0.5O12,
LLZTO) show an Li-ion conductivity of 10−4–10−3 S·cm−1 [24–26].
With ceramic garnet-based LICEs, lithium metal can potentially be used
as the anode to improve the energy density with a broadened electrochemical window. However, different from SPEs’ properties, the garnetbased LICEs are hard and brittle, leading to a poor contact with electrodes and high interfacial resistance [27]. Therefore, hybridizing
ceramic LICEs with SPEs to fully utilize the complementary properties
of the two is an appealing strategy for the development of solid-state
LMBs.
There are significant research and development activities in hybrid
polymer/ceramic LICEs in the literature. A common practice is to use
the ceramic phase as a Li-ion conductor as well as a filler to inhibit
polymer crystallization. For example, He et al. [28] designed a lowimpedance integrated all-solid-state lithium battery using a PL (PEOLiTFSI) matrix loaded with 10% LLZO nanowires. The melting of the PL
and the electrolyte at a high temperature makes the entire battery an
integrated whole, which not only effectively reduces the impedance at
the interface between the positive electrode and the electrolyte, but also
facilitates ion conduction inside the positive electrode. In addition, the
integrated structure can accommodate volume changes during electrochemical cycling, enhancing affinity and contact stability between
the positive electrode and the electrolyte. Zhang et al. [15] demonstrated a PVDF/LLZTO composite electrolyte by dispersing LLZTO
2. Experimental procedure
2.1. Preparation of Li6.5La3Zr1.5Ta0.5O12
The garnet Li6.5La3Zr1.5Ta0.5O12 was prepared by the conventional
solid-state reaction. LiOH·H2O (Aladdin, 99.9%), La2O3 (Aladdin,
99.99%, heated at 900 °C for 12 h prior to use), Ta2O5 (Aladdin, 99.5%)
and ZrO2 (Aladdin, 99%) powders were weighed in stoichiometric ratios. A 15% excess of LiOH·H2O was added to recompense the possible
Li loss during the high temperature sintering. The raw powders were
then ball-milled in isopropanol for 12 h. After drying, the mixed powders were sintered at 900 °C for 12 h to ensure that the cubic LLZTO
phase is obtained. The obtained cubic LLZTO powders were further
ball-milled for 12 h again and then pressed into pellets (ϕ13 mm). The
pellets were finally sintered at 1150 °C for 6 h. To avoid Al contamination during sintering process, all the samples were contained in
MgO crucibles with a cover of the parent powders. After the sintering is
completed, the ceramic pellets are moved to a glove box for storage.
2.2. Preparation of hybrid solid electrolyte (HSE) membranes
The Li-ion conducting polymer was prepared from PVDF-HFP
(Arkema, France, molecular weight about 1000 k) and lithium salt
LiTFSI or LiClO4. Before use, both PVDF-HFP and lithium salt were first
vacuum dried at 100 °C for 24 h, then dissolved in N,N-dimethylformamide (DMF) with a weight ratio of 3:1, followed by stirring
at 65 °C for 4 h to obtain a well-distributed solution. Subsequently,
LLZTO powder was added into the homogeneous solution with different
weight ratios (0–40%). Then the obtained suspension was constantly
stirred at 65 °C for another 12 h to ensure that the LLZTO powders were
completely dispersed in the suspension. Casting the degassed suspension onto a Teflon mold and dried at 120 °C for 48 h, resulting in
∼80 µm thick electrolyte membrane. During the drying process, the
mold was covered by a porous tin foil to reduce the unevenness of the
membrane caused by partial evaporation of the solution. Finally, the
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uniform HSE membranes were punched into ϕ16 mm disks and placed
in glove box for further characterization.
a cubic phase (c-LLZTO with space group Iad and lattice parameter of
12.889 Å) (JCPDS 80-0457). With the addition of LLZTO powders and
lithium salt LiTFSI, the crystallinity of PVDF-HFP is reduced, as evidenced by the weakening of the characteristic amorphous peak intensity. However, no obvious change in diffraction peaks was observed
for c-LLZTO before and after hybridizing, suggesting that c-LLZTO and
PVDF-HFP have a good compatibility during the preparation.
The microstructures of LLZTO and HSE membranes are shown in
Fig. 1b and c, respectively. The LLZTO grains are ∼200–300 nm and
uniformly distributed with F and Zr, implying that both polymer and
ceramic are well mixed. Some minor pores are found in HSE, which
may be attributed to the solvent evaporation [17]. With the increase in
LLZTO loading, the number of pores seems to increase because of the
aggregation of LLZTO powders, resulting in a decrease in miscibility
with the PVDF-HFP matrix. The aggregation of LLZTO powders can be
confirmed by the EDS elemental mapping of Zr shown in Fig. S1. It is
found that, initially, as the content of LLZTO increases, the atomic ratio
of Zr increases. However, when the content of Zr is excessively increased, the dispersion of elements of Zr becomes uneven and the agglomeration of Zr occur. Besides, the consistency of the polymer matrix
is weakened with LLZTO loadings, so the crystallinity of the polymer
electrolyte is reduced, which is consistent with the XRD analysis. As
shown in Fig. 1d, PVDF-HFP/LiTFSI-SPE membrane and PVDF-HFP/
LLZTO-HSE membrane are different in color. The change of film color
after adding LLZTO is due to the fact that La atom in LLZTO can
complex with N atom as well as C]O group of DMF along with alkaline-like characteristic showing up, and then lead to the dark-brown
conversion of film color [15]. It is also noted that the HSEs prepared are
very flexible and can be used to conform various shapes of batteries.
2.3. Sample characterizations
The garnet LLZTO and the resultant HSE membrane were characterize by X-Ray Diffraction (XRD, Rigaku D/max 2500 PC system)
with Cu Kα radiation in the range of 2θ = 10–60°. The morphology and
the compositions of the samples were obtained by scanning electron
microscope (SEM, Hitachi S4700) equipped with energy dispersive
spectrometer (EDS). FT-IR spectra were collected by a Thermo Nicolet
380 Spectrometer. The thermal stability of different HSEs was investigated by a thermo-gravimetric and differential Scanning calorimeter (TG-DSC, NETZSCH STA 449F3) with a heating rate of
10 °C·min−1 from room temperature to 800 °C under Ar atmosphere.
2.4. Electrochemical measurements
The ionic conductivity of HSE was calculated by electrochemical
impedance spectroscopy (EIS) measurement using PSM1735 Frequency
Response Analyzer (Newtons 4th Ltd). The cell was assembled with a
symmetrical cell configuration of SS/HSE/SS using stainless steel (SS)
as a blocking electrode, while Li ion blocking gold electrodes (sputtered
on both surfaces) were used for pellet samples. The measurements were
performed in the frequency range of 10–107 Hz with an AC amplitude
of 10 mV at temperature range of 20–80 °C. Before measurement, the
samples were first stabilized at the temperature in a drying oven for 1 h.
The ionic conductivity is calculated by the following formula:
σ=
d
RS
(1)
3.2. Thermal stability and mechanical properties of the HSEs
where d is the thickness of the solid electrolyte, R is the impedance, and
S is the contact area of the electrolyte and the electrode [29].
Linear sweep voltammetry (LSV) of Li/HSE/SS 2032-type coin cells
were carried out using an electrochemical workstation (CHI660E, CH
Instruments, Shanghai) within a voltage window of 2.5–6 V at a scan
rate of 1 mV·s−1 on.
The symmetrical Li/HSE/Li cells were assembled to measure the Liion transport number (tLi+) of HSE. The samples were first polarized by
a DC voltage of 50 mV at 80 °C to achieve the steady state. The bulk
resistance and resistance of solid electrolyte interphase (SEI) were then
obtained by AC impedance measurements. The resistance was determined by AC impedance with frequency range of 10–107 Hz. And the
tLi+ was obtained by the Bruce–Vincent–Evans equation [17,30,31]:
t Li+ =
Is (ΔV − Io Ro)
Io (ΔV − Is Rs )
The thermal stability of the HSEs was shown in Fig. 2. As shown in
Fig. 2(a), the HSE shows a better thermal stability than that of polymer
only PVDF-HFP/LiTFSI. The trace amount of mass loss of ∼7% occurs
below 300 °C, which is attributed to the water vapor loss from the hygroscopic HSE and partial decomposition of LiTFSI in the electrolyte
[3,32]. The initial thermal decomposition of the composite solid electrolyte occurred at 310 °C with a huge weight loss, due to the decomposition process of PVDF-HFP and LiTFSI. After the temperature is
raised to 800 °C, the weight of the PVDF-HFP/LiTFSI electrolyte film
remains 21.2%, while the PVDF-HFP/LiTFSI/LLZTO-HSE remains
33.6%. Therefore, the addition of inorganic LLZTO increases the
thermal stability of PVDF-HFP-based solid polymer electrolyte membrane, thus the stability of LMBs. Fig. 2(b) shows DSC curves of the
PVDF-HFP-based polymer.
A crystalline melting peak appeared at 160–180 °C, indicating that
the PVDF-HFP polymer membrane still maintains a reasonably good
crystallinity. As regarding the composite with LLZTO, the area of the
crystalline melting peak decreases, suggesting that LLZTO affects the
crystallinity of the polymer porous membrane, which can expand the
amorphous region of polymer and increase the mobility of the segment
[23].
Fig. 2(c) shows the FTIR spectra of the HSE and Fig. S2 shows the
enlarged view of the highlighted region. The peaks at 744–875 cm−1
can be appointed to the α-phase of the crystalline PVDF, whereas the
peak at 838 cm−1 was responsible for the amorphous phase of PVDFHFP [30,33,34]. The entire FTIR absorption profile of the HSE was similar to the one without LLZTO, indicating that LLZTO is physically
mixed with other components. Besides, the characteristic peak of PVDFHFP was weakened or even disappeared with the addition of LiTFSI and
LLZTO, suggests that there is an interaction among the functional group
of LLZTO, LiTFSI and the organic solvent DMF [15]. Besides, the introduction of LLZO particles can relax the chains of PVDF-HFP polymer,
which also may affect the intensity of the characteristic peak of PVDFHFP [23].
(2)
where Io, Is, Ro, Rs represent the initial, steady-state polarization current, initial resistance and steady-state resistance, respectively.
The electrochemical measurements of charge-discharge cycling test
were carried out on a standard coin cell (CR2032). LFP, acetylene black
and PVDF-HFP (weight ratio 8:1:1) were added to N-methylpyrrolidone
(NMP), and then a uniform slurry was obtained by grinding and ultrasonication. The obtained slurry was then spread on an aluminum foil
collector to serve as the positive electrode, followed by vacuum drying
at 100 °C for 14 h. The charge-discharge curve was test within a voltage
window of 2.7–4.2 V at various rates and temperatures. The mass
loading of LFP is about 1.5 mg·cm−2. Metal lithium is used as the anode
of the battery.
3. Results and discussion
3.1. Phase and microstructural examination of the prepared HSEs
The prepared HSEs were examined with XRD, and the results are
shown in Fig. 1a. The as-prepared LLZTO powders for the HSEs behaved
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Fig. 1. (a) XRD patterns of PVDF-HFP based membrane and LLZTO powders; (b) SEM image of the LLZTO powders. (c) SEM images of PVDF-HFP/LLZTO-HSEs (d)
Photographs of a piece of PVDF-HFP/LiTFSI-SPE membrane and PVDF-HFP/LLZTO-HSE membrane, and bended PVDF-HFP/LLZTO-HSE.
of LLZTO pellets and HSEs with different LLZTO loadings (from 0 to
100 wt%) are shown in Fig. 3(a) and (b), respectively. The Nyquist plots
of as-prepared dense LLZTO pellet are shown in Fig. S4. With the
temperature rises, the size of the semicircle gradually decreases to
eventual disappearance. In the range of 40–80 °C, only a straight line is
observed. The ionic conductivity of LLZTO is 0.57 mS·cm−1 at 20 °C.
With the raise of the content of LLZTO, the ionic conductivity of the
HSEs gradually increases and reaches a maximum at 12.5 wt%. This is
because with initial LLZTO loading increase, the interfacial area between PVDF-HFP and LLZTO particles increases, which is an effective
channel for lithium-ion transport. A further increase beyond 12.5 wt%
is ascribed to the aggregation of particles and increased impedance
between particles [36]. At 20 °C, the ionic conductivity of the HSE
(12.5 wt% LLZTO) can be as high as ∼8.80 × 10−5 S·cm−1. At higher
temperatures, the lattice vibrations within the solid electrolyte increase, so does the number of freely accessible vacancies and carriers,
resulting in a boosted ionic conductivity. The HSE ionic conductivities
can reach 1.61 × 10−3 S·cm−1 at 80 °C. The activation energy Ea was
obtained according to the Arrhenius equation. The calculation results
are summarized in Table S1. The lowest activation energy was observed
with LLZTO loadings of 12.5 wt%, which is consistent with the results
of ionic conductivity.
Solid electrolytes with good mechanical properties are critical in the
use of LMBs. Fig. 2(d) shows a typical stress-strain curve of PVDF-HFP/
LiTFSI and HSE. The PVDF-HFP/LiTFSI membrane has a tensile
strength of 8.82 MPa and Young’s modulus of 5.1 MPa. Due to the addition of LLZTO powders, the tensile strength and Young's modulus of
the HSE were significantly improved to 9.22 and 12.8 MPa, respectively, which was attributable to the strong adhesion between PVDFHFP polymer and ceramic LLZTO. At the same time, the elongation is
reduced because the slip between the polymer segments becomes difficult after the composite LLZTO, and the steric hindrance between the
molecular chains becomes relatively large.
3.3. Li-ion conductivity of the HSEs
The Li-ion conductivity is the most important parameter to evaluate
the electrolyte performance. The room temperature ionic conductivities
of the polymer electrolytes prepared by adding LiClO4 and LiTFSI were
1.77 × 10−6 and 2.75 × 10−6 S·cm−1 (see EIS curves in Fig. S3), respectively. The strong delocalization of the TFSI− anion weakens the
hydrogen bonding interaction with the cation, and has the advantages
of low viscosity, low melting point, high conductivity, etc. Therefore,
LiTFSI is chosen as a Li+-conductive salt in this study [35].
The temperature-dependent ionic conductivity and Arrhenius plots
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Fig. 2. (a) TGA curves and (b) DSC trace of the PVDF-HFP/LiTFSI/LLZTO and PVDF-HFP/LiTFSI electrolyte. (c) FTIR spectra of the PVDF-HFP/LiTFSI/LLZTO and
PVDF-HFP/LiTFSI electrolyte. (d) Stress-strain curves of PVDF-HFP/LiTFSI/LLZTO and PVDF-HFP/LiTFSI electrolyte.
which signifies the decomposition of the electrolyte. After introducing
lithium salt LiTFSI or/and LLZTO, the electrochemical stability window
becomes larger. The HSE decomposes voltage is greater than 5 V (vs.
Li+/Li) with LLZTO fillers and the PVDF-HFP/LiTFSI membrane has an
electrochemical window of about 4.25 V (vs. Li+/Li).
In HSEs, the ionic conductivity is mainly contributed from three
components: polymer matrix, dispersed inorganic particles and
ceramic-polymer interface [41]. Fig. 5 illustrates a possible mechanism
of increased lithium ionic conductivity by incorporating inorganic Liconducting particles. First, the interaction between inorganic particles
LLZTO, polymer chains and lithium salts LiTFSI can provide more
possible lithium ion transport pathways by adjusting the structure of
polymer chains. The inorganic particles are equivalent to the Lewisalkali, which can weaken the polymer cation association effects to form
an “ion-ceramic complex” system, thus facilitating dissociation of the
lithium salt, increasing the number of free carriers, and weakening the
mutual relationship between O and Li+. This makes lithium ions easier
to transport, thus Li+ conductivity and transference number are increased. On the other hand, the LLZTO powders in HSE acting as a
physical plasticizer can impede the closeness, accumulation, and crystallization of the polymer chains, resulting in a polymer with less
crystallinity and more amorphous region. The more the amorphous
region, the more interface can be incorporated. The interface enhances
the lithium salt to dissolve in a faster manner, i.e. more Li+ and TFSI−
can be formed more rapidly at the interface. Therefore, more Li+
conducting channels are introduced into the HSE, and the corresponding activation energy is reduced. The conductive pathways of
dispersed inorganic particles and ceramic-polymer interface of HSE are
introduced through the composition of LLZTO, which has a higher Li+
conductivity and transference number. In this way, the electrochemical
3.4. Li-ion transport number of the HSEs
Fig. 3(c) shows polarization curves and initial/steady-state impedance plots (inset) of the HSE at room temperature. The tLi+ of the
composite solid electrolyte is determined to be 0.47, which is higher
than those of a liquid electrolyte (0.35) [37] and pure PVDF-HFP
electrolyte (0.36) [38]. The significant improvement is attributed to the
LLZTO filler. The LLZTO is an active lithium ion conductor in which the
transport number is unity. Consequently, the incorporation of LLZTO
powder as inorganic filler not only improves the ionic conductivity but
also achieved a positive effect on the electrochemical window and lithium ion transference number [39]. Besides, it is reported that the
addition of LLZTO ceramic fillers can relax polymer chains. Therefore,
the segment motion is promoted under the interaction of inorganic
fillers and polymer chains, accelerating dynamic processes between the
segments [23]. The schematic illustration of the ionic conduction mechanism of HSE is shown in Fig. 4. This further highlights the role of the
LLZTO particles in the ion transport in the HSE.
3.5. Electrochemical properties of the hybrid solid electrolytes
A key factor to achieve a more stable and safer lithium battery is the
electrochemical stability of the electrolyte. When the battery is in operation, the electrolyte must remain chemically stable within the operating voltage window. The working voltage of commercial lithium
batteries is usually 4.2 V, thus the oxidation potential of the electrolyte
should be at least higher than 4.2 V to prevent electrolyte decomposition. LSV was used to evaluate the electrochemical window of electrolytes from 2.5 to 5.5 V. As shown in Fig. 3(d), an anodic current
increment is observed in the pure PVDF-HFP membrane at 4.2 V [40],
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Fig. 3. (a) Ionic conductivity of PVDF-HFP/LiTFSI/LLZTO-HSEs with different LLZTO loading vs temperature; (b) Arrhenius plots of PVDF-HFP/LiTFSI/LLZTO HSEs;
(c) current-time profile of a symmetrical Li|HSE|Li (HSE with 12.5 wt% LLZTO) cell after applying a DC voltage of 10 mV. The inset shows the Nyquist impedance
spectra of the cell before and after polarization; (d) comparison of liner sweep voltammograms of PVDF-HFP/LiTFSI/LLZTO HSE (HSE with 12.5 wt% LLZTO) and
PVDF-HFP/LiTFSI electrolyte at a scan rate of 1 mV s−1.
Fig. 4. Schematic illustration of a possible ionic conduction mechanism of HSE.
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improved after the addition of LLZTO. During the cycle, the specific
capacity increases first and then decreases. The increase in capacity of
the previous cycle may be an activation process of the battery. Fig. 6(b)
is a comparison of the rate performance of two types of composite solid
electrolytes. The specific capacities of PVDF-HFP/LiTFSI/LLZTO solid
electrolytes at different current densities are 150.2, 133.8, 100.8, 72.6,
and 153.4 mAh·g−1, respectively. And the specific capacities of the
PVDF-HFP/LiTFSI solid electrolytes were 135.6, 113.8, 83.4, 56.3, and
111.64 mAh·g−1, respectively. At higher charge and discharge rates,
the polarization voltage of the battery increases, resulting in a decrease
in specific capacity (Fig. S5). This is because the diffusion at the electrode/electrolyte interface is limited and cannot be charging and discharging at large currents [42]. Due to the migration path of lithium
ions in the system increases after the addition of an appropriate amount
of LLZTO, and the electrochemical cycle performance due to the increase in conductivity becomes better.
Because the solid electrolyte has a low ionic conductivity and large
interfacial impedance at room temperature. In this work, a 10 μL liquid
electrolyte (LiPF6, EC:DEC:DMC = 1:1:1) was added to the interface of
HSE/LFP to minimize the solid/solid interfacial impedance, so the
performance can be ensured at lower temperatures. The impedance
curves before and after the interface modification are supplied in Fig.
S6. The interface impedance values were 250 Ω and 1217 Ω, respectively. The impedance was greatly reduced after the interface was
modified by the electrolyte. The electrochemical performance is shown
in Fig. 7. Fig. 7(a) and (b) show the cycle performance at 0.2 C and rate
performance of the solid state lithium battery at high temperature
(55 °C). The initial discharge specific capacity of the battery of PVDFHFP/LiTFSI and PVDF-HFP/LiTFSI/LLZTO electrolytes were 155.7,
154.9 mAh·g−1, and 84.6, 128.4 mAh·g−1 after 200 cycles, respectively. And the specific capacities of PVDF-HFP/LiTFSI solid electrolytes at different current densities are 139.4, 127.4, 110.6, 79.4 and
79.4 mAh·g−1, respectively. The specific capacities of the PVDF-HFP/
LiTFSI/LLZTO solid electrolytes were 159.1, 142.5, 136.1, 123.8 and
157.0 mAh·g−1, respectively.
Fig. 7(c) and (d) show the electrochemical performance of the solid
state lithium battery at room temperature. The initial discharge specific
capacity of the battery of PVDF-HFP/LiTFSI and PVDF-HFP/LiTFSI/
LLZTO electrolytes were 127.9, 152.1mAh·g−1, and 109.9, 144.6
mAh·g−1 after 200 cycles. The specific capacities of PVDF-HFP/LiTFSI
solid electrolytes at different current densities were 140.4, 116.5, 95.2,
57.9 and 140.3 mAh·g−1, respectively. The specific capacities of the
PVDF-HFP/LiTFSI/LLZTO solid electrolytes were 158.7, 143.5, 129.8,
102.8 and 168.8 mAh·g−1, respectively. It is worth noting that the cycle
stability of the battery is poor at high temperatures. This may be due to
microscopic deformation of the electrolyte at a high temperature for a
long time, and a small amount of active material is detached from the
current collector, resulting in a decrease in specific capacity.
Fig. 5. Galvanostatic cycling curves of Li|HSE|Li symmetrical cell at a current
density of 0.05 mA cm−2 and 25 °C.
performance is expected to improve.
We further carried out the galvanostatic cycling experiments by
charging for 0.5 h and discharging for 0.5 h of a symmetric battery
consisting of Li metal anode. The goal was to evaluate the Li-ion
transport capability and electrochemical stability.
Fig. 5 shows that the lithium-symmetric cell assembled using a
PVDF-HFP/LiTFSI and PVDF-HFP/LiTFSI/LLZTO solid electrolyte.
After cycling for 350 h at current density of 0.05 mA·cm−2, the polarization voltage of symmetrical cell with PVDF-HFP/LiTFSI electrolyte is
gradually increases, and its increased from 46 mV to 285 mV. This indicates that the PVDF-HFP/LiTFSI polymer electrolyte is unstable, and
lithium dendrite growth and side reactions occur in the interface. The
symmetrical battery of PVDF-HFP/LiTFSI/LLZTO composite solid
electrolyte exhibits a very stable polarization voltage during the 500 h
cycle, and the polarization voltage is 24.6 mV. This shows that the HSE
can enable a reversible plating and striping of Li metal without obvious
lithium dendrite. Overall, the above electrochemical data show that the
HSE developed has potential to be used in solid-state LMBs with good
rate capacity and cycle performance.
The LFP and lithium metal were assembled with PVDF-HFP/LiTFSI
and PVDF-HFP/LiTFSI/LLZTO solid electrolytes to form an all-solid
battery at a high temperature (55 °C) for battery performance testing.
Fig. 6(a) is a performance cycle comparison of the current density of 0.2
C. The initial discharge specific capacity of the batteries corresponding
to the two types of solid electrolytes was 118.2, 123.6 mAh·g−1, and
78.4, 127.7 mAh·g−1 after 200 cycles, respectively. It can be clearly
seen that the specific capacity and cycle stability of the battery are
Fig. 6. The all solid-state battery Li/HSE/LiFePO4 (a) cycling performance at 0.2 C and 55 °C; (b) charge and discharge curves at various current densities at 55 °C.
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J. Lu, et al.
Fig. 7. The solid-state battery Li/HSE/LiFePO4 (a) cycling performance at 0.2 C and 55 °C; (b) charge and discharge curves at various current densities at 55 °C; (c)
cycling performance at 0.2 C and room temperature; (d) charge and discharge curves at various current densities room temperature.
Fig. 8. Long-term cycling performance at 0.5 C and room temperature.
The SEM/EDS results of the PVDF-HFP/LiTFSI/LLZTO solid electrolyte before and after long-term cycling at 0.5 C are shown in Fig. S7.
The morphology of the solid electrolyte film became relatively rough
after 500 cycles. This may be due to the side reaction of metallic lithium
with the group in PVDF-HFP [45]. However, the cross-sectional morphology and elemental distributions of HSE before and after the cycles
show no significant difference with uniform element distribution. This
indicates that the side reactions are mainly restricted on the surface of
the membrane.
The long-term cycle performance of the solid battery at room temperature is tested at a current density of 0.5 C, as shown in Fig. 8. The
cell exhibits a high Coulombic efficiency of 99.6% and outstanding
capacity retention after 500 cycles (1st: 101.4 mAh·g−1 and 500th:
110.9 mAh·g−1). This indicates that the addition of LLZTO particles
providing lithium-ion conduction to the HSE is very effective in inhibiting side reactions that lead to capacity decay [43]. In our experiments, when complex LLZTO was used to enhance Li ion conduction,
the increase in cycle capacity retention is resulted from the decrease of
the amount of TFSI− anion transferred to the Li/HSE interface during
cell cycling [44]. In addition, the outstanding capacity retention of the
cell is also related to the minimum loss of active material participating
in the reaction, stable SEI film and excellent stability of HSE during
cycling.
4. Conclusion
In summary, we have successfully synthesized Ta-doped cubic
Li6.5La3Zr1.5Ta0.5O12 (LLZTO) by conventional solid-state reaction, and
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J. Lu, et al.
fabricated a flexible HSE based on PVDF-HFP and LLZTO uniformly
mixed hybrid via a cost-effective solution-casting method. The formed
HSE exhibits an excellent electrochemical stability greater than 5 V, a
good ionic conductivity of 8.80 × 10−5 S·cm−1 at room temperature. In
a voltage range of 2.7–4.2 V and room temperature, the Li/HSE/LFP
battery retained a initial discharge capacity of 110.9 mAh·g−1 at 0.5 C
for 500 cycles. At the same time, the smaller polarization and stable
long cycle in the Li/HSE/Li battery proved to effectively inhibit lithium
dendrites. The high performance of cell with HSE was attributed to the
high ionic conductivity when LLZTO powders are participated in Li ion
conduction. These results demonstrate the potential of HSE for all solid
state LMBs in the future.
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Acknowledgements
This work is supported by the Shenzhen Fundamental Research
Program of Subject Distribution (JCYJ20170413102735544) and
Shenzhen International Collaborative Project (201809283000313).
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.cej.2019.02.148.
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