Nano Energy 70 (2020) 104520
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Full paper
A simple approach to minimize the first cycle irreversible loss of sodium
titanate anode towards the development of sodium-ion battery
Ananta Sarkar, C.V. Manohar, Sagar Mitra *
Electrochemical Energy Laboratory, Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, Maharashtra, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Zirconium-doped hydrogenated sodium
titanium oxide anode
High performance super stable anode
1st cycle irreversible loss
SEI mechanism
Sodium-ion battery
First cycle irreversible loss is a major problem of any kind of anode materials in sodium-ion battery systems
(SIBs). Here, we present a stable (4000 cycles), high capacity (237 mAh g 1) and high rate capable hydrogenated
metal-doped sodium titanium oxide (Na2Ti3O7) anode material for SIBs and that suffers from high 1st cycle
irreversible loss (53.86%) initially. Later, we adopted a simple chemical shorting approach where, sodium metal
was kept immediate tight contact with anode material in presence of electrolyte medium to reduce the irre­
versible loss. It was found that 60 min chemical shorting is enough to minimize the irreversible loss to 4.11%
from 53.6% and enhance the Coulombic efficiency to 84% from 36.22% respectively without compromising the
specific capacity and cycling stability. This approach is adopted in full-cell (sodium vanadium phosphate vs.
hydrogenated metal-doped Na2Ti3O7) formation and the study also provided similar improved electrochemical
performance. The current understanding and the approach provide a new direction to overcome the 1st cycle
irreversible loss of sodium titanate anode and can be utilized further to develop sustainable rechargeable sodiumion battery.
1. Introduction
Most of the anodes like carbonaceous, conversion, alloying and
intercalation materials have been facing a common problem of huge
irreversible capacity loss and low Coulombic efficiency at its first cycle
and subsequent initial cycles for sodium-ion battery electrochemistry
(SIBs) [1–7]. Therefore, it is essential to understand the reason behinds
its first cycle capacity loss and minimize the issue. The first cycle irre­
versible loss in SIBs is similar to lithium-ion batteries (LIBs) which have
been studied extensively. This irreversible reaction is mainly due to the
(1) decomposition of electrolyte by chemical or electrochemical reaction
of solvents and salt on anode surface known as solid electrolyte interface
(SEI) (observed almost all kind of anodes) [8,9] (2) electrochemical
reduction of solvent molecules with solvated ions between the in­
terlayers of the electrode by co-intercalation process (mainly observer in
layered structured materials) [10,11] or (3) irreversible storage of Li or
Na ions (metal oxide materials, alloying or conversion materials) [12,
13]. Recently many approaches have been taken to minimize this irre­
versible loss [14–16]. As an example, the proper selection of electrolyte
(with additive) can suppress this capacity loss, but in the process, the
irreversible loss can minimize up to a certain level [10,17–19]. Surface
modification of anode materials is another option which can effectively
reduce the loss [14,20]. However, the cycling stability issue is more
prominent here due to unstable SEI formation. It should be noted here
that the stable SEI layer is the most important parameter for getting a
stable long-term cycling performances. Another strategy is to mix extra
lithium/sodium on the cathode parts or taking extra cathode materials
to act as a lithium/sodium reservoir to compensate the Li or Na-ion loss
in the first cycle [15,16]. However, in this process, extra materials
(cathode or Na/Li) were added to the electrode which effectively re­
duces the overall energy density of the cells and increases the opera­
tional cost.
Few studies recently attempted to overcome the issue in SIBs by
developing a new sodium reservoir at the cathode side [14,21–23].
Singh et al. added NaN3 salt with P2–Na2/3[Fe1/2Mn1/2]O2 cathode
materials as additional sodium sources, however this NaN3 lead to N2
gas generation during the irreversible electrochemical reaction and also
reduces the overall energy density (as NaN3 has only 35 wt% sodium
source) [21]. Tarascon and his group used sodium metal as a
pre-sodiated reagent for SIBs. Sodium metal was mixed to the electrode
materials by ball-milling approach and observed it was a most chal­
lenging task because sodium metals tend to stick on the metallic surface
* Corresponding author.
E-mail address: sagar.mitra@iitb.ac.in (S. Mitra).
https://doi.org/10.1016/j.nanoen.2020.104520
Received 3 September 2019; Received in revised form 6 December 2019; Accepted 18 January 2020
Available online 21 January 2020
2211-2855/© 2020 Elsevier Ltd. All rights reserved.
A. Sarkar et al.
Nano Energy 70 (2020) 104520
due to its ductile nature [22]. Moreover, the first cycle irreversible loss
did not suppress significantly for sodium-ion full-cell configuration by
this process (36% 1st cycle irreversible loss was observed for
(Na1(Fe0.5Mn0.5)O2 þ 10% Na3P)/C) [22]. Another elegant way to
reduce this loss is by the pre-sodiation or pre-lithiation of the anode
materials. Previously, Kulova and Skundin attempted to eliminate the
irreversible capacity loss via immediate tight contact with lithium metal
and silicone or graphite in electrolyte medium. In that shorting method,
a thin passive layer was formed on the electrode surface and the dy­
namic of this thin formation depend on the masses of lithium and silicon
or graphite. However, by this process, they were unable to decrease the
irreversible capacity extensively [24–26]. Liu et al. reported the
pre-lithiated silicon anode by self-discharge mechanism [27]. In this
report, Si was placed in contact with lithium and the electrolyte so that it
can help to form SEI layer by lithium-ion consumption over the Si
negative electrode surface. However, the report described a costly pro­
cedure and wasted a lot of costly electrolyte in this process [27].
To study and understand the process better, we have undertaken an
intercalation-based sodium titanium oxide (NTO) as an anode material
as it does not suffer from much volume expansion or rate capability
problem and the safety issues. However, it is limited by its specific ca­
pacity, needs further study to enhance its specific capacity and stability
[5,28,29]. Herein, we have implemented a simple metal doping tech­
nique followed by the hydrogenation process to enhance the specific
capacity as well as cycling stability of the anode. Currently, the zirco­
nium doped hydrogenated Na2Ti3O7 (HNTOZr) anode showed an
excellent electrochemical performance vs. Naþ/Na. It can also deliver a
high discharge specific capacity of ~240 mAh g 1 at 200 mA g 1 current
rate and sustained up to 2500 cycles with more than 85% capacity
retention. However, it is also facing the same irreversible loss and low
Coulombic efficiency problem in its 1st cycle. To overcome those prob­
lems, we have adopted a simple but powerful chemical shorting process
of the anode materials where sodium metal was kept tight contact with
anode material in electrolyte medium at different time durations to
suppress the first cycle irreversible loss and enhance the initial
Coulombic efficiency of the cells. The enhancement in the first cycle
Coulombic efficiency was observed by 48% whereas first cycle irre­
versible loss is minimized to 3.64% form 53.86% for hydrogenated
zirconium doped NTO and the complete report will give the pieces of
evidence and display our claim. A details investigation of SEI composi­
tions and its formation mechanism for SIBs also studied. At the end, a
full-cells based on NVP and chemically modified metal-doped NTO are
constructed that showed excellent sodium-ion battery performance in
terms of cycle life and energy density.
2.1. Structural and morphological characterizations
First, a single-step hydrothermal synthesis process was adopted to
prepare the zirconium-doped NTO nanorods and followed by hydroge­
nation technique to synthesize the HNTOZr nanorods (details synthesis
procedure was discussed in the experimental section in SI). Fig. 1a shows
the XRD pattern of as-prepared titanium oxides; pure NTO without any
doping, NTO with zirconium doping and NTO with zirconium doping
followed by hydrogenation (hereafter described as NTO, NTOZr, and
HNTOZr respectively). The synthesized NTO nanorods have a mono­
clinic phase (space group P 1 21/m1) with an intense peak at (h00) di­
rection; exactly matches with its standard JCPDS data (file no
072–1248). However, a mixed-phase of Na2Ti3O7 (monoclinic) and
Na0.57Ti2O4 (orthorhombic) was observed with zirconium doped into
NTO matrix and (100) plane peak position was shifted towards lower
two-theta value and became broader, and the peaks at 18.48� 2θ value
became more intense corresponds to (200) plane. When larger size
(ionic radius, 86 p.m.) zirconium was doped into comparative smaller
size (ionic radius, 56 p.m.) of titanium site, create a huge local distortion
and electronic repulsion on the host which help to increase the bond
length and expand the crystal lattice [30].
Here, zirconium has an important role in converting the monoclinic
to the orthorhombic phase partially [31]. However, after hydrogenation
at 450 � C, this peak (100) divided into two parts and became less
intense, and on the other hand, (004) plane was disappeared [32,33].
Generally, hydrogenation create some oxygen vacancy in the crystal
lattice; as a result amorphous nature of the materials increases, peaks
become less intense, border and even some times divided into two parts
[33,34]. The lattice parameter of mixed-phase HNTOZr material was
calculated using FullProf Suite Software shown in Fig. 1b. It was
observed that the lattice parameters of monoclinic Na2Ti3O7 (phase 1) a
¼ 8.565 Å, b ¼ 3.809 Å, c ¼ 9.123 Å, α ¼ 90.0� , β ¼ 101.65� and γ ¼ 90� ;
slightly changed from standard JCPDS data; a ¼ 8.571 Å, b ¼ 3.804 Å, c
¼ 9.135 Å, α ¼ 90.0� , β ¼ 101.65� and γ ¼ 90� . Whereas the other
orthorhombic Na0.57Ti2O4 (phase 2) showed the lattice parameters in
the order of a ¼ 9.592 Å, b ¼ 11.681 Å, c ¼ 2.938 Å, α ¼ 90.0� , β ¼ 90.0�
and γ ¼ 90� ; significantly changed from standard JCPDS data; a ¼ 9.486
Å, b ¼ 11.307 Å, c ¼ 2.945 Å, α ¼ 90.0� , β ¼ 90.0� and γ ¼ 90� . However,
other characteristic peaks remain at the same positions. The corre­
sponding crystallographic structure with possible Naþ intercalation
position was presented in Fig. 1c [35].
The morphological and the micro-structure of as-prepared samples
were analyzed by FEG-SEM and FEG-TEM (Fig. 1d–k) respectively. The
morphology of the NTO did not change after doping or hydrogenation
process. The length of the nanorods is in the range of few micrometers
(1–6 μm), and the width is observed around 150–500 nm. From the FEGTEM images, it was observed that the layer structured nanorods are the
stack of several sheets, displayed in Fig. 1e and Fig. S1. One can describe
the growth process; initially small sheets are formed, and with time,
these sheets are combined together, and formed nanorods. The HR-TEM
images showed the interlayer d spacing is ~0.48 nm for NTOZr corre­
sponds to (004) plane whereas ~ 0.83 nm and ~0.30 nm d spacing
relate to (100) and (300) plane respectively for HNTOZr which is wellmatched with XRD analysis, presented in Fig. 1f,j. The SAED pattern for
both hydrogenated and non-hydrogenated NTO showed (in Fig. 1g,k)
the crystalline nature [36], however, some amorphous nature was
noticed in HR-TEM images and XRD pattern of hydrogenated sodium
titanium oxide. It was interesting to see that after hydrogenation, the
color of the sample was changed from whitish blue to white (Fig. S3a).
From the XRD, ICP and EDS elements mapping analysis, we estimated
the empirical formula of as synthesized HNTOZr is NaTi3.92Zr0.09O5.8.
Further, the X-ray photoelectron spectroscopy (XPS) data of asprepared materials were analyzed to understand the effect of both zir­
conium doping and hydrogenation on the oxidation state of the elements
present in sodium titanium oxides, shown in Fig. 2. Fig. 2a–c indicates
the Ti 2p absorption peaks of as prepared NTO, NTOZr, and HNTOZr
2. Results and discussion
First cycle irreversible loss in any anode materials is one of the major
problems for both LIBs and SIBs and causes poor Coulombic efficiency in
its initial cycles. Either, we have to add extra lithium or sodium source
(cathode materials) to the system, or we can sacrifice the same amount
of anode capacity to compensate this loss which leads to decrease the
overall energy density (both gravimetric and volumetric energy density)
during full-cell construction. Here, we have adopted a hydrogenation
and doping technique to enhance the specific capacity of NTO anode
material. Further, a simple chemical shorting approach was used to
overcome the first cycle irreversible loss of hydrogenated doped sodium
titanate anode and to improve the initial cycle Coulombic efficiency in
SIBs. The current report begins together with the synthesis approach,
characterizations and electrochemical performances of different hy­
drogenated sodium titanium oxides along with its sodiation mechanism.
In later part, we reveal the approach of minimizing the first cycle irre­
versible loss, and at the end, we discuss the SEI compositions and its
formation mechanism with the help of XPS analysis for SIBs.
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Fig. 1. Structural and morphological characterization of as-prepared zirconium-doped Na2Ti3O7 (NTO) with and without hydrogenation; (a) characteristic XRD
pattern of as-prepared different Na2Ti3O7 showed after zirconium-doped the single-phase NTO converted to mixed phase of Na2Ti3O7 (monoclinic) and Na0.57Ti2O4
(orthorhombic), (b) characteristic XRD pattern of mixed-phase HNTOZr analyzed by commercial FullProf Suite software, (c) typical monoclinic Na2Ti3O7 structure
with possible sodium intercalation position, (d) FEG-SEM image of NTOZr nanorods, (e) FEG-TEM image of NTOZr nanorods (f) high resolution TEM image of NTOZr,
(g) associated selected area electron diffraction (SAED) pattern of NTOZr, (h) FEG-SEM image of HNTOZr showing similar type nanorods morphology, (i) FEG-TEM
image of HNTOZr nanorods (j) high resolution TEM image of HNTOZr, and (k) associated SAED pattern of HNTOZr nanorods.
electrodes, respectively. The Ti 2p3/2 peak of NTO observed at 459.06 eV
reflecting the pure phase of Ti4þ state whereas, Ti 2p1/2 peak is noticed
at ~468.82 eV. However, after zirconium doping into the structure, a
new small peak of Ti3þ appeared at 458.73 eV, and Ti4þ absorption peak
was slightly shifted to the higher binding energy at 459.32 eV. The Ti
2p1/2 peak of NTOZr also reflected the co-existence of Ti3þ and Ti4þ
phase (shown in Fig. 2b) and confirming the Zr-doping on Ti site. The
ratio of these Ti3þ/Ti4þ (~1:6) was calculated based on the area under
the curve. When hydrogenation was done on NTOZr, Ti4þ absorption
peak again shifted back to lower binding energy at 459.27 eV, but the
Ti3þ absorption peak remained at the same position at 458.73 eV, and
the ratio of these two peaks (Ti3þ/Ti4þ) was observed ~ 1: 4.8, shown in
Fig. 2c. The O1s absorption peaks of different sodium titanium oxides
were displayed in Fig. 2d–f. O1s absorption peak of NTO showed
(Fig. 2d) the two metallic oxidation peaks. First O1s peak at 530.66 eV
corresponds to the metallic bond of Ti–O (Ti4þ) whereas, 2nd hump at
533.01 eV binding energy was associated with Na–O bond [37].
Fig. 2e and f showed an additional peak at 531.91 eV for NTOZr and
at 531.81 eV for HNTOZr which was actually representing the Ti3þ-O
binding energy, and a similar peak is also observed at the Ti 2p ab­
sorption peak position. The Na 1s absorption peaks of NTO, NTOZr, and
HNTOZr were also shown in Fig. S5a suggested that the position of Na 1s
was not changed significantly with doping or after hydrogenation of the
materials, suggested that the doping or hydrogenation did not happen at
sodium site. To confirm the zirconium doping, Zr 3d absorption spec­
trum was shown in Fig. S5b which shows two major peaks at 182.55 and
183.19 eV suggested the mixed oxidation state of Zr3þ and Zr4þ phase.
The XPS survey scan of as prepared NTO, NTOZr and HNTOZr displayed
in Fig. 2g–i. The survey confirmed all elements (Na, Ti, O, C) were
present in the electrode sample, and a small absorption peak of Zr 3d
was present in NTOZr and HNTOZr. Not only that after doping, existence
of Ti3þ phase in both Ti 2p and O1s spectrum confirm the doping of
zirconium on titanium site in sodium titanium oxide matrix.
2.2. Electrochemical performances
After the chemical and physical characterization, we are interested
to know the electrochemical performances of as prepared sodium tita­
nium oxides, and the tested results are shown in Fig. 3. Fig. 3a and b
revealed the cycling performances and the 1st cycle Galvanostatic
charge-discharge profile of different NTO electrode vs. Naþ/Na at 200
mA g 1 current density over 0.01 V–2.2 V potential window respectively
against sodium metal at 20 � C. It showed the initial specific discharge
capacity of ~233 mAh g 1, 556 mAh g 1, 392 mAh g 1, and 514 mAh
g 1 for NTO, NTOZr, HNTO, and HNTOZr respectively and after few
cycles and, the capacity was stable to ~65 mAh g 1, 120 mAh g 1, 155
mAh g 1, and 200 mAh g 1 respectively. However, after continuous 120
cycles, the electrodes without doping was started to decay and
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Fig. 2. X-ray photoelectron spectroscopy (XPS) of as-prepared different sodium titanium oxide; (a) Titanium 2p XPS spectrum of as-prepared NTO showing singlephase Ti4þ, (b) Ti 2p XPS spectrum of as-prepared NTOZr confirms coexisting of Ti4þ and Ti3þ, (c) Ti 2p XPS spectrum of HNTOZr also showing mixed phase of Ti4þ
and Ti3þ, (d) O1s absorption peak of NTO showing two metallic oxidation peaks, first one is due to Ti4þ-O bond, and another one is associated with Na–O, (e) O1s
absorption peak of NTOZr presenting three metallic oxidation peaks, first one is due to Ti4þ-O bond and 2nd one ascribed for Ti3þ-O other one is associated with
Na–O, (f) O1s XPS spectrum of HNTOZr also reveals three metallic oxidation peaks, Ti4þ-O, Ti3þ-O, and Na–O, and (g–i) corresponding XPS survey scan of NTO,
NTOZr, and HNTOZr respectively showing the presence of individual elements.
ultimately give up after 150 cycles whereas the doped samples are stable
up to 250 cycles under the same electrochemical environment. There­
fore, it was clearly observed that zirconium-doped hydrogenated sam­
ples not only help to increase the specific capacity, also improve the
cycling stability.
Here, zirconium has an important role to enhance the specific ca­
pacity and durability of the electrode. As the bigger size zirconium
doped in NTO materials, increase the interlayer d spacing which helps to
enhance the specific capacity and rate capability of the anode materials
and explains in our previous study on zirconium doped cathode [30].
The typical charge-discharge curve for sodium titanate was shown in
Fig. 3b. In the first discharge process, first discharge plateau at ~0.83 V
is appeared due to the formation of SEI layer on the surface of the so­
dium titanate electrode. The second plateau at ~0.28 V is observed due
to the intercalation of sodium-ion into sodium titanate matrix. However,
this second plateau of our HNTOZr electrode is not prominent, rather it
is a sloppy one due to intercalation as well as absorption of sodium-ion
into/on electrode processes happening in the same time. During charge
process, corresponding de-intercalation plateau was observed at ~0.53
V and a huge first cycle irreversible lost and low Coulombic efficiency
were observed here. To observe the effect of hydrogenation time, sam­
ples with varied hydrogenation time had undergone the Galvanostatic
charge-discharge process at 200 mA g 1. The initial specific discharge
capacity of HNTOZr is ~330 mAh g 1, 259 mAh g 1, 514 mAh g 1 and
645 mAh g 1 for the materials prepared for 6 h, 8 h, 10 h, and 12 h time
duration respectively (Fig. 3d). Although 12 h sample shows the initial
high discharge capacity however after few cycles, it drops to ~150 mAh
g 1 whereas the sample prepared for 10 h, it is stable to ~195 mAh g 1.
However, the anode materials prepared for 6 and 8 h duration, ended
the life after 250 cycles. The HNTOZr sample prepared for 10h and 12 h
showed high surface area (35.10 m2/g and 37.80 m2/g respectively
displayed in Table S3 (SI), provided better contact between the active
materials, carbon additives and electrolyte which enhanced the cycling
stability of the cell than other electrodes [38]. The electrochemical
impedance spectroscopy (EIS) also carried out to understand further.
Among all HNTOZr, the material prepared at 10 h showed minimum
charge transfer resistance (Rct) (Fig. S6 and Table S4). So, hereafter we
choose the zirconium-doped hydrogenated electrode which was syn­
thesized for 10 h at 220 � C for further electrochemical performances.
Fig. 4 illustrated the electrochemical performances of HNTOZr (10 h)
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Fig. 3. Electrochemical performances of different
sodium titanium oxides with an average active ma­
terials loading of 1.5 mg cm 2 vs. Naþ/Na at a con­
stant current rate of 200 mA g 1 over the potential
window of 0.01–2.0 V at 20 � C � 2 � C accuracy; (a)
comparison of cycling performance of NTO, NTOZr,
HNTO and HNTOZr anode against sodium metal
(Naþ/Na), (b) 1st cycle charge-discharge comparison
of NTO, NTOZr, HNTO and HNTOZr, (c) cycling
performance of HNTOZr anode prepared at different
time duration up to 300 cycles against sodium metal
(Naþ/Na), and (d) corresponding 1st cycle chargedischarge profile of HNTOZr-10h showed better
electrochemical performance. Hereafter, we per­
formed all the electrochemical and other tests using
10 h hydrogenated sample.
Fig. 4. Electrochemical performances of
HNTOZr (10h) vs. Naþ/Na over the potential
window of 0.01–2.0 V; (a) cycling perfor­
mance of HNTOZr at 200 mA g 1 current
density showing very stable performance up
to 2500 cycles with average Coulombic ef­
ficiency of 99.70%, (b) corresponding 1st
cycle charge-discharge profile of HNTOZr
display poor initial Coulombic efficiency and
high irreversible loss, (c) rate performance
of HNTOZr up to 2000 cycles also showing
prolonged stable performance, and (d) cor­
responding 1st cycle charge-discharge curve
at different current density. All the electro­
chemical tests were performed at 20 � C � 2
�
C accuracy.
density of 200 mA g 1, 500 mA g 1, 1000 mA g 1, and 2000 mA g 1
respectively. It is interesting to notice that at a very high current rate of
1000 mA g 1 and 2000 mA g 1, it can deliver a discharge capacity of
150 mAh g 1 and 128 mAh g 1 respectively. The doped hydrogenated
NTO was able to retain its 82.10%, 92.19% and 93.34% initial capacity
after 2000 cycles at 200 mA g 1, 1000 mA g 1, and 2000 mA g 1 current
density respectively. Although, the electrode performed and exhibited
an advanced electrochemical performance in terms of specific capacity,
high rate capability, and ultra-longed cycling stability but suffering from
huge 1st cycle irreversible loss (53.86%) which leads to a poor
Coulombic efficiency (36.22%) in its initial cycle shown in Fig. 4b.
Therefore, it is important to understand the reason for it and minimize
the irreversible loss by any smart strategy.
over 0.01–2.2 V potential windows. The cycling performance and
charge-discharge profile at 200 mA g 1 current density were shown in
Fig. 4 a,b. It showed the initial 2nd cycle specific discharge capacity
~237 mAh g 1 and dropped to 200 mAh g 1 after 10 cycles, and after
that it becomes stable. This anode can withstand up to 2500 cycles (200
days continuous charge-discharge) with a high average Coulombic ef­
ficiency of 99.70%. It can able to deliver a discharge capacity of ~180
mAh g 1 with 92% capacity retention after 1000 and provided ~145
mAh g 1 specific capacity (75% capacity retention) after 2500 cycles.
The cycling performance at different current rates were also revealed in
Fig. 4c up to 2000 cycles, and corresponding 1st cycle charge-discharge
curves (Fig. 4d). The first cycle discharge capacity was observed ~514
mAh g 1, 395 mAh g 1, 285 mAh g 1, and 208 mAh g 1 at the current
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2.3. Minimization of 1st cycle irreversible loss
and 80% (OCV 0.83 V) respectively. However, with the increasing the
shorting time to 120 min showed in Fig. 5 d,e, the OCV of the cell is
lowered down to 0.37 V, but the irreversible loss was not reduced
further (4.68%), and enhancement of the Coulombic efficiency was
(81.75%) not significant. From this analysis, it was clear that 60–90 min
chemical shorting is enough to minimize the irreversible to 4.11%. The
cycling performance of chemically processed HNTOZr was displayed in
Fig. S8a which provided an initial specific discharge capacity of ~244
mAh g 1 at a current density of 200 mA g 1, and it was stable after 10
cycles with a discharge capacity of 193 mAh g 1. It also provided a
specific discharge capacity of 186 mAh g 1 (96.37% capacity retention)
after 400 cycles with a high average Coulombic efficiency of 99.70%. It
can provide a long cycle life of 4000 cycles at higher current rate of
2500 mA g 1 with 90% capacity retention (shown in modified Fig. 5f).
So, it was clear that by this process, the cycling and the energy perfor­
mance can be improved by minimizing the irreversible loss.
To confirm our claim, the sodium-ion full-cell performance was
shown against sodium vanadium phosphate (NVP) at three different
conditions: (1) full-cell was made with NVP against fresh HNTOZr
To minimize the irreversible loss, we adopt a unique approach here.
In this process, the appropriate amount of electrolyte was added to the
electrode, and sandwiched the electrode with sodium foil, socked with
electrolyte and kept for chemical shorting for different times starting
from 0 to 120 min inside the Glovebox. After this chemical shorting
process, the electrochemical cell was assembled by using this shorting
electrode, with used electrolyte and new sodium foil. The whole process
was done inside the Glovebox and then taken out for the electrochemical
test. Fig. 5 exhibits the charge-discharge profile of chemically shorted
electrode at a different time of 0–120 min. The electrode without
adopting this process showed a high irreversible loss of 53.86% in its
initial cycle with a Coulombic efficiency of 36.22% (OCV is ~2.56 V),
shown in Fig. 5a. When the electrode was shorted for 30 min (OCV is
dropped to 0.98 V), the irreversible loss is minimized to 12.51% and
Coulombic efficiency is enhanced to 77.50%, (Fig. 5b). This loss is even
lowered to 4.11% and 3.64% when the electrodes were kept for 60 min
and 90 min with enhanced Coulombic efficiency of 84% (OCV 0.89 V)
Fig. 5. Electrochemical performance of chemical shorted HNTOZr, a strategy to minimize 1st cycle irreversible loss and enhancing its initial Coulombic efficiency at
20 � C � 2 � C accuracy; (a–e) first three cycle charge-discharge profile of HNTOZr (a) without chemical shorting shows high 1st cycle irreversible loss and low
Coulombic efficiency, (b) with 30 min chemical shorting showing reduction of this irreversible loss to 12.51%, (c) 60 min shorting further reduce to 4.11% and
enhance the Coulombic efficiency to 84%, (d) with 90 min shorting, (e) with 120 min chemical shorting showing chemical sodiation behaviour, and above all the
cycling tests were done at 200 mAg 1 current rate, (f) cycling performance of 60 min chemically shorted HNTOZr showing very stable performance up to 4000 cycles
at 2500 mA g 1 current density, similar to without shorted HNTOZr, (g–i) full-cell performances of various time chemically shorted HNTOZr against sodium va­
nadium phosphate (NVP), first two-cycle charge-discharge profile of; (g) NVP vs. HNTOZr without shorted, (h) NVP vs. HNTOZr with 90 min chemically shorted, and
(i) 10% extra NVP vs. HNTOZr with 90 min chemically shorted.
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for good anode material for any battery. The 1st oxidation peak was
observed at ~0.53 V for both shorted and HNTOZr without shorted,
corresponds to the oxidation of Ti3þ to Ti4þ shown in Fig. 6b and the
current intensity also reduced significantly. It was also noticed that
small peak at 0.83 V also appeared for initial few cycles and entirely
disappeared after 10th cycle for shorted HNTOZr which again suggested
that in the 1st cycle; although most of the SEI is formed but need few
more cycles to stabilize. It was interesting to see that after shorting, 1st
cycle irreversible loss was reduced significantly. However, the behav­
iour of the CV remained unchanged which was also noticed in Fig. 5. The
charge-discharge capacity was also calculated from the CV experiments
(Fig. 6a and b). It was found that the 1st and 2nd cycle specific discharge
capacity are ~620 and ~227 mAh g 1 respectively for without chemical
sodiated HNTOZr sample. However, for 60 min chemically sodiated
HNTOZr electrode showed 1st and 2nd cycle specific discharge capacity
of 293 and 212 mAh g 1 respectively. From the CV data, we also found
that the first cycle irreversible loss was reduce from 63.38% to 27.64%
and this reduction is significant and consistent with results of galvano­
static experiments. (This value is slightly changes because the CV was
done much slower rate compare with Galvanostatic charge-discharge
process). From Fig. 5h and i and Fig. 6a and b, it was clear that the 1st
irreversible loss is not only due to the electrolyte decomposition or
chemical SEI formation, and also due to the irreversible Naþ trap inside
the sodium titanium oxide matrix [12,39].
Fig. 6c displayed the EIS of without chemically sodiated HNTOZr at a
different potential, and Fig. 6d showed the Nyquist plot of chemically
sodiated HNTOZr at a different time from 0 to 120 min. The impedance
measurement was done with a fixed electrode area of 0.785 cm2 with 10
mV perturbation in the frequency range from 1 MHz to 100 mHz in
automatic swipe mode from high to low frequencies. Generally, in
Nyquist plot, the initial impedance (Re) is due to the Ohmic resistance of
the cell component as well as electrolyte resistance. Then the small
semicircle at higher frequency zone is due to the sodium-ion migration
across the surface of the electrode known as surface film resistance (Rs).
The second big semi-circle at middle-frequency zone is attributed for
charge transfer resistance (Rct) between the electrolyte and surface of
the electrode and the final impedance at very lower frequency zone look
like an inclined straight line, or curvature is known for solid-state
diffusion or Warburg resistance/factor (Wd) [40–42]. The
charge-transfer resistance (Rct) initially increased from OCV to 0.98 V
(286.10 to 738.9.0 Ω), then further increased at 0.89 V (1214 Ω);
hereafter the value is reduced further upon potential reduction (387.3,
electrode shown in Fig. 5g; (2) full-cell constructed with NVP vs. 90 min
chemically shorted HNTOZr (Fig. 5h); and (3) extra 10% NVP was taken
to make sodium-ion full-cell against 90 min chemically shorted HNTOZr
(Fig. 5i). In 1st case, full-cell delivered a specific discharge capacity of
~60 mAh g 1 at 50 mA g 1 current rate (specific capacity and current
rate were calculated on the active mass of cathode materials, and here,
active cathode to anode material mass ratio in the full-cell was 2:1) with
a high first cycle irreversible loss of 45%. It showed very poor Coulombic
efficiency of 46% (here Coulombic efficiency is discharge capacity by
charge capacity), and in the 2nd cycle, the discharge capacity was further
decreased to ~ 45 mAh g 1. However, in the 2nd case, it provided a
specific discharge capacity of 104 mAh g 1 at the same current rate with
an improved 1st cycle Coulombic efficiency of 72%. Here, the 1st cycle
irreversible loss was reduced to 24% only. On the other hand, when we
added 10% extra NVP on the system (3rd case), the 1st cycle irreversible
loss was further reduced to 13% and 1st cycle Coulombic efficiency was
gained to 82%. In this procedure, the first cycle specific capacity (116
mAh g 1) was achieved to the half-cell theoretical capacity of NVP. The
1st cycle irreversible loss in Fig. 5h and i is due to the Naþ trapped inside
the anode electrode which was not coming back during successive cycles
and maybe some interlayer phase is also forming in the cathodic side
[39]. So, by this method, these full-cells were reflecting the above
half-cell (chemically shorted HNTOZr vs. Naþ/Na) performances.
2.4. Chemical shorting effect on electrochemical performances
To understand the effect of chemical shorting, we have attempted the
electrochemical experiments first to understand the electrode/electro­
lyte interface stability and its chemical nature. The cyclic voltammetry
(CV) and electrochemical impedance spectroscopy (EIS) were analyzed
for both shorted and without shorted HNTOZr electrodes. Fig. 6a and b
revealed the CV of HNTOZr and 60 min chemically shorted HNTOZr
respectively at a scan rate of 0.05 mV s 1 over the potential window of
0.01–2.2 V. The first reduction peak observed at ~0.97 V for HNTOZr
electrode (Fig. 6a), is due to the electrolyte decomposition or SEI for­
mation on the electrode and disappeared after 1st cycle (although some
small peak observed up to 5th cycles shown in the zoom portion) [10,11,
30]. However, this reduction peak was shifted towards lower potential
~0.83 V after 1 h chemically shorting. The 2nd reduction peak remained
at the same position for both cases at ~0.44 V was due to the conversion
of Ti4þ to Ti3þ phase. After the 1st cycle, 2nd reduction peak shifted to the
potential at 0.28 V and remain fixed for further cycles which is desirable
Fig. 6. Cyclic voltammetry (CV) profile at a
scan rate of 0.05 mV s 1 over the potential
window 0.01–2.2 V vs. Naþ/Na of (a)
HNTOZr, (b) 60 min chemically shorted
HNTOZr, and (c) EIS of fresh HNTOZr elec­
trode at a particular potential) during 1st
discharge process at a very slow current rate
of 20 mA g 1, and (d) electrochemical
impedance spectroscopy (EIS) at their OCV
of various time chemically shorted HNTOZr
(potential where the chemically shorted
HNTOZr showed OCV) showing similar type
impedance behaviour to the 1st discharge
process.
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Nano Energy 70 (2020) 104520
339.7 and 214.9 Ω respectively), and shown in Fig. 6c and Table S5. In
the initial state of the discharge process, sodium-ion start to absorbed on
the electrode surface and also formed SEI layer (up to 0.89 V); as a result,
a sharp increase in charge transfer resistance was observed. However,
once the SEI formed, sodium-ion start to migrate inside the electrode
matrix, form a conductive phase which reduced the charge transfer
resistance upon further discharge (up to 0.01 V) [30,43]. Chemically,
sodiated HNTOZr electrode also showed the similar type of impedance
behaviour like fresh HNTOZr electrode (discuss in details in the SI,
Fig. S9). As sodiation time was increased from 0 (OCV - 2.56 V) to 30 (at
0.98 V) minutes, the Rct value increases (286.1 Ω to 717 Ω) and when it
was kept for 60 min (at 0.89 V) sodiation, the Rct value further increased
to 981.4 Ω. However, further increased the sodiation time 90 min (0.83
V) to 120 min (0.35 V) the overall impedance reduced further (309.5
and 241 Ω respectively), presented in Fig. 6d. CV curve also revealed
that the electrolyte decomposition is stared from 0.98 V and at this
particular potential, the overall impedance increased, and after
completion of this decomposition (at 0.84 V), the overall impedance
reduced further. All the EIS fitting parameters with corresponding cir­
cuit model were shown in Table S5 and Fig. S9 respectively.
To understand the effect of chemical sodiation on sodiation mecha­
nism, CV at different scan rates from 0.05 to 1.0 mV s 1 are employed as
displayed in Figs. S10a and b for both chemically shorted (60 min) and
without modified HNTOZr electrodes. Both the CV curves showed
similar characteristic behaviour. To understand the sodium insertion
mechanism, power law, ipeak ¼ aυb (where ipeak is cathodic current, υ is
scan rate, a and b is constant) was introduced here. By this power-law,
one can easily identify if the process is diffusion-controlled or
capacitance-controlled or a combination of both effects. If the value of b
~0.5, then the method is controlled by diffusion process whereas, if b is
~1.0, the process is capacitance controlled [44]. By the using of the
above power law, we calculated the value of b is ~0.80 for the fresh
HNTOZr electrode. However, it was ~0.74 for chemically shorted
HNTOZr, shown in Figs. S10c and d (where b is the slope of the curve).
So, the sodiation mechanism is controlled by both intercalation and
absorption process. However, intercalation process became more
dominant after shorting of HNTOZr chemically.
2.5. Chemical shorting effect on physical property of the electrode
materials
The morphological characterization (FEG-SEM and FEG-TEM) of
chemically shorted HNTOZr electrode with various time duration
(Fig. 7). Fig. 7a–e exhibited the FEG-SEM images of chemically shorted
HNTOZr electrode at 0 min, 30 min, 60 min, 90 min, and 120 min
respectively. The FEG-SEM image of fresh electrode revealed the sodium
titanium nanorods are well connected with conducting carbon additives
(Fig. 7a). However, the other SEM images showed porous and
Fig. 7. Physical characterization by FEG-SEM and FEG-TEM after chemically shorted HNTOZr electrode at various time; (a–f) FEG-SEM images of HNTOZr electrode
(a) without chemical shorted showing proper mixing of HNTOZr nanorods and conducting carbon, (b) with 30 min shorted, (c) with 60 min chemically shored, (d)
with 90 min shorted, (e) with 120 min shorted showing with increasing the sodiation time thickness of the deposited layer (SEI) increasing and covering the surface
of the electrode, (f) after 1st discharge viewing partial deposition of SEI layer; (g–j) FEG-TEM image of chemically shorted at various time (g) 30 min, (h) 60 min, (i)
90 min and (j) 120 min showing average SEI layer thickness ~2 nm, 7 nm, 10 nm and 11 nm respectively.
8
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Nano Energy 70 (2020) 104520
transparent sheets were deposited on the surface of the electrode and
thickness of this deposited layer was increased with shorting time.
Generally, when sodium was shorted with the electrode in the presence
of the electrolyte, immediately the sodium-ion was started to react with
electrolyte as well as electrode and decomposed product (SEI) deposited
on the surface of the electrode and with time, the deposited layer had
covered the entire surface of the electrode, and made this layer thicker.
When we keep sodium on electrolyte and put electrode on it without
contact with sodium foil, no deposition was observed, as well as no
improvement in 1st cycle Coulombic efficiency or no reduction in the 1st
cycle irreversible loss, (SI, Fig. S11). The same phenomena were also
observed by FEG-TEM images shown in Fig. 7f–i. The average thickness
of the SEI layer was noticed in 30 min chemically shorted HNTOZr was
~2 nm and 7.5 nm when it was shorted for 60 min. However, the SEI
layer thickness was increased to 10 nm with equal to or more than 90
min shorting (Fig. 7i,j and Fig. S12).
To understand the chemical shortening mechanism and the compo­
nents of SEI composition, a details ex-situ XPS of chemically sodiated
HNTOZr electrodes and HNTOZr electrode after 1st discharge process
was analyzed in details (Fig. 8). For more accurate SEI analysis, we did
not add any conducting carbon, and minimum amount of CMC binder
was mixed to the chemically shorted electrode. It was noticed earlier in
Fig. 2c that the pure HNTOZr materials have mainly Ti4þ phase. When it
was kept for 90 min chemical shorting, 43% of Ti4þ was converted to
Ti3þ oxidation state, and 47% of Ti4þ was converted to Ti3þ phase after
120 min chemical shorting, projected in Fig. 8a and b. Here, for all the
cases, Ti4þ 2p3/2 absorption peak position moved towards lower binding
energy (from 459.44 eV to 459.29 eV with 90 min, 120 min chemical
shorting). Whereas Ti3þ 2p3/2 peak position was shifted towards lower
binding energy (458.66 to 458.63 eV) with shorting time increasing
from 90 to 120 min (Fig. 8a and b). However, after 1st discharge process,
Ti4þ phase was converted to lower oxidation state Ti3þ phase
completely, and this Ti3þ 2p3/2 spectrum was further shifted towards
lower binding energy side (558.20 eV) present in Fig. 8c. Similarly,
during 120 min chemical shorting process, partial of the Ti4þ phase was
reduced to Ti3þ phase, and this Ti3þ 2p3/2 position was shifted towards
lower binding energy side (458.63 eV).
C1s absorption peaks of all above HNTOZr (Fig. 8d–f) provided in­
formation about the decomposed components. The fresh HNTOZr elec­
trode showed absorption peaks at 284.28 eV, 285.45 eV, 287.12 eV and
Fig. 8. XPS analysis of HNTOZr after chemical shorting and cycling process to analyze the SEI compositions; (a) Ti 2p absorption peaks of HNTOZr-90 min showing
mixed phase of Ti4þ and Ti3þ, (b) Ti 2p peak of HNTOZr after 120 min chemical sodiation presenting Ti3þ major phase, (c) Ti 2p peak of HNTOZr after 1st discharge
process showed Ti3þ phase; (d–f) C1s spectrum of (d) fresh HNTOZr electrode showed major C¼C bond, (e) HNTOZr after 120 min chemical sodiation (f) HNTOZr
after 1st discharge; (g–i) O1s absorption peak of HNTOZr at different conditions, (d) HNTOZr after 90 min chemical sodiation, (e) HNTOZr 120 min chemical
sodiation, and (f) HNTOZr after 1st discharge.
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Nano Energy 70 (2020) 104520
288.83 eV ascribed to the C¼C, O–C/C–H, C¼O and O–C¼O bond
respectively [45,46]. The most prominent peak due to C¼C sp2 bond at
284.28 eV is coming from the sp2 conducting carbon adding to enhance
the electronic conductivity of the electrode and others carbonate and
bicarbonate bond is due to the CMC binder present at the electrode.
However, after chemical shorting for 90 min, this C¼C sp2 peak intensity
(284.74 eV) was reduced significantly shown in Fig. S20. The other
characteristic peaks were noticed at 285.75 eV, 287.26 eV, 287.92 eV
and 289.0 eV corresponds to C–O or C–H, C¼O, O–C¼O, and C–F bonds
[46,47].
As the chemical shorting time increases to 120 min, C¼C sp2 peak
thick was disappeared as the SEI layer has entirely covered the surface of
the electrode as shown in Fig. 7e (as XPS is a surface technique, C¼C
peak did not detect). However, the other peaks were noticed at the
binding energy of 285.51 eV (C–H or C–O), 287.16 eV (C¼O), and
289.04 eV (O–C¼O). After 1st discharge process of the fresh electrode,
sp2 absorption peak became less prominent, as shown in Fig. 7f (after 1st
discharge the SEI did not deposit all over the surface of the electrode; as
a result, weak absorption peak due to C¼C bond was seen). Other C1s
peaks were little bit shifted towards lower binding energy and observed
at 285.34 eV (C–H or C–O), 286.47 eV (C¼O), 288.25 eV (O–C¼O) and
290.17 eV (C–F, as the electrolyte, contain FEC, formed C–F at higher
potential <1.0 V).
XPS analysis of oxygen (O1s) spectrum was shown to specify the
components formed after chemical shorting (Fig. 2g and h). After
chemical shorting, one extra O1s absorption peak appeared at 531.91
eV, and 531.91 eV for 60 and 90 min shorted HNTOZr electrode
respectively (was not observed in the case of fresh HNTOZr electrode in
Fig. 2g–i). The other O1s peak at lower binding energy attributed to
Ti4þ-O peak was observed at the binding energy of 530.69 eV and
530.65 eV for 60 min, 90 min chemically shorted HNTOZr respectively
(Fig. 2g and h). It was interesting to notice that after chemical sodiation,
O1s peak at 533.33 eV became more dominant. This higher binding
energy O1s peak is generally ascribed to the Na–O bond as well as ox­
ygen bond associated with carbonaceous materials (main decomposed
products) (Fig. 8g–i) [9,12,48]. Similar type of behaviour of O1s spec­
trum for fresh HNTOZr also notice after 1st discharge process. Here, O1s
peak displayed at 531.42 eV and 530.45 eV represented Ti3þ-O bond and
Ti4þ-O bond respectively. The Na 1s absorption position did not change
with chemical sodiation or electrochemical charge-discharge process
(even after 3000 cycles Na 1s position did not alter), shown in Fig. S21a.
The fluorine absorption peaks notice at 682.52 eV, and 686.78 eV rep­
resented C–F and Na–F bonds respectively (Fig. S21b). The XPS survey
scans of chemically shorted HNTOZr at different time displayed in
Fig. S22. These survey scan confirmed all elements (Na, Ti, O, C, F, Cl,
and Zr) were present in the electrodes. XPS analysis of HNTOZr elec­
trode after discharge-charge process with and without chemical shorting
process also shown in Fig. S24 (also discussed in details IN SI). The XPS
of HNTOZr electrode after 3000 cycles showed the major C 1s peaks of
C–H or C–O (285.0 eV), C¼O (286.57 eV), O–C¼O (288.57 eV) and C–F
(289.50 eV), shown in Fig. S24c. To know further about the SEI com­
positions, FEG-SEM with EDS and XPS of HNTOZr electrode after 3000
cycles were shown in Figs. S25–26. SEM images showed a porous thick
SEI layer (mainly composites of C, O, Na, F, Cl, Ti analysis from EDS
mapping) was formed. As we observed in Fig. S25, the HNTOZr
morphology after 3000 cycles did not change at all. However, the SEI
thickness was increased, and hence porosity decreases significantly so
that it was not porous enough to intercalate the larger size sodium-ion
into HNTOZr matrix and as a consequence the cell died.
impurities present. So, during the discharge process on the anode sur­
face, various type of reduction process is happening simultaneously that
compete with each other. This reaction highly depends on reduction
potential, activation energy, exchange current density as well as the
surface morphology on the anode electrode, temperature, concentration
of electrolyte salt and reduction current rate, etc. [46,51,52] It is also
very difficult to understand the SEI mechanism when all these factors are
combined together especially for SIB system where no significant liter­
ature are present.
The details SEI formation mechanism was shown in Fig. S27.
Initially, Na2O and NaClO4-x are precipitated on the electrode surface by
the decomposition of NaClO4 salts. Whereas, additive FEC was reacted
with H2O (trace amount of H2O presence in the salt) and Naþ to form
sodium carbonate (Na2CO3), sodium alkyl carbonate (ROCO2Na), HF
and other organic compounds. In the meantime; NaF, Na2O, Na2CO3,
NaHCO3, and other products are deposited on the electrode surface as a
SEI layer due to Naþ reduction with EC and PC in subsequent steps as
shown in Fig. S27. During this process, a sufficient amount of CO2 and
CO gas also produced. The SEI formation is started near 1.0 V and
continued to 0.2 V in different steps. Initially, when the working po­
tential is just below 1.0 V, salt and FEC additive are started to react and
form new species which is further decomposed and precipitated on the
electrode surface, and when its voltage is reduced ~0.9 to 0.8 V, elec­
trolyte (solvent) was starting to decompose with FEC and make more
thick SEI. Moreover, when the potential reached below 0.5 V, chemical
sodium intercalation process is started, and the SEI formation continued
until all the polarized sodium titanium oxide is covered with it.
So, from the above observation, it is clear that the 1st cycle irre­
versible loss was significantly reduced by simple chemical shorting
process. By adopting this process, the 1st cycle irreversible loss was
minimized to ca. 4.11% from 53.86% and initial cycle Coulombic effi­
ciency was enhanced from 36.22% to 84%. The chemical shorting pro­
cess also enhances the 1st cycle Coulombic efficiency (46% to 82%) and
1st cycle irreversible loss reduced to 45%–13% of the sodium-ion fullcell (NVP against chemically shorted HNTOZr). The 1st cycle irreversible
loss is mainly due to two reasons (1) irreversible sodium-ion trap inside
the anode (HNTOZr). Here, we find that some of the sodium-ions are
encapsulated inside the HNTOZr matrix which never back to the system
again and (2) sodium-ion consumption during the SEI formation which
is one of the most necessary conditions for long life stable performance.
The SEI is forming by following few steps; initially salts (NaClO4), ad­
ditive (FEC) and impurities (H2O) are decomposed to form new chemical
species during the anodic polarization, and then this new chemical
species further decompose with electrolyte (solvent, here, EC and PC)
and precipitated on the surface of the electrode. In the 1st discharge
process, most of the SEI is form yet it continued for further cycles to
make it smooth thick porous layer. However, we need a critical SEI layer
thickness for these prolonged stable performances. Beyond this, the SEI
layers are no more porous enough to transport the large size sodium-ion
inside the active anode materials which lead to the sudden death of the
battery.
3. Conclusion
In summary, we have synthesized one-dimensional Zr-doped HNTO
as a suitable anode material for SIBs by a simple hydrothermal process
followed by hydrogenation technique. The optimized HNTOZr nanorods
anode materials provided excellent electrochemical performance for
SIBs in terms of high average discharge capacity of ~200 mAh g 1 at a
current rate of 200 mA g 1 and prolonged cycling stability up to 2500
cycles with an average 99.70% Coulombic efficiency with excellent rate
capability. However, this anode material was suffered from high 1st
cycle irreversible loss (53.86%) with a low Coulombic efficiency of
36.22%. We successfully suppress this issue by simple chemical shorting
of the electrode with sodium metal in the presence of electrolyte me­
dium. By adopting this process, the 1st cycle irreversible loss was
2.6. SEI formation mechanism
From the above pieces of evidence, the SEI formation mechanism
was proposed with the help of existing LIB literature [45–47,49,50]. In
electrolyte there are lots of components; mainly salt (NaClO4), solvents
(EC and PC), additive (FEC) and a trace amount of water and air
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Nano Energy 70 (2020) 104520
minimized to 3.64% and Coulombic efficiency increases to 84% without
compromising the specific capacity and cycling stability. The shorting
process also improves the intercalation kinetics (became more diffusive
controlled after shorting). This approach also improved the full-cell
performance; construct with NVP vs. chemically shorted HNTOZr.
Here, the 1st cycle irreversible loss was minimized to 13% from 45% and
1st cycle Coulombic efficiency was improved to 82%. As chemical SEI
formation has an essential role towards this 1st cycle irreversible loss, a
detail investigation on decomposed components of chemically shorted
HNTOZr electrode as well after 1st discharge of fresh HNTOZr electrode
was analyzed by sophisticated XPS technique. We found NaF, Na2CO3,
NaHCO3, and other organic hydrocarbons are present and the main
components of this decomposed layers on the electrode surface after
chemical shorting. Initially, salts, additive, and impurities (H2O) were
decomposed to form new chemical species during the anodic polariza­
tion and then this new chemical species further decomposed with elec­
trolyte (solvent, here, EC and PC) and precipitated on the surface to form
SEI. This SEI formation was continued until the entire surface was
covered. This chemical shorting process was able to overcome the first
cycle irreversible loss which can enhance the overall energy density.
This simple method also helps to sodiated any cathode or anode mate­
rials (electrode does not contain sodium source) and opens up a new
direction for paring of sodium electrode for next-generation high energy
density batteries.
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Declaration of competing interest
The authors declare that the research was conducted in the absence
of any commercial or financial relationships that could be construed as a
potential conflict of interest.
Acknowledgments
The authors are thankful for the financial support provided by MHRD
(Ministry of Human Resource Development) and NCPRE (Grant No. 31/
09/2015-16/PVSE-R&D) funded by the Ministry of New Renewable
Energy, Govt. of India. The authors are indebted to SAIF, IIT-Bombay for
their assistance in FEG-TEM, NCPRE for FEG-SEM facility, and DESE for
XRD analysis and IRCC, IIT Bombay for their assistance in XPS and BET
studies. We are also grateful to Dr. Sudeep Sarkar and all the ECEL
members especially Md. Adil, Amlan Roy, Mohammad Furquan and
Divyamahalakshmi Muthuraj for their continuous support and
encouragement.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.nanoen.2020.104520.
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Ananta Sarkar received his master’s degree in chemical engi­
neering department from Jadavpur University, India, in 2013.
Recently, he obtained his Ph.D. degree (2014–2019) from the
Department of Energy Science and Engineering at Indian
Institute of Technology Bombay, India. His research interests
focus on the development of sodium-ion batteries and nextgeneration battery materials for commercialization of cell
technology for EV and energy storage applications.
C. V. Manohar received his Ph.D. degree (2014–2019) in IITBMonash Research Academy (a joint research collaboration be­
tween Indian Institute of Technology Bombay and Monash
University). He is currently working as Manager for Energy
Technology in GODI and his research work mainly focuses on
Lithium-ion batteries, sodium-ion batteries and next-generation
battery materials for commercialization of cell technology for
EV and energy storage applications.
Sagar Mitra is a Professor in the Department of Energy Science
and Engineering, Indian Institute of Technology Bombay,
Mumbai, India. He obtained his Ph.D. from the Indian Institute
of Science, Bangalore, India, in 2004. He worked as Post­
doctoral Fellow at UPS and UJV, France (2004–2006). He is the
recipient of “Principal Electrochemist” at Replisaurus Tech­
nologies, Sweden (2006–2009). He is currently leading the
battery research team in National Centre for Solar Photovoltaic
Research and Education (funded by Ministry of New and
Renewable Energy, Govt. of India) at IIT Bombay and his cur­
rent research mainly focused on advanced materials and tech­
nology development for different energy storage applications,
particularly in lithium-ion batteries and sodium-ion batteries.
12