Download (DMMP) Electrolytes Containing Dimethylmethylphosphonate

Improved Performance of LiNi0.5Mn1.5O4 Cathodes with
Electrolytes Containing Dimethylmethylphosphonate
(DMMP)
Mengqing Xu, Dongsheng Lu, Arnd Garsuch and Brett L. Lucht
J. Electrochem. Soc. 2012, Volume 159, Issue 12, Pages A2130-A2134.
doi: 10.1149/2.077212jes
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© 2012 The Electrochemical Society
A2130
Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012)
0013-4651/2012/159(12)/A2130/5/$28.00 © The Electrochemical Society
Improved Performance of LiNi0.5 Mn1.5 O4 Cathodes with
Electrolytes Containing Dimethylmethylphosphonate (DMMP)
Mengqing Xu,a,∗ Dongsheng Lu,a Arnd Garsuch,b and Brett L. Luchta,∗∗,z
a Department of Chemistry, University of Rhode Island, Kingston,
b BASF SE, GCN/E, Ludwigshafen am Rhein, 67056, Germany
Rhode Island 02881, USA
The cycling performance of Li/LiNi0.5 Mn1.5 O4 cells with 1.0 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC)
(3:7) with and without added dimethyl methylphosphonate (DMMP) (0.5–1.0%) was investigated. Addition of DMMP resulted in
improved capacity retention during cycling to high voltage (4.9 V vs Li). Ex-situ surface analysis of LiNi0.5 Mn1.5 O4 electrodes
after cycling via Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Infrared (IR) spectroscopy
suggest that addition of DMMP inhibits electrolyte decomposition on the surface of the cathode. Addition of DMMP also inhibits
the dissolution of Mn from LiNi0.5 Mn1.5 O4 particles stored in electrolyte at 85 ◦ C.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.077212jes] All rights reserved.
Manuscript submitted August 13, 2012; revised manuscript received September 10, 2012. Published October 23, 2012.
Lithium ion batteries have the best gravimetric and volumetric energy density of commercial rechargeable batteries. The cathodes are
typically composed of lithium transition metal oxides or phosphates
and the anode is graphite, which provides a cell with a high operating
voltage (3 ∼ 4.5 V) and high capacity (150 ∼ 240 mAh/g).1 Due to the
high energy density, lithium ion batteries are being extensively pursued for transportation applications, such as hybrid electric vehicles,
plug-in hybrid electric vehicles, and fully electric vehicles. However,
cost, safety, cycle life, energy and power are some of the major hurdles for implementation of lithium ion batteries into transportation
applications.2
In pursuit of higher energy density lithium ion batteries, there is
growing interest in cathode materials that operate at high voltages
(>4.5 V vs Li) and exhibit higher energy densities. Great progress
has been achieved in developing new cathode materials, including
LiNi0.5 Mn1.5 O4 (4.9 V)2,3 and Li2 MnO3 -LiMO2 (4.6 V).4,5 However,
a major difficulty in utilizing these high-voltage cathodes is the instability of the organic electrolytes in contact with the cathode surface
at high operating voltages (>4.5 V).2,6 Significant effort has been directed to inhibit the detrimental reactions of the LiNi0.5 Mn1.5 O4 with
electrolyte at high voltage, including surface coating with various
inorganic oxides, including Al2 O3 , ZnO, and Bi2 O3 .7–9 The surfacecoated cathodes have superior cyclability compared to the uncoated
material, however; the surface-coating method has a negative effect on
the discharge capacity of the material. Other methods of improving the
performance include doping with other transition metals such as Fe10
and Cr11 and the incorporation of electrolyte additives.12–14 The development of a thorough understanding of the reactions of electrolyte
with the surface of high voltage cathode materials and the development of novel electrolytes capable of long term reversible cycling to
high voltage is of great current interest.
Recently, Xing et al investigated the oxidative decomposition
mechanism of PC with and without PF6 − and ClO4 − .15 They found
that the presence of PF6 − and ClO4 − anions significantly reduces PC
oxidation stability, stabilizes the PC-anion oxidation decomposition
products, and changes the order of the oxidation decomposition paths.
Interestingly, they also found that HF and PF5 were detected upon the
initial step of PC-PF6 − oxidation while HClO4 formed during initial
oxidation of PC-ClO4 − .16 In addition, it has been reported that the
thermal instability of LiPF6 in carbonate solvents leads to the generation of acidic species including PF5 and HF.17 The presence of acidic
species in the electrolyte can lead to corrosion of the surface of the
metal oxide and dissolution of transition metals.18 This is especially
problematic with LiMn2 O4 or LiNi0.5 Mn1.5 O4 .18,19 However, the incorporation of Lewis basic species can inhibit the generation of acidic
species and metal oxide surface corrosion.18,20,21
∗
Electrochemical Society Student Member.
Electrochemical Society Active Member.
z
E-mail: [email protected]
∗∗
As a expansion of our investigation of Lewis basic additives
designed to thermally stabilize LiPF6 electrolytes,20,21 we have investigated, dimethyl methylphosphonate (DMMP), as an electrolyte
additive to improve the performance of LiNi0.5 Mn1.5 O4 cathode material cycled to high voltage (4.9 V vs Li). DMMP has previously
been investigated as a non-flammable co-solvent which is compatible
with the electrode materials.22–24 The electrochemical performance
of Li/LiNi0.5 Mn1.5 O4 cells with DMMP containing electrolyte has
been investigated and the surface of LiNi0.5 Mn1.5 O4 electrodes have
been characterized to provide a better understanding of differences in
performance.
Experimental
Battery grade solvents, ethylene carbonate (EC) and ethyl(methyl)-carbonate (EMC), and lithium hexafluorophosphate (LiPF6 )
were provided by Novolyte Technologies (U.S.). Dimethyl
methylphosphonate (DMMP) was purchased from Aldrich, followed
by distilling and soaking with 4Å molecule sieves before use. 1.0 M
LiPF6 EC/EMC (3/7, v/v) electrolyte was prepared as the standard
electrolyte (STD). The concentration of DMMP was 0.5% and 1%
(wt.), respectively. LiNi0.5 Mn1.5 O4 powder is provided by Hunan
Shanshan advanced materials Co. LTD (China).
The cathode electrode is composed of 89% LiNi0.5 Mn1.5 O4 , 6%
conductive carbon, and 5% PVDF. 2032-type coin cells were assembled with lithium foil anode, LiNi0.5 Mn1.5 O4 cathode, and Celgard
2325 separator. Each cell contains 30 μL electrolyte. Cells were built
in triplicate with good reproducibility. The cells were cycled with
a constant current-constant voltage charge and constant current discharge between 3.5 to 4.9 V with Arbin BT2000 cycler according
to following protocol: 1st cycle at C/20; 2nd and 3rd cycles at C/10;
remaining cycles at C/5. All cells were produced in triplicate and
representative data is provided. After 50 cycles the cells were opened
and the LiNi0.5 Mn1.5 O4 cathodes were extracted, washed with anhydrous DMC three times to remove the residual EC and LiPF6 salt, and
vacuum dried overnight at room temperature for XPS, SEM, and IR
analysis.
The samples for thermal storage were prepared in a high purity argon filled glove-box as described previously.17 The vials were charged
with 0.1 g LiNi0.5 Mn1.5 O4 powder followed by addition of 2 mL standard and 1% DMMP electrolyte, respectively. The vials were flame
sealed under reduced pressure. Care was given to avoid contamination of the vial walls near the sealing point. The sealed samples were
transferred to an oil bath of 85◦ C for 4 days. Samples were weighed
before and after thermal storage to confirm seal. After thermal storage, vials were opened in an argon filled glove-box. The solid cathode LiNi0.5 Mn1.5 O4 sample was separated from the electrolyte and
then washed with dimethyl carbonate (DMC) three times followed
by drying in vacuum. The resulting cathode particles were analyzed
by scanning electron microscopy (SEM) and X-ray photoelectron
Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012)
STD
0.5% DMMP
1% DMMP
120
98
96
94
100
92
80
STD
0.5% DMMP
1% DMMP
60
Efficiency, %
140
Capacity, mAh/g
Results and Discussion
100
160
90
88
0
5
10
15
20
25
30
35
40
45
A2131
50
Cycle Number
Figure 1. Cycling performance of Li/LiNi0.5 Mn1.5 O4 cells with standard
(STD) and DMMP containing electrolyte.
spectroscopy (XPS). The residual electrolyte solutions were analyzed
by ICP-MS to determine Mn and Ni content.
The XPS spectra were acquired with PHI 5500 system using Al Kα
radiation (hν = 1486 eV) under ultrahigh vacuum. Lithium was not
monitored due to its low inherent sensitivity and small change of binding energy. Calibration of XPS peak position was made by recording
XPS spectra for reference compounds, which are present on the electrode surface: LiF, PVDF, and lithium alkyl carbonate. The graphite
peak at 284.3 eV was used as a reference for the final adjustment
of the energy scale in the spectra. The spectra obtained were fitting
using XPS peak software (version 4.1). Linear syntheses of elemental
spectra were conducted using Gaussian-Lorentzian (80:20) curve fit
with Shirley background subtraction. Scanning electron spectroscopy
(SEM) was conducted on a JEOL-5900 SEM. FTIR spectra were acquired on a Bruker (Tensor 27) with attenuated total reflection (ATR)
mode with 4 cm−1 resolution and a germanium crystal and 128 scans.
Cycling performance of Li/LiNi0.5 Mn1.5 O4 cells.— The cycling
performance of Li/LiNi0.5 Mn1.5 O4 cells with standard electrolyte
(1.0 M LiPF6 in EC/EMC (3:7)) and electrolyte containing 0.5%
and 1% added DMMP after initial formation cycles are depicted in
Figure 1. The cells were cycled from 3.5 to 4.9 V vs. Li. The cell
with 0.5% DMMP electrolyte shows slightly higher initial discharge
capacity than the other cells, 129.1 mAh/g compared to 122.9 mAh/g
(standard) and 120.5 mAh/g (1% DMMP). This higher initial discharge capacity is ascribed to the higher coluombic efficiency of the
cell cycled with electrolyte containing 0.5% DMMP during formation
cycles. Cycling cells at room temperature results in moderate capacity
fade for all cells. However, the cells with DMMP added electrolyte
have superior capacity retention to the cells containing standard electrolyte after 50 cycles, 76.8%, 70.0%, and 64.8% capacity retention,
for 0.5% DMMP, 1% DMMP, and standard electrolyte, respectively.
Scanning electron microscopy (SEM) of LiNi0.5 Mn1.5 O4
electrodes.— The SEM images of LiNi0.5 Mn1.5 O4 cathodes fresh and
after 50 cycles with standard electrolyte and DMMP containing electrolytes are depicted in Figure 2. Aggregated cathode particles and
conductive carbon are present on the fresh electrode. The aggregated
particle is composed of primary particles with octahedral structure,
ranging from 1–5 μm (Figure 2a). After 50 cycles, the surface of
LiNi0.5 Mn1.5 O4 particles are altered. The cathode particles cycled with
standard electrolyte have a smooth surface and it is difficult to distinguish the primary particles (Figure 2b). The cells cycled with the
DMMP containing electrolytes have surfaces that more resemble the
fresh cathode that the cathode cycled with standard electrolyte.
X-ray photoelectron spectroscopy (XPS) analysis of
LiNi0.5 Mn1.5 O4 electrode.— In order to better understand the
changes to the surface of the LiNi0.5 Mn1.5 O4 electrode upon cycling
at high voltage, electrodes cycled with standard electrolyte and
DMMP containing electrolyte were analyzed by XPS. Table I
contains the elemental concentrations of LiNi0.5 Mn1.5 O4 electrode
before and after cycling. After 50 cycles, the concentrations of C, F
and Mn are decreased; while O and P are increased. The changes to
Figure 2. SEM images of LiNi0.5 Mn1.5 O4 electrodes: fresh electrode (a); after 50 cycles with
standard electrolyte (STD) (b); after 50 cycles
with 0.5% DMMP containing electrolyte (c); and
after 50 cycles with 1% DMMP containing electrolyte (d).
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Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012)
Table I. Relative elemental concentrations of LiNi0.5 Mn1.5 O4
electrode before and after cycling.
fresh
STD
0.5% DMMP
1% DMMP
C1s%
O1s%
F1s%
48.2
46.9
47.9
45.1
12.0
18.5
16.8
19.6
28.7
26.4
25.0
24.7
P2p%
Mn2p%
Ni2p%
2.3
0.7
0.3
7.5
1.5
4.4
5.3
3.5
4.4
5.2
4.9
1% DMMP
0.5% DMMP
STD
the surface concentrations are consistent with the presence of a thin
surface layer.
The XPS element spectra are provided in Figure 3. The C 1s XPS
spectrum of the fresh LiNi0.5 Mn1.5 O4 electrode contains three peaks.
The peak at 284.3 eV is assigned to conductive carbon, and the peaks
at 285.7 and 290.4 eV are characteristic of the PVDF binder. The F1s
peak of PVDF is also observed at 687.6 eV. The O 1s spectrum of
fresh LiNi0.5 Mn1.5 O4 electrode is dominated by metal oxide (529 eV),
and contains a very low concentration of Li2 CO3 (531.6 eV), which
is frequently present on fresh cathode particles.18
After 50 cycles, new species corresponding to the electrolyte decomposition products are formed on the electrode surface, as observed
in C 1s, O 1s, F 1s, and P 2p XPS spectra. Analysis of the XPS
spectra reveals significant difference between the electrode cycled in
standard electrolyte and DMMP added electrolyte. In addition to the
peaks present on the fresh electrode, new peaks consistent with C-O
(286 eV) and C=O (288 eV) containing species are present in the C 1s
spectrum of the LiNi0.5 Mn1.5 O4 electrode cycled with either standard
or DMMP containing electrolyte. The corresponding peaks for C=O
(532–533 eV) and C-O (533–534 eV) containing species are observed
in the O1s spectrum. C-O and C=O species are consistent with the
presence of lithium alkyl carbonates and polycarbonates. The intensity
of the peak of the metal oxide in the O 1s XPS spectrum is greater for
the electrodes cycled with electrolyte containing 0.5% DMMP or 1%
DMMP than electrodes cycled with standard electrolyte, suggesting a
thinner surface layer formed on the electrode surface with electrolytes
containing DMMP. The F 1s spectra contain new peaks at 684.5 eV
characteristic of LiF. The intensity of the LiF peak is strongest for
the electrode cycled with the standard electrolyte and is much weaker
for the electrodes cycled with DMMP containing electrolytes. The P
2p spectra are similar for all cycled electrodes and contain a peak at
2200
C-C
F-C
2000
1800
1600
1400
1200
1000
Wavenumbers, cm
Figure 4. FTIR-ATR spectra of fresh LiNi0.5 Mn1.5 O4 electrode, cycled with
standard electrolyte (STD), and DMMP containing electrolyte.
∼134 eV characteristic of Lix PFy Oz . The similarity of the P2p spectra
suggests that the DMMP does not react on the surface of the cathode.
The results discussed above suggest that the surface layer formed on
the LiNi0.5 Mn1.5 O4 electrode in DMMP containing electrolyte is thinner than that formed with standard electrolyte. The thinner surface
layer formed on LiNi0.5 Mn1.5 O4 electrode with DMMP containing
electrolyte correlates with the improvement of cycling performance
to 4.9 V as shown in Figure 1.
FTIR-ATR of LiNi0.5 Mn1.5 O4 electrode.— Ex-situ IR spectra of
LiNi0.5 Mn1.5 O4 electrodes are depicted in Figure 4. The IR spectrum of a fresh LiNi0.5 Mn1.5 O4 electrode and electrodes after cycling
are dominated by absorptions of the PVDF binder at 1400, 1180,
879 cm−1 . After 50 cycles, additional absorptions are present in the
IR spectra at 1768 and 1582 cm−1 corresponding to polycarbonates
and lithium alkyl carbonates, respectively,6 which are associated with
the oxidative decomposition products of the electrolyte at high voltage
and consistent with the XPS data.
O 1s
P 2p
O-M
LixPO yFz
C=O
fresh
C-O
LixPO yFz
C=O
800
-1
F 1s
C 1s
PVDF
fresh
LiF
LixPFy
STD
0.5% DMMP
1% DMMP
Figure 3. C 1s, F 1s, O 1s and P 2p XPS spectra of LiNi0.5 Mn1.5 O4 electrode before and after cycled with standard (STD) and DMMP added electrolyte.
Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012)
A2133
Figure 5. SEM images of LiNi0.5 Mn1.5 O4 powder before and after thermal storage: fresh powder (a); storage in 1.0 M LiPF6 EC:EMC (3/7) at 85◦ C for 4 days
(b); and storage in 1.0 M LiPF6 EC:EMC (3/7) + 1.0% DMMP at 85◦ C for 4 days (c).
Scanning electron microscopy (SEM) of LiNi0.5 Mn1.5 O4 particles
before and after thermal storage.— In order to better understand the
mechanism for the improvement of the cycling performance and inhibition of cathode surface film generation in the presence of DMMP,
the reactions of fresh uncycled cathode particles with electrolyte with
and without added DMMP at elevated temperature (85◦ C) was conducted. LiPF6 electrolytes are unstable at elevated temperature resulting the generation of PF5 , HF, and other acidic flourophophate
species.17 These acidic species can react with the surface of the metal
oxide cathode particles resulting in the deposition of electrolyte decomposition products and dissolution of the transition metal ions.18
Lewis basic species, related to DMMP, have been reported to inhibit
the thermal decomposition of LiPF6 and slow cathode surface reactions and transition metal dissolution.18,21 Figure 5 shows the SEM
images of LiNi0.5 Mn1.5 O4 powder before and after storage at 85◦ C
in the presence of electrolyte. The structure of the primary particles
is severely damaged upon storage in the presence of standard electrolyte at 85◦ C for 4 days (Figure 5b). The cathode surface appears
etched and significant deposition of electrolyte decomposition products has occurred. The etching of the metal oxide is ascribed to the
generation of HF, PF5 , and related fluorophosphates at elevated temperature due to the thermal dissociation of LiPF6 . The SEM images
of the LiNi0.5 Mn1.5 O4 particles stored at elevated temperature in the
presence of electrolyte containing DMMP electrolyte reveal much
less etching of the cathode surface and deposition of electrolyte decomposition products (Figure 5c). Addition of DMMP inhibits the
thermal decomposition of LiPF6 /carbonate electrolytes which likely
protects the cathode surface from damage during storage at elevated
temperature.22
C 1s
C-O
fresh
O 1s
C-H
C=O
Table II. Relative concentrations of elements LiNi0.5 Mn1.5 O4
powder before and after thermal storage.
C1s%
O1s%
16.1
31.6
10.7
45.9
31.0
31.3
fresh
STD
1% DMMP
F1s%
18.8
29.1
P2p%
Mn2p%
Ni2p%
2.1
1.0
31.6
10.4
18.3
6.4
6.1
9.6
XPS spectra of LiNi0.5 Mn1.5 O4 powder before and after thermal
storage.— The XPS spectra of fresh uncharged LiNi0.5 Mn1.5 O4 powder before and after storage at 85◦ C for 4 days with standard electrolyte and electrolyte containing 1% DMMP are provided in Figure 6.
The C 1s spectrum of the fresh sample contains the universal C-H
contamination peak at 284.8 eV, and some C-O and C=O containing
species. The O 1s spectrum of the fresh sample is dominated by the
metal oxide peak at 529.5 eV, but also contains low concentrations
of C-O and C=O containing species, consistent with the C1s spectra and the presence of Li2 CO3 a commonly found surface species
on lithiated metal oxides.18 After storage at elevated temperature,
the LiNi0.5 Mn1.5 O4 powder is covered by electrolyte decomposition
products, as shown in Figure 6, including C-O and C=O species characteristic of lithium alkyl carbonates from the decomposition of the
carbonate solvents and LiF and Lix POy Fz from the thermal decomposition of LiPF6 . The relative elemental concentrations presented in
Table II also suggest that the surface of LiNi0.5 Mn1.5 O4 powder is covered with electrolyte decomposition products. The concentration of F
is significantly increased; while the concentrations of O and Mn are
P 2p
F 1s
O-M
Li2CO3
O-C
LixPFy
LixPOyFz
LiF
STD
1% DMMP
292 290 288 286 284 282 280
Binding Energy (eV)
540 538 536 534 532 530 528 526
Binding Energy (eV)
696 694 692 690 688 686 684 682
Binding Energy (eV)
142 140 138 136 134 132 130
Binding Energy (eV)
Figure 6. XPS spectra of LiNi0.5 Mn1.5 O4 powder before and after thermal storage, fresh powder (top), storage in 1.0 M LiPF6 EC:EMC electrolyte (middle), and
storage in 1.0 M LiPF6 EC:EMC + 1% DMMP electrolyte (bottom) at 85◦ C for 4 days.
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Journal of The Electrochemical Society, 159 (12) A2130-A2134 (2012)
Table III. Concentration of Mn and Ni dissolution in the electrolyte
after thermal storage.
STD
1% DMMP
Mn dissolution (%)
Ni dissolution (%)
0.9
0.7
0.32
0.21
decreased. The metal oxide peaks of the sample after thermal storage
are still present, suggesting that the surface layer is thin, less than the
depth of electron penetration (<5 nm).
ICP-MS results.— After thermal storage, the electrolyte was analyzed with ICP-MS to monitor the concentration of Mn and Ni in the
electrolyte. The percentage of Mn and Ni dissolved in the standard
electrolyte is slightly higher than those in the 1% DMMP containing
electrolyte as shown in Table III, which is consistent with the results
discussed above, suggesting that DMMP can inhibit the dissolution
of Mn and Ni at elevated temperature in LiPF6 -based electrolyte.
Conclusions
The effect of the addition of a novel Lewis base, dimethyl
methylphosphonate (DMMP), to LiPF6 /carbonate electrolytes on the
cycling performance of Li/LiNi0.5 Mn1.5 O4 cells cycled to 4.9 V vs
Li has been investigated. Addition of DMMP improves the capacity
retention upon cycling. Ex-situ surface analysis of the cycled electrodes suggests that the addition of DMMP inhibits the decomposition of electrolyte on the cathode surface and Mn dissolution from
the cathode particles. The improved performance of the electrolyte is
attributed to better thermal stability of the LiPF6 electrolyte and the
inhibition of the generation of acidic decomposition products in the
presence of DMMP.22 DMMP is a promising additive for improving
the performance of high voltage spinel cathodes.
Technologies of the U.S. Department of Energy under Contract
No. DE-AC02-05CH11231, Subcontract No 6879235 under the Batteries for Advanced Transportation Technologies (BATT) Program.
We also thank BASF SE for support of this work. M. X. also acknowledges the financial support from the National Natural Science Foundation of China (21003054), Guangdong Natural Science
Foundation (10351063101000001, S2011040001731), and specialized research Fund for the Doctoral Program of Higher Education
(20104407120008).
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This work was supported by the Assistant Secretary for
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