A920
Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
0013-4651/2014/161(6)/A920/7/$31.00 © The Electrochemical Society
Effect of Residual Lithium Compounds on Layer Ni-Rich
Li[Ni0.7 Mn0.3 ]O2
Dae-Hyun Cho,a Chang-Heum Jo,a,∗ Woosuk Cho,b,∗∗ Young-Jun Kim,b Hitoshi Yashiro,c,∗∗
Yang-Kook Sun,d,∗∗ and Seung-Taek Myunga,∗∗,z
a Department of Nano Science and Technology, Sejong University, Seoul 143-747, South Korea
b Advanced Batteries Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi-do 463-816, Korea
c Department of Chemical Engineering, Iwate University, Morioka, Iwate 020-8551, Japan
d Department
of Energy Engineering, Hanyang University, Seoul 133-791, South Korea
In order to confirm reasons that deteriorate cathode performances, Ni-rich Li[Ni0.7 Mn0.3 ]O2 is modified by lithium isopropoxide to
artificially provide lithium excess environment by forming Li2 O on the surface of active materials. X-ray diffraction patterns indicate
that the lithium oxide coating does not affect structural change comparing to the bare material. Scanning electron microscopy and
transmission electron microscopy data show the presence of coating layers on the surface of Li[Ni0.7 Mn0.3 ]O2 . Electrochemical
tests demonstrate that the Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 exhibits a greater irreversible capacity with a small capacity because of the
presence of insulating layers composed of lithium compounds on the active materials since these layers delay facile Li+ diffusion.
Also, the Li2 O layer forms byproducts such as Li2 CO3 , LiOH, and LiF, as are proved by X-ray photoelectron spectroscopy and
time-of-flight secondary ion mass spectrometry. The presence of residual lithium tends to bond with hydrocarbons induced from
decomposition of electrolytic salt during electrochemical reactions. And the reaction, accelerated by the decomposition of electrolytic
salt that produces the byproducts, causes the formation of passive layers on the surface of active material. As a result, the new layers
consequently impede diffusion of lithium ions that deteriorate electrochemical properties.
© 2014 The Electrochemical Society. [DOI: 10.1149/2.042406jes] All rights reserved.
Manuscript submitted March 3, 2014; revised manuscript received April 14, 2014. Published April 23, 2014.
Rechargeable lithium batteries (LIBs) have been intensively investigated as power-assisting or power sources in automotive applications
such as electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles. Furthermore, rechargeable LIBs are considered
to be the most promising energy storage system for renewable energy systems operating on intermittent energy sources, such as wind
power plants and solar cells because of their high energy density
and long cycle life. These kinds of applications require high capacity, good capacity retention, and reliable safety. For these reasons,
LiCoO2 is not suitable due to lower capacity and structural instability caused by hexagonal to monoclinic structural change which
results in Co dissolution.1–3 Therefore, Ni-rich layered compounds
(Li[Ni1-x Mnx ]O2 ) are very attractive in terms of capacity and its retention upon cycling.4–6 Unlike LiCoO2 , however, control of cationic
stoichiometry, namely Li/Ni ratio to be one, is difficult, particularly
in the Li layer of [Li1-x NiII x ]3b [NiIII ]3a [O2 ]6c .7–10 To overcome this
drawback, Arai et al.11 suggested that addition of excessive lithium
is effective in reduction of cation mixing in the Li layer. The resulting structure retains the well-ordered layer structure with low
cation mixing, so that the materials could deliver better capacity upon
cycling. Since the excessive amount of lithium is necessary to produce highly ordered structure for Ni-rich layer compounds, unreacted
lithium ingredient can be remained on the surface of active materials,
presumably as an oxide form, Li2 O. Also, the outer part of the Li2 O
is contaminated with moisture and CO2 in air, forming LiOH and
Li2 CO3 .
For the aforementioned reasons, Ni-rich compounds usually exhibit highly alkaline state above pH 11 owing to the remained lithium
salts, when they are stabilized into the α-NaFeO2 layer structure. The
presence of residual lithium compounds may not be favored because
they cause side reactions through oxidative decomposition of LiOH
and Li2 CO3 at high voltage. This reaction is not reversible, causing
irreversible capacity. In this paper, we adopt Co-free Li[Ni0.7 Mn0.3 ]O2
cathode material so as to eliminate the effect of Co on capacity
that accelerates capacity fade due to dissolution of Co ions into the
electrolyte.12 Here, the as-synthesized Li[Ni0.7 Mn0.3 ]O2 is modified
by Li2 O to intentionally provide lithium-rich environment in the outer
surface. Also, we investigate how the residual lithium compounds
deteriorate the electrode performance during cycling.
∗
Electrochemical Society Student Member.
Electrochemical Society Active Member.
z
E-mail: [email protected]
∗∗
Experimental
First, (Ni0.7 Mn0.3 )(OH)2 precursor was prepared via coprecipitation following our prior works.13 An aqueous solution of
Ni(II)SO4 · 6H2 O (Samchun) and Mn(II)SO4 · 5H2 O (Junsei) (molar
ratio of Ni:Mn = 7:3) with concentration of 2.0 mol L−1 was dropped
into a continuously stirred tank reactor (CSTR) with 1000 rpm under
N2 atmosphere. At the same time, NaOH solution (2.0 mol L−1 ) and
NH4 OH solution (3.6 mol L−1 ) as chelating agents were also separately added into the reactor by adjusting the solution pH to 11.7
at 50◦ C. Then, the precipitated particles were filtered, washed, and
dried in an oven at 80◦ C for 24 h. The obtained (Ni0.7 Mn0.3 )(OH)2
was heated to produce dehydrated (Ni0.7 Mn0.3 )3 O4 at 500◦ C for 5 h.
The dehydrates were thoroughly mixed with an appropriate amount
of LiOH and calcined at 900◦ C for 15 h in air.
To prepare the environment in which the as-synthesized active
materials present with residual lithium compounds, the surfaces of
active materials were intentionally modified by excess amount of
lithium isopropoxide; for 10 g of active material, the added amount
of lithium isopropoxide is 0.7 g, which corresponds to ∼0.01 mol
of lithium isopropoxide. According to our preliminary experiment,
addition of more lithium isopropoxide resulted in low capacity due to
the thick coating layer and less amount of lithium isopropoxide below
0.007 mol did not show the presence of uniform Li2 O coating layer.
Lithium isopropoxide was dissolved in anhydrous ethanol, and the assynthesized Li[Ni0.7 Mn0.3 ]O2 was poured into the solution and stirred
until the solution completed evaporation of the solvent at 80◦ C in air.
Finally, the dried powder was heated at 400◦ C for 5 h in air to form
lithium oxide (approximately 0.0045 mol per 1 mol Li[Ni0.7 Mn0.3 ]O2
active material) on the surface of active materials.
X-ray diffractometry (XRD; Rint-2000, Rigaku) and transmission
electron microscopy (HR-TEM; JEM-3010, JEOL) were employed to
characterize the synthesized powders. Time-of-flight secondary ion
mass spectroscopy (ToF-SIMS; PHI TRIFT V, ULVAC-PHI) was also
used to confirm the presence of the lithium oxide layer on the surface
of Li[Ni0.7 Mn0.3 ]O2 powders. Also, X-ray photoelectron spectroscopy
(XPS; PHI 5600, Perkin-Elmer) measurements were conducted on the
surfaces of bare and coated Li[Ni0.7 Mn0.3 ]O2 materials. Macro-mode
(3 mm × 3 mm) Ar-ion etching was done to analyze the chemical
state and surface composition of the coated powders. The etching rate
was estimated as 4 Å min−1 for silica.
For the fabrication of cathodes, the as-synthesized and the
Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 powders were mixed with conductive
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Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
materials (Super-P and Ketjen black, 1:1 in weight) and polyvinylidene fluoride in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone.
The obtained slurries were applied onto Al foil and dried at 80◦ C in
air. The electrodes were transferred to a vacuum oven and dried at
110◦ C overnight prior to use. Coin-type cells (2032) were assembled
using Li metal (Honjo chemical) as the anode in an argon-filled glove
box. The electrolyte used was 1M LiPF6 in ethyl carbonate–dimethyl
carbonate (3:7 in volume, PanaX). The cells were charged and discharged between 3.0 and 4.3 V by applying a constant current (50 mA
g−1 ) for electrochemical tests at room temperature.
In situ XRD data were obtained using an Empyrean diffractometer
(PANalytical) equipped with monochromated Cu Kα radiation from
2θ = 15 to 70◦ , with a step size of 0.03◦ and a count time of 7s using
a lab-made in situ cell.14 The cell was charged and discharged with a
current density of 50 mA g−1 . The lattice parameters were calculated
by the following method: the positions of the individual peaks were
fitted with a psuedo-Voigt or Lorentz function, and typically 8 or 9
peak positions were input to fit the program which minimizes the
least squares difference between the calculated and measured peak
positions by adjusting the lattice constant and the vertical displacement
of the sample.
To identify the presence of byproducts on the surface of the active
materials after cycling, the cycled active materials were measured
using a ToF-SIMS surface analyzer operated at 10−9 Torr, equipped
with a liquid Bi+ ion source and pulse electron flooding. During the
analysis, the targets were bombarded by the 10 keV Bi+ beams with
a pulsed primary ion current varying from 0.3 to 0.5 pA. The total
collection time was 100s and rastered over a 120 μm × 120 μm
dimension.
Results and Discussion
Figure 1 shows Rietveld refinement results of bare and lithium
oxide-coated Li[Ni0.7 Mn0.3 ]O2 (approximately 0.0045 mole per
1 mole Li[Ni0.7 Mn0.3 ]O2 active material). The XRD patterns could be
identified as an α-NaFeO2 structure with R3m space group. Though
the Li[Ni0.7 Mn0.3 ]O2 was modified by foreign layers, there are no
significant differences in the XRD pattern compared to that of the
bare Li[Ni0.7 Mn0.3 ]O2 . The calculated lattice parameters also indicate no change in the crystal structure before and after the coating
Figure 1. Rietveld refinement results of XRD data for (a) bare and (b) Li2 Ocoated Li[Ni0.7 Mn0.3 ]O2 powder.
A921
Table I. Rietveld refinement results of XRD data for bare
Li[Ni0.7 Mn0.3 ]O2 .
Nominal Formula
Crystal system
Space group
Atom
Site
Li
Ni2
Ni1
Mn
O
a-axis / Å
c-axis / Å
Rwp /%
Rp /%
3a
3a
3b
3b
6c
x
y
0
0
0
0
0
0
0
0
0
0
Li[Ni0.7 Mn0.3 ]O2
Rhombohedral
R3m
z
g
B #x002F; Å2
1/2
0.941(2)
1/2
0.059(2)
0
0.7
0
0.3
0.258(3)
1
2.8804(2)
14.2543(10)
14.2
9.15
1.1
1.1
1.0
1.0
0.8
Table II. Rietveld refinement results of XRD data for Li2 O-coated
Li[Ni0.7 Mn0.3 ]O2 .
Nominal Formula
Crystal system
Space group
Atom
Site
Li1
Ni2
Ni1
Mn
O
a-axis / Å
c-axis / Å
Rwp /%
Rp /%
3a
3a
3b
3b
6c
x
y
0
0
0
0
0
0
0
0
0
0
Li[Ni0.7 Mn0.3 ]O2
Rhombohedral
R3m
z
g
1/2
0.941
1/2
0.059(2)
0
0.7
0
0.3
0.258(2)
1
2.8808(2)
14.2548(9)
11.8
7.46
B / Å2
1.1
1.1
1.0
1.0
0.8
(Tables I and II). The amount of Ni2+ occupying Li site (3b), due
to the similarity of ionic radius for Li+ (0.76 Å, coordination number: 6)15 and Ni2+ (0.69 Å, coordination number: 6)15 appears to be
identical for both materials, approximately 5.9% in Li site, resulting
in [Li0.941 Ni0.059 ][Ni0.7 Mn0.3 ]O2 . This is reasonable that the coating
layer is comprised of low crystalline product since the heating temperature of 400◦ C is too low to form crystalline phase but sufficient to
remove isopropoxide ingredient, resulting in formation of low crystalline Li2 O coating layer. And it is also thought that the lithium oxide
does incorporate into neither Li nor Ni site because of no change in
the lattice parameters (Tables I and II) but resides only on the surface
of the active materials. For convenience, in the following discussion,
the phases are still associated with their nominal chemical formula
Li[Ni0.7 Mn0.3 ]O2 , whatever the real cationic distribution.
To confirm morphology of the as-synthesized and the Li2 O-coated
coated materials, the powders were observed by SEM and TEM
(Figure 2). In the SEM images (Figs. 2a and 2b), the smooth surface of active materials (Fig. 2a) is covered by flake-like particles
after the Li2 O coating (Fig. 2b). From TEM observation (Fig. 2c),
even the bare materials display some sediment of foreign materials
on the outer surface, presumably lithium compounds such as Li2 O,
Li2 CO3 and LiOH derived from the excessive amount of LiOH addition for compensation of lithium evaporation to minimize the cation
mixing during calcination. Such foreign layers are readily noticed on
the surface of the active material after the Li2 O coating (Fig. 2d); the
newly formed thicker layers range from 10−40 nm in thickness for
the Li[Ni0.7 Mn0.3 ]O2 , and the resulting coating layers appear inhomogeneous but porous showing light contrast compared to the active
material.
Residual lithium oxide, Li2 O, on the surface of active materials is
readily combined with oxygen, carbon dioxide, and water molecules,
and forms compounds like LiOH and Li2 CO3 , as confirmed in ToFSIMS spectra (Figs. 3a and 3b). In particular, the Li2 O-coated sample
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A922
Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
Figure 2. SEM images of the (a) bare and (b) Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 ;
bright-field TEM images the (c) bare and (d) Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 .
The scale bar indicates 20 nm.
represents stronger spectrum intensity for the Li2 C+ (m = 31.01) and
Li2 OH+ (m = 31.03) fragments (Fig. 3b). The results indicate that
the outer surface of Li2 O coating layer was obviously transformed
to LiOH and Li2 CO3 layers after exposure in air. Hence, the bare
and Li2 O-coated materials were analyzed by pH, LiOH, and Li2 CO3
titrations (Fig. 3c). In the result of pH titration, the coated material
by Li2 O exhibits higher pH value than that of the bare. And through
the LiOH and Li2 CO3 titrations, it is confirmed that the Li2 O coating
induces more formation of LiOH and, in particular, Li2 CO3 on the
surface of active materials (Scheme 1).
The bare and the Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 /Li cells were continuously cycled at a constant current density of 50 mA g−1 at 25◦ C.
The first discharge capacity of the bare material appears evidently
higher than that of Li2 O-coated electrode (Fig. 4a). In addition, the
coulombic efficiency of the bare electrode at the first cycle is approximately 75.3%, whereas the coated material shows lower columbic
efficiency (67.7%) than that of the bare. The presence of thick and
non-uniform coating layers would cause lower discharge capacity and
efficiency for the Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 electrode. Decomposition or oxidation of those excessive lithium compounds derived from
the Li2 O coating is also considered for the low coulombic efficiency
at the first cycle. The capacity retentions are approximately 76% for
the bare and 70% for the Li2 O-coated electrodes (Figs. 4b-4d).
As observed in Fig. 4, deterioration of the electrode performance
is obvious in the virtual condition through the surface modification
by residual lithium compounds. Since the only difference between
Figure 3. ToF-SIMS spectra for Li2 OH+ and Li2 C+ of (a) bare
Li[Ni0.7 Mn0.3 ]O2 , (b) Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 materials before cycling,
and (c) pH measurement and titration results of LiOH and Li2 CO3 using bare
and Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 powders before electrochemical test.
two electrodes is the presence of residual lithium compounds on the
surface of active materials, the remained lithium compounds may
affect structural variation during Li+ insertion and extraction. Hence,
the bare and Li2 O-coated materials were examined by in situ XRD
measurement to follow the structural changes during the first cycle
(Fig. 5). In fact, both electrodes undergo similar structural evolution
during the charge and discharge. Some peaks gradually shift toward
higher angle in 2θ on charge that is due to structural changes from
H1 (hexagonal) to H2 (hexagonal phase) phase via the intermediate
M (monoclinic) phase.16–18 And the peaks return to the original peak
position on discharge without appearance of other phases, occurring
in a simple topotactic reaction (Figs. 5a and 5b).
Scheme 1. Surface change of Li[Ni0.7 Mn0.3 ]O2 materials after exposure in air.
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Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
A923
Figure 4. Charge and discharge curves of Li[Ni0.7 Mn0.3 ]O2 /Li cells at room temperature; (a) first charge-discharge curves of bare and Li2 O-coated
Li[Ni0.7 Mn0.3 ]O2 /Li cells; continuous charge and discharge curves of (b) bare, (c) Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 /Li cells; (d) cyclability plots of the cells
at 50 mA g−1 at room temperature. The presented curves are for 1st, 25th, 50th, 75th, and 100th cycles.
Interlayer distances were calculated from the in situ XRD data
shown in Fig. 5, and the results are plotted in Fig. 6. The resulting distances increase gradually during the charge because of the increase in
coulombic (electrostatic) repulsion force between the oxygen-oxygen
layers by ionicity of metal-oxygen bonding.19 Although the delivered
capacity was small for the Li2 O-coated electrode, the interlayer distances vary similarly to those of the bare electrode. From this point
of view, it is possible that the residual lithium compounds do not affect structural change but do interrupt Li+ diffusion because of the
thickness and insulating properties the lithium compounds. For the
reasons, the coated electrode would deliver less capacity than the bare
material.
The extensively cycled cells were disassembled, and both the bare
and Li2 O-coated electrodes were washed with salt-free dimethyl carbonate solution and subsequently dried in vacuum oven at 80◦ C. In
the TEM image of bare material, the sediments were, in part, detected
on the surface of active material after the cycling, and the original
smooth surface line of the particle was also kept (Fig. 7a). The coating layer on the surface of the coated material became thicker than
before cycling (Fig. 7b). It is likely that the coated lithium compounds
layers are altered through reaction with electrolyte during the cycling.
For the above reason, the extensively cycled bare and coated electrodes were analyzed by ToF-SIMS to unveil how the outermost original lithium compounds coating layers were altered. Four fragments
are found in the mass range of 30.6–31.4: P+ (m = 30.97): CF+ (m =
30.99), Li2 C+ (m = 31.01), and Li2 OH+ (m = 31.03). P+ is derived
from remained LiPF6 salt and CF+ is due to the presence of PVDF
binder in the electrode. The development of Li2 C+ intensity is evidently noticed after the cycling (Figs. 8a and 8b), compared to those
fresh electrodes (Figs. 3a and 3b). In comparison with the intensity
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A924
Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
Figure 6. Interlayer distances derived from the (003) peak for the bare and
the Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 from the in-situ XRD patterns in Figure 5.
Figure 5. In-situ XRD patterns of (a) bare and (b) Li2 O-coated
Li[Ni0.7 Mn0.3 ]O2 as a function of Li content during the first cycle. Al denotes an aluminum foil used as a current collector.
of the Li2 OH+ fragment, the intensity of Li2 C+ appears to be close
to that of Li2 OH+ for the bare electrode and the signal is somehow
stronger than that of Li2 OH+ for the Li2 O-coated electrode. This result indicates that Li2 CO3 layer resides just above the LiOH layer, as
follows:
2LiOH + CO2 → Li2 CO3 + H2 O
[1]
Aurbach’s group20 suggested that CO and CO2 are anodic oxidation products of ethylene carbonate and dimethylene carbonate, which
are used as solvents in the present study. Through the above reaction,
these oxidation byproducts are deposited on the outer surface of residual LiOH layer forming the Li2 CO3 layer but leaving water molecules
(eq. 1). As is clear from the XPS results, more amount of Li2 CO3 is
formed on the outermost surface of the Li2 O-coated electrode after the
extensive cycling test (Figs. 8e and 8f). Hence, the relative intensity of
Li2 OH+ diminishes compared to the fresh state. Etching the electrode
surface by Bi+ for 50s, the carbonate layer almost disappears, and
only the LiOH layer is observed for both electrodes, meaning that the
formed Li2 CO3 layer is not thick.
Commonly, electrolyte using LiPF6 salt, which is unstable in the
presence of H2 O molecules and at high temperature or high operation
voltage, is decomposed by the following equations:21,22
LiPF6 → LiF ↓ +PF5
[2]
PF5 + H2 O → POF3 + 2 HF
[3]
Figure 7. Bright-field TEM images of (a) bare and (b) Li2 O-coated
Li[Ni0.7 Mn0.3 ]O2 after extensively cycling. The scale bar indicates 50 nm.
and
2 POF3 + 3 Li2 O− → 6 LiF ↓ + P2 O5 ↓ (or Lix POF y )
[4]
The produced LiF is generally deposited on the surface of active
material. The LiF compound is not found on the XPS spectra for
both fresh electrodes (Figs. 9a and 9c). After both electrodes were
extensively cycled, the LiF is detected at 685 eV (Figs. 9b and 9d). In
particular, the relative intensity of LiF binding energy is significantly
higher for the coated material than that of bare one. It is believed
that the formation of LiF is highly dependent on the amount of presenting water molecules in the electrolyte as expected from the above
reactions. In this experiment, the difference is the presence of Li2 O
coating layer on the surface of active material. Through the anodic
decomposition of electrolytic solvent, the produced CO and CO2 tend
to bond with the hydrated Li2 O layer, namely LiOH, forming Li2 CO3
layer in the outermost surface, and this subsequently accompanies
the formation of water molecules. In comparison with the bare material, the formed Li2 CO3 layer seems thicker for the coated electrode,
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Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
A925
Figure 8. ToF-SIMS spectra for P+ , CF+ , Li2 OH+ and Li2 C+ of (a) bare, (b) Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 , (c) extensively cycled bare Li[Ni0.7 Mn0.3 ]O2
electrodes, (d) extensively cycled Li2 O-coated Li[Ni0.7 Mn0.3 ]O2 electrodes. XPS carbon spectra after extensively cycled (e) bare Li[Ni0.7 Mn0.3 ]O2 and
(f) Li2 O-coated Li[Ni0.7 Mn0. 3 ]O2 . The range from 284 eV to 285 eV indicates carbon. The range from 289 eV to 291.5 eV belongs to carbonate.
Figure 9. XPS spectra of fluorine; (a) before and (b) after extensively cycled bare Li[Ni0.7 Mn0.3 ]O2 ; (c) before and (d) after extensively cycled Li2 O-coated
Li[Ni0.7 Mn0. 3 ]O2 . The peak at 685 eV indicates lithium fluoride (LiF).
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A926
Journal of The Electrochemical Society, 161 (6) A920-A926 (2014)
The progressive formation of insulating LiF layer impedes lithium
diffusion and deteriorates the electrode performance during cycling.
Therefore, we suggest that the concentration of surface lithium should
be minimized to avoid these unfavorable effects.
Acknowledgments
Scheme 2. Effect of the residual lithium on the surface of Li[Ni0.7 Mn0.3 ]O2 .
which also means generation of more amounts of water molecules in
the electrolyte. This accelerates the decomposition of electrolytic salt,
LiPF6 , to lead to more formation of insulating LiF in the outermost
surface. Therefore, the XPS result indicates the further development
of LiF intensity for the Li2 O-coated electrode after the cycling test.
These layers, such as LiF, Li2 CO3 , and LiOH, coexist on the surface
of the active materials, so that they impede the diffusion of lithium
ions due to their insulating properties and thus deterioriate the electrochemical performances of active materials as described in Scheme 2.
These series of sequential occurrence are affected by the presence
of residual Li2 O. Therefore, efforts should be made to minimize the
amount of impurities on the surface of active materials to reduce
these side reactions occurring in the interface of active materials and
electrolyte during cycling. Surface modification capturing the residual Li2 O compound would be a possible way to reduce the level
of the residual lithium compounds on the surface of Ni-rich layer
compounds.
Conclusion
The effect of residual lithium on the surface of Li[Ni0.7 Mn0.3 ]O2
is investigated in an artificial environment provided by Li2 O coating.
Exposure of the coated materials in air induces notable increase in the
amount of LiOH as a major component and Li2 CO3 as a minor component. Because of these contaminations, the surface-modified electrode
shows unfavored electrode performances such as low capacity, large
irreversible capacity, and poor capacity retention. It is found that the
LiOH layer on the Li2 O is transformed to Li2 CO3 layers accompanied
by the formation of water. This water molecule also accelerates the
decomposition of LiPF6 salt, and this consequently gives rise to the
formation of LiF toward the outermost surface of the active material.
The authors thank Miwa Watanabe, Iwate University, for her helpful assistance in the experimental work. This work was partially
supported by the IT R&D program of MKE/KEIT [10041856] and
the secondary battery R&D program for leading green industry of
MKE/KEIT [10041094].
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