Nano Research
Nano Res
DOI
10.1007/s12274-014-0631-8
An effective method to reduce residual lithium
compounds on Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 active
material using phosphoric acid derived Li3PO4
nanolayer.
Chang-Heum Jo1, Dae-Hyun Cho1, Hyung-Joo Noh3, Hithshi Yashiro2, Yang-Kook Sun3(), Seung
Taek Myung1 ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0631-8
http://www.thenanoresearch.com on November 7 2014
© Tsinghua University Press 2014
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1
TABLE OF CONTENTS (TOC)
An effective method to reduce residual lithium
compounds on Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 active
material using phosphoric acid derived Li3PO4
nanolayer.
Chang-Heum Jo1, Dae-Hyun Cho1, Hyung-Joo Noh3,
Bare
Hithshi Yashiro2, Yang-Kook Sun3*, Seung Taek
Myung1*
1 “Sejong
University, South Korea”.
2
“Iwate University, Japan”.
3“Hanyang University, South Korea”.
Byproducts :
Reduced
Residual lithium
Water Absorption
Function
Li3PO4-coated
(LiOH, Li2CO3, LiF , etc)
HF Scavenging
Function
Protecting
Active Material
Surface of the Ni-rich material modified by H3PO4 induces the
formation of Li3PO4 nanolayer after reacting with residual LiOH and
Li2CO3 on the surface so that the amounts of residual lithium
compounds are significantly reduced, and this solves the main problem
considered as one of the degradation causes during electrochemical
cycling. In addition, the formed Li3PO4 surface layer has
multi-functions such as absorption of water in the electrolyte that
lowers HF level, HF scavenging, and protection of the active materials
from deteriorating side reactions with the electrolyte.
Seung-Taek Myung, http://dasan.sejong.ac.kr/~smyung/
1
Nano Res
DOI (automatically inserted by the publisher)
Research Article
An effective method to reduce residual lithium compounds on
Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 active material using phosphoric
acid derived Li3PO4 nanolayer.
Chang-Heum Jo1, Dae-Hyun Cho1, Hyung-Joo Noh3, Hithshi Yashiro2, Yang-Kook Sun3( ), Seung Taek
Myung1 ()
1 Departmen
of Nano Engineering, Sejong University,98 Gunja-dong, Gwangjin-gu, Seoul 143-747,South Korea.
of Chemical Engineering, Iwate University, Morioka, Iwate 020-8551, Japan.
3 Departmen of Energy Engineering, Hanyang University, Seongdong-gu, Seoul 133-791, South Korea.
2 Departmen
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT
Ni-rich Li[Ni0.6Co0.2Mn0.2]O2 surface is modified with H3PO4. After the coating at 80oC, the products are
heated further at a moderate temperature of 500oC in air, where the added H3PO4 transforms to Li3PO4
compounds after reacting with residual LiOH and Li2CO3 on the surface. A thin and uniform smooth
nanolayer (<10 nm) is observed on the surface of Li[Ni 0.6Co0.2Mn0.2]O2 as confirmed by transmission electron
microscopy (TEM). Time-of-flight secondary ion mass spectroscopic (ToF-SIMS) data exhibit the presence of
LiP+, LiPO+, and Li2PO2+ fragments, indicating the formation of Li 3PO4 coating layer on the surface of the
Li[Ni0.6Co0.2Mn0.2]O2. As a result, the amounts of residual lithium compounds, such as LiOH and Li 2CO3, are
significantly reduced through the formation of Li3PO4 from the starting H3PO4. As a result, the Li3PO4-coated
Li[Ni0.6Co0.2Mn0.2]O2 exhibits noticeable improvement in capacity retention and rate capability due to the
reduction of residual LiOH and Li 2CO3 compounds. Further investigation of the extensively cycled
electrodes by XRD, TEM, and ToF-SIMS demonstrate that the Li3PO4 coating layers have multi-functions:
absorption of water in the electrolyte that lowers HF level, HF scavenging, and protection of the active
materials from deteriorating side reactions with the electrolyte during extensive cycling, enabling high
capacity retention over 1000 cycles.
KEYWORDS
Li3PO4, Coating, Positive electrode, Lithium, Batteries
————————————
Address correspondence to [email protected], [email protected]
2
Introduction
Lithium ion batteries play an important role in
energy storage and conversion devices and are
promising power sources for (hybrid) electric
vehicles. For these reasons, improving electrode
performance are important to assure long cycling
life, thermal stability, and safety [1-7]. Recently,
Li[Ni1/3Co1/3Mn1/3]O2, which is composed of divalent
Ni, trivalent Co, and tetravalent Mn, has received
considerable attention due to high capacity by Ni 2+/4+
and Co3+/4+ redox and its thermal and structural
properties supported by tetravalent Mn [8-11], which
are superior to those of conventional LiCoO2.
However, recent technologies require more
electricity for efficient operation, so that high
capacity batteries, which warrant safety concerns, are
needed to fulfill user demand. For these reasons,
Ni-rich compounds are attractive due to their
reversible capacities [12-14]. However, these
materials have thermal and structural instabilities
caused by oxygen loss from their host structures at
highly delithiated states. Even worse, Ni-rich
compounds always contain a large amount of
residual lithium compounds like LiOH and Li 2CO3
[15]. The presence of lithium residues is not favored
because their oxidation results in the formation of
Li2O and CO2 gas during operation at higher
voltages, which lowers the coulombic efficiency
between the charge and discharge capacities [16].
Otherwise, the active materials do not deliver high
capacity due to severe cation mixing (occupation of
divalent Ni in the Li layer), which deteriorates the
electrode's capacity.
Application of hetero-materials, such as oxides
[17-19], phosphates [20-22], or fluorides [23-25], to
the surfaces of lithium transition metal oxide
particles can mitigate the problems mentioned
above. Surface modification improves electro
chemical properties with the use of very small
amounts of coating materials. The mechanism
behind improvements made to electrochemical and
thermal properties by surface modification has been
investigated to a limited extent. Several studies have
found that the coating materials preserved the active
materials by acting as fluorine-based protective
films. The films reduce the dissolution of the active
material by scavenging HF, and thereby diminish
impedance increases at the interfaces, such as Rfilm
and Rct [17,18]. Other studies have also proved
changes in the surface compositions and structures
of lithium transition metal oxides [21,22,26].
From the above review, it is necessary to reduce
the concentration of residual lithium on the surface
of Ni-rich active materials. The above-mentioned
coating media can chemically bond with the residual
lithium compounds on the surfaces of the active
materials
through
calcination
at
moderate
temperatures, though the resulting coating layers
exhibit low crystalline or amorphous characteristics.
In this case, the new compounds are able to reduce
the surface concentration of lithium. For example,
coating with Al2O3 would result in formation of
LiAlO2 compound on the outermost surfaces of the
active materials. Hence, two Al atoms react with two
Li atoms to from LiAlO2 (Al2O3 + Li2O  2LiAlO2).
The LiAlO2 is known to be an electric insulator, so
that crystalline LiAlO2 layers are not expected to aid
the electrode performances of bare materials.
Similarly, many coating media show electric and
ionic
insulating
properties.
Therefore,
reconsideration of the coating media is necessary to
decrease the residual lithium concentration on the
surface and to thereby improve the electrode
performance when using an ionic conductor.
For this reason, we modify the surface of Ni-rich
Li[Ni0.6Co0.2Mn0.2]O2 using phosphoric acid, H3PO4.
In this case, 3 Li atoms can be captured to form
Li3PO4 via the heat treatment from the residual
lithium
compounds
on
the
surface
of
Li[Ni0.6Co0.2Mn0.2]O2. Additionally, Li3PO4 is known
to be a ionic conductor (~6 x 10-8 S cm-1 [21, 27]), so
that its conducting properties are likely to assist
improvement in electrode performances. In this
paper, we introduce electrochemical properties and
the causes of improved properties when using
Li3PO4 coating layers derived from H3PO4 coatings.
1. Experimental Section
1.1. Synthesis of Li[Ni0.6Co0.2Mn0.2]O2
[Ni0.6Co0.2Mn0.2](OH)2 was prepared as follows. An
aqueous solution of NiSO4, CoSO4, and MnSO4 with
a concentration of 2.0 mol dm−3 was pumped into a
continuously stirred tank reactor under nitrogen
3
atmosphere. At the same time, a NaOH aqueous
solution of 2.0 mol dm−3 and the desired amount of
NH4OH aqueous solution (a chelating agent) were
also separately fed into the reactor. Then, the
[Ni0.6Co0.2Mn0.2](OH)2 particles obtained were
filtered, washed, and dried in air. The hydroxide and
LiOH powders at a molar ratio 1:1.05 were mixed
thoroughly. The excess Li was used to compensate
for the loss of Li during calcination. The mixture was
heated at 480oC for 5 h and calcined at 950oC for 10 h
in air, then slowly cooled to room temperature at a
rate of 1oC min−1.
1.2. Surface modification
H3PO4 (Kanto) was first completely dissolved in
anhydrous ethanol at room temperature. The
as-synthesized active material, Li[Ni 0.6Co0.2Mn0.2]O2,
was slowly poured into the solution. The starting
ratio of active material versus H3PO4 was 99:1 in
weight. Then, the solution containing the active
material was constantly stirred at 80°C, accompanied
by slow evaporation of the solvent. The
solution-treated, as-received, and bare Li[Ni0.6Co0.2
Mn0.2]O2 powders were fired at 500°C for 5 h in air.
1.3. Characterization
X-ray diffractometry (XRD, Rint-2000, Rigaku) and
Rietveld refinement were employed to characterize
the crystal structure of the produced powders. XRD
data were obtained at 2θ = 10-110o, with a step size of
0.03o and a count time of 8s. The FULL PROF
Rietveld program was used to analyze the powder
diffraction patterns. Field-emission scanning electron
microscopy (FE-SEM, HITACHI S-4700) and
high-resolution transmission electron microscopy
(HR-TEM, JEM-3010, JEOL) were employed to
characterize the shape of the prepared powder. The
chemical compositions of the final powders were
determined by atomic absorption spectroscopy
(AAS, Vario 6, Analytic Jena). Time-of-flight
secondary ion mass spectroscopy (ToF-SIMS, PHI
TRIFT V nanoTOF, ULVAC-PHI) was also used to
identify the coating layer on the surfaces of the
Li[Ni0.6Co0.2Mn0.2]O2 powders.
To fabricate positive electrodes, the prepared
active materials were mixed with conducting
materials (active material and Super P carbon
black:Ketjen black and polyvinylidene fluoride in an
80:10:10 weight ratio in N-methyl pyrrolidone (NMP)
solution). The obtained slurry was applied onto Al
foil and first dried at 80oC to remove the NMP and
consequently at 120°C overnight under vacuum prior
to use. Electrochemical cell tests were performed
after assembling a 2032 coin-type cell using Li metal
as the negative electrode in an argon-filled glove box.
The electrolyte solution used was 1 mol dm−3 LiPF6 in
ethylene carbonate-dimethyl carbonate (3:7 in
volume). The cells were charged and discharged
between 3.0 V and 4.3 V, and the cells for the C-rate
tests were also charged and discharged by applying
different current densities at room temperature. The
long cycle-life tests were performed in laminated
type full cell wrapped with Al pouch (thickness, 2
mm; width, 30 mm; and length, 40 mm). Mesocarbon
microbeads (MCMB, Osaka gas) was used as the
negative electrode. The electrolyte solution used was
1 mol dm−3 LiPF6 in ethylene carbonate-dimethyl
carbonate (3:7 in volume, PanaX). A preliminary cell
formation was performed for Li-ion cell: five cycles
were performed at room temperature at 0.01 – 0.5
C-rates in the voltage range of 3.0 - 4.2 V. The cells
were charged and discharged between 3 and 4.2 V by
applying constant current densities at 25oC.
AC-impedance measurements were carried out by
applying a 1 MHz to 1 mHz frequency range with an
ac-amplitude of 10 mV.
For HF titration, the cycled cells were
disassembled and all contents were washed with
salt-free solvent in the glove box for several days.
NaOH aqueous solution and bromothymol blue
were used as an indication solution for the titration
of the cycled electrolyte. The cycled electrolyte was
also analyzed to check for dissolved transition metal
ions by atomic absorption spectroscopy.
To analyze the byproducts on the surface of the
active materials before and after cycling, the
pristine powders and cycled electrodes were
examined using a time-of-flight-secondary ion mass
spectroscopy 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 240 s and the
beams were rastered over 400 m  400 m.
4
2. Result and Discussion
2.1. Surface modification of Li[Ni0.6Co0.2Mn0.2]O2.
Figure 1 shows XRD patterns of the bare and
surface-modified Li[Ni0.6Co0.2Mn0.2]O2. The chemical
composition of the product was determined to be
Li1.016[Ni0.596Co0.201Mn0.203]O2 by AAS. Here, the small
amount of excess Li was thought to be residual
lithium appearing on the surface of the active
material. Hence, the refinement was performed
assuming a chemical composition of Li 1.00[Ni0.596
Co0.201Mn0.203]O2 (hereafter referred to as Li[Ni0.6Co0.2
Mn0.2]O2). The diffraction patterns of the two samples
are confirmed to be a single phase which is identified
as hexagonal α-NaFeO2 structure with space group
of R m. Although the Li[Ni0.6Co0.2Mn0.2]O2 was
covered with hetero material, it was difficult to find
significant differences in the crystal structure after
the coating. As evident in Tables 1 and 2, the
calculated structural parameters, such as cation
mixing and lattice parameters, did not vary before
and after the surface modification. This is because
the coating material was not doped in the parent
oxide or did not form impurities. These results imply
that the mild coating conditions did not alter the
physical properties of the materials, and the coating
medium would be remained as low crystalline
product after the coating.
Morphological characterizations of bare and
coated powders are shown in Fig. 2. Both the bare
and coated Li[Ni0.6Co0.2Mn0.2]O2 had smooth edge
lines. There was no additional layer on the surface of
the bare particles (Fig. 2a), but the presence of a
coating layer with a thickness below 10 nm was
obvious on the surface of Li[Ni0.6Co0.2Mn0.2]O2 (Fig.
2b, c). In the macroscopic images (Fig. 2d and e), the
coated powders exhibited a spherical morphology
with an average secondary particle diameter of about
7 µ m. The particle in Fig. 2e was covered with
P-containing material.
ToF-SIMS analysis was performed to understand
the nature of the coating layer on the surface of
Li[Ni0.6Co0.2Mn0.2]O2 (Fig. 3). The method is not
useful for quantitative analysis, but this equipment
has an advanced sensitivity for detecting ions
qualitatively. Since the elemental mapping data
showed the presence of the element P (Fig. 2d), we
first analyzed H- and P-related fragments because
the starting coating material was H3PO4.
Unfortunately, fragments related to these two
elements did not exist in the spectra. However, we
found Li and P related fragments, showing LiP+ (m =
37.98), LiPO+ (m = 53.98) and Li2PO2+ (m = 76.99) (Fig.
3a-c), respectively, although these fragments were
not obvious on the bare Li[Ni0.6Co0.2Mn0.2]O2. As a
result, the coated material exhibited lower pH value
than that of the bare. And through the LiOH and
Li2CO3 titration, it was confirmed that the Li3PO4
coating was successful in reducing the formation of
LiOH and Li2CO3 on the surface of active materials
(Table 3). Based on these results, we can conclude
that the coating layer was composed of lithium
phosphate, Li3PO4.
003
003
a
b
104
104
30
40
50
60
70
Intensity / Count
80
90
208
20
207
10
0111
0012/024
021/116
100
113
108/110
90
107
80
105
70
006/102
101
60
Cu K 2 / Degree
208
50
207
40
0111
0012/024
021/116
30
107
105
20
113
108/110
006/102
101
10
100 110
Cu K 2 / Degree
Figure 1 Rietveld refinement patterns of XRD data for the (a) bare and (b) lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %)
5
Table 1 Rietveld refinement results of XRD data for bare
Table 2 Rietveld refinement results of XRD data for lithium
Li[Ni0.6Co0.2Mn0.2]O2.
phosphate-coated Li[Ni0.6Co0.2 Mn0.2]O2 (1 wt. %).
Formula
Li1.00[Ni0.596Co0.201Mn0.203]O2
Formula
Li1.00[Ni0.596Co0.201Mn0.203]O2
Crystal system
Rhombohedral
Crystal system
Rhombohedral
Space group
R m
Space group
R m
Atom
Site
x
y
z
Li1
3a
0
0
Ni1
3a
0
Co
3b
Mn
2
g
B/Å
Atom
Site
x
y
z
1/2
0.967(2)
1.0
Li1
3a
0
0
0
1/2
0.033(2)
1.0
Ni1
3a
0
0
0
0
0.201
0.9
Co
3b
3b
0
0
0
0.203
0.9
Mn
Ni2
3b
0
0
0
0.563(3)
0.9
Li2
3b
0
0
0
0.033(3)
O
6c
0
0
0.2586(1)
1
Cell parameters
2
g
B/Å
1/2
0.967(2)
1.0
0
1/2
0.033(2)
1.0
0
0
0
0.201
0.9
3b
0
0
0
0.203
0.9
Ni2
3b
0
0
0
0.563(3)
0.9
0.9
Li2
3b
0
0
0
0.033(3)
0.9
0.8
O
6c
0
0
0.2586(1)
1
0.8
a=b=2.8678(1)Å
Cell parameters
c=14.2123(7)Å
a=b=2.8671(1)Å
c=14.2122(7)Å
Rwp
9.76 (%)
Rwp
8.96 (%)
Rp
6.64 (%)
Rp
6.17 (%)
Although additional layers were not observed on
the surfaces of the bare powders (Fig. 2a), the
residual lithium compounds were presented as LiOH
and Li2CO3 as evident from the Li2OH+, LiCO2+, and
LiC+ fragments (Fig. 3d-f). According to our recent
work, residual lithium compounds impede facile
lithium ion migration, which in turn cause
polarization [16]. The amounts of residual lithium
increased with increasing Ni content in Li[Ni x[Co0.5
Mn0.5]1-x]O2 (x = 0.5-0.8) [15]. It is thought that the
residual lithium existed as a form of Li 2O in the
initial stage after the synthesis. The Li 2O residue and
moisture in the air combined to form LiOH, which
was detected as the Li2OH+ fragment (Fig. 3d). Also,
Li2CO3 was formed by joining with LiOH from the
surfaces of the particles and CO2 in the air [28].
Hence, the byproduct was detected as LiCO2+
fragments having Li-C-O bond (Fig. 3e and f). This
investigation shows the surface structure of the bare
material, namely, the Li2CO3 layer in the outermost
surface and the LiOH layer beneath it.
Interestingly, the intensities of the Li 2OH+, LiCO2+,
and LiC+ fragments were significantly reduced after
the coating, forming the Li3PO4 layer. The melting
point of H3PO4 is as low as 43oC, and the PO43- anion
thus would readily react with the residual lithium
compounds on the surface of Li[Ni0.6Co0.2Mn0.2]O2
during fluxing in the anhydrous ethanol at 80oC.
Through this process, the residual lithium forms
Li3PO4, and this, in turn, dramatically reduced the
residual lithium compounds on the surface of the
as-seen Li[Ni0.6Co0.2Mn0.2]O2 in Fig. 3d-f:
3Li2O + 2H3PO4 → 2Li3PO4 + 3H2O
3LiOH + H3PO4 → Li3PO4 + 3H2O
3Li2CO3 + H3PO4 → 2Li3PO4 + 3CO2 + 3H2O
(1),
(2),
(3).
The Li3PO4 layer formed by the above reaction was
detected as LiP+, LiPO+, and Li2PO2+ fragments (Fig.
3a-c), indicating that the amorphous or low
crystalline Li3PO4 phase was formed during the
heating process at 500 oC in air. Furthermore, three Li
atoms were trapped to produce the Li 3PO4 from the
H3PO4 starting material, and this simultaneously
decreases the residual lithium compounds after the
6
coating (Table 3). Compared to the bare sample, the
Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 exhibited a lower
presence of residual lithium compounds on the
surface, which was attributed to the consumption of
the residual lithium by the formation of Li3PO4.
Table 3 pH value and titration results of residual lithium for
bare and lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2
(1 wt. %). (unit : ppm)
pH
LiOH
Li2CO3
Total
Bare Li[Ni0.6Co0.2Mn0.2]O2
11.57
1736
3284
5220
Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2
10.86
1197
738
1935
a
The surface states of the bare and Li3PO4-coated
Li[Ni0.6Co0.2Mn0.2]O2 are illustrated on the basis of the
above results (Fig. 4). The surfaces of the bare
powders were covered with high amount of residual
lithium compounds that may cause a large resistance
due to the insulating characteristics of the
compounds [29]. On the other hand, the surfaces of
the Li3PO4-coated powders consisted mostly of
Li3PO4 layers formed via combination of the residual
lithium compounds and the phosphoric acid. It is
known that Li3PO4 is stable up to 6 V versus Li/Li +
and has excellent ionic conductivity [26]. From these
results, the nature and multiple functions of Li 3PO4
coating layers derived from H3PO4 can be
understood.
Active Material
b
Active Material
c
Active Material
50nm
50nm
10nm
e
d
6µm
P Ka1
Figure 2 TEM m
bright-field images of (a) bare Li[Ni0.6Co0.2Mn0.2]O2, (b) lithium phosphate-coated Li[Ni0.6Co0.2 Mn0.2]O2 and (c)
magnified lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %). (d) SEM images and (e) EDX mapping of P for the lithium
phosphate-coated Li[Ni0.6Co0.2Mn0.2] O2 (1 wt. %).
7
a
b
c
d
e
f
Figure 3 ToF-SIMS results of pristine bare and lithium phosphate-coated Li[Ni0.6Co0.2 Mn0.2]O2 (1 wt. %) powders ; (a) LiP+, (b)
LiPO+, (c) Li2PO2+, (d) Li2OH+, (e) LiCO2+, and (f) LiC+ fragments. (top: bare , bottom: lithium phosphate-coated Li[Ni0.6Co0.2
Mn0.2]O2)
500oC
Heat treatment
Active
Material
Lithium hydroxide
Lithium Carbonate
Li3PO4 Coating layer
Phosphate Anion (PO4-)
Active
Material
3Li2O + 2H3PO4 → 2Li3PO4 + 3H2O
3LiOH + H3PO4 → Li3PO4 + 3H2O
3Li2CO3 + H3PO4 → Li3PO4 + 3CO2 + 3H2O
Figure 4 Schematic illustration of chemistry on the surface for (a) bare Li[Ni 0.6Co0.2 Mn0.2]O2 and (b) lithium phosphate-coated
Li[Ni0.6Co0.2Mn0.2]O2.
8
2.2. Comparison of electrode performances
To observe the electrochemical properties of the
electrodes, cycling tests were carried out using both
the bare and the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2
electrodes (ca. 14 mg cm-2) by applying 280 mA g-1 (1
C-rate) at 25oC. The first charge and discharge curves
and their derivatives are shown in Fig. 5a and b. The
voltage profiles of the bare and Li 3PO4-coated
electrodes were almost identical. However, the
Li3PO4-coated electrode showed a slightly improved
coulombic efficiency, compared to the bare electrode.
0
3.6
3.4
-1
3.2
Bare
Bare
3.0
Li3PO4 coated Li[Ni0.6Co0.2Mn0.2]O2
Li3PO4 coated Li[Ni0.6Co0.2Mn0.2]O2
100
150
-1
200
3.0
3.5
-2
4.5
4.0
Voltage / V
4.2
+
c
4.0
d
3.8
4.0
3.8
3.6
1 cycle
10 cycle
50 cycle
75 cycle
100 cycle
150 cycle
1 cycle
10 cycle
50 cycle
75 cycle
100 cycle
150 cycle
3.4
3.2
3.0
4.2
+
50
0
20
40
60
80
100 120 140 160 180 200
0
20
40
60
-1
3.4
3.2
80
3.0
100 120 140 160 180 200
-1
Capacity / mAh g
Capacity / mAh g
180
f
e
-1
3.6
110
160
100
90
140
80
70
120
100
0
Bare
Bare
Li3PO4 coated Li[Ni0.6Co0.2Mn0.2]O2
Li3PO4 coated Li[Ni0.6Co0.2Mn0.2]O2
20
40
60
80
100
120
Voltage / V VS. Li/Li
0
Capacity / mAh g
Voltage / V VS. Li/Li
-1
3.8
2.8
Capacity / mAh g
1
dQ / dV / mAhg V
b
4.0
140
160
0
20
Cycle number
60
40
60
80
100
120
50
140
Coulombic efficiency / %
+
a
4.2
-1
2
4.4
Voltage / V VS. Li/Li
Also, there was no electrochemical activity of the
Li3PO4 coating medium, indicating that those
materials were not formed during the heat treatment
for the surface modification. because, if the
phosphorous was substituted for oxygen at the 6c
site or for the transition metal at the 3b sites, the
resulting voltage profiles should vary as seen in
previous reports [30, 31] due to the change in
interaction energies between the Li layer and the
transition metal layer in the host structure.
160
Cycle number
Figure 5 Comparison of (a) the first charge and discharge curves of bare and lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1
wt. %) and (b)the corresponding differentiated curves. Discharge curves of (c) bare and (d) lithium phosphate-coated Li[Ni0.6Co0.2
Mn0.2]O2 (1 wt. %) during 150cycles at room temperature. (e) The resulting cyclability and (f) coulombic efficiency of bare and
lithium phosphate-coated Li[Ni0.6 Co0.2Mn0.2]O2/Li cells at room temperature.
9
4.0
4.2
b
a
4.0
3.8
3.8
3.6
3.6
3.4
3.4
3.2
3.2
3.0
10C
2.8
2.6
0
50
7C
5C
100
3C
3.0
1C
2C
10C 5C 2C1C 0.1C
7C 3C
0.1C
150
200
0
50
-1
2C
-1
2.6
200
Capacity / mAh g
180
Capacity / mAh g
150
-1
Capacity / mAh g
1C
100
2.8
+
Voltage / V VS. Li/Li
+
4.2
high rates (over 3 C: 840 mA g-1). It is likely that the
presence of the Li3PO4 coating layers facilitated Li+
diffusion, even at a high rate, because of its ionic
conducting character [26]. In addition, the reduced
insulating residual lithium compounds caused by
the Li3PO4 coating likely assist for the better rate
capability. For these reasons, the Li 3PO4-coated
Li[Ni0.6 Co0.2Mn0.2]O2 could have better capacity and
retention than the bare at high rates.
Long term cycle test was conducted with a full-cell
using the MCMB negative electrode to confirm
effectiveness of Li3PO4-coated Li[Ni0.6 Co0.2Mn0.2]O2
electrode applying 1C (12 mA) for charge and 2C (24
mA) for discharge in the voltage range of 3 – 4.2 V
(Fig. 7). According to our prior report [17], capacity
fade is accelerated when cycled at higher rates. The
full cell showed outstanding capacity retention upon
1000 cycles, maintaining 95.6% of the first discharge
capacity (Fig. 7a). Also, there is no significant decay
in the operation voltage during cycling (Fig. 7b),
verifying the effectiveness of the presence of Li 3PO4
layer on the surface of active material.
Voltage / V VS. Li/Li
This result indicates that the coating material
resided only on the surface of the Li[Ni0.6Co0.2
Mn0.2]O2. Fig. 5c-f show the cycling performance of
the bare and Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 over
150 cycles. The bare Li[Ni0.6Co0.2Mn0.2]O2 experienced
a fast fade in capacity relative to the Li 3PO4-coated
Li[Ni0.6Co0.2Mn0.2]O2: approximately 76.1% of its
capacity retention for the bare electrode and 94.1% of
its capacity retention for in the Li3PO4-coated
electrode, showing good coulombic efficiency.
We also tested the rate capability of the bare and the
Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 (Fig. 6). The cells
were cycled at different current densities at 25 oC (1 C
= 280 mA g-1) between 3 V and 4.3 V, and were
charged galvanostatically with 0.5 C (140 mA g-1)
prior to each discharge. After the discharge at high
rates, the cells were buffered at 0.1 C to minimize
damages of electrodes. The lithium phosphate
-coated Li[Ni0.6 Co0.2Mn0.2]O2 delivered higher
capacity and good capacity retention at all currents
in comparison with the bare cell (Fig. 6). In addition,
the difference in capacity was more discernible at
c
3C
5C
160
7C
10C
140
120
Bare
Li3PO4 coated Li[Ni0.6Co0.2Mn0.2]O2
100
0
5
10
15
20
25
30
Cycle number
Figure 6 Rate capability of (a) bare and (b) lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 /Li cells at room temperature. (c) Rate
10
capability and cyclability of bare and lithium phosphate-coated Li[Ni0.6Co0.2 Mn0.2]O2 (1 wt. %)/Li cells at room temperature with
different discharge rates.
4.2
a
180
b
4.0
3.8
160
3.6
1
Cycle
200 Cycle
400 Cycle
600 Cycle
800 Cycle
1000 Cycle
140
120
100
0
200
400
600
800
1000
0
50
3.4
3.2
Voltage / V
Capacity / mAh g
-1
200
3.0
2.8
100
150
200
-1
Cycle number
Discharge capacity / mAhg
Figure 7 (a) cyclability and (b) charge and discharge curve for long term full-cell test of lith ium phosphate-coated Li[Ni0.6Co0.2
Mn0.2]O2 (1 wt. %)/MCMB cell.
Ac-impedance measurements were performed to
investigate the improved electrochemical properties
of the Li3PO4-coated Li[Ni0.6 Co0.2Mn0.2]O2 as a
function of cycle number at room temperature. Since
the Cole-Cole plots correspond to the high-to
-medium frequency region, the semicircles are
attributed to the resistance of the surface film on the
electrode (Fig. 8a and b). As the cycling progressed,
the surface film resistance of the bare cell increased
considerably in comparison with that of the
Li3PO4-coated cell. The increase of Rfilm for the bare
cell was almost six-fold of its initial Rfilm. It was
assumed that byproducts, which can impede Li +
diffusion, would be gradually formed in the
electrolyte as cycling progressed [32]. According to
prior reports [16, 33], the byproducts formed on the
surface of the active material are mainly composed of
LiF, LixPFy, and LixPFyOz-type compounds that are
highly resistive to migration of Li + ions. Also, the
residual lithium compounds presenting on the
surfaces of the active materials can readily react with
the electrolyte. It is anticipated that the above
-mentioned deteriorative decomposition reaction
would occur less progressive for the Li 3PO4-coated
Li[Ni0.6 Co0.2Mn0.2]O2, as seen from the slow increase
in Rfilm for the Li3PO4-coated Li[Ni0.6 Co0.2Mn0.2]O2
(Fig. 8b).
1200
0 cycle
1 cycle
10 cycle
20 cycle
50 cycle
100 cycle
150 cycle
-ZImg / 
1000
800
600
a
0 cycle
1 cycle
10 cycle
20 cycle
50 cycle
100 cycle
150 cycle
b
400
200
0
0
200
400
600
800
Zreal / 
1000
0
200
400
600
800
1000 1200
Zreal / 
Figure 8 Cole-Cole plots of (a) bare and (b) lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %) during 150 cycle.
11
2.3. Morphological and structural characterization
of post cycled electrodes
Since there were different cycling performance
tendencies, namely, the better capacity retention of
the Li3PO4-coated electrode, the extensively cycled
electrodes were examined by XRD (Fig. 9). The XRD
patterns were calibrated using the internal standard
50
60
70
80
90
Intensity / A.U.
208
40
027
30
0111
0012/024
021/116
20
107
100 110 10
113
018/110
90
105
80
006/102
101
70
208
60
027
50
0111
0012/024
40
113
018/110
107
105
30
b
104
a
006/102
101
20
003
104
003
10
of the Al current collector. The resulting XRD
patterns were not identical. Indeed, the widths of the
peaks appeared to be broader for the cycled bare
compared to the as-synthesized sample shown in Fig.
1a, and the diffraction intensity of the (003) peak was
significantly lower (Fig. 9a), which was ascribed to
structural disintegration.
100 110
Cu K 2 / Degree
Cu K 2 / Degree
Figure 9 Rietveld refinement patterns of XRD data for the (a) bare Li[Ni0.6Co0.2Mn0.2]O2 electrode and the (b) lithium
phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %) electrode after 150 cycling at room temperature.
Table 4 Rietveld refinement results of XRD data for bare
Table 5 Rietveld refinement results of XRD data for lithium
Li[Ni0.6Co0.2Mn0.2]O2 after 150 cycling.
phosphate-coated Li[Ni0.6Co0.2 Mn0.2]O2 after 150 cycling.
Formula
Li1.00[Ni0.596Co0.201Mn0.203]O2
Formula
Li1.00[Ni0.596Co0.201Mn0.203]O2
Crystal system
Rhombohedral
Crystal system
Rhombohedral
Space group
R m
Space group
R m
Atom
Site
x
y
z
Li1
3a
0
0
Ni1
3a
0
Co
3b
Mn
2
g
B/Å
Atom
Site
x
Y
z
1/2
0.828(2)
1.1
Li1
3a
0
0
0
1/2
0.172(1)
1.1
Ni1
3a
0
0
0
0
0.201
0.9
Co
3b
3b
0
0
0
0.203
0.9
Mn
Ni2
3b
0
0
0
0.428(3)
0.9
Li2
3b
0
0
0
0.172(3)
O
6c
0
0
0.2564(9)
1.000
Cell parameters
a=b=2.8582(2)Å
2
g
B/Å
1/2
0.935(2)
1.0
0
1/2
0.065(2)
1.0
0
0
0
0.201
0.8
3b
0
0
0
0.203
0.8
Ni2
3b
0
0
0
0.535(2)
0.8
0.9
Li2
3b
0
0
0
0.065(2)
0.8
0.8
O
6c
0
0
0.2578(6)
1.000
0.8
Cell parameters
c=14.2494(8)Å
a=b=2.8640(1)Å
c=14.2445(7)Å
Rwp
17.9 (%)
Rwp
13.8 (%)
Rp
17.0 (%)
Rp
12.7 (%)
12
The Rieveld refinement results of the XRD data
indicated that a high degree of cation mixing, as
inferred by the occupation of Ni 2+ in the Li layer at a
molar fraction of 17.2%, progressed in the Li layer of
the crystal structure, resulting in the gradual decline
in capacity upon cycling. The calculated lattice
parameters also deviated from those of the
as-synthesized sample (Table 1).
In contrast, almost no change was apparent in the
XRD pattern of the extensively cycled Li3PO4-coated
electrode (Fig. 9b), including the widths of the peaks
compared to the as-synthesized sample (Fig. 1b). The
original layer structure was maintained, showing
less variation in the a- and c-axes upon cycling, and
the occupation of Ni2+ (3.3% before cycling) in the Li
layer was approximately 6.5% (Table 4), which was
significantly lower than in the cycled bare electrode.
These results prove that the Li3PO4 coating obviously
improved the structural integrity of the layer
structure, resulting in less cation mixing after
extensive cycling.
Since the severe structural disintegration occurred
in the bare electrode after the extensive cycling test
compared to the Li3PO4-coated electrode, the cycled
cells were disassembled, and all components were
rinsed with salt-free dimethyl carbonate for a week.
It is generally accepted that HF generation is
common in alkyl carbonate electrolytes using LiPF6
salt [34, 35]. This salt is unstable at elevated
temperatures, at high operating voltages, and in the
presence of water molecules. Since the complete
removal of water molecules, which appear as
impurities at concentrations of under 30 ppm in
commercially-available electrolytes, is impossible,
HF formation is inevitable as a result of the
decomposition process of the electrolyte salt [34, 35].
The concentration of HF detected was 239 ppm for
the bare electrode (Fig. 10). Meanwhile, the HF
content was greatly reduced to 92 ppm for the coated
electrode. The HF in the electrolyte accelerated the
gradual degradation of the active materials by
dissolving the metallic ions. Because the bare
materials were directly exposed to the HF in the
electrolyte, higher amounts of transition metals were
dissolved into the electrolyte than from the
Li3PO4-coated electrode since the coating layer acted
as a shield against the deteriorative reaction with HF.
Hence, the original crystal structure could be
retained during the cycling test. In addition, the
variation in impedance suggests that the slow
increase in the resistance was closely related to the
structural integrity being supported by the Li 3PO4
coating layer on the surfaces of the active materials.
Figure 10 HF titration and transition metal dissolution results
for the electrolyte of 150 cycled bare and lithium phosphatecoated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %) cells.
To understand the role of Li3PO4 coating layer
affecting to the better electrode performance,
ToF-SIMS observations were carried out using the
cycled bare and Li3PO4-coated Li[Ni0.6 Co0.2Mn0.2]O2
electrodes (Fig. 11). Dissolution of the active
materials by HF occurs as follows [17]:
MnO + 2HF → MnF2 + H2O
CoO + 2HF → CoF2 + H2O
NiO + 2HF → NiF2 + H2O
(4),
(5),
(6).
The byproducts were detected as fragments having
Ni-F bonding (Fig. 11a, showing NiF+ fragment).
Indeed, HF is formed by a series of decomposition
processes in the LiPF6-based electrolyte. According to
reports [32, 36], de composition of the LiPF6 salt
occurs as follows:
LiPF6 →LiF + PF5,
PF5 + H2O → POF3 + 2HF,
POF3 + 3Li2O → 6LiF + P2O5 (or LixPOFy).
(7),
(8),
(9).
13
a
b
c
d
e
f
Figure 11 ToF-SIMS results of cycled bare electrode and lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %) electrode after
cycling: (a) NiF+, (b) LiF+, (c) LiPOF+, (d) PO2H2+, (e) POH2+, and (f) LiPO3H2+ fragments. (top: bare, bottom: lithium
phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2)
The LiPF6 salt is in equilibrium with LiF and PF5 in
the electrolyte solutions [37]. In addition, the
byproduct, PF5, is subject to reaction with water in
the electrolyte solution because the electrolyte
always contains a small amount of water as an
impurity. Consequently, noxious POF3 and HF,
which damage the active materials directly, are
produced through the reaction between PF5 and
H2O. These products attack both the active material
and the byproducts on the surface, generating LiF
(Fig. 11b) and LixPOFx (Fig. 11c, see the fragment of
LiPOF+). These byproducts were more evident on the
surface of the bare Li[Ni0.6 Co0.2Mn0.2]O2 because the
materials were directly exposed to HF.
The bare Li[Ni0.6 Co0.2Mn0.2]O2 also had surface
layers such as LiOH and Li2CO3. These compounds
can also be dissolved by HF attack, generating LiF:
Li2CO3 + 2HF → 2LiF + H2O + CO2
LiOH + 2HF → LiF + H2O
(10),
(11).
As seen in Fig. 11b, the LiF was detected more
clearly as LiF+ fragment on the surface of the bare
electrode. The LiF leads to increased impedance
because the insulating LiF tends to disturb Li+
migration.[38] Similar phenomena were also observed
in Figs. 8a and b that described a great increase in the
Rfilm for the bare but less serious for the
Li3PO4-coated electrodes during cycling. It is
confirmed in the ToF-SIMS spectra that the relative
intensity of the LiF+ fragment appeared to be small
for the coated electrode, presumably due to the
lower amounts of residual lithium compounds
because of their consumption during the formation
of Li3PO4 on the surface during coating (Fig. 3).
Therefore, it is obvious that the presence of Li3PO4
layer delayed decomposition of the electrolyte.
We also assumed that the Li3PO4 coating layers
functioned to scavenge the acidic HF as follows:
Li3PO4 + HF → LixHyPO4(or POxHy) + LiF
(11),
which involved the appearance of PO2H3+ (Fig. 11.d),
POH2+ (Fig. 11.e) and LiPO3H2+ (Fig. 11.f) fragments.
14
As more HF was created by the reaction of PF5 and
water, the acidity of the electrolyte would increase,
as follow:
2HF → H+ + FHF-
thought that the presence of Li3PO4 layers
contributes to the reduction in water in the
electrolyte by absorbing the presenting water in the
electrolyte that causes the formation of LixHyPO4 (or
POxHy), which, in turn, reduces generation of HF in
the electrolyte. In addition, the coating layers
scavenges HF, forming LixHyPO4 (or POxHy), which
suppresses HF propagation during cycling.
These functions of the Li3PO4 layers are confirmed
in Fig. 12 which shows TEM images of the
extensively-cycled
bare
and
Li3PO4-coated
Li[Ni0.6Co0.2Mn0.2]O2 electrodes. Some byproducts
were observed on the surfaces of the bare electrodes
due to their formation as a result of electrolyte
decomposition. The particles were damaged due to
HF attack during cycling (Fig. 12a and b). By
contrast, the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 kept
its original smooth edge with coating layers, leading
to higher capacity retention because the Li 3PO4
-coating layer acted as protecting and scavenging
layers against HF and water in the electrolyte during
extensive cycling (Fig. 12c and d). Role of the Li 3PO4
layers is proposed in Fig. 13.
(12).
Furthermore, we propose that the Li3PO4 layer also
scavenges water molecules in the electrolyte, as it is
well known that a phosphate moiety absorbs water
molecules [39]. Therefore, the outermost Li3PO4
layers would first react with water molecules in the
electrolyte. This hypothesis could be explained in the
following manner:
Li3PO4 + H2O → LixHyPO4 (or POxHy) + Li2O
(13).
This reaction is similar to that of the HF scavenging
process shown in Reaction (11). Hence, the removal
of water is important because it suppresses the
reaction between PF5 and water molecules which
creates HF. This can also explain why the electrolyte
decomposition, evidenced by the LiPOF+ fragment
(Fig. 11c), was delayed on the surface of the
Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2. Therefore, it is
a
b
Active
Material
Active
Material
Byproducts
20nm
20nm
c
Li3PO4 coating layer
20nm
Active
Material
Li3PO4 coating layer
10nm
d
Active
Material
Figure 12 TEM bright field images of extensively cycled bare and lithium phosphate-coated Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %)
electrode at room temperature. Bare electrode, (a) lower magnification and (b) higher magnification. Lithium phosphate-coated
electrode, (c) lower magnification and (d) higher magnification.
15
a
b
Figure 13 Schematic illustration of byproducts on the surface for 150 cycled (a) bare and (b) lithium phosphate-coated
Li[Ni0.6Co0.2Mn0.2]O2 (1 wt. %).
3. Conclusion
In this study, we attempted to modify the
Li[Ni0.6Co0.2Mn0.2]O2 surface with H3PO4, resulting in
Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2 through reaction
with the residual LiOH and Li2CO3 compounds
presented on the surfaces of the Li[Ni0.6Co0.2Mn0.2]O2
particles. This was effective in reducing the
concentration of residual LiOH and Li 2CO3 on the
surface of the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2. As
a result, the Li3PO4-coated Li[Ni0.6Co0.2Mn0.2]O2
performed better capacity retention, rate capabilities,
and lower impedance growth than the bare
Li[Ni0.6Co0.2Mn0.2]O2. These improvements were
mainly attributed to the presence of the Li 3PO4
coating layers: i) the lower concentrations of LiOH
and Li2CO3 which impeded Li+ migration, ii) the
water absorption of Li3PO4 which produced less HF
from the decomposition of LiPF6 salt, iii) HF
scavenging which reduced the level of HF in the
electrolyte, and iv) protection of active material from
HF attack during cycling. These multi-functions of
the Li3PO4 coating layer had positive effects on the
electrochemical performance and structural integrity
of the electrode materials. We believe that this
approach is applicable to high-capacity Ni-rich
compounds, which are expected to be used as power
sources for (hybrid) electric vehicles and energy
storage systems.
Acknowledgements
This work was partially supported by the IT R&D
program of MKE/KEIT [10041856, Technology
development for life improvement of high-Nicomposition cathodes at high temperatures (≧ 60oC)]
and the secondary battery R&D program for leading
green
industry
of
MKE/KEIT
[10041094,
Development of Co-free, high-thermal-stability, and
eco-friendly layered cathode materials for lithium
ion batteries].
16
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