Improving cyclability of 5 V cathodes by electrochemical surface modification

Article abstract of DOI:10.1246/cl.2004.616

A simple electrochemical procedure is proposed for surface modification of cathode materials. A metal oxide-retaining layer deposited electrochemically on the electroactive material avoids its direct contact with the electrolyte. For 5 V cathodes, which are extremely oxidant at charged state, this action significantly enhances the cyclability. In addition, the own stability of the metal oxide-retaining layer deposited on the cathode surface has a significant effect on the cyclability, which can be enhanced by inducing a mechanical force during the electrodeposition process.

Full text of DOI:10.1246/cl.2004.616

616
Chemistry Letters Vol.33, No.5 (2004)
Improving Cyclability of 5 V Cathodes by Electrochemical Surface Modification
Ali Eftekhariꢀ
Electrochemical Research Center, P. O. Box 19395-5139, Tehran, Iran
(Received February 9, 2004; CL-040143)
A simple electrochemical procedure is proposed for surface
short (<1 s), and this action cannot affect Li intercalating system
of the cathode.
Figure 1 compares cyclability of different 5 V cathode mate-
modification of cathode materials. A metal oxide-retaining layer
deposited electrochemically on the electroactive material avoids
its direct contact with the electrolyte. For 5 V cathodes, which
are extremely oxidant at charged state, this action significantly
enhances the cyclability. In addition, the own stability of the
metal oxide-retaining layer deposited on the cathode surface
has a significant effect on the cyclability, which can be enhanced
by inducing a mechanical force during the electrodeposition
process.
(A)
150
125
18.1%
100
33.1%
75
Surface modification is an efficient method to reduce capaci-
ty fading of cathode materials in lithium batteries. Capacity fad-
ing of 5 V cathode materials is due to the generation of extremely
active oxidant during high-voltage performance resulting in
electrolyte oxidation. Very recently, surface modification has
been applied to improve cyclability of 5 V cathode materials.1,2
However, further attention should be paid to this issue.
In the present paper, it is aimed to propose a simple method
for surface modification of cathode materials. This method is
typically examined for three well-known 5 V cathode materials,
namely LiNi0:5Mn1:5O4,1–4 LiFe0:5Mn1:5O4,5,6 and LiCoPO4.7,8
The first two belong to the class of LiMxMn2ꢁxO4 5 V cathode
materials, which is the most known class of 5 V cathode materi-
als. LiCoPO4 is an example of another class of 5 V cathode ma-
terials namely olivine-type compounds.
50
63.0%
25
0
0
0
10
10
10
20
20
20
30
40
40
40
50
50
50
60
60
60
Cycle Number
(B)
150
125
100
75
7.9%
21.7%
47.5%
Three known samples were synthesized according to the
methods reported in the literatures: LiNi0:5Mn1:5O4,1 LiFe0:5
-
50
Mn1:5O4,5,6 LiCoPO4.7,8 The cathode materials synthesized were
deposited onto an Al current collector with a 100 W sputtering
method to prepare film-based cathodes and examined in a
coin-type cell containing the cathode and a lithium foil as both
anode and reference electrode. The electrolyte solution was a
1 M LiPF6 in ethylene carbonate–dimethyl carbonate (EC/
DMC) 3:7. Charge–discharge profiles of the cathodes were re-
corded with a rate C/2 at room temperature using a battery cy-
cler to determine the specific capacity at each cycle number.
The first aim of the present study is to show the usefulness
(and indeed importance) of metal-oxide coating of 5 V cathode
materials to achieve acceptable cyclability. Secondly, it is aimed
to use a simple electrochemical procedure instead of current
physical techniques such as rf sputtering for the surface modifi-
cation of the cathode materials.
25
30
Cycle Number
(C)
150
100
50
15.4%
33.3%
90.6%
It has been reported9,10 that metal oxides can be incorporated
within LiMn2O4 film during an electrochemical process. Indeed,
this electrochemical process can also be used for the deposition
of a thin film of metal oxide on the cathode material. A thin film
of Al2O3 (ca. 10 nm) was deposited on the cathode surface from
an electrolyte solution containing 0.1 M Al(NO3)3 in 1 M LiOH
by applying a constant current density (ca. 1 mA/cm2). Since the
metal oxide is very thin, the time required for this process is very
0
30
Cycle Number
Figure 1. Cyclability data for different cathodes: (A)
LiNi0:5Mn1:5O4, (B) LiFe0:5Mn1:5O4, and (C) LiCoPO4. The da-
ta correspond to conventional cathodes ( ), cathodes coated by
Al2O3 ( ), and cathodes coated by Al2O3 under an applied cen-
trifugal force ( ).
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
617
rials with and without metal oxide-retaining layer. It is obvious
that the metal oxide-retaining layer significantly improves cycla-
bility of the 5 V cathodes, avoiding direct contact of active oxi-
dants generated during high-voltage performance.6 The results
reported in Figure 1 for different 5 V cathodes indicate that the
influence of the metal oxide-retaining layer is similar for all cas-
es. This satisfactorily provides a strong evidence for usefulness
of the procedure proposed for improving cyclability of 5 V cath-
ode materials.
In addition to simplicity of the electrochemical procedure
proposed for the surface modification of cathode materials, it
has another advantage, i.e. flexibility for specified purposes by
controlling electrodeposition process. For instance, inducing a
mechanical force can improve the electrodeposition process. In
a series of papers,11–15 it has been demonstrated that electrodepo-
sition under centrifugal forces can improve stability and proper-
ties of materials deposited. For instance, transition metal hexa-
cyanoferrate films deposited in the presence of gravitational13
or magnetic16 fields have enhanced stabilities. It is of great im-
portance for the surface modification of cathode materials, since
the stability of the own metal oxide-retaining layer deposited on
the cathode surface is proportional to the cathode cyclability. On
the other hand, it has been reported that electrodeposition under
centrifugal forces leads to higher electrical conductivity of ma-
terials deposited owing to lower interfacial resistance of the par-
ticles.15 It is also of interest for the surface modification of cath-
ode materials, since metal oxides are usually more resistive than
electroactive material and also there is a significant interfacial
resistance between the electroactive film and the metal oxide
layer.
materials. It also indicates the importance of interfacial resist-
ance for the surface modification of cathodes.
To understand the structure of the metal oxide layer depos-
ited onto the cathode surface, it is useful to investigate its depth
profile. Depth profiling by sequential Arþ-ion sputtering and
XPS analysis is a common way to determine surface layer thick-
ness and to measure elemental concentrations as a function of
depth. For this purpose, depth profile data were obtained by Ar
ion-beam sputtering (4 keV). Figure 2 displays depth profiles
of the Al2O3 layers deposited on the LiFe0:5Mn1:5O4 cathode
surface. It is obvious that aluminum also exists, out of the 10-
nm thickness of the own film, within the cathode material. It
might be attributed to the fact that electrolyte solution of the
plating bath fills empty pores within the cathode material, and
Al2O3 can be formed there.
The amount of aluminum is higher throughout the depth
profile for the Al2O3 layer electrodeposited under centrifugal
force. High amount of aluminum at the own Al2O3 layer is
due to a denser film with lesser impurities deposited under the
later condition. It has been described16 that inducing a mechan-
ical force during electrodeposition process increases the film
density and reduces the amount of impurities incorporated with-
in the electroactive material from the electrolyte. Moreover, the
mechanical force induced compelled the depositing particle to
diffuse inside the cathode material.
In conclusion, the experimental procedure proposed for the
surface modification cathode materials is an efficient method for
improving their cyclability. Such improvement leads 5 V cath-
ode materials toward practical performances. As stated, an im-
portant advantage of the procedure is its flexibility for control-
ling the surface modification. Therefore, further investigations
in this direction may lead to peculiar results.
To gain this possibility, the electrochemical deposition of
the metal oxide layer was performed in the presence of an ap-
plied centrifugal force. The detailed experimental procedure
for this action has been reported in the literature.11,12 A typical
centrifugal force of 210 g was used in the present study. The cy-
clability data of the 5 V cathode materials coated with a metal
oxide layer under applied centrifugal force are also illustrated
in Figure 1. It is obvious that electrodeposition of the metal ox-
ide-retaining layer under centrifugal force is an efficient ap-
proach to additionally improve cyclability of the 5 V cathode
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0
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Depth / nm
Figure 2. Depth analysis of the Al2O3 layer deposited without
) and with centrifugal force ( ) on the LiFe0:5Mn1:5O4 (as
a typical cathode).
(
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Published on the web (Advance View) April 24, 2004; DOI 10.1246/cl.2004.616