Solvothermal synthesis and electrochemical performance of Li 2MnSiO4/C cathode materials for lithium ion batteries

Article abstract of DOI:10.1016/j.jallcom.2014.06.086

Orthorhombic structure Li₂MnSiO₄/C with Pmn2 ₁ space group is synthesized by the solvothermal method. Carbon coating and Ni²⁺ doping are used to improve the electronic conductivity and the cycling performance of

Full text of DOI:10.1016/j.jallcom.2014.06.086

Journal of Alloys and Compounds 614 (2014) 271–276
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Solvothermal synthesis and electrochemical performance of Li2MnSiO4/C
cathode materials for lithium ion batteries
Yan-Chao Wang a,b, Shi-Xi Zhao a, , Peng-Yuan Zhai a,b, Fang Li a, Ce-Wen Nan b
⇑
a Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
b School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Orthorhombic structure Li2MnSiO4/C with Pmn21 space group is synthesized by the solvothermal
method. Carbon coating and Ni2+ doping are used to improve the electronic conductivity and the cycling
performance of Li2MnSiO4 cathode material, respectively. The particles of Li2MnSiO4/C are much smaller
and more uniform than those of Li2MnSiO4 due to the carbon coating. It is shown that Ni2+ has been
reduced into metal Ni during the synthesis process. The synthesized Ni-modified Li2MnSiO4/C (denoted
as (LMS@Ni)/C) cathode material exhibits better electrochemical performance in comparison with Li2-
MnSiO4/C, attributing to higher lithium ion diffusion coefficient as well as electronic conductivity. The
initial discharge capacity of (LMS@Ni)/C is 274.5 mA h gꢀ1 and the reversible capacity after 20 cycles is
119.8 mA h gꢀ1 at 25 °C.
Received 15 February 2014
Received in revised form 15 June 2014
Accepted 16 June 2014
Available online 24 June 2014
Keywords:
Cathode material
Li2MnSiO4
Solvothermal synthesis
Carbon coating
Ni-modified
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
However, Li2MnSiO4 as a cathode material for practical
applications still have several issues to be solved [15–19]. First,
it is difficult to obtain a phase-pure sample, some inactive
impurity phases such as MnO and Li2SiO3 always appear in
Lithium ion batteries are widely applied in energy storage and
conversion. The expanded applications of Lithium ion batteries in
automobiles, aerospace, and power-grid demand the long-life
and light-weight batteries, which largely depend on the choice of
the resultant. Second, its low electronic conductivity (ꢁ10ꢀ14
-
)
S cmꢀ1) and low lithium diffusion coefficient (ꢁ10ꢀ16 cm2 Sꢀ1
cathode materials [1–3]. ðXO4Þꢀ polyanion compounds have
severely limit the rate performance. Furthermore, its structural
instability caused by delithiation and Mn dissolution leads to
capacity rapidly fading during the cycling process. Therefore,
Li2MnSiO4 materials generally exhibit poor practical electro-
chemical performance. Effective strategies to improve the elec-
trochemical performance of Li2MnSiO4 include carbon coating,
n
attracted lots of attention for their unique properties as cathode
materials [4–6]. Of these materials, the olivine lithium iron phos-
phate (LiFePO4) has been widely studied due to its excellent struc-
ture stability, as well as inexpensive and environmental benign
characteristics [6,7]. Despite these advantages, it suffers from the
low electronic conductivity. To solve this issue, numerous strate-
gies have been utilized, including cationic doping, carbon coating
and nanoparticle synthesizing [6,8–10]. Nevertheless, the energy
density of LiFePO4 is relatively limited partially due to only one
lithium ion extraction from the host. Following the successful
development of LiFePO4 cathode, lithium transition metal silicates
(Li2MSiO4, M = Fe, Mn, Co, Ni) are regarded as materials of consid-
erable potential because of their high theoretical capacity
doping with other cations to partially replace Mn2+
, and
decreasing the particle size [20–23]. It has been demonstrated
that decreasing the particle size can shorten the pathway for
Li+ transfer, which will weaken the negative effect of low Li+
diffusion coefficient [20,24].
In recent years, various methods have been used to prepare Li2-
MnSiO4 materials, such as solid-state reaction, polyol method, and
sol–gel process [25–28]. In order to obtain samples with nano par-
ticle size and high surface area, the solvothermal method was used
to prepare Li2MnSiO4 with carbon coating in this study. To further
improve the cycling performance of Li2MnSiO4/C, we intended to
substitute Ni2+ for Mn2+ in Li2MnSiO4 to improve its structural sta-
bility. The effect of carbon coating and Ni2+ doping on the struc-
ture, morphology and electrochemical performance of Li2MnSiO4
was investigated.
(>300 mA h gꢀ1
)
[5,11–14]. And
a
reversible capacity of
333 mA h gꢀ1 can be demonstrated because two lithium ions can
be extracted from Li2MnSiO4, corresponding to Mn2+/Mn3+ and
Mn3+ /Mn4+ [12].
⇑
Corresponding author. Tel./fax: +86 0755 26036372.
E-mail address: zhaosx@sz.tsinghua.edu.cn (S.-X. Zhao).
http://dx.doi.org/10.1016/j.jallcom.2014.06.086
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

272
Y.-C. Wang et al. / Journal of Alloys and Compounds 614 (2014) 271–276
Fig. 1. XRD patterns of all samples.
2.3. Electrochemical measurements
Table 1
Lattice parameters of the Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C nanocomposites.
The electrochemical performance of as-prepared samples was characterized
with coin-type cells (CR2032). The cathode was fabricated by mixing active mate-
rial, acetylene black, and polyvinylidene fluoride with a weight ratio of 80:10:10
in N-methyl pyrrolidone (NMP) to form homogenous slurry. The slurry was then
pasted on an Al foil current collector, and the coated film was dried at 120 °C for
8 h. The resulting film was pressed, punched into circular discs with a diameter
a (Å)
b (Å)
c (Å)
Rwp (%)
Rp (%)
Li2MnSiO4
Li2MnSiO4/C
(LMS@Ni)/C
6.296(8)
6.298(9)
6.291(3)
5.377(7)
5.377(0)
5.380(7)
4.990(7)
4.984(3)
4.980(6)
19.98
7.78
10.92
13.12
5.56
7.50
of 15 mm, and the thickness of the cathode material was about 120
average loading of 1.6–2.8 mg cmꢀ2. The electrolyte was LiPF6 (1M) in ethylene car-
bonate and dimethyl carbonate (1:1 v/ ). All the cells were assembled in an argon-
lm with an
v
filled glove box with O2 and H2O levels below 2 ppm with lithium metal as the
anode. Electrochemical measurements were carried out using a battery test system
(C2001A, Land, China) in a voltage range of 1.5–4.8 V at 0.05 C (1 C = 333 mA gꢀ1).
The specific capacity of each sample was calculated based on the mass of Li2MnSiO4,
excluding the amount of carbon coating. Electrochemical impedance spectroscopy
(EIS) was measured on an electrochemical workstation (CHI660D, Chenhua, China)
in the frequency range from 1 mHz to 100 kHz.
2. Experimental
2.1. Preparation
Li2MnSiO4/C with Ni2+ doping was prepared via the solvothermal method by
using starch as the carbon source. Firstly, C19H42BrN (0.1 mmol, Aladdin, 99%)
was dissolved in methanol (45 ml). Secondly, C2H3O2Liꢂ2H2O (Aladdin, 99%), MnC4-
H6O4ꢂ4H2O (Aladdin, 99%), Si (OC2H5)4 (Aladdin, 99%) and NiC4H6O4ꢂ4H2O (Aladdin,
99%) were added into the above solution with the molar ratio of 2.2:0.95:1:0.05.
Finally, glacial acetic acid (1.5 mL) was added as a catalyst. After stirring for 6 h,
the homogeneous solution was loaded into a 100 ml Teflon-lined autoclave and
maintained at 120 °C for 20 h. The jelly-like product was dried at 60 °C 12 h in a
vacuum. The obtained precursor was thoroughly ground and mixed with starch sus-
pension with a weight ratio of Li2MnSiO4:C of 2:1. After stirring at 40 °C for 5 h, the
mixture was dried at 60 °C for 12 h. The dried product was heat treated at 450 °C for
2 h and then 700 °C for 10 h in Ar (95%)/H2 (5%) atmosphere to obtain Li2MnSiO4
with carbon coating and Ni2+ doping.
3. Results and discussion
3.1. XRD, SEM, TEM and TG analysis
The powder XRD patterns of the as-prepared Li2MnSiO4, Li2-
MnSiO4/C, and Li2MnSiO4/C with Ni2+ doping are shown in
Fig. 1(a). All diffraction patterns can be indexed to the orthorhom-
bic structure with a Pmn21 space group [29], and the broad diffrac-
tion peaks indicate the synthesized samples with low degree of
crystallization. In addition, there are some impurity phases in the
resultants (labeled in Fig. 1(a)) such as Li2SiO3 in Li2MnSiO4 and
MnO in Li2MnSiO4/C with Ni2+ doping. These impurities have also
been reported previously [16,28–31]. No impurity is observed in
the diffraction pattern of Li2MnSiO4/C sample, indicating that the
carbon coating can inhibit the formation of impurity phases. In
the diffraction pattern of Ni2+ doped sample, some characteristic
peaks of metal Ni are observed. This indicates that no Ni2+ ions
enter into the lattice. In order to confirm the presence of metal
Ni, the (Li2MnSiO4/C)+Ni nanocomposite as a reference sample
Similar processes were used to prepare Li2MnSiO4 and Li2MnSiO4/C materials
with a molar ratio of Li+:Mn2+:SiO44ꢀ of 2.2:1:1.
2.2. Physical characterization
X-ray diffraction (XRD, D/MAX-2500, Rigaku, Japan, Cu K
a radiation, 40 kV,
100 mA), scanning electron microscope (SEM, S-4800, Hitachi, Japan, 5.0 kV), and
transmission electron microscope (TEM, JEM-2011, JEOL, Japan, 200 kV) were used
to characterize the structure and the morphology of the prepared samples. The con-
tent of carbon was measured by thermogravimetry analysis (TG, STA 449 F3, Jupi-
ter) for the Li2MnSiO4/C nanocomposite. The specific surface area of each cathode
materials (denoted by as,BET) was obtained by analysis of nitrogen adsorption–
desorption isotherms recorded at 77 K on an adsorption analyzer (Belsorp-mini,
BEL, Japan).
Fig. 2. SEM images of (a) Li2MnSiO4, (b) Li2MnSiO4/C and (c) (LMS@Ni)/C.

Y.-C. Wang et al. / Journal of Alloys and Compounds 614 (2014) 271–276
273
Fig. 3. TEM images of (a) Li2MnSiO4/C and (b) (LMS@Ni)/C before the first cycle, and for (c) Li2MnSiO4/C and (d) (LMS@Ni)/C after the first cycle.
Table 2
Carbon content of Li2MnSiO4/C.
Samples
Theoretical
value (wt.%)
Measurements
(wt.%)
The loss
amount
(wt.%)
Error
values (wt.%)
Li2MnSiO4/C
33.3
15.9/96.2 = 16.5
16.8
2.2
Fig. 4. TG analysis of Li2MnSiO4/C heated from room temperature to 1000 °C in air.
was prepared by mixing the obtained Li2MnSiO4/C material with Ni
powders with a molar ratio of 1:0.05. It is shown that the diffrac-
tion peaks of Ni2+ doped sample are same with the characteristic
peaks of pure Ni in reference sample, confirming that Ni2+ has been
reduced to Ni in the Li2MnSiO4/C with Ni2+ doping sample. It was
found that the reduction of Ni2+ was caused by the reducing atmo-
sphere (Ar2 + H2) and carbon source during the synthesis process.
Then the synthesis of Ni-modified Li2MnSiO4/C (denoted as
(LMS@Ni)/C) nanocomposite was repeated by the solvothermal
method with a molar ratio of Li+:Mn2+: SiO44ꢀ:Ni2+ of 2.2:1:1:0.05
to study the effect of Ni on Li2MnSiO4/C. The result of XRD is iden-
tical with above.
Fig. 5. Cycling performance of Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C.
Fig. 1(b) presents the Rietveld plots of Li2MnSiO4, Li2MnSiO4/C
and (LMS@Ni)/C. The calculated lattice parameters are listed in
Table 1. The unit cell parameters of Li2MnSiO4/C and (LMS@Ni)/C
are almost identical, indicating that Ni does not occupy the lattice

274
Y.-C. Wang et al. / Journal of Alloys and Compounds 614 (2014) 271–276
Fig. 6. Charge/discharge profiles of (a) Li2MnSiO4/C and (b) (LMS@Ni)/C nanocomposites.
Fig. 7. (a) Nyquist plots of the Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C nanocomposite electrodes before cycling. (b) Linear relationship between Z0 and xꢀ1/2 in the low
frequency region. (c) Nyquist plots of the Li2MnSiO4/C and (LMS@Ni)/C nanocomposite electrodes after the first charge/discharge cycle. (d) Linear relationship between Z0 and
xꢀ1/2 in the low-frequency region.
of Li2MnSiO4/C. As a result, it is found that a straight substitution
Ni2+ for Mn2+ does not work via the solvothermal method.
Fig. 1(c) shows the XRD patterns of the (LMS@Ni)/C sample after
the first charge and discharge process. The typical diffraction peaks
belonging to Li2MnSiO4 cannot be observed, indicating that the
crystal structure of Li2MnSiO4 changes into amorphous during

Y.-C. Wang et al. / Journal of Alloys and Compounds 614 (2014) 271–276
275
the cycling process. However, the peaks of metal Ni can still be
3.3. EIS measurements
observed, indicating that metal Ni is stable during the charge–dis-
charge process.
Electrochemical impedance spectroscopy (EIS) measurements
have been performed to investigate the effect of Ni on the Li ion
migration dynamics. Fig. 7 shows the equivalent circuit and
Nyquist plots of the three samples tested at 25 °C [33]. All of the
Nyquist plots consist of a semicircle in the high to medium fre-
quency region and an inclined line in the low frequency region.
In the equivalent circuit, Re represents the electrolyte resistance,
corresponding to the intercept along the Z0 axis, and Rct represents
the charge transfer resistance between the electrolyte and elec-
trode material, corresponding to the diameter of the semicircle
on the Z0 axis. ZW is the Warburg coefficient (the Li+ diffusion in
the bulk electrode), corresponding to the slope of the inclined line
in þthe low frequency region. The lithium ion diffusion coefficient
(DLi) can be calculated as follows [34]:
SEM images of Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C are
shown in Fig. 2. The particle size of the Li2MnSiO4/C is confined
to 20–60 nm, much smaller and more uniform than that of the Li2-
MnSiO4 with size of 60–150 nm. This is attributed to the carbon
coating, which separates the base particles and inhibits the growth
of the particles during the calcinations process. The particle size of
the (LMS@Ni)/C is about 20–50 nm, indicating that a small amount
of Ni does not markedly affect the morphology of Li2MnSiO4/C.
Fig. 3 shows the TEM images of Li2MnSiO4/C and (LMS@Ni)/C.
Before cycling, the lattice fringes and carbon coating layer can be
observed clearly in Li2MnSiO4/C and (LMS@Ni)/C (Fig. 3(a) and
Fig. 3(b), respectively). After the first cycle, the lattice fringes have
become indistinct, as shown in Fig. 3(c) and Fig. 3(d). It indicates
that the crystal structure of Li2MnSiO4 undergoes the process of
amorphization, which is consistent with the XRD analysis.
Fig. 4 presents the TG data obtained for Li2MnSiO4/C. Water
evaporation happens below 250 °C, carbon oxidation of
C + O2 ? CO2 happens from 250 to 500 °C (the second mass
decrease in TG), and weight addition caused by the transition of
Mn2+ ? Mn3+ happens from 500 to 1000 °C. As shown in Table 2,
the content of carbon in Li2MnSiO4/C is 16.5 wt.%.
R2T2
2A2n4F4C2r2
DþLi
¼
ð1Þ
In this equation, A is the surface area of the cathode material
(listed in Table 3), n is the number of electrons per molecule
involved in electron transfer (2 here), F is the Faraday constant
(96,500 C molꢀ1), C is the molar concentration of Li ions (listed in
Table 3) [16], R is the gas constant (8.314 J Kꢀ1 molꢀ1), T is room
temperature (298 K), and
r is the slope of the fitting line.
Table 4 presents the parameters obtained from EIS analysis. Rct
value of Li2MnSiO4/C is much lower than that of Li2MnSiO4 before
the first cycle, indicating that the carbon coating can improve the
electronic conductivity of Li2MnSiO4. DþLi of Li2MnSiO4/C is
1.61 ꢃ 10ꢀ17 cm2 sꢀ1 before the first cycle, reducing to 1.28 ꢃ
10ꢀ19 cm2 sꢀ1 after the first cycle. However, DLþi of (LMS@Ni)/C is
2.96 ꢃ 10ꢀ17 cm2 sꢀ1 before the first cycle and 8.60 ꢃ 10ꢀ19 cm2
sꢀ1 after the first cycle, which is higher than that of Li2MnSiO4/C.
3.2. Electrochemical performance
Fig.
5 shows the cycling performance of Li2MnSiO4, Li2-
MnSiO4/C and (LMS@Ni)/C cathode materials, which was mea-
sured in the voltage range of 1.5–4.8 V at 0.05 C at 25 °C. The
initial discharge capacity of Li2MnSiO4/C (259.9 mA h gꢀ1
) is
much higher than that of Li2MnSiO4 (15.7 mA h gꢀ1), which dem-
onstrates that the carbon coating can effectively improve the
discharge performance of Li2MnSiO4. However, the discharge
capacity of Li2MnSiO4/C fades to 87.4 mA h gꢀ1 after 20 cycles.
The Ni-modified sample, (LMS@Ni)/C exhibits higher initial dis-
charge capacity of 274.5 mA h gꢀ1 and its discharge capacity
remains 119.8 mA h gꢀ1 after 20 cycles, demonstrating that the
presence of Ni can improve the discharge performance of Li2-
MnSiO4/C. Fig. 6(a) and (b) shows the charge–discharge curves
of Li2MnSiO4/C and (LMS@Ni)/C, respectively. There is no appar-
ent voltage plateau in the curves, which may be caused by the
low electrical conductivity and structural change of Li2MnSiO4
[27,32].
Rct value of (LMS@Ni)/C is 302.5
X, which is lower than that of
Li2MnSiO4/C (338.0 ) after the first cycle. The above results indi-
X
cate that the presence of Ni can improve the lithium ion diffusion
coefficient as well as electronic conductivity of Li2MnSiO4/C
cathode material.
4. Conclusions
Li2MnSiO4/C nanocomposite is prepared by the solvothermal
method followed by carbon coating process. It is shown that Li2-
MnSiO4/C cathode material with smaller particles and higher elec-
tronic conductivity exhibits better electrochemical performance
compared with the Li2MnSiO4 without carbon coating. The initial
Table 3
discharge capacity of Li2MnSiO4/C cathode is 259.9 mA h gꢀ1
,
The values of as,BET, A* and C* of Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C.
which is much higher than that of Li2MnSiO4 (15.7 mA h gꢀ1).
The presence of Ni improves the lithium ion diffusion coefficient
as well as the electronic conductivity, resulting in the enhanced
electrochemical performance of the (LMS@Ni)/C cathode material.
The results indicate that it may be a strategy to improve the elec-
trochemical performance for the cathode materials with poor con-
ductivity by adding nano-metal.
as,BET (m2 gꢀ1
)
A (cm2)
C (mol cmꢀ3
)
Li2MnSiO4
Li2MnSiO4/C
(LMS@Ni)/C
10.251
24.454
39.570
5.136
11.542
11.159
0.0196(6)
0.0196(8)
0.0197(0)
*A is the surface area of the cathode material, and C is the molar concentration of
Li ions.
Table 4
Impedance parameters of Li2MnSiO4, Li2MnSiO4/C and (LMS@Ni)/C cathodes before and after the first cycle.
Before the first cycle
After the first cycle
cm2 sꢀ1/2
)
DLi (cm2 sꢀ1
)
Rct (
X)
r
(X
cm2 sꢀ1/2
)
DLi (cm2 sꢀ1
)
þ
þ
Rct (X)
r
(X
Li2MnSiO4
Li2MnSiO4/C
(LMS@Ni)/C
97.40
76.47
79.29
205.57
51.57
39.34
5.11 ꢃ 10ꢀ18
1.61 ꢃ 10ꢀ17
2.96 ꢃ 10ꢀ17
–
–
–
338.0
302.5
577.72
230.75
1.28 ꢃ 10ꢀ19
8.60 ꢃ 10ꢀ19

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Y.-C. Wang et al. / Journal of Alloys and Compounds 614 (2014) 271–276
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This project was supported by the National Natural Science
Foundation of China (Grant No. 51372136), and Shenzhen Basic
Research Project (Grant No. JCYJ20130402145002372).
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