High performance composites of spinel LiMn2O4/3DG for lithium ion batteries

Article abstract of DOI:10.1039/c7ra12613a

A highly crystalline nanosized spinel LiMn₂O₄/3DG composite cathode material for high rate lithium ion batteries was successfully prepared by mixing spinel LiMn₂O₄ particles with reduced graphene oxide (3DG). Spinel LiMn₂O₄ and reduced three-dimensional graphene oxide were synthesized using a hydrothermal method and freeze-drying technology, respectively. The structure, morphology and electrochemical performance of the synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge techniques. The results showed that the LiMn₂O₄/3DG composites exhibited excellent rate capability and stable cycling performance. The discharge capacity was 131 mA h g^-1 and the capacity remains at 89.3% after 100 cycles at a 0.5 C rate, while the discharge capacity was 90 mA h g^-1 at 10 C. Compared with spinel LiMn₂O₄ materials, the LiMn₂O₄/3DG composites showed obvious improvement in electrochemical performance.

Full text of DOI:10.1039/c7ra12613a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
High performance composites of spinel LiMn2O4/
3DG for lithium ion batteries
Cite this: RSC Adv., 2018, 8, 877
X. D. Luo,ac Y. Z. Yin, M. Yuan,a W. Zeng,a G. Lin,a B. Huang,ac Y. W. Liac
b
*
ac
*
and S. H. Xiao
A highly crystalline nanosized spinel LiMn2O4/3DG composite cathode material for high rate lithium ion
batteries was successfully prepared by mixing spinel LiMn2O4 particles with reduced graphene oxide
(3DG). Spinel LiMn2O4 and reduced three-dimensional graphene oxide were synthesized using
a hydrothermal method and freeze-drying technology, respectively. The structure, morphology and
electrochemical performance of the synthesized materials were characterized by X-ray diffraction (XRD),
scanning electron microscopy (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy
(EIS) and galvanostatic charge–discharge techniques. The results showed that the LiMn2O4/3DG
composites exhibited excellent rate capability and stable cycling performance. The discharge capacity
was 131 mA h gꢀ1 and the capacity remains at 89.3% after 100 cycles at a 0.5 C rate, while the discharge
capacity was 90 mA h gꢀ1 at 10 C. Compared with spinel LiMn2O4 materials, the LiMn2O4/3DG
composites showed obvious improvement in electrochemical performance.
Received 20th November 2017
Accepted 11th December 2017
DOI: 10.1039/c7ra12613a
rsc.li/rsc-advances
some concerns, such as their large polarization at high charge–
discharge rates which results in lower power density, and their
1. Introduction
unstable spinel structure which causes poor cycling perfor-
mance.14–16 Based on this, various approaches have been
studied to improve their electrochemical performance.17,18
Reducing the particle size and optimizing the morphology were
considered good methods and have been widely applied in the
previous work,18–21 such as the sol–gel,22 precipitation23 and
hard-template routes,24 and the hydrothermal method.25,26 It
has also been well established that doping and coating were
desirable approaches to improve the power density.27–31 Yue
et al.32 employed hydrothermal treatment to synthesize
a LiMn2O4/C composite at a lower temperature than the
conventional calcination method, and the composite material
delivered a discharge capacity of 83 mA h gꢀ1 at a high current
density of 2 A gꢀ1. Bak et al.33 successfully synthesized a spinel
LiMn2O4/reduced graphene oxide hybrid via a microwave-
assisted hydrothermal method, achieving an excellent rate
capability. Zhan et al.34 used the hydrothermal method to
prepare a 3DG/LFP/C composite, and the electronic conductivity
and lithium ion diffusion rate were greatly enhanced.
Lithium ion batteries, with their high energy density, long cycle
life, lack of a memory effect, low self-discharge rate, environ-
mental friendliness and many other advantages, have become
the primary choice of energy storage device in portable elec-
tronics, and have been intensively investigated for use in high
power applications.1,2 LiMn2O4 is typically obtained by the
reaction of a mixture of lithium salt (e.g. CH3COOLi$2H2O) and
manganese oxides at around 750 ꢁC in air for many hours. The
high temperature could contribute to achieving highly crystal-
line spinel LiMn2O4, but it suffers from a problem by causing an
oxygen deciency, which would lead to faster capacity fading
during cycling.3–5 In recent years, many other techniques have
been reported in a large number of publications, such as
hydrothermal and sol–gel methods,2 the Pechni method,6 spray
drying,7 microemulsion8 and microwave hydrothermal tech-
niques,9 and controlled crystallization.10
Spinel LiMn2O4, as a most promising substitute for LiCoO2
in lithium ion battery cathode materials,11–13 is attracting more
and more attention from the public. However, there are also
Three-dimensional graphene, with its huge surface area,35–37
high number of three-dimensional porous channels and excel-
lent conductivity, was undoubtedly a good choice as a carbon
coating material. Using three-dimensional graphene was ex-
pected to improve the conductivity of the material38,39 and speed
up the diffusion rate of lithium ions, thus increasing the elec-
trochemical properties of the material.
aGuangxi Key Laboratory of Electrochemical and Magneto-chemical Functional
Materials, College of Chemistry and Bioengineering, Guilin University of
Technology, Guilin 541004, China. E-mail: 420466855@qq.com
bQinzhou University, Qinzhou Key Laboratory of Selenium-enriched Functional
Utilization of Biowaste Resources, College of Petroleum and Chemical Engineering,
Qinzhou 535011, Guangxi, China. E-mail: yinyanzhen2009@163.com
cGuilin University of Technology, Collaborative Innovation Center for Exploration of
Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi,
Guilin 541004, China
In this work, we designed and synthesized optimized spinel
LiMn2O4 and three-dimensional reduced graphene oxide using
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 877–884 | 877

View Article Online
RSC Advances
Paper
the hydrothermal method and freeze-drying technology, spectra were obtained using a Raman Spectrometer (LabRAM
respectively. Based on this, a well-coated nanostructured HR) with the laser as the excitation source at 532 nm.
LiMn2O4/3DG composite with excellent high rate capability and
stable cycling performance was synthesized successfully.
2.5. Electrochemical measurements
The electrochemical performance of the materials was assessed
2. Experimental
using 2016-type coin cells. Using the synthesized composite
2.1. Synthesis of 3DG
materials as the active material, the cathodes were prepared by
mixing the active material, poly(vinylidene diuoride) (PVDF)
and carbon black in a mass ratio of 8 : 1 : 1 in N-methyl-2-
pyrrolidone solvent. Metallic lithium foil was used as the
negative electrode. The electrolyte was 1 M LiPF6 solution in
ethylene carbonate (EC) and dimethyl carbonate (DMC) (with
a volume ratio of 1 : 1), and Celgard 2300 polyethylene lm was
used as the separator. The battery was assembled in a high
purity argon glove box, with relative water and oxygen standards
of less than 0.1 ppm. Electrochemical impedance spectroscopy
(EIS) and cyclic voltammetry (CV) were performed on an elec-
trochemical workstation (CHI760e, Shanghai Chenhua Co.,
Ltd., China). Charge/discharge tests and rate tests were per-
formed using the NEWARE battery test system (BTS-4000,
Shenzhen New will Co., Ltd., China) with different current
densities.
Graphite oxide (GO) gel was synthesized from natural graphite
using the modied Hummers method.40,41 The concentration
was controlled at 4 mg mLꢀ1. 10% mass content of NiCl2$6H2O
was added into 50 mL graphite oxide solution. Aer being
stirred until mixed evenly and subjected to ultrasound at room
temperature for 2 h, the mixed solution was put into a 100 mL
Teon-lined stainless steel autoclave at 180 ꢁC for 12 h, then
cooled to room temperature. The three-dimensional graphene
gel was taken out and cleaned with distilled water several times.
Aer freezing, the three-dimensional graphene was placed in
a freeze dryer for 72 h, and three-dimensional porous graphene
was collected.
2.2. Synthesis of the MnO2 material
MnSO4$H2O and Na2S2O8 were dissolved in deionized water in
a specic stoichiometric ratio. Aer fully mixing, the solution
was placed into a 100 mL Teon-lined stainless steel autoclave
at 100 ꢁC for 10 h. Aer ltration, it was washed three times with
deionized water and absolute ethanol, respectively. The washed
black precipitate was dried at 80 ꢁC for 20 h or longer to obtain
the MnO2 precursors.
3. Results and discussion
Fig. 1a shows the XRD patterns of graphite, graphite oxide and
3DG. It can be seen that the strong diffraction peak at 2q ¼ 26.5ꢁ
corresponding to graphite disappeared, and a diffraction peak
at about 2q ¼ 10.6ꢁ appeared, which is the characteristic peak of
GO.42 We can see that the diffraction peak shied to the right as
2.3. Synthesis of LiMn2O4/3DG
Based on the prepared MnO2 precursors, LiOH$H2O was
weighed and a specic proportion was used. Aer mixing in
ꢁ
absolute ethanol, the mixture was dried at 80 C. The mixture
was fully ground whilst adding absolute ethanol, then annealed
at 450 ꢁC for 6 h in a Muffle furnace. The temperature was then
raised to 750 ꢁC for 18 h with a heating rate of 5 ꢁC minꢀ1. Aer
cooling naturally to room temperature, the required spinel
LiMn2O4 sample was obtained, labeled as the S-0 sample. 0.25 g
3DG and 0.75 g spinel LiMn2O4 were weighed respectively
according to the mass ratio 1 : 3, then the mixture was fully
ground with a mortar and placed into the Muffle furnace at
200 ꢁC for 4 h. 3DG/LiMn2O4 composites with different 3DG
contents (15%, 25% and 35%) were prepared using the same
method, and they were labeled as S-1, S-2 and S-3, respectively.
2.4. Characterization
The crystal structures of the materials were characterized with
X-ray diffraction (XRD) measurements (Panalytical X’Pert PRO
MRD, Holland) with Cu Ka radiation operating at a continuous
scan of 2q ¼ 5–80ꢁ at a scan rate of 0.03ꢁ minꢀ1. The micro-
structure of the samples was investigated by eld emission
scanning electron microscopy (FE-SEM SU5000). The mass
percentage of 3DG in the composites was determined by ther-
mogravimetric analysis (TGA) with a SDT-Q600 simultaneous
Fig. 1 XRD patterns of (a) graphite, GO and 3DG, and (b) 3DG and the
thermogravimetric analyzer under an air atmosphere. Raman 3DG/LiMn2O4 composite.
878 | RSC Adv., 2018, 8, 877–884
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
graphite oxide was reduced, and a new diffraction peak for 3DG being reduced. The ID/IG value of the 3DG/LiMn2O4 composite
at 2q ¼ 24.2ꢁ appeared.43 This diffraction peak is relatively was greater than that of 3DG (1.12 > 1.02), indicating that the
broad, which means that the structure of GO was randomly 3DG/LiMn2O4 composite had a higher degree of edge defects.
orientated, and that the diffraction was reected from the side Generally, defects in the carbon materials would result in irre-
of the three-dimensional structure of GO.44 At the same time, versible lithium ion storage.49 Thus, the addition of 3DG to the
a weaker peak for graphene at about 2q ¼ 43ꢁ was found 3DG/LiMn2O4 composite led to a larger irreversible capacity
through careful observation, which is the characteristic peak for than that of LiMn2O4. In addition, the thermal stability of gra-
graphene based on the chemical reduction of GO. As seen from phene was one of the important indicators of whether GO was
Fig. 1b, the XRD pattern of 3DG/LiMn2O4 is in accordance with being reduced, so we conducted thermogravimetric tests on
the standard pattern of LiMn2O4 (JCPDS card no. 35-0782).32,38 graphene oxide and reduced graphene oxide samples at 20–
In addition, a weak diffraction peak at about 2q ¼ 24.2ꢁ corre- 750 ꢁC under an air atmosphere. Fig. 2b shows the TGA curves
sponding to 3DG appeared, which meant that the 3DG/LiMn2O4 of GO, 3DG, LiMn2O4 and the LiMn2O4/3DG (25% wt)
composites were successfully synthesized.
composite. We can see that graphene oxide showed obvious
To determine the state of the RGO, Raman spectroscopy was weight loss at 50–100 ꢁC and 150–200 ꢁC, respectively. The rst
used. As seen in Fig. 2a, GO, 3DG and the composite all showed stage was attributed to the weight loss of residual moisture in
two peaks at approximately 1345 and 1590 cm, which were graphene oxide, while the latter could be due to the mass weight
attributed to the D and G bands of carbonaceous materials, loss of oxygen-containing groups (hydroxyl, epoxy, carbonyl,
respectively.45 Typically, the D-peak represented a defect and an carboxyl, etc.) in graphene oxide.50,51 Compared to GO, the
irregular structure at the edge of the graphene, and the G-peak thermal stability of reduced graphene oxide was signicantly
illustrated the existence of graphitic carbon, representing an improved. This was because the more completely the reduction
ordered sp2 bond structure.46,47 The integrity and order of the reaction proceeded, the lower the content of residual labile
graphene crystal structure were characterized using the ratio of oxygen-containing groups. Combined with the TGA curve of
the intensity of the D peak and the G peak (ID/IG).48 If the ID/IG spinel LiMn2O4, the content of 3DG in the 3DG/LiMn2O4
value was higher, it meant that the graphene had a higher composite could be preliminarily calculated to be about 21.9%.
degree of edge defects and less graphitization of carbon. The ID/
SEM images of samples of GO, 3DG, spinel LiMn2O4 and the
IG value of GO was calculated to be 0.98. The edge defects composite are shown in Fig. 3. As seen from Fig. 3a–c, GO
increased and the average size of the sp2 region became smaller, consisted of a number of stacked single layers of graphene
causing an increase of the ID/IG value. This was a result of oxide, and the reduced three-dimensional graphene was
oxidized graphene lm fragmentation in the process of GO a porous material composed of graphene sheets which were
overlapping, wound and wrapped around each other, with the
hole diameter ranging from a few nanometers to tens of
microns. It was obvious from Fig. 3b that the monolayer three-
dimensional graphene was very thin and had a huge surface
area. Fig. 3d shows graphene aer grinding, the three-
dimensional structure of which has been entirely destroyed.
Fig. 3e and f shows SEM micrographs of spinel LiMn2O4 and the
3DG/LiMn2O4 composite, respectively. It shows that most of the
spinel LiMn2O4 nanoparticles have been embedded in the
porous graphene, and only a small portion has been sand-
wiched between the graphene sheets or exposed to the graphene
sheet, which might be due to the incomplete uniformity of the
compound of three-dimensional graphene and the spinel
LiMn2O4 nanoparticles.
Fig. 4a shows CV curves at 0.1 mV sꢀ1 of three different 3DG/
LiMn2O4 composite samples. Two well-dened redox peaks
appeared in the CV curves, corresponding to the two processes
during which lithium ions were embedded in and removed
from the lattice. It is clear from the gure that sample S-3,
compared to the other two samples, had the smallest peak
currents (Ip) according to the Randles–Sevcik equation:52,53
Ip ¼ (2.69 ꢂ 105)n2/3SD1/2 1/2
v C0
Ip is the peak current (A) in the above formula, n is the number
of electrons transferred during the reaction (for spinel LiMn2O4,
n ¼ 1), S denotes the surface area of the electrode material (SS-
Fig. 2 (a) Raman spectra and (b) TGA curves of GO, 3DG, LiMn2O4 and
the composite.
¼ 1.766, SS-1,2,3 ¼ 1.130 cm2), D represents the lithium ion
0
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 877–884 | 879

View Article Online
RSC Advances
Paper
Fig. 3 SEM images of (a) GO, (b and c) 3DG, (d) two-dimensional graphene, (e) spinel LiMn2O4 and (f) the 3DG/LiMn2O4 composite.
diffusion coefficient (cm2 sꢀ1), v is the potential scanning rate Different from samples S-1, S-2 and S-3, sample S-0 was tested
(v ¼ 0.1 mV sꢀ1) and C0 represents the initial concentration of on a larger specic surface area of the electrode material. The
lithium ions in the electrode (C0 ¼ 0.02378 mol cmꢀ3). The peak current reached its maximum value when the 3DG content
lithium ion diffusion coefficient was only closely related to the was 25% in the 3DG/LiMn2O4 composite, with the increase of
peak current when the other factors remained unchanged, and 3DG content from 15% to 35% corresponding to a larger
the larger the peak current, the greater the diffusion coefficient lithium ion diffusion coefficient. At the same time, we could see
of lithium ions. We could conclude that sample S-2 possessed that the peak current of all four samples gradually increased
the maximum diffusion coefficient of lithium ions, demon- with the increasing scanning rate, and the potential gap
strating a higher electrochemical activity. This may be due to between the redox peaks also increased, indicating greater
there being too much 3DG content in sample S-3, to a certain electrochemical polarization. Fig. 4f illustrates the relationship
extent, hindering the proliferation of lithium ions during the between the peak current (Ip) and the square root of the scan-
charge and discharge process. Fig. 4b–e shows CV curves of ning rate (v1/2) for samples S-0 and S-2. A good linear relation-
spinel LiMn2O4 and different 3DG contents of the 3DG/LiMn2O4 ship between them by tting the data is shown in the gure.
composites at scanning rates ranging from 0.1 to 0.5 mV sꢀ1
.
Through the above-mentioned Randles–Sevcik equation, the
880 | RSC Adv., 2018, 8, 877–884
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 4 (a) CV curves at 0.1 mV sꢀ1 of S-1, S-2 and S-3, (b–e) CV curves of spinel LiMn2O4 and 3DG/LiMn2O4 composite samples at scanning rates
ranging from 0.1 to 0.5 mV sꢀ1 and (f) relationship between the peak current (Ip) and the square root of the scanning rate (v1/2) for LiMn2O4 and S-
2 samples.
lithium ion diffusion coefficient can be calculated. The results We also found that the Li+ diffusion coefficient at the oxidation
are summarized in Table 1. It is apparent that the Li+ diffusion stage was greater than that at the corresponding reduction
coefficient of sample S-2 at every stage was greater than that of stage, indicating that lithium ions were more likely to be
S-0. This may result from the addition of three-dimensional extracted than inserted.54
graphene, which increased the number of lithium ion diffu-
The cycling performance of spinel LiMn2O4 and its
sion channels and improved the diffusion rate of lithium ions. composites with different 3DG contents over a potential window
of 3.0–4.4 V at a current density of 0.5 C are displayed in Fig. 5a.
It was clear that the discharge capacity of sample S-2 was higher
in each cycle than that of all the other samples. The rst
Table 1 Summary of the lithium ion diffusion coefficients for the
samples in Fig. 4f
discharge capacity of sample S-2 was 131 mA h gꢀ1, exhibiting
a higher discharge capacity of 117 mA h gꢀ1 aer 100 charge–
Diffusion coefficient (cm2 sꢀ1
)
discharge cycles. While S-0 had only an initial discharge specic
capacity of 126 mA h gꢀ1, the discharge capacity was maintained
at 110 mA h gꢀ1 aer 100 cycles. Compared to the capacity
retention rate of 87.3% for sample S-0, the capacity retention
rate of S-2 was 89.3% aer 100 cycles, which was a favorable
Oxidation
peak 1
Oxidation peak 2 Redox peak 1 Redox peak 2
S-0 1.28 ꢂ 10ꢀ11 7.18 ꢂ 10ꢀ12
S-2 1.40 ꢂ 10ꢀ11 7.60 ꢂ 10ꢀ12
1.18 ꢂ 10ꢀ11
1.27 ꢂ 10ꢀ11
6.25 ꢂ 10ꢀ12
8.07 ꢂ 10ꢀ12
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 877–884 | 881

View Article Online
RSC Advances
Paper
Fig. 5 (a) Cycling performance and (b) first charge–discharge profiles of LiMn2O4 and LiMn2O4/3DG at 0.5 C.
improvement, and it also conrmed that the 3DG/LiMn2O4
The electrochemical impedance spectra and tting curves of
composite materials had a better cycling stability than spinel the LiMn2O4 and LiMn2O4/3DG composite electrodes are dis-
LiMn2O4. Fig. 5b illustrates the rst charge and discharge played in Fig. 7. Both of the proles consist of a semicircular
curves of samples S-0 and S-2. Two pairs of obvious charge and curve from the high frequency region to the mid frequency
discharge platforms at about 3.9 V and 4.1 V in both samples region, and a straight line in the low frequency region. It was
correspond to the two-step embedding and removal of lithium believed that the semicircle in the high frequency region was
ions in different stages of the electrochemical reaction, which caused by charge transfer between the electrolyte and electrode
are also consistent with the CV curves. In detail, the two
discharge platforms of sample S-2 are longer and more even
than those of S-0, and this is also a result of smaller polariza-
tion. Sample S-2 therefore displayed better electrochemical
performance.
To further explore the electrochemical properties of spinel
LiMn2O4 and its composites, the rate performance charts of the
samples at different charge–discharge rates from 0.2 C to 10 C
and then back to 0.2 C were investigated and are shown in
Fig. 6a. It was found that sample S-2 delivered reversible
capacities of 133 mA h gꢀ1, 130 mA h gꢀ1, 128 mA h gꢀ1
,
124 mA h gꢀ1, 112 mA h gꢀ1, 90 mA h gꢀ1 and 128 mA h gꢀ1 at
current rates of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C and nally 0.2 C,
respectively, which were better than those of sample S-
0 (127 mA h gꢀ1, 121 mA h gꢀ1, 117 mA h gꢀ1, 110 mA h gꢀ1
,
80 mA h gꢀ1, 36 mA h gꢀ1 and 120 mA h gꢀ1). Signicantly, the
discharge specic capacity had been improved especially in
terms of the high charge and discharge rates. The specic
capacity of S-0 at 10 C was only 36 mA h gꢀ1, while sample S-2
achieved 90 mA h gꢀ1. The better electrochemical perfor-
mance of the LiMn2O4/3DG composite could be ascribed to the
effective three-dimensional conductive network of 3DG, making
the particles well connected. Simultaneously, the capacity
retention of sample S-2 reached up to 96.2% aer a series of
charge and discharge cycles, higher than that of spinel LiMn2O4
by 1.7%. As seen in Fig. 6b and c, the charge–discharge platform
shortens with an increase in rate, and the discharge specic
capacity is constantly declining. This may be due to the fact that
the lithium ion diffusion rate and electron transfer rate cannot
meet the requirements of high rates, resulting in increased
polarization. Different to sample S-0, the addition of 3DG in S-2
greatly improved the conductivity of the material, and the
stability of the material was enhanced to some extent, causing
a smaller polarization and a longer and more stable discharge
platform, thus resulting in an excellent higher rate
performance.
Fig.
6 (a) The rate profiles of LiMn2O4 and the LiMn2O4/3DG
composites, (b and c) charge–discharge profiles of LiMn2O4 and
LiMn2O4/3DG composite at various rates.
882 | RSC Adv., 2018, 8, 877–884
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Guangxi
Natural
Science
Foundation
(Grant
No.
2017GXNSFFA198007).
References
1 J. Y. Luo, X. L. Li and Y. Y. Xia, Electrochim. Acta, 2007, 52,
4525–4531.
2 Y. L. Cui, W. J. Bao, Z. Yuan, Q. C. Zhuang and Z. Sun, J. Solid
State Electrochem., 2012, 16, 1551–1559.
3 J. M. Tarascon and D. Guyomard, Electrochim. Acta, 1993, 38,
1221–1231.
4 T. Ohzuku, M. Kitagawa and T. Hirai, ChemInform, 1990, 21,
3169.
Fig. 7 EIS spectrum of the LiMn2O4 and LiMn2O4/3DG composite
electrodes.
5 J. C. Hunter, J. Solid State Chem., 1981, 39, 142–147.
6 W. Liu, G. C. Farrington, F. Chaput and B. Dunn, J.
Electrochem. Soc., 1996, 143, 879–884.
7 K. Vediappan, W. S. Kim and C. W. Lee, J. Nanosci.
Nanotechnol., 2011, 11, 680.
8 A. Subramania, N. Angayarkanni and T. Vasudevan, Mater.
Chem. Phys., 2007, 102, 19–23.
9 J. Feng, Z. Wang, B. Shen, L. Zhang, X. Yang and N. He, RSC
Adv., 2014, 4, 28683–28690.
10 G. Hu, J. Jiang, Z. Peng, K. Du, Y. Cao and J. Duan, J. Nanosci.
Nanotechnol., 2013, 13, 2262–2265.
11 X. M. He, J. J. Li, Y. Cai, Y. Wang, J. Ying, C. Jiang and
C. Wan, J. Solid State Electrochem., 2005, 9, 438–444.
12 X. Xiao, J. Lu and Y. Li, Nano Res., 2010, 3, 733–737.
interface, which was called charge-transfer resistance (RU),
while the straight line in the lower frequency region was due to
lithium ion diffusion in the crystal lattice of spinel LiMn2O4,
namely the Warburg impedance (Zw).55 By tting data, the
charge transfer impedance values of samples S-0 and S-2 were
determined to be 243 U and 180 U, respectively. Denitely,
sample S-2 had a smaller impedance of lithium ion intercala-
tion and de-intercalation of the crystal lattice than that of S-0,
indicating that the addition of three-dimensional graphene
improved the electronic conductivity of the material. It also
increased the number of three-dimensional channels, which
are benecial for lithium ions to migrate, consistent with the
SEM images and CV results. Therefore, by coating with three-
dimensional graphene, the electrochemical performance of
spinel LiMn2O4 has been greatly enhanced.
¨
13 T. J. Patey, R. Buchel, S. H. Ng, F. Krumeich, S. E. Pratsinis
´
and P. Novak, J. Power Sources, 2009, 189, 149–154.
14 L. J. Xi, H. E. Wang, Z. G. Lu, S. L. Yang, R. G. Ma, J. Q. Deng
and C. Y. Chung, J. Power Sources, 2012, 198, 251.
15 D. Guo, Z. Chang, B. Li, H. Tang and X. Z. Yuan, J. Solid State
Electrochem., 2013, 17, 2849.
16 J. Luo, L. Cheng and Y. Xia, Electrochem. Commun., 2007, 9,
1404–1409.
17 J. Jiang, Q. Liu, L. Xu, J. Xu, J. Du, X. He and D. Li, J. Nanosci.
Nanotechnol., 2016, 16, 12640–12643.
18 H. W. Lee, P. Muralidharan and D. K. Kim, J. Korean Ceram.
Soc., 2011, 48, 379–383.
19 N. N. Sinha and N. Munichandraiah, J. Indian Inst. Sci., 2009,
89, 381–392.
20 L. Xiao, Y. Guo, D. Qu, B. Deng, H. Liu and D. Tang, J. Power
Sources, 2013, 225, 286–292.
21 M. A. Kiani, M. F. Mousavi and M. S. Rahmanifar, Int. J.
Electrochem. Sci., 2011, 6, 2581–2595.
22 X. M. Liu, Z. D. Huang, S. Oh, P. C. Ma, P. C. Chan,
G. K. Vedam and J. K. Kim, J. Power Sources, 2010, 195,
4290–4296.
4. Conclusions
In conclusion, we have successfully synthesized nanoscale
LiMn2O4/3DG (15 wt%, 25 wt% and 35 wt%) composites using
a simple and low-cost route. XRD analysis conrmed that the
graphene peak successfully appeared in the diffraction pattern
of spinel LiMn2O4, but had no other side effects on the crystal
structure. The CV results showed that the LiMn2O4/3DG
(25 wt%) composite had the largest lithium ion diffusion coef-
cient. Beneting from the huge surface area and rich three-
dimensional porous channels of 3DG, the electronic conduc-
tivity of the material and lithium ion diffusion rate have been
greatly improved. This research showed that spinel LiMn2O4,
through three-dimensional graphene coating, could signi-
cantly enhance the rate performance and cycling performance,
which is undoubtedly a novel option to improve performance
for lithium ion battery cathode materials.
23 J. Xie, C. Y. Sun, T. J. Zhu, G. S. Cao, X. B. Zhao and
S. C. Zhang, J. Solid State Electrochem., 2013, 17, 2589–2594.
24 J. Chen, N. Zhao and J. Zhao, J. Solid State Electrochem., 2017,
1–8.
Conflicts of interest
There are no conicts to declare.
25 Q. Jiang, X. Wang and H. Zhang, J. Electron. Mater., 2016, 45,
4350–4356.
Acknowledgements
This work was nancially supported by the National Science 26 H. Zou, B. Wang, F. Wen and L. Chen, Ionics, 2017, 23, 1083–
Foundation of China (Grant No. 51364008, 51663020) and the
1090.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 877–884 | 883

View Article Online
RSC Advances
Paper
27 Y. Wang, Y. Wang, E. Hosono, K. Wang and H. Zhou, Angew. 42 Y. Liao, Y. Huang, D. Shu, Y. Zhong, J. Hao, C. He and
Chem., Int. Ed., 2008, 47, 7461–7465. X. Song, Electrochim. Acta, 2016, 194, 136–142.
28 G. Xu, Z. Liu, C. Zhang, G. Cui and L. Chen, J. Mater. Chem. A, 43 H. Liu, J. Zhang, B. Zhang, L. Shi, S. Tan and L. Huang,
2015, 3, 4092–4123. Electrochim. Acta, 2014, 138, 69–78.
29 P. Mohan and G. P. Kalaignan, J. Nanosci. Nanotechnol., 44 B. Zhang, T. Wang, S. Liu, S. Zhang, J. Qiu, Z. Chen and
2014, 14, 5028–5035. H. Cheng, Microporous Mesoporous Mater., 2006, 96, 79–83.
30 H. Zhang, Z. Li, S. Yu, Q. Xiao, G. Lei and Y. Ding, J. Power 45 S. Fu, L. Ma and M. Gan, J. Mater. Sci.: Mater. Electron., 2016,
Sources, 2016, 301, 376–385. 1–9.
31 S. Lee, Y. Cho, H. K. Song, K. T. Lee and J. Cho, Angew. Chem., 46 R. S. Diggikar, D. J. Late and B. B. Kale, RSC Adv., 2014, 4,
Int. Ed., 2012, 51, 8748–8752. 22551–22560.
32 H. Yue, X. Huang, D. Lv and Y. Yang, Electrochim. Acta, 2009, 47 H. Xue, T. Wang, H. Guo, X. Fan, Z. Zhu, X. Pan and J. He,
54, 5363–5367. RSC Adv., 2014, 4, 57724–57732.
33 S. M. Bak, K. W. Nam, C. W. Lee, K. H. Kim, H. C. Jung, 48 R. Du, X. Tian, J. Yao, Y. Sun, J. Jin, Y. Zhang and Y. Liu,
X. Q. Yang and K. B. Kim, J. Mater. Chem., 2011, 21, 17309–
17315.
34 T. T. Zhan, W. F. Jiang, C. Li, X. D. Luo, G. Lin, Y. W. Li and
S. H. Xiao, Electrochim. Acta, 2017, 246, 322–328.
35 T. Yang, Q. Guan, Q. Li, L. Meng, L. Wang, C. Liu and K. Jiao,
J. Mater. Chem. B, 2013, 1, 2926–2933.
Diamond Relat. Mater., 2016, 70, 186–193.
49 K. Zhu, Y. Zhang and H. Qiu, J. Alloys Compd., 2016, 675,
399–406.
50 W. Cheng, C. Ding and X. Nie, ACS Sustainable Chem. Eng.,
2017, 5, 5503–5511.
51 M. M. Viana, M. C. Lima, J. C. Forsythe, V. S. Gangoli,
M. Cho, Y. Cheng and V. Caliman, J. Braz. Chem. Soc.,
2015, 26, 978–984.
36 T. T. Chen, W. L. Song and L. Z. Fan, Electrochim. Acta, 2015,
165, 92–97.
37 V. H. Luan, H. N. Tien and T. H. Le, J. Mater. Chem. A, 2012, 52 Y. Mao, S. Xiao and J. Liu, Mater. Res. Bull., 2017, 96, 437–
1, 208–211. 442.
38 C. Jiang, Z. Tang, S. Deng, Y. Hong, S. Wang and Z. Zhang, 53 J. Chen, L. Yang, S. Fang, S. I. Hirano and K. Tachibana,
RSC Adv., 2017, 7, 3746–3751. J.ournal of Power Sources, 2012, 200, 59–66.
39 J. Zhang, B. Zhou and B. Zhao, Chin. J. Chem., 2017, 35, 1601– 54 M. Vujkovic, I. Stojkovic, N. Cvjeticanin and S. Mentus,
1610. Electrochim. Acta, 2013, 92, 248–256.
40 M. S. Poorali and M. M. Bagheri-Mohagheghi, J. Mater. Sci.: 55 N. Li, J. Liang, D. Wei, Y. Zhu and Y. Qian, Electrochim. Acta,
Mater. Electron., 2016, 27, 260–271. 2014, 123, 346–352.
´
´
´
´
´
´
41 N. Justh, B. Berke, K. Laszlo and I. M. Szilagyi, J. Therm. Anal.
Calorim., 2017, 1–6.
884 | RSC Adv., 2018, 8, 877–884
This journal is © The Royal Society of Chemistry 2018