Improved electrochemical properties of LiMn2O4 with the Bi and La co-doping for lithium-ion batteries

Article abstract of DOI:10.1039/c5ra13005k

A series of LiBi_xLa_xMn_2-2xO₄ (x = 0, 0.002, 0.005, 0.010, 0.020) samples were synthesized by solution combustion synthesis in combination with calcination. The phase structure and morphology of the products were characterized by X-ray diffraction, scanning electron microscopy, and transition electron microscopy. The results demonstrated that a single-phase LiMn₂O₄ spinel structure was obtained for the LiBi_xLa_xMn_2-2xO₄ (x = 0, 0.002, 0.005) samples, whereas impurities were observed for the LiBi_xLa_xMn_2-2xO₄ (x = 0.010, 0.020) samples as a result of the doping limit. The electrochemical properties were investigated by galvanostatic charge-discharge cycling and cycling voltammetry in a voltage range of 3.2-4.4 V. The substitution of Mn³⁺ by equimolar Bi³⁺ and La³⁺ could significantly improve the structural stability and suppress the Jahn-Teller distortion, thereby resulting in improved electrochemical properties for the Bi and La co-doped samples in contrast with the pristine LiMn₂O₄ sample. In particular, the LiBi_0.005La_0.005Mn_1.99O₄ sample delivered a high initial discharge capacity of 130.2 mA h g^-1 at 1C, and following 80 cycles, the capacity retention was as high as 95.0%. Moreover, it also presented the best rate capability among all the samples, in which a high discharge capacity of 98.3 mA h g^-1 was still maintained at a high rate of 7C compared with that of 75.8 mA h g^-1 for the pristine LiMn₂O₄ sample.

Full text of DOI:10.1039/c5ra13005k

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Improved electrochemical properties of LiMn2O4 with the Bi and La co-doping for lithium-ion
batteries
ChengꢀGong Han, Chunyu Zhu, Genki Saito, Tomohiro Akiyama∗
Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo 062ꢀ8628, Japan
Abstract: A series of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples were synthesized by solution combustion
synthesis in combination with calcination. The phase structure and morphology of the products were characterized by Xꢀray
diffraction, scanning electron microscopy, and transition electron microscopy. The results demonstrated that a singleꢀphase
LiMn2O4 spinel structure was obtained for the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005) samples, whereas impurities were observed
for the LiBixLaxMn2ꢀ2xO4 (x=0.010, 0.020) samples as a result of the doping limit. The electrochemical properties were investigated
by galvanostatic charge–discharge cycling and cycling voltammetry in a voltage range of 3.2–4.4 V. The substitution of Mn3+ by
equimolar Bi3+ and La3+ could significantly improve the structural stability and suppress the Jahn–Teller distortion, thereby
resulting in improved electrochemical properties for the Bi and La coꢀdoped samples in contrast with the pristine LiMn2O4 sample.
In particular, the LiBi0.005La0.005Mn1.99O4 sample delivered a high initial discharge capacity of 130.2 mAh/g at 1 C, and following
80 cycles, the capacity retention was as high as 95.0%. Moreover, it also presented the best rate capability among all the samples,
in which a high discharge capacity of 98.3 mAh/g was still maintained at a high rate of 7 C compared with that of 75.8 mAh/g for
the pristine LiMn2O4 sample.
Keywords: LiMn2O4, Cathode material, Bi and La doping, Liꢀion batteries
∗ Corresponding author. Tel.: +81 11 706 6842; fax: +81 11 726 0731.
Eꢀmail address: takiyama@eng.hokudai.ac.jp (T. Akiyama)
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1. Introduction
Rechargeable Liꢀion batteries are considered to be the most promising energyꢀstorage devices for powering electric
vehicles (EVs) and hybrid EVs because of their high volumetric/gravimetric energy density, and long cycling life 1ꢀ5
.
The low
cost, high safety, high capacity, and good rate capability of Liꢀion batteries adapted for further commercialized applications
are closely linked to the cathode, anode, and electrolyte materials used, especially cathode materials because they exhibit a
much lower capacity than anode materials 6
. Amongst many cathode materials, LiMn2O4 has been particularly highlighted as
an appropriate and attractive cathode because of its low cost, high safety, environmental friendliness, and good
electrochemical performance at high currents 7,8
.
Spinel LiMn2O4 material has a cubic structure with the space group of Fd3m,
in which Mn ions occupy the 16d octahedral sites while Li ions are located at the 8a tetrahedral sites and quickly diffuse
through crossꢀlinked 1×3 channels formed by the edgeꢀshared MnO6 frame. This supports good electrochemical
performance even at high current rates 7
. However, in the 4 V region, LiMn2O4 cathode materials undergo severe capacity
fading during repeated cycling. This mainly results from (1) Jahn–Teller distortion inducing the irreversible phase transition
from the cubic phase to tetrahedral phase; and (2) the dissolution of Mn2+ into the electrolyte solution resulting from the
disproportional reaction, 2Mn3+→Mn2+ + Mn4+
.
9,10
Up to now, two main strategies, surface modification11,12 and doping technology13,14, have been developed to solve the
aforementioned capacity fading issue. Surface modification can decrease the contact areas between the LiMn2O4 particles and
the electrolyte solution, and therefore inhibit the manganese dissolution. However, surface modification technology usually
impairs the capacity and scarcely alleviates the Jahn–Teller distortion 15
. Recently, the doping of one or more elements at the
manganese sites has been proposed as an effective method to alleviate capacity fading by reducing the Jahn–Teller distortion
and the dissolution of manganese in the electrolyte solution 14,16
.
The single element doping of LiMn2O4 has been widely
performed using a small amount of trivalent ions, such as Co3+,17 Al3+,18,19 Fe3+,20 Cr3+,21 Ga3+,22,23 Sm3+,24,25 La3+,26,27 resulting
in an improved cycling performance during the charge/discharge process. By contrast, LiMn2O4 with the substitution of
multiple ions has received more attention because of the occurrence of the synergistic effect, which improves the cycling life
.
28ꢀ31
Mohan et al. demonstrated that La and Sm coꢀsubstituted LiSmxLa0.2ꢀxMn1.80O4 (x=0.05, 0.10, 0.15) cathode materials
synthesized by the solꢀgel method exhibited enhanced structural stability of the octahedral sites in the spinel structure with
the substitution of a small amount of La and Sm. This resulted in improved capacity retentions of 90% and 82% for the
LiSm0.10La0.10Mn1.80O4 sample, compared with those of 74% and 60% for the pristine LiMn2O4 after 100 cycles at a current
density of 0.5 C at temperatures of 30 °C and 50 °C, respectively 32
. Iqbal et al. prepared La and Zn coꢀdoped LiMn2O4 spinel
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materials using a solꢀgel method 33
. The results revealed that the LiLa0.01Zn0.01Mn1.98O4 sample achieved a higher discharge
capacity of 92 mAh/g and 78% of the initial discharge capacity at 5 C compared to values of 51 mAh/g and 41% for the unꢀ
doped LiMn2O4 sample. This indicates that the La and Zn coꢀdoping stabilized the structural integrity of the spinel host by
suppressing the Jahn–Teller distortion and shortening the diffusion of the Li ions. La, a rare earth metal, can be selected as a
possible candidate to function as a substitute for manganese because of its large binding energy, therefore forming a pillar
and preventing the collapse of the cubic structure, and increasing the average valence of Mn, thereby reducing the
occurrence of the Jahn–Teller distortion 26,33,34
manganese as a dopant 35,36
.
Moreover, the heavy element, bismuth, has been found to partially substitute
.
From electrochemical investigations in a strong alkaline electrolyte (9M KOH), Schlörb et al.
reported that Biꢀdoped LiMn2O4 material achieved remarkably improved cyclability, whereas after 10 cycles, a rapid
decrease in capacity to 50% of the initial value was observed for the unꢀdoped LiMn2O4 sample 36
.
Following 100 cycles
measured at a constant current of 40 mA/g, Tan et al. demonstrated that Biꢀmodified LiMn2O4, synthesized by a solꢀgel
method, achieved a discharge capacity of 100 mAh/g and a capacity retention of 84.7%, compared with a capacity retention
of 74.2% for the unmodified sample 37
. When the heavy element, bismuth, is introduced to the spinel structure, it can result in
the hybridizing of the 6s and 6p orbitals of the bismuth ions with the adjacent 2p orbital of the oxygen in addition to the 3d
orbital of the manganese 38
.
Doped bismuth ions can increase the charge transfer from the oxygen anion to the manganese
cation because of the overlapping of the bismuth orbital with the oxygen and manganese orbitals. This probably results in a
wider conduction band, increasing the electronic conductivity and lithium diffusivity of the electrodes 38,39
.
Considering the
lack of studies on Bi and La coꢀdoping, Bi and La are introduced to partially substitute Mn in order to improve both the
stability of the spinel structure and the electrochemical performance during this study.
In this work, a novel solution combustion synthesis (SCS) method was employed because it can be used to homogeneously
dope elements in trace amounts and yield nanoꢀsized particles with high specific areas, thus benefitting the electrochemical
properties 40
Bi and La were selected as the doping elements to partially substitute Mn in order to improve the
.
electrochemical properties of the LiMn2O4 material. Considering the above, LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010,
0.020) samples were synthesized by the SCS method in combination with calcination. The effect of the Bi and La doping
amounts on the structure, morphology, and electrochemical performance was investigated in detail.
2. Experimental
2.1 Preparation and characterization of Bi and La coꢀdoped LiMn2O4 cathode materials.
Bi and La coꢀdoped LiMn2O4 samples were prepared by solution combustion synthesis (SCS) in combination with
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calcination. Stoichiometric amounts of oxidizers, lithium nitrate (LiNO3, 99.0%, Kishida Chemical Co., Ltd., Japan),
bismuth nitrate (Bi(NO3)3·5H2O, 99.5%, Kanto Chemical Co., Inc., Japan), lanthanum nitrate (La(NO3)3·6H2O, 99.9%,
Wako Pure Chemical Industries, Ltd., Japan), and manganese nitrate (Mn(NO3)2, 50% w/w aqueous solution, Alfa Aesar )
were dissolved in 5 ml of distilled water in terms of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020). A corresponding
amount of urea (NH2CONH2, 99.0%, Chameleon Reagent, Japan) as the reductant (with φ=0.5, φ is the ratio of the total
valence of reductants to the total valence of nitrate oxidizers) was also added into the above solutions. The homogenous solꢀ
gels were formed by evaporating the water at 350 rpm at a temperature of 90 °C. The obtained solꢀgels selfꢀignited in a
homemade combustion synthesis apparatus, which was placed into a constant temperature laboratory electric jacket that was
preꢀheated and maintained at a temperature of 400 °C. The power of the electric jacket was switched off once the
combustion reaction associated with the gas emission was observed. Following the SCS, the collected powders were further
calcined at 800 °C for 24 h at a rate of 5 °C/min in air to obtain the final powders.
The phase structure was characterized by powder Xꢀray diffraction (XRD, Cu Kα, Rigaku Miniflex). The morphology, size
of the powders, the energyꢀdispersive spectroscopy (EDS), the electron energyꢀloss spectroscopy (EELS), and the energy
dispersive Xꢀray spectroscopy (EDX) were determined by scanning electron microscopy (SEM, JEOL, JSMꢀ7001FA) and
transmission electron microscopy (TEM, JEOL JEMꢀ2010F).
2.2 Cell assembly and electrochemical measurement.
A twoꢀelectrode Swagelockꢀtype cell consisting of a working electrode and a lithium metal anode was employed. It was
assembled in an Arꢀfilled glove box, as described in our previous reports 40,41
The working electrode contained 80 wt.%
.
active materials, 10 wt.% polymer binder (polyvinylidene fluoride (PVDF)) dissolved in an Nꢀmethylꢀ2ꢀpyrrolidone (NMP)
solution, and 10 wt.% conductive carbon (acetylene black). The above mixture was magnetically stirred to form a
homogenous slurry. The slurry was cast on an aluminum foil current collector and subsequently dried in a vacuum oven at
110 °C over a period of 12 h. The dried aluminum foil electrode was punched into disks with a diameter of 10 mm and a
thickness of 0.1 mm. Meanwhile, a lithium metal disk with a diameter of 10 mm was selected as the counter and reference
electrode. A solution of 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/dimethyl carbonate (DMC)
(1:1 in volume) was used as the electrolyte while a celgard polypropylene membrane was employed as the separator. The
electrochemical measurements were performed galvanostatically within the voltage range of 3.2–4.4 V with a current
density of 1 C (a rate of 1 C corresponds to a full charge/discharge of 150 mAh/g in 1 h) at the temperature of 25 °C using a
battery tester (Arbin Instrument, MSTAT4, USA). Cyclic voltammetry (CV) was performed using the
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potentiostat/glavanostat apparatus (Autolab, PGSTAT128N).
3. Results and discussion
Fig. 1 shows the XRD patterns of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples, with the enlarged (111)
diffraction peak on the left. All the diffraction peaks of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005) sample well correlate with
the standard peaks of the spinelꢀstructure, indicating the successful synthesis of the singleꢀphase LiMn2O4 by the SCS in
combination with calcination. However, only the main diffraction peaks can be indexed to the LiMn2O4 phase for the
LiBixLaxMn2ꢀ2xO4 (x=0.010, 0.020) samples. The LaMnO3.15 impurity phase can be detected in the LiBi0.01La0.01Mn1.98O4 and
LiBi0.02La0.02Mn1.96O4 samples while an extra Bi2O3 impurity phase can be observed in the LiBi0.02La0.02Mn1.96O4 sample
because of the doping limit. The shift to a lower angle for all diffraction peaks, especially the (111) peak as the most intense
peak, is observed after the coꢀdoping of Bi and La based on the character of the cubic structure. The 2θ angle of the (111)
diffraction peak shifts to a lower angle as the x values increase, indicating the enlargement of the lattice parameters for the
Bi and La coꢀdoped LiMn2O4 samples. And the explanation will be shown in the following Fig. 2.
Fig. 2 displays the lattice parameters of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples. All the lattice
parameters are refined by MDI Jade Software. The enlarged lattice parameters of the LiBixLaxMn2ꢀ2xO4 samples can be
observed as x increases from 0 to 0.01, indicating that Mn3+ with a smaller ionic radius, 0.72 Å, is substituted by Bi3+ and
La3+ with the same ionic radius, 1.03 Å. However, as x increases from 0.01 to 0.02, the lattice parameters of the Bi and La
coꢀdoped samples are relatively constant, demonstrating that the amount of Bi and La doping attains a limit which results in
the LaMnO3.15 and Bi2O3 impurities shown in Fig. 1. The enlargement of the lattice parameters indicates that Bi and La enter
the spinel structure, probably benefiting the improved electrochemical properties of the LiMn2O4 materials as a result of the
enlarged Li+ diffusion channel during the intercalation/deꢀintercalation process.
Fig. 3 shows the SEM images of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples. All the samples exhibit
similar morphologies. There are no obvious changes in the morphologies or particle sizes of the Bi and La coꢀdoped samples,
indicating that the trace doping of Bi and La does not have a significant effect on the morphologies or particle sizes of the
coꢀdoped samples. The size of the primary particles in the inset is 100–400 nm. Asymmetrical spheres with a size of 3–7 ꢁm
can be observed in the secondary particles.
Fig. 4 demonstrates the discharge capacity as a function of the cycle number at a current density of 1 C in the voltage range
of 3.2–4.4 V for the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples. The pristine LiMn2O4 sample exhibits an
initial discharge capacity of 121.3 mAh/g and only maintains a capacity retention of 89.0% after 80 cycles. As the x value is
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increased from 0 to 0.002 and 0.005, the initial discharge capacity of the Bi and La coꢀdoped LiMn2O4 sample improves,
exhibiting values of 128.7 and 130.2 mAh/g for x=0.002 and 0.005, respectively. A remarkably improved capacity retention
of 95.0% and a discharge capacity of 123.7 mAh/g are delivered for the LiBi0.005La0.005Mn2O4 sample after 80 cycles,
indicating that a more stable structure is achieved during the Liꢀion intercalation/deꢀintercalation process compared with the
pristine LiMn2O4. Both this capacity retention and discharge capacity are higher than the values of 89.1% and 106 mAh/g
after 50 cycles at the current density of 1 C for the LiLa0.05Mn1.95O4 sample by the solꢀgel method,26 and the values of 91.6%
and 103.7 mAh/g after 100 cycles at the current density of 0.2 C for the LiLa0.05Mn1.95O4 sample by the solid state reaction.27
In addition, these values are also higher than that of 84.7% and 100 mAh/g after 100 cycles at the current density of near 0.3
C for the Biꢀmodified LiMn2O4 sample which a trace doping amount of Bi is verified by the increased lattice parameters
calculated from XRD results.37 The detailed dates of the current and reported works are also shown in the Table 1. This
result indicates that coꢀdoping is superior to singleꢀdoping for improving the electrochemical properties of LiMn2O4 material
because of their synergistic effects. These results can be attributed to the factors discussed below. The bond energy of LaꢀO
(786.2 KJ/mol) for the doped sample is greater than that of MnꢀO (402 KJ/mol), which can improve the spinel structure
during cycling. What’s more, both the dissolution of Mn3+ into the electrolyte and the Jahn–Teller distortion are inhibited
because the relatively partial amount of Mn3+ is reduced by the substitution of Bi3+ and La3+, as demonstrated in Fig. 7 (d).
Moreover, the diffusion of the Li ions for the insertion/extraction is enhanced probably owing to the amplified channels
resulting from the enlarged lattice parameters of the Bi and La coꢀdoped LiMn2O4 sample.15,32,39 On the other hand, the
improved cycling performance of the LiBi0.005La0.005Mn1.99O4 sample can also be confirmed by the less capacity fade of 0.08
mAh/g cycleꢀ1 than that of 0.19 mAh/g cycleꢀ1 for the pristine LiMn2O4 sample, displaying that the substitution of Mn by the
partial amounts of Bi and La is in favor of enhancing the cycling performance. The discharge capacity of the Bi and La coꢀ
doped LiMn2O4 with x values of 0.010 and 0.020 is less than that of the LiBi0.005La0.005Mn2O4 sample which can be
attributed to the appearance of the second phase. However, following the 80 cycles, a slightly improved capacity retention of
90.8% is achieved for the LiBi0.02La0.02Mn2O4 sample compared with that of the pristine LiMn2O4, which may be ascribed to
the coating effect of the Bi2O3 second phase on the surface of the particles 24
. The appropriate amounts of Bi and La doping
can improve the cycling performance of the spinel LiMn2O4 material.
The charge–discharge curves of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples for the 1st and 80th cycles
are shown in Fig. 5 (a) and (b), respectively, at a current density of 1 C between voltages of 3.2 and 4.4 V. Two obvious
plateaus can be observed in the voltage range of 3.9 to 4.2 V for all the charge–discharge curves, which can be attributed to
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32
the wellꢀdefined spinel structure of LiMn2O4
. These two plateaus correspond to two oxidation/reduction reaction stages
during the Li deꢀinsertion/insertion process. The first plateau at approximately 4.0 V results from the removal of Li+ ions
from half of the tetrahedral sites in which Li–Li interactions occur. The second plateau at approximately 4.1 V can be
attributed to the removal of Li+ ions from the other tetrahedral sites. Among these samples, the LiBi0.005La0.005Mn1.99O4
sample exhibits the highest discharge capacity for both the 1st and 80th cycles.
Fig. 6 presents the CV curves of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.005) samples in the voltage range of 3.2–4.4 V at a scan rate
of 0.1 mV/s. For comparison, the CV was only conducted for the pristine LiMn2O4 and LiBi0.005La0.005Mn1.99O4 samples.
Two pairs of clearly separated redox peaks were observed in the CV curves of the pristine LiMn2O4 and
LiBi0.005La0.005Mn1.99O4 samples. These can be ascribed to two typical reversible processes during the Li+ intercalation/deꢀ
intercalation cycling which correspond to the two plateaus in the charge–discharge curves of Fig. 5. The peak parameters of
the CV curves during the first cycle of the pristine LiMn2O4 and LiBi0.005La0.005Mn1.99O4 samples are listed in Table 2. A
cathodic peak of 3.94/4.06 V and anodic peaks of 4.07/4.20 V can be observed for the pristine LiMn2O4 sample while a
cathodic peak of 3.96/4.08 V and anodic peaks of 4.05/4.18 V can be observed for the LiBi0.005La0.005Mn1.99O4 sample. The
anodic peaks shift to a lower potential for the doped LiMn2O4 sample compared with the pristine LiMn2O4 sample,
indicating that a portion of Li+ ions can migrate under less energy from the distorted spinel structure of Mn3+ substituted by
Bi3+ and La3+
.
A larger cathodic potential difference, ꢂE1=0.13 V, and anodic potential difference, ꢂE2=0.14 V, are
32,42
exhibited for the pristine LiMn2O4 sample. However, clearly reduced values of ꢂE1=0.09 V and ꢂE2=0.10 V are observed for
the LiBi0.005La0.005Mn1.99O4 sample. This suggests that there is a reduced polarization of the Bi and La coꢀdoped LiMn2O4
material owing to faster intercalation/deꢀintercalation of Li+ ions into/from the spinel structure compared with the pristine
sample 24,33,43 The substitution of Mn3+ with Bi3+ and La3+ cations can maintain the stability of the spinel structure, therefore
.
improving the cycling performance of the materials during the charge/discharge process.
Fig. 7 (a) and (b) display the TEM image with the electron diffraction pattern of the [110] zone axis within the inset, and
the EDX analysis for the LiBi0.005La0.005Mn1.99O4 sample, respectively. The TEM image shows primary particles with a size
of approximately 300 nm, which corresponds to the SEM image in Fig. 3. As there is no evidence of the surface coating
layer exhibited in either the morphology shown in the TEM image or the single phase indexed from the XRD patterns in Fig.
1, it can be confirmed that the coꢀdoped Bi and La ions enter into the structure of the LiBi0.005La0.005Mn1.99O4 sample. The
electron diffraction pattern demonstrated in the inset exhibits typical spinel structure diffraction along the [110] direction,
correlating with Fig. 1. To analyze the change in the valence states before/after the La and Bi doping, the EELS data was
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measured for the obtained particles. Fig. 7 (c) and (d) show the oxygen (O) Kꢀedge and manganese (Mn) Lꢀedge EELS of
the LiBixLaxMn2ꢀ2xO4 material with x = 0 and 0.005, respectively. The spectra positions can be calibrated from their zeroꢀloss
peaks. The peaks of Mn–L3 and Mn–L2 result from the transition of electrons from 2p3/2 to 3d3/2 and 3d5/2, and from 2p1/2 to
3d3/2, respectively 44
valence state of Mn 45
.
These relative intensities can be determined by the unoccupied bands in the 3d orbitals and therefore the
The L3/L2 intensity ratio is related to the valence state of Mn 46,47
Compared with Fig. 7 (c) and (d),
.
.
following the Bi and La coꢀdoping there is a decrease in the L3/L2 intensity ratio which indicates an increase in the Mn
oxidation state. Specifically, following the Bi and La coꢀdoping, a relatively greater amount of Mn4+ appear, arising from the
substitution of Bi3+ and La3+ at the Mn3+ sites. In order to identify the existence and distribution of the Bi and La, the EDS of
the LiBi0.005La0.005Mn1.99O4 sample is shown in Fig. 8. The peaks relating to Bi and La confirm the existence of these two
elements while the element maps demonstrate the uniform distribution of the Bi and La in the LiBi0.005La0.005Mn1.99O4
sample.
Good rate capability, especially the discharge capacity at high current densities, is always required for high power
applications of Liꢀion batteries. Fig. 9 shows the rate capability of the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020)
samples in the voltage range of 3.2–4.3 V. All the samples exhibit decreased capacities as the current densities are increased,
which can be attributed to the increased ohmic and electrochemical polarization arising from the limited diffusion of the Li+
ions into/out of the spinel structure 15,48
The pristine LiMn2O4 sample exhibits an initial discharge capacity of 120.3 mAh/g
.
at a current density of 1 C and a dramatically decreased value of 75.8 mAh/g at a high current of 7 C. However, among all
the samples, the best rate capability was achieved in the LiBi0.005La0.005Mn1.99O4 sample, exhibiting discharge capacities of
127.6 (1 C), 120.8 (2 C), 115.1 (3 C), 111.0 (4 C), and 106.8 mAh/g (5 C). The high value of 98.3 mAh/g is still maintained
even at a high current of 7 C. The superior rate capability can be attributed to the broadened diffusion pathway of the Li+
ions during the intercalation/deꢀintercalation as a result of the enlarged lattice parameters and improved stable structure via
the substitution of Mn3+ with a suitable amount of Bi3+ and La3+ cations 15,32,39
.
Fig. 10 shows the discharge curves for the LiBi0.005La0.005Mn1.99O4 sample at various current densities. Two obvious
plateaus can be observed in the discharge curve at 1 C, 2 C, and 3 C; however, the plateaus gradually disappear to transform
into the slope at a high current density, such as 7 C. This can be attributed to the ohmic drop and the increase in the cell
polarization at high current densities 5
. The discharge capacity as a function of the cycle number at current densities of 1 C, 3
C, and 5 C between the voltage of 3.2 and 4.4 V is presented in Fig. 11 for the LiBi0.005La0.005Mn1.99O4 sample. The
LiBi0.005La0.005Mn1.99O4 sample exhibits good capacity retention at current densities of 3 C and 5 C. Even after 500 cycles, it
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delivers a discharge capacity of 105.9 mAh/g at 3 C and 101.1 mAh/g at 5 C, exhibiting the best highꢀrate capability.
4. Conclusions
A singleꢀphase spinel structure was synthesized for the LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005) samples via SCS followed by
calcination, while impurities were detected in the x=0.010, 0.020 samples because of the doping to an extent. As the x values
were increased, the increase in the lattice parameters indicated that Bi and La entered the spinel structure. Following 80
cycles, the LiBi0.005La0.005Mn1.99O4 sample delivered the highest initial discharge capacity of 130.2 mAh/g and 95% capacity
retention at 1 C compared with values of 121.3 mAh/g and 89% for the pristine LiMn2O4 sample. Even after 500 cycles,
high capacities of 105.9 mAh/g at 3 C and 101.1 mAh/g at 5 C were still maintained for the LiBi0.005La0.005Mn1.99O4 sample,
thus achieving the best highꢀrate capability. The improved electrochemical properties of the Bi and La coꢀdoped LiMn2O4
material can be attributed to the good stability of the spinel structure and the high level of diffusion of the Li ions in/out the
enlarged structure.
Acknowledgement
We would like to thank "Nanotechnology Platform" Program of the Ministry of Education for allowing us to use
transmission electron microscope observation, the partially financial support by the Japan Society for Promotion of Science
(JSPS) and Japan Science and Technology Agency (JST).
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Figures
Fig. 1 XRD patterns of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples with the enlarged (111) diffraction peak.
Fig. 2 Lattice parameters of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples. The crystal structure of LiMn2O4 is
shown in the inset. The enlargement of lattice parameters indicates that Bi and La enter into the spinel structure.
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Fig. 3 SEM images of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples.
Fig. 4 Discharge capacity as a function of the cycle number for LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples
at 1 C between 3.2 and 4.4 V.
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Table 1. Synthetic method and electrochemical properties of the current and reported works. SCS, SꢀG, and SSR represent
the solution combustion synthesis, solꢀgel, and solid state reaction, respectively.
Materials
Synthetic
method
SCS
Current
density
1 C
Voltage
(V)
Discharge
Capacity
retention
89.0%
Ref.
capacity( mAh/g)
116.3 (80 cycles)
123.7 (80 cycles)
106 (50 cycles)
103.7 (100 cycles)
100 (100 cycles)
LiMn2O4
3.2ꢀ4.4
3.2ꢀ4.4
2.75ꢀ4.5
3.0ꢀ4.3
3.5ꢀ4.5
This work
LiBi0.005La0.005Mn1.99O4
LiLa0.05Mn1.95O4
SCS
1 C
95.0%
This work
SꢀG
1 C
89.1%
26
27
37
LiLa0.05Mn1.95O4
SSR
0.2 C
~0.3 C
91.6%
Biꢀmodified LiMn2O4
SꢀG
84.7%
Fig. 5 Charge–discharge curves of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples at 1 C between 3.2 and 4.4 V.
(a) the 1st cycle; (b) the 80th cycle.
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Fig. 6 Cyclic voltammetry (CV) curves of LiBixLaxMn2ꢀ2xO4 (x=0, 0.005) samples in the voltage range of 3.2–4.4 V at a scan
rate of 0.1 mV/s.
Table 2. Peak parameters of CV curves at the 1st cycle for the pristine LiMn2O4 and LiBi0.005La0.005Mn1.99O4 samples. ꢂEp is
the separation between the anodic peak potential, Epa, and the cathodic peak potential, Epc.
Potential value (V)
Epa1
4.07
4.05
Epa2
4.20
4.18
Epc1
3.94
3.96
Epc2
4.06
4.08
ꢂEp1
0.13
0.09
ꢂEp2
0.14
0.10
x=0
x=0.005
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Fig. 7 The TEM image and electron diffraction pattern in [110] zone axis (a), and EDX analysis (b) of
LiBi0.005La0.005Mn1.99O4 sample, in addition to the comparison of EEL spectra for LiBixLaxMn2ꢀ2xO4 with x = 0 (c), and x =
0.005 (d).
Fig. 8 EDS analysis of LiBi0.005La0.005Mn1.99O4 sample.
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Fig. 9 Rate capability of LiBixLaxMn2ꢀ2xO4 (x=0, 0.002, 0.005, 0.010, 0.020) samples.
Fig. 10 Discharge curves at various current rates for LiBi0.005La0.005Mn1.99O4 sample.
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Fig. 11 Discharge capacity as a function of the cycle number for LiBi0.005La0.005Mn1.99O4 sample at 1 C, 3 C, and 5 C
between 3.2 and 4.4 V.
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Reference
1 M. Armand, J.M. Tarascon, Nature, 2008, 451, 652‒657.
2 P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed., 2008, 47, 2930‒2946.
3 J.B. Goodenough, Y. Kim, Chem. Mater., 2010, 22, 587‒603.
4 C.M. Hayner, X. Zhao, H.H. Kung, Annu. Rev. Chem. Biomol. Eng., 2012, 3, 445‒471.
5 Y.L. Ding, J. Xie, G.S. Cao, T.J. Zhu, H.M. Yu, X.B. Zhao, J. Phys. Chem. C, 2011, 115, 9821‒9825.
6 A. Manthiram, A. Vadivel Murugan, A. Sarkar, T. Muraliganth, Energy Environ. Sci., 2008, 1, 621‒638.
7 H.R. Lee, B. Lee, K.Y. Chung, B.W. Cho, K.ꢀY. Lee, S.H. Oh, Electrochim. Acta, 2014, 136, 396‒403.
8 R. Thirunakaran, R. Ravikumar, S. Vanitha, S. Gopukumar, A. Sivashanmugam, Electrochim. Acta, 2011, 58, 348‒358.
9 R.J. Gummow, A. Dekock, M.M. Thackeray, Solid State Ionics, 1994, 69, 59‒67.
10 Y.Y. Xia, Y.H. Zhou, M. Yoshio, J. Electrochem. Soc., 1997, 144, 2593‒2600.
11 T. Qiu, J. Wang, Y. Lu, W. Yang, Electrochim. Acta, 2014, 147, 626‒635.
12 S. Hu, Y. Li, F. Lai, X. Zhang, Q. Li, Y. Huang, X. Yuan, J. Chen, H. Wang, RSC Adv., 2015, 5, 17592‒17600.
13 Y. Xu, G. Chen, E. Fu, M. Zhou, M. Dunwell, L. Fei, S. Deng, P. Andersen, Y. Wang, Q. Jia, H. Luo, RSC Adv., 2013, 3,
18441‒18445.
14 H. Zhao, X. Liu, C. Cheng, Q. Li, Z. Zhang, Y. Wu, B. Chen, W. Xiong, J. Power Sources, 2015, 282, 118‒128.
15 W. Wen, B. Ju, X. Wang, C. Wu, H. Shu, X. Yang, Electrochim. Acta, 2014, 147, 271‒278.
16 R. Singhal, S.R. Das, M.S. Tomar, O. Ovideo, S. Nieto, R.E. Melgarejo, R.S. Katiyar, J. Power Sources, 2007, 164, 857‒861.
17 P. Arora, B.N. Popov, R.E. White, J. Electrochem. Soc., 1998, 145, 807‒815.
18 F.P. Nkosi, C.J. Jafta, M. Kebede, L. le Roux, M.K. Mathe, K.I. Ozoemena, RSC Adv., 2015, 5, 32256‒32262.
19 A.B. Yuan, L. Tian, W.M. Xu, Y.Q. Wang, J. Power Sources, 2010, 195, 5032‒5038.
20 Y. Shi, S. Zhu, C. Zhu, Y. Li, Z. Chen, D. Zhang, Electrochim. Acta, 2015, 154, 17‒23.
21 N. Jayaprakash, N. Kalaiselvi, Gangulibabu, D. Bhuvaneswari, J. Solid State Electrochem., 2011, 15, 1243‒1251.
22 A. Iturrondobeitia, A. Goñi, V. Palomares, I. Gil de Muro, L. Lezama, T. Rojo, J. Power Sources, 2012, 216, 482‒488.
23 D.ꢀQ. Liu, Z.ꢀZ. He, X.ꢀQ. Liu, J. Alloys Compd., 2007, 440, 69‒73.
24 M. Mo, K.S. Hui, X. Hong, J. Guo, C. Ye, A. Li, N. Hu, Z. Huang, J. Jiang, J. Liang, H. Chen, Appl. Surf. Sci., 2014, 290,
412‒418.
25 S.R.K. Balaji, D. Mutharasu, S. Shanmugan, N.S. Subramanian, K. Ramanathan, Ionics, 2010, 16, 351‒360.
26 D. Arumugam, G.P. Kalaignan, P. Manisankar, Solid State Ionics, 2008, 179, 580‒586.
17

RSC Advances
Page 18 of 18
View Article Online
DOI: 10.1039/C5RA13005K
27 H. Sun, Y. Chen, C. Xu, D. Zhu, L. Huang, J. Solid State Electrochem., 2012, 16, 1247‒1254.
28 R.M. Rojas, J.M. Amarilla, L. Pascual, J.M. Rojo, D. Kovacheva, K. Petrov, J. Power Sources, 2006, 160, 529‒535.
29 F. Bonino, S. Panero, D. Satolli, B. Scrosati, J. Power Sources, 2001, 97–98, 389‒392.
30 R. Alcantara, M. Jaraba, P. Lavela, J.M. Lloris, C.P. Vicente, J.L. Tirado, J. Electrochem. Soc., 2005, 152, A13‒A18.
31 H. Goktepe, H. Sahan, S. Patat, A. Ulgen, Ionics, 2009, 15,233‒239.
32 P. Mohan, B. Ranjith, G.P. Kalaignan, J. Solid State Electrochem., 2014, 18, 2183‒2192.
33 A. Iqbal, Y. Iqbal, L. Chang, S. Ahmed, Z.Y. Tang, Y. Gao, J. Nanopart. Res., (2012), 14, 1ꢀ14 .
34 H.W. Liu, L. Song, K.L. Zhang, Erꢀdoped LiMn2O4. Inorganic Materials, 2005, 41, 646‒649.
35 G.P. Nayaka, J. Manjanna, K.C. Anjaneya, P. Manikandan, P. Periasamy, V.S. Tripathi, Bull. Mater. Sci., 2014, 37, 705‒711.
36 H. Schlörb, M. Bungs, W. Plieth, Electrochim. Acta, 1997, 42, 2619‒2625.
37 C.L. Tan, H.J. Zhou, W.S. Li, X.H. Hou, D.S. Lue, M.Q. Xu, Q.M. Huang, J. Power Sources, 2008, 184, 408‒413.
38 H. Ahmad, N. Mohammad, Z. Sirus, World Appl. Sci. J., 2012, 19, 424‒430.
39 H.L. Wang, T.A. Tan, P. Yang, M.O. Lai, L. Lui, J. Phys. Chem. C, 2011, 115, 6102‒6110.
40 C.ꢀG. Han, C. Zhu, G. Saito, T. Akiyama, Adv. Powder Technol., 2015, 26, 665‒671.
41 C. Zhu, A. Nobuta, G. Saito, I. Nakatsugawa, T. Akiyama, Adv. Powder Technol., 2014, 25, 342‒347.
42 J.M. Tarascon, W.R. McKinnon, F. Coowar, T.N. Bowmer, G. Amatucci, D. Guyomard, J. Electrochem. Soc., 1994, 141,
1421‒1431.
43 J.L. Wang, Z.H. Li, J. Yang, J.J. Tang, J.J. Yu, W.B. Nie, G.T. Lei, Q.Z. Xiao, Electrochim. Acta, 2012, 75, 115‒122.
44 K.J. Carroll, D. Qian, C. Fell, S. Calvin, G.M. Veith, M. Chi, L. Baggetto, Y.S. Meng, Phys. Chem. Chem. Phys., 2013, 15,
11128‒11138.
45 R.D. Leapman, L.A. Grunes, P.L. Fejes, Phys. Rev. B, 1982, 26, 614‒635.
46 H.K. Schmid, W. Mader, Micron, 2006, 37, 426‒432.
47 T. Riedl, T. Gemming, K. Wetzig, Ultramicroscopy, 2006, 106, 284‒291.
48 H. Zhao, F. Li, X. Liu, C. Cheng, Z. Zhang, Y. Wu, W. Xiong, B. Chen, Electrochim. Acta, 2015, 151, 263‒269.
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