Enhancing Electrochemical Performance of LiMn2O4 Cathode

Research Article pubs.acs.org/journal/ascecg

Enhancing Electrochemical Performance of LiMn2O4 Cathode Material at Elevated Temperature by Uniform Nanosized TiO2 Coating Congcong Zhang,† Xiaoyu Liu,† Qili Su,† Jianhua Wu,‡ Tao Huang,† and Aishui Yu*,† †

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Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Institute of New Energy, Fudan University, No. 2205, Songhu Road, Yangpu District, Shanghai 200438, China ‡ Jiangmen KanHoo Industry Co., Ltd., No. 22, South Jiaoxing Road, Jianghai District, Jiangmen City, Guangdong 529040, China S Supporting Information *

ABSTRACT: The severe capacity fading of LiMn2O4 at elevated temperature hinders its wide application in lithium ion batteries despite several advantages over present cathode materials in terms of cost, rate capability, and environmental benignity. In this study, porous nanosized TiO2-coated LiMn2O4 is prepared via a modified sol−gel process of controlling hydrolysis and condensation of titanium tetrabutoxide in ethanol/ammonia mixtures, and the phenomenon of homogeneous nucleation has been almost entirely avoided. The X-ray diffraction patterns and transmission electron microscopy images show that a porous nanosized TiO2 layer is uniformly coated on the surface of spinel LiMn2O4. Electrochemical tests reveal that the optimal coating content is 3 wt % which shows remarkably improved capacity retentions at both room temperature of 25 °C and elevated temperature of 55 °C. Even after long-term charge and discharge cycles, the TiO2 layer is still robust enough to prevent LiMn2O4 particles from the attack of electrolyte. The inductively coupled plasma-atomic emission spectrometry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy results indicate that the obvious improvement of TiO2-coated LiMn2O4 electrodes is attributed to the suppression of Mn dissolution, as well as the enhancement of kinetics of Li+ diffusion. KEYWORDS: Lithium ion battery, Cathode material, Lithium manganese oxide, Surface modification, Improved electrochemical performance

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INTRODUCTION Lithium ion batteries (LIBs) with high energy density have been regarded as the most prospective power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs).1−4 In particular, spinel lithium manganese oxide (LiMn2O4) has attracted significant attention due to its nonpoisonous character, low cost, and unexceptionable rate capability compared with the commercial layered lithium metal oxide (LiMO2, M = Ni, Mn, and Co).5−7 In addition, the threedimensional channel (8a-16c-8a) favors of the Li+ insertion and extraction process. Unfortunately, LiMn2O4 is subjected to serious capacity deterioration at elevated temperature, hindering its widespread usage in lithium ion batteries. Previous studies show that several factors led to this problem such as (1) Jahn−Teller effect in deeply discharged state, (2) the slow dissolution of manganese stemming from the disproportional reaction of Mn3+ (2Mn3+ → Mn2+ + Mn4+), and (3) the decomposition of electrolyte at the upper voltage regions.8,9 To solve this capacity deterioration problem, many efforts have been investigated. Among them, surface modification has © 2016 American Chemical Society

demonstrated excellent performance because the coated protective layer can reduce the direct contact area between active material and electrolyte to minimize the dissolution of manganese species. To date, various oxides,10−14 fluorides,15−17 and phosphates18,19 have been applied to this method. Among the candidates for coating layers, nanosized TiO2 has great stability and special channels which could store Li+.20,21 Thus, we believe that TiO2 will result in significantly improved electrochemical performance for LiMn2O4. However, it is difficult to control the reaction kinetics, leading to nonuniformity of the coating layer that may strip off during long-term cycling. In addition, superfluous titanium oligomers may polymerize together and finally form TiO2 particles, which are also adverse for TiO2 coating, as shown in Figure 1a. Recently, the Zhao research group22 reported a versatile kinetics-controlled coating method to construct uniform porous Received: August 20, 2016 Revised: October 13, 2016 Published: November 3, 2016 640

DOI: 10.1021/acssuschemeng.6b02011 ACS Sustainable Chem. Eng. 2017, 5, 640−647

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram of the TiO2 coating process and illustration of TiO2 layer protecting LMO particles from destruction: (a) traditional chemical process and (b) kinetics-controlled coating method. photoelectron spectroscopy (XPS, PHI500C ESCA) with Al Kα radiation (hν = 1486.6 ev), and all the spectra were calibrated with C 1s peak at 284.8 eV. Electrochemical Measurements. The working electrodes were fabricated by mixing 80 wt % as-prepared particles, 10 wt % carbon conductive agents (Super P), and 10 wt % polyvinylidene fluoride (PVDF), which were all dissolved in N-methyl-pyrolline (NMP) and compressed onto the aluminum foil. After being thoroughly dried, the films were cut into wafers (12 mm × 12 mm) as cathode electrodes and dried overnight at 80 °C in a vacuum oven prior to use. To evaluate electrochemical performance of the samples, coin cells with CR2016 configuration were assembled in an argon-filled glovebox (Mikarouna, Superstar 1220/750/900), using Li metal as anode and Celgard 2300 as separator. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 w/w). The galvanostatic charge−discharge measurements were conducted on a battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) between 3.0 and 4.5 V (vs Li+/Li) at both 25 and 55 °C. Electrochemical impedance spectroscopy (EIS) measurements were performed on a Zahner IM6 electrochemical workstation (Zahner-Elektrik GmbH & Co. KG, Germany), and an ac voltage of 5 mV amplitude was applied over a frequency range from 100 kHz to 10 mHz at the open circuit potential after 1 cycle and 30 cycles. The EIS results were simulated using ZPVIEW software.

TiO2 shells for multifunctional core−shell structures. This approach is very simple and reproductive. Furthermore, the TiO2 shells have large mesoporosities, and the thickness is easily controllable. Herein, we propose a modified sol−gel method for preparation of TiO2-coated LiMn2O4 cathode material. This approach combines the advantages of stability of TiO2 and a simple operational process. The schematic diagram of the coating process is demonstrated in Figure 1b. With control of the hydrolysis and condensation of titanium tetrabutoxide in ethanol/ammonia mixtures, a porous nanosized TiO2 coating layer is uniformly coated on the surface of LiMn2O4 material, and the phenomenon of homogeneous nucleation has been almost entirely avoided. Moreover, the electrochemical performance of TiO2-coated LiMn2O4 is evaluated and receives great improvement.

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EXPERIMENTAL SECTION

Synthesis of TiO2-Coated LiMn2O4 Cathode Materials. Commercial LiMn2O4 (Carus Chemical Company, LMO hereafter) was used as the pristine material whose average particle size is about 10 μm, and the corresponding TiO2-coated LiMn2O4 particles were synthesized by a modified sol−gel method to control the hydrolysis and condensation of titanium tetrabutoxide (TBOT) in ethanol/ ammonia mixtures. The pristine LMO was immersed in ethanol together with some amount of concentrated ammonia solution. Then, the mixture was ultrasonically vibrated for 15 min and was continuously vigorously stirred at 45 °C for 24 h, while TBOT solution was slowly added drop by drop. The resultant products were centrifuged and washed several times, and then dried at 80 °C overnight. In the end, the obtained particles were calcined at 400 °C for 2 h in air to obtain TiO2-coated LMO particles. The desired amounts of TiO2 coating were 1, 3, and 5 wt %, and the corresponding TiO2-coated LMO products are abbreviated to 1-TiO2-LMO, 3-TiO2LMO, and 5-TiO2-LMO, respectively. Characterization. The crystalline structure of the samples was characterized by X-ray diffraction (XRD) on a Bruker D8 Advance Xray diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). The morphology of TiO2 coating layer was observed by means of transmission electron microscopy (TEM, JEM-2100F) and filedemission scanning electron microscopy (FE-SEM, Hitachi S-4800). Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Thermo E.IRISDuo) was used to measure the content of Mn. The surface chemical compositions of the samples were measured by X-ray

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RESULTS AND DISCUSSION The XRD patterns of pristine and TiO2-coated LMO are shown in Figure 2. All the diffraction peaks can be assigned to a welldefined spinel structure of LMO with the space group Fd3m (JCPDS 88-1749), suggesting that the bulk structure of the LMO has no significant change after surface modification. No obvious diffraction peaks of TiO2 are detected in the modified samples, which is likely due to the low weight ratio of TiO2.15 The lattice parameters and unit cell volume of samples shown in Table S1 are calculated by Jade 6.0. Both the pristine and surface-modified samples exhibit similar cell size, indicating that no Ti4+ incorporates into the bulk structure of LMO during the coating and sintering process and TiO2 may coat on the surface of the LMO particles.23 To distinguish the structure of the coating layer, TEM and HRTEM images of pristine LMO and 3-TiO2-LMO are offered in Figure 3. The pristine LMO shown in Figure 3a contains primary particles with diameter ranging from 50 to 80 nm and exhibits a very smooth edge line. The HRTEM image in Figure 641


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surface of spinel LMO, without incorporation of Ti4+ into the bulk structure of LMO, which is consistent with the XRD result. Furthermore, the coating layer is not particularly dense and is composed of porous networks that are connected with the three-dimensional channel of spinel LMO and can be used to store Li+. Therefore, this special structure is in favor of enhancing the kinetics of the Li+ diffusion. Figure 4 shows the cycling performance of the pristine and TiO2-coated LMO electrodes at 25 and 55 °C at 0.2 C for the first five cycles and 0.5 C for the subsequent cycles (1 C = 120 mA g−1). It can be distinctly observed that TiO2-coated LMO samples exhibit improved electrochemical performance at both room and elevated temperature. As shown in Figure 4a, the discharge capacity of pristine LMO fades from its initial capacity of 126.9 to 101.9 mAh g−1 after 250 cycles, corresponding to the capacity retention of 80.3%. On the contrary, TiO2-coated LMO samples not only deliver high initial discharge capacities (128.9, 126.4, and 125.3 mAh g−1 for 1-TiO2-LMO, 3-TiO2-LMO, and 5-TiO2-LMO, respectively) which are close to that of pristine sample but also exhibit remarkably improved capacity retention, 86.1%, 90.1%, and 88.1%, respectively. The better cycling performance of modified LMO indicates that the protective nanosized TiO2 layer can effectively reduce the contact area between active materials and electrolyte, resulting in fewer side reactions and Mn dissolution. As shown in Figure 4b, both pristine and TiO2-coated LMO samples show declined cycling performance at 55 °C, which may be due to the observation that such long-term charge and discharge cycles at elevated temperature lead to the decomposition of electrolyte, thus resulting in inferior capacity retention. The discharge capacity of pristine LMO sharply fades to 32.9 mAh g−1 after 250 cycles, while those of 1, 3, and 5 wt % TiO2-coated LMO only decrease to 54.8, 77.1, and 75 mAh g−1 at the 250th cycle, corresponding to capacity retentions of 44.2%, 62.0%, and 60.3%, respectively. These above results indicate that the sample with 3 wt % coating amount exhibits the best cycling properties at both room and elevated temperature. In a comparison with the optimal TiO2 coating content of 3 wt %, 1 wt % is only enough for the TiO2 layer to cover a fraction of the surface of active materials. As TiO2 increases away from the optimum coating content, the superfluous TiO2 may hinder the Li+ diffusion. Therefore, the optimal 3 wt % TiO2-coated sample is enough to alleviate the Mn dissolution by scavenging the harmful HF species,12 while it does not block the Li+ transportation during the charge and discharge processes. Furthermore, 3-TiO2-LMO shows more obvious improvement at 55 °C than at 25 °C. The capacity

Figure 2. XRD patterns of the pristine and TiO2-coated LMO particles.

Figure 3. TEM images of (a) pristine LMO and (b) 3-TiO2-LMO and HRTEM images of (c) pristine LMO and (d) 3-TiO2-LMO.

3c shows that the periodic lattice fringes with an interplanar spacing of 0.48 nm represent the (111) plane of spinel LMO (JCPDS 88-1749). As shown in Figure 3b,d, a porous nanosized layer with a thickness of approximately 5 nm is uniformly coated on the surface of LMO, and the (311) plane of cubic lattice of LMO could be clearly observed. In addition, the coating effect of 1-TiO2-LMO and 5-TiO2-LMO is also shown in Figure S1. The coating layer is also porous, and the thicknesses are about 3 and 7.5 nm, respectively. These TEM results indicate that the TiO2 coating layer resides only on the

Figure 4. Cycling performance of pristine and TiO2-coated samples in a voltage range 3.0−4.5 V at (a) 25 °C and (b) 55 °C. 642


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temperature. It can be drawn obviously that both of the two samples have similar charge−discharge profiles and exhibit two distinct charge−discharge plateaus at 4.1 and 4.0 V during the initial several cycles, indicating that the TiO2 coating layer does not affect the intrinsic charge−discharge behaviors of spinel LMO and the Li+ insertion and extraction occurs in two states.9 The voltage plateau at 4.1 V is attributed to the orderly Li+ intercalating in the tetrahedral (8a) sites, while 4.0 V is ascribed to the disorderly Li+ intercalating.30 As shown in Figure 6a, the cyclicity of the pristine LMO deteriorates seriously, and the plateau regions are gradually deformed upon cycling. According to the two peaks of the dQ/dV plot inset, the voltage drops of the two potential plateaus are about 6.5 and 8.1 mV, respectively. On the contrary, the two plateaus in discharge curves of 3-TiO2-LMO are shaped well and can be obviously recognized even after 100 cycles, and the voltage drop is only 1.7 and 1.6 mV. These results indicate that 3 wt % TiO2-coated LMO electrode has better reversibility than pristine electrode. This may be due to the fact that TiO2 surface layer with special porous structure enhances the structural stability of spinel LMO, thus facilitating Li+ insertion and extraction process. To intensively explore the origin of the improved electrochemical performance of the TiO2-coated LMO sample, EIS is performed for the pristine and 3-TiO2-LMO electrodes after different cycles at room temperature. As shown in Figure 7a,b, both samples show typical Nyquist features, including an intercept in the high frequency region of the Z′ real axis, semicircles in the high-middle frequency region, and an inclined line in the lower frequency region. The intercept represents ohmic resistance (Rs), while the semicircle reflects surface film resistance (Rsf) that covers the surface of cathode materials13 and the charge transfer resistance (Rct),31 and the inclined line is attributed to Warburg impedance (Zw), which is directly related to Li+ diffusion in the bulk electrode.16 For clearer depiction of the EIS curves, the Nyquist plot is fitted with the equivalent circuits in Figure 7c, and the fitting results of Rsf and Rct are presented in Table 1. As listed in Table 1, 3-TiO2-LMO shows a lower Rsf value (14.9 Ω) than the pristine LMO (23.6 Ω) after the first cycle. This may be attributed to a TiO2 coating layer, which reduces the contact area between the electrode and electrolyte, forming fewer undesired solid electrolyte interface (SEI) films.15,23 In addition, the Rsf values of the two samples become larger after 30 cycles; the pristine LMO exhibits a significant raise (29.4 Ω) while 3-TiO2-LMO shows minor changes (6.8 Ω). The result above proves that the TiO2 layer is favorable for alleviating Mn dissolution and deposition of byproducts that stem from electrolyte decomposition.16 In addition, the suppression of the impedance increase is beneficial for extending battery life during continuous charge−discharge cycles, which partly explains the improved cycling stability of 3-TiO2-LMO. For the two samples, the Rct values rapidly decrease after 30 cycles, likely due to electrochemical activation. Furthermore, 3-TiO2LMO shows a lower Rct value than the pristine LMO after 1 cycle and 30 cycles, which may be related to the fact that the porous TiO2 layer can enhance the kinetics of the Li+ diffusion. The smaller Rct can accelerate electrochemical reactions and result in better electrochemical performance. In order to study the surface chemistry of LMO electrodes and determine the role of the surface modification in enhancing the stability of LMO, XPS signals of C 1s, O 1s, and F 1s spectra are collected for the pristine and 3-TiO2-LMO electrodes before and after 250 charge−discharge cycles at

retention of 3-TiO2-LMO has doubled compared with that of the uncoated sample at 55 °C, while it only shows an enhancement of about 12.2% at 25 °C, implying that the nanosized TiO2 coating layer on the surface of LMO is more positive to improve the cycling performance at elevated temperature. The previous literature reported that the capacity decay of spinel LiMn2O4 was mainly attributed to the Mn dissolution during cycling.9,24−26 When using LiPF6-based organic electrolyte, HF is unavoidably generated from the interaction of LiPF6 and a small amount of water in electrolyte. The produced HF continuously attacks LMO particles according to the following reactions shown in eqs 1−3:27−29 LiPF6 → LiF + PF5

(1)

PF5 + H 2O → 2HF + PF3O

(2)

2LiMn2O4 + 4HF → 3λ‐MnO2 + 2LiF + 2MnF2 + 2H 2O

(3)

When the material is directly dipped in HF, these reactions occur easily and will be more severe at elevated temperature. Fortunately, TiO2-coated LMO has less chance to be exposed to HF because of the uniform nanosized TiO2 layer which covers the active materials, leading to less Mn dissolution. Therefore, a protective layer of TiO2 on LMO can effectively reduce the Mn dissolution, and greatly decrease the capacity fading, in agreement with the results of Figure 4. To compare the amounts of Mn dissolution, the content of manganese in cathode electrodes of the pristine and TiO2coated LMO is measured by ICP-AES before cycling and after 250 cycles at 55 °C, as shown in Figure 5. It is clear that less

Figure 5. Weight percent of Mn dissolution of pristine and TiO2coated electrodes after 250 cycles at 55 °C.

Mn is dissolved for TiO2-coated LMO samples and the amount of Mn dissolution decreases with the content of TiO2 coating from 1 to 5 wt %, which is in agreement with the expected role of the TiO2 layer that inhibits Mn dissolution. Therefore, the TiO2 coating layer on the surface of LMO particles can effectively enhance the electrochemical performance by suppressing Mn dissolution, especially at elevated temperature. Figure 6 shows the charge−discharge curves and corresponding differential capacity (dQ/dV) plots of the pristine and 3TiO2-LMO during the 6th, 60th, and 100th cycles at a discharge rate of 0.5 C between 3.0 and 4.5 V at room 643


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Figure 6. Charge−discharge curves of (a) pristine LMO and (b) 3-TiO2-LMO during the 6th, 60th, and 100th cycle at room temperature and corresponding dQ/dV plots of the circled parted (inset).

Figure 7. Nyquist plots of the pristine and 3-TiO2-LMO (a) after 1 cycle and (b) after 30 cycles at 0.5 C at room temperature, and (c) equivalent circuit performed to fit the Nyquist plots in parts a and b.

room temperature, as shown in Figure 8. After 250 cycles, the peaks of C 1s, O 1s, and F 1s spectra appear to shift more or less, which may be contributions from the different chemical environment.12 Because of the complication in XPS spectra from PVDF binder and Super P contained in the electrodes, only significant peaks are taken into account for comparative

Table 1. Impedance Parameters of Equivalent Circuits after 1 cycle

after 30 cycles

sample name

Rsf/Ω

Rct/Ω

Rsf/Ω

Rct/Ω

pristine LMO 3-TiO2-LMO

23.6 14.9

209.7 139.9

53.0 21.7

20.0 18.0

Figure 8. XPS results of the pristine and 3-TiO2-LMO electrodes before and after 250 charge−discharge cycles at room temperature. The signals for C 1s, O 1s, and F 1s are shown in parts a−c, respectively. 644


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Figure 9. FE-SEM images of the pristine LMO electrode (a) before cycling and (b) after 250 cycles at room temperature, and the 3-TiO2-LMO electrode (c) before cycling and (d) after 250 cycles at room temperature.

morphology of pristine LMO is smooth, as shown in Figure 9a. However, the pristine electrode after cycling shown in Figure 9b exhibits distinct difference. The LMO particles lose the smooth edges and abnormal surfaces formed on the pristine electrode which may derive from the byproducts of electrolyte decomposition,38 leading to the aggregation of particles. On the other hand, the modified electrode in Figure 9c shows the similar surface morphology compared to the pristine electrode before cycling. Even after long-term charge and discharge cycles, the 3-TiO2-LMO electrode shown in Figure 9d remains almost unchanged and maintains its original surface morphology. On the basis of the above results, we propose a schematic diagram of illustration of TiO2 layer protecting LMO particles from destruction, as shown in Figure 1. After traditional chemical processing, part of the surface of LMO particles is exposed to electrolyte, leading to inevitable destruction. However, the porous nanosized TiO2 layer prepared by the modified sol−gel method is robust enough to prevent LMO particles from the attack of electrolyte and is difficult to strip off from the LMO surface during long-term charge−discharge cycling.

study. It can be drawn from Figure 8 that the peak of CO in both the C 1s and O 1s spectra is related to Super P for the electrodes before cycling and species such as carbonates and polyethers for electrodes after cycling, while the peak of CO arises from species containing lithium alkyl carbonates and polycarbonates, which are the products from electrolyte decomposition.32,33 Also, both the pristine and 3-TiO2-LMO electrodes after cycling show peaks from LiF and LixPOyFz, which may also come from the electrolyte.34,35Table S2 summarizes the corresponding peak percent from XPS fitting, which shows that the increase of the peak intensities for CO/ CO in both the C 1s and O 1s spectra is greater in the pristine LMO electrode compared with that of the 3-TiO2LMO electrode after cycling. In addition, the peak intensity for LiF/LixPOyFz in the F 1s spectrum of the cycled 3-TiO2-LMO electrode is weaker than that of the pristine LMO. These results indicate that more organic and inorganic species including alkyl carbonates, polycarbonates, polyethers, LiF, and LixPOyFz are covered on the surface of the pristine LMO electrode, which is consistent with the reaction between the cathode materials and electrolyte. It is thus believed that the TiO2 coating layer can effectively reduce the contact area between active materials and electrolyte, in return increasing the structural stability of cathode materials. Meanwhile, it can also reduce the erosion of electrolyte and decrease the byproducts covered on the electrode surface, thus cutting down the interface impedance and enhancing the kinetics of the Li+ diffusion. In particular, one peak at 685.4 eV in the F 1s spectrum is distinctively observed for the 3-TiO2-LMO electrode after 250 cycles, which may be related to TiF4. As eq 4 shows, the TiO2 coating layer can scavenge the harmful HF species through the formation of the TiF4 layer.36,37 Furthermore, the formed metal fluoride layer is also resistant to HF and protects the active materials from HF attack. TiO2 + 4HF → TiF4 + 4H 2O

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CONCLUSION We developed a modified sol−gel method to prepare LiMn2O4 cathode material with a uniform and porous nanosized TiO2 layer coating. In comparison with the pristine LiMn2O4, 3 wt % TiO2-coated LiMn2O4 (3-TiO2-LMO) exhibits much better cycling performance at both 25 and 55 °C. The capacity retentions of the 3-TiO2-LMO are 90.1% and 62.0% after 250 cycles at 25 and 55 °C, respectively, while those of pristine LMO are 80.3% and 26.0%, respectively. The obvious effect of the TiO2 coating layer may be attributed to the following factors: (1) The special porous structure facilitates Li+ diffusion, and (2) the uniform nanosized layer can maintain the structural stability of the bulk LMO and effectively suppress the Mn dissolution. Even at elevated temperature, the TiO2 layer is robust enough to prevent LMO particles from the attack of electrolyte, resulting in coated LMO withstanding longer

(4)

Figure 9 gives the variations of morphology of the pristine and 3-TiO2-LMO electrodes after 250 charge−discharge cycles at room temperature. Before cell testing, the surface 645


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charge−discharge cycles. We believe that this modified method could have a great implication for the achievement of large-scale commercial applications for lithium ion batteries.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02011. Lattice parameters and unit cell volume for the samples obtained from the XRD patterns, HRTEM images of 1TiO2-LMO and 5-TiO2-LMO, and peak percent of the pristine and 3-TiO2-LMO electrodes before and after cycling (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-51630320. Fax: +86-21-51630320. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from the 973 Program (No. 2013CB934103) and the Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China.

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REFERENCES

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Enhancing Electrochemical Performance of LiMn2O4 Cathode

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