- Email: [email protected]
Nano-TiO2(B) coated LiMn2O4 as cathode materials for lithium-ion batteries at elevated temperatures
Electrochimica Acta 156 (2015) 121–126
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nano-TiO2(B) coated LiMn2O4 as cathode materials for lithium-ion batteries at elevated temperatures Yesheng Shang, Xiujing Lin, Xu Lu, Tao Huang, Aishui Yu * Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 November 2014 Received in revised form 6 January 2015 Accepted 6 January 2015 Available online 7 January 2015
In this study, nano-TiO2(B) coated LiMn2O4 was prepared via a two-step method, combining a hydrothermal method with electrostatic attraction. By adjusting the sintering time, porous and dense nano-TiO2(B) coating structure were formed on the surface of LiMn2O4 particles. Electrochemical test results showed that 2 wt.% porous TiO2(B) coated LiMn2O4 exhibited highest capacity retention at 77.4% after 300 cycles at 55 C and the best rate capability. Inductively coupled plasma spectroscopy(ICP), charge and discharge curves and electrochemical impedance spectroscopy (EIS) results revealed that the improved electrochemical performances was due to the suppression of the undesired SEI film, as well as suppression of Mn dissolution at the cathode and reduced polarization. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: spinel lithium manganese oxide lithium ion batteries surface coating
1. Introduction Spinel lithium manganese oxide (LiMn2O4, LMO hereafter) is regarded as a promising cathode material for lithium-ion battery devices [1–3]. However, LMO suffers from the drawback of poor cycling stability, which is mainly associated with manganese dissolution during battery operation, especially at elevated temperatures [4–6]. To overcome these problems, many researchers have attempted to improve the high temperature cyclic performance by forming a coating layer that would inhibit the Mn dissolution of LMO. In the aspect of surface coating, there are two basic points. Firstly, the coatings should be thermally and chemically stable during cycling. Secondly, surface coatings should not suppress the Li+ transportation, significantly. Among the candidates for coatings, titanium dioxide (TiO2) attracts the most attention due to its thermal stability, structural stability and fast kinetics. Lihong Yu et al. [7] have prepared TiO2-modified spinel LiMn2O4 by a simple sol–gel method to improve its electrochemical performance at elevated temperatures. Compared with the bulk TiO2 polymorph material, nano-TiO2(B) shows more open channels in the lattice, resulting in easier Li-ion access to the crystal structure [8], which make it a promising coating material for LMO. To the best of our
* Corresponding author. Tel.: +86 21 51630320; fax: +86 21 51630320. E-mail address: [email protected] (A. Yu). http://dx.doi.org/10.1016/j.electacta.2015.01.024 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
knowledge, TiO2(B) has never been applied as a surface coating of LMO yet. In addition to the candidate selection, the design of a coating structure is also important to Li+ transportation. It is believed that thick coatings may hinder the transportation of lithium ions, leading to the evident decay of the specific capacity [9]. As a result, most concerns were focused on the tuning of the amount or the thickness of coating layers [10,11]. However, limited information has been gathered thus far about the surface morphology of coated LiMn2O4 electrodes or their functions, although the surface morphology can be influenced by the synthesis and sintering processes. In this paper, we offer a novel two-step method for the preparation of a TiO2(B)-coated LiMn2O4 cathode material. By adjusting the heat treatment time, we successfully prepared LMO with two different coating structures .The electrochemical behavior is characterized, and the relationship between the coating structure and electrochemical performance is discussed in details. 2. Experimental Commercial Li1.06Al0.05Mg0.025Mn0.925O4 (Jiang Su Guo Tai Libode New Material Co., LTD) with an average particle size of 0.5 mm was used as the pristine material. Inductively coupled plasma spectroscopy (ICP) was applied to determine the chemical formula of the commercial material, and the result showed an exact formula of Li1.03Al0.037Mg0.023Mn0.938O4, which was still
122
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
regarded as Li1.06Al0.05Mg0.025Mn0.925O4 and was written as LiMn2O4 or LMO for short hereafter.
microscope (TEM, CM200FEG), while the phase formation and crystallinity were tested by the selected area electron diffraction (SAED).
2.1. Synthesis of TiO2(B) nanoparticles 2.3. Electrochemical measurements TiO2(B) nanoparticles were prepared using a hydrothermal method modified from a previously reported procedure [12]. In a typical synthesis, 0.36 g of Ti (99.7%, 200–300 mesh) was dissolved in an aqueous solution of H2O2 (30 mL, 30.0%) and NH4OH (10 mL, 35%) in an ice-water bath for 2 h with stirring. Then, 0.85 g of glycolic acid (99%,) was dissolved in the solution, which was maintained at 80 C for 2 h to eliminate excess H2O2 and NH3. The solution changed to a yellow gel. The gel was dissolved in ca. 60 mL of water to form a yellowish transparent solution followed by the addition of 1.0 g of H2SO4 (98.0%). The resulting solution was sealed in a Teflon-lined stainless steel autoclave and heated at 160 C for 30 min. The final powder was filtered, washed several times with distilled water and finally dispersed in 100 mL of water. 2.2. Synthesis of TiO2(B)-coated LiMn2O4 cathode material In addition, 10 mL, 20 mL and 30 mL of the above TiO2(B) suspension for 1 wt.%, 2 wt.% and 3 wt.% TiO2(B) coatings were diluted to 100 ml, respectively. As shown in Fig. 1(a), TiO2(B) and LiMn2O4 exhibited a maximum zeta potential difference when the pH was approximately 7. Thus, the pH value was adjusted to 7 by adding NH4OH(1 mL, 35%). The mixture was ultrasonically agitated for 30 min followed by mechanical stirring for 60 min, while 0.5 g of commercial LiMn2O4 was slowly added. During this process, the surface charges of LiMn2O4 and TiO2(B) were opposite in sign and thus, a junction between them was formed by the electrostatic attraction, as shown in Fig. 1b. The final powder was filtered and washed several times with distilled water, dried at 80 C for 2 h, and subsequently heat-treated in a furnace at 300 C for 2 h and 4 h in air, respectively. For convenience, the samples were marked with X (wt%)–T (h) in which X represented the coating amount and T represented the annealed time. Inductively coupled plasma spectroscopy (ICP, Thermo E.IRISDuo) was used to analyze the Mn contents. The crystalline structure of the samples was detected by X-ray diffraction (XRD) measurements, which were conducted on a Bruker D8 Advance X-ray diffractometer. In addition, the powder morphology was observed using a transmission electron
Fig. 1. (a) Zeta potential analysis of TiO2(B) and LiMn2O4; (b) schematic diagrams of the coating process of the TiO2(B) coated LiMn2O4 by electrostatic attraction forces.
The electrochemical performance of the prepared LiMn2O4 was investigated using coin cells assembled in an argon-filled glove box (SIMATIC OP7, MBRAUN). The cell was composed of a lithium anode and a cathode that was a mixture of the prepared LiMn2O4 (70%), Super P Carbon black (20%) and polytetrafluroethylene (PTFE) (Dupont) (10%). The mixture was rolled into a thin sheet with a uniform thickness and was cut into 10 mm 10 mm sections before being pressed into an aluminum mesh. The typical loading of the active material was approximately 10 mg cm2. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 w/w), with Celgard 2300, used as a separator. Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660 B electrochemical analysis instrument, and the results were fitted by Zview 6.0 software. The charge discharge and cycling properties were evaluated on a Land CT 2001 A electrochemical measurements system between 3 and 4.5 V at various rates (1C = 148 mAh g1). 3. Results and Discussion Fig. 2(a) shows the XRD patterns of the pristine LiMn2O4, as well as the 2 wt.% TiO2(B)-coated LiMn2O4 powders sintered at 300 C for 2 h and 4 h. All of the diffraction peaks corresponded to a welldefined spinel structure with the space group Fd3 m, which was in good agreement with JCPDS card 88–1749. This finding indicated that the bulk structure of the LiMn2O4 remained unchanged after surface modification and heat treatment. No significant peaks for TiO2(B) were detected, which may be explained by the fact that the minor amount (2 wt.%) of TiO2(B) in the coated samples could not be detected due to the limited accuracy of X-ray diffraction. The enlarged diffraction peak of (111) of Fig. 2(a) is displayed in Fig. 1(b). No significant shift was detected, which suggested that the lattice parameter of LMO remained unchanged and furthermore, no diffusion of Ti4+ into the LMO lattice occurred during the coating and sintering process. To distinguish the nanostructures on the TiO2(B) coated LiMn2O4, HRTEM micrographs were collected and are displayed in Fig. 3(a) and Fig. 3(b). It can be observed from Fig. 3(a) that TiO2(B) nanoparticles with diameter of 3–5 nm were uniformly coated on the surface of the LiMn2O4, which finally formed a 5–10 nm coating. The inset showed the SAED pattern of the core and in accordance with the calculated result, the well-resolved periodic lattice fringe indicated that the spacing of the observed lattice planes was approximately 0.48 nm, which was consistent with the interplanar distance of the (111) plane of spinel LiMn2O4 (JCPDS card no. 35–0728). In addition, the spacings of the lattice planes for several coating particles were calculated from the SAED pattern and were approximately 0.357 and 0.207 nm, which can be indexed as those of the (110) and (003) planes of the TiO2(B) phase (JCPDS card no.46–1237), respectively. No transition layer of Li-Mn-Ti-O can be clearly observed which agreed well with the above XRD results. The TEM and SAED analysis demonstrated that TiO2(B) particles were successfully coated on the surface of the LiMn2O4. The coatings were not that dense and the interface between the TiO2(B) nanoparticles was composed of a special network of channels that made the sample porous. As the sintering time increased to 4 h, the thickness and the phase formation of the TiO2(B) coatings remained unchanged, as shown in Fig. 3(b). However, it was obvious that TiO2(B) particles
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
123
Fig. 2. XRD patterns of (a) the pristine LiMn2O4, 2 wt.% TiO2(B)-coated LiMn2O4 powders sinterred at 300 C for 2 h and 4 h, (b) the enlarged (111) diffraction peak of (a).
tended to aggregate and the interface between the TiO2(B) particles became more ambiguous. Heat treatment had a marked effect on the crystalline grown and interface elimination. Herein, the surface coating layer became more dense and compact after sintering for 4 h. Considering the variety in coating structure, it was naturally expected that there would be a difference in the electrochemical performance of the Li/LiMn2O4 cells. Electrochemical tests were performed to evaluate the effect of the nano-TiO2(B) coating. Fig. 4(a) showed the cycling performance of pristine LiMn2O4, 1 wt.%, 2 wt.%, and 3 wt.% TiO2(B)-coated LiMn2O4 in the voltage range of 3–4.5 V at 55 C. For the pristine LiMn2O4, the specific discharge capacity faded to 66.1 mAh/g after 300 cycles, with a capacity loss of 37.0%. In the case of the TiO2(B) coating, the discharge capacities at the 300th cycle were 72.8, 77.4 and 69.5 mAh/g with capacity losses of 31.0%, 26.3% and 33.8%, for the coating contents of 1, 2 and 3 wt.%, respectively. The 2 wt. %-2 h TiO2(B) coating was the most effective for improving the cycling performance of LiMn2O4. These results indicated that an appropriate TiO2(B) coating could facilitate the diffusion of lithium ions. As the TiO2(B) increased beyond the optimal coating content, the excess insulating TiO2(B) hindered the transportation of lithium ions, leading to the decay of the specific capacity. In addition, the cathode electrodes of the bare LiMn2O4 and the TiO2(B) coated LiMn2O4 were examined by ICP before cyclings and after 300 cycles to measure the amounts of Mn dissolution, shown in Fig. 4(b). The weight percent of Mn dissolution was calculated by the following Eq. (1), Md % ¼
M0 M 1 100% M0
(1)
in which Md% stands for the weight percent of Mn dissolution, M0 stands for the mass of manganese element in the as-prepared electrode, M1 stands for the mass of manganese element in the cycled (after 300 cycled at elevated temperature) electrode. And both M0 and M1 were measured by ICP test. It was clear that the
weight percent of Mn dissolution were reduced by coating LiMn2O4 with TiO2(B), which agreed well with the expected role of the TiO2(B) coating that inhibited the Mn dissolution. Thus, it was demonstrated that a TiO2(B) coating on the surface of LiMn2O4 was an effective way to improve the high-temperature cyclic performance by inhibiting the Mn dissolution. The effect of the coating structure on the cycle performance was investigated as a function of the calcination time, as shown in Fig. 5(a). It was apparent that both 2 wt.%-2 h and 2 wt.%-4 h TiO2(B)-coated LiMn2O4 electrodes revealed better cycle retention than the pristine sample. This result indicated that the TiO2(B) surface coating had an effect on preventing the direct contact with the electrolyte and suppressed side reactions which finally led to capacity fading. More notably, the 2 wt.%-2 h sample showed even better cycle retention than 2 wt.%-4 h. In addition, Fig. 5(b) shows the coulombic efficiency for pristine and TiO2(B) coated samples during the initial 10 cycles. The initial coulombic efficiency for pristine, 2 wt.%-2 h and 2 wt.%-4 h was 95.7%, 96.2% and 96.1%, respectively. It was clear that all these samples remained high initial coulombic efficiency, which indicated that the cathode materials have good crystallinity, few defects and proper structure reversibility [13]. A slight higher efficiency could be seen in TiO2(B) coated samples than in pristine samples, which may be lie in that TiO2(B) coating could suppress the formation of a thick SEI film, thus reducing the lithium consumption during SEI forming process. In addition, a significant coulombic efficiency drop could be seen in the 6th cycle, whereas 0.5 C was applied for cycling instead of 0.1 C. This coulombic efficiency drop was caused by the increased polarization and similar result has been reported [13]. The better cycling performance of 2 wt.%-2 h may be attributed to the structure features. We believed that after sintering for 2 h, the interface between TiO2(B) nanoparticles composed a special network of channels that could facilitate the Li+ transport. However, as the sintering time increased to 4 h, the TiO2(B) nanoparticles aggregated, accompanied by the vanishing of the
Fig. 3. HRTEM morphology of 2 wt.% TiO2(B) coated LiMn2O4 sintered for (a) 2 h and (b) 4 h. The inset represents for the SAED patterns.
124
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
Fig. 4. Cycling performance of (a) pristine LMO and 1 wt.%, 2 wt.%, and 3 wt.% TiO2(B)-coated LMO sintered for 2 h and (b) weight percent of Mn dissolution of pristine and TiO2(B) coated LMO electrodes after 300 cycles in voltage range of 3–4.5 V at 55 C.
Fig. 5. (a) Cycling performance of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h in the voltage range of 3–4.5 V at 55 C, (b) Coulombic efficiency for pristine and TiO2(B) coated samples at first 10 cycles at 55 C.
interface, thus impeding the Li+ transport. The mechanics were further proven by the rate performance discussed below. Fig. 6(a) presents the rate capability of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature. It was clear that both TiO2(B)-coated electrodes exhibited a superior rate capability than the pristine sample. Furthermore, compared with 2 wt.%-2 h and 2 wt.%-4 h, minor improvements could be observed at low rates (0.1 C, 0.2 C, 0.5 C), whereas significant enhancements were detected at high rates (2 C and 5 C). These outcomes suggested that the electrode material with porous nanoscale coatings could improve the kinetic properties of the lithium-ion during intercalation and could easily deliver a relatively high discharge capacity, even under high currents. When the current rate returned to the low rate current (C/10) after
being charged and discharged at 5 C, the capacity of the sample was retained at almost the same value of the initial discharge capacity at C/10, which indicated that the cathodes had perfect structural reversibility at room temperature. Charge and discharge curves are displayed in Fig. 6(b). Two plateaus at approximately 4.15 V and 4.00 V were observed in all of the samples, showing the charge/discharge signature of the spinel LMO [14]. In particular, the polarizing voltages between the practical and theoretical discharge plateau (4.10 V) were only 0.16 V and 0.23 V for 2 wt. %-2 h and 2 wt.%-4 h, respectively, which were significantly lower than that of the pristine LTO electrode (0.37 V). This result demonstrated that the TiO2(B) coating could effectively enhance the kinetics of LiMn2O4, which was further confirmed by the EIS results, as described in the subsequent section.
Fig. 6. (a) Rate preformance of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature, (b) Discharge curves of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature.
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
125
Fig. 7. Nyquist plots of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h (a) after the 1st cycle and (b) after the 30th cycle at 0.5 C at room temperature.
Electrochemical impedance spectroscopy (EIS) experiments were performed to investigate the impedance variation of the cell prepared with pristine and TiO2-coated samples after different cycles. As shown in Fig. 7(a), we observed an intermediatefrequency semicircle and a low-frequency tail. The intermediatefrequency semicircle reflected the charge transfer resistance (Rct), and low-frequency tail was associated with Li+ ion diffusion in the particles of the electrode material [15]. In addition, an additional high-frequency part, ascribed to the formation of a solid electrolyte interphase (SEI) film on the electrode (RSEI) upon cycling, is also observed. Based on this mechanism, the equivalent circuits for the pristine and surface-modified LMO samples are given in Fig. 7. As reported, the cell impedance mainly depends on RSEI and Rct [16,17]. Therefore, we focused on the effect of the TiO2(B) coating layer on the resistance of the surface film and charge transfer. Furthermore, the Nyquist plots were then fitted to the model of Rs (Q1RSEI)(Q2Rct)W by Zview 6.0, and the fitting results of Rct and RSEI are presented in Table 1. As listed in Table 1, it was obvious that the 2 wt.%-2 h and 2 wt. %-4 h showed lower RSEI value (24.8 and 25.1 V,respectively) than pristine LMO (33.4 V) after the first cycle. In addition, after 30 cycles, 2 wt.%-2 h and 2 wt.%-4 h showed minor changes in the RSEI while pristine LMO exhibited a significant raise in RSEI. These results indicated that TiO2(B) coating could suppress the formation of the undesired SEI film and stabilize the interface during cycling. The Rct evolution is also shown in Table 1. For all of the samples, the Rct rapidly decreased after 30 cycles, which may be associated with electrochemical activation. Furthermore, the TiO2(B) coated LMO showed lower Rct values than the pristine LMO after the 1st and 30 cycles, which revealed that the TiO2(B) coating had an effect on restraining the increase of polarization. This phenomenon could be considered as one of the possible reasons for the rate capability enhancement by the TiO2(B) coating. Furthermore, 2 wt.%-2 h had even much lower Rct (21.8 V) than 2 wt.%-4 h (41.3 V) which corresponded to the better rate capability and lower voltage drop in Fig. 6. It was believed that with special porous structure, TiO2(B) coating could facilitate the Li ion transport thus impeding the polarization. These results were consistent with related studies in which it was demonstrated that nanoparticulate metal oxide coatings applied to 5 V spinel electrodes improved both cycling performance and rate capability at high temperatures by Table 1 The impedance parameters of equipment circuits. Samples
pristine LMO 2 wt.%-2 h 2 wt.%-4 h
After 1st cycle
After 30th cycle
RSEI(V)
Rct(V)
RSEI(V)
Rct(V)
33.4 24.8 25.1
279.1 218.7 230.3
44.0 28.1 29.7
48.5 21.8 41.3
suppressing the formation of thick SEI layers and enhancing charge-transfer kinetics [18,19]. 4. Conclusion Nano-TiO2(B) was successfully coated on commercial LiMn2O4 via a two-step method including a combined hydrothermal method and electrostatic attraction. TEM morphology showed that after 2 h of sintering, the coating was porous but then became dense and compact as the sintering time increased to 4 h. Electrochemical test results showed that with the porous 2 wt.% TiO2(B) coating, the material exhibited exhibited the best hightemperature performance in terms of cycle and rate capacity. Furthermore, the effect of the porous TiO2(B) coating was believed to be as follows: 1. the TiO2(B) coating suppresses the excessive growth of the SEI film and reduce the Mn dissolution and 2. the porous structure could facilitate the Li ion transport and reduce the polarization. This work provided us with a facile and novel solution to the problem of combining cycling performance at elevated temperatures and rate performance. Acknowledgements The authors acknowledge funding supports from 973 Program (2013CB934103) and Science & Technology Commission of Shanghai Municipality (12dz1200402 & 08DZ2270500), China. References [1] L. Feng, Y. Chang, L. Wu, T. Lu, Electrochemical behaviour of spine1 LiMn2O4 as positive electrode in rechargeable lithium cells, J. Power Sources 63 (1996) 149. [2] T. Tsumura, A. Shimizu, M. Inagaki, Lithium extraction/insertion from LiMn2O4-effect of crystallinity, Solid State Ionics 90 (1996) 197. [3] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359. [4] J.M. Tarascon, D. Guyomard, The Li1+xMn2O4/C rocking-chair system: a review, Electrochim. Acta 38 (1993) 1221. [5] D. Kim, S. Park, O.B. Chae, J.H.Y.-U. Ryu Kim, R.-Z. Yin, S.M. Oh, Re-Deposition of Manganese Species on Spinel LiMn2O4 Electrode after Mn Dissolution, J. Electrochem. Soc. 159 (2012) A193. [6] Y. Liu, X. Li, H. Guo, Z. Wang, Q. Hu, W. Peng, Y. Yang, Electrochemical performance and capacity fading reason of LiMn2O4/graphite batteries stored at room temperature, J. Power Sources 189 (2009) 721. [7] L. Yu, X. Qiu, J. Xi, W. Zhu, L. Chen, Enhanced hen, Enhanced hhigh-potential and elevated-temperature cycling stability of LiMn2O4 cathode by TiO2 modification for Li-ion battery, Electrochim. Acta 51 (2006) 6406. [8] A.R. Armstrong, G. Armstrong, J. Canales, P.G. Bruce, TiO2-B Nanowires, Angew. Chem. Int. Ed. 43 (2004) 2286. [9] C. Qing, Y. Bai, J. Yang, W. Zhang, Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating, Electrochim. Acta 56 (2011) 6612. [10] J. Zhao, Y. Wang, Atomic layer deposition of epitaxial ZrO2 coating on LiMn2O4 nanoparticles for high-rate lithium ion batteries at elevated temperature, Nano Energy 2 (2013) 882. [11] S. Zhao, Y.Q. Bai Chang, Y. Yang, W. Zhang, Surface modification of spinel LiMn2O4 with FeF3 for lithium ion batteries, Electrochim. Acta 108 (2013) 727.
126
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
[12] Y. Ren, L. Zheng, F. Pourpoint, A.R. Armstrong, C.P. Grey, P.G. Bruce, Nanoparticulate TiO2(B): An Anode for Lithium-Ion Batteries, Angew. Chem. Int. Ed. 51 (2012) 2164. [13] X. Zhang, H. Zheng, V. Battaglia, R.L. Axelbaum, Electrochemical performance of spinel LiMn2O4 cathode materials made by flame-assisted spray technology, J. Power Sources 196 (2011) 3640. [14] J. Luo, L. Cheng, Y. Xia, LiMn2O4 hollow nanosphere electrode material with excellent cycling reversibility and rate capability, Electrochem. Commun. 9 (2007) 1404. [15] H. Manjunathaa, K.C. Mahesha, G.S. Suresha, T.V. Venkateshab, The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy, Electrochim. Acta 56 (2011) 1439.
[16] Chen, J. Liu, K. Amine, Symmetric cell approach and impedance spectroscopy of high power lithium-ion batteries, J. Power Sources 96 (2001) 321. [17] J. Liu, A. Manthiram, Understanding the Improvement in the Electrochemical Properties of Surface Modified 5 V LiMn1. 42Ni0. 42Co0. 16O4 Spinel Cathodes in Lithium-ion Cells, Chemistry of Materials 21 (2009) 1695. [18] J. Liu, A. Manthiram, Improved Electrochemical Performance of the 5 V Spinel Cathode LiMn15Ni0.42Zn0.08O4 by Surface Modification, J. Electrochem. Soc 156 (2009) A66. [19] J. Liu, A. Manthiram, Effects of Surface-Film Formation on the Electrochemical Characteristics of LiMn2O4Cathodes of Lithium Ion Batteries, J. Electrochem. Soc 156 (2009) A709.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nano-TiO2(B) coated LiMn2O4 as cathode materials for lithium-ion batteries at elevated temperatures Yesheng Shang, Xiujing Lin, Xu Lu, Tao Huang, Aishui Yu * Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 November 2014 Received in revised form 6 January 2015 Accepted 6 January 2015 Available online 7 January 2015
In this study, nano-TiO2(B) coated LiMn2O4 was prepared via a two-step method, combining a hydrothermal method with electrostatic attraction. By adjusting the sintering time, porous and dense nano-TiO2(B) coating structure were formed on the surface of LiMn2O4 particles. Electrochemical test results showed that 2 wt.% porous TiO2(B) coated LiMn2O4 exhibited highest capacity retention at 77.4% after 300 cycles at 55 C and the best rate capability. Inductively coupled plasma spectroscopy(ICP), charge and discharge curves and electrochemical impedance spectroscopy (EIS) results revealed that the improved electrochemical performances was due to the suppression of the undesired SEI film, as well as suppression of Mn dissolution at the cathode and reduced polarization. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: spinel lithium manganese oxide lithium ion batteries surface coating
1. Introduction Spinel lithium manganese oxide (LiMn2O4, LMO hereafter) is regarded as a promising cathode material for lithium-ion battery devices [1–3]. However, LMO suffers from the drawback of poor cycling stability, which is mainly associated with manganese dissolution during battery operation, especially at elevated temperatures [4–6]. To overcome these problems, many researchers have attempted to improve the high temperature cyclic performance by forming a coating layer that would inhibit the Mn dissolution of LMO. In the aspect of surface coating, there are two basic points. Firstly, the coatings should be thermally and chemically stable during cycling. Secondly, surface coatings should not suppress the Li+ transportation, significantly. Among the candidates for coatings, titanium dioxide (TiO2) attracts the most attention due to its thermal stability, structural stability and fast kinetics. Lihong Yu et al. [7] have prepared TiO2-modified spinel LiMn2O4 by a simple sol–gel method to improve its electrochemical performance at elevated temperatures. Compared with the bulk TiO2 polymorph material, nano-TiO2(B) shows more open channels in the lattice, resulting in easier Li-ion access to the crystal structure [8], which make it a promising coating material for LMO. To the best of our
* Corresponding author. Tel.: +86 21 51630320; fax: +86 21 51630320. E-mail address: [email protected] (A. Yu). http://dx.doi.org/10.1016/j.electacta.2015.01.024 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
knowledge, TiO2(B) has never been applied as a surface coating of LMO yet. In addition to the candidate selection, the design of a coating structure is also important to Li+ transportation. It is believed that thick coatings may hinder the transportation of lithium ions, leading to the evident decay of the specific capacity [9]. As a result, most concerns were focused on the tuning of the amount or the thickness of coating layers [10,11]. However, limited information has been gathered thus far about the surface morphology of coated LiMn2O4 electrodes or their functions, although the surface morphology can be influenced by the synthesis and sintering processes. In this paper, we offer a novel two-step method for the preparation of a TiO2(B)-coated LiMn2O4 cathode material. By adjusting the heat treatment time, we successfully prepared LMO with two different coating structures .The electrochemical behavior is characterized, and the relationship between the coating structure and electrochemical performance is discussed in details. 2. Experimental Commercial Li1.06Al0.05Mg0.025Mn0.925O4 (Jiang Su Guo Tai Libode New Material Co., LTD) with an average particle size of 0.5 mm was used as the pristine material. Inductively coupled plasma spectroscopy (ICP) was applied to determine the chemical formula of the commercial material, and the result showed an exact formula of Li1.03Al0.037Mg0.023Mn0.938O4, which was still
122
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
regarded as Li1.06Al0.05Mg0.025Mn0.925O4 and was written as LiMn2O4 or LMO for short hereafter.
microscope (TEM, CM200FEG), while the phase formation and crystallinity were tested by the selected area electron diffraction (SAED).
2.1. Synthesis of TiO2(B) nanoparticles 2.3. Electrochemical measurements TiO2(B) nanoparticles were prepared using a hydrothermal method modified from a previously reported procedure [12]. In a typical synthesis, 0.36 g of Ti (99.7%, 200–300 mesh) was dissolved in an aqueous solution of H2O2 (30 mL, 30.0%) and NH4OH (10 mL, 35%) in an ice-water bath for 2 h with stirring. Then, 0.85 g of glycolic acid (99%,) was dissolved in the solution, which was maintained at 80 C for 2 h to eliminate excess H2O2 and NH3. The solution changed to a yellow gel. The gel was dissolved in ca. 60 mL of water to form a yellowish transparent solution followed by the addition of 1.0 g of H2SO4 (98.0%). The resulting solution was sealed in a Teflon-lined stainless steel autoclave and heated at 160 C for 30 min. The final powder was filtered, washed several times with distilled water and finally dispersed in 100 mL of water. 2.2. Synthesis of TiO2(B)-coated LiMn2O4 cathode material In addition, 10 mL, 20 mL and 30 mL of the above TiO2(B) suspension for 1 wt.%, 2 wt.% and 3 wt.% TiO2(B) coatings were diluted to 100 ml, respectively. As shown in Fig. 1(a), TiO2(B) and LiMn2O4 exhibited a maximum zeta potential difference when the pH was approximately 7. Thus, the pH value was adjusted to 7 by adding NH4OH(1 mL, 35%). The mixture was ultrasonically agitated for 30 min followed by mechanical stirring for 60 min, while 0.5 g of commercial LiMn2O4 was slowly added. During this process, the surface charges of LiMn2O4 and TiO2(B) were opposite in sign and thus, a junction between them was formed by the electrostatic attraction, as shown in Fig. 1b. The final powder was filtered and washed several times with distilled water, dried at 80 C for 2 h, and subsequently heat-treated in a furnace at 300 C for 2 h and 4 h in air, respectively. For convenience, the samples were marked with X (wt%)–T (h) in which X represented the coating amount and T represented the annealed time. Inductively coupled plasma spectroscopy (ICP, Thermo E.IRISDuo) was used to analyze the Mn contents. The crystalline structure of the samples was detected by X-ray diffraction (XRD) measurements, which were conducted on a Bruker D8 Advance X-ray diffractometer. In addition, the powder morphology was observed using a transmission electron
Fig. 1. (a) Zeta potential analysis of TiO2(B) and LiMn2O4; (b) schematic diagrams of the coating process of the TiO2(B) coated LiMn2O4 by electrostatic attraction forces.
The electrochemical performance of the prepared LiMn2O4 was investigated using coin cells assembled in an argon-filled glove box (SIMATIC OP7, MBRAUN). The cell was composed of a lithium anode and a cathode that was a mixture of the prepared LiMn2O4 (70%), Super P Carbon black (20%) and polytetrafluroethylene (PTFE) (Dupont) (10%). The mixture was rolled into a thin sheet with a uniform thickness and was cut into 10 mm 10 mm sections before being pressed into an aluminum mesh. The typical loading of the active material was approximately 10 mg cm2. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 w/w), with Celgard 2300, used as a separator. Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660 B electrochemical analysis instrument, and the results were fitted by Zview 6.0 software. The charge discharge and cycling properties were evaluated on a Land CT 2001 A electrochemical measurements system between 3 and 4.5 V at various rates (1C = 148 mAh g1). 3. Results and Discussion Fig. 2(a) shows the XRD patterns of the pristine LiMn2O4, as well as the 2 wt.% TiO2(B)-coated LiMn2O4 powders sintered at 300 C for 2 h and 4 h. All of the diffraction peaks corresponded to a welldefined spinel structure with the space group Fd3 m, which was in good agreement with JCPDS card 88–1749. This finding indicated that the bulk structure of the LiMn2O4 remained unchanged after surface modification and heat treatment. No significant peaks for TiO2(B) were detected, which may be explained by the fact that the minor amount (2 wt.%) of TiO2(B) in the coated samples could not be detected due to the limited accuracy of X-ray diffraction. The enlarged diffraction peak of (111) of Fig. 2(a) is displayed in Fig. 1(b). No significant shift was detected, which suggested that the lattice parameter of LMO remained unchanged and furthermore, no diffusion of Ti4+ into the LMO lattice occurred during the coating and sintering process. To distinguish the nanostructures on the TiO2(B) coated LiMn2O4, HRTEM micrographs were collected and are displayed in Fig. 3(a) and Fig. 3(b). It can be observed from Fig. 3(a) that TiO2(B) nanoparticles with diameter of 3–5 nm were uniformly coated on the surface of the LiMn2O4, which finally formed a 5–10 nm coating. The inset showed the SAED pattern of the core and in accordance with the calculated result, the well-resolved periodic lattice fringe indicated that the spacing of the observed lattice planes was approximately 0.48 nm, which was consistent with the interplanar distance of the (111) plane of spinel LiMn2O4 (JCPDS card no. 35–0728). In addition, the spacings of the lattice planes for several coating particles were calculated from the SAED pattern and were approximately 0.357 and 0.207 nm, which can be indexed as those of the (110) and (003) planes of the TiO2(B) phase (JCPDS card no.46–1237), respectively. No transition layer of Li-Mn-Ti-O can be clearly observed which agreed well with the above XRD results. The TEM and SAED analysis demonstrated that TiO2(B) particles were successfully coated on the surface of the LiMn2O4. The coatings were not that dense and the interface between the TiO2(B) nanoparticles was composed of a special network of channels that made the sample porous. As the sintering time increased to 4 h, the thickness and the phase formation of the TiO2(B) coatings remained unchanged, as shown in Fig. 3(b). However, it was obvious that TiO2(B) particles
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
123
Fig. 2. XRD patterns of (a) the pristine LiMn2O4, 2 wt.% TiO2(B)-coated LiMn2O4 powders sinterred at 300 C for 2 h and 4 h, (b) the enlarged (111) diffraction peak of (a).
tended to aggregate and the interface between the TiO2(B) particles became more ambiguous. Heat treatment had a marked effect on the crystalline grown and interface elimination. Herein, the surface coating layer became more dense and compact after sintering for 4 h. Considering the variety in coating structure, it was naturally expected that there would be a difference in the electrochemical performance of the Li/LiMn2O4 cells. Electrochemical tests were performed to evaluate the effect of the nano-TiO2(B) coating. Fig. 4(a) showed the cycling performance of pristine LiMn2O4, 1 wt.%, 2 wt.%, and 3 wt.% TiO2(B)-coated LiMn2O4 in the voltage range of 3–4.5 V at 55 C. For the pristine LiMn2O4, the specific discharge capacity faded to 66.1 mAh/g after 300 cycles, with a capacity loss of 37.0%. In the case of the TiO2(B) coating, the discharge capacities at the 300th cycle were 72.8, 77.4 and 69.5 mAh/g with capacity losses of 31.0%, 26.3% and 33.8%, for the coating contents of 1, 2 and 3 wt.%, respectively. The 2 wt. %-2 h TiO2(B) coating was the most effective for improving the cycling performance of LiMn2O4. These results indicated that an appropriate TiO2(B) coating could facilitate the diffusion of lithium ions. As the TiO2(B) increased beyond the optimal coating content, the excess insulating TiO2(B) hindered the transportation of lithium ions, leading to the decay of the specific capacity. In addition, the cathode electrodes of the bare LiMn2O4 and the TiO2(B) coated LiMn2O4 were examined by ICP before cyclings and after 300 cycles to measure the amounts of Mn dissolution, shown in Fig. 4(b). The weight percent of Mn dissolution was calculated by the following Eq. (1), Md % ¼
M0 M 1 100% M0
(1)
in which Md% stands for the weight percent of Mn dissolution, M0 stands for the mass of manganese element in the as-prepared electrode, M1 stands for the mass of manganese element in the cycled (after 300 cycled at elevated temperature) electrode. And both M0 and M1 were measured by ICP test. It was clear that the
weight percent of Mn dissolution were reduced by coating LiMn2O4 with TiO2(B), which agreed well with the expected role of the TiO2(B) coating that inhibited the Mn dissolution. Thus, it was demonstrated that a TiO2(B) coating on the surface of LiMn2O4 was an effective way to improve the high-temperature cyclic performance by inhibiting the Mn dissolution. The effect of the coating structure on the cycle performance was investigated as a function of the calcination time, as shown in Fig. 5(a). It was apparent that both 2 wt.%-2 h and 2 wt.%-4 h TiO2(B)-coated LiMn2O4 electrodes revealed better cycle retention than the pristine sample. This result indicated that the TiO2(B) surface coating had an effect on preventing the direct contact with the electrolyte and suppressed side reactions which finally led to capacity fading. More notably, the 2 wt.%-2 h sample showed even better cycle retention than 2 wt.%-4 h. In addition, Fig. 5(b) shows the coulombic efficiency for pristine and TiO2(B) coated samples during the initial 10 cycles. The initial coulombic efficiency for pristine, 2 wt.%-2 h and 2 wt.%-4 h was 95.7%, 96.2% and 96.1%, respectively. It was clear that all these samples remained high initial coulombic efficiency, which indicated that the cathode materials have good crystallinity, few defects and proper structure reversibility [13]. A slight higher efficiency could be seen in TiO2(B) coated samples than in pristine samples, which may be lie in that TiO2(B) coating could suppress the formation of a thick SEI film, thus reducing the lithium consumption during SEI forming process. In addition, a significant coulombic efficiency drop could be seen in the 6th cycle, whereas 0.5 C was applied for cycling instead of 0.1 C. This coulombic efficiency drop was caused by the increased polarization and similar result has been reported [13]. The better cycling performance of 2 wt.%-2 h may be attributed to the structure features. We believed that after sintering for 2 h, the interface between TiO2(B) nanoparticles composed a special network of channels that could facilitate the Li+ transport. However, as the sintering time increased to 4 h, the TiO2(B) nanoparticles aggregated, accompanied by the vanishing of the
Fig. 3. HRTEM morphology of 2 wt.% TiO2(B) coated LiMn2O4 sintered for (a) 2 h and (b) 4 h. The inset represents for the SAED patterns.
124
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
Fig. 4. Cycling performance of (a) pristine LMO and 1 wt.%, 2 wt.%, and 3 wt.% TiO2(B)-coated LMO sintered for 2 h and (b) weight percent of Mn dissolution of pristine and TiO2(B) coated LMO electrodes after 300 cycles in voltage range of 3–4.5 V at 55 C.
Fig. 5. (a) Cycling performance of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h in the voltage range of 3–4.5 V at 55 C, (b) Coulombic efficiency for pristine and TiO2(B) coated samples at first 10 cycles at 55 C.
interface, thus impeding the Li+ transport. The mechanics were further proven by the rate performance discussed below. Fig. 6(a) presents the rate capability of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature. It was clear that both TiO2(B)-coated electrodes exhibited a superior rate capability than the pristine sample. Furthermore, compared with 2 wt.%-2 h and 2 wt.%-4 h, minor improvements could be observed at low rates (0.1 C, 0.2 C, 0.5 C), whereas significant enhancements were detected at high rates (2 C and 5 C). These outcomes suggested that the electrode material with porous nanoscale coatings could improve the kinetic properties of the lithium-ion during intercalation and could easily deliver a relatively high discharge capacity, even under high currents. When the current rate returned to the low rate current (C/10) after
being charged and discharged at 5 C, the capacity of the sample was retained at almost the same value of the initial discharge capacity at C/10, which indicated that the cathodes had perfect structural reversibility at room temperature. Charge and discharge curves are displayed in Fig. 6(b). Two plateaus at approximately 4.15 V and 4.00 V were observed in all of the samples, showing the charge/discharge signature of the spinel LMO [14]. In particular, the polarizing voltages between the practical and theoretical discharge plateau (4.10 V) were only 0.16 V and 0.23 V for 2 wt. %-2 h and 2 wt.%-4 h, respectively, which were significantly lower than that of the pristine LTO electrode (0.37 V). This result demonstrated that the TiO2(B) coating could effectively enhance the kinetics of LiMn2O4, which was further confirmed by the EIS results, as described in the subsequent section.
Fig. 6. (a) Rate preformance of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature, (b) Discharge curves of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h at room temperature.
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
125
Fig. 7. Nyquist plots of pristine LMO and 2 wt.% TiO2(B)-coated LMO sintered for 2 h and 4 h (a) after the 1st cycle and (b) after the 30th cycle at 0.5 C at room temperature.
Electrochemical impedance spectroscopy (EIS) experiments were performed to investigate the impedance variation of the cell prepared with pristine and TiO2-coated samples after different cycles. As shown in Fig. 7(a), we observed an intermediatefrequency semicircle and a low-frequency tail. The intermediatefrequency semicircle reflected the charge transfer resistance (Rct), and low-frequency tail was associated with Li+ ion diffusion in the particles of the electrode material [15]. In addition, an additional high-frequency part, ascribed to the formation of a solid electrolyte interphase (SEI) film on the electrode (RSEI) upon cycling, is also observed. Based on this mechanism, the equivalent circuits for the pristine and surface-modified LMO samples are given in Fig. 7. As reported, the cell impedance mainly depends on RSEI and Rct [16,17]. Therefore, we focused on the effect of the TiO2(B) coating layer on the resistance of the surface film and charge transfer. Furthermore, the Nyquist plots were then fitted to the model of Rs (Q1RSEI)(Q2Rct)W by Zview 6.0, and the fitting results of Rct and RSEI are presented in Table 1. As listed in Table 1, it was obvious that the 2 wt.%-2 h and 2 wt. %-4 h showed lower RSEI value (24.8 and 25.1 V,respectively) than pristine LMO (33.4 V) after the first cycle. In addition, after 30 cycles, 2 wt.%-2 h and 2 wt.%-4 h showed minor changes in the RSEI while pristine LMO exhibited a significant raise in RSEI. These results indicated that TiO2(B) coating could suppress the formation of the undesired SEI film and stabilize the interface during cycling. The Rct evolution is also shown in Table 1. For all of the samples, the Rct rapidly decreased after 30 cycles, which may be associated with electrochemical activation. Furthermore, the TiO2(B) coated LMO showed lower Rct values than the pristine LMO after the 1st and 30 cycles, which revealed that the TiO2(B) coating had an effect on restraining the increase of polarization. This phenomenon could be considered as one of the possible reasons for the rate capability enhancement by the TiO2(B) coating. Furthermore, 2 wt.%-2 h had even much lower Rct (21.8 V) than 2 wt.%-4 h (41.3 V) which corresponded to the better rate capability and lower voltage drop in Fig. 6. It was believed that with special porous structure, TiO2(B) coating could facilitate the Li ion transport thus impeding the polarization. These results were consistent with related studies in which it was demonstrated that nanoparticulate metal oxide coatings applied to 5 V spinel electrodes improved both cycling performance and rate capability at high temperatures by Table 1 The impedance parameters of equipment circuits. Samples
pristine LMO 2 wt.%-2 h 2 wt.%-4 h
After 1st cycle
After 30th cycle
RSEI(V)
Rct(V)
RSEI(V)
Rct(V)
33.4 24.8 25.1
279.1 218.7 230.3
44.0 28.1 29.7
48.5 21.8 41.3
suppressing the formation of thick SEI layers and enhancing charge-transfer kinetics [18,19]. 4. Conclusion Nano-TiO2(B) was successfully coated on commercial LiMn2O4 via a two-step method including a combined hydrothermal method and electrostatic attraction. TEM morphology showed that after 2 h of sintering, the coating was porous but then became dense and compact as the sintering time increased to 4 h. Electrochemical test results showed that with the porous 2 wt.% TiO2(B) coating, the material exhibited exhibited the best hightemperature performance in terms of cycle and rate capacity. Furthermore, the effect of the porous TiO2(B) coating was believed to be as follows: 1. the TiO2(B) coating suppresses the excessive growth of the SEI film and reduce the Mn dissolution and 2. the porous structure could facilitate the Li ion transport and reduce the polarization. This work provided us with a facile and novel solution to the problem of combining cycling performance at elevated temperatures and rate performance. Acknowledgements The authors acknowledge funding supports from 973 Program (2013CB934103) and Science & Technology Commission of Shanghai Municipality (12dz1200402 & 08DZ2270500), China. References [1] L. Feng, Y. Chang, L. Wu, T. Lu, Electrochemical behaviour of spine1 LiMn2O4 as positive electrode in rechargeable lithium cells, J. Power Sources 63 (1996) 149. [2] T. Tsumura, A. Shimizu, M. Inagaki, Lithium extraction/insertion from LiMn2O4-effect of crystallinity, Solid State Ionics 90 (1996) 197. [3] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359. [4] J.M. Tarascon, D. Guyomard, The Li1+xMn2O4/C rocking-chair system: a review, Electrochim. Acta 38 (1993) 1221. [5] D. Kim, S. Park, O.B. Chae, J.H.Y.-U. Ryu Kim, R.-Z. Yin, S.M. Oh, Re-Deposition of Manganese Species on Spinel LiMn2O4 Electrode after Mn Dissolution, J. Electrochem. Soc. 159 (2012) A193. [6] Y. Liu, X. Li, H. Guo, Z. Wang, Q. Hu, W. Peng, Y. Yang, Electrochemical performance and capacity fading reason of LiMn2O4/graphite batteries stored at room temperature, J. Power Sources 189 (2009) 721. [7] L. Yu, X. Qiu, J. Xi, W. Zhu, L. Chen, Enhanced hen, Enhanced hhigh-potential and elevated-temperature cycling stability of LiMn2O4 cathode by TiO2 modification for Li-ion battery, Electrochim. Acta 51 (2006) 6406. [8] A.R. Armstrong, G. Armstrong, J. Canales, P.G. Bruce, TiO2-B Nanowires, Angew. Chem. Int. Ed. 43 (2004) 2286. [9] C. Qing, Y. Bai, J. Yang, W. Zhang, Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating, Electrochim. Acta 56 (2011) 6612. [10] J. Zhao, Y. Wang, Atomic layer deposition of epitaxial ZrO2 coating on LiMn2O4 nanoparticles for high-rate lithium ion batteries at elevated temperature, Nano Energy 2 (2013) 882. [11] S. Zhao, Y.Q. Bai Chang, Y. Yang, W. Zhang, Surface modification of spinel LiMn2O4 with FeF3 for lithium ion batteries, Electrochim. Acta 108 (2013) 727.
126
Y. Shang et al. / Electrochimica Acta 156 (2015) 121–126
[12] Y. Ren, L. Zheng, F. Pourpoint, A.R. Armstrong, C.P. Grey, P.G. Bruce, Nanoparticulate TiO2(B): An Anode for Lithium-Ion Batteries, Angew. Chem. Int. Ed. 51 (2012) 2164. [13] X. Zhang, H. Zheng, V. Battaglia, R.L. Axelbaum, Electrochemical performance of spinel LiMn2O4 cathode materials made by flame-assisted spray technology, J. Power Sources 196 (2011) 3640. [14] J. Luo, L. Cheng, Y. Xia, LiMn2O4 hollow nanosphere electrode material with excellent cycling reversibility and rate capability, Electrochem. Commun. 9 (2007) 1404. [15] H. Manjunathaa, K.C. Mahesha, G.S. Suresha, T.V. Venkateshab, The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy, Electrochim. Acta 56 (2011) 1439.
[16] Chen, J. Liu, K. Amine, Symmetric cell approach and impedance spectroscopy of high power lithium-ion batteries, J. Power Sources 96 (2001) 321. [17] J. Liu, A. Manthiram, Understanding the Improvement in the Electrochemical Properties of Surface Modified 5 V LiMn1. 42Ni0. 42Co0. 16O4 Spinel Cathodes in Lithium-ion Cells, Chemistry of Materials 21 (2009) 1695. [18] J. Liu, A. Manthiram, Improved Electrochemical Performance of the 5 V Spinel Cathode LiMn15Ni0.42Zn0.08O4 by Surface Modification, J. Electrochem. Soc 156 (2009) A66. [19] J. Liu, A. Manthiram, Effects of Surface-Film Formation on the Electrochemical Characteristics of LiMn2O4Cathodes of Lithium Ion Batteries, J. Electrochem. Soc 156 (2009) A709.