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The fluorine gradient-doped LiNi0.5Mn1.5O4 spinels are synthesized by a facile one-step method and the effect of heat treatment on their structure, morphology, and electrochemical performance are investigated. The results show that introduction of fluorine leads to a larger lattice parameter and particle size, and the formation of F-enriched surface. Whereas at 400 °C, the fluorine gradient-doped LiNi0.5Mn1.5O4 sample exhibits an improved long-term cycling stability and high rate performance, due to the suppression of the reaction between electrolyte and cathode, resulting in a decrease in the total resistance and the formation of a thin, uniform and smooth film on the surface. As a result of in situ XRD with charged pristine and the fluorine gradient-doped samples, the similar thermal-decomposition pathways from the charged spinel to the final NiMn2O4-type spinel structure with a small amount of NiMnO3 and α-Mn2O3 are observed. In addition, the disappearance temperature of the charged spinel structures is at about 280 °C for the fluorine gradient-doped sample, exhibiting an improved thermal stability of high voltage cathode material. These results show that fluorine gradient-doped LiNi0.5Mn1.5O4 sample is a promising positive electrode material for high performance lithium ion batteries.

A commercial Li4Ti5O12 material was modified by NH4F using a facile and dry method at a low temperature in air. X-ray diffraction reveals that the fluorination did not change the bulk structure of Li4Ti5O12. X-ray photoelectron spectroscopy demonstrates that LiF was formed at the surface and Ti4+ was partially changed into Ti3+. Microscopic images show that some nanoislands were formed on the surface, which enlarged the surface area. Consequently, the NH4F-modified Li4Ti5O12 material exhibited significantly enhanced capacities and rate capabilities, even at low temperatures. The discharge capacity was increased from 149 to 167 mA h g-1 at 1 C, and the capacity retention was increased from 17.8 to 52.0% at 15 C. The capacity retention of NH4F-modified Li4Ti5O12 was greater than that of Li4Ti5O12 at each low-temperature point. Additionally, the introduction of F can protect the Li4Ti5O12 material from side reactions with the electrolyte and the atmosphere, enhancing the surface stability and reducing the release of gaseous products. It is believed that the NH4F-modified Li4Ti5O12 with enhanced electrochemical performance is a promising anode material for lithium ion batteries. Furthermore, this facile surface fluorination strategy is amenable to large-scale production.

A series of Al-doped Li1+xNi0.5AlxMn1.5-xO4 cathode materials for lithium ion batteries are successfully synthesized via a co-precipitation method. Raman spectroscopy studies reveal that the cation-disordered tends to increase with the content of Al. The data of X-ray photoelectron spectroscopy (XPS) depth profiles reveal that the inert Al3+ ions segregates preferentially to the surface during the synthesis process. The results of the electrochemical tests suggest that the substitution of a small amount of Al has the ability to improve the rate capability of LiNi0.5Mn1.5O4 spinel with conventional electrolytes. Especially, the Al-doped (x = 0.06) sample delivers a long cycle-life at high rate (20 C). The enhanced performance is attributed to the formation of Al-enriched surface, providing a more stable interface with the electrolyte at high voltage (∼4.7 V), along with the stabilization of the spinel structure with a disordering of the cations and improved Li-ion diffusion, based on results of the potentiostatic intermittent titration technique.

A commercial Li

Rechargeable lithium-ion batteries (LIB) with high specific energy and high specific power, long life, low self-discharge, and wide work-temperature range, have been considered to be one of the most promising energy storage system. In daily use lithium-ion batteries are often exposed to great stress like high ambient temperature and high storage voltages, so calendar aging is permanently present, arising independently from other stress like cycling. This may influence electrochemical performance, kinetics and safety of the battery. Overcharge performance is an important feature of a battery. Most of these studies are emphasizing the importance of the overcharge performances of the battery materials. In present study, the heat-release and gas evolution during overcharge process for calendar aged 10 Ah LTO pouch batteries are investigated by several analytical methods, with the positive of a mixture of LiNi1/3Mn1/3Co1/3O2 and LiCoO2 ( m : m = 7:3) and the electrolyte of 1.2 M LiPF6 in 3:7 EC/EMC). Ten identical cells are stored at 55℃ and a voltage of 2.75 V (SOC100%). The batteries are stored at open circuit potential and the storage is over up to 5 months. The trace gas is observed when the storage time is arrived to 3 months. Seriously, the amount of evolution gas is large when up to 5 month. The evolved gase consists of carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4), propylene (C3H6), and Propane (C3H8), as shown in Fig. 1. Especially, the amount of CO2 gas is much higher than the other gas. The carbon dioxide (CO2) is mainly from the decomposition of EC [1]. Meanwhile, the residual capacity of the batteries is gradually decreased to 86% after stored with 5 months, accompanied by the increasing of the internal resistance. The LTO pouch batteries with the pristine, aging for 3 months, 4 months and 5months are test by overcharge under adiabatic condition. The batteries are charged with 1C from SOC 100% to a voltage of 4 V. All the LTO pouch batteries show no thermal runaway in the overcharge process. With the decrease of the residual capacity, the raising range of the temperature and average heat power of the battery in the overcharge process decreases, suggesting the overcharge property of LTO batteries aged for 5 months is improved, as shown in Fig.2. The internal gas of LTO pouch batteries with pristine and aged for 5 months after overcharged are collected by a micro-syringe, and analyzed by gas chromatography - mass spectroscopy, as shown in Fig. 1. The amount of methane (CH4) and ethane (C2H6) is obviously increasing after overcharge, which may be from the decomposition of EMC [2]. This work was supported by National 863 Program (No. 2013AA050902) and Shanghai technological innovation (No.14JC1491800). References [1] I. Belharouak, G.M. Koenig, Jr., T. Tan, H. Yumoto, N. Ota, K. Amine, J. Electrochem. Soc., 159 (8) A1165-A1170 (2012). [2] Y. Takei, K. Takeno, H. Morimoto, S. Tobishima, J. Power Sources 228 (2013) 32-38. Figure 1

Safety has been the primary issue in the use of the lithium ion technology in high energy and high power applications ranging from power tools to electric vehicles. The safety of lithium ion batteries is mainly derived from the thermal effects of the battery during normal and abnormal operation. Overheating may start exothermal reactions that release even more heat which in turn can lead to an accelerated process called a thermal runaway. In addition to the lithium ion batteries of the body factors (electrode materials [1], membrane [2] and electrolyte [3] and other internal materials) as well as the manufacturing process [4], the battery aging [5] process is also critical to the safety of the lithium ion batteries. An accelerating rate calorimeter is employed to study the heat release of 5 Ah lithium iron phosphate battery with different aging status under adiabatic condition. We design four aging methods, consisting of cycled at 1 C and 0.5 C, stored at 55 ℃ with SOC 100% and SOC 50%. Thermal behavior of lithium iron phosphate battery charged and discharged at 1C is investigated, and the influence of cycle number, cycle rate, storage time and storage temperature on the overcharge test of lithium iron phosphate battery was studied. The results show that the lithium iron phosphate batteries have no accidents such as leakage, explosion, burning etc, after 600 cycles at 1C or storage for 4 months at high temperature 55 ℃. With the increase of the cycle numbers, the average heat power of the battery charge-discharge and overcharge increases. At the same cycle numbers, the raising range of the temperature and average heat power of the battery cycled at 1 C is decreased, compared with that of battery cycled at 0.5 C. With the increase of the storage time at the high temperature, the average power of the battery charge-discharge and overcharge increase as well. Furthermore, The raising range of the temperature and average heat power of the lithium iron phosphate battery with 100% SOC full power of the battery is smaller than that of battery with 50% SOC. This work was supported by National 863 Program (No. 2013AA050902) and Shanghai Science and Technology Engineering Research Center Program (No. 13DZ2280200). Reference [1] Jiang J, Dahn J R. Electrochemistry Communications, 2004, 6: 39~43. [2] Chen J J, Wang S Q, Dandan Cai, Wang H H. Journal of Membrane Science, 2014, 449, 169-175. [3] Tyunina, Yu E, Chekunova, Marina D. Journal of Molecular Liquids, 2013.11, 187: 332~336. [4] Kise M, Yoshioka S,Hamand I.Electrochemistry, 2004,72(9)：641-646. [5] Sarre G, Blanchard P, Broussely M. Journal of Power Sources, 2004, 127(1): 65-71.

The high power and energy density of lithium ion batteries have made them one of the most attractive energy storage systems. Spinel type LiMn2O4 is considered for a promising material of Lithium ion batteries due to good safety, low cost, abundance, nontoxicity, and high-rate capability. However, poor cycle stability during repeated cycling occurs at room temperature and is more severe at high temperature, originating from the interplay of Jahn-Teller distortion of the lattice [1] and manganese dissolution [2], and retarding its implementation in commercial products. Lithium-rich layered oxide Li[Li0.167Ni0.225Co0.005Mn0.558]O2 possesses larger specific capacity and excellent cycle life at 2.75 ~ 4.2V, but slightly lower in high-rate discharge capability compared to LiMn2O4. Therefore, positive active material is the key in Lithium ion batteries for different application, and using mixed positive active material is an effective way to deal with the balance between energy density, power density and electrochemical performance for energy storage systems application. 18650 cells are subjected to evaluation tests using Li4Ti5O12 as the anode material, the mixture of LiMn2O4 and Li[Li0.167Ni0.225Co0.005Mn0.558]O2 as positive active material, 1.2 mol L-1 LiPF6 in mixture of EC:DMC:EMC (1:2:2 in weight) as electrolyte. The rate capability and cycling performance at different temperatures of the cells are investigated. For comparison, the 18650 LiMn2O4 / Li4Ti5O12 and Li[Li0.167Ni0.225Co0.005Mn0.558]O2 / Li4Ti5O12 cells are fabricated. The capacity plots of the three kinds of cells are shown in Fig.1. The voltage platform of Li[Li0.167Ni0.225Co0.005Mn0.558]O2 / Li4Ti5O12 cell is at around 2.2 V and 2.35 V, be lower than that of LiMn2O4 / Li4Ti5O12 cell of 2.5 V and 2.55 V, so the voltage platform of the cell using the mixture cathode material (LiMn2O4 : Li[Li0.167Ni0.225Co0.005Mn0.558]O2 =7:3 in weight) is slightly lower. This system with the mixture cathode material shows excellent capacity retention of 90% after 5000 cycles at 1C and room temperature. In particularly, the cyclic performance of the cells with the mixture cathode material is obviously improved with the capacity retention of 87% after 4000 cycles at 1C and 55 ◦C, compared with that of the LiMn2O4 / Li4Ti5O12 cell. As shown in Fig.2, the capacity of LiMn2O4 / Li4Ti5O12 cell is markedly fading before 100 cycles, owing to the manganese dissolution of LiMn2O4 at high temperature. But the fading trend is slower between 1000 cycles an 2700 cycles, and it may be due to the saturation of manganese dissolution, resulting from the high voltage platform of Li4Ti5O12 anode material with 1.55 V, unlike the carbon anode, for which significant deposition of manganese. In contrary, the capacity fading of the cell with the mixture cathode material is gentle, due to the introduction of Li[Li0.167Ni0.225Co0.005Mn0.558]O2. Furthermore, the improved rate performance of the cells with the mixture cathode material also is observed with capacity retention of 85 % at 5 C (0.3C = 100%), compared with that of Li[Li0.167Ni0.225Co0.005Mn0.558]O2 / Li4Ti5O12 cell. Therefore, the mixture of LiMn2O4 and Li[Li0.167Ni0.225Co0.005Mn0.558]O2 is a promising alternative material to LiMn2O4for lithium ion battery. This work was supported by National 863 Program (No. 2013AA050902) and Shanghai technological innovation (No.14JC1491800). References [1] K.Y. Chung, K. Kim, Electrochim. Acta 49 (2004) 3327-3337. [2] L. Yang, M. Takahashi, B.F. Wang, Electrochim. Acta 51 (2006) 3228-3234. Figure 1