Julie Trottier

Olivine Coated Spinel: 5-Volt System for High Energy Lithium Batteries

The cyclability of Li ion transfer in Li-ion rechargeable batteries depends mainly on the dimensional stability of the host material during insertion and deinsertion of Li+. Recently, it was found that redox reactions of non stoichiometric or doped LiMyMn2−yO4 (M=Li, Co, Cr, Ni, Al, etc.) spinels are much better than that of the pure LiMn2O4 ceramics, which exhibit a slight capacity fading. These materials have been investigated to improve the cycling performance of LiMn2O4, showing a tetragonal distortion induced by the excess of Mn Jahn–Teller ions in deeply discharged electrode. However, all the reported doping methods have led to a decreased specific charge compared to the undoped LiMn2O4 materials so far. The search of high-voltage material electrodes has been focused on two categories: the inverse spinels, e.g., LiMVO4, and the normal spinels, LiMyMn2−yO4. Recent investigations have shown that, among the Ni-substituted LiMn2O4 spinels, the composition LiNi0.5Mn1.5O4 possesses specific electrochemical characteristics such as a high capacity of 130–140 mAh/g associated with a highvoltage plateau in the 5-V range Gao et al. [1] investigated the origin of the voltage profile for LiNiyMn2−yO4. For the same system, Zhong et al. [2] showed the effects of the synthesis route (sol-gel vs. solid state) on some structural and electrochemical properties. The almost flat voltage profile was confirmed by Ohzuku et al. [3]. All these works only reported on structural and electrochemical information, Also the cycle life was find very poor due the instability of the electrolyte at high voltage (5V). The olivine coated spinel (5V) was not reported before. The stability of the electrolyte is one the most important key for cycling the cell at high voltage. The aim of this communication is the growth and characterization of the LMN : LiNi0.5Mn1.5O4 (spinel) materials prepared by different techniques. Because the standard electrolyte is very stable in contact with olivine, we applied the olivine coated spinel 5 V system for high energy lithium batteries. Figure 1 shows the voltage profile of LiFePO4 (LFP) coated LMN, the LFP show a voltage activity at 3.5 V and the spinel material at 4. 7 V. The capacity with non coated is 103 mAh/g , the first coulombic efficiency (1 CE) is 67 %, for the LFP coated LMN, the capacity is 120 mAh/g and the 1 CE is 76 %. The olivine coating increases the reversible capacity and also the 1 CE %. Figure 2 shows the comparison of the rate performances of uncoated and LFP coated spinel. The LFP coated spinel increases the capacity at high rate. The high power performance was improved with LFP-coated LMN. For the safety of point of view, the DSC peaks (not shown) is shifted to high temperature side by olivine coating spinel. LFP coating has more shifted than LiMnPO4, LiNiPO4, or LiCoPO4. Thermal stability is improved by LFP coating LMN.