Review on Challenges and Recent Advances in the Electrochemical Performance of High Capacity Li- and Mn-Rich Cathode Materials for Li-Ion Batteries

Li and Mn-rich layered oxides, xLi(2)MnO(3)(1-x)LiMO2 (M=Ni, Mn, Co), are promising cathode materials for Li-ion batteries because of their high specific capacity that can exceed 250 mA h g(-1). However, these materials suffer from high 1(st) cycle irreversible capacity, gradual capacity fading, low rate capability, a substantial charge-discharge voltage hysteresis, and a large average discharge voltage decay during cycling. The latter detrimental phenomenon is ascribed to irreversible structural transformations upon cycling of these cathodes related to potentials 4.5 V required for their charging. Transition metal inactivation along with impedance increase and partial layered-to-spinel transformation during cycling are possible reasons for the detrimental voltage fade. Doping of Li, Mn-rich materials by Na, Mg, Al, Fe, Co, Ru, etc. is useful for stabilizing capacity and mitigating the discharge-voltage decay of xLi(2)MnO(3)(1-x)LiMO2 electrodes. Surface modifications by thin coatings of Al2O3, V2O5, AlF3, AlPO4, etc. or by gas treatment (for instance, by NH3) can also enhance voltage and capacity stability during cycling. This paper describes the recent literature results and ongoing efforts from our groups to improve the performance of Li, Mn-rich materials. Focus is also on preparation of cobalt-free cathodes, which are integrated layered-spinel materials with high reversible capacity and stable performance.