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, xLi₂MnO₃·(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 xLi₂MnO₃·(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.