Modification of Electrochemical Behavior of Li-Rich Layered Oxide Cathode through Defect Consideration

Abstract

Cathode materials formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) with layered structure received much attention due to their surprisingly high reversible capacity for Li-ion battery applications.[1,2] For better understanding of such high-capacity layered cathode, a systematic study on powder morphology, crystal structures and reaction mechanisms of Li1.2Mn0.54Ni0.13Co0.13O2 during electrochemical charge/discharge cycles were conducted. First, the capacity of Li-rich layered cathode powder was strongly affected by its particle size. To obtain various particle sizes of cathode powder, different powder synthesis routes have been adopted. By usng sol-gel process, particle size of Li1.2Mn0.54Ni0.13Co0.13O2 powder obtained is around 200 nm. On the other hand, Li1.2Mn0.54Ni0.13Co0.13O2 powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place. Although Li-rich layer-structured cathode material show a reversible capacity as high as 250 mAh/g at low rate, its electrochemical properties such as capacity loss at first cycle, rate capability and capacity fading still need to be examined and characterized. In this study, Li1.2Mn0.54Ni0.13Co0.13O2 were investigated. The TEM structural analysis shows that the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.