Electrochemical behavior of lithium-rich layered oxide Li[LiNiMn]O cathode material for lithium-ion battery.

Lithium-rich layered oxide Li[LiNiMn]O, which also can be written as 0.6LiMnO·0.4LiNiMnO or 0.9Li[LiMn]O·0.4LiNiMnO, is synthesized using a solid-state reaction method. Its crystal structure and electrochemical behavior as the cathode material in lithium-ion batteries are studied. A reaction mechanism is proposed to interpret its unique electrochemical behavior shown in the first charge-discharge cycle. It includes four reactions: (1) LiNiMnO → Li + NiMnO + e, (2) Li[LiMn]O → Li + [LiMn]O + e, (3) [LiMn]O → 1/3 Li + 2/3 MnO + 2/3 O· + e, and (4) Li + NiMnO + e → LiNiMnO. The extraction of oxygen atoms (O·) in the reaction (3) results in the crystal structure rearrangement. Based on this hypothesis, it is found that the expected capacity of activated lithium-rich layered oxide xLiMnO·(1 − x)LiNiMnO (0 ≤ x ≤ 1) increases from 230 to 280 mAh g with increasing x value. Li[LiNiMn]O has an expected total first charge capacity of 396 mAh g, but its expected capacity is only 247 mAh g due to an initial capacity loss caused by the oxygen loss. Experimentally, within a charge-discharge voltage window from 2.0 to 4.8 V, Li[LiNiMn]O delivers a charge capacity of 310 mAh g and a discharge capacity of 215 mAh g, respectively, at 40 mA g during the first cycle. The electrochemical kinetic behavior of Li[LiNiMn]O is controlled by the charge-transfer process rather than by Li diffusion or blockage of solid-electrolyte interphase (SEI) layers at the end of Li extraction in the first charge. [ABSTRACT FROM AUTHOR]