Electrochemical behavior of lithium-rich layered oxide Li[Li0.23Ni0.15Mn0.62]O2 cathode material for lithium-ion battery

Abstract

Lithium-rich layered oxide Li[Li0.23Ni0.15Mn0.62]O2, which also can be written as 0.6Li2MnO3·0.4LiNi0.5Mn0.5O2 or 0.9Li[Li1/3Mn2/3]O2·0.4LiNi0.5Mn0.5O2, 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) LiNi0.5Mn0.5O2 → Li+ + Ni0.5Mn0.5O2 + e−, (2) Li[Li1/3Mn2/3]O2 → Li+ + [Li1/3Mn2/3]O2 + e−, (3) [Li1/3Mn2/3]O2 → 1/3 Li+ + 2/3 MnO2 + 2/3 O· + e−, and (4) Li+ + Ni0.2Mn0.8O2 + e− → LiNi0.2Mn0.8O2. 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 xLi2MnO3·(1 − x)LiNi0.5Mn0.5O2 (0 ≤ x ≤ 1) increases from 230 to 280 mAh g−1 with increasing x value. Li[Li0.23Ni0.15Mn0.62]O2 has an expected total first charge capacity of 396 mAh g−1, but its expected capacity is only 247 mAh g−1 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[Li0.23Ni0.15Mn0.62]O2 delivers a charge capacity of 310 mAh g−1 and a discharge capacity of 215 mAh g−1, respectively, at 40 mA g−1 during the first cycle. The electrochemical kinetic behavior of Li[Li0.23Ni0.15Mn0.62]O2 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.