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

Lithium-rich layered oxide Li[Li0.23Ni0.15Mn0.62]O-2, which also can be written as 0.6Li(2)MnO(3)center dot 0.4LiNi(0.5)Mn(0.5)O(2) or 0.9Li[Li1/3Mn2/3]O-2 center dot 0.4LiNi(0.5)Mn(0.5)O(2), 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 -> aEuro parts per thousand Li+ + Ni0.5Mn0.5O2 + e(-), (2) Li[Li1/3Mn2/3]O-2 -> aEuro parts per thousand Li+ + [Li1/3Mn2/3]O-2 + e(-), (3) [Li1/3Mn2/3]O-2 -> aEuro parts per thousand 1/3 Li+ + 2/3 MnO2 + 2/3 O center dot + e(-), and (4) Li+ + Ni0.2Mn0.8O2 + e(-) -> aEuro parts per thousand LiNi0.2Mn0.8O2. The extraction of oxygen atoms (O center dot) 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 xLi(2)MnO(3)center dot(1 -aEuro parts per thousand x)LiNi0.5Mn0.5O2 (0 a parts per thousand currency signaEuro parts per thousand x a parts per thousand currency signaEuro parts per thousand 1) increases from 230 to 280 mAh g(-1) with increasing x value. Li[Li0.23Ni0.15Mn0.62]O-2 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]O-2 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]O-2 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.