Structure and Delithiation/Lithiation of the Lithium-Rich Layered Oxide Li[Li_0.23Ni_0.15Mn_0.62]O₂ as Cathode Material

摘要

The typical lithium-rich layered oxide Li[Li0.23Ni0.15Mn0.62]O2 is synthesized via a solid state reaction, and is used as the cathode material for lithium-ion batteries. In situ X-ray diffraction analyses reveal the formation process of Li[Li0.23Ni0.15Mn0.62]O2 crystallites with C2/m monoclinic symmetry. The ?1 Li[Li0.23Ni0.15Mn0.62]O2 cathode delivers an initial discharge capacity of 221 mAh g and keeps a reversible capacity of 174 mAh g?1 after 50 cycles. A large capacity loss and the unique charge curve during the first cycle both indicate that Li[Li0.23Ni0.15Mn0.62]O2 was changed into Ni0.2Mn0.8O2 due to irreversible oxygen loss and “rich lithium” extraction. The cathode’s high capacity results from the reversible conversions between Ni0.2Mn0.8O2 and LiNi0.2Mn0.8O2. Both Ni0.2Mn0.8O2 and LiNi0.2Mn0.8O2 keep a stable framework during cycling due to two suggested reasons: (1) some transition metal atoms migrate into the lithium layers and play as the supports between the transition metal layers; (2) 1/4 Mn keeps a stable tetravalent state during cycling and then weakens the Jahn- 3+ Teller effect of Mn . The predicted reversible capacity of Li[Li(1?2x)/3NixMn(2?x)/3]O2 (0 < x < 0.5) increases from 229 to 280 mAh g?1 with increasing x value, and fairly coincides with the experimental results in this work and literature.