High-Temperature Treatment of Li-Rich Cathode Materials with Ammonia: Improved Capacity and Mean Voltage Stability during Cycling

Li-rich electrode materials of the family xLi₂MnO₃·(1−x)LiNi_aCo_bMn_cO₂ (a + b + c = 1) suffer a voltage fade upon cycling that limits their utilization in commercial batteries despite their extremely high discharge capacity, ≈250 mA h g⁻¹. Li-rich, 0.35Li₂MnO₃·0.65LiNi_0.35Mn_0.45Co_0.20O₂, is exposed to NH₃ at 400 °C, producing materials with improved characteristics: enhanced electrode capacity and a limited average voltage fade during 100 cycles in half cells versus Li. Three main changes caused by NH₃ treatment are established. First, a general bulk reduction of Co and Mn is observed via X-ray photoelectron spectroscopy and X-ray absorption near edge structure. Next, a structural rearrangement lowers the coordination number of CoO and MnO bonds, as well as formation of a surface spinel-like structure. Additionally, Li⁺ removal from the bulk causes the formation of surface LiOH, Li₂CO₃, and Li₂O. These structural and surface changes can enhance the voltage and capacity stability of the Li-rich material electrodes after moderate NH₃ treatment times of 1–2 h.