Stabilizing ultrahigh-nickel cobalt-free lithium layered oxide cathode via Mg doping: pillar and electromagnetic center roles

Abstract

Ultrahigh-nickel cobalt-free lithium layered oxide cathodes are widely researched and applied owing to their high capacity and low cost. Unfortunately, the ultrahigh nickel content implies deep lithium-ion extraction/insertion and intense redox reactions, while the cobalt-free results in poor electronic conductivity and serious Li⁺/Ni²⁺ disordering mixing. These factors severely threaten the cycle life and rate capability for battery. In this work, a Mg²⁺-doped LiNi_0.9Mn_0.1O₂ (NM91 and NM91-Mg) ultrahigh-nickel cobalt-free cathode is prepared to enhance lithium storage performance. Mg²⁺ doping plays a dual role as both an "electrostatic center" and a "pillar" within the lithium layers. Firstly, Mg²⁺ possesses more extranuclear electrons than Li⁺, resulting in stronger electrostatic interactions within the lithium layer. This enables it to act as an electrostatic center, inhibiting disordered cation migration. XRD refinement results confirm that Mg doping suppresses the detrimental Li⁺/Ni²⁺ mixing, thereby mitigating the negative effects associated with cobalt-free. Secondly, Mg²⁺ fixed within the lithium layers acts as a stable pillar species. It helps prevent the layered structure from collapsing during deep lithium-ion extraction. Battery in-situ XRD results show that Mg doping suppresses the harmful H2 to H3 phase transition, alleviating the negative effects caused by the ultrahigh nickel content. Consequently, NM91-Mg demonstrates significantly superior cycle stability (capacity retention for 100 cycles: 86.6 % vs. 81.1 %) and rate capability (discharge capacity at a high rate of 5 C: 98 mAh g^-1 vs. 89 mAh g^-1) compared to NM91. Remarkably, the constructed full battery also achieves a high capacity retention of 91.7 % after 100 cycles. Mg doping provides targeted improvements for ultrahigh-nickel cobalt-free cathode materials, bringing them closer to practical application.