Optimising the synthesis of LiNiO2: coprecipitation versus solid-state, and the effect of molybdenum doping

LiNiO₂ (LNO) was prepared by two synthesis techniques: solid-state (SS-LNO) and coprecipitation (C-LNO). The results showed that C-LNO could be synthesised in as little as 1 hour at 800 °C in O₂ to give a pristine material. The layered oxide structures of both materials have been investigated using PXRD, confirming that phase pure samples have been made. Electrochemical properties were explored over a range of voltage windows (2.7 - 4.1 V, 2.7 - 4.2 V, and 2.7 - 4.3 V vs. Li⁺/Li), to analyse how the H2-H3 phase transition impacts the cathode materials capacity retention. Electrochemical measurements showed that the initial discharge capacity and cycle stability are improved in C-LNO compared to SS-LNO, achieving 221 mAh g^-1 and 199 mAh g^-1 respectively in the voltage range 2.7 - 4.3 V (at 10 mA g^-1), with capacity retentions of 47% and 41% after 100 cycles. A Mo doped system, Li_1.03Mo_0.02Ni_0.95O₂ (Mo-LNO) was then prepared via the solid-state route. Mo-LNO showed an even higher initial discharge capacity of 240 mAh g^-1 between 2.7 - 4.3 V vs Li⁺/Li, with a slightly enhanced capacity retention of 52%. Through the investigation of the different voltage ranges it was shown that capacity fade can be minimised by cycling the materials below 4.2 V, (attributed to avoiding the detrimental H2-H3 phase transition) although this results in a lower discharge capacity. This is shown by the cycling of SS-LNO, C-LNO and Mo-LNO in the voltage window 2.7 - 4.1 V, where discharge capacities of 144 mAh g^-1, 168 mAh g^-1 and 177 mAh g^-1 were achieved with higher capacity retentions of 84%, 76% and 90% after 100 cycles respectively, the latter system showing promise as a cobalt-free cathode material.