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 degrees C in O-2 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 mA h g(-1) and 199 mA h 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, Li1.03Mo0.02Ni0.95O2 (Mo-LNO) was then prepared via the solid-state route. Mo-LNO showed an even higher initial discharge capacity of 240 mA h 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 mA h g(-1), 168 mA h g(-1) and 177 mA h 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.