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

Abstract

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 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, 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 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.