Comprehensive study of a versatile polyol synthesis approach for cathode materials for Li-ion batteries

This work reports a comprehensive study of a novel polyol method that can successfully synthesize layered LiNi_0.4Mn_0.4Co_0.2O₂, spinel LiNi_0.5Mn_1.5O₄, and olivine LiCoPO₄ cathode materials. When properly designed, polyol method offers many advantages such as low cost, ease of use, and proven scalability for industrial applications. Most importantly, the unique properties of polyol solvent allow for greater morphology control as shown by all the resulting materials exhibiting monodispersed nanoparticles morphology. This morphology contributes to improved lithium ion transport due to short diffusion lengths. Polyol-synthesized LiNi_0.4Mn_0.4Co_0.2O₂ delivers a reversible capacity of 101 and 82 mAh·g^-1 using high current rate of 5C and 10C, respectively. It also displays surprisingly high surface structure stability after charge-discharge processes. Each step of the reaction was investigated to understand the underlying polyol synthesis mechanism. A combination of in situ and ex situ studies reveal the structural and chemical transformation of Ni-Co alloy nanocrystals overwrapped by a Mn- and Li-embedded organic matrix to a series of intermediate phases, and then eventually to the desired layered oxide phase with a homogeneous distribution of Ni, Co, and Mn. We envisage that this type of analysis will promote the development of optimized synthesis protocols by establishing links between experimental factors and important structural and chemical properties of the desired product. The insights can open a new direction of research to synthesize high-performance intercalation compounds by allowing unprecedented control of intermediate phases using experimental parameters.