Surface and bulk defect formation during hydrothermal synthesis of LiCoPO4 crystals and their electrochemical implications

Abstract

Lithium cobalt phosphate (LiCoPO₄, LCP) is a high-voltage cathode material with a lot of promise in delivering high energy density in comparison to the established LiFePO₄ counterpart. However, the road to developing LCP is hampered not only by electrolyte interfacial reaction due to high-voltage but also by the lack of critical knowledge regarding material crystal properties linked to synthesis that limit the attainment of full discharge capacity. Herein, we study in-depth the synthesis of LCP by the hydrothermal method and its post-synthesis modifications by high-energy planetary milling and conductive carbon coating in order to shed light on the crystal chemistry affecting its electrochemical performance. Via adjusting the Li/Co molar ratio and pH of precursor solution, the supersaturation is controlled to achieve high-purity and well-crystalline LCP particles with sub-micron size. After carefully characterizing the hydrothermally synthesized LCP crystalline material, we discovered the presence of two types of defects, surface composition inhomogeneities and bulk cation mixing, which adversely affect the Li-ion intercalation kinetics and storage capacity. More specifically, we identified (i) the formation of undesired nano-scale Co(OH)₂ passivation layer on LCP surface and (ii) abundant anti-site defects blocking one-dimensional (1-D) Li-ion diffusion channels. These crystal defects we show to impose critical limitations to hydrothermally produced LCP materials in delivering near theoretical discharge capacities; hence on the basis of these new insights, alternative crystal engineering approaches need to be developed in pursuit of high-performance LCP cathodes.