Morphology controlled synthesis of LiFePO4/C nanoplates for Li-ion batteries

Abstract

Lithium iron phosphate, LiFePO₄ (LFP), is considered to be a potential cathode material for lithium-ion batteries but its rate performance is significantly restricted by sluggish kinetics of electrons and lithium ions. A simple solvothermal method has been described in this article to synthesize carbon coated LFP (LFP/C) nanoplates with varying thickness from 20 to 500 nm by using different iron precursors. The influence of solvents on the morphology of the LFP in the solvothermal synthesis is also investigated. A uniform carbon coverage at the surfaces has been achieved by a selective chelating carbonising source, D-gluconic acid lactone. The smallest dimension of the nanoplates has been found to be the b-axis where the Li⁺ ion diffuses quickly. The overall capacity and rate performance have, in general, been found to increase with the decrease of thickness of the nanoplates. Hierarchical LFP/C with ∼30 nm thickness shows the best electrochemical performance of 167 mA h g⁻¹, followed by spindle (<20 nm thickness but aggregated, 121 mA h g⁻¹), plates (200–300 nm thickness, 110 mA h g⁻¹) and diamond shaped LFP/C (300–500 nm thickness, 82 mA h g⁻¹) at a current rate of 17mA g⁻¹ (0.1C rate). The spindle shaped LFP/C shows unexpected electrochemical performance since the nanoplates are heavily agglomerated in the bulk which prevents access for the liquid electrolyte, as well as additive Super P carbon, between neighbouring nanoplates during the fabrication of the composite electrodes. Hence, only the peripheral plates of the spindle are actively involved in the insertion/extraction of Li⁺, while the core of the spindle shaped LFP/C is almost inactive, resulting in moderate storage behaviour.