Melt Synthesis of Lithium Manganese Iron Phosphate: Part I. Composition, Physical Properties, Structural Analysis, and Charge/Discharge Cycling

Abstract

Melt synthesis is a fast and simple process to make dense LiMnyFe1-yPO4 (LMFP with 0 ≤ y ≤ 1) from all-dry, low-cost precursors with zero waste. This study characterizes melt LMFP materials with 0%–100% Mn after particle size reduction by planetary milling and carbon coating with glucose. The melt LMFP samples show higher electrical conductivity at similar pellet density than LFP (0% Mn) and LMFP (79% Mn) reference samples made by traditional methods. The melt LMFP samples exhibit higher crystallinity than the reference samples and show no crystalline impurities. Their unit cell volume and crystallographic density scale with Mn content; the percentage of Fe and/or Mn in Li positions is below 1.5%, which is comparable to reference samples. Crystallite sizes of at least 100 to 175 nm are observed for melt LMFP, which is larger than the fine ∼50 nm crystallites of reference LMFP. Melt LFP shows specific discharge capacity and cycling stability comparable to reference LFP, but the melt LMFP samples with 25%–100% Mn shows worse performance than reference LMFP (79% Mn). Part two of this study will quantify the solid-state lithium diffusion coefficient in melt LMFP materials and correlate it to their electrochemical performance.