Melt Synthesis of Lithium Manganese Iron Phosphate: Part II. Particle Size, Electrochemical Performance, and Solid-State Lithium Diffusion

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. Part one of this study confirmed that highly crystalline and phase pure LMFP materials can be made by melt synthesis. This part shows that planetary milling can reduce the primary particle size of melt LMFP (0%–75% Mn) to ∼200 nm, which is smaller than the primary particles in commercial LFP reference material (0% Mn). However, further particle size reduction is needed to reach particle sizes below 70 nm observed in reference LMFP (79% Mn). Melt LFP shows almost identical specific capacity and charge/discharge voltage as reference LFP. Melt LMFP materials show a high voltage Mn plateau at ∼4 V associated with the Mn2+/3+ redox, the length of which scales with Mn content. The Mn plateau raises the average discharge voltage of LMFP; hence a minimum specific discharge capacity between 160 mAh g−1 (0% Mn) and 145 mAh g−1 (80% Mn) is sufficient to match the volumetric energy density of LFP. The Atlung Method for Intercalant Diffusion shows that the lithium diffusion coefficient in LMFP is ∼1 order of magnitude higher in the voltage region of the Fe2+/3+ redox couple (3.75–3.1 V vs Li+/Li) than in the voltage region of the Mn2+/3+ redox couple (4.3–3.75 V vs Li+/Li). This emphasizes the need for very small primary particles when making LMFP with relatively high Mn content.