INFLUENCE OF THE NATURE OF A BINDER ON THE ELECTROCHEMICAL PROPERTIES OF THE LiMn0.7Fe0.3PO4 CATHODE MATERIAL

Анотація

This study presents an electrochemical comparison of Li||LiMn₀.₇Fe₀.₃PO₄ coin cells using two different polymers as binders: PVDF and the water-soluble binder NV‑1A. The charge/discharge curves obtained at a current rate of 0.05 C exhibit two well-defined plateaus at approximately 3.5 V and 4.1 V, corresponding to the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox pairs, respectively. The capacity ratio of these plateaus is about 7:3, which matches the Mn/Fe ratio in the pristine material. The specific capacity of the coin cells with different binders falls within the range of 137–141 mAh·g⁻¹, which is consistent with the manufacturer’s specifications for the material. However, upon the first cycles, the coin cells with NV‑1A binder exhibit a slightly lower specific capacity. This is attributed to the possible formation of Li₃PO₄ particles on the electrode surface, which leads to a decrease in specific capacity upon the initial cycles but improves the overall stability upon prolonged cycling. The first-cycle coulombic efficiency is 96.0 % for PVDF and 94% for NV‑1A, increasing to 99.5% ± 0.1 % upon further cycling for both systems. Increasing the charge/discharge current density leads to a decrease in the specific capacity of Li||LiMn₀.₇Fe₀.₃PO₄ electrodes. This decrease is more pronounced for PVDF. At 0.5 C, the specific capacity reaches 68 mAh·g⁻¹ for PVDF and 85 mAh·g⁻¹ for NV‑1A. Cyclic stability tests reveal that, despite the lower initial capacity of the NV‑1A based coin cells, their capacity fade rate is only 0.1 % per cycle at 0.1 C, compared with 0.45 % for PVDF. Consequently, after 75 cycles, the NV‑1A coin cells exhibit superior capacity retention. These findings are further discussed in the context of electrode degradation mechanisms upon cycling.