Revisiting MnP4 as a negative electrode for Li-ion batteries: mechanism and performance

Abstract

MnP₄ has already been identified as a promising negative electrode for Li-ion batteries. Despite its interesting theoretical capacity, well above that of graphite, this material was not studied further due to its poor cyclability. MnP₄ was here prepared by ball milling and used as an active negative electrode material in optimised electrode formulation. This ball milled MnP₄ optimised electrode shows improving cycling performance, with a stable specific capacity of 600 mAh g⁻¹ over 60 cycles. The combination of operando X-ray diffraction, Mn K-edge X-ray absorption spectroscopy, and ³¹P and ⁷Li NMR analyses reveals a two-step reversible mechanism: Li insertion in MnP₄ forming amorphous “Li_xMnP₄” which is converted into Li₃P and Mn metal at low potential. Unlike in previous studies related to MnP₄, the MnP₄ CMC-based electrode shows neither crystalline Li₇MnP₄ formation at mid discharge, nor MnP₄ reformation upon charge. This modified reaction pathway appears to be beneficial for long-term capacity retention.