Revisiting MnP 4 as high-performance negative electrode for Li-ion batteries: mechanism and performance

Abstract

MnP 4 has been already 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. MnP 4 was here prepared by ball milling and used as active negative electrode material in optimised electrode formulation. This ball milled MnP 4 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 31 P and 7 Li NMR analyses reveals a two-step reversible mechanism: Li insertion in MnP 4 forming an amorphous "Li x MnP 4 " which is converted into Li 3 P and Mn metal at low potential. Unlike in previous works related to MnP 4 , the MnP 4 CMC-based electrode shows neither crystalline Li 7 MnP 4 formation at mid discharge, nor MnP 4 reformation upon charge. This modified reaction pathway appears to be beneficial for long-term capacity retention.