Limiting the aging process of spinel-based Li-ion cells by cathode sulfur doping

The degradation of stoichiometric lithium-manganese spinel (LiMn₂O₄, LMO) is a critical concern in the longevity and efficiency of eco-friendly, cheap lithium-ion batteries. The complex mechanisms of degradation, involving Jahn-Teller distortion of Mn³⁺, as well as Mn²⁺ dissolution, are well-known to be accelerated by reactive species like HF. This study explores sulfur substitution into the LMO’s oxygen sublattice as a targeted approach to mitigate these aging factors. The properties and stability in the electrochemical cell of the S-doped LMO were investigated and compared to unmodified LMO using electrothermal atomic absorption spectrometry, X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning and transmission electron microscopy as well as galvanostatic charge/discharge tests and electrochemical impedance spectroscopy. The sulfur doping was found to significantly increase the structural stability of the spinel and enhance the performance of the cells. These improvements were found to be caused by sulfur, which limited passive manganese dissolution and stretched the spinel structure diminishing diffusion constraints and reducing the susceptibility to mechanical strains. Furthermore, the S-doping was found to enhance both the passivation layer stability and PVDF binder durability. Although a full understanding of sulfur’s role in the sipenl-based cell aging process remains intricate, the findings affirm its role in limiting the degradation of the LiMn₂O₄, especially under severe working conditions. These insights into sulfur doping open new avenues for developing more robust and environmentally friendly LMO-based materials for large-scale battery systems.