On the Manganese Dissolution Process from LiMn2O4 Cathode Materials

Abstract

Transition metal (TM) dissolution is a process experienced by most cathode materials based on lithium transition metal oxides. Spinel LiMn2O4 (LMO) is the best-known cathode material that suffers from TM dissolution. Therefore, LMO is selected here to understand the dissolution process and derive an inductively coupled plasma optical emission spectroscopy (ICP-OES) method for quantifying dissolved metal ions. Furthermore, the LMO powder is coated with thin Al2O3 films of different thicknesses using atomic layer deposition (ALD) in an attempt to suppress the dissolution of Mn. Two different types of counter electrodes, lithium iron phosphate (LFP) and Li-metal, were used to investigate the role of the counter electrode on Mn dissolution. HF is identified as the lead cause of Mn dissolution, through comparisons of cells containing LiPF6 or LiClO4 based electrolytes. The results show that Li-metal counter electrode effectively minimizes the dissolution process via likely consuming HF and H2O impurity. In contradiction to the purpose of the protective Al2O3 thin film coating, surface coated LMO showed higher dissolution of Mn compared to pristine LMO, both in LFP||LMO and Li||LMO configurations. Al2O3 is proposed to generate H2O when reacts with HF. H2O could have the possibility to migrate back in the electrolyte and participate in the hydrolysis of LiPF6, resulting in more HF and thereby more Mn dissolution.