Most cathode materials based on lithium transition metal oxides suffer from transition metal (TM) dissolution. Due to the general migration and deposition of transition metals on the anode surface, this ageing mechanism affects not only the surface structure of cathodes but also the solid electrolyte interface (SEI) on the anode. This can lead to increased cell resistance and capacity loss. Coating cathode materials with thin protective layers has shown to be an effective approach for extending lifetime of cathodes. However, there is a basic lack of knowledge of the detailed mechanisms of protective coating. We have studied the impact of a popularly used coating material, Al₂O₃,¹ on inhibiting HF-induced TM dissolution from two common cathode materials, LiMn₂O₄ and LiNi_0.8Mn_0.1Co_0.1O₂.² Karl Fischer titration, fluoride selective probe, and inductively coupled plasma optical emission spectroscopy are used in tandem to detect the evolution of H₂O, HF and TM concentrations when the cathode materials in either lithiated or delithiated state come into contact with an aged ethylene carbonate, diethyl carbonate and lithium hexafluorophosphate (EC/DEC/LiPF₆) based electrolyte. The study demonstrates the importance of acid-base interactions in controlling TM dissolution in Li-ion batteries, as well as how coatings improve the cathode's chemical integrity towards an acidic electrolyte.

Furthermore, the role of EC and LiPF₆ are explored in the same perspective by altenative electrolyte solvents and salts, since electrolytes based on LiPF₆ in EC have been shown to be unstable at the high potentials.³ We will present results on galvanostatic cycling of cathode materials using EC/DEC and sulfolane/DEC based electrolytes containing LiPF₆, Lithium bis(fluorosulfonyl)imide (LiFSI), and Lithium bis(oxalato)borate (LiBOB). Lithium iron phosphate (LFP), with a high average potential of 3.45 V, was chosen as the counter electrode to restrict the reduction of TMs on the counter electrode. Having a counter electrode that provides similar environment to the investigated cathode materials helps avoid the influence of anode/electrolyte degradation species on the TM dissolution mechanism.

Acknowledgement:

The authors acknowledge financial support from the Swedish Energy Agency via project no. 45518-1.

References

1. D. S. Hall, R. Gauthier, A. Eldesoky, V. S. Murray, and J. R. Dahn, ACS Appl. Mater. Interfaces, 11, 14095–14100 (2019).
2. Y. Tesfamhret, H. Liu, Z. Chai, E. Berg, and R. Younesi, ChemElectroChem, 8, 1516–1523 (2021).
3. Y. Zhang, Y. Katayama, R. Tatara, L. Giordano, Y. Yu, D. Fraggedakis, G.J. Sun, F. Maglia, R. Jung, M.Z. Bazant, Y. Shao-Horn, Energy Environ. Sci., 13, 183–199 (2020).