Probing Electrochemical Reactions at the Electrode-Electrolyte Interface in Lithium Metal Battery, Combining Experimental Approach and Computational Calculation
Authors: Aoki Yasuhito , Oda Mami , Kojima Sachiko 
Published: December 2024

Lithium-ion batteries have attracted significant attention for applications such as energy supply for electric vehicles, large-scale stationary power sources, and energy storage for renewable energy. Research and development of various electrode materials continue for further increasing charge / discharge capacity, expanding operation lifetime. Theoretical capacity of lithium metal is 3860 mAh/g, which is approximately 10 times larger than that of graphite negative electrode (372 mAh/g), which is commonly used in conventional lithium-ion batteries. Therefore, lithium metal batteries, which combine lithium metal negative electrode with a commonly used Lithium nickel manganese cobalt oxides (NCM) positive electrode, are expected to significantly increase energy density and considered to be one of the promising next-generation batteries.

Improving the performance of the electrolyte solution is essential to fully utilize the newly developed electrode materials for both positive and negative electrodes. As lithium metal exhibits the most negative potential (-3.04 V v.s. SHE), the electrolyte solution must possess high reduction stability. At the same time, passivation film, solid electrolyte interface (SEI), formed on lithium metal also plays an important role to prevent further decomposition of electrolyte solution. Meanwhile, positive electrodes have oxidation potentials of 4.2 V or higher, and the reactivity at the positive electrode (cathode) and the electrolyte interface (CEI) is also a crucial factor influencing the performance of lithium metal batteries. Thus, controlling the reactions of the electrolyte solution at the electrode interface is a critical factor for operating lithium metal batteries stably and reliably over the long term.

In recent years, research and development of electrolyte solutions have focused on enhancing reduction and oxidation stability by exploring various types of solvents, electrolyte salts, and controlling their concentrations. Ether-based electrolyte solutions are known for their higher reduction stability comparing commercially available carbonate-based electrolyte solutions. However, the oxidative stability of conventional concentration ether-based electrolyte solution is quite low, leading the decomposition at oxidation potentials over 4 V. Strategies to improve the oxidative stability of ether-based electrolyte solutions have been extensively studied, and one of the candidates is increasing electrolyte salt concentration.

In this study, we have conducted electrochemical performance tests on lithium metal batteries using a concentrated electrolyte solution of ether-based solvents and LiFSI (Lithium bis(trifluoromethanesulfonyl)imide), with lithium metal and NCM positive electrode. As the solvation structures in the electrolyte solution directly affect the formation mechanism of SEI and CEI, the solvation structures are estimated by Raman spectroscopy, and their chemical stability and decomposition mechanisms are estimated by DFT calculations. We have conducted detailed analyses of reduction and oxidative decomposition reactions occurring at the electrode surface, and confirmed the applicability of the prediction combining Raman spectroscopy and DFT calculation.

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