Application of Highly Concentrated Electrolytes with Surfactant to High-Energy Lithium Metal Batteries with Pure Nickel Layered Oxides

1. Introduction
Li metal has gained intensive research interest as a promising negative electrode material for high-energy rechargeable batteries, owing to the high theoretical capacity (3860 mA h g^–1) and low standard electrode potential (–3.04 V vs. SHE). Highly concentrated electrolytes (HCEs) are expected to be viable electrolytes that achieve highly efficient Li deposition/stripping reactions.¹ However, the insufficient wettability of HCEs to conventional polyolefin separators hinders their use for practical applications. In this study, the use of a surfactant as an electrolyte additive is proposed as an effective approach to improve the electrolyte wettability. The surfactant-added HCEs allows stable operation of practical high-energy Li metal batteries with a high-loading pure Ni layered oxide positive electrode.
2. Experimental
1 wt% surfactant was added to a HCE (5.0 mol dm^-3 (M) LiN(SO₂F)₂ (LiFSA) dissolved in 1,2-dimethoxyethane (DME)). To evaluate the wettability of polyolefin separator to HCE with or without a surfactant, an electrolyte droplet was placed on the separator. To examine the reversibility of Li deposition/stripping reactions in HCE with or without surfactant, chronopotentiometry was performed on Li/Cu cells using two types of separators, polyolefin separator and aramid-coated polyolefin separator², respectively. Electrochemical performances of practical high-energy Li metal batteries composed of Li-deficient Li_0.975Ni_1.025O₂³ composite electrode with high mass loading (~20 mg cm^–2) and thin metallic Li foil (50 μm) were tested using 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) and 5 M LiFSA in DME with 1 wt% surfactant.
3. Results and discussions
A droplet of the HCE remains intact on the polyolefin separator with low porosity. In contrast, the addition of 1 wt% surfactant allows HCE to sufficiently wet the polyolefin separator because of an amphipathic structure with high affinity to both polyolefin separator and HCEs. In both HCEs with or without surfactant, highly reversible Li deposition/stripping behaviors with a high Coulombic efficiency of approximately 99% are obtained, and the maximum reversible Li deposition/striping capacity reaches over 10 mA h cm^–2.⁴ These results indicate that the addition of surfactant to HCEs has no detrimental effect on reversibility of Li deposition/stripping reaction. In the galvanostatic charge/discharge tests of Li/Li_0.975Ni_1.025O₂, the cell with 1 M LiPF₆ in EC/DMC shows an increase in polarization and severe capacity fading upon cycling. This degradation is caused by the decomposition of the electrolyte on the surface of the active material during charging.³ In contrast, the cell with surfactant-added HCE can be stably cycled without a polarization increase and achieves 99% capacity retention after 50 cycles. These results demonstrate that the surfactant-added HCE enables stable operation of high-energy-density lithium metal batteries with conventional polyolefin separators.
1. Y. Yamada and A. Yamada, J. Electrochem. Soc., 162, 14 (2015).
2. Y. Ugata et al., and N. Yabuuchi, Energy Adv., 2, 503 (2023).
3. I. Konuma et al., and N. Yabuuchi, Energy Stor. Mater., 66, 103200 (2024).
4. Y. Ugata, T. Kuriyama, T. Komagata et al., and N. Yabuuchi, submitted.