Understanding SEI evolution during the cycling test of anode-free lithium-metal batteries with LiDFOB salt

Abstract

Anode-free lithium-metal batteries (AFLMBs) have the potential to double the energy density of Li-ion batteries, but face the challenges of mossy dendritic lithium plating and an unstable solid electrolyte interphase (SEI). Previous studies have shown that the AFLMBs with an electrolyte containing lithium difluoro(oxalato)borate (LiDFOB) salt outperform those with lithium hexafluorophosphate (LiPF₆), but the mechanism behind this improvement is not fully understood. In this study, X-ray photoelectron spectroscopy (XPS) depth profile analysis and electrochemical impedance spectroscopy (EIS) were conducted to investigate the SEI on plated Li from the two conducting salts and their evolution in Cu‖NMC full cells during cycling. XPS results revealed that an inorganic-rich SEI layer is formed in the cell with LiDFOB-based electrolyte, with a low carbon/oxygen ratio of 0.56 compared to 1.42 in the LiPF₆-based cell. With the inorganic-rich SEI, a dense electroplated Li with a shining surface on the Cu substrate can be retained after ten cycles. The inorganic-rich SEI enhances the reversibility of Li plating and stripping, with a high average CE of ∼98% and a stable charge/discharge voltage profile. The changes in SEI resistance and cathode electrolyte interphase resistance are more prominent compared to the changes in solution and charge transfer resistances, which further validate the role of the passivation films on Li deposits and NMC cathode surfaces in stabilizing AFLMB cycling performance.

Fig. 1. XPS depth profile analysis of electroplated Li after the formation cycle, with etching times of 0, 6, and 12 minutes. C 1s XPS spectra of plated Li with (a) LiDFOB and (b) LiPF₆ electrolyte salt. O 1s XPS spectra of plated Li with (c) LiDFOB and (d) LiPF₆ electrolyte salt.

Fig. 2. (a) Arrhenius plot of 1 M LiPF₆ in EC : DEC (1 : 1, v/v) and 1 M LiDFOB in EC : DEC (1 : 1, v/v) electrolyte obtained with electrolyte conductivity measurement from 70 °C to −10 °C. The electrolyte conductivity was measured from −10 to 70 °C. (b) LSV scan of Li‖Cu half cell with 1 M LiPF₆ and 1 M LiDFOB in EC : DEC (1 : 1, v/v) electrolyte with a scan rate of 0.1 V s⁻¹. (c) Li plating overpotential at first discharge of Li‖Cu half cell with 1 M LiPF₆ and 1 M LiDFOB in EC : DEC (1 : 1, v/v) electrolyte at a current density of 1 mA cm⁻².

Fig. 3. (a–c) SEM images of plated Li on Cu foil after fully charged at different cycles with LiDFOB salt. (d) Illustration of dense Li plating with inorganic-rich SEI. (e–g) SEM image of plated Li on Cu foil after fully charged at different cycles with LiPF6 salt. (h) Illustration of dendritic Li plating with organic-rich SEI. All the cells are cycled from 2.8–4.5 V with a current density of 50 mA g⁻¹.

Fig. 4. Galvanostatic charge–discharge voltage profiles of Cu‖NMC full cell with 1 M (a) LiDFOB and (b) LiPF₆ in EC : DEC (1 : 1 v/v). (c) Cycling stability of Cu‖NMC full cell with 1 M LiDFOB and LiPF₆ in EC : DEC (1 : 1 v/v).

Fig. 5. Nyquist plots of the fully charged Cu‖NMC cells with (a) LiDFOB and (b) LiPF6 salt at the 1st, 5th, 10th, and 15th cycle and the fitting data of (c) solid electrolyte interphase resistance at the anode (R_SEI-anode) and (d) cathode electrolyte resistance (R_CEI-cathode). The fitting parameters are listed in Table S1.