Synergistic Effect of a Dual-Salt Liquid Electrolyte with a LiNO3 Functional Additive toward Stabilizing Thin-Film Li Metal Electrodes for Li Secondary Batteries

Li metal thickness has been considered a key factor in determining the electrochemical performance of Li metal anodes. The use of thin Li metal anodes is a prerequisite for increasing the energy density of Li secondary batteries intended for emerging large-scale electrical applications, such as electric vehicles and energy storage systems. To utilize thin (20 μm thick) Li metal anodes in Li metal secondary batteries, we investigated the synergistic effect of a functional additive (Li nitrate, LiNO₃) and a dual-salt electrolyte (DSE) system composed of Li bis(fluorosulfonyl)imide (LiTFSI) and Li bis(oxalate)borate (LiBOB). By controlling the amount of LiNO₃ in DSE, we found that DSE containing 0.05 M LiNO₃ (DSE-0.05 M LiNO₃) significantly improved the electrochemical performance of Li metal anodes. DSE-0.05 M LiNO₃ increased the cycling performance by 146.3% [under the conditions of a 1C rate (2.0 mA cm^-2), DSE alone maintained 80% of the initial discharge capacity up to the 205th cycle, whereas DSE-0.05 M LiNO₃ maintained 80% up to the 300th cycle] and increased the rate capability by 128.2% compared with DSE alone [the rate capability of DSE-0.05 M LiNO₃ = 50.4 mAh g^-1, and DSE = 39.3 mAh g^-1 under 7C rate conditions (14.0 mA cm^-2)]. After analyzing the Li metal surface using scanning electron microscopy and X-ray photoelectron spectroscopy, we were able to infer that the stabilized solid electrolyte interphase layer formed by the combination of LiNO₃ and the dual salt resulted in a uniform Li deposition during repeated Li plating/stripping processes.

Highly Stable Porous Polyimide Sponge as a Separator for Lithium-metal Secondary Batteries

To inhibit Li-dendrite growth on lithium (Li)-metal electrodes, which causes capacity deterioration and safety issues in Li-ion batteries, we prepared a porous polyimide (PI) sponge using a solution-processable high internal-phase emulsion technique with a water-soluble PI precursor solution; the process is not only simple but also environmentally friendly. The prepared PI sponge was processed into porous PI separators and used for Li-metal electrodes. The physical properties (e.g., thermal stability, liquid electrolyte uptake, and ionic conductivity) of the porous PI separators and their effect on the Li-metal anodes (e.g., self-discharge and open-circuit voltage properties after storage, cycle performance, rate capability, and morphological changes) were investigated. Owing to the thermally stable properties of the PI polymer, the porous PI separators demonstrated no dimensional changes up to 180 °C. In comparison with commercialized polyethylene (PE) separators, the porous PI separators exhibited improved wetting ability for liquid electrolytes; thus, the latter improved not only the physical properties (e.g., improved the electrolyte uptake and ionic conductivity) but also the electrochemical properties of Li-metal electrodes (e.g., maintained stable self-discharge capacity and open-circuit voltage features after storage and improved the cycle performance and rate capability) in comparison with PE separators.