Realization of a Long-Lifespan Thin Li Metal Batteries Capable of Self-Recovery with a Functional Separator
Authors: Jang Eunbin , Seo Harim , Yoo Jeeyoung 
Published: December 2024

Due to the rapid expansion of the electric vehicles (EVs) market, the demand for large-scale batteries is increasing. Particularly, EVs face several limitations in terms of battery volume and weight. Therefore, achieving high energy density in limited spaces requires a significant reduction in the volume of the anode. Recently, the research is underway on batteries that employ thin Li metal anodes, typically around 20-25μm thick, as the next generation of lithium-ion batteries. However, thin Li anodes present various challenges regarding their cycle life. Compared to Li metal, thin Li metal faces scarcity in lithium sources. Furthermore, due to the unstable formation and destruction of the Solid Electrolyte Interphase (SEI) during the plating-stripping process in Li metal anode charging and discharging, lithium ions and electrolyte are constantly consumed. These side reactions lead to decreased capacity and deteriorated lifespan characteristics. To maintain stable charging and discharging properties, it is necessary to minimize electrolyte decomposition by forming a stable SEI in the early cycle and continuously supplying consumed Li ions throughout the cycling process. Commonly used methods to supplement the limited lithium source of the thin Li anode and replenish consumed Li ions during charging and discharging represent the employing the high-concentration electrolytes (HCE). However, HCE have limits on the dissolved concentration of salts in the electrolyte, and the method of dissolving salts in the entire electrolyte is not attractive a cost perspective. Additionally, as the electrolyte concentration increases, viscosity also increases, leading to a decrease in ion mobility.

In this study, high-concentration lithium bis(fluoromethanesulfonyl)imide (LiFSI) salts are coated onto commercial separators to provide a locally high-concentration environment at the electrode/electrolyte interface. This coating layer dissolves as Li ions are consumed, replenishing the depleted Li ions. Furthermore, the presence of high-concentration Li ions in the electrolyte eliminates free solvent. Reducing free solvent suppresses electrolyte consumption further. Additionally, by adopting LiFSI as the salt, rapid decomposition of anions in the initial cycle facilitates the formation of a stable LiF layer. The densely formed LiF layer exhibits high mechanical strength and electrochemical stability, aiding in achieving long-term characteristics.

Li||Li symmetric cells were tested using commercial electrolytes, including 1M LiFSI in diethylene glycol dimethyl ether (DME) and 1M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/ diethylene carbonate (EC/DEC). Both electrolytes showed improved cycle stability and lower polarization in cells using an ionically conductive salt-coated separator (SDL) compared to polyethylene (PE) separators. Particularly, the nucleation polarization observed during the initial charging process, when Li ions are deposited onto bare lithium metal, significantly decreased in SDL cells, indicating better Li ion affinity on the lithium surface of SDL cells. Tafel plots were used to analyze the exchange current at the electrode/electrolyte interface. The exchange current for PE was 1.09 mA/cm2, while for SDL, it was 1.52 mA/cm2, suggesting faster interfacial kinetics in SDL compared to PE. The results were also evident in rate capability tests, with the largest capacity difference observed at 5C. Scanning electron microscope (SEM) images revealed distinctly different Li ion deposits on the Cu foil. PE showed vigorous dendrite growth with many voids, while SDL exhibited dense Li deposition, attributed to the presence of high-concentration salts at the interphase, suppressing dendrite growth and inducing uniform Li ion flux. The stable interfacial characteristics were analyzed using X-ray photoelectron spectroscopy (XPS). After 100 cycles, PE showed a high proportion of electrolyte decomposition components, while SDL exhibited a higher proportion of LiF formation. These results demonstrate coated separator with LiFSI effectively suppressed electrolyte decomposition. Additionally, boehmite was pre-coated on the PE separator to endow with coating adhesion, resulting in improved surface morphology. When various commercial electrolytes were applied after salt coating, significantly enhanced wettability was observed compared to PE. Ultimately, half-cells were constructed using thin-film Li metal for galvanostatic charge-discharge tests, achieving superior cycle stability of over 200 cycles with SDL.

Figure 1