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Li-metal batteries employing Li-metal anodes with the highest capacity density have attracted considerable attention. However, the Li-metal deposition is often nonuniform, which reduces the cycle performance of Li-metal batteries. The three-dimensionally ordered macroporous polyimide (3DOM PI) separator has provided much better Li deposition/dissolution cycle performance owing to its unique and ordered pore structure, but its low mechanical strength is an obstacle to practical use. In this study, we added polybenzimidazole (PBI) to increase the mechanical strength of the 3DOM PI separator. Both the 3DOM PI and 3DOM PI+PBI separators with pore sizes of 300 and 100 nm were prepared to investigate the effect of pore size on the mechanical strength of the separator. The 3DOM PI+PBI separators exhibited higher tensile strength, higher thermal stability, better Li deposition/dissolution cycle performance, and higher charge–discharge performance of the Li-metal battery than the 3DOM PI separators. Here, it was suggested that the increase in the mechanical strength of the separator prevented the compression of the pores in the separator by external pressure and provided a more uniform Li+ ion flux inside the separator.

In this study, lithium-sulfur (Li-S) cells, employing various electrolytes, were manufactures to investigate the impact of the electronic conductivity of S-carbon composite cathode layer, the Li6.25Al0.25La3Zr2O13 (Al-LLZO) solid electrolyte separator, and highly-concentrated liquid electrolytes on the electrochemical reaction of S-cathodes. Firstly, the composite particles comprising nanocarbon and S were optimized by using various nanocarbon materials. Then, the highly-concentrated electrolytes, containing sulfolane solvents, proved effective in suppressing the dissolution of discharge intermediates, such as Li2S8 and Li2S6. However, some irreversible behavior of S-cathode was still observed, attributed to the dissolution of discharge intermediates, and S itself, even within the highly concentrated electrolytes. To address this issue, an Al-LLZO solid electrolyte pellet was introduced as a separator in the Li-S cell. This separator served to prevent contact between discharge intermediates and Li-metal anode, thus minimizing chemical self-discharge of intermediates resulting from S-discharge. The use of the Al-LLZO solid electrolyte separator led to improved reversibility of the S-cathode. In fact, the discharge capacity retention was enhanced employing this cell configuration, combining the highly-concentrated electrolyte and the Al-LLZO separator. However, the cell impedance increased during the extended cycle testing, leading to degradation of the discharge capacity. This can be attributed to the loss of electronic conductivity within the cathode layer, caused by the volume change in S, and the formation of a resistive layer on the Al-LLZO solid electrolyte, stemming from chemical reactions with the discharge intermediates.

Highly concentrated ionic liquid electrolytes (ILEs) have garnered considerable attention because of the safety of Li metal batteries. However, the use of highly concentrated electrolytes including ILEs is limited by the low affinity to separators. The separator directly contacts the surface of the Li metal anode, which significantly affects the morphology of deposited Li metal. In this study, a three-dimensionally ordered macroporous polyimide (3DOM PI) was proposed as a high-affinity separator for highly concentrated ILE ([lithium bis(fluorosulfonyl)imide]1[N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide]1). The 3DOM PI separator is made of polyimide and has a high porosity with a 3D-ordered uniform pore structure. These properties resulted in a higher affinity for highly concentrated ILE and higher ionic conductivity compared with that of conventional separators, making it possible to improve the reversibility of Li deposition/dissolution and the rate capability of the full cells. Moreover, the 3DOM PI separator improved the uniform Li deposition/dissolution and enabled the non-dendritic deposition of Li metal. The 3DOM PI with ILE provided more stable Li metal deposition/dissolution (charge/discharge reaction).