A novel stable semi-solid electrolyte with hollow structured metal organic framework as framework for fast Li+ transferring in lithium metal batteries

Abstract

Semi-solid electrolytes tend to inherit the advantages of both solid-state electrolytes and liquid electrolytes. However, the challenge is to simultaneously enhance both ionic conductivity, transference number and long cycle performance of batteries. In this study, a semi-solid electrolyte was successfully synthesized with a hollow structured metal organic framework (HMOF) and a polymer binder (PVDF). The hollow structure of HMOF serves as a framework to encapsulate a large amount of lithium salt and electrolyte, reducing concentration polarization in lithium metal batteries. Its unique pore structure and abundant hydroxyl functional groups can restrict anion movement, thereby increasing the Li-ion transference number. On the other hand, the PVDF acts as a binder, making the electrolyte thinner, more flexible, and easier to process for broader applications. Furthermore, a Li@HMOF semi-solid electrolyte (LHMSSE) was fabricated by adding Bis (trifluoromethane) sulfonimide lithium salt (LiTFSI) into the HMOF(Li@HMOF), which exhibits a high ionic conductivity of 0.89 mS cm^-1 at 30 °C and a high Li-ion transference number of 0.75 at 30 °C. Notably, the Lithium symmetric battery of Li||LHMSSE ||Li can operate stably for over 1200 h at a current density of 0.2 mA cm^-2, and the lithium metal full battery of Li||LHMSSE||LFP (LiFePO₄) retains a capacity of 127.7 mAh g^-1 after 2000 cycles at 1C, with a capacity retention rate of 89.1 % and an average coulombic efficiency of 99.9 %. When paired with high-load LFP (10 mg cm^-2) or NCM811 (LiNi_0.8Co_0.1Mn_0.1O₂) (10 mg cm^-2) cathodes, a stable cycling performance can be achieved at a rate of 0.2C.

Keywords: Anion immobilization; Lithium metal batteries; Metal-organic frameworks; Semi-solid electrolytes.