A quasi-solid dual-salt polymer electrolyte for high performance sodium metal batteries

Abstract

Sodium metal batteries (NMBs) are promising alternatives to lithium batteries as intermittent energy storages systems for large-scale grid powers due to their low-cost and high natural abundance of Na. However, Na dendrite growth in the organic liquid electrolytes (OLEs) and leakage of liquid induces safety risks. One of the effective strategies to improve the safety of NMBs is to replace OLEs with quasi-solid electrolytes (QSEs). QSE contains a unique hybrid structure with a polymer host with one or more liquid plasticizers and Na salt. Although QSE can eliminate electrolyte leakage and reduce dendritic growth, the typical Na salts used in QSE still encounter problems such as risk of explosion (sodium perchlorate, NaClO₄), decomposition when exposed to moisture (sodium hexafluorophosphate, NaPF₆), and corrosion to the aluminum current collector (sodium bis(trifluoromethanesulfonyl)imide, NaTFSI). New strategies, such as developing new salts, or using mixed salts are needed to solve these problems. In this thesis, the effect of mixed salts NaTFSI and sodium-difluoro(oxalate)borate (NaDFOB) were studied in polymer-based QSE, and a free-standing dual-salt polymer electrolyte (DSPE) was obtained via a simple solution casting method. Several aspects related to DSPE were investigated: materials characterization of DSPE; the electrochemical performances of DSPE based on the molar ratio of NaDFOB to NaTFSI from 0, 5, 10, 20, 40, and 100 mol.%; and the interfacial analysis of electrode cycled with DSPE. It was found that DSPE with 20 mol.% of NaDFOB (DSPE-20) delivered a high ionic conductivity (0.3 mS cm^-1), an extended electrochemical window (4.5 V), and improved capacity retention in the battery of 98.3% in 100 cycles compared with 54.7% of using DSPE with 0 mol.% of NaDFOB (DSPE-0) and 95.6% of using DSPE with 100 mol.% of NaDFOB (DSPE-100). The improved performance of DSPE-20 was due to the high ionic conductivity given by NaTFSI, and a stable cathode electrolyte interphase (CEI) with Na_xBO_yF_z that decomposed by NaDFOB preferentially among the components in DSPE as confirmed by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and density functional theory (DFT) calculation.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Francesco CIUCCI (Supervisor)