Nonflammable phosphate electrolytes for high-safety lithium–sulfur batteries

Abstract

Lithium–sulfur battery is cost-effective and has a theoretical specific energy density of up to 2600 W·h·kg−1; moreover, the earth is rich in sulfur resources; therefore, it is considered a promising candidate for next-generation high-energy-density batteries. However, a few obstacles still persist in the process of commercialization, including the insulation of charge and discharge products, huge volume changes, flammable electrolytes, polysulfide “Shuttle Effect”, and unstable lithium metal anode. Among these challenges, safety is a main concern in the process of commercialization. Batteries using sulfurized polyacrylonitrile (PAN/S) as positive electrode active material have substantial advantages in terms of lifespan and a wider selection of electrolytes. As lithium polysulfide dissolution has not been considered, phosphate-based electrolytes can be used in this system because they also have good nonflammable properties. Phosphate-based flame retardants or nonflammable solvents can capture combustion free radicals and have excellent flame retardant effects as main solvents and flame retardant additives. This study explores the application of high-concentration triethyl phosphate (TEP) and tri (2,2,2-trifluoroethyl) phosphate (TFP) electrolytes in Li-PAN/S batteries to improve battery safety for next-generation high-energy-density secondary batteries. We investigate the effect of TEP and TFP-based electrolytes on the stability of lithium metal negative and sulfur positive electrodes as well as interfacial reactions during battery cycling. Charge–discharge and Li deposition/stripping tests were conducted using a high-concentration electrolyte with a molar ratio of 1∶2 between LiFSI and TEP. The results reveal that the electrolyte can achieve a 99.6% S utilization at 0.1 Cand stable Li deposition/stripping performance. However, the high viscosity and limited ionic conductivity of electrolyte limit its battery performance under high-loading electrode and high-rate conditions. By conducting electrochemical tests on TEP and TFP-based electrolytes with different TTE dilution ratios, we found that the charge–discharge overpotential of TFP-based electrolytes was higher, making it difficult to stably cycle in Li/Cu half cells. Furthermore, TFP molecules formed an unstable interface layer on the Li metal surface, which deteriorates the cell performance. After diluting TEP-based electrolyte with TTE at a volume fraction of 1∶2, excellent rate performance was achieved, with stable cycling of over 200 cycles at a rate of 1C and ultimately maintaining a discharge specific capacity of 1038.1 mA·h·g−1. These results highlight the potential of high-concentration TEP as a promising candidate for safer and higher-performing Li–S batteries.

Keywords

• nonflammable electrolytes
• localized high-concentration electrolytes
• phosphate
• lithium–sulfur batteries
• sulfurized polyacrylonitrile