# Weak-solvation dilution for interphase preservation and extreme low-temperature sodium-ion storage

## TL;DR

A new electrolyte strategy enables sodium-ion batteries to operate efficiently at extremely low temperatures by improving ion transport and interphase stability.

## Contribution

A weak-solvation dilution strategy using methyl difluoroacetate is introduced to enhance low-temperature performance in sodium-ion batteries.

## Key findings

- The optimized electrolyte achieves 60 Wh kg−1 energy density at −70°C using NFPP and hard carbon.
- Pouch cells retain 83% of room temperature capacity at −30°C and function at −50°C.
- The strategy improves interphase durability and enables high-power operation at low temperatures.

## Abstract

Sodium-ion batteries (SIBs) show significant promise for their abundance and potential fast kinetics. However, ether-based electrolytes are limited by low anodic stability, and carbonate electrolytes typically suffer from reduced conductivity and sluggish desolvation at lower temperatures. Here, a weak-solvation dilution strategy that utilizes methyl difluoroacetate (MDFA) as a weakly solvated diluting agent to prepare an optimized carbonate/ester hybrid electrolyte is reported. MDFA reduces the bulk viscosity and takes part in the solvation structures to promote bulk ionic transportation and desolvation. Meanwhile, the reduced interaction between sodium and the solvent system limits salt dissolution to improve the interphase durability. The optimized electrolyte has an energy density of 60 Wh kg−1 at −70°C with a full cell using Na4Fe3(PO4)2P2O7 (NFPP) and hard carbon (HC), which exceeds the current limitation of carbonate electrolytes. It also enables Na(Ni1/3Fe1/3Mn1/3)O2||HC (NNFMO||HC) pouch cells to maintain 83% of room temperature capacity at −30°C and function with an appliance at −50°C. This strategy provides the possibility of operating high-power SIBs under low temperatures, which could also be extended to other batteries.

A diluting agent features with improved ionic conduction at low temperatures is used to fabricate an ultra-low concentration electrolyte, which supports sodium-ion battery to work at −70°C.