# Enhanced sodium storage in hard carbon via solvent co-intercalation electrolyte enabling Ah-level pouch cells at low temperatures

## TL;DR

A new electrolyte design improves sodium ion battery performance at low temperatures by enabling efficient ion storage in hard carbon.

## Contribution

A co-intercalation ether electrolyte is introduced to bypass slow desolvation and enable low-temperature performance in sodium ion batteries.

## Key findings

- The co-intercalation electrolyte achieves an initial Coulombic efficiency of 80.5% at −50°C.
- An Ah-level full cell retains 107 Wh kg⁻¹ specific energy at −50°C.
- The electrolyte design overcomes the trade-off between low-temperature ion desolvation and diffusion.

## Abstract

Sodium ion batteries are attracting extensive interest due to their low cost and abundant sodium resources. However, sodium ion batteries still suffer from severe performance degradation at low temperatures due to the conflict between ion desolvation and diffusion. Herein, we design a co-intercalation ether electrolyte to achieve solvent co-intercalation in the hard carbon negative electrode, thereby bypassing the slow desolvation process while ensuring rapid ion diffusion in electrolyte and hard carbon. The optimized solvation structure also promotes the formation of a thin, inorganic-rich solid electrolyte interface, facilitating interfacial ion transport. As a result, the co-intercalation electrolyte enables hard carbon to deliver good low-temperature performance, with an initial Coulombic efficiency of 80.5% at −50°C (20 mA g-1) and a capacity retention of 93% after 200 cycles (100 mA g-1). Moreover, an Ah-level full cell retains cell stack-level (excluding packaging) specific energy of 163 Wh kg-1 at 25 °C and 107 Wh kg-1 at −50°C (100 mA g-1), demonstrating the practical feasibility of this strategy for wide-temperature sodium ion batteries. This work has the potential to overcome the long-standing trade-off between low-temperature ion desolvation and diffusion, offering an approach for electrolyte design toward wide-temperature sodium ion batteries.

Slow electrolyte desolvation can impact kinetics in hard carbon-based electrodes, limiting low-temperature cyclability. Here, authors design a co-intercalation electrolyte which bypasses slow desolvation, enabling an Ah-level pouch cell with a specific energy density of 107 Wh kg⁻¹ at −50 °C.