# AlF3 Mediated In‐Situ Cathode Interface Stabilization Enables High‐Rate and Long‐Life Na‐Ion Batteries at Elevated Temperature

## TL;DR

A new AlF3 coating stabilizes sodium-ion battery cathodes at high temperatures, enabling long-lasting and high-rate performance.

## Contribution

A dual-functional AlF3 coating is introduced to stabilize the cathode interface and improve conductivity in sodium-ion batteries.

## Key findings

- AlF3 coating forms a stable CEI composed of NaF/AlF3/NaAlF4, suppressing side reactions and enhancing Na+ diffusion.
- Modified NVOPF@C shows 84.9% capacity retention after 1000 cycles at 10 C at 55°C in half cells.
- The coating creates a continuous conductive network with F─Al─O bonds and carbon nanotubes for efficient electron transfer.

## Abstract

The instability of the cathode electrolyte interface (CEI) under high voltage and elevated temperature poses a major challenge to the practical application of Na3V2(PO4)2O2F (NVOPF) cathodes in sodium‐ion batteries (SIBs). To overcome this issue, we introduce a dual‐functional AlF3 coating that effectively stabilizes the interface while improving bulk electronic conductivity. The AlF3‐modified NVOPF@C exhibits exceptional cycling stability at 55°C, maintaining 84.9% capacity after 1000 cycles at 10 C in half cells and 96.8% at 5 C in full cells using hard carbon anodes. Detailed characterization reveals that the AlF3 coating promotes the formation of a robust, inorganic‐rich CEI, primarily composed of a NaF/AlF3/NaAlF4 ternary fluoride composite. This newly constructed CEI layer not only acts as a protective barrier to suppress detrimental interfacial side reactions but also serves as an efficient ionic conductor to facilitate Na+ diffusion. In addition, the AlF3 coating induces the creation of F─Al─O bridging bonds with surface oxygen groups, which collaborate with carbon nanotubes to establish a highly continuous conductive network that enables efficient electron transfer. These findings underscore the crucial significance of constructing a stable inorganic‐dominated CEI and continuous conductive pathways for developing high‐rate and thermally stable SIBs.

Ultrathin AlF3 coating facilitates the formation of an inorganic‐dominated cathode electrolyte interphase (CEI), predominantly composed of NaF/AlF3/NaAlF4. This CEI layer effectively suppresses detrimental interfacial side reactions while simultaneously enhancing the diffusion kinetics of Na+. As a result, the modified NVOPF exhibits outstanding long‐term cycling stability under high rates in both half and full cells, particularly at 55°C.