# Single‐Crystalline Borate Covalent Organic Frameworks for Solid‐State Lithium Metal Batteries

## TL;DR

This paper introduces a new single-crystalline borate COF material that shows excellent performance as a solid-state electrolyte for lithium metal batteries.

## Contribution

The synthesis of a single-crystalline 3D borate COF with high ionic conductivity and dendrite suppression in lithium metal batteries.

## Key findings

- The B-COF achieved 8.1 mS cm−1 ionic conductivity and 0.98 lithium-ion transference number at room temperature.
- Symmetric cells using B-COF enabled stable lithium deposition/stripping for 2000 hours without dendrite formation.
- Full cells with LiFePO4 cathodes retained 91.8% capacity after 600 cycles with 99.98% Coulombic efficiency.

## Abstract

To advance lithium metal batteries, novel solid‐state electrolytes are crucial. Covalent Organic Frameworks (COFs) are promising due to their crystalline, porous structure, light composition, strong bonds, high surface area, and stability. COFs can be engineered for enhanced ion conduction, with their porosity and ion‐functional groups enabling fast ion movement and uniform lithium deposition. We synthesized a single‐crystalline 3D borate COF (B‐COF), achieving an ionic conductivity of 8.1 mS cm−1 at room temperature and a lithium‐ion transference number of 0.98 in a quasi‐solid‐state. In symmetric cells, B‐COF supported stable lithium deposition/stripping for 2000 h, suppressing dendrite formation. Full cells with LiFePO4 cathodes cycled stably at 0.5C, delivering 147 mAh g−1 initial capacity, 91.8% retention, and 99.98% Coulombic efficiency over 600 cycles. These results highlight B‐COF's potential as a high‐performance solid electrolyte for lithium metal batteries.

Advancing lithium metal batteries demands novel solid‐state electrolytes. Covalent organic frameworks (COFs) offer high ion conduction via porosity and functional groups. We synthesized single‐crystalline 3D borate COF with 8.1 mS cm−1 conductivity and 0.98 Li+ transference. It enables dendrite‐free cycling for 2000 h and stable LiFePO4 cells over 600 cycles with 91.8% retention.