# Crystalline covalent organic polymer as an effective zincophilic protective layer to boost the performance of aqueous zinc-ion batteries

## TL;DR

A new protective layer made of a crystalline polymer improves the performance and lifespan of aqueous zinc-ion batteries by enabling uniform zinc deposition.

## Contribution

A novel zincophilic covalent organic polymer (CityU-51) is developed for dendrite-free zinc anodes in aqueous batteries.

## Key findings

- CityU-51 enables an ultra-long battery lifespan exceeding 6300 hours at 1 mA cm−2.
- The polymer allows stable operation for over 6000 hours at an ultrahigh current density of 30 mA cm−2.
- A full cell achieves 268.63 mAh g−1 capacity with 97% retention over 500 cycles.

## Abstract

Developing an effective layer to isolate zinc anodes from electrolytes, stop side reactions and help uniform zinc deposition in aqueous zinc-ion batteries (AZIBs) is very important and highly desirable. However, most reported protective layers have less effective ion transport channels and poor zincophilicity, leading to short cycling lifetimes. To address this issue, we try to employ tetrathiafulvalene (TTF) derivatives as building units to construct single crystals of new polymers for such protection because these polymers can provide ordered ion-transport channels and strong interactions between sulfur and zinc species. Here, single crystals of a one-dimensional TTF-based organic polymer with square-wave-shaped chains, designated as CityU-51, have been prepared through the assembly of 4,4′,5,5′-tetra(isoquinolin-6-yl)-2,2′-bi(1,3-dithiolylidene) and 1,4-bis(benzodioxaborole) benzene via the formation of B–N bonds. CityU-51 exhibits high zincophilicity as the TTF moieties interact strongly with zinc species, demonstrating exceptional performance in guiding uniform zinc deposition. Remarkably, the as-fabricated AZIBs utilizing CityU-51 as an anode protective layer exhibit an ultra-long lifespan exceeding 6300 hours at 1 mA cm−2. Moreover, the batteries can continuously work over 6000 hours even at an ultrahigh current density of 30 mA cm−2. A high capacity of 268.63 mAh g−1 of a full cell is observed, and a stable capacity retention ratio of 97% is maintained over 500 cycles. This study underscores the significance of functional B–N polymers and offers a novel coating option for dendrite-free anodes in AZIBs.

Single crystals of a 1D tetrathiafulvalene-based covalent organic polymer (CityU-51) serve as anode protective layer in aqueous Zn-ion batteries, achieving an ultra-long lifespan exceeding 6300 h at 1 mA cm2 and over 6000 h at 30 mA cm2.