Manipulating Coulombic Efficiency of Cathodes in Aqueous Zinc Batteries by Anion Chemistry

Electrolyte environments, including cations, anions, and solvents are critical for the performance delivery of cathodes of batteries. Most works focused on interactions between cations and cathode materials, in contrast, there is a lack of in-depth research on the correlation between anions and cathodes. Here, we systematically investigated how anions manipulate the coulombic efficiency (CE) of cathodes of zinc batteries. We take intercalation-type V2O5 and conversion-type I-2 cathodes as typical cases for profound studies. It was found that electronic properties of anions, including charge density and its distribution, can tune conversion or intercalation reactions, leading to significant CE differences. Using operando visual Raman microscopy and theoretical simulations, we confirm that competitive coordination between anions and I- can regulate CEs by modulating polyiodide diffusion rates in Zn-I-2 cells. In Zn-V2O5 cells, anion-tuned solvation structures vastly affect CEs through varying Zn2+ intercalation kinetics. Conversion I-2 cathode achieves a 99 % CE with highly electron-donating anions, while anions with preferable charge structures that interact strongly with Zn2+ afford an intercalation V2O5 a nearly 100 % CE. Understanding the mechanism of anion-governed CEs will help us evaluate compatibility of electrolytes with electrodes, thus providing a guideline for anion selection and electrolyte design for high-energy, long-cycling zinc batteries.