Quantifying and Suppressing Proton Intercalation to Enable High-Voltage Zn-Ion Batteries

Schematic of the Zn2+/H+ insertion mechanism in aqueous electrolyte: Zn2+ insertion dominates at high voltage, but H+ intercalation accounts for most of the observed capacity. The latter process exhibits poor reversibility at slow rates, as the precipitate detaches from the cathode surface.

Scientific Achievement

The relative contributions of Zn2+ and H+ insertion to the overall capacity
of VPO4F are differentiated and quantified in aqueous and hybrid electrolytes. Increasing the salt concentration and/or proportion of nonaqueous solvent successfully suppresses water dynamics, improving the anodic stability and promoting Zn2+ intercalation over competing processes.

Significance and Impact

To be implemented in commercial applications (e.g. grid storage), Zn-ion technology must improve its low-rate and/or long-term electrochemical performance, which is often diminished by competing H+ intercalation-water oxidation mechanisms. Here, a hybrid aqueous-nonaqueous electrolyte was designed to tune interfacial dynamics and favor Zn2+ insertion, which increases the energy density and improves the cyclability
of VPO4F at slow rates.

Research Details

  • The first successful demonstration of reversibile (de)insertion of a divalent ion in the high-voltage VPO4F tavorite structure was achieved.
  • The Zn2+/H+ co-insertion processes taking place during discharge of VPO4F in aqueous electrolytes were differentiated and quantified, where high-voltage activity resulted from Zn2+ insertion.
  • Similar to Li+ electrochemistry, two distinct phase transitions were observed upon Zn2+ insertion, depending on the degree of zincation.
  • By employing a hybrid electrolyte, the electrochemical activity of water was tailored to disfavor H+ intercalation and improve anodic stability.
  • When Zn2+ dominates electrochemistry, stable cycling at slow rates was observed and a high energy density of 237 Wh·kg-1 was attained.

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DOI: 10.1002/aenm.202102016

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