Screening metal cation additives driven by differential capacitance for Zn batteries

In electrochemical devices, the electric double layer (EDL) theory posits that the potential difference of the Helmholtz layer significantly influences the activation energy and reaction rate of the electrochemical reaction, which determines the uniformity of metal ion deposition. In this work, we compare the differential capacitance of metal cationic sulfate, along with factors such as cost as well as safety, and Ce(SO₄)₂ is selected to adjust the EDL at the Zn anode/electrolyte interface. Molecular dynamics simulations show the formation of a more compressed diffuse layer with the help of Ce⁴⁺, resulting in a decreased potential difference of the Helmholtz layer, an improved Zn²⁺ deposition overpotential and a reduced Zn electrode corrosion rate. In situ XRD and Raman spectroscopy reveal that Zn²⁺ deposition is favored along the (002) crystal plane, and hydrogen evolution is hindered. The Ce⁴⁺ modified electrolyte enables a Zn anode with high coulombic efficiency (99.6%) over 600 cycles at 10 mA cm⁻². The compressed EDL extends the lifespan of the Zn-Zn cell to 2500 hours at 5 mA cm⁻² and stable cycling for 300 cycles at 20 mA cm⁻² and 10 mA h cm⁻². Assembling Zn full cells with high-loading cathodes shows enhanced long-term cycling performance. Our findings reveal the potential of using high-valence metal cations like Ce⁴⁺ to regulate the EDL and uniform Zn²⁺ deposition in ZIBs, which suggests a promising direction for developing electrolytes for practical ZIBs.