Resolving the Solvation Structure and Transport Properties of Aqueous Zinc Electrolytes from Salt-in-Water to Water-in-Salt Using Neural Network Potential
Chuntian Cao, A. Kingan, Ryan C. Hill, Jason Kuang, Lei Wang, Chunyi Zhang, Matthew R. Carbone, Hubertus J. J. van Dam, Shinjae Yoo, Shan Yan, Esther S. Takeuchi, Kenneth J. Takeuchi, Xifan Wu, Milinda Abeykoon, Amy C. Marschilok, Deyu Lu
Abstract
<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <a:msub> <a:mrow> <a:mi>Zn</a:mi> <a:mi>Cl</a:mi> </a:mrow> <a:mn>2</a:mn> </a:msub> </a:math> solutions are promising electrolytes for aqueous zinc-ion batteries. Here, we report a joint computational and experimental study of the structural and dynamic properties of aqueous <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <d:msub> <d:mrow> <d:mi>Zn</d:mi> <d:mi>Cl</d:mi> </d:mrow> <d:mn>2</d:mn> </d:msub> </d:math> electrolytes with concentrations ranging from salt-in-water to water-in-salt (WIS). By developing a neural network potential (NNP) model, we perform molecular dynamics (MD) simulations with accuracy but at much larger lengths and longer timescales. The NNP predicted structures are validated by the structure factors measured by X-ray total scattering experiments. The MD trajectories provide a comprehensive and quantitative picture of the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <g:msup> <g:mi>Zn</g:mi> <g:mrow> <g:mn>2</g:mn> <g:mo>+</g:mo> </g:mrow> </g:msup> </g:math> solvation shell structures. Additionally, we find that the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <j:mrow> <j:mrow> <j:mi mathvariant="normal">O</j:mi> </j:mrow> </j:mrow> <j:mo>−</j:mo> <j:mrow> <j:mrow> <j:mi mathvariant="normal">H</j:mi> </j:mrow> </j:mrow> </j:math> covalent bonds in water are strengthened with increasing salt concentration, thus expanding the electrochemical stability window of aqueous electrolytes. In terms of dynamic properties, the calculated and experimentally measured conductivities are in good agreement. Through the analysis of the calculated cation transference number, we propose a three-stage charge carrier transport mechanism with increasing concentration: independent ion transport, strongly correlated ion transport, and small positive charge carrier diffusion through negatively charged polymeric clusters. Our study provides fundamental atomic scale insights into the structure and transport properties of the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <o:msub> <o:mrow> <o:mi>Zn</o:mi> <o:mi>Cl</o:mi> </o:mrow> <o:mn>2</o:mn> </o:msub> </o:math> electrolyte that can aid the optimization and development of WIS electrolytes.