image: Schematic illustration on allelic reconfiguration strategy and corresponding characterization.
Credit: ©Science China Press
In this "post lithium-ion battery" era, next-generation multivalent metal ion batteries, as a potential option for sustainable energy storage technology, have garnered intensive research attention in recent decade. Although multivalent charge carriers can facilitate multiple electrons transfer processes during electrochemical reactions, their intrinsic stronger electrostatic interactions with the host material would result in sluggish diffusion kinetics, thereby exhibiting limitations in terms of electrochemical performance.
To address the challenges, this study takes hydrated vanadate as the host material and, for the first time, prepares Cu/Zn solid-solution phase hosts with varying ratios. The layered crystal structure of this host, along with abundant interlayer confined species (H₂O and -OH), provides favorable diffusion pathways for charge carriers. Additionally, the introduction of active Cu further enhances the redox reaction activity, resulting in excellent reversible calcium-ion storage capacity in an organic electrolyte system.
Design Concept
Through a scalable co-precipitation method, Cu element is used to perform solid-solution substitution at Zn sites, forming a copper and zinc co-occupied host structures with both equal and unequal ratios. As shown in the figure, the primary reason for selecting Cu/Zn solid-solution is that, they both are transition metal elements in the ds block (like closest neighbor), exhibiting very similar properties in terms of outer electron configuration, atomic radius, and electronegativity. Consequently, the solid-solution phase crystal structure does not undergo significant lattice distortion, further ensuring excellent structural robustness.
Theoretical Simulation
Based on first-principles calculations, the influence of lattice water on the diffusion barrier of charge carriers was first analyzed, and a corresponding linear relationship was derived, indicating that the presence of lattice water plays a crucial role for diffusion kinetics of charge carriers. Secondly, the stabilizing effect of solid-solution substitution on interlayer lattice water was further explored, revealing the intrinsic reasons behind the "water-locking" mechanism of Cu/Zn solid-solution. Finally, by combining calculations of the electronic density of states, the mechanism by which solid-solution substitution enhances the intrinsic electronic conductivity of pyrovanadate was investigated.
Experimental Validation
By virtue of electrochemical performance, the capability of the solid-solution phase hydrated pyrovanadate host for calcium ion storage was evaluated. As observed in the figure below, the "copper" and "zinc" solid-solution phase materials can further activate the redox reaction plateau, thereby achieving reversible electrochemical calcium ion storage behavior. This validates the effectiveness and feasibility of as-proposed solid-solution design strategy for multivalent charge carrier hosts.