Unlocking High-Performance Quasi-Solid-State Multivalent Metal Ion Batteries

In the pursuit of advancing post-Li-ion battery technology, the development of rechargeable Mg batteries has shown tremendous promise due to their high theoretical capacity, material abundance, and low cost. However, the passivating nature of Mg, particularly in aqueous solutions, presents a significant obstacle. The practicality of Mg anodes within aqueous electrolytes has been largely dismissed due to the perceived lack of reversibility and narrow electrochemical window, leading researchers to focus on non-aqueous systems. Nonetheless, non-aqueous electrolytes also suffer from poor ionic conductivity, high cost, and safety hazards such as flammability and toxicity.<br/><br/>This work addresses these challenges by introducing a pioneering quasi-solid-state magnesium-ion battery (QSMB), designed to harness the advantages of both aqueous and non-aqueous systems. By immobilizing the electrolyte's water network, the polyethylene polymer polymer suppresses the hydrogen evolution reaction at the magnesium metal anode, and achieves a fully reversible Mg dissolution and deposition chemistry through a Cl-induced transformation of the impermeable passivation film into a conductive metallic oxide complex.<br/><br/>The quasi-solid-state electrolyte also plays a major role in attaining high voltage and capacity by facilitating true multivalent Mg ion storage. Through confining the hydrogen bond network, the electrolyte effectively impedes proton insertion which competes with multivalent metal-ion insertion in aqueous batteries. In this study, in-situ characterizations reveal that the electrolyte promotes the high-voltage (de-)intercalation of both Mg²⁺ and MgCl₃^- ions instead, delivering unparalleled battery performance.<br/><br/>The QSMB showcases a remarkable energy density of 264 Wh kg^-1—nearly five times higher than its aqueous Mg-ion counterparts, and a voltage plateau (2.6-2.2 V) that surpasses other Mg-ion batteries. Moreover, it retains an impressive 90% capacity after 900 cycles, even at subzero temperatures (-22°C). By leveraging the high ionic conductivity from aqueous systems and a broad electrochemical window from non-aqueous systems, the quasi-solid-state battery represents an innovative avenue for the design of high-performing Mg-ion batteries.<br/><br/>More importantly, this study provides invaluable insights into interphase passivation, metal stripping and plating mechanisms at the anode, and ion storage regulation at the cathode to produce high-energy, rechargeable, solid-state battery technologies. This quasi-solid-state approach could be extended to the design of other multivalent metal-ion batteries, including Zn-ion and Al-ion batteries, demonstrating the potential for broader applications.