Cation-Disordered Rocksalt Cathode for Anode-Free Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) are gaining attention as a safe, cost-effective, and environmentally friendly energy storage solution, largely due to the natural abundance of zinc and manganese, and the use of aqueous electrolytes. These electrolytes offer benefits such as fast ion transport and low toxicity, but their mildly acidic nature leads to side reactions at the zinc metal anode—particularly corrosion, byproduct formation, and hydrogen evolution. These processes begin upon cell assembly and continue even at rest, often overlooked due to the excess zinc commonly used in cells, with negative-to-positive (N/P) ratios exceeding 150. However, experiments show that over 70% of capacity can be lost after just five days of idle aging, indicating severe anode degradation and raising concerns about material waste and sustainability. Although strategies like artificial interfacial layers and electrolyte additives help reduce side reactions, they cannot fully prevent corrosion while a metallic zinc anode is present. A more transformative approach is the anode-free ZIB, in which the anode is simply a current collector, and zinc is electroplated and stripped during cycling. This configuration dramatically improves zinc utilization, reduces waste, and enhances energy density by lowering the N/P ratio to near zero. Yet, realizing such systems requires cathode materials that are pre-loaded with zinc in their pristine form—an essential criterion seen in anode-free lithium-ion cells using LiCoO₂. Most conventional ZIB cathodes, such as MnO₂ and V₂O₅, do not contain zinc in their initial state and would require pre-loading steps, which are impractical at scale. Few zinc-containing cathodes—e.g., ZnMn₂O₄, Zn₃V₃O₈, Zn₂MoO₃—exist, but each has drawbacks. ZnMn₂O₄ suffers from strong Zn²⁺ repulsion in its spinel structure unless modified. Vanadium-based compounds have toxicity and voltage limitations, and Prussian blue analogues like Zn₃[Fe(CN)₆]₂ offer insufficient capacity. In this study, we present ZnMnO₂, a zinc-containing cathode based on a cation-disordered rocksalt (DRX) structure, newly applied in aqueous ZIBs. DRX materials have shown promise in lithium-ion systems due to their high capacity and stability. ZnMnO₂ was synthesized and tested in 2 M ZnSO₄ with 0.2 M MnSO₄ electrolyte. It delivers a high initial charge capacity of 312.8 mAh g⁻¹ and an average discharge voltage of 1.36 V vs. Zn/Zn²⁺. Anode-free cells built with ZnMnO₂ showed stable cycling over 100 cycles without any modification to the electrolyte or anode. Characterization revealed that ZnMnO₂ undergoes a phase transition to a spinel structure during initial cycles, which then governs the battery's behavior. The dominant energy storage mechanism involves Mn dissolution and redeposition, alongside reversible Zn²⁺ intercalation. This work represents the first use of DRX materials in aqueous ZIBs and opens new pathways for cathode development. We also synthesized DRX ZnFeO₂, using earth-abundant Fe, which showed similarly promising results. These DRX-based cathodes, with intrinsic zinc content and chargeable structure, offer a viable route for the future of anode-free ZIBs.