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New energy storage methods are emerging to increase the energy density of state-of-the-art battery systems beyond conventional intercalation electrode materials. For instance, employing anion redox can yield higher capacities compared with transition metal redox alone. Anion redox in sulfides has been recognized since the early days of rechargeable battery research. Here, we study the effect of d–poverlap in controlling anion redox by shifting the metal dband position relative to the S pband. We aim to determine the effect of shifting the dband position on the electronic structure and, ultimately, on charge compensation. Two isostructural sulfides LiNaFeS2and LiNaCoS2are directly compared to the hypothesis that the Co material should yield more covalent metal–anion bonds. LiNaCoS2exhibits a multielectron capacity of ≥1.7 electrons per formula unit, but despite the lowered Co dband, the voltage of anion redox is close to that of LiNaFeS2. Interestingly, the material suffers from rapid capacity fade. Through a combination of solid-state nuclear magnetic resonance spectroscopy, Co and S X-ray absorption spectroscopy, X-ray diffraction, and partial density of states calculations, we demonstrate that oxidation of S nonbonding pstates to S22–occurs in early states of charge, which leads to an irreversible phase transition. We conclude that the lower energy of Co dbands increases their overlap with S pbands while maintaining S nonbonding pstates at the same higher energy level, thus causing no alteration in the oxidation potential. Further, the higher crystal field stabilization energy for octahedral coordination over tetrahedral coordination is proposed to cause the irreversible phase transition in LiNaCoS2.

Identifying positive electrode materials capable of reversible multivalent electrochemistry in electrolytes containing divalent ions such as Mg2+, Ca2+, and Zn2+at high operating potentials remains an ongoing challenge in “beyond lithium-ion” research. Herein, we explore the Zn2+charge-storage mechanism of a vanadium-based Na+superionic conductor (NASICON), Na3V2(PO4)3. By using X-ray synchrotron techniques to unravel potential-dependent structure–property relationships, we ascribe the reversible electrochemical behavior of Na3V2(PO4)3to a quasi-two-stage intercalation process that involves both Na+and Zn2+. Initial charging of Na3V2(PO4)3leads to a Na+-extracted phase corresponding to NaV2(PO4)3, whereas subsequent discharge results predominantly in Na+intercalation followed by Zn2+intercalation. Operando X-ray diffraction of Na3V2(PO4)3was used to study the phase changes associated with the first charge/discharge process, and ex situ measurements were used to precisely link the changes in the crystal structure to a quasi-two-stage intercalation of Na+and Zn2+. The corresponding changes in the V-oxidation state, V-O coordination, and the presence of Zn2+were confirmed by X-ray absorption spectroscopy. The results of this work present a comprehensive understanding of the charge-storage properties for a well-established NASICON structure that confers both the high capacity (∼100 mA h g–1) and high potential (1.35 and 1.1 V vs Zn/Zn2+).

Electrochemically-formed disordered rock salt compounds are an emerging class of Li-ion electrode materials for fast-charging energy storage. However, the specific factors that govern the formation process and the resulting charge storage performance are not well understood. Here, we characterize the transformation mechanism and charge storage properties of an electrochemically-formed disordered rock salt from V9Mo6O40(VMO). The crystal structure of VMO has similar motifs to that of α-V2O5, a well-studied analogue, but VMO has less mechanical flexibility due to additional corner-sharing octahedra in its structure. As a result, VMO undergoes a single-step transformation pathway, which we characterize through operando X-ray diffraction, and forms an unusual highly distorted lamellar microstructure, as we show with high-resolution transmission electron microscopy. The resulting LixVMO material shows fast charging and other electrochemical characteristics and performance typical of many nanomaterials, even though the material is composed of relatively large particles.