Interplay between Cation and Anion Redox in Ni-Based Disordered Rocksalt Cathodes

Abstract

The reversibility of the redox processes plays a crucial role in the electrochemical performance of lithium-excess cation-disordered rocksalt (DRX) cathodes. Here, we report a comprehensive analysis of the redox reactions in a representative Ni-based DRX cathode. The aim of this work is to elucidate the roles of multiple cations and anions in the charge compensation mechanism that is ultimately linked to the unique electrochemical performance of Ni-based DRX cathode. The low-voltage reduction reaction results in the low energy efficiency and strong voltage hysteresis. Our data reveal that the Mo migration between octahedral and tetrahedral sites enhances the O reduction potential, thus offering a potential strategy to improve energy 1 efficiency. This work highlights the important role that the electrochemically inactive transition metal plays in the redox chemistry and provides useful insights into the potential pathway to further address the challenges in Ni-based DRX systems.

High-Voltage Reactivity and Long-Term Stability of Cation-Disordered Rocksalt Cathodes

“…Li-excess cation-disordered rocksalt (DRX) is an emerging family of high-capacity cathodes for next-generation Li-ion batteries. , Upon introduction of an excess of Li into the disordered cation lattice (≥10% per formula unit), long-range Li + diffusion is enabled via percolating 0-TM octahedral–tetrahedral–octahedral (o-t-o) channels. , Facile Li + diffusion and the ability to utilize cationic and anionic redox processes have resulted in exceptionally high reversible capacities of ≲300 mAh g –1 , and high energy densities of ≲1000 Wh kg –1 , for several Li-excess DRX cathodes. − While such cation-disordered transition metal (TM) oxides can exhibit high initial capacities, their long-term performance suffers from a fast capacity decay upon cycling. − One major cause of such performance degradation for DRX oxides has been identified as the irreversible loss of O from the crystal lattice, leading to impedance buildup. , Over the past few years, partial fluorination of DRX oxides has been proven to be effective at improving their cycling performance by suppressing oxygen gas release and mitigating surface degradation processes. − However, even the best-performing DRX oxyfluoride cathodes still exhibit a gradual decrease in capacity upon extended cycling.…”

A Universal Strategy Based on Bridging Microstructure Engineering and Local Electronic Structure Manipulation for High-Performance Sodium Layered Oxide Cathodes

“…With the increasing depletion of fossil fuels, there is a general recognition of the urgent need to develop energy storage technologies with high energy efficiency and cost effectiveness. , Over the past three decades, lithium-ion batteries have been widely used in a variety of fields due to their high energy density among existing rechargeable batteries. − Nevertheless, the high cost and limited resources of lithium restrict its large-scale application. − In this background, sodium-ion batteries (SIBs) have been regarded as viable energy storage systems due to their low cost, widely distributed sodium resources, and environmental friendliness. − Especially, electrode materials have recently achieved rapid development, such as carbon for anode materials and Prussian blue compounds, polyanionic compounds, layered metal oxides, and organic compound material as cathode material for SIBs. − …”

Quantitative Decoupling of Oxygen‐Redox and Manganese‐Redox Voltage Hysteresis in a Cation‐Disordered Rock Salt Cathode

“…Among various redox-mechanism studies for alkali transitionmetal oxide cathodes, recent spectroscopic and ab initio research has provided evidence correlating voltage hysteresis with oxygen redox, in which the lattice oxygen is oxidized to form various species (e.g., trapped O 2 [13,16] or non-dimerized O − [17,18] ) accompanied with varying local coordination (e.g., TM migration, [12,19,20] defect formation, [21] or ligand-to-metal charge transfer [22] ). However, few studies can experimentally decouple and quantify the incremental capacity contributions from both TM and O redox (dq i in Equation 2), particularly if these redox processes simultaneously occur at a given voltage.…”

“…Voltage hysteresis can be clearly visualized by plotting charging and discharging voltage V against cumulative capacity q , which is defined here as the net amount of electrons transferred from the cathode to the external circuit, for incremental electric charge dq > 0 on charging ( Chg ) and dq < 0 on discharging ( Dchg ): $$$$\begin{equation} q = \int \limits _{Chg}dq + \int \limits _{Dchg}dq \end{equation}$$$$At a definite voltage V , incremental capacity dq is measurable by an externally connected potentiostat, but internally dq is ultimately redistributed and compensated by a set of redox‐active species, denoted with i , such as TM and O in the DRX material: $$$$\begin{equation} dq = \sum _{i} dq_{i} \end{equation}$$$$Among various redox‐mechanism studies for alkali transition‐metal oxide cathodes, recent spectroscopic and ab initio research has provided evidence correlating voltage hysteresis with oxygen redox, in which the lattice oxygen is oxidized to form various species (e.g., trapped O 2 [ 13,16 ] or non‐dimerized O − [ 17,18 ] ) accompanied with varying local coordination (e.g., TM migration, [ 12,19,20 ] defect formation, [ 21 ] or ligand‐to‐metal charge transfer [ 22 ] ). However, few studies can experimentally decouple and quantify the incremental capacity contributions from both TM and O redox ( dq i in Equation 2), particularly if these redox processes simultaneously occur at a given voltage.…”