Tara P. Mishra

Achieving high‐energy‐density and long‐term cycling stability in Mn‐redox‐based cathodes is a critical step toward advancing Ni‐ and Co‐free Li‐ion chemistries to diversify supply chains and lower battery costs. Here, we demonstrate an unusually high performance benchmark for Mn‐redox cathodes using near‐micron single‐crystalline δ‐phase cation‐disordered rocksalt (δ‐DRX), derived from Mn‐rich DRX, delivering ≈640 Wh kg ⁻¹ with ≈97% capacity retention over 400 cycles. The very high energy density and cycle life were achieved by using a high Mn content, to suppress oxygen redox activity, and careful control of the synthesis parameters to tune single crystal particle size. Using high‐throughput automated electron microscopy, we identify interparticle compositional heterogeneity present in the early stage of phase‐pure DRX formation as the origin of orthorhombic LiMnO ₂ impurity formation during cooling, a finding that rationalizes the long high‐temperature dwell times required and the associated particle coarsening in conventional synthesis. We demonstrate that an accelerated cooling strategy kinetically suppresses localized impurity formation, reducing long high‐temperature exposure, and enables downsizing of single‐crystalline Mn‐rich DRX particles to the near‐micron regime (D50 = 1.5 µm). This work provides a mechanistic basis for particle‐size control in Mn‐rich DRX synthesis, and presents δ‐DRX as a stable, sustainable, high‐performance, Ni‐ and Co‐free cathode material.

In materials formed by symmetry-reducing transformations such as δ-DRX, projection effects can hide domain boundaries or make them mimic disorder, limiting single-zone-axis STEM-HAADF characterization.

Lithium-ion (Li-ion) batteries are crucial to technological advancement due to their remarkable energy density and lifespan. However, traditional cathode materials, mainly based on layered and spinel structures, rely on scarce and contentious resources like nickel (Ni) and cobalt (Co). Disordered rocksalt (DRX) materials have surfaced as a novel category of high-capacity, earth-abundant cathodes. Since the discovery of DRX materials, advanced electron microscopy has been essential in unraveling the microstructural complexities of DRX cathodes. This presentation will explore the fundamental insights that electron microscopy has contributed to the study of DRX cathode materials. From short-range ordering to high-entropy DRX materials, electron microscopy has provided significant insights into the DRX platform. After a brief overview of DRX progress, we will examine the development of high Mn-content large particle (1 μm) DRX materials, which seems to be facilitated by the material's transformation to a spinel-like (δ-phase). This phase exhibits partial disorder across multiple length scales, posing challenges for classical characterization techniques. We employ scanning electron nano diffraction (SEND) with a pixelated detector to investigate the phase transformation of DRX cathodes to the δ-phase during chemical delithiation. Our findings indicate that the δ-phase is a partially disordered spinel with transition metals (Mn and Ti) displaying 16c/16d order over short coherence lengths. Additionally, high-resolution HAADF-STEM imaging reveals the presence of antiphase boundaries, which separate nanoscale (3 nm) spinel domains. These atomic-level insights help to elucidate the superior performance of the large particle chemically delithiated DRX cathodes.

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To address the growing demand for energy and support the shift toward transportation electrification and intermittent renewable energy, there is an urgent need for low‐cost, energy‐dense electrical storage. Research on Li‐ion electrode materials has predominantly focused on ordered materials with well‐defined lithium diffusion channels, limiting cathode design to resource‐constrained Ni‐ and Co‐based oxides and lower‐energy polyanion compounds. Recently, disordered rocksalts with lithium excess (DRX) have demonstrated high capacity and energy density when lithium excess and/or local ordering allow statistical percolation of lithium sites through the structure. This cation disorder can be induced by high temperature synthesis or mechanochemical synthesis methods for a broad range of compositions. DRX oxides and oxyfluorides containing Earth‐abundant transition metals have been prepared using various synthesis routes, including solid‐state, molten‐salt, and sol‐gel reactions. This review outlines DRX design principles and explains the effect of synthesis conditions on cation disorder and short‐range cation ordering (SRO), which determines the cycling stability and rate capability. In addition, strategies to enhance Li transport and capacity retention with Mn‐rich DRX possessing partial spinel‐like ordering are discussed. Finally, the review considers the optimization of carbon and electrolyte in DRX materials and addresses key challenges and opportunities for commercializing DRX cathodes.

Hydrogen holds great promise as a cleaner alternative to fossil fuels, but its efficient and affordable storage remains a significant challenge. Bimetallic systems, such as Pd and Ni, present a promising option for storing hydrogen. In this study, using the combination of different cutting-edge X-ray and electron techniques, we observed the transformations of Pd-Ni nanoparticles, which initially consist of a NiO-rich shell surrounding a Pd-rich core but undergo a major transformation when they interact with hydrogen. During hydrogen exposure, the Pd core breaks into smaller pockets, dramatically increasing its surface area and enhancing the hydrogen storage capacity, especially in nanoparticles with lower Pd content. The findings provide a deep understanding of the morphological changes at the atomic level during hydrogen storage and contribute to designing cost-effective hydrogen storage using multimetallic systems.

Mn‐rich disordered rocksalt materials with Li‐excess (DRX) materials have emerged as a promising class of earth‐abundant and energy‐dense next‐generation cathode materials for lithium‐ion batteries. Recently, an electrochemical transformation to a spinel‐like “δ” phase has been reported in Mn‐rich DRX materials, with improved capacity, rate capability, and cycling stability compared with previous DRX compositions. However, this transformation unfolds slowly over the course of cycling, complicating the development and understanding of these materials. In this work, it is reported that the transformation of Mn‐rich DRX materials to the promising δ phase can be promoted to occur much more rapidly by electrochemical pulsing at elevated temperature, rate, and voltage. To extend this concept, micron‐sized single‐crystal DRX particles are also transformed to the δ phase by the same method, possessing greatly improved cycling stability in the first demonstration of cycling for large, single‐crystal DRX particles. To shed light on the formation and specific structure of the δ phase, X‐ray diffraction, scanning electron nanodiffraction (SEND) and atomic resolution STEM‐HAADF are used to reveal a nanodomain spinel structure with minimal remnant disorder.

NASICON‐type Li conductors (Li‐NASICON) have traditionally been regarded as promising candidates for solid‐state Li‐air battery applications because of their stability in water and ambient air. However, the presence of water in the cathode of a Li‐air battery can induce a highly alkaline environment by modifying the discharge product from Li₂O₂ to LiOH which can potentially degrade cathode and separator materials. This study investigates the alkaline stability of common Li‐NASICON chemistries through a systematic experimental study of LiTi_xGe_2‐x(PO₄)₃ (LTGP) with varying x = 0–2.0. Density functional theory calculations are combined to gain a mechanistic understanding of the alkaline instability. It is demonstrated that the instability of LTGP in an alkaline environment is mainly driven by the dissolution of PO₄^3– groups, which subsequently precipitate as Li₃PO₄. The introduction of Ti facilitates the formation of a Ti‐rich compound on the surface that eventually passivates the material, but only after significant bulk degradation. Consequently, phosphate‐based Li‐NASICON materials exhibit limited alkaline stability, raising concerns about their viability in humid Li‐air batteries.