Na-ion technology is increasingly studied as a low-cost solution for grid storage applications. Many positive electrode materials have been reported, mainly among layered oxides and polyanionic compounds. The vanadium oxy/flurophosphate solid solution Na3V2(PO4)2F3-yO2y (0 ≤ y ≤ 1), in particular, has proven the ability to deliver ≈500 Wh kg-1, operating on the V3+/V4+ (y = 0) or V4+/V5+ redox couples (y = 1). This paper reports here on a significant increase in specific energy by enabling sodium insertion into Na3V2(PO4)2FO2 to reach Na4V2(PO4)2FO2 upon discharge. This occurs at ≈1.6 V and increases the theoretical specific energy to 600 Wh kg−1, rivaling that of several Li-ion battery cathodes. This improvement is achieved by the judicious modification of the composition either as O for F substitution, or Al for V substitution, both of which disrupt Na-ion ordering and thereby enable insertion of the 4th Na. This paper furthermore shows from operando X-Ray Diffraction (XRD) that this energy is obtained in the cycling range Na4V2(PO4)2FO2–NaV2(PO4)2FO2 with a very small overall volume change of 1.7%, which is one of the smallest volume changes for Na-ion cathodes and which is a crucial requisite for stable long-term cycling.

Disordered rocksalt (DRX) Li-rich transition metal (TM) oxides, especially those based on Mn, are prospective high-energy-density cathode materials for the next generation of Li-ion batteries that use earth abundant metals. Fluorine substitution on the oxygen sublattice has been shown to reduce oxygen redox by lowering the average anion valence and increasing the amount of redox-active TM, and simultaneously improve energy density, average voltage and capacity retention. While these benefits of fluorination are well established, it is not well understood how F affects Li transport and therefore the rate performance of Mn-based DRX cathodes. Herein, we investigate the effects of both F substitution and the accompanying Li-excess on Li migration barriers using first-principles calculations. We demonstrate that F has a small negative effect on Li migration barriers while Li-excess decreases Li migration barriers. Because fluorination enables more Li-excess, these results do not predict any detrimental impact on Li transport.

Understanding structural transformation at the surface of Ni-rich layered compounds is of particular importance for improving the performance of these cathode materials. In this Letter, we identify the surface phases using first-principles-based kinetic Monte Carlo simulations. We show that slow kinetics precludes the conventional Li 0.5 NiO 2 spinel to form from its layered parent phase at room temperature. Instead, we suggest that densified phases of the types Ni 0.25 NiO 2 and Ni 0.5 NiO 2 can form by Ni back diffusion from the surface owing to oxygen loss at highly charged states. Our conclusion is supported by the good agreement between the simulated STEM images and diffraction patterns and previously reported experimental data. While these phases can be mistaken for spinel and rock salt structures in STEM, they are noticeably different from these common structure types. We believe that these results clarify a long-standing puzzle about the nature of surface phases on this important class of battery materials.

In this work, we identify a new potential Mg battery cathode structure Mo₃(PO₄)₃O, which is predicted to exhibit ultra-fast Mg²⁺ diffusion and relatively high voltage based on first-principles density functional theory calculations. Nudged elastic band calculations reveal that the migration barrier of the percolation channel is only ∼80 meV, which is remarkably low, and comparable to the best Li-ion conductors. This low barrier is verified by ab initio molecular dynamics and kinetic Monte Carlo simulations. The voltage and specific energy are predicted to be ∼1.98 V and ∼173 W h kg^-1, respectively. If confirmed by experiments, this material would have the highest known Mg mobility among inorganic compounds.

Layered oxides have been the dominant cathodes in lithium-ion batteries, and among them, high-nickel (Ni) systems are attractive because of their high capacity. For practical use, synthetic control of stoichiometry and structural ordering is crucial but has been nontrivial due to the complexity inherent to synthesis reactions, which often proceed via nonequilibrium pathways. We report here a combined in situ synchrotron X-ray diffraction and ab initio study of solid-state synthesis of layered oxides starting from acetate precursors for LiCoO2and LiNiO2and their solid solution LiNi0.8Co0.2O2. While all three systems ultimately evolve into the same thermodynamically stable layered phase (R3¯ m), each chemistry involves distinct metastable intermediates. We explain the phase progressions using a structural template model, demonstrating that during the synthesis of LiCoO2, the formed metastable spinel polymorph (Li2Co2O4; Fd3¯ m) is a kinetically facile lithiation product of spinel Co3O4- the low-temperature (LT) intermediate from the decomposed Co-acetate. Similarly, in the Ni-based systems, the acetate decomposition products, rocksalts (Ni,Co)O, topotactically template the kinetic pathways of forming disordered rocksalts (Lix(Ni,Co)2-xO2; Fm3¯ m), consequently leading to off-stoichiometric Lix(Ni,Co)O2with undesired high Li/Ni mixing. These findings highlight new opportunities for engineering precursors to form LT intermediates that template the synthesis of target phases and structural properties.

First-principles based lattice models allow the modeling of ab initio thermodynamics of crystalline mixtures for applications such as the construction of phase diagrams and the identification of ground state atomic orderings. The recent development of compressive sensing approaches for the construction of lattice models has further enabled the systematic construction of sparse physical models without the need for human intuition other than requiring the compactness of effective cluster interactions. However, conventional compressive sensing based on L1-norm regularization is strictly only applicable to certain classes of optimization problems and is otherwise not guaranteed to generate optimally sparse and transferable results, so that the method can only be applied to some materials science applications. In this paper, we illustrate a more robust L0L1-norm compressive-sensing method that removes the limitations of conventional compressive sensing and generally results in sparser lattice models that are at least as predictive as those obtained from L1-norm compressive sensing. Apart from the theory, a practical implementation based on state-of-the-art mixed-integer quadratic programming (MIQP) is proposed. The robustness of our methodology is illustrated for four different transition-metal oxides with relevance as battery cathode materials: Li2xTi2(1-x)O2, Li2xNi2yO2, MgxCr2O4, and NaxCrO2. This method provides a practical and robust approach for the construction of sparser and more predictive lattice models, improving on the compressive sensing paradigm and making it applicable to a much broader range of applications.

The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions.

The safety issue of lithium-ion batteries is a crucial factor limiting their large-scale application. Therefore, it is of practical significance to evaluate the impact of their overcharge behavior because of the severe levels of oxygen release of cathode materials during this process. Herein, by combining a variety of in situ techniques of spectroscopy and electron microscopy, this work studies the structural degradation of LiNi0.8Co0.1Mn0.1O2 (NCM811) accompanying the oxygen release in the overcharge process. It is observed that a small amount of O2 evolves from the initial surface at ≈4.7 V. When charging to a higher voltage (≈5.5 V), a large amount of O2 evolves on the newly formed surface due to the occurrence of microcracks. Based on experimental results and theoretical calculations, it is determined that the oxygen release mainly occurs in the near-surface regions, where the remaining oxygen vacancies accumulate to create voids. To suppress the oxygen release, single-crystalline NCM811 with integrated structure is introduced and serves as a cathode, which can effectively inhibit morphology destruction and reduce the activation of lattice oxygen in the surface region. These findings provide a theoretical basis and effective strategy for improving the safety performance of Ni-rich cathode materials in practical applications.

In the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase. Understanding the competition between thermodynamics and kinetics is a fundamental step towards the rational synthesis of target materials. Here, we use in situ synchrotron X-ray diffraction to investigate the multistage crystallization pathways of the important two-layer (P2) sodium oxides Na_0.67MO₂ (M = Co, Mn). We observe a series of fast non-equilibrium phase transformations through metastable three-layer O3, O3' and P3 phases before formation of the equilibrium two-layer P2 polymorph. We present a theoretical framework to rationalize the observed phase progression, demonstrating that even though P2 is the equilibrium phase, compositionally unconstrained reactions between powder precursors favour the formation of non-equilibrium three-layered intermediates. These insights can guide the choice of precursors and parameters employed in the solid-state synthesis of ceramic materials, and constitutes a step forward in unravelling the complex interplay between thermodynamics and kinetics during materials synthesis.