The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

The scarce inventory of compounds that allow for diffusion of multivalent cations at reasonable rates poses a major impediment to the development of multivalent intercalation batteries. Here, we contrast the thermodynamics and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically accessible metastable phases of V2O5, ζ- and ε-V2O5, with the relevant parameters for the thermodynamically stable α-phase of V2O5 using density functional theory calculations. The metastability of the frameworks results in a higher open circuit voltage for multivalent ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted within these structures encounter suboptimal coordination environments and expanded transition states, which facilitate easier ion diffusion. Specifically, a nudged elastic band examination of ion diffusion pathways suggests that migration barriers are substantially diminished for Na- and Mg-ion diffusion in the metastable polymorphs: the predicted migration barriers for Mg ions in ζ-V2O5 and ε-V2O5 are 0.62–0.86 and 0.21–0.24 eV, respectively. More generally, the results indicate that topochemically derived metastable polymorphs represent an interesting class of compounds for realizing multivalent cation diffusion because many such compounds place cations in “frustrated” coordination environments that are known to be useful for realizing low diffusion barriers.

The invention of rechargeable batteries has dramatically changed our landscapes and lives, underpinning the explosive worldwide growth of consumer electronics, ushering in an unprecedented era of electric vehicles, and potentially paving the way for a much greener energy future. Unfortunately, current battery technologies suffer from a number of challenges, e.g., capacity loss and failure upon prolonged cycling, limited ion diffusion kinetics, and a rather sparse palette of high-performing electrode materials. Here, we discuss the origins of diffusion limitations in oxide materials using V2O5 as a model system. In particular, we discuss constrictions in ionic conduction pathways, narrow energy dispersion of conduction band states, and the stabilization and self-trapping of polarons as local phenomena that have substantial implications for introducing multiscale compositional and phase heterogeneities. Strategies for mitigating such limitations are discussed such as reducing diffusion path lengths and the design of metastable frameworks yielding frustrated coordination and decreased barriers for migration of polarons.

Organic metal halide perovskites have rapidly emerged as among the leading candidates for the next generation of photovoltaic and light-emitting devices. The band gap, exciton binding energy, and absorption cross-section of these materials are tunable to some extent by compositional variation. Dimensional confinement represents an attractive alternative to compositional variation for tuning these properties via quantum confinement close to the Boḧr radius. While the stabilization of few-layered nanoplatelets of methylammonium lead bromide has recently been demonstrated, mechanistic understanding of synthetic parameters resulting in dimensional confinement remains to be developed. Here we show that the layer thickness can be precisely modulated as a function of the chain length and concentration of the added alkylammonium cations. Surface capping ligands bind preferentially to sheets of corner sharing PbBr 6 octahedra and thereby buffer the extent of supersaturation of monomeric units enabling precise modulation of the layer-by-layer growth of 2D nanoplatelets. Crystal growth can be confined to yield nanoplatelets with tunable unit cell thickness spanning between one and six layers, which allows for precise tuning of the emissive properties of 2D perovskite nanoplatelets in the range of 430−520 nm depending on the layer thickness. The results suggest a generalizable strategy for tuning the layer thickness of these materials as a function of the alkyl chain length of the ligands.

Understanding the diffusion mechanisms of Li ions through host materials and the resulting phase evolution of intercalated phases is of paramount importance for designing electrode materials of rechargeable batteries. The formation of lithiation gradients and discrete domains during intercalation leads to the development of strain within the host material and is responsible for the observed capacities of most cathode materials being well below theoretically predicted values. Such mesoscale heterogeneity has also been implicated in the loss of capacity upon cycling. Due to their inherent complexity, the analysis of such heterogeneity is rather complex and precise understanding of the evolution of metal sites remains underexplored. In this work, we use phase-pure, single-crystalline V 2 O 5 nanowires with dimensions of 183 ± 50 nm and lengths spanning tens of microns as a model cathode material and demonstrate that V K-edge Xray absorption near-edge structure can be used as an effective probe of the local valence and geometry of vanadium sites upon lithiation. We demonstrate that a highly lithiated phase is nucleated and grows at the expense of a homogeneous low-lithiumcontent α-phase without mediation of a solid-solution with intermediate lithium content. Density functional theory calculations allow for assignment of the pre-edge feature to dipolar transitions that are particularly sensitive to the V 3d−O 2p hybridization of the vanadyl bond and the local geometry of the distorted [VO 5 ] square pyramid. The quantitative analysis of multiple vanadium sites and their evolution as a function of Li-ion content provides insight into the mechanism of phase evolution and the nature of lithiation gradients. The phase coexistence and segregation is further observed in scanning transmission X-ray microscopy images of individual lithiated V 2 O 5 nanowires. The mechanisms and the dynamics of nucleation and growth unraveled here are of great importance for the design and discovery of Li-ion cathode materials.