Defining Diffusion Pathways in Intercalation Cathode Materials: Some Lessons from V₂O₅ on Directing Cation Traffic

Abstract

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.

Mg2+storage and mobility in anatase TiO2: the role of frustrated coordination

“…59,60 Anatase TiO 2 has high thermal and chemical stability, does not readily undergo transformation to rutile, and is not generally considered 'metastable' in the same way as, for example, chemically de-intercalated tunnel bronze structures. 47 Anatase TiO 2 therefore provides a stable structure in which frustrated coordination of Mg (and Li-ions) occurs under dilute conditions and this leads to low migration barriers for the ions. The limitation of the anatase structure is therefore not its stability under cycling, but the loss of frustrated coordination at high Mg (or Li) concentrations.…”

“…6 We note that such an assumption neglects any contribution from Coulombic interactions between Mg ions and electrons localised upon Ti 3+ ions (small polarons), which are known to contribute to limited diffusion for Li-ion cathode materials 45 and may affect the mobility of Mg ions. 46,47 For comparison with anatase, the Mg diffusion barriers in the a and d polymorphs of V 2 O 5 , which is considered one of the most promising oxides for Mg ion battery cathodes 2 are $1100 and $600 meV respectively. 48,49 Similarly low barriers to that of anatase, of $600-800 meV have been identied in spinel-structured oxides.…”

Phase stability of intercalated V2O5 battery cathodes elucidated through the Goldschmidt tolerance factor

“…The coordination in the δ phase has also been described as 'tetrahedral', with the distances to the O3 ions considered too long to be real coordination lengths, leaving coordination to two O2 ions in one layer and two O1 ions in the adjacent layer. 28,33 The relative stability of the α and δ -phases is clearly linked with the size of the interlayer ion M: smaller M ions have a preference for the lower (4+2) coordination in the δ phase, and stabilise it, whilst larger M ions favour the higher coordination number (8) in the α phase.…”

“…23,24 Electronic conductivity and formation of stable localised lattice distortions (polarons) are additional factors proposed to affect mobility of intercalated ions in V 2 O 5 . [25][26][27][28][29] One key factor affecting intercalation in Mg x V 2 O 5 is the crystal structure at varying x compositions. The structure of V 2 O 5 has been investigated as a Li-ion battery cathode, and it is known to undergo multiple phase changes upon lithiation.…”

Strongly Coupled Pyridine‐V2O5·nH2O Nanowires with Intercalation Pseudocapacitance and Stabilized Layer for High Energy Sodium Ion Capacitors

“…Unfortunately, a large capacity loss of almost 40% after only 30 cycles was observed, which definitely hindered the application for layered V 2 O 5 · n H 2 O as cathode for high energy and power SICs. Several investigations indicate that the origins of capacity loss for vanadium oxides are the collapse of layer structure, self‐trapping of polarons, and decrease of conductivity during Na + (de)intercalations . Hence, metal ions preintercalation as “pillar effect” was proposed to enhance the layered structure and restrain the “lattice breathing”, which has shown an enhancement of the cycling stability .…”