TiNb2O7 is a Wadsley–Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in Li x TiNb2O7) is rapid with low activation barriers from NMR and D Li = 10–11 m2 s–1 at the temperature of the observed T 1 minima of 525–650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100–200 meV down the tunnels but ca. 700–1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in Li x TiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1–3 eV, indicating that this structure is specifically suited to Li+ mobility.

PNb 9 O 25 , a Wadsley-Roth compound whose structure is obtained by appropriate crystallographic shear of the ReO 3 structure, is a high-power electrode material that can reach 85 % of the equilibrium capacity in 30 minutes and 67% in 6 minutes. Here we show that multielectron redox, as observed through X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, and an insulator-to-metal transition upon lithium insertion, as suggested by a number of complementary techniques, contribute to the impressive performance. Chemically tuning the tetrahedral site between phosphorus and vanadium leads to significant changes in the electrochemistry and kinetics of lithium insertion in the structure, pointing to larger implications for the use of crystallographic shear phases as fast-charging electrode materials.

Whether attempting to eliminate parasitic Li metal plating on graphite (and other Li-ion anodes) or enabling stable, uniform Li metal formation in 'anode-free' Li battery configurations, the detection and characterization (morphology, microstructure, chemistry) of Li that cannot be reversibly cycled is essential to understand the behavior and degradation of rechargeable batteries. In this review, various approaches used to detect and characterize the formation of Li in batteries are discussed. Each technique has its unique set of advantages and limitations, and works towards solving only part of the full puzzle of battery degradation. Going forward, multimodal characterization holds the most promise towards addressing two pressing concerns in the implementation of the next generation of batteries in the transportation sector (viz. reducing recharging times and increasing the available capacity per recharge without sacrificing cycle life). Such characterizations involve combining several techniques (experimental-and/or modeling-based) in order to exploit their respective advantages and allow a more comprehensive view of cell degradation and the role of Li metal formation in it. It is also discussed which individual techniques, or combinations thereof, can be implemented in real-world battery management systems on-board electric vehicles for early detection of potential battery degradation that would lead to failure.

Wadsley–Roth crystallographic shear phases are a family of transition-metal oxides that show tremendous promise as electrode materials in Li-ion batteries. Despite their ability to intercalate lithium at high rates, little is known about their structural, thermodynamic, and electronic properties as a function of Li concentration. In this study, we use first-principles statistical mechanics methods to explore the lithium-site preference, lithiation strain, and electronic structure of PNb9O25, a Wadsley–Roth phase that has been shown to reversibly cycle at a rate of 60 C and that can accommodate more than one Li per Nb. We find that Li ions can occupy five symmetrically distinct interstitial sites within the PNb9O25 crystal structure, with three being pyramidal sites coordinated by five oxygen and two being window sites with square-planar oxygen coordination. The insertion of Li into PNb9O25 leads to a complex site filling sequence, with pyramidal sites preferred at low Li concentrations, followed by the filling of window sites at higher Li concentrations. Our findings are aided by neutron diffraction where pyramidal sites are found to be filled at low compositions. The order in which sites are filled is strongly influenced by the chemical strain due to Li insertion. The strain arises from the delocalization of donated electrons over the d orbitals of the structure’s edge-sharing niobium, which leads to a tetragonal distortion along the c-axis, thereby making vertical window sites favorable for Li occupancy at intermediate to high Li concentrations. Given the crystallographic similarities among different shear phases, we expect that the results of this study will also shed light on the electrochemical properties of other Wadsley–Roth chemistries.