Linda Wangoh

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

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.

Electron lone pair distortion facilitated metal-insulator transition in β-Pb0.33V2O5 nanowires

The electronic structure of β-Pb0.33V2O5 nanowires has been studied with x-ray photoelectron spectroscopy techniques. The recent synthesis of defect-free β-Pb0.33V2O5 nanowires resulted in the discovery of an abrupt voltage-induced metal insulator transition. First principle calculations predicted an additional V-O-Pb hybridized “in-gap” state unique to this vanadium bronze playing a significant role in facilitating the transition. We confirm the existence, energetic position, and orbital character of the “in-gap” state. Moreover, we reveal that this state is a hybridized Pb 6s–O 2p antibonding lone pair state resulting from the asymmetric coordination of the Pb2+ ions.

Correlating Lithium Hydroxyl Accumulation with Capacity Retention in V2O5 Aerogel Cathodes

V2O5 aerogels are capable of reversibly intercalating more than 5 Li(+)/V2O5 but suffer from lifetime issues due to their poor capacity retention upon cycling. We employed a range of material characterization and electrochemical techniques along with atomic pair distribution function, X-ray photoelectron spectroscopy, and density functional theory to determine the origin of the capacity fading in V2O5 aerogel cathodes. In addition to the expected vanadium redox due to intercalation, we observed LiOH species that formed upon discharge and were only partially removed after charging, resulting in an accumulation of electrochemically inactive LiOH over each cycle. Our results indicate that the tightly bound water that is necessary for maintaining the aerogel structure is also inherently responsible for the capacity fade.

Uniform second Li ion intercalation in solid state ϵ-LiVOPO4

Full, reversible intercalation of two Li+ has not yet been achieved in promising VOPO4 electrodes. A pronounced Li+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x–ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li+ intercalation is a prerequisite for the formation of intermediate phases Li1.50VOPO4 and Li1.75VOPO4. The evolution from LiVOPO4 to Li2VOPO4 via the intermediate phases is confirmed by direct comparison between O K–edge absorption spectroscopy and density functional theory.

Electrochemical Performance of Nanosized Disordered LiVOPO4

ε-LiVOPO4 is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO4, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by 7Li and 31P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO4 at C/50 (1 C = 153 mA h g–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO4/Li3VO4/acetylene black composite yields the high cycling capacity of 250 mA h g–1 at C/5 for over 70 cycles.