Amorphous VO2 : A Pseudocapacitive Platform for High-Rate Symmetric Batteries.

Among the various VO2 polymorphs, the layered compound, VO2 (B), has been the most widely investigated lithium-ion battery electrode material. For sodium-ion electrodes, however, an amorphous solid may be more advantageous as a result of the open framework to facilitate ion insertion and the ability to tolerate volumetric changes. Herein, it is shown that the Na+ insertion properties of amorphous VO2 (a-VO2 ) are superior to those of crystalline VO2 (B). Amorphous VO2 exhibits a linear voltage characteristic over a 3 V range (4.0 to 1.0 V vs Na/Na+ ) leading to a reversible capacity as high as 400 mAh g-1 and rapid redox kinetics, which is attributed to its pseudocapacitive nature. The linear voltage characteristic over 3 V affords the opportunity of fabricating a symmetric Na-ion battery in which the a-VO2 material serves as both the positive electrode and the negative electrode. Such a symmetric battery offers safer operation in terms of overcharging, overdischarging, polarity reversal, high charge/discharge current abuse, and long-term usage. The results suggest that amorphous transition metal oxides may offer advantageous attributes for rapid, safe, and energy-dense storage.

Fast-Charging Cathodes from Polymer-Templated Mesoporous LiVPO4F.

Fast-charging cathodes with high operating voltages are critical to the development of high energy and power density lithium-ion batteries. One route to fast-charging battery materials is through the formation of nanoporous networks, but these methods are often limited by the high calcination temperatures required for synthesis. Here, we report the synthesis of carbon-coated nanoporous LiVPO4F with excellent rate capabilities that can be stably cycled up to 4.6 V in standard LiPF6 electrolytes. During charge and discharge at 30C, 110 mAh/g (70% of theoretical capacity) was obtained, and only 9% of capacity was lost after 2000 cycles at 20C. These materials also showed excellent stability, with little self-discharge, an open-circuit voltage of 4.2 V, and a discharge capacity of 139 mAh/g obtained after holding for 12 h. Rate capabilities were further demonstrated in a proof-of-concept full cell made with a nanostructured Nb2O5. These devices were able to deliver 200 mAh/g at 1C and 100 mAh/g at 30C. Finally, operando X-ray diffraction and electrochemical kinetics were further used to provide insight into the nature of fast charging in these materials.

Differentiating Double-Layer, Psuedocapacitance, and Battery-like Mechanisms by Analyzing Impedance Measurements in Three Dimensions.

Electrochemical energy storage arises from processes that are broadly categorized as capacitive, pseudocapacitive, or battery-like. Advanced charge-storing materials that are designed to deliver high capacity at a high rate often exhibit a multiplicity of such mechanisms, which complicates the understanding of their charge-storage behavior. Herein, we apply a "3D Bode analysis" technique to identify key descriptors for fast Li-ion storage processes, where AC impedance data, such as the real capacitance (C') or phase angle (ϕ), are represented versus the frequency (f) and a third independent variable, the applied DC cell voltage. For double-layer processes, a near-constant C' or ϕ is supported across the entire voltage range, and the decrease in these values shows a near-linear decrease at higher f. For pseudocapacitance, an increase in C' is delivered, accompanied by high C' retention at higher f compared to double-layer processes. Interestingly, the lower ϕ values, where C' is highest, suggest that this is a key descriptor for pseudocapacitance, where high-rate charge storage is still facilitated within a kinetically limited regime. For battery-like processes, a high C' is only observed at the voltage at which the material stores charge, while outside that voltage, C' is negligible. The three-dimensional (3D) Bode analysis allows charge-storage dynamics to be mapped out in great detail with more delineation between mechanisms compared to the more frequently deployed kinetic analyses derived from cyclic voltammetry.

Irreversibility at macromolecular scales in the flake graphite of the lithium-ion battery anode.

Charging a commercial lithium-ion battery intercalates lithium into the graphite-based anode, creating various lithium carbide structures. Despite their economic importance, these structures and the dynamics of their charging-discharging transitions are not well-understood. We have videoed single microcrystals of high-quality, natural graphite undergoing multiple lithiation-delithiation cycles. Because the equilibrium lithium-carbide compounds corresponding to full, half, and one-third charge are gold, red, and blue respectively, video observations give direct insight into both the macromolecular structures and the kinematics of charging and discharging. We find that the transport during the first lithiation is slow and orderly, and follows the core-shell or shrinking annuli model with phase boundaries moving at constant velocities (i.e. non-diffusively). Subsequent lithiations are markedly different, showing transport that is both faster and disorderly, which indicates that the initially pristine graphite is irreversibly and considerably altered during the first cycle. In all cases deintercalation is not the time-reverse of intercalation. These findings both illustrate how lithium enters nearly defect-free host material, and highlight the differences between the idealized case and an actual, cycling graphite anode.

Designing Pseudocapacitance for Nb2O5/Carbide-Derived Carbon Electrodes and Hybrid Devices.

Composite structures for electrochemical energy storage are prepared on the basis of using the high-rate lithium ion insertion properties of Nb2O5. The Nb2O5 is anchored on reduced graphene oxide (rGO) by hydrothermal synthesis to improve the charge-transfer properties, and by controlling the surface charge, the resulting Nb2O5-rGO particles are attached to a high-surface-area carbide-derived carbon scaffold without blocking its exfoliated layers. The electrochemical results are analyzed using a recently published multiscale physics model that provides significant insights regarding charge storage kinetics. In particular, the composite electrode exhibits surface-confined charge storage at potentials of <1.7 V (vs Li/Li+), where faradaic processes dominate, and electrical double layer charge storage at potentials of >2.2 V. A hybrid device composed of the composite electrode with activated carbon as the positive electrode demonstrates increased energy density at power densities comparable to an activated carbon device, provided the hybrid device operates in the faradaic potential range.