Nanostructured Niobium Oxides as Negative Electrodes for Lithium and Sodium Ion Batteries

Over 30 years have passed since the commercialization of the lithium ion battery (LIB), which to date continues to present key challenges in energy/power, stability, and safety. Intense research efforts have made large strides in developing durable LIBs with high energy and power densities toward a wide range of applications from electric vehicles to large scale energy storage systems for renewable energy. Nevertheless, limited work has been done in the applications of amorphous oxide electrode materials due to the perception that amorphous materials are less electrically conductive than crystalline ones. Recent studies suggest that inducing crystallization of amorphous nanostructured oxides through electrochemical cycling can lead to materials with enhanced electrochemical charge storage performance. However, fundamental knowledge regarding the driving forces, thermodynamics, and nucleation and growth kinetics of the electrochemically-induced amorphous-to-crystalline (a-to-c) transformation remains limited. In this work we report a new nanostructured rock salt (RS) Nb₂O₅ electrode formed through operando electrochemical cycling of amorphous Nb₂O₅ with Li⁺. This new polymorph of Nb₂O₅ exhibits high capacity, superb rate capability, and great cycle life in LIBs, owing to the open framework of the cubic structure. We show, for the first time, that the insertion of three lithium ions into Nb₂O₅ (~ 1.5 electron transfers per Nb) is possible in the new RS-Nb₂O₅ (Li_xNb₂O₅, 0 ≤ x ≤ 3) for Li-ion storage. Utilizing the a-to-c approach in Nb₂O₅ electrodes permitted a much higher specific capacity because the system was naturally allowed to choose and optimize its crystalline structure through a process of self-organization. This work also presents a unique opportunity to utilize nanostructured Nb₂O₅ materials for the sodium ion battery (SIB) system, which is an attractive alternative to LIB due to a more sustainable outlook. The origin of this endeavor began by studying the degradation of sodium hexafluorophosphate (NaPF₆)-based non-aqueous electrolytes containing different solvent mixtures (e.g., cyclic and acyclic carbonates) in the presence of water, highlighting two electrolyte additives, 2,2,2-trifluoroethoxy-2,2,2-ethoxy phosphazene (FM2) and fluoroethylene carbonate (FEC). We found that FEC is not efficient to protect the electrodes from being exposed to HF, while FM2 mitigated HF formation. Initial insight into nanochanneled Nb₂O₅ negative electrodes for SIB have indicated material with larger pore size, smaller wall thickness, and more amorphous character perform at the highest capacity and at larger rates. Our efforts focus on developing a fundamental understanding of the self-organization of nanoscale amorphous transition metal oxides during cycling, and examining the physiochemical phenomena of this phase transformations for creating a powerful modular approach to designing improved battery materials with programmable physical and chemical properties. In addition, this work expands into exploring the effects of crystallinity on the performance of Nb₂O₅ electrodes used in SIBs. Our broader impact was aimed at profoundly transforming electrical energy storage research, fabrication, and applications to enable new design strategies for nanoscale oxide materials that go beyond current energy materials performance limits.