Nanostructured Niobium Oxides as Negative Electrodes for Lithium and Sodium Ion Batteries
Authors: Barnes Pete Levi 
Published: August 2021

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 (LixNb2O5, 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.