Mixed Valence Niobium Oxides for High Power 3D Microbattery Applications
Authors: Asfaw Habtom Desta , Edström Kristina , Nyholm Leif 
Published: February 2020

Research in microbatteries is stimulated by the need for miniature power sources for use in autonomous sensing, wireless communications, medical implants and other microscale devices (1-10 mm3).1, 2 In such applications wherein physical space is at premium, sufficiently small, yet powerful, energy storage devices are desired. Currently, either thin or thick film microbatteries are in common use; however, 2D geometries suffer from severe limitations in that they are unable to deliver both high power and high energy densities simultaneously. The requirement for high energy and power densities can be met using (1) high energy materials that are structurally and electronically suited for fast rate cycling (examples: b-TiO2, Nb2O5, α-MoO3…)3-5 and (2) 3D–structured electrodes which allow for the use of an increased amount of active material without affecting the power delivery.1, 2, 6-8 In the latter context, a variety of electrode architectures have been considered including various types of carbon foams.9-11 By virtue of its bicontinuous architecture and reasonably high electrical conductivity, a carbon foam electrode provides an effective conduit for the transport of electrons and an interconnected porosity for the percolation of the electrolyte throughout the electrode. As a result, an intimate contact can be maintained amongst the current collector (the carbon foam), the active material (deposited onto the carbon foam) and the electrolyte (flooding the voids) over a microscale length. This in turn ensures efficient utilization of the active material at the desired cycling rates. It should be noted, however, that practical applications require optimal porosity and void sizes to ensure an acceptable trade-off between power and energy densities. To this end, part of the research in microbatteries focuses on the development of suitable porous electrodes.
This presentation highlights the fabrication of 3D microstructured electrodes using emulsion-templated carbon foams as scaffold for niobium oxide nanoparticles. As compared to commercially available carbon foams, they have higher specific surface areas and void sizes that are two orders of magnitude smaller.10-12The void size distribution, surface areas and electrical conductivity of the carbon foams can be fine-tuned by optimizing the synthesis conditions. A brief discussion will be presented in this regard. Niobium is known to form both stoichiometric and non-stoichiometric oxides. The mixed valency present in the nonstoichiometric oxides is associated with increased electrical conductivity.13 The semiconducting nature of these oxides makes them ideal candidates for high power applications. In this particular case, the carbon foam has been used for dual purposes: (1) to entrap the oxide nanoparticles and (b) to promote the reduction of part of the Nb5+ to Nb4+ at elevated temperatures:
Nb2O5 + δC à NbO2.5-δ + δCO
Two types of mixed valence oxides have been synthesized and investigated for potential applications in high power 3D microbatteries. Detailed discussions on the structures and electrochemical behaviors of the oxides will be offered in the presentation.
Figure 1. (A) SEM of NbOx-coated carbon foam (B) CV of electrode in (A)
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