Elucidating the Role of Defects for Electrochemical Intercalation in Sodium Vanadium Oxide

Abstract

Na 1.25+x V 3 O 8 (with x < 0, = 0, and > 0) was synthesized via a wet chemical route involving the reduction of V 2 O 5 in oxalic acid and NaNO 3 followed by calcination. It was possible to control the sodium composition in the final product by adjusting the amount of sodium precursor added during synthesis. It was revealed that deficient and excessive sodium contents, with respect to the ideal stoichiometry, are accommodated or compensated by the respective generation of oxygen vacancies and partial transition metal reduction, or cation disordering. When examined as NIB electrode material, the superior performance of the cation disordered material with excessive sodium was clearly demonstrated, with more than 50% higher storage capacity and superior rate capacity and cyclic stability. The formation of oxygen vacancies initially seemed promising but was coupled with stability issues and capacity fading upon further cycling. The disparity in electrochemical performance was attributed to variations in the electronic distribution as promoted through Na−ion interactions and the direct influence of such on the oxygen framework (sublattice); these factors were determined to have significant impact on the migration energy and diffusion barriers. ■ INTRODUCTIONThe proliferation of electrical energy demand has driven the rapid progression of improved technologies related to energy distribution and storage. However, energy storage materials and devices have come to be viewed as a crux impeding advanced device development. Lithium-ion (Li-ion) batteries are a mature and robust technology because of their high energy density and portability. Despite their success in such application, Li-ion batteries (LIBs) are a poorly suited choice for large-scale energy storage applications given their high cost, associated with resource scarcity, as well as safety concerns. Conversely, sodium-ion (Na-ion) batteries have been gaining considerable traction as a realistic candidate for large-scale energy storage applications over the past several years.Na-ion batteries (NIB) are attractive because sodium resources are seemingly inexhaustible as well as ubiquitous and, therefore, cost considerably less (by a factor of roughly 30−40 times) than lithium; additionally, sodium does not undergo an alloying reaction with aluminum at low voltage, as is the case with lithium, meaning that aluminum can replace copper as the anodic current collector, which equates to an overall cell cost savings of ∼2%. 1,2 The lower operating voltage of Na-ion cells results in enhanced stability of the nonaqueous electrolyte but also manifests itself in lower energy density. The majority of the proposed electrode materials for Na-ion battery show similar or slightly lower specific capacity and redox potential than when used in Li-ion cells. Moreover, the accommodation of sodium in traditional host materials is difficult because the ionic radius and reduction potential of sodium are strikingly larger than that of lithium. Therefore, the de/sodiation process induces l...

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