Ruetschi taught us that electrolytic manganese oxide establishes a higher EMF versus zinc in aqueous alkaline cells because it is cation defective. Thackeray, David, and Goodenough showed us in 1982 that even low-cost, environmentally benign iron oxides can serve as reversible hosts in nonaqueous electrolytes for Li-ion insertion. Counterintuitive though it may be, the combination works in nonaqueous media: the absence of the redox-active transition metal cation (Mn+) in mixed ion/electron-conducting metal oxides, such as oxides of vanadium [2] and iron [3], can increase, not decrease, the capacity of the defective material to insert lithium and thereby increase the quantity of energy stored. Getting something (higher specific capacity and energy density) for nothing (removing Mn+ from a structural unit) offers a promising and still under-exploited phenomenon in EES. As highlighted with examples from past and ongoing work, three protocols will be described to incorporate cation vacancies into transition metal oxides to improve performance in both aqueous and nonaqueous energy storage: (1) inducing point defects in conventional oxides using appropriate atmospheric conditions with a driving force such as temperature; (2) substitutional doping of a highly oxidized cation into a metal-oxide framework; and (3) expressing insertion oxides in morphologies with high surface areas, such as aerogels, that increase the fraction of surface sites that favor formation of cation vacancies.

Something from nothing: Enhancing cathode electrochemical charge storage with cation vacancies

253rd National Meeting (2017)

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