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In situ visualization of Li/Ag2VP2O8 batteries revealing rate-dependent discharge mechanism

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Science  09 Jan 2015:
Vol. 347, Issue 6218, pp. 149-154
DOI: 10.1126/science.1257289

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Watching the silver lining inside

Some types of batteries contain both a transition metal reducible metal, such as the cathode material Ag2VP2O8. During operation, both Ag and V ions are reduced, and the Ag atoms can form wires to enhance the internal conductivity. Kirshenbaum et al. probe the discharge of a battery at different rates and track the formation of Ag atoms using in situ energy-dispersive x-ray diffraction (see the Perspective by Dudney and Li). They show how the discharge rate affects whether the Ag or V is preferentially reduced and also the distribution of the Ag atoms, and then correlate this to the loss of battery capacity at higher discharge rates.

Science, this issue p. 149; see also p. 131

Abstract

The functional capacity of a battery is observed to decrease, often quite dramatically, as discharge rate demands increase. These capacity losses have been attributed to limited ion access and low electrical conductivity, resulting in incomplete electrode use. A strategy to improve electronic conductivity is the design of bimetallic materials that generate a silver matrix in situ during cathode reduction. Ex situ x-ray absorption spectroscopy coupled with in situ energy-dispersive x-ray diffraction measurements on intact lithium/silver vanadium diphosphate (Li/Ag2VP2O8) electrochemical cells demonstrate that the metal center preferentially reduced and its location in the bimetallic cathode are rate-dependent, affecting cell impedance. This work illustrates that spatial imaging as a function of discharge rate can provide needed insights toward improving realizable capacity of bimetallic cathode systems.

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