Abstract
A percolating network of high electrical conductivity needed to operate electrodes at a fast rate can be formed by in situ reduction of Ag+ originating from mixed metal oxide lattices, but few studies have elucidated trends in this mechanism as a function of Ag+ concentration and structure. Candidates compared for the first time here are spinel Ag2MoO4, monoclinic and triclinic Ag2Mo2O7, and Ag2Mo3O10·2H2O, which have reduction potentials for Ag+ and Mo6+ strongly decoupled by up to ∼600 mV in aqueous zinc-ion electrolyte. Under these conditions, Ag0 is the first reduction product and a decrease of charge transfer resistance by ∼100× is observed within 2.5% consumption of total Ag+ independent of initial structure. However, resistance metrics alone poorly describe materials which are robust to reducing silver with high energy at faster rates. Instead, after accounting for crystallinity and morphology differences, we find that the acidity of the molybdate framework is responsible for a switch in charge balance mechanism from the bulk formation of a mixed ZnMoOx to pseudocapacitive Zn2+ precipitation, and that this mechanism switch is associated with minimized losses to rate, voltage and capacity yields as carbon/binder free electrodes relative to composites. The location of this acidity cutoff near the pH of the ZnSO4 electrolyte may suggest a design principle for future low-carbon electrodes beyond molybdate framework structures.
Highlights
Electrodes must increasingly support faster electron transport as the devices they power become more demanding
Given that vanadium oxide cathodes have shown tremendous promise in aqueous zinc-ion batteries (AZIB's)[7,8,9] recent efforts to incorporate reductive displacement cathodes in this new application have included Ag0-doped V2O5,10 Ag0.4V2O5,11 Ag1.2V3O8,12 Ag2V4O11,13 Ag0.33V2O5@V2O5$nH2O,14 b-AgVO3,15 and even Cu analogs in CuV2O6,16,17 Cu0.95V2O5,18 CuxV2O5,19 Cu0.34V2O5 and Cu3(OH)2V2O7.21,22 an early emphasis on reporting individual structural metrics – which we summarize in Table S1† – rather than critical comparisons has led to inconsistencies when viewed broadly that, to our knowledge, have not been addressed
For triclinic Triclinic Ag2Mo2O7 (t-2Mo), m2Mo was heated for 5 hours at 450 C in air; this synthesis improves upon prior reports which have prepared this polymorph via solid-state sintering of Ag2MoO4 or Ag2O with MoO3 requiring 2–4 days.[40,46]
Summary
Electrodes must increasingly support faster electron transport as the devices they power become more demanding Towards this goal, the development of materials containing Ag+ is pursued for fundamental interest because the reduced product can contain a network of atomically dispersed, conductive Ag0 particles that support fast electron transport throughout the bulk.[1,2] Minute amounts of dispersed Ag+ are known to signi cantly improve electronic transport; as example, cathodes of the silver–vanadium–phosphate-oxide type show percolation thresholds with 0.3% volume reduction of Ag+ concurrently resulting in 106 decrease in electrical resistance.[3,4,5] Silver vanadium oxide (SVO) cathodes remain the dominant material in implantable cardiac de brillator batteries due to this reductive displacement mechanism.[6]. Among many possiblities,[24] sample crystallinity and morphology may rank highly considering that the
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