Abstract
Ag0.50VOPO4·1.8H2O (silver vanadium phosphate, SVOP) demonstrates a counterintuitive higher initial loaded voltage under higher discharge current. Energy dispersive X-ray diffraction (EDXRD) from synchrotron radiation was used to create tomographic profiles of cathodes at various depths of discharge for two discharge rates. SVOP displays two reduction mechanisms, reduction of a vanadium center accompanied by lithiation of the structure, or reduction-displacement of a silver cation to form silver metal. In-situ EDXRD provides the opportunity to observe spatially resolved changes to the parent SVOP crystal and formation of Ag0 during reduction. At a C/170 discharge rate V5+ reduction is the preferred initial reaction resulting in higher initial loaded voltage. At a discharge rate of C/400 reduction of Ag+ with formation of conductive Ag0 occurs earlier during discharge. Discharge rate also affects the spatial location of reduction products. The faster discharge rate initiates reduction close to the current collector with non-uniform distribution of silver metal resulting in isolated cathode areas. The slower rate develops a more homogenous distribution of reduced SVOP and silver metal. This study illuminates the roles of electronic and ionic conductivity limitations within a cathode at the mesoscale and how they impact the course of reduction processes and loaded voltage.
Highlights
Polyanion-type materials such as LiFePO4 have been heavily researched as battery cathodes because of their impressive stability and high operating voltage relative to oxide based materials [1,2,3]
The results reported here use in-situ Energy dispersive X-ray diffraction (EDXRD) with spatial resolution to investigate the reduction of Ag0.50VOPO4·1.8H2O cathodes for the first time to probe the counterintuitive higher initial loaded voltage seen under higher current discharge
Elemental composition of synthesized SVOP material was determined from inductively coupled plasma optical emission spectroscopy (ICP-OES) and crystalline lattice water content was calculated from mass loss during heating to 580° C during thermogravimetric analysis (TGA)
Summary
Polyanion-type materials such as LiFePO4 have been heavily researched as battery cathodes because of their impressive stability and high operating voltage relative to oxide based materials [1,2,3]. Chemical stability of the polyanion can reduce cathode dissolution relative to oxides, extending the battery’s lifetime. For battery systems used to power implantable cardioverter defibrillators (ICD), silver vanadium phosphate (AgwVxPyOz, SVOP) materials have been shown to minimize cathode dissolution compared to the commercially utilized silver vanadium oxide cathode [4,5,6], creating the potential for improved ICD batteries with extended longevity [7,8,9,10,11,12,13,14,15]. As SVOP compounds are reduced, silver is reduced in-situ, forming a conductive network of silver nanoparticles that can improve electrical conductivity by ~15,000 fold [14]
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