Rate Dependent Multi-Mechanism Discharge of Ag_0.50VOPO₄·1.8H₂O: Insights from In Situ Energy Dispersive X-ray Diffraction

Abstract

Ag 0.50 VOPO 4 · 1.8H 2 O (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 Ag 0 during reduction. At a C/170 discharge rate V 5+ 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 Ag 0 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. Polyanion-type materials such as LiFePO 4 have been heavily researched as battery cathodes because of their impressive stability and high operating voltage relative to oxide based materials.1-3 The thermal stability of the polyanion framework improves battery safety especially under demanding conditions. Furthermore, chemical stability of the polyanion can reduce cathode dissolution relative to oxides, extending the battery's lifetime. Specifically, for battery systems used to power implantable cardioverter defibrillators (ICD), silver vanadium phosphate (Ag w V x P y O z , SVOP) materials have been shown to minimize cathode dissolution compared to the commercially utilized silver vanadium oxide cathode, 4-6 creating the potential for improved ICD batteries with extended longevity.7-15 However, a significant limitation of phosphate-based cathodes is their inherently poor electrical conductivity. Silver vanadium phosphates are able to overcome this issue due to the incorporation of silver. 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 In-situ energy dispersive X-ray diffraction (EDXRD) is a powerful technique that can be used to better understand the mechanisms of electrochemical discharge in SVOP materials. EDXRD utilizes white, collimated X-rays as the incident beam, which is scattered by the sample. 16 The resulting diffraction patterns are distributed in the selected energy range and captured by a detector held at a fixed angle. The light source is synchrotron based in order to emit the high-energy radiation n...

Material and Structural Design of Novel Binder Systems for High-Energy, High-Power Lithium-Ion Batteries

“…The above study is a good example showing that the combination of advanced characterizations and modeling calculations can shed light on rational design of binder coatings. To further understand the roles of polymer binders in battery electrodes, other advanced characterization techniques can be used such as X-ray microfluorescence for elemental mapping of the electrode, X-ray absorption spectroscopy for determining the oxidation states of the metal centers resident in the solid electrolyte interphase and at the electrode surface, energy-dispersive X-ray diffraction for achieving spatial resolution of the electrochemical reduction process within the electrode, transmission neutron diffraction analysis for mapping lateral inhomogeneity, and synchrotron radiation X-ray tomographic microscopy for dynamic mapping of changes in both electrode structure and chemical content with cycling . These tools can monitor the electrochemical processes within a battery electrode, which are closely related to binder properties.…”

Reduction of silver ions in molybdates: elucidation of framework acidity as the factor controlling charge balance mechanisms in aqueous zinc-ion electrolyte

“…8 , black traces) at 40 mA g −1 rates. Notably, the stand-alone pellets all exhibit an initial voltage drop followed by a recovery – a property well known in SVO cathodes and analogs 31,70,71 as the initial polarization being overcome by the in situ generation of a conductivity change provided by the Ag 0 network. We consider the following metrics:…”

“…9F), which has been used for other silver metal oxides. 1,3,31,72 In this circuit, R 1 is the solution resistance, the R 2 / CPE parallel models the anode and the R 3 /CPE parallel models the cathode. R 3 is of interest as it represents the interparticle charge transfer resistance, R ct .…”

“…For instance, lithiation of Ag x Mn 8 O 16 (1.0 # x # 1.8) does not form Ag 0 immediately, but rather Mn accepts the rst $4 electron equivalents. 28,29 Even with a lower reduction potential, V 5+ is still competitive with Ag + for early electrons in Ag 2 VO 2 PO 4 , 1 resulting in Ag 0 networks which may form differently as a function of rate 30 and other factors, 31 including the identity of the intercalating ion. Ag 0 appears immediately when Ag x Mn 8 O 16 is discharged in sodium batteries, 32 and reoxidation of Ag 0 which is not known in lithium batteries occurs in aqueous Zn systems.…”

Synthetic Control of Crystallite Size of Silver Vanadium Phosphorous Oxide (Ag0.50VOPO4·1.9H2O): Impact on Electrochemistry

“…(12) The rate of the discharge process significantly impacts whether the Ag extrusion and V reduction are sequential or concurrent (13), the distribution of silver within the cathode and the resulting electrode impedance where a faster rate of discharge resulted in non-homogeneous Ag 0 formation in the cathode with more Ag + reduction near the current collector. (14,15) At a slower rate the Ag 0 was much more evenly distributed throughout the cathode resulting in improved charge transfer resistance and a more complete utilization of active material.…”