Silver vanadium diphosphate Ag2VP2O8: Electrochemistry and characterization of reduced material providing mechanistic insights

Abstract

Silver vanadium phosphorous oxides (AgwVxPyOz) are notable battery cathode materials due to their high energy density and demonstrated ability to form in-situ Ag metal nanostructured electrically conductive networks within the cathode. While analogous silver vanadium diphosphate materials have been prepared, electrochemical evaluations of these diphosphate based materials have been limited. We report here the first electrochemical study of a silver vanadium diphosphate, Ag2VP2O8, where the structural differences associated with phosphorous oxides versus diphosphates profoundly affect the associated electrochemistry. Reminiscent of Ag2VO2PO4 reduction, in-situ formation of silver metal nanoparticles was observed with reduction of Ag2VP2O8. However, counter to Ag2VO2PO4 reduction, Ag2VP2O8 demonstrates a significant decrease in conductivity upon continued electrochemical reduction. Structural analysis contrasting the crystallography of the parent Ag2VP2O8 with that of the proposed Li2VP2O8 reduction product is employed to gain insight into the observed electrochemical reduction behavior, where the structural rigidity associated with the diphosphate anion may be associated with the observed particle fracturing upon deep electrochemical reduction. Further, the diphosphate anion structure may be associated with the high thermal stability of the partially reduced Ag2VP2O8 materials, which bodes well for enhanced safety of batteries incorporating this material.

In situ profiling of lithium/Ag2VP2O8primary batteries using energy dispersive X-ray diffraction

“…In our previous study of Ag 2 VP 2 O 8 , the electrochemical reduction mechanism was probed via ex situ X-ray powder diffraction of the cathode material at various states of discharge. 30 Up to 2.0 electron equivalents of discharge, formation of silver metal was evidenced by an increasing integral area of the peak at 38.11 twotheta, corresponding to (111) of Ag 0 . While the bulk ex situ XRD measurements in the prior report were useful to verify the formation of silver at early stages of reduction, it was not possible to determine the location of the Ag 0 , or the homogeneity of its distribution within the cathode.…”

“…The peak positions for Ag 2 VP 2 O 8 match previously reported results. 30,42 On the right side of Fig. 4 are intensity contour plots of 56 EDXRD spectra for cells discharged to (d) 0, (e) 0.1 and (f) 0.5 electron equivalents.…”

“…28,29 the investigation of Ag 2 VP 2 O 8 as a cathode material in Li based batteries showed that the material displays a reduction displacement mechanism similar to other silver vanadium phosphate materials. 30 The discharge process can be expressed as:…”

In situ visualization of Li/Ag 2 VP 2 O 8 batteries revealing rate-dependent discharge mechanism

“…Demonstrations include experimental and theoretical studies of more than 40 representative geometries, from single and multiple helices, toroids, and conical spirals to structures that resemble spherical baskets, cuboid cages, starbursts, flowers, scaffolds, fences, and frameworks, each with single-and/or multiple-level configurations. C ontrolled formation of 3D functional mesostructures is a topic of broad and increasing interest, particularly in the past decade (1)(2)(3)(4)(5)(6)(7)(8)(9). Uses of such structures have been envisioned in nearly every type of micro/ nanosystem technology, including biomedical devices (10)(11)(12), microelectromechanical components (13,14), photonics and optoelectronics (15)(16)(17), metamaterials (16,(18)(19)(20)(21), electronics (22,23), and energy storage (24, 25).…”

“…Such schemes provide access to only certain classes of geometries, through either rotations of rigid plates to yield tilted panels, rectangular cuboids, pyramids, or other hollow polyhedra, or rolling motions of flexible films to form tubes, scrolls, or related shapes with cylindrical symmetry [for reviews, see (1,9,13)]. Here, we present a different set of concepts in which strain relaxation in an elastomeric substrate simultaneously imparts forces at a collection of lithographically controlled locations on the surfaces of planar precursor structures.…”