www.acsprf.org

1.    Overview

The last decade of frenetic battery research has made clear the formidable challenges of safely achieving high power and energy densities in Li-ion batteries, which necessitate the optimal performance of disparate materials at the limits of their thermodynamic stability windows. Despite the high stakes, materials discovery approaches for developing cathode materials remain primarily empirical. A major impediment to the development of rational materials design and optimization approaches has been lack of fundamental mechanistic understanding of the transformations induced within materials upon lithiation (and subsequent delithiation). A major focus of our ACS PRF-funded research program has been to elucidate the influence of finite size, morphology, and stoichiometry on Li-ion insertion/extraction and diffusion processes in binary and ternary vanadium oxides, especially structures that crystallize in quasi-layered phases such V₂O₅ and M_xV₂O₅.

Over the last year, we have mounted a systematic effort aimed at growing well-defined V₂O₅ and M_xV₂O₅ nanostructures with a high degree of compositional, dimensional, and morphological control. We have further undertaken a comprehensive study of the geometric and electronic structure of these materials using both ensemble and individual nanowire spectroscopy and microscopy tools. Finally, we have performed a detailed mechanistic study of alterations to the geometric and electronic structure of Ag₂VO₂PO₄ upon cathodic discharge, using X-ray absorption spectroscopy to develop an orbital-specific picture of the redox chemistry accompanying electrochemical lithiation.

2.    Findings

2.1. Morphology Controlled Fabrication of V₂O₅ Nanostars: VO is the rare vanadium oxide to crystallize in a highly symmetric rock salt structure. In an article featured as the cover of CrystEngComm (Fig. 1), we have devised a novel hydrothermal synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of V₂O₅. A vapor transport process is used to first deposit a VO precursor onto a 100 nm Fe film. Selective etching of the deposited grains along high-surface-energy crystallographic facets yields V₂O₅ nanostructures that exhibit clear six-fold symmetry and retention of electronic structure. The lithiation/delithiation of these high-surface-area materials is currently being examined using Raman spectroscopy.

2.2. Single-Nanowire Measurements of β'-Cu_xV₂O₅ Nanowires: In keeping with our theme of combining ensemble characterization with individual nanowire electrical transport and electronic structure studies, we have examined the properties of β'-Cu_xV₂O₅ (x ~ 0.65) nanowires prepared by the hydrothermal reduction of CuV₂O₆. Studying single wires allows us to access the intrinsic structural and electronic properties of this 1D bronze material without obscuration from grain boundary connectivity or domain dynamics. Remarkably, we have observed a thermally induced transition to a metallic state characterized by very high conductivity in the 340—400 K range (Fig. 2) for single nanowires of these materials, likely arising from the melting of charge ordering (discrete V⁴⁺ and V⁵⁺ sites). The implications of a heating-induced enhancement in conductivity are significant for Li-ion batteries that need to maintain electroactivity at high ambient temperatures. Individual-nanowire scanning transmission X-ray microscopy measurements suggest that the magnitude of the phase transition depends sensitively on the precise stoichiometry of the nanowires.

Fig. 1. V2O5 nanostars prepared as part of the ACS-PRF-funded program have been featured as the cover of CrystEngComm.

Fig. 2. The electrical conductivity of a single β'-CuxV2O5 dramatically increases upon heating to a temperature of 395 K.

2.3. Mechanistic Insights into the Cathodic Discharge of Ag₂VO₂PO₄: Inspired primarily by the greater chemical stability of PO₄^3- constituent building blocks, substantial recent research attention has focused on the use of phosphate derivatives of transition metals as cathode materials. Indeed, the M_TM—O—P coupling of strongly covalent phosphate tetrahedra confers thermal and chemical stability to a variety of crystal structures. Ag₂VO₂PO₄ has attracted particular interest as a candidate cathode material for use in the next generation of lithium-based batteries owing to its bimetallic nature, which faciliates multielectron transfer processes. A remarkable 15,000-fold enhancement in conductivity of this material is observed upon lithiation due to the extrusion of a percolative Ag nanoparticle network. However, the concomitant amorphization of Ag₂VO₂PO₄ during electrochemical discharge precludes the use of standard diffraction tools and renders fundamental mechanistic studies of the lithiation process rather difficult.

X-ray absorption spectroscopy on the other hand poses no crystallinity requirements, and consequently in a comprehensive article published in the Journal of Physical Chemistry C, we have used a combination of V K-, V L-, Ag K-, and O K-edge X-ray absorption fine structure spectroscopy measurements to determine the local vanadium and silver oxidation states, local coordination geometry, and stoichiometry for Ag₂VO₂PO₄ samples with varying extents of electrochemical lithiation. Hard-X-ray V K-edge X-ray absorption near edge structure (XANES) and Ag K-edge XANES and extended X-ray absorption fine structure (EXAFS) measurements suggest the concurrent and not sequential reduction of the Ag⁺ and V⁵⁺ redox-active sites of Ag₂VO₂PO₄, although complete reduction of Ag+ to nanoparticulate Ag⁰ metal precedes the appearance of a significant concentration of V³⁺ species in the discharged phases. These measurements further enable a quantitative description of the stoichiometry and redox nature of samples at varying depths of discharge. Soft X-ray V L- and O K-edge NEXAFS spectroscopy in conjunction with density functional theory (DFT) calculations provide the first description of the electronic structure of Ag₂VO₂PO₄ and evidence the stability of phosphate groups to advanced states of discharge and reduced covalency of V—O linkages. These specific structural characteristics make this system an attractive cathode material. The NEXAFS measurements further provide an exquisitely detailed picture of orbital occupancies as a function of the depth of discharge and directly evidence V—O rehybridization accompanying the electrochemical lithiation of Ag₂VO₂PO₄. The sequence of V 3d states occupied upon electrochemical reduction from lower to higher energy follows: V 3d_zy then 3d_zx, and finally 3d_xy. The methodologies developed here are applicable in principle to any cathode material without any crystallinity requirements to obtain a detailed picture of how geometric and electronic structure varies as a function of electrochemical lithiation and will be furher used to develop detailed mechanistic undertstanding of lithiation/delithiation processes in binary and ternary vanadium oxides.