In Situ Electrochemical Scanning Probe Microscopy of Lithium Battery Cathode Materials: Vanadium Pentoxide (V 2 O 5 )
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EE7.10.1
In Situ Electrochemical Scanning Probe Microscopy of Lithium Battery Cathode Materials: Vanadium Pentoxide (V2O5) Joseph W. Bullard III and Richard L. Smith Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA ABSTRACT Atomic force microscopy was used to characterize the structural evolution of the V2O5(001) surface during the electrochemical cycling of lithium. With Li insertion, nanometer-scale pits develop at the V2O5(001) surface. The pits first appear as the composition of the crystal approaches Li0.0006V2O5. Pit nucleation and growth continue through further discharge, resulting in a micro-porous (001) surface morphology. During subsequent Li extraction, cracks develop along the V2O5 axis. Surface regions in the vicinity of these cracks “swell” during ensuing lithiation reactions, suggesting that the cracks locally facilitate Li uptake. INTRODUCTION Vanadium pentoxide (V2O5) readily takes part in reversible topotactic redox reactions and has attracted interest for a number of applications, including electrochromic windows and secondary batteries [1-5]. The oxide’s ability to reversibly intercalate ionic species derives from its open layered structure, which is composed of sheets of VO5 square pyramids that link by sharing corners and edges (Figure 1) [6,7]. Adjacent layers are held together by only van der Waals forces, enabling the structure to reversibly adjust to accommodate guest ions in the interlayer interstitial sites [1,2]. The electrochemical cycling of Li has been of particular interest, due to its relevance to secondary battery applications [1-5]. The phase relations in the Li/V2O5 system have been established, as have the structures of the various LixV2O5 phases [2,3]. Across the compositional range from x = 0 to x = 1, there are three distinct phases, with compositions of x ≤ 0.1, 0.35 ≤ x ≤ 0.5, and 0.9 ≤ x ≤ 1.0 [3]. Lithium uptake is generally accompanied by structural changes, such as changes in the interlayer spacing and the symmetry of the host crystal. In the initial solid solution phase LixV2O5 (x ≤ 0.1), Li+ ions are accommodated with only subtle increases in the b and c lattice parameters, which vary from those of V2O5 at x = 0 (Figure 1 caption) to b = 3.565 Å and c = 4.386 Å for x ≈ 0.1 [3].
[010] [100]
(a)
[001]
(b)
Figure 1. The V2O5 structure. V2O5 has orthorhombic symmetry (space group = Pmmn) and lattice parameters of a = 11.496 Å, b = 3.551 Å, and c = 4.357 Å [6].
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While a number of studies have examined the bulk structural chemistry and electrochemical behavior related to the Li/V2O5 system, little is known about the surface microstructural changes that accompany Li cycling. Such knowledge could shed light on a number of issues, for example the local mechanics of interfacial Li exchange. A microstructure-level picture might also aid in the interpretation of macroscopic performance characteristics, such capacity fade [5]. Moreover, with the growing interest in nanostructured and thin f
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