60th Annual Report on Research 2015

Our ACS PRF funded project has focused on the evolution of the electronic structure of promising materials for next generation Li-ion battery cathodes. Recently, nano-engineered LiFePO₄ olivine has replaced costly and thermally unstable LiCoO₂. The use of polyanions adversely reduces the specific capacity; one way to compensate for the loss of capacity is to develop a material containing a polyatomic anion (thermally stable) that is capable of inserting two lithium (or sodium) atoms per redox-active metal ion, such as VOPO₄. In general one would extend this to a system that is already thermal stable and environmentally benign that can accommodate more two or more lithium (or sodium) per redox-active metal, e.g. V₂O₅•nH₂O aerogels (or delta-phase V₄O₁₀•nH₂O).

Our research has focused on addressing the ultimate question: what is happening at the microscopic level when more than 2 Li per vanadium are intercalated into V₂O₅•nH₂O aerogels? In our second year report we summarize our x-ray photoemission spectroscopy (XPS) and pairs distribution function (PDF) measurements of V₂O₅•nH₂O aerogels as a function of electrochemical lithiation.

Results:

1) Role of tightly bound water species:

The presence of water has been shown to play an important role in determining the interlayer distance and electrochemical performance of V₂O₅•nH₂O aerogels. Surface water and adsorbed species on the aerogel can be eliminated by heating the aerogel to 220C without collapsing the sparse aerogel structure. Heating to 300C removes the remaining tightly bound water (i.e. crystalline water species within the interlayers) resulting in the formation of dense crystalline orthorhombic V₂O₅ (Fig. 1).

Fig. 1 Thermogravimetric analysis (TGA) and x-ray diffraction (XRD) of as-prepared aerogel as a function of heating. Heating to 220C results in V₂O₅•0.6H₂O aerogel, where only the interlayer H₂O remains. Heating to 300C results in the crystalline alpha-phase V₂O₅ phase.

Fig. 2: (Top) The first cycle discharge/curve curve of our V₂O₅•0.6H₂O aerogel cathodes. (Bottom) XPS analysis summarizing the evolution of the V⁵⁺ and LiOH species during the first cycle.

To determine how the Li⁺ reacts with the interlayer tightly bound water species, we examined the evolution of the chemical environment of electrochemically discharged and charged V₂O₅•nH₂O aerogels heat treated to 220°C using XPS. Our studies revealed that in addition to the expected reversible vanadium reduction and oxidation associated with the intercalation process; we observed the partially irreversible formation of LiOH species (Fig. 2). This is attributed to the reaction between Li⁺ and the tightly bound interlayer water, and the accumulation of inactive Li species during each cycle is considered responsible for the gradual capacity fade observed in our experiments. These studies are currently under review for publication.

2) Local structure of V₂O₅•nH₂O aerogels:

The xerogel structure was solved by Petkov et al. [J. Am. Chem. Soc. 2002] using atomic PDF i.e. the Fourier transform of the total scattering diffraction pattern. This method provides local, medium and long-range structural information because it is sensitive to Bragg and diffuse scattering contributions. This makes it ideal for studying materials with no long-range order, such as sparse aerogel and xerogel forms. The xerogel consists of a double layered structure with water molecules in between them by (as shown in Fig. 1), often referred to as delta-phase V₄O₁₀•nH₂O. The aerogel structure has not been explicitly solved but it is assumed to be similar to the xerogel structure. Moreover, its evolution of the local structure as a function of lithium content is not known.

We performed PDF experiments at NSLS-II at Brookhaven National Laboratory of out V₂O₅•0.6H₂O aerogel electrodes at various stages of discharge and charge. Our experimental data is currently being analyzed, and we are collaborating with Prof. Watson at Trinity College Dublin who is performing atomistic simulations of the structure. These calculations explicitly consider the weak dispersive forces associated with layered materials.

Additional studies:

Our research on the electronic structure of intercalated V₂O₅ nanowires.  We previously reported upon the nature of the lone pair distortion in Pb²⁺ intercalated V₂O₅ nanowires. [L. Wangoh et al., Appl. Phys. Lett. (2014)] We were able to show that the lone pair distortion is facilitated by the hybridization of the Pb 6s with the O 2p orbitals, which raises the valence band maximum (compared to V₂O₅). The modified band alignment was explored further for water splitting applications. This study is published as Integrating β-Pb0.33V2O5 Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer by Kate E. Pelcher, Christopher C. Milleville, Linda Wangoh, Saurabh Chauhan, Matthew R. Crawley, Peter M. Marley, Louis F. J. Piper, David F. Watson, and Sarbajit Banerjee in Chemistry of Materials, vol. 27 pp 2468–2479 (DOI: 10.1021/cm504574h). Other lone pair active ions (Sn²⁺) will be examined.