59th Annual Report on Research 2014

Our ACS PRF funded project 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.  However, 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 aerogels (or d-V₄O₁₀,). 

         Our research has focused on addressing the ultimate question: what is happening at the microscopic level when more than 2 Li (or Na) per vanadium are intercalated into d-V₄O₁₀?  Various structural and electronic phase transitions occur when metal ions are intercalated, which need to be understood. Our approach was to use x-ray spectroscopy to correlate the chemical composition, geometric phase and electronic structure as we intercalated metal ions (e.g. Li⁺).  This approach meant that we could directly compare our experimental results with first principles calculations to provide insight into the fundamental mechanisms. 

         In our first year report we summarize our studies of idealized systems to determine how the electronic structure is modified by chemical and structural changes associated with metal ion intercalation.  In addition to our target material system, d-V₄O₁₀, these studies included, vanadium dioxide (VO₂); vanadium oxide bronzes (M_xV₂O₅, where M included Li⁺, Na⁺, K⁺, Ag⁺, Pb²⁺, Sn²⁺); and manganese phosphates.   Our work on model systems provided the necessary platform for interpreting results from ex-situ studies of real cathode electrodes (at various stages of discharge).  In electrodes the active material we have found that this is complicated by chemical reactions can occur at the highest intercalation because of the interface with the electrolyte.  A unique aspect of our research was the use of synchrotron-based spectroscopy techniques; including x-ray absorption spectroscopy (XAS) and hard x-ray photoelectron spectroscopy (HAXPES).  These techniques provided a means of correlating the chemical, structural and electronic properties of V₂O₅ nano-structures (at various stages of intercalation). 

Findings:

Our results are categorized into: 1) correlating chemical composition with electronic structure in model systems; 2) correlating geometric phase with electronic structure in model systems; 3) what happens in real systems.

1)      Model (Composition) X-ray photoelectron spectroscopy (XPS) is a powerful tool for studying the electronic and chemical composition of solids.  Traditional XPS is extremely surface sensitive, which makes it difficult for studying vanadium oxides because surface preparation methods can easily reduce the vanadium ions i.e. not representative of the bulk material.  The situation is further complicated when considering real cathode materials, where the signal from buried nano-structures is attenuated.   HAXPES circumvents some of these issues by having a larger attenuation length and reduced sensitivity to carbon species. We have initially employed HAXPES to study model systems, such as β-M_xV₂O₅ bronzes and chemically lithiated d-V₄O₁₀ aerogels, before addressing real electrodes.  Using this HAXPES we were able to determine the electronic structure as a function of composition (metal intercalation).  Figure 1 displays the evolution of the V charge state and valence band structure for d-V₄O₁₀, γ-V₂O₅, β-Li_0.66V₂O₅, VO₂(B).  In addition to the filling of the V 3d orbital with vanadium reduction we are also sensitive to the bonding strength of the various polymorphs (with sparsely-packed d-V₄O₁₀ having the weakest V3d-O2p bonding). 

Figure 1: HAXPES of model vanadium oxides correlating the electronic structure with composition and polymorph

We extended our approach to employ HAXPES quantify the vanadium reduction with orbital filling irrespective of metal ion.  Figure 2 shows how the vanadium reduction and orbital filling correlated with the amount of intercalation determined from x-ray diffraction studies in a set of β-M_xV₂O₅ nanorods.  This work has provided accurate descriptions of the electronic structures of well-defined phases.  It provides the platform for successfully interpreting the evolution of the electronic structure of real cathode materials, which will be further addressed in our 2nd year.

Figure 2: Our HAXPES results quantifying the amount of V3d orbital filling with vanadium reduction in model β-M_xV₂O₅ nanorods.

2)       Model (Structure) We have also employed x-ray spectroscopy techniques (x-ray absorption and photoelectron spectroscopy) to correlate structural transitions and local distortions that can occur in these systems.  Our work included studies on the abrupt monoclinic-rutile transition in VO₂ and the lone-pair distortion in β-Pb_0.33V₂O₅.  In this case, x-ray spectroscopy techniques are sensitive to orbital signatures associated with the geometric structure.  This approach was extended to examine the thermal stability of MnPO₄, and the formation of Mn₂P₂O₇.

Figure 3: Our x-ray spectroscopy study of structural distortions in real cathode materials.  The inset shows first principles calculations of the polarons.

3)      Real Systems We extended our x-ray studies to include real cathode materials, initially with Li_xMnPO₄ nanoparticles at various stages of discharge.  Figure 3 displays the electron and hole polarons that result from the Jahn-Teller distortion (JTD) associated with the Mn³⁺ charge state.   The JTD severely reduces the electronic and ionic transport in the material, which explains its sluggish performance.  We have recently extended this work to include HAXPES/XPS/XAS of electrochemically lithiated V₄O₁₀ fabricated electrodes.  In these studies have observed the formation of Li oxide and carbonate species whenever the V³⁺ is present.  We are currently determining whether this is due to processing or a reaction process occurring at these voltages, and will address the origin of this phenomenon in year 2 of our ACS-PRF grant.

Impact:

Our ACS-PRF grant has already resulted in five publications with the project funded graduate students, Nicholas Quackenbush and Linda Wangoh.  It has provided the opportunity for us to pioneer the use of HAXPES for studying battery cathodes.  In addition to providing extensive synchrotron experience for my students, the knowledge and expertise gained in our first year of the ACS-PRF grant has helped us secure state and federal funding (NSF and DOE) in related areas of study in functional oxides and electrochemical storage.