Ieuan D. Seymour

Characterizing Oxygen Local Environments in Paramagnetic Battery Materials via 17O NMR and DFT Calculations

Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of (17)O NMR in the same materials has not yet been reported. In this work, we present a combined (17)O NMR and hybrid density functional theory study of the local O environments in Li2MnO3, a model compound for layered Li-ion batteries. After a simple (17)O enrichment procedure, we observed five resonances with large (17)O shifts ascribed to the Fermi contact interaction with directly bonded Mn(4+) ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the (17)O shifts of environments similar to the 4i and 8j sites in pristine Li2MnO3, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li2MnO3 structure. From the ratio of the intensities of the different (17)O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying (17)O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides.

Electrochemical Performance of Nanosized Disordered LiVOPO4

ε-LiVOPO4 is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO4, in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by 7Li and 31P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO4 at C/50 (1 C = 153 mA h g–1) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO4/Li3VO4/acetylene black composite yields the high cycling capacity of 250 mA h g–1 at C/5 for over 70 cycles.

Chapter 11:NMR Studies of Electrochemical Storage Materials

This chapter describes the application of solid-state NMR spectroscopy for investigating battery electrode materials at controlled state of charge. Magic-angle spinning NMR gives the highest possible chemical resolution, but only allows these often metastable electrode materials to be studied in an ex situ manner, i.e., outside the electrochemical cell, with a risk of oxidation and chemical relaxation. Complementary to the MAS NMR approach, we therefore explain the use of dedicated static NMR probes optimally designed for coupling to battery cyclers. This in situ approach allows electrode materials to be studied inside electrochemical cells during repeated charge and discharge cycles. As electrode materials are generally paramagnetic or conductive in, at least, certain charge states, one of the NMR challenges is to deal with the large line broadening and shifts. In conjunction with density functional theory computation described in this chapter, however, these paramagnetic and Knight shifts are, in fact, rich sources of detailed information about the underlying materials’ structure.

An ab initio investigation on the electronic structure, defect energetics, and magnesium kinetics in Mg3Bi2

We present a comprehensive ab initio investigation on Mg3Bi2, a promising Mg-ion battery anode material with high rate capacity. Through combined DFT (PBE, HSE06) and G0W0 electronic structure calculations, we find that Mg3Bi2 is likely to be a small band gap semiconductor. DFT-based defect formation energies indicate that Mg vacancies are likely to form in this material, with relativistic spin–orbit coupling significantly lowering the defect formation energies. We show that a transition state searching methodology based on the hybrid eigenvector-following approach can be used effectively to search for the transition states in cases where full spin–orbit coupling is included. Mg migration barriers found through this hybrid eigenvector-following approach indicate that spin–orbit coupling also lowers the migration barrier, decreasing it to a value of 0.34 eV with spin–orbit coupling. Finally, recent experimental results on Mg diffusion are compared to the DFT results and show good agreement. This work demonstrates that vacancy defects and the inclusion of relativistic spin–orbit coupling in the calculations have a profound effect in Mg diffusion in this material. It also sheds light on the importance of relativistic spin–orbit coupling in studying similar battery systems where heavy elements play a crucial role.

Identifying the chemical and structural irreversibility in LiNi0.8Co0.15Al0.05O2 – a model compound for classical layered intercalation

In this work, we extracted 95% of the electrochemically available Li from LiNi0.8Co0.15Al0.05O2 (NCA) by galvanostatically charging the NCA/MCMB full cell to 4.7 V. Joint powder X-ray and neutron diffraction (XRD & ND) studies were undertaken for NCA at highly charged states at the first cycle, and discharged states at different cycles. The results indicate that the bulk structure of NCA maintains the O3 structure up to the extraction of 0.90 Li per formula unit. In addition, we found that the transition metal layer becomes more disordered along the c-axis than along the a- and b-axes upon charging. This anisotropic disorder starts to develop no later than 4.3 V on charge and continues to grow until the end of charge. As Li is re-inserted during discharge, the structure that resembles the pristine NCA is recovered. The irreversible loss of Li and the migration of Ni to the Li layer have been quantified by the joint XRD and ND refinement and the results were further verified by solid state 7Li NMR and magnetic measurements. Our work clearly demonstrates that the NCA bulk retains a robust, single phase O3 structure throughout the wide delithiation range (up to 0.9 Li per formula unit of NCA) and is suitable for higher energy density usage with proper modifications.

Mixed X-Site Formate-Hypophosphite Hybrid Perovskites

Following the recent discovery of a new family of hybrid ABX3 perovskites where X=(H2 POO)- (hypophosphite), this work reports a facile synthesis for mixed X-site formate perovskites of composition [GUA]Mn(HCOO)3-x (H2 POO)x , with two crystallographically distinct, partially ordered intermediate phases with x=0.84 and 1.53, corresponding to ca. 30 and 50 mol % hypophosphite, respectively. These phases are characterised by single-crystal XRD and solid-state NMR spectroscopy, and their magnetic properties are reported.