The world increasingly depends on batteries to store renewable energy and use that same energy in our vehicles and portable communication devices. This puts exceeding pressure on global resources. We need batteries that charge faster and live longer, such that we can use less resources. Faster charge and longer life are currently limited by the negative electrode, typically graphite, because fast charging would push the potential into the regime of hazardous and cycle-life degrading lithium plating. The ideal potential for fast charge would be low, but just above the around 1 V reduction potential of the electrolyte. Niobium-based metal oxides have the optimal electronegativity to strike this balance, with a nominal potential around 1.6 V, charging rates >5C and a cycle-life projected over 10,000 cycles.
Chapter 1 shows that the exact potential can be tuned further by changing the average oxidation state through substitution of Nb5+ with for example W6+ or Ti4+. The range of average oxidation states then directly spans a material phase space classed by anion-to-cation ratios of 2.33 ≤ *y* 3-type blocks of corner-sharing octahedra that have ample window sites to rapidly intercalate many lithium-ions, interspaced with *y*=2.5 crystallographic shear planes of edge-sharing octahedra that add stability and electronic conductivity to the structure, and anchored at their corner by *y*=2 regions of tetrahedra or edge-sharing octahedra. The influence of this structure on cell performance is relatively unknown. Numerous publications exist on individual members of this Wadsley-Roth (WR) material family, but gaps in theory and varying experimental conditions make it impossible to compare.
The aim of this thesis is to provide a fair and fundamental comparison across this material class, relating compositional and structural properties to cell thermodynamics and kinetics that can then be used to optimise the material selection and model any full-scale cell geometry. In total 16 different compounds were synthesised with comparable geometrical parameters. Subsequently, they were fully parameterised with various electrochemical tests. Current theory is still too firmly based on traditional metal plate electrodes. Because the WR materials allow extreme conditions of high currents and could be tuned over an extensive structural and compositional range, their study forms an excellent opportunity to modernise the fundamental understanding of the thermodynamics and kinetics of intercalation lithium-ion batteries, in general, and in relation to structural and compositional parameters.
Chapter 2, on thermodynamics and energy density, introduces fundamental principles of configurational entropy to explain the steep bends at the cell potential ends and the detailed peaks in the cyclovoltammogram. Density function theory (DFT) exposed a site filling order and structural straightening. Via molecular orbital theory this was then related to enthalpic effects of relatively steeper potential regions due to progressively poorer charge-compensation and relatively poor shielding, but also relatively flatter potential regions related to metal-to-metal repulsion and pseudo Jahn-Teller effects at the block edge. Owing to their increased edge-sharing, low *y* materials could thus reach lower potentials without reaching the voltage cut-off earlier. Low *y* materials thus exhibit high energy density, particularly considering that they also consist of more lightweight elements. The structural straightening upon reduction was identified as the crucial mechanism that provides a competitive energy density to the WR material. The first cycle data and DFT also revealed the mechanism that tetrahedral linkages are irreversibly trapping lithium and that they can be left out of the structure to achieve nearly 100% first cycle efficiencies.
On the other hand, the study in Chapter 3 of their intercalation kinetics through temperature-dependent GITT and PEIS with novel application of the compensation effect shows that lower *y* is at the cost of lower entropy of the diffusion pathways, such that their intercalation diffusion coefficients are lower. In general, the compensation effect and the effect of entropy can not be underestimated, while the effect of activation enthalpy could be misleading. Various PEIS, cyclovoltammetry, PITT and GITT techniques had to be critically reviewed and stripped from metal-plate concepts, to identify the formation of film layers and the trends in diffusion. The charge transfer reaction rate and lithium intercalation diffusion were identified as the main contributors to loss, limiting the charge/discharge rate. However, this study observed that the chemical lithium intercalation diffusion coefficient increases with rate. This surprising effect is no longer adequately described by the conventional mass-transfer theory and suggests effects of non-equilibrium driving forces, excited lithium hopping, lattice vibrations and energy barrier softening. Such a mechanism is essential to explain the high rate performance of WR materials and intercalation materials in general and provides an important direction for future theory and experimental research.
All in all, this study showed a tradeoff between energy and rate, with TiNb2O7, Zn2Nb34O87 and PNb9O25 as winners. Independent of the tradeoff, performance could be further improved in the future with the substitution of lightweight cations, and by increasing the crystallographic entropy with multiple cations. In general, this work identified several new applications of theory to the modern battery cell, which will hopefully become more widely applied and further underpinned by in-situ direct observation methods on the particle level.
All the theory and full parameterisation methods above were combined into a full cell continuum model in Chapter 4, that not only validates these approaches but also allows the design, verification and prediction of any commercial format multilayer cell geometry. This paves the way for this new class of ultra fast-charge long-life batteries that can power more of the world, with fewer batteries.Echion Technologies Ltd
