R. Prasada Rao

Evaluation of undoped and M-doped TiO2, where M = Sn, Fe, Ni/Nb, Zr, V, and Mn, for lithium-ion battery applications prepared by the molten-salt method

The molten-salt method was used to synthesize a series of transition-metal containing titanium dioxides. Some of the transition metals were found to substitute into the TiO2 lattice, such as (Ti0.9Fe0.1)O2, (Ti0.9Zr0.1)O2, (Ti0.9V0.1)O2, and (Ti0.9Mn0.1)O2, while others were formed as composite electrodes (in addition to relatively minor substitutions), namely 0.1SnO2–0.9TiO2 and 0.05NiO–0.1Nb2O5–0.9TiO2. Although identical synthesis-conditions were used the different transition metals yielded different phases. A comparative study of the electrodes relating surface area and composition (via X-ray photoelectron spectroscopy, XPS), and electrochemical behaviour is presented in this work. Among the substituted single phase electrodes, (Ti0.9Zr0.1)O2 exhibited the best reversible capacity of ∼160 mA h g−1, at the end of the 60th cycle in the voltage range 1.0–2.6 V, with a capacity fade of 24% from the 2nd to the 60th cycle. Among the composite electrodes, 0.05NiO–0.1Nb2O5–0.9TiO2 shows the best performance which is comparable to pure TiO2 but with a slower capacity-fade on extended cycling. The worst performing electrode is (Ti0.9V0.1)O2 with a reversible capacity of only ∼70 mA h g−1 at the end of 70 cycles with a current density of 130 mA g−1 in the voltage range 1.0–2.6 V and a capacity drop of 52% from the 2nd to the 70th cycle. The composite 0.1SnO2–0.9TiO2 features the highest irreversible capacity-loss. Zr-substitution into TiO2 gives the best electrochemical performance.

Structural requirements for fast lithium ion migration in Li10GeP2S12

Dynamic lithium distribution, structural stability and ion transport mechanism in the new ultrafast ion conductor Li10GeP2S12 are clarified with the help of atomistic molecular dynamics simulations, revealing only a weak anisotropy of Li+ diffusion, and a coupling of Li+ cation diffusion to PS43− anion rotational mobility. The role of a previously overlooked Li site and the limited structural stability at elevated temperatures are discussed for the first time.

Structure property correlation in lithium borophosphate glasses

To investigate the influence of cation mobility variation due to the mixed glass former effect, 0.45Li2O-(0.55 − x) P2O5−x B2O3 glasses (0 ≤; x ≤ 0.55) are studied keeping the molar ratio of Li2O/(P2O5 + B2O3) constant. Addition of B2O3 into lithium phosphate glasses increases the glass transition temperature (Tg) and number density, decreases the molar volume, and generally renders the glasses more fragile. The glass system has been characterised experimentally by XRD, XPS and impedance studies and studied computationally by constant volume molecular dynamics (MD) simulations and bond valence (BV) method to identify the structural variation with increasing the B2O3 content, its consequence for Li+ ion mobility, as well as the distribution of bridging and non-bridging oxygen atoms. These studies indicate the increase of P-O-B bonds (up to Y = [B2O3]/([B2O3] + [P2O5]) ≈ 0.5 and B-O-B bonds, as well as the decrease of P-O-P bonds and non-bridging oxygens (NBOs) with rising B2O3 content. The system with Y ≈ 0.5 exhibits maximum ionic conductivity, 1.0 × 10−7 S cm−1, with activation energy 0.63 V. Findings are rationalised by a model of structure evolution with varying B2O3 content Y and an empirical model quantifying the effect of the various structural building blocks on the ionic conductivity in this mixed glass former system.

Understanding Ionic Conduction and Energy Storage Materials with Bond-Valence-Based Methods

The analysis and prediction of ion transport in solids from static and dynamic structure models has become an interesting application for the bond valence approach. Specific adaptations of the bond valence approach for this application area are discussed, and the resulting predictions are compared to those from alternative screening approaches. A particular advantage is that the bond-valence-based approach can be applied to both crystalline and glassy solids and that the level of computational effort can be easily adjusted to the level of detail required in the prediction from static pathway models for screening purposes to bond-valence-based molecular dynamics simulations for analyzing the coupling between the migration of the mobile species and rearrangements in the immobile substructure.

Mechanism of Ultrafast (Dis)charging of Li Ion Batteries by Heterogeneous Doping of LiFePO 4

Molecular dynamics (MD) simulations with a dedicated force-field and our bond valence (BV) pathway analysis have been employed to reproduce and explain the experimentally observed ultrafast Li+ transport in surface modified LixFePO4-δ as a consequence of heterogeneous doping, i.e. the Li+ redistribution in the vicinity of the interface between LixFePO4 and a pyrophosphate glass surface layer. Over the usual working temperature range of LIBs Li+ ion conductivity in the surface modified LixFePO4 phase is enhanced by 2-3 orders of magnitude, while the enhancement practically vanishes for T > 700K. Simulations for the bulk phase reproduce the experimental conductivities and the activation energy of 0.57eV (for x ≈ 1). A layer-by-layer analysis of structurally relaxed multilayer systems indicates a continuous variation of Li+ mobility with the distance from the interface and the maximum mobility close to the interface, but Li+ diffusion rate remains enhanced (compared to bulk values) even at the center of the simulated cathode material crystallites. Our BV migration pathway analysis in the dynamic local structure models shows that the ion mobility is related to the extension of unoccupied accessible pathway regions. The change in the extent of Li redistribution across the interface with the overall Li content constitutes a fast pseudo-capacitive (dis)charging contribution.

Structure and Ion Transport Pathways in 0.45Li 2 O-(0.55-x)P 2 O 5 -xB 2 O 3 Glasses

Lithium borophosphate glasses 0.45Li 2 O-(0.55-x)P 2 O 5 -xB 2 O 3 (where 0 ≤ x ≤ 0.40) were investigated focusing on the influence of cation mobility changes due to mixed glass former effect. It was found that glass transition temperature (Tg) increases and molar volume decreases with B 2 O 3 addition. X-ray photoelectron spectroscopy (XPS) spectra showed that besides P-O-P, B-O-B and P=O, P-O - , B-O - bond peaks, an intermediate O1s peak due to P-O-B bonds emerges in glasses with B 2 O 3 contents x ≥ 0.15. Molecular dynamics (MD) simulations for the same systems have been performed with an optimized potential, fitted to match bond lengths, coordination numbers and ionic conductivity (σ dc ). Structural effects on ion transport as the origin of the mixed glass former effect can be quantified by applying the bond valence analysis (BV) approach to the equilibrated MD trajectories.