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A highly reactive composite cathode material incorporating nano-particles of the popular Li-ion battery cathode material Li2FeSiO4 (LFS) is here studied to probe the activation of the controversial Fe3+/Fe4+ redox couple in exploiting the second Li-ion in the formula unit - for use in rechargeable Li-ion batteries. A novel form of in situ Mossbauer spectroscopy is used to monitor the oxidation state of the Fe-ions in symmetric LFS LFS cells. This is based on mapping the poorly resolvable Mossbauer spectra from the expected Fe3+/Fe4+ redox couple in the working electrode onto the highly resolvable Fe2+/Fe3+ spectra from the counter electrode. Comparison of such data from half-delithiated Li(1)Fe3+SiO4 parallel to Li(1)Fe3+SiO4 and almost lithium-free "Li(0)Fe4+SiO4 parallel to Li(0)Fe4+SiO4" symmetric cells is demonstrated - to distinguish the electrode reactions from the those involving the electrolyte. Lithium is shown to cycle reversibly in the symmetric cells. However, a large proportion of the cycled lithium (similar to 70%) does not derive from the bulk of the electrodes, but is rather a result of high-V electrolyte degradation, where charge balance is maintained by leaching lithium from the electrolyte and inserting it into the electrodes.

As a tribute to the major contribution made by Academician Gligor Jovanovski to the field of Mineralogy in Macedonia, this paper promotes the potential role that minerals can have as a future source of inspiration in identifying novel materials for sustainable energy storage in general, and for advanced Li-ion batteries in particular. We exemplify this by indicating the innovative use of polyanions in novel Li-ion battery cathode materials such as the olivine lithium iron phosphate (LiFePO4), and in an even newer material - the orthosilicate lithium iron silicate (Li2FeSiO4). Both materials have strong intrinsic links to mineralogy and - illustrate well how mineralogy can lead to new material breakthroughs in this and other areas of modern technology.

Density Functional Theory (DFT) has here been used to study the substitution of SiO44- for VO43- polyanions in the orthosilicate Li-ion battery cathode material Li2FeSiO4, in order to enhance electron transfer between the TM-ions and thereby achieve a capacity increase from the potential redox activity of the orthovanadate polyanion. Comparison of results for five different model structures for LiFeXO4, X = Si, P and V, reveals that VO43- substitution destabilizes the tetrahedral structures towards olivine- or spinel-type structures. Our modelling of lithiation of the hypothetical 100% substituted system LiFeVO4 to Li2FeVO4 predicts the reduction of V5+ in the VO43- anion to V4+ at a potential of 2.1 V. While complete delithiation of LiFeVO4 to FeVO4 is accompanied by Fe2+/Fe3+ oxidation at similar to 3.1 V. These lithiation and delithiation processes trigger changes in the unit-cell volume: -6% and +10%, respectively. Notably, only minor structural distortions were observed in both VO43- and the more exotic VO44- tetrahedra. Thermodynamically feasible VO43- substitution levels are also shown to be <30%. This is exemplified for a 12.5% VO4-substituted system which exhibits similar to 50% smaller band-gap and increased capacity at an average deintercalation potential of similar to 3.2 V compared to the un-substituted system.

Classical molecular dynamics modeling studies at 363 K are reported of the local atomic-level and macroscopic nanostructures of two well-known perfluorosulfonic acid proton exchange polymer membrane materials: Nation and Hyflon. The influence of the different side-chain lengths in the two polymers on local structure is relatively small: Hyflon exhibits slightly greater sulfonate-group clustering, while Nation has more isolated side chains with a higher degree of hydration around the SO3- side-chain ends. This results in shorter mean residence times for water molecules around the end groups in Nation. Hyflon also displays a lower degree of phase separation than Nafion. The velocities of the water molecules and hydronium ions are seen to increase steadily from the polymer backbone/water interface toward the center of the water channels. Because of its shorter side chains, the number of hydronium ions is similar to 50% higher at the center of the water channels in Hyflon, and their velocities are similar to 10% higher. The water and H3O+ diffusion coefficients are therefore higher in the shorter side-chain Hyflon system: 6.5 x 10(-6) cm(2)/s and 25.2 x 10(-6) cm(2)/s, respectively; the corresponding values for Nation are 6.1 x 10(-6) cm(2)/s and 21.3 x 10(-6) cm(2)/s, respectively. These calculated values compare well with experiment: 4 x 10(-6) cm(2)/s for vehicular H3O+ diffusion.

Molecular dynamics (MD) simulation has been used to probe ion-conduction mechanisms in crystalline LiPF₆.PEO₆ for smectic- and nematic-ordered models of methyl-terminated short-chain monodisperse poly(ethylene oxide) chains with the formula CH3-(OCH2CH2)23-OCH3; M_w = 1059. The effect of aliovalent substitution of the PF₆- anion by ca. 1% SiF₆^2- has also been studied. External electric fields in the range 3-6 x 10⁶ V m^-1 have been imposed along, and perpendicular to, the chain direction in an effort to promote ion transport during the short timespan of the simulation. Ion-migration barriers along the polymer channel are lower for the nematic models than for the smectic, with anions migrating along the channels more readily than Li-ions. Ion mobility within the smectic interface could also be confirmed, but at a higher field-strength threshold than along the chain direction. Li-ion migration within the smectic plane appears to be suppressed by ion pairing, while Li-ion transport across the smectic gap is facilitated by uncoordinated methoxy end-groups. Interstitial Li-ions introduced into the PEO channel through SiF₆^2- doping are also shown to enhance Li-ion conduction.

The Nafion, Dow and Aciplex systems – where the prime differences lies in the side-chain length – have been studied by molecular dynamics (MD) simulation under standard pressure and temperature conditions for two different levels of hydration: 5 and 15 water molecules per (H)SO3 end-group. Structural features such as water clustering, water-channel dimensions and topology, and the dynamics of the hydronium ions and water molecules have all been analysed in relation to the dynamical properties of the polymer backbone and side-chains. It is generally found that mobility is promoted by a high water content, with the side-chains participating actively in the H3O+/H2O transport mechanism. Nafion, whose side-chain length is intermediate of the three polymers studied, is found to have the most mobile polymer side-chains at the higher level of hydration, suggesting that there could be an optimal side-chain length in these systems. There are also some indications that the water-channel network connectivity is optimal for high water-content Nafion system, and that this could explain why Nafion appears to exhibit the most favourable overall hydronium/water mobility.

X-ray and neutron powder diffraction studies have been made of the single-phase systems LiCoxFe1−xPO4 (x = 0, 0.25, 0.40, 0.60 and 0.75) to establish how Co2+ substitutes into the LiFePO4 olivine structure. Rietveld refinement shows that all four substituted materials have the same olivine structure (space group: Pnma) with lithium occupying octahedral (4a) sites, and Co2+ replacing Fe2+ at the octahedral (4c) sites. The a and b cell parameters decrease while the c parameter increases on the addition of Co2+. There are certain indications of structural instability for high Co-content compositions.