Structure and lithium transport pathways in Li₂FeSiO₄ cathodes for lithium batteries

The importance of exploring new low-cost and safe cathodes for large-scale lithium batteries has led to increasing interest in Li(2)FeSiO(4). The structure of Li(2)FeSiO(4) undergoes significant change on cycling, from the as-prepared γ(s) form to an inverse β(II) polymorph; therefore it is important to establish the structure of the cycled material. In γ(s) half the LiO(4), FeO(4), and SiO(4) tetrahedra point in opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the inverse β(II) polymorph on cycling involves inversion of half the SiO(4), FeO(4), and LiO(4) tetrahedra, such that they all now point in the same direction, eliminating edge sharing between cation sites and flattening the oxygen layers. As a result of the structural changes, Li(+) transport paths and corresponding Li-Li separations in the cycled structure are quite different from the as-prepared material, as revealed here by computer modeling, and involve distinct zigzag paths between both Li sites and through intervening unoccupied octahedral sites that share faces with the LiO(4) tetrahedra

Lithium diffusion in Li₅FeO₄

The anti-fluorite type Li5FeO4 has attracted significant interest as a potential cathode material for Li ion batteries due to its high Li content and electrochemical performance. Atomic scale simulation techniques have been employed to study the defects and Li ion migration in Li5FeO4. The calculations suggest that the most favorable intrinsic defect type is calculated to be the cation anti-site defect, in which Li+ and Fe3+ ions exchange positions. Li Frenkel is also found to be lower in this material (0.85 eV/defect). Long range lithium diffusion paths were constructed in Li5FeO4 and it is confirmed that the lower migration paths are three dimensional with the lowest activation energy of migration at 0.45 eV. Here we show that doping by Si on the Fe site is energetically favourable and an efficient way to introduce a high concentration of lithium vacancies. The introduction of Si increases the migration energy barrier of Li in the vicinity of the dopant to 0.59 eV. Nevertheless, the introduction of Si is positive for the diffusivity as the migration energy barrier increase is lower less than that of the lithium Frenkel process, therefore the activation energy of Li diffusion

Defects, Dopants and Sodium Mobility in Na₂MnSiO₄

Sodium manganese orthosilicate, Na2MnSiO4, is a promising positive electrode material in rechargeable sodium ion batteries. Atomistic scale simulations are used to study the defects, doping behaviour and sodium migration paths in Na2MnSiO4. The most favourable intrinsic defect type is the cation anti-site (0.44 eV/defect), in which, Na and Mn exchange their positions. The second most favourable defect energy process is found to be the Na Frenkel (1.60 eV/defect) indicating that Na diffusion is assisted by the formation of Na vacancies via the vacancy mechanism. Long range sodium paths via vacancy mechanism were constructed and it is confirmed that the lowest activation energy (0.81 eV) migration path is three dimensional with zig-zag pattern. Subvalent doping by Al on the Si site is energetically favourable suggesting that this defect engineering stratergy to increase the Na content in Na2MnSiO4 warrants experimental verification

A₂MnXO₄ Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Four new manganese germanates and silicates, A₂MnGeO₄ (A = Li, Na) and A₂MnSiO₄ (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li₂MnGeO₄ is orthorhombic (<i>Pmn</i>2₁) layered, isostructural with Li₂CdGeO₄, and the three other compounds are monoclinic (<i>Pn</i>) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (<i>Pn</i> A₂MXO₄, <i>Pna</i>2₁ and <i>Pbca</i> AMO₂), average bond angles on bridging oxygens (here, Mn–O–X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (<i>Fd</i>3̅<i>m</i>) structure typified by CsAlO₂. The sublattices of the magnetic Mn²⁺ ions in both structure types under study (<i>Pmn</i>2₁ and <i>Pn</i>) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li₂MnSiO₄ were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag₂MnSiO₄ remains paramagnetic down to 2 K, but A₂MnXO₄ (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at <i>T</i> < <i>T</i>_N for all AFM phases. We constructed the magnetic phase diagrams for A₂MnXO₄ (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis

Structural polymorphism in Li₂CoSiO₄ intercalation electrodes: a combined diffraction and NMR study

Li2CoSiO4 was prepared in three polymorphic forms. The βII (Pmn21) polymorph was obtained by hydrothermal synthesis (150 °C), and subsequent heat treatments yielded the βI (Pbn21) form (700 °C) and the γ0 (P21/n) form (1100° then quenching from 850 °C). Rietveld refinement of X-ray and neutron powder diffraction patterns reveal considerable Li/Co mixing for βII, very moderate mixing for βI, and no mixing for γ0. 7Li MAS NMR spectra have been recorded for the three forms. The mechanism of the Fermi contact interaction that leads to negatively shifted signals is as yet unexplained, but the nature and the number of signals were analyzed in relation to the site occupancies for each compound. The agreement is good for βII, although the extent of disorder leads to very poorly defined NMR signals; it is reasonable (although not fully quantitative) for βI, where well-defined NMR signals can be assigned to definite environments; finally, the γ0 sample surprisingly leads to a single rather broad NMR signal, whereas two well-defined and rather different environments are present in the structure deduced from diffraction