John O. Thomas

Uppsala University, Uppsala, Uppsala, Sweden

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Publications (53)138.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A mathematical model describing ionic transport in a 3D-microbattery (3D-MB) electrolyte is developed here using finite element methodology. The model is then exploited to study a 3D-MB based on an interdigitated plate (“trench”) architecture for a 10μm-thick electrolyte layer separating 10μm-thick graphite anode and LiCoO2 cathode plates. The effect of varying plate length, end-shape and electronic conductivity is also modelled. It is shown that the 3D-MB architecture gives rise to qualitatively non-uniform current densities, leading to sub-optimal surface utilization. This can, in turn, be optimized by varying electrode geometries and/or material properties.
    Solid State Ionics 01/2011; 192(1):279-283. · 2.05 Impact Factor
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    ABSTRACT: Nanostructured Li2FeSiO4/C was synthesized by high-energy ball-milling and the amorphous citrate-assisted techniques. Similar redox behaviour is observed for samples prepared by the amorphous citrate-assisted route followed by a 4h heat treatment: 0.3V polarization and more sloping behaviour was observed when cycling between 2.0V and 3.7V at 60°C; lower capacity fade is also observed compared to Li2FeSiO4/C prepared by the solid-state reaction technique. A discharge capacity of 102mAhg−1 is obtained for samples prepared by the high-energy ball-milling method, while capacities decrease from 95 to 77mAhg−1 using the amorphous citrate method for heat-treatment times increasing successively from 4h to 18h.
    Solid State Ionics 01/2011; 192(1):356-359. · 2.05 Impact Factor
  • Anti Liivat, John O. Thomas
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    ABSTRACT: The orthosilicate family of materials Li2MSiO4 for M=Fe, Mn and Co are coming to be seen as potentially cheap cathode materials for large-scale Li-ion batteries, not least through the possibility for significant capacity gains if more than one Li-ion can be removed per formula unit. To gain insights into possible Li-ion migration pathways and diffusion barriers for Li-ions, model systems for LixFeSiO4 (x≈1, 2) are here studied using the Density Functional Theory (DFT) approach. Li-ion and ion-vacancy migration barriers are calculated for a number of model systems. The results help explain why the Li/Fe site-mixing observed during electrochemical cycling of Li2FeSiO4 does not lead to any noticeable loss in cell performance, despite the increased tortuosity introduced into the Li-migration pathways by this ion-mixing process.
    Solid State Ionics 01/2011; 192(1):58-64. · 2.05 Impact Factor
  • Anti Liivat, John O Thomas
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    ABSTRACT: Computational Materials Science, 50 (2010) 191-197. doi:10.1016/j.commatsci.2010.07.025
    Computational Materials Science 11/2010; 50(1):191-197. · 1.88 Impact Factor
  • Daniel Brandell, Jaanus Karo, John O. Thomas
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    ABSTRACT: A diffraction profile is here derived from classical Molecular Dynamics (MD) simulation for the hydrated perfluorosulphonic acid fuel-cell membrane material Nafion® at 363 K using a 76 Å × 76 Å × 76 Å box. The MD simulation reproduces the phase-separated nanoscale structure of Nafion® and water channels. No specific structural features, such as a characteristic channel diameter, could be distinguished. Nevertheless, the envelope of the simulated diffraction profile based on 6000 MD “snapshots” reproduced well the key features of the experimental SAXS profile. This draws into questions previous interpretations of diffraction data for the Nafion® system which involve simplistic notions of channel- and cluster-diameter.
    Journal of Power Sources 09/2010; · 5.26 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 07/2010; 32(30).
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    ABSTRACT: 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: Nafion 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 Nafion has more isolated side chains with a higher degree of hydration around the SO(3)(-) side-chain ends. This results in shorter mean residence times for water molecules around the end groups in Nafion. 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 approximately 50% higher at the center of the water channels in Hyflon, and their velocities are approximately 10% higher. The water and H(3)O(+) 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 Nafion 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 H(3)O(+) diffusion.
    The Journal of Physical Chemistry B 05/2010; 114(18):6056-64. · 3.61 Impact Factor
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    ABSTRACT: DFT calculations are presented which probe the effect of low-concentration Mn substitution of the Fe-sites in Li2FeSiO4: the promising new and potentially cheap cathode material for upscaled Li-ion battery applications. The LixFe0.875Mn0.125SiO4 system investigated could be achieved by replacing 12.5% of the Fe-sites in 2 × 2 × 1 and 2 × 2 × 2 supercells by Mn ions. The evolution of Bader charges and partial densities of states (DOS) have been followed under a stepwise delithiation process. A clear structural distortion is seen to occur at the Mn-site on delithiation, suggesting possible structural instability. Oxidation of Mn beyond 3+ is calculated to occur at potentials in excess of 4.7 V, implying that oxidation of well separated (>10Å), low-concentration Mn ions to Mn4+ is energetically unfavourable in the LixFe0.875Mn0.125SiO4 structure. This, together with previous DFT results for higher levels of Mn substitution into Li2FeSiO4, indicates that capacity increase in Li2Fe1 − yMnySiO4 through a >1 electron redox reaction may not be so readily attainable in practice, either for high or low Mn concentrations.
    Computational Materials Science. 01/2010; 47(3):678-684.
  • Anti Liivat, John O. Thomas
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    ABSTRACT: 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 2.1V. While complete delithiation of LiFeVO4 to FeVO4 is accompanied by Fe2+/Fe3+ oxidation at ∼3.1V. 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
    Computational Materials Science - COMPUT MATER SCI. 01/2010; 50(1):191-197.
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    ABSTRACT: The first discharge of the Li+ ion anode material LiSn2(PO4)3 was investigated with Mössbauer spectroscopy and electrochemical techniques. Mössbauer spectroscopy provided insight into the structure of the tin atoms of the fully discharged anode materials. Spectra consist of overlapping peaks, which are assigned to noncrystalline β-Sn and Li–Sn alloy domains. An analysis of the relative intensities of the Mössbauer spectra shows the relative abundance of β-Sn increases at the expense of the Li–Sn alloy as the discharge rate increases. Cell polarization occurs at higher discharge rates, leading to inefficient electrode utilization and poor cycling performance. Sluggish Li+ ion diffusion through the amorphous Li3PO4 network that is formed early in the discharge process might be responsible for the poor electrochemical performance and the accumulation of unalloyed tin.
    Journal of Solid State Electrochemistry 01/2009; 13(8):1267-1272. · 2.28 Impact Factor
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    ABSTRACT: X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li2FeSiO4 cathodes extracted from Li-ion batteries in both a charged and discharged state. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate (DEC) as organic solvents. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. However, solvent reaction products from EC were found together with lithium carbonate. A LiPF6-based electrolyte, on the other hand, showed inferior salt stability with LixPFy, LixPOyFz and LiF species formed on the surface. Solvent reaction products together with lithium carbonate were also found. There are also indications that Li2FeSiO4 is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF6. A better understanding of the surface chemistry of carbon-coated Li2FeSiO4 after the first cycles in a Li-ion battery has thus been achieved, thereby facilitating the optimisation of Li-ion batteries based on this potentially cheap and electrochemically most promising cathode material giving excellent capacity retention: <3% drop over 120 cycles.
    Journal of Materials Chemistry 01/2009; 19(1). · 6.63 Impact Factor
  • Electrochemical and Solid State Letters - ELECTROCHEM SOLID STATE LETT. 01/2009; 12(5).
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    ABSTRACT: 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)SO(3) 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 H(3)O(+)/H(2)O 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.
    Journal of Molecular Modeling 11/2007; 13(10):1039-46. · 1.98 Impact Factor
  • Miguel A Zendejas, John O Thomas
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    ABSTRACT: Solid electrolytes are ionic solids which have unusually high ionic conductivities, comparable to those of molten salts. The characteristic features of the conducting phase of these materials include low activation energy for translational motion, and an open crystal structure involving an interconnected network of vacant sites available to one or more ionic species. The beta-aluminas are one of the most widely investigated family of solid electrolytes, permitting fast ion conduction for monovalent (including protonic), divalent and even trivalent cation species. The conduction is restricted to a layer containing mobile ions and bridging oxygens, which serve to support the spinel blocks. This makes it an ideal system in which to study two-dimensional diffusion in a real situation by computer simulation methods. We illustrate the usefulness of molecular dynamics simulation (MDS) in investigating the atomistic processes responsible for conduction, emphasizing the important role played by the collective movement of the ions. A detailed analysis of this motion is presented. This allows us to clarify long-standing questions concerning the mechanism of diffusion for the mobile sodium ions.
    Physica Scripta 01/2007; 1990(T33):235. · 1.03 Impact Factor
  • Miguel A Zendejas, John O Thomas
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    ABSTRACT: Results of a molecular dynamics simulation (MDS) of a Mg2+ stabilized form of the well known solid electrolyte Na+ beta-alumina (idealized formula: Na1+xAl11-xMgxO17 for x = 0.22) are presented. The simulated crystal is stable over a wide temperature range, while still exhibiting a high degree of mobility for the mobile Na+ ion species. Analysis of Na+ trajectories is able to identify the conduction mode prevalent at a given temperature. This changes dramatically as the temperature increases; going from an almost conventional hopping motion at low T to highly correlated motion at high T, where a number of ions move over large distances. Remarkably, the transition is found to coincide with a region of reduced overall mobility, with the diffusion constant passing through a local minimum. The mechanisms underlying such phenomena are exposed, and comparisons made with our earlier MDS study of the normal form of Na+ beta-alumina, where the charge compensation is achieved through the introduction of an extra oxygen into the conduction plane (the Roth defect). It is shown that significantly different levels of diffusivity can result from different compensation mechanisms.
    Physica Scripta 08/2006; 47(3):440. · 1.03 Impact Factor
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    ABSTRACT: Photoelectron spectroscopy (PES) has been used to characterise the surface of Li 2 FeSiO 4 cathodes extracted from lithium-ion batteries. Pristine, uncycled, air-exposed electrodes were first analysed and found to carry significantly greater amounts of Li 2 CO 3 on their surfaces than electrodes stored under inert atmosphere. The surface film formed on electrochemical cycling of Li 2 FeSiO 4 electrodes at 60 uC using a LiN(SO 2 CF 3) 2 salt based electrolyte revealed high salt stability and only small amounts of solvent reaction products. These were mainly of Li-carboxylate type; neither carbonates nor LiF were found. The excellent capacity retention (,3% over 120 cycles) and minimal irreversible capacity during the first cycle are probably a direct result of this very thin surface film. Li 2 FeSiO 4 must therefore be seen as a most promising (and potentially cheap) positive electrode material for future large-scale Li-ion battery applications.
    Journal of Materials Chemistry 07/2006; 16:3483–3488. · 6.63 Impact Factor
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    ABSTRACT: The lithium extraction and insertion mechanism in the cathode material Li2FeSiO4 has been monitored by in situ X-ray diffraction and Mössbauer spectroscopy during the first two cycles. The residual amounts of Li2FeSiO4 and LiFeSiO4 in the fully charged and discharged states are 5% and 10%, respectively, on the basis of both Mössbauer spectroscopy and powder XRD studies; this is also in good agreement with the results of electrochemical measurements. The observed lowering of the potential plateau from 3.10 to 2.80 V during the first cycle can be explained by a structural rearrangement in which some of the Li ions (in the 4b site) and Fe ions (in the 2a site) become interchanged.
    Journal of Materials Chemistry 06/2006; 16(23):2266-2272. · 6.63 Impact Factor
  • Anton Nytén, John O. Thomas
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    ABSTRACT: 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.
    Solid State Ionics 01/2006; 177(15):1327-1330. · 2.05 Impact Factor
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    ABSTRACT: A density functional theory (DFT) calculation is reported for the novel Li-ion battery cathode material lithium iron silicate (Li2FeSiO4) and for three possible Li arrangements in the delithiated structure (LiFeSiO4). Relevant battery-related properties have been derived: average voltage (2.77V vs. Li/Li+), energy density (1200Wh/l) and specific energy (440Wh/kg). Lattice constants and atomic fractional coordinates are also given for each case. The calculated values are in good agreement with recent experimental values (A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun., 7 (2005) 156). Voltages were calculated (again vs. Li/Li+) for the three different Li arrangements in LiFeSiO4; these differed by 0.22V – a difference which could perhaps be related to the experimentally observed 0.30V drop in voltage between the first and subsequent charge cycles.
    Electrochemistry Communications - ELECTROCHEM COMMUN. 01/2006; 8(5):797-800.
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    ABSTRACT: The pore structure and morphology of direct methanol fuel cell electrodes are characterized using mercury intrusion porosimetry and scanning electron microscopy. It is found that the pore size distributions of printed primer and catalyst layers are largely dictated by the powders used to make the printing ink. The extent to which the pore structure is modified by changing several parameters in the membrane electrode assembly (MEA) manufacturing process is discussed. The pore structure of the printed layers is found to be invariant with respect to changes in powder loading or in choice of printing substrate, and is relatively undisturbed by MEA hot-pressing. Changing the source of the primer powder and adding a pore-forming agent to the catalyst ink are found to be successful methods of creating a more open pore structure in the printed layers.
    Journal of The Electrochemical Society. 08/2005; 152(9):A1844-A1850.

Publication Stats

276 Citations
138.37 Total Impact Points

Institutions

  • 1992–2011
    • Uppsala University
      • Department of Chemistry - Ångström Laboratory
      Uppsala, Uppsala, Sweden
  • 2007
    • Virginia Polytechnic Institute and State University
      Blacksburg, Virginia, United States
  • 2005
    • University of Tartu
      • Institute of Physics
      Tartu, Tartumaa, Estonia