Browsing by Author "Rao, RP"

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    Formation and conductivity studies of lithium argyrodite solid electrolytes using in-situ neutron diffraction
    (Elsevier, 2013-01-10) Rao, RP; Sharma, N; Peterson, VK; Adams, S
    Lithium-ion conducting argyrodites Li6PS5X (X = Cl, Br, I) are a promising class of fast-ion conductors for all-solid state Li-ion batteries. To gain a deeper insight into the phase formation of Li6PS5Cl, in situ neutron diffraction studies are carried out on a stoichiometric ball-milled precursor mixture during thermal treatment. The evolution of the S2 −/Cl− anion disorder and its correlation with ionic conductivity are reported here. In contrast to earlier reports, an argyrodite phase is found to form between 80 and 150 °C, but the phase shows only moderate conductivity when crystallized at such low temperatures and further thermal treatment is required to access the highly conducting phase. The maximum room-temperature ionic conductivity of 1.1 × 10− 3 S/cm is observed for samples annealed at intermediate temperatures (250 °C). When ball-milled glass-ceramic precursors for Li6PS5Cl are crystallized with a constant slow heating rate, the initially formed argyrodite phase is found to be Li7PS6, which is then gradually converted into Li6PS5Cl at higher temperatures. The industrial requirements for minimizing cost by using lower annealing temperatures thus need to be balanced with the requirements of obtaining the highest conducting composition of the phase for performance in all-solid state batteries. © 2012, Elsevier B.V.
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    In situ neutron diffraction monitoring of Li7La3Zr2O12 formation: towards a rational synthesis of garnet solid electrolytes
    (American Chemical Society, 2015-04-01) Rao, RP; Gu, W; Sharma, N; Peterson, VK; Avdeev, M; Adams, S
    The favorable combination of fast-ionic conductivity and high electrochemical stability of Li-stuffed garnet type Li7La3Zr2O12 (LLZ) makes this material a promising candidate for applications as a solid-state electrolyte in high-energy-density batteries. However, a widespread technical use of LLZ is impeded by difficulty in reliable formation and densification of the pure fast-ion conducting phase. The present study of the phase-formation process enables rational fabrication procedures to be devised based on a thorough understanding of the complex phase formation of LLZ. In situ neutron powder diffraction monitoring of the phase formation revealed an influence of the partial melting of precursors on the formation of the fast-ion conducting phase, indicating that in the typical synthesis route LLZ is not formed in a solid-state reaction but from a partial carbonate melt that decomposes on further heating. The cooling rate critically influences lithium ordering and ionic conductivity. © 2015 American Chemical Society
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    Revisiting the layered Na3Fe3 (PO4) 4 phosphate sodium insertion compound: structure, magnetic and electrochemical study
    (IOP Publishing, 2019-11-18) Shinde, GS; Gond, R; Avdeev, M; Ling, CD; Rao, RP; Adams, S; Barpanda, P
    Layered sodium iron phosphate phase [Na3Fe3(PO4)4] was synthesized by solution combustion synthesis method, marking the first attempt of solvothermal synthesis of this phase. Its crystal structure was verified by synchrotron and neutron powder diffraction. Rietveld analyses proved the phase purity and formation of monoclinic framework with C2/c symmetry. It undergoes an antiferromagnetic ordering ~27 K. This combustion prepared nanoscale Na3Fe3(PO4)4 compound was found to be electrochemically active with a stepwise voltage profile involving an Fe3+/Fe2+ redox activity centred at 2.43 V vs. Na/Na+. Despite various cathode optimization, only 1.8 Na+ per formula unit could be reversibly inserted into the Na3Fe3(PO4)4 framework leading to capacity close to 50 mAh g−1. This limited electrochemical activity can be rooted to (i) relatively large diffusion barrier (ca. 0.28 eV) as per Bond valence site energy (BVSE) calculations and (ii) possible structural instability during (de)sodiation reaction. © 2019 The Author(s). CC-BY licence - Published by IOP Publishing Ltd
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    Screening of the alkali-metal ion containing materials from the Inorganic Crystal Structure Database (ICSD) for high ionic conductivity pathways using the bond valence method
    (Elsevier Science BV, 2012-10-04) Avdeev, M; Sale, M; Adams, S; Rao, RP
    High ionic conductivity is one of the key characteristics of electrolytes and electrode materials directly affecting performance of electrochemical devices in which they are used. In the case of inorganic crystalline solid electrolytes and insertion cathodes the topology and geometry of crystal structure essentially defines ionic conductivity and charge–discharge rates. We employed the bond valence method to identify materials with crystal structures featuring infinite networks of pathways of suitable size that is a prerequisite for fast ion transport. Taking advantage of the method low computational cost, we carried out exhaustive analysis of similar to 13,000 entries of the Inorganic Crystal Structure Database and ranked the materials based on the fraction of crystal structure space with low bond-valence mismatch. The results may be used as a guide for further theoretical and experimental studies of promising compositions. © 2012, Elsevier Ltd.
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    Structural evolution of NASICON-type Li1+xAlxGe2−x(PO4)3 using in situ synchrotron x-ray powder diffraction
    (Royal Society of Chemistry, 2016-04-04) Safanama, D; Sharma, N; Rao, RP; Brand, HEA; Adams, S
    Fast Li-ion conducting Li1+xAlxGe2-x(PO4)3 or LAGP ceramics are the most commonly used anode-protecting membranes in new generation Li-air batteries. The electrochemical properties of this solid membrane (electrolyte) are highly dependent on the purity of the phase and the actual amount of Al incorporated into the structure which often deviates from the synthetic inputs for different annealing conditions. Hence, optimizing the annealing temperature range is of great importance to achieve desirable phases and therefore optimized properties. Here in situ synchrotron X-ray diffraction is carried out during the synthesis of LAGP. Starting with ball-milled and calcined LAGP glass powders we observe the structural evolution during the glass to ceramic transition. Sequential Rietveld refinements show that the dominant Al-poor LGP phase transforms into an Al-incorporated LAGP structure at temperatures higher than 800 °C. The c lattice parameter is found to be highly dependent on the temperature and also the amount of Al incorporated into the structure. The relationship between the c lattice parameter and Al concentration in LAGP is evaluated and the correlation can be used to allow the estimation of Al doping. Thus this work allows the lattice parameter to "fingerprint" the dopant concentration. © 2016 The Royal Society of Chemistry.
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    Temperature dependence of structure and ionic conductivity of LiTa2PO8 ceramics
    (American Chemical Society, 2022-11-30) Dai, R; Avdeev, M; Kim, SJ; Rao, RP; Adams, St
    LiTa2PO8 has recently been reported as a new fast Li-ion conducting structure type within the series of Lix(MO6/2)m(TO4/2)n polyanion oxides. Here, we demonstrate the preparation of LiTa2PO8 by solid-state syntheses, clarify the temperature dependence of lithium distribution and ionic conductivity, and study the structural stability, densification, and achievable total conductivity as a function of sintering conditions synergizing experimental neutron and X-ray powder diffraction and electrochemical studies with computational energy landscape analyses and molecular dynamics simulations. A total room temperature conductivity of 0.7 mS cm-1 with an activation energy of 0.27 eV is achieved after sintering at 1323 K for 10 h. Spark plasma sintering yields high densification >98%, highly reproducible bulk conductivities of 2.8 mS cm-1, in agreement with our bond valence site energy-based pathway predictions, and total conductivities of 0.6 mS cm-1 within minutes. Powder diffraction studies from 3 to 1273 K reveal a reversible flipping of the monoclinic angle from above to below 90° close to room temperature as a consequence of rearrangements of the mobile ions that change the detailed pathway topology. A consistent model of the temperature-dependent Li redistribution, conductivity anisotropy, and transport mechanism is derived from a synopsis of diffraction experiments, experimental conductivity studies, and simulations. Due to the limited electrochemical window of Lix(TaO6/2)2(PO4/2)1 (LTPO), a direct contact with Li metal or high voltage cathode materials leads to degradation, but as demonstrated in this work, semi-solid-state batteries, where LTPO is protected from direct contact with lithium by organic buffer layers, achieve stable cycling. © 2022 American Chemical Society