Abstract
The structure and Li+ ion dynamics of a new class of ABO3 perovskite with Li on both the A- and B-sites are described. La3Li3W2O12 is synthesized by solid state reaction at 900 °C and shown by powder X-ray diffraction to adopt the structure of a monoclinic double perovskite (A2)BB′O6, (La1.5Li0.5)WLiO6, with rock salt order of W6+ and Li+ on the B-site. High resolution powder neutron diffraction locates A-site Li in a distorted tetrahedron displaced from the conventional perovskite A-site, which differs considerably from the sites occupied by Li in the well studied La2/3–xLi3xTiO3 family. This is confirmed by the observation of a lower coordinated Li+ ion in the 6Li magic angle spinning nuclear magnetic resonance (NMR) spectra, in addition to the B-site LiO6, and supported computationally by density functional theory (DFT), which also suggests local order of A-site La3+ and Li+. DFT shows that the vacancies necessary for transport can arise from Frenkel or La excess defects, with an energetic cost of ∼0.4 eV/vacancy in both cases. Ab initio molecular dynamics establishes that the Li+ ion dynamics occur by a pathway involving a series of multiple localized Li hops between two neighboring A-sites with an overall energy barrier of ∼0.25 eV, with additional possible pathways involving Li exchange between the A- and B-sites. A similar activation energy for Li+ ion mobility (∼0.3 eV) was obtained from variable temperature 6Li and 7Li line narrowing and relaxometry NMR experiments, suggesting that the barrier to Li hopping between sites in La3Li3W2O12 is comparable to the best oxide Li+ ion conductors. AC impedance-derived conductivities confirm that Li+ ions are mobile but that the long-range Li+ diffusion has a higher barrier (∼0.5 eV) which may be associated with blocking of transport by A-site La3+ ions.
Highlights
Replacing liquid electrolytes with solid ceramics or polymers in next-generation battery technologies is of growing interest due to the major impact that will result from the increase in lifetime and safety, higher power outputs and higher energy densities expected in energy storage appliances.[1]
Initial synthetic effort concentrated on the introduction of A-site vacancies into SrLi0.4W0.6O324 with the aim of subsequently filling them partially with Li. This was addressed by replacing Sr2+ with 2/3 La3+ to form the series Sr1−3x/2LaxLi0.4W0.6O3. These reactions led to multiphase products, but it was noted that solid state reactions at the target composition La2/3Li0.4W0.6O3 with a large amount of excess Li to avoid Li loss during synthesis resulted in the formation of Li4WO5 and a new phase matching a monoclinic distorted perovskite by indexing the Powder X-ray diffraction (PXRD) pattern
Initial Inductively coupled plasma optical emission spectrometry (ICP-OES) and TEM-EDX analysis suggested that this phase had a composition close to La3Li3W2O12, and subsequent synthesis at 900 °C for 65 h from oxide starting materials at this nominal composition resulted in phase pure samples without the need for the removal of impurities by washing
Summary
Replacing liquid electrolytes with solid ceramics or polymers in next-generation battery technologies is of growing interest due to the major impact that will result from the increase in lifetime and safety, higher power outputs and higher energy densities expected in energy storage appliances.[1]. A major goal of the field is to synthesize lithium-based ceramics with ionic conductivities that equal or surpass current liquid electrolytes. The fastest crystalline inorganic Li+ ion conductors reported to date are sulfides such as Li10GeP2S12,2 with a room temperature conductivity of 0.012 S/cm, and the related Li11Si2PS12.3 The high conductivities arise from the presence of many potential sites for the Li cations that provide low energy pathways through these structures Despite their high Li+ conductivities, the practical applications of these compounds are limited by their tendency to decompose under ambient conditions, producing H2S when in contact with moisture, which necessitates handling in inert atmospheres.[4] Oxides are generally more chemically stable, and the doped garnet Li7La3Zr2O12 and the ABO3 perovskite La2/3−xLi3xTiO3 (LLTO; A = La, Li, B = Ti) systems have respectable conductivities in the region of 10−3 S/cm which arise from motion of the Li occupying the A-site between the O12 anion cages that define these sites.[5,6] bulk ionic conductivity in LLTO may be limited by the presence of La3+ rich and La3+ poor A-site layers within the crystal structure, arising from the 2D diffusion of Li+ ions at lower temperature and 3D diffusion at higher temperature.[7−9] In addition, the grain boundary conductivity is 1 order of magnitude lower than
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