The Role of Defects in Li 3 ClO Solid Electrolyte: Calculations and Experiments
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The Role of Defects in Li3ClO Solid Electrolyte: Calculations and Experiments M. Helena Braga1,a, Verena Stockhausen1, Joana C.E. Oliveira1,b, Jorge A. Ferreira2 1
Engineering Physics Department, FEUP, Porto University, R. Dr. Roberto Frias, s/n, 4200-465,
Porto, Portugal and CEMUCa and CFPb. 2
Energy and Geology National Laboratory, LNEG, R. da Amieira, S. Mamede Infesta, Portugal.
ABSTRACT We have analyzed the hopping movement of a new ionic solid electrolyte by calculating defect formation energies and activation barriers. The role of the lattice during diffusion was established. Thermodynamic properties were determined by means of first principles and phonon calculations at working temperatures. The new solid electrolyte, an antiperovskite, Li3-2xMxAO (in which M is a higher valent cation like Ca2+ or Mg2+ and A is a halide like Cl- or Br- or a mixture of halides), was studied either pure or doped. Moreover, we present experimental ionic conductivity data for these novel solid state ionic conductors for the doped and the pure solid electrolyte from room temperature and up to ~253 °C. In this paper, we compare the ionic conductivity of the latter solid electrolyte with other fast ionic conductors. INTRODUCTION Li-ion batteries are appealing for a prolific variety of applications as they provide higher energy density when compared to other rechargeable batteries. The use of Li metal as negative electrode improves the specific capacity but raises safety issues due to the reactions that take place with the gel-liquid electrolyte. Dendrites tend to grow establishing a preferred conduction channel leading to shortcuts. One of the possible ways to overcome this problem is to use solid electrolytes. A solid electrolyte material must have high ionic conductivity and be a good electronic insulator. Gel-liquid electrolytes usually present electronic band gaps around 4 eV [1]. Ionic conductivity occurs when ions hop from site to site throughout the crystal structure; therefore it is necessary to have partial occupancy of energetically equivalent or near-equivalent sites. In favorable structures, the defects may be mobile, leading to high ionic conductivity. There is a small group of materials in which defect creation by doping is unnecessary since, in the parent stoichiometric crystal there is already extensive disorder in the mobile ion sublattice above 0 K. An example of the latter solid electrolytes is the high temperature polymorph, α-AgI [2]. The number of stoichiometric compounds that present technically useful order-disorder transitions is extremely limited; therefore most attempts for designing more efficient solid electrolytes rely on chemical doping. An antiperovskite compound has the same crystal structure of a perovskite (cubic, Pm-3m) but with inverted charge. NaMgF3 is a perovskite in which Na+ occupies the center of the cubic structure, Mg2+ the center of the octahedra and F- the vertices as shown in figure 1. In Li3ClO
antiperovskite, the center of the cube is occupied by Cl- and the center of the octa
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