First principles study of defects in solid electrolyte lithium thiophosphate Li 7 P 3 S 11
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First principles study of defects in solid electrolyte lithium thiophosphate Li7P3S11 Ka Xionga*, Weichao Wanga, Roberto Longo Pazosa and Kyeongjae Choa,b,† a
Materials Science & Engineering Dept, The University of Texas at Dallas, Richardson, TX 75080, USA b Physics Dept, The University of Texas at Dallas, Richardson, TX 75080, USA
*[email protected] † kjcho @utdallas.edu ABSTRACT We investigate the electronic structure of interstitial Li and Li vacancy in Li7P3S11 by first principles calculations. We find that Li7P3S11 is a good insulator with a wide band gap of 3.5 eV. We find that the Li vacancy and interstitial Li+ ion do not introduce states in the band gap hence they do not deteriorate the electronic properties of Li7P3S11. The calculated formation energies of Li vacancies are much larger than those of Li interstitials, indicating that the ion conductivity may arise from the migration of interstitial Li. INTRODUCTION Inorganic solid electrolytes have attracted much attention for being used in lithium batteries to replace conventional liquid electrolytes to achieve better safety and reliability [1-2]. This has led to intensive research during the last three decades and many electrolyte candidates have been proposed such as lithium phosphate oxynitride (LiPON) and Li2S-P2S5-based glasses such as lithium thiophosphate. LiPON has been used as commercial solid electrolyte in thin-film batteries [3-6]. However, its ionic conductivity is rather low (10-6 S cm-1). To improve the performance of the battery cell, the electrolyte material should have high ionic conductivity. Moreover, a good electrolyte should also satisfy two other requirements: it should have low electric conductivity (good insulator) and it should be stable in contact with both cathode and anode electrodes. Li2S-P2S5-based glasses are of interest mainly because of their high ionic conductivity [710]. The high ionic conductivity is due to the formation of superionic crystalline phases that are precipitated by careful heat treatment. Lithium thiophosphate Li7P3S11, a superionic conductor, has been reported to have high ionic conductivity up to 4.1x10-3 S cm-1 at room temperature [1113]. To date, many experimental efforts have been dedicated to understand and improve the properties of these materials, but there are few theoretical studies [14]. Clearly, a practical use of solid electrolytes will require significant research efforts for fundamental understanding of the material properties at atomic scales. In this work, we use first principles calculations to investigate the electronic structures and stability of important defects (interstitial Li and Li vacancy) in Li7P3S11. COMPUTATIONAL METHODS We employ the total energy plane-wave basis code VASP [15]. The pseudopotential is generated using the projected augmented wave (PAW) method. An energy cutoff of 400 eV and a 4x4x4 k-point Monkhorst-Pack grid are used. For defect calculations, we construct supercells containing 84 atoms. In our supercell, we study both interstitial Li (denoted as Lii) and
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