Systematic Search for Lithium Ion Conducting Compounds by Screening of Compositions Combined with Atomistic Simulation
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Systematic Search for Lithium Ion Conducting Compounds by Screening of Compositions Combined with Atomistic Simulation Daniel Mutter1,2, Daniel F. Urban2, and Christian Elsässer1,2 1
Freiburg Materials Research Center (FMF), Albert-Ludwigs-Universität Freiburg, StefanMeier-Str. 21, 79104 Freiburg, Germany 2
Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstr. 11, 79108 Freiburg, Germany
ABSTRACT Replacing liquid by solid state electrolytes has the potential to significantly improve current Li ion batteries concerning performance and safety. The material class NZP, based on the compound NaZr2(PO4)3, exhibits a structural framework suitable for ionic conduction. In this work, a systematic compositional screening and simulation approach, combining classical molecular-dynamics, first-principles calculations, and structural analysis was applied, with which a set of new Li ion conducting NZP compounds could be identified. INTRODUCTION A promising route to significantly improve current Li ion battery technology in terms of performance, reliability, safety, and cost is the application of bulk inorganic compounds as ionconducting electrolytes instead of using salts in organic liquids [1,2]. For this purpose, suitable solid-state materials are needed, which show structural stability over a broad temperature range, no detrimental electrochemical reactions with the electrodes within the applied voltage range, and most importantly, high mobility for Li ions at zero electronic conductivity. Moreover, in order to be of practical relevance, a solid electrolyte should be composed of earth-abundant and non-toxic elements. Owing to their unique framework of corner-sharing [X2(LO4)3]k- units (“lanterns”), so-called NASICON compounds, crystallizing in the structure of NaZr2(PO4)3 (NZP, [3]) with space group R3̅c, exhibit a three-dimensional network of channels enabling fast migration of cations M+ such as Na+ or Li+ (see Figure 1). These cations can occupy two distinct symmetry inequivalent positions in the rhombohedral unit cell of the crystal, namely the positions M1 (Wyckoff position 6b) and M2 (18e), in a concentration range of M+ between zero and four cations per formula unit (f. u.). The amount depends on the negative charge k of one lantern unit, which is determined by the formal oxidation states of the elements on the positions X and L. The compound LiTi2(PO4)3 (LTP) with Li+, Ti4+ and P5+ on the M1, X and L sites, respectively, shows a high conductivity of Li+ ions of about 10-4 S/cm at room temperature [4]. Partial substitution of Ti ions by trivalent Al ions can increase this value significantly [4,5]. A variety of other elements on the X positions were investigated experimentally, but in many cases, structural distortions by slight rearrangements of the oxygen tetrahedra and octahedra coordinating the L and X cations, respectively were observed [6] at elevated temperatures, which deteriorated the ionic conductivity. By means of first-principles calculations based on density functional theory (DFT), activation e
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