Mineral derived lithium solid electrolyte
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Mineral derived lithium solid electrolyte Bo Wang Imerys, San Jose, CA 95134, U.S.A.
ABSTRACT
Lithium solid electrolyte with NASICON structure in the form of Li1+2xAlxTi2-xSixP3-xO12 solid solution has been prepared by high temperature solid state reaction using low cost kaolin as the starting material. The crystal structure of the solid solution was investigated by powder Xray diffraction. The AC impedance measurements indicate that ionic conductivity increased by more than one order of magnitude when a small amount of Al3+ and Si4+ ions were incorporated into the LiTi2(PO4)3 crystal structure. The significant improvement on ionic conductivity can be attributed to the increased interstitial Li+ ions in the crystal structure. The highest ionic conductivity was found in Li1.2Al0.1Ti1.9Si0.1P2.9O12: 8.3 x 10-5 S·cm-1 at room temperature (21°C) and 1.5 x 10-3 S·cm-1 at 100°C.
INTRODUCTION Lithium solid electrolytes based on NASICON type materials LiM2(PO4)3 (M = Ge, Ti, Sn, Zr and Hf) have attracted great interest for applications in all-solid-state batteries [1-6]. The structure of these NASICON-related phosphate compounds is characterized by M2(PO4)3 units of corner-sharing MO6 octahedra and PO4 tetrahedra. These basic structural units join by additional corner sharing to form a three-dimensional interconnected framework structure. The mobile lithium ions are distributed in the threedimensionally connected channels of the network structure. Studies have shown that ionic conductivity of LiTi2(PO4)3 can be enhanced when Ti4+ ions are partially substituted with trivalent ions such as Al 3+ ions [7-12]. Partial substitution of P5+ ions with tetravalent ions such as Si4+ ions is also effective to increase ionic conductivity [13]. Due to the high lithium ionic conductivity and stability in air and water, NASICON type LiTi2(PO4)3 based solid electrolytes have been used in all-solid-state lithium batteries [14] and lithium-air batteries [15]. Kaolin is a natural aluminosilicate clay mineral with chemical formula of Al4Si4O10(OH)8 [16]. One of the industrial applications is to use kaolin as a raw material for ceramics [17]. In this study, kaolin is used to substitute Ti4+ and P5+ ions in LiTi2(PO4)3 with Al3+and Si4+ ions to enhance ionic conductivity. In this paper, we report the crystal structure and the ionic conductivity of kaolin derived Li1+2xAlxTi2-xSixP3-xO12 solid solution.
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EXPERIMENTAL DETAILS A refined kaolin sample from a deposit in Georgia, USA (Imerys) was used as a starting material. Table 1 lists the chemical composition of kaolin determined by X-Ray fluorescence (XRF). All Li1+2xAlxTi2-xSixP3-xO12 solid solution samples were prepared by solid-state reactions from appropriate amounts of the stoichiometric starting materials of Li2CO3 (Alfa Aesar, ACS, 99.0% min),
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