Grain Boundary Conductivity in Crystalline LiTi2(PO4)3

Abstract

Abstract : Recently, there has been a renewed interest in the development of high energy Li-Air batteries. One configuration involves the use of a Li anode in a non-aqueous electrolyte, which is separated from an aqueous electrolyte containing the air cathode by a solid state Li-ion conducting membrane. Several solid state polycrystalline Li-ion conductors, based on pervoskite (1,2), garnet (3,4) and NASICON (Na super ion conductor) (5 through 12) structures, are under consideration as possible membrane materials. One of most widely investigated crystalline Liion conducting membrane materials, based on the NASICON structure, is LiTi2(PO4)3 (5 through 10). In order to sinter crystalline LiTi2(PO4)3 to the high relative densities required for use as a membrane and increase Li-ion conductivity, two approaches have been undertaken (5 through 10). The first it to use a doped material, LiMxTi2 x(PO4)3 (where M=Al, Sc, Y and La) (5,6,9,10). The second is to use LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3 (5,7,8). Previous investigations have suggested the total Li-ion conductivity, based on analysis of ac impedance data, of M-doped LiTi2(PO4)3 (where M=Al, Sc, Y and La) and LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3 was controlled by Li-ion grain boundary conductivity, which is about 1 to 2 orders of magnitude lower compared to Li-ion bulk conductivity (5 through 10). It has been suggested that for both polycrystalline M-doped LiTi2(PO4)3 (where M=Al, Sc, Y and La) and LiTi2(PO4)3 containing a small amount of Li2O or Li3PO4 or Li3BO3, that both approaches lead to the formation of a continous amorphous film around the grains (5 through 10). It is the transport of Li-ions through this amorphous film which controls the sintering rate (i.e., densification) and grain boundary Li-ion conductivity, and hence, total Li-ion conductivity of the material (5 through 10).