Electrical Properties Of Solid State Lithium Ion Conductors Based On Li_3xLa_2/3-xTi O₃

Though broadly used in many aspects, lithium ion batteries still face some major technique problems. For example, caused by liquid electrolyte leakage, explosions of cellphone and electrical vehicles have often been reported. The application of inorganic solid state lithium ion electrolyte is a solution to this problem. However, the electrical conductivity of solid state lithium ion electrolytes is usually lower than that of liquid electrolyte materials.The objective of this work is to understand the lithium ion conduction in the grain bulk and grain boundaries of Li3xLa2/3-xTiO3(LLTO). Li3xLa2/3-x TiO3 ceramics with different lithium contents were prepared with solid phase method. XRD analysis was taken to reveal the crystal structure and the lattice parameters of LLTO. Electrochemical impedance spectroscopy(EIS) was taken to measure the electrical properties. By comparing electrical conductivity, the optimal lithium ion concentration in LLTO was determined. Considering the loss of lithium in the sintering process, the actual lithium content in the samples should be less than the nominal values.Sr-doped LLTO ceramics, Li0.36SryLa0.5467-2/3yTiO3, were investigated to understand how Sr doping affects the phase and lattice parameters of LLTO ceramics. Lattice parameter determines the size of the bottleneck for lithium ion transport.According to microstructure investigations by SEM and TEM, the grain boundaries of LLTO ceramics are free of second phase. While the grain boundary conductivity is still3-5 orders of magnitude lower than the bulk conductivity. Such a low grain boundary conductivity can be successfully explained by the space-charge theory. The space-charge potential is positive, causing the depletion of Li+ in the space-charge layer, which accounts for the low grain boundary conductivity.