Enhancing the ionic conductivity of solid state electrolytes by oxide glass doping.

摘要

As the increasing use of lithium ion secondary batteries in practical application, solid state electrolyte attracted lots of attention since it's a crucial part for the lithium ion secondary batteries. Compare with liquid electrolyte, the advantage of solid state electrolyte covering no electron leakage, safety, chemical and thermal stability, high energy density and so on. However, one disadvantage of inorganic solid state electrolyte is the low ionic conductivity which is primarily due to its low grain boundary conductivity. Among all of the solid state electrolyte candidates for lithium battery applications, the perovskite structure lanthanum lithium titanate (La2/3-x □1/3-2xLi3xTiO3) has the best lithium ionic conductivity, which can reach 10-3S/cm in the grain bulk, but typically only 10-6S/cm at the grain boundary.;It has been found that the addition of amorphous phases doped into the LLTO grain boundary can significantly improve its overall conductivity. Several candidates for use as a suitable, high conducting amorphous dopant with the best properties were considered. The sulfide glass system is widely recognized as being the highest lithium ion conducting amorphous system. However, the difficulties associated with its fabrication and instability in air along with possible reactions with oxide materials are major drawbacks to its application as a dopant in the LLTO system. These are not problems found in the oxide glass system and so we looked at several oxide glass formers. Previously, people introduced boron trioxide, silica and alumina as single composition amorphous phase dopants into LLTO in an effort to improve its grain boundary conductivity. To obtain further enhancements to the grain boundary ionic conductivity, composite composition oxide glasses were investigated. As it turns out, some of these systems can reach the ionic conductivity of LLTO grain bulk.;In the present work, we used the melt quench method to make different compositions of the 0.4Li2O+0.25B2O3+(0.35-x)SiO 2+xAl2O3 glass system and doped these into the LLTO grain boundary to enhance its ionic conductivity. SiO2 is generally used as a base glass former. Substitution of Si by Al is believed to decrease the activation energy of the glass, and furthermore, B2O 3 and Al2O3 can be treated as a network former of alkali glass. Our result shows that, at room temperature, the best conductivity can reach 3.4x10-4S/cm at the grain boundary. The amount of Al2O3 was found to play a crucial role in the conductivity of this glass system. To gain further understanding, we examined the morphology of the LLTO/oxide glass pellet cross section and tested the density of these pellets to confirm the amount of oxide glass necessary to fully fill space at the LLTO grain boundary.