Biopolymer Sol-Gel Synthesis of Anion/Cation Doped Garnet Electrolyte Materials for Solid-State Batteries

Abstract

Garnet solid-state electrolytes have attracted great interest due to its high Li ion conductivity and good electrochemical stability with electrode materials [1]. To enable the All-Solid-State Batteries, several issues need to be addressed, such as to increase the Li+ ion ionic conductivity, lower the interfacial resistance between electrolyte and electrode materials and prevent the dendrite growth of Li metal through electrolyte materials. There has been significant work investigating doping strategies to improve the conductivity of Li5La3Nb(Ta)2O12 and Li7La3Zr2O12 systems [2-5], with the key conclusions being that Li ion contents of the order of 6.4-6.7 are optimal. In this work, we show that Ce and Zn can be successfully doped into the structure with extensive solid solutions, leading to improved conductivities and lower activation energies compared to the undoped samples. Unlike cation doping, there has been little research on anion doping strategies. Both experimental and computational work [6] has shown that the incorporation of F to give Li7-xLa3Zr2O12-xFx leads to a change from a tetragonal to a cubic cell, along with an enhancement in Li ion conductivity. Specifically, the synthesis of F-doped Li7La3Zr2O12 has been examined through solid state synthesis route using polymer reagents (PTFE) as the fluorine source, a route that we have previously shown to be a very flexible means of F incorporation in solids [7]. As well as the preparation of the above systems via standard solid state synthesis, we present results on the use of a novel low temperature biopolymer sol-gel route. Both undoped and Al-doped Li7La3Zr2O12 has been made successfully at 700 oC in air, with comparable conductivity to the literature. The detailed characterisation of these systems is reported.