Development of sputtered nitrogen-doped Li1+xAlxGe2-x(PO4)3 thin films for solid state batteries

Nitrogen-doped Li 1+x Al x Ge 2-x (PO 4 ) 3 (LAGP) thin films were prepared by magnetron sputtering in a mixture of Ar + N 2 using an LAGP powder target. The as-deposited films were amorphous, but could be crystallised into the NASICON LAGP phase after annealing at temperatures above 550 °C. The introduction of nitrogen to the sputtering gas has two effects on the deposited films; incorporation of a low concentration of nitrogen into the LAGP phase but also reduction of the rate of deposition. The former leads to improvements in Li ion conductivity whereas the latter can cause porosity and discontinuities in the films and limits their application in solid state devices. Up to 23% nitrogen in the sputtering gas the first effect is dominant and the total ionic conductivity improves without introducing morphological defects. However if the nitrogen content is increased further, the porosity decreases the measured conductivity. Optimised nitrogen doping in the sputtered LAGP films results in ionic conductivities as high as 2.3 × 10 −4 S cm −1 in films only 1 μm thick (2.7 times higher than undoped LAGP films) and activation energies below 0.38 eV. • N-doped Li 1+x Al x Ge 2-x (PO 4 ) 3 thin films were prepared by sputtering in a mixture of Ar + N 2 using an LAGP-powder target. • The microstructure and properties of the films were characterized to understand how nitrogen is incorporated to the films. • The entire process was optimised especially in terms of nitrogen content for high ionic conductivities in the films. • A conductivity value of 2.3 × 10 −4 S cm −1 at 20 °C (2.7 times higher than undoped films) was achieved for N-doped LAGP films.