High-performing iron phosphate for enhanced lithium ion solid state batteries as grown by atomic layer deposition

Atomic layer deposition (ALD) is an excellent tool for realisation of uniform coating of cathode materials on highly 3D-nanostructured microbatteries. We have developed an ALD-process for deposition of iron phosphate, FePO4, as a cathode material and characterised its electrochemical properties towards a lithium metal anode. Thin films were deposited between 196 and 376 °C using the precursor pairs: trimethyl phosphate (TMP, Me3PO4) with both H2O and ozone (O3) as an oxygen source, and Fe(thd)3 (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedionate) with O3. The as-deposited films are amorphous and crystallize to trigonal FePO4 after heat treatment in air at 600 °C. The amorphous FePO4 films were characterised electrochemically proving exceptional cyclability and capacities almost reaching the 100% theoretical value (178 mA h g−1) for 1 hour charge–discharge rates.