Improving the performance of titania nanotube battery materials by surface modification with lithium phosphate.

Self-organized TiO2 nanotubes ranging from amorphous to anatase structures were obtained by anodization procedures and thermal treatments at 500°C. Then electrolytic Li3PO4 films were successfully deposited on the nanotube array by an electrochemical procedure consisting in proton reduction with subsequent increase in pH, hydrogen phosphate dissociation and Li3PO4 deposition on the surface of the cathode. The Li3PO4 polymorph (γ or β) in the deposit could be tailored by modifying the electrodeposition parameters, such as time or current density, as determined by X-ray patterns. The morphological analysis evidenced the formation of a 3D nanostructure consisting of Li3PO4 coating the TiO2 nanotube array. The anode-solid electrolyte stacking was tested in lithium half cells. Interestingly, the electrochemical performances revealed a better cycling stability for samples containing low amount of lithium phosphate, which is deposited for short times and low current densities. These results suggested the possibility of fabricating 3D Li-ion batteries. nt-TiO2/γ-Li3PO4/LiFePO4 full cells were cycled at different rates in the C/5-5C range. This cathode-limited microbattery delivered a reversible gravimetric capacity of 110 mA h g(-1) and a capacity retention of 75 % after 190 cycles at 5C.