Influence of titanium nitride interlayer on the morphology, structure and electrochemical performance of magnetron-sputtered lithium iron phosphate thin films

• Formation of iron rich surface particles due to Ti and LFP interdiffusion.
• TiN interlayer effectively blocks interdiffusion.
• Very smooth and homogeneous surfaces are obtained with TiN interlayer.
• Crystallization behaviour is improved by TiN interlayer.

Pure LiFePO4 (LFP) thin films with different thicknesses are deposited at room temperature by a radio frequency (RF) magnetron-sputtering process. Ti foils with and without titanium nitride (TiN) coating as well as thermally oxidized Si wafers coated with Ti or TiN are used as substrates. In a subsequent annealing step, LiFePO4 thin films are crystallized at 500 °C. The interaction between Ti and LiFePO4 as well as between TiN and LiFePO4 is characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), secondary ion mass spectrometry (SIMS), cyclic voltammetry (CV) and galvanostatic measurements. A severe diffusion of Ti into LiFePO4 is found and leading to the formation of impurity phases which resulting in disturbing crystallization behaviour and rough surfaces. Moreover, 80 nm LiFePO4 thin films do not show the desired electrochemical characteristics when they are deposited on Ti foils directly. By using a TiN interlayer, the diffusion of Ti into LiFePO4 can be blocked resulting in smooth morphologies and improving crystallisation behaviour. Impurity phases do not develop and all samples exhibit the expected electrochemical characteristics. Therefore, TiN is a promising candidate for the use as a current collector in all-solid-state batteries with LiFePO4 electrodes.

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