Study of the mechanism of lithium insertion and depletion in lithium iron phosphate thin films

摘要

The mechanism of lithium insertion and depletion in the cathode material lithium iron phosphate (LFP) has been research subject for over a decade. Although being widely-used by now, the exact mechanism still remains unclear. Thin film technology is a method of reducing the complexity of a system in order to study these mechanisms. Firstly, this work presents the preparation of LFP thin films with pulsed laser deposition (PLD). Galvanostatic cycling of thin films prepared on metal substrates was possible for 100 cycles without significant capacity loss. Films prepared on silicon substrates were smooth and covering. Chemical delithiation is compared to electrochemical delithiation. Thin films could still be cycled electrochemically after chemical delithiation. TEM images revealed the size of the primary particles to be less than 20 nm.The preparation process was followed by a detailed electrochemical study via galvanostatic intermittent titration technique (GITT). The study reveals the vanishing of the miscibility gap in nanosized LFP. This effect is attributed to the small size of the primary particles in the film. Furthermore, after the application of a current pulse a concentration gradient remains inside the film even after long relaxation times. Diffusion coefficients of the single-phase LFP were determined. A model for the mechanism of delithiation of the thin film is proposed supporting the model of meta-stable intermediate phases as has been suggested for LFP nanoparticles.An outlook toward a cell for in operando spectroscopy is shown in the last chapter. Proof of principle studies have been conducted and a design for a cell is proposed.