Trivalent Cation Dopants and their Influence on the Inductive Effect in Lithium Iron Phosphate

Lithium-ion batteries are an integral part of everyday life, being found in most portable electronics and even some larger electronic devices, such as electric bicycles. They are one of the most important tools of the revolution against fossil fuels and, therefore, a lot of research efforts are focused on improving their utility, efficiency, and general applicability.

Lithium iron phosphate (LFP) is one of the most universally employed cathode materials due to its environmental benignity, cost effectiveness, and non-toxicity. However, despite decades of research on the material, it still presents shortcomings such as low ionic and electronic conductivity, that prevent it from being implemented in large-scale grid applications.

One of the most effective methods of overcoming these limitations is cation doping. Several trivalent cations from across the periodic table (Al, Sc, V, Ho, Yb) were selected to dope LFP at different concentrations, and the physical, structural, and electrochemical effects of these materials are investigated.

In this thesis, various structural and electrochemical analyses are carried out to assess the phase purity, Li-ion diffusion, oxidation states, and local environments of the doped and undoped materials. The discharge capacity is not seen to improve with the presence of the dopants at varying concentrations; however interesting results, such as an increase in electrochemical stability for the holmium-doped LFP, are seen when compared to the undoped analogue.

The redox activity of phosphorus within the phosphate polyanion is explored in a number of samples, namely a commercial sample of LFP (Gelon LFP), in-house synthesised LFP and 1% holmium-doped LFP, and an optimised sample of 20% vanadium-doped LFP. X-ray absorption spectroscopy is performed ex situ on electrodes of these materials at different states-of-charge and various elemental edges are probed in order to gain insight into the redox activities of the different elements. Phosphorus is confirmed to be redox active in both the doped and undoped LFP samples, with the presence of the holmium slightly affecting the polarisation of the phosphate when compared to the undoped analogue.