LiFePO₄-Based Composite Electrode Material: Synthetic Approaches, Peculiarities of the Structure, and Regularities of Ionic Transport Processes

This article discusses approaches to the synthesis of composite cathode materials based on lithium-intercalated transition metal phosphates (by the example of iron–lithium phosphate). Among various methods and approaches to the synthesis, solid-state synthesis with a preliminary mechanochemical treatment of a mixture of starting materials was chosen as a variant of the compromising of simplicity and processability. In a series of experiments, different factors were sequentially varied in order to identify their influence on the properties of the final product. The article further presents the results of a systematic study of the electrochemical properties of LiFePO₄-based material using a set of electrochemical methods: galvanostatic and potentiostatic intermittent titration and cyclic voltammetry. An original approach to graphical presentation of the potential–lithium ion concentration dependence in the LiFePO₄ electrode was developed: for Li_xFePO₄ solid solutions, the dependence was plotted against the lithium ion concentration; in the case of Li_1 – xFePO₄ solid solutions, against the lithium vacancies’ concentration. This approach allowed correctly calculating and introducing the correction parameter z into the modified Randles–Ševčik equation, in order to determine the diffusion coefficient according to the cyclic voltammetry method. Analysis of the galvanostatic and potentiostatic intermittent titration transients was carried out in terms of the developed models that allow describing lithium transport in the diffusion layer with a permeable inner boundary (a mathematical model of heterogeneous system with two phases in equilibrium and an interphase boundary). A good agreement has been found between theoretically calculated and experimental current and potential transients. When varying the composition of the electrode with respect to lithium, kinetic parameters of lithium intercalation were determined: the lithium diffusion coefficient and the phase transition constant.