Synthesis of modified lithium iron phosphate and its electrochemical properties

Lithium-ion batteries (LIBs) are used to power portable electronic devices and electric vehicles (EVs). Cobalt has been an essential element on popular cathode active materials of LIBs since the commercialization of LiCoO2 (LCO) by Sony in 1991. However, most cobalt reserves and processing facilities are in Democratic Republic of Congo (DRC) and China. This creates geopolitical risks and restricts supply expansion. On top of that, child labour is used to mine cobalt in DRC. Consequently, cobalt free and low-cobalt materials should be and are developed and used for commercial applications. One of the common cobalt free materials used today is lithium iron phosphate, also known as LiFePO4 (LFP). LFP was identified as a cathode material for LIBs by Padhi, Nanjundaswamy, and Goodenough in 1997. Main advantages of LFP are its flat voltage profile, low cost, abundant material supply, environmental friendliness (nontoxic and cobalt free), and thermal stability. The downsides include relatively low theoretical capacity, low energy density, low electronic conductivity, and low ionic diffusivity. This study focuses on synthesis of modified LFP and its electrochemical properties. Common and novel synthesis methods are reviewed shortly. Especially, performance increasing modifications are reviewed. These include coating, control of particle size and particle morphology, and doping in which ions are used to replace atoms in LFP. Effects of the modifications are discussed through the paper with most time spent on doping and comparison of the effects induced by doping with different elements. In the end the dopants are ranked according to their effects on performance, price, and sustainability of a battery. To understand the modifications and their effect on electrochemical performance the structure of the material must be studied and understood. Because of this a chapter to crystallography and structure of LFP is included. Similarly, some basic information about common electrochemical characterization techniques is included to help the reader understand the plots and values produced by them. These include cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), electrode setups, and terminology.