Polyanionic sodium iron phosphate cathodes for sodium ion batteries

Abstract

In recent years the development of more sustainable batteries alternatives to the commonly used lithium-ion batteries (LIBs) became more important. The reason for this is the limitation of lithium (Li) in the earth’s crust and the rising prices for raw materials. Sodium-ion batteries (SIBs) got more in the center of attention because of the abundance and the lower cost of sodium (Na) and the relative similarity to LIBs. This thesis is a review about the recent development of polyanionic phosphate cathode materials containing sodium and iron for future sodium-ion batteries. It also provides an introduction in the rocking chair mechanism of sodium-ion batteries, that work similar to lithium-ion batteries, explanations of commonly used terminology regarding battery compounds, an overview of electrochemical methods for the evaluation of the performance of a material and ways to enhance the performance. The thesis covers cathode materials such as, maricite NaFePO4, olivine NaFePO4, NASICON Na3Fe2(PO4)3, NaFeP2O7, Na2FeP2O7, Na4Fe3(PO4)2P2O7 and Na2FePO4F are discussed. In addition to the description of their frameworks, many conventional synthesis methods like the sol-gel method, the solid-state method, hydrothermal method, as well as electrochemical synthesis methods for the thermodynamically unstable olivine structure of NaFePO4, will be explained on an example from the literature. Although, these cathode materials are not yet developed so far that they can replace LIBs, they might play a role in the near future for energy storage and other fields that don’t require a high energy density.