고성능 리튬이차전지용 올리빈계 양극소재에 관한 연구

Abstract

Lithium iron phosphate (LFP) has attracted much attention as a cathode electrode material for next-generation lithium rechargeable battery system because of its superior chemical/thermal stability, long term cycle life from rigid crystal structure, suitable energy density for using large scale energy storage system and use of low-cost element, iron. In the ideal case, the lithium iron phosphate could can release and insert lithium ions on their crystal structure with a theoretical gravimetric energy density specific capacity of 580 Wh kg-1(theoretical capacity of ~ 169 mAh g-1, Fe2+/3+ redox voltage of 3.42 V (vs. Li/Li+)) through the one dimensional lithium diffusion channels being along the [010] direction of crystal structure (Pnma). However, the presence of immobile defects in the diffusion paths, which may originate from impurities or Li-Fe cation site exchange defect (anti-site defect), can significantly retard the mobility of ions of lithium iron phosphate. In particular, crystals with only one-dimensional diffusion pathways, such as olivine-type materials lithium iron phosphate, are detrimental with the presence of defects. Depending on synthesis process, approximately 0.5–7 % of the Li-Feanti-site defect is present in crystal structure, which results in immobile Fe ions in the [010] lithium ion diffusion channel. According to report, the presence of 0.1 % anti-site defects in a micron-sized particle reduces its energy density to almost half of theoretical capacity and decreases the lithium ionic conductivity by two or three orders of magnitude.

Chapter 2 introduce a new method to remove anti-site defects in olivine crystals using electrochemical charge carrier injection process at a room temperature. The Fe anti-site defects in LiFePO4 are effectively reduced by the electrochemical recombination of Li/Fe anti-sites. The healed crystal structure of lithium iron phosphate recovers its specific capacity and high-power capabilities. In this chapter, various configuration of anti-site defects and its recombination mechanisms are discussed.

Chapter 3 and 4 deals with a new type lithium-excess composition lithium iron phosphate having zero Fe anti-site in their crystal structure. It is confirmed that the Fe anti-site defects are completely removed in lithium-excess composition lithium iron phosphate due to its unique Fe oxidation state, showing superior rate capability and long term cycle ability. In this chapter, not only the structural characterization of lithium-excess lithium iron phosphate with zero Fe anti-site but also electrochemical behavior arising from thermodynamic and kinetic properties, especially Spinodal decomposition behavior and memory effect, will be discussed.