Tailoring olivine cathode electrode materials for high performance lithium secondary battery

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 강기석.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.Chapter 1. General Introduction 1 1.1 Preface 1 1.1.1 General background 1 1.1.2 Spinodal decomposition behavior on electrode 4 1.1.3 References 8 Chapter 2. Anti-site reordering in LiFePO4 using charge carrier injection 11 2.1 Introduction 11 2.2 Experimental 15 2.3 Results and Discussion 18 2.3.1 Anti-site configuration & possible recombination process 18 2.3.2 Charge carrier injection method to reduce anti-site concentration 25 2.4 Conclusion 43 2.5 References 44 Chapter 3. Lithium-excess olivine for zero FeLi defect 51 3.1 Introduction 51 3.2 Experimental 55 3.3 Results and Discussion 57 3.3.1 Structural characterization of lithium-excess olivine 57 3.3.2 Local atomic configuration and origin of zero Fe anti-site of lithium-excess olivine 75 3.3.3 New [101] diffusion path of lithium-excess olivine 91 3.3.4 Electrochemical properties 101 3.4 Conclusion 103 3.5 References 104 Chapter 4. Thermodynamic and kinetic issues 121 4.1 Hysteresis gap and Memory effect of LiFePO4 121 4.1.1 Experimental section 121 4.1.2 Lowed Spinodal decomposition barrier 122 4.1.3 Less memory effect at new LFP system 125 4.2 Conclusion 145 4.3 References 146 Chapter 5. Summary 157 Chapter 6. Abstract in Korean 159Docto