S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Tailoring phosphate cathode materials for high performance lithium rechargeable battery
- 공과대학 재료공학부(하이브리드 재료)
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부(하이브리드 재료), 2014. 2. 강기석.
- Electrochemical energy storage systems have attracted tremendous interests due to skyrocketing growth of sustainable and environmental-friendly energy market in the world. Lithium rechargeable batteries have been widely used as the energy storage system for portable electronic due to its high energy/power density and long cycle life. Recently, intensive research efforts
have focused on the development of Li rechargeable battery for large scale applications such as electric vehicles. Conventional cathode materials such as lithium transition metal oxides (LiMO2, M = transition metals) possess intrinsic chemical instability at overcharged state. They release oxygen from the crystal structure or experience irreversible phase transformation at elevated temperature, which consequently raises safety concerns during operation. In this respect, numerous studies have been carried out in order to find a safe and stable cathode material. Among many candidates, phosphate materials have been considered as the best candidate of energy storage system for large-scale applications due to its high structural stability and safety by strong P-O covalent bonding, potentially low production cost, high energy density, and excellent cyclability. In this thesis, various phosphate electrode materials such as olivine, NASICON, and alluaudite are investigated.
Olivine studies consist of three parts: (i) the improvement of electrochemical performance of LiMnPO4, (ii) the particle size effect on phase stability on the LiMnPO4, and (iii) thermal stability of binary olivine LiFe1-xMnxPO4. In the (i) part, we demonstrate that the electrochemical properties of LiMnPO4 can be significantly improved by doping small amount of Fe and Mg. The presence of Fe and Mg in LiMnPO4 provide multiple nucleation sites, unlike pure LiMnPO4, thus the power capability of a LiMnPO4 was significantly enhanced by easier Li+ de/intercalation resulted from multiple nucleation sites. Fe-Mg co-doped LiFe0.05Mn0.05Mn0.9PO4 indicates a discharge capacity of ~140mAh g-1 at C/5 rate (1C = 170mA g-1) and the high capacity is retained at even higher current rates (110mAh g-1 at 3C), which has yet to be achieved in LiMnPO4 without nucleation enhancers. In the (ii) part, the particle size effect on the phase stability of delithiated LiMnPO4 is elucidated using temperature-controlled in situ X-ray diffraction. The phase stability of LixMnPO4 (x < 1) at high temperature was significantly influenced by the particle size. While LixMnPO4 phase having large particle size (larger than 200nm) decomposed into Mn2P2O7 at the temperature above 200◦C, delithiated LixMnPO4 was transformed into Mn3(PO4)2 above 200◦C when the delithiated LixMnPO4 has a small particle size (ca. 50 nm). This indicates that the phase transformation of a LiMnPO4-based battery is influenced by the particle size. In the (iii) part, the phase stability of LiFe1-xMnxPO4 is extensively studied. It is identified that the thermal stability of partially delithiated Li1-yFe1-xMnxPO4 is sensitively affected by the Fe/Mn ratio of Li1-yFe1-xMnxPO4. While Fe-rich material in Li1-yFe1-xMnxPO4 readily formed a solid solution phase of Li1-yFe1-xMnxPO4 near room temperature or with only slight heating, the Mn-rich material in Li1-yFe1-xMnxPO4 retained its two-phase characteristic up to ~250oC before decomposition into non-olivine phases. The decomposition mechanism of fully delithiated Li1-yFe1-xMnxPO4 is more sensitively affected by the Fe/Mn ratio in the crystal. It is identified that the decomposition temperature of Li1-yFe1-xMnxPO4 is higher when Fe/Mn ratios in the structure increase.
The NASICON study is focused on the improvement of electrochemical performances of monoclinic Li3V2(PO4)3. The power capability of the Li3V2(PO4)3 electrode can be greatly improved by a simple low-temperature coating of a conducting polymer, namely, poly(3,4-ethylenedioxythiophene) (PEDOT). The carbon-free Li3V2(PO4)3/PEDOT electrode delivered ca. 120 mAh g-1, more than 90% of its theoretical capacity of 133mAh g-1, at a rate of 10C (where 1C = 133 mA g-1). The capacity retention at this rate was 97% after 100 cycles. At 30C, Li3V2(PO4)3/PEDOT delivered ca. 110mAh g-1. This remarkable power and cycle stability achieved by a simple coating process places this 4V-class electrode among the most promising electrode candidates for next-generation batteries.
In the alluaudite study, we focus the non-olivine LiFePO4 with alluaudite structure. Non-olivine LiFePO4 with an alluaudite structure were successfully prepared for the first time via soft chemical ion-exchange of the novel alluaudite Na0.67FePO4. The open crystal structure of alluaudite LiFePO4 allows fast lithium motion as evidenced from the reversible electrochemical cycling in a lithium rechargeable battery system. About 0.8 Li in LiFePO4 could be extracted and reinserted in a highly reversible manner. The charge/discharge profile reveal that the one-phase (solid-solution) reaction based lithiation and delithiation occurs within the alluaudite crystal structure contrary to the well-known two-phase reaction in the olivine LiFePO4.
In conclusion, phosphate electrode materials such as olivine, NASICON, and alluaudite have the excellent characteristics enough to be used as the cathode material for large-scale applications. We believe that the results reported herein provide a new basis and strategy for phosphate cathode materials, which can be used as better-performing lithium rechargeable batteries.