S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Phase evolution study of lithium-rich composite oxide for Li-ion batteries
리튬 과잉 복합 양극 소재 상형성 기구 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2017. 2. 김현이.
- The composite material LixNi0.25Co0.10Mn0.65O(3.4+x)/2 (x=1.6, 1.4, 1.2, 1.0, 0.8) were synthesized and characterized for their structural, morphological, and performance as cathode materials in Li-ion batteries. The Rietveld refinement results indicate the presence of two phases at high lithium levels (x=1.6 and 1.4): Li2MnO3 (C2/m) and LiMO2 (M = Ni, Co, Mn) (R3¯m)
the latter contains Ni2+ and Ni3+. At low lithium levels (x=1.2, 1.0, and 0.8) an additional spinel phase LiM2O4 (Fd3¯m) emerges, which is known to affect the electrochemical performance of the oxide. Structural analysis reveals that the spinel phase contains mixed transition metals Ni, Co, and Mn as [Li+,Co2+][Ni2+,Co3+,Mn4+]2O4. A low lithium level is found to induce primary particle growth, as well as Co and Ni segregation within the secondary particles. These results are expected to contribute to material optimization and commercialization of lithium-rich oxide cathodes.
The composite material Li2MnO3·Li(Ni,Co,Mn)O2·LiM0.5Mn1.5O4 (M = Mn, Ni, Co) were synthesized and characterized for their structural, morphological, and performance as cathode materials in Li-ion batteries. XRD analysis indicates the presence of Li2MnO2 (C2/m), Li(Ni,Co,Mn)O2 (R3¯m), and spinel phase LiM0.5Mn1.5O4 (M = Mn, Ni, Co) (Fd3¯m). In LiM0.5Mn1.5O4 (M = Mn) (Fd3¯m) composition spinel LiMn2O4 phase is embedded. At 20mol% embedding additionally LiNi0.5Mn1.5O4 phase is also detected. Rocksalt NiO phase is formed in LiM0.5Mn1.5O4 (M = Ni) composition even in oxygen atmosphere. LiM0.5Mn1.5O4 (M = Co) composite composition is homogeneously synthesized even in 20mol% embedding. By embedding LiCo0.5Mn1.5O4 phase the electrochemical performance in 18650 full cell using graphite anode is improved. Spinel embedding in lithium-rich composite oxide can improve the electrochemical performance through structural stability. The phase content, crystal size and lattice parameters were analyzed through the Rietveld refinement in LiM0.5Mn1.5O4 (M = Co) composite composition. Spinel embedding induces primary particle growth during heat-treatment, as well as Co and Ni segregation within the secondary particles. Ab initio calculation shows that spinel embedding in lithium-rich composite oxide can lower the formation energy by stabilizing the structure. The phase evolution process was analyzed during high temperature XRD method.