공침법을 이용한 리튬이차전지용 층상계 양극 활물질의 전기화학적, 구조적 특성에 관한 연구

Abstract

In Part ?T, Highly crystalline Li[Ni1-x-yCoxMny]O2 (x + y ?? 0.5) (Li[Ni0.6Co0.2Mn0.2]O2, Li[Ni0.55Co0.15Mn0.3]O2, and Li[Ni0.5Co0.25Mn0.25]O2) were synthesized through a coprecipitation method. The capacities of the prepared samples were proportional to the amount of Ni in the host structure. The thermogravimetric analysis (TGA) and in situ high-temperature-X-ray diffraction (HT-XRD) analysis revealed that changes in the amount of manganese ions in the host structure profoundly affect the structural stability of the samples with x + y ?? 0.5. Li[Ni0.55Co0.15Mn0.3]O2, containing the highest manganese content (y = 0.3), showed the most stable structural integrity among the samples as confirmed by in situ HT-XRD. The electrochemical performances of the samples in Ni amount (0.5 ?? 1 - x - y ?? 0.6) with the variation of Co (0.15 ?? x ?? 0.25) did not significantly vary under the test conditions (3.0 - 4.3 V). The small increase of Mn ions plays an important role in preservation of its initial structural symmetry during the high-temperature heating as well as electrochemical cycling. Furthermore, the structural stability has a relationship with the thermal stability and the electrochemical stability, especially at an elevated temperature (55oC). On the basis of the differential scanning calorimetry and TGA results, the Li[Ni0.55Co0.15Mn0.3]O2 sample demonstrated improved thermal stability compared to the other samples.

In Part ?U, A high energy functional cathode material with an average composition of Li[Ni0.72Co0.18Mn0.10]O2, comprising mainly of a core material Li[Ni0.8Co0.2]O2 encapsulated completely within a stable manganese rich concentration-gradient shell was successfully synthesized by a co-precipitation process. The Li[Ni0.72Co0.18Mn0.10]O2 with a concentration-gradient shell had a shell thickness of about 1 ??m and an outer shell composition rich in manganese of Li[Ni0.55Co0.15Mn0.30]O2. The core material can deliver a very high capacity of over 200 mAh g-1, while the manganese rich concentration- gradient shell improve the cycling and thermal stability of the material. These improvements were caused by a gradual and continuous increase of the stable tetravalent Mn in the concentration-gradient shell layer. The electrochemical and thermal properties of this cathode material were found to be far superior to those of the core Li[Ni0.8Co0.2]O2 material alone. Electron microscopy also revealed that the original crystal structure of this material remained intact after cycling.