Co 치환과 BiOF 코팅을 이용한 5V spinel Li[Ni0.5Mn1.5]O4 양극 활물질의 전기화학적 특성 향상에 관한 연구

Abstract

In Part Ⅰ, Effect of Co substitution for Mn on Li[Ni0.5CoxMn1.5-x]O4 was investigated. Coprecipitation was employed to synthesize the [Ni0.25CoxMn0.75-x](OH)2 as a precursor and it was fired with LiOH at 900 oC for 24 h in air. From Rietveld refinement of X-ray diffraction data and scanning electron microscopic examinations, it was found that the as-synthesized Li[Ni0.5CoxMn1.5-x]O4 were crystallized in cubic spinel structure with Fd3m space group and the final products presented spherical secondary morphology (3 μm in average). Electrochemical investigation revealed that Co3+ replacement for Mn4+ gave rise to improved rate and cycling performances probably due to the improved electronic conductivity and structural stability achieved by the presence of Co3+ in the spinel Li[Ni0.5CoxMn1.5-x]O4.

In Part Ⅱ, The electrochemical properties of BiOF-coated 5V spinel Li[Ni0.5Mn1.5]O4 were investigated at elevated temperatures (55 oC). As observed by scanning and transmission electron microscopy, BiOF nanolayers with ~ 10nm thickness were coated on the surface of Li[Ni0.5Mn1.5]O4. The BiOF coating layer protected the surface of the active materials from HF generated by the decomposition of LiPF6 in the electrolyte during electrochemical cycling. The dissolution of transition metal elements was also suppressed upon cycling. Therefore, the capacity retention of the BiOF-coated Li[Ni0.5Mn1.5]O4 was obviously improved compared to the pristine Li[Ni0.5Mn1.5]O4 at 55 oC.