리튬이온전지 양극활물질 Ni-rich NCM의 합성과 전기화학적 특성
Synthesis and Electrochemical Performance of Ni-rich NCM Cathode Materials for Lithium-Ion Batteries
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층상구조의 Ni-rich NCM계 양극활물질 Li[NixCo(1-x)/2Mn(1-x)/2]O2 (x ≥ 0.6)은 LiCoO2 대비 높은 에너지밀도와 가격 경쟁력의 장점을 가진다. Ni 함량에 비례하여 가역 방전용량이 증가하는 장점이 있는 반면, 합성 중에 발생하는 양이온 혼합으로 인해 안정적인 전기화학성능을 구현하기 어려운 문제가 있다. 본 연구에서는 합성 분위기, 리튬 원료물질, 합성 시간, 합성 온도, Li/M (M=transition metal) 비율 등의 다양한 합성조건을 변수로 하여 Ni 함량 증가에 따라 최적의 층상구조 Ni-rich NCM을 각각 합성하고 이에 대한 전기화학성능을 보고하였다. Li[Ni0.6Co0.2Mn0.2]O2 (NCM6)을 기준으로 Ni 함량이 증가한 Li[Ni0.7Co0.15Mn0.15]O2 (NCM7)와 Li[Ni0.8Co0.1Mn0.1]O2 (NCM8)의 합성시 전이금속 중 Ni의 비율이 증가함에 따라 양이온 혼합이 증가하는 것이 관찰되었고, 이는 전기화학 성능에 부정적인 영향을 끼치는 것으로 확인되었다. Ni 비율별 NCM에 대한 연구결과 비율 내 확인한 최적의 조건에서 NCM6은 180 mAh·g-1, 96.2% (50회), NCM7은 187 mAh·g-1, 94.7%(50회), NCM8은 201 mAh·g-1, 92.7% (50회)의 초기 방전용량 및 수명평가 후 용량유지율 값을 각각 구현하였다.
Abstract
Layered Ni-rich NCM cathode materials Li[NixCo(1-x)/2Mn(1-x)/2]O2 (x ≥ 0.6) have advantages of high energy density and cost competitive over LiCoO2. The discharge capacity of NCM increases proportionally to the Ni contents. However, there is a problem that it is difficult to realize the stable electrochemical performance due to cation mixing. In this study, synthesis conditions for the layered Ni-rich NCMs are investigated to achieve deliver the ones having good electrochemical performances. Synthesis parameters are atmosphere, lithium source, synthesis time, synthesis temperature and Li/M (M=transition metal) ratio. The degree of cation mixing gets worse as the Ni content is increased from Li[Ni0.6Co0.2Mn0.2]O2 (NCM6) to Li[Ni0.8Co0.1Mn0.1]O2(NCM8). It is confirmed that higher level of cation mixing affects negatively on the electrochemical performance of NCMs. Optimum synthesis conditions are explored for NCMx (x=6, 7, 8) in order to reduce the cation mixing. Under optimized conditions for three representative NCMx, a high initial discharge capacity and a good cycle life are obtained for 180 mAh·g-1, 96.2% (50cycle) in NCM6, 187 mAh·g-1, 94.7% (50 cycle) in NCM7, and 201 mAh·g-1, 92.7% (50 cycle) in NCM8, respectively.
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