Lithium ion batteries (LIBs) exhibit outstanding performance and have been applied in smart phones, electric vehicles, etc. Since successful commercialization of LIBs, price of lithium resources have been increasing as well and post-LIB systems have drawn significant attention of researchers. Among many potential alternatives, sodium ion batteries (SIBs) have gained much attention due to natural abundance and uniform, world-wide distribution of sodium resource, which are especially attractive feature for large-scale energy storage application. Transition metal oxides ($NaMO_2$, M=single or mixture of transition metal elements) are promising candidate for electrode materials for SIBs due to high theoretical capacity, simple and scalable synthesis method, and appropriate operating voltage. In order to achieve develop high performance TM oxides, it is critical to understand the crystal structure, phase transformation behavior, and ion intercalation properties within the structure. One of the common approaches for improving electrochemical properties of transition metal oxides is varying the composition of transition metal elements. Another approach is substitution of electrochemically inactive, non-transition metal. In this work, non-transition metals, $Ca^{2+}$ and $Li^+$, were substituted on Fe/Mn-based transition metal oxides and their effects on crystal structure, phase transformation behavior, and Na intercalation were studied by electrochemical analysis, Rietveld refinement of synchrotron and neutron diffraction patterns, etc. $Ca^{2+}$ was substituted on alkali site and its effect on the structure and electrochemical performance strongly depended on layered structure. The substitution site of $Li^+$ was controlled by controlling synthesis condition. Rietveld refinement of diffraction patterns and theoretical calculation revealed that substitution of small $Li^+$ ion at alkali site assists intercalation of $Na^+$ and stabilized the structure. When $Li^+$ was substituted on P2/O3 composite, electrochemical performance indicated additional capacity other than TM redox was attributed to Li substitution at P2/O3 biphasic structure. Synergetic effect of Li substitution in layered structure and P2/O3 biphasic structure resulted high capacity beyond theoretical limit, stable cycle life, and superior rate capability.
우수한 성능을 가지는 리튬이온전지(LIBs)는 다양한 전자기기에 사용되어 왔다. 리튬 원료의 가격이 빠르게 상승하며, 차세대 이차전지 연구도 활발해지고 있다. 차세대 이차전지 중 나트륨이온전지(SIBs)는 원료가 풍부하고 널리 분포하기 때문에 대규모 에너지 저장 장치 분야의 리튬이온전지 대체재로 주목 받고 있다. 층상구조 산화물은 전기화학적 특성이 우수하고 쉽게 합성이 가능한 양극 소재로 다양한 연구가 진행되어 왔다. 고성능의 층상구조 산화물 개발을 위해서는 결정 구조, 충/방전 과정에서 층상구조의 변화, 이온의 삽입/탈리 거동에 대한 이해가 필수적이다. 기존 연구에서는 전이금속 층에 다양한 전이금속을 치환하는 방법이 주로 보고된 바 있다. 본 연구에서는 전이금속이 아닌 비전이금속을 층상구조에 치환하여 비전이금속 치환이 결정 구조, 층상구조의 변화, 이온 삽입/탈리 거동에 미치는 영향을 연구하였다. Fe/Mn 기반의 층상구조 산화물의 알칼리 층 혹은 전이금속층에 칼슘 혹은 리튬을 치환하고 전기화학적 분석법, 회절 패턴 분석, 실시간 분석 등을 진행하여 치환 원소 및 치환 위치에 따른 영향을 연구하였다.