Investigation of new cathode materials for rechargeable magnesium-ion batteries

Abstract

이차전지는 화석연료 의존도를 낮춰 에너지 자립도를 높이고 이산화탄소 배출량을 획기적으로 저감할 수 있는 핵심 녹색기술이다. 그럼에도 불구하고, 현 리튬이온 전지의 이론적 한계는 셀 기준 300 Wh/kg 정도이며 이론적 한계까지 개발된다 해도 자동차용으로 요구하는 500Wh/kg에 훨씬 못 미친다. 이에 2025년 이후를 대비할 수 있는 높은 에너지 밀도 및 높은 가격 경쟁력을 가지는 혁신 전지의 개발이 요구되고 있다. 마그네슘 이온 (Mg2+) 전지는 이론적 관점에선 리튬 이차전지에 비해 2배의 용량을 가지는 혁신 미래 전지로 여겨지나 낮은 마그네슘 확산성 때문에 전극재료의 개발이 어려워 전 세계적으로 연구결과가 거의 없는 미개척 분야이다. 이 논문에서는 새로운 마그네슘 전지용 양극재 Prussian blue, NH4V4O10, H2V3O8 및 Na0.04MoO3∙(H2O)0.49 에 대하여 전기화학적 분석 및 구조적 특성을 연구하였다. 재료의 형태 분석 및 원소분석은 SEM, TEM, TG, EDX, ICP, FTIR 및 XPS 와 같은 기법을 사용하여 분석을 하였으며, 전기화학적 분석은Cyclic voltammetry, Galvanostatic charge/discharge를 이용하여 물질의 용량, 전압, 반응 메커니즘 (diffusion controlled reaction, surface limited reaction), 및 확산성에 대한 연구를 하였다. 구조 결정학을 기초로 하여 반응 메커니즘을 하였으며, powder X-ray diffraction기법을 사용하여 3차원 전자 밀도 맵으로 변환 후 분석하였다. 또한 Bond valence sum maps 계산을 통하여 마그네슘의 확산 경로를 분석 및 제시하였다. |Magnesium-ion batteries (MIBs) have received attention, as a multivalent-ion battery candidate for post LIBs. Compared to LIBs, such systeMaster are expected to provide a lower cost because of the substantially higher abundance of magnesium resources on Earth, a potentially higher capacity because of a two-electron reaction per ion, and the increasing energy density comes from the magnesium metal anode (Mg: 3833 mAh cm−3, graphite: 777 mAh cm−3) with a higher safety because of a lack of dendrite formation. However, the lack of cathode materials with considerable capacities in conventional nonaqueous electrolyte is one of the great challenges for their practical applications. This dissertation discusses the development of new host materials for magnesium-ion batteries, their electrochemical performances, and understating their intercalation mechanism through ab initio structure determination via powder X-ray diffraction profile. Besides, to unveil the working mechanism, 3D bond-valence sum difference map calculation, elemental analyses (by ICP, EDX, XPS, FTIR), and morphology characterizations (by SEM and TEM) and electrochemical analyses (galvanostatic test, cyclic voltammetry) were performed. (1) In the first section, the magnesium intercalation capability of Prussian blue (PB) analogue, potassi-um nickel hexacyanoferrate K0.86Ni[Fe(CN)6]0.954(H2O)0.766 (KNF-086), is demonstrated as a cathode material for rechargeable magnesium-ion batteries using a conventional organic electrolyte. K1.51Ni[Fe(CN)6]0.954(H2O)0.766 is synthesized first, and potassium ions are electrochemically extracted to pre-pare the KNF-086 cathode. The electrochemical test cell is composed of KNF-086 as the working electrode, activated carbon as the counter and reference electrode, and 0.5 M Mg(ClO4)2 in acetonitrile as the electro-lyte. The cell shows a magnesium insertion reaction with a discharge capacity of 48.3 mAh g−1 at a 0.2 C rate, and a discharge voltage at 2.99 V (vs. Mg2+/Mg) according to the CV reduction peak current that is the high-est among the cathode materials ever reported for magnesium-ion batteries. Elemental analysis and Fourier electron-density map analysis from powder X-ray diffraction data confirm that the magnesium-inserted phase is Mg0.27K0.86Ni[Fe(CN)6]0.954(H2O)0.766 (MKNF-086), and the magnesium ions in MKNF-086 are positioned at the center of the large interstitial cavities of cubic PB. Compared to KNF-086, MKNF-086 exhibits a de-creased unit cell parameter (0.8%) and volume (2.4%). These results demonstrate that a PB analogue, potas-sium nickel hexacyanoferrate, could be utilized as a potential cathode material for conventional organic electrolyte-based magnesium-ion batteries. (2) In the second topic, we present high magnesium-ion storage performance and evidence for the elec-trochemical magnesiation of ammonium vanadium bronze NH4V4O10, as a cathode material for MIBs. NH4V4O10 was synthesized via a conventional hydrothermal reaction. It shows reversible magnesiation with an initial discharge capacity of 174.8 mAh g−1, and the average discharge voltage of ~2.31 V (vs. Mg/Mg2+) using 0.5 M Mg(ClO4)2 in acetonitrile as the electrolyte. Cyclic voltammetry, galvanostatic, discharge–charge, FTIR, XPS, Powder XRD, and elemental analyses unequivocally show evidences for the reversible magnesiation of the material and suggests that keeping the ammonium ions in the interlayer space of NH4V4O10 could be crucial for the structural stability with a sacrifice of initial capacity but much enhanced retention capacity. This is the first demonstration of electrochemical magnesiation with a high capacity above 2 V (vs. Mg/Mg2+) using a conventional organic electrolyte with a relatively low water concentration. (3) In the third section, we report H2V3O8, or V3O7∙H2O, as a high-energy cathode material for MIBs. It exhibits reversible magnesiation-demagnesiation behavior with an initial discharge capacity of 231 mAh g−1 at 60 °C, and an average discharge voltage of ~1.9 V vs. Mg/Mg2+ in an electrolyte of 0.5 M Mg(ClO4)2 in acetonitrile, resulting in a high energy density of 440 Wh kg−1. The structural water remains stable during cycling. The crystal structure for Mg0.97H2V3O8 is determined for the first time. Bond valence sum difference mapping shows facile conduction pathways for Mg ions in the structure. The high performance of this mate-rial with its distinct crystal structure employing water–metal bonding and hydrogen bonding provides in-sights to search for new oxide-based stable and high-energy materials for MIBs. (4) In the last section, Magnesium batteries have received attention as a multiply charged reaction for future energy storage. To date, most known oxide host materials for magnesium batteries are not working at room temperature as well as in nonaqueous electrolytes. Herein we first reported a unique host material, Na0.04MoO3∙(H2O)0.49, prepared via chemical reduction method from alpha-MoO3 for magnesium intercalation reaction in a 0.5 M Mg(ClO4)2/AN electrolyte at 25 oC. The electrochemical properties show a discharge ca-pacity of 157.4 mAh g−1 at 0.2 C rate and an average discharge voltage at 2.16 V (vs. Mg/Mg2+) for the re-versible magnesium insertion/extraction. The Na0.04MoO3∙(H2O)0.49 cathode exhibits capacity retention of 93.4% at the 100th cycle versus the first cycle at 2 C rate. Our results clearly demonstrate the material as a potential cathode material for magnesium batteries in a nonaqueous and ambient temperature for the first time.