３．分子性２次電池における固体電気化学と物性科学
Solid-state electrochemistry and materials science in rechargeable moleculebased batteries
金属クラスター錯体や金属有機構造体（MOF）、共有結合構造体(COF)などの分子性物質を構成成分とする２次電池の研究を進めている。多電子反応による高容量化とともに、クラスター構造やポーラス構造を利してイオン伝導を確保することによって、エネルギー密度とパワー密度の両両方を実現することができるだろう。我々は、このような固体電気化学反応を分子論的に理解したうえでの物質開拓を論理的に進めるため、X 線吸収スペクトルや NMR などの電気化学反応下のオペランド計測も開発している。さらに、固体電気化学反応を利用して結晶構造を保ったままバレンス制御することによって、固体物性を自由に制御する研究も進めている。

We have been developing molecule-based rechargeable batteries, using transition-metalcluster complexes, metal-organic frameworks (MOF) and covalent-organic frameworks(COFs). It is expected that their multi-step redox reactions and high ionic conductivities dueto their cluster or porous structures would realize both high-energy and high-power densities.
To develop material research in a logical way, we have developed various in operando
measurement systems under electrochemistry for XAFS, NMR, etc. We are also trying tocontrol the valence of these molecule-based materials by solid-state electrochemical
reactions, to modulate the physical properties freely.

Fig. 1. We fabricated a rechargeable molecular cluster battery, based on a cathode active
material, [Mn12O12(CH3COO)16(H2O)4]. The charging/discharging experiments revealed
rechargeable battery performance with a capacity of ca. 90 Ah/kg, while the first discharging
process exhibited an extremely high value of 200-250 Ah/kg.

Fig. 2. We carried out in situ Mn K-edge X-ray absorption fine structure (XAFS) studies ofMn12 molecular cluster batteries to clarify the battery reaction mechanism. The XAFS results
confirm that super-reduced species [Mn12]8- were formed by solid-state electrochemistryduring charging/discharging.

Fig. 3. We carried out in operando XAFS studies on the cathode materials of the POM-MCBs.
We found that this battery includes a solid-state electrochemical reaction between the original[PMo12O40]3- and the super-reduced state [PMo12O40]27-, which results in a twenty-fourelectron reduction/oxidation. The molecular structure of [PMo12O40]27-, estimated fromEXAFS analyses, showed that Mo4+ metal-metal bonded triangles are formed due to thelarge number of additional electrons in the super-reduced state.

Fig. 4. POM/SWNT hybrid materials, in which the POM molecules are individually adsorbed
onto the surfaces of SWNTs, can be used as a cathode-active material in rechargeable
lithium batteries. This POM/SWNT battery exhibits a very high capacity (> 300 Ah/kg) with ashort charging/discharging time (< 2 hours), suggesting smooth electron transfer between
the POMs and the electrode through SWNTs, and smooth access of lithium ions to the POMs.

Fig. 5. Metal-organic frameworks (MOFs) are well known for their application to various typesof energy storage; nevertheless, their potential in electron storage has scarcely been
investigated. We discovered a new electrochemical mechanism, namely “bipolar charging”
mechanism by analyzing the solid-state electrochemical process of a flexible redox active
MOF. In a single redox cycle, not only the Li-ions but also the bulky anions are separatelyintercalated into the pores of the MOF and contribute to the total capacity. With this “bipolarcharging” mechanism, a general synthetic strategy could be proposed. Furthermore, MOF
materials employing this mechanism may exhibit remarkable reactivity and high cyclicstability, and be adopted as versatile electrode materials in various battery architectures.

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