In-situ XAFS studies of Mn12 molecular-cluster batteries: super-reduced Mn12 clusters in solid-state electrochemistry

In situ X-ray absorption fine structure (XAFS) analyses were performed on rechargeable molecular cluster batteries (MCBs), which were formed by a lithium anode and cathode-active material, [Mn(12)O(12)(CH(3)CH(2)C(CH(3))(2)COO)(16)(H(2)O)(4)] with tert-pentyl carboxylate ligand (abbreviated as Mn12tPe), and with eight Mn(3+) and four Mn(4+) centers. This mixed valence cluster compound is used in an effort to develop a reusable in situ battery cell that is suitable for such long-term performance tests. The Mn12tPe MCBs exhibit a large capacity of approximately 210 Ah kg(-1) in the voltage range V=4.0-2.0 V. The X-ray absorption near-edge structure (XANES) spectra exhibit a systematic change during the charging/discharging with an isosbestic point at 6555 eV, which strongly suggests that only either the Mn(3+) or Mn(4+) ions in the Mn12 skeleton are involved in this battery reaction. The averaged manganese valence, determined from the absorption-edge energy, decreased monotonically from 3.3 to 2.5 in the first half of the discharging (4.0>V>2.8 V), but changed little in the second half (2.8>V>2.0 V). The former valence change indicates a reduction of the initial [Mn12](0) state by approximately ten electrons, which corresponds well with the half value of the observed capacity. Therefore, the large capacity of the Mn12 MCBs can be understood as being due to a combination of the redox change of the manganese ions and presumably a capacitance effect. The extended X-ray absorption fine structure (EXAFS) indicates a gradual increase of the Mn(2+) sites in the first half of the discharging, which is consistent with the XANES spectra. It can be concluded that the Mn12tPe MCBs would include a solid-state electrochemical reaction, mainly between the neutral state [Mn12](0) and the super-reduced state [Mn12](8-) that is obtained by a local reduction of the eight Mn(3+) ions in Mn12 toward Mn(2+) ions.