The inelastic neutron scattering (ZNS) spectra from 16 to 4000 cm(-1) are presented for lamellar-like manganese dioxides at 30 K: delta -MnO2 and chemically modified Bi-delta -MnO2 at various reduction degrees (Bi-delta -MnO1.96, Bi-delta -MnO1.88, Bi-delta -MnO1.78, Bi-delta -MnO1.70, and Bi-delta -MnO1.51). Three different types of protons are distinguished: water-like entities at 540 cm(-1), oxonium-like entities at 815 cm(-1), and free protonic entities giving a continuum with constant intensity. The nonreduced samples contain equimolecular mixtures of water-lace and oxonium-like entities, and there is no visible modification of the spectra attributable to the Bi entities. As the reduction degree increases, the band intensity at 540 cm(-1) decreases and that at 815 cm(-1) increases. Free protons are virtually unaffected. Even for the most reduced sample, there is no evidence for any feature specific to protons inserted in the [MnO6] layers. TRIO processes can be distinguished: the removal of water molecules by the solvent on the one hand and the protonation of water-like entities on the other. (C) 2000 The Electrochemical Society. S0013-4651(00)01-079-X. All rights reserved.

Several kinds of MnO2 were progressively reduced by cinnamic alcohol (CA) and aqueous hydrazine solutions (AHS) to compare changes in their structure. With α-MnO2 stabilized by NH+4, the maximum homogeneous degree of H-insertion (MHID) is only 0.62H per Mn, which involves the filling of each NH+4-free tunnel by four protons. This MHID value is consistent with the discharge capacity during the electrochemical reduction in 1m KOH solution and in nonaqueous media (~0.65 and ~0.63 faradays per Mn, respectively). This result shows that Li+ and H+ ions occupy the same sites. The lowest degrees of oxidation are obtained when AHS are used, resulting in progressive appearance of a spinel structure which replaces the original lattice. For degrees of reduction x lower than MnO1.33, pyrochroite exists in a poorly crystallized form since it is not observed in the XRD patterns. The XRD patterns of γ-MnO2 reduced to MnO1.12 usually show the spinel structure while the patterns of the Bi-doped MnO2 reduced to MnO1.14 exhibit peaks corresponding to pyrochroite and bismite (Bi2O3). Thus, the presence of Bi3+ hinders the formation of the nonelectroactive compound Mn3O4 or γ-Mn2O3, but the mechanism to explain this cannot be determined by XRD data alone.

The common manganese dioxides IC2 and Tekkosha are rechargeable only for 18 and 10 cycles, respectively, when the depth of discharge is similar to 0.5 e/Mn. This low cycle life is due to the production of Mn3O4 during cycling. When the oxygen evolution is combined with electrochemical oxidation, these samples become rechargeable for 60-70 cycles. The increase in the cycle life is due to the oxidation of Mn3O4 to the initial MnO2 by oxidizing species generated near the Mn3O4 sites during oxygen evolution by overcharging. The overrecharging has a positive effect when the duration of the recharge stage does not exceed similar to 50% of that of the discharge stage. Beyond this value the cycle life decreases.

The spectra of five samples recorded at 20 K with the NERA spectrometer from ∼ 0 to 1600 cm −1 (at JINR, Dubna, Russian Federation) are reported: chemical γ -MnO 2 (CMD SEDEMA WSA), electrochemical γ -MnO 2 (EMD TEKKOSHA), lamellar Bi + δ -MnO 2 , δ -MnO 2 2H 2 O and NH 4 + α -MnO 2 . The relative concentrations of the protonic species are estimated.

Recent work on γ-MnO2 using Inelastic Neutron Scattering shows the presence of several types of proton in the lattice. These types of proton include; first, a hydride-like H+ associated with specific lattice sites; second, a molecule-like (H+)4 which charge compensates Mn4+ vacancies; third, a free H+ component. The idea of free is developed into the measurement of momentum distributions. A Neutron Compton Scattering spectrometer, suitable for the measurement of atomic momentum distributions, is outlined very briefly. The first observation of the atomic momentum of a tunnelling proton is presented.

The inelastic neutron scattering spectra of two untreated gamma-MnO2 samples (chemical manganese dioxide or CMD, and electrodeposited manganese dioxide or EMD) at 20 K reveal that the observed intensities are proportional to the amount of Mn4+ vacancies in the lattice. Localized protons give broad bands between 500 and 1100 cm-1. In addition, the broad scattering continua extending over the whole energy transfer range are interpreted in terms of proton recoil (Compton effect). These almost free protons experience a very shallow potential barrier (approximately 30 cm-1). The spectra of the chemically reduced materials (EMD and CMD) and manganite at 20 K are compared. They show three main peaks at approximately 1110, 2200, and 3100 cm-1 which are assigned to the 0 --> 1, 0 --> 2, and 0 --> 3 transitions of almost degenerate isotropic oscillators. The intensity ratio I02/I01 is consistent with a reduced mass of approximately 1 amu for the oscillators. Therefore, the existence of covalently bound MnOOH entities in these samples is ruled out. Most of the inserted protons are located at the center of the oxygen octahedra in the channels. Force field calculations including inelastic neutron scattering (INS) band intensities and phonon-wings are in good agreement with the observations. For the INS spectrum of manganite, the main band at 1124 cm-1 can be decomposed into two components at 1131 and 1095 cm-1 with an intensity ratio I1131/I1095 approximately 2. This splitting is assigned to a small anisotropy of the proton site. The chemically reduced samples (CMD and EMD) show much broader bands which can be decomposed into several Gaussian components. The main components are tentatively assigned to protons inserted in sites nearest neighbor to the vacancies (approximately 900 cm-1), next nearest neighbors (approximately 1000 cm-1), and sites further away from the vacancies (approximately 1110 cm-1).

The inelastic-neutron scattering spectra of partially reduced electrodeposited MnO2 samples (MnO2Hy with y = 0. 04, 0.34, 0.42, 0.56, 0.72, and 0.96) at 20 K are reported. Band decomposition in the 600 to 1200 cm-1 region reveals four components at almost fixed frequencies for all the samples. On the basis of their relative intensities for the different reduction rates, these components are assigned to: single protons in sites surrounded by empty sites at approximately 750 cm-1, protons in sites nearest neighbor to the Mn4+ vacancies at approximately 890 cm-1, protons in sites next nearest neighbor to the Mn4+ vacancies at approximately 1010 cm-1, and, finally, at 1120 cm-1 protons further away from the vacancies in the ramsdellite/pyrolusite-like channels of the intergrowth structure. All these sites are populated simultaneously, but at different rates, as the proton insertion degree increases. The variation of the integrated intensity of the four components with the reduction degree is discussed in terms of dynamical coupling with the charge compensating protons in the vacancies. The spectra in the 30 to 500 cm-1 region reveal different dynamical regimes for the lattice modes.

The inelastic neutron-scattering spectra of γ-MnO2 are reported at 20, 100, 200 and 300 K. The difference spectra (100-20 K and 200-20 K) show bands at 40, 90, 185 and 345 cm−1 which are not observed at 20 or 300 K. These bands correspond to the J = 0, 1 2 and 3 levels of a freely rotating spherical top with a rotational constant B = 25 cm−1. This is assignment to (H+)4 entities in the lattice vacancies. The frequency shift of the J=0 level is interpreted in terms of inversion, i.e., out-of-phase rotation and antitranslation of two H+…H+ pairs inside the (H+)4 entities. At 20 K the free rotation disppears. An equilibrium between two configurations for the four photons associated with the vacancy is proposed. At 300 K the spectrum indicates a gas of free particles.

The inelastic neutron scattering spectra of chemically reduced γ−MnO2 (MnO2H0.96) show three main peaks at 1120, 2200 and 3100 cm-1 which are assigned to the 0→1, 0→2 and 0→3 transitions of a degenerate isotropic oscillator. The protons are located at the centre of the oxygen octahedra.