| Abstract | Six synthetic manganese dioxides (coded SBPA, Faradiser WSZ, IBA no.14, IBA no.19, R2 and Faradiser M) covering the range of crystal structures exhibited by battery active materials were studied by XRD (X-Ray Diffraction) and FTIR (Fourier Transform Infra-Red) spectroscopy when H was inserted with organic reductants. Initial x-ray diffraction patterns (i.e. before H insertion) indicated a structural series, the γ-γt-MnO2 series. where γ-MnO2 designates the phase defined by de Wolff's intergrowth model. This model described γ-MnO2 as a random alternation of two types of layers derived
from the known structures of the minerals ramsdellite and pyrolusite. γ-MnO2 may be characterised by one parameter, p, the pyrolusite type layer fraction. p=0 corresponds to ramsdellite whereas p=1 corresponds to pyrolusite. The additional structural defect responsible for the γ-γt-MnO2 series was found to be that of microtwinning in accordance with the model proposed by Pannetier et al. Material SBPA possessed a low degree of microtwinning with p=0.204±O.OO5 whereas typical battery active manganese dioxides IBA no.19 and R2 possessed x-ray diffraction patterns consistent with a high degree of microtwinning and p-0.2. The other materials, except Faradiser M, displayed
x-ray diffraction patterns intermediate between those of SBPA and IBA no. 19/R2. Faradiser M possessed a high degree of microtwinning but with p-0.7.
The most H inserted compounds (i.e. of composition MnOOH1.0) also formed an analogous structural series. the δ-δt-MnOOH series. δ-MnOOH may be described as a random alternation of two types of layers derived from the known structures of the minerals groutite and manganite which are reduced isostructural derivatives of ramsdellite and pyrolusite. δ-MnOOH was successfully analyzed for the concentration of (presumed) manganite type layer defects (m) in an analogous manner to that required to determine p, which confirmed the existence of reduced intergrowth structures. m was found to be less than p unless the H insertion reaction temperature was lowered (-2°C) in which case m=p. This was consistent with the observed precipitation of small amounts of γ-MnOOH when H was inserted with chemical reductants in non-aqueous solvents.
The level of H insertion may be represented by the formula r in MnOOHr, where -0.1≤r≤1.0 (r starts at a value greater than zero due to the non-stoichiometry of battery active materials). H insertion into EMD (Electrodeposited Manganese Dioxide) R2 led to approximate isotropic lattice expansion in the H insertion region 0.11≤r≤0.80. This observation was consistent with a homogeneous solid state reduction with formation of a solid solution in which H+ and e- were mobile. In the region 0.80≤r≤1.01'new' non-moving peaks emerged characteristic of the final product while the original lines continued to move. The line shift indicated anisotropic lattice expansion. The 'new' non moving peaks could not be interpreted on the basis that a new phase was emerging. An explanation for this behaviour based on the properties of the defect crystal structure of the original material has been found. It involved identification of a type of x-ray line shift characteristic of random layer structures. The effects observed were consistent with random precipitation of δ-MnOOH micro-domains within the solid solution. The 'new lines' which emerged represented a re-emergence of lines originally overlapped with other lines due to the particular effects of microtwinning. The appearance of microdomains of the end product crystallizing within the solid solution implied that H+ and e- were no longer mobile in the crystal structure but located or 'frozen' in position. The presence of 'frozen' H (i.e δ-MnOOH micro-domains) was supported by measurements of FTIR band areas at wavenumber regions where OH vibration modes occurred. The onset of OH vibration modes with increasing H insertion supported the interpretation of the x-ray patterns.
Interpretation of the FTIR spectra of material SBPA indicated no OH bond formation in the H insertion region 0.068≤r≤0.35 and OH bond formation in the region 0.35≤r≤0.882. Examination of the XRD patterns indicated heterogeneous solid state reduction had occurred in the H insertion region 0.40≤r≤0.882. The onset of OH bond formation at r=0.35 was interpreted as a necessary precursor to heterogeneous reduction starting at r=OAO. Heterogeneous reduction was presumed to have occurred by H location in an outer particulate layer which propagated into the bulk.
H insertion into the remaining materials was interpreted in a similar manner. That is beyond a certain r in MnOOHr H location, as indicated by OH bond formation, led to crystallization of δ-MnOOH micro-domains which either randomly precipitated in the solid solution or they associated in such a way that led to heterogeneous reduction. The r in MnOOHr at which H started to locate appeared to depend on the relative rates at which H was inserted and diffused into the solid. H location occurred at r-0.20 for Faradiser M in contrast to R2 in which it located at r=0.80.
A complementary study of the stability of the H inserted compounds in 7M KOH was carried out. Results from potential measurements, x-ray diffraction and SEM were obtained. Over a period of six weeks potential measurements indicated development of a heterogeneous potential at deep H insertion levels. X-ray diffraction and SEM signified formation of δ-MnO2 and γ-MnOOH. The results confirm the proposition of Holton et al. that H inserted compounds are unstable in KOH and disproportionate into δ-MnO2 and γ–MnOOH. This work removes doubt concerning the above proposition since
formation of δ-MnO2 n their work was not simply the result of instability. The H insertion level at which instability was observed appeared related to the formation of δ-MnOOH micro-domains, particularly for R2 in which instability occurred between 0.80≤r≤1.0. This coincides with the oxidation state, MnO1.6, beyond which alkaline manganese batteries cease to deliver useful power on prolonged intermittent discharge. Faradiser M possessed the largest instability region (0.48≤r≤1.0) and would, on this basis, be unsuitable for alkaline manganese batteries. |
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