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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.2 Concepción jun. 2001

http://dx.doi.org/10.4067/S0366-16442001000200010 

HOLLANDITE CATHODES FOR LITHIUM ION BATTERIES. 1.
SYNTHESIS AND CHARACTERIZATION OF THE MM'Mn7O16
COMPOUNDS (M= K, Ba; M' = Mg, Mn, Fe, Ni).

SALVADOR BARBATO1, AMBROSIO RESTOVIC1, JUAN LUIS GAUTIER2 *

1 Departamento de Química, Facultad de Ciencias Básicas, Universidad de Antofagasta,
Av. Angamos 601, Antofagasta, Chile.
2 Departamento de Química de los Materiales, Facultad de Química y Biología,
Universidad de Santiago de Chile, Av. L. B. O'Higgins 3363, Santiago, Chile.
(Received: November 22, 2000 - Accepted: March 02, 2001)
* Corresponding author. E-mail: jgautier@lauca.usach.cl, Fax: 56-2- 6812108

ABSTRACT

BaMMn7O16 (M = Mg, Mn, Fe, Ni) and AMn8O16 (A = Ba, K2) hollandite oxides were synthesized in air from metal carbonates and a mixture of simple oxides using the thermal decomposition in solid phase method at 850 C. The oxides BaMn8O16 and BaFeMn7O16 crystallized in the hollandite monoclinic phase, S.G. I2/m and S.G. P21/n respectively, whereas K2Mn8O16 was tetragonal, S.G. I4/m. Two new phases are reported: BaMgMn7O16 and BaNiMn7O16 which crystallize in the hollandite monoclinic phase structure. The ionic distributions were proposed on the basis of several physicochemical analyses comporting XRD, GTA, magnetic susceptibility and oxidation power measurements. Barium and potassium manganites showed Mn3+ and Mn4+ in octahedral sites whereas BaMMn7O16 (M = Mg, Ni) exhibited Mn2+ and Mn4+ in sites B. On the other hand, iron-manganese hollandite, BaFeMn7O16, shows Fe3+ ions besides Mn3+ and Mn4+ cations.

Keywords: Hollandite, Cationic distribution, Mixed oxides, Manganates, Ionic conductor

RESUMEN

Se sintetizaron a 850 oC en aire , óxidos de estructura holandita de fórmula BaMMn7O16 (M = Mg, Mn, Fe, Ni) y AMn8O16 (A = Ba,K2), usando como precursores carbonatos metálicos y una mezcla de óxidos simples, mediante la técnica de descomposición térmica en fase sólida. Los óxidos BaMn8O16 and BaFeMn7O16 cristalizaron en la fase holandita monoclínica, grupo espacial I2/m y P21/n respectivamente, mientras que el óxido K2Mn8O16 presenta una estructura holandita tetragonal, grupo espacial I4/m. Se informa de dos nuevos óxidos, BaMgMn7O16 y BaNiMn7O16, que cristalizaron en la estructura holandita monoclínica. Se proponen las respectivas distribuciones iónicas sobre la base de análisis fisicoquímicos como: DRX, ATG, susceptibilidad magnética y poder de oxidación. Manganitas de bario y de potasio presentan en los sitios octaédricos de la estructura los iones Mn3+ y Mn4+ mientras que los óxidos de fórmula general BaMMn7O16 (M = Mg, Ni) muestran iones Mn2+ and Mn4+ en tales sitios. Por otra parte, el óxido BaFeMn7O16 presenta además de los iones Mn3+ y Mn4+, los iones Fe3+ en los sitios octaédricos B estructurales.

Palabras claves: Holandita, Distribución catiónica, Oxidos mixtos, Manganatos, Conductor iónico.

INTRODUCTION

The system A- (M, B)-O (A is a monovalent o divalent metal, M and B are transition metals) with hollandite structure consists of a layered or tunneled framework, where mobile cations reside in between the layers or in the tunnels. Solid electrolytes with hollandite structure having 2D or 3D conductivities have been extensively studied because they act as fast ionic conductors for secondary batteries (1-3) and can be useful in the immobilization of radioactive wastes (4). Oxides with hollandite structure of the type AxB6O16 (x £ 2) includes double chains of edge-sharing BO6 octahedra which share corners with other double chains to give a framework structure containing 4 square tunnels in z-direction (Fig. 1). A represents ions in the tunnel cavities of the structure, and B are smaller and highly charged cations occupying octahedral sites. The symmetry can be tetragonal (I4/m) or monoclinic (I2m, b = 90 to 91.5). When the cationic ratio rA/rB < 2.08 the hollandite is monoclinic; it is tetragonal when that ratio is > 2.08. In previous work we have shown the effect of Ti substitution by Al to decrease the performance of the ion lithium cells with BaTi8O16 and BaAl2Ti6O16 as the anodes(6). In this work, we have also showed that BaTi8O16 is tetragonal whereas BaMn8O16 is monoclinic. The effect of large ions such as Ti3+ on the B sites of the BaTi8O16 increases the unit cell volume thus facilitating the Li+ insertion.


 

 

 

Fig. 1 Scheme of the hollandite structure (hkl 001) showing shared octahedral via edges and corners, plus the tunnel cavities [after 5].

 

 

 

The objective of this work was to synthesize and examine how the barium hollandite structure responds to various combinations of either M2+ or M3+ and Mn4+ ions on the B sites through the probable ionic distribution. The system studied is BaMMn7O16 (M = Mg, Fe, Mn, Ni). The effect on the A-sites tunnel was studied substituting barium metal by potassium metal.

EXPERIMENTAL

Synthesis.

The compounds BaMMn7O16 (M = Mg, Mn, Fe, Ni) and K2Mn8O16 were obtained through the solid phase reaction using metallic oxides and carbonates as precursors. For the simple oxides: Fe2O3 (Carlo Erba, R.P.), MnO2 (I.C.No 1, ICS, Cleveland), NiO (Merck p.a.), MgO (Baker, U.S.P. F.C.C.) and the carbonates: BaCO3 (Baker A.C.S.), K2CO3 (Merck p.a.) the stoichiometric amounts were determined from the reactions 1 to 5.

BaCO3(s) + 8MnO2(s) ® BaMn8O16(s) + CO2(g) + 1/2O2(g) (1)
BaCO3(s) + 7MnO2(s) + 1/2 Fe2O3(s) ® BaFeMn7O16(s) + CO2(g) + 1/4O2(g) (2)
BaCO3(s) + 7MnO2(s) + NiO(s) ® BaNiMn7O16(s) + CO2(g) (3)
BaCO3(s) + 7MnO2(s) + MgO(s) ® BaMgMn7O16(s) + CO2(g) (4)
K2CO3(s) + 8MnO2(s) ® K2Mn8O16(s) + CO2(g) + 1/2O2(g) (5)

The starting materials were first mixed using wet grinding to 0.18 mm diameter by means a small ball mill, then pelletized (1 ton cm-2) and calcinated in air inside a horizontal furnace at 850 oC, for 48 h. During this preparation, each pellet was grounded several times using an agata mortar, to ensure a homogeneous reaction. Before the calcinations, all samples were thermally treated at 800 oC for 24 h and then cooled at 80 oC h-1 in order to obtain thermodynamically equilibrated structures. The selected temperature determined from GTA was the same as that already reported (6) for BaMn8O16.

The samples were stored an argon atmosphere at room temperature. The actual metal ratio in the oxides was controlled by atomic absorption spectroscopy (Perkin Elmer 460).

Characterization.

GTA measurements of synthesized samples were performed in a GTA-DTA Setaram apparatus containing a tungsten furnace and an electronic balance (Ugine-Eyraud). The measurement conditions were: temperature range 40 to 1500 oC in 10-5 mmHg vacuum (Edwards diffusion pump), at 5 oC min-1 heating rate. The samples were pelletized at 5-ton cm-2. Phase purity and the hollandite structure were analyzed by X-ray diffractometry (XRD) at 25 oC with a Siemens powder diffractometer (S5000), using CuKa radiation (l = 0.229092 nm) and power of 40 kV x 30 mA, with a diffracting monochromatic beam over the range 15 ­70o2q with a step interval 0.02o2q and a count time of 1s per step. The cell parameters were obtained using a personal computer with Itera software (7) on the basis of a minimum value of the R factor, taking into account the hkl distances and intensities observed and calculated as the corresponding statistical weight of each diffraction line.

The oxidation power (eq mol-1) was determined by chemical reduction of the Mn+ cations (with n3) using VOSO4 as a reducing agent according to reaction (6). In this method, which has been widely used by us (8,9), approximately 100 mg of the oxide was mixed with 50 mL of VOSO4 sulfuric acid solution (to avoid cationic hydrolysis) and the mixture was heated at 90 oC until complete reduction of the oxide. After that, the solution was titrated with 1N KMnO4 solution, which had been already standardized with oxalate solution. The standard deviation was estimated to be ±0.03.

Mn+(solid) + (n-2)VO2+(aq) + 3(n-2) H2O ® M2+(aq) + (n-2) V(OH)4+ (aq) + 2(n-2) H+(aq) (6)

In order to determine the magnetic moment of the oxide (m), we carried out magnetic susceptibility measurements as a function of temperature using a Faraday type magnetic balance. The magnetic susceptibilities c were corrected for the intrinsic diamagnetism. The method consists of measuring the force F acting on a mass m of an oxide exhibiting magnetic susceptibility, which is placed in a magnetic field H whose gradient is ðH/ðx, according to the relationship F = mc H ðH/ðx. The calculation of the Curie constant, Cm, has been made as described in ref. [10], from plots of c -1= T/Cm + 1/co - m /(T-q), with q being the Weiss temperature, co = - q /Cm and m the magnetic moment of the oxide. The oxide cationic distributions resulting from the magnetic study were computed from the best fit with the theoretical value of Cm (molar Curie constant): Cm = S xi Ci, where Ci is an individual ionic Curie constant taken from the literature (11). As reference compound we used HgCo (SCN) 4, which has a magnetic susceptibility that is a function of the temperature c = 16.44 . 10-6 ­ 5 . 10-8 (T-298).

RESULTS and DISCUSSION

Hollandite characterization

The oxides BaMn8O16 and BaFeMn7O16 crystallized in the hollandite monoclinic phase, S.G. I2/m and S.G. P21/n respectively, whereas K2Mn8O16 appeared tetragonal, S.G. I4/m. No parasitic phases were detected by XRD. The XRD results agree with those shown on JCPDS 38-476, 12-514 and 29-1020. The X-ray diffractograms of the BaNiMn7O16 and the BaMgMn7O16 oxides (Figs. 2 and 3) are reported for the first time, and the compounds can be considered having the monoclinic phase. A remarkable common feature of the X-ray spectra is the good definition at least 40 diffraction lines. However, the broad diffraction peaks indicate poor crystallinity. The crystallographic data of the barium iron manganese oxide (JCPDS 12-514) was used as reference. Both oxides exhibit also the hollandite monoclinic phase, S.G. P21/n. To facilitate a comparison between the tetragonal and the monoclinic hollandites, a transformation (12) P21/n ® I2/m was made. K2Mn8O16 oxide showed as the principal plane hkl 211 (dhkl = 0.2404nm), BaMn8O16 hkl 301 (dhkl = 0.3138 nm), BaFeMn7O16 hkl 301,103 (dhkl = 0.3148 nm, 0.3092 nm), BaNiMn7O16 hkl 301 (dhkl = 0.3132 nm) and BaMgMn7O16 hkl 221 (dhkl = 0.2405 nm). Hollandites with large A ions and small B cations are generally tetragonal. A tetragonal-to-monoclinic transformation in hollandites occurs because the tunnel ions are unable to support the octahedral limits, and these collapse onto the tunnel ions. However, the change in symmetry depends also on a next-nearest-neighbor interaction between the A and B ions. Table I and table II show the X-ray diffraction data of BaNiMn7O16 and BaMgMn7O16, respectively. It is clear that the cation A does not change, and that the symmetry is conserved. The observed cell parameters and unit-cell volumes of the hollandites studied are displayed on Table III. These results show that the nature of the cations placed in octahedral coordination with the oxygen (BO6 octahedra) affects the cell parameters. The unit cell volume for BaMMn7O16 (M = Mg, Fe, Ni) increases continuously with an increasing ionic radius of the dopant ion M. The b and c parameters increase with rM2+ but the a-parameter shows a high value when Fe3+ is the dopant ion. This can be related with the distance (13), Fe-O (0.2025 nm in Fe2O3), Ni-O (0.2084 nm in NiO) and Mg-O (0.205 nm in MgO). However, it is also important to consider that the covalence of the cation M affects the M-O distance in octahedral sites.


Fig 2. XRD pattern of the oxide BaNiMn7O16


Fig 3. XRD pattern of the oxide BaMgMn7O16




GTA results are shown on Fig. 4. The thermograms exhibited two weight loss at different temperatures. The first between 300 OC and 800 oC which corresponds to hydration due to H2O/OH in surface and an important weight loss (DP/P) between 800 oC and 1300 oC - 1400 oC due to O2 evolution with formation of BaMnO3 (or K2MnO3) according to reactions 7 to 11.


Fig 4. GTA of hollandites in vacuum. (1) BaMnMn7O16, (2) K2Mn8O16, (3) BaMgMn7O16, (4) BaNiMn7O16, (5) BaFeMn7O16

Table IV shows the total DP/P observed and calculated that corresponds to the reactions indicated below.

BaMn8O16(s) ® BaMnO3(s) + 7 MnO(s) + 3 O2(g)     (7)
K2Mn8O16 (s) ® K2MnO3(s) + 7 MnO(s) + 3 O2(g)     (8)
BaMgMn7O16(s) ® BaMnO3(s) + MgMnO3(s) + 5 MnO + 3 O2(g) (9)
BaFeMn7O16(s) ® BaMnO3(s) + FeMnO3(s) + 5 MnO + 3 O2(g) (10)
BaNiMn7O16(s) ® BaMnO3(s) + NiMnO3 (s) + 5 MnO + 3 O2(g) (11)


Table V shows the oxidation power of the oxides BaMMn7O16 as a function of the substitution cation which was added M = Mg, Fe, Mn, Ni. It appears that the amount of equivalents per mol of oxide (eq mol-1) do not change with M, suggesting that the average oxidation state of the cations placed in the octahedral position is the same and equal to 14 eq mol-1.


Using the molar magnetic susceptibilities measured as a function of T (Fig. 5 is shown as an example) it was possible to obtain the effective magnetic moment of the oxide meff from Cm value obtained using the Curie-Weiss law. The experimental values of the meff showed in table V are in agreement with the existence of metals with high valencies in high spin state and indicate that the Mn is present preferably as the Mn4+ ion in the octahedral sites of the structure.


Fig 5. Magnetic susceptibility as a function of temperature (1) BaMgMn7O16 (2) BaNiMn7O16

Cationic distribution.

We have used the methodology already reported by us (14) which use physicochemical measurements as the basis to propose the oxide cation distribution. The proposed cation distribution must show a good agreement between the experimental and the calculated parameters, such as the magnetic moment meff, and the oxidation power eq mol-1. Table VI gathers the cationic distributions proposed. The potassium and barium manganates showed both Mn3+ and Mn4+ cations in octahedral sites. K2Mn8O16 showed an experimental Curie constant Cm = 17.6 emu which is in agreement with the calculated Cm = 17.3 emu considering the individual Ci values, Mn3+ = 3, Mn4+ = 1.876. In the case of BaMMn7O16 (M = Mg, Ni) using Ci = 3 for Ni2+, both cationic distributions are compatible with only Mn4+ ions in B sites. However, in the case of iron and BaFeMn7O16, and upon considering Ci = 4.34 emu for Fe3+ ion, we strongly feel that the redox couple Mn4+/Mn3+ does exist. Regarding the oxidation power values, all ionic structures exhibited the same value (14 eq mol-1) according to the formula proposed.


CONCLUSIONS

We have synthesized oxides with hollandite structure using thermal decomposition in air of metal carbonates and mixed simple oxides at 850 oC. We report two new compounds; BaMgMn7O16 and BaNiMn7O16 which were obtained from barium carbonate and mixed g-manganese bioxide and magnesium (or nickel) oxide. These compounds have monoclinic phase structure. A tetragonal structure was obtained for K2MnMn7O16 whereas BaMnMn7O16 exhibited monoclinic structure. The unit cell volume depends on the radius of the

substituting M cation on BaMMn7O16 (M = Mg, Mn, Fe, Ni). The ionic distribution obtained by means of several analyses showed the presence of Mn4+ ion at the octahedral sites in all cases besides the corresponding M2+ or M3+ substituting M ion.

ACKNOWLEDGEMENTS

Financial assistance from FONDECYT (projects 1950542 and 1990951) is gratefully acknowledged.

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