Sintered ceramics of Ca 0.9A 0.1MnO 3-δ(A = La, Nd, Sm, Gd and Y) were studied on their cathode properties in LiOHaq. solution. After firing, the samples were obtained as high conductivity sintered (porous) materials composed of an orthorhombic perovskite-type phase. Next, charge discharge performances of the electrodes consisting of the sintered sample were investigated. The discharge capacity of Ca 0.9Y 0.1MnO 3-δwas 185 mAh·g -1on the 1st cycling, and the 1st charging was possible by 130 mAh·g -1. However, the 2nd discharge capacity remarkably decreased to lower than 50 mAh·g -1. Considering no obvious charging property on the previous La-substituted sample of Ca 0.9La0.1MnO 3-δ, it would mean that change of the substituent for CaMnO 3 affects the electrochemical property. The roll of lithium ions, the effect of the cut-off potential range on the cycle performance would be discussed leading to the charge/discharge results of the cell (-)Zn/LiOHaq./Ca 0.9Y 0.1MnO 3-δ(+).
When lanthanum is partially substituted for calcium sites in the perovskite‑type oxide of CaMnO3, high electronic conductivity is observed at room temperature due to the valence change of manganese and the easy change of oxygen content in the lattice [
M. Manickam et al. have already carried out the investigation of batteries using LiOHaq. and MnO2 [
The samples were prepared from reagent grade powders of CaCO3 and Mn2O3 and some rare earth oxides A2O3 (A = La, Nd, Sm, Gd and Y) with 99.9% purity. These materials were weighed in the defined molar ratios, mixed, temporarily pressed into pellets, and pre-fired in an alumina boat at 1200˚C in air for 10 h. To obtain porous samples for electrochemical measurements, the pre-fired samples were crushed, mixed with NH4HCO3, pressed (600 kg∙cm−3) into disks (ca. 12 mm diameter by 1 mm thickness) and sintered again at 1300˚C in air for 10 h. The crystal structure of the samples was checked by X-ray diffraction (XRD) and the sample morphology by scanning electron microscopy (SEM).Electrical conductivity of the sintered porous samples was measured by the 2-terminal ac method with 10 kHz signal in the temperature range from room temperature to 1000˚C in air. The oxygen content, i.e. 3 − δ in Ca0.9A0.1MnO3−δ at room temperature was calculated from the TGA result measured between room temperature and 1000 ˚C using the starting powder mixture.
To investigate the cathodic properties of the sample in LiOHaq. solution, the sintered sample disc (ca. 0.50 g) including no graphite was fixed on a platinum plate with a Pt lead in a polyethylene cell. This was immersed in 5% LiOHaq. solution and the current of 5 mA·g−1 was passed through the test electrode (cathode material) and a counter electrode (platinum plate). In each case, the cathode potential was measured in the window range from −0.80 to +0.60 V against an Hg/HgO electrode at 30˚C. From the discharge curves, the discharge capacities of the cathode materials were calculated.
The oxides shown by Ca1−xAxMnO3−δ are generally known to have fairly wide solid solution ranges. In the present experiment, the sample compositions were fixed to be x = 0.10 for various A elements. As a result, sintered (porous) samples were confirmed to be composed of an orthorhombic perovskite‑type phase in all cases. Of course, the crystal lattices somewhat changed depending on the ionic radius of A element; Ca0.9Y0.1MnO3−δ showed the most packed unit lattice among them and Ca0.9La0.1MnO3−δ the most open parallel structure, which coincided with the size of substituting elements [
The electrodes, constructed with porous ceramic oxides, showed stable EMFs’ over 200 mV (vs. Hg/HgO) in 5% LiOH solution at 30˚C.
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When the sample composition was represented as CaMnO3, the electrode reaction in an alkaline solution would be generally shown in the followings,
If LiOH solution was used, the next reaction could also be expected,
In any case, the discharge capacity due to the change of Mn4+ to Mn3+ should be 187.5 mAh·g−1 at maximum. If some manganese were substituted by a rare earth element, tri-valent manganese could be formed to reduce the content of tetra-valent manganese, which would cut down the total discharge capacity. As we observed the actual discharge capacity of 200 mAh·g−1 exceeding the maximum, we thought that the prolonged discharge time was principally due to the change of Mn4+ down to Mn2+, where the lithium insertion might be favorable in addition to hydrogen insertion. Then, we tried to check the lithium insertion into the bulk sample by ICP using the fully rinsed sample after discharge. Unfortunately, we could not strictly determine whether the obtained signal of lithium was based on the sample bulk or in the LiOH adhered tightly in the sample pore (surface). From the discharge result of the sample showing the discharge capacity less than 2 mAh·g−1 done in 1 M LiClO4/PC solution by 10 mA·g−1 as an additional experiment, the effect of lithium insertion itself would be undesirable. Of course, the XRD diffraction analysis was carried out, the result of which is mentioned in the following paragraph.
Next, we compared the performances within the samples of Ca0.9La0.1MnO3−δ and Ca0.9Y0.1MnO3−δ
Furthermore, these phenomena were recognized by SEM pictures; the sample situation was fairly different before and after discharging as seen in
In order to check whether the ceramic oxide Ca0.9Y0.1MnO3−δ could be employed as an actual battery material or not, a cell was set up using this oxide as a cathode material, a zinc metal as an anode material and 5% LiOHaq. solution as an electrolyte solution.
The sintered ceramic Ca0.9A0.1MnO3−δ (A = La, Nd, Sm, Gd and Y) showed properties as a cathode material with no conductive powder such as graphite in 5% LiOHaq. solution. The discharge capacity changed depending on
the A elements and that Ca0.9Y0.1MnO3−δ was 190 mAh∙g−1, which was greater than 165 mAh∙g−1 of Ca0.9La0.1MnO3−δ in the previous experiment using 15% KOH aq. solution. This result shows the effect of lithium ions to promote the discharge reaction. On the other hand, the discharge capacity indicated that manganese was reduced partly from Mn4+ to Mn2+ in the oxide. By changing the cutoff potential shallow to depress formation of the compound including Mn2+, the 2nd or more discharge/charge was found to be available. Finally the cell of using zinc like (−)Zn/LiOHaq./Ca0.9Y0.1MnO3−δ(+) was found to work as a primary battery with ca. 1.0 V plateau voltage.