Solid-state electrolyte for use in lithium-air batteries or in lithium-water batteries

Abstract

The invention relates to solid-state electrolytes for use in lithium-air batteries or in lithium-water batteries. It is the object of the invention to provide solid electrolyte for use in lithium-air batteries or lithium-water batteries, with the solid electrolyte having sufficient strength, good conductivity for lithium ions, imperviousness for gas and water resistance and being inexpensive in manufacture. The solid-state electrolyte in accordance with the invention has an open-pore ceramic carrier substrate. In this respect, at least one layer which is conductive for lithium ions, which has an electrical conductivity of at least 10.sup.5 Scm.sup.1 and which is gas-impervious is formed on the surface facing the cathode. In this respect, the carrier substrate has greater mechanical strength and a larger layer thickness than the at least one layer.

Claims

1. A solid-state electrolyte for use in lithium-air batteries or lithium-water batteries, comprising a multi-layer structure for providing mechanical stability and lithium ion conductivity wherein the function of mechanical stability is on one layer and the function of lithium ion conductivity is separated on at least one second layer, said one layer comprising an open-pore ceramic carrier substrate comprising a ceramic material which is selected from Al.sub.2O.sub.3, ZrO.sub.2, MgAl.sub.2O.sub.4, SiC and Si.sub.3N.sub.4 for providing the mechanical stability function and wherein the pores of said carrier substrate have a pore size in the range of 1 m and 10 m, and said at least one second layer which is conductive for lithium ions for providing the lithium ion conductivity function, said second layer having an electrical conductivity of at least 10.sup.5 Scm.sup.1, which is gas-impervious and which is sintered on a surface of said ceramic carrier substrate and presents a face open to a cathode of a battery, and wherein said carrier substrate layer has greater mechanical strength and a larger layer thickness than the at least one second layer.

2. A solid-state electrolyte in accordance with claim 1, characterized in that the carrier substrate has a porosity of at least 15% and a maximum of 60%; and the substrate thickness is in the range from 20 m-500 m, and wherein the at least one second layer has a thickness between 10 m and 50 m.

3. A solid-state electrolyte in accordance with claim 1, characterized in that the thickness of the carrier substrate is at least twice as large as the thickness of the at least one second layer.

4. A solid-state electrolyte in accordance with claim 1, characterized in that the carrier substrate presents a face open to an anode of a battery and has at least two layers each having a different porosity or, starting from an anode-side surface, has a graduated porosity, which reduces in size in the direction of the ion-conductive second layer.

5. A solid-state electrolyte in accordance with claim 1, characterized in that the pores of the carrier substrate are infiltrated with a liquid electrolyte conductive for lithium ions.

6. A solid-state electrolyte in accordance with claim 1, characterized in that the at least one surface of the carrier substrate is coated over the full surface with the at least one gas-impervious second layer.

7. A solid-state electrolyte in accordance with claim 1, characterized in that said at least one second layer is partly infiltrated into the carrier substrate.

8. A solid-state electrolyte for use in lithium-air batteries or lithium-water batteries, comprising a multi-later structure for providing mechanical stability and lithium ion conductivity wherein the function of mechanical stability is on one layer and the function of lithium ion conductivity is separated on a least one second layer, said one layer comprising an open-pore ceramic carrier substrate comprising a ceramic material which is selected from Al.sub.2O.sub.3, ZrO.sub.2, MgAl.sub.2O.sub.4, SiC and Si.sub.3N.sub.4 for providing the mechanical stability function, said carrier substrate having a porosity of at least 15% and a maximum of 60%; the pores of said substrate having a pore size in the range of 1 m-10 m; and the substrate thickness is in the range from 20 m-500 m, and said at least one second layer which is conductive for lithium ions for providing the lithium ion conductivity function, said second layer having an electrical conductivity of at least 10.sup.5 Scm.sup.1, which is gas-impervious and which is sintered on a surface of said ceramic carrier substrate and presents a face open to a cathode of a battery, said carrier substrate layer having a greater mechanical strength and having a thickness at least twice as large as the thickness of said at least one second layer.

Description

(1) The invention will be explained in more detail by way of example in the following.

(2) There are shown:

(3) FIG. 1 in diagram form, densities of sintering bodies of the compounds Li02 and Li04 in dependence on the maximum sintering temperature;

(4) FIG. 2 a comparison of the temperature-dependent sintering shrinkages of the compounds Li02 and Li04 measured by means of hot-stage microscopy;

(5) FIG. 3 specific electrical conductivities at room temperature of the compounds listed in Table 1 in dependence on the sintering temperature of powder compacts; and

(6) FIG. 4 temperature-dependent specific conductivities and activating energies of selected compounds from Table 1 after sintering at air at 1000 C.

(7) FIG. 5 is a schematic rendering of the electrolyte layers in a battery environment.

(8) The samples named in the following are designated by Li0 and an additional digit.

(9) Within the framework of the embodiment described in the following, on the one hand, the manufacture of glasses, their processing into powders and sintered powder compacts as well as the measurement of the ion conductivity by means of impedance spectroscopy are described.

(10) Furthermore, the processing of two selected powders into pastes suitable for screen printing, the coating of porous ceramic substrates as well as the co-firing of the layers and the corresponding electrical conductivity measurements are shown. In this respect, value was placed on the fact that one powder has a good sintering capability and one powder has a worse or insufficient sintering capability.

(11) Table 1 shows a selection of material compositions which were melted to glasses, further processed and characterized in accordance with the specification described in the following. With the exception of Li06, they are compositions of the system Li.sub.2OAl.sub.2O.sub.3P.sub.2O.sub.5P.sub.2O.sub.5. Li06 additionally comprises SiO.sub.2.

(12) TABLE-US-00001 TABLE 1 shows compositions of Li.sub.2OAl.sub.2O.sub.3TiO.sub.2(SiO.sub.2)P.sub.2O.sub.5 glasses. Glass des. Li02 Li03 Li04 Li06 Li09 Oxide Proportions of the oxides in mol % Li.sub.2O 14 16.25 20 18.6 16.25 Al.sub.2O.sub.3 9 3.75 7.5 3.5 3.75 TiO.sub.2 38 42.5 35 39.5 42.5 P.sub.2O.sub.5 39 37.5 37.5 31.4 37.5 SiO.sub.2 7 Total 100 100 100 100 100

(13) In the glass manufacture, the chemicals listed in the following were used in the quality analytical purity: Li.sub.2CO.sub.3 (Baker, Mallinckrodt) Al(OH).sub.3 (VWR) TiO.sub.2 (Sigma Aldrich) (NH.sub.4).sub.2HPO.sub.4 (Sigma Aldrich) AlPO.sub.4 (Alpha Aesar)

(14) The mixture of the raw substances took place in a screw-closable plastic container by means of a tubular mixer for approx. 30 minutes. The raw substance blends manufactured in this manner were filled into an Al.sub.2O.sub.3 crucible and were initially precalcinated at temperatures between 400 C. and 600 C. at air for at least 5 h and were subsequently melted to glasses at temperatures between 1300 C. and 1400 C. with dwell times of 2 h at a maximum temperature of 1400 C. at air. The casting of the glass melts took place either in deionized water (RT) or on a dry steel plate (RT). On a casting of the melt in water, the glass frit obtained can be dried in a drying cupboard at 150 for 12 h at air.

(15) The glass frits were initially comminuted in a disk rocker mill to a screen fraction <500 m for the further processing of the glasses. The final grinding of the precomminuted frits is carried out at air in a planetary ball mill down to typical particle diameters of d.sub.10<1 m, d.sub.50=5 m-10 m and d.sub.90<25 m corresponding to the mass portions (Q.sub.3) (measurement of the particle size distribution using laser diffraction).

(16) The further processing can take place by cold pressing and sintering. The powders were pressed uniaxially into cylindrical samples having heights between 8 mm-10 mm and a diameter of 6 mm for the manufacture of solid sample bodies. The pressing force amounted to 2.5 kN. On a subsequent heat treatment carried out under atmospheric conditions, the sample bodies were sintered at air.

(17) A typical oven profile: 25 C.-2K/min could be operated until the reaching of the maximum temperature to the amount of 1200 C. The maximum temperature was held for 1 h and cooling then takes place at a rate of 2 K/min down to 25 C.

(18) FIG. 1 shows by way of example the behavior of compacts of the glass powders Li02 and Li04 in dependence on the maximum sintering temperature. Whereas the composition Li02 does not show any real sintering shrinkage or densification independently of the selected sintering temperature up to a maximum 1000 C., the density of Li04 increases constantly from a sintering temperature of 700 C. up to 1000 C. The composition Li02 can thus be called non-sinter active and the composition Li04 sinter active under atmospheric conditions.

(19) This behavior is confirmed with reference to shrinkage curves (FIG. 2) which were measured by means of optical dilatometry in a hot-stage microscope. While a linear sintering shrinkage of around 8.5% occurred with Li04 up to a temperature of 1000 C., in the case of Li02 only a slight increase of 8.Math.10-6 ppm/K was measured in the temperature range from 100 C. to 1000 C.

(20) The phase compositions shown in Table 2 resulted from quantitative Rietveld X-ray analyses of Li02 and Li04 samples after sintering.

(21) TABLE-US-00002 TABLE 2 Phase compositions of the crystallized compounds Li02 and Li04 using X-rays and Rietveld analyses Crystalline Sample Sintering temperature/ C. phases des. 500 C. 700 C. 1000 C. Li.sub.1+xT.sub.2xAl.sub.x(PO.sub.4).sub.3/ Li04 77.8 86.0 91.5 Mass % AlPO.sub.4/Mass % 11.8 <1 Li.sub.1+xT.sub.2xAl.sub.x(PO.sub.4).sub.3/ Li02 69.6 68.4 71.9 Mass % AlPO.sub.4/Mass % 21.4 22.8 22.3

(22) In the case of Li04, the portion of AlPO.sub.4 still present at 500 C. becomes lower as the sintering temperature increases and is no longer detectable by X-rays at temperatures above 700 C. The sintering density of this material also increases in this temperature interval. In contrast to this, the composition Li02 contains a much higher portion of AlPO.sub.4 which also does not reduce as the sintering temperature increases. A real densification of this material is not measurable.

(23) Subsequent to the sintering of the compacts, the respective end faces were sputtered with gold and the electrical conductivity was determined using impedance spectroscopy at room temperature in the frequency range from 1 Hz to 1 MHz and at an amplitude of 100 mV. The respective measurement at the maximum of the phase angle was used as the resistance value. The reciprocal of the specific resistance value represents the specific conductivity of the samples. FIG. 3 shows the specific electrical conductivities measured for the different glasses in dependence on the maximum sintering temperature. It is shown in comparison with FIG. 1 that the less sinter active material Li02 has the smallest electrical conductivity. With the other materials (Li03, Li04, L06 and Li09), the specific electrical conductivity increases by several orders of magnitude as the sintering temperature rises. Maximum electrical conductivities of more than 10.sup.4 Scm.sup.1 are achieved with the materials Li09 and Li03 at a sintering temperature of at least 1000 C.

(24) The specific electrical conductivities were furthermore measured using impedance spectroscopy in a temperature range between room temperature and 500 C. with selected samples which were sintered at a maximum temperature of 1000 C. FIG. 3 shows the plot of the temperature-dependent specific conductivities in accordance with the Arrhenius law.

(25) The activation energies for the electrical conductivities of the materials can be derived from this plot. The measured activation energies lie between 0.27 eV and 0.57 eV on a mechanism running via ion conduction.

(26) Powders of the materials Li02 and Li04 were used for a paste manufacture for the application as a layer to a microporous substrate after preparation in accordance with the above statements while using organic binding agents customary in thick film technology and a solvent type. The solid content of the pastes was in the range between 75% by mass and 85% by mass. The pastes were applied over the whole surface to film-cast Al.sub.2O.sub.3 substrates with a size of 22 cm.sup.2, a thickness of 500 m, porosities between 30% by volume and 38% by volume and mean pore diameters between 4 m and 8 m. The application of the pastes took place by means of double screen printing, with a drying of the first layer (30 minutes, 120 C., air) taking place between the two screen printing steps. In this way, layer thicknesses were applied between 30 m and 40 m in the non-sintered state onto the porous Al.sub.2O.sub.3 substrates. Subsequent to the drying process, the printed substrates were subjected to a firing process with a debinding step. In this respect, heating rates and cooling rates between 5 and 10 K/min and maximum temperatures between 1000 C. and 1100 C. (dwell time at the maximum temperature 1 h to 3 h) were used. The debinding was integrated into this co-firing process and took place at 500 C. at air for 2 h. The crystalline phase composition of the co-fired thick layers on the porous carrier substrates was checked qualitatively by means of radiography and was evaluated as identical to the phases found for the compounds Li02 and Li04 (cf. Table 2 in this respect). The imperviousness of the co-fired substrates was checked via the mass spectroscopic measurement of the helium leak rate (apparatus: Helium Leak Locator, Oerlikon). Leak rates of more than 10.sup.3 mbar.sup.1 were measured for the substrates coated with Li02, which allows a conclusion of non-impervious layers. Leak rates of less than 10.sup.8 1 mbar.sup.1 were measured for the substrates coated with Li04, which allows a conclusion of impervious layers. The measured leak rates relate to a circular measurement surface having a circumference of 1.55 cm. The results are in agreement with the sintering behavior of the compounds Li02 and Li04 as solid powder compacts.

(27) The determination of the electrical conductivity of these coated samples took place via electrochemical impedance spectroscopy. For this purpose, circular samples having a diameter of 1.27 mm were prepared from the sintered and coated 22 cm.sup.2 substrates. These samples were installed in a 2-electrode assembly cell (Swagelok cell) for this measurement. Two round stainless steel platelets, which act as inert in the given electrochemical conditions (blocking electrodes), are used as the electrodes. The round sample pieces were placed between these electrodes and were saturated with some drops of electrolyte (LP40, commercially available battery electrolyte 1M LiPF.sub.6 in EC:DEC 1:1) (approx. 40-60 mg). The electrical contacting takes place on one side directly through the accumulator of the cell and on the other side via a spring which ensures a uniform contact pressure on the samples.

(28) The measurement took place at a potentiostat having an integrated FRA (Gamry Reference 600). Since measurement took place in 2-electrode arrangements, the reference electrode had to be placed on the counter-electrode. The impedance measurement was carried out at an amplitude of 100 mV with a resting potential of the cell; the examined frequency range was between 10.sup.5 Hz and 1 Hz. The ohmic resistance portion at high frequencies (approx. 10.sup.4 Hz-10.sup.5 Hz) was used for the calculation of the electrical conductivity.

(29) Table 3 lists the results of the impedance-spectroscopic measurements at the coated substrates. The porous layers formed from Li02 show clearly higher electrical conductivities than the impervious layers based on Li04. This is explained by the fact that the liquid electrolyte has saturated the porous Li04 layer and that thereby the conductivity of the electrolyte is predominantly measured. In contrast to this, in the case of the substrates coated with Li04, specific electrical conductivities are measured which are only slightly below the electrical conductivities for the solid Li04 sintering bodies shown in FIG. 3.

(30) TABLE-US-00003 TABLE 3 provides specific conductivities of the porous Al.sub.2O.sub.3 substrates coated with Li02 and Li04 measured using impedance spectroscopy. Specific conductivity at the respective sintering temperature/mS cm.sup.1 Lithium-ion 800 C. 1000 C. 1200 C. conductive layer Li04 (thickness 1 .Math. 10.sup.3 4 .Math. 10.sup.3 2 .Math. 10.sup.2 approx. 30 m) Li02 (thickness 8 .Math. 10.sup.2 7 .Math. 10.sup.2 approx. 40 m)