Electrode unit for an electrochemical device

Abstract

An electrode unit for an electrochemical device, comprising (i) a solid electrolyte which divides a space for molten cathode material, selected from the group consisting of elemental sulfur and polysulfide of the alkali metal anode material, and a space for molten alkali metal anode material, and (ii) a porous solid state electrode directly adjacent to the solid electrolyte within the space for the cathode material, with a non-electron-conducting intermediate layer S present between the solid state electrode and the solid electrolyte, wherein this intermediate layer S has a thickness in the range from 0.5 to 5 mm and, before the first charge of the electrochemical device, has been impregnated fully with a polysulfide composition, comprising (A) pure polysulfides Met.sub.2S.sub.x with Met=alkali metal of the alkali metal anode material selected from lithium, sodium, potassium, and x is dependent on the alkali metal and is 2, 3, 4 or 5 for Na and is 2, 3, 4, 5, 6, 7, 8 for Li and is 2, 3, 4, 5, 6 for K, or (B) mixtures of the polysulfides of one and the same alkali metal from (A) with one another.

Claims

1. An electrode unit for an electrochemical device, comprising (i) a solid electrolyte which divides a space for molten cathode material, selected from the group consisting of elemental sulfur and polysulfide of the alkali metal anode material, and a space for molten alkali metal anode material and (ii) a porous solid state electrode adjacent to the solid electrolyte within the space for the cathode material, with a non-electron-conducting intermediate layer S present between the solid state electrode and the solid electrolyte, wherein this intermediate layer S has a thickness in the range from 1.0 to 5 mm and, before the first charge of the electrochemical device, has been impregnated fully with a polysulfide composition, comprising (A) pure polysulfides Met.sub.2S.sub.x with Met=alkali metal of the alkali metal anode material selected from lithium, sodium, potassium, and x is dependent on the alkali metal and is 2, 3, 4 or 5 for Na and is 2, 3, 4, 5, 6, 7, 8 for Li and is 2, 3, 4, 5, 6 for K, or (B) mixtures of the polysulfides of one and the same alkali metal from (A) with one another; wherein the solid electrolyte is a polycrystalline ceramic material having an ion conductivity for the alkali metal ions that correspond to the alkali metal anode material; wherein only the intermediate layer S is between the porous solid state electrode and the solid electrolyte.

2. The electrode unit according to claim 1, wherein the basis of the non-electron-conducting intermediate layer S is a flat structure of fibers selected from alumina (Al.sub.2O.sub.3), silicon dioxide, mixed oxides of aluminum with silicon, silicates and aluminosilicates.

3. The electrode unit according to claim 1, wherein the solid electrolyte is a cylindrical shaped body closed at one end.

4. An electrochemical device comprising the electrode unit as defined in claim 1.

5. The electrochemical device according to claim 4, wherein the electrochemical device is a sodium-sulfur cell.

6. A process for first charging of the electrochemical device as defined in claim 4, which comprises initially charging the space for the molten cathode material with a polysulfide compound (I) as a melt, comprising: (A) pure polysulfides Met2Sx with Met=alkali metal of the desired alkali metal anode material, selected from lithium, sodium, potassium, x is dependent on the alkali metal and is 2, 3, 4 or 5 for Na and is 2, 3, 4, 5, 6, 7, 8 for Li and is 2, 3, 4, 5, 6 for K, or (B) mixtures of the polysulfides of one and the same alkali metal from (A) with one another and/or in each case with elemental sulfur or (C) mixtures of the particular alkali metal sulfide Met2S with elemental sulfur and/or the polysulfides Met2Sx mentioned in (A) or (B), and additionally installing, in the space for the molten anode material, an electron-conducting device in such a way that it touches the surface of the solid electrolyte facing the anode material at least in the lower region, connects cathode space and anode space to an electrical circuit and sends an electrical current through this electrochemical device, such that the polysulfide compound (I) is cleaved electrolytically, forming elemental sulfur in the cathode space and metallic alkali metal in the anode space.

7. A process for producing a non-electron-conducting intermediate layer S in the electrode unit as defined in claim 1, which comprises subjecting a porous starting material that forms the non-electron-conducting intermediate layer S to a pressure of less than 1 atm and impregnating it with a molten polysulfide composition of the alkali metal that forms the alkali metal anode material comprising (A) pure polysulfides Met2Sx with Met=alkali metal of the alkali metal anode material selected from lithium, sodium, potassium, and x is dependent on the alkali metal and is 2, 3, 4 or 5 for Na and is 2, 3, 4, 5, 6, 7, 8 for Li and is 2, 3, 4, 5, 6 for K, or (B) mixtures of the polysulfides of one and the same alkali metal from (A) with one another.

Description

EXAMPLES

Example 1: For Comparison

(1) Deliberate Destruction of a Sodium-Sulfur CellWithout Intermediate Layer Sin the Charged State

(2) A standard sodium-sulfur cell (central sodium cell) was constructed from a cylindrical solid electrolyte composed of beta-alumina, closed at the bottom and having an internal diameter of 5.6 cm, a wall thickness of 0.2 cm and a length of 50 cm, in which was disposed, in an axially central position, a solid cylindrical displacer body made of 1.4404 stainless steel (diameter 5.5 cm, length 45.5 cm), which formed an annular gap between the outer surface of the displacer body and the inner surface of the solid electrolyte, the annular gap being the anode space. Directly on the outer surface of the solid electrolyte were a 5 mm-thick layer of graphite felt electrode and a device for making electrical contact with this electrode, i.e. a current collector. This electrode unit/solid electrolyte was accommodated in a virtually axially central position in a cylindrical metal housing made of stainless steel having an internal diameter of 10.8 cm and the space between the outer surface of the solid electrolyte and the metal housing was the cathode space.

(3) The cell was heated to 300 C. The cathode space was evacuated with the aid of a vacuum pump and then filled with about 5 kg of molten liquid sulfur. The anode space was not under reduced pressure and was charged with 45 g of molten sodium from an external reservoir vessel via an overflow system. Measurement points for temperature and pressure were arranged at different points in the cathode space. At the base of the metal housing was mounted a T-shaped line with a bursting disk that bursts at 10 bar gauge in the vertical leg thereof. The horizontal leg of the line, which can be shut off, was utilized for filling of the cathode space with sulfur.

(4) A hydraulic pump was used to pump high-boiling oil, which is virtually inert with respect to sodium under the conditions, into the anode space that had been filled virtually completely with liquid sodium, and hence pressure was exerted on the inner surface of the solid electrolyte too, At a pressure of about 80 bar, the solid electrolyte was destroyed by fracture. When the solid electrolyte fractured, sodium and sulfur came into direct contact and reacted vigorously to form heat and pressure.

(5) The temperature in the upper part of the cell rose to more than 1200 C. within the first second after the fracture of the solid electrolyte. The abrupt temperature jumps at some points in the cell led to vaporization within milliseconds of such a large amount of sulfur that local pressures of 10 bar occurred. As a result, the total pressure within the cell reached at least 11 bar during the first second, a pressure at which the bursting disk burst.

Example 2: Inventive

(6) Deliberate destruction of a sodium-sulfur cellwith intermediate layer Sin the charged state

(7) The experimental setup was analogous to example 1, except that a 1 mm-thick layer of matted polycrystalline alumina fibers, commercially available as Saffil Paper from Saffil, was on the outer surface of the solid electrolyte. This layer was joined directly by a 5 mm-thick layer of graphite felt electrode, which was provided with a device for making electrical contact with this electrode.

(8) The cell was heated to 300 C. The cathode space was brought to a pressure of about 20 mbar (abs.) with the aid of a vacuum pump and then charged with about 5 kg of molten liquid Na.sub.2S.sub.5 (disodium pentasulfide), and hence the intermediate layer S and the porous solid state electrode were impregnated with Na.sub.2S.sub.5. In the nitrogen-filled anode space, there was no sodium at first (at the start of the experiment); it was then filled with sodium by sending an electrical current through the cell and electrochemically decomposing the disodium pentasulfide. In this way, the cell was charged.

(9) Measurement points for temperature and pressure were arranged at different points in the cathode space.

(10) The cell was charged up to 80%, i.e. 80% of the disodium pentasulfide (Na.sub.2S.sub.5) introduced at the start were converted electrochemically to elemental sodium and elemental sulfur.

(11) The solid electrolyte was then destroyed by means of a hydraulic pressure of 80 bar, as described above in example 1, and a less vigorous reaction was observed.

(12) The temperature rise within the cell proceeded gradually over a few minutes and only rose to about 470 C. at a few points. The pressure within the cell rose only by 0.6 bar (abs.) within one minute, and the bursting disk remained intact. The pressure buildup within the cell was still within the range of normal operating pressure of a sodium-sulfur cell.

(13) This experiment showed that the intermediate layer S prevents an uncontrolled and explosive reaction in the destruction of a solid electrolyte of a sodium-sulfur cell, and hence increases the safety of such a cell.