Field of the Invention

The present invention relates to an electric energy storage system, particularly to a cost-effective system for rechargeable batteries consisting of solid components and working at ambient temperatures.

Background of the Invention

The modern, liberalized electric power grid in developed countries comprises many independent players, both consumers and suppliers. The electricity consumption substantially changes during the day and during the year, but also the supply fluctuates, often in an unpredictable way, particularly from the broadly supported and often subsidized renewable sources, comprising random factors such as weather, the most important among the sources nowadays being solar photovoltaic and wind farms. It is known that predictability of the solar and wind farm output is very low. The electricity production needs to be scaled up and down by enormous increments in enormously short periods to meet the momentary jumps in consumption and supply, keeping the grid in equilibrium, which is sometimes hardly achievable with classic fossil turbines, necessitating to disconnect some of the consumers or some of the suppliers, resulting in damages. A strong need is therefore felt for energy storage systems which would allow to store electrical energy during the excessive supply and to release it during the excessive demand.

Pumped hydroelectric storage is nowadays the largest-capacity form of grid energy storage, but the geographical conditions enabling this form are not generally available, and even where available - the ecological concerns often hinder its full use. Electrochemical energy storage has, therefore, become an important candidate, and much developmental efforts have been invested into rechargeable batteries. Lithium batteries have been generally used, mostly as lithium-ion batteries; later lithium-sulfur batteries appeared, exhibiting high energy density. The expensive lithium is replaced by sodium in sodium-sulfur battery, which usually employ liquid sodium and liquid sulfur. This type of battery has a high energy density, high efficiency of charge/discharge, and a long cycle life, and it is made from relatively inexpensive materials. As many as 1,500 charge and discharge cycles may be attained with sodium- sulfur battery, while providing an output in a range of megawatts for several hours. The main disadvantage of the sulfur-sodium battery is a high operating temperature of up to 350°C, and potential hazard represented by the liquid sodium which can explode in contact with air and moisture. It is therefore an object of the invention to provide an electrochemical storage system employing the advantages of the sodium- sulfur battery, but lacking the drawbacks of the known techniques.

It is another object of this invention to provide an electrochemical system for rechargeable battery employing sulfur and sodium sulfide and working at ambient temperature.

It is still another object of this invention to provide an electrochemical system for rechargeable battery working at ambient temperature and being cost-effective.

It is a further object of this invention to provide an electrochemical system for rechargeable battery, exhibiting a high energy density and being capable of at least 1,500 charge and discharge cycles, such as at least 2,000.

It is a still further object of this invention to provide a rechargeable battery comprising sulfur and sodium sulfide, working at ambient temperature and exhibiting suitable charge/discharge parameters. This invention also aims at providing a rechargeable battery essentially comprising solid components and working at ambient temperature.

Other objects and advantages of present invention will appear as description proceeds.

Summary of the Invention

The present invention provides a rechargeable battery comprising i) cathode comprising iron or carbon in contact with a solid cathode mixture; ii) a solid cathode mixture comprising iron sulfide, carbon, and sulfur; iii) anode in contact with a solid anode mixture; and iv) a solid anode mixture adjacent to said solid cathode mixture, either in contact with said cathode mixture or separated from it by a porous film separator, the anode mixture comprising sodium polysulfide, aluminum oxide, and sodium chloride; the battery essentially consisting of solid phases and working at a temperature lower than 80°C. Said anode comprises a metal selected from the group consisting of aluminum, magnesium, titanium, niobium, tantalum, and a mixture thereof. In one embodiment, anode contains aluminum. Cathode may comprise carbon, iron, iron alloys, iron with carbon admixtures - usually up to 1.8%, and steels. Said carbon in the cathode or in the cathode mixture may be graphite. In a preferred embodiment of the invention, said anode mixture comprises a mixture of sulfur and sodium sulfide having a formula Na ₂ S _n , wherein n is a natural number; particularly, the mixture comprises sodium poly sulfides having a formula Na ₂ S _n wherein n is equal to or greater than 3. Said separator is selected, for example,from polymeric film, thin fibrous layer, cloth, or paper, and is impregnated with sodium polysulfide. In one embodiment of the invention, said sodium chloride may be at least partially replaced with potassium chloride; in another embodiment, said aluminum oxide may be at least partially replaced with magnesium oxide. The rechargeable battery of the invention preferably comprises only solid components and operates at ambient temperature. The battery of the invention preferably exhibits a lifetime of at least 1500 cycles or more, such as at least 2,000 or at least 2,500 or at least 3,000 or at least 4,000 or at least 5,000 cycles. The battery in a preferred embodiment may exhibit a lifetime of at least 6000 or at least 7000 or at least 8000 or at least 9000 or at least 10,000 cycles. When describing, in the present text, a battery operating at ambient temperature, the intention is that the outer temperature outside the battery is ambient; of course, the internal temperature inside the battery may somewhat increase; said ambient temperature is the temperature in which the user of the battery lives, usually between 0 and 40°C, more usually between 15 and 30°C.

The rechargeable battery of the invention preferably comprises only solid phases constituted by components including iron sulfide, carbon, sulfur, sodium polysulfide, aluminum oxide, and sodium chloride, wherein sulfur is reduced and polysulfide formed during the discharging cycle, the battery performing at ambient temperature safely during at least 10,000 discharging/charging cycles. Said cathode mixture preferably comprises sulfur and iron sulfide in an approximate molar ratio of 1:2. Said sulfur and iron sulfide constitute between 75 and 99% of the cathode mixture. Said cathode mixture preferably comprises carbon constituting from 1 to 25% of the cathode mixture. Said cathode mixture may comprise up to 1% polyethylene as a binder. The rechargeable battery in one embodiment of the invention comprises an anode mixture in which sulfur and sodium polysulfide constitute between 30 and 85%, preferably between 50 and 70%; aluminum and/or magnesium oxides constitutes between 25 and 50%, preferably between 30 and 45% of the anode mixture; and sodium and/or potassium chloride constitutes from 1 to 15%, preferably between 3 and 9% of the anode mixture. The invention provides a method of preparing a cost effective and safe rechargeable battery working at ambient temperature, comprising steps of i) preparing a cathode mixture comprising sulfur, iron sulfide, carbon, optionally PE, and a solvent, wherein said sulfur and iron sulfide are present in an approximate molar ratio of 1:2, said sulfur and iron sulfide constituting between 75 and 99% of the dry cathode mixture, said carbon constituting from 1 to 25% of the dry cathode mixture, and said PE constituting up to 1% of the dry cathode mixture, said solvent constituting from 10 to 20% of the dry cathode mixture; ii) preparing an anode mixture comprising sulfur, sodium sulfides, alkaline metal chloride, metal oxide, and a solvent, said sulfur and sodium sulfides constituting between 30 and 85%, preferably between 50 and 70% of the dry anode mixture; in a preferred embodiment, the anode mixture comprises sulfur S and sodium sulfide Na ₂ S in a molar ratio of 2:1; said metal oxide, preferably aluminum and/or magnesium oxide, constituting between 25 and 50%, preferably between 30 and 45% of the dry anode mixture, and said metal chloride, preferably sodium and/or potassium chloride, constituting from 1 to 15%, preferably between 3 and 9% of the dry anode mixture, said solvent constituting from 10 to 20% of the dry cathode mixture; iii) placing said cathode mixture into a steel casing; iv) placing said anode mixture adjacent to said cathode mixture in said steel casing and electrically conductively contacting said cathode mixture but not said steel casing; v) removing said solvents from said mixtures, wherein said step of removing the solvents may be performed separately for the cathode mixture and the anode mixture anywhere after steps i) and iv); vii) covering said anode mixture with an aluminum closure; viii) compressing said cathodic and anodic mixtures between said steel casing and said aluminum cover in a pressure-controlling manner so that the predetermined voltage is produced between said steel casing and said aluminum cover; thereby obtaining a cost- effective rechargeable battery, consisting only of solid phases and working safely at ambient temperature; in one embodiment of the invention, the rechargeable battery works at least 2,000 discharging/recharging cycles, such as at least 5,000 cycles. In a preferred embodiment of the invention, said step of placing the anode mixture adjacent to the cathode mixture comprises placing between the cathode mixture and anode mixture a porous separator impregnated with sodium polysulfide in a solvent and dried, the film being permeable for all present ionic species. In a preferred embodiment of the invention, provided is a method of preparing a cost effective, solid state, and safe rechargeable battery working at ambient temperature, comprising a step of homogenizing a mixture comprising sulfur and disodium sulfide in solvent, thereby forming or enabling the formation, in the anode mixture, of sodium polysulfide in situ.

The invention is directed to an cost-effective and safe electrochemical system, which can be up- scaled as needed, for energy storage and for work at ambient temperature, comprising a first dry powder and a second dry powder, the first powder comprising iron sulfide, carbon, and sulfur, and the second powder comprising sodium polysulfide, aluminum oxide, and sodium chloride, wherein sulfur is reduced and polysulfide formed during the discharging cycle.

Brief Description of the Drawings

The above and other characteristics and advantages of the invention will be more readily apparent through the following examples, and with reference to the appended drawings, wherein:

Fig. 1. is a schematic representation of a cathodic system and an anodic system according to one embodiment of the invention; 1 is cathode system consisting of cathode comprising iron or carbon 2 and cathode mixture 3 comprising FeS, carbon, and sulfur; 4 is a separator made of a fabric layer impregnated with sodium polysulfide; 5 with 6 is anode system consisting of aluminum anode 6 and anode mixture 5 comprising Na ₂ S _n , NaCl, and A1 ₂ 0 ₃ ; Fig. 2. shows current profile of a rechargeable battery according to the invention during charging;

Fig. 3. shows current profile of a rechargeable battery according to the invention during discharging at a load of 55 Ohm; and

Fig. 4. is a scheme of an experimental battery comprising anode and cathode systems according to the invention; 1 is a carbon steel housing, serving as cathode, covered by an isolating material, 2 is isolating material such as glass, 3 is cathode mixture, 4 is separator, 5 is anode mixture, 6 is aluminum anode; during the preparation of the experimental battery, members from 2 to 6 are placed inside housing 1 and covered by gasket 7 and cover 8, wherein bolts 9 press the housing and the cover together; bolts 10 serve as anode plug and for regulating the compression of the anode and cathode mixtures for achieving optimal current collection.

Detailed Description of the Invention

The invention relates to electrochemistry field, in particular to the electrochemical accumulator device, such as secondary cell (rechargeable battery). It has now been surprisingly found that it is possible to employ the components of sodium-sulfur battery in an solid state secondary cell working essentially at ambient temperature. The new battery consists of a cathode mixture being constituted by a solid mix of iron sulfide, sulfur, and graphite as volumetric current collector, and an anode mixture being constituted by solid mix of aluminum oxide, sodium chloride, and sodium polysulfide, said polysulfide preferably prepared in situ from sulfur and disodium sulfide. Said solid mix may comprise a powder comprising, for example, particles having a size of less than 100 micron, or it may comprise a pressed powder or a powder compressed till the state of a solid block; solvents may be added in order to wet the components, followed by drying, in order to improve the process of preparation and pressing. Resulting cell is a technologically simple and cheap rechargeable battery with long lifetime, without need to operate it at temperatures of 300 to 350°C as published sodium-sulfur batteries. Moreover, the tests of batteries according to the invention have not shown any degradation in terms of performance or any corrosion on electrodes. The performance parameters of the battery during the tests provided a lifetime assessment of more than 5000 discharge/recharge cycles, which is more than usually observed lifetime of up to 4000 cycles. Both the materials and the process ensure low cost of the new battery. Exemplified secondary batteries were characterized, among others, by charge/discharge performance. The electromotive force of the battery according to some embodiments is 1.77V, and nominal voltage 1.2V.

The invention relates to a new rechargeable battery, which comprises only solid components, operates at room temperature, is cost-effective, can be up- scaled, and will allow a lifetime of more than 5000 cycles, and possibly even more than 10,000 cycles.

The energy storage system of the invention comprises a cathode system and an anode system; the cathode system consists of cathode comprising iron or carbon, and cathode mixture comprising iron sulfide, carbon and sulfur; the anode system consists of aluminum anode, and anode mixture comprising sodium sulfides, aluminum oxide, and sodium chloride. In a preferred embodiment, the energy storage system, such as a secondary battery of the invention may be represented by the following formula:

[cathode/(FeS, C, S)]/[Na ₂ S _n , NaCl, Al ₂ 0 ₃ /anode], in which brackets separate anode and cathode systems, slash separates phases, and wherein FeS stands for iron sulfide, C represents graphite of other carbon powder form, S represents elemental sulfur, and n is a natural number equal to or greater than 3. In one embodiment of the invention, NaCl (sodium chloride) may be at least partially replaced with KC1 (potassium chloride) and A1 ₂ 0 ₃ (aluminum oxide) may be at least partially replaced with MgO (magnesium oxide), a battery being represented by the following formula:

[cathode/(FeS, C, S)]/[Na ₂ S _n , NaCl, KC1, A1 ₂ 0 ₃ , MgO/anode].

In a preferred embodiment, the energy storage system of the invention comprises a cathode system and an anode system; the cathode system consists of cathode comprising iron or carbon, and cathode mixture comprising iron sulfide, carbon and sulfur; the anode system consists of anode comprising a metal selected from aluminum, magnesium, titanium, niobium, tantalum, and a mixture thereof, and anode mixture comprises sodium polysulfides, aluminum oxide, and sodium chloride, wherein said sodium polysulfides are prepared in situ from Na ₂ S and S. In a preferred embodiment, the energy storage system, such as a secondary battery of the invention may be represented by the following formula:

[cathode/(FeS, C, S)]/[Na ₂ S _n , NaCl, Al ₂ 0 ₃ /anode], wherein S represents elemental sulfur, and Na ₂ S _n represents a mixture of Na ₂ S _n in which n is a natural number equal to or greater than 3. In one embodiment of the invention, NaCl may be at least partially replaced with KC1 and A1 ₂ 0 ₃ with MgO.

In one aspect of the invention, provided is an energy storage system comprising a cathode system and an anode system; the cathode system consists of cathode comprising iron or carbon, and cathode mixture comprising iron sulfide, carbon and sulfur; the anode system consists of anode comprising aluminum, magnesium, titanium, niobium, tantalum, or a mixture thereof, and anode mixture comprises sodium polysulfides, aluminum oxide, and sodium chloride, wherein said cathode mixture and said anode mixture are separated by a film or a thin layer impregnated with polysulfide, the layer possibly being a thin polymeric layer, a fibrous layer or a fabric preventing the cathode and anode mixtures from mixing during the cell preparation. The separator is easily penetrable for any ionic species. In a preferred embodiment, the energy storage system, such as a secondary battery of the invention may be represented by the following formula:

[cathode/(FeS, C, S)]/film/[ Na ₂ S _n , NaCl, Al ₂ 0 ₃ /anode], wherein Na ₂ S _n represents a mixture of various Na ₂ S _n in which n is a natural number equal to or greater than 3.

The cathode mixture according to the invention comprises sulfur (S), iron sulfide (FeS), and a conductive and electrolytically inert material such as carbon, wherein said S and FeS are preferably present in a molar ratio of about 1:2, for example in a weight ratio of between 1:6 and 1:5. Said S with said FeS preferably constitute between 75 and 99% of the cathode mixture, and said conductive material constitutes from 1 to 25% of the cathode mixture. In one embodiment, the cathode mixture comprises up to 1% polyethylene (PE) as binder.

The anode mixture according to the invention preferably comprises sodium polysulfides, alkaline metal chloride, and metal oxide. Sodium polysulfides constitute between 30 and 85% of the dry anode mixture, preferably between 50 and 70%; said metal oxide, preferably aluminum and/or magnesium oxide, constitute between 25 and 50%, preferably between 30 and 45%, and said metal chloride, preferably sodium and/or potassium chloride, from 1 to 15%, preferably between 3 and 9%. In a preferred embodiment of the invention, said polysulfides are provided as Na ₂ S ₃ . In one embodiment, said sodium polysulfides comprise disodium trisulfide, disodium tetrasulfide, and disodium pentasulfide.

The anode mixture and the cathode mixture in an electrochemical storage system according to the invention are essentially solid materials working at a temperature not higher than 80°C. The secondary cell of the invention, ready to work, comprises essentially dry components. The invention is directed to a method of preparing a secondary cell, comprising preparing a raw cathodic mixture, a raw separator, and a raw anodic mixture, wherein said raw components comprise solvents assisting in the preparation of homogeneous phases, most of said solvents being removed after incasing cathodic mixture, anodic mixture, and separator, and compressing them to a ready-to-work configuration. Solvents employed in the method of the invention may comprise acetone, tetrahydrofuran, toluene, tetrachloromethane, xylene, white spirit, and other. In one embodiment, the cathode mixture may be prepared while employing a solvent. In another embodiment, the cathode mixture is prepared by employing a binder, such as PE. When a binder is used, for example PE is added to the mixture in form of powder, followed by heating at 100-110°C and compressed with a pressure of about 50 bar.

The separator may be any non-conductive material that can be impregnated by sodium polysulfide, the material including paper, cotton cloth or other types of cloth or fabric or thin fibrous layer, or a polymeric layer comprising nylon, polyethylene terephthalate, polytetrafluoroethylene, and other materials. The main role of the separator is preventing any mixing between the cathodic and anodic mixtures during the preparation of the secondary cell of the invention, as the mixtures during the cell preparation are wet and may be liquid or paste-like.

In one aspect of the invention, provided is a method of preparing a cost effective secondary cell having a high energy density, consisting of solid components and working at low temperatures, possibly at ambient temperature, the method comprising steps of: i) preparing a cathode mixture comprising S, FeS, C, optionally PE, and a solvent, wherein said S and FeS are present in an approximately 1:2 molar ratio, for example in a weight ratio of between 1:6 and 1:5, and said S with said FeS preferably constituting between 75 and 99% of the dry cathode mixture, said C constituting from 1 to 25% of the dry cathode mixture, and said PE constituting up to 1% of the dry cathode mixture, said solvent constituting from 10 to 20% of the dry cathode mixture; ii) preparing an anode mixture comprising sulfur, sodium sulfides, alkaline metal chloride, metal oxide, and a solvent, said sulfur and sodium sulfides constituting between 30 and 85%, preferably between 50 and 70% of the dry anode mixture; said metal oxide, preferably aluminum and/or magnesium oxide, constituting between 25 and 50%, preferably between 30 and 45% of the dry anode mixture, and said metal chloride, preferably sodium and/or potassium chloride, constituting from 1 to 15%, preferably between 3 and 9% of the dry anode mixture, said solvent constituting from 10 to 20% of the dry cathode mixture; iii) placing said cathode mixture into a steel casing; iv) placing said anode mixture adjacent to said cathode mixture in said steel casing and electrically conductively contacting said cathode mixture but not said steel casing; v) removing said solvents from said mixtures; vi) covering said anode mixture with an aluminum closure; vii) compressing said cathodic and anodic mixtures between said steel casing and said aluminum cover in a pressure-controlling manner so that the predetermined voltage is produced between said steel casing and said aluminum cover; wherein said step v) of removing the solvents may be performed separately for the cathode mixture and the anode mixture anywhere after steps i) and iv). An example of solvent for the cathode mixture is acetone, which wets the components in a desired way. An example of solvent for the anode mixture is tetrahydrofuran; it dissolves slightly Na ₂ S and very slightly S, and it enables their reaction resulting in Na ₂ S ₃ , taking about 15 minutes.

In one preferred embodiment, said step of placing the anode mixture adjacent to the cathode mixture comprises placing between the cathode mixture and anode mixture a porous separator impregnated with sodium polysulfide in a solvent and dried, the film being permeable for all present ionic species; said solvent may comprise, for example, tetrahydrofuran, toluene, or xylene. Removing solvent from the film may be performed similarly as removing the solvents from the cathode and anode mixtures in said step v).

The invention is directed to an energy storage system comprising cathode system and anode system represented, in a preferred embodiment, by the following formula:

[cathode/(FeS, C, S)]/film/[Na ₂ S _n , NaCl-KCl, Al ₂ 0 ₃ -MgO/anode], wherein cathode comprises carbon or iron, C corresponds to conductive inert material such as carbon powder, possibly comprising graphite or other carbon species, Na ₂ S _n corresponds to a mixture of Na ₂ S _n species wherein n is a natural number equal to or greater than 3, Al ₂ 0 ₃ -MgO stands for aluminium oxide or magnesium oxide or a mixture thereof, NaCl-KCl stands for sodium chloride or potassium chloride or a mixture thereof, and anode comprises a metal selected from aluminum, magnesium, titanium, niobium, tantalum, or a mixture thereof. Said film separator is preferably impregnated with a solution containing Na ₂ S _n and a solvent such as tetrahydrofuran, and it is dried before closing the cathode mixture in a closed cell.

The terms cathode and anode are employed for the electrodes through which electrons enter and leave, respectively, the secondary cell during discharging (=working) cycle.

The invention will be further described and illustrated in the following examples.

Examples

Laboratory experimental battery

Purpose of the laboratory experimental battery was to check influence of the compositions of the cathode and anode systems on the secondary cell characteristics. The experimental battery is schematically depicted in Fig. 1. It consisted of carbon steel housing (1), the internal surfaces of which were covered by isolation material such as glass (2). The housing served as cathode, and accommodated cathode mixture (3), separator (4), anode mixture (5), and aluminium cover (6) which served as anode. Isolating gasket (7) and strong cover (8) were pressed to the housing by bolts (9). Regulating bolts (10) served for compressing the cathode and anode mixtures to allow good current collection, one of the bolts served as an anode plug. Several of employed cathode compositions, anode compositions, and separators are presented below.

Cathode mixture 1

FeS - 44 g

S - 8 g

C - 12 g

acetone - 10 g

Anode mixture 1

Na ₂ S - 23 g

S - 18 g

AIA - 25 g

NaCl - 2 g

Tetrahydrofuran - 8 g

Anode mixture 2

Na ₂ S - 23 g

S - 18 g

AIA - 25 g

KC1 - 5 g

tetrahydrofuran 8 g

Anode mixture 3

Na ₂ S - 23 g S - 18 g

MgO - 15 g

NaCl - 2 g

Tetrahydrofuran - 8 g

Separators 1 to 3

paper

cotton cloth

Nylon

Assembly steps

Commercially available materials were employed. Metal housing was washed with water and acetone and dried. Cathode mixture, including the solvent was placed onto the bottom of the housing, and the solvent was removed from it by hot air in a sealed container. Separator, paper sheet for example, is placed on cathode system. Anode mixture, including solvent, sodium polysulfide, sodium chloride, and suspension of alumina, was added onto the dry cathode mixture, and was dried in said sealed container. Both cathodic and anodic mixtures were obtained by mechanical stirring with solvents till receiving homogenous suspensions; after drying, the mixtures had the form of solid block.

The gasket and aluminum cover were assembled and the bolts were carefully tightened gradually with tightening torque of 2 Nm, 4 Nm, and 6 Nm. Initial short circuit current was measured, and a positive value indicated correct assembly. Examples of the experimental battery discharge and charge are presented in Fig. 2 and 3, respectively, the battery corresponding to a rechargeable battery according to one embodiment of the invention, combining cathode mixture 1 and anode mixture 1 described above. The tests of batteries according to the invention have not shown any degradation in terms of performance or any corrosion on electrodes; the low current density and lower heat evolution further contributes to lowered destruction processes. Also, it is believed that there are no secondary processes/reactions at the actual working temperatures, which would pollute the cathode and anode mixtures during the discharge or charge cycle. The performance parameters of the battery during the tests provided a lifetime assessment of more than 5000 discharge/recharge cycles. While this invention has been described in terms of some specific examples, many modifications and variations are possible. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.