WO2008141865A1

WO2008141865A1 - Electrochemical energy accumulator and method for the operation thereof

Info

Publication number: WO2008141865A1
Application number: PCT/EP2008/054229
Authority: WO
Inventors: Florian Wahl; Martin Holger Koenigsmann
Original assignee: Robert Bosch Gmbh
Priority date: 2007-05-23
Filing date: 2008-04-08
Publication date: 2008-11-27
Also published as: DE102007023896A1

Abstract

The invention relates to an electrochemical energy accumulator (1) having at least one cell (3) comprising at least one cathode (7), one anode (5), and an electrolyte (17) allowing the flow of current from the anode (5) to the cathode (7). The electrochemical energy accumulator (1) further comprises at least two storage tanks (11) for accommodating one electrolyte each or at least one storage container (11) for accommodating at least a component of the electrolyte (17), wherein the storage tanks (11) are provided for accommodating various electrolytes (17) or various components of the electrolytes for different operating states. The invention further relates to a method for the operation of an electrochemical energy accumulator (1), wherein, depending on the operating state, at least one component of the electrolytes (17) is removed from the cell (3), added to the cell (3), or exchanged.

Description

description

title

Electrochemical energy storage and method for its operation

State of the art

The invention relates to an electrochemical energy store according to the preamble of claim 1. Furthermore, the invention relates to a method for operating an electrochemical energy store.

In order to achieve high energy densities, lithium-ion batteries are currently used. These have energy densities of up to approximately 200 Ah / kg as high-energy batteries. As high-performance batteries, energy densities up to 100 Ah / kg are achieved.

The temperature range in which lithium ion accumulators work effectively and safely is limited. This is generally in the range between -10 ⁰ C and 50 ⁰ C. However, this temperature range is not sufficient in particular for applications in electric and hybrid vehicles. A vehicle must be able to operate safely and reliably even at low temperatures, for example after long periods in winter or at high temperatures, for example in the summer after long periods on heated asphalt. For this reason, it is desired to provide accumulators that can be safely and reliably operated in a temperature range of -30 ⁰ C to +80 ⁰ C.

Disclosure of the invention

Advantages of the invention

An inventively designed electrochemical energy storage comprises at least one cell with at least one cathode, an anode and an electrolyte, which allows nen flow of current from the anode to the cathode. The electrochemical energy store further comprises at least two storage chambers for receiving in each case an electrolyte or at least one storage container for receiving a component of an electrolyte, wherein the storage chambers for receiving different electrolytes or various basic constituents of the electrolyte are provided for different operating states.

Since essentially the electrolyte is responsible for the temperature limit within which an electrochemical energy store can be actuated, depending on the formulation of the electrolyte, the electrochemical energy store can each be operated in specific temperature ranges. As soon as the temperature is outside the limits for the respective electrolyte, the performance of the electrochemical energy store drops significantly and, in addition, harmful side reactions can occur which shorten the life of the electrochemical energy store. The inventive solution, in which at least two reservoirs are provided for receiving in each case an electrolyte, it is possible in a simple manner to use the electrolyte as a function of the operating temperature of the electrochemical energy store. Thus, on the one hand, if the reservoirs each contain an electrolyte, first pump the electrolyte contained in the energy store into one of the storage reservoirs and then infest the energy storage device with another electrolyte from a second reservoir. If the reservoir contains only one component of the electrolyte, then this component is added or removed according to the temperature at which the electrochemical energy storage is operated.

The inventively designed electrochemical energy storage can always operate this with the optimal for the current temperature conditions electrolyte mixture. So it is for example possible to operate the electrochemical energy storage within the required by the automotive industry temperature range from -30 ⁰ C to +80 ⁰ C at top speed. Harmful side reactions, which may occur, for example, during operation of the energy store outside the temperatures intended for the electrolyte, are minimized. As a result, the life of the electrochemical energy storage is increased. In addition, safety-critical situations, such as the so-called "thermal runaway" of the battery, which can lead to fire phenomena or explosions, for example, are less likely to occur, meaning that the battery will become more secure due to the temperature-adapted electrolyte composition.

In order to be able to pump the electrolyte from the energy store into a storage container or to convey it from a storage container into the energy store, the storage containers are each preferably connected to the cell via a pump. In order that the electrolyte contained in the reservoir or the components of the electrolyte contained in the reservoir can not flow into the cells, if this is undesirable, the reservoirs are connected in a preferred embodiment via a distribution and closing device to the cells. The distribution and closing device comprises, for example, valves with which the storage containers can be closed with respect to the cell. Alternatively, it is also possible for the distribution and closing device to comprise, for example, selective membranes with which the storage containers can be closed relative to the cell. In particular, when the reservoir contain components of the electrolyte, the distribution and closing device preferably comprises selective membranes with which the reservoir can be closed. The membranes are selected so that they each pass only the component contained in the reservoir. In this way, it is also possible that the component can be recycled from the electrolyte back into the reservoir when the ambient conditions so require. Suitable membranes with which the reservoir can be closed are, for example, piezomembranes.

The distribution and closing device, via which the storage containers are connected to the cell, is preferably connected to a control system. By detecting the temperature can be released by means of the control system of the reservoir containing the suitable ambient conditions for the electrolyte or the component of the electrolyte that must be supplied. In this way, you can always realize the right electrolyte or composition in the cell. Accordingly, the electrolyte contained in the cell can be removed by the control system before supplying an electrolyte suitable for the ambient conditions. Alternatively, it is also possible to remove components that are not needed again from the cell.

In order to enable a change of the electrolyte during the operation of the electrochemical energy storage, for example, when the electrochemical energy storage is used in a motor vehicle, during a ride with high Leistungsausufung, as is the case for example, or after a long service life in winter In which a cryogenic electrolyte must be replaced with a mixture for higher temperatures, the addition, removal or replacement of the at least one constituent of the electrolyte takes place successively for individual cells during operation. The advantage of this approach is that there is no significant reduction in performance, since not all of the electrolyte is pumped out for a short time and then replaced by a new electrolyte. -A-

The operating state of the electrochemical energy store is preferably determined by the ambient temperature and the temperature in the cell of the electrochemical energy store.

In a particularly preferred embodiment, the electrochemical energy store is a lithium-ion accumulator.

Particularly preferred as an electrolyte for use at high temperatures, ie at temperatures in the range of -5 to 80 ⁰ C, is an electrolyte in which LiBOB (lithium bisoxalatoborat) is dissolved in ethylene carbonate and additionally a fire retardant, for example triethyl phosphate, contains.

For use at low temperatures, ie at temperatures in the range of -40 to 10 ⁰ C, the usually used as a solvent ethylene carbonate is the limiting factor. The ethylene carbonate is necessary in particular for the construction of the protective layer on the electrodes. However, if the protective layer is intact, the ethylene carbonate may be substituted for operation at low temperatures. A solvent that can be used at lower temperatures requires a lower melting and boiling point. Suitable solvents are, for example, methyl formate, diethyl carbonate, ethyl acetate, methylburyrate, ethyl butyrate and many esters, for example tetrahydrofuran and some of its derivatives. As a salt for the electrolyte is suitable at lower temperatures, for example, the currently commonly used LiPF. ₆ Another, possibly better suitable conducting salt is also LiBF ₄ .

Preferred as the electrolyte at low temperatures is, for example, LiPF ₆ or LiBF ₄ dissolved in methyl formate or diethyl carbonate. In addition, the electrolyte may also contain a small proportion of ethylene carbonate. The proportion of ethylene carbonate is preferably in the range from 0 to 30% by volume, in particular in the range from 0 to 10% by volume.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

Show it: Figure 1 is a schematic representation of an inventive energy storage in a first embodiment, Figure 2 is a schematic representation of an inventive energy storage in a second embodiment.

Embodiments of the invention

FIG. 1 schematically shows an electrochemical energy store designed according to the invention in a first embodiment.

An electrochemical energy store 1 comprises a plurality of cells 3. Each cell 3 represents a galvanic unit in which electricity is generated by an electrochemical reaction. For this purpose, each cell 3 comprises at least one anode 5 and at least one cathode 7. The anode 5 and the cathode 7 are separated from each other by a separator 9.

Furthermore, each cell 3 contains an electrolyte, which is not shown here. According to the invention, the electrolyte is liquid. In general, the electrolyte comprises a solvent having a high electrical constant to dissolve salts and as low a viscosity as possible to facilitate ion transport, and at least one salt which is dissociated in solution in the solvent.

If the electrochemical energy store is a lithium-ion accumulator, the anode 5 is, for example, an anode customary for lithium-ion accumulators, as is known to the person skilled in the art. A suitable anode 5 contains, for example, a carbon-based intercalation compound, an alloy of lithium with tin and / or silicon, optionally also in a carbon matrix, metallic lithium or lithium titanate. The cathode is a common for lithium-ion batteries cathode, as is known in the art. Suitable materials for the cathode are, for example, lithium cobalt oxide, lithium nickel oxide, lithium cobalt nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron oxide, lithium manganese dioxide; Lithium manganese oxide and mixed oxides of lithium manganese oxide; Lithium iron phosphate, lithium manganese phosphate, lithium cobalt phosphate and lithium nickel phosphate. The preferred cathode material used is lithium cobalt oxide, lithium nickel oxide, lithium cobalt nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium iron phosphate and lithium manganese phosphate. AIs separator 9 is likewise suitable for any separator known to the person skilled in the art, as used in lithium-ion accumulators. The separator 9 is usually a semi-permeable membrane that is permeable to lithium ions.

Suitable materials for the separator are, for example, polypropylene, polyethylene, fluorinated hydrocarbons, ceramic-coated hydrocarbons, fiberglass, cellulose-based materials or mixtures of the aforementioned materials. Preferred materials for the separator are polyethylene and polypropylene.

According to the invention, the electrochemical energy store 1 comprises reservoir 11 for accommodating in each case an electrolyte or constituents of an electrolyte. In the embodiment shown here, the electrochemical energy storage device 1 comprises three reservoirs 11. When the reservoirs 11 are provided to receive an electrolyte, at least two reservoirs 11 are required according to the invention, each of which can accommodate a different electrolyte. If the reservoir 11 are provided for receiving only one or more components of the electrolyte, it may be sufficient if only one reservoir 11 is provided.

The storage containers 11 are each connected to each cell 3 of the electrochemical energy store 1 via a connection which contains at least one closing element. The closing elements are designed so that the connection to each cell 3 can be closed or released separately with the storage container 11, so that the cells 3 can each be filled or emptied independently of each other. The closing elements and the connections from the storage containers 11 to the individual cells 3 are in the embodiment shown here in a distribution and closing device 13th

The reservoir 11 each contain different electrolytes that can be used in different operating conditions of the electrochemical energy storage device 1. Usually, the electrolytes are selected as a function of the operating temperature of the electrochemical energy store 1 and the ambient temperature. Thus it is preferred if one of the reservoir 11 contains an electrolyte which can be used at low temperatures and another reservoir 11 contains an electrolyte which can be used at high temperatures.

Alternatively, it is also possible that the reservoir 11 contains a component of the electrolyte which is added in order to allow safe operation of the electrochemical energy store 1 at certain temperatures, wherein this component is the Elekt- stabilized in the specified temperature range. If the operating conditions take different values, for example, the temperature rises, it is possible to remove this component again from the electrolyte and returned to the reservoir 11. In order to make this possible, it is preferred if the distribution and closing device 13 contains a closing element which comprises a selective membrane which is permeable to the component to be fed into the electrolyte or to be removed from the electrolyte and to the other constituents not the electrolyte.

If the reservoirs 11 each contain an electrolyte for different operating conditions, it is preferred if the distribution and closing device 13 further contains a pump element with which the electrolyte can be pumped out of the cells 3 into an empty reservoir 11 before another electrolyte is supplied to the cells 3.

The limiting factor for the use of the electrolyte at high temperatures is the commonly used conductive salt LiPF ₆ . In order to be able to operate the electrochemical energy store 1 at high temperatures, it is therefore preferable if the conducting salt LiPF _{6 is} replaced by a salt which permits safe operation of the electrochemical energy store 1 even at high temperatures. A suitable conducting salt is, for example, LiBOB (lithium bisoxalatoborate) or LiBF ₄ . However, it is preferred if only one salt is used to avoid mixed salts. In addition to the change of the conductive salt, it is also possible to exchange the solvent. Usually, the solvent used is ethylene carbonate. Since this has a high flash point, the use of ethylene carbonate at high temperatures is not a risk. To increase the reliability of the electrochemical energy store 1 when operating at high temperatures, it is additionally possible to use flame retardants, which due to their sometimes very high boiling point or Melting point at the temperatures at which electrochemical energy storage 1 are commonly used, can not be used. Such flame retardants are, for example, hexamethoxycyclophosphazenes, alkyl phosphates, for example trimethyl phosphate, ethylene ethyl phosphate, methyl nonafluorobutyl ether. By using the flame retardants, the reliability of the electrochemical energy store can be significantly improved, especially when operating at higher temperatures. In addition, some of the flame retardants, for example, alkyl phosphates, for example ethylene ethyl phosphate, improve a protective layer that forms on the electrodes. This also improves the aging stability of the electrochemical energy store 1. Particularly preferred as an electrolyte for use at high temperatures, ie at temperatures in the range of 50 to 90 ⁰ C is an electrolyte in which LiBOB is dissolved in ethylene carbonate, and additionally contains a fire retardant, for example trimethyl phosphate.

For use at low temperatures, ie at temperatures in the range of -40 to 0 ⁰ C, the ethylene carbonate used as solvent is the limiting factor. The ethylene carbonate is necessary in particular for the construction of the protective layer on the electrodes. However, if the protective layer is intact, the ethylene carbonate may be substituted for operation at low temperatures. A solvent that can be used at lower temperatures requires a low melting and boiling point. Suitable solvents are, for example, methyl formate, diethyl carbonate, ethyl acetate, methyl butyrate, ethyl butyrate and many esters, for example tetrahydrofuran and some of its derivatives. At low temperatures, for example, the generally used LiPF ₆ is suitable as a salt for the electrolyte. Another, possibly better suitable conducting salt is also LiBF ₄ .

In order to control the exchange of the electrolyte or the supply or removal of components of the electrolyte, the distribution and closing device 13 is preferably connected to a control unit 15. The control unit 15 controls the opening or closing of the connections from the storage container 11 into the individual cells 3. For this purpose, the ambient temperature and the operating temperature of the electrochemical energy store 1 are monitored in the control unit 15, for example. Depending on the ambient temperature and the operating temperature of the electrochemical energy storage device 1, it is selected with which electrolyte the cells 3 are operated. If a wrong electrolyte is contained in the cells, this is first pumped out of the cells 3 in the reservoir 11. Subsequently, the correct electrolyte for the corresponding operating conditions is pumped from a reservoir 11 into the cells 3.

If only components of the electrolyte are contained in the reservoir 11, depending on the operating temperature, either components are transported from the reservoir 11 into the cells 3 or back from the cell 3 into the reservoir 11. In order to enable a change of the electrolyte in the individual cells 3, even during ongoing operation, it is preferred if the electrolyte is exchanged from the cells 3 successively cell by cell. That is, first of all, the electrolyte is removed from a first cell 3 into a storage container 11 and another electrolyte is supplied from another storage container 11 into the cell 3. Once this process is completed, the electrolyte in a second cell is replaced in the same way. Of course, it is also possible to remove the electrolyte from one cell 3, while another electrolyte is simultaneously fed into another cell 3. Furthermore, it is also possible to exchange the electrolyte in all cells 3 at the same time.

2 shows an inventively designed electrochemical energy storage device 1 is shown in a second embodiment.

The electrochemical energy store 1 shown in FIG. 2 differs from that of FIG. 1 in that the storage containers 11 are not arranged on one side, but on different sides of the electrochemical energy store 1. If an operation of the electrochemical energy store 1 is provided with only two different electrolytes is, two reservoirs 11 are sufficient. It is necessary that each reservoir 11 has a volume which is sufficient to receive the entire electrolyte 17 from the electrochemical energy storage device 1. It is either possible that each cell 3 of the electrochemical energy storage device 1 has its own reservoir 11 for the electrolyte 17 or there are provided for several cells 3 common reservoir 11. In particular, it is preferred if the entire electrochemical energy storage device 1 has only one storage reservoir 11 for each electrolyte, which feeds all the cells 3.

In order to exchange the electrolyte 17, the electrolyte 17 is first pumped out of the cells 3 into an empty storage container 11. This takes place via the distribution and closing device 13, with which the storage container 11 can be closed relative to the cells 3. In addition, a pump can be accommodated in the distribution and closing device 13 with which the electrolyte 17 can be pumped out of the cells 3 into the storage container 11 or pumped out of the storage containers 11 into the cells 3. Once the cell 3 is emptied, i. is free of electrolyte is transported from a full reservoir 11, another electrolyte via the distribution and closing device 13 in the cell 3. As soon as the anode 5 and the cathode 7 are in contact with the electrolyte 17, a flow of current is possible. However, optimum operation is possible only when the anode 5 and the cathode 7 are completely covered by electrolytes 17.

Claims

claims

1. An electrochemical energy store, comprising at least one cell (3) with at least one cathode (7), an anode (5) and an electrolyte (17), which allows a current flow from the anode (5) to the cathode (7), characterized in that the electrochemical energy store 1 furthermore comprises at least two storage containers (11) for receiving in each case an electrolyte (17) or at least one storage container (11) for receiving at least one component of an electrolyte, the storage containers (11) receiving different electrolytes (17). or different components of the electrolyte are provided for different operating conditions.

2. Electrochemical energy storage device according to claim 1, characterized in that the storage container (11) are each connected via a pump to the cell (3).

3. Electrochemical energy storage according to claim 1 or 2, characterized in that the storage container (11) via a distribution and closing device (13) with the cells (3) are connected.

4. Electrochemical energy store according to claim 3, characterized in that the distribution and closing device (13) contains valves with which the cells (3) relative to the storage containers (11) are closable.

5. Electrochemical energy store according to claim 3, characterized in that the distribution and closing device (13) contains selective membranes, with which the

Reservoir (11) relative to the cell (3) are closable.

6. Electrochemical energy store according to one of claims 1 to 5, characterized in that the electrochemical energy store (1) further comprises a control unit (15), which controls an exchange of the electrolyte (17) or of components of the electrolyte.

7. Electrochemical energy store according to one of claims 1 to 6, characterized in that the electrochemical energy store (1) is a lithium-ion accumulator.

8. A method for operating an electrochemical energy store (1) according to one of claims 1 to 7, wherein depending on the operating state, at least one component of Electrolyte (17) is removed from the cell (3) is added to the cell (3) or replaced.

9. The method according to claim 8, characterized in that in an electrochemical see energy storage (1) with a plurality of cells (3) adding, removing or replacing the at least one component of the electrolyte (17) during operation successively for each individual cells ( 3) is performed.

10. The method according to claim 8 or 9, characterized in that the operating state by ambient temperature and temperature in the cell (3) of the electrochemical energy store (1) is determined.