Lithium-ion cell

Abstract

Lithium-ion cell, comprehensive - two opposing working electrodes (12, 14) of different polarity, between which in an electrolyte space (16) a separator (18) is arranged which electronically insulates the working electrodes (12, 14) from each other and is permeable to lithium ions, and - a lithium-containing reservoir electrode (182) which is in electronically insulating, lithium-ion-exchanging contact with the electrolyte space (16), wherein a voltage between the reservoir electrode (182) and the working electrode (12, 14) can be measured and a voltage between the reservoir electrode (182) and the working electrode (12, 14) can be applied by means of a measuring and control circuit (22) connecting the reservoir electrode (182) with at least one of the working electrodes (12, 14). and wherein the reservoir electrode (182), comprising an electrically conductive polymer material (183) onto which a lithium-containing deposit material (184) is deposited, is porous and is arranged between two electronically insulating and lithium-ion permeable insulating layers (181) of the separator (18), characterized by that the application material (184) contains lithium iron phosphate LiFePO4.

Description

Field of invention

[0001] The invention relates to a lithium-ion cell comprising - two opposing working electrodes of different polarity, between which a separator that electronically insulates the working electrodes from each other and is permeable to lithium ions is arranged in an electrolyte space, and - a lithium-containing reservoir electrode that is in electronically insulating, lithium-ion-exchanging contact with the electrolyte space, wherein a voltage between the reservoir electrode and the working electrode can be measured and a voltage between the reservoir electrode and the working electrode can be applied by means of a measuring and control circuit connecting the reservoir electrode with at least one of the working electrodes and wherein the reservoir electrode, comprising an electrically conductive polymer material onto which a lithium-containing deposition material is applied, is porous and is arranged between two electronically insulating and lithium-ion permeable insulating layers of the separator. State of the art

[0002] Such lithium-ion cells are known from WO 2015 / 074 065 A1

[0003] Lithium-ion cells are well-known as modern, high-performance energy storage devices for electronic devices and for vehicles with purely electric or hybrid drives. The advantages of lithium-ion cells, whose operating principle is based on the migration of lithium ions between the two working electrodes in an electrolyte that does not itself participate in the electrochemical reactions at the working electrodes, lie primarily in their high energy density and their ability to withstand a very high number of charge and discharge cycles. The typical structure of a lithium-ion cell includes two working electrodes that are capable of binding or intercalating lithium ions.To prevent an electrical short circuit between the working electrodes, a separator is arranged in the electrolyte-filled space between them. This separator provides electrical insulation while allowing lithium ions to pass through. The passage of such high-density ion currents is necessary to enable a correspondingly high battery current. Typically, the separator consists of one or more layers of a porous, electrically insulating polymer material, such as polyethylene or polypropylene, or a mixture thereof. The porosity is designed to minimize the restriction of lithium ion migration.

[0004] It has been shown that lithium-ion cells suffer a significant loss of capacity over their lifetime, with this effect being more pronounced in the early stages of the cell's lifespan than in later stages. The main cause of this capacity loss is the formation of a lithium-containing intermediate layer between the negative electrode and the electrolyte, also known to those skilled in the art as the SEI (Solid Electrolyte Interface). This intermediate layer stores lithium ions that are then no longer available for the electrochemical process. Furthermore, various parasitic reactions are known to consume lithium, which is then no longer available for cell operation.

[0005] From US patent 7,726,975 B2, it is known to couple a reservoir electrode to the electrolyte chamber perpendicular to the two working electrodes and the separator via its own separator. This reservoir electrode fulfills two functions. First, it can be used as a reference electrode, the voltage difference of which relative to the working electrodes can be measured by means of a measuring and control circuit. From this, the person skilled in the art can draw conclusions about the state of charge of the cell, in particular about the current and potential binding or intercalation capacity for lithium ions at the working electrodes. This makes it possible, in particular, to determine whether and to what extent lithium originally present in the cell has been removed from the electrochemical process, which can be attributed in particular to the effects explained above.By applying a suitable voltage between the reservoir electrode and a working electrode, an electronic current can be generated as a countermeasure, flowing from the reservoir electrode via the measuring and control circuitry to the working electrode. This results in an ionic current flowing from the reservoir electrode, through its separator, to the working electrode. In other words, lithium ions are introduced from the reservoir electrode into the electrolyte space, where they are available for further electrochemical reactions and can replace the lithium bound in the SEI or consumed by parasitic reactions. The lifespan of the lithium-ion cell is thus significantly extended.

[0006] A disadvantage of this well-known approach is the unfortunate spatial arrangement of the reservoir electrode relative to the working electrodes, which complicates the compact construction of lithium-ion cells in common formats. In particular, designing a lithium-ion cell in the usual stacked or spiral arrangement would require the reservoir electrode, positioned perpendicular to the working electrodes, to be very small, resulting in a correspondingly low storage capacity for reservoir lithium.

[0007] To solve this problem, the aforementioned generic WO 2015 / 074065 A1 proposes designing the reservoir or reference electrode as a porous, electrically conductive, and ion-permeable intermediate layer of the separator. Specifically, it suggests using an electrically conductive, microporous polymer membrane embedded with metallic lithium. However, metallic lithium can only be processed under a protective atmosphere, making the production of such reservoir electrodes complex and expensive.

[0008] DE 10 2013 224 294 A1 also discloses a separator with a conductive, porous insert used to detect dendrites growing from an electrode and penetrating the separator. Upon contact with the conductive layer, a short circuit occurs between it and the electrode, which can be detected by monitoring the voltage between the two. Furthermore, the conductive insert is coated with lithium-containing material similar to the actual lithium electrode, enabling it to serve as a reference electrode that does not participate in the actual electrochemical process. Task

[0009] The object of the present invention is to enable the cheaper production of a generic lithium-ion cell. Description of the invention

[0010] This problem is solved in conjunction with the features of the preamble of claim 1 by the fact that the application material comprises lithium iron phosphate LiFePO4.

[0011] Preferred embodiments of the invention are the subject of the dependent claims.

[0012] As is generally known from the prior art and also provided for in the context of the present invention, the reservoir electrode is integrated into the separator between the working electrodes. In other words, the separator between the working electrodes is functionalized as a reservoir electrode on the one hand and a reference electrode on the other.

[0013] As explained above, the performance of a lithium-ion cell depends crucially on the ion current flowing as freely as possible between the working electrodes. This is readily achieved with separators made of porous insulating layers. This also applies to separators consisting of several such insulating layers, between which a porous reservoir layer, likewise unimpeded by the ion current, is embedded. This reservoir layer does not contribute to the primary function of the separator, namely the ionically permeable and electronically insulating separation of the working electrodes. This is also unnecessary, as this task is performed by the insulating layers. The reservoir layer merely provides lithium and must not further impede the ion current, which is ensured by its (sufficiently high) porosity.The reservoir electrode thus has approximately the same surface area available as each working electrode, allowing a considerable amount of reservoir lithium to be stored here. This reservoir lithium can be replenished over the cell's lifetime in a manner known in principle to replace any lithium lost. Accordingly, the overall lifetime of the lithium-ion cell according to the invention is also extended compared to the prior art.

[0014] Naturally, it is necessary that the reservoir electrode as a whole be electrically conductive to enable a functional connection to the measuring and control circuit. As is generally known from the prior art and also provided for in the context of the present invention, the reservoir electrode comprises an electrically conductive polymer material onto which a lithium-containing deposition material is applied. Suitable electrically conductive polymer materials include, for example, polyaniline, polypyrrole, or polythiophene, which are preferably used individually or in mixtures. According to the invention, lithium iron phosphate (LiFePO4) is used as the lithium-containing deposition material. This material can be provided, in particular, in the form of nanoparticles with which the conductive polymer layer can be coated or which can be embedded in the conductive polymer layer.LiFePO4 is particularly interesting for use in the present invention because of its ability to provide a constant voltage over a wide operating range (lithium concentration range). However, a disadvantage of LiFePO4 is its comparatively low energy density. In this respect, classic lithium metal oxides, such as NMC (lithium nickel manganese cobalt oxide), would be preferable due to their higher energy density, but this is not provided for in the scope of the invention. Lithium metal has the highest energy density, but it cannot be processed in the presence of oxygen; however, if it is processed under a protective gas atmosphere, it can certainly be used in the scope of the present invention. The specific application method for the LiFePO4 onto the conductive polymer layer is of minor importance for the present invention. Besides the already mentioned embedding of nanoparticles, other methods are known to those skilled in the art.Vapor deposition, spraying, melting and other methods are known.

[0015] The polymer material of the reservoir electrode and / or the insulating layers are preferably used in the form of porous membranes. Such porous membranes can be formed, for example, as stretched films. The mechanical stress applied during the stretching of a film allows pores of precisely controllable size to be created within the film.

[0016] Further features and advantages of the invention will become apparent from the following detailed description and the drawings. Brief description of the drawings

[0017] It shows: Fig. 1 A schematic representation of a lithium-ion cell according to the invention. Detailed description of preferred embodiments

[0018] Fig.Figure 1 shows a schematic representation of a lithium-ion cell 10 according to the invention. The cell 10 comprises a first, negative working electrode 12 and a second, positive working electrode 14. Between the working electrodes 12 and 14 is an electrolyte chamber 16, which is filled with an electrolyte that also impregnates the working electrodes 12 and 14. A separator 18 is arranged in the electrolyte chamber 16, the primary function of which is to electronically isolate the working electrodes 12 and 14 from each other while allowing a current of lithium ions to flow through the electrolyte chamber 16. The working electrodes 12 and 14 are made of materials that allow reversible binding or intercalation of lithium ions that can move freely in the electrolyte. Various materials are known to those skilled in the art, the different properties of which affect the operating characteristics of the cell 10.

[0019] As explained at the beginning, during the operation of cell 10, especially during its first charge and discharge cycles, an intermediate layer 20 can form between the first electrode 12 and the electrolyte space, whereby lithium ions are stored in layer 20 and removed from the electrochemical process.

[0020] To replace lithium ions lost in this way or otherwise, the separator 18 is specially designed. In the illustrated embodiment, it comprises two outer insulating layers 181, preferably made of an electronically insulating polymer permeable to lithium ions, in particular polyethylene or polypropylene. The insulating layers are preferably formed as stretched films. The insulating layers 181 provide electronic isolation of the working electrodes 12, 14.

[0021] A reservoir electrode 182 is arranged between the insulating layers 181. In the illustrated embodiment, this reservoir electrode is designed as an electrically conductive polymer layer 183 in which a lithium-containing deposition material 184 is embedded. According to the invention, the lithium-containing deposition material 184 consists of lithium iron phosphate, e.g., in the form of embedded nanoparticles.

[0022] The reservoir electrode 182 is connected to the working electrodes 12, 14 via a measuring and control circuit 22. The measuring and control circuit 22 is designed to measure a voltage between the reservoir electrode 182 and one of the working electrodes 12, 14, indicated by the voltmeter symbol "V". Furthermore, it is possible to apply a voltage U between the reservoir electrode 18 and one of the working electrodes 12, 14 using the measuring and control circuit 22. This induces an electron current from the reservoir electrode 18 to one of the working electrodes 12, 14 via the measuring and control circuit 22, resulting in a corresponding lithium ion current from the reservoir electrode 18 into the electrolyte space 16. In this way, lithium stored in the intermediate layer 20 can be replenished.The required voltage can be determined in terms of magnitude and duration based on a prior voltage measurement between reservoir electrode 18 and working electrodes 12, 14, with reservoir electrode 18 serving as the reference electrode.

[0023] Of course, the embodiments discussed in the detailed description and shown in the figures represent only illustrative examples of the present invention. In light of the disclosure herein, a wide range of variations is available to those skilled in the art. In particular, they can certainly vary the specific design of the reservoir electrode. For example, embodiments are conceivable in which an electrically conductive substrate is coated on only one side with the lithium-containing deposition material. Reference symbol list 10 lithium-ion cells 12 First working electrode 14 Second working electrode 16 Electrolyte space 18 Separator 181 Insulation layer 182 Reservoir electrode 183 Electrically conductive polymer layer 184 Lithium-containing application material 20 Intermediate shift 22 Measuring and control circuit

Claims

[1] Lithium-ion cell, comprising - two opposing working electrodes (12, 14) of different polarity, between which in an electrolyte space (16) a separator (18) is arranged which electronically insulates the working electrodes (12, 14) from each other and is permeable to lithium ions, and - a lithium-containing reservoir electrode (182) which is in electronically insulating, lithium-ion-exchanging contact with the electrolyte space (16), wherein a voltage between the reservoir electrode (182) and the working electrode (12, 14) can be measured and a voltage between the reservoir electrode (182) and the working electrode (12, 14) can be applied by means of a measuring and control circuit (22) connecting the reservoir electrode (182) with at least one of the working electrodes (12, 14). and wherein the reservoir electrode (182), comprising an electrically conductive polymer material (183) onto which a lithium-containing deposit material (184) is deposited, is porous and is arranged between two electronically insulating and lithium-ion permeable insulating layers (181) of the separator (18), characterized by , that the application material (184) contains lithium iron phosphate LiFePO4. [2] Lithium-ion cell according to claim 1, characterized by , that the electrically conductive polymer material (183) comprises a polyaniline, a polypyrrole or a polythiophene. [3] Lithium-ion cell according to any of the preceding claims, characterized by , that the insulating layers (181) consist of polyethylene or polypropylene. [4] Lithium-ion cell according to any of the preceding claims, characterized by, that the polymer material of the reservoir electrode (182) and / or the insulating layers (181) are designed as porous membranes. [5] Lithium-ion cell according to claim 4, characterized by , that the polymer material of the reservoir electrode (182) and / or the insulating layers (181) are formed as stretched films.

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