A composite diaphragm and a method for detecting lithium dendrites in a battery

Abstract

The present invention discloses a composite diaphragm, comprising ≥1 porous polymer film, 0-2 inorganic porous insulating layers, and a conductive network layer; the inorganic porous insulating layer is disposed on the outer side of the outermost porous polymer film; the conductive network layer is disposed between two polymer film layers or between a polymer film and an inorganic porous insulating layer, the thickness of the conductive network layer being 0.1-10 μm, and the pore size of the conductive network layer being 10-300 nm; the detection of lithium dendrites in the battery is achieved by monitoring the electrical signal of the conductive network layer in the battery. The present invention also discloses a method for detecting lithium dendrites in a battery, which determines whether lithium dendrites have reached the conductive network layer by monitoring the voltage difference or conduction between the conductive network layer and the negative electrode in the composite diaphragm. The present invention can accurately and efficiently identify the lithium plating process, effectively improving the safety performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to a composite diaphragm and a method for detecting lithium dendrites of a battery. Background Art

[0002] With the large-scale application of power batteries, safety issues have also emerged one after another. Whether it is lithium iron phosphate batteries with higher intrinsic material safety or ternary batteries, there have been many cases of automobile spontaneous combustion accidents. These accidents do not only occur when the batteries are charging. Some new energy vehicles will also catch fire while in operation or when parked. The main reason is that as the batteries become larger, the unevenness of the electrode leads to local lithium deintercalation and inconsistent impedance, which in turn leads to local lithium deposition at the negative electrode. After repeated charge and discharge cycles, lithium dendrites gradually form. As the lithium dendrites continue to grow, they pierce the diaphragm and produce an internal short circuit, causing a violent exothermic reaction and even fire and explosion. In addition, for electric vehicles used in northern winter, low temperatures and high charging rates can also produce lithium dendrites. Therefore, for electric vehicles with a capacity of hundreds of kWh and a weight of hundreds of kilograms, it is very important and urgent to dynamically detect the lithium deposition status of the battery, issue a warning in the first time, and repair and replace the battery to prevent spontaneous combustion.

[0003] Researchers can currently only use various methods to reduce the probability of lithium dendrites, but they cannot completely prevent them. Furthermore, there are often no warning signs before lithium dendrites cause internal short circuits, making them difficult to detect. Existing lithium dendrite detection methods suffer from complex detection systems and high costs, making them difficult to implement in practice. Furthermore, existing detection methods struggle to accurately identify localized lithium deposition. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned defects and provide a composite diaphragm and a method for detecting lithium dendrites in a battery, thereby solving the technical problem that lithium deposition in existing lithium-ion batteries is difficult to accurately identify. The present invention can accurately and efficiently identify the lithium deposition process and effectively improve the safety performance of the battery.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] A composite diaphragm comprising ≥1 porous polymer film, 0-2 inorganic porous insulating layers and at least one conductive network layer;

[0007] The inorganic porous insulating layer is arranged on the outer side of the outermost porous polymer film;

[0008] The conductive network layer is arranged between two polymer films or between a polymer film and an inorganic porous insulating layer. The thickness of the conductive network layer is 0.1 to 10 μm, and the pore size of the conductive network layer is 10 to 300 nm.

[0009] By monitoring the electrical signals of the conductive network layer in the battery, the detection of lithium dendrites in the battery can be achieved.

[0010] Furthermore, the material used for the conductive network layer is one or more of a metal material, a carbon material, a metal oxide or a conductive organic matter;

[0011] The conductive network layer is prepared by screen stretching, punching, electrostatic spinning, vapor deposition, printing, electroplating, roller coating or spraying.

[0012] Furthermore, the micropores of the conductive network layer are curved holes; and the axes of the micropores of the conductive network layer are curves.

[0013] Furthermore, the number of the conductive network layer is 1 to 2 layers;

[0014] There is a conductive network layer, which is separated from the negative electrode of the battery by only an inorganic porous insulating layer or a porous polymer film.

[0015] Furthermore, the material used for the porous polymer film is one or more of PE, PP, PI, PET, PVDF or cellulose;

[0016] The thickness of the single-layer porous polymer film is 5 to 10 μm;

[0017] The thickness of the inorganic porous insulating layer is 1 to 5 μm;

[0018] The material used for the inorganic porous insulating layer is one or more of Al2O3, Al(OH)3, SiO2, silicate, metaaluminate or metasilicate.

[0019] A method for detecting lithium dendrites in a battery, wherein the battery comprises the composite separator;

[0020] The battery lithium dendrite detection method includes:

[0021] Monitor the voltage difference between the conductive network layer in the composite separator and the negative electrode;

[0022] The voltage difference is used to determine whether the lithium dendrite has reached the conductive network layer. Specifically:

[0023] When -5mV≤voltage difference≤5mV, and the voltage difference fluctuation is ≤2mV, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced;

[0024] When the voltage difference is less than -5mV or the voltage difference is greater than 5mV, or the voltage difference fluctuation is greater than 2mV, it is judged that the lithium dendrites have not reached the conductive network layer and the battery does not need to be replaced;

[0025] or,

[0026] Monitoring the voltage difference between the positive electrode and the conductive network layer in the composite separator;

[0027] When OCV-5mV≤voltage difference≤OCV+5mV, and the voltage difference fluctuation is ≤2mV, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced. Otherwise, no replacement is required.

[0028] or,

[0029] Monitor the conductivity between the negative electrode and the conductive network layer in the composite diaphragm. Once it is detected that the negative electrode and the conductive network layer in the composite diaphragm are electrically connected, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced. Otherwise, no replacement is required.

[0030] Furthermore, the conductive network layer is connected to an external signal processor by using a wire or a metal battery shell, and the voltage difference between the conductive network layer in the composite diaphragm and the negative electrode is monitored by the external signal processor.

[0031] Furthermore, when the metal battery shell serves as the positive electrode or negative electrode, a third electrode is provided, and the conductive network layer is connected to the third electrode using a wire. An external signal processor collects signals from the third electrode and the negative electrode to monitor the voltage difference between the conductive network layer and the negative electrode in the composite diaphragm.

[0032] Furthermore, the method of connecting the conductive network layer with the external signal processor using the metal battery housing is as follows:

[0033] After the battery cells are stacked or wound, wrap the composite diaphragm around the battery cells several times;

[0034] Cutting the composite diaphragm, heating the tail of the composite diaphragm to shrink the porous polymer film, exposing the conductive network layer; installing the battery cell and the composite diaphragm into a metal battery casing, and making electrical contact between the conductive network layer and the metal battery casing;

[0035] The external signal processor is connected to the metal battery housing.

[0036] Furthermore, when the composite diaphragm includes an inorganic porous insulating layer, the inorganic porous insulating layer in the heated area is removed before heating to shrink the porous polymer film to prevent the inorganic porous insulating layer from obstructing electrical contact between the conductive network layer and the outer shell.

[0037] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0038] (1) The present invention creatively proposes a composite diaphragm for detecting lithium dendrites in batteries. The composite diaphragm can be used to monitor lithium deposition without blind spots inside the battery, effectively enhancing the early warning capability of lithium dendrites.

[0039] (2) The present invention proposes a method for detecting lithium dendrites in a battery. By directly detecting whether the battery is undergoing lithium deposition, the method can more accurately monitor whether the battery is undergoing lithium deposition without requiring additional algorithms or data. The method can monitor the battery in real time and provide a lithium deposition signal in a timely manner.

[0040] (3) The present invention provides a specific method for collecting composite diaphragm signals, which is simple to operate, highly applicable, and conducive to large-scale application. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of a composite diaphragm comprising two polymer layers according to the present invention;

[0042] Figure 2 Schematic diagram of a composite diaphragm comprising three polymer layers according to the present invention;

[0043] Figure 3 Schematic diagram of a composite diaphragm comprising an inorganic porous insulating layer according to the present invention;

[0044] Figure 4 Schematic diagram of a pole piece assembly prepared from the composite diaphragm of the present invention;

[0045] In the figure, 11-porous polymer film, 12-conductive network layer, 13-inorganic porous insulating layer;

[0046] 10-composite diaphragm, 20-negative electrode, 21-negative electrode ear, 30-positive electrode, 31-positive electrode ear, 100-electrode piece group. DETAILED DESCRIPTION

[0047] The following detailed description of the present invention will make the features and advantages of the present invention more clear and explicit.

[0048] The word "exemplary" is used exclusively herein to mean "serving as an example, example, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless otherwise indicated.

[0049] The present invention designs a composite diaphragm with a conductive network layer embedded inside. When lithium dendrites precipitate and grow and contact the conductive network layer in the diaphragm, a detection signal can be obtained and a warning can be given in time.

[0050] The present invention provides a composite diaphragm, comprising a plurality of porous polymer films, with a conductive network layer provided between the plurality of porous polymer films. The conductive network layer has a porous structure, and the thickness of the conductive network layer is 0.1 to 10 μm, preferably 0.5 to 2 μm. If the thickness is too thin, the strength will be too low, and the conductive network layer will be easily broken, resulting in a reduction in the monitoring area. If the thickness is too thick, not only will the thickness and weight of the diaphragm increase, but the diaphragm will also become hard, affecting production efficiency. The conductive network pore size is 10 to 300 nm, preferably 20 to 100 nm. The pore size is comparable to the size of lithium dendrites, and the negative electrode surface is fully covered, allowing accurate identification of precipitated lithium dendrites. If the pore size is too small, it will make lithium ion transmission difficult, resulting in increased internal resistance of the battery and decreased rate performance. If the pore size is too large, lithium dendrites will penetrate the diaphragm without contacting the conductive network layer, resulting in the inability to identify the generation of lithium dendrites.

[0051] Since the conductive network layer is in a grid shape and has a certain toughness and strength, it can not only increase the strength and tensile properties of the composite diaphragm, but also to a certain extent prevent lithium dendrites from penetrating the diaphragm and causing internal short circuits, thereby delaying the occurrence of short circuits.

[0052] When multiple conductive network layers are provided, or the micropores of the conductive network layer are curved holes with complex paths, the accuracy of lithium plating detection can be increased.

[0053] Preferably, the polymer material is at least one of PE, PP, PI, PET, PVDF or cellulose.

[0054] Preferably, when the multi-layer porous polymer film has two layers, the conductive network layer is between the two polymer layers.

[0055] Preferably, when the multilayer porous polymer film is three layers, the conductive network layer is located between any two adjacent layers of the three polymer films. Setting one conductive network layer can realize the monitoring of lithium dendrites, and setting two layers can improve the monitoring accuracy.

[0056] Preferably, one or both sides of the composite diaphragm are further coated with an inorganic porous insulating layer.

[0057] Preferably, the inorganic porous insulating layer has a thickness of 1-5 μm and is made of at least one of Al 2 O 3 , Al(OH) 3 , SiO 2 , silicate, metaaluminate, and metasilicate.

[0058] Preferably, the thickness of a single layer of the polymer film is 5 to 10 μm.

[0059] Preferably, the conductive network layer is made of various conductive metals, including but not limited to copper, aluminum, iron, titanium, tungsten, palladium, silver, platinum, gold, etc., as well as metal oxides, carbon materials, semiconductor materials, conductive organic matter, etc.

[0060] Preferably, the conductive network layer may be in the form of, but not limited to, screen stretching, punching, electrostatic spinning, vapor deposition, printing, electroplating, roller coating, and spraying.

[0061] An application of a composite diaphragm in a lithium-ion battery, specifically a method for detecting lithium dendrites, wherein the lithium-ion battery comprises the composite diaphragm, positive and negative electrode sheets, an electrolyte, a battery casing, etc.

[0062] The conductive network layer in the composite diaphragm is connected to the external signal processor through a wire or a metal shell to transmit signals;

[0063] By monitoring the voltage difference between the conductive network layer in the composite diaphragm and the negative electrode, it is determined whether the lithium dendrites have reached the conductive network layer. Specifically:

[0064] When -5mV≤voltage difference≤5mV, and the voltage difference fluctuation is ≤2mV, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced;

[0065] When the voltage difference is less than -5mV or the voltage difference is greater than 5mV, or the voltage difference fluctuation is greater than 2mV, it is judged that the lithium dendrites have not reached the conductive network layer (or lithium dendrites are not detected), and there is no need to replace the battery;

[0066] The battery lithium dendrite detection method also includes: monitoring the voltage difference between the positive electrode and the conductive network layer in the composite diaphragm; when OCV-5mV≤voltage difference≤OCV+5mV, and the voltage difference fluctuation is ≤2mV, it is judged that the lithium dendrites have reached the conductive network layer and the battery needs to be replaced, otherwise no replacement is required.

[0067] The method for detecting lithium dendrites in a battery also includes: detecting the conductivity between the negative electrode and the conductive network layer in the composite diaphragm. Once it is detected that the negative electrode and the conductive network layer in the composite diaphragm are electrically connected, it is determined that lithium dendrites have reached the conductive network layer and the battery needs to be replaced. Otherwise, no replacement is required.

[0068] Specifically, the negative electrode of the battery and the conductive network layer in the diaphragm can be connected through a signal processor, and whether electrical conduction occurs between the negative electrode and the conductive network layer of the diaphragm can be determined by sending and receiving electrical signals, thereby determining whether lithium dendrites occur.

[0069] Preferably, the conductive network layer is connected to the battery casing by wrapping the separator around the cell twice after lamination or winding. The separator is then cut and the end of the separator is heated to shrink the polymer, exposing the conductive network. This ensures electrical contact between the conductive network and the metal casing when inserting the battery into the casing. For batteries with an inorganic porous insulating layer on the surface, the inorganic porous insulating layer must be removed from the heated area before heating to prevent the insulating layer from obstructing electrical contact between the conductive network layer and the outer casing.

[0070] Preferably, for a battery structure in which the housing serves as the positive electrode or the negative electrode, a third electrode is provided, and the conductive network layer is connected to the third electrode by wire welding, etc. The signal processor collects information of the third electrode and the negative electrode.

[0071] like Figures 1 to 3 Three specific embodiments of the present invention are shown, wherein 11 is a porous polymer film, 12 is a conductive network layer, and 13 is an inorganic porous insulating layer. Figure 1 The composite diaphragm 10 is composed of two layers of porous polymer films 11 and a conductive network layer 12, and the conductive network layer 12 is located between the two layers of porous polymer films 11 to prevent the conductive network layer from contacting the positive or negative electrode surface after the battery is manufactured and affecting the monitoring results. The preferred polymer material is one or two of PE, PP, and PI. Figure 2 The composite diaphragm is composed of three layers of porous polymer films 11 and one layer of conductive network layer 12. The three layers of porous polymer films are respectively recorded as the first porous polymer film, the second porous polymer film and the third porous polymer film. By designing three layers of porous polymer films and combining the different physical properties of different materials, a safety protection effect can be achieved. The conductive network layer 12 is located between the first porous polymer film and the second porous polymer film, and the first porous polymer film faces the negative electrode. Therefore, there is only the first porous polymer film between the negative electrode and the conductive network layer, which can identify the appearance of lithium dendrites earlier. There are also the second porous polymer layer and the third porous polymer layer between the conductive network layer and the positive electrode, which can better protect the lithium dendrites from contacting the positive electrode and preventing short circuits. Preferably, the first porous polymer film and the third porous polymer film are made of PE, and the second porous polymer film is made of PP. Preferably, the first porous polymer film and the first porous polymer film are made of PP, and the second porous polymer film is made of PI. As Figure 3 The composite separator is composed of two porous polymer films 11 and a conductive network layer 12. In addition, an inorganic porous insulating layer 13 is provided on the outside of the porous polymer film 11. Preferably, the polymer layer is one or both of PE and PP, and the inorganic porous insulating layer is alumina ceramic. Preferably, when preparing the battery cell, the inorganic porous insulating layer 13 faces the positive electrode.

[0072] The conductive network layer in the composite diaphragm transmits signals through a wire or a shell. Preferably, the method for connecting the conductive network layer to the shell is to wrap the core twice after the core is laminated or wound, cut the diaphragm, and heat the tail of the diaphragm to shrink the polymer, thereby exposing the conductive network. When the battery is put into the shell, ensure that the conductive network is in electrical contact with the metal shell. For batteries containing an inorganic porous insulating layer on the surface, the inorganic porous insulating layer in the heated area needs to be removed before heating (the removal method is preferably adhesive removal) to prevent the insulating layer from obstructing the electrical contact between the conductive network layer and the outer shell; preferably, for a battery structure in which the outer shell is used as the positive or negative electrode, a third electrode is provided, and the conductive network layer is connected to the third electrode by means of wire welding or the like. The signal processor collects information from the third electrode and the negative electrode. By monitoring the voltage and conduction of the conductive network layer and the negative electrode in the diaphragm, it can be determined that lithium dendrites have entered the diaphragm and reached the conductive network. The battery needs to be repaired and replaced immediately. Otherwise, continued use will continue to grow until it pierces the diaphragm and causes an internal short circuit, thereby causing a fire and explosion.

[0073] At the same time, the conductive network layer is not limited to between the two polymer layers of the diaphragm, and can be carried on the diaphragm in any form to play the same role. For example, the diaphragm is a single-layer PE or PP diaphragm, and a conductive network layer is set on one side, and then an inorganic insulating layer such as an Al2O3 ceramic particle layer or boehmite is coated on the conductive network, which can play the same role and is also within the scope of protection of the present invention.

[0074] Example 1

[0075] A composite diaphragm consists of two polymer layers and a conductive network layer. The first polymer layer and the second polymer layer are both made of PE and are 7um thick. The conductive network layer is made of copper and is 1um thick. The total thickness of the diaphragm is 15um, and the pore size of the conductive network is 10 to 300nm.

[0076] The preparation method of the electrode group, single cell and battery system is as follows:

[0077] like Figure 4 , the positive electrode 30 and the negative electrode 20 are prepared respectively according to conventional methods, the positive electrode 30 and the negative electrode 20 are connected to the positive electrode ear 31 and the negative electrode ear 21 respectively, the positive electrode 30 is NCM, and the negative electrode 20 is graphite, and the positive and negative electrodes are separated by a composite separator 10 and wound to form a pole piece group 100. In particular, after the battery cell is wound, the separator is wrapped around the battery cell twice, the separator is cut, and the tail of the separator is heated to shrink the polymer, thereby exposing the conductive network layer 12, and ensuring that the conductive network layer 12 is in electrical contact with the metal shell when the battery cell is put into the shell.

[0078] The conventional process is followed to complete the liquid filling, formation, capacity separation, aging and sorting to produce a 20Ah single cell.

[0079] The sorted batteries are connected in series and parallel to make 400V, 50kWh battery modules.

[0080] Install a BMS module, specifically to monitor the battery casing and negative electrode electrical signals, and issue a lithium plating safety warning when lithium dendrites are detected.

[0081] Example 2

[0082] A composite diaphragm consists of two polymer layers and a conductive network layer. The first polymer layer is made of PE and the second polymer layer is made of PP, both with a thickness of 7um. The conductive network layer is made of copper with a thickness of 1um. The total thickness of the diaphragm is 15um.

[0083] The electrode assembly, single cell and battery system were prepared according to Example 1. When preparing the electrode assembly, the PP layer faced the positive electrode.

[0084] Example 3

[0085] A composite diaphragm consists of three polymer layers and one conductive network layer. The first polymer layer and the third polymer layer are both made of PP and are 6um thick. The second polymer layer is made of PE and is 6um thick. The conductive network layer is arranged between the first polymer layer and the second polymer layer. The conductive network layer is made of copper and is 1um thick. The total thickness of the diaphragm is 19um.

[0086] The electrode assembly, single cell and battery system were prepared according to Example 1. In particular, when preparing the electrode assembly, the first polymer layer PE was facing the negative electrode.

[0087] Example 4

[0088] A composite diaphragm consists of two polymer layers and a conductive network layer. The first polymer layer and the second polymer layer are both made of PE and are 8um thick. The conductive network layer is made of copper and is 2um thick. There is also an inorganic porous insulating layer on the outside of the first polymer layer. The material is Al2O3 ceramic and is 2um thick. The total thickness of the diaphragm is 20um.

[0089] The electrode assembly, single cell and battery system were prepared according to the method of Example 1. In particular, the ceramic layer was aligned with the positive electrode during the preparation of the electrode assembly.

[0090] Example 5

[0091] A composite diaphragm consists of a polymer layer, a conductive network layer and a ceramic layer. The first polymer layer is made of PE and is 8um thick. The conductive network layer is made of copper and is 1um thick. There is also an inorganic porous insulating layer on the outside of the conductive network layer. The material is Al2O3 ceramic and is 3um thick. The total thickness of the diaphragm is 12um.

[0092] The electrode assembly, single cell and battery system were prepared according to Example 1.

[0093] The different diaphragm forms in the above embodiments 1 to 5 meet different needs. The principle of detecting lithium dendrites is the same and can be applied to various battery forms. For 18650, soft-pack batteries, etc. that cannot use the shell as a monitoring signal output, a third electrode can be set, or a wire can be used to realize signal output.

[0094] Example 6

[0095] A composite diaphragm, a pole piece assembly, a single cell and a battery system were prepared according to Example 1. The difference between this example and Example 2 is that copper enameled wire is used to electrically connect the conductive network layer, and the copper wire is led out of the shell when the pole piece assembly is placed in the shell.

[0096] Comparative Example 1

[0097] The electrode assembly, single cell and battery system were prepared according to Example 1, except that the separator was a common 14 μm PE separator.

[0098] Comparative Example 2

[0099] The electrode assembly, single cell and battery system were prepared according to Example 1, except that the pore size of the conductive network layer was 5 nm.

[0100] Comparative Example 3

[0101] The electrode assembly, single cell and battery system were prepared according to Example 1, except that the pore size of the conductive network layer was 500 nm.

[0102] The 20Ah single cells of Examples 1 to 6 and Comparative Examples 1 to 3 were subjected to overcharging cycles to artificially increase the rate of lithium deposition and the generation of lithium dendrites. The number of cycles required to obtain a lithium dendrite signal was tested. The test results are shown in Table 1. The lithium dendrite signal in Table 1 is the signal when the conductive network layer is connected to the negative electrode.

[0103] Table 1 Test results

[0104] serial number Obtain lithium dendrite signal cycle number Number of fire or explosion cycles Example 1 343 546 Example 2 335 575 Example 3 316 544 Example 4 332 623 Example 5 351 485 Example 6 340 536 Comparative Example 1 / 447 Comparative Example 2 310 584 Comparative Example 3 460 468

[0105] It can be seen that the built-in conductive network layer of the diaphragm can identify the precipitation of lithium dendrites in advance. If the battery cannot be replaced in time, with further circulation and the continuous growth of lithium dendrites, the lithium dendrites will pierce the diaphragm and cause an internal short circuit, causing fire or explosion.

[0106] In Comparative Example 1, there was no warning sign, but the fire and explosion occurred during the cycle. Therefore, stopping use immediately upon receiving the lithium dendrite signal and treating the problematic battery can avoid subsequent fire and explosion.

[0107] Although Comparative Example 2 can obtain the signal of lithium dendrite precipitation earlier, its internal resistance is much higher than that of other batteries due to the small pore size of the conductive network layer used, and its rate performance is poor, and its application prospects are not good.

[0108] Comparative Example 3 obtained the signal of lithium dendrites the latest, and the battery caught fire and exploded shortly after the signal was obtained. This was because the aperture of the conductive network layer was too large, and the early lithium dendrites were not identified. As the battery continued to cycle, the lithium dendrites further grew and could not be detected, increasing the risk of missed detection.

[0109] In addition, it can be found that the time of the lithium dendrite signal obtained in different embodiments and the time of the short circuit are inconsistent, which is mainly affected by the thickness, porosity, material and whether the diaphragm contains a ceramic layer. This is because the thicker the diaphragm, the smaller the pore size, or the diaphragm with a ceramic layer has better tolerance to dendrites, but with continuous growth, it will eventually lead to an internal short circuit. At the same time, it can be found that the battery using a composite diaphragm containing a conductive network layer has more cycles of fire or explosion than the battery without the composite diaphragm, indicating that the conductive network layer itself can also hinder the lithium dendrites from penetrating the diaphragm to a certain extent, delaying the occurrence of battery short circuit. In addition, the ceramic layer cannot avoid the short circuit caused by dendrites. Therefore, for application fields such as power batteries that have extremely high requirements for battery safety, it is necessary to strengthen the monitoring and early warning capabilities of lithium dendrites. The present invention adopts a direct detection method with high reliability and important prospects and significance.

[0110] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

[0111] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A composite diaphragm, characterized in that: It includes ≥1 layer of porous polymer film, 0 to 2 layers of inorganic porous insulating layer and at least one conductive network layer; The inorganic porous insulating layer is arranged on the outer side of the outermost porous polymer film; The conductive network layer is arranged between two polymer films or between a polymer film and an inorganic porous insulating layer. The thickness of the conductive network layer is 0.1 to 10 μm, and the pore size of the conductive network layer is 10 to 300 nm. By monitoring the electrical signals of the conductive network layer in the battery, the detection of lithium dendrites in the battery can be achieved.

2. A composite diaphragm according to claim 1, characterized in that: The material used for the conductive network layer is one or more of metal materials, carbon materials, metal oxides or conductive organic substances; The conductive network layer is prepared by screen stretching, punching, electrostatic spinning, vapor deposition, printing, electroplating, roller coating or spraying.

3. A composite diaphragm according to claim 1, characterized in that: The micropores of the conductive network layer are curved holes; the axes of the micropores of the conductive network layer are curved lines.

4. A composite diaphragm according to claim 1, characterized in that: The number of conductive network layers is 1 to 2; There is a conductive network layer, which is separated from the negative electrode of the battery by only an inorganic porous insulating layer or a porous polymer film.

5. The composite diaphragm according to claim 1, characterized in that: The material used for the porous polymer film is one or more of PE, PP, PI, PET, PVDF or cellulose; The thickness of the single-layer porous polymer film is 5 to 10 μm; The thickness of the inorganic porous insulating layer is 1 to 5 μm; The material used for the inorganic porous insulating layer is one or more of Al2O3, Al(OH)3, SiO2, silicate, metaaluminate or metasilicate.

6. A method for detecting lithium dendrites in a battery, characterized in that: The battery comprises the composite separator according to any one of claims 1 to 5; The battery lithium dendrite detection method includes: Monitor the voltage difference between the conductive network layer in the composite separator and the negative electrode; The voltage difference is used to determine whether the lithium dendrite has reached the conductive network layer. Specifically: When -5mV≤voltage difference≤5mV, and the voltage difference fluctuation is ≤2mV, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced; When the voltage difference is less than -5mV, the voltage difference is greater than 5mV, or the voltage difference fluctuation is greater than 2mV, it is determined that the lithium dendrites have not reached the conductive network layer and the battery does not need to be replaced; or, Monitoring the voltage difference between the positive electrode and the conductive network layer in the composite separator; When OCV-5mV≤voltage difference≤OCV+5mV, and the voltage difference fluctuation is ≤2mV, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced. Otherwise, no replacement is required. or, Monitor the conductivity between the negative electrode and the conductive network layer in the composite diaphragm. Once it is detected that the negative electrode and the conductive network layer in the composite diaphragm are electrically connected, it is judged that lithium dendrites have reached the conductive network layer and the battery needs to be replaced. Otherwise, no replacement is required.

7. The method for detecting lithium dendrites in a battery according to claim 6, wherein: The conductive network layer is connected to an external signal processor by using a wire or a metal battery shell, and the voltage difference between the conductive network layer in the composite diaphragm and the negative electrode is monitored by the external signal processor.

8. The method for detecting lithium dendrites in a battery according to claim 7, wherein: When the metal battery shell is used as the positive electrode or negative electrode, a third electrode is set, and the conductive network layer is connected to the third electrode using a wire. The external signal processor collects signals from the third electrode and the negative electrode to monitor the voltage difference between the conductive network layer and the negative electrode in the composite diaphragm.

9. The method for detecting lithium dendrites in a battery according to claim 7, wherein: The method of connecting the conductive network layer to the external signal processor using the metal battery shell is as follows: After the battery cells are stacked or wound, wrap the composite diaphragm around the battery cells several times; Cutting the composite diaphragm, heating the tail of the composite diaphragm to shrink the porous polymer film, exposing the conductive network layer; installing the battery cell and the composite diaphragm into a metal battery casing, and making electrical contact between the conductive network layer and the metal battery casing; The external signal processor is connected to the metal battery housing.

10. The method for detecting lithium dendrites in a battery according to claim 9, wherein: When the composite separator includes an inorganic porous insulating layer, the inorganic porous insulating layer in the heated area is removed before heating to shrink the porous polymer film to prevent the inorganic porous insulating layer from obstructing electrical contact between the conductive network layer and the outer shell.

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