Composite diaphragm and lithium battery

Abstract

The utility model provides a composite diaphragm which comprises a first diaphragm and a second diaphragm which are sequentially arranged, the first diaphragm is a non-woven fabric diaphragm or a cellulose diaphragm, and the second diaphragm is a glass fiber diaphragm; the porosity of the first diaphragm is not higher than that of the second diaphragm. According to the utility model, the first diaphragm and the second diaphragm with specific porosity are matched to form the composite diaphragm, and the first diaphragm and the second diaphragm can play a synergistic role, so that the electrolyte wettability of the composite diaphragm is effectively improved on the premise of ensuring that the composite diaphragm has good mechanical performance; and the low-temperature pulse performance and the lithium ion transmission dynamic characteristic of the lithium battery applying the lithium ion battery are improved.

Description

Technical Field

[0001] This utility model belongs to the field of lithium battery technology, specifically, it relates to a composite separator and a lithium battery. Background Technology

[0002] Currently, lithium batteries have limited internal space and a limited amount of electrolyte that can be added. During discharge, the positive electrode gradually expands, absorbing the remaining free electrolyte. The single polypropylene separator currently in use has weak competitive adsorption of electrolyte, resulting in insufficient free electrolyte inside the battery in the later stages of discharge, difficulties in ion migration in low-temperature, high-current pulse discharge applications, and fluctuations in the discharge curve. At the same time, the single polypropylene separator has a limited absorption capacity for electrolyte, which cannot effectively improve the battery's electrolyte retention, affecting the battery's low-temperature pulse performance and low-temperature discharge capacity.

[0003] Currently, various application scenarios worldwide, such as high-latitude regions and extreme icy and snowy weather, place high demands on the pulse performance of batteries in low-temperature environments, requiring batteries to maintain pulse discharge for 5 hours in applications at -40°C. Previous research has revealed that the separator in the battery not only separates the positive and negative electrodes to prevent short circuits, transports lithium ions, and ensures battery safety, but also plays a crucial role in increasing the battery's liquid retention capacity and improving its low-temperature discharge performance. Therefore, researching and developing a separator with high liquid retention capacity to improve the sustained pulse performance of lithium batteries at low temperatures is a technical problem urgently needing to be solved by those skilled in the art. Utility Model Content

[0004] The purpose of this invention is to provide a composite separator that has good mechanical strength and liquid retention capacity, enabling lithium batteries using it to have excellent safety performance and low-temperature discharge performance.

[0005] According to one aspect of the present invention, a composite diaphragm is provided, comprising a first diaphragm and a second diaphragm arranged sequentially, wherein the first diaphragm is a non-woven fabric diaphragm or a cellulose diaphragm, and the second diaphragm is a glass fiber diaphragm; the porosity of the first diaphragm is not higher than that of the second diaphragm.

[0006] In the composite separator provided by this invention, the nonwoven fabric separator has a three-dimensional network structure, giving it excellent liquid storage and locking properties; the cellulose separator surface contains hydrophilic functional groups such as hydroxyl groups, which have good affinity for the electrolyte and can provide a stable transport channel for lithium ions and electrons; the glass fiber separator surface contains abundant hydrophilic functional groups such as hydroxyl groups, and the silicate hydrogen bond structure in the glass fiber separator further enhances its hydrophilicity. Therefore, the glass fiber separator has a good electrolyte wetting rate and electrolyte absorption rate, which can quickly adsorb the electrolyte. This is beneficial for competing with the expanding positive electrode for electrolyte in the later stage of discharge, avoiding the problem of difficult ion migration in low-temperature high-current pulse discharge application scenarios, and improving the low-temperature pulse performance and low-temperature discharge capacity of the battery; moreover, the glass fiber separator has good mechanical strength, which can effectively improve the mechanical properties and stability of the composite separator, thereby improving the safety performance of the lithium battery.

[0007] Based on this, by combining a first separator (non-woven fabric separator or cellulose separator) with a second separator (glass fiber separator) as a composite separator, and further controlling the porosity of the first and second separators, the first and second separators can work synergistically. While ensuring that the composite separator has good mechanical properties, the electrolyte wettability of the composite separator is effectively improved, thereby enhancing the low-temperature pulse performance and lithium-ion transport kinetics of the lithium battery using it.

[0008] Preferably, the porosity of the first diaphragm is 50% to 90%, and the porosity of the second diaphragm is 50% to 90%. The porosity of the first diaphragm can be 50%, 60%, 70%, 80%, 90%, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. Similarly, the porosity of the second diaphragm can be 50%, 60%, 70%, 80%, 90%, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0009] Preferably, the porosity of the first separator is 50%–70%, and the porosity of the second separator is 80%–90%. Controlling the porosity of the first separator helps to improve its mechanical strength while ensuring good liquid retention capacity, avoiding the problem of reduced mechanical properties and puncture strength due to excessive porosity, which degrades battery safety. The second separator has good mechanical properties; controlling its porosity further improves its electrolyte wettability and storage capacity while ensuring good mechanical strength, reducing the risk of poor electrolyte wettability due to low porosity. Thus, the composite separator prepared by combining the first and second separators possesses excellent liquid retention capacity, overall stability, and mechanical properties, resulting in an overall improvement in the lithium-ion transport kinetics and safety performance of lithium batteries using it.

[0010] Preferably, the glass fiber diaphragm comprises glass fiber filaments with an aspect ratio of (500–2000):1. The aspect ratio of the glass fiber filaments can be 500:1, 1000:1, 1500:1, 2000:1, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. By controlling the aspect ratio of the glass fiber filaments, it is beneficial to improve the overall strength of the composite diaphragm, reduce local stress concentration, and simultaneously improve the dispersion and uniformity of the glass fiber filaments, maintaining good electrolyte wettability and thus enhancing the overall performance of the composite diaphragm.

[0011] Preferably, the average diameter of the glass fiber filaments is 0.5μm to 8μm, and the average length of the glass fiber filaments is 0.5mm to 2mm. The average diameter of the glass fiber filaments can be 0.5μm, 2μm, 4μm, 6μm, 8μm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The average length of the glass fiber filaments can be 0.5mm, 1mm, 2μm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0012] Preferably, the thickness ratio of the first diaphragm to the second diaphragm is (0.05–0.2):(0.1–0.4). The thickness ratio can be 0.05:0.1, 0.1:0.1, 0.05:0.4, 0.1:0.4, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. By controlling the thickness ratio of the first diaphragm to the second diaphragm, it is beneficial to improve the wettability and electrolyte retention capacity of the composite diaphragm while ensuring good mechanical strength.

[0013] Preferably, the thickness of the first diaphragm is 0.05 mm to 0.2 mm, and the thickness of the second diaphragm is 0.1 mm to 0.4 mm. The thickness of the first diaphragm can be 0.05 mm, 0.1 mm, 0.2 mm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The thickness of the second diaphragm can be 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0014] Preferably, the first diaphragm is a nonwoven fabric diaphragm, which is a polyethylene (PE) nonwoven fabric diaphragm, a polypropylene (PP) nonwoven fabric diaphragm, a polyimide (PI) nonwoven fabric diaphragm, or a polyethylene terephthalate (PET) nonwoven fabric diaphragm.

[0015] According to one aspect of the present invention, the first diaphragm is a cellulose diaphragm, which is a cellulose acetate diaphragm, a methylcellulose diaphragm, an ethylcellulose diaphragm, a carboxymethylcellulose diaphragm, or a hydroxycellulose diaphragm.

[0016] According to another aspect of the present invention, a lithium battery is provided, the lithium battery comprising the above-described composite separator.

[0017] Preferably, the lithium battery further includes a manganese dioxide positive electrode and a lithium metal negative electrode. Attached Figure Description

[0018] Figure 1 The diagram shows the structural schematics of the diaphragms prepared in Examples 1-9 and Comparative Examples 3-5.

[0019] Figure 2 This is a schematic diagram of the diaphragm prepared in Comparative Example 1.

[0020] Figure 3 This is a schematic diagram of the diaphragm prepared in Comparative Example 2.

[0021] In the above figures, the correspondence between the technical features and the reference numerals is as follows: 1 represents the first diaphragm, and 2 represents the second diaphragm. Detailed Implementation

[0022] The technical features of the present invention will be further clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0023] Example 1

[0024] This embodiment provides a composite membrane, the preparation method of which includes the following steps:

[0025] (1) Preparation of the first diaphragm 1

[0026] Polypropylene is used as raw material to prepare a polypropylene nonwoven membrane using a melt-blown process. The prepared polypropylene nonwoven membrane is used as the first membrane 1. The thickness of the first membrane 1 is 0.1 mm and the porosity is 70%.

[0027] (2) Preparation of the second diaphragm 2

[0028] Borosilicate glass fiber filaments (with an average diameter of 1 μm and an average length of 1.5 mm) were used as raw materials to prepare a glass fiber diaphragm using a wet process. The prepared glass fiber diaphragm was used as a second diaphragm 2 with a thickness of 0.2 mm and a porosity of 90%.

[0029] (3) Preparation of composite membrane

[0030] The first diaphragm 1 and the second diaphragm 2 are stacked, punched, and assembled.

[0031] The sample obtained in this embodiment is as follows: Figure 1 The composite diaphragm shown has a structure that, in the thickness direction, includes a first diaphragm 1 and a second diaphragm 2 in sequence.

[0032] Example 2

[0033] This embodiment prepares a composite membrane according to Embodiment 1. The difference between this embodiment and Embodiment 1 is that the porosity of the first membrane 1 in this embodiment is 50%. Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Embodiment 1.

[0034] Example 3

[0035] This embodiment prepares a composite membrane according to Embodiment 1. The difference between this embodiment and Embodiment 1 is that the porosity of the first membrane 1 in this embodiment is 90%. Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Embodiment 1.

[0036] Example 4

[0037] This embodiment prepares a composite membrane according to Embodiment 1. The difference between this embodiment and Embodiment 1 is that the porosity of the second membrane 2 in this embodiment is 70%. Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Embodiment 1.

[0038] Example 5

[0039] This embodiment prepares a composite membrane according to Embodiment 1. The difference between this embodiment and Embodiment 1 is that the porosity of the second membrane 2 in this embodiment is 80%. Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Embodiment 1.

[0040] Example 6

[0041] This embodiment refers to Example 1 for preparing the composite diaphragm. The difference between this embodiment and Example 1 is that in the process of preparing the second diaphragm 2, the average diameter of the borosilicate glass fiber used in this embodiment is 0.5 μm and the average length is 1.5 mm (length-to-diameter ratio of 3000:1). Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0042] Example 7

[0043] This embodiment refers to Example 1 for preparing the composite diaphragm. The difference between this embodiment and Example 1 is that in the process of preparing the second diaphragm 2, the average diameter of the borosilicate glass fiber used in this embodiment is 2 μm and the average length is 1.5 mm (length-to-diameter ratio is 750:1). Apart from the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0044] Example 8

[0045] This embodiment prepares a composite membrane according to Embodiment 1. The difference between this embodiment and Embodiment 1 is that the thickness of the first membrane 1 is 0.2 mm and the thickness of the second membrane 2 is 0.1 mm. Apart from the above differences, the materials and processes used in this embodiment are strictly consistent with those in Embodiment 1.

[0046] Example 9

[0047] This embodiment prepares a composite membrane according to Example 1. The difference between this embodiment and Example 1 is that in this embodiment, an ethyl cellulose membrane is used instead of the polypropylene nonwoven membrane in Example 1 as the first membrane 1. Apart from the above differences, the materials and processes used in this embodiment are strictly consistent with those in Example 1.

[0048] The preparation method of the ethyl cellulose membrane in this embodiment is as follows: using ethyl cellulose as raw material, the ethyl cellulose membrane is prepared by wet process, and the obtained ethyl cellulose membrane is used as the first membrane 1. The thickness of the first membrane 1 is 0.1 mm and the porosity is 70%.

[0049] Comparative Example 1

[0050] This comparative example prepares a composite membrane according to Example 1. The difference between this comparative example and Example 1 is that in the preparation of the composite membrane, a first membrane 1 of the same thickness is used instead of the second membrane 2 in Example 1. Apart from the above differences, the materials and processes used in this comparative example are strictly consistent with those in Example 1.

[0051] This comparative example yields the following: Figure 2 The composite diaphragm shown (double-layer first diaphragm) has a structure that includes a first diaphragm 1 and a first diaphragm 1 in the thickness direction.

[0052] Comparative Example 2

[0053] This comparative example prepares a composite membrane according to Example 1. The difference between this comparative example and Example 1 is that in the preparation of the composite membrane, a second membrane 2 of the same thickness is used instead of the first membrane 1 in Example 1. Apart from the above differences, the materials and processes used in this comparative example are strictly consistent with those in Example 1.

[0054] This comparative example yields the following: Figure 3 The composite diaphragm shown (double-layer second diaphragm) has a structure that includes a second diaphragm 2 and a second diaphragm 2 in the thickness direction.

[0055] Comparative Example 3

[0056] This comparative example prepared a composite membrane according to Example 1. The difference between this comparative example and Example 1 is that the porosity of the first membrane 1 in this comparative example is 45%. Apart from the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0057] Comparative Example 4

[0058] This comparative example prepared a composite membrane according to Example 1. The difference between this comparative example and Example 1 is that the porosity of the first membrane 1 in this comparative example is 95%. Apart from the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0059] Comparative Example 5

[0060] This comparative example prepared a composite membrane according to Example 1. The difference between this comparative example and Example 1 is that the porosity of the second membrane 2 in this comparative example is 45%. Apart from the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0061] Test Example 1

[0062] 1. Participants:

[0063] CR2032 batteries were prepared using the composite separators obtained in Examples 1-9 and Comparative Examples 1-5. The composite separators and CR2032 batteries obtained in Examples 1-9 and Comparative Examples 1-5 were used as the test objects in this test example.

[0064] 2. Test items:

[0065] (1) Battery liquid retention: Liquid retention = weight of battery after assembly - weight of cell before liquid injection.

[0066] (2) Low temperature pulse performance: Take 5 pcs of batteries and put them in a -40℃ low temperature discharge chamber for 0.5h. Then start the discharge cabinet and discharge in the following mode: 15mA discharge for 6ms, repeat 5 times after 50ms interval, standby for 20s as one cycle, and continue for 5h. Test the load voltage of the battery at the start of -40℃ and after 5h of pulse.

[0067] (3) Mechanical strength: The composite separators prepared by Examples 1 to 9 with a width of 50 mm and a length of 150 mm were used to test the mechanical strength of the test objects using a universal testing machine. The mechanical strength affects the cutting speed of the separator during battery production and is mainly determined by the first or second separator with lower tensile strength in the composite separator.

[0068] 3. Test Results:

[0069] Table 1 Performance test results of Examples 1-9 and Comparative Examples 1-5

[0070]

[0071] The test results are shown in Table 1. The performance test results of Example 1 were compared with those of Comparative Examples 1 and 2. Table 1 shows that, under the same conditions of other materials and operations in battery preparation, the composite separator used in Comparative Example 1 was a double-layer first separator, while the composite separator used in Comparative Example 2 was a double-layer second separator. Therefore, the electrolyte retention and low-temperature pulse performance of the resulting batteries were lower than those of Example 1. This indicates that, compared to Comparative Examples 1 and 2, Example 1, by combining a first separator (non-woven fabric separator or cellulose separator) with a second separator (glass fiber separator) as a composite separator and further controlling the porosity of the first and second separators, allows the first and second separators to work synergistically. While ensuring good mechanical properties, this effectively improves the electrolyte wettability of the composite separator, thereby enhancing the low-temperature pulse performance and lithium-ion transport kinetics of the lithium battery using it.

[0072] Comparing the performance test results of Example 1 with those of Comparative Examples 3-5, it can be found that the electrolyte retention and low-temperature pulse performance of the batteries prepared in Comparative Examples 3 and 5 are lower than those of Example 1. This is because, under the same conditions of other materials and operations in battery preparation, the porosity of the first separator 1 in Comparative Example 3 and the second separator 2 in Comparative Example 5 are too low, resulting in poor electrolyte wettability of the composite separator and hindering the provision of a good transport channel for lithium ions, thus worsening the electrolyte retention and low-temperature pulse performance of the battery. Simultaneously, the increased spinning density of the composite separator improves its mechanical strength. Comparing the performance test results of Example 1 with those of Comparative Example 4, it can be found that the electrolyte retention and low-temperature pulse performance of the battery prepared in Comparative Example 4 are higher than those of Example 1. This is because, under the same conditions of other materials and operations in battery preparation, the high porosity of the first separator 1 in Comparative Example 4 facilitates uniform lithium ion transport in the composite separator, thus improving the electrolyte retention and low-temperature pulse performance of the battery. However, the reduced spinning density of the composite separator lowers its mechanical strength.

[0073] The performance test results of Example 1 were compared with those of Examples 2-5. Table 1 shows that, under the same conditions of other materials and operations used in battery preparation, the porosity of the first separator 1 in Example 3 was higher than 70%, resulting in a composite separator with lower mechanical properties than that of Example 1. Similarly, the porosity of the second separator 2 in Example 4 was lower than 80%, resulting in a battery with lower liquid retention and lower low-temperature pulse performance than that of Example 1. This indicates that, compared to Examples 3 and 4, Example 1, by further optimizing the porosity of the first and second separators, facilitates the creation of a composite separator that combines excellent liquid retention capacity with overall stability and mechanical properties, thereby improving the overall lithium-ion transport kinetics and safety performance of the lithium-ion battery using it.

[0074] The performance test results of Example 1 were compared with those of Examples 6-7. Table 1 shows that, under the same conditions of other materials and operations in battery preparation, the aspect ratios of the glass fibers in the first separator 1 of Examples 6 and 7 were higher than 2000:1 and lower than 500:1, respectively. Consequently, the mechanical properties of the resulting composite separator were lower than those of Example 1, and the electrolyte retention and low-temperature pulse performance of the resulting batteries were also lower than those of Example 1. This indicates that, compared to Examples 6-7, Example 1, by controlling the aspect ratio of the glass fibers, is beneficial in improving the overall strength of the composite separator, reducing local stress concentration, and simultaneously improving the dispersion and uniformity of the glass fibers, thus maintaining good electrolyte wettability and enhancing the overall performance of the composite separator.

[0075] The above embodiments are only used to illustrate the technical solutions of this utility model and are not intended to limit the scope of protection of this utility model. Although the utility model has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this utility model, but such modifications or substitutions are all within the scope of protection of this utility model.

Claims

1. A composite diaphragm, characterized in that, The composite membrane includes a first membrane and a second membrane arranged sequentially. The first membrane is a non-woven fabric membrane or a cellulose membrane, and the second membrane is a glass fiber membrane. The porosity of the first membrane is not higher than that of the second membrane.

2. The composite diaphragm as described in claim 1, characterized in that, The porosity of the first diaphragm is 50% to 90%, and the porosity of the second diaphragm is 50% to 90%.

3. The composite diaphragm as described in claim 2, characterized in that, The porosity of the first diaphragm is 50% to 70%, and the porosity of the second diaphragm is 80% to 90%.

4. The composite diaphragm as described in claim 1, characterized in that, The glass fiber diaphragm comprises glass fiber filaments, wherein the aspect ratio of the glass fiber filaments is (1000-2000):

1.

5. The composite diaphragm as described in claim 4, characterized in that, The average diameter of the glass fiber filament is 0.5μm to 8μm, and the average length of the glass fiber filament is 0.5mm to 2mm.

6. The composite diaphragm as described in claim 1, characterized in that, The thickness ratio of the first diaphragm to the second diaphragm is (0.025~0.1):(0.1~0.4).

7. The composite diaphragm as described in claim 5, characterized in that, The thickness of the first diaphragm is 0.025 mm to 0.2 mm, and the thickness of the second diaphragm is 0.1 mm to 0.4 mm.

8. The composite diaphragm as described in claim 1, characterized in that, The first diaphragm is a non-woven fabric diaphragm, which is a polyethylene non-woven fabric diaphragm, a polypropylene non-woven fabric diaphragm, a polyimide non-woven fabric diaphragm, or a polyethylene terephthalate non-woven fabric diaphragm.

9. The composite diaphragm as described in claim 1, characterized in that, The first diaphragm is a cellulose diaphragm, which is a cellulose acetate diaphragm, a methylcellulose diaphragm, an ethylcellulose diaphragm, a carboxymethylcellulose diaphragm, or a hydroxycellulose diaphragm.

10. A lithium battery, characterized in that, The lithium battery includes the composite separator as described in any one of claims 1 to 9.

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