Multilayer composite diaphragm and application thereof

Abstract

The invention discloses a multi-layer composite diaphragm and application thereof. The multi-layer composite diaphragm sequentially comprises a 12 + / -1m wet-process PE porous base membrane, a positive electrode and a negative electrode in the thickness direction, the 0.8-1.2 m lithium supplementing and hole plugging layer is prepared from Li2O. 2B2O3 glass, LiTFSI (LiTFSI) and PVDF (Polyvinylidene Fluoride); the 0.4-0.6 m copper passivation layer is composed of methyl benzotriazole or 1, 2, 4-triazole and PVDF (Polyvinylidene Fluoride); the 2.5-3.5 m ceramic layer is composed of Al2O3 and PVDF (Polyvinylidene Fluoride); according to the invention, the LBO glass can release Li < + > at a low potential through the lithium-supplementing hole-blocking layer, so that a lithium-deficient area on the surface of the reclaimed material is filled; meanwhile, the molten glass phase adheres to diaphragm microdefects, and an initial short-circuit channel caused by copper spines is physically blocked; through a copper passivation layer, methylbenzotriazole and Cu < 2 + > form a stable Cu (BTA) 2 complex, and the copper deposition potential is subjected to negative shift gt; the voltage is 200 mV, and the longitudinal growth of dendritic crystals is inhibited; gt is provided by the high-rigidity Al2O3 through the ceramic layer; 5 N / m needling strength is achieved, and the material serves as the last mechanical barrier material.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, specifically to a multilayer composite separator and its applications. Background Technology

[0002] With the peak of new energy vehicle power battery retirement approaching, the lithium iron phosphate (LiFePO4) recycling process, centered on "mechanical sorting-acid leaching-regeneration roasting," is rapidly gaining popularity. During the electrode crushing, screening, and reactivation processes, metal fragments such as Cu, Fe, and Al are inevitably introduced into the recycled material, with copper content typically ranging from 3 to 10 ppm. These metal fragments form rigid "micro-thorns" in subsequent homogenization, coating, and rolling processes, easily penetrating the separator, leading to: (1) If the micro-short circuit rate increases during the formation stage (≥10%), the battery will be scrapped directly; (2) The self-discharge rate after 30 days of storage at room temperature is >2%, which cannot meet the requirements for backup power storage; (3) After 100 cycles at room temperature (0.5°C), the capacity retention rate is less than 98%, and a "plummeting" decay occurs.

[0003] Existing technologies mainly employ two approaches: (1) Deep impurity removal at the electrode end: The copper content is reduced to <3ppm by acid washing, electrolysis or magnetic separation, but the processing cost increases by ≥3000 yuan / ton. Moreover, the removal efficiency of non-magnetic Fe and Al particles is low, and 2-5ppm of foreign matter remains, which cannot completely eliminate the risk of "micro-sting". (2) Passive protection at the diaphragm end: A 2–4 µm Al2O3 ceramic layer is coated on one or both sides of a 9–12 µm PE base film, and an organic passivation layer (aramid, PVDF-HFP, benzotriazole derivative) is added as needed. However, once the ceramic layer is partially punctured, the passivation layer on the outside of the ceramic cannot block the penetrated copper dendrites, and the short circuit rate is still ≥10%. In addition, the surface of the recycled material is deficient in lithium and has many lattice defects, with an initial efficiency of ≤90%. The existing diaphragm does not have the function of "lithium replenishment-pore plugging", and it is difficult to solve the three major pain points of micro short circuit, self-discharge and cycle drain at the same time.

[0004] Therefore, there is an urgent need for a composite separator that can combine the functions of "mechanical blocking, chemical passivation, and lithium replenishment and pore plugging" without increasing the cost of electrode impurity removal, and is compatible with recycled LFP with Cu≤10 ppm, so as to achieve: 0% formation short circuit rate; self-discharge rate <2% after 30 days of storage at room temperature; and capacity retention rate >98% after 100 cycles at 25℃ and 0.5C. Summary of the Invention

[0005] In view of this, the present invention proposes a multilayer composite separator and its application, which enables lithium iron phosphate recycled material with Cu content ≤10ppm to be directly used in backup power storage batteries without additional copper removal.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: In a first aspect, the present invention provides a multilayer composite membrane comprising, in sequence along the thickness direction, a wet-process PE porous base membrane, a lithium-filling and pore-blocking layer, a copper passivation layer, and a ceramic layer.

[0007] Furthermore, the thickness of the wet-process PE porous base membrane is 12±1 µm.

[0008] Furthermore, the thickness of the lithium replenishment and plugging layer is 0.8-1.2 µm, and the lithium replenishment and plugging layer is composed of D50=150–300nm, 1.0wt% Li2O·2B2O3 glass, 0.5wt% LiTFSI and 1.5wt% PVDF, and the solvent N-methylpyrrolidone.

[0009] Furthermore, the thickness of the copper passivation layer is 0.4-0.6 µm, and the copper passivation layer is composed of methylbenzotriazole or 0.3-0.6 wt% 1,2,4-triazole and 2.0 wt% PVDF and the solvent N-methylpyrrolidone.

[0010] Furthermore, the ceramic layer has a thickness of 2.5-3.5 µm and is composed of D50=500nm, 95wt% Al2O3 and 5wt% PVDF.

[0011] Secondly, the present invention provides an application of the above-mentioned multilayer composite separator in the field of battery material technology.

[0012] The beneficial effects of this invention are: (1) The present invention uses a lithium-filling and hole-blocking layer to allow LBO glass to release Li⁺ at low potential, filling the lithium-deficient area on the surface of recycled material; at the same time, after the glass phase melts, it adheres to the micro-defects of the separator, physically sealing the initial short-circuit channel caused by copper “micro-thorns”.

[0013] (2) In this invention, through a copper passivation layer, methylbenzotriazole and Cu 2+ A stable Cu(BTA)2 complex is formed, which negatively shifts the copper deposition potential by >200 mV, inhibiting dendrite longitudinal growth; the ceramic layer: high-rigidity Al2O3 provides >5 N / µm needle penetration strength, serving as the final mechanical barrier material.

[0014] (3) This invention reduces the formation short-circuit rate of recycled LFP with Cu≤10 ppm to 0% for the first time; the self-discharge rate after 30 days of storage at room temperature is ≤2%; the capacity retention rate after 100 cycles at room temperature of 0.5C is ≥98%, which is the same as that of virgin material; the copper removal process of the electrode is eliminated, and the added value of recycled material is ≥3000 yuan / ton, with significant economic and environmental benefits; the coating sequence is changed from "ceramic priority" to "function priority", providing a general design idea for high impurity system of recycled material. Attached Figure Description

[0015] Figure 1 These are charging curves for Embodiment 1 and Comparative Examples 1-2 of the present invention; Figure 2 The discharge curves are for Embodiment 1 and Comparative Examples 1-2 of the present invention; Figure 3 This is a table of full-cell performance test data obtained in Examples 2 and 3 and Comparative Examples 3-6 of the present invention; Detailed Implementation

[0016] To provide a more detailed understanding of the features and technical content of this invention, the implementation of this invention will be described in detail below with reference to the accompanying drawings. The accompanying drawings are for reference only and are not intended to limit this invention.

[0017] Example 1: Preparation of multilayer composite membranes: a) Wet-process PE porous base membrane: 12 µm wet-process PE porous base membrane, porosity 42%, puncture strength 4.8 N; b) First lithium-filling and pore-filling layer (LBO layer): Slurry composition—D50=200 nm, 1.0 wt% Li2O·2B2O3 glass + 0.5 wt% LiTFSI + 1.5 wt% PVDF, balance NMP; microgravure coating, wet thickness 1.0 µm, drying at 80 ℃×5min, dry film thickness 0.8 µm; c) Second copper passivation layer: Slurry composition—0.5 wt % methylbenzotriazole + 2.0 wt % PVDF, balance N-methylpyrrolidone (NMP); wet thickness 0.5 µm, dried at 80 ℃ for 5 min, dry film thickness 0.4 µm; d) Third ceramic layer: Slurry composition—D50=500 nm, 95 wt % Al2O3 + 5 wt % PVDF; wet thickness 3µm, dried at 120 ℃×10 min, dry film thickness 2.5 µm; The total dry film thickness is 14.5 µm; the coating sequence must be wet PE porous base film → lithium plugging layer → copper passivation layer → ceramic layer, and each layer must be dried and cooled to ≤40℃ before proceeding to the next coating layer.

[0018] Positive electrode preparation: 91 wt% recycled LiFePO4, containing 8.0 ppm Cu, 5 wt% conductive carbon black (SP), and 4 wt% PVDF, was slurried with NMP and coated onto carbon-coated aluminum foil, then stamped into φ14 mm positive electrode sheets.

[0019] Half-cell assembly: In an Ar glove box, the CR2032 type half-cell is packaged in the following order: "positive electrode shell → positive electrode sheet → 3 drops of electrolyte → multilayer composite separator → lithium sheet → gasket → negative electrode shell", with the separator coating facing the positive electrode.

[0020] Performance testing: 0.1 C constant current charge and discharge, 25 ℃, voltage range 2.0-3.8 V; initial charge capacity 165.1 mAh / g, initial discharge specific capacity 164.3 mAh / g, coulombic efficiency 99.5%; no "bulging" abnormality in the charging curve, no abnormality compared to normal batteries.

[0021] Example 2: Positive electrode preparation: 96.5 wt% recycled LiFePO4, containing 8 ppm copper (Cu), 1.5 wt% conductive carbon black, and 2.0 wt% PVDF. After NMP slurry preparation, the mixture was coated onto carbon-coated aluminum foil and rolled to a compaction density of 2.35 g / cm³. -3 The positive electrode plate.

[0022] Diaphragm preparation: Same as in Example 1.

[0023] Full cell assembly: The negative electrode is artificial graphite, and the electrolyte is conventional LiPF6. A 50 Ah pouch cell is prepared according to the conventional process, with the separator coating facing the positive electrode.

[0024] Example 3: Positive electrode preparation: Same as in Example 2, except that recycled LiFePO4 is used instead, with a copper content of Cu: 10 ppm.

[0025] Separator preparation and full cell assembly: Same as in Example 2.

[0026] Comparative Example 1: A membrane with only a ceramic layer Membrane preparation: 12 μm wet-process PE porous base membrane → direct coating of 3 μm Al2O3 ceramic, composition and drying conditions are the same as the ceramic layer in Example 1, with a total thickness of 12 μm.

[0027] Battery assembly: Same as in Example 1, only the separator is replaced.

[0028] Performance testing: The initial charge capacity at 0.1 C was 219.1 mAh / g, far exceeding the theoretical value of 170 mAh / g for LiFePO4. The initial discharge capacity was only 142.3 mAh / g, with a coulombic efficiency of 65%. The charging curve showed a significant upward tilt at the end of the charging process, indicating that Cu micro-dendries penetrated the ceramic layer, forming a continuous micro-short circuit. The charging capacity was consumed by side reactions, resulting in a "falsely high" capacity followed by a "real low" capacity.

[0029] Comparative Example 2: A membrane consisting only of a ceramic layer and a copper purification layer Membrane preparation: 12μm wet-process PE porous base membrane → first coat with a 0.5 μm copper passivation layer → then coat with a 3 μm ceramic layer, for a total thickness of 13.5 μm; the composition of the copper passivation layer and the ceramic layer and the drying conditions are the same as in Example 1.

[0030] Battery assembly: Same as in Example 1, only the separator is replaced.

[0031] Performance testing: The initial charge capacity at 0.1 C was 167.3 mAh / g, the initial discharge capacity was 149.9 mAh / g, and the coulombic efficiency was approximately 89.58%. The tail-end curve amplitude at the end of the charge was smaller than that of Comparative Example 1. The copper passivation layer can partially suppress Cu dendrites, but due to the lack of an LBO lithium replenishment and pore-blocking layer, the internal resistance of the separator is high, the interface defects are not repaired, the short circuit rate is not reduced to 0%, and the capacity retention rate is still not up to standard.

[0032] Comparative Example 3: Preparation of positive electrode: Same as in Example 2.

[0033] Membrane preparation: The difference from Example 2 is that only the lithium-filling and pore-blocking layer is omitted. The other materials, processes and test conditions are completely the same. The total thickness is 12.9 µm. The coating sequence is wet PE porous base film → copper passivation layer → ceramic layer.

[0034] Full battery assembly: Same as Example 2.

[0035] Comparative Example 4: Preparation of positive electrode: Same as in Example 2.

[0036] Separator preparation: The lithium-filling and pore-blocking layer is placed after the copper passivation layer, i.e. away from the positive electrode side, forming the sequence of "wet PE porous base film → copper passivation layer → lithium-filling and pore-blocking layer → ceramic layer". The remaining parameters are the same as in Example 2.

[0037] Full battery assembly: Same as Example 2.

[0038] Comparative Example 5: Positive electrode preparation: virgin LiFePO4 with a copper content of 0 ppm was used to verify the performance boundary of the membrane system under the "copper-free" baseline. All other conditions were the same as in Example 2, except that the positive electrode material was changed.

[0039] Membrane preparation: 12 µm wet-process PE porous base membrane + ceramic layer.

[0040] Full battery assembly: Same as Example 2.

[0041] Comparative Example 6: Preparation of positive electrode: Same as in Example 2.

[0042] Membrane preparation: 12 µm wet-process PE porous base membrane + ceramic layer.

[0043] Full battery assembly: Same as Example 2.

[0044] The batteries prepared in Example 1 and Comparative Examples 1 and 2 are half-cells, demonstrating the significant role of the separator prepared in Example 1. To further prove the role of the separator, the batteries prepared in Examples 2 and 3 and Comparative Examples 3-6 are full-cells. The performance test data of the full-cell batteries prepared in Examples 2 and 3 and Comparative Examples 3-6 are shown in Table 1.

[0045] The testing method is as follows: K-value test conditions: Store at 45℃ for 48 hours + stand at 25℃ for 24 hours, and calculate according to ΔV / Δt.

[0046] 30-day storage voltage drop: The battery at 5% SOC was stored for 30 days at an ambient temperature of 25℃±5℃. The voltage drop before (OCV1) and after (OCV2) storage was measured as OCV2-OCV1.

[0047] 30-day storage capacity retention: After the battery is left to stand for 30 days at 100% SOC under an ambient temperature of 25℃±5℃, the battery capacity before (Q1) and after (Q2) is tested. Capacity retention = Q2 / Q1.

[0048] Cyclic test: 25 ℃, 0.5 C charge / discharge, 2.5–3.6 V, 100 cycles. Table 1. Performance test data of full cells prepared in Examples 2 and 3 and Comparative Examples 3-6 As shown in Table 1, " / " indicates that no valid data was obtained due to short circuit during formation. In comparison, it can be seen that Example 2, through the synergistic effect of "pre-placed lithium plugging layer + mid-placed copper passivation layer + post-placed ceramic layer", has made the Cu ≤10 ppm recycled material system on par with virgin material in terms of the two core indicators of self-discharge and cycle life, verifying the universal value of the present invention for high impurity scenarios of recycled materials. In contrast, the comparative example does not have any problems with conventional PE for virgin material without copper, but the recycled material containing Cu: 8ppm + ceramic layer and different coating sequences all have abnormal self-discharge.

Claims

1. A multilayer composite diaphragm, characterized in that, Along the thickness direction, it consists of a wet-process PE porous base film, a lithium-filling and pore-blocking layer, a copper passivation layer, and a ceramic layer.

2. The multilayer composite diaphragm according to claim 1, characterized in that, The thickness of the wet-process PE porous base membrane is 12±1 µm.

3. The multilayer composite diaphragm according to claim 1, characterized in that, The thickness of the lithium-filling and plugging layer is 0.8-1.2 µm. The lithium-filling and plugging layer is composed of D50=150–300nm, 1.0wt% Li2O·2B2O3 glass, 0.5wt% LiTFSI and 1.5wt% PVDF, and the solvent N-methylpyrrolidone.

4. The multilayer composite diaphragm according to claim 1, characterized in that, The thickness of the copper passivation layer is 0.4-0.6 µm, and the copper passivation layer is composed of methylbenzotriazole or 0.3-0.6 wt% 1,2,4-triazole and 2.0 wt% PVDF and the solvent N-methylpyrrolidone.

5. A multilayer composite diaphragm according to claim 1, characterized in that, The ceramic layer has a thickness of 2.5-3.5 µm and is composed of D50=500nm, 95wt% Al2O3 and 5wt% PVDF.

6. The application of a multilayer composite separator as described in any one of claims 1 to 5 in the field of battery materials technology.

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