Lithium-carbon composite negative pole piece and secondary battery

Abstract

The invention discloses a lithium-carbon composite negative electrode plate and a secondary battery, and belongs to the technical field of batteries. The lithium-carbon composite negative electrode plate provided by the invention comprises a first porous carbon layer and second porous carbon layers arranged on the two surfaces of the first porous carbon layer respectively, and the porosity P1 of the first porous carbon layer is smaller than the porosity P2 of the second porous carbon layers; the negative pole piece with the specific porosity and the sandwich structure design has good mechanical strength, can provide enough space for metal lithium deposition when being subsequently applied to the secondary battery, reduces the volume expansion of the negative pole piece, and improves the safety performance of the secondary battery; meanwhile, the structure of the negative pole piece can also provide good electronic conductivity and store more electrolyte, so that electrolyte infiltration is facilitated, ion transmission impedance is reduced, and the cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a lithium-carbon composite negative electrode plate and a secondary battery. Background Art

[0002] With the continuous advancement of technologies such as electric vehicles, advanced energy storage, and smart consumer terminals, people have put forward higher demands on the energy density and cycle performance of batteries. Lithium metal anodes, due to their advantages such as high gram capacity and low potential, have demonstrated irreplaceable competitiveness in improving battery energy density. However, lithium metal anodes face many problems that seriously restrict their large-scale application. For example, lithium metal has active chemical properties and continuously reacts with the electrolyte, resulting in rapid consumption of electrolyte and lithium and the formation of a large amount of by-products, increased battery impedance, and rapid cycle decay. For example, lithium metal is unevenly deposited and forms dendrites, which not only causes severe volume expansion of the anode, but also dendrites are prone to puncturing the diaphragm and falling off, posing serious safety risks. Summary of the Invention

[0003] The purpose of this application is to solve the problem in the prior art that the volume expansion of lithium metal negative electrodes is obvious and the cycle performance of the secondary batteries prepared is poor, and to propose a lithium-carbon composite negative electrode plate and a secondary battery.

[0004] To achieve the above-mentioned object, the first aspect of the present application provides a lithium-carbon composite negative electrode plate, wherein the negative electrode plate comprises a porous carbon current collector and metallic lithium;

[0005] The porous carbon current collector includes a first porous carbon layer and second porous carbon layers respectively disposed on two surfaces of the first porous carbon layer;

[0006] The porosity P1 of the first porous carbon layer and the porosity P2 of the second porous carbon layer satisfy: P1<P2.

[0007] As an embodiment of the present application, the thickness H1 of the first porous carbon layer and the thickness H2 of the second porous carbon layer satisfy: 0.1≤H1 / H2≤2.

[0008] As an embodiment of the present application, the negative electrode sheet satisfies: 0<A≤0.045;

[0009] Where A = m / (H1×P1+2H2×P2);

[0010] m mg / cm 2 It is the mass per unit area of ​​metallic lithium in the negative electrode.

[0011] As an embodiment of the present application, 10%<P1≤50%.

[0012] As an embodiment of the present application, 50%<P2≤90%.

[0013] As an embodiment of the present application, 1 μm≤H1≤50 μm.

[0014] As an embodiment of the present application, 4.5 μm ≤ H2 ≤ 50 μm.

[0015] As an embodiment of the present application, 0 mg / cm 2 <m≤1.335mg / cm 2 .

[0016] As an embodiment of the present application, the thickness H of the porous carbon current collector satisfies 10 μm≤H≤100 μm.

[0017] As an embodiment of the present application, the first porous carbon layer includes a first carbon fiber skeleton, and the porosity K1 of the first carbon fiber skeleton satisfies 10%≤K1<50% and K1<P1.

[0018] As an embodiment of the present application, the second porous carbon layer includes a second carbon fiber skeleton, and the porosity K2 of the second carbon fiber skeleton satisfies 1%≤K2<10%.

[0019] As an embodiment of the present application, the average diameter of the first carbon fiber skeleton is D1, and the first carbon fiber skeleton satisfies 1≤D1 / K1≤20.

[0020] As an embodiment of the present application, the average diameter of the second carbon fiber skeleton is D2, and the second carbon fiber skeleton satisfies 10≤D2 / K2<50.

[0021] As an embodiment of the present application, the D1 satisfies 0.1 μm≤D1≤5 μm.

[0022] As an embodiment of the present application, the D2 satisfies 0.1 μm≤D2≤5 μm.

[0023] In a second aspect of the present application, a secondary battery is provided, wherein the secondary battery comprises the negative electrode sheet described in the present application.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] The lithium-carbon composite negative electrode plate provided in the present application includes a first porous carbon layer and a second porous carbon layer respectively arranged on the two surfaces of the first porous carbon layer, and the porosity P1 of the first porous carbon layer is smaller than the porosity P2 of the second porous carbon layer. The negative electrode plate with a specific porosity and a "sandwich" structure design has good mechanical strength. When subsequently used in a secondary battery, it can provide sufficient space for the deposition of metallic lithium and reduce the volume expansion of the negative electrode plate. At the same time, the structure of the negative electrode plate can also provide good electronic conductivity characteristics and store more electrolyte, which is beneficial to electrolyte infiltration, reduces ion transfer impedance, and improves the cycle performance of the secondary battery. DETAILED DESCRIPTION

[0026] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0027] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0028] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0029] The reagents and instruments used in this application without manufacturer indication are all conventional products that can be purchased commercially.

[0030] In one embodiment of the present application, the present application proposes a lithium-carbon composite negative electrode plate, wherein the negative electrode plate includes a porous carbon current collector and metallic lithium;

[0031] The porous carbon current collector includes a first porous carbon layer and second porous carbon layers respectively disposed on two surfaces of the first porous carbon layer;

[0032] The porosity P1 of the first porous carbon layer and the porosity P2 of the second porous carbon layer satisfy: P1<P2.

[0033] The research in this application found that the lithium-carbon composite negative electrode plate provided in this application includes a first porous carbon layer and a second porous carbon layer respectively arranged on the two surfaces of the first porous carbon layer, and the porosity P1 of the first porous carbon layer is smaller than the porosity P2 of the second porous carbon layer. The negative electrode plate with a specific porosity size and a "sandwich" structure design has good mechanical strength. When subsequently used in a secondary battery, it can provide sufficient space for the deposition of metallic lithium and reduce the volume expansion of the negative electrode plate. At the same time, the structure of the negative electrode plate can also provide good electronic conductivity characteristics and store more electrolyte, which is beneficial to electrolyte infiltration, reduces ion transfer impedance, and improves the cycle performance of the secondary battery.

[0034] Specifically, the porosity P1 of the first porous carbon layer of the negative electrode provided herein is smaller than the porosity P2 of the second porous carbon layer. Firstly, the relatively small porosity of the first porous carbon layer can provide a rich conductive fiber network, excellent electronic conductivity, and good mechanical strength. Secondly, the relatively large porosity of the second porous carbon layer can facilitate electrolyte infiltration into the interior of the negative electrode, thereby reducing ion transfer impedance; it can also provide space for lithium deposition, thereby reducing the volume expansion of the negative electrode. Thirdly, the porous carbon current collector provided herein is a self-supporting structure that not only provides space for lithium deposition but also acts as an electron flow. Furthermore, the porous carbon skeleton has a low density, high porosity, and low overall weight, thereby reducing the problem of low energy density in secondary batteries caused by the excessive weight of the negative electrode using conventional copper foil current collectors. Fourthly, the negative electrode provided herein also includes metallic lithium, which can ensure that the subsequent secondary battery has a high initial charge and discharge efficiency and provide sufficient lithium source replenishment, thereby improving the secondary battery's cycle performance.

[0035] It should be noted that the test method for the porosity P1 of the first porous carbon layer and the porosity P2 of the second porous carbon layer is as follows: the first porous carbon layer and the second porous carbon layer have a clear boundary and are easy to mechanically peel off, specifically as follows: take a porous carbon negative electrode with a size of not less than 5cm×5cm, use tweezers to peel off the first porous carbon layer and the second porous carbon layer; then take a flat area and punch the porous carbon layer into discs with a diameter of 2cm; use a micrometer to measure the thickness H1 (cm) of the first porous carbon and the thickness H2 (cm) of the second porous carbon layer; after drying the above-mentioned porous carbon layer at 100℃ for 2h, use a mercury intrusion instrument to measure them respectively, and keep the mercury pressure at 5Mpa, and obtain the mercury intrusion volume V1 (cm) respectively. 3 ) and V2(cm 3 ); Finally, the porosity was calculated by the formula: porosity = V / (πH), and the test was repeated for three groups to obtain the average value.

[0036] It should be noted that the second porous carbon layers respectively disposed on both surfaces of the first porous carbon layer have the same thickness. The thickness H2 of the second porous carbon layer in this application refers to the second porous carbon layer on one side.

[0037] In one embodiment, the thickness H1 of the first porous carbon layer and the thickness H2 of the second porous carbon layer satisfy: 0.1≤H1 / H2≤2.

[0038] Exemplarily, H1 / H2 may be any point value or any two point range value between 0.1-2, such as 0.1, 0.3, 0.5, 0.8, 1, 1.3, 1.5, 1.8, 2, etc.

[0039] In one embodiment, the thickness H1 of the first porous carbon layer and the thickness H2 of the second porous carbon layer satisfy: 0.25≤H1 / H2≤0.5, for example, 0.25, 0.28, 0.3, 0.32, 0.35, 0.38, 0.4, 0.42, 0.45, 0.48, 0.5, etc.

[0040] The research in this application found that H1 / H2 reflects the ratio between the first porous carbon layer and the second porous carbon layer to a certain extent. Due to the difference in porosity between the first porous carbon layer and the second porous carbon layer, the mechanical properties of the negative electrode and the utilization rate of the pores are different. When the thickness ratio of the first porous carbon layer and the second porous carbon layer is further selected to be 0.1-2, especially 0.25-0.5, while ensuring that the obtained negative electrode has good mechanical properties and does not affect processing and use, it also has a good pore structure, which can not only meet the lithium deposition requirements, reduce the volume expansion of the negative electrode, and improve the safety performance of the secondary battery prepared subsequently; it can also make the electrolyte in the secondary battery prepared subsequently effectively infiltrated, thereby reducing the ion transfer resistance and improving the cycle performance of the secondary battery.

[0041] In one embodiment, the negative electrode plate satisfies: 0<A≤0.045;

[0042] Where A = m / (H1×P1+2H2×P2);

[0043] m mg / cm 2 It is the mass per unit area of ​​metallic lithium in the negative electrode.

[0044] The research of this application found that the metallic lithium in the negative electrode plate provided by this application is attached to the inside and surface of the porous current collector. By further controlling the unit area mass of the metallic lithium and the parameters of the first porous carbon layer and the second porous carbon layer to satisfy the above-mentioned relationship, the obtained secondary battery has sufficient lithium source supplement, which can improve the cycle performance, and also has sufficient space for lithium deposition, which can improve the safety performance.

[0045] Illustratively, A can be any point value or any two point range value between 0-0.045, such as 0.001, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.45, etc.

[0046] In one embodiment, the negative electrode plate satisfies 0.004≤A≤0.008. For example, it can be 0.004, 0.005, 0.006, 0.007, 0.008, etc. The present application has found that when the negative electrode plate further satisfies 0.004≤A≤0.008, the overall performance of the secondary battery obtained is better.

[0047] In one embodiment, 10%<P1≤50%.

[0048] Exemplarily, P1 can be any point value between 10% and 50% or any two point range value, such as 11%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, etc.

[0049] In one embodiment, 20%≤P1≤45%, for example, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 45%, etc.

[0050] The research in this application found that the porosity P1 in the first porous carbon layer is within the above range, especially when 20% ≤ P1 ≤ 45%, it is located in the center of the negative electrode plate. In the secondary battery prepared subsequently, the diffusion impedance of the electrolyte to it is relatively large. Therefore, by selecting the porosity within the above range, the adsorption amount of the electrolyte can be increased to a certain extent, and at the same time, lithium nucleation sites can be provided, thereby improving the utilization rate of the pores in the first porous carbon layer; thereby improving the overall performance of the secondary battery.

[0051] In one embodiment, 50%<P2≤90%.

[0052] Exemplarily, P2 can be any point value between 50%-90% or any two-point range value, such as 51%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc.

[0053] In one embodiment, 58%≤P2≤77%, for example, it may be 58%, 60%, 62%, 64%, 66%, 68%, 70%, 72%, 74%, 76%, 77%, etc.

[0054] The research in this application found that the porosity P2 in the second porous carbon layer is within the above range, especially when 58% ≤ P2 ≤ 77%, it is located on the surface of the negative electrode plate. In the secondary battery prepared subsequently, the diffusion impedance of the electrolyte to it is relatively small. Therefore, by selecting the porosity within the above range, the mechanical properties of the negative electrode plate can be improved to a certain extent, avoiding affecting the processability of the negative electrode plate and the subsequent secondary battery. At the same time, it can also avoid the rapid deposition of lithium first, which leads to a weakening of the ability of the electrolyte to enter the first porous carbon layer; thereby ensuring the comprehensive performance of the secondary battery prepared.

[0055] In one embodiment, 1 μm≤H1≤50 μm.

[0056] Exemplarily, H1 may be any point value or any two point range value between 1 μm and 50 μm, such as 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, etc.

[0057] In one embodiment, 6 μm ≤ H1 ≤ 16 μm, for example, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, etc.

[0058] The present application study found that by further selecting 1μm≤H1≤50μm, especially 6μm≤H1≤16μm, a negative electrode sheet within a suitable thickness range can be obtained, thereby improving the overall performance of the secondary battery.

[0059] In one embodiment, 4.5 μm≤H2≤50 μm.

[0060] Exemplarily, H2 can be any point value or any two point range values ​​between 4.5μm and 50μm, such as 4.5μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, etc.

[0061] In one embodiment, 20 μm ≤ H2 ≤ 32 μm, for example, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, 32 μm, etc.

[0062] The present application study found that by further selecting 4.5μm≤H2≤50μm, especially 20μm≤H2≤32μm, a negative electrode sheet within a suitable thickness range can be obtained, thereby improving the overall performance of the secondary battery.

[0063] In one embodiment, 0 mg / cm 2 <m≤1.335mg / cm 2 .

[0064] It should be noted that the test method for the unit area mass m of metallic lithium is as follows: first, the lithium-carbon composite negative electrode sheet is punched into a disc with a diameter of 10 mm and used as the working electrode, the counter electrode is a lithium sheet with a diameter of 12 mm, the electrolyte is a carbonate electrolyte (LiPF6 concentration is 1 mol / L, the organic solvent is a mixture of ethylene carbonate EC and diethyl carbonate DEC in a volume ratio of 3:7, and the mass content of fluoroethylene carbonate FEC in the electrolyte is 5%), and they are assembled into a button cell; then the button cell is charged at room temperature with a current density of 0.1 mA / cm 2 , the cut-off condition voltage ≥ 1.0V, after the test is completed, the charging capacity CmAh is obtained; finally, the mass of metal lithium per unit area is calculated according to the following formula: m = 0.33Cmg / cm 2 .

[0065] For example, m may be 0 mg / cm 2 -1.335mg / cm 2 Any point value or any range value between two points, such as 0.001mg / cm 2 , 0.01mg / cm 2 , 0.05mg / cm 2 , 0.02mg / cm 2 , 0.04mg / cm 2 , 0.06mg / cm 2 , 0.08mg / cm 2 , 0.1mg / cm 2 , 0.2mg / cm 2 , 0.4mg / cm 2 , 0.6mg / cm 2 , 0.8mg / cm 2 , 1mg / cm 2 , 1.2mg / cm 2 , 1.3mg / cm 2 、1.335mg / cm 2 wait.

[0066] The research in this application found that when the mass per unit area of ​​metallic lithium is within the above range, it can achieve appropriate deposition of lithium on the basis of improving the initial charge and discharge efficiency of the secondary battery prepared subsequently, thereby improving the cycle performance and safety performance of the secondary battery.

[0067] In one embodiment, the thickness H of the porous carbon current collector satisfies 10 μm≤H≤100 μm.

[0068] Exemplarily, H can be any point value or any two point range value between 10μm-100μm, such as 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, 100μm, etc.

[0069] In one embodiment, 50 μm ≤ H ≤ 80 μm, for example, 50 μm, 53 μm, 58 μm, 63 μm, 68 μm, 73 μm, 78 μm, 80 μm, etc.

[0070] The present application has found that further selecting the thickness of the negative electrode sheet within the above range can strike a balance between the mass and energy density of the negative electrode sheet, thereby improving the overall performance of the secondary battery.

[0071] In one embodiment, the first porous carbon layer includes a first carbon fiber skeleton, and the porosity K1 of the first carbon fiber skeleton satisfies 10%≤K1<50% and K1<P1.

[0072] Exemplarily, K1 may be any point value between 10% and 50% or any two point range value, such as 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 49%, etc.

[0073] In one embodiment, 12%≤K1≤25%, for example, it may be 12%, 14%, 16%, 18%, 20%, 22%, 25%, etc.

[0074] The present application found that by controlling the porosity K1 of the first carbon fiber skeleton in the first porous carbon layer within the above range, the ratio of macropores to small pores in the first porous carbon layer can be made within an appropriate range, wherein the macropores are pores with a pore size of 0.5-1000 μm and an average pore diameter of 10-100 μm, which are formed by the mutual stacking of the first carbon fiber skeletons, and the small pores are formed when preparing the first porous carbon layer, with a pore size of less than 0.5 μm and an average pore diameter of 0.1-0.3 μm; by limiting K1 within the above range, the volume ratio of macropores to small pores can be met to be 0.1-1, so that the overall porosity of the first porous carbon layer is relatively small, providing good mechanical properties for the negative electrode plate and providing sufficient storage space for the electrolyte, and the small pores therein can also serve as defect sites to induce lithium deposition, thereby improving the cycle performance and safety performance of the secondary battery. In addition, the porosity of the first carbon fiber skeleton in the first porous carbon layer is within the above range, which can also store more electrolyte, reduce ion transfer impedance, and thus improve the cycle performance and safety performance of the secondary battery.

[0075] In one embodiment, the second porous carbon layer includes a second carbon fiber skeleton, and the porosity K2 of the second carbon fiber skeleton satisfies 1%≤K2<10%.

[0076] Exemplarily, K2 may be any point value or any two point range value between 1% and 10%, such as 1.1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0077] In one embodiment, 3%≤K2≤6%, for example, 3%, 4%, 5%, 6%, etc.

[0078] The research in this application found that by controlling the porosity K2 of the second carbon fiber skeleton in the second porous carbon layer within the above range, the ratio of large pores to small pores in the second porous carbon layer can be kept within an appropriate range. Specifically, the volume ratio of large pores to small pores is 10-1000, so that the overall porosity of the second porous carbon layer is larger, which is beneficial for the subsequent electrolyte in the secondary battery to infiltrate the interior of the negative electrode sheet, reduce the ion transfer impedance, and the rich pore structure is also beneficial to lithium deposition, thereby improving the cycle performance and safety performance of the secondary battery.

[0079] It should be noted that the testing method for the porosity of the first carbon fiber skeleton and the second carbon fiber skeleton is: use tweezers to peel off the first porous carbon layer and the second porous carbon layer; then use a crusher to break them into particles with a particle diameter of 1-5 mm; after drying the particles at 100°C for 2 hours, use a nitrogen adsorption instrument to perform a porosity test to obtain the porosity K1 of the first porous carbon fiber skeleton and the porosity K2 of the second porous carbon fiber skeleton.

[0080] In one embodiment, the average diameter of the first carbon fiber skeleton is D1, and the first carbon fiber skeleton satisfies 1≤D1 / K1≤20.

[0081] Exemplarily, D1 / K1 can be any point value or any two-point range value between 1-20, such as 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, etc.

[0082] In one embodiment, 4≤D1 / K1≤14. For example, it can be 4, 5, 7, 9, 11, 13, 14, etc.

[0083] The research in this application found that by further limiting the ratio of the average diameter and porosity of the first carbon fiber skeleton to 1≤D1 / K1≤20, especially when it is 4≤D1 / K1≤14, it is possible to ensure that the first porous carbon layer has a higher porosity, adsorbs sufficient electrolyte, and reduces impedance; and can provide more space for lithium deposition; in addition, it is possible to maintain the first porous carbon layer with a higher mechanical strength, ensure the overall structural stability of the electrode, and thus make the overall performance of the secondary battery better.

[0084] In one embodiment, the average diameter of the second carbon fiber skeleton is D2, and the second carbon fiber skeleton satisfies 10≤D2 / K2<50.

[0085] Exemplarily, D2 / K2 can be any point value or any two point range value between 10-50, such as 10, 15, 20, 25, 30, 35, 40, 45, 49, etc.

[0086] In one embodiment, 16≤D2 / K2≤34. For example, it may be 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, etc.

[0087] The research in this application found that by further limiting the ratio of the average diameter and porosity of the second carbon fiber skeleton to 10≤D2 / K2<50, especially when it is 16≤D2 / K2≤34, it is possible to ensure that the second carbon layer has a high porosity, which can meet the deposition of more lithium. At the same time, the relatively small porosity of the carbon fiber skeleton can maintain the high electronic conductivity of the carbon fiber and improve the rate performance, thereby making the overall performance of the obtained secondary battery better.

[0088] In one embodiment, the D1 satisfies 0.1 μm≤D1≤5 μm.

[0089] In one embodiment, D2 satisfies 0.1 μm≤D2≤5 μm.

[0090] Exemplarily, D1 and D2 can each independently be any point value or any two point range values ​​between 0.1-5, such as 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.

[0091] It should be noted that the test method for the average pore size of the first carbon fiber skeleton and the second carbon fiber skeleton is the same as the above-mentioned test method for the porosity of the corresponding carbon fiber skeleton, and the pore size of the corresponding carbon fiber skeleton is tested using a nitrogen adsorption instrument.

[0092] The research in this application found that by further selecting the average pore size of the first carbon fiber skeleton and the second carbon fiber skeleton within the above range, it can be beneficial for lithium metal to be preferentially deposited in the pores of the skeleton, thereby improving the deposition uniformity and pore utilization, thereby making the overall performance of the secondary battery obtained better.

[0093] In one embodiment, the method for preparing the lithium-carbon composite negative electrode sheet comprises the following steps:

[0094] (1) dispersing a first polymer as a carbon source and a first pore-forming agent in a first solvent to obtain a first spinning solution;

[0095] (2) dispersing a second polymer serving as a carbon source and a second pore-forming agent in a second solvent to obtain a second spinning solution;

[0096] (3) electrospinning the first spinning solution and then drying it to obtain a first porous carbon layer prepolymer, and then electrospinning the second spinning solution on both sides of the first porous carbon layer prepolymer and then drying it to obtain a composite prepolymer;

[0097] (4) The composite prepolymer is carbonized and then composited with metallic lithium to obtain a lithium-carbon composite negative electrode.

[0098] The preparation method of the negative electrode plate provided in the present application is simple to operate, and pores are formed in the carbon skeleton by adding a porogen, while a porous carbon current collector is prepared using electrospinning technology, with high preparation efficiency.

[0099] In some embodiments, the first polymer and the second polymer are each independently selected from at least one of polyacrylonitrile (PAN), asphalt, polypropylene (PP), polyethylene (PE), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polymethyl methacrylate (PMMA), and polyimide (PI).

[0100] In some embodiments, the number average molecular weight of the first polymer and the second polymer are each independently 100,000 to 5,000,000.

[0101] In some embodiments, the first pore-forming agent and the second pore-forming agent are each independently selected from at least one of polystyrene, ammonium carbonate, sodium bicarbonate, ammonium chloride, and starch.

[0102] In some embodiments, the first solvent and the second solvent are each independently selected from at least one of water, ethanol, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N-methylpyrrolidone (NMP).

[0103] In some embodiments, in step (1), in the first spinning solution, the mass volume concentration of the first polymer is 0.05 g / mL-0.2 g / mL, and the mass volume concentration of the first porogen is 0.005 g / mL-0.4 g / mL.

[0104] In some embodiments, in step (1), in the second spinning solution, the mass volume concentration of the second polymer is 0.05 g / mL-0.2 g / mL, and the mass volume concentration of the second porogen is 0.001 g / mL-0.1 g / mL.

[0105] It should be noted that the diameter and porosity of the first carbon fiber skeleton can be adjusted by adjusting the mass volume concentration of the first polymer in the first spinning solution, the type of the first polymer, and the electrospinning parameters; the porosity of the first porous carbon layer can be adjusted by adjusting the mass volume concentration of the first polymer and the first porogen.

[0106] The porosity of the second porous carbon layer, the diameter of the second carbon fiber skeleton and the porosity are adjusted in the same manner as the porosity of the first porous carbon layer and the diameter of the first carbon fiber skeleton.

[0107] In some embodiments, in the steps (1-3), the porosity of the porous carbon layer, the thickness of the porous carbon layer, the diameter of the porous carbon fiber skeleton and the porosity of the porous carbon fiber skeleton can be flexibly controlled by changing the corresponding spinning solution parameters and electrospinning parameters, and have the following rules: ① The mass volume concentration of the polymer and the pore-forming agent in the spinning solution will affect the porosity of the porous carbon fiber skeleton. To a certain extent, the higher the amount of the pore-forming agent, the greater the porosity; the type of pore-forming agent will affect the porosity in the carbon fiber skeleton, and the porosity corresponding to the inorganic small molecule pore-forming agent is smaller; ② The diameter of the carbon fiber skeleton depends on the type of polymer, The mass volume concentration of substances in the spinning solution and electrospinning parameters: To a certain extent, the greater the mass volume concentration of substances in the spinning solution, the larger the diameter of the electrospinning nozzle, the lower the voltage, and the larger the diameter of the obtained carbon fiber skeleton; otherwise, the smaller it is; ③ The porosity of the porous carbon layer depends on the electrospinning parameters: the larger the diameter of the electrospinning nozzle, the lower the voltage, the slower the drum speed, the larger the diameter of the spun fiber, and the denser the stacking, the smaller the porosity of the porous carbon layer; otherwise, the larger it is; ④ The thickness of the porous carbon layer depends on the electrospinning parameters: under the same conditions, the longer the spinning time, the thicker the carbon layer; otherwise, the thickness is smaller.

[0108] In some embodiments, in step (4), the carbonization treatment is carried out in an inert gas environment, at a temperature of 700-900° C., and for a time of 1-3 h.

[0109] In some embodiments, in step (4), the method of compounding with metallic lithium includes but is not limited to lithium metal melting method, rolling compounding method, in-situ electrochemical deposition method, evaporation / sputtering method, etc.

[0110] In one embodiment of the present application, the present application provides a secondary battery, and the secondary battery includes the negative electrode plate described in the present application.

[0111] In one embodiment, the secondary battery further includes a positive electrode plate, a separator, and an electrolyte.

[0112] In one embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a positive electrode active material. This application does not limit the positive electrode active material, and any known positive electrode active material may be used.

[0113] Illustratively, the positive electrode active material may be at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron manganese phosphate, and the like.

[0114] In one embodiment, the electrolyte further comprises an organic solvent, a lithium salt, and an additive. The present application has no limitation on the organic solvent, lithium salt, and additive in the electrolyte, and any known organic solvent, lithium salt, and additive can be used.

[0115] Illustratively, the organic solvent includes at least one of ethylene carbonate (EC), propylene carbonate, diethyl carbonate (DEC), ethyl methyl carbonate, and dimethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide; and the additive includes fluoroethylene carbonate (FEC).

[0116] In one embodiment, the separator of the secondary battery is disposed between the positive electrode and the negative electrode.

[0117] Example 1

[0118] The present invention provides a lithium-carbon composite negative electrode sheet and a secondary battery. The preparation method of the lithium-carbon composite negative electrode sheet and the secondary battery includes the following steps:

[0119] (1) Preparation of negative electrode sheet

[0120] S1. Dispersing a first polymer (PVDF-HFP, 10 g, number average molecular weight of 600,000, purchased from Zhongnai 2707 of Sinochem Blue Sky) as a carbon source and a first pore-forming agent (ammonium bicarbonate, 2 g) in a first solvent (water, 100 mL) and stirring uniformly to obtain a first spinning solution;

[0121] S2. A second polymer (PVDF-HFP, 10 g, number average molecular weight of 600,000, purchased from Zhonghua Lantian Zhongnai 2707) as a carbon source and a second pore-forming agent (ammonium bicarbonate, 0.5 g) were dispersed in a second solvent (water, 100 mL) and stirred to obtain a second spinning solution;

[0122] S3, electrospinning the first spinning solution (parameters: voltage 10 kV, distance from nozzle to drum 15 cm, drum speed 500 r / min, time 5 min) and then drying at 80° C. to obtain a first porous carbon layer prepolymer, and then electrospinning the second spinning solution on both sides of the first porous carbon layer prepolymer (parameters: voltage 10 kV, distance from nozzle to drum 15 cm, drum speed 500 r / min, time 10 min) and then drying at 80° C. to obtain a composite prepolymer;

[0123] S4, placing the composite prepolymer in a nitrogen environment and carbonizing it at 800°C for 2 hours, then covering it on the surface of heated molten metal lithium for lithium composite, and finally taking it out to obtain a lithium-carbon composite negative electrode;

[0124] (2) Preparation of positive electrode sheet

[0125] Lithium cobalt oxide, polyvinylidene fluoride (PVDF), and Super P were added to N-methylpyrrolidone (NMP) in a mass ratio of 90:5:5, stirred and mixed for 4 hours, and then the mixed slurry was evenly coated on the surface of aluminum foil and dried at 80°C in a vacuum oven for 8 hours to obtain a positive electrode sheet.

[0126] (3) Electrolyte

[0127] The electrolyte used is as follows: lithium salt LiPF6, solvent (ethylene carbonate (EC): diethyl carbonate (DEC): fluoroethylene carbonate (FEC)) = 30:70:5, mass ratio), additive FEC mass concentration of 10%, lithium salt molar concentration of 1 mol / L;

[0128] (4) Preparation of secondary batteries

[0129] The prepared positive electrode sheet, negative electrode sheet, separator (polyethylene porous polymer film) and other battery components are assembled, and a secondary battery is obtained through processes such as shaping, baking, packaging, liquid injection, formation, and capacity division.

[0130] Examples 2-5

[0131] The embodiment of the present application provides a negative electrode plate and a secondary battery. The difference from Example 1 is that the amount of porogen added to the first spinning solution and the second spinning solution and the electrospinning parameters are adjusted to achieve the parameters in Table 1-2.

[0132] Examples 6-8

[0133] The embodiment of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the electrospinning parameters are adjusted to achieve the parameters in Table 1-2.

[0134] Example 9

[0135] The embodiment of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the amount of the porogen added to the second spinning solution and the electrospinning parameters are adjusted to achieve the parameters in Table 1-2.

[0136] Example 10

[0137] The embodiment of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the amount of the porogen added to the first spinning solution and the electrospinning parameters are adjusted to achieve the parameters in Table 1-2.

[0138] Examples 11-12

[0139] The embodiment of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the electrospinning parameters are adjusted to achieve the parameters in Table 1-2.

[0140] Examples 13-15

[0141] The embodiment of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the composite amount of metallic lithium is adjusted to achieve the parameters in Table 1-2.

[0142] Comparative Example 1

[0143] The comparative example of the present application provides a negative electrode plate and a secondary battery. The difference from Example 1 is that the negative electrode plate is a lithium metal foil with a thickness of 20 μm composited on the two surfaces of a copper foil with a thickness of 10 μm.

[0144] Comparative Example 2

[0145] The comparative example of the present application provides a negative electrode plate and a secondary battery, which differ from Example 1 in that it only has a first porous carbon layer.

[0146] Comparative Example 3

[0147] The comparative example of the present application provides a negative electrode plate and a secondary battery, which differ from Example 1 in that it only has a second porous carbon layer.

[0148] Comparative Example 4

[0149] The comparative example of the present application provides a negative electrode plate and a secondary battery, which differs from Example 1 in that the first porous carbon layer is disposed on both surfaces of the second porous carbon layer.

[0150] The porosity P1 of the first porous carbon layer, the porosity P2 of the second porous carbon layer, the thickness H1 of the first porous carbon layer, the thickness H2 of the second porous carbon layer, H1 / H2, the unit area mass m of the metallic lithium, A, the thickness H of the porous carbon current collector, the porosity K1 of the first carbon fiber skeleton, the porosity K2 of the second carbon fiber skeleton, the average diameter D1 of the first carbon fiber skeleton, the average diameter D2 of the second carbon fiber skeleton, D1 / K1, and D2 / K2 parameters in the embodiments and comparative examples are shown in Table 1-2;

[0151] Table 1

[0152] P1 / ％ P2 / ％ H1 / μm H2 / μm H1 / H2 <![CDATA[m / mg / cm 2 ]]> A Example 1 30 70 10 20 0.5 0.2 0.0065 Example 2 40 77 20 15 1.3 0.2 0.0065 Example 3 20 66.7 6 22 0.3 0.2 0.0065 Example 4 11 51 1 4.5 0.2 0.2 0.043 Example 5 50 90 50 25 2 0.2 0.0029 Example 6 30 70 16 32 0.5 0.2 0.0040 Example 7 30 70 4 23 0.2 0.2 0.0060 Example 8 30 70 30 10 3.0 0.2 0.0087 Example 9 30 77 10 20 0.5 0.2 0.0059 Example 10 45 70 10 20 0.5 0.2 0.0062 Example 11 30 70 10 20 0.5 0.2 0.0065 Example 12 30 70 10 20 0.5 0.2 0.0065 Example 13 30 70 10 20 0.5 0.5 0.016 Example 14 30 70 10 20 0.5 0.05 0.0016 Example 15 30 70 10 20 0.5 1.335 0.043 Comparative Example 1 / / / / / / / Comparative Example 2 30 / 50 / / 0.2 0.013 Comparative Example 3 / 70 / 50 / 0.2 0.0029 Comparative Example 4 70 30 10 20 0.5 0.2 0.011

[0153] Table 2

[0154]

[0155]

[0156] The cycle performance and safety performance of the secondary batteries prepared in the examples and comparative examples are shown in Table 3; wherein the test method includes the following steps:

[0157] (1) Cyclic performance test

[0158] Take 5 secondary batteries prepared in each of the examples and comparative examples, and repeatedly charge and discharge the secondary batteries through the following steps, and calculate the cycle capacity retention rate of the secondary batteries:

[0159] First, in an environment of 25°C, the first charge and discharge were performed. Constant current and constant voltage charging was performed at a charging current of 0.1C (i.e., the current value at which the theoretical capacity is completely discharged within 10 hours) until the upper limit voltage reached 4.5V. Then, constant current discharge was performed at a discharge current of 1C until the final voltage reached 3V. The discharge capacity of the first cycle was recorded. Then, charge and discharge cycles were repeated until the cycle capacity retention rate reached 80%.

[0160] Cycle capacity retention rate = (discharge capacity of the cycle / discharge capacity of the first cycle) × 100%.

[0161] (2) Pole expansion rate measurement

[0162] Initial thickness: Before assembling the lithium-carbon composite negative electrode sheet into a secondary battery, use a micrometer to measure the thickness of the center area of ​​the negative electrode sheet. Take 3 points and measure 3 times. The average thickness is taken as the initial thickness T0;

[0163] Thickness after expansion: After the secondary battery is fully charged, disassemble and remove the negative electrode; use a micrometer to measure the thickness of the center area of ​​the electrode, select 3 points, measure 3 times, and the average thickness is the thickness after expansion T1;

[0164] Calculation of expansion rate: (T1-T0) / T0×100%.

[0165] Table 3

[0166]

[0167] As can be seen from Table 3, when the lithium-carbon composite negative electrode sheet prepared by the method provided by the present application is used in the preparation of a secondary battery, the obtained secondary battery has excellent cycle performance and the obtained negative electrode sheet has a small expansion rate; specifically, the obtained secondary battery has an 80% capacity retention rate for more than 106 cycles, and the expansion rate of the negative electrode sheet is less than 78.7%;

[0168] It can be seen from Examples 1-15 and Comparative Example 1 that the negative electrode sheet provided in the present application has an excellent effect of improving the cycle performance and gram capacity of the secondary rechargeable battery compared to the traditional negative electrode sheet, and can also effectively reduce the expansion rate of the negative electrode sheet; it can be seen from Examples 1-15 and Comparative Examples 2-4 that the layered structure of the negative electrode sheet of the present application plays a key role in achieving the effect of the present application. When it is not the structure in the present application, the effect of the present application cannot be achieved.

[0169] Finally, it should be noted that the above embodiments are intended to illustrate the technical solutions of the present application rather than to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A lithium-carbon composite negative electrode plate, characterized in that: The negative electrode plate includes a porous carbon current collector and metallic lithium; The porous carbon current collector includes a first porous carbon layer and second porous carbon layers respectively disposed on two surfaces of the first porous carbon layer; The porosity P1 of the first porous carbon layer and the porosity P2 of the second porous carbon layer satisfy: P1<P2.

2. The negative electrode sheet according to claim 1, characterized in that: The thickness H1 of the first porous carbon layer and the thickness H2 of the second porous carbon layer satisfy: 0.1≤H1 / H2≤2.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: The negative electrode plate satisfies: 0<A≤0.045; Where A = m / (H1×P1+2H2×P2); m mg / cm 2 It is the mass per unit area of ​​metallic lithium in the negative electrode.

4. The negative electrode sheet according to claim 3, characterized in that: At least one of the following is met: (1)10％＜P1≤50％； (2)50％＜P2≤90％； (3) 1μm≤H1≤50μm; (4) 4.5 μm ≤ H2 ≤ 50 μm; (5)0mg / cm 2 <m≤1.335mg / cm 2 。 5. The negative electrode sheet according to claim 1, characterized in that: The thickness H of the porous carbon current collector satisfies 10 μm≤H≤100 μm.

6. The negative electrode sheet according to claim 1, characterized in that: The first porous carbon layer includes a first carbon fiber skeleton, and the porosity K1 of the first carbon fiber skeleton satisfies 10%≤K1<50% and K1<P1.

7. The negative electrode sheet according to claim 1, characterized in that: The second porous carbon layer includes a second carbon fiber skeleton, and the porosity K2 of the second carbon fiber skeleton satisfies 1%≤K2<10%.

8. The negative electrode sheet according to claim 6 or 7, characterized in that: The average diameter of the first carbon fiber skeleton is D1, and the first carbon fiber skeleton satisfies 1≤D1 / K1≤20; And / or, the average diameter of the second carbon fiber skeleton is D2, and the second carbon fiber skeleton satisfies 10≤D2 / K2<50.

9. The negative electrode sheet according to claim 8, characterized in that: The D1 satisfies 0.1 μm≤D1≤5 μm; And / or, D2 satisfies 0.1 μm≤D2≤5 μm.

10. A secondary battery, characterized in that: The secondary battery comprises the negative electrode sheet according to any one of claims 1 to 9.

Images