Lithium ion battery and method for predicting, regulating and controlling hole impedance of negative pole piece

Abstract

The invention discloses a lithium ion battery and a method for predicting, regulating and controlling hole impedance of a negative pole piece. The lithium ion battery comprises a battery cell, the battery cell consists of a positive pole piece, a negative pole piece, a diaphragm and an electrolyte; the negative electrode plate is obtained by coating at least one surface of a negative electrode current collector with a negative electrode coating material and performing hot rolling, the negative electrode coating material comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder, and the negative electrode binder is formed by compounding a first binder and a second binder; wherein the hole impedance RA of the negative pole piece meets the following relational expression: RA = e (a / (a + b) + c) + 5 * d / e is greater than or equal to 2.0 and less than or equal to 15.0; wherein the peel strength YAN / m of the negative pole piece meets the following relational expression: YA = 200 * b + 2 * lgt + 5.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a lithium ion battery and a method for predicting and regulating the pore impedance of a negative electrode plate in a lithium ion battery. Background Art

[0002] Silicon-based negative electrode materials have become a hot topic in the current research of high energy density lithium-ion batteries because of their ultra-high theoretical gram capacity (the theoretical gram capacity can reach 4200mAh / g) and low delithiation potential (about 0.4V). However, due to the alloying reaction mechanism between silicon and lithium, silicon undergoes significant volume expansion (up to 300%) during the charge and discharge process. This expansion is accompanied by the gradual accumulation of internal stress, which will cause relative displacement between silicon particles, thereby reducing the contact tightness between silicon particles and binders, conductive agents and current collectors. More seriously, this poor contact may cause the active material to fall off from the surface of the current collector, thereby significantly reducing the cycle stability of the battery. Compared with traditional binders such as polystyrene-butadiene rubber (SBR), polyacrylic acid (PAA) binders contain rich carboxyl functional groups (-COOH). These carboxyl groups can form strong interactions with hydroxyl (-OH) and carboxyl groups on the surface of silicon particles through hydrogen bonds, which can enhance the contact between silicon particles and between silicon particles and conductive agents and current collectors, thereby reducing the shedding of silicon and graphite particles during charge and discharge.

[0003] However, generally speaking, the Young's modulus of polyacrylic acid (PAA) is 2 to 3 orders of magnitude higher than that of SBR, which makes polyacrylic acid (PAA) less flexible, easily leading to cracks and breakage problems during processing, and poor processing stability, thus affecting the overall performance of the electrode. Summary of the invention

[0004] The purpose of the present invention is to provide a lithium ion battery to solve the problems that the use of traditional SBR binders in negative electrode coating materials easily causes the negative electrode coating materials to fall off from the surface of the negative electrode current collector, resulting in a decrease in the overall performance of the battery, and the use of polyacrylic acid binders in negative electrode coating materials easily causes poor processing stability, which also affects the overall performance of the battery.

[0005] To achieve the above object, the present invention is implemented by the following technical solutions:

[0006] The present invention provides a lithium-ion battery, characterized in that the lithium-ion battery comprises a battery cell; the battery cell comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode coating material coated on at least one surface of the positive electrode current collector, the positive electrode coating material comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder; the negative electrode sheet is obtained by coating a negative electrode coating material on at least one surface of a negative electrode current collector and undergoing hot rolling, the negative electrode coating material comprises a negative electrode active material, a negative electrode conductive agent and a negative electrode binder, the negative electrode binder is composited by a first binder and a second binder, and the first binder is a styrene-butadiene rubber binder, and the second binder is a polyacrylic acid binder;

[0007] Among them, the hole impedance R of the negative electrode plate A (Ω·cm 2 ) satisfies the following relationship: 2.0≤R A =e (a / (a+b)+c) +5×d / e≤15.0; where: a represents the mass percentage of the first binder in the negative electrode coating material; b represents the mass percentage of the second binder in the negative electrode coating material; c represents the mass percentage of silicon in the negative electrode active material in the negative electrode coating material; d represents the single-sided surface density of the negative electrode sheet, and its unit is g / cm 2 ; e represents the thickness of the negative electrode coating material on the negative electrode sheet, and its unit is cm;

[0008] Among them, the peel strength Y of the negative electrode sheet A (N / m) satisfies the following relationship: Y A =200×b+2×lg t+5; wherein: b represents the mass percentage of the second binder in the negative electrode coating material; t represents the temperature of hot rolling ℃.

[0009] Specifically, the positive electrode current collector includes a coated area and an uncoated area of ​​the positive electrode coating material, and the uncoated area of ​​the positive electrode coating material accounts for 2.0 to 12.0% of the area of ​​the positive electrode current collector; the negative electrode current collector includes a coated area and an uncoated area of ​​the negative electrode coating material, and the uncoated area of ​​the negative electrode coating material accounts for 2.0 to 15.0% of the area of ​​the negative electrode current collector.

[0010] Furthermore, in a lithium-ion battery, the negative electrode coating material also includes a thickener.

[0011] Furthermore, in a lithium-ion battery, the thickener is sodium carboxymethyl cellulose, and the mass percentage of the thickener in the negative electrode coating material is 0.5-1.5%.

[0012] Furthermore, in a lithium-ion battery, the first binder is selected from styrene-butadiene rubber and / or polystyrene-acrylic acid; the second binder is selected from polyacrylic acid and / or polyacrylic acid-acrylonitrile.

[0013] Furthermore, in a lithium-ion battery: the mass percentage a of the first binder in the negative electrode coating material satisfies 0.5%≤a≤2.5%; the mass percentage b of the second binder in the negative electrode coating material satisfies 0.5%≤b≤3.0%; the mass percentage a of the first binder and the mass percentage b of the second binder satisfy 1.0%≤a+b≤3.5%.

[0014] Furthermore, in a lithium-ion battery, the mass percentage c of the silicon element in the negative electrode active material in the negative electrode coating material satisfies 5.0%≤c≤35.0%.

[0015] Furthermore, a lithium-ion battery: the single-surface density d of the negative electrode sheet satisfies 1×10 -3 g / cm 2 ≤d≤10×10 -3 g / cm 2 ; The thickness e of the negative electrode coating material on the negative electrode sheet satisfies 3×10 -3 cm≤e≤1×10 -2 cm.

[0016] Furthermore, in a lithium-ion battery, the rolling temperature t satisfies 120°C≤t≤160°C.

[0017] Furthermore, in a lithium-ion battery, the capacity N / P ratio of the negative electrode plate to the positive electrode plate is (1.02-1.12):1.

[0018] Furthermore, a lithium-ion battery: the chemical formula of the positive electrode active material is expressed as Li 1 Ni x Co y Mn z M w O 2 ; Wherein, 0.3≤x≤0.95, 0.1≤y<0.45, 0.05≤z<0.45, 0≤w≤0.25, x+y+z+w=1.0; M represents any one or more combinations of Zr, W, Ti, Al, Sr, B, and Nd elements.

[0019] Furthermore, in a lithium-ion battery, the negative electrode active material is selected from at least one of silicon carbon, artificial graphite, natural graphite, soft carbon or hard carbon; and the negative electrode conductive agent is a combination of carbon nanotubes and conductive carbon black.

[0020] Further, a lithium-ion battery: the electrolyte includes a lithium salt, a solvent and an additive; the lithium salt includes any one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; the additive includes any one or more combinations of propylene carbonate, butylene carbonate, ethyl acetate, ethyl methyl carbonate and fluoroethylene carbonate; the solvent includes any one or more combinations of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate. Preferably, the electrolyte additive helps to reduce the battery membrane resistance and improve the electrochemical performance of the lithium-ion battery.

[0021] Furthermore, a lithium-ion battery is provided: the lithium-ion battery is a cylindrical battery with a diameter of 15.0 to 70.0 mm and a length of 60.0 to 145.0 mm.

[0022] The present invention also provides a method for predicting and regulating the pore impedance of the negative electrode sheet in the above-mentioned lithium-ion battery, the method comprising: according to the pore impedance R of the negative electrode sheet A (Ω·cm 2 ) satisfies the following relationship: 2.0≤R A =e (a / (a+b)+c) +5×d / e≤15.0; by adjusting the mass percentage a of the first binder in the negative electrode coating material, adjusting the mass percentage b of the second binder in the negative electrode coating material, adjusting the mass percentage c of the silicon element in the negative electrode active material in the negative electrode coating material, adjusting the single-sided surface density d of the negative electrode plate, and adjusting the thickness e of the negative electrode coating material on the negative electrode plate, the pore impedance R of the negative electrode plate in the lithium-ion battery is predicted and controlled A .

[0023] The present invention also provides an electrical device, which comprises the lithium-ion battery mentioned above.

[0024] Beneficial effects of the present invention:

[0025] (1) The present invention uses a composite binder system of styrene-butadiene rubber binder (SBR) and polyacrylic acid binder (PAA) and combines it with a hot rolling process to enhance the bonding strength between the negative electrode active material, the negative electrode conductive agent and the negative electrode current collector, thereby improving the flexibility and processing stability of the negative electrode plate; specifically, polyacrylic acid (PAA) as a binder can not only significantly enhance the bonding strength between the negative electrode active material, the negative electrode conductive agent and the negative electrode current collector, but also effectively promote the effective transmission of lithium ions in the pores of the negative electrode plate; while the styrene-butadiene rubber binder (SBR) provides the necessary processing toughness, improves the mechanical stability of the plate, and effectively resists the stress caused by volume expansion during the charge and discharge process. During the hot rolling process, the temperature is close to the glass transition temperature of PAA, so that the PAA chain segments partially melt and rearrange, thereby enhancing its contact with the negative electrode active material, the negative electrode conductive agent and the negative electrode current collector, which helps to improve the electron transmission efficiency. In addition, cross-linking will also occur between the PAA segments (the cross-linking process is ), which helps to form a three-dimensional network structure and further improve the toughness of the electrode. At the same time, the hot rolling process can also change the pore morphology in the electrode, making its distribution and connectivity more uniform, and increasing the migration rate of lithium ions between the pores.

[0026] (2) In the lithium-ion battery provided by the present invention, a new mathematical formula is constructed to describe the relationship between the pore impedance of the negative electrode plate and the mass percentage a of SBR, the mass percentage b of PAA, the mass percentage c of silicon in the negative electrode active material, the single-surface surface density d of the negative electrode plate, and the thickness e of the negative electrode coating material on the negative electrode plate. By regulating the above variables, the pore impedance R of the negative electrode plate can be effectively predicted and optimized. A , and the pore impedance and the rate of lithium-ion batteries have a strong correlation, so the mathematical formula constructed by the present invention can be beneficial to the pore impedance R of the negative electrode sheet A The optimized design can improve the efficiency of lithium-ion battery design. The present invention combines the composite binder system with the hot rolling process, which not only improves the overall performance of silicon-based negative electrode lithium-ion batteries, but also provides a feasible solution for predicting and regulating the hole impedance of the negative electrode sheet.

[0027] (3) The average hole impedance R of the negative electrode obtained through actual testing B and the hole impedance R of the negative electrode sheet predicted by the mathematical formula constructed by the present invention A The deviation of the results is small, which shows the accuracy of the prediction results constructed by the mathematical formula of the present invention. DETAILED DESCRIPTION

[0028] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Example 1

[0030] This embodiment 1 provides a lithium-ion battery, which includes a battery cell; the battery cell is composed of a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte;

[0031] The positive electrode sheet is composed of a positive electrode current collector (aluminum foil) and a positive electrode coating material coated on both surfaces of the positive electrode current collector. The positive electrode coating material includes a positive electrode active material (Li 1 Ni 0.8 Co 0.1 Mn 0.1 O 2 ), positive electrode conductive agent (conductive carbon black and carbon nanotubes) and positive electrode binder (polyvinylidene fluoride (PVDF));

[0032] The negative electrode sheet is obtained by coating the negative electrode coating material on both surfaces of the negative electrode current collector (copper foil) and hot rolling at 140° C. The negative electrode coating material includes a negative electrode active material (silicon carbon), a negative electrode conductive agent (conductive carbon black and carbon nanotubes), a negative electrode binder (composite of SBR and binder PAA) and a thickener (sodium carboxymethyl cellulose);

[0033] Among them, the hole impedance R of the negative electrode plate A (Ω·cm 2 ) satisfies the following relationship:

[0034] 2.0≤R A =e (a / (a+b)+c) +5×d / e≤15.0; where: a represents the mass percentage of the first binder in the negative electrode coating material; b represents the mass percentage of the second binder in the negative electrode coating material; c represents the mass percentage of silicon in the negative electrode active material in the negative electrode coating material; d represents the single surface density of the negative electrode sheet g / cm 2 ; e represents the thickness of the negative electrode coating material on the negative electrode sheet in cm;

[0035] Among them, the peel strength Y of the negative electrode sheet A (N / m) satisfies the following relationship:

[0036] YA =200×b+2×lg t+5; wherein: b represents the mass percentage of the second binder in the negative electrode coating material; t represents the temperature of hot rolling ℃.

[0037] The method for manufacturing a lithium-ion battery provided in the above embodiment 1 comprises the following specific steps:

[0038] 1. Production of positive electrode:

[0039] Take the positive electrode active material (Li 1 Ni 0.8 Co 0.1 Mn 0.1 O 2 ), conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) are fully stirred and mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96.0:1.0:1.0:2.0 to obtain a positive electrode coating material, and then the positive electrode coating material is evenly coated on a 15.0 μm thick positive electrode collector (aluminum foil), and finally a positive electrode sheet is obtained after drying and cold pressing;

[0040] 2. Production of negative electrode:

[0041] The negative electrode active material (silicon carbon material (Si content is 10.0wt%, the balance is graphite)), conductive carbon black, carbon nanotubes, thickener sodium carboxymethyl cellulose, SBR binder (first binder) and PAA binder (second binder) are mixed in a mass ratio of 96.0:0.5:0.5:0.5:0.5:2.0, and after being fully stirred, added into deionized water and stirred to form a negative electrode coating material, which is then coated on the negative electrode collector, and after drying and hot rolling at 140°C (t=140°C), a negative electrode sheet is formed. The single-sided surface density d of the obtained negative electrode sheet is 4×10 -3 g / cm 3 The thickness of the negative electrode coating material on the negative electrode sheet is 5×10 - 3 cm;

[0042] Thus, it can be calculated that in Example 1, the mass percentage a of the first binder in the negative electrode coating material is 0.5%, the mass percentage b of the second binder in the negative electrode coating material is 2.0%, and the mass percentage c of the silicon element in the negative electrode active material in the negative electrode coating material is 9.6%;

[0043] 3. Preparation of electrolyte:

[0044] Dissolving lithium hexafluorophosphate in an organic solvent to obtain an electrolyte with a concentration of 1.0 mol / L; wherein the organic solvent is a mixture of ethylene carbonate, fluoroethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 15:15:20:50;

[0045] 4. Diaphragm:

[0046] The diaphragm is made of a material with high porosity, wherein the base film in the diaphragm is a PE base film with a thickness of 9.0 μm, and both sides of the base film are coated with a ceramic coating with a thickness of 1.0 μm and a PVDF coating with a thickness of 1.0 μm respectively;

[0047] 5. Assembly of lithium-ion batteries:

[0048] The positive electrode sheet and the negative electrode sheet are rolled and slit respectively, and then wound together with the separator to obtain a 21700 cylindrical battery cell. The battery cell is then welded to the connecting sheet and loaded into a battery shell. After completing the injection, sealing and formation processes, the lithium-ion battery of Example 1 is obtained, and the shell of the lithium-ion battery is a cylinder with the dimensional parameters of diameter: 21.0 mm and length 70.0 mm.

[0049] Specifically, the method for characterizing the mass percentage of the first binder (SBR) and the second binder (PAA) in the negative electrode coating material in the above-mentioned embodiment 1 comprises the following steps:

[0050] ① First, the negative electrode sheet rinsed with dimethyl carbonate and vacuum dried was placed in an ethyl acetate solution and subjected to ultrasonic treatment for 1 hour;

[0051] ② Subsequently, after centrifugation and filtration, infrared spectroscopy is used to determine the type of binder in the electrode. The C=C vibration of the benzene ring in the first binder usually has an obvious absorption peak near 1500cm-1; the second binder will have a strong carbonyl (C=O) vibration absorption peak near 1700cm-1 in the infrared spectrum; in addition, the OH stretching vibration of the -COOH functional group usually appears in the region of 3200-3600cm-1, and this region will also show a broad absorption peak; taking SBR and PAA as an example, if the presence of SBR and PAA is detected, thermogravimetric analysis (TG) will be further used to measure the relationship between the mass change of the negative electrode during heating and the temperature change to determine the mass percentage of SBR and PAA; since the thermal decomposition temperature of SBR is much higher than that of PAA, standard samples of commercial SBR and PAA can be used for TG testing to determine their respective decomposition temperature ranges, thereby calculating the content of SBR and PAA in the negative electrode coating material.

[0052] Specifically, the test method for the thickness e of the negative electrode coating material on the negative electrode sheet and the surface density d of the negative electrode sheet in the above-mentioned embodiment 1 is:

[0053] ① First, the negative electrode sheet of Example 1, which had been rinsed with dimethyl carbonate and vacuum dried, was cut into 6 square samples of standard size (2.0 cm×2.0 cm);

[0054] ② Then, the negative electrode coating material coated on the front and back surfaces of three of the square samples was wiped off, and then the samples were rinsed with ethanol, dried and weighed, and the average mass M1 of the three samples after the negative electrode coating material was wiped off was calculated, and the average thickness L1 of the samples was measured using a spiral micrometer;

[0055] ③Finally, weigh the mass of the other three square samples of the negative electrode coating material on the surface that have not been wiped off and calculate the average mass M2. At the same time, measure the average thickness L2 of these three samples, and calculate the thickness e of the negative electrode coating material on the negative electrode sheet, e=L2-L1, unit cm; calculate the surface density d of the negative electrode sheet: Unit: g / cm 2 .

[0056] Example 2

[0057] This embodiment 2 provides a lithium ion battery. The difference between this embodiment 2 and embodiment 1 is that in embodiment 2, the mass percentage a of the first binder is 1.0%, the mass percentage b of the second binder is 1.5%, and the other conditions are the same as those in embodiment 1.

[0058] Example 3

[0059] This embodiment 3 provides a lithium ion battery. The difference between this embodiment 3 and embodiment 1 is that in embodiment 3, the mass percentage a of the first binder is 1.5%, the mass percentage b of the second binder is 1.0%, and the other conditions are the same as those in embodiment 1.

[0060] Example 4

[0061] This embodiment 4 provides a lithium ion battery. The difference between this embodiment 4 and embodiment 1 is that the mass percentage c of silicon element in the negative electrode active material in the negative electrode coating material in embodiment 4 is 15.0%, and the other conditions are the same as those in embodiment 1.

[0062] Example 5

[0063] This embodiment 5 provides a lithium ion battery. The difference between this embodiment 5 and embodiment 1 is that the mass percentage c of silicon element in the negative electrode active material in the negative electrode coating material in embodiment 5 is 20.0%, and the other conditions are the same as those in embodiment 1.

[0064] Example 6

[0065] This embodiment 6 provides a lithium ion battery. The difference between this embodiment 6 and embodiment 1 is that the mass percentage c of silicon element in the negative electrode active material in the negative electrode coating material in embodiment 6 is 30.0%, and the other conditions are the same as those in embodiment 1.

[0066] Example 7

[0067] This embodiment 7 provides a lithium ion battery. The difference between this embodiment 7 and embodiment 1 is that the surface density d of the negative electrode sheet in embodiment 7 is 5×10 -3 g / cm 2 , the other conditions are the same as those in Example 1.

[0068] Example 8

[0069] This embodiment 8 provides a lithium ion battery. The difference between this embodiment 8 and embodiment 1 is that the surface density d of the negative electrode sheet in embodiment 8 is 6×10 -3 g / cm 2 , the other conditions are the same as those in Example 1.

[0070] Example 9

[0071] This embodiment 9 provides a lithium ion battery. The difference between this embodiment 9 and embodiment 1 is that the surface density d of the negative electrode sheet in embodiment 9 is 7×10 -3 g / cm 2 , the other conditions are the same as those in Example 1.

[0072] Example 10

[0073] This embodiment 10 provides a lithium ion battery. The difference between this embodiment 10 and embodiment 1 is that the thickness e of the negative electrode coating material on the negative electrode plate in embodiment 10 is 4.5×10 -3 cm, and the other conditions are the same as those in Example 1.

[0074] Embodiment 11

[0075] This embodiment 11 provides a lithium ion battery. The difference between this embodiment 11 and embodiment 1 is that the thickness e of the negative electrode coating material on the negative electrode plate in embodiment 11 is 4.0×10 -3 cm, and the other conditions are the same as those in Example 1.

[0076] Example 12

[0077] This embodiment 12 provides a lithium ion battery. The difference between this embodiment 12 and embodiment 1 is that the thickness e of the negative electrode coating material on the negative electrode plate in embodiment 12 is 3.5×10 -3cm, and the other conditions are the same as those in Example 1.

[0078] Comparative Example 1

[0079] The difference between Comparative Example 1 and Example 1 is that the first binder (SBR) is not added to the negative electrode coating material of Comparative Example 1, and the mass percentage b of the added second binder (PAA) is 2.5%, and the other conditions are the same as those of Example 1.

[0080] Comparative Example 2

[0081] The difference between Comparative Example 2 and Example 1 is that the second binder (PAA) is not added to the negative electrode coating material of Comparative Example 2, the mass percentage a of the added first binder (SBR) is 2.5%, and the other conditions are the same as those of Example 1.

[0082] Comparative Example 3

[0083] The difference between Comparative Example 3 and Example 1 is that the surface density d of the negative electrode sheet in Comparative Example 3 is 1×10 -2 g / cm 2 , which is greater than the surface density d of the negative electrode plate in Example 1, and the other conditions are the same as in Example 1.

[0084] Comparative Example 4

[0085] The difference between Comparative Example 4 and Example 1 is that the thickness e of the negative electrode coating material on the negative electrode sheet in Comparative Example 4 is 2×10 -3 cm, which is less than the thickness e of the negative electrode coating material on the negative electrode plate in Example 1. The rolling temperature t in Comparative Example 4 is 180° C., and the other conditions are the same as those in Example 1.

[0086] Comparative Example 5

[0087] Comparative Example 5 provides a lithium ion battery. The difference between Comparative Example 5 and Example 10 is that the rolling temperature t in Comparative Example 5 is 20° C., and the other conditions of Comparative Example 5 are the same as those of Example 10.

[0088] Comparative Example 6

[0089] Comparative Example 6 provides a lithium ion battery. The difference between Comparative Example 6 and Example 11 is that the rolling temperature t in Comparative Example 6 is 60° C., and the other conditions of Comparative Example 6 are the same as those of Example 11.

[0090] Comparative Example 7

[0091] Comparative Example 7 provides a lithium ion battery. The difference between Comparative Example 7 and Example 12 is that the rolling temperature t in Comparative Example 7 is 100° C., and the other conditions of Comparative Example 7 are the same as those of Example 12.

[0092] Example 13

[0093] This Example 13 provides an electrical device, which includes the lithium-ion battery of Example 1.

[0094] Example 14

[0095] This Example 14 provides a method for predicting and regulating the pore impedance of the negative electrode plate in a lithium-ion battery. The method is as follows: According to the following relational expression satisfied by the pore impedance R A : 2.0 ≤ R A = e (a / (a+b)+c) + 5×d / e ≤ 15.0; By adjusting the mass percentage a of the first binder in the negative electrode coating material, adjusting the mass percentage b of the second binder in the negative electrode coating material, adjusting the mass percentage c of silicon element in the negative electrode active material in the negative electrode coating material, adjusting the single-sided surface density d of the negative electrode plate, and adjusting the thickness e of the negative electrode coating material on the negative electrode plate, to predict and regulate the pore impedance R A of the negative electrode plate in the lithium-ion battery. And there is a strong correlation between the pore impedance and the rate performance of the lithium-ion battery. Therefore, this method is convenient for optimizing the design of the negative electrode plate, reducing the pore impedance of the negative electrode plate, thereby improving the cycle stability and rate performance of the battery.

[0096] Test:

[0097] (1) The test method for the pore impedance of the negative electrode plate includes the following steps:

[0098] ① Preparation of a symmetric soft-pack battery: Discharge the lithium-ion cylindrical battery to 2.5V. Inside the glove box, carefully disassemble the battery and take out the negative electrode plate of the battery cell. Immerse the negative electrode plate disassembled from the battery cell in dimethyl carbonate for 5 minutes to remove possible impurities and residues. Then place the electrode plate in a vacuum drying oven at 80°C for 12 hours to ensure its full drying. After drying, cut the negative electrode plate into a head, a middle part, and a tail, and scrape off some active substances, and then weld the tab for later use; Next, cut out a circular hole with a diameter of 14.0 mm from the high-temperature insulating tape and stick it tightly on the separator to play a confinement role; Subsequently, stack the negative electrode plate, the separator with the tape attached, and another negative electrode plate in sequence, and weld the negative electrode tab; Finally, perform electrolyte injection (the electrolyte is obtained by dissolving LiPF 6 in a mixed solvent of ethylene carbonate and ethyl methyl carbonate. The concentration of LiPF 6 in this electrolyte is 1.0 mol / L, and the volume ratio of ethylene carbonate to ethyl methyl carbonate is 1:1) and sealing treatment, so as to finally obtain a symmetric soft-pack battery; Make 3 symmetric soft-pack batteries from the head, the middle part, and the tail respectively;

[0099] ②Pore impedance test method:

[0100] Use an electrochemical workstation to perform electrochemical impedance spectroscopy (EIS) tests on the above 3 symmetric soft-pack batteries. The test frequency range is 1.0 Hz to 100 kHz, and the test temperature is 25 °C. By analyzing the EIS data, the pore impedance values of the 3 symmetric soft-pack batteries can be obtained, denoted as R 1 , R 2 and R 3 respectively. Calculate the average value R B =(R 1 +R 2 +R 3 ) / 3.

[0101] Thus, according to the pore impedance mathematical formula of the negative electrode sheet constructed by the present invention, the pore impedance R A of the negative electrode sheets of Examples 1 to 12 and Comparative Examples 1 to 7 predicted, and the average pore impedance R B of the negative electrode sheets of Examples 1 to 12 and Comparative Examples 1 to 7 actually measured by the above pore impedance test method are shown in Table 1 below:

[0102]

[0103]

[0104] From the test results of Examples 1 to 3 in Table 1, it can be seen that as the mass percentage b of the second binder (PAA) in the negative electrode coating material decreases, the pore impedance of the negative electrode sheet gradually increases. This is because PAA contains richer carboxyl (-COOH) functional groups compared to the first binder (SBR), and the H + / Li + exchange reaction between it and lithium ions (Li + ) can effectively improve the migration rate of lithium ions, thereby reducing the pore impedance.

[0105] From the test results of Examples 4 to 6 in Table 1, it can be seen that as the mass percentage c of silicon element in the negative electrode coating material increases, the pore impedance of the negative electrode sheet shows an increasing trend. This is mainly attributed to the fact that as the silicon element content increases, the negative electrode sheet will experience greater volume expansion during charge and discharge, which may lead to the destruction of the pore structure, thereby increasing the pore impedance.

[0106] From the test results of Examples 7 to 9 in Table 1, it can be seen that as the surface density d of the negative electrode sheet increases, the pore impedance of the negative electrode sheet also gradually increases.

[0107] It can be seen from the test results of Examples 10 to 12 in Table 1 that when the surface density d of the negative electrode plate is fixed and the thickness e of the negative electrode coating material on the negative electrode plate is further reduced, the pore impedance will also increase significantly. This is because increasing the surface density d or reducing the thickness e of the negative electrode coating material will lead to an increase in the apparent compaction degree of the negative electrode plate, thereby compressing the pores, which has an adverse effect on the diffusion of lithium ions.

[0108] It can be seen from the test results of Comparative Examples 1 to 4 in Table 1 that not adding SBR or PAA and too high pole piece single surface density d or too small negative electrode coating material thickness e will lead to a significant increase in the pore impedance of the negative electrode piece, and will lead to the pore impedance R predicted by the mathematical formula constructed by the present invention. A Compared with the measured hole impedance R B There is a large deviation.

[0109] It can be seen from the test results of Comparative Examples 5 to 7 in Table 1 that as the hot rolling temperature t of the negative electrode sheet decreases, the pore impedance of the negative electrode sheet shows an increasing trend. This is mainly attributed to the fact that the temperature decreases, which weakens the movement of the PAA segments and makes it difficult to form a uniform pore structure, resulting in an increase in pore impedance. Therefore, it is crucial to select a suitable binder ratio, a low silicon content, a suitable electrode sheet surface density, thickness and rolling temperature to optimize the pore impedance of the negative electrode sheet. Based on the above analysis, Example 1 has the optimal binder ratio, silicon content, electrode sheet surface density d and thickness e, and therefore exhibits the lowest electrode sheet pore impedance.

[0110] It can be seen from the test results of Examples 1 to 12 in Table 1 that, under limited conditions, the pore impedance of the negative electrode sheet of the above embodiments all conforms to the estimation formula of pore impedance, which is consistent with the arithmetic average pore impedance R obtained by actual test. B The deviation value is less than 10.0%, which indicates the accuracy of the mathematical formula constructed by the present invention in predicting the pore impedance results. Therefore, the mathematical formula constructed by the present invention can more accurately predict the pore impedance of the lithium-ion battery, thereby improving the efficiency of the battery design.

[0111] (2) A method for testing the peel strength of a negative electrode sheet comprises the following steps:

[0112] ① First, the negative electrode sheets disassembled from the battery cells are cleaned with dimethyl carbonate and vacuum dried, and then cut into long strips of standard size;

[0113] ② Then, use double-sided tape to fix the long sample on a flat thin steel plate, make sure the tape is attached to the center of the steel plate, and smooth it hard to eliminate bubbles;

[0114] ③ Then, after peeling off the protective layer of the double-sided tape, stick the long strip of the electrode to be tested on the double-sided tape, and use a pressure roller to evenly roll the sample to enhance adhesion;

[0115] ④Finally, tear off the unpasted end, bend the torn electrode piece naturally upward, and clamp it in the upper fixture of the tensile testing machine for a 180° peeling test. Record the tensile force curve. In the stable stage where the tensile force change does not exceed 10%, select this segment of data to calculate the average tensile force, and divide it by the electrode piece width to obtain the peel strength of the battery electrode piece.

[0116] Therefore, the peel strength Y of the negative electrode sheets of Examples 1 to 12 and Comparative Examples 1 to 7 predicted by the mathematical formula for the peel strength of the negative electrode sheet constructed according to the present invention is A The peel strength Y of the negative electrode sheets of Examples 1 to 12 and Comparative Examples 1 to 7 measured by the above-mentioned negative electrode sheet peel strength test method is B The results are shown in Table 2 below:

[0117]

[0118]

[0119] It can be seen from the test results of Examples 1 to 3 in Table 2 that increasing the content of PAA can increase the bonding strength between the active particles and the foil. It can be seen from Comparative Examples 5 to 7 that the reduction of the hot rolling temperature is not conducive to the formation of a tight bond between the active particles and the foil. The lower rolling temperature will lead to a decrease in the fluidity of the PAA binder, thereby affecting its distribution in the pole piece and reducing its bonding strength with the silicon particles, the conductive agent and the foil. It can be seen from Comparative Example 4 that a rolling temperature of 180°C will seriously reduce the peel strength of the pole piece, which is attributed to the fact that the rolling temperature of 180°C may cause the PAA chain segment to partially decompose, thereby weakening the bonding force.

[0120] It can be seen from the peel strength test results of Examples 1 to 12 in Table 2 that the negative electrode sheet peel strength Y estimated by the negative electrode sheet peel strength formula constructed by the present invention is A Compared with the measured negative electrode peel strength Y B The result deviation value is less than 10%, which shows that the peel strength Y predicted by the mathematical formula for the peel strength of the negative electrode sheet constructed by the present invention is A The accuracy of the results, therefore, the mathematical formula constructed by the present invention can more accurately predict the peel strength of the negative electrode sheet in the lithium-ion battery, and the peel strength has a strong correlation with the performance of the battery. Therefore, the peel strength of the negative electrode sheet can be intuitively designed according to the mathematical formula of the electrode sheet peel strength constructed by the present invention, thereby improving the efficiency of battery design.

[0121] (3) Rate performance test method:

[0122] Place the battery in a 25°C incubator for 4 hours and test it as follows:

[0123] ① Under the condition of 0.1C, constant current and constant voltage are charged to 4.2V, the cut-off current is 0.01C, and it is left to stand for 10 minutes. The capacity charged to 4.2V by constant current is counted as Q1;

[0124] ②Discharge at a constant current of 0.1C until the battery is cut off at 2.5V, the cut-off current is 0.01C, and then leave it for 10 minutes;

[0125] ③ Under the condition of 1C, charge to 4.2V with constant current and constant voltage, the cut-off current is 0.01C, and stand for 10 minutes. The capacity charged to 4.2V with constant current is counted as Q2;

[0126] ④Discharge at a constant current of 0.1C until the battery is cut off at 2.5V, the cut-off current is 0.01C, and let it stand for 10 minutes;

[0127] ⑤ Under the condition of 2C, constant current and constant voltage are charged to 4.2V, the cut-off current is 0.01C, and it is left to stand for 10 minutes. The capacity charged to 4.2V by constant current is counted as Q3;

[0128] ⑥Discharge at a constant current of 0.1C until the battery is cut off at 2.5V, the cut-off current is 0.01C, and let stand for 10 minutes;

[0129] ⑦ Under the condition of 3C, constant current and constant voltage are charged to 4.2V, the cut-off current is 0.01C, and it is left to stand for 10 minutes. The capacity charged to 4.2V by constant current is counted as Q4;

[0130] ⑧Discharge at a constant current of 0.1C until the battery is cut off at 2.5V, the cut-off current is 0.01C, and then leave it for 10 minutes;

[0131] ⑨Under the condition of 4C, charge to 4.2V with constant current and constant voltage, the cut-off current is 0.01C, and let stand for 10 minutes. The capacity charged to 4.2V with constant current is counted as Q5;

[0132] ⑩Discharge at a constant current of 0.1C until the cutoff is 2.5V, the cutoff current is 0.01C, and let it stand for 10 minutes.

[0133] The calculation method of capacity retention rate is: QX / Q1*100, where X can be 2, 3, 4, or 5.

[0134] The capacity retention rates of the batteries of Examples 1 to 12 and Comparative Examples 1 to 4 in the constant current section under different rate conditions of 1C, 2C, 3C and 4C were statistically analyzed and compared, and the results are shown in Table 3 below:

[0135]

[0136]

[0137] It can be seen from the test results in Table 3 that the lower the pore impedance of the negative electrode, the higher the capacity retention rate during the 1C, 2C, 3C, and 4C rate charge and discharge processes, and the better rate performance can be shown. This phenomenon can be attributed to several key advantages brought about by the reduction of pore impedance: ① Lower pore impedance means that lithium ions can diffuse more quickly inside the pores of the electrode, reducing the resistance encountered by lithium ions during the charge and discharge process, which helps to improve the rate performance of the battery; ② The optimization of the pore structure helps to improve the wettability of the electrolyte to the active material, thereby improving the insertion and extraction efficiency of lithium ions; ③ Lower pore impedance is usually associated with better interface contact quality. Good contact can reduce the contact resistance between the active material and the conductive agent and the current collector foil, thereby improving the rate performance of the battery.

[0138] (4) Cyclic performance test method:

[0139] Place the battery in a 25°C constant temperature box for 4 hours and test it according to the following steps:

[0140] ① First cycle constant current and constant voltage charging: charge at a constant current of 0.1C to 4.2V, then switch to constant voltage charging until the current drops to 0.01C;

[0141] ②After charging is completed, let it stand for 10 minutes;

[0142] ③ Perform constant current discharge at a rate of 0.1C to 2.5V;

[0143] ④ Repeat the above charging and discharging process: charge at a constant current rate of 1C to 4.2V; let it stand for 10 minutes again, and discharge at a constant current rate of 1C to 2.5V;

[0144] ⑤ Repeat the above charging and discharging process for a total of 200 cycles.

[0145] After 100, 150 and 200 cycles, the capacity retention rates of the batteries of Examples 1 to 12 and Comparative Examples 1 to 4 were counted and compared, and the results are shown in Table 4 below:

[0146]

[0147]

[0148] It can be seen from the test results in Table 4 that the lower the pore impedance of the negative electrode sheet and the greater the peel strength, the higher the capacity retention rate and the better the cycle stability during 100, 150 and 200 cycles of charge and discharge. This is because smaller pore impedance means a lower degree of polarization of the battery during charge and discharge, thereby improving the transmission efficiency of lithium ions.

[0149] In addition, the greater peel strength indicates a closer contact between the active material, conductive agent, binder, and current collector foil, which helps to improve electronic conductivity and reduce the risk of material dropout in subsequent cycles. In summary, the low pore impedance and high peel strength of the negative electrode sheet jointly promote the effective transmission of lithium ions during the charge and discharge process, improve the interface contact quality, enhance the structural stability, and accelerate the chemical reaction kinetics. These factors work together to achieve higher capacity retention and better cycle stability during 100, 150, and 200 cycles of charge and discharge.

[0150] In summary, the mathematical formula constructed by the present invention can predict the pore impedance of the negative electrode sheet in a lithium-ion battery by adjusting the binder ratio, silicon content, electrode sheet surface density and thickness of the negative electrode coating material, and the pore impedance has a strong correlation with the rate of the lithium-ion battery, which can be beneficial to optimize the design of the negative electrode sheet and reduce the resistance, thereby improving the cycle stability and rate performance of the battery.

[0151] The mathematical formula constructed by the present invention can effectively predict and optimize the hole impedance of the negative electrode sheet by regulating the variables such as the binder ratio, silicon content, and the surface density of the electrode sheet and the thickness of the negative electrode coating material, thereby improving the efficiency of the battery design. At the same time, the hot rolling process in the present invention can partially melt and rearrange the binder PAA chain segments, thereby enhancing its contact with silicon particles, conductive agents and current collector foils, and can also form a certain three-dimensional cross-linked network structure to improve the peel strength of the electrode sheet.

[0152] The present invention combines a composite binder system with a hot rolling process, and predicts the peel strength of the negative electrode sheet by constructing a negative electrode sheet peel strength formula, so that the peel strength of the negative electrode sheet can be directly estimated, so that the negative electrode sheet has a higher peel strength in theoretical design, and a higher peel strength is also the basis for improving the battery rate performance. In summary, this application can facilitate the design of the pore impedance and peel strength of the negative electrode sheet by constructing a negative electrode sheet pore impedance prediction formula and a negative electrode sheet peel strength estimation formula, thereby improving the design efficiency of lithium-ion batteries. The present invention not only improves the overall performance of silicon-based negative electrode lithium-ion batteries, but also provides a feasible solution for predicting and regulating the pore impedance of negative electrode sheets.

[0153] The above are preferred embodiments of the present invention and are only used to explain the present invention, not to limit the present invention. Any obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A lithium ion battery, characterized in that: The lithium-ion battery comprises a battery cell; the battery cell is composed of a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; The positive electrode sheet is composed of a positive electrode current collector and a positive electrode coating material coated on at least one surface of the positive electrode current collector, wherein the positive electrode coating material includes a positive electrode active material, a positive electrode conductive agent and a positive electrode binder; The negative electrode sheet is obtained by coating a negative electrode coating material on at least one surface of a negative electrode current collector and then hot rolling, wherein the negative electrode coating material includes a negative electrode active material, a negative electrode conductive agent and a negative electrode binder, wherein the negative electrode binder is composited by a first binder and a second binder, wherein the first binder is a styrene-butadiene rubber binder and the second binder is a polyacrylic acid binder; Among them, the hole impedance R of the negative electrode plate A (Ω·cm 2 ) satisfies the following relationship: 2.0≤R A =e (a / (a+b)+c) +5×d / e≤15.0; where: a represents the mass percentage of the first binder in the negative electrode coating material; b represents the mass percentage of the second binder in the negative electrode coating material; c represents the mass percentage of silicon in the negative electrode active material in the negative electrode coating material; d represents the single surface density of the negative electrode sheet g / cm 2 ; e represents the thickness of the negative electrode coating material on the negative electrode sheet in cm; Among them, the peel strength Y of the negative electrode sheet A (N / m) satisfies the following relationship: Y A =200×b+2×lg t+5; wherein: b represents the mass percentage of the second binder in the negative electrode coating material; t represents the temperature of hot rolling ℃.

2. A lithium ion battery according to claim 1, characterized in that: The negative electrode coating material also includes a thickener.

3. A lithium ion battery according to claim 2, characterized in that: The thickener is sodium carboxymethyl cellulose, and the mass percentage of the thickener in the negative electrode coating material is 0.5-1.5%.

4. A lithium ion battery according to claim 1, characterized in that: The first binder is selected from styrene-butadiene rubber and / or polystyrene-acrylic acid; the second binder is selected from polyacrylic acid and / or polyacrylic acid-acrylonitrile.

5. A lithium ion battery according to claim 1 or 4, characterized in that: The mass percentage a of the first binder in the negative electrode coating material satisfies 0.5%≤a≤2.5%; the mass percentage b of the second binder in the negative electrode coating material satisfies 0.5%≤b≤3.0%; the mass percentage a of the first binder and the mass percentage b of the second binder satisfy 1.0%≤a+b≤3.5%.

6. A lithium ion battery according to claim 1, characterized in that: The mass percentage c of the silicon element in the negative electrode active material in the negative electrode coating material satisfies 5.0%≤c≤35.0%.

7. A lithium ion battery according to claim 1, characterized in that: The single-surface density d of the negative electrode sheet satisfies 1×10 -3 g / cm 2 ≤d≤10×10 -3 g / cm 2 ; The thickness e of the negative electrode coating material on the negative electrode sheet satisfies 3×10 - 3 cm≤e≤1×10 -2 cm.

8. A lithium ion battery according to claim 1, characterized in that: The temperature t of the rolling satisfies 120°C≤t≤160°C.

9. A lithium ion battery according to claim 1, characterized in that: The capacity N / P ratio of the negative electrode plate to the positive electrode plate is (1.02-1.12):

1.

10. A lithium ion battery according to claim 1, characterized in that: The chemical formula of the positive electrode active material is Li1Ni x Co y Mn z M w O2; Among them, 0.3≤x≤0.95, 0.1≤y<0.45, 0.05≤z<0.45, 0≤w≤0.25, x+y+z+w=1.0; M represents any one or more combinations of Zr, W, Ti, Al, Sr, B, and Nd elements.

11. A lithium ion battery according to claim 1, characterized in that: The negative electrode active material is selected from at least one of silicon carbon, artificial graphite, natural graphite, soft carbon or hard carbon; the negative electrode conductive agent is selected from a combination of carbon nanotubes and conductive carbon black.

12. A lithium ion battery according to claim 1, characterized in that: The electrolyte comprises a lithium salt, a solvent and an additive; The lithium salt includes any one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate, lithium difluorooxalatoborate and lithium bis(trifluoromethylsulfonyl)imide; The additive includes any one or more combinations of propylene carbonate, butylene carbonate, ethyl acetate, ethyl methyl carbonate and fluoroethylene carbonate; The solvent includes any one or more combinations of dimethyl carbonate, diethyl carbonate, ethylene carbonate and ethyl methyl carbonate.

13. A lithium ion battery according to claim 1, characterized in that: The lithium ion battery is a cylindrical battery with a diameter of 15.0 to 70.0 mm and a length of 60.0 to 145.0 mm.

14. A method for predicting and controlling the pore impedance of the negative electrode sheet in the lithium ion battery according to any one of claims 1 to 13, characterized in that: The method is: according to the hole impedance R of the negative electrode sheet A Ω·cm 2 Satisfies the following relationship: 2.0≤R A =e (a / (a+b)+c) +5×d / e≤15.0; by adjusting the mass percentage a of the first binder in the negative electrode coating material, adjusting the mass percentage b of the second binder in the negative electrode coating material, adjusting the mass percentage c of the silicon element in the negative electrode active material in the negative electrode coating material, adjusting the single-sided surface density d of the negative electrode plate, and adjusting the thickness e of the negative electrode coating material on the negative electrode plate, the pore impedance R of the negative electrode plate in the lithium-ion battery is predicted and controlled A .

15. An electrical device, characterized in that: The electrical device comprises the lithium ion battery according to any one of claims 1 to 13.

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