CN116247279A - Secondary battery - Google Patents

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a secondary battery, which comprises a positive plate and a negative plate, wherein the positive plate comprises a positive current collector and a positive coating arranged on the surface of the positive current collector, the positive coating comprises a positive conductive agent, and the specific surface area of the positive conductive agent is 55-90 m ² The structural oil absorption value of the positive electrode conductive agent is 250-400ml/100g, the negative electrode plate comprises a negative electrode current collector and a negative electrode coating arranged on the surface of the negative electrode current collector, the negative electrode coating comprises a negative electrode conductive agent, and the specific surface area of the negative electrode conductive agent is 55-90 m ² The structural degree oil absorption value of the negative electrode conductive agent is 150-250ml/100 g. The secondary battery of the invention uses different conductive agents in the negative electrode plate and the positive electrode plate, namelyThe cost is reduced, and the ion conduction and the electron conduction are also increased, so that the impedance is reduced.

Description

a secondary battery

technical field

The invention belongs to the technical field of secondary batteries, and in particular relates to a secondary battery.

Background technique

Lithium-ion batteries have outstanding advantages such as high energy density, high operating voltage, low self-discharge rate, and environmental friendliness, and can be used as ideal batteries for motors. Compared with ordinary electric vehicle lithium-ion batteries, batteries need to constantly pursue high power and long cycle life. The battery rate performance obtained by the method of the prior art is low, and the high-temperature cycle is poor. The main reasons are the poor conductivity of the positive electrode material, poor electronic conductivity, poor liquid retention, poor ion conduction between the electrolyte and the material, and large ionic resistance. On the other hand, the conductive agent is not easy to disperse uniformly in the negative electrode water system, resulting in poor electronic conductivity between materials and thus poor kinetic properties. Therefore, a method is needed to solve the above technical problems.

Contents of the invention

One of the purposes of the present invention is to provide a secondary battery in view of the deficiencies of the prior art, using a conductive material with a low oil absorption value and a low degree of graphitization in a negative electrode sheet with good liquid retention and high conductivity. agent to reduce costs; use a positive electrode conductive agent with high oil absorption value and high graphitization degree in the positive electrode sheet with poor liquid retention and low conductivity to increase the ion conduction and electron conduction between the positive electrode materials and reduce the impedance. Thus, a battery with excellent power performance and cycle performance is obtained.

In order to achieve the above object, the present invention adopts the following technical solutions:

A secondary battery, comprising a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode collector and a positive electrode coating arranged on at least one surface of the positive electrode collector, the positive electrode coating includes a positive electrode conductive agent, and the ratio of the positive electrode conductive agent The surface area is 55-90m ² /g, and the structural oil absorption value of the positive electrode conductive agent is 250-400ml/100g. The negative electrode sheet includes a negative electrode current collector and a negative electrode coating arranged on at least one surface of the negative electrode current collector. The negative electrode coating The layer includes a negative electrode conductive agent, the specific surface area of the negative electrode conductive agent is 55-90m ² /g, and the structural oil absorption value of the negative electrode conductive agent is 150-250ml/100g.

Preferably, the weight fraction of the positive electrode conductive agent in the positive electrode coating is 1.0%-6.0%.

Preferably, the Raman spectrum ID/IG ratio of the positive electrode conductive agent is 0.9˜1.3.

Preferably, the weight fraction of the negative electrode conductive agent in the negative electrode coating is 0.5% to 3.0%.

Preferably, the Raman spectrum ID/IG ratio of the negative electrode conductive agent is 1.3-1.8.

Preferably, the positive electrode coating also includes a positive electrode active material, and the positive electrode active material includes nickel cobalt lithium manganate ternary material modified by doping coating or not, iron phosphate coated with carbon or not At least one or more of lithium materials, lithium manganese iron phosphate materials, lithium manganese oxide materials, and lithium cobalt oxide materials.

Preferably, the chemical formula of the nickel-cobalt lithium manganese oxide ternary material is Li _x Ni _a Co _b Mn _c O ₂ , wherein, 0.85<x<1.2, 0≤a≤1, 0≤b≤1, 0≤c ≤1, a+b+c=1.

Preferably, the chemical formula of the lithium manganese iron phosphate material is Li _x Mn _a Fe _b PO ₄ , where 0.85<x<1.2, 0≤a≤1, 0≤b≤1, a+b=1.

Preferably, the positive electrode conductive agent is at least one of carbon, carbon black, carbon nanotubes, graphite, soft carbon, hard carbon, amorphous carbon, etc. and/or the negative electrode conductive agent is carbon, carbon black , carbon nanotubes, graphite, soft carbon, hard carbon, amorphous carbon, etc. at least one.

Preferably, the negative electrode coating further includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, silicon element, silicon oxide, and tin element.

Preferably, the positive electrode coating further includes a binder, the negative electrode coating further includes a binder, and the binder includes styrene-butadiene rubber, polyacryloyl, polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene One of acrylonitrile and polyimide.

Compared with the prior art, the beneficial effect of the present invention is that: a secondary battery of the present invention has higher rate performance and longer cycle life. In the lithium ion battery of the present invention, the conductive agent with low oil absorption value and low graphitization degree is used in the negative electrode sheet with good liquid retention and high conductivity, so as to reduce the cost. Use a positive electrode conductive agent with high oil absorption value and high degree of graphitization in the positive electrode sheet with poor liquid retention and low conductivity to increase the ion conduction and electron conduction between the positive electrode and positive electrode materials, and reduce the impedance, thereby obtaining excellent power. performance and cycle performance of the battery.

Detailed ways

The present invention will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

A secondary battery, comprising a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode collector and a positive electrode coating disposed on the positive electrode collector, the positive electrode coating includes a positive electrode conductive agent, and the specific surface area of the positive electrode conductive agent is 55 ～90m ² /g, the structural oil absorption value of the positive electrode conductive agent is 250～400ml/100g, the negative electrode sheet includes the negative electrode current collector and the negative electrode coating arranged on the negative electrode current collector, the negative electrode coating includes the negative electrode conductive agent, The specific surface area of the negative electrode conductive agent is 55-90m ² /g, and the structured oil absorption value of the negative electrode conductive agent is 150-250ml/100g.

Due to the low conductivity and poor liquid absorption capacity of lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium iron phosphorus oxide and lithium iron manganese phosphorus oxide as the positive electrode material of lithium ion battery, this makes The internal resistance of the entire electrode increases, the utilization rate of the active material is low, the polarization increases when the charge and discharge current increases, and the cycle life of the battery decreases. In order to ensure that the battery electrode has good charge and discharge performance, a certain amount of conductive agents such as furnace black, acetylene black, and carbon nanotubes are usually added when making the positive electrode sheet to increase the gap between the active material and the current collector and between the active material particles. The electrical conductivity and the ability of the pole piece to absorb electrolyte can improve the electronic conductance and ion conductance of the battery, and improve the charge and discharge efficiency. For anode materials such as graphite and natural graphite, although they have certain conductivity and liquid absorption capacity, the rate performance of the battery is reduced due to the gaps between graphite particles when the battery is charged and discharged at a high rate. Therefore, conductive agents such as conductive carbon black and acetylene black with smaller particles (before adding the electrode slurry, its particle diameter is usually 40-400 nanometers) are usually added to the electrode slurry when making pole pieces. Conductive agents such as conductive carbon black and acetylene black can collect micro-current between graphite particles and between graphite and current collectors, reduce the contact resistance of electrodes, and accelerate the migration rate of electrons to effectively increase the lithium ion density. The migration rate in the electrode material increases the conductivity of the pole piece.

When making the pole piece, mix the active material, conductive agent, binder, and solvent in proportion, load the uniformly mixed electrode slurry on the positive and negative electrode current collectors, and obtain the positive and negative pole pieces after drying, rolling and cutting . Then, the pole piece and the isolation film are wound to form a winding core. Use aluminum-plastic film to wrap the rolling core to make a semi-finished cell, inject electrolyte, and obtain a finished lithium-ion battery through the steps of formation and capacity separation, and test the electrical performance.

In actual production and application, it is found that the ionic conductance and electronic conductance of the battery are important factors affecting the rate performance of the battery. The reason may be that the positive electrode sheet has poor liquid absorption capacity and poor conductivity. As a lithium-ion battery positive electrode material Lithium nickel cobalt manganese oxide, lithium cobalt manganese oxide, lithium manganese oxide, lithium iron phosphorus oxide and lithium iron manganese phosphorus oxide itself have low conductivity and poor liquid absorption capacity, which increases the internal resistance of the entire electrode and improves the activity. The material utilization rate is low, the polarization increases when the charge and discharge current increases, and the battery cycle life decreases.

Therefore, the object of the present invention is to overcome the disadvantages of poor rate performance and poor cycle performance in the prior art, and a secondary battery of the present invention has higher rate performance and longer cycle life. In the lithium ion battery of the present invention, the conductive agent with low oil absorption value and low graphitization degree is used in the negative electrode sheet with good liquid retention and high conductivity, so as to reduce the cost. Use a positive electrode conductive agent with high oil absorption value and high degree of graphitization in the positive electrode sheet with poor liquid retention and low conductivity to increase the ion conduction and electron conduction between the positive electrode and positive electrode materials, and reduce the impedance, thereby obtaining excellent power. performance and cycle performance of the battery.

In order to realize the rational application of the conductive agent, we need to select the appropriate positive and negative conductive agent materials. The conductive carbon can be selected from at least one of activated carbon, carbon black, carbon nanotubes, graphite, soft carbon, hard carbon, amorphous carbon, etc.; the positive electrode conductive agent can be selected with a specific surface area of 55-90㎡/g, oil absorption The value is 250-400ml/100g, Raman: ID/IG0.9-1.3; the specific surface area of the negative electrode conductive carbon black is 55-90㎡/g, the oil absorption value of the structure is 150-250ml/100g, Raman: ID/IG1 .3-1.8.

The above-mentioned positive and negative electrode sheets are wound with a separator to form a core and assembled into a semi-finished battery. After the electrolyte is injected, the finished lithium-ion battery is obtained through processes such as formation and volume separation. Compared to out-of-range designs, it has superior dynamics and high cycle life.

In some embodiments, the weight fraction of the positive electrode conductive agent in the positive electrode coating is 1.0%-6.0%. Preferably, the weight fraction of the positive electrode conductive agent in the positive electrode coating is 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%.

In some embodiments, the Raman spectrum ID/IG ratio of the positive electrode conductive agent is 0.9˜1.3. Preferably, the Raman spectrum ID/IG ratio of the positive electrode conductive agent is 0.9, 1.0, 1.1, 1.2, 1.3.

In some embodiments, the weight fraction of the negative electrode conductive agent in the negative electrode coating is 0.5%-3.0%. Preferably, the weight fraction of the negative electrode conductive agent in the negative electrode coating is 0.5%, 0.8%, 1.1%, 1.3%, 1.5%, 1.8%, 2.0%, 2.4%, 2.8%, 3.0%.

In some embodiments, the Raman spectrum ID/IG ratio of the negative electrode conductive agent is 1.3-1.8. Preferably, the Raman spectrum ID/IG ratio of the negative electrode conductive agent is 1.3, 1.4, 1.5, 1.6, 1.7, 1.8.

In some embodiments, the positive electrode coating also includes a positive electrode active material, the positive electrode active material includes nickel cobalt lithium manganate ternary material modified by doping coating or not, carbon coating or not At least one or more of lithium iron phosphate materials, lithium manganese iron phosphate materials, lithium manganese oxide materials, and lithium cobalt oxide materials.

In some embodiments, the chemical formula of the nickel-cobalt lithium manganate ternary material is Li _x Ni _a Co _b Mn _c O ₂ , wherein, 0.85<x<1.2, 0≤a≤1, 0≤b≤1, 0≤c≤1, a+b+c=1.

In some embodiments, the negative electrode coating further includes a negative electrode active material, and the negative electrode active material includes one or more of artificial graphite, natural graphite, silicon element, silicon oxide, and tin element.

In some embodiments, the positive electrode coating further includes a binder, the negative electrode coating further includes a binder, and the binder includes styrene-butadiene rubber, polyacryloyl, polyvinylidene fluoride, polytetrafluoroethylene One of vinyl, polyacrylonitrile, polyimide.

In a secondary battery of the present invention, the positive electrode has a certain specific surface area and a positive electrode conductive agent with a large structural oil absorption value, so that the positive electrode has a certain ability to absorb liquid and retain liquid, better wettability, and improve ion conductivity. The negative electrode uses a negative electrode conductive agent with a certain specific surface area and a low structure and low oil absorption value to reduce costs. The secondary battery prepared by the cooperation of the positive electrode sheet and the negative electrode sheet has higher rate performance and longer cycle life.

According to the physical characteristics of conductive carbon, conductive carbon is divided into conductive carbon 1-6, and the properties are listed in Table 1.

Table 1

Example 1

Mix positive electrode material powder, conductive carbon, carbon nanotubes and PVDF in a specified ratio, then add NMP into a high-speed mixer and mix uniformly to form a slurry with a solid content of 74%. The slurry was coated on one side of an aluminum foil with a thickness of 13 microns using a transfer coater, and dried to keep the coating weight per unit area at 20.0 mg/cm2 after drying. Then apply the same process on the other side of the aluminum foil and dry to obtain a semi-finished positive electrode sheet.

Artificial graphite powder, conductive carbon, carbon nanotubes, CMC and SBR were mixed in a specified ratio, and then deionized water was added to a high-speed mixer and uniformly mixed to form a slurry with a solid content of 48%. The slurry was coated on one side of a copper foil with a thickness of 8 microns using a transfer coater, and dried to keep the coating weight per unit area at 18.1 mg/cm2 after drying. Then apply the same process on the other side of the copper foil and dry to obtain a semi-finished negative electrode sheet.

Process and weld the bare metal foil part of the above-mentioned pole piece to form a pole lug, and then wind it with the separator to form a core. Use the aluminum-plastic film to wrap the rolling core to make a semi-finished cell, and then inject the electrolyte with an injection coefficient of 4g/Ah. After the steps of formation and capacity separation, the finished lithium-ion battery is obtained.

Slurry performance test

1. Slurry fineness test

Use the QXD scraper fineness meter of Tianjin Jingke Material Testing Machine to test the fineness of the positive electrode slurry according to GB/T1724-89. The smaller the fineness of the slurry, the smaller the particle material in the slurry. The better the degree of dispersion.

2. Slurry rheological performance test

Using the PhysicaMCR301 rotary rheological tester of Anton Paar Company, the rheological properties of the positive electrode slurry were tested according to the American Society for Testing and Materials standard ASTMD7175-2005. The rheological properties of the slurry can be tested to see whether the fluidity is good or bad.

Electrode performance test

1. Positive plate resistivity test

Use the ACCFilm film resistance meter to directly measure the overall resistivity of the pole piece. At this time, the measured resistance includes the resistance of the probe itself, the contact resistance between the probe and the coating, the coating resistance, the contact resistance between the coating and the current collector, and the current collector itself. Resistance, during the measurement process, the main parameters include loading current and pressure applied by the probe, and the corresponding test standard is GB/T19587-2017. The test principle is Ohm's law: R=U/I. If the conductive agent is unevenly dispersed in the dispersant and forms a larger agglomeration, the conductive agent on the resulting electrode sheet will also be unevenly dispersed, resulting in less or no conductive agent in the local area, thereby reducing the resistivity of the electrode sheet. increase. Therefore, the smaller the resistivity, the more uniform the dispersion of the conductive agent on the pole piece.

Battery performance test

1. Liquid retention coefficient

The battery injection coefficient is 4g/Ah, and the injection volume is the same. Part of the electrolyte will be lost in the second seal after formation. The actual electrolyte retention coefficient is the value of the actual electrolyte mass in the final battery compared to the actual capacity of the battery. The larger the liquid retention coefficient of the battery, the more electrolyte is absorbed by the electrode, and the amount of electrolyte required can be relatively reduced, which reduces the cost and the electrode has good liquid absorption capacity and relatively low ionic resistance, which has advantages in rate performance.

2. IMP

After the battery preparation is completed, the composition and volume are placed for 48 hours to test the impedance with a DC resistance instrument.

3. EIS

CHI660 chemical workstation was used to test the AC impedance of the lithium battery, the test frequency was 0.03Hz-100kHz, and the zview software was used to perform fitting analysis on the tested AC impedance spectrum.

4. Magnification performance

First test the 1C capacity, put the lithium-ion battery in a constant temperature environment of 25°C for 1 hour, use the nominal capacity to charge and discharge at 1C/1C, charge at a constant current of 1C to a voltage of 4.3V, and then charge at a constant voltage of 4.3V The lower charging cut-off current is 0.05C, and after standing for 15 minutes, discharge to 2.8V at a discharge current of 1C, and the discharge capacity is 1C capacity C0; 4C rate test: charge at a constant current to a voltage of 4.3 at a charging current of 1C0 After V, charge at a constant voltage of 4.3V with a cut-off current of 0.05C. After standing for 15 minutes, discharge at a constant current of 4C0 to 2.8V, and the discharge capacity is C1.

cycle performance

3. 45 degree 3C/3C cycle performance test:

First, charge and discharge for the first time in an environment of 45°C, charge at a constant current of 1C to a voltage of 4.3V, and then charge at a constant voltage of 4.3V with a cut-off current of 0.05C. After standing for 15 minutes, charge at 1C Under constant current discharge to 2.8V, record the discharge capacity of the first cycle as C1; then charge and discharge cycles until the capacity decays to 80% C1, and record the cycle circle corresponding to the discharge capacity retention rate of 80% number.

4. Cycle DCR and DCR growth test:

After charging the 1C1 constant current in the first circle of 45-degree cycle in 3 to a voltage of 4.3V, the cut-off current of charging at a constant voltage of 4.3V is 0.05C0, and after standing for 15 minutes, the voltage V3, 1C0 constant current discharge 30s before the start of discharge for 30s After recording the voltage V4, the discharge DCR1 of the first cycle (capacity is maintained at 100%) is (V3-V4)/C1. The same test shows the DCR2 after the cycle capacity decays by 80%, and the cycle DCR increases as (DCR2-DCR1)/DCR1× 100%.

Prepare Examples 2-10 with reference to Example 1, and record the corresponding performance test results in Table 2.

Table 2

It can be concluded from the above Table 2 that when both the positive electrode and the negative electrode use conductive carbon with less surface oxygen content, such as in Examples 1, 4, and 6, the performance of the prepared battery positive electrode is better, but the performance of the negative electrode is poor, resulting in overall The performance of the battery is poor, the liquid retention coefficient is between 3.74 and 3.78, the overall capacity is on the low side, between 107 and 118, and the capacity retention rate is between 550 and 603. When both the positive electrode and the negative electrode use conductive carbon with a low degree of graphitization, as in Examples 2, 7, and 9, the performance of the prepared battery positive electrode is poor, the electrode resistance is between 2.82 and 3.09, and the performance of the overall battery is not good. When the capacity retention rate drops to 80%, the number of charge-discharge cycles is 507-542 cycles. When preparing the battery, the positive electrode uses conductive carbon with less surface oxygen content, and the negative electrode uses conductive carbon with more surface oxygen content, as in Examples 3, 5, 8, and 10. The prepared battery performance is better. The charge-discharge cycle test was carried out at 45 degrees Celsius with 2C rate charge/2C rate discharge. After 912 cycles of charge and discharge, it still has an 80% capacity retention rate.

According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also change and modify the above embodiment. Therefore, the present invention is not limited to the above-mentioned specific implementation manners, and any obvious improvement, substitution or modification made by those skilled in the art on the basis of the present invention shall fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims (10)

1. A secondary battery, characterized in that, comprises a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode coating arranged on the positive electrode current collector, the positive electrode coating includes a positive electrode conductive agent, and the positive electrode conductive The specific surface area of the agent is 55-90m ² /g, and the oil absorption value of the positive electrode conductive agent is 250-400ml/100g. The negative electrode sheet includes a negative electrode current collector and a negative electrode coating arranged on the surface of the negative electrode current collector. The negative electrode coating The negative electrode conductive agent is included, the specific surface area of the negative electrode conductive agent is 55-90m ² /g, and the oil absorption value of the negative electrode conductive agent is 150-250ml/100g. 2 . The secondary battery according to claim 1 , wherein the weight fraction of the positive electrode conductive agent in the positive electrode coating is 1.0%˜6.0%. 3. The secondary battery according to claim 1 or 2, characterized in that the Raman spectrum ID/IG ratio of the positive electrode conductive agent is 0.9-1.3. 4 . The secondary battery according to claim 1 , wherein the weight fraction of the negative electrode conductive agent in the negative electrode coating is 0.5% to 3.0%. 5. The secondary battery according to claim 1 or 4, characterized in that the Raman spectrum ID/IG ratio of the negative electrode conductive agent is 1.3-1.8. 6. The secondary battery according to claim 1, wherein the positive electrode coating also includes a positive electrode active material, and the positive electrode active material includes nickel cobalt lithium manganese oxide through or without doping coating modification At least one or more of ternary materials, lithium iron phosphate materials with or without carbon coating, lithium iron manganese phosphate materials, lithium manganese oxide materials, and lithium cobalt oxide materials. 7. The secondary battery according to claim 1, wherein both the positive electrode conductive agent and the negative electrode conductive agent comprise carbon black. 8. The secondary battery according to claim 7, wherein the positive electrode conductive agent and the negative electrode conductive agent further comprise at least one of activated carbon, carbon nanotubes, graphite, soft carbon, hard carbon, and amorphous carbon. 9. The secondary battery according to claim 6, wherein the chemical formula of the nickel-cobalt lithium manganate ternary material is Li _x Ni _a Co _b Mn _c O ₂ , wherein, 0.85<x<1.2, 0 ≤a≤1, 0≤b≤1, 0≤c≤1, a+b+c=1. 10. The secondary battery according to claim 1, wherein the negative electrode coating also includes a negative electrode active material, and the negative electrode active material includes artificial graphite, natural graphite, silicon element, silicon oxide, and tin element. one or more of.