CN116013697B - Lithium ion capacitor and preparation method thereof - Google Patents

Abstract

The invention provides a lithium ion capacitor and a preparation method thereof. The lithium ion capacitor comprises an anode, a cathode, electrolyte and a diaphragm; the positive electrode comprises activated carbon, and the negative electrode comprises lithium titanate and a hard carbon material; wherein the median particle size of the hard carbon material is at least 5 times the median particle size of the lithium titanate; the lithium titanate and the hard carbon material are stirred and mixed to form a negative electrode active material, wherein the weight ratio of the lithium titanate in the negative electrode active material is more than or equal to 58%. According to the invention, the lithium titanate and the hard carbon are uniformly mixed by utilizing the size difference of the hard carbon and the lithium titanate through a stirring pre-dry mixing method, so that lithium titanate particles can be stably and uniformly embedded between gaps of a hard carbon material, and the power characteristic and the cycle life of a lithium ion capacitor are improved; the structure can also remarkably improve the usage ratio of lithium titanate, further improve the power density of the lithium ion capacitor and improve the discharge performance.

Description

A lithium ion capacitor and a method for preparing the same

Technical Field

The present invention relates to the technical field of energy storage materials, in particular to a lithium ion capacitor and a preparation method thereof.

Background Art

The braking and energy recovery systems of green and environmentally friendly transportation vehicles such as new energy electric vehicles, high-speed railways and subways urgently need electrochemical energy storage devices with high power density, high energy density, high safety and long life. Among the energy storage devices that combine power and energy, lithium-ion capacitors have superior performance. In terms of structure, lithium-ion capacitors cover the negative electrode materials of lithium-ion batteries (such as graphite, hard carbon, lithium titanate, silicon-based materials, etc.), the carbon electrode materials of supercapacitors (such as activated carbon) and lithium-ion electrolytes. In terms of reaction principle, lithium-ion capacitors maintain dynamic balance through the electrochemical reaction of lithium ions (Li ⁺ ) in the electrolyte at the negative electrode and the double electric layer formed by the negative charges in the electrolyte (such as PF ₆ ^- ) on the surface of the carbon electrode to achieve electrical energy storage. Therefore, lithium-ion capacitors have the characteristics of high power density, long life, high safety, green environmental protection and wide temperature range of double-layer capacitors, and have the characteristics of high energy density and high voltage of lithium-ion batteries.

Double-layer capacitors store charge on the surface and near the surface of carbon electrodes, which can achieve rapid charge response and a power density of up to 1000 to 10000 W/kg. Lithium-ion capacitors store energy through electrochemical reactions at the negative electrode and physical adsorption at the positive electrode. The charge transfer rate is relatively slow. Although the power density is better than that of lithium-ion batteries, it cannot match double-layer capacitors and cannot withstand large current bombardment continuously. At present, the power density of commercial lithium-ion capacitors is generally 200 to 1000 W/kg, and they still need to be used in combination with double-layer capacitors in high-power situations, which greatly limits the development of lithium-ion capacitors. In addition, the energy density of commercial lithium-ion capacitors is 10 to 40 Wh/kg, which has gradually failed to meet the needs of high-energy fields. Therefore, in order to meet market demand, it has become more urgent to improve the power density and energy density of lithium-ion capacitors.

Lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is a typical battery-type negative electrode material with a theoretical specific capacity of up to 175mAh/g, a typical spinel structure and "zero strain" characteristics. During electrochemical reactions, the change rate of lithium titanate lattice parameters is ≤0.2%, with almost no volume expansion, and can withstand large current bombardment. In addition, lithium titanate has a unique three-dimensional diffusion channel, which can achieve fast charging and discharging of lithium ions inside the material, and is more compatible with activated carbon that rapidly stores energy by adsorbing ions, and the constructed device has excellent power characteristics. In addition, the discharge platform of lithium titanate is higher (1.55V) than the formation potential of lithium dendrites, and no dendrites are generated on the electrode surface, which can prevent dendrites from piercing the diaphragm. Compared with hard carbon, graphite and other electrodes, lithium titanate has higher safety.

However, lithium titanate-based electrodes have two fatal defects: (1) During the charge and discharge process, the electrolyte solvent will react on the lithium titanate electrode, and the high lithium insertion potential will result in the inability to form a SEI (solid electrolyte interface) protective film on the surface of the lithium titanate negative electrode. The lack of protection of the lithium titanate negative electrode causes the electrolyte to be continuously reduced, forming a large amount of _H2 , CO and _CO2 and other gases inside the device, which ultimately leads to device bulging, leakage, and a sharp reduction in cycle life. The lithium titanate electrode sheet made with sodium carboxymethyl cellulose (CMC) as a thickener, styrene butadiene rubber (SBR) as a binder, and water as a solvent has a more serious gas production phenomenon. Therefore, patents CN106169375A, CN105047418A, CN105609321A, etc. use polyvinylidene fluoride (PVDF) as a binder and N-methylpyrrolidone (NMP) as a solvent to make oil-based lithium titanate-based electrode sheets. The monomer produced by this negative electrode produces less gas and has better performance, but the cost is high and the environmental pollution it produces is serious, making it unsuitable for industrial production. (2) The low electronic conductivity and small lithium ion diffusion coefficient of lithium titanate have seriously hindered the development of lithium titanate-type lithium ion capacitors. In view of this, when lithium titanate is used as the negative electrode active material and water is used as the solvent, it is necessary to introduce a substance that can form an SEI film, has excellent conductivity, and has a large lithium ion diffusion coefficient to compound with lithium titanate in order to improve the cycle life and power characteristics of lithium titanate-based lithium ion capacitors.

Among the materials that can be composited with lithium titanate, hard-to-graphitize carbon (hard carbon) is a good choice, with the following four unique advantages: (1) It is composed of open pores, closed pores, carbon layers, heteroatoms and non-graphitizable carbon atoms, and has high ionic and electronic conductivity; (2) It has diverse energy storage mechanisms, shows isotropy, and is often used in high-power devices; (3) It has dispersed crystals, stable structures, and interlayer spacing greater than that of graphite, making it easy for lithium ions to be deintercalated, which is convenient for improving the output power of the device; (4) It has a high reversible specific capacity (400-800mAh/g) and can form an SEI film. Since lithium ions need to overcome a large potential barrier when deintercalating in hard carbon materials, the rate performance of lithium ion capacitors is poor, while lithium titanate can overcome these defects. When hard carbon and lithium titanate are composited, the diffusion rate of ions and electrons can be improved, which is more compatible with the activated carbon positive electrode that performs rapid adsorption and desorption. Therefore, whether hard carbon and lithium titanate can be evenly mixed and the weight ratio of the two are crucial, which determines the cycle life and power characteristics of lithium ion capacitors.

In the prior art, in the existing hard carbon modified _Li4Ti5O12 negative _electrode active material, such as the Chinese patent CN109741956A, _a modified hard carbon material with lithium titanate coated on the surface of the hard carbon material is added, resulting in the lithium titanate being exposed on the outer surface, making it difficult to form an SEI film, and the produced monomer has serious gas production and rapid capacity decay; Chinese patent CN105190811A discloses a composite layer formed by a mixture of hard carbon material and titanium oxide (lithium titanate) loaded on the negative electrode plate, but the energy density and power density of the capacitor prepared therefrom still need to be further improved, and it also requires the addition of a specific electrolyte, the preparation process is cumbersome, and the cost is relatively high.

At present, lithium titanate and hard carbon composites are often coated on the surface of lithium titanate by hydrothermal and calcination methods. This method is not only complicated to operate and has cumbersome procedures, but also cannot achieve 100% coating. Therefore, Chinese patent CN107680830A uses a ball mill to mechanically mix lithium titanate and hard carbon at a speed of 300-400r/min, so that lithium titanate particles are evenly embedded in the gaps of hard carbon materials, lithium titanate is introduced into carbon materials, and the lithium ion diffusion coefficient is improved. However, this method has some problems: (a) During ball milling, the friction loss of the machine body is very large, which may contaminate the product, mix metal impurities, and affect the leakage current and self-discharge of the monomer; (b) The particle size of lithium titanate and hard carbon materials cannot be accurately controlled by the ball mill, and the size of different batches of materials may be inconsistent due to reasons such as grinding time, machine aging, and zirconium ball wear; (c) During ball milling, the crystal structure of hard carbon and lithium titanate may be destroyed, especially the side with larger particle size, which will be continuously worn by force. Moreover, the weight of hard carbon in the negative electrode material obtained by this method is greater than 90%, and the weight of lithium titanate is less than 10%, resulting in that the energy density and power density of the obtained capacitor still need to be further improved.

Therefore, it is necessary to provide a lithium ion capacitor with a simple preparation process and excellent electrochemical performance.

Summary of the invention

The purpose of the present invention is to provide a lithium ion capacitor with a simple preparation process and excellent electrochemical performance. The present invention can evenly embed lithium titanate in the hard carbon material by selecting a combination of hard carbon material and lithium titanate with a specific particle size and dosage, and a pre-mixing method with stirring, so as to form a special "sea island structure", which can effectively protect the negative electrode material, alleviate the gas production problem of lithium titanate, and extend the cycle life of lithium titanate-based lithium ion capacitors; it can also further increase the amount of lithium titanate added and improve the energy density of the capacitor.

Another object of the present invention is to provide a method for preparing the lithium ion capacitor.

To achieve the above object, the present invention adopts the following technical solution:

A lithium ion capacitor, comprising a positive electrode, a negative electrode, an electrolyte and a separator; the positive electrode comprises activated carbon (AC), and the negative electrode comprises lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and a hard carbon material (HC);

The median particle size (D ₅₀ ) of the hard carbon material is at least 5 times the median particle size of the lithium titanate; the lithium titanate and the hard carbon material are stirred and mixed to form a composite negative electrode active material, in which the weight proportion of lithium titanate is ≥58%.

The present invention cleverly utilizes the size difference of 6 to 10 times between hard carbon and lithium titanate, and uses stirring to pre-mix, so that relatively small lithium titanate particles can be embedded between hard carbon to form an "island structure", and the crystal structure and layered structure of the larger hard carbon material are not destroyed, ensuring that the hard carbon material can be used as a protective layer for the negative electrode active material lithium titanate, which is more conducive to the formation of SEI protective film, reducing the gas production of lithium ion capacitors, and facilitating homogenization to obtain a uniform slurry in a short time. Specifically: the hard carbon material has a larger interlayer spacing (≥0.37nm), which can accommodate more electrolytes, forming a multi-layer protective wall between lithium titanate and the electrolyte, so that a SEI film is formed on the electrode surface, preventing the electrolyte from being continuously reduced and reducing gas generation; the hard carbon material has a unique structure, rich defect structure, diverse energy storage mechanism, and high reversible specific capacity (400 to 800mAh/g), which helps to improve the performance of lithium ion capacitors.

Preferably, in the composite negative electrode active material formed by the lithium titanate and the hard carbon material, the weight proportion of lithium titanate is 60-75%. If the weight proportion of lithium titanate is too small, the initial discharge capacity of the obtained lithium ion capacitor is low. On the other hand, since lithium ions need to overcome a large potential barrier when deintercalating in the hard carbon material, the rate performance of the lithium ion capacitor is poor. If the weight proportion of lithium titanate is too large, although the discharge performance is closer to pure lithium titanate, the capacity retention rate of the obtained lithium ion capacitor will show a downward trend due to the reduction in the proportion of the hard carbon protective layer. Within the above range of the present invention, a lithium ion capacitor with good initial discharge performance and cycle performance can be obtained.

Preferably, the median particle size of the hard carbon material is 6 to 10 times the median particle size of the lithium titanate. A certain size difference can make the "island structure" more evenly distributed, which is beneficial to cycle stability. When the size difference increases to a certain extent, under the same proportion conditions, the gaps between the hard carbon materials will increase, which will cause too much lithium titanate to gather together, resulting in defects in the protective structure of lithium titanate, and causing the cycle stability of the obtained lithium ion capacitor to show a certain downward trend. Therefore, within the above-mentioned appropriate range of the present invention, the overall performance of the obtained capacitor is excellent.

Preferably, the particle size of the hard carbon material is 8 to 11 μm; the median particle size of the lithium titanate is 1.2 to 1.6 μm. If the particle size of the hard carbon material is too large, large gaps are likely to form at the contact interface between the hard carbon materials, and the protective effect of the formed hard carbon protective layer is poor; if the particle size of the lithium titanate is too small, the specific surface area is large, and agglomeration is likely to occur. In addition, dust may be generated during the stirring process due to the small mass. Within the above-mentioned suitable range of the present invention, a capacitor with excellent comprehensive performance can be obtained.

Preferably, the tap density of the hard carbon material is 0.78 to 0.95 g/cm ³ ; the tap density of the lithium titanate is 1.86 to 1.90 g/cm ^3. In order to maximize the advantages of hard carbon and lithium titanate materials and avoid disadvantages, when mixing, a material with a suitable tap density can be selected so that the tap density of lithium titanate is significantly higher than that of the hard carbon material (that is, for the same mass of hard carbon and lithium titanate materials, the volume of hard carbon is larger and more widely distributed, which can ensure that the lithium titanate is surrounded by the hard carbon material). Within the above-mentioned tap density range of the present invention, the volume of hard carbon per unit mass is 1.9 to 2.5 times that of lithium titanate. Within this parameter range, the overall performance of the obtained capacitor is excellent.

It should be noted that, in order to prevent dust from spreading in the present invention, before the two powders are dry-mixed, the vacuum valve is first closed, a small amount of ethanol is added, and alcohol can be sprayed into the mixing chamber from the endoscope port at irregular intervals according to the mixing conditions.

In the present invention, the positive electrode comprises an activated carbon layer formed on a positive electrode current collector. The activated carbon layer further comprises conductive carbon black (Super P), a thickener, a binder and a dispersant. The positive electrode current collector is aluminum foil.

The negative electrode comprises a negative electrode active material layer formed on a negative electrode current collector. The negative electrode active material layer further comprises conductive carbon black, a thickener, a binder and a dispersant. The negative electrode current collector is a copper foil.

The thickener is sodium carboxymethyl cellulose (CMC); the binder is styrene-butadiene rubber (SBR); and the dispersant is N-methyl pyrrolidone (NMP).

The separator is a cellulose separator or cellulose separator paper, which is low in price and environmentally friendly.

The electrolyte includes but is not limited to lithium hexafluorophosphate (LiPF ₆ ) solution. As the electrolyte concentration in the electrolyte increases, a higher electrolyte concentration helps to improve the energy density and power density of the lithium ion capacitor, but too high a concentration is not conducive to capacity retention and electrolyte infiltration. Therefore, the concentration of LiPF ₆ in the lithium hexafluorophosphate solution is 1 to 1.5 mol/L, and the conductivity is 5 to 10 mS/cm.

The AC//Li ₄ Ti ₅ O ₁₂ /HC lithium ion capacitor of the present invention maintains a dynamic balance through the electrochemical reaction of lithium ions (Li ⁺ ) in the electrolyte in lithium titanate and hard carbon, and the double electric layer formed by negative charges (such as PF ₆ ^- ) in the electrolyte on the surface of the activated carbon electrode to achieve electrical energy storage. During the first charge, within the voltage range of 0.05 to 1.0 V, lithium ions store energy by adsorbing positive charges (Li ⁺ ) at defect sites of the hard carbon material, and can form a partial double electric layer capacitor with the surface of the activated carbon positive electrode, which can effectively improve the capacity and power performance of the lithium ion capacitor.

The method for preparing the lithium ion capacitor is also within the protection scope of the present invention, and comprises the following steps:

S1. Preparation of positive and negative electrode sheets

The activated carbon, conductive carbon black, thickener and binder are first mixed evenly, and then an aqueous solution containing a dispersant is added, and after mixing evenly, an activated carbon slurry is obtained, and the activated carbon slurry is coated on the positive electrode current collector to form an activated carbon layer, and after drying, a positive electrode sheet is obtained;

Meanwhile, after the negative electrode active material, conductive carbon black, thickener and binder are uniformly mixed, an aqueous solution with a dispersant is added to fully dissolve the mixture to obtain a negative electrode active material slurry, the negative electrode active material slurry is coated on a negative electrode current collector to form a negative electrode active material layer (AC//Li ₄ Ti ₅ O ₁₂ /HC), and the negative electrode sheet is obtained after drying;

S2. Assemble lithium-ion capacitor

The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence and rolled into a battery cell. After the battery cell is dried, an electrolyte is added to obtain the lithium ion capacitor.

Based on the size difference between hard carbon and lithium titanate, the present invention pre-dry mixes lithium titanate and hard carbon by stirring to evenly mix, so that lithium titanate particles can be stably and evenly embedded in the gaps between hard carbon materials, thereby improving the power characteristics and cycle life of lithium ion capacitors. At the same time, when slurrying, the dispersant is added after carbon black, which can significantly improve the dispersibility of the positive electrode slurry and further improve the discharge capacity and capacity retention rate of lithium ion capacitors.

Preferably, in the activated carbon slurry, the weight ratio of activated carbon: conductive carbon black: thickener: binder = 84:10: (1.4-1.6): (4.4-4.6), by optimizing the ratio of activated carbon to thickener and binder, and supplemented with a trace amount of dispersant, a slurry with higher viscosity can be prepared, which can increase the shear frequency of the dispersant on the particles, reduce the fineness of the slurry, significantly improve the dispersion performance of the activated carbon, and improve the energy density, power density and cycle performance of the lithium ion capacitor. The viscosity of the activated carbon slurry is ≥3200mPa.s, and correspondingly, the fineness of the activated carbon slurry is ≤40μm.

Preferably, in the negative electrode active material slurry, the negative electrode active material: conductive carbon black: thickener: binder = 84:10: (1.4-1.6): (4.4-4.6), and the slurry with a specific viscosity is further diluted by adding an aqueous solution of a dispersant. The viscosity of the negative electrode active material slurry is ≥1900mPa.s, and the fineness is ≤50μm.

Preferably, the concentration of the dispersant in its aqueous solution is 2-7 wt %.

Preferably, the drying in step S1. is vacuum drying.

Preferably, the coating in step S1. is wet coating.

Preferably, the thickness of the activated carbon layer is 250-330 μm, and the surface density is ≥0.012 g/cm ² , more preferably 0.013-0.014 g/cm ² .

The thickness of the negative electrode active material layer is 140-210 μm, and the surface density is ≥0.012 g/cm ² , and more preferably 0.013-0.014 g/cm ² . Within the appropriate positive and negative electrode dosage range, it is beneficial to obtain a lithium ion capacitor with good cycle stability and longer life.

Preferably, the length of the positive electrode sheet and/or the negative electrode sheet is 130-160 mm.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention utilizes the size difference between hard carbon and lithium titanate to uniformly mix lithium titanate and hard carbon by stirring in advance dry mixing method, so that lithium titanate particles can be stably and evenly embedded in the gaps between hard carbon materials, so that a dense SEI film is formed on the electrode surface, thereby improving the power characteristics and cycle life of lithium ion capacitors; this structure can also significantly increase the proportion of lithium titanate used, further improve the power density of lithium ion capacitors, and enhance discharge performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the structure of the negative electrode sheet of the present invention.

FIG2 shows the negative electrode sheets obtained in Example 1 (FIG. a), Example 2 (FIG. b) and Comparative Example 1 (FIG. c). It can be seen that as the hard carbon material in the negative electrode active material increases, the color of the sheet gradually becomes darker.

FIG3 is a time-voltage curve diagram of the lithium ion capacitor obtained in Example 1 at different rates.

FIG4 is a performance test diagram of the lithium ion capacitor of Example 1 at 30C rate and 1000 cycles in different operating voltage ranges. It can be seen that this lithium ion capacitor is more suitable for an operating voltage of 3.8 to 2.0 V. When operating in this voltage range, the capacity is higher and the retention rate is better.

FIG5 is a performance curve of the lithium ion capacitor of Example 1 after 10,000 cycles at 30C. It can be seen that the cycle performance of the lithium ion capacitor is stable. After 10,000 cycles at 30C, the discharge capacity is still as high as about 20F, and the capacity retention rate is as high as 85.1%.

FIG6 shows the change of the DC internal resistance of the lithium ion capacitor of Example 1 after 10,000 cycles at 30C and 45C. It can be seen that after 10,000 cycles at 30C, the internal resistance is 115 mΩ, which is 1.43 times that before the cycle; after 10,000 cycles at 45C, the internal resistance becomes 121 mΩ, which is 1.51 times that before the cycle.

FIG. 7 is a cycle performance curve of lithium ion capacitors made in Examples 1, 2, 3 of the present invention and Comparative Example 1 without introducing hard carbon at a rate of 10C.

FIG8 is a cycle performance curve of lithium ion capacitors made in Examples 1, 2, 3 of the present invention and Comparative Example 1 without introducing hard carbon at a rate of 30C.

FIG. 9 is a cycle performance curve of lithium ion capacitors made in Examples 1, 2, 3 of the present invention and Comparative Example 1 without introducing hard carbon at a rate of 45C.

DETAILED DESCRIPTION

For better explanation of the purpose, technical scheme and advantage of the present invention, the present invention will be further described below in conjunction with specific embodiment and accompanying drawing, but embodiment does not limit the present invention in any form.Unless otherwise specified, the reagent, method and equipment adopted in the present invention are conventional reagents, methods and equipment in the art.Unless otherwise specified, the reagents and materials used in the present invention are commercially available.

Example 1

This embodiment provides a lithium ion capacitor, which is prepared according to a method comprising the following steps:

S1. Preparation of positive and negative electrode sheets

Activated carbon (average particle size = 7.0 μm) was used as the active material of the positive electrode. CMC, Super P, activated carbon (AC) and SBR were added to the material tank of an electric stirrer in a weight ratio of 1.5:10:84:4.5 to prepare a positive electrode active slurry with a viscosity of 3300 mPa.s and a fineness of 40 μm. (In the preparation of the positive electrode slurry, in order to improve the dispersion uniformity, a dispersant NMP was also added. Specifically, the dispersant was dissolved in water to prepare a dispersant solution with a concentration of 2 wt%. The dispersant solution was divided into two parts, which were added before adding CMC and before adding conductive carbon black respectively); then the obtained positive electrode active slurry was evenly coated on a 20 μm thick aluminum foil, the coating thickness was 260 μm, and the coating surface density was 0.0136 g/cm ² , and then vacuum dried, rolled, and cut into positive electrode sheets with a length of 150 mm for standby use;

Lithium titanate (median particle size D ₅₀ of 1.4 μm, interlayer spacing d ₁₁₁ =0.48 nm) and hard carbon powder (median particle size D ₅₀ of 9.0 μm, interlayer spacing d ₀₀₂ =0.38 nm) were added to a planetary mixer at a weight ratio of 67:33, and dry mixed (first stirred at a speed of 35 r/min for 30 min, and then stirred at a speed of 80 r/min for 120 min), and mixed evenly to obtain a negative electrode active material (the tap density of the hard carbon was 0.91 g/cm ³ , and the tap density of the lithium titanate was 1.88 g/cm ³ ); then, CMC, Super P, negative electrode active material and SBR are added into the material tank of the electric stirrer, and after dry mixing, a 6wt% NMP aqueous solution is added as a solvent to prepare a negative electrode active slurry with a viscosity of 2220mPa.s and a fineness of 50μm, which is evenly coated on a 8μm thick copper foil, with a coating thickness of 153μm and a surface density of 0.0133g/ ^cm2 , and then vacuum dried, rolled, and cut into negative electrode sheets with a length of 140mm for standby use (see Figure 1 for a schematic diagram of the structure of the negative electrode sheet);

S2. Assemble lithium-ion capacitor

The positive electrode sheet, the negative electrode sheet and the cellulose separator paper are wound into a lead-type battery cell, and then the battery cell is vacuum dried and injected with a lithium ion electrolyte (lithium ion electrolyte is LiPF ₆ ) with a concentration of 1.0 mol/L and a conductivity of 9.15 mS/cm to prepare most lithium ion capacitors.

Example 2

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the negative electrode active material, the weight ratio of lithium titanate and hard carbon powder is 70:30, the obtained slurry viscosity is 2000mPa.s, and the surface density of the negative electrode sheet is 0.0138g/ ^cm2 .

Example 3

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the negative electrode active material, the weight ratio of lithium titanate and hard carbon powder is 60:40, the obtained slurry viscosity is 2280 mPa.s, and the surface density of the negative electrode sheet is 0.0130 g/cm ² .

Example 4

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the median particle size of the lithium titanate material used is 1.2 μm, that is, the ratio of the median particle size of the hard carbon material (D _50(HC) ) to the median particle size of lithium titanate (D _{50(Li4Ti5O12)} ) is 7.5.

Example 5

The present embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the median particle size of the lithium titanate material used is 1.2 μm, and the median particle size of the hard carbon material is 11 μm, that is, the ratio of the median particle size of the hard carbon material (D _50(HC) ) to the median particle size of lithium titanate (D _{50(Li4Ti5O12)} ) is 9.2.

Example 6

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the negative electrode active material: the hard carbon material used has a tap density of 0.79 g/cm ³ , and the lithium titanate material used has a tap density of 1.88 g/cm ³ .

Example 7

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the negative electrode active material: the tap density of the hard carbon material used is 0.85 g/cm ³ , and the tap density of the lithium titanate material used is 1.90 g/cm ³ .

Example 8

The present embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that when preparing the activated carbon slurry, the conductive carbon black is added last. Since the carbon black is light and difficult to disperse, it is not added after the dispersant NMP is added. NMP does not play the role of a dispersant. Therefore, the viscosity of the obtained activated carbon slurry is relatively large, which is 4500mPa.s, and the fineness is slightly poor, which is 70μm.

Example 9

The present embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the activated carbon slurry, the weight ratio of CMC, Super P, activated carbon (AC) and SBR is 1.6:10:84:4.4, and the obtained activated carbon slurry has a viscosity of 3716 mPa.s and a fineness of 40 μm.

Example 10

The present embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that in the activated carbon slurry, the weight ratio of CMC, Super P, activated carbon (AC) and SBR is 1.6:10:82:6.4, and the obtained active slurry has a viscosity of 4320 mPa.s and a fineness of 50 μm.

Embodiment 11

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the thickness of the coating on the positive electrode sheet is 320 μm, and the surface density is 0.0163 g/cm ² .

Example 12

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the thickness of the coating on the negative electrode plate is 175 μm, and the surface density is 0.0155 g/cm ² .

Example 13

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that after cutting, the length of the positive electrode sheet is 132 mm.

Embodiment 14

This embodiment provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that after cutting, the length of the positive electrode sheet is 160 mm.

Comparative Example 1

This comparative example provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that no hard carbon material is added to the negative electrode active material.

Comparative Example 2

This comparative example provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the median particle size of the hard carbon material used is 5.5 μm, that is, the ratio of the median particle size of the hard carbon material (D _50(HC) ) to the median particle size of lithium titanate (D _{50(Li4Ti5O12)} ) is 4.

Comparative Example 3

This comparative example provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the preparation method of the negative electrode active material slurry is different (lithium titanate and hard carbon are not pre-dry mixed). The preparation method of the negative electrode active material slurry of this example is specifically as follows:

According to the dosage ratio of Example 1, CMC, Super P, lithium titanate, hard carbon powder and SBR are added to the material tank of the electric stirrer, and then a 6wt% NMP aqueous solution is added as a solvent, and stirring is started for mixing. After mixing according to the beating time of Example 1, a negative electrode active slurry with a viscosity of 2600mPa.s and a fineness of 70μm is obtained.

Comparative Example 4

This comparative example provides a lithium ion capacitor, which is prepared according to the preparation method of Example 1. The difference from Example 1 is that the negative electrode active material is pre-mixed by ball milling and then mixed by a stirrer to obtain a negative electrode active slurry with a viscosity of 1500mPa.s and a fineness of 25μm.

Performance Testing

The performance of the lithium ion capacitors obtained in the above examples and comparative examples was characterized, and the specific test items, test methods and results are as follows:

1. Test the discharge performance of the lithium ion capacitor: The test results are shown in Figure 3 and Table 1; Figure 3 is the time-voltage curve of the lithium ion capacitor obtained in Example 1 at 30C and 45C rates within the working voltage range of 3.8 to 2.0V. It can be seen that at 30C, the discharge time of the lithium ion capacitor is 49s, and at 45C, the discharge time is only 36s, proving that the lithium ion capacitor has a higher power characteristic. The discharge capacity corresponding to different rates can also be tested (see Table 1). The test results of other embodiments and comparative examples are shown in Table 1;

2. Cycle performance: Test the capacity retention rate and internal resistance change of lithium-ion capacitors after 1000 and 10000 cycles at 30C, as well as the first discharge performance at different rates. The test results are shown in Figures 4 to 9 and Table 1;

3. Calculate the energy density and power density of the capacitor. The test results are shown in Table 1.

Table 1 Test results of lithium ion capacitors obtained in the embodiments and comparative examples

From the above results we can see that:

The lithium ion capacitor prepared in the embodiment of the present invention has a small DC internal resistance (all below 97mΩ, and can be as low as 78mΩ), a high capacity retention rate at a high coulomb rate (all above 86.6%, and can be as high as 93.4%), and excellent discharge performance at a high rate. In addition, it has a higher energy density and power density at the same volume.

The lithium ion capacitor prepared by the present invention has excellent comprehensive discharge performance. The AC//Li ₄ Ti ₅ O ₁₂ /HC wound lithium ion capacitor prepared by the present invention exhibits almost the same discharge capacity when a rate of 10C to 45C is applied (the discharge capacity at 45C changes within 10% relative to 10C), can withstand a higher continuous discharge rate, has excellent rate performance, and is significantly better than the maximum continuous discharge rate (8 to 14C) that commercially available lithium ion capacitors can withstand; the lithium ion capacitor has a stable structure, a small internal resistance, and a small change in internal resistance after a long cycle. After 10,000 cycles at a large rate of more than 30C, the change in internal resistance is ≤1.55 times; the lithium ion capacitor has excellent reversibility, and the charge and discharge efficiency of different cycle times is ≥100%, showing a unique characteristic that the discharge capacity is greater than the charging capacity, and is a typical device dominated by pseudocapacitive behavior.

The results of Examples 1 to 3 show that as the weight proportion of lithium titanate in the negative electrode active material increases, the initial discharge capacity of the obtained lithium ion capacitor shows an upward trend, but when the weight proportion of lithium titanate increases to a certain amount, the proportion of lithium titanate continues to increase, although the discharge performance is closer to pure lithium titanate, the capacity retention rate of the obtained lithium ion capacitor will show a downward trend due to the reduction in the proportion of the hard carbon protective layer. Within the above range of the present invention, a lithium ion capacitor with good initial discharge performance and cycle performance can be obtained.

The results of Example 1, Examples 4-5, and Comparative Example 2 show that there is a certain size difference between lithium titanate and hard carbon materials, which can make the "island structure" more evenly distributed, which is beneficial to cycle stability; when the size difference increases to a certain extent, under the same proportion conditions, the gaps between the hard carbon materials will increase, which will lead to defects in the protective structure of lithium titanate, causing the cycle stability of the obtained lithium ion capacitor to show a certain downward trend. The size difference between lithium titanate and hard carbon materials in Comparative Example 2 is small, and a stable embedded "island structure" cannot be formed, and the initial discharge performance and cycle stability of the obtained capacitor are significantly deteriorated.

The results of Examples 1 and 6-7 show that when the tap density of the hard carbon material is 0.78-0.95 g/cm ³ and the tap density of lithium titanate is 1.86-1.90 g/cm ³ , this combination is beneficial to the cycle stability of the lithium ion capacitor.

The results of Examples 1 and 8 to 10 show that the appropriate ratio of the raw materials and the appropriate order of addition in the activated carbon slurry can further improve the performance of the activated carbon electrode, and thus can improve the discharge capacity and capacity retention rate of the obtained lithium ion capacitor.

The results of Example 1 and Examples 11 to 14 show that the length and thickness ratio of the positive and negative electrodes will also have a certain impact on the capacity and capacity retention rate of the lithium ion capacitor, but within the above range of the present invention, a lithium ion capacitor with good discharge performance and life can be obtained.

The results of Example 1 and Comparative Examples 3 to 4 show that lithium titanate and hard carbon are evenly mixed by a specific pre-dry mixing method, which reduces the dispersion and stirring time during slurrying, so that lithium titanate particles are stably and evenly embedded in the gaps between the hard carbon materials, thereby improving the power characteristics and cycle life of lithium ion capacitors.

The results of Example 1 and Comparative Example 1 show that the introduction of hard carbon helps to improve the cycle performance and electron transfer rate of lithium ion capacitors.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims (10)

1. A lithium ion capacitor, characterized in that the lithium ion capacitor comprises a positive electrode, a negative electrode, an electrolyte and a separator; the positive electrode comprises activated carbon, and the negative electrode comprises lithium titanate and a hard carbon material; The median particle size of the hard carbon material is at least 5 times the median particle size of the lithium titanate; the lithium titanate and the hard carbon material are stirred and mixed to form a negative electrode active material, first at a speed of 35 r/min for 30 min, and then at a speed of 80 r/min for 120 min; In the negative electrode active material, the weight proportion of lithium titanate is ≥58%. 2 . The lithium ion capacitor according to claim 1 , wherein the weight proportion of lithium titanate in the composite negative electrode active material formed by the lithium titanate and the hard carbon material is 60-75%. 3 . The lithium ion capacitor according to claim 1 , wherein the median particle size of the hard carbon material is 6 to 10 times the median particle size of the lithium titanate. 4 . The lithium ion capacitor according to claim 1 , wherein the median particle size of the hard carbon material is 8 to 11 μm; and the median particle size of the lithium titanate is 1.2 to 1.6 μm. 5 . The lithium ion capacitor according to claim 1 , wherein the tap density of the hard carbon material is 0.78 to 0.95 g/cm ³ ; and the tap density of the lithium titanate is 1.86 to 1.90 g/cm ³ . 6 . The lithium ion capacitor according to claim 1 , wherein the positive electrode comprises an activated carbon layer formed on a positive electrode current collector; and the negative electrode comprises a negative electrode active material layer formed on a negative electrode current collector. 7. The lithium ion capacitor according to claim 6, characterized in that it includes at least one of the following features: 1) The positive electrode current collector is aluminum foil; 2) The negative electrode current collector is copper foil; 3) The activated carbon layer and the negative electrode active material layer further include conductive carbon black, a thickener, a binder and a dispersant. 8. The method for preparing a lithium ion capacitor according to any one of claims 1 to 7, characterized in that it comprises the following steps: S1. Preparation of positive and negative electrode sheets The activated carbon, conductive carbon black, thickener and binder are first mixed evenly, and then an aqueous solution containing a dispersant is added, and the mixture is evenly mixed to obtain an activated carbon slurry, and the activated carbon slurry is coated on the positive electrode current collector to form an activated carbon layer, and the positive electrode sheet is obtained after drying; Meanwhile, after the negative electrode active material, conductive carbon black, thickener and binder are uniformly mixed, an aqueous solution with a dispersant is added to fully dissolve the mixture to obtain a negative electrode active material slurry, the negative electrode active material slurry is coated on the negative electrode current collector to form a negative electrode active material layer, and the negative electrode sheet is obtained after drying; S2. Assemble lithium-ion capacitor The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence and rolled into a battery cell. After the battery cell is dried, an electrolyte is added to obtain the lithium ion capacitor. 9. The preparation method according to claim 8, characterized in that the viscosity of the activated carbon slurry is ≥3200 mPa.s. 10 . The preparation method according to claim 8 , characterized in that the thickness of the activated carbon layer is 250 to 330 μm; the thickness of the negative electrode active material layer is 140 to 210 μm.