A negative electrode sheet and its preparation method and battery

Abstract

This invention relates to a negative electrode sheet, its preparation method, and a battery. The negative electrode sheet includes a current collector and an active material layer disposed on at least one surface of the current collector. A coating is disposed on the surface of the active material layer away from the current collector. The coating material includes modified lithium titanate. In this invention, a coating containing modified lithium titanate is disposed on the surface of the active material, which can effectively improve the safety of the battery cell during overcharging. The modified lithium titanate coating not only improves the chemical stability of the negative electrode interface and reduces internal heat generation during overcharging, but also effectively suppresses the excessive lithium plating caused by overcharging by the structural stability of the modified lithium titanate coating, thereby inhibiting lithium dendrite growth and greatly reducing short circuits caused by microperforations in the separator.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a negative electrode sheet, its preparation method, and a battery. Background Technology

[0002] The most important technical indicators for power batteries include energy density, safety performance, cycle life, manufacturing cost, and adaptability to multiple environments.

[0003] Ternary batteries once rivaled lithium iron phosphate batteries due to their advantages such as high energy density, excellent low-temperature adaptability, and good cycle life. However, lithium iron phosphate batteries have gained more market share due to their lower cost and greater safety and reliability. In the next few years, the lithium battery industry will focus more on cost reduction and safety improvement.

[0004] Lithium-ion battery safety encompasses not only limiting misuse but also, and perhaps more importantly, ensuring reliability, specifically eliminating abnormal states such as spontaneous combustion and sudden power loss. These abnormal states are triggered by three main factors: internal short circuits, high-temperature induction, and voltage exceeding the operating threshold. Ultimately, these factors boil down to the internal temperature of the battery cell. As the internal temperature rises, the cell sequentially experiences decomposition of the negative electrode SEI film, intensified side reactions between the negative electrode and electrolyte, followed by melting of the separator base film, electrolyte vaporization, thermal decomposition of the positive electrode, and electrolyte decomposition. This eventually leads to a large-scale short circuit within the cell, culminating in thermal runaway, resulting in black smoke, combustion, and explosion. To address the initial issues of negative electrode SEI film decomposition and intensified side reactions between the negative electrode and electrolyte, raising the reaction temperature threshold and reducing the degree of reaction are feasible approaches.

[0005] Currently, graphite-based anode materials have advantages such as easy processing, high specific capacity, wide discharge range, and affordable price. However, their rate performance is poor, and the lithium deintercalation potential of graphite anodes based on the intercalation-deintercalation mechanism is approximately 0.1V vs. Li / Li. + The lithium dendrite growth potential is slightly higher than that of metallic lithium (0V), meaning that lithium plating is highly likely to occur at the graphite anode. The growth of lithium dendrites can cause the separator to puncture, which can easily lead to safety issues.

[0006] Therefore, how to overcome the defect of easy lithium plating in graphite anodes and thus improve safety is a technical problem that urgently needs to be considered in the field of graphite anode technology.

[0007] It is worth noting that carbon-coated lithium titanate anode materials exhibit a high operating voltage of 1.5–1.7V, and the octahedral structure composed of titanium-oxygen bonds provides ample interstitial space to supply Li. + The storage and transfer of energy are important; however, the higher operating voltage also means a lower total output voltage, which in turn results in lower energy density. In addition, the low intrinsic electronic conductivity of lithium titanate is a bottleneck that limits its rate capability. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a negative electrode sheet, its preparation method and battery. A coating containing modified lithium titanate is formed on the surface of the active material. The modified lithium titanate has an octahedral structure formed by titanium ions and six surrounding oxygen ions. The crystal framework remains unchanged during charging and discharging, ensuring the stability and zero-strain characteristics of the negative electrode during the lithium insertion / extraction process, thereby improving the safety performance of the negative electrode sheet. Furthermore, the modified lithium titanate can reduce the lithium insertion working potential and optimize the electronic conductivity compared to lithium titanate, thereby improving the rate capability of the negative electrode sheet.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector and an active material layer disposed on at least one surface of the current collector, wherein a coating is disposed on the surface of the active material layer on the side away from the current collector;

[0011] The coating material includes modified lithium titanate.

[0012] In this invention, a coating containing modified lithium titanate is provided on the surface of the active material, which can effectively improve the safety of the battery cell during overcharging. The modified lithium titanate coating can not only improve the chemical stability of the negative electrode interface and reduce the internal heat generation during overcharging, but also effectively suppress the excessive lithium deposition caused by overcharging by the structural stability of the modified lithium titanate coating, thereby inhibiting the growth of lithium dendrites and greatly reducing the short circuit caused by the microperforation of the separator.

[0013] The modified lithium titanate provided by this invention has an octahedral structure formed by titanium ions and six surrounding oxygen ions. The crystal framework remains unchanged during charging and discharging, ensuring the stability and zero-strain characteristics of the negative electrode during the lithium insertion / extraction process, thereby improving the safety performance of the negative electrode. Furthermore, compared with lithium titanate, the modified lithium titanate can reduce the lithium insertion working potential and optimize the electronic conductivity, thereby improving the rate capability of the negative electrode.

[0014] In this invention, the active material layer and the coating are disposed on the upper and lower surfaces of the current collector or on one side of the current collector.

[0015] Preferably, the active material of the active material layer includes any one or a combination of at least two of natural graphite, artificial graphite, soft carbon, hard carbon, or mesophase carbon microparticles. Typical but non-limiting combinations include combinations of natural graphite and artificial graphite, combinations of artificial graphite and soft carbon, combinations of soft carbon and hard carbon, combinations of hard carbon and mesophase carbon microparticles, combinations of natural graphite, artificial graphite, and soft carbon, combinations of artificial graphite, soft carbon, and hard carbon, and combinations of soft carbon, hard carbon, and mesophase carbon microparticles.

[0016] Preferably, the thickness of one side of the active material layer is 15 to 100 μm, for example, it can be 15 μm, 30 μm, 50 μm, 80 μm or 100 μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] The single-sided thickness here refers to the thickness of the active material layer on one surface of the current collector. When the active material layer is disposed on both surfaces of the current collector, the double-sided thickness is 30 to 200 μm.

[0018] Preferably, the areal density of the active material layer on one side is 20–160 g / m³. 2 For example, it could be 20g / m 2 50g / m 2 80g / m 2 100g / m 2 Or 160g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0019] The single-sided areal density here refers to the areal density of the active material layer on one surface of the current collector. When the active material layer is disposed on both surfaces of the current collector, the double-sided areal density is 40–320 g / m². 2 .

[0020] Preferably, the modified lithium titanate has a perovskite structure.

[0021] Preferably, the modified lithium titanate has the chemical formula A. x Li y TiO3, 3x+y=2.

[0022] Preferably, element A includes any one or at least two combinations of La, Y, Sc, Al, Ga, In, or Tl. Typical but non-limiting combinations include combinations of La and Y, Y and Sc, Sc and Al, Al and Ga, Ga and In, In and Tl, La, Y and Sc, Y, Sc and Al, Sc, Al and Ga, Al, Ga and In, and Ga, In and Tl.

[0023] Using A x Li y Although TiO3 material will reduce the overall specific capacity of the negative electrode to some extent, due to A x Li yTiO3 materials do not form an SEI film on their surface, and their reduction potential is higher than that of Li / Li+. This means that the material does not have any side reactions with the electrolyte during charging and discharging, which improves the chemical stability of the entire interface and thus improves the initial coulombic efficiency. The improvement in initial coulombic efficiency means a reduction in irreversible lithium loss, which ultimately helps to improve the cycle life of the single cell.

[0024] In addition, A x Li y In the TiO3 perovskite structure, the partial substitution of A ions maintains the basic crystal structure, but the difference in ionic radius introduces crystal structure defects. These defects, when properly designed, cause changes in the material's properties, not only in the vs. Li / Li ratio. + The reduction in operating potential also optimizes electronic conductivity, ultimately resulting in higher specific capacity and superior rate capability.

[0025] A x Li y The TiO3 perovskite structure utilizes its octahedral structure to ensure crystal stability during ion insertion / extraction transfer. The proportion of A-site substituent ions is 0.2 ≤ x < 0.6, for example, it can be 0.2, 0.3, 0.4, 0.5, or 0.55, but is not limited to the listed values; other unlisted values ​​within the range are also applicable. Adjusting the proportion of A-site substituent ions can achieve vs. Li / Li ratios. + The lower operating potential and better electronic conductivity allow for the improvement of rate capability and safety while maintaining high specific capacity.

[0026] In this invention, A x Li y Modified lithium titanate with a TiO3 perovskite structure is combined with graphite in a layered manner to serve as a negative electrode. This improves the safety performance, rate capability, and cycle life of lithium-ion batteries using the negative electrode without reducing energy density.

[0027] Preferably, the thickness of the coating does not exceed 40% of the thickness of the active material layer, for example, it can be 5%, 10%, 20%, 30% or 40%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 8 to 16%.

[0028] When the coating thickness is within the range described in this invention, the specific capacity of the negative electrode is guaranteed, and its safety performance is improved. When the coating thickness is too large, the overall specific capacity of the negative electrode will be greatly reduced, and the overall performance of the battery will be poor. When the coating thickness is too small, the improvement effect is not obvious.

[0029] Preferably, the areal density of the coating is 10–50 g / m³. 2For example, it could be 10g / m 2 20g / m 2 30g / m 2 40g / m 2 Or 50g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0030] The active material layer includes an active material, a conductive agent, and a binder.

[0031] Preferably, the conductive agent comprises carbon black and / or carbon nanotubes.

[0032] Preferably, the adhesive is any one or a combination of at least two of sodium carboxymethyl cellulose, styrene-butadiene rubber, or polyvinylidene fluoride. Typical but non-limiting combinations include combinations of sodium carboxymethyl cellulose and styrene-butadiene rubber, combinations of styrene-butadiene rubber and polyvinylidene fluoride, and combinations of sodium carboxymethyl cellulose and polyvinylidene fluoride.

[0033] Preferably, the active material in the active material layer has a mass percentage of 80% to 100% and not 100%, for example, it can be 80%, 85%, 90%, 95% or 99%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, in the active material layer, the mass percentage of the conductive agent is 0 to 10% and not 0, for example, it can be 2%, 4%, 6%, 8% or 10%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, in the active material layer, the mass percentage of the adhesive is 0 to 10% and not 0, for example, it can be 2%, 4%, 6%, 8% or 10%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the coating further includes a conductive agent and a binder.

[0037] Preferably, the mass ratio of the modified lithium titanate, conductive agent, and binder is (94-96):(2-3):(2-3), for example, it can be 95:2:3, 95:3:2, 95:2.5:2.5, 94:3:3 or 96:2:2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0038] Preferably, the adhesive is any one or a combination of at least two of sodium carboxymethyl cellulose, styrene-butadiene rubber, or polyvinylidene fluoride. Typical but non-limiting combinations include combinations of sodium carboxymethyl cellulose and styrene-butadiene rubber, combinations of styrene-butadiene rubber and polyvinylidene fluoride, and combinations of sodium carboxymethyl cellulose and polyvinylidene fluoride.

[0039] Preferably, the conductive agent comprises carbon black and / or carbon nanotubes.

[0040] Secondly, the present invention provides a method for preparing the negative electrode sheet described in the first aspect, the method comprising the following steps:

[0041] (1) The active material layer slurry is coated onto the surface of the current collector and dried to obtain the active material layer;

[0042] (2) The coating slurry is coated on the surface of the active material layer and dried to obtain the negative electrode sheet.

[0043] Preferably, the active material layer slurry in step (1) includes an active material, a conductive agent, a binder, and a solvent.

[0044] Preferably, the solvent is deionized water and / or N-methylpyrrolidone.

[0045] Preferably, the drying temperature in step (1) is 70 to 110°C, for example, 70°C, 80°C, 90°C, 100°C or 110°C, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] Preferably, the drying time in step (1) is 30 to 60 minutes, for example, 30 minutes, 35 minutes, 40 minutes, 50 minutes or 60 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Preferably, the coating slurry in step (2) includes modified lithium titanate, a conductive agent, a binder, and a solvent.

[0048] Preferably, the solvent is deionized water and / or N-methylpyrrolidone.

[0049] Preferably, the drying temperature in step (2) is 80 to 120°C, for example, it can be 80°C, 90°C, 100°C, 110°C or 120°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0050] Preferably, the drying time in step (2) is 30 to 60 minutes, for example, 30 minutes, 35 minutes, 40 minutes, 50 minutes or 60 minutes, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] Preferably, the method for preparing the modified lithium titanate includes:

[0052] A mixture of lithium source, titanium source, modified element source and solvent is ball-milled and then dried to obtain ball milling material;

[0053] The obtained ball milling material was calcined to obtain the modified lithium titanate.

[0054] Preferably, the source of the modifying element is an oxide of the modifying element.

[0055] Preferably, the modifying element includes any one or at least two combinations of La, Y, Sc, Al, Ga, In or Tl. Typical but non-limiting combinations include combinations of La and Y, Y and Sc, Sc and Al, Al and Ga, Ga and In, In and Tl, La, Y and Sc, Y, Sc and Al, Sc, Al and Ga, Al, Ga and In, and Ga, In and Tl.

[0056] Preferably, the solvent is any one or a combination of at least two of ethanol, isopropanol, 1-butanol, 2-butanol, isobutanol, isoamyl alcohol, ethyl acetate, methyl acetate, propyl acetate, acetone, or methyl ethyl ketone. Typical but non-limiting combinations include combinations of ethanol and isopropanol, isopropanol and 1-butanol, 1-butanol and 2-butanol, 2-butanol and isobutanol, isobutanol and isoamyl alcohol, isoamyl alcohol and ethyl acetate, ethyl acetate and methyl acetate, methyl acetate and propyl acetate... Combinations of esters, combinations of propyl acetate and acetone, combinations of acetone and methyl ethyl ketone, combinations of ethanol, isopropanol and 1-butanol, combinations of isopropanol, 1-butanol and 2-butanol, combinations of 1-butanol, 2-butanol and isobutanol, combinations of 2-butanol, isobutanol and isoamyl alcohol, combinations of isobutanol, isoamyl alcohol and ethyl acetate, combinations of isoamyl alcohol, ethyl acetate and methyl acetate, combinations of ethyl acetate, methyl acetate and propyl acetate, combinations of methyl acetate, propyl acetate and acetone, and combinations of propyl acetate, acetone and methyl ethyl ketone.

[0057] Preferably, the ball mill is a double planetary ball mill.

[0058] Preferably, the revolution speed of the ball mill is 150-400 rpm, for example, 150 rpm, 200 rpm, 300 rpm, 350 rpm, or 400 rpm, but not limited to the listed values; other unlisted values ​​within this range are also applicable. The rotation speed is 400-1000 rpm, for example, 400 rpm, 600 rpm, 800 rpm, 900 rpm, or 1000 rpm, but not limited to the listed values; other unlisted values ​​within this range are also applicable. The ratio of the ball mill's rotation speed to its revolution speed is ≥2.

[0059] Preferably, the ball milling time is 3 to 5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] Preferably, the drying temperature is 100-130°C, for example, 100°C, 110°C, 120°C, 125°C or 130°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0061] Preferably, the drying time is 10 to 15 hours, for example, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours or 15 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0062] Preferably, the calcination includes primary calcination and secondary calcination.

[0063] Preferably, the temperature of the first calcination is 700-900℃, for example, 700℃, 750℃, 800℃, 850℃ or 900℃, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0064] Preferably, the calcination time is 7 to 9 hours, for example, 7 hours, 7.5 hours, 8 hours, 8.5 hours or 9 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0065] Preferably, the temperature of the secondary calcination is 1100-1300℃, for example, it can be 1100℃, 1150℃, 1200℃, 1250℃ or 1300℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] Preferably, the secondary calcination time is 10 to 15 hours, for example, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours or 15 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0067] Thirdly, the present invention provides a battery containing a negative electrode sheet as described in the first aspect, or the negative electrode sheet in the battery is prepared by the preparation method described in the second aspect.

[0068] Based on the above technical solutions, the beneficial effects of the present invention are as follows:

[0069] In this invention, a coating containing modified lithium titanate is provided on the surface of the active material, which can effectively improve the safety of the battery cell during overcharging. The modified lithium titanate coating can not only improve the chemical stability of the negative electrode interface and reduce the internal heat generation during overcharging, but also effectively suppress the excessive lithium plating caused by overcharging by the structural stability of the modified lithium titanate coating, thereby inhibiting the growth of lithium dendrites and greatly reducing the short circuit caused by the microperforation of the separator.

[0070] The modified lithium titanate provided by this invention has an octahedral structure formed by titanium ions and six surrounding oxygen ions. The crystal framework remains unchanged during charging and discharging, ensuring the stability and zero-strain characteristics of the negative electrode during the lithium insertion / extraction process, thereby improving the safety performance of the negative electrode. Furthermore, compared with lithium titanate, the modified lithium titanate can reduce the lithium insertion working potential and optimize the electronic conductivity, thereby improving the rate capability of the negative electrode. Attached Figure Description

[0071] Figure 1 This is a schematic diagram of the negative electrode sheet provided in Example 1.

[0072] in:

[0073] 11-Current collector, 12-Active material layer, 13-Coating. Detailed Implementation

[0074] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0075] Example 1

[0076] This embodiment provides a negative electrode sheet, such as Figure 1 As shown, the negative electrode includes a copper foil 11 and an active material layer 12 disposed on the upper and lower surfaces of the copper foil 11, and a coating 13 is disposed on the side of the active material layer 12 away from the copper foil 11.

[0077] The active material layer 12 is made of natural graphite, with a single-sided thickness of 50 μm on both the upper and lower surfaces. The coating layer 13 is made of modified lithium titanate (La). 0.5 Li 0.5 TiO3, with a single-sided coating thickness of 7μm on both sides.

[0078] The method for preparing the negative electrode sheet includes the following steps:

[0079] (1) Mix natural graphite, carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber in deionized water at a mass ratio of 95:2:1.5:1.5 to obtain an active material layer slurry. Coat the upper and lower surfaces of copper foil 11 with the obtained active material layer slurry and dry at 80°C for 45 min to obtain active material layer 12.

[0080] (2) Modified lithium titanate La was mixed in a mass ratio of 95:3:2. 0.5 Li 0.5 TiO3, carbon black, and polyvinylidene fluoride are mixed in N-methylpyrrolidone to obtain a coating slurry. The obtained coating slurry is coated on the surface of the active material layer 12 and dried at 100°C for 45 min to obtain the negative electrode sheet.

[0081] Among them, the modified lithium titanate La 0.5 Li 0.5 TiO3 was prepared by the following method:

[0082] Take La 0.5 Li 0.5 TiO3 was mixed with lithium titanate, titanium dioxide and lanthanum oxide in ethanol in a molar ratio, and then subjected to double planetary ball milling for 4 h and dried at 120 °C for 12 h to obtain the ball milling material.

[0083] The obtained ball milling material was calcined at 800℃ for 8 hours and then calcined at 1200℃ for 13 hours to obtain the modified lithium titanate.

[0084] Example 2

[0085] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is Y. 0.56 Li 0.32 TiO3.

[0086] Example 3

[0087] This embodiment provides a negative electrode, which differs from Embodiment 1 in that the modified lithium titanate is Ga. 0.48 Li 0.56 TiO3.

[0088] Example 4

[0089] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is Sc. 0.5 Li 0.5 TiO3.

[0090] Example 5

[0091] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is Al.0.5 Li 0.5 TiO3.

[0092] Example 6

[0093] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is In. 0.5 Li 0.5 TiO3.

[0094] Example 7

[0095] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is Tl. 0.5 Li 0.5 TiO3.

[0096] Example 8

[0097] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the thickness of the coating on both sides is 22 μm.

[0098] Example 9

[0099] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the thickness of the coating on both sides is 9μm.

[0100] Example 10

[0101] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the thickness of the coating on both sides is 3μm.

[0102] Example 11

[0103] This embodiment provides a negative electrode sheet, which differs from Embodiment 1 in that the modified lithium titanate is La. 0.1 Li 1.7 TiO3.

[0104] Example 12

[0105] This embodiment provides a negative electrode, which differs from Embodiment 1 in that the modified lithium titanate is La. 0.6 Li 0.2 TiO3.

[0106] Comparative Example 1

[0107] This comparative example provides a negative electrode sheet, which differs from Example 1 in that it has no coating.

[0108] The obtained negative electrode sheet, positive electrode sheet, and separator are wound to form a core. The core is then encapsulated in an aluminum shell to obtain a square battery. The moisture is removed by vacuum baking, and electrolyte is injected. The square battery is then subjected to formation, aging, and capacity grading to obtain the finished battery.

[0109] The positive electrode sheet is prepared as follows: lithium nickel cobalt manganese oxide, conductive carbon black / carbon nanotubes, and polyvinylidene fluoride are mixed and dispersed in N-methylpyrrolidone at a mass ratio of 96.5:2:1.5 to obtain a positive electrode active layer slurry. The slurry is then uniformly coated on the upper and lower surfaces of an aluminum foil, dried in a vacuum oven at 100°C for 45 minutes until constant weight, and then compacted by a roller press to obtain a positive electrode sheet including a positive electrode active material layer.

[0110] The specific capacity and initial coulombic efficiency of the finished square battery were tested using the Landon CT-4008T-5V50mA 8-channel test system with a voltage window of 0V to 2.0V.

[0111] The negative electrode sheets obtained in Examples 1-12 retained the active material layer and coating on only one side, while the active material layer and coating on the other side were wiped off. The negative electrode sheet obtained in Comparative Example 1 retained the active material layer on only one side, while the active material layer on the other side was wiped off. Similarly, the positive electrode sheet retained the positive active material layer on only one side, while the positive active material layer on the other side was wiped off. The negative and positive electrode sheets after wiping were used to assemble a button cell with a separator and placed at room temperature for 12 hours (to allow the electrolyte to fully wet the electrode sheet). The electrode sheet was then placed on a test system and discharged at a constant current of 0.1C to 5mV, placed for 5 minutes, and then charged at a constant current of 0.1C to 1.5V. The ratio of the charging capacity to the discharging capacity of the first charge and discharge is the initial coulombic efficiency (the higher the coulombic efficiency, the less the negative electrode consumes the first irreversible active lithium). By calculating the ratio of the initial charging capacity of the half-button cell to the mass of the negative electrode sheet, the specific capacity of the negative electrode sheet can be obtained (the larger the specific capacity, the greater the ability of the negative electrode to insert and extract active lithium).

[0112] The resulting prismatic batteries were subjected to overcharge testing. The testing procedure was as follows: the battery was charged at a constant current of 1C to 4.2V, then charged at a constant voltage of 0.1C. Afterwards, the battery cell was placed in an explosion-proof chamber in a safety laboratory, and a temperature sensor was attached to the large surface of the cell. It was then continuously charged at a constant current of 1C to 5V, followed by constant voltage for 20 minutes. Whether the cell experienced thermal runaway was defined as whether it opened its valve, emitted smoke, or caught fire. Simultaneously, the temperature of the large surface of the cell was recorded at the moment the voltage reached 5V or the valve opened (if thermal runaway occurred before the cell reached 5V, the temperature of the large surface of the cell at the moment of thermal runaway was recorded; if the cell did not open its valve when overcharged to 5V, the temperature of the large surface of the cell at 5V was recorded). Each example of the test included 5 samples. Runaway included situations such as valve opening, black smoke, or combustion.

[0113] The test results are shown in Table 1.

[0114] Table 1

[0115]

[0116] The following conclusions can be drawn from Table 1:

[0117] (1) As can be seen from the comparison between Examples 1-7 and Comparative Example 1, a coating containing modified lithium titanate is provided on the surface of the active material. The structure of the modified lithium titanate is an octahedral structure formed by titanium ions and six surrounding oxygen ions. The crystal skeleton remains unchanged during the charging and discharging process, thereby ensuring the stability and zero strain characteristics of the negative electrode during the lithium insertion and extraction process, thus improving the safety performance of the negative electrode sheet. Furthermore, the modified lithium titanate can reduce the lithium insertion working potential and optimize the electronic conductivity compared to lithium titanate, thereby improving the rate capability of the negative electrode sheet.

[0118] (2) As can be seen from the comparison between Examples 8-10 and Example 1, the coating thickness affects the electrical performance and safety performance of the negative electrode sheet. When the coating thickness is within the range described in this invention, the specific capacity of the negative electrode sheet is guaranteed and its safety performance is improved. When the coating thickness is too large, the overall specific capacity of the negative electrode sheet will be greatly reduced and the overall performance of the battery will be poor. When the coating thickness is too small, the improvement effect is not obvious.

[0119] (3) A comparison of Examples 11 and 12 with Example 1 shows that adjusting the ratio of A-site substituent ions (0.2 ≤ x < 0.6) is beneficial to achieving its vs. Li / Li ratio. + Lowering the operating potential and improving electronic conductivity can maximize both the specific capacity and the rate capability and safety. An excessively high X ratio causes significant distortion to the octahedral structure, making it prone to collapse during overcharging and over-discharging, leading to uncontrolled electrode interface degradation. Conversely, an excessively low X ratio makes it difficult to lower the lithium intercalation operating potential, thus limiting the overall specific capacity of the negative electrode. Therefore, considering all factors, 0.2 ≤ x < 0.6 is the preferred range, and further optimization requires more experimental and data support.

[0120] In summary, a coating containing modified lithium titanate is applied to the surface of the active material. The modified lithium titanate retains the zero-strain characteristics of lithium titanate, and its structure is an octahedral structure formed by titanium ions and six surrounding oxygen ions. The crystal framework remains unchanged during charging and discharging, thereby ensuring the stability and zero-strain characteristics of the negative electrode during the lithium insertion / extraction process, thus improving the safety performance of the negative electrode sheet.

[0121] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode includes a current collector and an active material layer disposed on at least one surface of the current collector, with a coating disposed on the side of the active material layer away from the current collector. The coating material includes modified lithium titanate; The chemical formula of the modified lithium titanate is A. x Li y TiO3, 3x+y=2, 0.2≤x<0.6; The A x Li y The A element in TiO3 includes any one or a combination of at least two of La, Y, Sc, Al, Ga, In, or Tl; The modified lithium titanate has a perovskite structure.

2. The negative electrode sheet according to claim 1, characterized in that, The active material of the active material layer includes any one or a combination of at least two of the following: natural graphite, artificial graphite, soft carbon, hard carbon, or mesophase carbon particles.

3. The negative electrode sheet according to claim 1, characterized in that, The thickness of the active material layer on one side is 15~100μm; And / or, the areal density of the active material layer on one side is 20~160 g / m². 2 .

4. The negative electrode sheet according to claim 1, characterized in that, The thickness of the coating does not exceed 8-16% of the thickness of the active material layer; And / or, the areal density of the coating is 10~50 g / m³. 2 .

5. The negative electrode sheet according to claim 1, characterized in that, The active material layer includes an active material, a first conductive agent, and a first binder.

6. The negative electrode sheet according to claim 5, characterized in that, The first conductive agent includes carbon black and / or carbon nanotubes; And / or, the first adhesive is any one or a combination of at least two of sodium carboxymethyl cellulose, styrene-butadiene rubber, or polyvinylidene fluoride.

7. The negative electrode sheet according to claim 5, characterized in that, In the active material layer, the mass percentage of the active material is 80-100% but not 100%. And / or, in the active material layer, the mass percentage of the first conductive agent is 0~10% and not 0; And / or, in the active material layer, the mass percentage of the first adhesive is 0 to 10% and not 0.

8. The negative electrode sheet according to claim 1, characterized in that, The coating also includes a second conductive agent and a second adhesive.

9. The negative electrode sheet according to claim 8, characterized in that, The mass ratio of the modified lithium titanate, the second conductive agent, and the second binder is (94~96):(2~3):(2~3).

10. The negative electrode sheet according to claim 8, characterized in that, The second adhesive is any one or a combination of at least two of sodium carboxymethyl cellulose, styrene-butadiene rubber, or polyvinylidene fluoride; And / or, the second conductive agent comprises carbon black and / or carbon nanotubes.

11. A method for preparing the negative electrode sheet according to any one of claims 1-10, characterized in that, The preparation method includes the following steps: (1) The active material layer slurry is coated onto the surface of the current collector and dried to obtain the active material layer; (2) The coating slurry is coated on the surface of the active material layer and dried to obtain the negative electrode sheet.

12. The preparation method according to claim 11, characterized in that, The active material layer slurry in step (1) includes an active material, a first conductive agent, a first binder, and a first solvent.

13. The preparation method according to claim 12, characterized in that, The first solvent is deionized water and / or N-methylpyrrolidone.

14. The preparation method according to claim 11, characterized in that, The drying temperature in step (1) is 70~110℃; And / or, the drying time in step (1) is 30~60 min; And / or, the drying temperature in step (2) is 80~120℃; And / or, the drying time in step (2) is 30~60 min.

15. The preparation method according to claim 11, characterized in that, The coating slurry in step (2) includes modified lithium titanate, a second conductive agent, a second binder, and a second solvent.

16. The preparation method according to claim 15, characterized in that, The second solvent is deionized water and / or N-methylpyrrolidone.

17. The preparation method according to claim 15, characterized in that, The method for preparing the modified lithium titanate includes: A mixture of lithium source, titanium source, modified element source and third solvent is ball-milled and then dried to obtain ball-milled material; The obtained ball milling material was calcined to obtain the modified lithium titanate.

18. The preparation method according to claim 17, characterized in that, The source of the modifying element is an oxide of the modifying element.

19. The preparation method according to claim 17, characterized in that, The third solvent is any one or a combination of at least two of the following: ethanol, isopropanol, 1-butanol, 2-butanol, isobutanol, isoamyl alcohol, ethyl acetate, methyl acetate, propyl acetate, acetone, or methyl ethyl ketone.

20. The preparation method according to claim 17, characterized in that, The ball mill is a double planetary ball mill; And / or, the ball mill has an orbital speed of 150~400 rpm and a rotational speed of 400~1000 rpm; And / or, the ball milling time is 3-5 hours; And / or, the drying temperature is 100~130℃; And / or, the drying time is 10-15 hours.

21. The preparation method according to claim 17, characterized in that, The calcination includes primary calcination and secondary calcination.

22. The preparation method according to claim 21, characterized in that, The temperature of the first calcination is 700~900℃; And / or, the calcination time for one calcination is 7-9 hours; And / or, the temperature of the secondary calcination is 1100~1300℃; And / or, the secondary calcination time is 10~15h.

23. A battery, characterized in that, The battery contains a negative electrode sheet as described in any one of claims 1-10, or the negative electrode sheet in the battery is prepared by the preparation method described in any one of claims 11-22.

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