Lithium titanate composite material and preparation method thereof, negative electrode sheet and preparation method thereof, and lithium battery and battery pack

Abstract

The application relates to the field of lithium ion batteries, and discloses a lithium titanate composite material and a preparation method thereof, a negative electrode sheet and a preparation method thereof, a lithium battery and a battery pack. A nitrogen-doped carbon intermediate layer and a two-dimensional MXene material outer layer are sequentially coated on the surface of lithium titanate particles, the Mxene material outer layer is bridged between adjacent particles through hydrogen bonds, thereby constructing a "core-shell-bridge" three-dimensional conductive network structure, the electronic conductivity of the lithium titanate composite material is significantly improved, the electronic / ion transmission efficiency of the lithium titanate negative electrode is improved, the lithium titanate negative electrode has excellent rate performance and cycle performance, the interface impedance is reduced through the synergistic optimization of the three-dimensional composite material and the electrolyte, polarization is inhibited, the lithium battery still has high capacity and long cycle stability under an ultrahigh rate, the problem of discontinuous conductive networks and high interface impedance in the prior art is solved, and the battery pack formed by the battery can achieve excellent rate performance and cycle performance under an ultrahigh rate.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, and in particular to a three-dimensional lithium titanate composite material and its preparation method, as well as a negative electrode sheet and its preparation method, and lithium batteries and battery packs. Background Technology

[0002] With the rapid development of electric vehicles and energy storage systems, the demand for high-rate charge / discharge batteries is increasing. Traditional lithium-ion batteries mostly use graphite anodes, but lithium dendrites are prone to precipitation at high rates (≥20C), leading to short cycle life and safety hazards.

[0003] Lithium titanate (LTO) has become an ideal anode material for high-power batteries due to its "zero-strain" characteristics, high cycle stability (>20,000 cycles), and excellent safety. However, traditional LTO batteries are limited by their low intrinsic electronic conductivity (~10). -13 (S / cm) and insufficient lithium-ion diffusion rate (~10) -9 cm 2 Due to the inherent limitations of LTO (LTO per second), it is difficult to achieve high-rate charge and discharge exceeding 20C. While existing technologies can improve rate performance by using nano-sized LTO particles or surface-coated carbon materials, these methods tend to increase interfacial side reactions, leading to poor high-rate cycle performance and making it difficult to simultaneously meet the requirements of high-rate charging and high-rate discharging. Summary of the Invention

[0004] In view of this, the purpose of this application is to provide a three-dimensional lithium titanate composite material and a method for preparing the same, which significantly improves the electrical conductivity of the lithium titanate three-dimensional composite material and has excellent rate performance and cycling performance at ultra-high rates of 20C.

[0005] Another objective of this application is to provide a negative electrode sheet based on the above-mentioned lithium titanate three-dimensional composite material, a method for preparing the same, a lithium-ion battery, and a battery pack composed thereof.

[0006] In order to solve the above-mentioned technical problems / achieve the above-mentioned objectives, or at least partially solve the above-mentioned technical problems / achieve the above-mentioned objectives, as a first aspect of this application, a three-dimensional lithium titanate composite material is provided, comprising a plurality of lithium titanate composite material particles, wherein the lithium titanate composite material particles include a lithium titanate core layer, a nitrogen-doped carbon intermediate layer, and an MXene material outer layer; the MXene material outer layer bridges adjacent lithium titanate composite material particles through hydrogen bonds.

[0007] Optionally, the thickness of the nitrogen-doped carbon intermediate layer is 5-10 nm, and the thickness of the MXene material outer layer is 1-2 μm.

[0008] Optionally, the MXene material is Ti3C2T. mWhere T represents a surface functional group, including one or more of -O, -OH, -F, and -Cl, and m represents the coverage degree of the surface functional group, 0 < m < 2.

[0009] As a second aspect of this application, a method for preparing the lithium titanate three-dimensional composite material described in this application is provided, comprising:

[0010] A precursor is obtained by hydrothermal reaction of lithium titanate and carbon source. The precursor is then treated at high temperature in a nitrogen atmosphere to obtain nitrogen-doped carbon-coated lithium titanate.

[0011] MXene material was obtained by etching using MAX phase ceramics as raw material.

[0012] The nitrogen-doped carbon-coated lithium titanate and MXene material solution are thoroughly mixed and dried to obtain the three-dimensional lithium titanate composite material.

[0013] As a third aspect of this application, a negative electrode sheet is provided, including a current collector and a negative electrode material coated on at least one surface of the current collector, said negative electrode material including the lithium titanate three-dimensional composite material described in this application.

[0014] As a fourth aspect of this application, a method for preparing the negative electrode sheet described in this application is provided, comprising:

[0015] The lithium titanate three-dimensional composite material, conductive agent, and binder are used to form a negative electrode slurry, which is then coated onto at least one surface of the current collector.

[0016] The current collector coated with negative electrode slurry is rolled under three gradient pressures, and then the electrode is annealed to obtain the negative electrode.

[0017] Optionally, the three-gradient pressure rolling includes:

[0018] The first rolling process applies a pressure of 5-8 MPa and a rolling speed of 10-15 m / min.

[0019] The second rolling process applies a pressure of 10-12 MPa and a rolling speed of 5-10 m / min, while the rollers are heated to 50±5℃.

[0020] The third rolling process applies a pressure of 15-18 MPa and a rolling speed of 3-5 m / min, while the rollers are heated to 70±5℃.

[0021] Optionally, the annealing includes:

[0022] In a protective gas atmosphere, heat to 180-220℃ and hold for 1-3 hours, then cool down.

[0023] As a fourth aspect of this application, a lithium-ion battery that directly converts chemical energy into electrical energy is provided, including a positive electrode, a negative electrode as described in this application, a separator, and an electrolyte.

[0024] Optionally, the electrolyte includes an electrolyte salt, an additive, and a solvent, wherein the electrolyte salt includes lithium hexafluorophosphate and lithium difluorooxalate borate, and the additive includes lithium difluorophosphate and fluoroethylene carbonate.

[0025] In a fifth aspect of this application, a battery pack that directly converts chemical energy into electrical energy is provided, comprising a plurality of lithium-ion batteries as described in this application, wherein the plurality of lithium-ion batteries are electrically connected in series and / or in parallel.

[0026] This application involves sequentially coating the surface of lithium titanate particles with a nitrogen-doped carbon intermediate layer and a two-dimensional MXene material outer layer. The MXene material outer layer is formed by bridging adjacent particles with hydrogen bonds, thereby constructing a "core-shell-bridge" three-dimensional conductive network structure and increasing the electronic conductivity of the lithium titanate three-dimensional composite material to 10. -2 The S / cm ratio improves the electron / ion transport efficiency of the lithium titanate anode, enabling lithium batteries using it as the anode active material to exhibit excellent rate performance and cycle performance. Furthermore, through synergistic optimization of three-dimensional composite materials and electrolyte, the interfacial impedance is reduced and polarization is suppressed, allowing the lithium battery to maintain high capacity and long cycle stability at ≥20C rates. At the same time, it solves the problems of discontinuous conductive networks and high interfacial impedance in existing lithium titanate composite materials. Battery packs composed of these batteries in series / parallel can achieve excellent rate performance and cycle performance at ultra-high rates. Attached Figure Description

[0027] Figure 1 The diagram shown is a structural schematic of the lithium titanate three-dimensional composite material of this application;

[0028] Figure 2 The figure shows a comparison of the room temperature 20C charging and 20C discharging cycle curves of soft-pack batteries assembled with different groups of lithium titanate materials. Detailed Implementation

[0029] This application discloses a three-dimensional lithium titanate composite material and its preparation method, as well as a negative electrode sheet, its preparation method, and a lithium battery. Those skilled in the art can refer to the content of this application and appropriately modify the process parameters to achieve the desired results. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this application. The products, processes, and applications described in this application have been described through preferred embodiments. Those skilled in the art can obviously modify or appropriately change and combine the preparation methods described herein without departing from the content, spirit, and scope of this application to realize and apply the technology of this application. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0030] It should be noted that, in this document, relational terms such as "first" and "second," "step 1" and "step 2," and "(1)" and "(2)" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Moreover, the embodiments and features described in this application can be combined with each other without conflict.

[0031] In the first aspect of this application, a three-dimensional lithium titanate composite material is provided, comprising a plurality of lithium titanate composite material particles. Each lithium titanate composite material particle includes a lithium titanate core layer, a nitrogen-doped carbon intermediate layer, and an MXene material outer layer. The MXene material outer layer bridges adjacent lithium titanate composite material particles via hydrogen bonds. A schematic diagram of the structure is shown below. Figure 1In the lithium titanate three-dimensional composite material of this application, the nitrogen-doped carbon intermediate layer provides an electronic pathway, and the MXene material outer layer acts as a "bridge" to connect the electronic pathways of each lithium titanate particle, which greatly improves the electron / ion transport efficiency and makes up for the discontinuity of the conductive network of lithium titanate and the surface-coated carbon material. At the same time, the "core-shell-bridge" structure of the lithium titanate three-dimensional composite material of this application can reduce the interface impedance and suppress polarization, thereby improving the high-rate cycling performance. It can simultaneously meet the requirements of high-rate charging and high-rate discharging, and achieve excellent results such as capacity retention of ≥90% at 20C (compared with 1C) and capacity decay of <5% after 1000 cycles of 20C charge and discharge.

[0032] In some embodiments of this application, the thickness of the nitrogen-doped carbon intermediate layer is 5-10 nm, for example, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any value between the two; the thickness of the MXene material outer layer is 1-2 μm, for example, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, or any value between the two; furthermore, the layer can be of homogeneous or non-homogeneous thickness. In other embodiments of this application, the lithium titanate has a particle size of 150-250 nm and an average particle size of 200 nm.

[0033] MXene materials are a class of transition metal carbides, nitrides, or carbonitrides with a two-dimensional layered structure. They belong to a new type of inorganic compound materials, and their general structural formula is M. n+1 X n T m Where M represents a metallic element, primarily a transition metal, such as at least one of Ti, Nb, V, Mo, Zr, Cr, W, and Ta; X represents C and / or N; n is 1, 2, 3, or 4; T m The term "m" represents the functional groups on the material surface, including one or more of -O, -OH, -F, and -Cl. "m" represents the coverage degree of surface functional groups, where 0 < m < 2, and m = 2 represents theoretical 100% coverage. Typically, 1 ≤ m < 2, and more preferably 1.5 ≤ m ≤ 1.8. This is a common range measured by characterization techniques such as X-ray photoelectron spectroscopy (XPS) in most samples prepared by wet chemical etching (such as HF or LiF-HCl). In some embodiments of this application, the MXene material is Ti3C2T. m , 1.5≤m≤1.8.

[0034] In a second aspect of this application, a method for preparing the lithium titanate three-dimensional composite material described in this application is provided, comprising:

[0035] A precursor is obtained by hydrothermal reaction of lithium titanate and carbon source. The precursor is then treated at high temperature in a nitrogen atmosphere to obtain nitrogen-doped carbon-coated lithium titanate.

[0036] MXene material was obtained by etching using MAX phase ceramics as raw material.

[0037] The nitrogen-doped carbon-coated lithium titanate and MXene material solution are thoroughly mixed and dried to obtain the three-dimensional lithium titanate composite material.

[0038] In some embodiments of this application, the hydrothermal reaction can be carried out by dispersing lithium titanate in a carbon source solution. The hydrothermal reaction can be improved by adding dispersants, ultrasound, stirring and other techniques. The carbon source includes, but is not limited to, one or more of glucose, sucrose, chitosan, citric acid and urea. The carbon source concentration can be dynamically adjusted according to the coating thickness of the nitrogen-doped carbon intermediate layer. Taking glucose solution as an example, its molar concentration is 0.08-0.1 mol / L. The dispersant includes, but is not limited to, polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS).

[0039] In some embodiments of this application, the high-temperature treatment includes heat treatment at 500-700°C for 1-5 hours, and the flow rate of the nitrogen atmosphere can be selected as 40-60 mL / min.

[0040] MAX phase ceramics are a class of ternary layered carbide or nitride ceramic materials, and are an important raw material for the synthesis of MXene materials. Their general chemical formula is M. n+1 AX n The meanings of M, X, and n are consistent with those in MXene materials. A represents a main group element, primarily group IIIA or IVA elements, such as Al, Si, Ga, Ge, Sn, and In. Multilayer MXene materials can be obtained by etching the A layer in MAX phase ceramics. The etched multilayer MXene materials need to be ultrasonically exfoliated with an intercalating agent (such as tetrapropylammonium hydroxide) to obtain few-layer / monolayer nanosheets. In the etching method, fluoride solutions (such as HF or LiF-HCl) are commonly used to remove the A layer, retaining the MX framework and forming surface functional groups. The functional groups mainly include -O, -OH, and a small amount of -F. In some embodiments of this application, Ti3AlC2MAX phase is used as raw material. The Al layer is removed by etching with hydrofluoric acid, tetrapropylammonium hydroxide (TPAOH) intercalating agent is added, and ultrasonic exfoliation is performed to obtain few-layer MXene materials.

[0041] In some embodiments of this application, the nitrogen-doped carbon-coated lithium titanate and MXene material solutions are thoroughly mixed by means including but not limited to stirring, ultrasonication, or a combination thereof; the mass ratio of the nitrogen-doped carbon-coated lithium titanate and MXene material can be selected as (5-15):1, for example 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1 or any ratio between the two.

[0042] In a third aspect of this application, a negative electrode sheet is provided, comprising a current collector and a negative electrode material coated on at least one surface of the current collector, said negative electrode material comprising the lithium titanate three-dimensional composite material described in this application.

[0043] In some embodiments of this application, the negative electrode material includes the lithium titanate three-dimensional composite material described in this application, as well as a binder and a conductive agent.

[0044] In some embodiments of this application, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0045] In some embodiments of this application, the adhesive includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0046] In some other embodiments of this application, the negative electrode material includes the lithium titanate three-dimensional composite material described in this application, as well as conductive carbon nanotubes, conductive carbon black, and binder PVDF, with their weight ratios being 91-97:1-3:1-3:1-3.

[0047] In some embodiments of this application, the current collector may be a metal foil or a composite current collector. For example, copper foil, aluminum foil, etc., may be used as the metal foil.

[0048] In a fourth aspect of this application, a method for preparing the negative electrode sheet described in this application is provided, comprising:

[0049] The lithium titanate three-dimensional composite material, conductive agent, and binder are used to form a negative electrode slurry, which is then coated onto at least one surface of the current collector.

[0050] The current collector coated with negative electrode slurry is rolled under three gradient pressures, and then the electrode is annealed to obtain the negative electrode.

[0051] In the preparation of the negative electrode sheet in this application, a three-dimensional lithium titanate composite material is mixed with a conductive agent (carbon nanotubes + conductive carbon black) and a binder (such as PVDF) in a certain proportion, coated on an aluminum foil current collector, and then subjected to gradient rolling and low-temperature annealing to promote the molecular chain reconstruction of PVDF, improve the mechanical strength of the electrode sheet, stabilize the interfacial bonding between Maxine material and nitrogen-doped carbon-coated lithium titanate, reduce contact resistance, and maintain the electrode sheet as a porous electrode structure with a porosity of 30%-40%, thus balancing the lithium-ion transport rate and electronic conductivity.

[0052] In some embodiments of this application, the three-gradient pressure rolling includes:

[0053] The first rolling press applies a pressure of 5~8MPa and a rolling speed of 10~15m / min. The primary purpose of this stage is not final compaction, but to prepare for the subsequent high-pressure stage. Its main functions include: (1) breaking up the loose agglomerates of active materials, conductive agents and binders in the coated electrode to make them more evenly distributed; (2) expelling the air mixed in with the slurry and during the coating process to avoid the formation of cavitation or pinholes in the subsequent high-pressure stage; (3) reducing the electrode thickness evenly and uniformly to provide a consistent and stable foundation for the application of higher pressure in the future and to prevent stress concentration.

[0054] Heating should be avoided during the first rolling stage. Heating will cause the binder to soften prematurely and become adhesive, which will cause the aggregates to stick together instead of breaking them up, thus failing to achieve the goal of homogenization. At the same time, the surface binder softens and prematurely closes the pores on the electrode surface.

[0055] The intended purpose can be better achieved under the pressure and speed of the first rolling stage in this application. If the pressure is too high, it will be over-compacted, prematurely sealing the pores on the electrode surface and blocking the escape channels for internal air and residual solvent (NMP). If the rolling speed is too fast, the electrode will pass through the pressure for too short a time, resulting in insufficient stress transfer and uneven compaction in the thickness direction.

[0056] The second rolling process applies a pressure of 10-12 MPa at a rolling speed of 5.0-10.0 m / min, while the rollers are heated to 50±5℃. The purpose of this stage is to build a continuous conductive network, ensuring close contact between the active material, conductive agent, and binder. If the pressure is too high at this stage, approaching or reaching the final strength, it will cause irreversible mechanical damage to the material, resulting in microcracks in the active material particles and destroying the Mxene sheets and the three-dimensional conductive network. If the temperature is too high, the binder will soften prematurely and excessively, easily sticking to the rollers and causing thickness control failure. If the temperature is too low, the binder will not soften sufficiently, resulting in poor flowability and inability to fully encapsulate the particles and fill the pores, leading to poor physical contact between particles and high contact resistance.

[0057] The third rolling process applies a pressure of 15-18 MPa at a rolling speed of 3.0-5.0 m / min, while the rollers are heated to 70±5℃. The purpose of this stage is to precisely control the final thickness and porosity to achieve interface stability and strong bonding. If the pressure is too high at this stage, the compaction density will reach its limit, severely compressing or even closing the lithium-ion transport channels, resulting in a sharp drop in rate performance; excessive stress at the current collector interface may cause microscopic stretching or even fatigue of the aluminum foil, leading to electrode embrittlement. If the temperature is too high, the edges of the Mxene sheets will begin to oxidize to form TiO2 under high temperature and pressure, reducing conductivity; excessive pyrolysis of the binder will reduce adhesion; if the rolling speed is too fast, residual stress will exist inside the electrode, the binder will not be fully shaped, and the bonding strength will not reach its optimal level.

[0058] In addition, three-stage rolling can produce a better negative electrode sheet, but the first, second, and fourth rolling processes all have adverse effects to varying degrees. Among them, the adverse effects of the first rolling process include: (1) the instantaneous high pressure crushes the active material particles and the Mxene bridging network. (2) the pore structure deteriorates, forming a large number of closed "dead pores" and blocking the lithium-ion transport channels. (3) the binder does not have time to flow and rearrange, resulting in weak interfacial bonding. (4) lithium-ion migration is hindered, resulting in poor rate performance; the active particle structure is destroyed, accelerating cycle decay.

[0059] The adverse effects of secondary rolling include: (1) If the pre-pressing is insufficient, the inside of the electrode may not be fully compacted, forming an uneven structure with dense outer layer and sparse inner layer; (2) After a single high-pressure final pressing, the thickness rebound is still significant; (3) The pressure and temperature of the pre-pressing and final pressing need to be precisely matched, and the fault tolerance rate is low.

[0060] The adverse effects of four-time rolling: (1) repeated rolling leads to damage to the active material lattice and breakage of the conductive agent; (2) excessive loss of porosity, severe compression of lithium ion channels, and difficulty in electrolyte wetting; (3) low efficiency and high cost; (4) may lead to hardening and embrittlement of the electrode, decreased flexibility, and easy generation of cracks when the charge and discharge volume changes.

[0061] In some embodiments of this application, the annealing includes:

[0062] In a protective gas atmosphere, the temperature is raised to 180-220℃ and held for 1-3 hours, then cooled. The annealing temperature can be selected from 180℃, 190℃, 200℃, 210℃, 220℃, or any value between two of these. The annealing holding time can be selected from 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or any value between two of these.

[0063] In some other embodiments of this application, the annealing is carried out by heating to 180-220°C in two stages. In the first stage, the temperature is first raised to 100-120°C and held for 0.5-1 hour. In the second stage, the temperature is raised to 180-220°C and held for 1-3 hours.

[0064] In some embodiments of this application, the protective gas atmosphere includes, but is not limited to, nitrogen and inert gases, to prevent electrode oxidation. In other embodiments of this application, the protective gas atmosphere is a nitrogen atmosphere with a flow rate of 10-20 L / min.

[0065] In some embodiments of this application, the cooling can be carried out naturally to below 80°C, and the annealed negative electrode sheet can be removed to assemble the battery.

[0066] In a fourth aspect of this application, a lithium-ion battery is provided, including a positive electrode, a negative electrode as described in this application, a separator, and an electrolyte.

[0067] This application does not impose specific limitations on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte can be liquid, gel, or all-solid. In some embodiments of this application, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt, additives, and a solvent. In other embodiments of this application, the electrolyte salt may include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, or lithium tetrafluorooxalate phosphate. In other embodiments of this application, the solvent may include at least one selected from ethylene carbonate, propylene carbonate, diethyl carbonate, ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, ethylene glycol dimethyl ether, methyl ethyl sulfone, or diethyl sulfone. The additive includes lithium difluorophosphate and fluoroethylene carbonate.

[0068] In some embodiments of this application, the electrolyte salt comprises lithium hexafluorophosphate (LiPF6) and lithium difluorooxalate borate (LiDFOB) in a molar ratio of (1-2):1; the solvent comprises ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1-3:4-6:1-3; the electrolyte contains 1-3% fluoroethylene carbonate (FEC) and 1-3% lithium difluorophosphate (LiPO2F2); the total lithium salt concentration is 1.0-1.5 mol / L.

[0069] In some embodiments of this application, the positive electrode, the negative electrode, and the separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0070] In some embodiments of this application, the lithium-ion battery may include an outer packaging. This outer packaging can be used to encapsulate the aforementioned electrode components and electrolyte. In other embodiments of this application, the outer packaging of the lithium-ion battery may be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the lithium-ion battery may also be a soft pack, such as a pouch.

[0071] In a fifth aspect of this application, a battery pack that directly converts chemical energy into electrical energy is provided, comprising a plurality of lithium-ion batteries as described in this application, wherein the plurality of lithium-ion batteries are electrically connected in series and / or in parallel.

[0072] In some embodiments of this application, the battery pack may further include one or more of a battery management system, a thermal management component, and a housing structure.

[0073] In a sixth aspect of this application, an electrical device is provided, including the lithium-ion battery described in this application. The lithium-ion battery provides electrical energy to the electrical device and can also be used as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0074] In the comparative experiments provided in this application, unless otherwise specified, all experimental conditions and materials are kept consistent to ensure comparability. Unless otherwise specified, all experimental materials and reagents used in the examples are commercially available.

[0075] The following provides further details regarding the lithium titanate three-dimensional composite material, its preparation method, the negative electrode sheet, its preparation method, and the lithium battery provided in this application.

[0076] Example 1:

[0077] Step 1: Preparation of three-dimensional composite materials

[0078] (1) Preparation of nitrogen-doped carbon-coated LTO

[0079] LTO powder with an average particle size of 200 nm was dispersed in a glucose solution (concentration 0.1 mol / L). 0.5 wt% polyvinylpyrrolidone (PVP) was added as a dispersant. The mixture was mechanically stirred at 1000 rpm for 30 min, followed by sonication at 40 kHz for 1 h to form a homogeneous suspension. The suspension was transferred to a polytetrafluoroethylene-lined reactor and hydrothermally reacted at 180 °C for 12 h. After natural cooling to room temperature, the mixture was washed alternately with deionized water and ethanol by centrifugation (8000 rpm, 5 min / time, repeated 5 times) to remove unreacted glucose and dispersant. The mixture was then vacuum dried at 60 °C for 12 h to obtain a black precursor powder. The precursor was placed in a tube furnace and heated to 600 °C at a rate of 5 °C / min under a nitrogen atmosphere (flow rate 50 mL / min), held for 2 h, and then naturally cooled to obtain nitrogen-doped carbon-coated LTO (LTO@NC).

[0080] (2) Mixing process of LTO@NC and MXene suspension

[0081] Using Ti3AlC2MAX phase as raw material, the Al layer was removed by etching with hydrofluoric acid (40% concentration) for 48 h; the Al layer was removed by centrifugation with deionized water until pH>6, tetrapropylammonium hydroxide (TPAOH) intercalating agent was added, and ultrasonic exfoliation (200 W, 1 h) was performed to obtain a few-layer MXene colloidal solution.

[0082] MXene was dispersed in deionized water and the concentration was adjusted to 5 mg / mL. The solution was then sonicated (40 kHz, 30 min) to ensure uniform dispersion of the film.

[0083] LTO@NC and MXene suspension (concentration 5 mg / mL) were mixed at a mass ratio of 9:1 (9g LTO@NC: 1g MXene dry weight), and magnetically stirred at 500 rpm for 2 h. Following this, the mixture was ultrasonically dispersed at 40 kHa for 1 h, allowing MXene sheets to tightly coat the LTO@NC surface through hydrogen bonds and van der Waals forces. Finally, the mixture was vacuum dried at 60℃ for 24 h until the water content was <0.5 wt%, yielding a three-dimensional LTO@NC / MXene composite material. The obtained three-dimensional composite material exhibits a nitrogen-doped carbon interlayer of 5-10 nm and an outer MXene layer of 1-2 μm thickness, possessing a core-shell-bridge structure. The powder conductivity, measured using the four-probe method, is 1.5 × 10⁻⁶. -2 S / cm.

[0084] Step 2: Electrolyte preparation

[0085] LiPF6 and LiDFOB were dissolved in EC / DMC / EMC (volume ratio 3:5:2) solvent at a molar ratio of 1:1. 2% FEC and 1% LiPO2F2 were added and mixed thoroughly to achieve a lithium salt concentration of 1.2 mol / L.

[0086] Step 3: Electrode and battery assembly

[0087] (1) Preparation of negative electrode: LTO@NC / MXene, carbon nanotubes, conductive carbon black and binder are prepared into a slurry in a ratio of 94:1:2.5:2.5 and coated on aluminum foil (coating amount 10 mg / cm²).

[0088] (2) Electrode rolling

[0089] A multi-roll continuous roller press (at least 3 roller pressing units) is used. The rollers are made of high-hardness tungsten carbide (hardness ≥90HRA) with a surface roughness Ra≤0.1μm. The rolling temperature range is (room temperature ~80℃). Rolling is performed in three stages.

[0090] The first rolling process applies a pressure of 5 MPa at a speed of 15 m / min, reducing the electrode porosity from 50% to approximately 45%. The electrode then enters the second rolling unit, where the rolling speed is increased to 8.0 m / min, the rollers are heated to 50°C, and the rolling pressure is adjusted to 11 MPa. This thermal softening effect reduces the elastic modulus of the PVDF binder, further reducing the electrode porosity to approximately 38%, and establishing a preliminary conductive network of carbon nanotubes and conductive carbon black. The electrode then enters the third rolling unit, where the pressure increases to 16 MPa, the rolling speed decreases to 5 m / min, and the roller temperature is raised to 70°C. This promotes the extension of the PVDF molecular chains and their filling of voids, reducing the electrode porosity to approximately 35%, resulting in a final electrode compaction density of 2.3 g / cm³. 3 Peel strength 1.8 N / cm.

[0091] (3) Electrode annealing

[0092] Cut the electrode sheet into 82mm wide pieces. The electrodes, 85mm long, are laid flat on a special fixture with a spacing of ≥5mm. The furnace door is closed, and a vacuum is first drawn to -0.1MPa, followed by the introduction of high-purity nitrogen (≥99.999%). This process is repeated three times to replace any residual air. Temperature control is set as follows: First stage: from room temperature to 120℃, heating rate 3℃ / min, holding for 30min to evaporate residual solvent; Second stage: from 120℃ to the target temperature (180℃-220℃), preferably 200℃, heating rate 2℃ / min, holding for 1-3h, preferably 2h, to promote binder crosslinking. Nitrogen gas is introduced throughout the process (flow rate 10-20L / min) to maintain a slight positive pressure (0.1-0.2MPa) to prevent electrode oxidation. After annealing, the electrodes are cooled to below 80℃ in the furnace and then removed.

[0093] (4) Preparation of positive electrode sheet

[0094] LiNi 0.5 Co 0.2 Mn 0.3 O2, carbon nanotubes, conductive carbon black, and binder are mixed in a ratio of 94:1:3:2 to form a slurry, which is then coated onto aluminum foil. The same rolling process is used to reduce the porosity of the electrode to 35%. The electrode is then annealed using the same annealing process as the negative electrode and assembled into a soft-pack battery (rated capacity 2 Ah). The separator is a 16 μm thick PP membrane with a porosity of 50%, and the electrolyte prepared above is injected into it.

[0095] Test results:

[0096] 20C discharge capacity: 147.5 mAh / g (1C capacity is 155 mAh / g), retention rate 95.16%; after 1000 cycles of 20C charge and 20C discharge, the capacity decays by 3.2%. Figure 2 As shown.

[0097] Example 2:

[0098] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0099] (1) In the preparation of nitrogen-doped carbon-coated LTO, the glucose solution concentration was 0.08 mol / L; the dispersant was 0.3 wt% SDS; the temperature was increased to 550℃ at 3℃ / min and held for 3h.

[0100] (2) In the mixing process of LTO@NC and MXene suspension, LTO@NC and MXene suspension (concentration 6 mg / mL) are mixed at a mass ratio of 8:1 (8g LTO@NC: 1g MXene dry weight);

[0101] (3) In the preparation of electrolyte, LiPF6 and LiDFOB are dissolved in EC / DMC / EMC (volume ratio 3:5:2) solvent at a molar ratio of 1.5:1, and 2% FEC and 1.5% LiPO2F2 are added and mixed evenly. The lithium salt concentration is 1.3 mol / L.

[0102] (4) In the assembly of electrode sheets and batteries, LTO@NC / MXene powder, carbon nanotubes, conductive carbon black and binder are prepared into a slurry in a ratio of 94:1:2:3;

[0103] During the electrode rolling process, a pressure of 6 MPa is applied during the first rolling process, and a pressure of 14 MPa is applied during the second rolling process.

[0104] Test results:

[0105] 20C discharge capacity: 142 mAh / g (1C capacity is 155 mAh / g), retention rate 91.6%; after 1000 cycles of 20C charge and 20C discharge, the capacity decays by 4.78%, see [reference needed]. Figure 2 .

[0106] Example 3:

[0107] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0108] (1) In the preparation of electrolyte, LiPF6 and LiDFOB are dissolved in EC / DMC / EMC (volume ratio 2:6:2) solvent at a molar ratio of 2:1, and 3% FEC and 1.5% LiPO2F2 are added and mixed evenly. The lithium salt concentration is 1.5 mol / L.

[0109] (2) In the assembly of electrode sheets and batteries, LTO@NC / MXene powder, carbon nanotubes, conductive carbon black and binder are prepared into a slurry in a ratio of 93:2:2:3;

[0110] During the electrode rolling process, a pressure of 7 MPa is applied during the first rolling, 16 MPa during the second rolling, and 18 MPa during the third rolling. The final porosity of the electrode is approximately 30%.

[0111] During electrode annealing, the second stage annealing temperature was 220℃, the time was 1.5h, and the nitrogen flow rate was 15L / min.

[0112] (3) In the preparation of the positive electrode, LiNi 0.5 Co 0.2 Mn 0.3 O2, carbon nanotubes, Ketjen black, and binder are mixed in a ratio of 95:1:2:2 to form a slurry, which is then coated onto aluminum foil. The diaphragm is a PE diaphragm (12μm) coated with a 3μm thick Al2O3 layer and has a porosity of 45%.

[0113] Test results:

[0114] 20C discharge capacity: 140 mAh / g (1C capacity is 155 mAh / g), retention rate 90.3%; after 1000 cycles of 20C charge and 20C discharge, the capacity decays by 6.14%, see [reference needed]. Figure 2 .

[0115] Comparative Example 1:

[0116] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0117] The negative electrode sheet was prepared directly using LTO powder with an average particle size of 500 nm and the battery was assembled.

[0118] Test results:

[0119] 20C discharge capacity: 138 mAh / g (1C capacity is 155 mAh / g), retention rate 89%; after 1000 cycles of 20C charge and 20C discharge, the capacity decays by 13.9%, see [reference needed]. Figure 2 .

[0120] Comparative Example 2:

[0121] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0122] The negative electrode sheet was prepared by directly using nitrogen-doped carbon coated LTO and then assembling the battery.

[0123] Test results:

[0124] 20C discharge capacity: 115 mAh / g (1C capacity is 155 mAh / g), retention rate 74.2%; after 1000 cycles of 20C charge and 20C discharge, the capacity decays by 20%, see [reference needed]. Figure 2 .

[0125] Comparative Example 3:

[0126] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0127] The first rolling process applies a pressure of 12 MPa at a rolling speed of 2.0 m / min. The electrode then enters the second rolling unit, where the rolling speed decreases to 5 m / min, the rollers are heated to 70°C, and the rolling pressure is adjusted to 8 MPa. The electrode then enters the third rolling unit, where the pressure increases to 20 MPa, the rolling speed is 20 m / min, and the roller temperature is raised to 85°C. The final electrode compaction density is 2.4 g / cm³. 3 Peel strength 3.0 N / cm.

[0128] Test results:

[0129] 20C discharge capacity: 84 mAh / g (1C capacity is 155 mAh / g), retention rate 54.19%; after 313 cycles of 20C charge and 20C discharge, the capacity decayed by 27.13%, see [reference needed]. Figure 2 .

[0130] Comparative Example 4:

[0131] The battery was prepared and assembled according to the scheme of Example 1, with the following differences:

[0132] The electrode annealing process is not performed.

[0133] Test results:

[0134] 20C discharge capacity: 87 mAh / g (1C capacity is 155 mAh / g), retention rate 56.13%; after 313 cycles of 20C charge and 20C discharge, the capacity decayed by 20.36%, see [reference needed]. Figure 2 .

[0135] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A negative electrode sheet, characterized in that, The device includes a current collector and a negative electrode material coated on at least one surface of the current collector, the negative electrode material including a lithium titanate three-dimensional composite material; the lithium titanate three-dimensional composite material includes a plurality of lithium titanate composite material particles, the lithium titanate composite material particles including a lithium titanate core layer, a nitrogen-doped carbon intermediate layer and an MXene material outer layer; the MXene material outer layer bridges adjacent lithium titanate composite material particles through hydrogen bonds; The negative electrode sheet is prepared according to the following method: The lithium titanate three-dimensional composite material, conductive agent, and binder are used to form a negative electrode slurry, which is then coated on at least one surface of the current collector. The current collector coated with negative electrode slurry is rolled under three gradient pressures to anneal the electrode sheet, thereby obtaining the negative electrode sheet. The three-gradient pressure rolling includes: The first rolling process applies a pressure of 5-8 MPa and a rolling speed of 10-15 m / min. The second rolling process applies a pressure of 10-12 MPa and a rolling speed of 5-10 m / min, while the rollers are heated to 50±5℃. The third rolling process applies a pressure of 15-18 MPa and a rolling speed of 3-5 m / min, while the rollers are heated to 70±5℃.

2. The negative electrode sheet according to claim 1, characterized in that, The thickness of the nitrogen-doped carbon intermediate layer is 5-10 nm, and the thickness of the MXene material outer layer is 1-2 μm.

3. The negative electrode sheet according to claim 1, characterized in that, The MXene material is Ti3C2T. m Where T represents a surface functional group, including one or more of -O, -OH, -F, and -Cl, and m represents the coverage degree of the surface functional group, 0 < m < 2.

4. A method for preparing a negative electrode sheet as described in any one of claims 1-3, characterized in that, include: A negative electrode slurry is made by combining lithium titanate three-dimensional composite material, conductive agent and binder, and then coated on at least one surface of the current collector. The current collector coated with negative electrode slurry is rolled under three gradient pressures to anneal the electrode sheet, thereby obtaining the negative electrode sheet. The three-gradient pressure rolling includes: The first rolling process applies a pressure of 5-8 MPa and a rolling speed of 10-15 m / min. The second rolling process applies a pressure of 10-12 MPa and a rolling speed of 5-10 m / min, while the rollers are heated to 50±5℃. The third rolling process applies a pressure of 15-18 MPa and a rolling speed of 3-5 m / min, while the rollers are heated to 70±5℃.

5. The preparation method according to claim 4, characterized in that, The annealing includes: In a protective gas atmosphere, heat to 180-220℃ and hold for 1-3 hours, then cool down.

6. A lithium-ion battery that directly converts chemical energy into electrical energy, characterized in that, It includes a positive electrode, a negative electrode as described in claim 1, a separator, and an electrolyte.

7. A battery pack that directly converts chemical energy into electrical energy, characterized in that, The invention includes multiple lithium-ion batteries as described in claim 6, wherein the multiple lithium-ion batteries are electrically connected in series and / or in parallel.

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