Graphite negative electrode material and application thereof

Abstract

The invention provides a graphite negative electrode material and application thereof.The graphite negative electrode material comprises graphite and a lithium ion fast ion conductor, the lithium ion fast ion conductor is modified on the outer surface of the graphite through a covalent bond or a physical adsorption effect, and the work function of the lithium ion fast ion conductor is lower than 6 eV.

Description

Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion batteries, and in particular, to a graphite negative electrode material and applications thereof. Background Art

[0002] Currently, the most widely used lithium-ion batteries typically use graphite as the anode material and ethylene carbonate (EC) as the main solvent in the electrolyte. However, at low temperatures (-20°C), especially under low-temperature fast charging conditions (>2C), lithium ion transport is hindered and the electrolyte viscosity increases, causing the performance of these batteries to deteriorate significantly.

[0003] Lithium ion transport in anode materials can be divided into four stages: 1) transport of solvated lithium ions in the electrolyte; 2) desolvation of solvated lithium ions at the anode-electrolyte interface; 3) diffusion of lithium ions in the solid electrolyte interface (SEI) film on the graphite surface; and 4) diffusion of lithium ions within graphite particles. At low temperatures (-20°C), the increased viscosity of EC-based electrolytes hinders lithium ion transport and increases the contact resistance at the electrode-electrolyte interface. Therefore, the use of low-melting-point electrolytes is considered an ideal solution. Furthermore, the desolvation process of solvated lithium ions becomes slow at low temperatures, hindering the migration of lithium ions into the SEI and graphite particles. Simultaneously, the ion diffusion capacity of the SEI decreases, further hindering lithium ion transport. Ultimately, this results in a sharp drop in battery power and energy density at low temperatures. Furthermore, low-melting-point solvents typically interact strongly with lithium ions, which can lead to co-intercalation of lithium ions and solvent into graphite, potentially causing battery failure. What is even more complicated is how to achieve a lower melting point electrolyte, easier lithium ion desolvation process and faster lithium ion diffusion in the same battery system. These problems are still difficult problems that have not been solved by industry and academia. Summary of the Invention

[0004] In view of this, the present disclosure provides a graphite anode material and its application to address the key issues of poor desolvation and diffusion of ions at the graphite anode interface, which prevents stable circulation in an electrolyte system dominated by a low-melting-point solvent, leading to poor low-temperature performance of lithium-ion batteries. The present disclosure aims to at least partially address the above technical issues. The technical solutions provided by the present disclosure are as follows:

[0005] As one aspect of the present disclosure, there is provided a graphite negative electrode material, comprising:

[0006] graphite;

[0007] Lithium ion fast ion conductor, modified on the outer surface of graphite by covalent bonding or physical adsorption;

[0008] Among them, the work function of lithium-ion fast ion conductors is lower than 6eV.

[0009] According to an embodiment of the present disclosure, the content of the lithium ion fast ion conductor is 0.1-10 wt%.

[0010] According to an embodiment of the present disclosure, the content of the lithium ion fast ion conductor is 2-5 wt%.

[0011] According to an embodiment of the present disclosure, the lithium ion fast ion conductor with a work function lower than 6 eV is selected from any one or more of oxides, sulfides, hydrides, halides, borates, phosphates and lithium phosphorus oxynitride of lithium.

[0012] According to an embodiment of the present disclosure, the oxide of lithium is selected from any one or more of perovskite material lithium lanthanum titanium oxide, sodium ion conductor type mineral lithium titanium phosphate, lithium ion conductor type mineral, garnet type mineral lithium lanthanum zirconium oxide;

[0013] The sulfide of lithium is selected from any one or two of lithium sulfide-diphosphorus pentasulfide, lithium sulfide-diphosphorus pentasulfide-metal sulfide;

[0014] The hydride of lithium is selected from any one or more of lithium borohydride, lithium borohydride-lithium chloride, lithium borohydride-lithium bromide, lithium borohydride-lithium iodide, doped lithium borohydride, lithium borohydride ammonia complex, lithium amide, lithium aluminum hydride, lithium imide;

[0015] The halide of lithium is selected from any one or more of lithium iodide, lithium zinc iodide and lithium chloride oxide;

[0016] The borate and / or phosphate of lithium is selected from any one or more of lithium tetraborate, lithium phosphate and lithium oxide-boron oxide-diphosphorus pentoxide;

[0017] The lithium ion conductor type mineral is selected from any one or more of lithium zinc germanate, lithium phosphide, lithium stannide, lithium antimonide and lithium bismuthide.

[0018] As another aspect of the present disclosure, a lithium ion battery is provided, comprising the above-mentioned graphite negative electrode material.

[0019] According to an embodiment of the present disclosure, the lithium ion battery further comprises:

[0020] An electrolyte system, the electrolyte system comprising an organic solvent and a lithium salt;

[0021] The lithium ion battery can maintain a reversible capacity of 70% or more under low temperature conditions, the low temperature conditions being -40-0°C.

[0022] According to an embodiment of the present disclosure, the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium difluoro oxalate borate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethanesulfonyl)imide or lithium tetrafluoroborate;

[0023] The concentration of the lithium salt is 0.6-1.6 mol / L -1 .

[0024] According to the embodiment of the present disclosure, the organic solvent includes the first organic solvent and / or the second organic solvent;

[0025] The first organic solvent is a low-melting-point organic solvent, and is selected from any one or more of propylene carbonate (-49℃), 1,3-dioxolane (-95℃), ethylene glycol dimethyl ether (-58℃), diethylene glycol dimethyl ether (-64℃), triethylene glycol dimethyl ether (-45℃), tetraethylene glycol dimethyl ether (-30℃), diethyl ether (-117℃), acetonitrile (-45℃), tetrahydrofuran (-108℃), and 2-methyltetrahydrofuran (-136℃);

[0026] The second organic solvent is selected from any one or more of vinyl carbonate, dimethyl carbonate, and vinyl ethylene carbonate;

[0027] The low-melting-point organic solvent has a melting point of -136 to -30℃.

[0028] According to the embodiment of the present disclosure, the volume ratio of the low-melting-point organic solvent in the organic solvent is 10-100%.

[0029] Based on the above technical solution, the graphite negative electrode material and application thereof provided by the present disclosure select a lithium ion fast ion conductor with a low work function (less than 6 eV) as a modification material, which is coated on the outer surface of the graphite material, and is modified at the sheet layer boundary of the graphite particles through covalent bonds (X-C and / or X-O-C, where X represents the lithium ion fast ion conductor) or physical adsorption, so as to form a solid electrolyte interface film containing the fast ion conductor on the outer surface of the graphite in situ or after prelithiation, promote the desolvation of lithium ions, and promote the rapid diffusion of lithium ions through the solid electrolyte interface film into the graphite particles. In the embodiment of the present disclosure, the lithium ion fast ion conductor with a low work function is used as a modifier of the graphite boundary, which can act as an electron donor, effectively adsorb solvated lithium ions at the negative electrolyte interface, weaken the binding force between lithium ions and solvent molecules, promote the desolvation process of lithium ions, and enable the graphite negative electrode material to also work stably in an electrolyte with a low melting point (-136 to -30℃). At the same time, the interface film formed by the lithium ion fast ion conductor can promote the diffusion of lithium ions and improve the diffusion capacity of lithium ions at low temperatures. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 FIG. 1 is a scanning electron microscope image of the graphite negative electrode material in Example 1 of the present disclosure;

[0031] Figure 2 FIG. 2 is a scanning electron microscope image of the graphite material in Example 1 of the present disclosure;

[0032] Figure 3 A charge-discharge cycle test graph for the graphite negative electrode material in Embodiment 4 of the present disclosure;

[0033] Figure 4 A charge-discharge curve graph for the graphite negative electrode material in Embodiment 6 of the present disclosure;

[0034] Figure 5 A charge-discharge curve graph for the graphite negative electrode material in Embodiment 7 of the present disclosure under low temperature conditions. DETAILED DESCRIPTION

[0035] In order to make the objects, technical solutions, and advantages of the present disclosure clearer, the following will be further described in detail with specific embodiments and with reference to the drawings.

[0036] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it will be apparent to one skilled in the art that one or more embodiments can be practiced without these specific details. In addition, in the following description, descriptions of well-known structures and techniques have been omitted to avoid unnecessarily obscuring the concept of the present disclosure.

[0037] The terms used herein are merely used to describe specific embodiments and are not intended to limit the present disclosure. The terms "include", "comprise" and the like used herein indicate the presence of the features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0038] All terms used herein (including technical and scientific terms) have meanings commonly understood by one of ordinary skill in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having meanings consistent with the context of the present specification, and should not be interpreted in an idealized or overly formal manner.

[0039] In the case of using expressions similar to "at least one of A, B, and C, etc.", in general, it should be interpreted as having a meaning that includes one or more of the corresponding items (for example, "a system having at least one of A, B, and C" should include a system having A alone, a system having B alone, a system having C alone, a system having A and B together, a system having A and C together, a system having B and C together, and / or a system having A, B, and C together, etc.) as understood by one of ordinary skill in the art.

[0040] In the process of realizing the present disclosure, it is found that in the lithium ion battery electrolyte, solvents with low melting point such as propylene carbonate (PC, -49℃), 1,3-dioxolane (DOL, -95℃) and ethylene glycol dimethyl ether (DME, -58℃) and the like have strong interaction with lithium ions, which easily causes lithium ions and solvents to co-intercalate into graphite, leading to interlayer peeling of graphite and thus causing the battery to fail.

[0041] At present, in order to weaken the interaction between lithium ions and solvent molecules, the related technology mainly focuses on introducing a cosolvent or a higher concentration of lithium salt to change the solvent structure of the electrolyte. For example, an electrolyte system using difluoroethylene carbonate (DFEC) as a film-forming additive can form a stable passivation film on the surface of graphite, which can achieve stable cycling for 500 cycles in an electrolyte containing 30% PC and reach a discharge capacity of 86% at -20℃, but it still cannot realize low-temperature charging. For another example: by adding a fluorinated ethylene carbonate graphite-based film-forming additive and a benzene-based compound to inhibit PC co-intercalation additives, the first discharge capacity reaches 1230.2 mAh (94.6% of the design capacity) at room temperature in an electrolyte containing 35% PC, but there is no cycle stability and low-temperature capacity retention. In addition, there is a method of using a high concentration of lithium salt (3.2 mol L -1 ) to make graphite reversibly de-intercalate lithium in PC electrolyte, but its cycle stability is poor, and its capacity decreases from the initial 140 mAh g -1 to 110 mAh g -1 after 50 cycles. In addition, there is a method of using a weakly solvated electrolyte of a cyclic hydrocarbon-based solvent and an inert diluent to increase the steric hindrance effect by introducing a bulky cyclic hydrocarbon group to reduce the coordination ability of the ether-based solvent with lithium ions, thereby enhancing the compatibility of graphite with the ether-based electrolyte, so that the full battery matching the composition of lithium iron phosphate has a capacity retention rate of 96% after 100 cycles. However, the stability of this ether-based electrolyte system is still inferior to that of the conventional electrolyte system of lithium ion batteries.

[0042] In summary, the additives in the above-mentioned methods inevitably affect the solvent structure and thus affect the stability of the positive and negative electrodes of the battery, and the SEI in the cosolvent or higher concentration lithium salt system usually limits the ion diffusion ability, resulting in poor low-temperature performance.

[0043] Therefore, in order to solve the problems of the prior art and related technology, the present disclosure constructs an artificial interface modifier on the surface of graphite, so that the obtained graphite negative electrode material can promote lithium ion desolvation and ion diffusion at the electrode-electrolyte interface, improve the performance of lithium ion batteries under low temperature conditions, and achieve compatibility with low melting point solvents without relying on electrolyte additives or high concentration of salt assistance.

[0044] Therefore, the present disclosure provides a graphite negative electrode material and an application thereof. Specifically, as an aspect of the present disclosure, a graphite negative electrode material is provided, comprising:

[0045] Graphite;

[0046] A lithium ion fast ion conductor modified on the outer surface of the graphite through covalent bonding or physical adsorption;

[0047] The work function of the lithium ion fast ion conductor is less than 6 eV.

[0048] According to an embodiment of the present disclosure, by introducing a lithium ion fast ion conductor with a low work function (less than 6 eV) as a modifier of the graphite boundary, the desolvation ability and diffusion rate of lithium ions at the electrode surface interface can be improved, and the lithium ion battery formed thereby can achieve good low-temperature fast charging performance and cycle stability. Specifically, the lithium ion fast ion conductor with a low work function can act as an electron donor and can produce a strong interaction force with solvated lithium ions in the battery, thereby binding with lithium ions. At the same time, the lithium ion fast ion conductor with a low work function has a weak interaction with solvent molecules, thereby weakening the binding force between lithium ions and solvent molecules, and further promoting the desolvation process of lithium ions, so that lithium ions can quickly diffuse in the solid electrolyte membrane, rather than solvated lithium ions being transported in the solid electrolyte membrane. Moreover, the lithium ion fast ion conductor is modified in the form of nanoparticles at the boundary of the graphite layers in the graphite material, which can effectively expose the graphite, promote the desolvation of lithium ions and the diffusion of lithium ions on the surface of the graphite, improve the diffusion ability of lithium ions at low temperatures, and enable the graphite negative electrode material to work stably in an electrolyte with a low melting point.

[0049] According to an embodiment of the present disclosure, when the work function of the lithium ion fast ion conductor is higher than 6 eV, such as lithium carbonate (6.1 eV) and lithium fluoride (7.6 eV), the lithium ion desolvation process cannot be significantly promoted, and thus the low-temperature fast charging effect cannot be exhibited. A higher work function of the lithium ion fast ion conductor can result in an increased interaction between lithium ions and solvent molecules in the electrolyte. Under low-temperature conditions, this interaction makes it more difficult for lithium ions to separate from the solvent molecules, thereby hindering the effective migration and transport of lithium ions and slowing down the diffusion rate of lithium ions inside the battery. Moreover, the increased viscosity of the electrolyte can further reduce the charge and discharge efficiency of the battery and affect the battery performance.

[0050] According to an embodiment of the present disclosure, the graphite negative electrode material can be obtained by mixing and stirring the graphite material with the lithium ion fast ion conductor. The lithium ion fast ion conductor is coated on the outer surface of the graphite by a physical method of mechanical mixing. Alternatively, the graphite negative electrode material can also be obtained by a chemical method, for example, a suitable compound is selected according to the required lithium ion fast ion conductor, and the compound is mixed with the graphite material so that the compound can cover the surface of the graphite material, and then the mixture is subjected to pre-lithiation treatment to synthesize the lithium ion fast ion conductor on the surface of the graphite material. In this process, a covalent bond is formed between the graphite material and the lithium ion fast ion conductor, so that the lithium ion fast ion conductor is effectively modified on the surface of the graphite material.

[0051] According to an embodiment of the present disclosure, the graphite material can include any one of commercialized products such as natural graphite, artificial graphite, etc.

[0052] According to an embodiment of the present disclosure, the content of the lithium ion fast ion conductor is 0.1-10 wt%, for example, it can be 0.1 wt%, 0.5 wt%, 4.5 wt%, 6.8 wt%, 9.5 wt%, etc., but is not limited to the listed values, and other values not listed in this range are also applicable. When the content of the lithium ion fast ion conductor in the graphite negative electrode material is too high, the adsorption-desorption process between the graphite and the lithium ion will be affected, resulting in a decrease in the capacity of the battery. By controlling the content of the lithium ion fast ion conductor, the battery performance can be optimized, the graphite material can be fully exposed, and the problem of capacity decrease of the material caused by too high content of the lithium ion fast ion conductor can be avoided.

[0053] According to an embodiment of the present disclosure, preferably, the content of the lithium ion fast ion conductor is 2-5 wt%. By controlling the content of the lithium ion fast ion conductor, the transmission effect of the lithium ion can be improved without seriously wasting the capacity of the material.

[0054] According to an embodiment of the present disclosure, the lithium ion fast ion conductor with a work function lower than 6 eV is selected from any one or more of oxides, sulfides, hydrides, halides, borates, phosphates, and lithium phosphorus oxynitride of lithium.

[0055] Specifically, the oxides of lithium are selected from any one or more of perovskite lithium lanthanum titanium oxide, sodium ion conductor type mineral lithium titanium phosphate, lithium ion conductor type mineral, and garnet type mineral lithium lanthanum zirconium oxide.

[0056] The sulfides of lithium are selected from any one or two of lithium sulfide-pentasulfide diphosphorus, lithium sulfide-pentasulfide diphosphorus-metal sulfide, wherein the metal sulfide can be cadmium sulfide, tin sulfide, or antimony sulfide.

[0057] The hydride of lithium is selected from any one or more of lithium borohydride, lithium borohydride-lithium chloride, lithium borohydride-lithium bromide, lithium borohydride-lithium iodide, doped lithium borohydride, lithium borohydride ammine complex, lithium amide, lithium aluminum hydride, lithium imide;

[0058] The halide of lithium is selected from any one or more of lithium iodide, lithium zinc iodide, and lithium chloride oxide, and can also be selected from other materials in spinel-type minerals and anti-perovskite materials;

[0059] The borate and / or phosphate of lithium is selected from any one or more of lithium tetraborate, lithium phosphate, and lithium oxide-lithium oxide-boron oxide-phosphorus pentoxide;

[0060] The lithium ion conductor-type mineral is selected from any one or more of lithium zinc germanate, lithium phosphide, lithium stannide, lithium antimonide, and lithium bismuthide.

[0061] In embodiments of the present disclosure, the aforementioned lithium ion fast ion conductor with a low work function (less than 6 eV) can effectively reduce the migration energy barrier of lithium ions in the electrolyte, accelerate the transmission speed of lithium ions, and weaken the binding force between lithium ions and solvent molecules, allowing the lithium ions to remove the solvent molecules and further diffuse to the surface of the graphite, thereby improving the performance of the battery.

[0062] According to embodiments of the present disclosure, the lithium ion fast ion conductor is preferably any one or more of lithium stannide, lithium iodide, and lithium phosphate. These materials have stable chemical properties and high ion conductivity, and can be used in lithium ion batteries to significantly improve the energy density, cycle stability, and safety performance of the battery.

[0063] As another aspect of the present disclosure, a lithium ion battery is disclosed, comprising the aforementioned graphite negative electrode material.

[0064] According to embodiments of the present disclosure, the aforementioned graphite negative electrode material is made into a substrate and used as a battery negative electrode to form a lithium ion battery, which can improve the desolvation ability and diffusion rate of lithium ions at the electrode surface interface, allowing the lithium ions to have good diffusion ability at low temperatures, achieving good low-temperature fast-charging performance and cycle stability.

[0065] According to embodiments of the present disclosure, the lithium ion battery further comprises:

[0066] An electrolyte system, the electrolyte system comprising an organic solvent and a lithium salt;

[0067] The lithium ion battery can maintain a reversible capacity of more than 70% under low-temperature conditions, and the low-temperature conditions are -40-0°C.

[0068] According to embodiments of the present disclosure, the graphite negative electrode material provided by the present disclosure has strong applicability, can be compatible with various electrolyte systems, and can also be stably operated under low temperature conditions. Compared with the reversible capacity at room temperature, the lithium ion battery using the above graphite negative electrode material can still maintain more than 70% of the reversible capacity under low temperature conditions of-20℃.

[0069] According to embodiments of the present disclosure, the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, or lithium tetrafluoroborate.

[0070] According to embodiments of the present disclosure, in the lithium ion battery, the positive electrode and the negative electrode are connected by the electrolyte for lithium ion conduction, and the concentration of the lithium salt in the electrolyte system is 0.6-1.6 mol / L. -1 For example, it can be 0.8 mol / L, -1 1.0 mol / L, -1 1.2 mol / L, -1 1.5 mol / L, -1 and the like. The presence of the lithium salt enables the lithium ion to shuttle effectively between the positive electrode and the negative electrode, thereby realizing the normal operation of the battery. Preferably, the concentration of the lithium salt is 1 mol / L, -1 which can ensure good mass transfer efficiency and battery performance.

[0071] According to embodiments of the present disclosure, the organic solvent includes a first organic solvent and / or a second organic solvent; wherein the first organic solvent is a low-melting-point organic solvent, and the melting point of the low-melting-point organic solvent is-136-30℃, for example, it can be-125℃, -95℃, -80℃, -75℃, -38℃, and the like, but is not limited to the listed values, and other values not listed in this range are also applicable. The first organic solvent can be selected from any one or more of propylene carbonate (-49℃), 1,3-dioxolane (-95℃), ethylene glycol dimethyl ether (-58℃), diethylene glycol dimethyl ether (-64℃), triethylene glycol dimethyl ether (-45℃), tetraethylene glycol dimethyl ether (-30℃), diethyl ether (-117℃), acetonitrile (-45℃), tetrahydrofuran (-108℃), and 2-methyltetrahydrofuran (-136℃).

[0072] According to the embodiments of the present disclosure, the first organic solvent has a low melting point, is traditionally considered as a low-melting-point solvent incompatible with graphite materials, has a strong solvation feature, has a strong binding capacity with lithium ions, and can form a stable solvated complex, which makes it difficult for lithium ions to migrate and transport in the electrolyte. However, the graphite negative electrode material provided by the present disclosure can still achieve stable circulation in the electrolyte containing the above-mentioned low-melting-point organic solvent, and the reversible capacity thereof can not only reach more than 90% of the theoretical capacity, but also maintain a capacity retention rate of more than 90% after 200 cycles. Tests show that the graphite negative electrode can still maintain a capacity retention rate of more than 80% at room temperature with a current density of 2C. In addition, the material also has good low-temperature performance, and can still maintain a reversible capacity of more than 70% (compared with room temperature) at an extreme low temperature of -20℃, and will not deposit lithium during low-temperature fast charging.

[0073] According to the embodiments of the present disclosure, the second organic solvent is a common organic solvent, i.e., a non-low-melting-point organic solvent with a weak binding capacity with lithium ions, and the second organic solvent is selected from any one or more of vinyl carbonate, dimethyl carbonate, and vinyl ethylene carbonate. When the organic solvent selected in the electrolyte system is only these common organic solvents, the graphite negative electrode material can exhibit good low-temperature fast charging performance in the second organic solvent and can still operate stably in the range of -20 to 45℃.

[0074] According to the embodiments of the present disclosure, the volume ratio of the low-melting-point organic solvent (the first organic solvent) in the organic solvent is 10 to 100%, for example, can be 10%, 20%, 40%, 60%, 80%, etc., but is not limited to the listed values, and other values not listed in the range are also applicable. The graphite negative electrode material provided by the present disclosure is suitable for electrolyte systems composed of various low-melting-point organic solvents, and the proportion of the low-melting-point organic solvent in the total amount of the solvent can be adjusted according to the actual situation to adapt the electrolyte to different temperature environments. Even if all the organic solvents in the electrolyte system are selected as low-melting-point organic solvents, good low-temperature fast charging performance and cycle stability can also be achieved.

[0075] In order to make the purpose, technical scheme and advantages of the present disclosure clearer and more definite, the technical scheme and principles of the present disclosure will be further described and explained below by combining specific embodiments with the drawings. It should be noted that the following specific embodiments are only used as examples, and the protection scope of the present disclosure is not limited thereto.

[0076] Unless otherwise specified, the test materials and reagents used in the following embodiments can be obtained from commercial channels. If the specific technology or condition is not specified in the embodiments, it is a conventional method, which can be performed according to the technology or condition described in the literature in the art or according to the product instruction.

[0077] Example 1

[0078] The tin powder and graphite powder were mixed in a mass ratio of 8:92, and then subjected to mechanical shock treatment under an argon protective atmosphere for 18 hours, so as to uniformly coat the tin powder on the graphite material. The tin-coated graphite material was then made into a sheet, and a lithium stannide (work function < 3 eV) fast ionic conductor was formed by pre-lithiation to coat the graphite surface of the graphite anode material.

[0079] Figure 1 A scanning electron microscope image of the graphite anode material in Example 1 of the present disclosure; Figure 2 A scanning electron microscope image of the graphite material in Example 1 of the present disclosure.

[0080] From Figure 1 It can be seen that, compared with the graphite material in Figure 2 , Figure 1 The particles of the lithium stannide fast ionic conductor are uniformly distributed and coated on the outer surface of the graphite particles.

[0081] Example 2

[0082] The antimony powder and graphite powder were mixed in a mass ratio of 8:92, and then subjected to mechanical shock treatment under an argon protective atmosphere for 18 hours, so as to uniformly coat the antimony powder on the graphite material. The antimony-coated graphite material was then made into a sheet, and a lithium antimonide (work function < 3 eV) fast ionic conductor was formed by pre-lithiation to coat the graphite surface of the graphite anode material.

[0083] Example 3

[0084] The lithium iodide (work function < 3 eV) powder and graphite powder were mixed in a mass ratio of 8:92, and then subjected to mechanical shock treatment under an argon protective atmosphere for 3 hours. Acetonitrile solvent was then added and stirred for 24 hours, and then subjected to suction filtration and drying. The lithium iodide-coated graphite surface of the graphite anode material was obtained after the sheet was made and pre-lithiation treatment.

[0085] Comparative Example 1

[0086] The pure graphite sample was made into a sheet, and a lithium carbonate (work function of 6.1 eV) coated graphite anode material was obtained after pre-lithiation solution treatment by adding 10 wt% ethylene carbonate (VC).

[0087] Example 4

[0088] The graphite anode material prepared in Example 1 was used to prepare an electrode sheet, and a lithium half-cell was assembled and subjected to charge and discharge tests. The charge and discharge interval was 0.01-1 V, and the electrolyte system used was a PC-based electrolyte, which was prepared from propylene carbonate (PC) solvent and 1 mol L -1of lithium hexafluorophosphate (LiPF6) and is marked as: 1M LiPF6+PC. The charge-discharge cycle test is carried out at a current density of 0.2C, and the voltage range is 0.01-1V. The cycle test result is shown in Figure 3 .

[0089] Figure 3 The charge-discharge cycle test graph of the graphite negative electrode material in Example 4 of the present disclosure.

[0090] The Figure 3 It can be seen that the graphite negative electrode material provided by the present disclosure can be stably cycled in a low-melting-point electrolyte, and the reversible capacity is more than 90% of the theoretical capacity. The capacity retention rate after 200 cycles is high, the initial capacity is 327mAh g -1 , and the capacity after cycling is 306mAh g -1 , which is more than 90%.

[0091] Example 5

[0092] The graphite negative electrode materials prepared in Examples 1-3 and Comparative Example 1 are used as electrode sheets, and lithium half-batteries are assembled. The charge-discharge tests are carried out in EC-based electrolyte and PC-based electrolyte, and the performance of the graphite negative electrode material is evaluated. The charge-discharge interval is 0.01-1V.

[0093] The preparation process of the EC-based electrolyte and the PC-based electrolyte is as follows:

[0094] Preparation of EC-based electrolyte: ethylene carbonate (EC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1:1, and 1mol L -1 of lithium hexafluorophosphate (LiPF6) is added, and is marked as: 1M LiPF6+EC:EMC:DMC=1:1:1 (volume ratio).

[0095] PC-based electrolyte: 1mol L -1 of lithium hexafluorophosphate (LiPF6) is added to propylene carbonate (PC) solvent, and is marked as: 1M LiPF6+PC; or ethylene carbonate (PC), methyl ethyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1:1, and 1mol L -1 of lithium hexafluorophosphate (LiPF6) is added to form a PC-based electrolyte, and is marked as: 1M LiPF6+PC:EMC:DMC=1:1:1 (volume ratio).

[0096] The performance results obtained after the charge-discharge test are shown in Table 1 below. The graphite negative electrode material obtained in Comparative Example 1 directly failed after the first discharge in the PC-based electrolyte 1M LiPF6+PC:EMC:DMC=1:1:1 (volume ratio), and the graphite electrode sheet was found to be broken after the battery was disassembled after cycling.

[0097] As can be seen from Table 1, in the 1M LiPF6+PC electrolyte, 1M LiPF6+PC:EMC:DMC=1:1:1 (volume ratio) electrolyte and 1M LiPF6+EC:EMC:DMC=1:1:1 (volume ratio) electrolyte, the graphite negative electrode material provided by the present disclosure can exhibit a high reversible capacity, and can also maintain a high reversible specific capacity in a low melting point electrolyte. The graphite material not coated with a low work function lithium ion fast ion conductor will intercalate solvent molecules in the electrolyte incompatible with graphite.

[0098] Table 1

[0099]

[0100] Example 6

[0101] 1 mol L -1 Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to the ethylene glycol dimethyl ether (DME) solvent to form a DME electrolyte system, which is marked as: 1M LiTFSI+DME.

[0102] The graphite negative electrode materials in Example 1, Example 2 and Comparative Example 1 and the pure graphite material were assembled into lithium half-batteries in the DME electrolyte system for testing, and the charge-discharge interval was set to 0.01-1V. The test results are shown in Figure 4 .

[0103] Figure 4 The charge-discharge curve of the graphite negative electrode material in Example 6 of the present disclosure.

[0104] From Figure 4 It can be seen that in the DME electrolyte system, the reversible specific capacity of the pure graphite material is 131.5mAh g -1 , the reversible specific capacity of the graphite negative electrode material formed by the lithium carbonate coated graphite material is 77.2mAh g -1 It can be seen that the charge-discharge reversible capacity of the graphite negative electrode material formed by the lithium carbonate coated graphite material is provided by the solvent intercalation. The reversible specific capacity of the graphite negative electrode material formed by the lithium tin coated graphite material in Example 1 is 338.1mAh g -1 , and the reversible specific capacity of the graphite negative electrode material formed by the lithium antimony coated graphite material in Example 2 is 318.8mAh g -1It can be seen that, compared with the graphite material without coating and the graphite material coated with other types of lithium ion conductors, the graphite negative electrode material provided by the present disclosure can exhibit better fast charging performance in a low-melting-point electrolyte incompatible with graphite.

[0105] Example 7

[0106] The graphite negative electrode material formed by coating the graphite with lithium stannate in Example 1 and the graphite negative electrode material formed by coating the graphite with lithium antimonate in Example 2 were respectively used as the electrode sheet, propylene carbonate (PC) and dioxolane (DOL) were mixed in a volume ratio of 1:1, and 1 mol / L lithium bis (fluorosulfonyl) imide (LiTFSI) was added to form a PC / DOL electrolyte, marked as 1M LiTFSI+PC / DOL=1:1. In the PC / DOL electrolyte, the ternary material NCM523 positive electrode was matched to assemble a full battery, and the reversible specific capacity of low-temperature charging and discharging was tested in a temperature environment of-20°C. -1 The graphite negative electrode material formed by coating the graphite with lithium stannate in Example 1 and the graphite negative electrode material formed by coating the graphite with lithium antimonate in Example 2 were respectively used as the electrode sheet, propylene carbonate (PC) and dioxolane (DOL) were mixed in a volume ratio of 1:1, and 1 mol / L lithium bis (fluorosulfonyl) imide (LiTFSI) was added to form a PC / DOL electrolyte, marked as 1M LiTFSI+PC / DOL=1:1. In the PC / DOL electrolyte, the ternary material NCM523 positive electrode was matched to assemble a full battery, and the reversible specific capacity of low-temperature charging and discharging was tested in a temperature environment of-20°C.

[0107] Meanwhile, the graphite negative electrode material formed by coating the graphite with lithium carbonate in Comparative Example 1 and the pure graphite material were used as the electrode sheet, and the ternary material NCM523 positive electrode was matched to assemble a full battery, and the reversible specific capacity of low-temperature charging and discharging was tested in a temperature environment of-20°C in the EC-based electrolyte 1M LiPF6+EC:EMC:DMC=1:1:1.

[0108] Figure 5 The charge-discharge curve of the graphite negative electrode material under low-temperature conditions in Example 7 of the present disclosure.

[0109] From Figure 5 It can be seen that the graphite negative electrode material formed by coating the graphite with lithium stannate in Example 1 and the graphite negative electrode material formed by coating the graphite with lithium antimonate in Example 2 have reversible specific capacities of 106.5 mAh / g -1 and 107.5 mAh / g -1 respectively in the PC-based electrolyte under low-temperature conditions of-20°C. -1 -1

[0110] ​​Based on the above technical scheme, the graphite negative electrode material formed by coating the graphite material with the lithium ion fast ion conductor with a low work function has good compatibility with various types of electrolyte under the condition of no other additives and EC solvent, can exhibit a higher reversible capacity, indicating that the material combined with a low-melting-point electrolyte has the potential to achieve low-temperature charging. Further, in the low-melting-point electrolyte incompatible with graphite, the material can achieve more excellent low-temperature performance, i.e., can still maintain more than 70% of the reversible capacity at room temperature at-20℃, and will not deposit lithium during low-temperature rapid charging, exhibiting high cycle stability.

[0111] The above specific embodiments further specifically describe the purposes, technical solutions and beneficial effects of the present disclosure, and it should be understood that the above are only specific embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present disclosure shall be included in the protection scope of the present disclosure.

Claims

1.A graphite negative material, comprising: graphite; a lithium ion fast ion conductor modified on the outer surface of the graphite by covalent bond or physical adsorption; wherein the work function of the lithium ion fast ion conductor is less than 6 eV. 2.The graphite negative material according to claim 1, wherein the content of the lithium ion fast ion conductor is 0.1-10 wt%. 3.The graphite negative material according to claim 2, wherein the content of the lithium ion fast ion conductor is 2-5 wt%. 4.The graphite negative material according to claim 1, wherein the lithium ion fast ion conductor with a work function less than 6 eV is selected from any one or more of lithium oxides, lithium sulfides, lithium hydrides, lithium halides, lithium borates, lithium phosphates and lithium phosphorus oxynitride. 5.The graphite negative material according to claim 4, wherein the lithium oxides are selected from any one or more of perovskite material lithium lanthanum titanium oxide, sodium ion conductor type mineral lithium titanium phosphate, lithium ion conductor type mineral, garnet type mineral lithium lanthanum zirconium oxide; the lithium sulfides are selected from any one or two of lithium sulfide-diphosphorus pentasulfide, lithium sulfide-diphosphorus pentasulfide-metal sulfide; the lithium hydrides are selected from any one or more of lithium borohydride, lithium borohydride-lithium chloride, lithium borohydride-lithium bromide, lithium borohydride-lithium iodide, doped lithium borohydride, lithium borohydride-ammonia complex, lithium amide, lithium aluminum hydride, lithium imide; the lithium halides are selected from any one or more of lithium iodide, lithium zinc iodide and lithium chloride oxide; the lithium borates and / or lithium phosphates are selected from any one or more of lithium tetraborate, lithium phosphate and lithium oxide-boron oxide-diphosphorus pentoxide; the lithium ion conductor type mineral is selected from any one or more of lithium zinc germanate, lithium phosphide, lithium stannide, lithium antimonide and lithium bismuthide. 6.A lithium ion battery comprising the graphite negative material according to any one of claims 1-5. 7.The lithium ion battery according to claim 6, further comprising: an electrolyte system comprising an organic solvent and a lithium salt; the lithium ion battery can maintain a reversible capacity of 70% or more under low temperature conditions of-40-0℃. 8.The lithium ion battery according to claim 7, wherein the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium perchlorate, lithium difluoro oxalate borate, lithium bisfluorosulfonylimide, lithium bis(trifluoromethanesulfonyl)imide or lithium tetrafluoroborate; The concentration of the lithium salt is 0.6-1.6 mol L -1 . 9.The lithium ion battery according to claim 7, wherein the organic solvent comprises a first organic solvent and / or a second organic solvent; wherein the first organic solvent is a low melting point organic solvent selected from any one or more of propylene carbonate, 1,3-dioxolane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethyl ether, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran; the second organic solvent is selected from any one or more of vinyl carbonate, dimethyl carbonate, vinyl ethylene carbonate; the melting point of the low melting point organic solvent is-136-30℃. 10.The lithium ion battery according to claim 9, wherein The low-melting organic solvent has a volume ratio of 10-100% in the organic solvent. The low-melting organic solvent has a volume ratio of 10-100% in the organic solvent.

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