Lithium ion battery negative electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a lithium ion battery negative electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of new energy materials. The lithium ion battery negative electrode material provided by the invention has a double-layer core-shell structure, the inner core is graphite, the middle layer is a phosphorus-sulfur co-coating layer, and the outer layer is a carbon layer; the carbon layer is a pyrolysis product of ionic liquid. The lithium ion battery negative electrode material provided by the invention has excellent low-temperature performance and fast charging performance. The invention also provides a preparation method of the lithium ion battery negative electrode material, and a lithium ion battery comprising the lithium ion battery negative electrode material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy materials, and in particular to a lithium ion battery negative electrode material, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] With the rapid development of electric vehicles and portable electronic devices, lithium ion battery technology is rapidly developing, and products are rapidly iterated and updated. The market side of lithium ion batteries has higher performance requirements, such as charging time (ultra-fast charging application scenarios), low-temperature endurance, etc. At present, graphite is still the mainstream negative electrode material due to its mature preparation process, stable performance, and low lithium intercalation potential, but it has the following bottlenecks: the lithium ion diffusion kinetics between graphite layers is slow, which limits the fast charging performance; the graphite layer is prone to peeling or pulverization during the cycle process, affecting the cycle life; the diffusion rate of lithium ions between graphite layers at low temperature is significantly reduced, resulting in a sharp drop in capacity.

[0003] In the traditional technology, carbon coating or phosphorus coating modification methods are often used to improve the performance of graphite. Among them, the preparation of phosphorus-coated graphite usually involves three steps of phosphorus source introduction, heat treatment and structure regulation. Common methods include liquid coating, ball milling assisted coating and chemical vapor deposition (CVD), all of which involve high-temperature heat treatment. If there is no heat treatment, the performance will decrease significantly. For example, there is a patent that introduces a phosphorus source and graphite powder in a certain mass ratio under an argon atmosphere to obtain a phosphorus-coated graphite negative electrode by mechanical shaking. Although this method is simple, the structure strength of the material obtained by this coating method is low, which can easily lead to structure collapse during charging and discharging, degrade the electrical performance of the lithium ion battery, and cause safety problems (the phosphorus source is directly exposed to the outermost layer). Carbon coating usually uses biomass materials as raw materials to obtain a carbon coating layer by carbonization; for example, there is a patent that prepares a biomass-based phosphorus-coated ultra-low temperature lithium ion battery negative electrode material through two-step carbonization; the preparation process is redundant, and the required carbonization temperature is relatively high (750-1500℃) and the time is relatively long (5-24h), which results in high energy consumption, and the P coating is uneven, which significantly degrades the cycle performance of the material. According to the above analysis, the traditional coating has the problems of poor uniformity, complex process (such as chemical vapor deposition) or high energy consumption of high-temperature treatment; most importantly, the electrochemical performance of the modified graphite prepared by the traditional technology is still not ideal.

[0004] Therefore, it is urgent to develop a graphite negative electrode with a simple preparation method and excellent low-temperature performance and fast charging performance. SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application proposes a lithium ion battery negative electrode material, which can effectively improve the low-temperature performance and fast charging performance of graphite materials.

[0006] The application further provides a preparation method of the lithium ion battery negative electrode material.

[0007] The application further provides a lithium ion battery comprising the lithium ion battery negative electrode material or the lithium ion battery negative electrode material prepared by the preparation method.

[0008] According to the embodiment of the first aspect of the application, a lithium ion battery negative electrode material is provided, which has a double-layer core-shell structure, wherein the inner core is graphite, the intermediate layer is a phosphorus-sulfur co-coating layer, and the outer layer is a carbon layer; the carbon layer is a pyrolysis product of an ionic liquid.

[0009] The lithium ion battery negative electrode material according to the embodiment of the application has at least the following beneficial effects:

[0010] In the conventional technology, P is directly coated on the graphite, and uniform coating cannot be achieved. The application researches and finds that this is because the affinity of P and C is relatively poor, which leads to the P atoms being more inclined to form island-like aggregation on the end face of the graphite, and it is difficult to form a uniform coating layer. In the application, sulfur (S) is introduced into the intermediate layer. Because the affinity of S and C is good, and S and P can easily form a bond, S "bridges" C and P, so that the P atoms can be uniformly deposited. The uniformly coated intermediate layer significantly improves the fast charging performance of the lithium ion battery negative electrode material through the following mechanisms: the intermediate layer has high conductivity, which can reduce the charge mass transfer impedance (Rct) of the lithium ion battery negative electrode active material; in the intermediate layer, the alloying reaction of phosphorus (such as Li + + P + e - → Li3P) provides additional lithium storage sites and shortens the lithium ion diffusion path; secondly, the intermediate layer can also guide the uniform embedding of lithium ions and inhibit local polarization; overall, the intermediate layer enhances the lithium ion diffusion dynamics: the intermediate layer reacts with the electrolyte to form a stable SEI film (rich in Li3P, LiPO2, etc.), reduces the irreversible capacity loss, and the uniform SEI film can effectively inhibit the growth of lithium dendrites.

[0011] Further, in the lithium ion battery negative electrode material provided by the application, the carbon layer has the functions of improving the electronic conductivity and inhibiting the side reaction with the electrolyte.

[0012] The intermediate layer and the carbon layer together have the function of buffering the volume change of the lithium ion battery negative electrode material during the charging and discharging process, which can maintain the structural integrity of the inner core graphite layer and reduce the particle shedding / structural damage.

[0013] In summary, in the lithium ion battery negative electrode material provided by the application, the carbon layer and the intermediate layer are combined, which significantly improves the low-temperature performance and fast charging performance of the obtained lithium ion battery negative electrode material, as well as other comprehensive electrochemical performance.

[0014] According to some embodiments of the present application, the mass percentage of sulfur in the intermediate layer is 0.5-2.5%.

[0015] According to some embodiments of the present application, the mass of the core accounts for 80-98% of the total mass of the core and the intermediate layer.

[0016] According to some embodiments of the present application, the intermediate layer comprises S layer and P layer arranged in sequence from the core to the outer layer. There is no obvious physical division between the S layer and the P layer, but they are bridged and combined by P-S bonds.

[0017] According to some embodiments of the present application, the carbon layer is doped with N and P at the same time.

[0018] According to some embodiments of the present application, the specific surface area of the lithium ion battery negative electrode material is 1-2m 2 g -1 .

[0019] According to some embodiments of the present application, the initial efficiency of the lithium ion battery negative electrode material at 0.2C is ≥93%.

[0020] According to some embodiments of the present application, the reversible gram capacity of the lithium ion battery negative electrode material at 0.2C is ≥355mAh / g.

[0021] According to some embodiments of the present application, the ratio of the reversible gram capacity of the lithium ion battery negative electrode material at 4C and 0.2C is ≥93.5%.

[0022] According to some embodiments of the present application, the capacity retention rate of the lithium ion battery negative electrode material after 300 cycles at 4C is ≥92%.

[0023] According to some embodiments of the present application, the DCR of the lithium ion battery negative electrode material at -20℃ is ≤235mΩ@0.4C 10s.

[0024] According to some embodiments of the second aspect of the present application, a preparation method of the lithium ion battery negative electrode material according to the embodiments of the first aspect of the present application is provided, and the preparation method comprises the following steps:

[0025] S1. Mixing a phosphorus source, a sulfur source and graphite in a protective atmosphere, and calcining;

[0026] S2. Mixing the product obtained in step S1 and an ionic liquid in a protective atmosphere, and then performing heat treatment.

[0027] Since the preparation method adopts all the technical solutions of the lithium ion battery negative electrode material according to the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments.

[0028] The preparation method can obtain a lithium ion battery negative electrode material with a complete intermediate layer and outer layer coating, stable structure, excellent fast charging and low temperature performance.

[0029] Since the heat treatment in step S2 is also required, the calcination temperature in step S1 does not need to be very high. Meanwhile, the ionic liquid is used as the carbon source for the outer layer in the present application, and the imidazole ring thereof can act as a soft template on the carbon skeleton in the graphitization process, thereby significantly reducing the heat treatment temperature in step S2 compared with traditional organic carbon sources (such as gelatin), and achieving the effect of energy saving. That is, the preparation method provided by the present application significantly reduces the required heat energy consumption through the selection of raw materials and the design of steps.

[0030] The carbon layer obtained by using the ionic liquid provided by the present application as the carbon source has high disorder degree (isotropic), wide carbon layer lattice spacing, doping of heteroatoms (co-doping of N and P), fast lithium ion diffusion rate, many active sites on the surface of the carbon layer for lithium ion intercalation, additional lithium storage sites, shortened lithium ion diffusion path, and the carbon layer can also guide uniform intercalation of lithium ions due to the doping of heteroatoms.

[0031] Through the heat treatment in step S2, the intermediate layer and the outer layer can be bridged by P-C / P-O-C covalent bonds, and the uniformity of the carbon layer coating is further guided. In use, the ionic liquid is in a liquid state, thereby further improving the uniformity of the carbon layer coating.

[0032] Overall, the preparation method provided by the present application is simple, easy to implement, low in energy consumption, and can improve the low temperature and fast charging performance of the obtained lithium ion battery negative electrode material.

[0033] According to some embodiments of the present application, in step S1, the mass percentage of the graphite in the obtained mixture is 80-98%.

[0034] According to some embodiments of the present application, in step S1, the mass of the sulfur source accounts for 0.5-2.5% of the total mass of the phosphorus source and the sulfur source.

[0035] According to some embodiments of the present application, in step S1, the mixing method comprises ball milling, and the ball milling satisfies at least one of the following conditions:

[0036] (a) the rotation speed of the ball milling is 200-400 rpm;

[0037] (b) the time length of the ball milling is 0.5-12 h.

[0038] According to some embodiments of the present application, in step S1, the calcining comprises a first constant temperature stage and a second constant temperature stage in sequence; the temperature of the first constant temperature stage is 400-500°C; and the temperature of the second constant temperature stage is 250-300°C. In the first constant temperature stage, S and P are both in gaseous state essentially, and after the first constant temperature stage ends and the temperature starts to decrease, S starts to deposit below 400°C and is more inclined to deposit on the surface of the core graphite due to the surface energy, and P starts to deposit when the temperature reaches the temperature of the second constant temperature stage; thus, the intermediate layer actually comprises S layers and P layers deposited in sequence, wherein the S layers are in contact with the core graphite; further, by setting the holding temperature platform of the first constant temperature stage and the second constant temperature stage, the deposition uniformity of the S layers and the P layers can be further improved. Further, according to the above explanation, the calcining process in step S1 is actually a CVD process.

[0039] According to some embodiments of the present application, in step S1, the duration of the first constant temperature stage is 2-5h.

[0040] According to some embodiments of the present application, in step S1, the duration of the second constant temperature stage is 12-24h.

[0041] According to some embodiments of the present application, in step S1, the heating rate of the calcining is 1-5°C / min.

[0042] According to some embodiments of the present application, in step S2, the temperature of the heat treatment is 200-800°C. The ionic liquid can obtain a good graphite structure at a heat treatment temperature of about 250°C, and thus a temperature range of 200-300°C can be selected in actual production.

[0043] According to some embodiments of the present application, in step S2, the duration of the heat treatment is 2-4h.

[0044] According to some embodiments of the present application, in step S2, the mass ratio of the ionic liquid to the product obtained in step S1 is 1-6:1.

[0045] According to some embodiments of the present application, in step S2, the ionic liquid comprises organic cations and inorganic anions; the organic cations contain imidazole rings or pyridine rings; and the inorganic anions contain phosphate groups.

[0046] According to some embodiments of the present application, in step S2, the phosphate groups contain at least one of phosphate groups, monohydrogen phosphate groups and dihydrogen phosphate groups.

[0047] According to some embodiments of the present application, in step S2, the ionic liquid comprises 1-butyl-3-methylimidazolium dihydrogen phosphate (chemical formula as follows):

[0048]

[0049] According to some embodiments of the present application, in step S2, the mixing method comprises stirring, wherein the stirring speed is 150-200 rpm, and the stirring time is 3-6 hours. After mixing, the obtained mixture is in a viscous and uniform state.

[0050] In the present application, the protective atmosphere is at least one of nitrogen, argon and helium. For example, in actual production, argon can be selected according to stability and cost.

[0051] According to embodiments of the third aspect of the present application, a lithium ion battery is provided, which comprises the lithium ion battery negative electrode material according to embodiments of the first aspect of the present application, or the lithium ion battery negative electrode material prepared by the preparation method according to embodiments of the second aspect of the present application.

[0052] Since the lithium ion battery adopts the lithium ion battery negative electrode material or the preparation method according to the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments.

[0053] According to some embodiments of the present application, the lithium ion battery comprises at least one of a soft pack battery, a button cell, a cylindrical battery and a square cell.

[0054] According to some embodiments of the present application, the lithium ion battery comprises at least one of a full cell, a half cell and a symmetric cell.

[0055] According to some embodiments of the present application, when the lithium ion battery is a full cell, the lithium ion battery comprises a positive electrode active material; the positive electrode active material comprises at least one of a polyanion material, a layered material and a spinel material; in actual production, the positive electrode active material is not strictly limited, and can be selected from commercially available materials according to the design performance of the lithium ion battery.

[0056] Other features and advantages of the present application will be set forth in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or can be learned by practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0057] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which:

[0058] Figure 1 is a flowchart and principle schematic diagram of Example 1 of the present application.

[0059] Figure 2 is a Raman spectrum of the product obtained in step S1 of Example 1 and Comparative Example 2 of the present application.

[0060] Figure 3 is a SEM image of the product obtained in step S1 of Example 1 of the present application.

[0061] Figure 4 is a SEM image of the product obtained in step S1 of Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0062] The concept and the technical effects of the present application will be described in detail below with reference to the embodiments, so as to fully understand the objects, features and effects of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0063] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0064] According to the embodiments of the first aspect of the present application, a lithium ion battery negative electrode material is provided, which has a double-layer core-shell structure, wherein the inner core is graphite, the intermediate layer is a phosphorus-sulfur co-coated layer, and the outer layer is a carbon layer; the carbon layer is a pyrolysis product of an ionic liquid.

[0065] According to some embodiments of the present application, in the intermediate layer, the mass percentage of sulfur is 0.5-2.5%. For example, it can be specifically 0.5%, 1%, 1.5%, 2%, 2.5%, or a range value formed by any two of them.

[0066] According to some embodiments of the present application, the mass of the inner core accounts for 80-98% of the total mass of the inner core and the intermediate layer. For example, it can be specifically 80%, 85%, 90%, 95%, 98%, or a range value formed by any two of them.

[0067] According to some embodiments of the present application, in the direction from the inner core to the outer layer, the intermediate layer comprises S layer and P layer arranged in sequence. And there is no obvious physical division limit between the S layer and the P layer, but they are bridged and combined through P-S bond.

[0068] According to some embodiments of the present application, the carbon layer is simultaneously doped with N and P.

[0069] According to some embodiments of the present application, the specific surface area of the lithium ion battery negative electrode material is 1-2 m 2 g -1 . For example, it can be specifically 1 m 2 g -1 , 1.1 m 2 g -1 , 1.2 m 2 g -1 , 1.3 m 2 g -1 , 1.4 m 2 g -1 , 1.5 m 2 g -1 , 1.8 m 2 g -1 , 2 m 2 g -1 , or a range value formed by any two of them.

[0070] According to some embodiments of the present application, the 0.2C initial efficiency of the lithium ion battery negative electrode material is ≥93%. For example, it can be specifically 93%, 94%, 95%, 96%, or a range value formed by any two of them.

[0071] According to some embodiments of the present application, the 0.2C reversible capacity of the lithium ion battery negative electrode material is ≥355 mAh / g. For example, it can be specifically 355 mAh / g, 356 mAh / g, 360 mAh / g, 365 mAh / g, 370 mAh / g, or a range value formed by any two of them.

[0072] According to some embodiments of the present application, the ratio of the reversible capacities of the lithium ion battery negative electrode material at 4C and 0.2C is ≥93.5%. For example, it can be specifically 93.5%, 94%, 94.5%, 95%, or a range value formed by any two of them.

[0073] According to some embodiments of the present application, the capacity retention rate of the lithium ion battery negative electrode material at 4C after 300 cycles is ≥92%. For example, it can be specifically 92%, 93%, 94%, 95%, 96%, or a range value formed by any two of them.

[0074] According to some embodiments of the present application, the DCR of the lithium ion battery negative electrode material at -20°C is ≤235 mΩ@0.4C 10s. For example, it can be specifically 235 mΩ@0.4C 10s, 230 mΩ@0.4C 10s, 225 mΩ@0.4C 10s, 220 mΩ@0.4C 10s, 215 mΩ@0.4C 10s, 210 mΩ@0.4C 10s, or a range value formed by any two of them.

[0075] According to embodiments of the second aspect of the present application, there are provided methods for preparing the negative electrode material of the lithium ion battery of the first aspect of the present application, the method comprising the following steps:

[0076] S1. mixing a phosphorus source, a sulfur source and graphite in a protective atmosphere, and calcining;

[0077] S2. mixing the product obtained in step S1 and an ionic liquid in a protective atmosphere, and heat treating.

[0078] According to some embodiments of the present application, in step S1, the mass percentage of graphite in the obtained mixture is 80-98%. For example, it can be specifically 80%, 85%, 90%, 95%, 98%, or a range value formed by any two of them.

[0079] According to some embodiments of the present application, in step S1, the mass of the sulfur source accounts for 0.5-2.5% of the total mass of the phosphorus source and the sulfur source. For example, it can be specifically 0.5%, 1%, 1.5%, 2%, 2.5%, or a range value formed by any two of them.

[0080] According to some embodiments of the present application, in step S1, the mixing method comprises ball milling, which satisfies at least one of the following conditions:

[0081] (a) the rotation speed of ball milling is 200-400 rpm; for example, it can be specifically 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, or a range value formed by any two of them.

[0082] (b) the duration of ball milling is 0.5-12 h. For example, it can be specifically 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, or a range value formed by any two of them.

[0083] According to some embodiments of the present application, in step S1, the calcining comprises a first constant temperature and a second constant temperature performed in sequence; the temperature of the first constant temperature is 400-500°C; the temperature of the second constant temperature is 250-300°C.

[0084] According to some embodiments of the present application, in step S1, the temperature of the first constant temperature is 425-475°C. For example, it can be specifically 425°C, 450°C, 475°C, or a range value formed by any two of them.

[0085] According to some embodiments of the present application, in step S1, the duration of the first constant temperature is 2-5 h. For example, it can be specifically 2 h, 3 h, 4 h, 5 h, or a range value formed by any two of them.

[0086] According to some embodiments of the present application, in step S1, the temperature of the second constant temperature section is 260-290℃. For example, it can be specifically 260℃, 270℃, 280℃, 290℃, or a range value formed by any two of them.

[0087] According to some embodiments of the present application, in step S1, the duration of the second constant temperature section is 12-24h. For example, it can be specifically 12h, 14h, 16h, 18h, 20h, 22h, 24h, or a range value formed by any two of them.

[0088] According to some embodiments of the present application, in step S1, the heating rate of calcination is 1-5℃ / min. For example, it can be specifically 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or a range value formed by any two of them.

[0089] According to some embodiments of the present application, in step S2, the temperature of heat treatment is 200-800℃. For example, it can be specifically 200℃, 250℃, 300℃, 350℃, 400℃, 500℃, 600℃, 700℃, 800℃, or a range value formed by any two of them.

[0090] According to some embodiments of the present application, in step S2, the duration of heat treatment is 2-4h. For example, it can be specifically 2h, 2.5h, 3h, 3.5h, 4h, or a range value formed by any two of them.

[0091] According to some embodiments of the present application, in step S2, the mass ratio of the ionic liquid to the product obtained in step S1 is 1-6:1. For example, it can be specifically 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, or a range value formed by any two of them.

[0092] According to some embodiments of the present application, in step S2, the ionic liquid comprises organic cations and inorganic anions; the organic cations contain imidazole rings or pyridine rings; the inorganic anions comprise phosphates.

[0093] According to some embodiments of the present application, in step S2, the phosphates comprise at least one of phosphate, monohydrogen phosphate and dihydrogen phosphate.

[0094] According to some embodiments of the present application, in step S2, the ionic liquid comprises 1-butyl-3-methylimidazolium dihydrogen phosphate (chemical formula as follows);

[0095]

[0096] According to some embodiments of the present application, in step S2, the mixing manner comprises stirring, wherein the stirring speed is 150-200 rpm, and the stirring time is 3-6 h. Further specifically, the stirring speed can be 150 rpm, 180 rpm, 200 rpm, or a range value formed by any two of them; and the stirring time can be 3 h, 4 h, 5 h, 6 h, or a range value formed by any two of them.

[0097] In the present application, the protective atmosphere is at least one of nitrogen, argon and helium. For example, in actual production, argon can be selected according to stability and cost.

[0098] According to the embodiments of the third aspect of the present application, a lithium ion battery is provided, which comprises the lithium ion battery negative electrode material of the embodiments of the first aspect of the present application, or the lithium ion battery negative electrode material prepared by the preparation method of the embodiments of the second aspect of the present application.

[0099] According to some embodiments of the present application, the lithium ion battery comprises at least one of a soft-pack battery, a button cell, a cylindrical battery and a square cell.

[0100] According to some embodiments of the present application, the lithium ion battery comprises at least one of a full cell, a half cell and a symmetric cell.

[0101] According to some embodiments of the present application, when the lithium ion battery is a full cell, the lithium ion battery comprises a positive electrode active material; and the positive electrode active material comprises at least one of a polyanion material, a layered material and a spinel material. For example, it can be specifically lithium iron phosphate, lithium cobaltate or lithium nickel cobalt manganese oxide.

[0102] Embodiment 1

[0103] The present example provides a lithium ion battery negative electrode material and a preparation method thereof, which is prepared by the process provided in the present application. Figure 1 According to the process provided in the present application, a lithium ion battery negative electrode material is prepared by the process of CVD-liquid coating-carbonization. The specific steps are as follows:

[0104] S1. The phosphorus source white phosphorus, sulfur monomer and graphite (purchased from Shanghai Sun Sun Technology Co., Ltd., parameters see Table 1, Comparative Example 1) are mixed, and dispersed at a speed of 250 rpm for about 3 h under the protection of inert gas (argon); wherein the mass ratio of graphite / (P+S) is 9:1, and the mass ratio of S / (S+P) is 1%;

[0105] The obtained mixture is heated to 450℃ at a rate of 3℃ / min under argon atmosphere, and slowly cooled to 280℃ at a rate of 3℃ / min after constant temperature for 3 h, and slowly cooled to room temperature after constant temperature at 280℃ for about 20 h;

[0106] S2. Add 1-butyl-3-methylimidazolium dihydrogen phosphate ionic liquid to the product obtained in step S1, continue to stir slowly at 180 rpm for 6 h; wherein the mass ratio of 1-butyl-3-methylimidazolium dihydrogen phosphate and the product obtained in step S1 is 6:1;

[0107] The obtained mixture is transferred into a boat and heat treated at 250℃ for about 3h, and then crushed and sieved after cooling to room temperature.

[0108] In the detailed description, about means that the error range is within ±2%, and the performance of the obtained product will not be significantly affected within this range, and other positions are explained as such.

[0109] Example 2

[0110] In this example, a lithium ion battery negative electrode material is prepared, which is different from example 1 in that:

[0111] In step S1, the mass ratio of graphite / (P+S) is 8:2.

[0112] Example 3

[0113] In this example, a lithium ion battery negative electrode material is prepared, which is different from example 1 in that:

[0114] In step S1, the mass ratio of S / (S+P) is 2%.

[0115] Comparative Example 1

[0116] In this example, a lithium ion battery negative electrode material is prepared, which is different from example 1 in that:

[0117] Comparative Example 2

[0118] In this example, a lithium ion battery negative electrode material is prepared, which is different from example 1 in that:

[0119] In step S1, no elemental sulfur is added, and the amount of white phosphorus is increased correspondingly, so that the mass ratio of the inner core to the intermediate layer is 9:1.

[0120] The specific steps of this example are:

[0121] S1. Mix graphite (same as example 1) and white phosphorus according to a mass ratio of 9:1, and disperse for 3h under inert gas protection environment at a rotation speed of 250rpm to obtain a phosphorus-graphite mixture;

[0122] Heat the obtained mixture to 450℃ at a rate of 3℃ / min under Ar atmosphere, and then slowly cool to 280℃ at a rate of 3℃ / min after keeping the temperature constant for 3h, and then slowly cool to room temperature after keeping the temperature constant at 280℃ for about 20h;

[0123] S2. Add 1-butyl-3-methylimidazolium dihydrogen phosphate ionic liquid to the product obtained in step S1, and continue to stir slowly at a rotation speed of 180 rpm for 6 h; wherein the mass ratio of 1-butyl-3-methylimidazolium dihydrogen phosphate to the product obtained in step S1 is 6:1;

[0124] The obtained mixture is transferred into a boat, and heat treatment is performed at 250℃ for about 3 h. After cooling to room temperature, crushing and sieving are performed.

[0125] Comparative Example 3

[0126] In this example, a lithium ion battery negative electrode material is prepared, which is different from Example 1 in that:

[0127] In step S2, the ionic liquid is replaced with an equal mass of pectin, and the mixing and heat treatment conditions in step S2 are adjusted correspondingly.

[0128] The specific steps of this example are as follows:

[0129] S1. Mix the phosphorus source white phosphorus, sulfur and graphite (same as in Example 1) under the protection of an inert gas (argon) environment, and disperse at a rotation speed of 250 rpm for about 3 h; wherein the mass ratio of graphite / (P+S) is 9:1, and the mass ratio of S / (S+P) is 1%.

[0130] The obtained mixture is heated to 450℃ at a rate of 3℃ / min under an argon atmosphere, and after constant temperature for 3 h, it is slowly cooled to 280℃ at a rate of 3℃ / min. After constant temperature at 280℃ for about 20 h, it is slowly cooled to room temperature;

[0131] S2. Mix the product obtained in step S1 with a pectin aqueous solution, wherein the mass ratio of pectin to the product obtained in step S1 is 6:1; in the pectin aqueous solution, the ratio of solute to solvent is 1 g / 200 mL; the mixing method is 600 rpm stirring for 30 min.

[0132] Freeze-dry the obtained mixture;

[0133] Transfer the dried substance into a boat, and heat from room temperature to 400℃ at a heating rate of 3℃ / min, and keep constant temperature for 30 min to perform carbonization treatment on the pectin; then naturally cool to room temperature.

[0134] Application Example 1

[0135] A button-shaped lithium ion battery is provided in this example, and the raw materials used in this example are all commercially available unless otherwise specified. The working electrode is composed of a current collector (copper foil) and a coating layer loaded on the surface of the current collector, and the coating layer includes 97.6% by mass of a negative active material (lithium ion battery negative material from the examples or comparative examples), 1.2% by mass of a binder (SBR), 0.4% by mass of a conductive agent (SP), and 0.8% by mass of a thickening agent (CMC). The above raw materials are mixed uniformly according to the mass ratio, and after pulping, they are coated on the surface of the copper foil, dried in an oven, and then rolled, with a compaction density of 1.60 g / cm 3 The counter electrode is a lithium metal sheet; the separator is a polypropylene separator; the electrolyte used is LPF6 dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DEC), and methyl ethyl carbonate (EMC) at a mass ratio of 1:1:1, with a lithium salt concentration of 1 mol / L.

[0136] Application Example 2

[0137] A soft package-shaped lithium ion battery is provided in this example, and the lithium ion battery is a lithium iron phosphate / graphite (lithium ion battery negative material from the examples or comparative examples) system with a designed N / P value of 1.105; it is produced by Weili Electric Vehicle Co., Ltd. and the model number is 3268130. Among them,

[0138] The positive electrode is composed of a positive electrode current collector (aluminum foil) and a positive electrode coating layer loaded on the surface of the positive electrode current collector, and the positive electrode coating layer includes 96% by mass of a positive active material (lithium iron phosphate from Changzhou Lithium Source LFP-D05), 2% by mass of a binder PVDF, and 2% by mass of a conductive agent SP. The components of the positive electrode coating layer are mixed, pulped, and then coated on the aluminum foil, dried in an oven, and then rolled, with a compaction density of 2.65 g / cm 3 to obtain the required positive electrode.

[0139] The negative electrode is composed of a negative electrode current collector (copper foil) and a negative electrode coating layer loaded on the surface of the negative electrode current collector, and the negative active material (lithium ion battery negative material from the examples or comparative examples), styrene-butadiene rubber (SBR), sodium methyl cellulose (CMC), and Super P are mixed uniformly according to a mass ratio of 97.6:1.2:0.8:0.4, pulped, and then coated on the copper foil, dried in an oven, and then rolled, with a compaction density of 1.60 g / cm 3 to obtain the required negative electrode.

[0140] The separator is a polypropylene separator.

[0141] The electrolyte used is LPF6 dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a mass ratio of 1:1:1, and the lithium salt concentration is 1 mol / L.

[0142] A small soft-pack battery with a design capacity of 2.5 Ah was prepared by the "Z" letter method. The electrolyte was injected in an argon glove box, the injection coefficient was 2.9, and the battery was packaged after standing at 45°C for 48 h and used for testing.

[0143] Test Example

[0144] The first aspect of this example tests the physical and chemical properties of the lithium ion battery negative electrode material obtained in the examples and comparative examples. Specifically, the specific surface area is tested by BET, the properties of the product obtained in step S1 are tested by Raman, and the morphology is tested by SEM. Among them, the Raman test results show that in the product obtained in step S1 of the examples, there is a clear P-S bond; which shows that S in the intermediate layer plays a good bridging role, connecting the graphite of the core and P of the intermediate layer, and has a significant gain effect on improving the uniform distribution of P and obtaining a lithium ion battery negative electrode material with more stable structure and better electrochemical performance. The test results of examples 1-2 are comparable, and the Raman test results of examples 1 and comparative example 2 are shown in Figure 2 , wherein P-S-graphite represents example 1, P-graphite represents comparative example 2, and Pristine graphite represents the raw material graphite used in example 1. Regarding the morphology, the morphology of the product obtained in step S1 is mainly tested, and the results show that the product obtained in step S1 of example 1 has a uniform and smooth surface, i.e., the deposition of P is uniform; while the product obtained in step S1 of comparative example 2 has a rough surface, i.e., the uniformity of P deposition is significantly reduced; comparing the results of example 1 and comparative example 2, it can be seen that P is uniformly coated on the surface of graphite under the induction of S, and S indeed has a significant induction effect on the uniform deposition of P; the SEM results of the products obtained in step S2 of examples 1 and comparative example 2 are shown in Figures 3-4 . The test results of the specific surface area are shown in Table 1.

[0145] In the second aspect of this example, the initial efficiency and capacity of the lithium ion battery obtained in application example 1 are tested, wherein the test voltage is 0.005-2V, and the rate is 0.2C, wherein 1C=372mAh / g, and the current density of 1C is 3.5mA / cm 2 . The test results are shown in Table 1.

[0146] In the third aspect of this example, the capacity of the lithium-ion battery obtained in Application Example 2 was tested under 0.2C and 4C conditions, and the ratio of the reversible capacity at 4C and 0.2C (4C capacity retention rate) and the capacity retention rate after 300 cycles at 4C (compared to the capacity of the first cycle at 4C) were calculated. The specific test results are shown in Table 1.

[0147] In the fourth aspect of this example, the low-temperature resistance (DCR) of the lithium-ion battery anode materials obtained in the examples and comparative examples was tested using the lithium-ion battery obtained in Example 2. The test temperature was -20°C, the state of charge was adjusted to 50% SOC, the pulse current ratio was 0.4C, and the pulse duration was 10s. The specific test results are shown in Table 1.

[0148] Table 1. Performance of the lithium-ion battery anode materials obtained in the examples and comparative examples.

[0149] Sample Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Example 2 Example 3 Specific surface area m 2 g -1 ]] 2.31 3.27 24.1 1.37 1.12 1.30 Initial capacity % 91.5 94.1 83.7 93.5 95.0 92.9 0.2 C capacity mAh / g 350.0 356.4 341.8 355.3 368.2 358.0 4 C capacity retention % 89.1 91.8 92.0 94.3 93.7 92.8 Cycle capacity retention % 89.6 91.7 89.3 95.6 92.0 91.3 DCR mΩ 368.8 301.5 285.4 221.7 219.1 233.3

[0150] Comparing the results of Example 1, Comparative Example 1, and Comparative Example 3, it can be seen that if a traditional organic carbon source is used instead of the ionic liquid specified in this invention, the specific surface area of ​​the resulting lithium-ion battery anode material will be significantly increased. Furthermore, the oxygen element in the organic carbon source cannot be completely removed, and these oxygen-containing functional groups will significantly degrade the initial efficiency of the resulting lithium-ion battery anode material. However, when using an ionic liquid as the carbon source, the initial efficiency is not significantly degraded and may even be improved to some extent. In addition, when using an ionic liquid as the carbon source, after the heat treatment in step S2, the resulting carbon layer has high disorder (isotropy), wide carbon layer spacing, heteroatom doping, fast lithium-ion diffusion rate, and many active sites on the carbon layer surface for lithium-ion intercalation, providing additional lithium storage sites, shortening the lithium-ion diffusion path, thereby further improving capacity, rate capability, cycle life, and other performance, and significantly reducing impedance.

[0151] Comparing the results of Example 1, Comparative Example 2, and Example 3, it can be seen that introducing S into the intermediate layer can guide the uniform deposition of P, thereby reducing the specific surface area of ​​the obtained lithium-ion battery anode material to a certain extent, significantly improving its rate performance and long-cycle performance, and significantly reducing impedance. Moreover, as the S content in the intermediate layer increases, the rate performance and cycle performance of the obtained lithium-ion battery anode material show a trend of first increasing and then decreasing, and correspondingly, the DCR value shows a trend of first decreasing and then increasing. Within the experimental range, the capacity and initial efficiency do not change significantly with the change of S content.

[0152] Comparing the results of Examples 1 and 2, it can be seen that as the thickness of the intermediate layer increases, due to the contribution of P, the initial efficiency and capacity of the obtained lithium-ion battery anode material are improved to a certain extent, and the impedance will decrease to a certain extent; however, the rate capability and long cycle capability will decrease at the same time.

[0153] The results of Comparative Examples 1-3 show that the carbon-phosphorus coated negative electrode material bridged by S has superior electrochemical performance and structural stability.

[0154] In conclusion, the lithium ion battery negative electrode material provided by the application has the advantages of simple preparation process, low required temperature, and easy large-scale production and use, due to the design of the double-layer core-shell structure and the design of the components in the intermediate layer, which significantly improves the uniformity of P coating in the intermediate layer, and inhibits the side reaction between the lithium ion battery negative electrode material and the electrolyte. Due to the above advantages, the lithium ion battery including the lithium ion battery negative electrode material is expected to have wide application in the fields of energy storage technology, power battery technology and communication electronics.

[0155] The embodiments of the application are described in detail above with reference to the drawings, but the application is not limited to the above embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the purpose of the application. In addition, the embodiments of the application and the features in the embodiments can be combined with each other without conflict.

Claims

1. A lithium-ion battery anode material, characterized in that, The lithium ion battery negative electrode material has a double-layer core-shell structure, wherein the inner core is graphite, the intermediate layer is a phosphorus-sulfur co-coating layer, and the outer layer is a carbon layer; the carbon layer is a pyrolysis product of an ionic liquid.

2. The lithium-ion battery anode material of claim 1, wherein, In the intermediate layer, the mass percentage of sulfur is 0.5-2.5%.

3. The lithium-ion battery anode material of claim 1, wherein, The mass of the inner core accounts for 80-98% of the total mass of the inner core and the intermediate layer. And / or, the carbon layer is simultaneously doped with N and P.

4. A method for producing a negative electrode material for a lithium ion battery as claimed in any one of claims 1 to 3, characterized by, The preparation method comprises the following steps: S1. In a protective atmosphere, mix a phosphorus source, a sulfur source, and graphite, and calcine; S2. In a protective atmosphere, mix the product obtained in step S1 and the ionic liquid, and then perform heat treatment.

5. The preparation method according to claim 4, characterized in that, In step S1, the mass percentage of the graphite in the obtained mixture is 80-98%; and / or, in step S1, the mass of the sulfur source accounts for 0.5-2.5% of the total mass of the phosphorus source and the sulfur source.

6. The preparation method according to claim 4, characterized in that, In step S1, the calcination comprises a first constant-temperature stage and a second constant-temperature stage performed in sequence; the temperature of the first constant-temperature stage is 400-500℃; and the temperature of the second constant-temperature stage is 250-300℃.

7. The preparation method according to claim 4, characterized in that, In step S2, the temperature of the heat treatment is 200-800℃; and / or, in step S2, the duration of the heat treatment is 2-4h.

8. The preparation method according to claim 4, characterized in that, In step S2, the mass ratio of the ionic liquid to the product obtained in step S1 is 1-6:1; and / or, the ionic liquid comprises organic cations and inorganic anions; the organic cations contain imidazole rings or pyridine rings; and the inorganic anions comprise phosphates.

9. The production method according to claim 8, characterized by, In step S2, the phosphates comprise at least one of phosphate, monohydrogen phosphate, and dihydrogen phosphate; And / or, the ionic liquid comprises 1-butyl-3-methylimidazolium dihydrogen phosphate.

10. A lithium-ion battery, characterized by, The lithium ion battery comprises the lithium ion battery negative electrode material according to any one of claims 1-3, or comprises the lithium ion battery negative electrode material prepared by the preparation method according to any one of claims 4-9.

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