Negative electrode material, preparation method thereof and lithium ion battery

Abstract

The invention provides a negative electrode material, a preparation method thereof and a lithium ion battery, and belongs to the technical field of new energy storage materials. The negative electrode material comprises a phosphorus-carbon carrier and a lithium phosphate covering layer, wherein the lithium phosphate covering layer at least partially covers the surface of the phosphorus-carbon carrier; and compared with the mass of the phosphorus-carbon carrier, the mass content of lithium phosphate in the lithium phosphate covering layer is 0.5-5%. The lithium phosphate covering layer formed by the invention can participate in the formation of a solid electrolyte interface film on the surface of a negative electrode in the cycle process of the battery, prevents the electrolyte from further reacting with a negative electrode material, is beneficial to protecting the electrode, can allow lithium ions to pass through and is beneficial to improving the stability of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy storage materials, and in particular to negative electrode materials and preparation methods thereof, and lithium ion batteries. Background Art

[0002] With the popularization of lithium-ion battery applications, higher requirements have been placed on the performance indicators of lithium-ion batteries. During the charging / discharging process of lithium-ion batteries, as the lithium insertion / extraction reaction continues, the structure and volume of the negative electrode sheet are more likely to undergo drastic changes, causing the interface structure to be destroyed, affecting the reversible capacity, cycle life, and rate performance of lithium-ion batteries.

[0003] In order to improve the above-mentioned performance of lithium ions, composite negative electrode materials are often used in related technologies. However, the volume change of composite negative electrode materials during battery cycling is relatively drastic.

[0004] Therefore, it is necessary to find a negative electrode material that can reduce the changes in the structure and volume of the negative electrode sheet while maintaining good cycle performance. Summary of the invention

[0005] In view of this, in order to at least partially solve at least one of the above-mentioned technical problems, the present invention provides a negative electrode material and a preparation method thereof, and a lithium ion battery.

[0006] According to an embodiment of one aspect of the present invention, a negative electrode material is provided, comprising: a phosphorus-carbon carrier, and a lithium phosphate covering layer, wherein the lithium phosphate covering layer at least partially covers the surface of the phosphorus-carbon carrier; compared with the mass of the phosphorus-carbon carrier, the mass content of lithium phosphate in the lithium phosphate covering layer is 0.5~5%.

[0007] In some exemplary embodiments, the mass content of lithium phosphate in the lithium phosphate covering layer is 1-2% compared to the mass of the phosphorus-carbon carrier.

[0008] In some exemplary embodiments, in the phosphorus-carbon carrier, the mass content of phosphorus is 5-80%; in the phosphorus-carbon carrier, the mass content of carbon is 20-95%.

[0009] In some exemplary embodiments, the phosphorus in the phosphorus-carbon carrier includes at least one of red phosphorus, black phosphorus, and white phosphorus; and the carbon in the phosphorus-carbon carrier includes at least one of a carbon carrier having a layered structure and a carbon carrier having a porous structure.

[0010] According to an embodiment of another aspect of the present invention, there is provided a method for preparing a negative electrode material as described above, comprising: mixing a phosphorus-carbon carrier material with a lithium alkali to form a mixture powder; mixing the mixture powder with a phosphoric acid solution so that the lithium alkali on the surface of the mixture powder reacts with the phosphoric acid to generate a phosphorus-carbon carrier material whose surface is at least partially covered with a lithium phosphate covering layer; and subjecting the phosphorus-carbon carrier material whose surface is at least partially covered with a lithium phosphate covering layer to a first drying treatment to obtain a negative electrode material.

[0011] In some exemplary embodiments, mixing the phosphorus-carbon carrier material with lithium alkali to form a mixture powder includes: grinding and mixing the phosphorus-carbon carrier material and the lithium alkali to form a mixture powder; or immersing the phosphorus-carbon carrier material in a lithium alkali solution and performing a second drying treatment to form a mixture powder.

[0012] In some exemplary embodiments, the lithium base includes at least one of lithium hydroxide or lithium carbonate.

[0013] In some exemplary embodiments, the first drying process is freeze drying or spray drying; when the first drying process is spray drying, the spray drying temperature is 150-170° C., and the spray drying atmosphere is an inert atmosphere.

[0014] In some exemplary embodiments, the molar ratio of lithium ions in the lithium base to phosphate ions in the phosphoric acid solution is 3:1; the conditions of the second drying treatment are: drying at 75-85° C. in a vacuum environment for 11-13 hours.

[0015] According to another aspect of the embodiments of the present invention, there is provided a lithium ion battery, comprising a negative electrode, wherein the negative electrode comprises the negative electrode material as described above, a conductive agent and a binder.

[0016] According to the negative electrode material of the embodiment of the present invention, by forming a lithium phosphate covering layer uniformly covering the surface of the phosphorus-carbon carrier, it is possible to effectively buffer the changes in the structure and volume of the surface of the negative electrode sheet, thereby improving the cycle stability and life of the battery. During the battery cycle, the lithium phosphate in the lithium phosphate covering layer participates in the formation of the solid electrolyte interface (SEI) film on the surface of the negative electrode sheet, which can provide a more solid and stable SEI film, improve the cycle stability of the negative electrode sheet during the battery cycle, and thus improve the various performances of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0018] Figure 1 A flow chart showing a method for preparing a negative electrode material according to an embodiment of the present invention;

[0019] Figure 2A transmission electron microscope image of the phosphorus-carbon composite 1 covered with lithium phosphate according to Example 1 of the present invention is shown;

[0020] Figure 3 shows a scanning electron microscope image of the phosphorus-carbon composite 1 covered with lithium phosphate according to Example 1 of the present invention;

[0021] Figure 4 The scanning electron microscope image of the mixture powder of Comparative Example 1 of the present invention is shown;

[0022] Figure 5 shows an X-ray photoelectron spectrum of the phosphorus-carbon composite 2 covered with lithium phosphate according to Example 2 of the present invention; and

[0023] Figure 6 A comparison chart of the rate performance of Examples 2 to 3 of the present invention and the control group 2 is shown. DETAILED DESCRIPTION

[0024] Below, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of concepts of the present invention.

[0025] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The term "comprising" used herein indicates the existence of features, steps, operations, but does not exclude the existence or addition of one or more other features.

[0026] In the case of using expressions such as "at least one of A, B, and C, etc.", it should generally be interpreted in accordance with the meaning of the expression generally understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.). In the case of using expressions such as "at least one of A, B, or C, etc.", it should generally be interpreted in accordance with the meaning of the expression generally understood by those skilled in the art (for example, "a system having at least one of A, B, or C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0027] In order to reduce the changes in the structure and volume of the negative electrode during the charging and discharging process, related technologies often use carbon materials, metals or metal oxides, high molecular polymers, graphene and other materials to coat the carrier material. However, the introduction of the above materials has caused a serious obstacle to the formation of the SEI film on the surface of the negative electrode, making it difficult to achieve the process of lithium ion embedding and extraction, thereby reducing the battery's cycle performance and rate performance.

[0028] In the process of realizing the concept of the present invention, it was found that by forming a lithium phosphate covering layer on the surface of the phosphorus-carbon carrier, the presence of lithium phosphate is beneficial to ensuring the transmission of lithium ions in the SEI film on the surface of the negative electrode material during the cycle, while taking into account the fact that the structure and volume of the negative electrode material change less.

[0029] Specifically, according to an embodiment of one aspect of the present invention, a negative electrode material is provided, including: a phosphorus-carbon carrier, and a lithium phosphate covering layer, wherein the lithium phosphate covering layer at least partially covers the surface of the phosphorus-carbon carrier; compared with the mass of the phosphorus-carbon carrier, the mass content of lithium phosphate in the lithium phosphate covering layer is 0.5~5%.

[0030] It should be noted that among phosphorus-carbon carriers, phosphorus has a higher theoretical capacity, but poor electrical conductivity. Combining phosphorus with carbon materials with good electrical conductivity to form a phosphorus-carbon carrier helps to improve the conductivity of phosphorus and promote the rapid transfer of electrons.

[0031] According to an embodiment of the present invention, by forming a lithium phosphate covering layer on the surface of a phosphorus-carbon carrier, the lithium phosphate is relatively stable in nature, and can form an SEI film on the surface of the negative electrode sheet during the battery cycle operation, which helps to stabilize the electrode-electrolyte interface, reduce the occurrence of unnecessary side reactions, and can effectively alleviate the volume expansion of the negative electrode sheet, which helps to improve the safety and cycle stability of the battery. At the same time, the presence of lithium phosphate is conducive to improving the interface transmission of lithium ions on the surface of the negative electrode material, improving the rate performance of the battery, and further improving the cycle performance and fast charging capability of the phosphorus-carbon negative electrode battery. The present invention combines the phosphorus-carbon carrier and the lithium phosphate covering layer, and the synergistic effect helps to improve the relevant performance of the battery.

[0032] It should be noted that the mass content of lithium phosphate in the lithium phosphate covering layer is 0.5-5% compared to the mass of the phosphorus-carbon carrier. The above arrangement is helpful to form a better lithium ion diffusion situation. If the mass content of lithium phosphate is too little, it is difficult to form a uniform coverage, and it is difficult to promote the diffusion of lithium ions; if the mass content of lithium phosphate is too much, the SEI film will be too thick during the battery cycle, so that the diffusion of lithium ions is hindered.

[0033] For example, the mass content of lithium phosphate may be 0.5%, 1%, 2%, 3%, 4% or 5%, etc.

[0034] In the present invention, "at least partially covering" can be understood as the lithium phosphate covering layer forming an independent film layer, which completely covers the surface of the phosphorus-carbon carrier; it can also be understood as the lithium phosphate covering layer is a plurality of sheet-like covering structures, which partially cover the surface of the phosphorus-carbon carrier, in which case there may be a small portion of the phosphorus-carbon carrier directly exposed to the electrolyte. In both of the above cases, a better lithium ion diffusion condition can be formed while effectively alleviating the volume expansion of the negative electrode sheet, which is not particularly limited here.

[0035] Compared with carbon coating in related technologies, such as coating with polymers, graphene and other materials, the present invention does not affect the structure and morphology of the negative electrode material itself by introducing a lithium phosphate covering layer, and the lithium phosphate therein can directly participate in the formation of the SEI film as part of the interface composition, taking into account the stability of the negative electrode material interface and the diffusion capacity of lithium ions.

[0036] Further preferably, the mass content of lithium phosphate in the lithium phosphate covering layer is 1-2% compared to the mass of the phosphorus-carbon carrier, for example, 1%, 1.5% or 2%. Further adjusting the mass content of lithium phosphate to the above range helps to further improve the diffusion capacity of lithium ions and improve the relevant performance of the battery.

[0037] It should be noted that lithium phosphate participates in the formation of SEI film during the battery cycle, and the thickness of the formed SEI film is 100-120nm, for example, 100nm, 110nm or 120nm. The formed SEI film can prevent the electrolyte from penetrating into the negative electrode material, while taking into account a better diffusion path of lithium ions.

[0038] In some illustrative embodiments, in the phosphorus-carbon carrier, the mass content of phosphorus is 5-80%, and in the phosphorus-carbon carrier, the mass content of carbon is 20-95%. As set above, it helps to optimize the conductivity and capacity of the negative electrode material. Specifically, setting the mass content of carbon as above helps to enhance the overall conductivity of the phosphorus-carbon carrier and improve the working efficiency and power density of the battery. Setting the mass content of phosphorus as above helps to increase the energy density of the negative electrode material as low as possible while ensuring good conductivity, which helps to increase the total capacity of the battery.

[0039] It should be noted that during the charge and discharge process, phosphorus will expand in volume. The present invention helps to absorb the volume change of phosphorus during the cycle through carbon by adjusting the ratio of the above-mentioned phosphorus and carbon, which helps to improve the cycle stability and service life of the negative electrode sheet.

[0040] For example, the mass content of phosphorus may be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70% or 80%, etc. The mass content of carbon may be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95%, etc.

[0041] In some illustrative embodiments, the phosphorus in the phosphorus-carbon carrier includes at least one of red phosphorus, black phosphorus, and white phosphorus, wherein red phosphorus has relatively stable performance and is environmentally friendly; black phosphorus has a layered structure similar to graphite, has good conductivity and high theoretical capacity; white phosphorus has high activity, but due to its certain toxicity, it can be mixed with other phosphorus as needed, and is not particularly limited here. The carbon in the phosphorus-carbon carrier includes at least one of a carbon carrier with a layered structure and a carbon carrier with a porous structure. The carbon carrier with a layered structure, such as graphene, can provide a large surface area and excellent conductivity, which is conducive to the penetration of electrolyte and the rapid diffusion of lithium ions, thereby improving the charge and discharge efficiency and cycle life of the battery.

[0042] It should be noted that during the experiment, it was found that replacing lithium phosphate with lithium carbonate also has a certain improvement effect. Due to the better stability and ionic conductivity of lithium phosphate, the performance of the battery is relatively better.

[0043] According to another embodiment of the present invention, a method for preparing the negative electrode material as described above is provided. Figure 1 A flow chart showing a method for preparing a negative electrode material according to an embodiment of the present invention is shown. Figure 1 As shown, the method includes operations S101 to S103.

[0044] In operation S101, a phosphorus-carbon support material is mixed with a lithium base to form a mixture powder.

[0045] In operation S102, the mixture powder is mixed with a phosphoric acid solution so that the lithium alkali on the surface of the mixture powder reacts with the phosphoric acid to generate a phosphorus-carbon support material whose surface is at least partially covered with a lithium phosphate covering layer.

[0046] In operation S103, the phosphorus-carbon support material at least partially covered with a lithium phosphate covering layer is subjected to a first drying process to obtain a negative electrode material.

[0047] According to an embodiment of the present invention, a lithium phosphate covering layer is generated in situ by a neutralization reaction between phosphoric acid and lithium alkali, and the phosphorus-carbon carrier material is coated to form a uniformly covered lithium phosphate, thereby avoiding insufficient coverage or uneven coverage thickness. The preparation method of the present invention is simple to operate, highly efficient, and low in cost, and is suitable for large-scale production.

[0048] It should be noted that when lithium phosphate and phosphorus-carbon carrier were composited, relevant preliminary experiments were conducted. Attempts were made to physically mix lithium phosphate and phosphorus-carbon materials directly, but the experiment found that such a mixture was difficult to form a uniform lithium phosphate coating on the surface of the phosphorus-carbon carrier, and the overall performance improvement of the battery was relatively limited.

[0049] In some exemplary embodiments, the phosphorus-carbon support material can be prepared, for example, by vapor deposition or solid phase ball milling.

[0050] In some illustrative embodiments, operation S101 may specifically include grinding and mixing the phosphorus-carbon carrier material and lithium alkali to form a mixture powder. Grinding and mixing helps to fully mix the two and improve the consistency and stability of the negative electrode material. It may also include immersing the phosphorus-carbon carrier material in a lithium alkali solution and performing a second drying treatment to form a mixture powder. The immersion treatment helps the lithium salt to penetrate deeply into the surface and pores of the phosphorus-carbon carrier. The above two methods not only help to form a SEI film during the cycle, but also help to adjust the electronic structure of the phosphorus-carbon carrier, further reducing the diffusion resistance of lithium ions.

[0051] In some exemplary embodiments, the lithium base includes at least one of lithium hydroxide or lithium carbonate. Lithium hydroxide helps to improve the electrochemical performance of the negative electrode material, and lithium carbonate helps to improve the thermal stability of the negative electrode material.

[0052] In some exemplary embodiments, the first drying process is freeze drying or spray drying, which helps to reduce the loss of lithium phosphate that may occur in other ways during the drying process (such as loss of lithium phosphate caused by miscibility with the solvent, etc.). When the first drying process is spray drying, the temperature of the spray drying is 150-170°C, for example, 150°C, 160°C or 170°C, and the atmosphere of the spray drying is an inert atmosphere.

[0053] In some exemplary embodiments, the molar ratio of lithium ions in the lithium base to phosphate ions in the phosphoric acid solution is 3:1, so as to avoid excess of the two raw materials, so that lithium ions and phosphate ions can be fully prepared to form lithium phosphate. The conditions for the second drying treatment are: at 75-85°C, for example, 75°C, 80°C or 85°C, preferably 80°C, and drying under vacuum for 11-13h, for example, 11h, 12h or 13h, preferably 12h.

[0054] According to another aspect of the embodiments of the present invention, there is provided a lithium ion battery, comprising a negative electrode, wherein the negative electrode comprises the negative electrode material as described above, a conductive agent and a binder.

[0055] According to an embodiment of the present invention, by using the above-mentioned negative electrode material, it is helpful to form a more uniform SEI film when the lithium-ion battery is cyclically operated, and the volume expansion of the negative electrode can be alleviated while reducing or avoiding the penetration of the electrolyte into the negative electrode. At the same time, the presence of the lithium phosphate covering layer is beneficial to provide an interface transmission path for lithium ions, thereby improving the various performances of the lithium-ion battery.

[0056] In the present invention, the material properties of the prepared negative electrode (such as reversible capacity, cycle life, rate performance, etc.) are significantly improved, and the prepared negative electrode exhibits relatively excellent electrochemical performance in the lithium-ion battery system. In the half-cell test assembled with lithium metal as the counter electrode, the phosphorus-carbon negative electrode containing lithium phosphate exhibits a rate performance far higher than that of the ordinary negative electrode, and the gram capacity is more than doubled at a current density of 8A / g.

[0057] The present invention is further described below by embodiments and related test experiments and results thereof. In the detailed description below, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present invention. However, it is obvious that one or more embodiments may also be implemented without these specific details. And, in the absence of conflict, the details in the following embodiments may be combined into other feasible embodiments at will.

[0058] It should be noted that the following specific examples are only for illustration, and the protection scope of the present invention is not limited thereto. The chemicals and raw materials used in the following examples are all commercially available or homemade by recognized processing methods.

[0059] Embodiment 1:

[0060] The following is the preparation process of the lithium phosphate-covered phosphorus-carbon composite 1.

[0061] Preparation of phosphorus-carbon complex:

[0062] Take 20g of a phosphorus-carbon composite (referred to as control group 1) prepared in advance by a vapor deposition process, wherein the components are 50wt% of porous carbon and 50wt% of red phosphorus.

[0063] Preparation of negative electrode materials:

[0064] 124 mg of LiOH powder was weighed into a beaker, and 100 mL of deionized water was added and stirred to dissolve. 20 g of the prepared phosphorus-carbon composite was transferred to a beaker and stirred to uniformly disperse it in the LiOH solution to obtain a mixed dispersion.

[0065] The mixed dispersion was spray dried at 160° C. in a nitrogen atmosphere to allow LiOH to be uniformly attached to the surface of the phosphorus-carbon composite. The dried mixture powder was collected and 10 g of the mixture powder was weighed for later use.

[0066] Weigh 100 mg of concentrated phosphoric acid (85% mass fraction), dilute and dissolve it in 100 mL of deionized water, then add 10 g of the mixture powder, stir and disperse it evenly, and the phosphoric acid reacts with the LiOH on the surface of the mixture powder to generate a lithium phosphate dispersion.

[0067] The lithium phosphate dispersion is transferred into a plastic centrifuge tube, which is quickly frozen using liquid nitrogen. The centrifuge tube is then placed in a freeze dryer, and the lithium phosphate-covered phosphorus-carbon complex 1 is collected through a freeze drying process.

[0068] The prepared lithium phosphate-covered phosphorus-carbon composite 1 was subjected to characterization tests. Figure 2 FIG. 1 shows a transmission electron microscope image of the phosphorus-carbon composite 1 covered with lithium phosphate according to Example 1 of the present invention. Figure 2 As shown, the phosphorus-carbon composite 1 covered with lithium phosphate has a diffraction pattern of Li3PO4 component. Figure 3 FIG. 1 shows a scanning electron microscope image of the phosphorus-carbon composite 1 covered with lithium phosphate according to Example 1 of the present invention. Figure 3 As shown, observing its surface morphology, it can be seen that the surface is relatively smooth without obvious particles, indicating that the coverage is relatively uniform.

[0069] Comparative Example 1:

[0070] 0.1 g of lithium phosphate powder was directly dissolved in 3 L of deionized water, and 10 g of the phosphorus-carbon composite prepared in Example was added, and stirred and dispersed evenly to obtain a mixed dispersion. The mixed dispersion was dried using the same spray drying conditions as Example 1, and the dried mixture powder was collected.

[0071] The mixed powder was subjected to characterization tests. Figure 4 The SEM image of the mixture powder of Comparative Example 1 of the present invention is shown. Figure 4 As shown, observing its morphology, there are obvious lithium phosphate particles on the surface, and the distribution of lithium phosphate on the surface of the phosphorus-carbon composite is uneven.

[0072] Embodiment 2:

[0073] The following is the preparation process of the lithium phosphate-covered phosphorus-carbon composite 2.

[0074] Take 10g of a phosphorus-carbon composite (referred to as control group 2) prepared in advance by a solid phase ball milling process, wherein the components are 50wt% of graphite carbon and 50wt% of red phosphorus.

[0075] Preparation of negative electrode materials:

[0076] Weigh 96 mg of Li2CO3 powder and mix it with 10 g of phosphorus-carbon composite to obtain a mixed sample. Transfer the mixed sample evenly to two 50 mL ball mill jars. Grind and mix the mixed sample using a planetary ball mill at a speed of 200 rpm for 4 hours to make Li2CO3 evenly adhere to the surface of the phosphorus-carbon composite, and collect the mixed powder for later use.

[0077] The mixed powder was added into an aqueous solution containing 85 mg of H3PO4 and treated with an ultrasonic machine for 30 minutes to make it evenly dispersed. At the same time, H3PO4 reacted with Li2CO3 on the surface of the powder to generate a Li3PO4 dispersion.

[0078] The Li3PO4 dispersion was spray dried at 160°C in a nitrogen atmosphere to make the Li3PO4 evenly adhere to the surface of the phosphorus-carbon composite, and the phosphorus-carbon composite 2 covered with lithium phosphate was collected after drying.

[0079] The prepared lithium phosphate-covered phosphorus-carbon composite 2 was subjected to characterization tests. Figure 5 The X-ray photoelectron spectroscopy (XPS) diagram of the phosphorus-carbon composite 2 covered with lithium phosphate according to Example 2 of the present invention is shown. Figure 5 As shown, it can be seen from the P2p spectrum that there is a peak signal of the Li3PO4 component, which confirms the existence of Li3PO4.

[0080] Embodiment 3:

[0081] The following is the preparation process of lithium phosphate-covered phosphorus-carbon composites 3~7.

[0082] The preparation process of the phosphorus-carbon composite of this embodiment 3 is the same as that of embodiment 2, and will not be repeated here.

[0083] Preparation of negative electrode materials:

[0084] By changing the amount of Li2CO3 and H3PO4, phosphorus-carbon composites covered with different lithium phosphates were prepared.

[0085] Using (48 mg of Li2CO3 + 42.5 mg of H3PO4), (192 mg of Li2CO3 + 170 mg of H3PO4), (288 mg of Li2CO3 + 255 mg of H3PO4), (384 mg of Li2CO3 + 340 mg of H3PO4), and (480 mg of Li2CO3 + 425 mg of H3PO4) precursors, phosphorus-carbon composites coated with Li3PO4 with mass fractions of 0.5%, 2%, 3%, 4%, and 5% were prepared, respectively. The phosphorus-carbon composite 3 (3-1) with a mass fraction of 0.5% was marked as lithium phosphate-covered, the phosphorus-carbon composite 4 (3-2) with a mass fraction of 2% was marked as lithium phosphate-covered, the phosphorus-carbon composite 5 (3-3) with a mass fraction of 3% was marked as lithium phosphate-covered, the phosphorus-carbon composite 6 (3-4) with a mass fraction of 4% was marked as lithium phosphate-covered, and the phosphorus-carbon composite 7 (3-5) with a mass fraction of 5% was marked as lithium phosphate-covered.

[0086] The phosphorus-carbon composite samples of Examples 1 to 3 and Control Groups 1 to 2 were mixed with Ketjen black and polyvinylidene fluoride (PVDF) at a mass ratio of 90:5:5, and dispersed in N-methylpyrrolidone (NMP) solvent to prepare an electrode slurry. The electrode slurry was coated and transferred to the surface of the copper foil current collector, and then vacuum dried at 80°C for 12 hours. After drying, it was taken out after natural cooling in a drying oven, and cut into phosphorus-carbon negative electrode sheets with a diameter of 10 mm. The active material surface loading was calculated by weighing and was close to 2 mg / cm 2 .

[0087] The above groups of phosphorus-carbon negative electrode sheets were used as working electrodes, metal lithium foil with a thickness of 300 μm was used as the counter electrode, 25 μL of lithium-ion battery electrolyte was added, and a polyethylene separator with a thickness of 16 μm was used to assemble a CR2032 standard button battery.

[0088] Perform electrochemical impedance spectroscopy (EIS) tests on each group of batteries, analyze and record the ohmic resistance (R bulk ) and the charge transfer resistance (R ct ), as shown in Table 1. As can be seen from Table 1, compared with the batteries assembled from samples 1 and 2 of the control group, the ohmic resistance R bulk is almost unchanged, while the charge transfer resistance R ct It has decreased significantly, indicating that the introduction of Li3PO4 reduces the reaction resistance of lithium ions on the surface of the material, which is conducive to the rapid delithiation / insertion reaction of the negative electrode.

[0089] Table 1: Impedance test results of phosphorus-carbon negative electrode half-cell

[0090]

[0091] For each group of button batteries assembled, rate performance tests were conducted at current densities of 0.5, 1.0, 2.0, 4.0, 6.0, and 8.0 A / g, respectively. The charge and discharge voltage range of the battery was 0.01-2.00 V, and the average discharge specific capacity at different rate currents was recorded. As shown in Table 2 below, it can be seen that compared with the batteries assembled from samples of control group 1 and control group 2, the specific capacity of the batteries assembled from materials containing lithium phosphate on the surface (Example 1 to Example 3) at different current densities above 0.5 A / g is better than that of the batteries of control group 1 and control group 2, indicating that the introduction of lithium phosphate improves the rate performance of the phosphorus-carbon composite.

[0092] Table 2: Gram capacity of phosphorus-carbon anode half-cell at different current densities (unit: mAh / g)

[0093]

[0094] The rate performance of the batteries of Examples 2-3 and Control Group 2 was compared, and the data were organized into a dotted line graph. Figure 6 The figure shows the rate performance comparison of Examples 2 to 3 of the present invention and Control Group 2. Figure 6 As shown in the figure, the rate performance of the material without Li3PO4 on the surface decays very quickly. At current densities of 4A / g and above, the gap between the gram capacity and the samples containing Li3PO4 gradually increases. At a current density of 8A / g, the gram capacity is less than 50% of the samples in the other groups. For samples with different Li3PO4 contents, as the lithium phosphate content increases from 0 to 2%, the battery performance gradually improves. Figure 6 It can be clearly seen that the 2% content is better than the 1% content, and then better than the 0.5% and 0% content; the 1% and 2% content samples have better performance in terms of gram capacity at high rates. When the lithium phosphate is further increased, the battery performance gradually decreases, that is, the 3% content is better than the 4% content, and the 4% content is better than the 5%.

[0095] During the experiments related to the present invention, it was found that when the content was higher than 5%, due to the excessive lithium phosphate content, the SEI film formed during the cycle was thicker, which in turn hindered the diffusion of lithium ions and caused the battery performance to further deteriorate.

[0096] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A negative electrode material comprising: A phosphorus-carbon carrier, and a lithium phosphate covering layer, wherein the lithium phosphate covering layer at least partially covers the surface of the phosphorus-carbon carrier; Compared with the mass of the phosphorus-carbon carrier, the mass content of lithium phosphate in the lithium phosphate covering layer is 0.5-5%.

2. The negative electrode material according to claim 1, wherein Compared with the mass of the phosphorus-carbon carrier, the mass content of lithium phosphate in the lithium phosphate covering layer is 1~2%.

3. The negative electrode material according to claim 1 or 2, wherein In the phosphorus-carbon carrier, the mass content of phosphorus is 5-80%; In the phosphorus-carbon carrier, the mass content of carbon is 20-95%.

4. The negative electrode material according to claim 3, wherein The phosphorus in the phosphorus-carbon carrier includes at least one of red phosphorus, black phosphorus and white phosphorus; The carbon in the phosphorus-carbon carrier includes at least one of a carbon carrier with a layered structure and a carbon carrier with a porous structure.

5. A method for preparing a negative electrode material according to any one of claims 1 to 4, comprising: mixing a phosphorus-carbon support material with a lithium base to form a mixture powder; The mixed powder is mixed with a phosphoric acid solution so that the lithium alkali on the surface of the mixed powder reacts with the phosphoric acid to generate a phosphorus-carbon carrier material with a surface at least partially covered with a lithium phosphate covering layer; The phosphorus-carbon carrier material whose surface is at least partially covered with a lithium phosphate covering layer is subjected to a first drying treatment to obtain a negative electrode material.

6. The preparation method according to claim 5, wherein: The step of mixing the phosphorus-carbon carrier material with the lithium base to form a mixture powder comprises: Grinding and mixing the phosphorus-carbon support material and the lithium base to form a mixture powder; or The phosphorus-carbon support material is immersed in a lithium alkali solution and subjected to a second drying process to form a mixture powder.

7. The preparation method according to claim 5 or 6, wherein: The lithium base includes at least one of lithium hydroxide or lithium carbonate.

8. The preparation method according to claim 5, wherein The first drying process is freeze drying or spray drying; When the first drying treatment is spray drying, the spray drying temperature is 150 to 170° C., and the spray drying atmosphere is an inert atmosphere.

9. The preparation method according to claim 6, wherein: The molar ratio of lithium ions in the lithium base to phosphate ions in the phosphoric acid solution is 3:1; The conditions of the second drying treatment are: drying at 75-85° C. in a vacuum environment for 11-13 hours.

10. A lithium ion battery comprising a negative electrode, wherein the negative electrode comprises the negative electrode material according to any one of claims 1 to 4, a conductive agent and a binder.

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