Negative electrode material and preparation method thereof, secondary battery and electronic equipment

Abstract

The invention provides a negative electrode material and a preparation method thereof, a secondary battery and electronic equipment, the negative electrode material comprises silicon carbon particles, the silicon carbon particles comprise metal simple substances, and the metal simple substances comprise at least one of Sn or Ge. According to the negative electrode material provided by the invention, the DCR performance and the thermal storage performance of the secondary battery under the low SOC can be improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode material, a method for preparing the negative electrode material, a secondary battery using the negative electrode material, and an electronic device using the secondary battery. Background Art

[0002] Lithium-ion batteries have been widely used in power vehicles, consumer electronics and large-scale energy storage due to their ultra-high energy density, long cycle life and low self-discharge characteristics. However, the negative electrode of traditional commercial lithium-ion batteries is basically limited to graphite materials, which have low capacity (LiC6, 372mAh g -1 ) has greatly hindered the improvement of energy density. In recent years, silicon-based materials have been widely used for their high specific capacity (Li 15 Si4, 3579mAh g -1 ), rich resource reserves, reasonable lithium platform (<0.4V vs.Li / Li + ) has gradually entered the industrialization track of lithium-ion battery negative electrodes and is considered to be the most promising negative electrode material for the next generation.

[0003] Despite the above advantages, silicon-based materials will undergo severe volume changes during the lithiation / delithiation process, resulting in a decrease in the cycle performance of lithium-ion batteries, and causing the negative electrode active material particles in the electrode to pulverize and fail, hindering the full play of the electrochemical performance of silicon-based materials. In response to this problem, the prior art usually nanosizes silicon-based materials and composites them with porous carbon. Although this can prevent the negative electrode active material particles from breaking to a certain extent and improve the cycle performance, due to the low intrinsic ion / conductivity of silicon and the low ion conductivity of porous carbon, especially the high DC internal resistance (DCR) at low state of charge (SOC), the polarization of lithium-ion batteries is large and the charge and discharge rate performance is poor. Summary of the invention

[0004] The present application provides a negative electrode material and a preparation method thereof, a secondary battery and an electronic device.

[0005] In a first aspect, the present application provides a negative electrode material, the negative electrode material includes silicon-carbon particles, the silicon-carbon particles include a metal element, and the metal element includes at least one of Sn or Ge.

[0006] In the present application, the silicon-carbon particles in the negative electrode material are composited with metal elements, and the silicon-carbon particles contain silicon. During the heating process of preparing the silicon-carbon particles, part of the metal elements (such as Sn or Ge) will react with silicon to form Sn-Si alloy or Ge-Si alloy, which significantly improves the ionic conductivity and electronic conductivity of the negative electrode material, and the metal elements are also beneficial to improve the ionic conductivity of the negative electrode material under low SOC of silicon, and improve the DCR performance of the secondary battery under low SOC; at the same time, Sn-Si alloy or Ge-Si alloy is formed in the negative electrode material, and the metal in the alloy is dispersed in the Si domain in an atomic form. After the lithium reaction of the secondary battery, the metal is dispersed in the silicon-lithium alloy with a larger expansion as a buffer matrix, which alleviates the volume change of the negative electrode material during the charge and discharge process. Sn-Li and / or Ge-Li use a higher redox potential to increase the decomposition energy barrier of the silicon-lithium alloy and the electrolyte reaction, thereby improving the thermal storage performance of the negative electrode material.

[0007] Based on the first aspect, in some possible implementations, based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, 2≤M≤10. This is beneficial to improving the ionic conductivity and electronic conductivity of the silicon-carbon particles, and is beneficial to improving the specific capacity of the negative electrode material. The above-mentioned suitable content of the metal is beneficial to forming an alloy with silicon and acting as a buffer matrix, thereby improving the thermal storage performance of the negative electrode material.

[0008] Based on the first aspect, in some possible embodiments, the silicon-carbon particles include porous carbon, silicon and a carbon layer, the porous carbon has pores, the silicon is located in the pores of the porous carbon, and the carbon layer is located on the surface of the porous carbon. The porous carbon serves as the skeleton of the silicon-carbon particles, so that the silicon-carbon particles have good electrical conductivity and cycle stability. The carbon layer can improve the electrical conductivity of the silicon-carbon particles, which is conducive to further improving the DCR performance of the secondary battery at low SOC.

[0009] Based on the first aspect, in some possible implementations, based on the mass of the silicon-carbon particles, the mass percentage of silicon is S%, 5≤S / M≤30. The silicon-carbon particles give full play to the capacity contribution of silicon, so that the silicon-carbon particles have a better gram capacity, and are also conducive to improving the ionic conductivity and electronic conductivity of the silicon-carbon particles. The metal element and Si generate a metal silicon alloy with an appropriate amount to improve the DCR performance of the secondary battery at low SOC; after lithiation, the appropriate amount of metal is dispersed in the silicon-lithium alloy, which is conducive to making the secondary battery have better thermal storage performance.

[0010] Based on the first aspect, in some possible implementations, based on the mass of the silicon-carbon particles, the mass percentage of silicon is S%, and 6≤S / M≤11. This is beneficial to further improve the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0011] Based on the first aspect, in some possible embodiments, the differential capacity curve of the delithiation of the negative electrode material has a characteristic peak of delithiation of the metal element at 400mV to 900mV. The lithiation / delithiation of the metal in the alloy formed by the above metal and silicon can improve the overall ion network continuity of the negative electrode material, which is conducive to improving the rapid occurrence of the delithiation / insertion reaction of the negative electrode material, thereby improving the DCR performance of the secondary battery at low SOC; the appropriate lithium-treated metal is dispersed in the Si-Li alloy to ensure that the negative electrode material has good thermal storage performance.

[0012] Based on the first aspect, in some possible embodiments, the silicon-carbon particles have a surface area and an internal area, and based on the mass of the surface area, the mass percentage of the metal element in the metal element is M1%; based on the mass of the internal area, the mass percentage of the metal element in the metal element is M2%, wherein the surface area refers to the area from 0nm to 500nm from the surface of the silicon-carbon particles, and the internal area refers to the area in the silicon-carbon particles other than the surface area, and |M1-M2|≤10. The uniform distribution of metals in the silicon-carbon particles can reduce the volume change of the negative electrode material during the charging and discharging process, and also help to alleviate the volume expansion of Si and metals (Sn or Ge) during the lithiation and delithiation process, thereby improving the DCR performance of the secondary battery at low SOC. The uniform distribution of metal elements in the silicon-carbon particles can reduce the local heat accumulation or aggregation of the silicon-carbon particles during the charging and discharging process of the secondary battery, thereby reducing the risk of thermal runaway and improving the safety and thermal storage performance of the secondary battery.

[0013] The second aspect of the present application provides a method for preparing silicon-carbon particles in a negative electrode material, comprising the following steps: S1) dissolving an acid solution or an alkaline solution, a resin, and a metal compound in ethanol and dispersing and mixing, and drying to obtain a mixture, wherein the metal compound includes at least one of the elements Sn or Ge, and the mass ratio of the resin to the metal compound is (10-80): 1; S2) carbonizing and activating the mixture to obtain porous carbon containing metal oxides; S3) reducing the porous carbon, and then using chemical vapor deposition to deposit silicon and carbon-coat the porous carbon to obtain silicon-carbon particles. The silicon-carbon particles obtained in the above preparation process contain Sn or Ge single substances, which can improve the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0014] The third aspect of the present application provides a secondary battery, comprising a negative electrode plate, a positive electrode plate and an electrolyte, wherein the negative electrode plate further comprises the negative electrode material or the negative electrode material obtained by the preparation method. The negative electrode plate comprising the silicon-carbon particles improves the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0015] Based on the third aspect, in some possible embodiments, the electrolyte includes a compound of formula I, and the mass percentage of the compound of formula I is B%, based on the mass of the electrolyte, 0.1≤B≤2,

[0016] It is beneficial to further improve the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0017] Based on the third aspect, in some possible implementations, based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, 2≤M / B≤10. This is beneficial to further improve the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0018] The fourth aspect of the present application provides an electronic device, including a secondary battery, which powers a load in the electronic device. The secondary battery including a negative electrode plate has excellent high-temperature storage performance and DCR performance under low SOC, which is beneficial to improving the service life of the electronic device and its applicability in high-temperature environments. DETAILED DESCRIPTION

[0019] The technical scheme in the embodiment of the present application is described clearly and in detail below. Obviously, the described embodiment is a part of the embodiment of the present application, rather than all the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0020] An embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0021] The outer shell can be a packaging bag encapsulated by an encapsulation film (such as an aluminum-plastic film), such as a soft-pack battery. In other embodiments, the secondary battery can also be a steel shell battery, an aluminum shell battery, etc.

[0022] The electrode assembly includes a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet. The electrode assembly may be a laminated structure, which is formed by alternating the positive electrode sheet, the separator and the negative electrode sheet. In other embodiments, the electrode assembly may also be a wound structure, which is formed by stacking the positive electrode sheet, the separator and the negative electrode sheet and then winding them.

[0023] Negative electrode

[0024] The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector may be made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, composite current collector or carbon-based current collector. The negative electrode material layer includes a negative electrode material.

[0025] An embodiment of the present application provides a negative electrode material, the negative electrode material includes silicon-carbon particles, the silicon-carbon particles include a metal element, and the metal element includes at least one of Sn or Ge.

[0026] In the present application, the silicon-carbon particles in the negative electrode material are composited with metal elements, and the silicon-carbon particles contain silicon. During the heating process of preparing the silicon-carbon particles, part of the metal elements (such as Sn or Ge) will react with silicon to form Sn-Si alloy or Ge-Si alloy, which significantly improves the ionic conductivity and electronic conductivity of the negative electrode material, and the metal elements are also beneficial to improve the ionic conductivity of the negative electrode material under low SOC of silicon, and improve the DCR performance of the secondary battery under low SOC; at the same time, Sn-Si alloy or Ge-Si alloy is formed in the negative electrode material, and the metal in the alloy is dispersed in the Si domain in an atomic form. After the lithium reaction of the secondary battery, the metal is dispersed in the silicon-lithium alloy with a larger expansion as a buffer matrix, which alleviates the volume change of the negative electrode material during the charge and discharge process. Sn-Li and / or Ge-Li use a higher oxidation / reduction potential to increase the decomposition energy barrier of the silicon-lithium alloy and the electrolyte reaction, thereby improving the thermal storage performance of the negative electrode material.

[0027] In some embodiments, the silicon carbon particles include Sn, or Ge, or both Sn and Ge.

[0028] In some embodiments, based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, 2≤M≤10. The negative electrode material contains a suitable amount of metal element to improve the ionic conductivity and electronic conductivity of the silicon-carbon particles, and is beneficial to improve the specific capacity of the negative electrode material. The above-mentioned suitable content of metal is beneficial to form an alloy with silicon and act as a buffer matrix, thereby improving the thermal storage performance of the negative electrode material. In some embodiments, the mass percentage M of the metal element can be 2, 3, 4, 5, 6, 7, 8, 9, 10 or any value within the range of any two of the above values.

[0029] In some embodiments, the silicon-carbon particles include porous carbon, silicon and a carbon layer, the porous carbon has pores, the silicon is located in the pores of the porous carbon, and the carbon layer is located on the surface of the porous carbon. Elemental silicon is evenly dispersed in the pores of the porous carbon, and the porous carbon serves as the skeleton of the silicon-carbon particles, so that the silicon-carbon particles have good electrical conductivity and cycle stability. And porous carbon, as a matrix for silicon deposition, can inhibit the volume expansion of silicon during lithium insertion, thereby reducing expansion stress. At the same time, the pore structure in the porous carbon substrate can also disperse the expansion stress of the silicon material and improve the cycle performance and expansion performance of the silicon-carbon particles. On the one hand, the carbon layer can reduce the specific surface area of ​​the silicon-carbon particles, reduce the side reactions of the silicon-carbon particles with the electrolyte, and improve its first coulomb efficiency. On the other hand, it can further improve the conductivity of the silicon-carbon particles, which is conducive to further improving the DCR performance of the secondary battery at low SOC.

[0030] In some embodiments, based on the mass of the silicon-carbon particles, the mass percentage of silicon is S%, 5≤S / M≤30. When the content ratio of the metal element and silicon in the silicon-carbon particles satisfies the above relationship, the silicon-carbon particles give full play to the capacity contribution of silicon, so that the silicon-carbon particles have a better gram capacity, and are also beneficial to improve the ionic conductivity and electronic conductivity of the silicon-carbon particles. The metal element and Si generate a metal silicon alloy with an appropriate amount to improve the DCR performance of the secondary battery at low SOC, and satisfy the above relationship, so that in the silicon-carbon particles, the silicon element and the metal element meet the appropriate ratio range, and the appropriate amount of metal is dispersed in the silicon-lithium alloy after lithiation, which is beneficial to make the secondary battery have better thermal storage performance. In some embodiments, the ratio of S / M can be 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, 20, 25, 28, 30 or any value within the range composed of any two of the above values.

[0031] In some embodiments, based on the mass of the silicon-carbon particles, the mass percentage of silicon is S%, 6≤S / M≤11, which can further improve the DCR performance and thermal storage performance of the secondary battery at low SOC.

[0032] In some embodiments, the differential capacity curve of the delithiation of the negative electrode material has a characteristic peak of delithiation of a metal element at 400mV to 900mV. The differential capacity-voltage curve of the delithiation of the negative electrode material can be understood as, with the charge and discharge specific capacity of the negative electrode material as the horizontal coordinate and the voltage as the vertical coordinate, the charge and discharge curve of the negative electrode material can be obtained, and then the first-order derivative of the delithiation specific capacity of the negative electrode material with respect to the voltage is taken as the vertical coordinate, and the voltage is the horizontal coordinate, and the differential capacity-voltage curve of the delithiation of the negative electrode material can be obtained. The differential capacity-voltage curve can reflect the capacity contained in the negative electrode material within the unit voltage range. If the capacity at a certain voltage platform is high, it means that a lot of capacity will be contributed within a very small voltage fluctuation range, and a characteristic peak will be shown on the curve, each characteristic peak represents an electrochemical reaction, and the peak height of each characteristic peak also indicates the size of the corresponding electrochemical reaction's contribution to the capacity. The differential capacity curve of the delithiation of the negative electrode material has a characteristic peak of delithiation of a metal element at 400mV to 900mV, indicating a delithiation reaction of lithium precipitation in the negative electrode material. In the above-mentioned delithiation characteristic peak, on the one hand, the metal element (such as Sn, or Ge, or Sn and Ge) participates in the lithiation / delithiation process of the negative electrode material. The lithiation / delithiation of the metal in the alloy formed by the above-mentioned metal and silicon can improve the overall ion network continuity of the negative electrode material, and the ionic conductivity and electronic conductivity of Sn-Li or Ge-Li are greater than those of Si-Li, which is conducive to improving the rapid occurrence of delithiation / lithiation reactions of negative electrode materials, thereby improving the DCR performance of secondary batteries at low SOC. On the other hand, the main delithiation potential of Sn-Li or Ge-Li alloys is located at the delithiation potential of Si-Li alloys at low SOC, which can reduce the overall DCR performance of secondary batteries at low SOC. On the basis of the above, the appropriate lithiated metal is dispersed in the Si-Li alloy to ensure that the negative electrode material has good thermal storage performance.

[0033] In some embodiments, the silicon-carbon particles have a surface area and an internal area, and based on the mass of the surface area, the mass percentage of the metal element in the metal element is M1%; based on the mass of the internal area, the mass percentage of the metal element in the metal element is M2%, wherein the surface area refers to the area from 0nm to 500nm from the surface of the silicon-carbon particles, and the internal area refers to the area in the silicon-carbon particles other than the surface area, and |M1-M2|≤10. When |M1-M2| satisfies the above range, it means that the content of the metal elements in the surface area and the internal area of ​​the silicon-carbon particles is uniformly distributed, and the metal in the silicon-carbon particles is uniformly distributed as a whole. On the one hand, the uniformly distributed metal in the silicon-carbon particles can reduce the volume change of the negative electrode material during the charging and discharging process, and is also conducive to alleviating the volume expansion of Si and metal (Sn or Ge) during the lithiation and delithiation process, and can alleviate the expansion stress of the metal during the charging and discharging process, reduce the risk of silicon-carbon particles breaking or pulverizing, thereby improving the cycle performance of the negative electrode material and improving the DCR performance of the secondary battery at low SOC. On the second hand, the uniformly distributed metals in the silicon-carbon particles are conducive to improving the conductivity of the overall negative electrode material, improving the uniformity of the ion conductive network and the electronic conductive network of the negative electrode material, and promoting the rapid transmission of lithium ions, thereby improving the DCR performance of the secondary battery at low SOC. On the third hand, the relative metal content in the surface area and the internal area is similar, the overall uniformity is good, and the overall structure of the interior and surface of the silicon-carbon particles is stable, which can improve the uniformity and stability of the SEI film interface formation, reduce the interface impedance, and reduce the growth rate of the SEI film, thereby improving the safety and thermal storage performance of the secondary battery. On the fourth hand, the uniform distribution of metal elements in the silicon-carbon particles is conducive to improving the phenomenon that the local excessive lithiation of the silicon-carbon particles in the area with high metal (Sn or Ge) content leads to irreversible phase change and capacity loss, thereby improving the safety and thermal storage performance of the secondary battery. On the fifth hand, the uniform distribution of metal elements in the silicon-carbon particles can reduce the local heat accumulation or aggregation of the silicon-carbon particles during the charging and discharging process of the secondary battery, thereby reducing the risk of thermal runaway and improving the safety and thermal storage performance of the secondary battery.

[0034] In some embodiments, |M1-M2| can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or any value within the range formed by any two of the above values.

[0035] In some embodiments, the electrolyte includes a compound of formula I, and the mass percentage of the compound of formula I is B%, based on the mass of the electrolyte, 0.1≤B≤2,

[0036] The electrolyte contains the compound of formula I, which can improve the DCR performance and heat storage performance of the secondary battery at low SOC. The content of the compound of formula I in the electrolyte is within the above range, which is conducive to further improving the DCR performance and heat storage performance of the secondary battery at low SOC. In some embodiments, B can be 0.1, 0.2, 0.5, 0.7, 0.9, 1.0, 1.2, 1.5, 1.8, 2 or any value within the range composed of any two of the above values.

[0037] In some embodiments, based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, 2≤M / B≤10. The content of the metal element and the content of the compound of formula I in the electrolyte satisfy the above relationship, which is beneficial to further improve the DCR performance and thermal storage performance of the secondary battery at low SOC. In some embodiments, the ratio of M / B can be 2, 3, 4, 5, 6, 7, 8, 9, 10 or any value within the range of any two of the above values.

[0038] The present application also provides a method for preparing silicon-carbon particles in a negative electrode material, comprising the following steps:

[0039] S1) dissolving and dispersing a resin, a metal compound and an acid solution in an ethanol / water system, mixing and drying to obtain a mixture, wherein the metal compound includes at least one of the elements Sn or Ge, and the mass ratio of the resin to the metal compound is (10-80):1.

[0040] In some embodiments, the resin may be a phenolic resin, and the metal compound may be at least one of stannous chloride, tin chloride, and tin dioxide, and may be used with an acid solution to achieve uniform dissolution, and the acid solution may be a hydrochloric acid solution. The metal compound may also be at least one of potassium germanate or germanium dioxide, and may be used with an alkaline solution to achieve uniform dissolution, and the alkaline solution may be a potassium hydroxide solution.

[0041] In some embodiments, the concentration of the acid / base solution is 1 to 12 mol / L, and the mass ratio of the metal compound to the acid / base solution is (10 to 50): (0.1 to 1). The metal compound is fully dissolved in the acid / base solution, the resin is dissolved in ethanol, and in the ethanol / water system, the metal compound and the resin are fully mixed and uniform. In some embodiments, the concentration of the acid / base solution can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L, 12 mol / L, or any value within the range of any two of the above values. In some embodiments, the mass ratio of the metal compound to the acid / base solution can be 10:0.1, 10:1, 50:0.1, 50:1, or any ratio within the range of any two of the above values.

[0042] In some embodiments, the mass ratio of the resin to the metal compound is (10-80):1. The mass ratio of the resin to the metal compound within the above range is conducive to making the silicon-carbon particles contain an appropriate amount of metal and facilitating the metal to be uniformly distributed in the silicon-carbon particles. In some embodiments, the mass ratio of the resin to the metal compound can be 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 80:1 or any ratio within the range formed by any two of the above values.

[0043] In this step, drying can be performed by direct drying or by spray drying to solidify and dry the mixture, so as to obtain the mixture. The commonly used solidification conditions in the prior art can be applied, which will not be described in detail here.

[0044] In this step, the method of forming the mixture includes but is not limited to high-speed dispersion and ball milling.

[0045] S2) carbonizing and activating the mixture to obtain porous carbon containing metal oxides.

[0046] Specifically, under an inert atmosphere (for example, but not limited to a nitrogen atmosphere), the spray-dried mixture is heated to 700°C to 1100°C, carbonized for 1h to 5h, and activated to form a porous carbon skeleton containing metal oxides after cooling to room temperature (such as 25°C). The activation can activate the carbonized structure by an activation gas of carbon dioxide or water vapor. The activation temperature is 700°C to 1100°C, the activation time is 6h to 24h, and the flow rate of the activated gas is 50sccm to 300sccm. In some embodiments, the activation temperature can be 700°C, 800°C, 900°C, 1000°C, 1100°C, or any value within the range of any two of the above values. The activation time can be 6h, 7h, 8h, 9h, 10h, 13h, 16h, 19h, 20h, 22h, 24h, or any value within the range of any two of the above values. The flow rate of the activation gas may be 50 sccm, 100 sccm, 200 sccm, 250 sccm, 300 sccm, or any value within a range formed by any two of the foregoing values.

[0047] S3) performing reduction treatment on the porous carbon, and then performing silicon deposition and carbon coating on the porous carbon by chemical vapor deposition to obtain silicon-carbon particles.

[0048] Specifically, after the porous carbon is slowly heated to 600°C to 800°C under an inert atmosphere (for example, but not limited to an argon atmosphere), the atmosphere is switched to an argon / hydrogen mixture, the reduction reaction time is 1h to 3h, and the flow rate of the argon / hydrogen mixture is 100sccm to 500sccm. The argon / hydrogen mixture contains 2% to 20% hydrogen and 80% to 98% argon by mass. Then, the atmosphere is switched to a silane mixture for deposition for 1h to 20h to form a nucleus, and the deposition temperature can be 500°C to 1000°C. Among them, the heating rate of the slow heating can be but not limited to 0.5°C / min to 5°C / min, the silane mixture contains 2% to 20% silane by mass and 80% to 98% inert gas (for example, but not limited to argon), and the flow rate of the silane mixture is 100sccm to 500sccm. In some embodiments, the temperature of the reduction reaction can be 600° C., 650° C., 700° C., 750° C., 800° C., or any value within the range formed by any two of the above values. The time of the reduction reaction can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, or any value within the range formed by any two of the above values. The flow rate of the argon / hydrogen mixture can be 100 sccm, 200 sccm, 300 sccm, 400 sccm, 500 sccm, or any value within the range formed by any two of the above values.

[0049] In some embodiments, the time for depositing the silane mixture to form silicon can be 1h, 3h, 5h, 6h, 7h, 8h, 9h, 10h, 13h, 16h, 19h, 20h or any value within the range formed by any two of the above values; the deposition temperature can be 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C or any value within the range formed by any two of the above values.

[0050] Then, the silane mixed gas atmosphere is switched to an alkane mixed gas at a temperature of 500°C to 1000°C for 2 hours to 20 hours to form a carbon layer covering the core body, and then the alkane mixed gas atmosphere is switched to an inert atmosphere (for example, but not limited to a nitrogen atmosphere) and cooled to room temperature to finally obtain silicon-carbon particles. The flow rate of the alkane mixed gas is 100 sccm to 500 sccm, and the alkane mixed gas contains 5% to 100% of alkanes (for example, but not limited to acetylene) and 0% to 95% of inert gas (for example, but not limited to argon) by mass percentage. In some embodiments, the temperature for forming the carbon layer can be 500°C, 600°C, 700°C, 800°C, 900°C, 1000°C or any value within the range of any two of the above values. The time for forming the carbon layer covering the core body can be 2h, 3h, 5h, 6h, 7h, 8h, 9h, 10h, 13h, 16h, 19h, 20h or any value within the range formed by any two of the above values. The flow rate of the alkane mixed gas can be 100sccm, 200sccm, 300sccm, 400sccm, 500sccm or any value within the range formed by any two of the above values.

[0051] The negative electrode material layer also includes a binder and a conductive agent. The binder is used to bind the negative electrode material particles to facilitate the formation of a film layer, and can also improve the bonding force between the negative electrode material layer and the negative electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylic (ester) styrene butadiene rubber, epoxy resin or nylon, etc.

[0052] Conductive agent in the negative electrode material layer, the conductive agent includes but is not limited to carbon-based materials, metal-based materials, conductive polymers or any combination thereof. In some embodiments, carbon-based materials may include but are not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, metal-based materials may include but are not limited to metal powder or metal fiber, such as copper, nickel, aluminum or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0053] The negative electrode active material includes graphite, which includes at least one of natural graphite and artificial graphite. Since graphite has a certain degree of flexibility, it can alleviate the overall volume expansion of the negative electrode material layer when combined with silicon-carbon particles. At the same time, graphite and silicon-carbon particles can be used as negative electrode materials at the same time to fully utilize the advantages of both silicon-carbon particles and graphite to achieve better electrochemical performance.

[0054] Isolation film

[0055] The material and shape of the separator used in the secondary battery of the present application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0056] For example, the isolation film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be selected.

[0057] A surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride and poly (vinylidene fluoride-hexafluoropropylene).

[0058] Electrolyte

[0059] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt and an optional additive. The organic solvent in the electrolyte of the present application may be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. The electrolyte used in the electrolyte of the present application is not limited, and it may be any electrolyte known in the prior art. The additive of the electrolyte of the present application may be any additive known in the prior art that can be used as an electrolyte additive.

[0060] In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate or ethyl propionate. In some embodiments, the organic solvent includes an ether solvent, for example, at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bistrifluoromethanesulfonyl imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl) imide Li(N(SO2F)2)(LiFSI), lithium bis(oxalatoborate) LiB(C2O4)2 (LiBOB) or lithium di(oxalatoborate) LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0061] Positive electrode

[0062] The positive electrode plate includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector may use aluminum foil or nickel foil, etc. The positive active layer contains a positive active material, and the positive active material includes a compound that reversibly embeds and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive active material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese and nickel. In some embodiments, the positive active material may include but is not limited to lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganese oxide, spinel lithium nickel manganese oxide and lithium titanate.

[0063] The positive electrode active layer also includes a binder to bond the positive electrode active material particles to facilitate the formation of a film layer, and at the same time, it can also improve the bonding force between the positive electrode active layer and the positive electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylic (ester) styrene butadiene rubber, epoxy resin or nylon, etc. At least one of the above.

[0064] The positive electrode active layer may also include a conductive material, including but not limited to a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material may include but is not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0065] The above-mentioned secondary battery is applied to electronic equipment to power the load in the electronic equipment. Moreover, the above-mentioned secondary battery containing the negative electrode plate has excellent DCR performance and heat storage performance under low SOC, which is conducive to improving the service life of the electronic equipment and the applicability in high temperature environment. Among them, the electronic equipment may include but is not limited to laptop computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and lithium-ion capacitors, etc.

[0066] The present application is described below by specific examples and comparative examples. Those skilled in the art should understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application.

[0067] Example 1-1

[0068] (1) Preparation of silicon carbon particles:

[0069] 714 g of phenolic resin (solid content 70%), 10 g of stannous chloride dihydrate, and 2 mL of 12 mol / L hydrochloric acid () were dispersed and stirred in 200 mL of ethanol system, and then spray-dried for curing and drying.

[0070] Under nitrogen, the cured mixture was heated to 800°C and carbonized for 1 hour, and then activated by introducing carbon dioxide at a flow rate of 300 sccm at 900°C for 6 hours to form porous carbon containing metal tin oxide.

[0071] Under nitrogen, the porous carbon is slowly heated to 800°C, and then the atmosphere is switched to an argon / hydrogen mixture (the mass ratio of argon / hydrogen is 9:1), and the reduction reaction is carried out for 1 hour, and the flow rate of the argon / hydrogen mixture is 200sccm. Then, chemical vapor deposition is used, and the porous carbon skeleton continues to be heated to 500°C at a rate of 2°C / min under an argon atmosphere, and then the atmosphere is switched to a silane mixture (20% silane and 80% argon by mass percentage), and deposited at 500°C for 10 hours to form a nucleus, wherein the flow rate of the silane mixture is 200sccm. Then the atmosphere is switched to an acetylene mixture (20% acetylene and 80% argon by mass percentage) and continued to be deposited at 500°C for 4 hours, and then switched to nitrogen, and the flow rate of the acetylene mixture is 200sccm, and the temperature is cooled to room temperature (such as 25°C) to obtain silicon-carbon particles.

[0072] (2) Preparation of negative electrode sheet: negative electrode active material (artificial graphite and the above silicon-carbon particles are mixed in a weight ratio of 80:20), carbon nanotubes, polymethyl acrylate and sodium carboxymethyl cellulose in a weight ratio of 97:1:1:1, deionized water is added as a solvent, and a negative electrode slurry with a solid content of 40wt% is prepared. The negative electrode slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and dried at 85°C to obtain a negative electrode sheet with a negative electrode material layer coated on one side. The coating weight of the negative electrode material layer during coating is 10mg / cm 2 Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet with a negative electrode material layer coated on both sides. Then, after cold pressing, cutting, slitting, and drying at 120°C under vacuum conditions for 12 hours, a negative electrode sheet with a specification of 78mm×875mm is obtained, which is welded to the pole ear for standby use. Among them, the compaction density of the negative electrode material layer after cold pressing is 1.7g / cm 3 ; The negative electrode active material includes artificial graphite and the silicon-carbon composite material prepared above, and the mass ratio of artificial graphite to silicon-carbon particles is 80:20.

[0073] (3) Preparation of positive electrode sheet: The positive electrode active material lithium cobalt oxide, conductive carbon black Super P, and positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a positive electrode slurry with a solid content of 72wt%, and the positive electrode slurry was obtained after vacuum stirring. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12μm, and dried at 85°C to obtain a positive electrode sheet with a single-sided coating of a positive electrode material layer. The coating weight of the positive electrode material layer during coating was 19mg / cm 2Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided positive electrode material layer. Then, after cold pressing, cutting, slitting, and drying at 85°C under vacuum conditions for 4 hours, a positive electrode sheet with a specification of 74mm×867mm is obtained, which is welded to the pole ear for standby use. Among them, the compaction density of the positive electrode material layer after cold pressing is 4.2g / cm 3 .

[0074] (4) Preparation of electrolyte: Under dry argon environment, LiPF6 is added to a mixed solution of 1,3-propane sultone (1,3-PS), propyl propionate (PP), ethyl propionate (EP), fluoroethylene carbonate (FEC), propylene carbonate (PC), and ethylene carbonate (EC), wherein, based on the mass of the electrolyte, the mass proportion of LiPF6 is 12.5%, the mass proportion of 1,3-PS is 3%, the mass proportion of PP is 45%, the mass proportion of EP is 15%, the mass proportion of FEC is 10%, and the remainder is PC and EC, and the mass ratio of PC to EC is 1:1.

[0075] (5) Preparation of isolation membrane: A porous polyethylene film with a thickness of 7 μm (provided by Celgard Company) was used as the isolation membrane.

[0076] (6) Preparation of lithium-ion batteries: stack the positive electrode sheet, separator, negative electrode sheet, and separator in order, so that the separator is between the positive electrode sheet and the negative electrode sheet to play an isolating role, and wind to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film packaging bag, dry it at 80°C, inject the electrolyte, and go through vacuum packaging, standing, formation, shaping, and capacity testing to obtain a soft-pack lithium-ion battery.

[0077] Example 1-2 to Example 1-7

[0078] The difference between Examples 1-2 to 1-7 and Example 1-1 is that the mass ratio of the resin to the metal compound during the preparation of the silicon-carbon particles, or the time for coating the carbon layer on the surface of the core body is changed, and the remaining steps are the same as Example 1-1. The specific preparation parameters can be referred to Tables 1 and 2.

[0079] Example 2-1 to Example 2-8

[0080] The difference between Example 2-1 to Example 2-8 and Example 1-2 is that the preparation parameters of deposited silicon in the process of preparing silicon-carbon particles are changed, and the remaining steps are the same as Example 1-1. The specific preparation parameters can be referred to Table 3.

[0081] Comparative Example 1

[0082] The difference between Comparative Example 1 and Example 1-1 is that no metal compound is added. For specific preparation parameters, please refer to Table 1. The rest is the same as Example 1-1.

[0083] Example 3-1 to Example 3-7

[0084] The difference between Examples 3-1 to 3-7 and Example 2-5 is that the compound of Formula I is further added and the mass ratio of the compound of Formula I is adjusted, and the mass ratio of the resin and the metal compound in the process of preparing the silicon-carbon particles is changed to adjust the metal content in the silicon-carbon particles. In addition to adjusting the parameters of the relevant electrolyte components according to Table 4, the mass ratio of PC and EC is adaptively increased or decreased, and the mass ratio of PC and EC remains unchanged. The rest is the same as Example 2-5.

[0085] Related test methods for negative electrode materials

[0086] (1) Test method for mass percentage of metal elements:

[0087] Take 2g of silicon-carbon granular material and place it in a PTFE beaker, add 10mL of concentrated nitric acid and 2mL of hydrofluoric acid, place it on a 220-degree flat heater, heat and digest until almost evaporated, then slowly add 10mL of nitric acid to continue heating and dissolving. Then filter and separate with deionized water, collect the filtrate, and then perform a constant volume ICP element test to obtain the mass ratio of Sn and Ge.

[0088] (2) Test method for the mass percentage of silicon in silicon-carbon particles:

[0089] The mass percentage of Si element in the negative electrode active material is determined by alkaline etching and high temperature oxidation test. Specifically, 5g of negative electrode material and 50g of sodium hydroxide are added to a crucible, heated to 400°C and kept warm for 1h, then cooled to below 100°C, the reactants are put into 80°C deionized water, and washed and filtered with 1M hydrochloric acid solution and deionized water several times. The filtrate is collected and then subjected to constant volume ICP element test to obtain the mass percentage of Si.

[0090] (3) Test method for the mass percentage of metal elements in the surface area and internal area of ​​silicon-carbon particles:

[0091] The negative electrode sheet was cut into sections by ion milling to obtain CP samples. The internal and surface areas of the silicon-carbon particles were calibrated. The SEM-EDX line scan was used to take multiple areas (5 times, taking the average value) to obtain the metal element content of the surface and external areas of the silicon-carbon particles.

[0092] Lithium-ion battery performance test:

[0093] (1) DCR of lithium-ion batteries

[0094] At 25°C, the lithium-ion battery was charged to 4.53V at a constant current rate of 0.2C, and then charged to 0.05C at a constant voltage. Then, it was discharged at a constant current rate of 0.1C for 10 seconds (the sampling method was 100ms), and at a constant current rate of 0.7C for 360 seconds (the sampling method was 100ms). After relaxation for 120 minutes, the above charge and discharge operations were cycled 10 times, and the DCR data at 10% SOC was read and recorded.

[0095] (2) Thermal storage performance of lithium-ion batteries

[0096] At 25°C, the lithium-ion full battery was discharged to 3.0V at a current density of 0.2C. Then it was charged to 4.53V at a current density of 0.2C and then charged to a current density of 0.05C at a constant voltage. At this time, the SOC of the lithium-ion battery was 100% and the initial thickness of the test was T0. Then the lithium-ion full battery was placed in a test box and the temperature was raised to 80°C for 24 hours. After cooling to 25°C, the thickness of the lithium-ion full battery was tested using a laser thickness gauge and the thickness was T1.

[0097] The high temperature storage thickness expansion ratio is: (T1-T0) / T0. The preparation conditions and test results of each embodiment and comparative example are recorded in Tables 1 to 4.

[0098] Table 1

[0099]

[0100]

[0101] Table 2

[0102]

[0103] Combined with Table 1 and Table 2, compared with the comparative example, in Examples 1-1 to 1-7, when the silicon-carbon particle material contains Sn, Ge, or Sn and Ge single substances, the DCR performance and thermal storage performance of the secondary battery at low SOC can be improved. In Example 1-3, the mass ratio of Sn to Ge in the surface area is 1.2:1, and the mass ratio of Sn to Ge in the internal area is 1.2:1.

[0104] In the process of preparing silicon-carbon particles, the mass ratio of the resin to the metal compound or the time of coating the carbon layer on the surface of the core is changed. When the mass percentage of the metal element and the difference between the surface area and the internal area meet a specific range, the DCR performance and thermal storage performance of the secondary battery at low SOC can be further improved.

[0105] Table 3

[0106]

[0107]

[0108] In Table 3, in the process of preparing silicon-carbon particles, the temperature, time or flow rate of depositing silicon in the process of preparing silicon-carbon particles is changed. When the ratio S / M of silicon content to metal content meets a specific range, the DCR performance and thermal storage performance of the secondary battery at low SOC can be further improved.

[0109] Table 4

[0110]

[0111] Combined with Table 4, in lithium-ion batteries, by changing the composition and mass proportion of the compound of formula I in the electrolyte, when the compound of formula I in the electrolyte is within an appropriate mass proportion range, and the ratio M / B of the content of the metal element to the compound of formula I in the silicon-carbon particles meets a specific range, the DCR performance and thermal storage performance of the secondary battery at low SOC are further improved.

[0112] The above disclosure is only a preferred implementation mode of the present application, and certainly cannot be used to limit the present application. Therefore, equivalent changes made according to the present application are still within the scope covered by the present application.

Claims

1. A negative electrode material, characterized in that: The negative electrode material includes silicon-carbon particles, the silicon-carbon particles include a metal element, and the metal element includes at least one of Sn or Ge.

2. The negative electrode material according to claim 1, characterized in that Based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, 2≤M≤10.

3. The negative electrode material according to claim 1, characterized in that The silicon-carbon particles include porous carbon, silicon and a carbon layer. The porous carbon has pores. The silicon is located in the pores of the porous carbon. The carbon layer is located on the surface of the porous carbon.

4. The negative electrode material according to claim 3, characterized in that Based on the mass of the silicon-carbon particles, the mass percentage of the silicon is S%, and 5≤S / M≤30.

5. The negative electrode material according to claim 1, characterized in that The negative electrode material satisfies at least one of the following conditions: (1) Based on the mass of the silicon-carbon particles, the mass percentage of the silicon is S%, 6≤S / M≤11; (2) The differential capacity curve of the negative electrode material after delithiation has a delithiation characteristic peak of the metal element at 400 mV to 900 mV; (3) The silicon-carbon particles have a surface region and an internal region, and based on the mass of the surface region, the mass percentage of the metal element in the metal single substance is M1%; based on the mass of the internal region, the mass percentage of the metal element in the metal single substance is M2%, wherein the surface region refers to a region 0nm to 500nm away from the surface of the silicon-carbon particles, and the internal region refers to a region of the silicon-carbon particles excluding the surface region, and |M1-M2|≤10.

6. A method for preparing silicon-carbon particles in a negative electrode material as claimed in any one of claims 1 to 5, characterized in that: The following steps are involved: S1) dissolving an acid solution or an alkaline solution, a resin, and a metal compound in ethanol, dispersing and mixing, and drying to obtain a mixture, wherein the metal compound includes at least one of the elements Sn or Ge, and the mass ratio of the resin to the metal compound is (10-80):1; S2) carbonizing and activating the mixture to obtain porous carbon containing metal oxides; S3) performing a reduction treatment on the porous carbon, and then performing silicon deposition and carbon coating on the porous carbon by chemical vapor deposition to obtain the silicon-carbon particles.

7. A secondary battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, characterized in that: The negative electrode sheet further comprises the negative electrode material according to any one of claims 1 to 5 or the negative electrode material obtained by the preparation method according to claim 6.

8. The secondary battery according to claim 7, characterized in that The electrolyte comprises a compound of formula I, and based on the mass of the electrolyte, the mass percentage of the compound of formula I is B%, 0.1≤B≤2, 9. The secondary battery according to claim 8, characterized in that Based on the mass of the silicon-carbon particles, the mass percentage of the metal element is M%, and 2≤M / B≤10.

10. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 7 to 9.

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