Negative electrode material, method for preparing the same, secondary battery, and electronic device

Abstract

A kind of negative material and its preparation method, secondary battery and electronic device, negative material includes silicon carbon particles, the cross section of silicon carbon particles includes surface area and internal area, surface area contains first substance, first substance has ion conduction ability;The hole area ratio of surface area is greater than the hole area ratio of internal area.By regulating the hole area distribution of the surface area and internal area of silicon carbon particles and the cooperation of first substance, the expansion of silicon material can be effectively buffered, and an efficient lithium ion path is constructed in the surface area, which improves the kinetic performance and expansion inhibition performance of the secondary battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of energy storage, and particularly relates to a negative electrode material and a preparation method thereof, a secondary battery and an electronic device. BACKGROUND

[0002] As a kind of repeatable charge-discharge energy storage device, the secondary battery plays a vital role in modern society and is widely used in portable electronic devices, electric vehicles, energy storage systems and other fields. The basic principle is to realize the storage and release of electric energy through the lithium ion insertion and extraction process between the positive and negative electrode materials. In the secondary battery, the negative electrode material serves as the carrier of lithium ions and plays a decisive role in the overall performance of the battery. The ideal negative electrode material should have high specific capacity, good cycle stability, high coulomb efficiency and excellent electronic and ionic conductivity.

[0003] However, with the continuous development of secondary battery technology, improving its kinetic performance and inhibiting the swelling performance have become urgent technical problems. Among them, the improvement of the kinetic performance is directly related to the charge and discharge rate and efficiency of the battery, and is crucial for meeting the fast charging and high power application requirements. The swelling problem, especially in the silicon-containing battery system, due to the precipitation and dissolution reaction of silicon-containing materials during the charge and discharge process, will cause significant changes in the volume of the negative electrode, and further increase the internal stress of the battery, which may even cause short circuit, failure and other safety problems. Therefore, how to improve the kinetic performance and inhibit the swelling performance of the secondary battery is a key problem for the continuous progress and wide application of the secondary battery technology. SUMMARY

[0004] Therefore, the application provides a negative electrode material and a preparation method thereof, a secondary battery and an electronic device, which can buffer the expansion of silicon materials by adjusting the pore area of the surface region and the internal region of the silicon-carbon particles and cooperating with the first substance, and construct an efficient lithium ion channel in the surface region, thereby improving the kinetic performance and inhibiting the swelling performance of the secondary battery.

[0005] In a first aspect, the present application provides a negative electrode material, which comprises silicon-carbon particles, a cross section of the silicon-carbon particles comprises a surface region and an internal region, the surface region contains a first substance, the first substance has ion conduction capability; the first substance contains M elements, the M elements are selected from at least one of S elements, N elements, La elements, Ti elements, Zr elements, Al elements, and P elements; a pore area ratio of the surface region is greater than that of the internal region; the surface region refers to a region 0 nm to 500 nm away from the surface of the silicon-carbon particles, and the internal region refers to a region 600 nm to 1200 nm away from the surface of the silicon-carbon particles. By controlling the pore area ratio of the surface region in the cross section of the silicon-carbon particles to be greater than that of the internal region, that is, controlling the pore channels in the surface region of the silicon-carbon particles to have a larger pore size, when nano-silicon is deposited, silane is difficult to be adsorbed in the larger pore channels, and the smaller specific surface area caused by the larger pore size also reduces the contact between the surface region and the silane gas, which is not conducive to the adhesion and deposition of nano-silicon, so as to reduce the deposition amount of nano-silicon in the surface region, so that more nano-silicon is deposited in the internal region deep in the porous carbon, thereby facilitating the improvement of the buffering effect of the volume expansion of nano-silicon in the process of lithium extraction and insertion, the inhibition effect of the expansion of the negative electrode material, the reduction of the side reaction with the electrolyte, and the reduction of the damage to the surface of the silicon-carbon particles; and the first substance with ion conduction capability is easy to combine in the larger pore channels of the surface region, which can construct a continuous and efficient lithium ion channel in the surface region of the silicon-carbon particles, improve the kinetic performance of the negative electrode material, and the lithium ion channel in the surface region also helps to disperse the expansion stress, while avoiding the direct contact between nano-silicon and the electrolyte to cause severe side reactions, reducing the accumulation of by-products, so that the secondary battery exhibits excellent rate performance and inhibition performance.

[0006] In some embodiments, the silicon-carbon particles comprise porous carbon, the porous carbon comprises primary pores and secondary pores, the pore size of the primary pores is D1 nm, 0.5≤D1<2, the pore size of the secondary pores is D2 nm, 2≤D1≤15; based on the sum of the pore volume of the primary pores and the pore volume of the secondary pores, the pore volume ratio of the secondary pores is P%, 2≤P≤20. The present application regulates the pore volume ratio of the secondary pores in the sum of the pore volume of the primary pores and the pore volume of the secondary pores in the porous carbon in the above range, that is, the pore volume ratio of the primary pores is 80% to 98%, which is much higher than that of the secondary pores, which can provide a higher silicon deposition capacity, thereby improving the lithium storage capacity of the silicon-carbon particles; in combination with the pore area ratio of the surface region being greater than that of the internal region, it is beneficial for more nano-silicon to be deposited in the deep region of the porous carbon more than 500 nm, to improve the buffering effect of the volume expansion of nano-silicon, and the first substance is more deposited in the surface region to improve the conduction capability of the ion channel in the surface region, thereby further improving the rate performance and inhibition performance of the secondary battery.

[0007] In some embodiments, 4≤P≤15. When the porous carbon satisfies this condition, the rate performance of the secondary battery can be further improved and the swelling performance can be inhibited.

[0008] In some embodiments, the first substance is selected from at least one of lithium aluminate, lithium phosphate, Li x1 PO y1 N z1 , wherein 2.8≤x1≤3.3, 3.0≤y1≤3.8, 0.16≤z1≤0.46, Li7La3Zr2O 12 , Li 3x2 La 2 / 3-x2 TiO3, wherein 0.06≤x2≤0.16, lithium thiophosphate, Li 1+x3 Al x3 Ti 2-x3 (PO4)3, wherein 0≤x3≤0.7, Li6PS5Cl. The above-mentioned first substance has high ionic conductivity, can form a continuous and efficient lithium ion passage in the surface region of the silicon-carbon particle, thereby shortening the diffusion path of lithium ions in the silicon-carbon particle, accelerating the speed of lithium ion intercalation and deintercalation, and also can serve as a buffer layer to reduce the volume expansion of the silicon-carbon particle, improve the wettability of the electrolyte to the silicon-carbon particle, thereby further improving the rate performance of the secondary battery and inhibiting the swelling performance.

[0009] In some embodiments, the mass percentage of the M element is C%, 0.3≤C≤6, based on the mass of the silicon-carbon particle; preferably, 0.6≤C≤4. When the mass percentage of the M element satisfies the above-mentioned range, the conduction path and structural stability of the lithium ion passage can be further optimized, the diffusion rate of lithium ions in the surface region of the silicon-carbon particle can be improved, and the rate performance of the secondary battery and the inhibition of the swelling performance can be improved.

[0010] In some embodiments, the silicon-carbon particle comprises porous carbon and nanosilicon located in the pores of the porous carbon; the surface of the silicon-carbon particle is provided with a carbon layer, and the thickness of the carbon layer is T nm, 5≤T≤80; preferably, 10≤T≤50. By controlling at least part of the surface of the silicon-carbon particle to have a carbon layer, the present application can form an excellent electronic passage with the internal porous carbon, can improve both the ionic conductivity and the electronic conductivity of the silicon-carbon particle in cooperation with the first substance, and can improve the mechanical strength and toughness of the silicon-carbon particle, improve the buffering effect on the expansion of nanosilicon during charging and discharging, and improve the structural stability of the silicon-carbon particle, thereby further improving the rate performance of the secondary battery and inhibiting the swelling performance.

[0011] In some embodiments, 20≤C×T≤60. When 20≤C×T≤60 is satisfied, the mutual cooperation effect of the carbon layer and the first substance can be improved, and the rate performance of the secondary battery and the inhibition of the swelling performance can be further improved.

[0012] The application also provides a preparation method of the foregoing negative electrode material, at least comprising the following steps:

[0013] Step S1, calcining the precursor material at T1 ℃ in an inert gas protection atmosphere for t1 hours to obtain carbonized material; 550≤T1≤1600, 2≤t1≤10;

[0014] Step S2, after crushing the carbonized material, introducing a first activation gas at T2 ℃ for t2 hours to obtain first activated material; 780≤T2≤950, 8≤t2≤20; the first activation gas is at least one of water vapor and carbon dioxide;

[0015] Step S3, introducing a second activation gas at T3 ℃ in an inert gas protection atmosphere for t3 hours to obtain porous carbon; 850≤T3≤1300, 2≤t3≤8; the second activation gas is at least one of water vapor, carbon dioxide or oxygen;

[0016] Step S4, introducing silane gas at T4 ℃ for t4 hours to obtain silicon deposition particles; 380≤T4≤600, 2≤t4≤5;

[0017] Step S5, transferring the silicon deposition particles to a dispersion medium in an inert gas protection atmosphere, adding a source of element M, then dispersing, and then evaporating the dispersion medium in a vacuum environment to obtain silicon-carbon particles.

[0018] Based on the above scheme, the present application controls the pore area ratio of the surface region of the silicon-carbon particles to be greater than that of the internal region by twice activation treatment to make the pore channels in the surface region have a larger pore diameter, and sets the first substance containing element M in the surface region, so as to construct an efficient lithium ion channel and improve the buffering effect on the volume expansion of nano-silicon, thereby further improving the rate performance and expansion inhibition performance of the secondary battery.

[0019] In some embodiments, the preparation method further comprises: step S6, heating the silicon-carbon particles to T5 ℃ in an inert gas protection atmosphere, then introducing acetylene gas, and keeping the temperature for t5 hours; 500≤T5≤700, 2≤t5≤8. By step S4, a carbon layer can be set on the surface of the silicon-carbon particles, which can further improve the rate performance and expansion inhibition performance of the secondary battery in cooperation with the first substance.

[0020] The application also provides a secondary battery, comprising a positive electrode, a negative electrode and an electrolyte; the negative electrode comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector; the negative electrode material layer comprises the foregoing negative electrode material or the negative electrode material prepared by the foregoing preparation method.

[0021] In some embodiments, the electrolyte comprises at least one of ethyl acetate or fluoroethyl acetate; the electrolyte satisfies at least one of the following conditions based on the mass of the electrolyte:

[0022] (1) the mass percentage of ethyl acetate is E%, 1≤E≤20; preferably, 5≤E≤16;

[0023] (2) the mass percentage of fluoroethyl acetate is F%, 15≤F≤50; preferably, 20≤F≤30.

[0024] Based on the above scheme, the M element in the first substance can induce the formation of a solid electrolyte interface film (SEI film) rich in inorganic matter on the surface of the silicon-carbon particles from ethyl acetate and fluoroethyl acetate, reduce the consumption of lithium ions in the electrolyte, and improve the density and stability of the SEI film, thereby buffering the volume expansion of the silicon-carbon particles during charging and discharging. The fluoroethyl acetate can further improve the flexibility of the SEI film, further adapting to the volume change of the nano-silicon. The SEI film rich in Li element is also beneficial to improving the interfacial compatibility between the negative electrode and the electrolyte, improving the wettability of the electrolyte to the above-mentioned silicon-carbon particles, promoting the transmission of lithium ions on the interface between the silicon-carbon particles and the electrolyte, and thereby improving the expansion inhibition performance and rate performance of the secondary battery.

[0025] In some embodiments, the electrolyte comprises a first lithium salt selected from at least one of lithium bis(trifluoromethylsulfonylimide), lithium bis(fluorosulfonylimide), or lithium trifluoromethanesulfonate; the mass percentage of the first lithium salt in the electrolyte is S1%, 0.01≤S1≤1, based on the mass of the electrolyte. The present application provides more free lithium ions through the first lithium salt, which cooperates with the first substance in the surface region of the silicon-carbon particles to further improve the ionic conductivity of the secondary battery system, while also being beneficial to improving the wettability of the electrolyte to the silicon-carbon particles and participating in the formation of a more stable SEI film rich in LiF on the surface of the silicon-carbon particles, buffering the volume expansion of the silicon-carbon particles, thereby further improving the expansion inhibition performance and rate performance of the secondary battery.

[0026] The present application further provides an electronic device comprising the aforementioned secondary battery. Based on the aforementioned secondary battery, the electronic device of the present application can adapt to the condition of fast charging and discharging, and achieve excellent service life and safety. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A pore size distribution curve of a negative electrode material provided for Example 1-1 of the present application. DETAILED DESCRIPTION

[0028] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application and not to limit the present application.

[0029] In a first aspect, the present application provides a negative electrode material, comprising silicon-carbon particles, a cross-section of the silicon-carbon particles comprising a surface region and an internal region, the surface region containing a first substance, the first substance having ion conduction ability; the first substance containing an M element, the M element being selected from at least one of S element, N element, La element, Ti element, Zr element, Al element, and P element; the pore area proportion of the surface region in the cross-section of the silicon-carbon particles being greater than the pore area proportion of the internal region; the surface region referring to a region 0-500 nm away from the surface of the silicon-carbon particles, and the internal region referring to a region 600-1200 nm away from the surface of the silicon-carbon particles. By controlling the pore area proportion of the surface region to be greater than the pore area proportion of the internal region, i.e., controlling the pore channels in the surface region to have a larger pore size, the silane is difficult to be adsorbed in the larger pore channels during the deposition of nano-silicon, and the smaller specific surface area caused by the larger pore size also reduces the contact between the surface region and the silane gas, which is not conducive to the adhesion and deposition of nano-silicon, thus the deposition amount of nano-silicon in the surface region can be reduced, and nano-silicon is more likely to be deposited in the internal region deep in the porous carbon, thereby facilitating the improvement of the buffering effect of the volume expansion of nano-silicon during the deintercalation of lithium, the inhibition effect of the expansion of the negative electrode material, the reduction of the side reaction with the electrolyte, and the reduction of the damage to the surface of the silicon-carbon particles; and the first substance having ion conduction ability is easy to be combined in the larger pore channels of the surface region, which can construct a continuous and efficient lithium ion path in the surface region of the silicon-carbon particles, improve the kinetic performance of the negative electrode material, and the lithium ion path in the surface region also helps to disperse the expansion stress, while avoiding the severe side reaction caused by the direct contact of nano-silicon with the electrolyte, reducing the accumulation of by-products, and making the secondary battery exhibit excellent rate performance and inhibition performance.

[0030] In the present application, a thin sample of the silicon-carbon particles can be obtained by a focused ion beam (FIB), the thickness of the thin sample being 50-100 nm, and then the thin sample is observed by a transmission electron microscope (TEM) to compare the pore area of the surface region and the internal region.

[0031] In some embodiments, the silicon-carbon particles comprise porous carbon, the porous carbon comprises primary pores and secondary pores, the primary pores have a pore size of D1 nm, 0.5≤D1<2, the secondary pores have a pore size of D2 nm, 2≤D1≤15; based on the sum of the pore volume of the primary pores and the pore volume of the secondary pores, the pore volume percentage of the secondary pores is P%, 2≤P≤20. Preferably, 4≤P≤15. Exemplarily, P can be 2, 4, 5, 6, 9, 10, 11, 12, 14, 15, 17, 18, 19, 20 or a value within a range consisting of any two of them. The present application controls the pore volume percentage of the secondary pores in the sum of the pore volume of the primary pores and the pore volume of the secondary pores in the porous carbon in the above range, that is, the pore volume percentage of the rest of the primary pores is 80% to 98%, the pore volume percentage of the primary pores is much higher than that of the secondary pores, which can provide a higher silicon deposition capacity, thereby improving the lithium storage capacity of the silicon-carbon particles; in combination with the pore area percentage of the surface region being greater than that of the internal region, it is beneficial for more nano-silicon to be deposited in the internal region of the porous carbon beyond the 500 nm deep region, improving the buffering effect on the volume expansion of nano-silicon, more of the first substance being deposited in the surface region, improving the conduction capacity of the ion channel in the surface region, thereby further improving the rate performance and expansion inhibition performance of the secondary battery.

[0032] In some embodiments, the first substance is selected from lithium aluminate, lithium phosphate, Li x1 PO y1 N z1 , wherein 2.8≤x1≤3.3, 3.0≤y1≤3.8, 0.16≤z1≤0.46, Li7La3Zr2O 12 , Li 3x2 La 2 / 3-x2 TiO3, wherein 0.06≤x2≤0.16, lithium thiophosphate, Li 1+x3 Al x3 Ti 2-x3 (PO4)3, wherein 0≤x3≤0.7, Li6PS5Cl. The above first substance has high ionic conductivity, which can form a continuous and efficient lithium ion channel in the surface region of the silicon-carbon particles, thereby shortening the diffusion path of lithium ions in the silicon-carbon particles, accelerating the speed of lithium ion intercalation and deintercalation, and also can act as a buffer layer to reduce the volume expansion of the silicon-carbon particles, improve the wettability of the electrolyte to the silicon-carbon particles, thereby being beneficial to further improve the rate performance and expansion inhibition performance of the secondary battery.

[0033] In some embodiments, the mass percentage of the M element is C%, 0.3≤C≤6, based on the mass of the silicon-carbon particles; preferably, 0.6≤C≤4. Exemplarily, C can be 0.5, 0.7, 1.3, 2.4, 3.3, 4.1, 4.3, 5.5, 5.9, 6.0, or a value within a range defined by any two of them. When the mass percentage of the M element satisfies the above range, the conduction path and structural stability of the lithium ion channel can be further optimized, the diffusion rate of lithium ions in the surface region of the silicon-carbon particles can be improved, and the rate performance and swelling performance of the secondary battery can be improved.

[0034] In some embodiments, the silicon-carbon particles comprise porous carbon and nanosilicon located in the pores of the porous carbon; a carbon layer is arranged on the surface of the silicon-carbon particles, and the thickness of the carbon layer is T nm, 5≤T≤80; preferably, 10≤T≤50. Exemplarily, T can be 5, 8, 15, 22, 27, 31, 35, 44, 48, 53, 59, 66, 71, 80, or a value within a range defined by any two of them. By controlling at least part of the surface of the silicon-carbon particles to have a carbon layer, the present application can form an excellent electronic channel with the internal porous carbon, can improve the ionic conductivity and electronic conductivity of the silicon-carbon particles in cooperation with the first substance, and can improve the mechanical strength and toughness of the silicon-carbon particles, improve the buffering effect of the nanosilicon expansion during the charging and discharging process, and improve the structural stability of the silicon-carbon particles, thereby further improving the rate performance and swelling performance of the secondary battery.

[0035] In some embodiments, 20≤C×T≤60. Exemplarily, the value of C×T is 20, 23, 25, 29, 33, 37, 40, 41, 43, 44, 46, 47, 48, 50, 51, 53, 55, 56, 57, 59, 60, or a value within a range defined by any two of them. When the above range is satisfied, the mutual cooperation effect of the carbon layer and the first substance can be improved, and the rate performance and swelling performance of the secondary battery can be further improved.

[0036] The present application also provides a preparation method of the foregoing negative electrode material, at least comprising the following steps:

[0037] Step S1, calcining the precursor material at T1℃ in an inert gas protection atmosphere for t1 hours to obtain carbonized material; 550≤T1≤1600, 2≤t1≤10;

[0038] Step S2, after crushing the carbonized material, introducing a first activation gas at T2℃, and reacting for t2 hours to obtain a first activated material; 780≤T2≤950, 8≤t2≤20; the first activation gas is at least one of water vapor and carbon dioxide;

[0039] Step S3, the first activated material is heated to T3℃ under an inert gas protection atmosphere, and a second activating gas is introduced, and the reaction is carried out for t3 hours to obtain the porous carbon; 850≤T3≤1300, 2≤t3≤8; the second activating gas is at least one of water vapor, carbon dioxide or oxygen;

[0040] Step S4, the porous carbon is introduced into silane gas at T4℃, and is kept for t4 hours to obtain the silicon deposition particles; 380≤T4≤600, 2≤t4≤5.

[0041] Step S5, the silicon deposition particles are transferred into a dispersion medium under an inert gas protection atmosphere, a source of element M is added for dispersion, and then the dispersion medium is evaporated under a vacuum environment to obtain the silicon-carbon particles.

[0042] In some embodiments, the dispersion medium comprises at least one of 1,2-dimethoxyethane, tetrahydrofuran, acetonitrile, ethyl propionate, ethyl acetate, 2-methyltetrahydrofuran, and ethanol.

[0043] In some example embodiments, the preparation of the porous carbon comprises:

[0044] The precursor material is carbonized at 550-1600℃ for 2-10 hours under a nitrogen atmosphere to obtain carbonized material. Then the carbonized material is placed in a reaction furnace, heated to 780-950℃, and then water vapor is introduced for pore formation. After keeping for 8-20 hours, the first activated material is obtained. Then the first activated material is heated to 850-1300℃ under a nitrogen atmosphere, and then water vapor is introduced. After keeping for 2-8 hours, the porous carbon is obtained. Through the two-step activation process, the proportion of the pore area of the surface region in the cross section of the silicon-carbon particles is controlled to be greater than the proportion of the pore area of the internal region.

[0045] In some embodiments, the precursor is selected from at least one of a biomass precursor, a sugar precursor, a synthetic resin precursor, or a pitch precursor.

[0046] In some example embodiments, the preparation of the silicon-carbon particles comprises:

[0047] The porous carbon is placed in a fluidized bed device, nitrogen is introduced into the interior to completely fluidize, and then heated to 380-600℃; for example, the heating rate is 5℃ / min; after the temperature in the fluidized bed stabilizes, silane gas is introduced, and kept for 2-5 hours to obtain the silicon deposition particles, and then switch to nitrogen and gradually reduce to room temperature. The silicon deposition particles are transferred into anhydrous acetonitrile under nitrogen protection, and Li2S and P2S5 with a molar ratio of 7:3 are added thereto, stirred uniformly, and then heated and evaporated in a vacuum environment to obtain the silicon-carbon particles.

[0048] Based on the above scheme, the present application can make the pores in the surface region have a larger pore size through twice activation treatment, so as to control the pore area ratio of the surface region in the cross section of the silicon-carbon particles to be greater than that of the internal region, and set the first substance containing the M element in the surface region, so as to construct an efficient lithium ion channel and improve the buffering effect on the volume expansion of the nano-silicon, thereby further improving the rate performance and inhibiting the expansion performance of the secondary battery.

[0049] In some example embodiments, the preparation method further comprises: step S6, heating the silicon-carbon particles to T6 ℃ under nitrogen protection, then introducing acetylene gas, and maintaining for t6 h; 500≤T6≤700, 2≤t6≤8. Through step S6, a carbon layer can be arranged on the surface of the silicon-carbon particles, which can further improve the rate performance and inhibit the expansion performance of the secondary battery after cooperating with the first substance.

[0050] In some embodiments, step S6 comprises: transferring the silicon-carbon particles obtained in step S3 into a rotary furnace under nitrogen protection, introducing nitrogen for a period of time, then heating to 550 ℃ at a speed of 10 ℃ / min, continuously introducing acetylene gas, maintaining for 2-8 h, then cutting off the acetylene gas and cooling down, and the obtained material is the silicon-carbon particles with a carbon layer arranged on the surface.

[0051] The present application also provides a secondary battery, which comprises a positive electrode, a negative electrode and an electrolyte.

[0052] Secondary battery

[0053] The secondary battery of the present application is not particularly limited. According to the type of the electron transport substance, it is classified into various categories. For example, when the electron transport substance is lithium (Li, including ions), the secondary battery is a lithium ion battery; when the electron transport substance is sodium (Na, including ions), the secondary battery is a sodium ion battery.

[0054] According to an embodiment of the present application, the secondary battery can include a battery cell and an electrolyte. The battery cell can include a packaging material and an electrode assembly disposed inside the packaging material, and the electrolyte can be filled in an internal space formed by the packaging material. The packaging material can protect the electrode assembly from external impact and prevent the electrolyte from leaking to the outside. According to the shape of the packaging material, the battery cell can be classified into a prismatic, cylindrical or pouch type.

[0055] The electrode assembly includes a positive electrode, a negative electrode and a separator, as well as other components known in the art in a secondary battery, and the present application does not limit the above-mentioned other components. Among them, the separator can be between the positive electrode and the negative electrode.

[0056] The method for manufacturing the secondary battery is not particularly limited in the present application, and for example, can include the steps of stacking a positive electrode, a separator, and a negative electrode in this order, and performing a winding, folding, or the like as necessary to obtain an electrode assembly, placing the electrode assembly in a packaging material, injecting an electrolyte into the packaging material, and sealing to obtain a secondary battery.

[0057] positive electrode

[0058] In the present application, the positive electrode is not particularly limited as long as the object of the present application can be achieved. The positive electrode includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector; the "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector in the thickness direction thereof, or can be located on both surfaces of the positive electrode current collector in the thickness direction thereof. It should be noted that the "surface" herein can be the entire area of the surface of the positive electrode current collector, or can be a partial area of the surface of the positive electrode current collector, and the present application is not particularly limited as long as the object of the present application can be achieved.

[0059] The type, size, and shape of the positive electrode current collector are not particularly limited in the present application as long as it does not cause a chemical change in the battery cell and has electrical conductivity. For example, the positive electrode current collector can use, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or a substance for surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, or silver, or the like. In the present application, the positive electrode current collector can also contain a non-metallic element, for example, the non-metallic element includes at least one of fluorine, phosphorus, boron, chlorine, silicon, and sulfur.

[0060] The positive electrode current collector can have an appropriate thickness as necessary. Although not particularly limited, the positive electrode current collector can have a thickness in the range of 1 μm to 500 μm, or can have a thickness in the range of 1 μm to 300 μm, or can have a thickness in the range of 1 μm to 100 μm, or can have a thickness in the range of 1 μm to 50 μm, or can have a thickness in the range of 1 μm to 20 μm.

[0061] Unless otherwise specifically stated, the terms thickness (or height), width, and length used in the present application refer to average values, and can be measured by a measuring instrument that can measure the thickness (or height), width, and length, respectively, and according to the method in the art.

[0062] The positive electrode current collector can form fine concavities and convexities on the surface, thereby further enhancing the adhesion to the positive electrode material layer. For example, the positive electrode current collector can be in a form selected from one or more of a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0063] In the present application, the positive electrode material layer includes a positive electrode active material, and the present application does not have a particular limitation on the type of positive electrode active material as long as the purpose of the present application can be achieved. For example, the positive electrode active material can include at least one of lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2(NCM 955 ), NCM 811 , NCM 622 , NCM 523 , NCM 111 ), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel lithium manganate, spinel lithium nickel manganate, and lithium titanate. In the present application, the positive electrode active material can also include a non-metallic element, for example, the non-metallic element includes at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In the present application, the thickness of the positive electrode current collector and the positive electrode material layer is not particularly limited as long as the purpose of the present application can be achieved.

[0064] In some embodiments, the positive electrode material layer can also include a positive electrode binder. The present application does not have a particular limitation on the type of positive electrode binder as long as the purpose of the present application can be achieved, for example, the positive electrode binder can include, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyolefin, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified styrene butadiene rubber (SBR), polyolefin ester, polyolefin alcohol, polyacrylic acid, or polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene or polypropylene.

[0065] In some embodiments, the positive electrode material layer can further include a conductive agent. The present application does not have a particular limitation on the type of conductive agent in the positive electrode material layer as long as the purpose of the present application can be achieved. In some exemplary embodiments, the conductive agent includes a carbon-based material, such as graphite, for example, natural graphite or artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, or carbon fiber; a conductive polymer, for example, a polyphenylene derivative; a conductive metal oxide, for example, zinc oxide, titanium oxide, etc.; a conductive whisker, for example, potassium titanate, etc.; or a mixture formed by any combination of these substances.

[0066] In the present application, the positive electrode material layer can be formed by applying a positive electrode slurry to at least one side of the positive electrode current collector and drying, and calendering can be performed after drying as necessary. The positive electrode slurry contains the positive electrode active material, the positive electrode binder, and the conductive agent described above. In addition, the positive electrode slurry can also contain a solvent, and the kind of the solvent is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the solvent can use N-methyl-2-pyrrolidone.

[0067] The mass ratio of the positive electrode active material, the conductive agent, and the positive electrode binder in the positive electrode material layer is not particularly limited in the present application, and can be selected by a person skilled in the art as necessary as long as the object of the present application can be achieved, and these mass ratios can apply the known mass ratios.

[0068] Negative electrode

[0069] The negative electrode is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the negative electrode contains a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. In the present application, the negative electrode material layer can be provided on one surface in the thickness direction of the negative electrode current collector, or can be provided on both surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the negative electrode current collector, or can be a partial area of the negative electrode current collector, and the present application is not particularly limited as long as the object of the present application can be achieved.

[0070] The kind, size, and shape of the negative electrode current collector are not particularly limited in the present application as long as it does not cause chemical changes in the battery and has electrical conductivity. For example, the negative electrode current collector can use, for example, stainless steel, copper, nickel, titanium, calcined carbon, or a substance for surface treatment of the surface of copper or stainless steel with carbon, nickel, titanium, or silver, etc.

[0071] The negative electrode current collector can have an appropriate thickness as necessary. Although not particularly limited, the negative electrode current collector can have a thickness in the range of 1 μm to 500 μm, or can have a thickness in the range of 1 μm to 300 μm, or can have a thickness in the range of 1 μm to 100 μm, or can have a thickness in the range of 1 μm to 50 μm, or can have a thickness in the range of 1 μm to 20 μm, or can have a thickness in the range of 5 μm to 10 μm.

[0072] The negative electrode current collector can form fine concave-convex on the surface, so that the adhesion with the negative electrode material layer can be further enhanced. For example, the negative electrode current collector can be in the form of one or more selected from the group consisting of a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0073] In some embodiments, the negative electrode material can further include other materials, for example, carbon materials including but not limited to graphite (artificial graphite, natural graphite, or graphitized carbon fiber) or amorphous carbon, and the like; metals that can be alloyed with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys, or alloys thereof with lithium; SiO β (0 < β ≤ 2), SnO, SnO2, vanadium oxides, lithium vanadium oxides, and the like, metal oxides that can be doped or undoped with lithium or alloys thereof with lithium; or composites containing the metal and carbon materials such as Si-C composites or Sn-C composites; or lithium titanate with spinel structure TiO2-Li4Ti5O 12 , and any one of them or a mixture of two or more thereof can be used. Specifically, carbon materials can use low-crystalline carbon and high-crystalline carbon, and the like. Representative low-crystalline carbon is soft carbon and hard carbon. Examples of high-crystalline carbon can be amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, primary graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, carbon microsphere (mesocarbon microbead), mesophase pitch, and high-temperature calcined carbon such as petroleum and coal-based coke (coke derived from petroleum or coal tar pitch), and the like.

[0074] The negative electrode material layer in the present application further includes a negative electrode binder. The kind of the negative electrode binder in the present application is not particularly limited as long as the purpose of the present application can be achieved, for example, the negative electrode binder can include but is not limited to at least one selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol, styrene butadiene rubber (SBR), polyethylene oxide, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, cellulose acetate, diacetyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyarylate, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0075] The kind of the conductive agent in the negative electrode material layer is not particularly limited in the present application, as long as the purpose of the present application can be achieved. In some exemplary embodiments, the conductive agent includes a carbon-based material, such as graphite, e.g. natural graphite or artificial graphite, carbon black, e.g. acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; a metal-based material, such as metal powder or metal fiber of copper, nickel, aluminum, silver, etc.; a conductive polymer, such as polyphenylene derivative; a conductive metal oxide, such as zinc oxide, titanium oxide, etc.; a conductive whisker, such as potassium titanate, etc.; or a mixture formed by any combination of these substances.

[0076] The mass ratio of the negative electrode material, the negative electrode conductive agent, and the negative electrode binder in the negative electrode material layer is not particularly limited in the present application, and can be selected by a person skilled in the art according to actual needs, as long as the purpose of the present application can be achieved. These mass ratios can use the known mass ratios.

[0077] In the present application, the negative electrode material layer can be formed by coating a negative electrode slurry on at least one side of the negative electrode current collector and drying, and calendering can be performed after drying as needed. The negative electrode slurry contains the above-mentioned negative electrode material and negative electrode binder, and can further contain a conductive agent as needed. In addition, the negative electrode slurry can also contain a solvent, and the kind of the solvent is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the solvent can use deionized water.

[0078] Separator

[0079] The separator of the present application is a film for preventing short circuit between the positive electrode and the negative electrode while allowing the electronic transport substance to pass through, and the separator is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g. polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, aramid; and the type of the separator can include at least one of woven film, non-woven film, microporous film, composite film, calendered film, and spunlaced film.

[0080] According to some embodiments of the present application, the separator can include a base material layer and a surface treatment layer. The base material layer can be a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Optionally, 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 can be used.

[0081] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, which can be a polymer layer or an inorganic layer, or a layer formed by mixing polymer and inorganic material. For example, the inorganic layer includes inorganic particles and a binder, which are not particularly limited in the present application, and can include at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium sulfate. The binder is not particularly limited in the present application, and can be at least one of the positive electrode binder or the negative electrode binder described above. The polymer layer includes a polymer, which is not particularly limited in the present application, and can include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the separator is not particularly limited as long as the object of the present application is achieved, and can be, for example, 5 μm to 500 μm.

[0082] Electrolyte

[0083] In the present application, the electrolyte refers to a medium that causes the movement of an electron transport substance to smoothly proceed the electrochemical reaction of the positive electrode and the negative electrode. The electrolyte can use a commonly used organic liquid electrolyte, an inorganic liquid electrolyte, a gel-type polymer electrolyte, a molten-type inorganic electrolyte, etc., but is not limited thereto. A solid-state electrolyte such as a gel-type polymer electrolyte can also be used instead of the electrolyte. A battery using a solid-state electrolyte is generally referred to as a solid-state battery or a full solid-state battery. The liquid electrolyte (electrolyte) generally includes a non-aqueous solvent and a lithium salt.

[0084] In some embodiments, the electrolyte includes at least one of ethyl acetate or fluoroethyl acetate; the mass ratio of ethyl acetate is E% based on the mass of the electrolyte, 1≤E≤20; preferably, 5≤E≤16; for example, E can be a value within a range consisting of 1, 2, 3, 4, 6, 8, 9, 10, 13, 14, 15, 16, 18, 20, or any two thereof. The mass ratio of fluoroethyl acetate is F% based on the mass of the electrolyte, 15≤F≤50; preferably, 20≤F≤30; for example, F can be a value within a range consisting of 15, 16, 19, 22, 25, 28, 31, 33, 36, 37, 41, 42, 45, 47, 50, or any two thereof.

[0085] Based on the above scheme, the M element in the first substance can induce the formation of a solid electrolyte interface film (SEI film) rich in inorganic matter on the surface of the silicon-carbon particles from ethyl acetate and ethyl fluoroacetate, reduce the consumption of lithium ions in the electrolyte, and improve the density and stability of the SEI film, thereby buffering the volume expansion of the silicon-carbon particles during the charging and discharging process. The ethyl fluoroacetate can further improve the flexibility of the SEI film, further adapting to the volume change of the nano-silicon. The SEI film rich in Li element is also beneficial to improving the interfacial compatibility between the negative electrode and the electrolyte, improving the wettability of the electrolyte to the above-mentioned silicon-carbon particles, promoting the transmission of lithium ions on the interface between the silicon-carbon particles and the electrolyte, and thereby improving the expansion inhibition performance and rate performance of the secondary battery.

[0086] In some embodiments, the electrolyte comprises a first lithium salt selected from at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bisfluorosulfonylimide, or lithium trifluoromethanesulfonate; and the mass fraction of the first lithium salt in the electrolyte is S1%, 0.01≤S1≤1. Illustratively, S1 can be 0.01, 0.07, 0.13, 0.24, 0.27, 0.32, 0.42, 0.49, 0.55, 0.66, 0.73, 0.83, 0.86, 0.92, 1, or a value within a range formed by any two of them. The present application provides more free lithium ions through the first lithium salt, which, in cooperation with the first substance on the surface area of the silicon-carbon particles, can further improve the ionic conductivity of the secondary battery system, while also being beneficial to improving the wettability of the electrolyte to the silicon-carbon particles and forming a more stable SEI film rich in LiF on the surface of the silicon-carbon particles, buffering the volume expansion of the silicon-carbon particles, thereby further improving the expansion inhibition performance and rate performance of the secondary battery.

[0087] According to some embodiments of the present application, the lithium salt can also include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate {LiB(C2O4)2, LiBOB}, lithium difluoro(oxalato)borate {LiBF2(C2O4), LiDFOB}, LiNO3, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiC(SO2CF3)3, Li2SiF6, or lithium difluoroborate. The content of the lithium salt in the electrolyte is not limited in the present application, as long as the purpose of the present application can be achieved.

[0088] The non-aqueous solvent is not particularly limited. For example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluoro-carbonate compound. The chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or ethyl methyl carbonate (EMC). The cyclic carbonate can include, but is not limited to, at least one of propylene carbonate (PC), butylene carbonate, or vinyl ethylene carbonate. The fluoro-carbonate compound can include, but is not limited to, at least one of 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The carboxylate compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (1,2-dimethoxy ethane, DME), dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-diethoxy ethane, 1-ethoxy-1-methoxy ethane, 2-methyl tetrahydrofuran, or tetrahydrofuran.

[0089] Electronic device

[0090] The present application further provides an electronic device including the secondary battery described above. Based on the secondary battery described above, the electronic device of the present application can adapt to the condition of fast charge and discharge, and achieve excellent service life and safety.

[0091] The electronic device of the present application is not particularly limited, and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input type computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0092] Measurement method

[0093] The present application can be measured by the following method, and the physical properties in the following examples and comparative examples are measured by the following method.

[0094] Pore size distribution, pore volume test

[0095] The porous carbon is tested by using a physical adsorption instrument (model: ipore 620), and the process includes: taking 0.15 g of porous carbon as a sample and placing it in a sample tube, first degassing at 200°C for 6h, then testing the adsorption amount of argon by the sample under different pressures, thereby drawing the isothermal adsorption curve of the sample, and then using BET fitting to calculate the specific surface area of the sample, using non-local density functional theory (NLDFT) fitting to calculate the pore volume and pore size distribution of the sample, and then calculating the pore volume ratio of the first type of pores and the second type of pores based on the pore volume.

[0096] M element and mass ratio test

[0097] The ICP method is used for element content analysis, and the ICP-OES (PE Avio 200) device is used to measure the mass ratio of M elements in the silicon-carbon particles.

[0098] Carbon layer thickness test

[0099] After obtaining a thin sample of the silicon-carbon particles by FIB, the thickness of the carbon layer is measured by TEM, and the thickness of the carbon layer is measured at 10 different positions such as edges and centers and the arithmetic mean value is calculated as the thickness of the carbon layer.

[0100] Ion conductivity test

[0101] The ion conductivity test is performed by using the GITT method, and the process includes: 1) mixing the silicon-carbon particles, the binder (polyacrylate), and the conductive agent (carbon nanotubes) in a ratio of 8:1:1 to make a slurry, then coating, drying, cold pressing, punching, and assembling into a lithium half-battery; 2) performing the first cycle of the half-battery at a rate of 0.1C, with a voltage range of 0.005V to 2.0V; 3) discharging the half-battery at a rate of 0.1C to 0.005V, then discharging at a rate of 0.1C for 10 min, standing for 1h, and repeating the above steps until the voltage is ≤0.005V; 4) extracting the steady-state voltage and calculating the ion conductivity (ion diffusion coefficient) D using the following formula:

[0102]

[0103] Cycling expansion performance test

[0104] The lithium ion full battery is placed in a thermostat at 25℃±1℃ for 30 minutes, charged at a current of 0.5C to 4.35V, then charged at a constant voltage of 4.35V to 0.025C, and then placed for 5 minutes, and then discharged at 0.5C to 3.0V, which is a one-time charge-discharge cycle process, and the initial thickness H0 of the lithium ion full battery is recorded. Then, according to the above cycle process, 600 cycles are cycled. The thickness H1 after the 600th cycle is recorded.

[0105] The cycle expansion rate = H0 / H1 x 100%.

[0106] Rate performance test

[0107] The lithium ion full battery is placed at a test temperature of 25℃ for 5 minutes, then charged at a current of 0.5C to 4.35V, then charged at a constant voltage of 4.35V to 0.025C; placed for 5 minutes, then discharged at a current of 0.2C to 3.0V, and the 0.2C discharge capacity is recorded; then placed for 5 minutes, repeat the above charging process, and then discharged at a current of 2C, and the 2C discharge capacity is recorded.

[0108] The rate capacity retention rate = 2C discharge capacity / 0.2C discharge capacity x 100%.

[0109] The following examples are used to illustrate the application. Unless otherwise specified, the raw materials used in the following examples are all from ordinary commercial products, and the devices or equipment used are all purchased from conventional market channels.

[0110] Example 1-1

[0111] Method for preparing silicon-carbon particles:

[0112] Step S1, phenolic resin is used as a precursor material and carbonized at 900℃ for 2h under a nitrogen atmosphere to obtain carbonized material.

[0113] Step S2, then the carbonized material is placed in a reaction furnace and heated to 860℃, then water vapor is introduced to form pores, and after 14h of heat preservation, a first activated material is obtained.

[0114] Step S3, then the first activated material is heated to 1100℃ under a nitrogen atmosphere, then water vapor is introduced, and after 5h of heat preservation, a porous carbon is obtained.

[0115] Step S4, the porous carbon is placed in a fluidized bed device, nitrogen is introduced into the interior to completely fluidize, and then heated to 500℃ at a heating rate of 5℃ / min; after the temperature in the fluidized bed stabilizes, silane gas is introduced, and after 3 hours of heat preservation, silicon particles are obtained, and then nitrogen is switched and gradually reduced to room temperature.

[0116] Step S5, the silicon particles are transferred into anhydrous acetonitrile under nitrogen protection, Li2S and P2S5 are added in a molar ratio of 7:3, and the mixture is stirred and then heated and evaporated under vacuum to obtain silicon-carbon particles.

[0117] Preparation of the negative electrode:

[0118] The silicon-carbon particles are mixed with artificial graphite at a mass ratio of 10:90 to obtain a negative electrode active material. The negative electrode active material (95wt%), carbon nanotubes (0.5wt%), polymethyl acrylate (3.5%), and sodium carboxymethyl cellulose (1%) are mixed, then deionized water is added and stirred to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on one surface of a copper foil, and after drying, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. The coated copper foil is dried, pressed, and cut into a specified size, and the tabs are welded to obtain the negative electrode.

[0119] Preparation of the positive electrode:

[0120] Lithium cobalt oxide (97wt%), conductive carbon black (1.5wt%), and polyvinylidene fluoride (1.5wt%) are mixed in N-methyl pyrrolidone to prepare a positive electrode slurry. The positive electrode slurry is uniformly coated on one surface of an aluminum foil, and after drying, the coating steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. The coated aluminum foil is dried, pressed, and cut into a specified size, and the tabs are welded to obtain the positive electrode.

[0121] Preparation of the separator: a 12μm thick polyethylene (PE) microporous film is selected as the separator.

[0122] Preparation of the electrolyte:

[0123] In an argon atmosphere glove box with a water content of less than 10ppm, ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a mass ratio of 1:1 to obtain a base solvent, then ethyl fluoroacetate and lithium hexafluorophosphate are added to the base solvent, and the mixture is stirred to obtain an electrolyte. Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate is 12.5%, the mass content of ethyl fluoroacetate is 20%, and the rest is the base solvent.

[0124] Preparation of the lithium ion full cell:

[0125] The positive electrode, the separator, and the negative electrode are sequentially stacked with the separator between the positive electrode and the negative electrode to play a separating role, and the electrode assembly is obtained by winding. The electrode assembly is placed in an outer packaging aluminum plastic film, baked, and then the above electrolyte is injected. After vacuum packaging, standing, formation, shaping, capacity testing, and other processes, the lithium ion full cell is obtained.

[0126] Examples 1-2 to 1-21, Comparative Examples 1-1 to 1-5

[0127] The difference from Example 1-1 is only that the mass percentage C% of M element, the pore volume percentage P% of the second type of pores and the thickness of the carbon layer are adjusted according to Table 1. The specific adjustment parameters and performance test results are shown in Table 1. In Table 1, the pore area percentage of the surface region is greater than that of the internal region by the second activation of step S3, the mass percentage C% of M element is increased by increasing the amount of M source added; the pore volume percentage P% of the second type of pores in the porous carbon is increased by increasing the reaction temperature in step S3 and prolonging the reaction time; the pore area percentage of the surface region is smaller than that of the internal region, that is, the pore volume percentage of the larger pore diameter pores in the surface region is smaller, so the M element combined in the surface region is also smaller.

[0128] The thin slice samples of the silicon-carbon particles of Example 1-1 and Comparative Example 1-1 are prepared by focused ion beam (FIB), and the thin slice samples are observed by transmission electron microscope (TEM). By comparing the pore area of the surface region and the internal region, it can be known that the pore area of the surface region of the thin slice sample of the silicon-carbon particles of Example 1-1 is greater than that of the internal region, and the pore area of the surface region of the thin slice sample of the silicon-carbon particles of Comparative Example 1-1 is smaller than that of the internal region.

[0129] The pore size distribution curve of the porous carbon of Example 1-1 is tested by a physical adsorption instrument as shown in Figure 1 It can be known that the porous carbon includes the first type of pores and the second type of pores, the pore diameter of the first type of pores is D1 nm, 0.5≤D1<2, the pore diameter of the second type of pores is D2 nm, 2≤D1≤15, and the pore volume percentage P% of the second type of pores based on the sum of the pore volume of the first type of pores and the pore volume of the second type of pores is 9%.

[0130] The preparation method of the silicon-carbon particles of Examples 1-16 to 1-21 further comprises: step S6, transferring the silicon-carbon particles into a rotary furnace under the protection of nitrogen, increasing the temperature to 550℃ at a speed of 10℃ / min after passing in nitrogen for a period of time, continuously passing in acetylene gas, cutting off the acetylene gas after heat preservation for a period of time and cooling down, to obtain silicon-carbon particles provided with a carbon layer on the surface. The heat preservation time in step S6 is increased to further increase the thickness T nm of the carbon layer.

[0131] Table 1

[0132]

[0133]

[0134] As can be seen from Table 1, the present application controls the pore area of the surface area in the cross section of the silicon-carbon particles to be greater than the pore area of the internal area, cooperates with the first substance containing the M element, and can make the secondary battery exhibit excellent rate performance and swelling suppression performance. In particular, when the mass percentage C% of the M element in the silicon-carbon particles is further controlled to satisfy 0.3≤C≤6, the secondary battery can exhibit more excellent rate performance and swelling suppression performance. In particular, when 0.6≤C≤4 is satisfied, the swelling suppression performance and rate performance of the secondary battery can be further improved.

[0135] In particular, in the porous carbon of the silicon-carbon particles, the pore volume percentage P% of the secondary pores based on the sum of the pore volume of the first type of pores and the pore volume of the second type of pores satisfies 2≤P≤20, which can further improve the swelling suppression performance and rate performance of the secondary battery. In particular, when 4≤P≤15 is satisfied, the rate performance and swelling suppression performance of the secondary battery can be further improved.

[0136] In particular, when a carbon layer is further provided on the surface of the silicon-carbon particles, and the thickness T nm of the carbon layer is controlled to satisfy 5≤T≤80, the rate performance and swelling suppression performance of the secondary battery can be further improved. In particular, when 10≤T≤50 is satisfied, the rate performance and swelling suppression performance of the secondary battery can be further improved. In particular, when 20≤C×T≤60 is satisfied, the mutual cooperation effect of the carbon layer and the first substance can be improved, and the rate performance and swelling suppression performance of the secondary battery can be further improved.

[0137] Examples 2-1 to 2-13

[0138] The difference compared with Example 1-19 is only that the types and mass percentages of ethyl acetate, fluoroethyl acetate and the first lithium salt in the electrolyte are adjusted according to Table 2, and the specific adjustment parameters and performance test results are shown in Table 2. At the same time, the content of the base solvent is adjusted adaptively, wherein the mass ratio of EC and DEC is unchanged.

[0139] Table 2

[0140]

[0141]

[0142] As can be seen from Table 2, the mass percentage F% of ethyl fluoroacetate in the electrolyte satisfies 15≤F≤50, which is conducive to improving the expansion inhibition performance and rate performance of the secondary battery. In particular, when 20≤F≤30 is satisfied, the expansion inhibition performance and rate performance of the secondary battery can be further improved. In particular, when ethyl acetate is further added to the electrolyte, and the mass percentage E% of ethyl acetate satisfies 1≤E≤20, the expansion inhibition performance and rate performance of the secondary battery can be further improved. In particular, when 5≤E≤16 is satisfied, the expansion inhibition performance and rate performance of the secondary battery can be further improved.

[0143] In particular, the first lithium salt is added to the electrolyte, and the mass percentage S1% satisfies 0.01≤S1≤1, which can further improve the expansion inhibition performance and rate performance of the secondary battery.

[0144] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the principles of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material includes silicon-carbon particles, the cross-section of which includes a surface region and an internal region. The surface region contains a first substance, which has ion conductivity. The first substance contains element M, which is selected from at least one of elements S, N, La, Ti, Zr, Al, P, and Cl. The proportion of the hole area in the surface region is greater than the proportion of the hole area in the internal region; The surface region refers to the area from 0 nm to 500 nm away from the surface of the silicon-carbon particles, and the inner region refers to the area from 600 nm to 1200 nm away from the surface of the silicon-carbon particles.

2. The negative electrode material according to claim 1, characterized in that, The silicon-carbon particles contain porous carbon, which includes Class I pores and Class II pores. The pore diameter of the Class I pores is D1 nm, 0.5 ≤ D1 < 2, and the pore diameter of the Class II pores is D2 nm, 2 ≤ D1 ≤ 15. Based on the sum of the pore volumes of the Class I pores and the Class II pores, the pore volume percentage of the Class II pores is P%, 2 ≤ P ≤ 20.

3. The negative electrode material according to claim 2, characterized in that, 4≤P≤15。 4. The negative electrode material according to any one of claims 1 to 3, characterized in that, The first substance is selected from lithium alumina, lithium phosphate, and Li. x1 PO y1 N z1 Where 2.8≤x1≤3.3, 3.0≤y1≤3.8, 0.16≤z1≤0.46, Li7La3Zr2O 12 Li 3x2 La 2 / 3-x2 TiO3, where 0.06≤x2≤0.16, lithium thiophosphate, Li 1+x3 Al x3 Ti 2-x3 (PO4)3, wherein 0≤x3≤0.7, and at least one of Li6PS5Cl.

5. The negative electrode material according to any one of claims 1 to 3, characterized in that, Based on the mass of the silicon-carbon particles, the mass percentage of element M is C%, 0.3 ≤ C ≤ 6.

6. The negative electrode material according to claim 5, characterized in that, 0.6≤C≤4。 7. The negative electrode material according to claim 5, characterized in that, The silicon-carbon particles comprise porous carbon and nano-silicon located within the pores of the porous carbon; a carbon layer is disposed on the surface of the silicon-carbon particles, the thickness of the carbon layer being T nm, where 5 ≤ T ≤ 80.

8. The negative electrode material according to claim 7, characterized in that, 10≤T≤50。 9. The negative electrode material according to claim 7, characterized in that, 20≤C×T≤60.

10. The method for preparing the negative electrode material according to any one of claims 1 to 9, characterized in that, At least the following steps are included: Step S1: Calcine the precursor material at T1℃ in an inert gas protective atmosphere for t1 hours to obtain carbonized material; 550≤T1≤1600, 2≤t1≤10; Step S2: After the carbonized material is crushed, a first activating gas is introduced at T2℃ and reacted for t2 hours to obtain the first activated material; 780≤T2≤950, 8≤t2≤20; the first activating gas is at least one of water vapor and carbon dioxide; Step S3: The first activated material is heated to T3°C in an inert gas protective atmosphere, and a second activated gas is introduced. The reaction is carried out for t3 hours to obtain porous carbon; 850≤T3≤1300, 2≤t3≤8; the second activated gas is at least one of water vapor, carbon dioxide or oxygen. Step S4: Pass the porous carbon into silane gas at T4°C and keep it at that temperature for t4 hours to obtain precipitated silicon particles; 380≤T4≤600, 2≤t4≤5; Step S5: Transfer the silicon particles to a dispersion medium in an inert gas protective atmosphere, add an M element source and disperse, then place the dispersion medium in a vacuum environment to evaporate, thereby obtaining the silicon-carbon particles.

11. The preparation method according to claim 10, characterized in that, The preparation method further includes: Step S6: Heat the silicon-carbon particles to T5°C in an inert gas protective atmosphere, then introduce acetylene gas and keep them at that temperature for t5 hours; 500≤T5≤700, 2≤t5≤8.

12. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte; the negative electrode 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 material layer comprises the negative electrode material according to any one of claims 1 to 9 or the negative electrode material prepared by the preparation method according to claim 10 or 11.

13. The secondary battery according to claim 12, characterized in that, The electrolyte includes at least one of ethyl acetate or ethyl fluoroacetate; Based on the mass of the electrolyte, the electrolyte satisfies at least one of the following conditions: (1) The mass percentage of the ethyl acetate is E%, 1≤E≤20; (2) The mass percentage of the fluoroethyl acetate is F%, 15≤F≤50.

14. The secondary battery according to claim 13, characterized in that, Based on the mass of the electrolyte, the electrolyte satisfies at least one of the following conditions: （1）5≤E≤16； （2）20≤F≤30。 15. The secondary battery according to claim 12, characterized in that, The electrolyte includes a first lithium salt, which is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, or lithium trifluoromethanesulfonate. Based on the mass of the electrolyte, the mass percentage of the first lithium salt is S1%, where 0.01 ≤ S1 ≤ 1.

16. An electronic device, characterized in that, The secondary battery includes any one of claims 12 to 15.

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