Negative electrode composite, negative electrode sheet, secondary battery, and electric device

Abstract

The application provides a negative electrode composite material, a negative electrode sheet, a secondary battery and an electric device. The negative electrode composite material comprises graphite and hard carbon, wherein the D50 particle size of the graphite is D Gr , the D50 particle size of the hard carbon is D HC , and D HC ≤0.366D Gr ; the D HC and the D Gr are measured in the same unit. The above negative electrode composite material can not only simultaneously exert the electrochemical performance advantages of graphite materials and hard carbon materials, but also has high uniformity of pore distribution in the negative electrode composite material and high compaction density due to the specific particle size matching, and can be used to provide a secondary battery with fast charging performance, high safety performance and high energy density.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode composite material, a negative electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] Graphite material is widely used as a negative electrode active material due to its high initial coulomb efficiency, low lithium intercalation potential and strong cycle stability, but active ions can only be deintercalated / intercalated from the end face of graphite grains, resulting in a low active diffusion coefficient and poor kinetics, and the phenomenon of lithium precipitation on the negative electrode is prone to occur when the battery is rapidly charged. Hard carbon material is a carbon material that is difficult to graphitize, and its structure contains many small graphite sheet structures, as well as amorphous regions and microporous structures, which is a short-range disorder and long-range order structure, and its interlayer spacing is larger than that of graphite, and for active ions, it is an isotropic material with good kinetics. However, the specific surface area of hard carbon material is large, and the surface active sites are too many, resulting in too large irreversible capacity, low initial coulomb efficiency, affecting the energy density of the battery, fast capacity decay, and difficult to use as a main material.

[0003] In the industry, a battery with fast charging capability and high energy density is obtained by compounding graphite material and hard carbon material, but they do not explore the particle size adaptability of graphite material and hard carbon material, and the particle size of graphite and hard carbon is not properly matched, resulting in uneven distribution of voids in the composite material, which not only seriously affects the compaction density of the material, but also is not conducive to the infiltration of the electrolyte into the composite material and the subsequent transmission of active ions, and even causes uneven lithium intercalation of the negative electrode sheet, resulting in lithium precipitation on the negative electrode. SUMMARY

[0004] Therefore, the present application provides a negative electrode composite material which can not only simultaneously exert the electrochemical performance advantages of graphite material and hard carbon material, but also has a high degree of uniformity of pore distribution inside the negative electrode composite material due to the specific particle size matching, and has a high compaction density, which can be used to provide a secondary battery with fast charging performance, high safety performance and high energy density.

[0005] The first aspect of the present application provides a negative electrode composite material, comprising graphite and hard carbon, wherein the D50 particle size of the graphite is D Gr , the D50 particle size of the hard carbon is D HC , and D HC ≤0.366D Gr ; and the D HC and the D Gr are measured in the same unit.

[0006] Because the D50 particle sizes of the graphite and the hard carbon satisfy D HC ≤0.366D GrTherefore, in the negative electrode composite material, the hard carbon can be uniformly distributed in the pores formed by the stacking of graphite particles, so that the material has good rate performance and can achieve high compaction density; at the same time, the negative electrode composite material has high uniformity of pore distribution, and the electrolyte seepage channels and active ion diffusion channels are uniformly distributed in the material, so that when the negative electrode composite material is applied to the negative electrode sheet, the risk of lithium precipitation in the negative electrode can be significantly reduced.

[0007] The second aspect of the present application provides a negative electrode sheet comprising the negative electrode composite material provided in the first aspect of the present application. Due to the negative electrode composite material provided in the first aspect of the present application, the electrode sheet can be used to provide a secondary battery with fast charging performance, high safety performance and high energy density.

[0008] The third aspect of the present application provides a secondary battery comprising the negative electrode sheet provided in the second aspect of the present application. The secondary battery can achieve fast charging, high safety performance and high energy density.

[0009] The fourth aspect of the present application provides a power consumption device comprising the secondary battery provided in the third aspect of the present application. The power consumption device has good endurance and fast charging capability. BRIEF DESCRIPTION OF DRAWINGS

[0010] Figure 1 The powder compaction density curve of the negative electrode composite material or the negative electrode material provided for Examples 1-2 and Comparative Examples 1-3;

[0011] Figure 2 The electrolyte infiltration curve of the negative electrode sheet provided for Examples 1-2 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0012] The negative electrode composite material provided in the embodiments of the present application comprises graphite and hard carbon, the D50 particle size of the graphite is D Gr , the D50 particle size of the hard carbon is D HC , and D HC ≤0.366D Gr ; the D HC and the D Gr are measured in the same unit.

[0013] After a large number of theoretical model designs and experiments, the applicant found that when the D50 particle sizes of the graphite and the hard carbon satisfy D HC ≤0.366D GrAt this time, the hard carbon can be uniformly distributed in the pores formed by the stacking of graphite particles, and the internal pore distribution of the negative electrode composite is high, which can not only improve the wettability of the electrolyte to the negative electrode composite, but also build sufficient and uniformly distributed active ion diffusion channels in the internal of the negative electrode composite, so that the hard carbon material can fully exert its kinetic performance, which is conducive to achieving the fast charging performance of the negative electrode composite, and can greatly reduce the risk of lithium precipitation of the negative electrode composite caused by uneven lithium intercalation, and improve the safety performance of the final battery. In addition, the negative electrode composite has a reasonable pore distribution, which is also conducive to the deintercalation / intercalation of active ions of the graphite material, so as to fully exert the high initial coulombic efficiency of the graphite.

[0014] In the present application, the morphology of graphite and hard carbon is not specifically limited, and can be a granular material. Both graphite and hard carbon can be composed of primary particles and secondary particles, or can be composed of secondary particles only. Among them, the secondary particles refer to particles formed by aggregation of multiple primary particles. Both the primary particles and the secondary particles satisfy the D50 particle size defined in the present application.

[0015] In the present application, the D50 of graphite and hard carbon can be randomly sampled from the negative electrode sheet, and the particle size of graphite or hard carbon particles in the sample is observed under a scanning electron microscope (SEM) (the number of graphite or hard carbon particles in the sample is generally more than 500, preferably more than 1000), and the particle size corresponding to the cumulative volume percentage of 50% of the graphite or hard carbon particles is determined.

[0016] In some embodiments of the present application, 0.232D Gr ≤D HC ≤0.366D Gr At this time, the hard carbon material can be uniformly distributed in the pores formed by the stacking of graphite particles, and the internal pore distribution of the negative electrode composite is high, which can not only improve the wettability of the electrolyte to the negative electrode composite, but also build sufficient and uniformly distributed active ion diffusion channels in the internal of the negative electrode composite, so that the hard carbon material can fully exert its kinetic performance, which is conducive to achieving the fast charging performance of the negative electrode composite, and can greatly reduce the risk of lithium precipitation of the negative electrode composite caused by uneven lithium intercalation, and improve the safety performance of the final battery. In addition, the negative electrode composite has a reasonable pore distribution, which is also conducive to the deintercalation / intercalation of active ions of the graphite material, so as to fully exert the high initial coulombic efficiency of the graphite.

[0017] In some embodiments of the present application, the D50 particle size D HC may be in the range of 1 μm-7.5 μm. Exemplarily, the D50 particle size D HCmay be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, etc. At this time, it is not only conducive to the full mixing of hard carbon and graphite, but also the hard carbon can be evenly distributed in the pores of the graphite stack, further improving the compaction density of the material, optimizing the pore distribution in the negative electrode composite material, thereby further reducing the risk of lithium precipitation in the negative electrode, also conducive to controlling the specific surface area of the hard carbon in a more appropriate range, and also conducive to regulating the average lithium intercalation potential of the hard carbon material to be lower.

[0018] In some embodiments of the present application, the specific surface area of the hard carbon is in the range of 2 m 2 / g-12 m 2 / g. For example, the specific surface area of the hard carbon can be 0.1 m 2 / g, 0.2 m 2 / g, 0.5 m 2 / g, 0.8 m 2 / g, 0.9 m 2 / g, 1 m 2 / g, 2 m 2 / g, 5 m 2 / g, 7.5 m 2 / g, 10 m 2 / g, 15 m 2 / g, 17.5 m 2 / g, 18 m 2 / g, 19 m 2 / g, 20 m 2 / g, 25 m 2 / g, 30 m 2 / g, etc. At this time, it is not only conducive to the hard carbon material to exert its kinetic performance advantage, but also can effectively reduce its irreversible capacity loss, and also conducive to regulating the average lithium intercalation potential of the hard carbon material to be lower, thereby further improving the rate performance and energy density of the negative electrode composite material.

[0019] In some embodiments of the present application, the interlayer spacing of the hard carbon is in the range of 0.338 nm-0.430 nm. The "interlayer spacing" here refers to the distance between layers, which can be represented by d 002-HC (002) crystal face spacing d 002-HCIn the range of 0.338 nm-0.430 nm. In the present application, the interlayer spacing of the hard carbon can be determined by X-ray diffraction (XRD) test. Exemplarily, the interlayer spacing of the hard carbon can be 0.338 nm, 0.340 nm, 0.341 nm, 0.342 nm, 0.342 nm, 0.345 nm, 0.350 nm, 0.355 nm, 0.360 nm, 0.365 nm, 0.370 nm, 0.375 nm, 0.380 nm, 0.385 nm, 0.390 nm, 0.400 nm, 0.410 nm, 0.420 nm, 0.430 nm, etc. At this time, it is beneficial to the deintercalation / intercalation of active ions in the hard carbon, thereby being beneficial to ensuring that the rate performance of the negative electrode composite material is good, and further being beneficial to ensuring the fast charging performance of the negative electrode composite material. In addition, controlling the interlayer spacing of the hard carbon material in the above range is also beneficial to improving the cycle performance of the hard carbon.

[0020] In some embodiments of the present application, the average lithium intercalation potential of the hard carbon is less than or equal to 0.4 V. In some specific embodiments, the average lithium intercalation potential of the hard carbon is in the range of 0.01 V-0.3 V. Exemplarily, the average lithium intercalation potential of the hard carbon can be 0.01 V, 0.02 V, 0.05 V, 0.1 V, 0.15 V, 0.2 V, 0.25 V, 0.3 V, 0.35 V, 0.4 V, etc. The average working voltage of the battery is equivalent to the difference between the lithium intercalation potentials of the positive and negative electrodes, so controlling the average lithium intercalation potential of the hard carbon to be lower is beneficial to improving the average working voltage of the battery, thereby further improving the energy density of the battery.

[0021] In some embodiments of the present application, the D50 particle size D Gr may be in the range of 8 μm-20 μm. Exemplarily, the D50 particle size D Gr of the graphite can be 8 μm, 8.5 μm, 9 μm, 10 μm, 15 μm, 15.5 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, etc. Controlling the D50 particle size D Gr of the graphite in the above range is beneficial to constructing pores with a suitable size, beneficial to the compounding of the hard carbon material, further improving the compaction density of the negative electrode composite material, also beneficial to ensuring that the specific surface area is suitable, and more beneficial to the deintercalation / intercalation of active ions, so that the electrochemical kinetic performance thereof is suitable, thereby further improving the electrochemical performance of the negative electrode composite material in all aspects.

[0022] In some embodiments of the present application, the interlayer spacing of the graphite is in the range of 0.336 nm-0.388 nm. Here, the "interlayer spacing" refers to the distance between layers, which can be represented by d 002-Gr 002), that is, the (002) interplanar spacing d002-Gr The interlayer spacing of graphite is within the range of 0.336 nm to 0.388 nm. In this application, the interlayer spacing of graphite can be determined by XRD testing. For example, the interlayer spacing of graphite can be 0.366 nm, 0.368 nm, 0.370 nm, 0.375 nm, 0.380 nm, 0.385 nm, 0.388 nm, etc. Controlling the interlayer spacing of graphite within the above range is more conducive to the deintercalation / intercalation of active ions, thereby further improving the rate performance and fast charging performance of the negative electrode composite material.

[0023] In some embodiments of this application, the specific surface area of ​​graphite is 0.1 m². 2 / g-5.0m 2 The specific surface area of ​​graphite can be in the range of / g. In some specific embodiments, the specific surface area of ​​graphite can be 0.5m². 2 / g-3.0m 2 Within the range of / g. For example, the specific surface area of ​​graphite can be 0.1m³. 2 / g, 0.3m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.5m 2 / g, 2.0m 2 / g, 2.2m 2 / g, 2.5m 2 / g, 2.8m 2 / g, 3.0m 2 / g etc. Controlling the specific surface area of ​​graphite within the above range is beneficial for the deintercalation / intercalation of active ions, thereby further improving the rate performance and fast charging performance of the negative electrode composite material.

[0024] In some embodiments of this application, the mass percentage of graphite in the negative electrode composite material is in the range of 85%-99%. Controlling the mass percentage of graphite within this range ensures high final battery compaction, high initial coulombic efficiency, high energy density, and also enables the battery to achieve high rate performance and fast charging capability. Specifically, when a negative electrode lithium supplement material (such as Li₂O, Li₅FeO₄, Li₂NiO₂, etc.) is added to the negative electrode composite material, the mass percentage of graphite in the negative electrode composite material can be in the range of 50%-99%.

[0025] This application does not limit the preparation method of the negative electrode composite material. In some specific embodiments, the above-mentioned negative electrode composite material can be obtained by uniformly mixing graphite and hard carbon that meet the particle size requirements.

[0026] The embodiment of the present application also provides a negative electrode sheet, which comprises the negative electrode composite provided by the embodiment of the present application. Generally, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface of the negative electrode current collector.

[0027] In some specific embodiments, the negative electrode active material layer comprises the negative electrode composite provided by the embodiment of the present application, and a certain amount of a binder and an optional conductive agent. The binder and the conductive agent are both materials known to those skilled in the art. Since the negative electrode composite provided by the present application is used as the negative electrode active material, the electrode sheet can be used to provide a secondary battery with fast charging performance, high safety performance and high energy density.

[0028] The embodiment of the present application also provides a secondary battery, which comprises the negative electrode sheet provided by the embodiment of the present application. The secondary battery can achieve fast charging, high safety performance and high energy density.

[0029] In the present application, the secondary battery can be a liquid battery using a liquid electrolyte, or a semi-solid battery using a semi-solid electrolyte, or a solid battery using a solid electrolyte. In some embodiments, the secondary liquid battery can comprise a positive electrode sheet, a negative electrode sheet, and a separator and an electrolyte arranged between the positive electrode sheet and the negative electrode sheet. In other embodiments, the secondary semi-solid battery can comprise a positive electrode sheet, a negative electrode sheet, and a semi-solid electrolyte arranged between the positive electrode sheet and the negative electrode sheet. In still other embodiments, the solid battery can comprise a positive electrode sheet, a negative electrode sheet, and a solid electrolyte arranged between the positive electrode sheet and the negative electrode sheet.

[0030] The secondary battery described above can be a lithium ion battery.

[0031] The present application also provides an electric device comprising the secondary battery described above.

[0032] In the present application, the electric device using the secondary battery described above is not particularly limited. Exemplarily, the electric device includes but is not limited to a mobile phone, a notebook computer, a tablet computer, a camera, a television, a radio, a wearable device (such as a smart watch, a smart bracelet, a head-mounted stereo headset, a Bluetooth headset), a backup power supply, a new energy vehicle, an electric tool, a household large storage battery, etc.

[0033] Since the secondary battery described above is used, the electric device has a battery with strong endurance, good cycle performance and fast charging function.

[0034] The technical solutions of the present application are described in further detail in the following embodiments.

[0035] Embodiment 1

[0036] A negative electrode composite material, comprising 95 wt.% of graphite and 5 wt.% of hard carbon, wherein the D50 particle size D Gr of the graphite is 13.37 pm, the D50 particle size D HC of the hard carbon is 4.88 pm (0.1-20); the specific surface area of the graphite is 1.16 m 2 / g, the specific surface area of the hard carbon is 2.73 m 2 / g, the interlayer spacing of the graphite is 0.336 nm, and the interlayer spacing of the hard carbon is 0.373 nm; and the average lithium intercalation potential of the hard carbon is 0.195 V.

[0037] Examples 2-10

[0038] The parameters of the negative electrode composite materials provided by Examples 2-10 are summarized in Table 1 below.

[0039] Table 1: Summary of parameters of negative electrode composite materials provided by various examples

[0040]

[0041] To highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0042] Comparative Example 1

[0043] A negative electrode material, comprising 100 wt.% of graphite, wherein the D50 particle size D Gr of the graphite is 13.37 pm, and the specific surface area of the graphite is 1.16 m 2 / g, and the interlayer spacing of the graphite is 0.336 nm.

[0044] Comparative Example 2

[0045] A negative electrode material, comprising 100 wt.% of hard carbon, wherein the D50 particle size D HC of the hard carbon is 4.88 pm, the specific surface area of the hard carbon is 2.73 m 2 / g, the interlayer spacing of the hard carbon is 0.373 nm, and the average lithium intercalation potential of the hard carbon is 0.195 V.

[0046] Comparative Example 3

[0047] A negative electrode composite material, comprising 95 wt.% of graphite and 5 wt.% of hard carbon, wherein the D50 particle size D Gr of the graphite is 13.37 pm, the D50 particle size D HC of the hard carbon is 7.9 pm; the specific surface area of the graphite is 1.16 m 2 / g, the specific surface area of the hard carbon is 3.62 m 2The interlayer spacing of the graphite is 0.336 nm, and the interlayer spacing of the hard carbon is 0.375 nm; the average lithium intercalation potential of the hard carbon is 0.187 V.

[0048] Performance test

[0049] (1) Powder compaction density test: a certain mass (denoted as m) of the negative electrode composite material or negative electrode material provided in the examples and comparative examples was weighed in a tabletting mold, a certain pressure was applied to the mold, the pressure was maintained for a period of time, and after the pressure was released, the thickness L and area S of the tablet were measured. The compaction density = (m / S) / L, and the unit of the compaction density is g / cm 3 . The test results of some examples are summarized in Figure 2 . Some results are summarized in Figure 1 .

[0050] (2) Electrochemical performance test

[0051] ① Preparation of negative electrode sheet: the negative electrode composite material or negative electrode material, conductive agent (specifically Super P), and binder carboxymethyl cellulose sodium, binder styrene-butadiene rubber were mixed in a mass ratio of 95:2:1.5:1.5, and then added to a solvent-water to obtain a negative electrode slurry; the negative electrode slurry was uniformly coated on the opposite two sides of the negative electrode current collector-copper foil, and then subjected to rolling, drying, and slitting to obtain a negative electrode sheet.

[0052] ② Preparation of positive electrode sheet: the positive electrode active material (specifically lithium cobaltate), conductive agent (specifically Super P or one or more of carbon nanotubes), and binder (specifically polyvinylidene fluoride) were mixed in a mass ratio of 8:1:1, and then added to a solvent (specifically N-methyl pyrrolidone) to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on the opposite two sides of the positive electrode current collector-aluminum foil, and then subjected to rolling, drying, and slitting to obtain a positive electrode sheet.

[0053] ③ The negative electrode sheet prepared in step ①, the separator (specifically a polypropylene-polyethylene-polypropylene three-layer composite separator), and the positive electrode sheet prepared in step ② were sequentially stacked to obtain a dry battery, and the separator was placed between the positive electrode sheet and the negative electrode sheet to serve as a separation function; and an organic solution of lithium hexafluorophosphate with a concentration of 1 M was injected, wherein the organic solution was ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DEC) in a volume ratio of 2:3:5, and then a formation process was performed to obtain a soft-pack battery.

[0054] ④ Electrochemical performance test: the initial thickness of the soft-pack battery prepared above was first measured, the capacity of the battery was tested at a small current (0.2C), and the corresponding energy density was calculated. The energy density is equal to the battery capacity x average working voltage / battery volume, and the unit is Wh / L. The results are summarized in Table 2.

[0055] Cycling was performed at 25°C according to the following charge-discharge protocol:

[0056] 1) Constant current-constant voltage charging to 4.15 V at a current density of 5 C, 3.7 C cut-off;

[0057] 2) Constant current-constant voltage charging to 4.25 V at a current density of 3.7 C, 3 C cut-off;

[0058] 3) Constant current-constant voltage charging to 4.45 V at a current density of 3 C, 2 C cut-off;

[0059] 4) Constant current-constant voltage charging to 4.48 V at a current density of 2 C, 0.35 C cut-off;

[0060] 5) Discharging to 3.0 V at a current density of 1 C, and then to 3.0 V at 0.5 C;

[0061] The above steps 1) to 5) were repeated for one cycle, and the battery was cycled for 800 cycles. The thickness of the battery after 800 cycles was recorded, the capacity retention rate and the thickness expansion rate of the battery were calculated, and the battery was disassembled to observe the lithium precipitation. The results are summarized in Table 2.

[0062] (3) Electrolyte wettability test of negative electrode composite material

[0063] The negative electrode sheets prepared in the above electrochemical test ① were directly subjected to static contact angle test: the negative electrode sheets prepared in each example and comparative example were fixed on the sample table of the static contact angle tester, and a fixed amount of electrolyte (the electrolyte used in electrochemical test ③) was dropped on the upper surface of the negative electrode sheet using a syringe; wherein, for each negative electrode sheet, the dropping height of the electrolyte was equal. The morphology at different time points after the electrolyte was dropped on the upper surface of the negative electrode sheet was captured by a high-speed camera, and the contact angle between the electrolyte droplet and the upper surface of the negative electrode sheet was measured by software, and a contact angle-time change curve was drawn to obtain the wettability curve of each negative electrode sheet. The test results of some examples are summarized in Table 3. Figure 2

[0064] Table 2 Summary of electrochemical performance test of batteries of each example and comparative example

[0065]

[0066] ​As can be seen from the data in Table 2, the battery prepared in the embodiments of the present application not only does not have the negative electrode lithium precipitation after 800 cycles in the case of achieving a large energy density, but also has a good capacity cycle retention rate and a low thickness expansion rate. It can be seen that the negative electrode composite material provided by the present application can greatly reduce the risk of lithium precipitation caused by uneven lithium intercalation of the negative electrode composite material, improve the safety performance of the final battery, and also can play the excellent electrochemical performance of graphite.

[0067] The above is an exemplary embodiment of the present application, and it should be pointed out that those skilled in the art can make some improvements and refinements without departing from the principles of the present application, and these improvements and refinements are also considered within the protection scope of the present application.

Claims

1. A negative electrode composite material, characterized in that, The negative electrode composite material comprises graphite and hard carbon, wherein the D50 particle size of the graphite is D. Gr The D50 particle size of the hard carbon is D HC 0.232 D Gr ≤D HC ≤0.366D Gr The D HC With the D Gr Counted in the same unit; The hard carbon has an average lithium intercalation potential of less than or equal to 0.4 V, and a specific surface area of ​​2 m². 2 / g-10m 2 Within the range of / g, the interlayer spacing of the hard carbon is in the range of 0.338nm-0.430nm; The interlayer spacing of the graphite is in the range of 0.336 nm to 0.388 nm, and the specific surface area of ​​the graphite is 0.5 m². 2 / g-3.0m 2 Within the range of / g.

2. The negative electrode composite material according to claim 1, characterized in that, 8μm≤D Gr ≤20μm。 3. The negative electrode composite material according to claim 1, characterized in that, The mass percentage of graphite in the negative electrode composite material is in the range of 85%-99%.

4. The negative electrode composite material according to claim 1, characterized in that, 1μm≤D HC ≤7.5μm。 5. A negative electrode sheet, characterized in that, The negative electrode sheet comprises the negative electrode composite material as described in any one of claims 1-4.

6. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in claim 5.

7. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 6.

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