Anode materials for lithium-ion batteries

Abstract

One embodiment of the present invention provides a Si-carbon composite anode material that has high efficiency and capacity while having an improved cycle life by adding nitrogen to the Si-carbon composite anode material to compensate for the low conductivity of the Si raw material, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode material for lithium ion batteries and a method for producing the same. [Background technology]

[0002] Lithium-ion batteries are currently the most widely used secondary battery system in portable electronic communication devices, electric vehicles, and even energy storage devices. Compared to commercially available aqueous secondary batteries (e.g., Ni-Cd and Ni-MH), lithium-ion batteries have attracted attention due to their advantages, including high energy density, operating voltage, and relatively low self-discharge rate. However, when considering more efficient use of portable devices and improved energy performance in electric vehicles, improving their electrochemical properties remains a technical challenge. For this reason, significant research and development is currently underway on the four major raw materials: cathode, anode, electrolyte, and separator.

[0003] Among these raw materials, graphite-based materials that exhibit excellent capacity retention and efficiency have been commercially available for the anode. However, the graphite-based materials have relatively low theoretical capacity (LiC6: 372mAh / g) and discharge capacity ratio, which means that they are somewhat insufficient to meet the high energy and high power density characteristics of batteries required in the market.

[0004] Therefore, many researchers are interested in the group IV elements (Si, Ge, Sn) on the periodic table, and Si in particular has a very high theoretical capacity (Li 15 Si4 has been attracting attention as a very attractive material due to its high capacity (3600mAh / g) and low operating voltage (~0.1V vs. Li / Li+). However, Si undergoes large volume expansion and contraction due to its reaction with lithium during charging and discharging, which can cause the silicon active material powder to pulverize and poor electrical contact between the silicon active material powder and the current collector. This phenomenon can cause the capacity of lithium-ion batteries to drop rapidly as the charge-discharge cycle progresses.

[0005] To overcome these problems, Patent Document 1 provides an anode material containing Si particles dispersed in SiO2, which exhibits higher capacity and improved cycle life characteristics than when Si is used alone. However, in addition to Si and SiO2, an intermediate phase called SiOx (0.5≦x<2) is formed within the anode material. Some of the oxygen in this SiOx reacts with Li to form the stable phase Li2O. This results in irreversible capacity, which deteriorates the cycle life characteristics of the anode material.

[0006] To solve these problems, recent research has been actively conducted to improve reversibility by combining Si and carbon. However, such Si-carbon composite anode materials have the problem of being unable to achieve high capacity due to the expansion of Si nanoparticles caused by repeated charge and discharge. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2011-0029087 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention provides a Si-carbon composite anode material having long cycle life, high capacity and high efficiency characteristics, and a method for manufacturing the same. [Means for solving the problem]

[0009] One embodiment of the present invention provides a negative electrode material for a lithium ion battery, comprising: Si nanoparticles; a carbon-based material; a carbon matrix; and 0.5 to 5 wt % of nitrogen.

[0010] The carbon-based material can be a coal-based or petroleum-based pitch containing graphite, CNTs, and beta resin.

[0011] The carbon matrix may be carbon derived from at least one of petroleum-based and coal-based pitches, coal tar, PAA, and PVA, each having a softening point of 250° C. or less.

[0012] The coal-based or petroleum-based pitch may have a fixed carbon ratio of 10% or more and may contain beta resin.

[0013] The nitrogen contained in the negative electrode material may be wholly or partially inserted or doped into the Si nanoparticles, carbonaceous material, and carbon matrix.

[0014] The negative electrode material may further include a conductive material, which may be graphite, CNT, or graphene.

[0015] The median particle size of the negative electrode material may be 7 to 15 μm.

[0016] The negative electrode material may contain 10 to 70% by weight of silicon (Si) and 20% by weight or more of fixed carbon (C).

[0017] Another embodiment of the present invention provides a negative electrode for a lithium ion battery, comprising the above-described negative electrode material.

[0018] The negative electrode may have a capacity of 1600 mAh / g or more, an initial efficiency of 87.5% or more, and an expansion rate of 57% or less.

[0019] Another embodiment of the present invention provides a method for manufacturing an anode material for a lithium-ion battery, the method comprising the steps of: preparing Si nanoparticles and a carbon-based material; mixing the Si nanoparticles and the carbon-based material to obtain a mixture; spray-drying the mixture to synthesize an Si-carbon composite; mixing the Si-carbon composite with a carbon matrix precursor and a nitrogen-based additive; press-molding the mixture; sintering the pressed mixture at a high temperature of 800°C or higher but lower than 1100°C; and crushing and classifying the sintered mixture. The manufactured anode material can contain 0.5 to 5 wt. % nitrogen.

[0020] The median particle size of the negative electrode material produced by the above-mentioned production method can be 7 to 15 μm.

[0021] The Si nanoparticles can be prepared by wet milling methods such as dry milling, bead milling, and ball milling, deposition methods (thermal evaporation, plasma deposition, etc.) under vacuum, electromagnetic melting, and co-evaporation.

[0022] The wet grinding method can be carried out using an organic solvent including an aqueous solvent such as EtOH or IPA.

[0023] The median particle size of the Si nanoparticles may be 30 to 500 nm.

[0024] The median particle size of the Si-carbon composite may be 6 to 20 μm.

[0025] The nitrogen-based additive may be an amine-based, amide-based, aromatic-N-based, carbamates-based, enamine-based, nitramine-based, nitrile-based, imine-based, nitrosamine-based, or organic nitrate-based additive.

[0026] The step of mixing the carbon matrix precursor and the nitrogen-based additive with the Si-carbon composite may be performed using a mechanofusion, VC, or planetary mixer.

[0027] After the grinding and poling, an additional coating process step may be included.

[0028] The negative electrode material produced by the above-mentioned production method can contain 10 to 70% by weight of silicon (Si) and 20% by weight or more of fixed carbon (C). [Effects of the Invention]

[0029] According to one embodiment of the present invention, nitrogen is added to the Si-carbon composite anode material to reduce the low conductivity (10 -4 By complementing the Si-carbon composite anode material with high efficiency and capacity while having improved cycle life, a method for manufacturing the same can be provided. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a graph showing the relationship between the capacity (mAh / g) and the N content (%) measured in <Evaluation of the composite alone>. [Figure 2] 1 is a graph showing the relationship between the initial efficiency (%) and the N content (%) measured in <Evaluation of the composite alone>. [Figure 3] 1 is a graph showing the expansion rate (%) as a function of the N content (%) measured in <Graphite Comprehensive Evaluation>. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to further explain the present invention to those having average knowledge in the art.

[0032] In this specification, unless specifically stated to the contrary, the term "comprises" is used to indicate that other elements may be further included rather than excluding other elements.

[0033] In the present specification, unless otherwise specified, the unit of % means % by weight.

[0034] An anode material according to one embodiment of the present invention may include Si nanoparticles; a carbon-based material; a carbon matrix; and 0.5 to 5 wt % of nitrogen.

[0035] That is, one embodiment of the present invention relates to a silicon-based anode material containing Si nanoparticles. The inclusion of Si nanoparticles in the anode material allows for a battery with a higher capacity than carbon-based anode materials. In addition, in one embodiment of the present invention, the Si nanoparticles may be fine particles with a median particle size (D50) of 30 to 500 nm. The median particle size refers to the particle size at the center of the particle size distribution obtained when measured using a Beckmann Coulter particle size analyzer. For surface stability, the Si nanoparticles may include a carbon coating layer.

[0036] The carbonaceous material contained in the anode material prevents the expansion of Si nanoparticles and ensures the conductivity of the anode material. The carbonaceous material may include oriented graphite and CNT, as well as coal- or petroleum-based pitch containing beta resin.

[0037] The carbon matrix ensures good formation of the Si-carbon composite on the carbon support layer, improves electrical contact between Si particles present inside and outside the Si-carbon composite, and ensures compactness within the anode material. While not necessarily limited thereto, the carbon matrix contained in the anode material may include carbon derived from at least one of petroleum-based or coal-based pitch, coal tar, PAA, and PVA, each of which has a softening point of 250°C or less. In this case, the carbon may be carbonized carbon. Preferably, the coal-based or petroleum-based pitch has a fixed carbon ratio of 10% or more and may include beta resin.

[0038] The negative electrode material may contain 0.5 to 5 wt. % nitrogen. As described above, the addition of nitrogen can improve the electrical conductivity of Si nanoparticles. If the nitrogen content is less than 0.5%, this effect cannot be achieved, and a more preferable lower limit is 0.6%. On the other hand, if the nitrogen content exceeds 5%, the absolute amount of Si and graphite capable of achieving capacity decreases, making it difficult to achieve high capacity for the negative electrode material. A more preferable upper limit is 4.5%.

[0039] In this case, nitrogen can be doped by various methods, and examples thereof include a method of adding an organic or inorganic compound containing nitrogen and then heat treating the compound, and a doping method using nitrogen plasma.

[0040] In addition, the nitrogen contained in the negative electrode material may be present in a form that is pre-intercalated or doped into the Si nanoparticles, carbonaceous material, and carbon matrix, in whole or in part.

[0041] The negative electrode material may further include a conductive material having excellent conductivity in order to improve cycling efficiency during charging and discharging, and preferably includes graphite, CNT, or graphene.

[0042] The negative electrode material may have a median particle size of 7 to 15 μm. If the median particle size exceeds 15 μm, the electrode may expand excessively during charge and discharge, resulting in a shortened electrode life. A more preferred upper limit is 12 μm, and an even more preferred upper limit is 11 μm. On the other hand, if the median particle size is less than 7 μm, the specific surface area increases due to an increase in the number of particles, which may result in a problem of reduced charge and discharge efficiency. A more preferred lower limit is 8 μm.

[0043] According to one embodiment, the negative electrode material of the present invention may contain 10 to 70 wt % of Si and 20 wt % or more of fixed carbon.

[0044] If the Si content is more than 70 wt%, electrode expansion due to repeated charge / discharge cycles and a resulting shortening of cycle life may occur. A more preferred upper limit is 68%, and an even more preferred upper limit is 65%. On the other hand, to provide a high-capacity Si-carbon composite anode material, it is preferred that the Si nanoparticles comprise 10 wt% or more of Si nanoparticles. A more preferred lower limit is 20%, and an even more preferred lower limit is 55%.

[0045] The negative electrode material may contain 20% by weight or more of fixed carbon. If the fixed carbon content is less than 20%, a large number of pores may be generated, causing side reactions with the electrolyte, which may result in a shortened service life of the negative electrode material. A more preferred lower limit is 25%, and an even more preferred lower limit is 30%.

[0046] A lithium ion negative electrode comprising the Si-carbon composite negative electrode material according to the present invention exhibits high capacity and excellent cycle life characteristics. Preferably, the lithium ion negative electrode comprising the Si-carbon composite negative electrode material according to the present invention has a capacity of 1600 mAh / g or more, an initial efficiency of 87.5% or more, and an expansion rate of 57% or less.

[0047] The method for manufacturing the anode material according to the present invention may include the steps of preparing Si nanoparticles and a carbon-based material, mixing the Si nanoparticles and the carbon-based material to obtain a mixture, spray-drying the mixture to prepare a Si-carbon composite, mixing the Si-carbon composite with a carbon matrix precursor and a nitrogen-based additive, and then press-molding the mixture, firing the pressed composite at a high temperature of 800°C or higher but lower than 1100°C, and pulverizing and classifying the fired composite. The manufactured anode material may also contain 0.5 to 5 wt% nitrogen.

[0048] The Si nanoparticles can be manufactured by wet milling methods such as dry milling, bead milling, and ball milling, deposition methods (thermal evaporation, plasma deposition, and the like) under vacuum, electromagnetic melting, and co-evaporation. Preferably, a wet milling method can be used, which can minimize the degree of oxidation of the Si nanoparticles and facilitates particle size control.

[0049] The wet grinding method can be carried out using an organic solvent including an aqueous solvent, and in a preferred embodiment, EtOH or IPA can be used to prevent oxidation of Si.

[0050] The median particle size of the Si nanoparticles may be 30 to 500 nm. If the particle size of the Si nanoparticles exceeds 500 nm, the capacity can be increased, but the electrode life may be shortened due to electrode expansion. A more preferable upper limit is 250 nm, and an even more preferable upper limit is 200 nm. On the other hand, if the particle size of the Si nanoparticles is less than 30 nm, the high capacity characteristics of the negative electrode material cannot be ensured. More preferably, the median particle size of the Si nanoparticles may be 50 nm or more.

[0051] The Si nanoparticles and the carbon-based material can then be mixed in the presence of a solvent, and the mixture can be spray-dried to form a Si-carbon composite, followed by drying the solvent. In one embodiment of the present invention, the solvent can be the same as that used to grind the Si nanoparticles, but is not necessarily limited thereto.

[0052] In a preferred embodiment, the median particle size of the Si-carbon composite formed after mixing may be 6 to 20 μm. If the particle size of the Si-carbon composite exceeds 20 μm, significant volume expansion and contraction may occur, which may result in deterioration of the cycle life characteristics of the electrode. A more preferred upper limit is 15 μm, and an even more preferred upper limit is 12 μm. On the other hand, if the particle size of the Si-carbon composite is less than 6 μm, the efficiency may decrease and the charge / discharge capacity may drop sharply. More preferably, the particle size of the Si-carbon composite may be 8 μm or more. Variables that affect the size of the Si-carbon composite may include the spray-drying conditions (solid content ratio, spray-drying rotation speed) and the particle size of the graphite to be mixed.

[0053] After spray drying, the Si-carbon composite can be mixed with a carbon matrix precursor and a nitrogen-based additive.

[0054] The carbon matrix precursor may be at least one of petroleum-based and coal-based pitches, coal tar, PAA, and PVA, each having a softening point of 250°C or less, and the petroleum-based and coal-based pitches may have a fixed carbon ratio of 10% or more and may contain beta resin.

[0055] The nitrogen-based additives include all organic substances containing nitrogen, and are not necessarily limited thereto. However, the nitrogen-based additives may be amine-based, amide-based, aromatic-N-based, carbamates-based, enamine-based, nitramine-based, nitrile-based, imine-based, nitrosamine-based, or organic nitrate-based.

[0056] Furthermore, the Si-carbon precursor, the carbon matrix precursor, and the nitrogen-based additive can be uniformly dispersed by the above-mentioned mixing. The mixing method is not particularly limited as long as the above-mentioned object can be achieved, but in one embodiment, the mixing can be performed using a mechanofusion, VC, or planetary mixer.

[0057] After the mixing, the mixture can be pressure-molded. This pressure-molding can minimize pores present inside the Si nanoparticle-carbon precursor and increase the bonding strength of each component particle, resulting in a high-density anode material. The pressure and pressure time in the pressure-molding step are not particularly limited as long as the above-mentioned objectives can be achieved.

[0058] After the pressure molding, the material can be fired at a high temperature of 800°C or higher but lower than 1100°C. The high-temperature firing removes volatiles and forms a carbon support layer. At temperatures lower than 800°C, this effect cannot be ensured. A more preferred lower limit is 850°C, and an even more preferred lower limit is 900°C. However, at temperatures above 1100°C, side reactions of the Si nanoparticles may cause a decrease in charge / discharge capacity and efficiency. A more preferred upper limit is 1000°C, and an even more preferred upper limit is 950°C.

[0059] After the high-temperature firing, the negative electrode material for lithium ion batteries having a median particle size of 7 to 15 μm can be manufactured through the steps of pulverization and classification.

[0060] Meanwhile, in one embodiment of the present invention, the method may further include coating the surfaces of the pulverized particles after the pulverization and classification. The additional coating protects the partially exposed Si surface and fills the remaining pores, thereby ensuring long-term service life.

[0061] The negative electrode material produced by the above-mentioned production method can contain 10 to 70% by weight of silicon (Si) and 20% by weight or more of fixed carbon (C). [Example]

[0062] (Example) In this experiment, Si nanoparticles were prepared in a slurry state using a wet bead mill with EtOH as a solvent. The median particle size of the Si nanoparticles was 100 nm. The Si nanoparticles were then mixed with graphite and spray-dried to produce Si-graphite composites. The particle size of the Si-graphite composite was 18-20 μm based on D50. When mixing the product with pitch, hexametylenetetramine (HMT) or melamine was added for nitrogen doping in the amounts listed in Table 1, and the mixture was mixed using a mechanofusion process. A compact was then produced using a pressure molding machine, and the compact was heat-treated at approximately 1000°C in an inert atmosphere. The density of the compact was 1-1.3 g / cc. After the heat treatment, the final product was obtained by crushing, classifying, and performing an additional carbon coating process. The particle size of the final product was confirmed to be approximately 7-15 μm based on D50. The composition ratio was 45 to 48 wt % in the case of Si, and 20 wt % or more in the case of fixed carbon.

[0063] The nitrogen content of the resulting final product was measured using a Nitrogen / Oxygen determinator (LECO) analyzer and is shown in Table 1.

[0064] <Evaluation of the complex alone> After obtaining the above negative electrode active material, the Si-carbon composite negative electrode active material was synthesized for electrochemical property analysis. The loading amount was 5 mg / cm. 2 A CR2032-type coin half cell was fabricated by rolling a coating onto a Cu current collector to achieve an electrode density of 1.2-1.3 g / cc. Charge-discharge tests were then conducted at an operating voltage range of 0.005 V to 1.0 V. The binder used for electrode fabrication was a PAA (polyacrylic acid) system, and the electrolyte was an additive-free EC:DEC = 1:1 (1.0 M LiPF6). The charge-discharge current was measured at 0.1 C for the initial cycle. The capacity and initial efficiency measured through the above process are shown in Table 1.

[0065] <Graphite Mixing Evaluation> A coin half cell was fabricated by mixing 80 wt% commercial natural graphite and 20 wt% synthetic Si-carbon composite anode material, achieving an anode capacity of 600 mAh / g. Its cathode was commercial LCO. The capacity and efficiency of the anode were measured at 0.1 C (charge) / 0.1 C (discharge), and its long-term life was measured at 0.5 C (charge) / 0.5 C (discharge). The measured capacity and initial efficiency are shown in Table 1. The anode composition was active material: conductive material: CMC: SBR = 96.1:1:1.4:1.5, with a rolling density of 1.4 g / cc. The electrolyte used was 1.0 M LiPF6 with additives of 1.0% VC and 10% FEC, and an EC:DEC = 1:1 ratio. To measure the expansion rate of the composite electrode, a coin half cell was used to complete the 50th cycle, and then fully charged to 0.005 V (0.005 C cut-off). The coin cell was then disassembled and the thickness of the charged electrode was measured to compare the change in expansion rate. The results are shown in Table 1.

[0066] [Table 1]

[0067] 1 to 3, when the nitrogen content was increased to 5 wt %, the capacity decreased slightly, but the initial efficiency increased and the expansion rate decreased.

[0068] However, in the case of anodes containing anode materials with a nitrogen content exceeding 5%, the capacity and initial efficiency dropped sharply, and the expansion characteristics also deteriorated sharply.

[0069] In the case of Comparative Example 1, since no nitrogen is contained, the low electrical conductivity of Si cannot be compensated for, and it is confirmed that the initial efficiency is low and the expansion coefficient is high.

[0070] In addition, in Comparative Examples 2 and 3, the nitrogen content exceeded 5%, and the capacity and initial efficiency decreased sharply. This is because when the nitrogen content exceeds a certain level, the absolute amounts of Si and graphite that can express capacity decrease.

[0071] On the other hand, Examples 1 to 8 of the present invention contained a nitrogen content of 0.5 to 5% by weight, and it was confirmed that the electrodes were able to have high capacity and high efficiency characteristics while enabling the mitigation of the expansion of Si particles.

Claims

1. Si nanoparticles; carbon-based materials; a carbon matrix; and A negative electrode material for a lithium ion battery, comprising 0.5 to 5% by weight of nitrogen.

2. The negative electrode material for a lithium ion battery according to claim 1 , wherein the carbon-based material comprises a coal-based or petroleum-based pitch containing graphite, CNTs, and beta resin.

3. 2. The negative electrode material for a lithium ion battery according to claim 1, wherein the carbon matrix is ​​carbon derived from at least one of petroleum-based and coal-based pitches, coal tar, polyamino acid (PAA), and polyvinyl alcohol (PVA), each of which has a softening point of 250°C or less.

4. 4. The negative electrode material for a lithium ion battery according to claim 3, wherein the coal-based or petroleum-based pitch has a fixed carbon ratio of 10% or more and contains beta resin.

5. 2. The negative electrode material for a lithium ion battery according to claim 1, wherein all or a part of the nitrogen contained in the negative electrode material is pre-intercalated or doped into the Si nanoparticles, the carbon-based material, and the carbon matrix.

6. The negative electrode material for a lithium ion battery according to claim 1 , further comprising a conductive material.

7. The negative electrode material for a lithium ion battery according to claim 6, wherein the conductive material is graphite, CNT, or graphene.

8. The negative electrode material for lithium ion batteries according to any one of claims 1 to 7, wherein the median particle size of the negative electrode material is 7 to 15 µm.

9. The negative electrode material for lithium ion batteries according to any one of claims 1 to 7, comprising 10 to 70% by weight of silicon (Si) and 20% by weight or more of fixed carbon (C).

10. Preparing Si nanoparticles and a carbon-based material; mixing the Si nanoparticles with the carbon-based material to obtain a mixture; spray-drying the mixture to synthesize a Si-carbon composite; mixing the Si-carbon composite with a carbon matrix precursor and a nitrogen-based additive; a step of compressing and molding the mixture after the mixing; After the pressing, high-temperature firing is performed at 800°C or more and less than 1100°C; and After the high-temperature firing, the step of crushing and classifying the product is included. The method for producing an anode material for a lithium ion battery, wherein the produced anode material contains 0.5 to 5 wt % of nitrogen.

11. 11. The method for producing an anode material for a lithium ion battery according to claim 10, wherein the Si nanoparticles are produced through a wet pulverization method such as a dry milling process, bead milling, or ball milling, a deposition method (thermal deposition, plasma deposition, etc.) produced under a vacuum atmosphere, an electromagnetic melting method, or a co-evaporation method.

12. The method for producing a negative electrode material for a lithium ion battery according to claim 10, wherein the median particle size of the Si nanoparticles is 30 to 500 nm.

13. The method for producing a negative electrode material for a lithium ion battery according to claim 10, wherein the median particle size of the Si-carbon composite is 6 to 20 μm.

14. The nitrogen-based additive is an amine-based, amide-based, aromatic-N-based, carbamate-based, enamine-based, nitramine-based, nitrile-based, imine-based, nitrosamine-based, or organic nitrate-based additive.

15. The method for producing a negative electrode material for a lithium ion battery according to claim 10, further comprising an additional coating process step after the grinding and polarization.

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