JP7388330B2 - Composite materials and lithium-ion secondary batteries - Google Patents

Description

The present invention relates to a composite material, for example, a composite material used for a negative electrode of a lithium ion secondary battery, and a lithium ion secondary battery.

Currently, lithium ion secondary batteries are widely used in portable electronic devices such as mobile phones, notebook computers, and digital cameras, electric vehicles, and the like. Research is underway to use silicon-based materials with high capacity as negative electrode materials for such lithium ion secondary batteries.

Patent Document 1 describes powder particles of a silicon material in which silicon is doped with one or more of boron, phosphorus, nitrogen, antimony, arsenic, aluminum, gallium, or indium, and the resistance value in the form of a wafer or ingot is 10 Ωcm or less. A silicon conductive material coating with a low resistivity whose surface is coated with carbon containing a large amount of crystalline component is disclosed. Patent Document 1 describes that good cycle characteristics can be obtained by using this silicon conductive material coating as a negative electrode active material of a lithium ion secondary battery.

Patent Document 2 discloses that silicon-based active material particles capable of intercalating and deintercalating lithium ions are heated while being supplied with a nitrogen source in a heating device, thereby making the silicon-based active material particles contain nitrogen. In the nitrogen introduction step, the nitrogen content in the silicon-based active material particles is controlled in the range of 100 ppm or more and 50,000 ppm or less, and the carbon source is vented into the heating device together with the nitrogen source, A method for producing a negative electrode material for a lithium ion secondary battery is disclosed in which a carbon film is formed on the surface of silicon-based active material particles. According to this manufacturing method, it is possible to control the nitrogen content in the silicon-based active material particles and at the same time form a conductive carbon film on the surface of the silicon-based active material particles, easily producing lithium ion with high capacity and excellent cycle characteristics. It is said that negative electrode material for secondary batteries can be obtained.

Patent Document 3 discloses a negative electrode active material including a carbon material containing boron and a silicon material containing at least one of silicon and silicon oxide but not containing boron. Patent Document 3 describes that this negative electrode active material makes it possible to realize a lithium ion secondary battery with excellent cycle characteristics without impairing the high discharge capacity density of silicon or silicon oxide.

Japanese Patent Application Publication No. 2004-296161 Japanese Patent Application Publication No. 2015-204192 JP2017-216227A

However, as described later, the silicon conductive material coating described in Patent Document 1, the negative electrode material for lithium ion secondary batteries described in Patent Document 2, and the negative electrode active material described in Patent Document 3 are also has low electronic conductivity.

The silicon conductive material coating described in Patent Document 1 mainly contains Group 13 (B, Al, Ga, and In) and Group 15 (N, P, As) in order to improve the electronic conductivity of silicon. and Sb) are doped. For example, B, which is a typical doping element of Group 13, has one fewer electron than carbon atom, which is a Group 14 element. Therefore, in the case of a substitutional solid solution in which carbon atoms in the hexagonal mesh plane are substituted, B behaves as a hole, so that the electronic conductivity is improved. On the other hand, N, which is a typical doping element of Group 15, has one more electron than carbon atoms and acts as a donor, thereby improving electronic conductivity. In this way, when silicon is doped with Group 13 and Group 15 elements, the effect of improving electron conductivity is offset by the inclusion of elements that behave as holes and elements that behave as donors. There are concerns.

Furthermore, in the silicon conductive material coating described in Patent Document 1, doping is performed during the growth of the silicon single crystal. Specifically, when refining metallic silicon, a salt made of the above-mentioned doping element is added at the same time as oxygen gas is blown into the silicon metal. In this case, the substances that form salts with the respective metal ions remain as impurities, resulting in a decrease in electronic conductivity.

In addition, in the silicon conductive material coating described in Patent Document 1, methods for growing silicon single crystals include magnetic field pulling method, Czochralski method, floating zone melting method, etc. The process is generally carried out at elevated temperatures. In that case, since B, P, N, etc. to be doped are easily oxidized, the actual amount of doping is thought to be small.

Furthermore, in the silicon conductive material coating described in Patent Document 1, carbon coating is performed by thermochemical vapor deposition using an aliphatic, alicyclic hydrocarbon, or an aromatic hydrocarbon as the carbon source. It is stated that the optimum temperature range at this time is preferably 1000 to 1400°C. However, at such high temperatures, thermal oxidation of silicon, which is the base particle, cannot be avoided, and when silicon is oxidized to SiO ₂ , electronic conductivity is significantly reduced.

In the negative electrode material for lithium ion secondary batteries described in Patent Document 2, nitrogen is added to the silicon-based active material particles by heating the silicon-based active material particles at 1000° C. while supplying a nitrogen source in a heating device. I'm letting you do it. However, when heating at such a high temperature, thermal oxidation of silicon, which is a base particle, cannot be avoided, and when silicon is oxidized to SiO ₂ , electronic conductivity is significantly reduced.

Furthermore, in the negative electrode material for lithium ion secondary batteries described in Patent Document 2, a carbon film is formed on the surface of silicon-based active material particles by a thermochemical vapor deposition method, and the temperature at that time is 900 to 1100°C. considered desirable. However, at such high temperatures, as described above, thermal oxidation of silicon, which is the base particle, cannot be avoided, resulting in a decrease in electronic conductivity.

In the negative electrode active material described in Patent Document 3, since the carbon material contains both boron and nitrogen, it has the same effect of improving electronic conductivity as the silicon conductive material coating described in Patent Document 1. There are concerns that they will be offset.

Here, Patent Document 3 describes that a carbon material is produced by heating petroleum coke powder at 2800° C. for 1 hour in an argon atmosphere. When heat treatment is performed at such a very high temperature, the amorphous portions that disappear during the heat treatment remain as scattering marks and exist in the carbon material as micropores. Although the electronic conductivity of the carbon material obtained by high-temperature heat treatment improves, the specific surface area becomes extremely large.

As the specific surface area of the negative electrode active material increases, as described in JP-A-9-199126, there are more starting points for undesirable reactions such as side reactions with the electrolyte, and the interface between the electrolyte and the electrode increases. It is known that an excessive SEI (Solid Electrolyte Interface) film is formed. When the SEI film is excessively formed, the irreversible capacity of the battery increases and the discharge capacity decreases.

The present invention solves the above problems, and aims to provide a composite material with high electronic conductivity and a small specific surface area, and a lithium ion secondary battery equipped with a negative electrode containing such a composite material. shall be.

The composite material of the present invention is
silicon oxide,
a carbonaceous material covering the silicon oxide;
Equipped with
The carbonaceous material is characterized in that it is doped with nitrogen.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode, and an electrolyte, and the negative electrode includes the above-mentioned composite material.

The composite material of the present invention includes silicon oxide and a carbonaceous material covering the silicon oxide, and the carbonaceous material has a structure in which nitrogen is doped, so that it has high electronic conductivity and a specific surface area. It has small characteristics.

In the lithium ion secondary battery of the present invention, since the negative electrode includes the above-mentioned composite material, the discharge capacity is improved, and thereby the charging and discharging efficiency is also improved.

1 is a cross-sectional view schematically showing the structure of a composite material of the present invention. FIG. 3 illustrates various substitutional types of nitrogen-doped carbonaceous materials. (a) shows a bright field image of the composite material of Example 1 by STEM-EDX, and (b) shows an element mapping image of C by STEM-EDX. (a) shows a bright field image of the composite material of Example 5 by STEM-EDX, and (b) shows an element mapping image of C by STEM-EDX. (a) to (e) are FE-SEM observation photographs of the composite materials of Examples 1 to 5, respectively. (a) to (c) are FE-SEM observation photographs of the materials of Comparative Examples 1 to 3, respectively.

Embodiments of the present invention will be shown below, and features of the present invention will be specifically explained.

FIG. 1 is a cross-sectional view schematically showing the structure of a composite material 10 of the present invention. Composite material 10 of the present invention includes silicon oxide 1 and carbonaceous material 2 covering silicon oxide 1. The carbonaceous material 2 is doped with nitrogen. That is, the composite material 10 of the present invention includes silicon oxide 1 and a carbonaceous material 2 covering the silicon oxide 1, and the carbonaceous material 2 is doped with nitrogen (hereinafter referred to as the requirement of the present invention). requirements). As will be described later, by doping the carbonaceous material 2 with nitrogen, the volume resistivity can be reduced without performing heat treatment at a high temperature. Further, the doped nitrogen blocks the pores of the carbonaceous material 2, thereby suppressing an increase in the specific surface area.

The composite material 10 of the present invention is used, for example, as a negative electrode material for a lithium ion secondary battery. A lithium ion secondary battery using the composite material 10 of the present invention as a negative electrode material has an improved discharge capacity as described later.

Here, in the present invention, "doped with nitrogen" means that when the surface of the carbonaceous material 2 is analyzed by X-ray photoelectron spectroscopy (XPS), a surface analysis (Nls spectrum) at a depth of several nm This means that N is detected in .

Note that chemical shifts occur due to differences in the bonding state and substitution position of N, and by evaluating this, it is possible to identify and quantitatively separate the position in the skeleton at which N is substituted with C. FIG. 2 shows various substitution types of nitrogen-doped carbonaceous materials. Among the various substitution types shown in FIG. 2, the pyridine-like substitution type is said to be effective in improving electronic conductivity. The pyridine-like substituted type is a substituted type in which N is bonded to C2 in a six-membered ring structure, as shown in FIG.

<Example 1>
(Operation A1)
Sucrose (Kishida Chemical Co., Ltd.) and dicyandiamide (Fuji Film Wako Pure Chemical Industries, Ltd.) were mixed in pure water and dissolved by stirring with a magnetic stirrer, so that 0.5M of sucrose and 0.25M of dicyandiamide were dissolved. A 100 mL aqueous solution was prepared.

(Operation A2)
A silicon oxide dispersion was prepared by adding 10 g of silicon oxide (5 μm) to the aqueous solution prepared in Operation A1 and performing a dispersion treatment for 8 minutes using a digital ultrasonic cleaner.

(Operation A3)
The silicon oxide dispersion prepared in Step A2 was added to the reaction vessel "autoclave unit", and water was poured under airtight conditions at a maximum temperature of 200°C and a holding time of 5 hours, while stirring at a speed of 300 rpm using a stirring blade. caused a thermal reaction.

(Operation A4)
After cooling the autoclave unit, the slurry inside the unit was added to a disposable cup. Subsequently, the slurry was separated into the synthesized composite material and the aqueous solution by filtration. Specifically, the composite material was washed six times with 50 mL of pure water until the filtrate became transparent, and then washed six times with 50 mL of ethanol until the filtrate became transparent. Then, the composite material left on the filter paper was collected using a spatula or the like. Subsequently, the recovered composite material was dried at 50° C. for 3 hours under atmospheric conditions using a constant temperature dryer.

(Operation A5)
The composite material obtained through the process of Operation A4 was crushed in an agate mortar.

(Operation A6)
The composite material powder obtained through the process of step A5 was subjected to focused ion beam processing using FE-TEM/EDX (multifunctional electron microscope "JEM-F200" manufactured by JEOL Ltd., Hitachi High-Technology Corporation). STEM-EDX mapping analysis was performed using an energy-dispersive A bright field image obtained by STEM-EDX is shown in FIG. 3(a), and a C elemental mapping image obtained by STEM-EDX is shown in FIG. 3(b).

As shown in FIGS. 3(a) and 3(b), the composite material of Example 1 has a structure in which a carbonaceous material coats silicon oxide. In the example shown in FIG. 3(a), the thickness of the carbonaceous material covering silicon oxide is 20 nm or more and 123 nm or less.

(Operation A7)
The specific surface area of the composite material powder obtained in step A5 was measured by the BET 1-point method using a specific surface area measuring device "BELSORP MR6" manufactured by Microtrac Bell Co., Ltd. .

(Operation A8)
The composite material powder obtained in step A5 is pressed, and a wide scan spectrum is measured on its surface using an XPS (X-ray photoelectron spectroscopy) device "VersaProbe" manufactured by ULVAC-PHI. Qualitative and semi-quantitative analyzes were performed from the measurements. The measurement area is within a circle with a diameter of 100 μm, and the analysis depth is several nm.

(Operation A9)
The volume resistivity of the powder when a load is applied was measured using a powder resistance measurement system "MCP-PD51" manufactured by Mitsubishi Chemical Analytech Co., Ltd. for the composite material powder obtained in step A5. The comb density was measured.

<Example 2>
The powder obtained in Operation A5 of Example 1 was added to an alumina crucible and heat-treated in a firing furnace. The heat treatment conditions were as follows: the maximum temperature was 400°C, the holding time was 1 hour, the temperature increase rate was 8°C/min, and the heat treatment was performed in an atmosphere in which nitrogen (N ₂ ) gas was constantly flowing at 5 L/min. A composite material was created. Further, the produced composite material was subjected to the above-mentioned operations A7, A8, and A9.

<Example 3>
The composite material of Example 3 was produced in the same manner as the method of producing the composite material of Example 2, except that the maximum temperature under the heat treatment conditions of Example 2 was 800°C. Further, the produced composite material was subjected to the above-mentioned operations A7, A8, and A9.

<Example 4>
The composite material of Example 4 was produced in the same manner as the method of producing the composite material of Example 2, except that the maximum temperature under the heat treatment conditions of Example 2 was 500°C. Further, the produced composite material was subjected to the above-mentioned operations A7, A8, and A9.

<Example 5>
The composite material of Example 5 was produced in the same manner as the method of producing the composite material of Example 2, except that the maximum temperature under the heat treatment conditions of Example 2 was 600°C. Further, the produced composite material was subjected to the above-described operations A6, A7, A8, and A9. A bright field image of the composite material of Example 5 by STEM-EDX is shown in FIG. 4(a), and an elemental mapping image of C by STEM-EDX is shown in FIG. 4(b).

The composite materials of Examples 1 to 5 described above are composite materials that meet the requirements of the present invention. FIGS. 5(a) to 5(e) show photographs of the composite materials of Examples 1 to 5 taken by FE-SEM (field emission scanning electron microscope), respectively.

Examples 6 to 8 described below are examples of lithium ion secondary batteries using composite materials that meet the requirements of the present invention as negative electrode materials. As described below, the lithium ion secondary batteries of Examples 6 to 8 included a positive electrode, a negative electrode, and an electrolyte, and the negative electrode contained a composite material that met the requirements of the present invention.

<Example 6>
(Operation B1)
28.5% by mass of the composite material powder of Example 3, 66.5% by mass of artificial graphite (10 μm), 1% by mass of carbon black, and 4% by mass of polyvinylidene fluoride were mixed, and further, N-methylpyrrolidone was added. A slurry was prepared by stirring with a hybrid mixer.

(Operation B2)
The slurry obtained through the process of Operation B1 was applied onto a 20 μm thick copper foil using a desktop coater, and vacuum dried at 120° C. for 1 hour. Thereafter, a negative electrode was produced by applying pressure with a press using a hydraulic cylinder.

(Operation B3)
The negative electrode produced in Operation B2 was punched into a circular shape with a diameter of 17 mm using an ultra-precision hand punch. Next, the punched negative electrode and a 300 μm lower plate were laminated on the negative electrode case. Thereafter, a separator was punched into a circular shape with a diameter of 17.5 mm inside the negative electrode case and sealed with a gasket.

(Operation B4)
An electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ), which is a non-aqueous electrolyte, at a concentration of 1M in a mixed solution of ethylene carbonate and diethyl carbonate at a volume ratio of 1:1.

(Operation B5)
The electrolytic solution prepared in operation B4 was impregnated onto the separator, and the electrolytic solution penetrated into the voids in the negative electrode.

(Operation B6)
The lithium foil used for the positive electrode was punched out into a circular shape with a diameter of 16.5 mm. Next, a positive electrode was laminated on the separator impregnated with the electrolytic solution, and an aluminum upper plate having a thickness of 12 μm was further laminated.

(Operation B7)
A coin cell having a diameter of 20 mm and a thickness of 1.6 mm was produced by joining the positive electrode case to the negative electrode case with a gasket placed around the periphery and sealing the case using a caulking machine. This coin cell is a lithium ion secondary battery.

(Operation B8)
An initial charging/discharging test was conducted on the coin cell obtained through the process of Operation B7 using a battery charging/discharging device "HJ1010mSM8A" and "HJ1005SM8A" manufactured by Hokuto Denko under the following conditions. First, charging was performed at a constant current of 0.2 mA/cm ² until the voltage of the coin cell reached 0V, and after reaching 0V, charging was performed by reducing the current so as to maintain the cell voltage at 0V. Thereafter, charging was terminated when the current value fell below 0.04 mA. Discharge was performed at a constant current of 0.2 mA/cm ² and was terminated when the cell voltage reached 1.5 V, and the discharge capacity was determined. The above charge/discharge test was repeated again to determine the first charge capacity, first discharge capacity, first charge/discharge efficiency, second charge capacity, second discharge capacity, and second charge/discharge efficiency. The initial charge/discharge efficiency is the ratio of the initial discharge capacity to the initial charge capacity (initial discharge capacity/initial charge capacity×100). Further, the second charge/discharge efficiency is the ratio of the second discharge capacity to the second charge capacity (second discharge capacity/second charge capacity×100).

<Example 7>
The coin cell of Example 7 was produced in the same manner as the method of producing the coin cell of Example 6, except that the composite material of Example 4 was used as the negative electrode active material. This coin cell is also a lithium ion secondary battery.

<Example 8>
The coin cell of Example 8 was produced in the same manner as the method of producing the coin cell of Example 6, except that the composite material of Example 5 was used as the negative electrode active material. This coin cell is also a lithium ion secondary battery.

<Comparative example 1>
A composite material of Comparative Example 1 was produced in the same manner as the method of producing the composite material of Example 4, except that in operation A1 described above, an aqueous solution was produced without adding dicyandiamide. Since dicyandiamide was not added, in the composite material of Comparative Example 1, the carbonaceous material covering the silicon oxide was not doped with nitrogen. Further, the produced composite material was subjected to the above-mentioned operations A7, A8, and A9.

<Comparative example 2>
A composite material of Comparative Example 2 was produced in the same manner as the method of producing the composite material of Example 5, except that in operation A1, an aqueous solution was produced without adding dicyandiamide. Since dicyandiamide was not added, in the composite material of Comparative Example 2, the carbonaceous material covering the silicon oxide was not doped with nitrogen. Further, the produced composite material was subjected to the above-mentioned operations A7, A8, and A9.

<Comparative example 3>
The silicon oxide (5 μm) used in producing the composite material of Example 1 was prepared as a sample of Comparative Example 3. Further, the prepared silicon oxide was subjected to the above-described operations A7, A8, and A9.

<Comparative example 4>
The silicon oxide (5 μm) used in producing the composite material of Example 1 was added to an alumina crucible and heat-treated in a firing furnace. The heat treatment conditions were a maximum temperature of 600°C, a holding time of 1 hour, a temperature increase rate of 8°C/min, and a heat treatment in an atmosphere in which nitrogen (N ₂ ) gas was constantly flowing at 5 L/min. The material was prepared. This material does not have silicon oxide coated with a carbonaceous material. Further, the produced material was subjected to the above-mentioned operations A7 and A9.

The materials of Comparative Examples 1 to 4 described above are materials that do not meet the requirements of the present invention. FIGS. 6(a) to 6(c) show photographs taken by FE-SEM (field emission scanning electron microscope) of the materials of Comparative Examples 1 to 3, respectively.

Comparative Examples 5 and 6 described below are examples of lithium ion secondary batteries using materials that do not meet the requirements of the present invention as negative electrode materials.

<Comparative example 5>
A coin cell of Comparative Example 5 was produced in the same manner as the method of producing the coin cell of Example 6, except that the composite material of Comparative Example 2 was used as the negative electrode active material. This coin cell is a lithium ion secondary battery. Further, the produced coin cell was subjected to the process of operation B8 described above.

<Comparative example 6>
A coin cell of Comparative Example 6 was produced in the same manner as the method of producing the coin cell of Example 6, except that silicon oxide of Comparative Example 3 was used as the negative electrode active material. Further, the produced coin cell was subjected to the process of operation B8 described above.

Table 1 shows the specific surface areas determined by the process of Operation A7 for the composite materials of Examples 1 to 5 and the materials of Comparative Examples 1 to 4 described above.

Further, Table 2 shows the results of qualitative and semi-quantitative analysis by XPS obtained by the process of operation A8 for the composite materials of Examples 1 to 5 and the materials of Comparative Examples 1 to 3 described above.

As shown in Table 2, the amount of nitrogen contained in the composite materials of Examples 1 to 5 is 3.0 atomic % or more and 8.5 atomic % or less. More specifically, the amount of nitrogen contained in the composite material is 3.1 atomic % or more and 8.1 atomic % or less.

Further, Table 3 shows the volume resistivity and bulk density determined by the process of Operation A9 for the composite materials of Examples 1 to 5 and the materials of Comparative Examples 1 to 4 described above.

In Table 3, the volume resistivity of the material of Comparative Example 4 is expressed as "O.L", which means over-range. That is, the volume resistivity of the material of Comparative Example 4 was so high that it was not possible to determine its value.

Table 4 summarizes the specific surface area, volume resistivity, and compatibility of specific surface area and volume resistivity of the composite materials of Examples 1 to 5 and the materials of Comparative Examples 1 to 4 described above.

Compatibility of specific surface area and volume resistivity means that the compatibility is good when the specific surface area is 80 m ² /g or less and the volume resistivity is less than 1.0 × 10 ⁹ Ω・cm. The other cases were marked as "x", which means poor compatibility.

As shown in Table 4, the compatibility of specific surface area and volume resistivity of all the composite materials of Examples 1 to 5 was "○". On the other hand, all of the materials of Comparative Examples 1 to 4 were rated "x" in terms of compatibility between specific surface area and volume resistivity. That is, a composite material that satisfies the requirements of the present invention has high electronic conductivity and a small specific surface area.

Further, among the composite materials of Examples 1 to 5, the composite materials of Examples 3 to 5 have a volume resistivity of 10×10 ² Ω·cm or less, which is higher in electronic conductivity. As shown in Table 2, in the composite materials of Examples 3 to 5, the amount of nitrogen contained in the composite material is 3.0 at % or more and 7.0 at % or less. Therefore, the amount of nitrogen contained in the composite material is preferably 3.0 atomic % or more and 7.0 atomic % or less. More specifically, the amount of nitrogen contained in the composite material is preferably 3.1 atomic % or more and 6.2 atomic % or less.

Note that the composite materials of Examples 2 to 5 were obtained by heat-treating the prepared composite materials, but the composite materials of Examples 3 to 5, which were heated at a temperature of 500°C or higher and 800°C or lower, were heated at 400°C. The volume resistivity was considerably lower than that of the composite material of Example 2. Therefore, the temperature at which the produced composite material is heated is preferably higher than 400°C, more preferably 500°C or higher and 800°C or lower.

Further, among the composite materials of Examples 1 to 5, the composite materials of Examples 4 and 5 have a specific surface area of 45 m ² /g or less, and a volume resistivity of 10×10 ² Ω·cm or less, It has higher electronic conductivity and smaller specific surface area. As shown in Table 2, in the composite materials of Examples 4 and 5, the amount of nitrogen contained in the composite material is 4.0 atomic % or more and 6.5 atomic % or less. Therefore, the amount of nitrogen contained in the composite material is more preferably 4.0 atomic % or more and 6.5 atomic % or less. More specifically, the amount of nitrogen contained in the composite material is more preferably 4.4 atomic % or more and 6.2 atomic % or less.

Among the materials of Comparative Examples 1 to 4, the material of Comparative Example 4 was obtained by heat-treating silicon oxide not covered with a carbonaceous material at 600°C. Therefore, due to oxidation of silicon oxide, the volume resistivity became so large that it became impossible to measure it.

On the other hand, in the composite material of Example 5, in which silicon oxide coated with a nitrogen-doped carbonaceous material was heat-treated at 600°C, the same as in Comparative Example 4, the oxidation of silicon oxide was suppressed, so that the volume The resistivity was extremely small at 3.2×10 Ω·cm.

As mentioned above, the composite material of the present invention in which silicon oxide is coated with a nitrogen-doped carbonaceous material has a low volume resistivity. Normally, in order to reduce the volume resistivity of a composite material to the desired value, high-temperature heat treatment of 800°C or higher is required after the composite material is manufactured, but silicon oxide is coated with a nitrogen-doped carbonaceous material. By doing so, it becomes possible to set the temperature during the heat treatment to a low temperature of 800° C. or lower (for example, 600° C. or lower). Thereby, oxidation of silicon oxide due to high-temperature heating can be suppressed, and a decrease in electronic conductivity can be suppressed. In addition, since the doped nitrogen closes the pores of the carbonaceous material, an increase in specific surface area due to heat treatment can be suppressed.

The first charge capacity, first discharge capacity, first charge/discharge efficiency, second charge capacity, second discharge capacity, and second charge/discharge efficiency obtained for the coin cells of Examples 6 to 8 and Comparative Examples 5 to 6 described above are shown. 5.

The coin cell of Example 7 using the composite material of Example 4, which has both low volume resistivity and low specific surface area, as the negative electrode active material, and the composite material of Example 5, which has both low volume resistivity and low specific surface area, as the negative electrode active material. The coin cell of Example 8 used had a higher initial discharge capacity than the coin cell of Example 6 and the coin cells of Comparative Examples 5 and 6, and therefore had a high initial charge/discharge efficiency of 80% or more. In addition, the second charge/discharge efficiency was as high as 97% or more.

On the other hand, in the coin cell of Comparative Example 5 in which the composite material of Comparative Example 2, which has a large specific surface area, was used as the negative electrode active material, the initial discharge capacity was lower than that of the coin cells of Examples 6 to 8, and the initial charge/discharge efficiency was also 62. It was as low as .8%. Furthermore, the second discharge capacity was also lower. This is because during the first charge and discharge, there are many starting points for undesirable reactions such as side reactions during decomposition of the electrolyte, and an excessive SEI film is formed at the interface between the electrolyte and the electrode. It is assumed that this is the case.

Furthermore, in the coin cell of Comparative Example 6 in which the composite material of Comparative Example 3, which has a small specific surface area but a high volume resistivity, was used as the negative electrode active material, electron conductivity within the electrode was low, so electron movement was small. Therefore, compared to the coin cells of Examples 6 to 8, the first discharge capacity and second discharge capacity are lower.

The present invention is not limited to the above embodiments, and various applications and modifications can be made within the scope of the present invention.

1 Silicon oxide 2 Carbonaceous material 10 Composite material

Claims (4)

silicon oxide,
a carbonaceous material covering the silicon oxide;
Equipped with
The carbonaceous material is a composite material doped with nitrogen,
A composite material characterized in that the amount of nitrogen contained in the composite material is 3.0 atomic % or more and 8.5 atomic % or less . silicon oxide,
a carbonaceous material covering the silicon oxide;
Equipped with
The carbonaceous material is a composite material doped with nitrogen,
A composite material characterized in that the amount of nitrogen contained in the composite material is 3.0 atomic % or more and 7.0 atomic % or less. silicon oxide,
a carbonaceous material covering the silicon oxide;
Equipped with
The carbonaceous material is a composite material doped with nitrogen,
A composite material characterized in that the amount of nitrogen contained in the composite material is 4.0 atomic % or more and 6.5 atomic % or less. A lithium ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte,
A lithium ion secondary battery, wherein the negative electrode contains the composite material according to any one of claims 1 to 3 .

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