Patent Application
20130149606
The present invention relates to a negative-electrode material powder, which can be used to obtain a lithium-ion secondary battery that is durable at practical level with large discharge capacity and satisfactory cycle characteristics. The present invention also relates to a negative electrode for a lithium-ion secondary battery and a negative electrode for a capacitor using this negative-electrode material powder, and a lithium-ion secondary battery and a capacitor.
In accordance with recent noticeable developments in the portable electronic equipment, communication equipment and the like, development of secondary batteries with high energy density is strongly demanded from the viewpoint of the economic efficiency and reduction in size and weight of the equipment. Currently available secondary batteries with high energy density include a nickel-cadmium battery, a nickel hydrogen-battery, a lithium-ion secondary battery, a polymer battery and the like. Among these batteries, the demand for the lithium-ion secondary battery is strongly growing in the power source market due to its dramatically enhanced life and capacity, compared with the nickel cadmium battery or nickel-hydrogen battery.
The positive electrode 1 includes a counter electrode case 1a, a counter electrode current collector 1b and a counter electrode 1c, where lithium cobaltate (LiCoO2) or manganese spinel (LiMn2O4) is mainly used for the counter electrode 1c. The negative electrode 2 includes a working electrode case 2a, a working electrode current collector 2b and a working electrode 2c, where a negative-electrode material used for the working electrode 2c generally includes an active material capable of occluding and releasing lithium ions (negative electrode active material), a conductive auxiliary agent, and a binder.
As a negative electrode active material for lithium-ion secondary battery, conventionally, a carbon-based material is used. As a new negative-electrode active material which can enhance the capacity of lithium-ion secondary battery more than in the past, a composite oxide of lithium and boron, a composite oxide of lithium and transition metal (V, Fe, Cr, Mo, Ni, etc.), a compound including Si, Ge or Sn, N and O, Si particle having a surface coated with a carbon layer by chemical vapor deposition, and the like are proposed.
However, each of these negative electrode active materials enhances the expansion or contraction thereof during occlusion and release of lithium ions, although it can improve the charging and discharging capacity to enhance the energy density. Therefore, lithium-ion secondary batteries using these negative electrode active materials are insufficient in the sustainability of discharge capacity against the repeated charging and discharging (hereinafter referred to as “cycle characteristics”).
On the other hand, it is conventionally attempted to use powders of silicon oxide represented by SiOx (0<x≦2), such as SiO, as the negative electrode active material. The silicon oxide can be a negative electrode active material with larger effective charging and discharging capacity, since the deterioration during charging and discharging such as the collapse of a crystal structure due to the occlusion and release of lithium ions or the generation of an irreversible substance is small. Therefore, the silicon oxide can be used as the negative electrode active material to obtain a lithium-ion secondary battery which is higher in capacity than the case using carbon and is more satisfactory in cycle characteristics than the case using a high-capacity negative-electrode material such as Si or Sn alloy.
When the silicon oxide powder is used as the negative electrode active material, in general, a carbon powder or the like is mixed thereto as a conductive auxiliary agent for compensating the low electric conductance of the silicon oxide. This allows securement of the electric conductivity in the vicinity of the portion of contact between the silicon oxide powder and the conductive auxiliary agent. However, in portions away from the contact portion, the silicon oxide powder is less likely to function as the negative electrode active material since the electric conductivity cannot be secured.
In order to solve this problem, proposed in Patent Literature 1 is a conductive silicon composite for non-aqueous electrolyte secondary battery negative-electrode material including a film of carbon formed on the surface of a particle having a structure in which microcrystals of silicon are dispersed in silicon dioxide (conductive silicon composite), and a method for producing the same.
According to the method proposed in Patent Literature 1, the formation of a uniform carbon film on the conductive silicon composite allows the securement of sufficient electric conductivity. However, according to the present inventors' examinations, a lithium-ion secondary battery using the conductive silicon composite of Patent Literature 1 had a problem such that since the silicon dioxide with microcrystals of silicon dispersed therein is used as the negative-electrode material, the expansion and contraction are intensified in the occlusion and release of lithium ions during charging and discharging, causing a sudden drop of capacity at a certain point of time when the charging and discharging cycle is repeated. Further, the lithium-ion secondary battery was not sufficient in discharge capacity and cycle characteristics.
To solve this problem, the present inventors made various examinations, particularly, on silicon oxide that is regarded as being a negative-electrode material powder (negative electrode active material) capable of enhancing the capacity of lithium-ion secondary battery. As a result, it was found that the deterioration of initial efficiency (the ratio of discharge capacity to charge capacity in first charging and discharging after shipment for a lithium-ion secondary battery (in initial charging and discharging)) is due to the formation of Li4SiO4, shown by the following expression (1). On the right-hand side of the expression (1), a first term Li22Si5 is a component bearing a reversible capacity, and a second term Li4SiO4 is a component bearing an irreversible capacity. Li4SiO4 cannot release lithium ions.
SiOx(44−x)10Li++(44−x)10e−→(4−x)/20Li22Si5+x/4Li4SiO4 (1)
The present inventors' examinations revealed that when silicon oxide (SiOx) is used as the negative-electrode material powder, and x=1, the lithium-ion secondary battery theoretically has characteristics of reversible capacity of 2007 mAh/g and initial efficiency of 76%. Since the reversible capacity was less than or comparable to about 1500 mAh/g in a conventional lithium-ion secondary battery using silicon oxide as the negative-electrode material powder, there is still room for improvement with respect to the reversible capacity of the lithium-ion secondary battery using silicon oxide as the negative-electrode material powder.
The present inventors' further examinations revealed a defect in a negative-electrode material powder having a carbon film which is formed on the surface of silicon oxide powder under the conditions described in Patent Literature 1, wherein the cycle characteristics of a resulting lithium-ion secondary battery is poor due to high crystallinity of the carbon film. This is considered to be attributable to the fact that the higher the crystallinity of the carbon film to be formed on the surface of silicon oxide powder is, the smaller the speed of receiving lithium ions is, and the lower the capability of relaxing the expansion and contraction of silicon oxide is.
In view of this problem, the present invention has an object to provide a negative-electrode material powder for a lithium-ion secondary battery which is durable at practical level with large discharge capacity and satisfactory cycle characteristics, a negative electrode for a lithium-ion secondary battery and a negative electrode for a capacitor using this negative-electrode material powder, and a lithium-ion secondary battery and a capacitor.
In order to solve the above-mentioned problem, the present inventors reviewed deterring the crystallinity of carbon film. As a result, it was found that carbon low in crystallinity can be obtained by setting the treatment temperature for carbon film formation to a temperature of not lower than 700° C. and not higher than 750° C., which is lower than the temperature range as being 800° C. or higher described in Patent Literature 1. However, it was also found that when the carbon film-forming treatment is performed at not lower than 700° C. and not higher than 750° C., tar components composed of macromolecular hydrocarbon, which are generated in thermal decomposition of organic matter as a carbon source, are apt to be left on the carbon film, and further the tar components are, as described later, correlated with the characteristics of lithium-ion secondary battery such that the larger the residual amount of the tar components, the poorer the cycle characteristics of the lithium-ion secondary battery.
This correlation results from that the existence of tar components facilitates the separation of the carbon film from the silicon oxide powder during charging and discharging of the lithium-ion secondary battery, and the separation progresses in accordance with increase in the number of charge/discharge cycles to deteriorate the performance of the battery. Additionally, the existence of tar components leads to increase in the irreversible capacity of the lithium-ion secondary battery since the tar components react with lithium and, furthermore, carbon containing the tar components causes an energy loss due to its high electric resistance.
Therefore, the present inventors examined about a method for removing the tar components, paying attention to the heating of carbon film-formed silicon oxide powder under vacuum (hereinafter also referred to as “vacuum treatment”), and confirmed that the tar components can be removed by this treatment. However, in the course of the examination, a problem occurred such that an extremely high heating temperature in the vacuum treatment leads to reduction in battery capacity since SiC is generated in the vicinity of the interface between silicon oxide and carbon film, and the amount of Si contributable to the battery capacity is reduced due to the generation of SiC. With respect to this problem, the present inventors found that the problem can be prevented by setting the heating temperature in the range of not lower than 600° C. to not higher than 750° C.
The present invention has been achieved based on the above-mentioned findings, and the summaries thereof lie in a negative-electrode material powder for lithium-ion secondary battery in the following (1) to (6), a negative electrode for lithium-ion secondary battery of the following (7), a negative electrode for capacitor of the following (8), a lithium-ion secondary battery of the following (9), and a capacitor of the following (10).
(1) A negative-electrode material powder for a lithium-ion secondary battery having a conductive carbon film on the surface of a lower silicon oxide powder, characterized in that the total content of tar components measured by TPD-MS is not less than 1 ppm by mass and not more than 4000 ppm by mass, and peaks exist at 1350 cm−1 and 1580 cm−1 in the Raman spectrum, while the peak at 1580 cm−1 has a half-value width of not less than 50 cm−1 and not more than 100 cm−1.
(2) The negative-electrode material powder for the lithium-ion secondary battery according to (1), characterized in that a specific surface area measured by the BET method is not less than 0.3 m2/g and not more than 40 m2/g.
(3) The negative-electrode material powder for the lithium-ion secondary battery according to (1) or (2), characterized in that in the Raman spectrum, the ratio I1350/I1580 of the height (I1350) of the peak at 1350 cm−1 to the height (I1580) of the peak at 1580 cm−1 satisfies 0.1<I1350/I1580<1.4.
(4) The negative-electrode material powder for the lithium-ion secondary battery according to any one of (1) to (3), characterized in that the proportion of the conductive carbon film is not less than 0.2 mass % and not more than 10 mass %.
(5) The negative-electrode material powder for the lithium-ion secondary battery according to any one of (1) to (4), characterized in that a specific resistance is not more than 40000 Ωcm.
(6) The negative-electrode material powder for the lithium-ion secondary battery according to any one of (1) to (5), characterized in that a maximum value P1 of SiOx-derived halos which appear at 2θ=10 to 30° and a value P2 of the strongest line peak of Si (111) which appears at 2θ=28.4±0.3°, in measurement by XRD using CuKα ray, satisfy a relationship of P2/P1<0.01.
(7) A negative electrode for a lithium-ion secondary battery, using the negative-electrode material powder for the lithium-ion secondary battery according to any one of (1) to (6).
(8) A negative electrode for a capacitor, using the negative-electrode material powder for the lithium-ion secondary battery according to any one of (1) to (6).
(9) A lithium-ion secondary battery, using the negative electrode for the lithium-ion secondary battery according to (7).
(10) A capacitor, using the negative electrode for the capacitor according to (8).
In the present invention, the “lower silicon oxide powder” means a powder of SiOx which satisfies 0.4≦x≦1.2. The measuring method of the x of SiOx, the measuring method of the specific surface area by the BET method and the measuring method of the tar component content by TPD-MS will be described later.
With respect to the lower silicon oxide powder, as described later, the wording of “including a conductive carbon film formed on the surface” indicates that the value of mole ratio Si/C of Si to C based on surface analysis using an X-ray photoelectron spectrometer is 0.02 or less, or means a state such that most of the surface of the lower silicon oxide powder is covered with C with almost no exposure of Si.
The negative-electrode material powder for a lithium-ion secondary battery and the negative electrode for a lithium-ion secondary battery or negative electrode for a capacitor of the present invention can be used to obtain a lithium-ion secondary battery or a capacitor which are durable at practical level with large discharge capacity and satisfactory cycle characteristics. The lithium-ion secondary battery and capacitor of the present invention are large in discharge capacity and satisfactory in cycle characteristics.
The negative-electrode material powder for a lithium-ion secondary battery of the present invention is a negative-electrode material powder for a lithium-ion secondary battery having a conductive carbon film formed on the surface of a lower silicon oxide powder, in which the total content of tar components measured by TPD-MS is not less than 1 ppm by mass and not more than 4000 ppm by mass, and peaks exist at 1350 cm−1 and 1580 cm−1 in the Raman spectrum, while the peak at 1580 cm−1 has a half-value width of not less than 50 cm−1 and not more than 100 cm−1.
The lower silicon oxide powder is, as described above, a powder of SiOx which satisfies 0.4≦x≦1.2. The reason for setting the x to this range is that when the value of x is below 0.4, the deterioration associated with charging and discharging cycles becomes serious in a lithium-ion secondary battery and a capacitor each using the negative-electrode material powder of the present invention, and when the value exceeds 1.2, the battery capacity is reduced. The x preferably satisfies 0.8≦x≦1.05.
By forming the conductive carbon film on the lower silicon oxide powder that is an insulator, the discharge capacity of a lithium-ion secondary battery using this lower silicon oxide powder as the negative-electrode material powder can be improved.
In the negative-electrode material powder for a lithium-ion secondary battery of the present invention, the total content of tar components is not less than 1 ppm by mass and not more than 4000 ppm by mass. When the total content of tar components is higher than 4000 ppm by mass, a resulting lithium-ion secondary battery is insufficient in the resistance to the expansion and contraction of negative electrode associated with charging and discharging, and inferior in cycle characteristics. On the other hand, when the total content is 4000 ppm by mass or less, a lithium-ion secondary battery satisfactory in initial efficiency and cycle characteristics can be obtained, and the cycle characteristics are particularly enhanced. The initial efficiency and cycle characteristics are further enhanced at 2000 ppm by mass or less. On the other hand, when the total content of tar components is less than 1 ppm by mass, the vacuum treatment of the negative-electrode material powder for lithium-ion secondary battery is prolonged, leading to increase in manufacturing cost. Therefore, the total content of tar components is more preferably set to not less than 40 ppm by mass.
The “peaks exist at 1350 cm−1 and 1580 cm−1 in the Raman spectrum” means that carbon exists in a sample being measured. The peak at 1350 cm−1 is a peak derived from disorder in crystal structure of carbon, which is called D band, and means that carbon is almost amorphous.
The peak at 1580 cm−1 is a peak derived from graphite structure, which is called G band, and the larger the half-value width is, the lower the crystallinity of graphite is. Carbon having high crystallinity in which the half-value width of the peak at 1580 cm−1 is less than 50 cm−1 causes the deterioration of the cycle characteristics of the lithium-ion secondary battery due to low capability of relaxing the expansion and contraction of silicon oxide in addition to low receiving rate of lithium ions. On the other hand, when the half-value width exceeds 100 cm−1, the electric conductivity of the carbon film is reduced. The half-value width of the peak at 1580 cm−1 is thus preferably not less than 60 cm−1 and not more than 90 cm−1.
It is preferred that in the Raman spectrum, the ratio I1350/I1580 of the height I1350 of the peak at 1350 cm−1 to the height I1580 of the peak at 1580 cm−1 satisfies 0.1<I1350/I1580<1.4. When I1350/I1580 is 0.1 or less, the cycle characteristics of lithium-ion secondary battery are deteriorated due to low capability of relaxing the expansion and contraction of silicon oxide in addition to low receiving rate of lithium ions, which result from high crystallinity of the carbon film. On the other hand, when the ratio is 1.4 or more, the electric conductivity of the carbon film is deteriorated.
In the Raman spectrum, as shown in
The end points of upper limit and lower limit of the baseline are concretely determined as follows. In the peak at 1350 cm−1, the upper limit of the baseline is 1450 cm−1, the lower limit is 1250 cm−1, the intensity at the upper limit is an average value of intensity in the range of 1450±5 cm−1, and the intensity at the lower limit is an average value of intensity in the range of 1550±5 cm−1. In the peak at 1350 cm−1, the upper limit of the baseline is 1650 cm−1, the lower limit is 1550 cm−1, the intensity at the upper limit is an average value of intensity in the range of 1650±5 cm−1, and the intensity at the lower limit is an average value of intensity in the range of 1550±5 cm−1.
The Raman spectrum shown in
In the negative-electrode material powder for lithium-ion secondary battery of the present invention, the specific surface area measured by the BET method is preferably not less than 0.3 m2/g and not more than 40 m2/g, more preferably, not less than 0.3 m2/g and not more than 5.0 m2/g. A small specific surface area of the negative-electrode material powder allows inhibition of the generation of the irreversible capacity component on the electrode surface during initial charging and discharging. When the specific surface area is 40 m2/g or less in powder having a particle size of about 10 μm, satisfactory performance of lithium-ion secondary battery can be secured with the generation of the irreversible capacity component being sufficiently reduced. When the specific surface area is 5.0 m2/g or less, the performance of lithium-ion secondary battery can be further enhanced. However, the production of powder having a specific surface area smaller than 0.3 m2/g is difficult to industrialize from an economic viewpoint.
The ratio of the conductive carbon film (hereinafter referred to as “carbon coating rate”) in the negative-electrode material powder for lithium-ion secondary battery is preferably not less than 0.2 mass % and not more than 10 mass %. This is based on the following reason.
Although the carbon film also contributes to the charging and discharging capacity of a lithium-ion secondary battery similarly to the lower silicon oxide, the charging and discharging capacity per unit mass thereof is small, compared with the lower silicon oxide. Therefore, the carbon coating rate of the negative-electrode material powder for lithium-ion secondary battery is preferred to be not more than 10 mass % from the viewpoint of securing the charging and discharging capacity of a lithium-ion secondary battery. On the other hand, when the carbon coating rate is smaller than 0.2 mass %, the conductivity-imparting effect by the conductive carbon film cannot be obtained, and a lithium-ion secondary battery using such a negative-electrode material powder hardly acts as a battery.
The specific resistance of the negative-electrode material powder for a lithium-ion secondary battery is preferably 40000 Ωcm or less. This is because when the specific resistance is larger than 40000 Ωcm, the powder hardly acts as the electrode active material of a lithium-ion secondary battery. The lower limit of the specific resistance does not particularly have to be specified since the smaller the specific resistance, the more satisfactory the electric conductivity, and a condition suitable for the electrode active material for a lithium-ion secondary battery is provided.
In the negative-electrode material powder for a lithium-ion secondary battery, it is preferred that a maximum value P1 of SiOx-incurred halos which appear at 10°≦2θ≦30° and a value P2 of the strongest line peak of Si (111) which appears at 2θ=28.4±0.3° in measurement by an X-ray refraction device (XRD) using CuKα ray satisfy a relationship of P2/P1<0.01, or that the negative-electrode material powder is amorphous. This is because it is acceptable for the lithium ion secondary battery that the lower silicon oxide powder in the negative-electrode material powder is amorphous.
The average particle size of the negative-electrode material powder for a lithium-ion secondary battery is preferably not less than 1 μm and not more than 15 μm, more preferably not less than 3 μm and not more than 12 μm. When the average particle size is too small, homogenous slurry cannot be formed in the production of the electrode, and the powder is apt to come off from a current collector. On the other hand, when the average particle size is too large, the preparation of an electrode film constituting the working electrode 2c shown in
The x of SiOx is the mole ratio (O/Si) of O content to Si content in the negative-electrode material powder for a lithium-ion secondary battery, and it can be calculated by use of the O content and Si content which are measured, for example, by the following measuring method.
The O content in the negative-electrode material powder for a lithium-ion secondary battery is calculated from the O content in a sample which is quantitatively evaluated by analyzing the sample of 10 mg by an inert gas fusion/infrared absorption method by use of an oxygen concentration analyzer (by LECO, TC 436).
The Si content in the negative-electrode material powder for a lithium-ion secondary battery is calculated from the Si content of a sample under quantitative evaluation by adding nitric acid and fluoric acid to the sample to dissolve the sample and analyzing the resulting solution by an ICP emission spectrophotometer (made by SHIMADZU). In this method, Si, SiO and SiO2 are dissolved, and Si constituting them can be detected.
In the negative-electrode material powder for a lithium-ion secondary battery of the present invention, the wording of “including a conductive carbon film formed on the surface of a lower silicon oxide powder” means that the mole ratio Si/C of Si to C is 0.02 or less when a lower silicon oxide powder subjected to the forming treatment of the conductive carbon film undergoes surface-analysis by an X-ray photoelectron spectrometer (XPS) using AlKα ray (1486.6 eV). The measurement conditions of XPS are shown in Table 1. The “Si/C is 0.02 or less” means a state such that most of the surface of the lower silicon oxide powder is covered with C, with almost no exposure of Si.
The specific surface area of a lower silicon oxide powder with a conductive carbon film being formed can be measured by the following BET method. A sample of 0.5 g is put in a glass cell and dried under reduced pressure at 200° C. for about 5 hours. The specific surface area is calculated from a nitrogen gas adsorption isotherm at liquid nitrogen temperature (−196° C.) which is measured with respect to this sample. The measurement condition is shown in Table 2.
The residual amount of tar component in the negative-electrode material powder for a lithium-ion secondary battery can be measured by the following TPD-MS (Temperature Programmed Desorption-Mass Spectroscopy). A sample of 50 mg is put in a silica-made cell and heated from room temperature to 1000° C. at a rate of 10 K/min in a helium gas flow of 50 mL/min. The gas being generated is analyzed by a mass analyzer (by SHIMADZU, GC/MS QP5050A).
The tar component means a high-molecular-weight component such as aromatic hydrocarbon, which is produced when the gas of hydrocarbon or organic matter is thermally decomposed. In the present invention, the total amount of constituents having molecular weights of 57, 106, 178, 202, 252 and 276 is regarded as the residual amount of tar component (refer to Table 5 to be described later). The typical chemical species of each molecular weight is xylene for 106, phenanthrene and antracene for 178, pyrene for 202, perylene and benzopyrene for 252, and pentacene and picene for 276.
The Raman spectrum is measured under conditions shown in Table 3 by use of a Raman spectrometric device.
The carbon coating rate is calculated from the mass of the negative-electrode material powder for a lithium-ion secondary battery and a result of carbon amount which is quantitatively evaluated by analyzing CO2 gas by an oxygen stream combustion-infrared absorption method by use of a carbon concentration analyzing device (by LECO, CS 400). A ceramic crucible is used as the crucible, copper is used as the combustion improver, and the analysis time is 40 seconds.
The specific resistance ρ (Ωcm) of the negative-electrode material powder for a lithium-ion secondary battery is calculated using the following equation (2).
ρ=R×A/L (2)
where R: electric resistance of sample (Ω), A: bottom area of sample (cm2), and L: thickness of sample (cm).
The electric resistance of a sample can be measured, for example, by a two-terminal method using a digital multimeter (by Iwatsu Test Instruments Corp., VOAC 7513). In this case, the sample of 0.20 g is filled in a powder resistance measuring tool (tool portion: made of stainless with an inside diameter of 20 mm, frame portion: made of polytetrafluoroethylene) and molded by pressurization at 20 kgf/cm2 for 60 seconds, and the thickness of the molded sample is measured using a micrometer.
The raw material chamber 6 is a cylindrical body, and includes a cylindrical raw material container 8 disposed at the center thereof and a heating source 10 disposed to surround the raw material container 8. As the heating source 10, for example, an electric heater can be used.
The precipitation chamber 7 is a cylindrical body disposed so that its axis coincides with that of the raw material container 8. The inner periphery of the precipitation chamber 7 is provided with a precipitation substrate 11 made of stainless steel to deposit gaseous silicon oxide generated by sublimation in the raw material chamber 6 thereon.
A vacuum device (not shown) for discharging the atmospheric gas is connected to the vacuum chamber 5 housing the raw material chamber 6 and the precipitation chamber 7 to discharge the gas in the direction of arrow A.
When a lower silicon oxide is produced using the production unit shown in
The formation of a conductive carbon film onto the surface of the lower silicon oxide powder is performed by means of CVD or the like. Concretely, the formation is performed using a rotary kiln as the device, and a mixed gas of hydrocarbon gas or an organic matter-containing gas, which is the carbon source, and inert gas as the gas.
When organic matter other than hydrocarbon is used as the carbon source, the amount of Si contributable to the occlusion and release of lithium ions is reduced since a component other than C and H, such as O or N, reacts with the silicon oxide to generate SiO2 or SiN, and the capacity of the lithium-ion secondary battery is thus reduced. Therefore, hydrocarbon gas composed of only C and H is preferred as the carbon source. When the hydrocarbon gas is used as the carbon source, aromatic series composed of only C and H are generated as the tar component, with constituents having molecular weights of 57, 106, 178, 202, 252 and 276 being main.
The treatment temperature for the formation of the conductive carbon film is not lower than 700° C. and not higher than 750° C. The treatment time is set, according to the thickness of a conductive carbon film to be formed, in the range of not less than 20 minutes to not more than 120 minutes. This treatment condition is within the extent that a conductive carbon film with low crystallinity can be obtained. It is also within the extent that the generation of SiC in the vicinity of the interface between the surface of the lower silicon oxide powder and the carbon film can be suppressed.
The lower silicon oxide powder with the thus-formed conductive carbon film is subjected to vacuum treatment, in which the powder is retained at a temperature of not lower than 600° C. and not higher than 750 ° C. for not less than 10 minutes under vacuum. The vacuum treatment is performed with the lower silicon oxide powder being stored in a vacuum tank, and the internal pressure of the vacuum tank is maintained at 1 Pa or less by use of an oil diffusion pump. The internal pressure is measured using a Pirani gauge.
The tar component produced during the formation of the carbon film can be volatilized and removed from the carbon film by this vacuum treatment. When the heating and retention temperature is in the above-mentioned range, the generation of SiC in the vicinity of the boundary between the silicon oxide and the carbon film is suppressed.
A configuration example of a coin-shaped lithium-ion secondary battery using the negative-electrode material powder for a lithium-ion secondary battery and negative electrode for a lithium-ion secondary battery of the present invention will be described in reference to
The negative-electrode material used for the working electrode 2c constituting the negative electrode 2 or the negative electrode for a lithium-ion secondary battery of the present invention is constituted using the negative-electrode material powder for a lithium-ion secondary battery of the present invention. Concretely, it can include the negative-electrode material powder for a lithium-ion secondary battery of the present invention that is an active material, other active materials, a conductive auxiliary agent and a binder. Among the constituent materials in the negative-electrode material, the proportion of the negative-electrode material powder for a lithium-ion secondary battery of the present invention to the total of the constituent materials except the binder is set to 20 mass % or more. The active materials other than the negative-electrode material powder for a lithium-ion secondary battery of the present invention do not necessarily have to be added. As the conductive auxiliary agent, for example, acetylene black or carbon black can be used, and as the binder, for example, polyacrylic acid (PAA) or polyvinylidene fluoride can be used.
The lithium-ion secondary battery of the present invention is durable at practical level with large discharge capacity and satisfactory cycle characteristics, since the above-mentioned negative-electrode material powder for a lithium-ion secondary battery and negative electrode for a lithium-ion secondary battery of the present invention are used.
The negative-electrode material powder of the present invention and the negative electrode using the same can be applied also to a capacitor.
For confirming the effects of the present invention, the following test using a lithium-ion secondary battery was carried out, and results thereof were evaluated.
The coin shape shown in
The negative electrode 2 is described first. Mixed granulated raw materials obtained by blending silicon powder and silicon dioxide powder in a predetermined proportion followed by mixing, granulating and drying were used as the raw materials, and a lower silicon oxide was precipitated on a precipitation substrate by use of the equipment shown in
A conductive carbon film was formed on the surface of this lower silicon oxide powder. For the formation of the carbon film, a rotary kiln was used as the device, and mixed gas of C4H10 (isobutane) and Ar was used as the gas. The treatment temperature for carbon film formation and the carbon coating rate are as shown in Table 4.
The lower silicon oxide powder with the conductive carbon film being formed was further subjected to vacuum treatment and made into a negative-electrode material powder for a lithium-ion secondary battery. The conditions shown in Table 4 were used as the vacuum treatment conditions (retention temperature and retention time), and the internal pressure of the vacuum tank was kept at 1 Pa or less by use of an oil diffusion pump. In the lower silicon oxide powder with the conductive carbon film being formed which was subjected to vacuum treatment, the specific surface area measured by the BET method was 3 m2/g, and the value of the above-mentioned P2/P1 measured by XRD was P2/P1=0.009.
Test Nos. 1 to 3 are Examples adopting a treatment temperature for carbon film formation of 700° C. and a vacuum treatment condition of 15 minutes or more at 750° C., or Inventive examples in which the total content of tar components in negative-electrode material powder for a lithium-ion secondary battery and the half-value width of the peak at 1580 cm−1 in the Raman spectrum satisfied the provision of the present invention. The total content of tar components was measured by TPD-MS. The molecular weight-based contents of tar components in the negative-electrode material powder for a lithium-ion secondary battery of Test No. 1 are shown in Table 5.
Test Nos. 4 and 5 are Examples adopting the same conditions as the Inventive examples except that the vacuum treatment was not performed or that the retention time of vacuum treatment was short, and Comparative examples in which the total content of tar components in the negative-electrode material powder for the lithium-ion secondary battery did not satisfy the provision of the present invention.
Test Nos. 6 and 7 are Examples adopting the same conditions as the Inventive examples except that the treatment temperature for carbon film formation was high such as 900° C. or 1100° C. and that the vacuum treatment was not performed, and Comparative examples in which the half-value width of the peak at 1580 cm−1 in the Raman spectrum in the negative-electrode material powder for the lithium-ion secondary battery did not satisfy the provision of the present invention.
Slurry was prepared by adding n-methylpyrrolidone to a mixture composed of 65 mass % of the negative-electrode material powder for lithium-ion secondary battery, 10 mass % of acetylene black and 25 mass % of PAA. This slurry was applied to a copper foil of 20 μm in thickness followed by drying for 30 minutes in an atmosphere of 120° C., and the resulting copper foil was punched into a size having an area of 1 cm2 per one side to form the negative electrode 2.
The counter electrode 1c was made of lithium foil. As the electrolyte, a solution in which LiPF6 (lithium hexafluorophosphate) was dissolved to a mixture of EC (ethylene carbonate) and DEC (diethyl carbonate) with a volume ratio of 1:1 so as to have a ratio of 1 mol/L was used. As the separator, a polyethylene porous film of 30 μm in thickness was used.
A secondary battery charging and discharging test device (by NAGANO) was used for the charging and discharging test. Charging was performed with a constant current of 1 mA until the voltage between both electrodes of the lithium-ion secondary battery reaches 0 V, and performed while maintaining 0 V after the voltage reached 0 V. Thereafter, the charging was terminated when the current value fell below 20 μA. Discharging was performed with a constant current of 1 mA until the voltage between both the electrodes of the lithium-ion secondary battery reached 1.5 V. Ten cycles of the above-mentioned charging and discharging were carried out as the test.
Lithium-ion secondary batteries produced in the above-mentioned conditions were subjected to the charging and discharging test, and evaluated with the sustainability rate of cycle capacity as an index. The specific resistance value of the negative-electrode material powder for the lithium-ion secondary battery was also measured. These values are shown in Table 4 together with the test conditions. The sustainability rate of cycle capacity is a value obtained by dividing the discharge capacity in the 10-th cycle by the initial discharge capacity, and the larger this value is, the more satisfactory the cycle characteristics is.
In Test Nos. 1 to 3 which are Inventive examples, the sustainability rate of cycle capacity was excellent such as 89.3% or more. This is attributable to that the retention time in the vacuum treatment was sufficiently long such as 15 minutes or more, and the retention temperature was low such as 750° C. The total content of tar components was reduced to 1917 ppm by mass or less due to the sufficiently long retention time in the vacuum treatment, and the detachment of the carbon film during charging and discharging was suppressed with good adhesion between silicon oxide and the carbon film. The suppression of the generation of SiC in the vicinity of the interface between silicon oxide and the carbon film was assumed to be due to the low retention temperature in the vacuum treatment. Further, due to the low treatment temperature for carbon film formation, the carbon film being formed had low crystallinity with the half-value width of the peak at 1580 cm−1 in the Raman spectrum of 69.8 cm−1 or more, and the capability of relaxing the expansion and contraction of silicon oxide was enhanced.
It was confirmed that each of the lithium-ion secondary batteries of Test Nos. 1 to 3 had an excellent initial discharge capacity such as 1750 mAh/g or more.
In Test Nos. 4 and 5 which are Comparative examples, the sustainability rate of cycle capacity was 71.6% or 78.7%, which is inferior to those of the Inventive examples, although the half-value width of the peak at 1580 cm−1 in Raman spectrum is about 70 cm−1 that is equivalent to the Inventive examples. This is attributable to that the total content of tar components was large such as 7339 ppm by mass or 4800 ppm by mass because the vacuum treatment is not performed or the retention time thereof is insufficient.
In Test Nos. 6 and 7 which are Comparative examples, the sustainability rate of cycle capacity was 82.5% or 79.8%, which is inferior to those of the Inventive examples, although the total content of tar components was low such as 614 ppm by mass or 88 ppm by mass because the treatment temperature for carbon film formation was as high as 900° C. or 1100° C. This is attributable to that the crystallinity of the carbon film being formed was high with the half-value width of the peak at 1580 cm−1 in the Raman spectrum being narrow such as 46.8 cm−1 or 38.1 cm−1.
The negative-electrode material powder for a lithium-ion secondary battery and the negative electrode for a lithium-ion secondary battery or negative electrode for a capacitor of the present invention can be used to obtain a lithium-ion secondary battery or a capacitor which are durable at practical level with large discharge capacity and satisfactory cycle characteristics. The lithium-ion secondary battery and capacitor of the present invention are large in discharge capacity and satisfactory in cycle characteristics. Accordingly, the present invention is a useful technique in the field of secondary batteries and capacitors.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-188053 | Aug 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/004348 | 7/29/2011 | WO | 00 | 2/22/2013 |
