JP5646188B2 - Negative electrode active material for lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode active material for a lithium ion secondary battery excellent in charge / discharge cycle characteristics.

  Conventionally, as a negative electrode active material of a lithium ion secondary battery, various carbon materials such as artificial graphite, natural graphite, and hard carbon having excellent cycle life and safety have been used. In addition, the performance has been improved by improving the utilization rate of these carbon materials and increasing the packing density per electrode volume. However, in such a lithium ion secondary battery using a carbon material as the negative electrode active material, a practical capacity close to the theoretical capacity of graphite (372 mAh / g) has been achieved, and the improvement in packing density has also approached the limit. Increasing the capacity beyond this is becoming difficult.

  For this reason, in order to further increase the capacity of lithium ion secondary batteries, it has been studied to use a material having a capacity higher than that of a carbon material as a negative electrode active material. Attention has been focused on metal-based materials including Si, Sn and the like having a very large charge / discharge capacity (Patent Document 1).

Japanese Patent No. 2999741

  However, since these metal-based materials undergo significant changes in volume during charge and discharge, the negative electrode active material layer made of these metal-based materials tends to collapse and maintain physical and electrical coupling within the negative electrode. Have difficulty.

  Therefore, in view of the above-described situation, the present invention provides a negative electrode active material for a lithium ion secondary battery that can alleviate the effect of volume change during charge / discharge and exhibit good charge / discharge cycle characteristics. Let it be an issue.

  That is, the negative electrode active material for a lithium ion secondary battery according to the present invention is a negative electrode active material for a lithium ion secondary battery made of a material containing at least silicon and oxygen in constituent elements, and has a thickness of 30 to 500 nm. It is made of scale-like powder having an average major axis / thickness ratio of 10 to 100, and has an oxygen content of 5 to 38 wt%. In the present invention, scaly means a long shape whose major axis is 10 times or more the thickness, and the shape can be measured using, for example, an electron microscope or a particle size distribution meter. Moreover, the oxygen content of the negative electrode active material according to the present invention can be measured using, for example, an inert gas melting infrared absorption method.

  Since the negative electrode active material according to the present invention is composed of scaly powder having the above-mentioned size and shape, the negative electrode active materials are oriented in parallel with each other in the negative electrode, so that the pressure and strain in the negative electrode are uniformly dispersed. For this reason, the influence by the volume change at the time of charging / discharging can be relieve | moderated effectively. In addition, since the negative electrode active material according to the present invention has an oxygen content of 5 to 38 wt%, the electron conductivity is maintained high and the charge / discharge capacity retention rate is excellent.

  The average major axis of the scaly powder is preferably 1 to 20 μm.

  In the present invention, the substance containing at least silicon and oxygen in the constituent elements is preferably a substance in which the oxygen element is present in a non-equilibrium state, and specifically comprises an amorphous material composed of silicon and oxygen. One in which particles of silicon alone are dispersed in the matrix can be mentioned.

  The lithium ion secondary battery including the negative electrode containing the negative electrode active material, the conductive agent and the binder according to the present invention is also one aspect of the present invention.

  In the lithium ion secondary battery according to the present invention, the conductive agent is preferably made of a carbon material, and the binder is preferably made of an organic material having an imide bond.

  According to the present invention having such a configuration, a lithium ion secondary battery having good charge / discharge cycle characteristics can be obtained.

  A lithium ion secondary battery according to an embodiment of the present invention will be described below.

  The lithium ion secondary battery according to the present embodiment takes, for example, a coin, a button, a sheet, a cylinder, a flat shape, a square shape, and the like from a positive electrode, a negative electrode, an electrolyte, a separator provided between the positive electrode and the negative electrode, and the like. It is configured.

  The negative electrode in the present embodiment uses a material (hereinafter also referred to as SiOx) containing at least silicon (Si) and oxygen (O) in constituent elements as a negative electrode active material. As the SiOx, those in which oxygen element is present in a non-equilibrium state are preferable. Specifically, silicon single particles are dispersed in an amorphous matrix composed of silicon and oxygen. Is mentioned. Such a material has a larger capacity than carbon materials such as graphite, has excellent life characteristics, has a long life, and has a smaller volume change rate during charge / discharge than silicon alone, and has excellent charge / discharge cycle characteristics. .

  The SiOx has a scale-like powder shape with a thickness of 30 to 500 nm and an average major axis / thickness ratio of 10 to 100. Since SiOx used in the present invention has a scale-like shape as described above, most of it is oriented in a direction parallel to the current collector when the negative electrode is produced. Therefore, the pressure and strain due to the volume change at the time of charge / discharge are uniformly dispersed in the negative electrode, and the physical and electrical coupling in the negative electrode is maintained even after repeated charge / discharge.

  Among these negative electrode active materials, those having an average major axis of 1 to 20 μm are preferable. If the average major axis is less than 1 μm, the effect of volume change due to expansion / contraction during charge / discharge increases, and the charge / discharge capacity tends to decrease. On the other hand, if the average major axis exceeds 20 μm, the effect of volume change during charge / discharge Thus, cracks are likely to occur in the active material powder, and the electrolyte is decomposed on the newly formed crack surface at that time, and the charge / discharge cycle characteristics are likely to be deteriorated. The powder shape of the SiOx can be measured using, for example, an electron microscope or a particle size distribution meter.

  The oxygen content of the SiOx is 5 to 38 wt%, preferably 10 to 30 wt%, more preferably 15 to 25 wt%. When the oxygen content exceeds 38 wt%, the electron conductivity is lowered, and due to this, the charge / discharge capacity is also lowered beyond the allowable range. On the other hand, when the oxygen content is less than 5 wt%, the charge / discharge capacity retention rate is lowered, and the charge / discharge cycle characteristics are poor. The oxygen content of the SiOx can be measured using, for example, an inert gas melting infrared absorption method.

  The SiOx may contain impurities in addition to silicon and oxygen as long as the characteristics are not hindered.

  Such scaly SiOx can be produced, for example, by the following method. In other words, a metal silicon alone or a mixture of metal silicon and silicon monoxide and / or silicon dioxide is used as a raw material in an atmosphere in which the oxygen partial pressure is appropriately adjusted using a sputtering method, a vacuum deposition method, or the like. After depositing the oxide of the composition to the above thickness on the surface of the material, the obtained deposited film is peeled off from the substrate and pulverized, whereby the scaly SiOx as described above can be obtained.

  Examples of the substrate include resin films such as polyethylene terephthalate, polyethylene, and polypropylene; metal foils such as copper and stainless steel.

  Moreover, as a method of grind | pulverizing the obtained deposited film, the method of wet-grinding in an organic solvent using a ball mill is mentioned, for example.

  For the negative electrode, in addition to the SiOx that is a negative electrode active material, for example, additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent may be appropriately selected and blended.

  The conductive agent is not particularly limited, and a known conductive agent can be used as appropriate. Among them, a fine structure such as carbon black, flaky graphite, and carbon nanofiber can be obtained by providing excellent conductivity. A powdery carbon material is preferably used. These electrically conductive agents may be used independently and 2 or more types may be used together.

  The binder is not particularly limited, and examples thereof include polyvinylidene fluoride, polyethylene, styrene butadiene rubber, polyimide, polyamideimide, polybenzimidazole, and the like, among others, polyimide, polyamideimide, polybenzimidazole, and the like. An organic material having an imide bond with high mechanical strength is preferably used. These binders may be used independently and 2 or more types may be used together.

  In order to produce the negative electrode, for example, a mixture of the negative electrode active material and various additives including the conductive agent and the binder is added to a solvent such as water or an organic solvent to form a slurry or a paste. The obtained slurry or paste is applied to a current collector using a doctor blade method or the like, and the dried one is adjusted with a roll press to obtain a negative electrode.

  Examples of the current collector include a foil, a sheet or a net made of copper, nickel, stainless steel, or the like: a sheet or a net made of carbon fiber. Instead of using the current collector, the negative electrode may be formed by compacting into a pellet.

  Examples of the positive electrode include Li-containing Ti, Mo, W, Nb, V, Mn, Fe, Cr, Ni, Co, and other transition metal oxides, sulfides, vanadium oxides, conjugated polymers, and the like. The thing which uses an organic electroconductive material, a chevrel phase compound, etc. as an active material is mentioned. These positive electrode active materials may be used independently and 2 or more types may be used together.

  In the positive electrode, in addition to the powder made of the active material, for example, additives such as a conductive agent, a binder, a filler, a dispersant, and an ionic conductive agent may be appropriately selected and blended.

  Examples of the conductive agent include graphite, carbon black, acetylene black, ketjen black, carbon fiber, and metal powder. Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, and polyethylene. Is mentioned. These additives may be used independently and 2 or more types may be used together.

  In order to manufacture the positive electrode, for example, a mixture of a positive electrode active material and various additives can be added to a solvent to form a slurry or paste, and the positive electrode can be manufactured using the same method as the above-described negative electrode.

  Examples of the electrolyte include a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent, a polymer electrolyte, an inorganic solid electrolyte, a composite material of a polymer electrolyte and an inorganic solid electrolyte, and the like.

  Examples of the solvent for the non-aqueous electrolyte include chain esters such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate: γ-lactones such as γ-butyllactone: 1,2-dimethoxy Chain ethers such as ethane, 1,2-diethoxyethane, and ethoxymethoxyethane: Cyclic ethers of tetrahydrofuran: Nitriles such as acetonitrile. These solvents may be used independently and 2 or more types may be used together.

Examples of the lithium salt that is a solute of the non-aqueous electrolyte include LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiSbF ₆ , LiSCN, LiCl, LiC ₆ H ₅ SO ₃ , Examples thereof include LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC ₄ P ₉ SO ₃ .

  As the separator, for example, a porous film made of a polyolefin such as polypropylene or polyethylene, or a porous material such as a glass filter or a nonwoven fabric can be used.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

<Manufacture of negative electrode active material>
Example 1
A polyethylene terephthalate film having a thickness of 100 μm was used as a base material, and a SiOx film was formed to a thickness of 200 nm on the base material surface by RF sputtering. The target was 6N pure metal silicon, and the distance between the substrate and the target was 65 mm. Moreover, argon-0.1% oxygen mixed gas was used as the sputtering gas, and the internal pressure during the reaction was adjusted to 0.1 Pa. Furthermore, the high frequency output was 400 W. Thereafter, the obtained SiOx film was peeled off from the base material, wet-pulverized for 30 minutes using ethanol as a solvent using a ball mill, and dried to obtain active material powder A. When the major axis of the obtained active material powder A was measured from an electron micrograph, an average of 10 μm and a distribution of 5 to 20 μm were shown. There was no powder cracking in the thickness direction, and all the powders maintained the thickness of 200 nm, which was the thickness at the time of film formation, so the ratio of average major axis / thickness was 50. The amount of oxygen contained in this active material powder A was measured by an inert gas melting infrared absorption method and found to be 5.0 wt%.

(Example 2)
The SiOx film formed and peeled off under the same conditions as in Example 1 was wet pulverized for 5 hours using ethanol as a solvent using a ball mill, and dried to obtain active material powder B. The shape of the obtained active material powder B showed a distribution of 2 to 8 μm with an average major axis of 4 μm. Similarly to the active material powder A, the active material powder B does not have a powder that is cracked in the thickness direction, and all the powders maintained the thickness at the time of film formation of 200 nm. It was 20. The amount of oxygen contained in this active material powder B was 5.8 wt%.

Example 3
An active material powder C was obtained in the same manner as in Example 1 except that argon-1.0% oxygen mixed gas was used as the sputtering gas during film formation. The shape of the obtained active material powder C was almost the same as that of the active material powder A. The amount of oxygen contained in this active material powder C was 9.8 wt%.

Example 4
An active material powder D was obtained in the same manner as in Example 1 except that an argon-3.0% oxygen mixed gas was used as the sputtering gas during film formation. The shape of the obtained active material powder D was almost the same as that of the active material powder A. The amount of oxygen contained in this active material powder D was 20.4 wt%.

(Example 5)
An active material powder E was obtained in the same manner as in Example 1 except that an argon-5.0% oxygen mixed gas was used as the sputtering gas during film formation. The shape of the obtained active material powder E was almost the same as that of the active material powder A. The amount of oxygen contained in this active material powder E was 29.1 wt%.

(Comparative Example 1)
The active material powder F was obtained in the same manner as in Example 1 except that the film was formed to a thickness of 5 μm. The shape of the obtained active material powder F is almost a block (short plate shape), the thickness is 1 to 3 μm, the major axis is 5 μm on average and 2 to 10 μm in distribution, and the average major axis / thickness ratio is 1.7. ~ 5. The amount of oxygen contained in this active material powder F was 5.0 wt%.

(Comparative Example 2)
The SiOx film formed under the same conditions as in Example 1 was peeled off and dry pulverized in a mortar to obtain an active material powder G. The shape of the obtained active material powder G showed a distribution of 35 to 100 μm with an average major axis of 50 μm. Similarly to the active material powder A, the active material powder G has no powder cracking in the thickness direction, and all the powders maintained the thickness at the time of film formation of 200 nm. 250. The amount of oxygen contained in this active material powder G was 5.0 wt%.

(Comparative Example 3)
An active material powder H was obtained in the same manner as in Example 1 except that an argon-7.0% oxygen mixed gas was used as a sputtering gas during film formation. The shape of the obtained active material powder H was almost the same as that of the active material powder A. The amount of oxygen contained in this active material powder H was 39.1 wt%.

<Evaluation of battery characteristics>
The battery characteristics of the lithium ion secondary battery using the active material powder obtained in each example and comparative example were evaluated as follows. To the obtained active material powder 80 wt%, Denka Black (registered trademark, manufactured by Denki Kagaku Kogyo) 10 wt%, polyamideimide N-methylpyrrolidone solution is added 10 wt% as polyamideimide solids, and N-methylpyrrolidone is added as a solvent. In addition, it was mixed in the form of a slurry. This was applied to a copper foil having a thickness of 10 μm so as to be 5 mg / cm ² , dried at 130 ° C., and further heat-cured at 250 ° C., and punched into a disk having a diameter of 13 mm so as to have a predetermined thickness. To prepare an electrode. Using this electrode, a coin cell using metallic lithium as a counter electrode was produced, and the battery characteristics were evaluated. In the coin cell, a polyethylene porous film having a thickness of 20 μm was used as a separator, and ethylene carbonate (EC) / diethyl carbonate (DEC) = 3/7 LiPF ₆ 1.2M was used as an electrolytic solution.

  The manufactured coin cell was charged to 0.005 V at a constant current of 0.05 C, and then 0.05 C discharge was performed to a discharge starting voltage of 1.4 V, and the discharge capacity at the first cycle was measured. Thereafter, a cycle of charging to 0.005 V at a constant current of 0.5 C and discharging at 0.5 C to 1.4 V was repeated 50 cycles. The results are shown in Table 1 below.

  In Examples 1 to 5, the discharge capacity at the first cycle decreased as the amount of oxygen contained in SiOx increased, but the discharge capacity retention rate during the charge / discharge cycle was improved and good battery characteristics were obtained. It was confirmed that On the other hand, in Comparative Example 1, since the shape of the active material powder was not scaly, the volume change due to the expansion and contraction of the active material powder was large in the charge / discharge cycle, and as a result, the deterioration of the discharge capacity increased. In Comparative Example 2, the active material powder has a scaly shape, but the length in the major axis direction is excessive, and therefore the active material powder is significantly cracked due to the volume change during charging and discharging. However, the electrolytic solution was decomposed on the newly formed surface formed at that time, and the charge / discharge cycle characteristics deteriorated. Further, in Comparative Example 3, since the amount of oxygen in SiOx was large, the electron conductivity was lowered, and the charge / discharge capacity was significantly lowered.

Claims (5)

A negative electrode active material for a lithium ion secondary battery composed of scaly SiOx powder containing silicon and oxygen in constituent elements,
The oxygen content is 5 to 38 wt%,
The scale-like SiOx powder has a thickness of 30 to 500 nm and an average major axis / thickness ratio of 10 to 100, and a negative electrode active material for a lithium ion secondary battery.   The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the average major axis is 1 to 20 μm.   A lithium ion secondary battery comprising a negative electrode containing the negative electrode active material according to claim 1, a conductive agent, and a binder.   The lithium ion secondary battery according to claim 3, wherein the conductive agent is made of a carbon material. The lithium ion secondary battery according to claim 3 or 4, wherein the binder is made of an organic material having an imide bond.