JP5343516B2 - Negative electrode for lithium secondary battery, lithium secondary battery, and method for producing negative electrode for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make cycle characteristics better through restraint of degradation of electrode energy density of a lithium secondary battery. <P>SOLUTION: The lithium secondary battery 20 is provided with a negative electrode for lithium secondary battery 23 equipped with a negative electrode collector 11 with its surface made coarse, a negative electrode active material layer 12 mainly composed of amorphous silicon and uniformly formed on a surface of the negative electrode collector 11 and a negative electrode binder 13 formed on a surface of the negative electrode active material layer 12. By using the negative electrode collector 11 with its surface coarse, stress concentration in the negative electrode active material layer generated by expansion and contraction of the negative electrode active material accompanying with charging and discharging is alleviated, thus pulverization or peeling of the negative electrode active material layer are restrained. Further, since the negative electrode binder 13 is formed on the surface of the negative electrode active material layer, the negative electrode active material layer is hardly dropped off from the negative electrode collector so as to restrain degradation of conductivity. Moreover, since unnecessary volume part without active material does no exist on the surface of the negative electrode collector 11, degradation of electrode energy density can be restrained. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a negative electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing a negative electrode for a lithium secondary battery.

  Conventionally, it has been proposed to increase the capacity of a lithium secondary battery by using a group 14 element typified by silicon or a material that forms an intermetallic compound with these oxides as a negative electrode active material. However, since these materials have a large volume change due to insertion and extraction of lithium during charging and discharging, cracking of active material particles, peeling from the current collector, and the like are likely to occur, and the cycle characteristics deteriorate.

Therefore, in Patent Document 1, for example, in a lithium secondary battery using silicon as a negative electrode active material, an active material containing silicon is formed in a plurality of island regions on a current collector so as to cover the plurality of island regions. By forming a conductive protection film, the conductive protection film absorbs the expansion and contraction of the active material that accompanies charge and discharge, suppresses peeling of the active material from the current collector, and controls deterioration of cycle characteristics. Proposed.
JP 2008-117574 A

  However, in the negative electrode described in Patent Document 1, the cycle characteristics are improved due to the presence of the conductive protective film in the voids between the island regions, but there is an unnecessary volume portion having no active material on the current collector surface. Therefore, there has been a problem that the electrode energy density is lowered.

  This invention is made | formed in view of such a subject, manufacture of the negative electrode for lithium secondary batteries, lithium secondary battery, and negative electrode for lithium secondary batteries which suppresses the fall of an electrode energy density and has favorable cycling characteristics. It aims to provide a method.

  In order to achieve the above object, the present inventors uniformly formed a negative electrode active material layer mainly composed of amorphous silicon on the surface of the roughened negative electrode current collector, and further formed on the surface. When a negative electrode for a lithium secondary battery in which a negative electrode binder is formed and a lithium secondary battery using the negative electrode as a negative electrode is prepared, it is found that the reduction in electrode energy density can be suppressed and cycle characteristics can be improved. The present invention has been completed.

  That is, the negative electrode for a lithium secondary battery according to the present invention includes a negative electrode current collector having a roughened surface, and a negative electrode active material having amorphous silicon as a main component and uniformly formed on the surface of the negative electrode current collector. And a negative electrode binder formed on the surface of the negative electrode active material layer.

  Moreover, the lithium secondary battery of the present invention includes a positive electrode containing a positive electrode active material, the above-described negative electrode for a lithium secondary battery, an ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions, It is equipped with.

  Furthermore, in the method for producing a negative electrode for a lithium secondary battery according to the present invention, a negative electrode active material layer in which a negative electrode active material layer containing amorphous silicon as a main component is uniformly formed on a negative electrode current collector whose surface is roughened. It includes a material layer forming step and a negative electrode binder forming step of forming a negative electrode binder on the surface of the negative electrode active material layer.

  In this negative electrode for a lithium secondary battery, a lithium secondary battery, and a method for producing a lithium secondary battery, cycle characteristics can be improved. The reason why such an effect is obtained is that, by forming a negative electrode active material layer on the roughened negative electrode current collector, the negative electrode active material layer generated by expansion and contraction of the negative electrode active material accompanying charge / discharge It is conceivable that the stress concentration is relaxed and the pulverization and peeling of the negative electrode active material layer are suppressed. Furthermore, it is conceivable that by forming a negative electrode binder on the surface of the negative electrode active material layer, the negative electrode active material layer is less likely to drop off from the negative electrode current collector, and the decrease in conductivity is suppressed. Moreover, in this negative electrode for lithium secondary batteries, since the unnecessary volume part which does not have an active material does not exist in the collector surface, it is thought that the fall of an electrode energy density can be suppressed.

  The negative electrode for a lithium secondary battery according to the present invention includes a negative electrode current collector having a roughened surface, a negative electrode active material layer formed mainly of amorphous silicon and uniformly formed on the surface of the negative electrode current collector, And a negative electrode binder formed on the surface of the negative electrode active material layer. The negative electrode for a lithium secondary battery can be used as a negative electrode for a lithium secondary battery such as a lithium ion secondary battery or a lithium air battery. Among these, it is preferable to utilize for the negative electrode of a lithium ion secondary battery.

  In the negative electrode for a lithium secondary battery of the present invention, the negative electrode current collector has a roughened surface. The negative electrode current collector is not particularly limited as long as it is formed of a conductive material. For example, a foil formed of a metal such as copper, stainless steel, or nickel-plated steel can be used. The surface may be roughened, but the surface roughness Rzjis (μm) is preferably 0.4 μm or more, and more preferably 4.5 μm or more and 7 μm or less. When the surface roughness Rzjis is 0.4 μm or more, it is easy to ensure the thickness of the negative electrode active material layer, and when it is 4.5 μm or more, it is easy to secure the thickness of the negative electrode active material layer. Further, in order to form irregularities on the surface of the current collector, the thickness of the negative electrode current collector corresponding to the unevenness is required, but when the surface roughness Rzjis is 7 μm or less, the thickness of the negative electrode current collector is about 30 μm. Since it can suppress, the fall of an electrode energy density can be suppressed. Here, the surface roughness Rzjis is also referred to as ten-point average roughness, and refers to the surface roughness determined based on JIS-B0601: 2001 Annex 1 (reference).

In the negative electrode for a lithium secondary battery of the present invention, the negative electrode active material layer is mainly composed of amorphous silicon and is uniformly formed on the negative electrode current collector. Here, uniform formation means that it is not formed in an island region on the negative electrode current collector. That is, it is sufficient that the negative electrode current collector surface is formed to have a certain extent, and it may not completely cover the current collector surface, or the composition and thickness may be non-uniform. However, it may have a multilayer structure or an inclined material structure in the thickness direction. Note that the term “mainly composed of amorphous silicon” includes not only the case of being made of only amorphous silicon but also the case of containing other than amorphous silicon (such as impurities and additives). The negative electrode active material layer is preferably formed so that the thickness t (μm) satisfies t ≦ Rzjis / 2. In general, silicon is said to have a volume change of about 400% due to charging / discharging, and on average, it is considered to change about 1.6 times per side. Considering the porosity (density) of amorphous silicon, it is preferable to satisfy t ≦ Rzjis / 1.85, and it is more preferable to satisfy t ≦ Rzjis / 2. In this way, the volume change of the amorphous silicon layer can be kept within the range of the surface roughness Rzjis of the negative electrode current collector, that is, between the irregularities on the surface, even in repeated charge / discharge cycles. It is considered that peeling between the current collector and the negative electrode active material layer can be suppressed. If the thickness of the amorphous silicon layer is 0.8 μm or more, the electrode energy density can be sufficiently increased, and if it is 5 μm or less, the particle diameter of the amorphous silicon becomes too large in the negative electrode active material layer. Since the cycle characteristics do not deteriorate, the thickness is preferably 0.8 μm or more and 5 μm or less. Further, the amorphous silicon as the main component of the negative electrode active material layer preferably has a density of 2.1 g / cm ³ or more, and in particular, the density of the negative electrode active material in contact with the negative electrode current collector is 2.1 g. / Cm ³ or more is preferable. When the density of the amorphous silicon is 2.1 g / cm ³ or more, conductivity is ensured and deterioration of cycle characteristics can be suppressed. On the other hand, it is preferable that an amorphous silicon layer having a density of 2.0 g / cm ³ or more and less than 2.1 g / cm ³ is formed on the outermost layer of the negative electrode active material layer. By doing so, the negative electrode binder is more easily penetrated into the negative electrode active material layer, and therefore, it is expected to have an effect on improving the cycle characteristics. In this case, in order to ensure conductivity, the thickness of the amorphous silicon having a density of 2.0 g / cm ³ or more and less than 2.1 g / cm ³ is preferably 1 μm or less, and about 0.2 μm. Is more preferable. Since the density of silicon is 2.33 g / cm ³ , it is clear that the density of amorphous silicon is less than this.

  In the negative electrode for a lithium secondary battery of the present invention, a negative electrode binder is formed on the surface of the negative electrode active material layer. The negative electrode binder only needs to be formed so as to substantially cover the surface of the negative electrode active material layer. The negative electrode binder may be formed so as to cover the entire surface, or there may be a portion that is not partially covered. Further, a part of the negative electrode binder may penetrate into the negative electrode active material layer. By forming the negative electrode binder on the surface of the negative electrode active material layer, it is possible to prevent the negative electrode active material layer from falling off due to charge / discharge. In addition, it is expected that the binder component enters the negative electrode active material layer as the volume of the negative electrode active material changes due to charge / discharge, and the stress concentration generated in the negative electrode active material layer is alleviated. The negative electrode binder should just be formed so that it may become 5% or more and 100% or less with respect to an amorphous silicon weight. If the amount of the negative electrode binder is 5% or more, the effect of adding the binder is obtained, and if it is 100% or less, the decrease in the electrode energy density can be suppressed. Moreover, if it is 50% or less, since a high capacity | capacitance maintenance factor is obtained, it is preferable. Furthermore, if it is 20% or less, the load characteristics after the cycle are also good, which is more preferable. As the negative electrode binder, polyimide resin, polyvinylidene fluoride, polytetrafluoroethylene, or the like can be used, and it is preferably thermoplastic.

  In the negative electrode for a lithium secondary battery of the present invention, it is preferable that the entire negative electrode for a lithium secondary battery is heat-treated after the negative electrode binder is formed on the surface of the negative electrode active material layer. When the negative electrode binder of the present invention is heat-treated at a temperature higher than the melting point, the binding force increases and the cycle characteristics are further improved. For example, polyimide is preferably heat treated at 300 ° C. to 500 ° C., and preferably heat treated at 500 ° C. or lower in order to avoid thermal decomposition. Polyvinylidene fluoride is preferably heat-treated at 160 ° C. or higher, but is preferably heat-treated at 350 ° C. or lower because it is thermally decomposed at 350 ° C. or higher. The melting point of polytetrafluoroethylene is preferably 327 ° C. and heat-treated at a temperature higher than this temperature. The atmosphere during the heat treatment is preferably performed under an inert gas atmosphere such as a vacuum, a nitrogen atmosphere, or an argon atmosphere. It may be performed in a reducing atmosphere such as a hydrogen atmosphere.

  A lithium secondary battery of the present invention includes a positive electrode including a positive electrode active material, a negative electrode for a lithium secondary battery described above, and an ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions. It is.

  The positive electrode of the lithium secondary battery of the present invention is obtained by, for example, applying a paste-like positive electrode mixture by mixing a positive electrode active material with a conductive material and a binder and adding an appropriate solvent onto the surface of the positive electrode current collector. And it can compress and form so that an electrode density may be raised as needed. The positive electrode current collector is not particularly limited as long as it is formed of a conductive material. For example, a foil or mesh formed of a metal such as aluminum, copper, stainless steel, or nickel-plated steel may be used. it can. As the positive electrode active material, an oxide containing lithium and a transition metal element or a polyanionic compound can be used. Specifically, for example, lithium cobalt composite oxide, lithium nickel composite oxide, lithium manganese composite oxide, lithium iron composite phosphorus oxide, and the like can be given. The conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more of carbon powder materials such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are mixed. Things can be used. The binder plays a role of connecting the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene is used. be able to. As the solvent for dispersing the positive electrode active material, the conductive material, and the binder, for example, an organic solvent such as N-methyl-2-pyrrolidone can be used.

In the lithium secondary battery of the present invention, as the ion conductive medium, a nonaqueous electrolytic solution, an ionic liquid, a gel electrolyte, a solid electrolyte, or the like in which a supporting salt is dissolved in an organic solvent can be used. Examples of the supporting salt include known LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ F ₅ SO ₂ ), and the like. Supporting salts can be used. The concentration of the supporting salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2M. As an organic solvent, for example, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), diethyl carbonate (DEC), dimethyl carbonate (DMC) and the like are used for conventional secondary batteries and capacitors. An organic solvent is mentioned. These may be used alone or in combination. Further, the ionic liquid is not particularly limited, but 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide, 1-ethyl-3-butylimidazolium tetrafluoroborate, or the like is used. Can do. The gel electrolyte is not particularly limited. For example, a polymer such as polyvinylidene fluoride, polyethylene glycol, or polyacrylonitrile, or a saccharide such as an amino acid derivative or sorbitol derivative is added with an electrolyte containing a supporting salt. And a gel electrolyte. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes. Well-known inorganic solid electrolytes include, for example, Li nitrides, halides, oxyacid salts, and the like. Among _{them, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, xLi 3 PO 4 - (1-x) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2, sulfide Examples thereof include phosphorus compounds. These may be used alone or in combination. Examples of the organic solid electrolyte include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyvinylidene fluoride, polyphosphazene, polyethylene sulfide, polyhexafluoropropylene, and derivatives thereof. These may be used alone or in combination.

  The lithium secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it is a composition that can withstand the usage range of the secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a microporous film of an olefin resin such as polyethylene or polypropylene Is mentioned. These may be used alone or in combination.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. An example of this lithium ion secondary battery is shown in FIG. FIG. 1 is a cross-sectional view schematically showing the configuration of the coin-type battery 20. The coin-type battery 20 includes a cup-shaped battery case 21, a positive electrode 22 provided inside the battery case 21, a negative electrode 23 provided at a position facing the positive electrode 22 via a separator 24, A non-aqueous electrolyte solution 27 containing a supporting salt, a gasket 25 formed of an insulating material, and a sealing plate 26 disposed in the opening of the battery case 21 and sealing the battery case 21 via the gasket 25. Yes. The positive electrode 22 includes a positive electrode current collector 14 and a positive electrode active material 15. In addition, the negative electrode 23 is composed of a negative electrode active material layer 12 composed mainly of amorphous silicon and uniformly formed on the surface of the negative electrode current collector 11 whose surface is roughened, and the negative electrode active material layer 12. And a negative electrode binder 13 formed on the surface.

  The method for producing a negative electrode for a lithium secondary battery of the present invention comprises (1) a roughening step for roughening the surface of the negative electrode current collector, and (2) a negative electrode current collector having a roughened surface. A negative electrode active material layer forming step for uniformly forming a negative electrode active material layer mainly composed of amorphous silicon; (3) a negative electrode binder forming step for forming a negative electrode binder on the surface of the negative electrode active material layer; and (4). A heat treatment step of performing a heat treatment at a temperature equal to or higher than the melting point of the negative electrode binder after the negative electrode binder forming step may be included.

(1) Roughening step The negative electrode current collector having a roughened surface used in the method for producing a negative electrode for a lithium secondary battery may be produced by a roughening step. The material of the negative electrode current collector is not particularly limited as long as it is formed of a conductive material. For example, a foil formed of a metal such as copper, stainless steel, or nickel-plated steel can be used. The roughening step is not particularly limited as long as the surface of the negative electrode current collector is roughened. For example, the surface of the negative electrode current collector may be precipitated by an electrolytic method or the like, and the surface thereof may be roughened. Thereby, the surface roughness Rzjis can be controlled. As described above, the surface roughness is preferably such that Rzjis (μm) is 0.4 μm or more, and more preferably 4.5 μm or more and 7 μm or less.

(2) Negative electrode active material layer formation process Next, the negative electrode active material layer which has an amorphous silicon as a main component is uniformly formed in the surface of the negative electrode collector whose surface was roughened. A method for forming the negative electrode active material layer is not particularly limited, but a method of directly depositing amorphous silicon on the surface of the negative electrode current collector without using a binder, for example, a sputtering method, a PVD method, a CVD method, a liquid It is preferable to form by one or more methods of the group consisting of a rapid cooling method, an electrodeposition method, a vapor deposition method, a thermal spraying method, or a plating method. Among these, it is more preferable to use a sputtering method. Further, as described above, the negative electrode active material layer is preferably formed so as to satisfy t ≦ Rzjis / 1.85, and more preferably formed so as to satisfy t ≦ Rzjis / 2. In this way, the volume change of the amorphous silicon layer can be kept within the range of the surface roughness Rzjis of the negative electrode current collector, that is, between the irregularities on the surface, even in repeated charge / discharge cycles. It is considered that peeling between the current collector and the negative electrode active material layer can be suppressed. If the thickness of the amorphous silicon layer is 0.8 μm or more, the electrode energy density can be sufficiently increased, and if it is 5 μm or less, the particle diameter of the amorphous silicon becomes too large in the negative electrode active material layer. Since the cycle characteristics are not deteriorated, it is preferably formed to be 0.8 μm or more and 5 μm or less. Further, the amorphous silicon as the main component of the negative electrode active material layer is preferably formed so as to have a density of 2.1 g / cm ³ or more, particularly the density of the negative electrode active material in contact with the negative electrode current collector. Is preferably formed to be 2.1 g / cm ³ or more. When the density of the amorphous silicon is 2.1 g / cm ³ or more, conductivity is ensured and deterioration of cycle characteristics can be suppressed. On the other hand, the outermost layer of the anode active material layer is preferably the density to form an amorphous silicon layer is less than 2.0 g / cm ³ or more 2.1 g / cm ^3. Since the negative electrode binder is more easily penetrated into the negative electrode active material layer, it is expected to be effective in improving cycle characteristics. In this case, in order to ensure conductivity, the amorphous silicon having a density of 2.0 g / cm ³ or more and less than 2.1 g / cm ³ is preferably formed to have a thickness of 1 μm or less, and 0.2 μm. It is more preferable to form so that it may become a grade.

(3) Negative electrode binder formation process Next, a negative electrode binder is formed in the surface of the above-mentioned negative electrode active material layer. For example, a doctor blade method or the like can be used to form the negative electrode binder. The negative electrode binder may be formed so as to substantially cover the surface of the negative electrode active material layer, and may be entirely covered or may be partially uncovered. A part of the negative electrode binder may penetrate into the negative electrode active material layer. By forming the negative electrode binder on the negative electrode active material layer, it is possible to prevent the negative electrode active material layer from dropping off due to charge / discharge. In addition, it is expected that the binder component enters the negative electrode active material layer as the volume of the negative electrode active material changes due to charge / discharge, and the stress concentration generated in the negative electrode active material layer is alleviated. The negative electrode binder may be formed so as to be 5% or more and 100% or less with respect to the amorphous silicon weight. When the amount of the negative electrode binder is 5% or more, the effect of adding the binder is obtained, and when the amount is 100% or less, the decrease in the electrode energy density can be suppressed. Moreover, if it is 50% or less, since a high capacity | capacitance maintenance factor is obtained, it is preferable. Furthermore, if it is 20% or less, the load characteristics after the cycle are also good, which is more preferable. As the negative electrode binder, polyimide resin, polyvinylidene fluoride, polytetrafluoroethylene, or the like can be used, and it is preferably thermoplastic.

(4) Heat treatment step The negative electrode for a lithium secondary battery in which the negative electrode binder is formed is preferably heat-treated at a temperature higher than the melting point of the negative electrode binder. When the negative electrode binder of the present invention is heat-treated at a temperature higher than the melting point, the binding force increases and the cycle characteristics are further improved. For example, polyimide is preferably heat treated at 300 ° C. to 500 ° C., and is preferably heat treated at 500 ° C. or lower in order to avoid thermal decomposition. Polyvinylidene fluoride is preferably heat-treated at 160 ° C. or higher, but is preferably heat-treated at 350 ° C. or lower because it is thermally decomposed at 350 ° C. or higher. Polytetrafluoroethylene has a melting point of 327 ° C. and is preferably heat-treated at a temperature higher than this temperature. The atmosphere during the heat treatment is preferably performed in an inert gas atmosphere such as a vacuum, a nitrogen atmosphere, or an argon atmosphere. You may carry out in reducing environment, such as hydrogen atmosphere.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the above-described embodiment, the manufacturing method includes a roughening step, but this step may be omitted by using a roughened negative electrode current collector. In the embodiment described above, the manufacturing method includes a heat treatment step, but this step may be omitted.

[Experiment 1]
A negative electrode for a lithium secondary battery was produced as follows. First, as a negative electrode current collector, an electrolytic foil having a thickness of 30 μm and a diameter of 100 mm made of roughened copper having a surface roughness Rzjis of 7 μm by depositing copper on the surface by an electrolytic method was prepared. A negative electrode active material layer made of amorphous silicon having a density of 2.1 g / cm ³ is formed on the negative electrode current collector made of the electrolytic foil by using a sputtering apparatus (load lock type sputtering film forming apparatus manufactured by Tokki Co., Ltd.). The film was deposited to a thickness of 3 μm (t ≦ Rzjis / 2). The deposition conditions at this time are shown in Table 1. Specifically, after the inside of the chamber of the sputtering apparatus was evacuated to 5 × 10 ⁻⁵ Pa, argon was introduced into the chamber, and the gas pressure was stabilized so that the gas pressure in the chamber was 0.5 Pa. Thereafter, a high frequency voltage was applied to a silicon sputtering source for a predetermined time by a high frequency power source in a state where the gas pressure in the chamber was stable, and amorphous silicon was deposited on the negative electrode current collector. For the measurement of the density of amorphous silicon, sputtering is performed on a small piece of glass (20 mm × 20 mm) under the above-mentioned conditions, and the thickness of silicon is measured. Separately, the same is applied to a large aluminum foil (100 mmφ). Sputtering was performed under the conditions, the weight of the deposited silicon was measured, and the thickness, area and weight were calculated. Note that the density of the amorphous silicon described in this specification was measured according to this method.

Next, the negative electrode active material layer was obtained by using an N-methylpyrrolidone (NMP) solution containing 12% polyvinylidene fluoride (PVdF) on the electrode obtained by depositing amorphous silicon on the negative electrode current collector. The solution was dropped so as to be 5% based on the weight, and was uniformly dispersed on the electrode using a doctor blade. Thereafter, the electrode was dried under reduced pressure (5 Pa or less) at 80 ° C. for 3 hours or longer, and the obtained electrode was cut out to an area of 2.05 cm ² to obtain the negative electrode of Experimental Example 1.

  Next, a bipolar cell was produced as follows. First, lithium hexafluorophosphate is added to a nonaqueous solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 30:70 so as to be 1 mol / L, and nonaqueous electrolysis is performed. A liquid was prepared. A non-aqueous electrolyte prepared using the above negative electrode as a working electrode, a lithium foil (thickness of 300 μm) having the same area as the counter electrode, and a separator (Tonyo Tapils) between the working electrode and the counter electrode. A bipolar cell was fabricated by satisfying

Next, a charge / discharge test was performed as follows. First, using the produced bipolar cell, the working electrode was reduced (charged) to 0.01 V at 0.1 C (0.4 mA), and then the working electrode was oxidized (discharged) to 1.5 V at 0.1 C. The initial charge capacity Qcp (mAh) and the initial discharge capacity Qdp (mAh) were measured, and the initial charge / discharge efficiency (%) represented by (Qdp / Qcp) × 100 was calculated. Subsequently, 50 cycles of charge and discharge were performed at 0.2 C (0.8 mA), and the discharge capacity Qd1 (mAh) at the first cycle of the charge and discharge cycle and the discharge capacity Qd50 (mAh) at the 50th cycle were measured. The capacity retention ratio (%) represented by Qd50 / Qd1) × 100 was calculated. Thereafter, load characteristics of 1 C (4 mA) and 0.05 C (0.2 mA) are measured at 20 ° C., and the ratio of the charge capacity at the 1 C current value to the charge capacity at the 0.05 C current value (hereinafter referred to as 1 C / 0). .05C value). The results are shown in Table 2. Table 2 shows the copper foil thickness (μm) and surface roughness Rzjis (μm) of the negative electrode current collector, the density (g / cm ³ ) and thickness (μm) of the negative electrode active material layer, and the negative electrode active material of the negative electrode binder The ratio (%) to the layer weight and the heat treatment temperature (° C.) are shown. Further, initial charge / discharge efficiency (%), cycle capacity retention rate (%), and 1C / 0.05C values were shown. Table 2 also shows data of Experimental Examples 2 to 19 described later.

[Experiment 2]
The negative electrode and the bipolar cell were formed in the same manner as in Experimental Example 1, except that the thickness of the copper foil as the negative electrode current collector was 18 μm, the surface roughness Rzjis was 4.5 μm, and the thickness of the negative electrode active material layer was 2 μm. 1C current value is 2.7 mA, 0.2C current value is 0.54 mA, 0.1C current value is 0.27 mA, 0.05C current value is the same as Example 1 except that the current value is 0.135 mA. The electrochemical properties were evaluated under the conditions.

[Experiment 3]
A negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 1 except that the thickness of the negative electrode active material layer was 1 μm, and the 1C current value was 1.6 mA, the 0.2C current value was 0.32 mA, and 0.1 C. The electrochemical characteristics were evaluated under the same conditions as in Experimental Example 1 except that the current value was 0.16 mA and the 0.05C current value was 0.08 mA.

[Experimental Example 4]
The negative electrode active material layer has a two-layer structure, and amorphous silicon having a density of 2.1 g / cm ³ is deposited as a first layer on the surface of the copper foil to a thickness of 0.8 μm, and a second layer is formed on the surface. As a result, amorphous silicon having a density of 2.0 g / cm ³ was deposited to a thickness of 0.2 μm. Table 1 shows the deposition conditions of the amorphous silicon at this time. Otherwise, a negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 1, and the 1C current value was 1.5 mA, the 0.2C current value was 0.3 mA, the 0.1C current value was 0.15 mA, and the 0.05C current. The electrochemical characteristics were evaluated under the same conditions as in Experimental Example 1 except that the value was 0.075 mA.

[Experimental Example 5]
A negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 4 except that the electrode after forming the negative electrode binder was heat-treated at 300 ° C. for 12 hours in an argon stream, and electrochemical characteristics were evaluated under the same conditions as in Experimental Example 4. .

[Experimental Examples 6 to 8]
The negative electrode and bipolar cell of Experimental Example 6 were prepared in the same manner as in Experimental Example 3, except that the electrode after forming the negative electrode binder was heat-treated at 300 ° C. for 12 hours in an argon stream. Moreover, the negative electrode and bipolar cell of Experimental example 7 were created similarly to Experimental example 3 except having heat-processed the electrode after negative electrode binder formation at 200 degreeC in argon stream for 12 hours. Further, the negative electrode and the bipolar cell of Experimental Example 8 were prepared in the same manner as in Experimental Example 3, except that the electrode after the formation of the negative electrode binder was heat-treated at 140 ° C. for 12 hours in an argon stream. Using these, the electrochemical characteristics were evaluated under the same conditions as in Experimental Example 3.

[Experimental Examples 9 to 12]
A negative electrode and a bipolar cell of Experimental Example 9 were prepared in the same manner as in Experimental Example 3 except that PVdF was formed to be 10% with respect to the weight of the negative electrode active material layer. Further, the negative electrode and the bipolar cell of Experimental Example 10 were prepared in the same manner as in Experimental Example 3, except that PVdF was formed to be 20% with respect to the weight of the negative electrode active material layer. Moreover, the negative electrode and bipolar cell of Experimental example 11 were created similarly to Experimental example 3 except having formed so that PVdF might be 50% with respect to the negative electrode active material layer weight. Moreover, the negative electrode and bipolar cell of Experimental example 12 were created similarly to Experimental example 3 except having formed so that PVdF might be 100% with respect to the negative electrode active material layer weight. Using these, the electrochemical characteristics were evaluated under the same conditions as in Experimental Example 3.

[Experimental Examples 13 to 16]
A negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 1 except that no negative electrode binder was formed, and electrochemical characteristics were evaluated under the same conditions as in Experimental Example 1 (Experimental Example 13). Moreover, except not forming a negative electrode binder, the negative electrode and the bipolar cell were created similarly to Experimental Example 2, and the electrochemical characteristics were evaluated on the same conditions as Experimental Example 2 (Experimental Example 14). Moreover, a negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 3 except that no negative electrode binder was formed, and electrochemical characteristics were evaluated under the same conditions as in Experimental Example 3 (Experimental Example 15). In addition, a negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 4 except that no negative electrode binder was formed, and electrochemical characteristics were evaluated under the same conditions as in Experimental Example 4 (Experimental Example 16).

[Experimental Examples 17 and 18]
The negative electrode and the bipolar cell of Experimental Example 17 were prepared in the same manner as Experimental Example 15 except that the surface roughness Rzjis of the negative electrode current collector was 0.25 μm. Further, the negative electrode and the bipolar cell of Experimental Example 18 were prepared in the same manner as in Experimental Example 15 except that the amorphous silicon density of the negative electrode active material layer was 2.0 g / cm ³ . Using these, the electrochemical characteristics were evaluated under the same conditions as in Experimental Example 3.

[Experimental Example 19]
A negative electrode and a bipolar cell were prepared in the same manner as in Experimental Example 15 except that the thickness of the negative electrode active material layer was 5 μm, and the 1C current value was 6.5 mA, the 0.2C current value was 1.3 mA, and 0.1 C. The electrochemical characteristics were evaluated under the same conditions as in Experimental Example 3 except that the current value was 0.65 mA and the 0.05C current value was 0.325 mA.

[Test results]
According to the results of Experimental Examples 1 to 19, Experimental Examples 1 to 12 in which the negative electrode binder (PVdF) was formed on the surface of the negative electrode active material layer were compared with Experimental Examples 13 to 19 in which the negative electrode binder was not formed. Was good. This is presumably because the negative electrode active material layer was less likely to fall off the negative electrode current collector and the decrease in conductivity was suppressed by forming the negative electrode binder on the surface of the negative electrode active material layer. Here, in each of Experimental Examples 1 to 8 in which the PVdF amount was 5%, not only the capacity retention rate but also the cycle post-load characteristics were good. On the other hand, in Experimental Examples 9 to 11 in which the PVdF amount was increased to 10 to 50%, the capacity retention rate was equal to or higher than that of Experimental Example 3 in which the PVdF amount was 5%, but the cycle post-load characteristics (1C / 0. 05C value) showed a low value. However, in a device that does not require a large current, it may be preferable if the capacity retention rate is good even if the post-cycle load characteristics (1C / 0.05C value) are low. The negative electrode current collector had a surface roughness of 4.5 μm and 7 μm, and had a good capacity retention ratio and good load characteristics after cycling. Moreover, when the negative electrode active material layer thickness (t) and the surface roughness (Rzjis) of the negative electrode current collector satisfy t ≦ Rzjis / 2, the capacity retention ratio and the cycle post-load characteristics were good. In Experimental Examples 5 to 8 in which heat treatment was performed, Experimental Example 8 in which heat treatment was performed at 140 ° C. lower than the melting point of PVdF was the same result as Experimental Example 3 in which heat treatment was not performed. In Experimental Examples 5 to 7, which were heat-treated at 300 ° C., the capacity retention ratio and the cycle post-load characteristics were greatly improved. This is presumably because the binding force increased due to the heat treatment of the negative electrode binder at a temperature higher than the melting point, and the loss of the negative electrode active material was suppressed. The density of the negative electrode active material layer was good when the density of the negative electrode active material layer in contact with the negative electrode current collector was 2.1 g / cm ³ or more, but the density of the outermost layer was low (2.0 g / cm ^In the experimental example 4 in which the amorphous silicon of ³ ) was formed, the cycle post-load characteristics were improved to be equal to those after the heat treatment without the above heat treatment. Among the experimental examples in which the negative electrode binder was not formed, the negative electrode current collector had a small surface roughness Rzjis (0.25 μm). Even in the case of forming the binder, it is presumed that the roughness Rzjis of the negative electrode current collector surface should be larger than 0.25 μm.

2 is a cross-sectional view illustrating an outline of a configuration illustrating an example of a coin-type battery 20. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 Negative electrode collector, 12 Negative electrode active material layer, 13 Negative electrode binder, 14 Positive electrode collector, 15 Positive electrode active material, 20 Coin type battery, 21 Battery case, 22 Positive electrode, 23 Negative electrode, 24 Separator, 25 Gasket, 26 Sealing Plate, 27 non-aqueous electrolyte.

Claims (7)

A negative electrode current collector having a roughened surface;
A negative electrode active material layer composed mainly of amorphous silicon and uniformly formed on the surface of the negative electrode current collector;
A negative electrode binder formed on the surface of the negative electrode active material layer;
The Bei example, the is heat treated at a temperature above the melting point of the negative electrode binder, a negative electrode for a lithium secondary battery. The negative electrode for a rechargeable lithium battery, the negative active negative electrode for a lithium secondary battery of claim 1 density of the outermost surface layer is less than 2.0 g / cm ³ or more 2.1 g / cm ³ of the material layer. In the negative electrode for a lithium secondary battery, the negative electrode current collector has a surface roughness Rzjis of 4.5 μm ≦ Rzjis ≦ 7 μm, a thickness of the negative electrode active material layer is Rzjis / 2 or less, and the negative electrode active material at least the negative density of the negative electrode active material electrode current collector to have contact is 2.1 g / cm ³ or more, according to claim 1 or 2 negative electrode for a lithium secondary battery according to the layers. A positive electrode including a positive electrode active material;
The negative electrode for a lithium secondary battery according to any one of claims 1 to 3 ,
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
Lithium secondary battery comprising A negative electrode active material layer forming step of uniformly forming a negative electrode active material layer mainly composed of amorphous silicon on a negative electrode current collector having a roughened surface;
A negative electrode binder forming step of forming a negative electrode binder on the negative electrode active material layer surface;
After the negative electrode binder forming step, a heat treatment step of performing a heat treatment at a temperature equal to or higher than the melting point of the negative electrode binder,
The manufacturing method of the negative electrode for lithium secondary batteries containing this. Wherein the negative electrode active material layer forming step, the density in the outermost layer of the negative electrode active material layer to form the anode active material layer is less than 2.0 g / cm ³ or more 2.1 g / cm ^3, according to claim 5 A method for producing a negative electrode for a lithium secondary battery. In the negative electrode active material layer forming step, a negative electrode current collector satisfying a surface roughness Rzjis of 4.5 μm ≦ Rzjis ≦ 7 μm is used, and the negative electrode active material layer has a thickness of Rzjis / 2 or less and at least the negative electrode current collector. The manufacturing method of the negative electrode for lithium secondary batteries of Claim 5 or 6 which forms a negative electrode active material layer so that the density of the negative electrode active material which touches a body may be 2.1 g / cm < ³ > or more.

Images