JP2010073402A - Method for manufacturing negative electrode for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce irreversible capacity and improve charge and discharge cycle characteristics in a lithium secondary battery. <P>SOLUTION: The method for manufacturing a negative electrode for the lithium secondary battery includes a process (A) in which an active material film 53 containing silicon is formed on a current collector 51, a process (B) in which lithium 55 is given to the active material film 53 in pressure-reduced state in a chamber, a process (C) in which, after the process (B), carbon dioxide is supplied into the chamber of pressure-reduced state and the pressure in the chamber is increased, and a process (D) in which, after the process (C), the current collector 51 with the active material film 53 formed is held in a prescribed temperature in the atmosphere containing carbon dioxide. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a negative electrode for a lithium secondary battery, and more particularly to a method for manufacturing a negative electrode in which a silicon thin film or a thin film containing silicon as a main component is formed on a current collector.

  Lithium ion secondary batteries are widely used, for example, as a power source for driving electronic devices, and a further increase in capacity is desired.

  Various negative electrode active materials have been studied in order to further increase the capacity of batteries. As a negative electrode active material having a high capacity, an alloy or oxide such as silicon or tin which is a material forming an intermetallic compound with lithium is promising. For example, the theoretical discharge capacity of silicon is about 4199 mAh / g, which is 11 times the theoretical discharge capacity of graphite currently used mainly for negative electrodes.

  However, when the negative electrode active material as described above is used, it expands and contracts greatly due to the reaction of insertion and desorption of lithium ions with respect to the negative electrode active material during charge and discharge. Therefore, when a negative electrode for a lithium secondary battery is configured using the negative electrode active material, the negative electrode active material is pulverized or the negative electrode active material is desorbed from the current collector due to repeated charge and discharge. May cause degradation. Further, the capacity that cannot be taken out of the capacity charged during the initial charge, that is, the difference between the initial charge capacity and the initial discharge capacity (hereinafter referred to as “irreversible capacity”) is large. There is also a problem that the advantage of having a high theoretical discharge capacity cannot be fully utilized.

  In order to solve these problems, a method has been proposed in which lithium is occluded in advance in the active material film containing the negative electrode active material. For example, in Patent Document 1, after an active material film made of a compound of silicon or tin is formed, lithium is deposited on the active material film by using a vapor phase method such as a vacuum evaporation method. Has been proposed to diffuse. In the example described in Patent Document 1, lithium is deposited on the active material film in a vacuum chamber, and subsequently argon gas is injected into the vacuum chamber as a purge gas, and then the obtained negative electrode is taken out from the vacuum chamber. Yes. In this specification, “purge gas” means a gas supplied into a vacuum chamber in a reduced pressure state in order to increase the pressure in the vacuum chamber (chamber) to, for example, atmospheric pressure.

  According to the method in which lithium is occluded in advance in the active material film, not only can the irreversible capacity during the initial charge / discharge be reduced, but also the stress due to the expansion / contraction of the active material accompanying charge / discharge can be relieved, so the charge / discharge cycle The deterioration of characteristics can be suppressed. Further, if the amount of occlusion of lithium is controlled so that electrically active lithium remains in the negative electrode after discharge, an increase in the potential of the negative electrode at the end of discharge can be suppressed, and charge / discharge cycle characteristics can be improved.

  However, when lithium is occluded in advance, the negative electrode is in a charged state, so that the surface of the active material film may be oxidized to form a film such as lithium oxide or lithium hydroxide. Such a coating increases the impedance inside the battery and causes deterioration of battery characteristics. Moreover, since the thickness of the coating film to be formed varies depending on the environment in which the battery is handled and the storage conditions, the characteristics may vary among individuals.

On the other hand, Patent Document 2 proposes a method in which lithium is stored in the active material film in advance and a film containing lithium carbonate is formed on the active material film surface. In this method, after depositing lithium on an active material film (silicon film), an active gas containing carbon dioxide is brought into contact with the active material film to form a film on the surface of the active material film. The coating is formed by depositing lithium in a vacuum chamber by a vacuum vapor deposition method and then flowing a gas in which carbon dioxide and argon are mixed at a volume ratio of 20:80 into the vacuum chamber.
JP 2005-85632 A JP 2005-216601 A

  In the method described in Patent Document 2, when the negative electrode is taken out from the vacuum chamber, a gas containing carbon dioxide and argon gas is allowed to flow into the vacuum chamber as a purge gas, and in this purge step, the coating containing lithium carbonate is formed. Forming. However, as a result of investigation by the inventor, there is a possibility that a film that can sufficiently prevent surface oxidation of the active material film cannot be formed in the purge process of Patent Document 2. A film containing lithium carbonate is formed by a chemical reaction. However, in the purge process, it is difficult to reliably form a film containing lithium carbonate by accurately controlling conditions such as temperature and time for reaction.

  Further, in Patent Document 2, silicon or tin is used as an active material, but when an oxide such as silicon oxide is used instead as an active material, a film containing lithium carbonate is reliably obtained by the method of Patent Document 2. It is particularly difficult to form the film. The reason for this will be described later.

  As described above, according to the conventional method, there is a possibility that the surface oxidation of the active material film that occurs when the lithium is occluded in advance may not be sufficiently suppressed, which is caused by the surface oxidation of the active material film while reducing irreversible capacity. It was difficult to suppress the deterioration of charge / discharge cycle characteristics.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to reduce irreversible capacity and improve charge / discharge cycle characteristics in a lithium secondary battery while suppressing an increase in impedance inside the battery. It is in.

  The method for producing a negative electrode for a lithium secondary battery according to the present invention includes a step (A) of forming an active material film containing silicon on a current collector, and applying lithium to the active material film in a reduced pressure state in a chamber. After step (B) and step (B), carbon dioxide gas is supplied to the reduced-pressure chamber to increase the pressure in the chamber, and after step (C), carbon dioxide gas And (D) holding the current collector on which the active material film is formed at a predetermined temperature in an atmosphere including

  According to the present invention, lithium can be stored in the active material film in advance, so that irreversible capacity can be reduced. Further, the lithium carbonate film having a sufficient thickness can be formed more uniformly on the surface of the active material film by the steps (C) and (D). Therefore, it is possible to suppress deterioration of battery characteristics due to an increase in impedance inside the battery and variations in characteristics among individual batteries, and as a result, charge / discharge cycle characteristics can be greatly improved.

  According to the method for producing a negative electrode for a lithium secondary battery of the present invention, lithium can be occluded in advance in the active material film, and a uniform surface modification layer can be reliably formed by the active material film. Therefore, irreversible capacity can be reduced and battery characteristics such as charge / discharge cycle characteristics can be improved. Moreover, such a negative electrode can be easily manufactured without complicating the manufacturing process.

  When the present invention is applied to a negative electrode using silicon oxide as a negative electrode active material, a particularly remarkable effect is obtained.

<Method for producing negative electrode>
Hereinafter, a method for producing a negative electrode for a lithium secondary battery (hereinafter, “negative electrode”) according to a preferred embodiment of the present invention will be described with reference to the drawings.

  FIG. 1A to FIG. 1C are process cross-sectional views for explaining a method for manufacturing a negative electrode of the present embodiment.

  First, as shown in FIG. 1A, an active material film 53 containing silicon is formed on a current collector 51 such as a copper foil.

  A method for forming the active material film 53 containing silicon is not particularly limited, but a vapor phase method such as a vacuum evaporation method or a sputtering method is preferably used. This is because the active material film 53 formed by the vapor phase method has higher adhesion than an active material film formed by a method other than the vapor phase method such as coating, and is difficult to peel from the current collector 51. Instead of using a vapor phase method, a particulate negative electrode material, a conductive agent, a binder, and the like are dispersed in a dispersion medium to form a slurry, and the slurry is applied on a current collector and dried to obtain an active material. The film 53 may be formed.

  The active material film 53 preferably contains silicon and oxygen. For example, the active material film 53 may be a film containing silicon oxide (SiOx, 0 <x <2) as a main component. More preferably, it has a composition represented by SiOx (0.2 ≦ x ≦ 1.5). In general, when silicon oxide is used as the active material, the irreversible capacity increases as the oxygen ratio in the silicon oxide increases. Therefore, when the molar ratio of oxygen to silicon (hereinafter also simply referred to as “oxygen ratio”) x in the active material film 53 is 0.2 or more, the effect of reducing the irreversible capacity by preliminarily occluding lithium 55 increases. Because. On the other hand, if the oxygen ratio x is greater than 1.5, even if the lithium 55 is occluded in advance, the irreversible capacity is large and the capacity as a battery is reduced.

  Next, as shown in FIG. 1B, lithium 55 is applied to the active material film 53 in a chamber under reduced pressure. For example, lithium 55 may be deposited on the active material film 53 by a vacuum process. At this time, the active material film 53 absorbs the deposited lithium 55 and expands to become an active material film 53a.

  A method for applying lithium 55 to the active material film 53 is not particularly limited. Lithium 55 can also be applied by attaching a lithium foil to the surface of the active material film 53, but it is preferable to use a vacuum process such as a vapor deposition method because the effects of the present invention can be obtained more significantly. When the vapor deposition method is used, lithium 55 can be applied more uniformly than the method of attaching a lithium foil, and the amount of lithium 55 deposited can be easily controlled.

  In addition, when performing the method which affixes lithium foil, it is thought that it is preferable to affix by the transcription | transfer with a roll etc. in a vacuum or inert gas.

The amount of lithium 55 applied to the active material film 53 is preferably adjusted so that the potential with respect to (Li / Li ⁺ ) after application is 0.3 V or more and 0.7 V or less. Thereby, charging / discharging cycle characteristics can be improved more effectively.

  Subsequently, carbon dioxide gas is supplied as a purge gas into the vacuum chamber. Thereby, the surface of the active material film 53a after the lithium 55 is applied first comes into contact with the purge gas. As a result, as shown in FIG. 1C, the lithium 55 reacts with the carbon dioxide gas on the surface of the active material film 53a to form a lithium carbonate film 55a. The coating 55a is considered to be formed by releasing a part of the lithium 55 occluded in the active material film 53a and reacting with carbon dioxide gas.

  The purge gas in the present embodiment is preferably a gas mainly composed of carbon dioxide gas. “Containing carbon dioxide as the main component” means, for example, that the volume ratio of carbon dioxide is greater than 80%. As a result, the active material film 53a provided with lithium 55 and having high activity can be uniformly treated with carbon dioxide gas. Furthermore, when a gas other than carbon dioxide comes into contact with the surface of the active material film 53a in a highly active state, there is a possibility that another reaction occurs. However, the active material film 53a is treated with carbon dioxide without exposing it to other gases. Therefore, reactions other than the reaction that generates lithium carbonate hardly occur. Note that it is particularly preferable to use a gas substantially consisting of only carbon dioxide as the purge gas because reactions other than the reaction that produces lithium carbonate can be more effectively suppressed.

  Thereafter, the current collector 51 on which the coating 55a and the active material film 53a are formed is held for a certain period of time in an atmosphere containing carbon dioxide gas (also referred to as “aging”). As a result, the reaction for generating lithium carbonate can be sufficiently advanced, and a coating having a sufficient thickness can be formed more uniformly on the entire surface of the active material film. Can be improved. Thus, the active material layer 60 which consists of the film 55a and the active material film 53a is formed, and the negative electrode 100 is obtained.

  The volume ratio of carbon dioxide in the atmosphere in which the aging process is performed is preferably greater than 20%, more preferably 80% or more. More preferably, the aging process is performed in an atmosphere substantially consisting of only carbon dioxide. Thereby, the aging time can be shortened. Moreover, you may carry out in the mixed gas atmosphere containing a carbon dioxide gas, In that case, it is preferable to use the mixed gas of a carbon dioxide gas and inert gas, such as argon and neon. This is because the mixed gas with the inert gas can exert only the effect of carbon dioxide.

  In this step, the temperature (holding temperature) maintained in an atmosphere containing carbon dioxide gas is a temperature range in which an unintended reaction such as softening of the current collector 51 and desorption of oxygen from the active material film 53a does not occur. What is necessary is just to be selected suitably. However, it is preferable that it is 25 degreeC or more and 200 degrees C or less. From the viewpoint of productivity, it is preferable to hold (i.e., perform heat treatment) in a heated state at a temperature of 150 ° C. or higher (for example, about 200 ° C.). This is because the reaction rate increases when the holding temperature is increased. On the other hand, when an electrolytic copper foil is used as the current collector 51, if the holding temperature exceeds 200 ° C., there is a problem that the current collector 51 begins to soften.

  The time (holding time) for holding in an atmosphere containing carbon dioxide gas is not particularly limited, but it is preferably 5 minutes or longer because the surface oxidation preventing effect by the coating 55a can be remarkably improved. On the other hand, if the holding time exceeds 24 hours, the productivity may be significantly lowered, and therefore it is preferably 24 hours or less. More preferably, it is 1 hour or less. If it exceeds 1 hour, there is no adverse effect on the characteristics of the active material film 53a, but the degree of improvement of the above effect per unit time is drastically reduced, which is not preferable from the viewpoint of productivity. Depending on the holding temperature, the holding time may be 1 minute.

  In the present embodiment, after the lithium 55 is applied in the chamber, the purge process is performed without removing the current collector 51 from the chamber. Accordingly, the active material film 53a provided with lithium 55 can be brought into contact with the carbon dioxide gas without being exposed to another atmosphere, so that the surface of the active material film 53a can be more reliably modified. After the purge process, the current collector 51 may be taken out of the chamber and the aging process may be performed in a heating furnace, or the aging process may be performed in the same chamber as the purge process. After the lithium 55 is applied in the chamber, if the current collector 51 is removed from the chamber and the purge process and the aging process are performed, the active material film 53a is brought into contact with carbon dioxide gas without being exposed to other atmospheres. Therefore, the reaction other than the reaction that generates lithium carbonate can be more effectively suppressed.

  The active material film 53 in FIG. 1A is a continuous film formed on the surface of the current collector 51. For example, the active material film 53 may have a structure in which a plurality of particulate active materials are deposited. Alternatively, the active material film 53 may have a plurality of columnar active materials arranged at intervals on the surface of the current collector 51. Here, both the particulate active material and the columnar active material are referred to as “active material bodies”.

  Next, the effect by this invention is demonstrated in detail, referring the schematic diagram of an active material body. FIG. 2A is a schematic enlarged sectional view showing a part of the active material film 53 shown in FIG. Here, only one active material body constituting the active material film 53 is shown in an enlarged manner.

  Each active material body 57 shown to Fig.2 (a) consists of silicon oxides, for example. Although not shown, in many cases, a natural oxide film made of silicon dioxide is formed on the surface of the active material body 57 or a film having a higher oxygen ratio than the inside of the active material body 57 is formed. it is conceivable that.

  When lithium is applied to such an active material body 57, as shown in FIG. 2 (b), each active material body 57 may occlude and expand lithium, and may crack from the surface toward the inside. . At this time, a new surface (a surface newly appearing from the inside of the active material body 57 to the surface) is generated by the cracking of the active material body 57.

  As a result of investigation by the present inventor, in order to modify the surface of the active material body 57a as shown in FIG. 2B and prevent the formation of a film such as lithium oxide or lithium hydroxide as described above. In other words, it has been found that it is important to first contact carbon dioxide with the new surface of the active material member 57a. For this purpose, it is necessary to supply carbon dioxide gas into the decompressed chamber after the application of lithium. As a result, the carbon dioxide gas first contacts the surface (including the new surface) of the active material body 57a, and lithium carbonate is generated.

  As a result, as shown in FIG. 2C, a lithium carbonate coating 55a is formed so as to cover the surface of the active material member 57a. At this time, the lithium carbonate coating 55a can also be formed on the new surface caused by the crack. However, there is a possibility that the thickness of the coating film 55a is not sufficient or the coating film 55a is not formed even on the inner surface of the crack by simply flowing the carbon dioxide gas into the chamber as a purge gas.

  Thereafter, although not shown, the negative electrode after the coating 55a is formed is held at a predetermined temperature in a carbon dioxide gas atmosphere. Accordingly, the reaction between carbon dioxide gas and lithium can be further advanced to form a lithium carbonate coating 55a having a sufficient thickness on the surface (including the new surface) of the active material body 57a.

  As described above, according to the present invention, the active material body 57a can be preliminarily occluded with lithium, and the lithium carbonate coating 55a can be reliably formed on the surface thereof, so that the surface oxidation of the active material body 57a is suppressed while suppressing the irreversible capacity. It is possible to more effectively suppress the increase in impedance and variations due to the conventional method. As a result, the charge / discharge cycle characteristics can be improved.

  The active material contained in the active material body 57 (that is, the active material contained in the active material film 53 shown in FIG. 1A) may be silicon (Si) alone, but is silicon oxide (SiOx). It is preferable that a particularly remarkable effect can be obtained. Since a battery using silicon oxide as an active material has a larger irreversible capacity than a battery using silicon alone as an active material, more lithium is previously stored in the active material body 57 containing silicon oxide. There is a need. In the step shown in FIG. 2B, if a large amount of lithium is occluded, the volume expansion of the active material body 57a increases and more cracks may occur. For example, in the conventional method disclosed in Patent Document 2, when many cracks occur in the active material film and the area of the new surface increases, a film containing lithium carbonate is reliably formed on the new surface of the active material film. It was extremely difficult to form. In contrast, in the present embodiment, the lithium carbonate coating 55a can be reliably formed on the new surface by the purge process and the aging process, so that the surface of the active material body 57a can be effectively modified.

  When silicon oxide is used, as described above, many new surfaces are generated by the active material body 57. Therefore, the current collector is heated (for example, 150 ° C. or higher) when held in a carbon dioxide atmosphere, and lithium and It is preferable to increase the reaction rate with carbon dioxide.

<Battery configuration and manufacturing method>
Hereinafter, the configuration of a lithium ion secondary battery obtained by applying the negative electrode 100 of this embodiment will be described with reference to the drawings, taking a stacked battery as an example.

  FIG. 3 is a schematic cross-sectional view of the lithium secondary battery of the present embodiment. The lithium ion secondary battery 200 includes a positive electrode 25, an electrode plate group including two negative electrodes 27 arranged with the positive electrode 25 interposed therebetween, and a separator 16 provided between each negative electrode 27 and the positive electrode 25, and an electrode An aluminum laminate exterior body 21 that accommodates the plate group is provided.

  The positive electrode 25 includes a positive electrode current collector 14 and a positive electrode active material layer 15 provided on both surfaces thereof. The negative electrode 27 includes a negative electrode current collector 17 and a negative electrode active material layer 18 formed on the surface thereof. The negative electrode active material layer 18 has the same configuration as the active material layer 60 described above with reference to FIG. That is, it has an active material film containing lithium and a lithium carbonate film formed on the surface thereof. The negative electrode active material layer 18 is disposed so as to face the positive electrode active material layer 15 with the separator 16 interposed therebetween.

  The positive electrode current collector 14 and the negative electrode current collector 17 are respectively connected to one end of the positive electrode lead 19 and the negative electrode lead 20, and the other end of the positive electrode lead 19 and the negative electrode lead 20 is led out of the exterior body 21. . The separator 16 is impregnated with an electrolyte having lithium ion conductivity. The negative electrode 27, the positive electrode 25, and the separator 16 are housed inside the outer package 21 and sealed together with an electrolyte having lithium ion conductivity. The arrangement of the negative electrode 27 and the positive electrode 25 may be reversed.

  The negative electrode for a lithium secondary battery of the present invention is not limited to the stacked battery shown in FIG. 3, but can be applied to a cylindrical battery or a square battery having a wound electrode group.

Since the present invention is characterized by the configuration of the negative electrode, the components other than the negative electrode 27 in the lithium ion secondary battery 200 are not particularly limited. For example, lithium-containing transition metal oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium manganate (LiMn ₂ O ₄ ) can be used for the positive electrode active material layer 15. It is not limited to this. Moreover, the positive electrode active material layer 15 may be comprised only with a positive electrode active material, and may be comprised with the mixture containing a positive electrode active material, a binder, and a electrically conductive agent. For the positive electrode current collector 14, Al, an Al alloy, Ti, or the like can be used.

  Various lithium ion conductive solid electrolytes and nonaqueous electrolytes are used as the lithium ion conductive electrolyte. As the non-aqueous electrolyte, a solution obtained by dissolving a lithium salt in a non-aqueous solvent is preferably used. The composition of the nonaqueous electrolytic solution is not particularly limited.

  The separator 16 and the outer package 21 are not particularly limited, and materials used in various forms of lithium secondary batteries can be used without particular limitation.

  Next, a method for manufacturing the battery of this embodiment will be described with reference to FIG.

  First, an active material film is formed on the negative electrode current collector by the method described above with reference to FIGS. 1A to 1C (S1). Next, a negative electrode active material layer is formed by the method described above with reference to FIG. 1 (S2).

  Subsequently, a positive electrode active material layer is formed on the positive electrode current collector to obtain a positive electrode (S3). The method for producing the positive electrode is not particularly limited, and a known method may be used. Further, an electrolytic solution is prepared (S4).

  Then, a positive electrode, a negative electrode, and a separator are laminated | stacked so that a positive electrode active material layer and a negative electrode active material layer may oppose through a separator, and an electrode group is obtained (S5). The obtained electrode plate group is sealed in a container together with the electrolyte (S6). In this way, a battery is manufactured.

(Examples and Comparative Examples)
Examples according to the present invention will be described below. In this example, electrodes 1 to 3 were produced using a vacuum process. For comparison, electrodes A to D of comparative examples were produced under conditions different from those of the present invention. Furthermore, sample cells for evaluation tests were respectively produced using the electrodes 1 to 3 of the examples and the electrodes A to D of the comparative examples, and the characteristics thereof were evaluated.

(1) Production of electrodes 1 to 3 of Examples (1-1) Formation of active material film containing silicon as main component In this example, silicon is used as the main component on the surface of the current collector by vacuum deposition. An active material film was formed.

  FIG. 5 is a schematic cross-sectional view of the vapor deposition apparatus used in this example. The vapor deposition apparatus 300 includes a chamber 2 and a vacuum exhaust apparatus 4 that exhausts the chamber. In the chamber, an unwinding can roller 7a, a winding can roller 7b, a can 5, an evaporation source 1, and a nozzle 3 for introducing oxygen into the chamber 2 are provided. The evaporation source 1 has a crucible for storing the vapor deposition material and an electron beam heating means for heating and evaporating the vapor deposition material. The current collector 6 travels along the can 5 from the unwinding can roller 7a and is wound up by the winding can roller 7b. The evaporation source 1 is disposed below the can 5 and supplies evaporated particles to the surface of the current collector 6 running on the can 5.

  First, the current collector 6 was unwound and mounted on the can roller 7a. In this example, a sheet-like electrolytic copper foil (manufactured by Furukawa Circuit Foil Co., Ltd.) having a width of 10 cm, a thickness of 35 μm, and a length of 50 m was used as the current collector 6. In addition, a silicon single crystal having a purity of 99.9999% (manufactured by Shin-Etsu Chemical Co., Ltd.) was placed in the crucible of the evaporation source 1.

Next, after evacuating until the pressure in the chamber 2 becomes 3 × 10 ⁻⁶ torr, oxygen gas is introduced into the chamber 2 from the nozzle 3, and the degree of vacuum in the chamber 2 is 2 × 10 ⁻⁴ torr. It was controlled to become. As the oxygen gas supplied from the nozzle 3, oxygen gas having a purity of 99.7% (manufactured by Nippon Oxygen Co., Ltd.) was used.

  Thereafter, the current collector 6 was caused to travel at a speed of 2 cm per minute from the unwinding can roller 7a through the can 5 to the winding can roller 7b. At this time, the silicon single crystal of the vapor deposition source 1 was irradiated with an electron beam to evaporate the silicon and enter the surface of the current collector 6 on the can 5. The acceleration voltage of the electron beam applied to the vapor deposition material was set to 10 kV, and the emission was set to 500 mA. The silicon vapor is supplied to the surface of the current collector 6 under the oxygen atmosphere controlled as described above, and as a result, a deposited film (active material film) made of silicon oxide is formed on the surface of the current collector 6. It was done. The deposition rate was set to 2 μm / min. In this way, a current collector 6 'on which an active material film was formed was obtained.

The thickness of the active material film formed in this example was 10 μm. Further, when the composition of the active material film was measured by ICP emission analysis, it was SiO _0.5 .

  Thereafter, the sheet-like current collector 6 ′ on which the active material film was formed was cut into a predetermined size (30 mm × 40 mm) to obtain seven sample electrodes. Using four of these, electrodes 1 to 3 of Examples were produced by the method described below. The remaining three were used for producing negative electrodes A to D of comparative examples.

(1-2) Lithium Application to Active Material Film Next, lithium was deposited on the surface of the active material film of the sample electrode obtained in (1-1) above by a vacuum deposition method. The main vapor deposition step and the subsequent steps were performed in a dry atmosphere with a dew point of −40 ° C. unless otherwise specified.

  FIG. 6 is a schematic cross-sectional view of a vapor deposition apparatus used for vapor deposition of lithium in this example. The vapor deposition device 400 is provided in the chamber 13, a vacuum exhaust device (rotary pump) 11 for exhausting the chamber 13, a heater (not shown) for heating the chamber 13, and the chamber 13. And a fixing base 12 for installing a current collector (sample electrode) 10 on which a material film is formed. Further, an evaporation source having a resistance heating boat 8 and heating means (not shown) is disposed vertically below the fixed base 12. Here, a tantalum boat was used as the resistance heating boat 8, and lithium (manufactured by Honjo Chemical Co., Ltd.) 9 having a purity of 99.97% was accommodated in the resistance heating boat 8 as a vapor deposition material.

After setting the sample electrode 10 on the fixed base 12 of the vapor deposition apparatus 400, the pressure in the chamber 13 was adjusted to 2 × 10 ⁻⁴ torr.

  Subsequently, lithium 9 was heated and evaporated by a heating means (not shown), and was deposited on the active material film of the sample electrode 10. The amount of lithium deposited was 12 μm in terms of thickness. All this lithium was once stored in the active material film.

(1-3) Formation of coating film containing lithium carbonate by carbon dioxide purge After depositing lithium by the step (1-2), as a purge gas when opening the chamber 13 of the vapor deposition apparatus 400, a purity of 99.995% High purity carbon dioxide gas (manufactured by Daio Corporation) was used. The rate at which carbon dioxide gas flows into the chamber 13 was (inflow rate) 1 liter / min. After the pressure in the chamber 13 reached atmospheric pressure, the carbon dioxide gas flow was performed for 1 minute at the same flow rate. Thereby, a part of lithium on the active material film and carbon dioxide gas reacted, and a film containing lithium carbonate was formed on the surface of the active material film.

(1-4) Holding Step in Carbon Dioxide Atmosphere The sample electrode on which the film was formed in the above (1-3) was taken out from the chamber 13 and allowed to stand in the heating furnace. The pressure was reduced to 5 × 10 ⁻³ torr. Thereafter, carbon dioxide having a purity of 99.995% was introduced into the heating furnace and sealed so that the pressure in the heating furnace was 1.0 atm.

  Next, the temperature in the heating furnace was heated to a predetermined temperature (holding temperature) at a rate of temperature increase of 10 ° C./min, and held in that state for a predetermined time. Here, as shown in Table 1, three types of electrodes 1 to 3 having different holding times and temperatures were produced.

  After the electrode 1 was manufactured, the electrode 1 was taken out from the heating furnace immediately after the heater of the heating furnace was stopped. After the electrodes 2 and 3 were manufactured, the heater was stopped and the temperature in the heating furnace was 100 ° C. After spontaneous cooling, the electrodes 2 and 3 were taken out.

  Here, the holding process is performed using a heating furnace, but the main holding process may be performed in the chin bar 13 without taking out the sample electrode from the chamber 13. In that case, the purge gas (carbon dioxide gas) introduced into the chamber 13 in all steps can be used as the atmospheric gas as it is.

(2) Preparation of Comparative Example Electrodes A to D Electrodes A to D of Comparative Example were prepared by changing the conditions in the purge process and the holding process. Table 1 shows the conditions for forming each electrode.

  A method for manufacturing the electrode A will be specifically described. First, after applying lithium to the active material film in the same manner as in the example, argon with a purity of 6N (manufactured by Daio Corporation) was allowed to flow into the chamber as a purge gas. The purge gas inflow rate and the purge gas flow time were the same as the carbon dioxide gas inflow rate and flow time in the above-described embodiment. Next, the purged sample electrode was held at a temperature of 25 ° C. for 1 hour while the chamber was kept in a dry atmosphere. The pressure in the chamber was 1.0 atm. In this way, an electrode A was obtained.

  The electrode B was produced in the same manner as the electrode A except that the holding temperature was 200 ° C. in the heating furnace. The electrode C was produced in the same manner as the electrode 3 except that argon gas was used as the purge gas. Furthermore, the production of the electrode D was performed in the same manner as the electrode 1 except that the holding step was performed in a dry atmosphere.

(3) Production of positive electrode 3 parts by weight of acetylene black as a conductive material was mixed with 100 parts by weight of lithium cobalt oxide (LiCoO ₂ ) having an average particle diameter of 5 μm to obtain a mixture. The resulting mixture is kneaded by adding 4 parts by weight of an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF), which is a binder, in terms of PVdF weight to obtain a paste-like positive electrode mixture. Obtained. This positive electrode mixture was applied to both surfaces of a current collector (thickness: 15 μm) made of aluminum foil (thickness: 85 μm on one side), dried and rolled to obtain a positive electrode.

(4) Production of sample cell Using the electrodes 1 to 3 of the examples produced in the above (1) and (2) and the electrodes A to D of the comparative examples, and the positive electrode produced in the above (3), A stacked sample cell was produced. The configuration of these sample cells was the same as that described above with reference to FIG. Referring to FIG. 3 again, a method for manufacturing the sample cell will be described.

  First, each electrode after the holding step produced by the above methods (1) and (2) is cut out to a size of 15 mm × 15 mm, and a nickel negative electrode lead 20 is joined to the end portion thereof by spot welding. A negative electrode 27 was prepared.

  Next, the positive electrode produced as described above was cut out in a size of 14.5 mm × 14.5 m, and a positive electrode lead 19 made of aluminum was joined to the end of the current collector by spot welding to obtain a positive electrode 25 for a cell.

Separator 16 (thickness: 16 μm) made of a polyethylene microporous film was disposed on both surfaces of cell positive electrode 25, and cell negative electrode 27 was further disposed on the outside thereof. It was fixed with a polypropylene adhesive tape so that the sandwiched positive electrode for cells 25 was not displaced, and a stack was obtained. The stack was accommodated in an aluminum laminate outer package (pouch) 21, 1 cm ^{3 of} an electrolytic solution was added, and the stack was sealed with a heat seal. In the electrolyte solution, LiPF ₆ (manufactured by Mitsubishi Chemical) was used as an electrolyte salt in a mixed solvent in which ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DEC) were mixed at a volume ratio of 3: 5: 2. What was dissolved 1M was used. As the aluminum laminate outer package 21, one made by Showa Denko Packaging (thickness: 95 μm) was used. In this way, a sample cell was obtained.

(5) Evaluation of sample cell The sample cell obtained by the said process (4) was taken out from dry atmosphere, and the charging / discharging cycle test was done on the following conditions with respect to each.

During charging: constant current and constant voltage charging, 15 mA,
End voltage 4.2V cut off, end current 0.75mA cut off Rest time: 10 minutes Discharge: constant current discharge, 3mA, end voltage 2.0V cut off Rest time: 10 minutes

  The charge / discharge cycle under the above conditions was defined as one cycle, and the charge / discharge cycle was repeated, and the number of cycles until the battery capacity deteriorated to 60% or less of the initial capacity was determined. The results are shown in Table 1. Note that the initial capacities of the sample cells of Examples and Comparative Examples were both about 15 mAh.

  As is apparent from Table 1, when comparing the number of cycles until the battery capacity is reduced to 60%, the sample cell having electrodes A to C formed using argon gas as the purge gas, regardless of the aging treatment conditions. In addition, the sample cell having the electrodes 1 to 3 and the electrode D formed using carbon dioxide gas as the purge gas showed better charge / discharge cycle characteristics.

  The reason why the charge / discharge cycle characteristics can be improved by using carbon dioxide as the purge gas is considered as follows. After applying lithium to the active material film containing silicon, the new surface of the active material generated by the application of lithium is first exposed to a purge gas. When carbon dioxide gas is used as the purge gas, lithium applied to the active material film comes into contact with the carbon dioxide gas to generate lithium carbonate, and a film containing lithium carbonate (surface modification layer) is formed on the active material film surface. As a result, surface oxidation of the active material film is less likely to occur, and deterioration of charge / discharge cycle characteristics can be suppressed.

  On the other hand, when the newly formed surface caused by the application of lithium, like the electrode C, first comes into contact with the argon gas, even if the new surface is brought into contact with the carbon dioxide gas in the subsequent aging treatment, the lithium carbonate coating is formed in the aging treatment step. The reaction that forms is unlikely to occur. This is apparent from the fact that the characteristics of the sample cell using the electrode C are substantially the same as the characteristics of the sample cell using the electrode B subjected to aging treatment in a dry atmosphere.

  Further, by comparing the characteristics of the sample cells having the electrodes 1 to 3 and the electrode D, even when carbon dioxide is used as the purge gas, when aging treatment is performed in a dry atmosphere (electrode D), aging is performed in the carbon dioxide atmosphere. It has been found that the cycle characteristics are significantly lower than when the treatment is performed (electrodes 1 to 3).

  Furthermore, when the holding time in the aging treatment is the same, the electrode 3 subjected to the aging treatment at a high holding temperature (200 ° C.) can realize higher charge / discharge cycle characteristics than the electrode 1 subjected to the aging treatment at 25 ° C. I understood. On the other hand, when the holding temperature in the aging treatment is the same, it was found that the electrode 3 that was held for 1 hour can realize higher charge / discharge cycle characteristics than the electrode 2 that was held for 5 minutes. Moreover, it turned out that charging / discharging cycling characteristics can be improved dramatically by raising holding temperature especially.

  From these results, it was confirmed that in order to realize a high charge / discharge cycle, it is important not only to use carbon dioxide as a purge gas but also to perform an aging treatment in a carbon dioxide atmosphere.

  When aging is performed in a carbon dioxide atmosphere after the purge step with carbon dioxide, the thickness of the surface modification layer on the surface of the active material film increases. That is, it is considered that the formation of the surface modification layer on the active material film proceeds, and a more uniform and thick surface modification layer is surely obtained. Thereby, it is possible to obtain a surface modification effect that was not sufficiently obtained only by the purge process. Note that the progress of the formation of the surface modification layer during the aging treatment is apparent from the fact that the effect of the surface modification varies depending on the retention time and the retention temperature.

  The present invention can be applied to various forms of lithium secondary batteries. The shape of the lithium secondary battery to which the present invention is applicable is not particularly limited, and may be any shape such as a coin shape, a button shape, a sheet shape, a cylindrical shape, a flat shape, and a square shape. Further, the form of the electrode plate group including the positive electrode, the negative electrode, and the separator may be a wound type or a laminated type. The size of the battery may be small for a small portable device or large for an electric vehicle. The lithium secondary battery of the present invention can be used for a power source of, for example, a portable information terminal, a portable electronic device, a small electric power storage device for home use, a motorcycle, an electric vehicle, a hybrid electric vehicle, etc., but the application is not particularly limited.

(A)-(c) is typical process sectional drawing for demonstrating the manufacturing method of the negative electrode of embodiment by this invention. (A)-(c) is typical process sectional drawing for demonstrating the manufacturing method of the negative electrode of embodiment by this invention, and is a typical enlarged view which shows a part of active material film | membrane. It is typical sectional drawing of the lithium ion secondary battery of embodiment of this invention. 3 is a flowchart of a method for manufacturing a battery according to an embodiment of the present invention. In the Example of this invention, it is typical sectional drawing of the vapor deposition apparatus used in order to form an active material film. In the Example of this invention, it is typical sectional drawing of the vapor deposition apparatus used in order to provide lithium to an active material film.

Explanation of symbols

1 Evaporation source 2 Chamber (vacuum tank)
DESCRIPTION OF SYMBOLS 3 Gas introduction nozzle 4 Vacuum exhaust apparatus 5 Can 6, 6 'Current collector 7a Unwinding can roller 7b Winding can roller 8 Resistance heating boat 9 Vapor deposition material (lithium)
10 Current collector with active material film 11 Vacuum exhaust system (rotary pump)
12 Fixing base (support plate)
13 Chamber (vacuum tank)
DESCRIPTION OF SYMBOLS 51 Current collector 53 Active material film 53a Active material film | membrane to which lithium was provided 55 Lithium 55a Coating containing lithium carbonate 57 Active material body 60 Active material layer 14 Positive electrode current collector 15 Positive electrode active material layer 16 Separator 17 Negative electrode current collector 18 Negative electrode active material layer 19 Lead for positive electrode 20 Lead for negative electrode 21 Aluminum laminate 25 Positive electrode 27 Negative electrode 100 Negative electrode 200 Battery 300, 400 Deposition device

Claims (9)

Forming an active material film containing silicon on the current collector (A);
In the chamber, applying lithium to the active material film in a reduced pressure state (B);
After the step (B), supplying a carbon dioxide gas to the decompressed chamber to increase the pressure in the chamber (C);
After the step (C), producing a negative electrode for a lithium secondary battery comprising the step (D) of holding the current collector on which the active material film is formed at a predetermined temperature in an atmosphere containing carbon dioxide gas Method.   2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein in the step (D), the time for holding the current collector on which the active material film is formed is 5 minutes or more and 24 hours or less.   In the said process (D), the said predetermined temperature is 25 degreeC or more and 200 degrees C or less, The manufacturing method of the negative electrode for lithium secondary batteries of Claim 1.   2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the step (C) is a step of increasing substantially the pressure in the chamber by substantially supplying only carbon dioxide gas to the decompressed chamber. .   2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein in the step (D), the concentration of carbon dioxide contained in the atmosphere is higher than 20%.   The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the active material film has a composition represented by SiOx (0.2 ≦ x ≦ 1.5).   The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the step (B) is a step of depositing lithium on the active material film using a vacuum deposition method. 2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein the potential with respect to (Li / Li ⁺ ) is 0.3 V or more and 0.7 V or less after the lithium is applied. A positive electrode capable of inserting and extracting lithium ions;
A negative electrode for a lithium secondary battery produced by the method according to any one of claims 1 to 8,
A separator disposed between the positive electrode and the negative electrode for a lithium secondary battery;
A lithium ion secondary battery comprising an electrolyte having lithium ion conductivity.

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