JP2007172991A - Negative electrode for lithium secondary battery, method for manufacturing same, and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery negative electrode, a manufacturing method for it, and a lithium secondary battery using the lithium secondary battery negative electrode for preventing deformation of a collector due to repeated charging/discharging and separation of an active material thin film from the collector. <P>SOLUTION: In this lithium secondary battery negative electrode, a thin film 3 formed of an active material storing/releasing lithium is layered on the main surface of the collector 1. In the status before charging/discharging, a low density area, which is ranged in a net shape in the surface direction parallel to the main surface of the collector 1 is formed in the thin film 3 to be extended in the thickness direction of the thin film 3. The low density area 2 is provided with a first curved part 2a, which is curved to swell in one direction of the surface direction in the thin film thickness direction, and a second curved part 2b curved to swell in the direction opposite to one direction continuously from the first curved part 2a. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

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

  In recent years, a lithium secondary battery that uses a non-aqueous electrolyte and charges and discharges by moving lithium ions between a positive electrode and a negative electrode has been used as one of the new secondary batteries with high output and high energy density. Yes.

  As such a negative electrode for a lithium secondary battery, a material using a material alloyed with lithium as a negative electrode active material has been studied. In particular, silicon has a large theoretical capacity and is promising as a negative electrode for a battery exhibiting a high capacity, and various secondary batteries using this as a negative electrode have been proposed.

  The present applicant uses silicon or the like as an active material, and as a negative electrode for a lithium secondary battery exhibiting good charge / discharge cycle characteristics, a microcrystal or a thin film on a current collector by a thin film formation method such as a CVD method, a sputtering method, or a vapor deposition method. A negative electrode for a lithium secondary battery in which an amorphous material is formed has been proposed (for example, Patent Document 1).

  In such a negative electrode for a lithium secondary battery, forming the active material thin film at high speed is effective in reducing the manufacturing cost. As a method for forming an active material thin film, there are a method of forming a thin film from a gas phase such as a CVD method, a sputtering method, a vapor deposition method, a thermal spraying method, a method of forming a thin film from a liquid phase such as a plating method, and the like. Among these, the vapor deposition method is a method for forming an active material thin film that can achieve a high thin film formation rate and can obtain a negative electrode for a lithium secondary battery excellent in charge / discharge cycle characteristics (for example, Patent Document 2). ).

  However, a material that is alloyed with lithium, such as silicon, expands and contracts the volume of the active material when occluding and releasing lithium. Therefore, there is a possibility that the active material may peel off from the current collector due to charge / discharge. The current collector may be deformed such as wrinkles. When the active material is peeled off or the current collector is deformed, the charge / discharge cycle characteristics are deteriorated and the energy density per unit volume of the battery is lowered.

  In order to solve such a problem, by forming irregularities on the surface of the current collector and forming an active material thin film thereon, as it goes toward the valleys of the irregularities on the current collector surface in the thickness direction of the active material thin film A negative electrode in which a gap having a wide width is formed has been proposed (for example, Patent Document 3).

  In the negative electrode having this structure, the stress due to the expansion and contraction of the active material thin film accompanying the charge / discharge reaction can be relieved by the voids, and the active material peeling and the current collector deformation can be suppressed.

Moreover, in order to solve the above-mentioned problem, a negative electrode having a structure in which an active material thin film is formed in a columnar shape and the major axis direction of the columnar part is inclined with respect to a plane perpendicular to the main surface of the negative electrode material has been proposed ( For example, Patent Document 4). In this document, the stress generated by the expansion and contraction of the active material thin film accompanying the charge / discharge reaction works to disperse the stress in the direction parallel to the main surface of the negative electrode material and in the direction perpendicular thereto. It is described that the deformation of can be suppressed. However, even with such an electrode structure, sufficiently good charge / discharge cycle characteristics are not obtained.
International Publication No. 01/029912 Pamphlet JP 2002-289181 A JP 2002-313319 A JP-A-2005-196970

  An object of the present invention is for a lithium secondary battery capable of suppressing deformation of a current collector due to repeated charge / discharge and separation of an active material from the current collector, and obtaining good charge / discharge cycle characteristics. An object of the present invention is to provide a negative electrode, a method for producing the same, and a lithium secondary battery using the negative electrode.

  The negative electrode for a lithium secondary battery of the present invention is a negative electrode for a lithium secondary battery in which a thin film made of an active material that occludes and releases lithium is deposited on the main surface of the current collector. Immediately after the formation of the thin film, a low density region connected in a network shape in a plane direction parallel to the main surface of the current collector is formed in the thin film so as to extend in the thickness direction of the thin film. A first bending portion that curves so as to swell in one direction of the surface direction, and a second bending portion that curves so as to swell in a direction opposite to the one direction following the first bending portion; It is characterized by having.

  In the active material thin film of the negative electrode for a lithium secondary battery of the present invention, in a state before charging / discharging, a low density region connected in a mesh shape in a plane direction parallel to the main surface of the current collector is formed in the thickness direction of the thin film. A first curved portion that is formed so as to extend so that the low density region swells in one direction of the surface in the thickness direction of the thin film, and the one direction following the first curved portion. And a second bending portion that curves so as to swell in the opposite direction. By forming the low density region having the first curved portion that swells in one direction and the second curved portion that swells in the opposite direction as described above, the stress generated by the expansion and contraction of the active material thin film accompanying the charge / discharge reaction is generated. It is considered that the active material is dispersed more effectively in the direction parallel to the direction parallel to the main surface of the current collector, thereby suppressing the peeling of the active material and the deformation of the current collector.

  Moreover, in the negative electrode for lithium secondary batteries of this invention, it is preferable that the cavity is formed in the boundary part of the 1st curved part of a low density area | region, and a 2nd curved part. By forming such a cavity, stress due to expansion and contraction of the active material thin film accompanying the charge / discharge reaction can be relieved, and active material peeling and current collector deformation can be further suppressed.

  The manufacturing method of this invention is a method which can manufacture the negative electrode for lithium secondary batteries of the said invention. That is, the production method of the present invention is a method for producing a negative electrode for a lithium secondary battery according to the present invention by depositing vapor deposition material particles on a main surface of a current collector by vapor deposition to form a thin film. The angle at which the material particles are incident on the main surface of the current collector is defined as an angle with respect to the normal of the main surface of the current collector. From an angle of 70 ° or more in one direction, an angle of 70 ° or more in a direction opposite to the one direction. Gradually changing with the progress of thin film formation until the first thin film forming step of forming the thin film while forming the first curved portion, and the angle at which the vapor deposition material particles enter the main surface of the current collector, By gradually changing the angle of the main surface of the current collector from 70 ° or more in the opposite direction to 70 ° or more in the one direction as the thin film is formed, the second curve is obtained. Thin while forming part It is characterized in that it comprises a second thin film forming step of forming a.

  According to the present invention, the negative electrode for a lithium secondary battery of the present invention can be manufactured by a vapor deposition method. Since it can be formed by a vapor deposition method, a thin film can be deposited at a high speed.

  In the first thin film forming step of the manufacturing method of the present invention, the first curvature of the low density region is obtained by gradually changing the angle at which the vapor deposition material particles enter the main surface of the current collector as the thin film is formed. A thin film is formed while forming the part. Specifically, as the angle with respect to the normal line of the current collector main surface, the vapor deposition material particles move from an angle of 70 ° or more in one direction to an angle of 70 ° or more in the opposite direction to the one direction. The incident angle is changed. The angle of 70 ° or more is preferably an angle within a range of 70 ° to 90 °.

  In the second thin film forming step, contrary to the first thin film forming step, the angle at which the vapor deposition material particles enter from the reverse direction to the one direction is changed. Specifically, the angle at which the vapor deposition material particles enter from the angle of 70 ° or more in the reverse direction to the angle of 70 ° or more in the one direction is gradually changed as the thin film is formed, thereby the second curved portion. Form. As an angle of 70 ° or more, an angle within a range of 70 ° to 90 ° is preferable.

  According to the manufacturing method of the present invention, the thin film is deposited on the main surface of the current collector while forming the first curved portion and the second curved portion in the low density region as described above. In addition, the negative electrode for a lithium secondary battery of the present invention can be efficiently formed by a vapor deposition method having a high thin film formation rate.

  In the manufacturing method of the present invention, while moving the current collector along the outer peripheral surface of the roller, the thin film is deposited on the main surface of the current collector by passing it over the vapor deposition source for evaporating the vapor deposition material particles. It is preferable to form them. In such a case, the first thin film forming step is a step of forming a thin film while moving the current collector in the first direction, and the second thin film forming step is opposite to the first direction. It is preferable to form the thin film while moving the current collector in the second direction.

  In this way, the first thin film forming step and the second thin film forming step can be realized by moving the current collector by reciprocating along the outer peripheral surface of the roller and passing it over the vapor deposition source. Thus, the negative electrode for a lithium secondary battery of the present invention can be easily produced.

  The current collector used in the present invention preferably has irregularities formed on the main surface on which the thin film is deposited. As the degree of unevenness of the main surface, the arithmetic average roughness Ra is preferably 0.1 μm or more. The arithmetic average roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994). The arithmetic average roughness Ra can be measured by, for example, a stylus type surface roughness meter. By depositing a thin film on the main surface of the current collector having such large unevenness, unevenness corresponding to the unevenness of the current collector surface can be formed on the surface of the thin film. The low density region in the present invention is formed together with the deposition of the thin film upward from the concave and convex valleys of the main surface of the current collector.

  By forming the active material thin film on the main surface having large irregularities, voids can be formed around the convex portions of the current collector main surface. Such voids can relieve stress due to expansion and contraction of the active material thin film accompanying the charge / discharge reaction, and can further suppress peeling of the active material and deformation of the current collector.

  The vapor deposition method in the present invention is not particularly limited as long as the film formation rate is high, and vacuum vapor deposition methods such as an electron beam vapor deposition method and other vapor deposition methods can be employed.

  The material used as the negative electrode active material in the present invention is not particularly limited as long as it can occlude and release lithium, but a material that occludes lithium by alloying with lithium is preferably used. Examples of such materials include silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, potassium, and indium. Among these, silicon is particularly preferably used because of its high theoretical capacity. Since silicon is difficult to be pulverized, it is preferably amorphous silicon or microcrystalline silicon.

  The current collector used in the present invention is preferably formed from a metal that is not alloyed with lithium. Examples of such a material include copper, an alloy containing copper, nickel, and stainless steel. The current collector may be a laminate of two or more of these materials.

  A lithium secondary battery according to the present invention includes the above-described negative electrode for a lithium secondary battery according to the present invention, a positive electrode, and a nonaqueous electrolyte.

  Since the lithium secondary battery of the present invention uses the above-described negative electrode for a lithium secondary battery of the present invention, it can exhibit good charge / discharge cycle characteristics.

The solvent of the nonaqueous electrolyte used in the lithium secondary battery of the present invention is not particularly limited, but cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl A mixed solvent with a chain carbonate such as carbonate is exemplified. Further, mixed solvents of the above cyclic carbonate and ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane are also exemplified. The solutes of the nonaqueous electrolyte include LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C _{4 F 9 SO 2), LiC} (CF 3 SO 2) 3, LiC (C 2 F 5 SO 2) 3, LiAsF 6, LiClO 4, Li 2 B 10 Cl 10, Li 2 B 12 Cl 12 and the like and their Mixtures are exemplified. In particular, LiXF _y (wherein X is P, As, Sb, B, Bi, Al, Ga, or In, y is 6 when X is P, As, or Sb, and X is Bi, Al, Ga or y when in is 4), lithium perfluoroalkyl sulfonic acid imide _{LiN (C m F 2m + 1} SO 2) (C n F 2n + 1 SO 2) ( wherein, m and n are each independently, to an integer of 1 to 4) or lithium perfluoroalkyl sulfonic acid methide _{LiN (C p F 2p + 1} SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) ( wherein Among them, solutes such as p, q and r are each independently an integer of 1 to 4 are preferably used. Among these, LiPF ₆ is particularly preferably used. Further, as the electrolyte, a gel polymer electrolyte in which a polymer electrolyte such as polyethylene oxide or polyacrylonitrile is impregnated with an electrolytic solution is exemplified. The electrolyte of the lithium secondary battery of the present invention is not limited as long as the lithium compound as a solute that exhibits ionic conductivity and the solvent that dissolves and retains the lithium compound do not decompose at the time of battery charging, discharging, or storage. Can be used.

Examples of the positive electrode material of the lithium secondary battery of the present invention include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiMnO ₂ , LiCo _0.5 Ni _0.5 O ₂ , LiNi _0.7 Co _0.2 Mn _0.1 O ₂ and other lithium-containing transition metal oxides. Examples thereof include metal oxides that do not contain lithium, such as MnO ₂ . In addition, any substance that electrochemically inserts and desorbs lithium can be used without limitation.

  ADVANTAGE OF THE INVENTION According to this invention, the deformation | transformation of the electrical power collector by repetition of charging / discharging and peeling of the active material thin film from an electrical power collector can be suppressed, and favorable charging / discharging cycling characteristics can be acquired.

  Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications within a range that does not change the gist thereof. It is.

  FIG. 1 is a schematic cross-sectional view showing a negative electrode for a lithium secondary battery in one embodiment according to the present invention. FIG. 1 schematically shows a negative electrode for a lithium secondary battery produced in Example 1 described later. FIG. 3 is a scanning electron microscope (SEM) photograph showing a cross section of the negative electrode for a lithium secondary battery produced in Example 1, and FIG. 4 shows the surface of the negative electrode for a lithium secondary battery when viewed from above. It is a scanning electron microscope (SEM) photograph.

  As shown in FIG. 1, an active material thin film 3 is formed on the main surface of the current collector 1. Concavities and convexities are formed on the main surface of the current collector 1, and there are convex portions 1a. In the active material thin film 3, a low density region 2 is formed so as to extend in the thickness direction of the thin film.

  As shown in FIG. 4, the low density region is connected in a mesh shape in a plane direction parallel to the main surface of the current collector 1. As shown in FIGS. 1 and 3, the low-density region connected in a mesh shape is formed in the thin film so as to extend in the thickness direction of the thin film.

  As shown in FIG. 1, in the thickness direction of the thin film 3, the low density region 2 has a first curved portion 2a that is curved so as to swell in one direction in the plane direction, and one direction following the first curved portion 2a. And a second curved portion 2b that curves so as to swell in the opposite direction. 3 shows a state viewed from the opposite side (that is, the back side) to FIG. 1, and therefore the bending directions of the first bending portion 2a and the second bending portion 2b in the low density region 2 are opposite to each other. ing. That is, in FIG. 1, the first curved portion 2a is curved so as to swell toward the left side of the drawing, and the second curved portion 2b is curved so as to swell toward the right side of the drawing, whereas in FIG. The curved portion 2a is curved so as to swell toward the right side of the drawing, and the second curved portion 2b is curved so as to swell toward the left side of the drawing. Also in the schematic diagrams and SEM photographs of Examples and Comparative Examples shown below, the bulging direction of the curved portion is similarly reversed.

  According to the present invention, the low-density region 2 formed in the thin film 3 is formed so as to have the first curved portion 2a and the second curved portion 2b, whereby charge / discharge cycle characteristics can be improved. This is because the stress generated by the expansion and contraction of the active material thin film accompanying the charge / discharge reaction is effectively dispersed in the direction perpendicular to the direction parallel to the thin film in the thin film due to the presence of the first curved portion and the second curved portion. For this reason, it is considered that peeling of the active material and deformation of the current collector are suppressed, and charge / discharge cycle characteristics are thereby improved.

  In the embodiment shown in FIGS. 1 and 3, the active material thin film 3 is formed by being deposited in two stages, as will be described later. That is, after a thin film is formed on the main surface of the current collector 1 in the first thin film forming process, the thin film is deposited thereon in the second thin film forming process to form the entire thin film. Reference numeral 2c shown in FIG. 1 indicates the surface of the thin film produced in the first thin film forming step. Thus, the thin film is deposited in the second thin film forming step on the surface 2c of the thin film formed in the first thin film forming step. As shown in FIG. 1, since the surface 2c of the thin film produced in the first thin film formation step is formed in a bowl shape, it is considered that the relationship with the thin film formed thereon is strengthened. Thus, when thin films are stacked and formed in a plurality of thin film forming steps, the thin films can be connected in a bowl shape at the boundary portion, thereby improving the adhesion in the thickness direction of the thin film, It is considered that the peeling of the active material and the deformation of the current collector are suppressed.

  Further, as shown in FIG. 1, a large cavity 5 that can be clearly recognized by a scanning electron microscope is formed at the boundary between the first curved portion 2a and the second curved portion 2b of the low density region 2. Yes. Such a cavity 5 is considered to be formed because a portion where vapor deposition material particles are difficult to deposit is formed when vapor deposition material particles are deposited. By forming such a cavity 5 in the thin film 3, stress due to expansion and contraction of the active material thin film 3 due to the charge / discharge reaction can be relieved by the cavity 5, thereby separating the active material and collecting current. Deformation of the body can be further suppressed.

  Further, as shown in FIG. 1, a gap 4 is also formed in the vicinity of the convex portion 1 a of the current collector 1. Such a void seems to be formed because a portion where vapor deposition material particles are difficult to deposit is generated by the convex portion 1a. The presence of such voids 4 can also relieve stress due to the expansion and contraction of the active material thin film accompanying the charge / discharge reaction, and can further suppress the peeling of the active material and the deformation of the current collector.

  FIG. 2 is a schematic cross-sectional view showing a thin film forming apparatus that can be used for manufacturing a negative electrode for a lithium secondary battery of the present invention.

  As shown in FIG. 2, a rotatable roller 27 is provided in the chamber, and a base material 24 that is a current collector is installed so as to be movable along the outer peripheral surface of the roller 27. Yes. The substrate 24 is wound around a roller 25 provided on one side of the roller 27 and a roller 26 provided on the other side, and the substrate 24 has an arrow A direction and an arrow in the direction opposite to the arrow A direction. It is provided so as to be movable in the B direction. The roller 27 is a coolable roller, and can cool the base material 24 when forming a thin film.

  Below the roller 27, a crucible 22 as an evaporation source is provided, and an electron beam gun 23 for irradiating the crucible 22 with an electron beam is provided. When the electron beam 29 emitted from the electron beam gun 23 is irradiated onto the vapor deposition material 21 in the crucible 22, the particles of the vapor deposition material 21 evaporate, and the vapor deposition material particles are deposited on the substrate 24. Is formed.

  A shielding plate 28 is provided in a region where the crucible 22 and the roller 27 face each other in order to limit a region where the vapor deposition material 21 is deposited on the base material 24.

  The center of the crucible 22 is provided at a position away from the center of the roller 27 by a distance X as shown in FIG. Moreover, each is arrange | positioned so that it may become the distance Y between the lowest part of the roller 27, and the front-end | tip part with the crucible 22. FIG.

  D <b> 1 to D <b> 5 on the outer peripheral surface of the roller 27 indicate positions when the vapor deposition material 21 is vapor deposited on the base material 24. When the base material 24 moves in the arrow A direction, the base material 24 sequentially passes through the positions of D1, D2, D3, D4, and D5. Moreover, when the base material 24 moves in the arrow B direction, the base material 24 sequentially passes through the positions of D5, D4, D3, D2, and D1.

  When the base material 24 is moved in the direction of arrow A, vapor deposition material particles are deposited on the base material 24 from the position D2, and formation of a thin film is started. The formation of the thin film continues through the position of D3 to the position of D4. At the position of D2, the angle at which the vapor deposition material particles enter the base material, that is, the current collector 24 is 90 ° as the angle with respect to the normal line of the main surface of the base material. The position is an angle of 90 ° in the reverse direction. Accordingly, when the substrate 24 moves in the direction of arrow A, the angle at which the vapor deposition material particles enter the current collector gradually changes from 90 ° in one direction to 90 ° in the reverse direction.

  When the substrate 24 moves in the direction of arrow B, which is the reverse direction, the formation of the thin film is started from the position D4, and the formation of the thin film continues through the position D3 to the position D2. In this case, the thin film is deposited while the angle at which the vapor deposition material particles enter the current collector 24 gradually changes from the 90 ° angle in the reverse direction to the 90 ° angle in one direction.

  For example, the thin film of the lower half portion of the active material thin film 3 shown in FIG. 1 can be formed while the base material passes through the positions D1 to D5 while moving the base material 24 in the arrow A direction. In this way, a thin film is formed on the base material 24 by the first thin film forming process, and after winding the base material 24 from the roller 25 to the roller 26, the roller 25 and the roller 26 are rotated in the opposite directions, The material 24 is moved in the direction of arrow B, a thin film is deposited on the base material 24 while the base material 24 passes through the positions D5 to D1, and the upper half portion of the active material thin film 3 is placed on the second thin film. It can be deposited in the formation process.

  The maximum angle of incidence of the vapor deposition material particles in the region between D1 and D5 can be adjusted by adjusting the position of the crucible 22 changing the distance between X and Y, for example.

(Examples 1-2 and Comparative Examples 1-3)
(Production of negative electrode)
As a base material (current collector), a rolled copper foil (thickness of 26 μm) having a roughened surface by depositing copper on the surface by an electrolytic method was used. The arithmetic average roughness Ra of the used collector surface is 0.5 μm. The arithmetic average roughness Ra is defined in Japanese Industrial Standard (JIS B 0601-1994), and can be measured by, for example, a stylus type surface roughness meter.

  A silicon thin film was formed on the substrate (current collector) using the thin film forming apparatus shown in FIG. As the vapor deposition material 21, silicon having a purity of 99.99% was used, and as the crucible 22, a water-cooled copper crucible was used.

  The horizontal distance X between the center position of the vapor deposition material 21 and the center position of the roller 27 was set to be 80 mm. Further, the distance Y between the lower end of the roller 27 and the tip of the crucible 22 was set to be 200 mm. As the roller 27, a roller having a diameter of 800 mm was used.

  A silicon thin film was formed on the base material under the conditions of the maximum incident angle, the number of passages, the base material traveling speed, and the deposition speed shown in Table 1, and negative electrodes of Examples 1-2 and Comparative Examples 1-3 were prepared. . The “maximum incident angle” in Table 1 is the maximum angle when the vapor deposition material particles are incident on the main surface of the substrate in the region of D1 to D5 in FIG. 2, and is an angle with respect to the normal line of the main surface. In Example 1, Example 2, and Comparative Example 1, the incident angle of the vapor deposition material particles at the positions D2 and D4. In Comparative Example 2 and Comparative Example 3, the maximum incident angle shown in Table 1 is obtained by adjusting the position of the shielding plate 28 shown in FIG. In Comparative Example 2 and Comparative Example 3, 55 ° is the maximum incident angle on the D1 side, and 40 ° is the maximum incident angle on the D5 side.

  Further, the “number of passages” in Table 1 is the number of times the base material passes over the crucible which is the vapor deposition source. In Example 1, after depositing a thin film by moving the base material in the direction of arrow A, The substrate is moved in the direction of arrow B to deposit a thin film. In the second embodiment, the number of passages is set to 4 by changing the moving direction of the arrow A, arrow B, arrow A, and arrow B directions four times. In Comparative Example 1, the thin film is formed by passing only once in the arrow A direction. Similarly, in Comparative Example 2, the silicon thin film is deposited only once in the direction of arrow A. In Comparative Example 3, as in Example 1, the silicon thin film is deposited by passing twice in the directions of arrow A and arrow B.

  The thickness of the silicon thin film to be deposited was 8 μm in all cases. The power of the electron gun at the time of forming the silicon thin film was 11 kW, and the thickness of the silicon thin film was adjusted by the traveling speed of the base material when depositing the silicon thin film.

  “Deposition rate (μm · m / min)” shown in Table 1 indicates the deposited film thickness of the silicon thin film when the running speed of the substrate is 1 m / min. The deposition rate when the maximum incident angle was 90 ° was 1.6 μm · m / min, which was about 1.7 times that when the maximum incident angle was 55 ° and 40 °.

The silicon thin film formed in each of the above examples and comparative examples was subjected to Raman spectroscopic analysis to identify its crystallinity. As a result, in all of the examples and comparative examples, a peak in the vicinity of 480 cm ⁻¹ was substantially recognized, but a peak in the vicinity of 520 cm ⁻¹ was not substantially recognized. Therefore, the formed silicon thin film was amorphous. The quality was confirmed.

[Observation of negative electrode cross section]
The cross section of the negative electrode after silicon thin film deposition in the negative electrode produced in Examples 1-2 and Comparative Examples 1-3 was observed with a scanning electron microscope (JSM-6500F manufactured by JEOL Datum Co., Ltd.). A cross section polisher (SM-09010, manufactured by JEOL Datum Co., Ltd.) was used for producing the cross section.

  FIG. 3 is an SEM photograph showing a cross section of the negative electrode of Example 1, and FIG. 4 is an SEM photograph showing the negative electrode surface seen from above. FIG. 1 is a diagram schematically showing a cross section of the negative electrode of Example 1. As described above, FIG. 1 is a view observed from the direction of the back side of FIG. 3, and the bending directions of the first bending portion and the second bending portion are shown in the opposite direction to FIG. 3.

  6 is a SEM photograph showing a cross section of the negative electrode of Example 2, and FIG. 7 is a SEM photograph showing the surface of the negative electrode as viewed from above. FIG. 5 is a diagram schematically showing a cross section of the negative electrode of Example 2. Also in FIG. 5, the bending direction of the bending portion is shown in the opposite direction to FIG.

  FIG. 9 is an SEM photograph showing a cross section of the negative electrode of Comparative Example 1, and FIG. 10 is an SEM photograph showing the surface of the negative electrode viewed from above. FIG. 8 is a diagram schematically showing a cross section of the negative electrode of Comparative Example 1. Also in FIG. 8, the direction in which the bending portion swells is shown in the direction opposite to the bending portion in FIG.

  FIG. 11 is a diagram schematically showing a cross section of the negative electrode of Comparative Example 2.

  FIG. 12 is a diagram schematically showing a cross section of the negative electrode of Comparative Example 3.

  FIG. 13 is an SEM photograph showing a cross section of the negative electrode of Comparative Example 3, and FIG. 14 is an SEM photograph showing the surface of the negative electrode viewed from above.

  As shown in FIGS. 1 and 3, in the negative electrode of Example 1, the first curved portion 2 a and the second curved portion 2 b are formed in the low density region 2 formed in the thickness direction of the thin film 3. A cavity 5 is formed at a boundary portion between the first curved portion 2a and the second curved portion 2b.

  As shown in FIGS. 5 and 6, in the negative electrode of Example 2, the first curved portion 2a is formed from the current collector 1 side in the low density region 2 formed in the thickness direction of the thin film 3, and subsequently. The second curved portion 2b is formed, and the first curved portion 2a and the second curved portion 2b are further formed thereon. The first first curved portion and the second curved portion are formed when the current collector is first moved in the direction of the arrow A and the direction of the arrow B to deposit a thin film, and are formed thereon. The first bending portion 2a and the second bending portion 2b are formed in the thin film formation in the movement of the current collector in the second arrow A direction and the arrow B direction.

  In the second embodiment, as shown in FIGS. 5 and 6, the cavities 5 are respectively formed in the boundary portions between the curved portions. A gap 4 is also formed in the vicinity of the convex portion 1 a on the main surface of the current collector 1.

  As shown in FIGS. 8 and 9, in the negative electrode of Comparative Example 1, the low density region 2 extending in the thickness direction of the thin film 3 is formed with only a curved portion 2 a that swells in one direction. This is presumably because the thin film is formed only when the current collector is moved in the direction of arrow A. A gap 4 is formed in the vicinity of the convex portion 1 a on the main surface of the current collector 1, but no cavity 5 is formed in the thin film 3.

  As shown in FIG. 11, in the negative electrode of Comparative Example 2, the low density region 2 is formed to extend straight in the thickness direction of the thin film 3, and no curved portion is formed.

  As shown in FIGS. 12 and 13, also in Comparative Example 3, the low density region 2 is formed so as to extend straight in the thickness direction of the thin film 3, and no curved portion is formed.

  Further, as shown in FIGS. 11 and 12, in Comparative Example 2 and Comparative Example 3, no cavity 5 is formed in the thin film 3.

  The cavity 5 formed in the active material thin film of Examples 1 and 2 is such a cavity 5 where the vapor deposition material particles are not deposited due to the incident angle of the vapor deposition material particles when depositing the vapor deposition material particles. It seems to be. The size of the cavity 5 was about 2 μm at the maximum portion.

<Evaluation of charge / discharge cycle characteristics>
Using the negative electrodes of Examples 1 and 2 and Comparative Examples 1 to 3, a three-electrode beaker cell was prepared and the charge / discharge cycle characteristics were evaluated.

(Preparation of electrolyte)
LiPF ₆ was dissolved in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 so as to be 1 mol / liter to prepare an electrolytic solution.

[Preparation of beaker cell]
Using the negative electrodes (size 20 mm × 20 mm) of Examples 1-2 and Comparative Examples 1-3 as working electrodes, a three-electrode beaker cell as shown in FIG. 15 was produced.

  As shown in FIG. 15, the beaker cell is configured by immersing a counter electrode 43, a working electrode 44, and a reference electrode 45 in an electrolytic solution 42 placed in a container 41. As the electrolytic solution 42, the above electrolytic solution was used, and as the counter electrode 43 and the reference electrode 45, lithium metal was used.

[Evaluation of charge / discharge characteristics]
The prepared beaker cell was charged with a constant current of 4 mA until the potential of the working electrode reached 0 V (vs. Li ^/ Li ⁺ ), and then the potential of the working electrode was 2 V (vs. Li / Li at a constant current of 4 mA. Li ⁺ ) was discharged, and this was regarded as one cycle of charge / discharge, and the discharge capacity per unit area in the first and tenth cycles was measured. Here, the reduction of the working electrode is charging, and the oxidation of the working electrode is discharging. From the discharge capacity at the first cycle and the discharge capacity at the 10th cycle, the capacity retention rate was calculated by the following formula and shown in Table 1.
Capacity retention rate (%) = (discharge capacity at the 10th cycle / discharge capacity at the first cycle) × 100

<Deformation of current collector and evaluation of adhesion of active material thin film to current collector>
The negative electrode after the above charge / discharge test was observed with the naked eye, and the deformation of the current collector and the adhesion of the active material to the current collector were observed.

  In all the negative electrodes of Examples 1-2 and Comparative Examples 1-3, almost no deformation such as wrinkles was observed on the current collector.

  In the negative electrodes of Examples 1 and 2 and Comparative Examples 2 and 3, peeling of the active material from the current collector was not observed. However, in the negative electrode of Comparative Example 1, the active material was active in about a half region in the electrode. Material delamination was observed.

  As shown in Table 1, the negative electrodes of Examples 1 and 2 have better capacity retention ratios than the negative electrodes of Comparative Examples 1 to 3. As described above, since the first curved portion and the second curved portion are formed in the low density region of the active material thin film, the stress due to the expansion and contraction of the active material thin film accompanying the charge / discharge reaction is This is considered to be because the active material is effectively dispersed in the thickness direction and the surface direction, so that peeling of the active material and deformation of the current collector are suppressed, and good charge / discharge cycle characteristics are obtained. Further, as described above, when the active material thin film is deposited in a plurality of times, the thin film deposition portion is connected in a saddle shape at the boundary surface, thereby improving the adhesion in the thickness direction of the active material. Is considered to further suppress the peeling of the active material and improve the charge / discharge cycle characteristics.

  Further, as described above, since a cavity is formed at the boundary portion of the curved portion of the low density region, the cavity portion can effectively relieve stress due to expansion and contraction of the active material thin film accompanying the charge / discharge reaction. This is also considered to improve the charge / discharge cycle characteristics. In particular, the negative electrode of Example 2 has a larger number of cavities than that of Example 1, and the reason why the capacity retention rate is higher than that of Example 1 is considered to be due to this reason.

  As described above, the negative electrode for a lithium secondary battery according to the present invention can suppress deformation of the current collector due to repeated charge and discharge, and peeling of the active material thin film from the current collector, and is favorable. It shows the charge / discharge cycle characteristics.

The schematic diagram which shows the cross section of the negative electrode for lithium secondary batteries of one Example according to this invention. 1 is a schematic cross-sectional view showing a thin film forming apparatus used for depositing an active material thin film on a current collector in an embodiment according to the present invention. The scanning electron micrograph which shows the cross section of the negative electrode for lithium secondary batteries in one Example according to this invention. The scanning electron micrograph which shows the surface of the negative electrode for lithium secondary batteries in one Example according to this invention. The schematic diagram which shows the cross section of the negative electrode for lithium secondary batteries of the other Example according to this invention. The scanning electron micrograph which shows the cross section of the negative electrode for lithium secondary batteries of the other Example according to this invention. The scanning electron micrograph which shows the surface of the negative electrode for lithium secondary batteries of the other Example according to this invention. The schematic diagram which shows the cross section of the negative electrode for lithium secondary batteries of the comparative example 1. FIG. The scanning electron micrograph which shows the cross section of the negative electrode for lithium secondary batteries of the comparative example 1. The scanning electron micrograph which shows the surface of the negative electrode for lithium secondary batteries of the comparative example 1. The schematic diagram which shows the cross section of the negative electrode for lithium secondary batteries of the comparative example 2. FIG. FIG. 4 is a schematic diagram showing a cross section of a negative electrode for a lithium secondary battery of Comparative Example 3. 6 is a scanning electron micrograph showing a cross section of a negative electrode for a lithium secondary battery of Comparative Example 3. 4 is a scanning electron micrograph showing the surface of a negative electrode for a lithium secondary battery of Comparative Example 3. The typical sectional view showing a three electrode type beaker cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Current collector 1a ... Convex part on the main surface of a current collector 2 ... Low density region 2a ... First curved portion 2b in low density region 2b ... Second curved portion in low density region 2c ... Border portion of thin film DESCRIPTION OF SYMBOLS 3 ... Active material thin film 4 ... Gap 5 ... Cavity 21 ... Evaporation material 22 ... Evaporation source 23 ... Electron beam gun 24 ... Base material (current collector)
25, 26 ... roller 27 ... roller 28 ... shielding plate 29 ... electron beam

Claims (5)

A lithium secondary battery negative electrode in which a thin film made of an active material that occludes and releases lithium is deposited on a main surface of a current collector,
In a state before charge and discharge, a low density region connected in a mesh shape in a plane direction parallel to the main surface of the current collector is formed in the thin film so as to extend in the thickness direction of the thin film, In the thickness direction of the thin film, the density region is curved so as to swell in one direction of the surface direction, and subsequently curved so as to swell in a direction opposite to the one direction following the first curved portion. A negative electrode for a lithium secondary battery, comprising a second curved portion.   2. The negative electrode for a lithium secondary battery according to claim 1, wherein a cavity is formed in a boundary portion between the first curved portion and the second curved portion in the low density region. A method for producing a negative electrode for a lithium secondary battery according to claim 1 or 2, wherein vapor deposition material particles are deposited on a main surface of the current collector by a vapor deposition method to form the thin film.
The angle at which the vapor deposition material particles enter the main surface of the current collector is defined as an angle with respect to the normal of the main surface, from an angle of 70 ° or more in one direction to an angle of 70 ° or more in the opposite direction to the one direction. A first thin film forming step of forming the thin film while forming the first curved portion by gradually changing with the progress of thin film formation;
Forming a thin film from an angle of 70 ° or more in the reverse direction to an angle of 70 ° or more in the one direction, where the angle at which the vapor deposition material particles enter the main surface of the current collector is an angle with respect to the normal of the main surface And a second thin film forming step of forming the thin film while forming the second curved portion by gradually changing the process as the process proceeds. A method for producing a negative electrode for a lithium secondary battery, comprising: A manufacturing method for forming the thin film by depositing the thin film on the main surface of the current collector by passing the current collector along the outer peripheral surface of a roller and passing it over an evaporation source for evaporating the vapor deposition material particles Because
The first thin film forming step is a step of forming a thin film while moving the current collector in a first direction, and the second thin film forming step is a step of moving the current collector into the first direction. 4. The method for producing a negative electrode for a lithium secondary battery according to claim 3, wherein the thin film is formed while being moved in a second direction which is the reverse direction.   A lithium secondary battery comprising the negative electrode according to claim 1, a positive electrode, and a nonaqueous electrolyte.