JP2009043523A - Method of manufacturing negative electrode for lithium secondary battery and negative electrode for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To overcome the problems that, since a negative electrode material made of a metal element or its alloy expands and contracts in a great extent at reaction of occlusion/release of lithium ion, electric joint with a collector can not be maintained, leading to decrease of a capacity. <P>SOLUTION: The negative electrode for a lithium secondary battery is manufactured by a method of depositing lithium and silicon as active materials for the negative electrode for a lithium secondary battery aslant at the same time on the surface of a collector 3 with its surface roughened with ruggednesses. With the negative electrode for the lithium secondary battery manufactured in this method, columnar negative electrode active material particles 21 are in a more expanded state than at initial formation, so that there is no risk of collision among the columnar negative electrode active material particles 21 or break of the collector, giving birth to the negative electrode with excellent cycle characteristics. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a negative electrode for a lithium secondary battery excellent in cycle characteristics, and a negative electrode for a lithium secondary battery produced by the production method.

  In recent years, the demand for portable communication devices has been increasing due to downsizing and diversification of applications. In addition, secondary batteries used in portable communication devices are required to have improved performance such as increased discharge capacity and extended life. In particular, lithium secondary batteries with high energy density have significantly increased production.

Many of the lithium secondary batteries currently on the market are configured using a Li-containing composite oxide such as LiCoO ₂ for the positive electrode and graphite for the negative electrode. Since the negative electrode material using graphite can occlude lithium ions only up to the composition represented by LiC ₆ , the capacity per volume of graphite as the negative electrode material has a maximum value of 372 mAh / g. Since this value is only about 1/5 of the theoretical capacity of metallic lithium, the current lithium secondary battery configuration has been limited in improving energy density.

Accordingly, a metal element such as Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, Bi, or an alloy thereof, which is known to be able to reversibly store and release lithium ions, is a lithium secondary battery. It is being used as a negative electrode material. Wherein the metal element or the theoretical capacity of lithium ions per volume of their alloys, is, for example, Al is 2167mAh / ^cm 3, Si is 2377mAh / ^cm 3, Ge is 2344mAh / ^cm 3, Sn is 1982mAh / ^cm 3, Pb is 1720 mAh / cm ³ , Sb is 1679 mAh / cm ³ , and Bi is 1768 mAh / cm ³ . That is, in comparison with a carbonaceous material using graphite, the capacity per volume can be increased by using the metal element or the alloy thereof as a negative electrode material. However, on the other hand, the above metal elements or their alloys have a large expansion / contraction generated during the lithium ion storage / release reaction, so that the electrical connection with the current collector cannot be maintained and the capacity is reduced. There is a problem of decreasing.

In response to the problem, columnar negative electrode active material particles are formed on the surface of the current collector by irradiating the current obtained by heating and generating the negative electrode active material from an oblique direction onto a current collector that has been roughened by providing unevenness in advance. An electrode that forms a close contact has been studied. When the columnar negative electrode active material grains are formed in close contact with each other, by utilizing the uneven mask effect provided on the surface of the current collector, a certain interval is provided between the columnar negative electrode active material grains adjacent to each other. Collisions between the negative electrode active material particles can be avoided (see, for example, Patent Document 1).
JP-A-2005-196970

  According to the above-described conventional negative electrode manufacturing method, by controlling the roughened state of the current collector surface on which the columnar negative electrode active material particles are formed, an appropriate interval is provided between the columnar negative electrode active material particles adjacent to each other. The collision between the columnar negative electrode active material particles during expansion can be avoided to some extent.

However, from the viewpoint of securing the space necessary for the columnar negative electrode active material particles to expand, there are cases where it is not possible to sufficiently cope with the control of the roughened state of the current collector surface alone. In particular, since the mask effect is not obtained in the direction perpendicular to the inclination direction of the current collector surface, and less expansion space can be secured than in the direction parallel to the inclination direction, the columnar negative electrode active material during expansion There is a problem that it is not possible to secure a sufficient interval for avoiding collision between grains.

  In order to solve the above-described problems, the present invention provides an active material for a lithium secondary battery negative electrode on a surface of a current collector that is roughened by providing irregularities on the surface, and is oblique to the surface of the current collector. A method for producing a negative electrode for a lithium secondary battery, wherein the active materials are lithium and silicon, and the lithium and silicon are vapor-deposited on a current collector at the same time.

  By the production method of the present invention, a negative electrode for a lithium secondary battery in which columnar negative electrode active material grains having substantially the same state as silicon expanded and occluded lithium ions is formed on the current collector surface. Can do.

  According to the method for producing a negative electrode for a lithium secondary battery of the present invention, even if the columnar negative electrode active material particles whose volume has been shrunk by discharging lithium ions during discharging, occlude lithium ions again during expansion and expand. In addition, it is possible to manufacture a negative electrode for a lithium secondary battery excellent in cycle characteristics, in which collision between columnar negative electrode active material particles hardly occurs, and sufficient adhesion between the current collector and the columnar negative electrode active material particles can be secured. .

  The best mode for carrying out the present invention will be described below with reference to the drawings.

(Embodiment 1)
<Production apparatus for negative electrode 20 for lithium secondary battery>
FIG. 1 is a schematic diagram of a vapor deposition device 13 that is a device for producing a negative electrode 20 for a lithium secondary battery according to Embodiment 1 of the present invention. As shown in FIG. 1, a vapor deposition apparatus 13 that is a manufacturing apparatus of a negative electrode 20 for a lithium secondary battery in Embodiment 1 includes a vacuum vessel 4 and a main valve 5 provided in a pipe 14 connected to the vacuum vessel 4. The high-vacuum pump 6 and the low-vacuum pump 7 are configured to depressurize the vacuum vessel 4 through the main valve 5.

The high vacuum pump 6 preferably has an ultimate vacuum of 10 ⁻⁴ Pa or less, and more preferably 10 ⁻⁶ Pa or less. Further, the low vacuum pump 7 may be any as long as the back pressure of the high vacuum pump 6 can be maintained.

  At the bottom of the vacuum vessel 4, a lithium evaporation source 1 for evaporating lithium and a silicon evaporation source 2 for evaporating silicon are provided via a shielding plate 8. The shielding plate 8 prevents cross talk between evaporated lithium and silicon. A crystal oscillation type lithium evaporation rate measurement rate monitor 9 and a silicon evaporation rate measurement rate monitor 10 are installed obliquely above the lithium evaporation source 1 and the silicon evaporation source 2, respectively. In addition, the evaporation rate of silicon from the silicon evaporation source 2 is detected.

  A current collector holder 12 is provided on the top of the vacuum vessel 4, and the current collector 3 is installed. A shutter 11 is provided directly under the current collector holder 12 to prevent lithium and silicon from adhering to the current collector 3 within the stabilization time. Here, the stabilization time indicates the time required for the lithium evaporation source 1 and the silicon evaporation source 2 to stabilize the evaporation rate of lithium and silicon.

The current collector holder 12 can change the installation surface of the current collector 3 at an arbitrary angle with respect to the vertical direction. In the present embodiment, the angle θ formed by the normal line from the deposition surface of the current collector 3 and the straight line connecting the current collector 3 and the midpoint between the evaporation surfaces of the lithium evaporation source 1 and the silicon evaporation source 2 is adjusted. (A detailed description of the configuration is omitted).

  The lithium evaporation source 1 may be any one that can stably heat lithium up to about 600 ° C., and for example, a Knudsen cell equipped with a cartridge-type heater can be used (detailed description of the configuration is omitted). To do).

  The silicon evaporation source 2 may be any material that can stably heat silicon. For example, an electron beam gun heating type copper crucible can be used. The copper crucible has a water-cooling function so that it does not break even if silicon is dissolved. The electron beam gun only needs to have an acceleration voltage of 5 to 10 kV and an output of an irradiation current of about 0.3 to 1 A. For example, a JEBG-303UA type electron gun manufactured by JEOL Ltd. can be used (detailed). The description of the configuration is omitted).

<The manufacturing method and preferable manufacturing conditions of the negative electrode 20 for lithium secondary batteries>
FIG. 2 is a schematic cross-sectional view of negative electrode 20 for a lithium secondary battery according to Embodiment 1 of the present invention. The manufacturing method and preferable manufacturing conditions of the negative electrode 20 for a lithium secondary battery in the first embodiment will be described below.

  First, the current collector 3 roughened by providing unevenness on the surface is placed in the current collector holder 12 of the vapor deposition apparatus 13.

  The surface roughening of the current collector 3 in Embodiment 1 is necessary for improving the adhesion between the current collector 3 and the negative electrode active material particles, and for forming the formed negative electrode active material particles in a columnar shape. . As an example of the current collector 3, a copper foil surface having a thickness of 35 μm with irregularities and a surface roughness Rz of 10 μm with a 10-point average roughness (for example, manufactured by Furukawa Circuit Foil Co., Ltd.) is preferable. The roughness Rz should be 5 μm or more in terms of 10-point average roughness. Here, the surface roughness Rz indicates a ten-point average roughness defined in JIS B0601: 1994. It should be noted that the composition of the current collector 3 is problematic even if it contains elements that do not react with lithium such as zirconium and titanium, and inevitable elements such as oxygen, selenium, and tellurium in addition to copper as the main component. Absent.

  The angle of the installation surface of the current collector 3 in the current collector holder 12 affects the shape of the columnar negative electrode active material particles 21 formed on the surface of the current collector 3. Therefore, in order to form the columnar negative electrode active material particles 21 having a good shape on the surface of the current collector 3, the lithium evaporation source 1 and the straight line connecting the silicon evaporation source 2 and the current collector 3 and the vapor deposition of the current collector 3 are performed. It is preferable to install the current collector 3 in the current collector holder 12 so that the angle θ formed with the normal from the surface is 30 ° ≦ θ ≦ 80 °.

  The columnar negative electrode active material grains 21 have a mask effect caused by the angle θ with the current collector 3 having a concavo-convex shape formed on the surface in advance, and vapors of lithium and silicon are preferentially deposited on the convex portions on the surface of the current collector 3. It is formed by doing. When the surface unevenness is small or the angle θ is small, the mask effect is weakened, and the negative electrode active material tends to be a continuous layer rather than a columnar shape. Therefore, when the negative electrode active material shrinks during discharge, the columnar negative electrode There is a possibility that the active material grains 21 themselves crack and reduce the capacity. Therefore, in order to obtain a preferable columnar shape for the columnar negative electrode active material grains 21, as the degree of unevenness on the surface of the current collector 3, for example, the surface roughness Rz at 10-point average roughness is 5 μm or more, and the angle θ is 30. It should be more than °. However, since the adhesion rate of lithium and silicon to the current collector 3 decreases as the angle θ is increased, the angle θ is preferably 80 ° or less.

  As described above, the columnar negative electrode active material particles 21 are formed obliquely on the surface of the current collector 3, so that voids that can expand around the columnar negative electrode active material particles 21 can exist. Therefore, even when the battery is further expanded, such as when the battery charge / discharge cycle is repeated or a high charge voltage is applied, the current collector 3 can be prevented from being broken due to contact between the columnar negative electrode active material particles 21.

Next, lithium and silicon are inserted into the lithium evaporation source 1 and the silicon evaporation source 2, respectively, and the high vacuum pump 6 and the low vacuum pump 7 are operated to depressurize the vacuum vessel 4. Thereafter, for example, argon is introduced into the vacuum vessel 4 and the pressure in the vacuum vessel 4 is adjusted to 1 × 10 ⁻⁴ Pa.

  Then, by applying AC power to the lithium evaporation source 1, lithium is heated and evaporated. At the same time, the silicon evaporation source 2 is irradiated with electrons to evaporate silicon by heating and melting. At this time, the lithium evaporation rate from the lithium evaporation source 1 and the silicon evaporation rate from the silicon evaporation source 2 are managed using the rate monitor 9 for measuring the lithium evaporation rate and the rate monitor 10 for measuring the silicon evaporation rate, respectively.

  The lithium evaporation rate and the silicon evaporation rate are measured by the rate monitor 9 for measuring the lithium evaporation rate and the rate monitor 10 for measuring the silicon evaporation rate, and the amount of evaporation of lithium from the lithium evaporation source 1 and the amount of silicon from the silicon evaporation source 2 are measured. Is confirmed to be a predetermined amount, the shutter 18 is opened, and the columnar negative electrode active material grains 21 are formed on the surface of the current collector 3.

The lithium evaporation rate measurement rate monitor 9, the silicon evaporation rate measurement rate monitor 10, the amount of lithium evaporation that reaches the current collector 3, and the amount of silicon evaporation are greatly affected by the residual gas in the respective arrival paths. Therefore, it is necessary to keep the gas type and gas pressure in the vacuum vessel 4 constant. Therefore, the argon introduced into the vacuum vessel 4 is preferably kept constant in the range of 10 ⁻⁴ Pa to 1 × 10 ⁻² Pa.

  In the first embodiment, the composition ratio R of lithium and silicon in the columnar negative electrode active material grains 21 formed on the surface of the current collector 3 is expressed as “number of moles of lithium to be deposited / number of moles of silicon to be deposited”, that is, , “Number of moles of lithium in columnar negative electrode active material grains 21 / number of moles of silicon”. It is preferable to control the lithium evaporation rate from the lithium evaporation source 1 and the silicon evaporation rate from the silicon evaporation source 2 so that the composition ratio R becomes 3.3 ≦ R ≦ 4.4.

  In general, when a lithium secondary battery is formed using a negative electrode, the lithium secondary battery is formed after the lithium ions are once released from the columnar negative electrode active material particles to shrink the volume. Then, it charges again and occludes lithium ion to a columnar negative electrode active material particle, and expands a volume.

  When the columnar negative electrode active material particles 21 are formed with a composition ratio R of less than 3.3, the columnar negative electrode active material particles 21 are not in a sufficiently expanded state, and thus may further expand during the charging process of the lithium secondary battery. is there. In this case, the capacity | capacitance of the negative electrode 20 for lithium secondary batteries falls by the columnar negative electrode active material particle | grains 21 which collide or contact, and fracture | ruptures from the electrical power collector 3, etc. in this case.

  However, when the composition ratio R is 3.3 ≦ R ≦ 4.4, the columnar negative electrode active material particles 21 are formed by occlusion of lithium ions, that is, substantially expanded, that is, by vapor deposition of lithium and silicon. It just returns to the shape (size) close to the columnar negative electrode active material particles 21 made. Therefore, since the collision between the columnar negative electrode active material particles 21 can be alleviated, the columnar negative electrode active material particles 21 are not easily broken, and sufficient adhesion between the current collector 3 and the columnar negative electrode active material particles 21 is ensured. A negative electrode for a lithium secondary battery having excellent cycle characteristics even during charge and discharge is possible.

  Thus, by setting the composition ratio R of lithium and silicon of the columnar negative electrode active material particles 21 to 3.3 ≦ R ≦ 4.4, the columnar negative electrode active material particles 21 expand even when charging and discharging are repeated. Therefore, good cycle characteristics can be maintained.

The amount of the columnar negative electrode active material particles 21 formed on the surface of the current collector 3 should be set according to the target battery capacity. For example, in the case of a negative electrode for a 18650 size cylindrical lithium secondary battery, the lithium evaporation rate from the lithium evaporation source 1 is set so that the target negative electrode plate has a capacity of about 4 to 6 mAh / cm ^2. It is preferable to adjust the silicon evaporation rate from the silicon evaporation source 2, the opening / closing time of the shutter 11, and the like.

<Configuration of lithium secondary battery>
A coin-type battery 30 will be described as a lithium secondary battery using the negative electrode 20 for a lithium secondary battery of the first embodiment.

  FIG. 3 is a schematic cross-sectional view showing the structure of a coin-type battery 30 using the negative electrode 20 for a lithium secondary battery according to Embodiment 1 of the present invention. The coin-type battery 30 includes an electrode group including a positive electrode 32, a negative electrode 20, and a separator 33 interposed therebetween, and is sealed with a gasket 35.

  The positive electrode 32 is formed using metallic lithium and is electrically connected to a positive electrode case 31 that also serves as a positive electrode terminal. The negative electrode 20 is the negative electrode 20 for a lithium secondary battery shown in the first embodiment, and is electrically connected to a sealing plate 36 that also serves as a negative electrode terminal. The positive electrode 32 and the negative electrode 20 are impregnated with an electrolyte having lithium ion conductivity.

  The negative electrode 20 is not limited by the shape of the lithium secondary battery, and can be applied to any one of buttons, sheets, cylinders, flats, squares, etc. in addition to the coin type. Needless to say, as the positive electrode, electrolyte, separator, and the like of the lithium secondary battery, those used in the current lithium secondary battery can be used.

  Next, specific examples of the present invention will be described. As an evaluation cell, a coin-type battery 30 using the negative electrode 20 for a lithium secondary battery was produced.

Example 1
Concavities and convexities were provided on an electrolytic copper foil having a core material thickness of 10 μm, and current collector 3 having a maximum height between the irregularities of 8 μm was formed. The normal line from the vapor deposition surface of the current collector 3 and the angle θ of the straight line connecting the current collector 3 and the midpoint between the evaporation surfaces of the lithium evaporation source 1 and the silicon evaporation source 2 form a predetermined angle. 1 in the vapor deposition apparatus 13. Metal lithium (purity 99.9%) 50 g and silicon (purity 99.9999%) 100 g were inserted into the lithium evaporation source 1 and the silicon evaporation source 2, respectively, and the inside of the vacuum vessel 4 was decompressed to 7 × 10 ⁻⁵ Pa. . Thereafter, argon was introduced into the vacuum vessel 4 through a mass flow controller, and the pressure in the vacuum vessel 4 was adjusted to 1 × 10 ⁻⁴ Pa.

  AC power of 80 V and 42 A was applied to the lithium evaporation source 1 to heat the lithium. At this time, the temperature of lithium was 524 ° C. The silicon evaporation source 2 was irradiated with electrons of 10 kV and 400 mA to heat and melt the silicon. Further, it was left for about 15 minutes until the evaporation rate of lithium or silicon was stabilized.

  The lithium evaporation rate measurement rate monitor 9 and the silicon evaporation rate measurement rate monitor 10 measure the lithium evaporation rate and the silicon evaporation rate, and the composition ratio R of lithium and silicon deposited on the current collector 3 on the substrate surface is 4 was confirmed, the shutter 11 was opened, and the formation of the columnar negative electrode active material grains 21 on the surface of the current collector 3 was started. Then, after 6 hours, the shutter 11 was closed, the formation of the columnar negative electrode active material grains 21 was completed, and the negative electrode 20 for a lithium secondary battery was produced.

(Example 2)
Columnar negative electrode active material grains 21 were formed on the surface of the current collector 3 under the same conditions as in Example 1 except that the composition ratio R of lithium and silicon deposited on the current collector 3 was 3.3, and the lithium A negative electrode 20 for a secondary battery was produced.

(Comparative Example 1)
Columnar negative electrode active material particles 21 were formed on the surface of the current collector 3 under the same conditions as in Example 1 except that only silicon was vapor-deposited on the current collector 3 to produce a negative electrode 20 for a lithium secondary battery. .

(Comparative Example 2)
Columnar negative electrode active material particles 21 were formed on the surface of the current collector 3 under the same conditions as in Example 1 except that the composition ratio R of lithium and silicon deposited on the current collector 3 was set to 3, and lithium secondary A negative electrode 20 for a battery was produced.

  The composition ratio R of lithium and silicon contained in each columnar negative electrode active material grain 21 formed as described above was measured and confirmed by high frequency inductively coupled plasma emission spectroscopy. Table 1 shows the measurement results.

(Characteristic comparison)
Using the negative electrode 20 for a lithium secondary battery produced in Example 1, Example 2, Comparative Example 1 and Comparative Example 2, an evaluation coin-type battery 30 was produced, and a charge / discharge cycle test was performed. The conditions of the cycle test were 100 μA for both the charging current and the discharging current, a charging stop voltage of 1 V, a discharge stopping voltage of 0 V, a rest time of 10 minutes between charging and discharging, and a measurement environment temperature of 20 ° C. As the order of charge and discharge in the cycle test, after the charge until the terminal voltage of the evaluation cell reaches the charge stop voltage, the discharge until the terminal voltage reaches 1V and the charge until the terminal voltage reaches 0V are alternately repeated. .

  The capacity maintenance rate of each coin-type battery 30 during the cycle test was measured, and the characteristics were compared. Here, the capacity retention rate is the ratio of the measured discharge capacity to the maximum discharge capacity in each cycle, with the maximum discharge capacity observed in the cycle test as the reference capacity.

In Example 1, Example 2, Comparative Example 1, and Comparative Example 2, the negative electrode 20 for a lithium secondary battery was cut into a circle having a diameter of approximately 11.3 mm. As the electrolytic solution, a solution obtained by dissolving 1 mol of LiPF _{6 in} 1 liter of a mixed solvent of 30% by volume of ethylene carbonate, 50% by volume of methyl ethyl carbonate, and 20% by volume of diethyl carbonate was used.

FIG. 4 is a relationship diagram between the number of charge / discharge cycles of each evaluation cell and the capacity maintenance rate in the cycle test. As shown in FIG. 4, in the case of Comparative Example 1 and Comparative Example 2, the capacity retention rate at the seventh cycle is rapidly decreased to 70% or less, whereas in the case of Example 1 and Example 2. Then
Maintains over 90% until the 7th cycle. This is because the columnar negative electrode active material particles 21 are formed by occluding lithium in silicon, whereby the expansion rate at the time of maximum expansion of the columnar negative electrode active material particles 21 is suppressed, that is, adjacent columnar negative electrode active material particles 21. It is suppressed that the columnar negative electrode active material particles 21 are broken and fall off from the current collector 3 due to the collision between each other. As a result, it is considered that the adhesion between the columnar negative electrode active material particles 21 and the current collector 3 was secured.

  INDUSTRIAL APPLICATION This invention is useful for the negative electrode for lithium secondary batteries which improves the cycling characteristics of a lithium secondary battery, and its manufacturing method.

Schematic of negative electrode manufacturing apparatus for lithium secondary battery in Embodiment 1 of the present invention Schematic sectional view of a negative electrode for a lithium secondary battery in Embodiment 1 of the present invention Schematic sectional view of a coin-type battery using the negative electrode for a lithium secondary battery of Embodiment 1 of the present invention Relationship diagram between the number of charge / discharge cycles and capacity maintenance rate of each evaluation cell in cycle test

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium evaporation source 2 Silicon evaporation source 3 Current collector 4 Vacuum container 5 Main valve 6 High vacuum pump 7 Low vacuum pump 8 Shield plate 9 Rate monitor for measuring lithium evaporation rate 10 Rate monitor for measuring silicon evaporation rate 11 Shutter 12 Current collector holder 13 Vapor deposition device 14 Piping 20 Negative electrode 21 Columnar negative electrode active material particle 30 Coin type battery 31 Positive electrode case 32 Positive electrode 33 Separator 35 Gasket 36 Sealing plate

Claims (3)

On the surface of the current collector roughened with unevenness on the surface,
A method for producing a negative electrode for a lithium secondary battery, wherein an active material for a lithium secondary battery negative electrode is obliquely deposited on the surface of the current collector,
The said active material is lithium and silicon, The manufacturing method of the negative electrode for lithium secondary batteries which vapor-deposits the said lithium and the said silicon on the said collector simultaneously. 2. The method for producing a negative electrode for a lithium secondary battery according to claim 1, wherein a composition ratio R of the lithium to be deposited and the silicon is 3.3 ≦ R ≦ 4.4.
Here, R represents the number of moles of lithium to be deposited / the number of moles of silicon to be deposited. The negative electrode for lithium secondary batteries manufactured by the manufacturing method of the negative electrode for lithium secondary batteries of Claim 1 or Claim 2.

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