KR20180064197A - Method for manufacturing negative electrode for rechargable lithium battery, negative electrode for rechargeable lithium battery manufactured by the same, and rechargeable lithium battery including the same - Google Patents

Abstract

The present invention relates to a method of manufacturing a negative electrode for a lithium secondary battery, comprising the following steps: forming a negative electrode active material layer by coating a negative electrode active material on a current collector; and coating a protective film on the negative electrode active material layer, wherein the protective film is coated through a thermal deposition process. According to the present invention, a negative electrode for a lithium secondary battery manufactured by the method, and the lithium secondary battery including the same can preventing the generation of lithium dentrite and can enhance the charge / discharge life of the battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery manufactured using the same, and a lithium secondary battery including the negative electrode. 2. Description of the Related Art INCLUDING THE SAME}

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

2. Description of the Related Art Recently, lithium secondary batteries have attracted attention as power sources for driving portable electronic devices and electric vehicles. The use of the organic electrolyte makes it possible to exhibit a discharge voltage which is twice as high as that of a battery using an existing alkaline aqueous solution and exhibit high power.

A lithium secondary battery mounted on an electric automobile or the like requires an increase in energy density through weight reduction and volume reduction. For this purpose, the development of anode materials using lithium is continuing.

The capacity of the carbon anode material is ~ 370 mAh / g, while the capacity of the lithium anode material is ~ 3,860 mAh / g, which is close to 10 times the theoretical capacity. The thickness of the negative electrode including the carbon-based negative electrode material is about 50 to 60 占 퐉, and the thickness of the negative electrode including the lithium negative electrode material is about 5 to 20 占 퐉. Therefore, when the lithium negative electrode material is included, the volume of the lithium secondary battery can be reduced by at least 1/3.

However, when lithium is used as an anode material, a problem arises that 1) dendrite is formed, 2) dead lithium occurs, and 3) the volume expands.

Lithium dendrite is generated on the surface of the lithium negative electrode during charging and discharging at a high current density. These lithium dendrites penetrate the separator to cause a short circuit with the anode, or the surface (SEI) due to the reaction between the anode and the electrolyte is continuously formed due to an increase in surface area, resulting in electrolyte depletion and generation of dead lithium . This reduces the cycle efficiency of the battery.

When replacing the carbonaceous anode material with lithium in a lithium ion battery, it is necessary to suppress the formation of lithium dendrite formed during charging and discharging although the energy density per volume and volume can be increased.

The present invention provides a method for manufacturing a negative electrode for a lithium secondary battery comprising a protective film for inhibiting the formation of lithium dendrite, a negative electrode for a lithium secondary battery manufactured from the negative electrode, and a lithium secondary battery comprising the same.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not intended to limit the invention to the precise form disclosed. It can be understood.

The negative electrode active material layer may be formed by coating a negative electrode active material on a current collector according to an embodiment of the present invention and a protective layer may be coated on the negative electrode active material layer. .

The protective film may be formed to a thickness of about 10 nm to about 500 nm.

At least one of the step of forming the anode active material layer and the step of forming the protective film may be performed in vacuum.

The step of forming the protective film may be performed by opening the chamber.

The current collector may be at least one selected from the group consisting of copper, gold, nickel, and alloys thereof.

The negative electrode active material may be at least one selected from the group consisting of lithium, a lithium alloy, and a carbonaceous material.

The protective film may be lithium halide.

The negative active material layer is formed using a first boat, the protective film is formed using a second boat, and a partition wall may be positioned between the first boat and the second boat.

The negative active material layer may be in direct contact with the protective film.

A cathode for a lithium secondary battery according to an embodiment can be manufactured using the above-described method.

A lithium secondary battery according to an embodiment includes a cathode, a cathode, and a separator disposed between the anode and the cathode, and the cathode can be manufactured using the above-described method.

According to the above-described embodiments, it is possible to provide a method of manufacturing a negative electrode for a lithium secondary battery in which a protective film is uniformly formed. The negative electrode and the lithium secondary battery for a lithium secondary battery manufactured from the method can suppress lithium dendrite formation, Can be increased.

1 is a schematic cross-sectional image of a cathode for a lithium secondary battery according to the present invention.
2 is a schematic cross-sectional image according to a method for manufacturing a negative electrode for a lithium secondary battery of the present invention.
3 is a graph showing a full stripping test result of Example 1. Fig.
Figs. 4 to 10 are graphs showing charging / discharging evaluations of batteries according to Examples and Comparative Examples. Fig.
11 to 13 are graphs of ESCA depth profile measurement according to the embodiment and the comparative example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, the well-known functions or constructions will not be described in order to clarify the present invention.

In order to clearly illustrate the present disclosure, portions that are not related to the description are omitted, and the same or similar components are denoted by the same reference numerals throughout the specification. In addition, since the sizes and thicknesses of the individual components shown in the drawings are arbitrarily shown for convenience of description, the present invention is not necessarily limited thereto.

In the drawings, the thickness is enlarged to clearly represent the layers and regions. The thickness of some layers and regions is exaggerated for convenience of explanation in the drawings. Whenever a portion such as a layer, film, region, plate, or the like is referred to as being "on" or "on" another portion, it includes not only the case where it is "directly on" another portion but also the case where there is another portion in between.

The lithium secondary battery can be classified into a lithium ion battery (hereinafter referred to as a "lithium secondary battery"), a lithium ion polymer battery, and a lithium polymer battery depending on the type of separation membrane and electrolyte used. Dolls, pouches, etc., and can be divided into a bulk type and a thin film type depending on the size. The structure and the manufacturing method of these cells are well known in the art, and detailed description thereof will be omitted.

More specifically, the lithium secondary battery may be constructed by stacking a cathode, an anode, and a separator in this order, and then winding the battery in a spiral wound state.

The negative electrode includes a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer may include a negative electrode active material. Specifically, the negative electrode according to an embodiment of the present invention may include a current collector, a negative electrode active material layer formed on the current collector, and a protective film formed on the negative electrode active material layer.

The negative electrode according to one embodiment may be manufactured by forming a negative electrode active material layer by coating a negative electrode active material on the current collector, and forming a protective film on the negative electrode active material layer.

The negative electrode current collector may include, but is not limited to, at least one selected from the group including copper, gold, nickel, and alloys thereof.

Examples of the negative electrode active material include a material capable of reversibly intercalating / deintercalating lithium ions, lithium, a lithium alloy, a carbon material, a material doped and dedoped to lithium, and a transition metal oxide It can be one.

The negative electrode active material layer may be formed by coating the negative electrode active material directly on the current collector. However, the present invention is not limited thereto. For example, the negative electrode active material may be coated on a separate support and dried. A method of laminating the whole phase may be used. Here, the support may be any material capable of supporting the negative electrode active material layer, and may be, for example, a mylar film or a polyethylene terephthalate (PET) film.

The protective film may include lithium halide, and may be, for example, lithium fluoride (LiF). Lithium halide has a large bandgap and is generally used in UV optics. Its melting point is relatively low at 845 ℃, which is suitable for vacuum thermal evaporation.

The protective layer according to one embodiment may be formed to have a uniform thickness using a thermal deposition process. The protective film may be uniformly formed to a thickness of about 10 nm to about 500 nm. The protective film may be in direct contact with the negative active material layer, and no foreign matter or foreign matter layer is located between the protective film and the negative active material layer.

The protective film prevents deterioration of the performance of the battery due to the formation of lithium dendrite and stably exhibits high capacity characteristics even after repeated charging and discharging, thereby providing a lithium secondary battery excellent in life characteristics and stability.

The step of forming the protective film may employ a thermal evaporation process. The thermal evaporation process may be carried out under the same or not the same conditions as those for forming the negative electrode active material according to the embodiment. For example, in the case of a thermal deposition process in a vacuum state, it is advantageous to control the thickness and the surface of the protective film, and the manufacturing process can be simple.

When a protective film is formed in a non-vacuum state, the lithium thin film reacts with nitrogen, oxygen, minute water, etc. in the atmosphere to form a film of Li ₃ N, LiOH, Li ₂ O, Li ₂ CO ₃ or the like. The reliability and performance of the battery may be deteriorated by the coating. The thus formed film can act as a resistive layer and can neglect the growth of lithium dendrites.

Hereinafter, the anode and the separator of the lithium secondary battery other than the above-described cathode will be schematically described.

The positive electrode according to an embodiment of the present invention includes a current collector and a positive electrode active material layer formed on the current collector.

As the cathode active material, a compound capable of reversible intercalation and deintercalation of lithium (a lithiated intercalation compound) can be used. Specifically, it may include at least one selected from the group including aluminum, cobalt, manganese, nickel, and combinations thereof.

The cathode active material layer may further include at least one of a binder and a conductive material.

The binder serves to adhere the positive electrode active material particles to each other well and adhere the positive electrode active material to the current collector well. Examples of the binder include polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinylchloride, Polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic resin, polyvinyl chloride, Styrene-butadiene rubber, epoxy resin, nylon, and the like.

The conductive material is used for imparting conductivity to the anode. Any conductive material can be used without causing any chemical change in the battery. Examples of the conductive material include natural graphite, artificial graphite, carbon black, acetylene black, Metal powders such as black, carbon fiber, copper, nickel, aluminum, and silver, metal fibers, and the like, and conductive materials such as polyphenylene derivatives may be used alone or in combination.

The negative electrode and the positive electrode are prepared by mixing an active material, a conductive material and a binder in a solvent to prepare an active material composition, and applying the composition to an electric current collector. The method of manufacturing the electrode is well known in the art, and therefore, a detailed description thereof will be omitted herein. As the solvent, N-methylpyrrolidone or the like can be used, but it is not limited thereto.

Depending on the type of the lithium secondary battery, the separator may be positioned between the anode and the cathode.

The separator may be polyethylene, polypropylene, polyvinylidene fluoride or a multilayer film of two or more thereof. The separator may be a polyethylene / polypropylene bilayer separator, a polyethylene / polypropylene / polyethylene triple separator, a polypropylene / polyethylene / polypropylene A three-layer separation film, or the like can be used.

A + is an alkali metal cation such as Li +, Na +, K +, or an ion composed of a combination thereof, and B- is an ion selected from the group consisting of PF6-, BF4- A salt comprising an ion such as Cl-, Br-, I-, ClO4-, AsF6-, CH3CO2-, CF3SO3-, N (CF3SO2) 2-, C (CF2SO2) 3- or a combination thereof (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran , An organic solvent composed of N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone (? -Butyrolactone) or a mixture thereof. But is not limited to.

Hereinafter, the performance of the battery according to the present invention will be described with reference to Comparative Example 1 and Examples 1 to 8.

Comparative Example  One

A copper collector having a thickness of 20 μm was used as a substrate. 1.7 g of lithium (Honzo metal, Japan) was charged into a molybdenum boat, and 10 μm thick lithium was uniformly deposited on the copper collector using a vacuum thermal evaporation method in an area of 5 cm × 5 cm.

The initial vacuum degree was maintained at 7 x 10 ^-7 torr or less, and the deposition pressure was 6 to 8 x 10 ^-6 torr. The deposition rate of lithium was kept constant at 3 nm / sec, and the substrate was rotated at 4 rpm to improve the uniformity of the lithium thin film. The process was conducted in a dry room where the dew point was maintained at -45 캜 or lower, and lithium was prepared after ventilation using the atmosphere in the dry room.

Example  1: ex-situ LiF  Thin film coating 16nm

The lithium thin film of 10 mu m formed in Comparative Example 1 was coated with lithium fluoride (LiF), which is one of the lithium halides. The initial vacuum degree for depositing lithium fluoride (TASCO, 3N, 3-6 mm) was the same as that for the deposition of lithium thin film, and a 16 nm LiF coating layer was deposited at a deposition rate of 3 nm / min. The same as Comparative Example 1 except for this.

Example  2: ex-situ LiF  Thin film coating 50nm

Under the same conditions as in Example 1, 50 nm lithium fluoride was deposited on the lithium thin film.

Example  3: ex-situ LiF  Thin film coating 80nm

Under the same conditions as in Example 1, a LiF coating layer of 80 nm was deposited on the Li thin film.

Example  4: in-situ LiF  Thin film coating 16nm

Under the same conditions as in Example 1, a lithium thin film of 10 nm was deposited on a copper substrate, and then lithium fluoride was deposited to a thickness of 16 nm immediately while maintaining a vacuum.

Example  5: in-situ LiF  Thin film coating 50nm

Under the same conditions as in Example 4, 50 nm of lithium fluoride was deposited on a 10-um lithium thin film.

Example  6: in-situ LiF  Thin film coating 80nm

Under the same test conditions as in Example 4, 80 nm of lithium fluoride was deposited on a 10 um lithium thin film.

The above-described Comparative Example 1 and Examples 1 to 6 are summarized in Table 1 below.

Lithium thin film Board Shield Remarks Comparative Example 1 Li 10um Copper 20um none Example 1 Li 10um Copper 20um LiF 16 nm LiF deposition after chamber opening Example 2 Li 10um Copper 20um LiF 50 nm LiF deposition after chamber opening Example 3 Li 10um Copper 20um LiF 80 nm LiF deposition after chamber opening Example 4 Li 10um Copper 20um LiF 16 nm Continuous LiF deposition in vacuum Example 5 Li 10um Copper 20um LiF 50 nm Continuous LiF deposition in vacuum Example 6 Li 10um Copper 20um LiF 80 nm Continuous LiF deposition in vacuum

Coin cell  Manufacturing and electrochemical performance evaluation

A coin cell was prepared using the lithium thin films of Comparative Example 1 and Examples 1 to 6 described above and electrochemical performance was evaluated. Coin 2032. The positive electrode was formed by punching the lithium thin film deposited on the copper current collector to a diameter of 14 mm and the negative electrode was punched with a lithium foil (Honjo) having a thickness of 500 μm to a diameter of 16 mm. Asymmetric cell was fabricated by separating PP (polypropylene, 25um, celgard) separator between anode and cathode. As the electrolyte, 1M LiPF ₆ (Solbrain) was dissolved in EC: EMC = 3: 7 (v / v) and no additional additives were used.

On the other hand, before evaluating the electrochemical performance before and after the application of the coating film, we examined whether the lithium thin film deposited can be used as a cathode. Specifically, the capacity expression ratio (%) was measured by stripping all of the lithium thin films at a relatively low current density. The capacity development rate was measured before and after deposition on a lithium thin film deposited to a thickness of 10 탆, and the weight of lithium deposited from the weight difference was calculated. From this, the theoretical capacity (Q ₀ ) was calculated and the actual capacity (Q ₁ ) was measured from the full stripping test.

As shown in FIG. 3, the capacity test result for the capacity was theoretically calculated to be 1.931 mAh / cm ² from the thickness of the deposited lithium thin film. Actually, the measured value was 1.929 mAh / cm ² , Respectively. At this time, a constant current of 0.166 mA / cm < ² > was applied under the measurement conditions, and the measurement was terminated when the voltage reached 1 V. [

To evaluate the electrochemical performance, a charge / discharge test was carried out at a current density of 1 mA / cm ^2. The charge / discharge time was 60 minutes, and the total charge / discharge depth was 1 mAh / cm ² . The breakdown voltage was set at + 1V, -1V.

FIGS. 4 to 10 are graphs of charge / discharge test results of the lithium thin film according to Comparative Example 1 and Examples 1 to 6, respectively. The charge / discharge depth criterion is a condition in which lithium is charged to the surface of the lithium thin film and discharged again at the time of initial charging.

Until the initial 10 cycles, Comparative Example 1 and Examples 1 to 6 show a similar voltage profile without a large increase in resistance, and it can be seen that charging / discharging is smooth.

On the other hand, there is no large resistance change at the time of charging from the 10th cycle on the basis of the lithium thin film (anode) of 10um, but it is understood that the voltage is rapidly increased at the ending stage during discharging. Lithium is stripped from the existing lithium thin film during charging and discharging, and the resistance suddenly increases due to the rapid change of the specific surface area due to the non-uniform stripping and the rapid growth of the thin film between the cathode and the electrolyte .

The cycle life was improved by 14 cycles in Example 3 compared to Comparative Example 1, which means that the mobility of lithium ions from the uniformly coated protective film is improved. In particular, Example 6 confirmed that the cycle life was improved up to 17 times. The cycle life according to Comparative Example 1 and Examples 1 to 6 is as summarized in Table 2 below.

Evaluation Downtime cycle Comparative Example 1 21 hours 10.5 Example 1 24 hours 12 Example 2 26 hours 13 Example 3 28 hours 14 Example 4 26 hours 13 Example 5 29 hours 14.5 Example 6 34 hours 17

FIG. 11 is Comparative Example 1, FIG. 12 is Example 3 where LiF is ex-situ coated with LiF at a thickness of 80 nm, and FIG. 13 is a graph showing ESCA depth profile measurement results of Example 6 coated in- to be. For the elemental analysis, X-ray was analyzed by irradiating monochromatic Al 15kV 15mA to about 1.1mm ² area. For the depth profile, the ion gun was irradiated with 3kV Ar ⁺ ion Was sputtered in an area of 3 mm ² . At this time, the sputtering rate was based on Ta ₂ O _{5 of} 3.6 nm / min.

According to the measurement results of the ESCA depth profiles, in Comparative Example 1, it was confirmed that C and O are located on the surface portion. It is believed that Li ₂ CO ₃ , LiOH and the like due to atmospheric exposure to lithium after the deposition of lithium exist, and Li ₂ O is present in the lower part. In the case of Comparative Example 1, it was confirmed that pure lithium was located while O was decreased in the depth direction.

Referring to FIG. 12, it can be seen that F is detected at the surface portion and LiF is coated, and Li ₂ O exists at the lower portion of the LiF coated surface as O rapidly increases at the portion where F disappears And it was found.

On the other hand, according to Example 6 shown in FIG. 13, it was confirmed that when LiF was coated in situ, F was largely detected on the surface portion and oxygen was hardly detected in a position where the depth decreased in the depth direction. That is, it was confirmed that the Li electrode directly contacting the LiF layer was located.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, It is obvious to those who have. Accordingly, it should be understood that such modifications or alterations should not be understood individually from the technical spirit and viewpoint of the present invention, and that modified embodiments fall within the scope of the claims of the present invention.

Claims (11)

Forming a negative electrode active material layer by coating a negative electrode active material on the current collector, and
And coating a protective film on the negative active material layer
Wherein the coating of the protective layer is performed by a thermal deposition process.
The method of claim 1,
Wherein the protective film is formed to a thickness of about 10 nm to about 500 nm.
The method of claim 1,
Wherein at least one of the step of forming the negative electrode active material layer and the step of forming the protective film is performed in a vacuum.
4. The method of claim 3,
Wherein the forming of the protective film is performed by opening the chamber.
The method of claim 1,
Wherein the current collector is at least one selected from the group consisting of copper, gold, nickel, and alloys thereof.
The method of claim 1,
Wherein the negative electrode active material is at least one selected from the group consisting of lithium, a lithium alloy, and a carbonaceous material.
The method of claim 1,
Wherein the protective film is lithium halide.
The method of claim 1,
Wherein the negative active material layer is formed using a first boat, the protective film is formed using a second boat, and the partition is positioned between the first boat and the second boat.
The method of claim 1,
Wherein the negative active material layer is in direct contact with the protective film.
An anode for a lithium secondary battery formed by using the method for manufacturing a cathode for a lithium secondary battery according to any one of claims 1 to 9.
anode,
Cathode, and
And a separator disposed between the anode and the cathode
The lithium secondary battery according to any one of claims 1 to 9, wherein the negative electrode is formed using the method for manufacturing a negative electrode for a lithium secondary battery.

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