TECHNICAL FIELD

This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0012626, filed on Jan. 31, 2019, and the entire contents of the Korean patent application are incorporated herein by reference.

The present invention relates to a method of pre-lithiating a negative electrode for a secondary battery, and more particularly, to a pre-lithiation method of negative electrode by immersing the negative electrode in an electrolyte solution to be impregnated, and applying vacuum after impregnation, and then performing present invention, before assembling a lithium secondary battery.

BACKGROUND ART

The present invention relates to a negative electrode active material for a secondary battery, a negative electrode including the same and a method for manufacturing the same. More specifically, the present invention relates to a negative electrode active material having improved cycle swelling and high rate charging property, a negative electrode, and a method for manufacturing the same.

As the price of energy sources increases due to depletion of fossil fuels and the interest in environmental pollution increases, the demand for environmentally friendly alternative energy sources becomes an indispensable factor for future life. Especially, as technology development and demand for mobile devices are increasing, demand for secondary batteries as energy sources is rapidly increasing.

Typically, in terms of the shape of the battery, there is a high demand for a prismatic secondary battery and a pouch-type secondary battery that can be applied to products such as mobile phones with a small thickness. In terms of materials, there is a high demand for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries having high energy density, discharge voltage, and output stability.

Generally, in order to prepare a secondary battery, first, a positive electrode and a negative electrode are formed by applying an electrode mixture containing an electrode active material to a surface of a current collector, then a separate is interposed therebetween to thereby make an electrode assembly, which is then mounted in a cylindrical or rectangular metal can or inside a pouch-type case of an aluminum laminate sheet, and a liquid electrolyte is injected or impregnated into the electrode assembly or a solid electrolyte to prepare a secondary battery.

Further, secondary batteries are classified according to the structure of the electrode assembly having a positive electrode/separator/negative electrode structure. Representative examples thereof include a jelly-roll (wound) electrode assembly in which long sheet type positive electrodes and negative electrodes are wound with a separator interposed therebetween, a stacked electrode assembly in which a plurality of positive and negative electrodes cut in a predetermined size unit are sequentially stacked with a separator interposed therebetween, and a stacked/foldable electrode assembly in which bi-cells or full cells, in which positive and negative electrodes of a predetermined unit are stacked with a separator interposed therebetween, are wound with a separator sheet.

On the other hand, the electrode generates a current through the exchange of ions, and the positive electrode and negative electrode constituting the electrode has a structure in which the electrode active material is applied to the electrode current collector made of metal.

Among them, in the case of the negative electrode, in the related art, lithium metal was used as a negative electrode in the secondary battery. However, as the short circuit of the battery due to the formation of dendrites and the risk of explosion caused by this are known, lithium metal is being replaced with a silicon-based compound or a carbon-based compound capable of reversible intercalation and desorption of lithium ions while maintaining structural and electrical properties.

However, the theoretical capacity density of carbon as an active material used as a negative electrode is 372 mAh/g (833 mAh/cm3). Therefore, in order to improve the energy density of the negative electrode, silicon (Si), tin (Sn), oxides and alloys thereof which are alloyed with lithium are considered as negative electrode materials. Among them, silicon-based materials have attracted attention due to their low cost and high capacity (4200 mAh/g).

However, the silicon has a problem in that volume change (shrinkage or expansion) occurs during the insertion/removal of lithium ions, resulting in deterioration of mechanical stability, and as a result, cycle characteristics are impaired and the initial irreversible capacity becomes large. Therefore, it is necessary to develop a material having structural stability, which is excellent in stability when used as an active material of an electrochemical device, and capable of ensuring cycle characteristics.

In order to solve the above problems, a method of pre-lithiating a negative electrode including a silicon-based negative electrode active material is known. As a pre-lithiation method, known methods include a method of producing an electrode by lithiating a negative electrode active material by a physicochemical method and a method of electrochemically pre-lithiating a negative electrode.

Korean Patent Publication No. 10-2015-0014676 discloses a method for pre-lithiating the surface of a negative electrode. Specifically, a method of pre-lithiating a surface of a negative electrode by immersing a roll, which is generated by rolling the negative electrode with metallic lithium on both sides of the negative electrode, in the electrolyte, is disclosed. Through the above method, a stable film is formed on the surface of the negative electrode and the film formation reaction that occurs during initial charging is performed in advance, so that no capacity drop occurs.

However, in the case of the above prior art, since the bubbles and moisture present in the micropores in the electrode are not removed in the process of immersing and impregnating the electrode in the electrolyte solution, it is difficult for lithium ions to enter into a space between the active materials in the subsequent pre-lithiation process, and thus pre-lithiation may take a long time, and the thickness of the electrode may become thick due to bubbles and moisture.

Therefore, there is a need for technology development for solving the problem.

DISCLOSURE

Technical Problem

The present invention was created to solve the above problems, and an object of the present invention is to provide a method for pre-lithiating a negative electrode for a secondary battery, which shortens the time required for the pre-lithiation process and minimizes the volume change of the electrode after pre-lithiation, a negative electrode for a secondary battery manufactured according to the above method and its manufacturing method.

Technical Solution

The method of pre-lithiating a negative electrode for a secondary battery according to the present invention includes:

immersing the negative electrode for a secondary battery in an electrolyte to perform electrolyte impregnation; and

pre-lithiating the negative electrode.

Immersing the negative electrode for the secondary battery in an electrolyte includes: injecting the prepared negative electrode into the electrolyte bath containing the electrolyte; and removing air bubbles and moisture in the negative electrode by applying a vacuum to the electrolyte bath in which the negative electrode is immersed.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the vacuum may be repeatedly applied twice or more.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, in the vacuum application step, each vacuum may be applied for a time of 1 second to 60 seconds.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the vacuum may be applied at an interval of 1 second to 60 seconds.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the vacuum may be applied to the electrolyte bath for a total time of 30 seconds to 300 seconds.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the vacuum may be performed within a time of 5 minutes to 60 minutes after the negative electrode is injected into the electrolyte bath.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, pre-lithiating the negative electrode includes introducing lithium ions onto the negative electrode active material by a physicochemical method or electrochemical method.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, pre-lithiating the negative electrode further include a stabilization step of leaving the lithiated negative electrode.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the physicochemical method may be a method in which lithium metal is brought into direct contact with the negative electrode.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, contact time between the negative electrode and lithium metal may be 30 minutes to 10 hours.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, the electrochemical method may be a method of lithiating the surface of the negative electrode by applying current or voltage to the negative electrode.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, CC-charging may be performed with a current amount of 0.01 C to 0.5 C, and CV-charging may be performed when reaching 0.001V to 0.01V.

Further, in the method for pre-lithiating the negative electrode for a secondary battery according to the present invention, a time for immersing the negative electrode in the electrolyte may be 6 hours to 48 hours.

In addition, the present invention provides a method for manufacturing a negative electrode for a secondary battery, and the method includes:

preparing a negative electrode slurry by mixing a negative electrode active material, a conductive material, and a binder;

coating the negative electrode slurry on a current collector;

rolling the current collector coated with the negative electrode slurry and punching and drying the current collector to be unit electrodes; and

pre-lithiating the unit electrodes according to the pre-lithiation method.

In addition, the present invention provides a secondary battery including the negative electrode for the secondary battery, and the negative electrode is as described above.

Advantageous Effects

When using the method of pre-lithiating the negative electrode for a secondary battery provided by the present invention, it is possible to shorten the time required for the pre-lithiation process and minimize the volume change of the electrode after pre-lithiation. In addition, the contact loss of the electrode is reduced, and accordingly, a secondary battery having excellent cycle performance and capacity retention can be provided.

