Lithium secondary battery and method for producing the same

Abstract

Disclosed is a method for producing a lithium secondary battery including forming an electrode assembly using a cathode, an anode and a separator, introducing the electrode assembly into a battery case, injecting an electrolyte into the battery case, and sealing the battery case, wherein, during assembly of the electrode assembly, insulating particles are dispersed on part of the surface of the separator, or at least one of the cathode and the anode contacting the separator. The step of dispersing insulating particles on the part of the surface of the separator or at least one of the cathode and the anode contacting the separator during battery assembly can considerably reduce short-circuits in a lithium secondary battery caused by intrinsic and extrinsic factors and thus low-voltage defects, and thereby significantly improve yield of a lithium secondary battery.

Claims

1. A method for producing a lithium secondary battery, the method consisting of: (a) distributing or scattering an insulating material directly onto a surface of at least one of a preformed cathode, a preformed anode or a polymer film separator to form a layer consisting of the insulating material, wherein the insulating material consists of powder and is selected from the group consisting of MgO, TiO.sub.2, Li.sub.4Ti.sub.5O.sub.12, ZrO.sub.2, InSnO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, GeO.sub.2, MoO, SnO, Cr.sub.2O.sub.3 and Sb.sub.2O.sub.3—SnO.sub.2, wherein the insulating material partially covers the surface, and wherein the layer formed by the distributing or the scattering of the insulating material has a thickness of 20 nm-5 μm; (b) forming an electrode assembly having the preformed cathode, the preformed anode and the polymer film separator by interposing the polymer film separator between the preformed cathode and the preformed anode and fusing the polymer film separator, the preformed cathode and the preformed anode at a hot fusion temperature to form the electrode assembly after the distributing or the scattering of the insulating material onto the surface in step (a); (c) introducing the electrode assembly formed in step (b) into a casing and injecting an electrolyte; (d) sealing the casing to finish assembly of the battery; (e) optionally performing formation of the battery by repeating charge/discharge cycles after the assemblage of the battery in order to activate the battery; and (f) optionally performing aging of the battery to stabilize the activated battery.

2. The method as claimed in claim 1, wherein the insulating material has a particle diameter of 20 nm-1 μm.

3. The method as claimed in claim 1, wherein the distribution or the scattering of the insulating material directly onto the surface is via a fine net or sieve vibration system.

4. The method as claimed in claim 1, wherein the insulating material is used in an amount of 2 μg/cm.sup.2˜50 mg/cm.sup.2 per unit area.

5. The method as claimed in claim 1, wherein the hot fusion is performed at a temperature of 60° C.-100° C.

6. The method of claim 1, wherein the polymer film separator is a microporous polymer separator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other-objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a schematic view showing the method for producing a lithium secondary battery according to the present invention; and

(3) FIG. 2 is a sectional view showing the structure of a lithium secondary battery obtained by assembling electrodes with a separator via hot fusion after distributing insulating powder onto an anode or a cathode.

BEST MODE FOR CARRYING OUT THE INVENTION

(4) Reference will now be made in detail to the preferred embodiments of the present invention. It is to be understood that the following examples are illustrative only and the present invention is not limited thereto.

Example 1˜3

Example 1: Manufacture of Lithium Secondary Battery

(5) First, 90 wt % of LiMn.sub.2O.sub.4, 5 wt % of Super-P as a conductive agent and 5 wt % of PVDF (polyvinylidene difluoride) as a binder were mixed, and NMP (N-methyl pyrrolidone) was added thereto to provide slurry. Next, the slurry was coated onto aluminum (Al) foil, followed by drying at 130° C. to provide a cathode.

(6) Then, 90 wt % of hard carbon as an anode active material, 9 wt % of PVDF as a binder and 1 wt % of Super-P was mixed, and NMP was added thereto to provide slurry. The slurry was coated onto copper (Cu) foil, followed by drying at 130° C. to provide an anode.

(7) As an electrolyte, EC/EMC (ethyl carbonate/ethyl methyl carbonate) solution containing 1M LiPF.sub.6 dissolved therein was used.

(8) Before laminating the preformed cathode and anode with each other, TiO.sub.2 insulating powder was uniformly scattered onto the surface of the anode in an amount of about 10 parts by weight by using a sieve vibration system. Next, both electrodes and a separator were laminated and the resultant laminate was passed through a roll laminator at 100° C. to compress the laminate. Then, the compressed laminates were stacked to form a bicell, which, in turn, was introduced into a battery casing. Finally, the preformed electrolyte was injected thereto.

Example 2

(9) A lithium secondary battery was provided in the same manner as described in Example 1, except that TiO.sub.2 insulating powder was scattered onto the surface of the cathode instead of the anode.

Example 3

(10) A lithium secondary battery was provided in the same manner as described in Example 1, except that TiO.sub.2 insulating powder was scattered onto the surface of the separator instead of the anode.

Example 4

(11) (Manufacture of Electrodes for Full Cell)

(12) A cathode and an anode, each disposed at the outermost part of the outermost full cell, were provided as single side-coated electrodes by coating electrode slurry merely onto one surface of aluminum foil. A cathode and an anode disposed at the inner part were provided as double side-coated electrodes by coating electrode slurry onto both surfaces of aluminum foil.

(13) (Manufacture of Separator)

(14) A multilayer polymer film was provided by using a microporous polypropylene film as a first polymer separator and polyvinylidene fluoride-chlorotrifluoroethylene copolymer 32008 available from Solvey Polymer Co. as a second gelled polymer.

(15) (Manufacture of Full Cell Disposed at Inner Part)

(16) A double side-coated cathode and anode were cut while leaving a tab portion. TiO.sub.2 powder was scattered onto the cathode in an amount of about 10 parts by weight, and the separator was interposed between the cathode and the anode. Then, the resultant laminate was passed through a roll laminator at 100° C. to perform hot fusion between each electrode and the separator and to provide an inner full cell.

(17) (Manufacture of Full Cell Disposed at Outermost Part)

(18) A single side-coated cathode and anode were cut while leaving a tab portion. TiO.sub.2 powder was scattered onto the cathode in an amount of about 10 parts by weight, and the separator was interposed between the cathode and the anode. Then, the resultant laminate was passed through a roll laminator at 100° C. to perform hot fusion between each electrode and the separator and to provide the outermost full cell.

(19) (Lamination of Full Cells)

(20) The full cells obtained as described above were stacked in the order of the outermost full cell, the inner full cell and the outermost full cell. At this time, the current collector of the single side-coated electrode was positioned at the outermost part. Then, TiO.sub.2 powder was scattered onto the interface between the full cells in an amount of about 10 parts by weight, and the separator was interposed between the full cells. Then, the resultant laminate was passed through a roll laminator at 100° C. to perform hot fusion. The resultant electrode assembly was introduced into a battery casing, and the preliminarily formed electrolyte was injected thereto.

Comparative Example 1

(21) A lithium secondary battery was provided in the same manner as described in Example 1, except that TiO.sub.2 insulating powder was not scattered onto the surface of an electrode.

Experimental Example 1: Evaluation of Defects in Lithium Secondary Batteries

(22) The following test was performed to evaluate generation of a short circuit and low-voltage defects in the lithium secondary batteries obtained according to Examples 1˜3 and Comparative Example 1.

(23) 1-1. Evaluation of Short Circuit Generation

(24) Electric resistance of each bicell was measured after the electrode lamination step during the manufacture of each battery to determine generation of a short circuit. Herein, the electric resistance of each bicell was measured between the cathode terminal and the anode terminal. An electric resistance of less than 100 Mohm was regarded as a short circuit. The results are shown in the following Table 1.

(25) 1-2. Evaluation of Low-Voltage Defect Generation

(26) Each battery was subjected to the initial charge/discharge cycle by charging it to 4.2 V at 500 mA and discharging it to a terminal voltage of 2.5V at a current of 3000 mA. Next, each battery was subjected to five charge/discharge cycles including charging to a charge cut off voltage of 4.2V at a current of 3000 mA and discharging to a terminal voltage of 2.5V. After the fifth cycle, discharge capacity was measured. Then, the voltage of each battery was measured in a 50% charged state, and a voltage drop was measured after two weeks in a 50% charged state. After two weeks, a voltage drop of 20 mV or higher was regarded as a low-voltage defect.

(27) After the test, it could be seen that the lithium secondary batteries according to Examples 1˜3 showed a significantly reduced generation of a short circuit and low-voltage defect, as compared to the battery obtained via a conventional process according to Comparative Example 1 (see Table 1).

(28) TABLE-US-00001 TABLE 1 Low-voltage defect Short generation generation after assemblage (%) (%) Ex. 1 0.5 1.9 Ex. 2 1.4 4.1 Ex. 3 1.8 6.3 Comp. Ex. 1 5.4 8.5

Experimental Example 2: Evaluation of Quality of Lithium Secondary Battery

(29) The lithium secondary batteries obtained according to Examples 1˜3 and Comparative Example 1 were compared to each other in terms of capacity.

(30) Each battery was subjected to charge/discharge cycles in a range of 4.2V to 2.5V at a current of 1 C. The capacity of the battery using no insulating powder according to Comparative Example 1 at the fifth cycle was taken as 100%. The capacity of each of the batteries according to Examples 1˜3 was compared to the capacity of the battery according to Comparative Example 1 to determine the percent capacity ratio (capacity %).

(31) Additionally, each battery was charged to 100%, discharged at a rate of 1 C for 30 minutes to be set under SOC 50 state, and then allowed to rest for 1 hour. Herein, the final OCV after the 1-hour rest was taken as V.sub.1. Next, a current was passed through each battery at a rate of 20 C for 10 seconds, and the voltage was taken as V.sub.2. The electric resistance of each battery was calculated by using a variance in voltage (ΔV=V.sub.1−V.sub.2), on the basis of the formula of [resistance=ΔV/20CA]. The percent capacity ratio and resistance of each battery are shown in the following Table 2.

(32) After the test, it could be seen that the lithium secondary battery including insulating powder scattered onto an electrode and/or separator according to the present invention showed capacity and resistance comparable to those of the conventional battery (see Table 2).

(33) TABLE-US-00002 TABLE 2 Capacity (%) Resistance (%) Ex. 1 99.10 99.94 Ex. 2 99.65 99.23 Ex. 3 99.91 99.47 Comp. Ex. 1 100 100

INDUSTRIAL APPLICABILITY

(34) As can be seen from the foregoing, the method for manufacturing a lithium secondary battery, including a step of scattering insulating powder partially or totally onto the surface of a separator or the surface of at least one electrode facing to the separator according to the present invention, significantly reduces generation of an internal short circuit between both electrodes caused by internal or external factors during the assemblage of a battery and generation of low-voltage defects, and thus significantly improves yield of desired batteries.

(35) Although several preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.