SECONDARY BATTERY WITH IMPROVED HIGH-TEMPERATURE AND LOW-TEMPERATURE PROPERTIES

Abstract

A secondary battery includes: a cathode; an anode; and an electrolyte, wherein the cathode includes a cathode current collector; a carbon layer including a binder, and carbon; and an active material layer, and the electrolyte includes lithium hexafluorophosphate (LiPF.sub.6) and lithium bis (fluorosulfonyl)imide (LiFSI). The secondary battery according to the present invention may have improved high-temperature and low-temperature properties, and may inhibit corrosion of a cathode to have increased life.

Claims

1. A secondary battery comprising: a cathode including a cathode current collector, a carbon layer and an active material layer which are sequentially arranged; an anode; and an electrolyte, wherein, the carbon layer includes carbon and N-methyl-2-pyrrolidone (NMP) insoluble binder at a weight ratio of 1:0.5 to 1.2, wherein the N-methyl-2-pyrrolidone (NMP) insoluble binder is a polyacrylate-based binder, the carbon layer does not include an N-methyl-2-pyrrolidone (NMP) soluble binder, the active material layer includes an active material and N-methyl-2-pyrrolidone (NMP), and the electrolyte includes LiFSI and LiPF.sub.6 at a weight ratio of from 1:0.5 to 1.2.

2. The secondary battery of claim 1, wherein the carbon layer does not include the active material.

3. The secondary battery of claim 1, wherein the carbon is selected from the group consisting of graphite, carbon black, carbon nanotube, grapheme and Ketjen black.

4. The secondary battery of claim 3, wherein the carbon black is Denka black.

5. The secondary battery of claim 1, wherein the carbon layer prevents direct contact between the active material layer and. the cathode current collector, thereby preventing corrosion of the cathode.

6. The secondary battery of claim 1, wherein swelling of the carbon layer is prevented when the secondary battery is left at 70° C. for 14 days.

7. The secondary battery of claim 1, wherein the electrolyte comprises ethylene carbonate, diethyl carbonate, and dimethyl carbonate.

Description

EXAMPLE 1

[0034] Manufacture of Cathode

[0035] An aluminum foil cathode current collector was prepared. Graphite powder and poly(acrylic acid) (PAA) were mixed at a weight ratio of 1:0.5 to prepare a slurry, and the slurry was applied and dried onto a cathode current collector, thereby manufacturing the cathode current collector coated with a carbon layer.

[0036] 90% by weight of LiMn.sub.2O.sub.4 active material, 5% by weight of a graphite conductive material, 5% by weight of a polyvinylidene fluoride binder were mixed in an N-methyl pyrrolidone solvent to prepare a cathode active material slurry. The cathode active material slurry was applied and dried onto a current collector for a cathode to manufacture a cathode.

[0037] Electrolyte

[0038] A mixture obtained by mixing ethylene carbonate including LiPF.sub.6 and LiFSI at 2 mol/L, diethyl carbonate, and dimethyl carbonate at a volume ratio of 2:1:2 was used as the electrolyte. Here, LiFSI and LiPF.sub.6 were included at a weight ratio of 1:0.5.

[0039] Manufacture of Secondary Battery

[0040] An anode was prepared by using a silicon-graphite complex anode active material and a copper foil as an anode current collector. The cathode, the electrolyte, the anode, and a general separator were used to manufacture a secondary battery.

EXAMPLE 2

[0041] A secondary battery was manufactured by the same method as Example 1 except for using LiFSI and LiPF.sub.6 at a weight ratio of 1:1.2 in the electrolyte.

EXAMPLE 3

[0042] A secondary battery was manufactured by the same method as Example 1 except for using PVA instead of using PAA, as the binder of the carbon layer.

Comparative Example 1

[0043] A secondary battery was manufactured by the same method as Example 1 except for using LiFSI only in the electrolyte, without using LiPF.sub.6 (that is, ethylene carbonate including LiFSI at 2 mol/L was used).

Comparative Example 2

[0044] A secondary battery was manufactured by the same method as Example 1 except for using LiPF.sub.6 only in the electrolyte, without using LiFSI (that is, ethylene carbonate including LiPF.sub.6 at 2 mol/L was used).

Comparative Example 3

[0045] A secondary battery was manufactured by the same method as Example 1 except for using LiFSI and LiPF.sub.6 at a weight ratio of 1:0.1 in the electrolyte.

[0046] Comparative Example 4

[0047] A secondary battery was manufactured by the same method as Example 1 except for using LiFSI and LiPF.sub.6 at a weight ratio of 1:2.5 in the electrolyte.

Comparative Example 5

[0048] A secondary battery was manufactured by the same method as Example 1 except for using PVDF instead of using PAA, as the binder of the carbon. layer.

Comparative Example 6

[0049] A secondary battery was manufactured by the same method as Example 1 except for directly applying a cathode active material slurry to the cathode current collector, and without using the carbon layer.

Experimental Example 1

[0050] Low-temperature output of each of the secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 was evaluated. A specific evaluation method is as follows. Voltage (V1) was measured by lowering a temperature to be −30° C. in a state where SOC (state of charge) of each secondary battery was maintained at 30%, and maintaining the secondary battery for 4 hours. After the secondary battery was discharged at 30 A for 10 seconds, voltage (V2) was measured. Then, a straight line connecting two points of (Current, Voltage)=(0,V1) and (30,V2) was drawn, and a current (Imin) at the moment at which an extended line of the drawn straight line contacts 2.5V which is a lower limit voltage was read. Here, a low-temperature output was calculated by 2.5V×current (Imin).

[0051] As a result, the secondary batteries of Examples 1 to 3 exhibited good low-temperature output at a low-temperature such as −30° C., and the secondary batteries of Comparative Examples 1 and 3 also exhibited excellent low-temperature output. However, the secondary batteries of Comparative Examples 2 and 4 exhibited lowered conductivity of lithium ions at a low-temperature, and the cathode of the secondary battery of Comparative Example 6 was corroded (Table 1).

[0052] Experimental Example 2

[0053] High-temperature storage property of each of the secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 6 was evaluated. A specific evaluation method is as follows. The batteries were charged with SOC of 95%, and left at 70° C. for 14 days. Then, capacity retention ratio of each secondary battery, and whether or not gas occurs in the batteries were confirmed (Table 1).

[0054] As a result, the secondary batteries of Examples 1 to 3 and Comparative Examples 4 and 5 exhibited good results even at a high-temperature of 70° C. However, it was confirmed that the electrolyte solvent and the LiPF.sub.6 salt were degraded at a high-temperature in the secondary batteries of Comparative Example 1 and 3, and particularly, in Comparative Example 1, swelling occurred severely at a temperature of 70° C. or more. In addition, corrosion of the cathode in the secondary battery of Comparative Example 6 was confirmed (Table 1).

[0055] Experimental Example 3

[0056] After the secondary batteries of Examples 1 to 3 and

[0057] Comparative Example 1 to 6 were continuously charged and discharged up to 200 cycles under 0.5 C charge and 1.0 C discharge conditions at room. temperature (25° C.), capacity retention ratio after 200 cycles was evaluated. The capacity retention ratio was shown as a relative ratio of a capacity after 200 cycles to a capacity at the first cycle.

[0058] As a result, the secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 4 exhibited good capacity retention ratio. However, it was confirmed that corrosion of the cathode occurred in the secondary batteries of Comparative Examples 5 and 6, and particularly, in Comparative Example 5, swelling of the carbon layer occurred (Table 1).

TABLE-US-00001 TABLE 1 Capacity Whether or Retention not Swelling Ratio (%) Occurs Low- during High- during High- Temperature Temperature Temperature Cycle Output (W) Storage at Storage at Life at −30° C. 70° C. 70° C. (%) Example 1 100 91 No Occur 95 Example 2 93 87 No Occur 94 Example 3 95 89 No Occur 94 Comparative 110 70 Occur 93 Example 1. Severely Comparative 85 81 No Occur 93 Example 2 Comparative 103 75 Occur 93 Example 3 Slightly Comparative 87 84 No Occur 94 Example 4 Comparative 101 87 No Occur 86 Example 5 Comparative 88 79 Occur 89 Example 6 Slightly