Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery comprising: a solid electrolyte layer; an anode layer disposed on one surface of the solid electrolyte layer; and a cathode layer disposed on the opposite side of the anode layer with respect to the solid electrolyte layer, the solid electrolyte layer including a first polymer solid electrolyte layer and a second polymer solid electrolyte layer, the first polymer solid electrolyte layer contains an inorganic filler and a first polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt, and the second polymer solid electrolyte layer contains a second polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt but does not contain an inorganic filler. The first polymer electrolyte layer includes a first surface and a second surface having different surface roughness in the thickness direction, the surface roughness of the first surface is greater than the surface roughness of the second surface, the first surface is disposed on the second polymer solid electrolyte layer side, and the inorganic filler is barium titanate.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery. Background Art

[0002] The demand for secondary batteries is increasing in various fields such as PCs, mobile phones, electric vehicles, and energy storage devices. Among secondary batteries, lithium secondary batteries in particular have a higher capacity density than other secondary batteries and operate at high voltages.

[0003] A lithium secondary battery generally consists of an anode (reduction electrode, cathode), a cathode (oxidation electrode, anode), and an electrolyte containing a lithium salt disposed between the anode and the cathode. The electrolyte is either a non-aqueous liquid electrolyte or a solid electrolyte. The non-aqueous liquid electrolyte penetrates into the interior of the anode. Therefore, the non-aqueous liquid electrolyte can easily form an interface between the active material of the anode and the electrolyte, thereby imparting high electrical performance.

[0004] However, since the non-aqueous liquid electrolyte uses a flammable organic solvent, it is prone to catching fire due to overcurrent caused by a short circuit (short), etc. Therefore, the non-aqueous liquid electrolyte requires a separate safety device, selection of special battery materials, etc., and limits the battery structure design. This is the reason for the increase in the manufacturing cost and the decrease in the productivity of lithium secondary batteries.

[0005] An all-solid-state battery uses a solid electrolyte in place of a liquid electrolyte. The all-solid-state battery does not have the disadvantages caused by the use of a flammable organic solvent. Therefore, the all-solid-state battery has the advantages of low manufacturing cost and excellent productivity. In addition, the all-solid-state battery has a simple structure. Therefore, the advantages brought by the structure of the all-solid-state battery are excellent stability and high capacity and output.

[0006] The types of solid electrolytes include sulfide solid electrolytes, oxide solid electrolytes, polymer solid electrolytes, etc. Sulfide solid electrolytes and oxide solid electrolytes must be compressed at high temperature and high pressure due to their high interfacial resistance. Polymer solid electrolytes can be manufactured under normal conditions (room temperature and atmospheric pressure), which is advantageous.

[0007] However, due to the high crystallinity of the polymer, the ionic conductivity of the polymer solid electrolyte is lower than that of the non-aqueous liquid electrolyte. Also, the interfacial resistance between the polymer solid electrolyte and the electrode is high. If the contact area between the polymer solid electrolyte and the electrode increases, the interfacial resistance between the electrode and the electrolyte decreases. A polymer solid electrolyte with a low interfacial resistance can exhibit an ionic mobility equivalent to that of a non-aqueous liquid electrolyte, and ultimately can extend the battery life. Summary of the Invention

[0008] Technical Problem

[0009] The present invention aims to increase the lifespan of a lithium secondary battery by improving its conductivity and reducing the interfacial resistance.

[0010] Method for Solving the Problem

[0011] The lithium secondary battery of the present invention includes: a solid electrolyte layer; an anode layer disposed on one surface of the solid electrolyte layer; and a cathode layer disposed on the opposite side of the anode layer with respect to the solid electrolyte layer. The solid electrolyte layer includes a first polymer solid electrolyte layer and a second polymer solid electrolyte layer. The first polymer solid electrolyte layer includes a first polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt, and an inorganic filler. The second polymer solid electrolyte layer includes a second polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt but does not include an inorganic filler. The first polymer electrolyte layer includes a first surface and a second surface with different surface roughnesses in the thickness direction. The surface roughness of the first surface is greater than that of the second surface. The first surface is disposed on the side of the second polymer solid electrolyte layer. The inorganic filler is barium titanate.

[0012] Advantages of the Invention

[0013] The lithium secondary battery of the present invention has high conductivity and low interfacial resistance, and thus has a long lifespan. Description of the Drawings

[0014] Figure 1 and Figure 2 is the process of carrying out the examples.

[0015] Figure 3 is the capacity retention rate of Example 1 and Comparative Example 1.

[0016] Figure 4 is the impedance of Example 2 and Comparative Example 2. Detailed Description of the Invention

[0017] The present invention is a lithium secondary battery.

[0018] The lithium secondary battery of the present invention includes a solid electrolyte layer, an anode layer, and a cathode layer. The anode layer is disposed on one surface of the solid electrolyte layer, and the cathode layer is disposed on the opposite side of the anode layer with respect to the solid electrolyte layer. That is, the anode layer, the solid electrolyte layer, and the cathode layer are disposed in the above order or the reverse order.

[0019] The solid electrolyte layer has a specific structure. The solid electrolyte layer includes a plurality of independent polymer solid electrolyte layers. The solid electrolyte layer includes at least two polymer solid electrolyte layers, namely, a first polymer solid electrolyte layer and a second polymer solid electrolyte layer.

[0020] The above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer have the composition of a polymer solid electrolyte. Each of the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer contains a polymer component, a plasticizer, and a lithium salt.

[0021] In a specific example, the components contained in the above-mentioned first polymer solid electrolyte layer and the components contained in the above-mentioned second polymer solid electrolyte layer may be the same as or different from each other.

[0022] In a specific example, the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer may be adjacent to each other. In this case, there may be no additional layer between the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer.

[0023] In a specific example, there may be an additional layer between the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer.

[0024] The difference between the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer lies in whether they contain inorganic fillers.

[0025] The above-mentioned first polymer solid electrolyte layer contains inorganic fillers. The above-mentioned second polymer solid electrolyte layer does not contain inorganic fillers.

[0026] In this specification, when it is said that a certain component does not contain a specific component, it means that other components other than the specific component may exist in the above-mentioned component.

[0027] The above-mentioned inorganic filler is barium titanate (BaTiO3). Since barium titanate is a high dielectric constant substance, the ionic conductivity can be significantly increased when barium titanate is introduced compared with other inorganic substances (for example: Al2O3).

[0028] As the inorganic filler, barium titanate with a particle size of 100 nm or less can be used. The smaller the particle size of the inorganic filler, the larger the surface area, so the smaller it is used, the more beneficial it is.

[0029] If only one polymer solid electrolyte layer containing inorganic fillers is provided between the anode and the cathode, the contact area between the polymer solid electrolyte layer and the electrode will decrease. This is because if the inorganic filler is introduced into the polymer solid electrolyte layer, the surface roughness of the polymer solid electrolyte layer increases. Here, it is necessary to introduce a layer that does not contain inorganic fillers such as the second polymer electrolyte layer to reduce the increased surface roughness of the polymer solid electrolyte layer.

[0030] The first polymer electrolyte layer includes inorganic fillers, and thus has surfaces with different surface roughnesses in the thickness direction, namely a first surface and a second surface. Here, the surface roughness of the first surface is greater than that of the second surface. If the surface with a lower surface roughness is adjacent to the electrode and the polymer electrolyte layer without inorganic fillers is introduced to the surface with a higher surface roughness, the problem of reduction in the electrode contact area caused by the introduction of inorganic fillers can be reduced. That is, the first surface is disposed on the side of the second polymer electrolyte layer. That is, the lithium-ion battery of the present invention has a laminated structure of an anode layer (cathode layer) / the second surface of the first polymer electrolyte layer / the first surface of the first polymer electrolyte layer / the second polymer electrolyte layer / the cathode layer (anode layer).

[0031] In a specific example, the first polymer solid electrolyte layer may be adjacent to the cathode layer. The first polymer solid electrolyte layer may be disposed on the side of the cathode layer. The first polymer solid electrolyte layer may be closer to the cathode layer than the second solid electrolyte layer.

[0032] In a specific example, the second polymer solid electrolyte layer may be adjacent to the anode layer. The second polymer solid electrolyte layer may be disposed on the side of the anode layer. The second polymer solid electrolyte layer may be closer to the anode layer than the first solid electrolyte layer.

[0033] In a specific example, a specific substance may be used as the active material of the anode layer. Specifically, the anode layer may contain the compound represented by the following Chemical Formula 1:

[0034] [Chemical Formula 1]

[0035] LiNi a Co b Mn c O2 (0.6 ≤ a ≤ 0.9, a + b + c = 1).

[0036] Preferably, in the above Chemical Formula 2, a may be close to 0.8, and b and c may each be close to 0.1. More preferably, the anode layer may contain a substance known as NCM811.

[0037] In a specific example, the lithium secondary battery of the present invention may be a lithium metal battery. The lithium metal battery uses lithium metal as the cathode. That is, in a specific example, the cathode layer may contain lithium metal.

[0038] The type of the polymer component is not particularly limited and can be appropriately selected. For example, the above polymer component can be selected from polyethylene oxide (PEO), polyvinyl chloride (PVC), poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride - hexafluoropropylene) (PVDF - HFP), etc. Preferably, the above polymer component can be PAN. In a specific example, in the above first polymer solid electrolyte layer and the above second polymer solid electrolyte layer, the above polymer component can be polyacrylonitrile respectively.

[0039] The content of the polymer component is not particularly limited either. The present invention can be appropriately adjusted within the range that can at least exert the function of the polymer electrolyte by having appropriate flexibility and liquid content. In a specific example, in the above first polymer solid electrolyte layer and the above second polymer solid electrolyte layer, the content of the above polymer component can be within the range of 18% to 21% by weight respectively.

[0040] The plasticizer increases the amorphous region by reducing the crystallinity of the polymer component in the polymer solid electrolyte. The plasticizer can be a nitrile - based compound selected from glutaric nitrile, succinonitrile, acetonitrile, adiponitrile, methoxyacetonitrile, etc. Preferably, the plasticizer is valeronitrile. In a specific example, in the above first polymer solid electrolyte layer and the above second polymer solid electrolyte layer, the plasticizer can be valeronitrile respectively.

[0041] The content of the plasticizer is not particularly limited and can be appropriately adjusted within the range that can exert the performance of the present invention. In a specific example, in the above first polymer solid electrolyte layer and the above second polymer solid electrolyte layer, the content of the above plasticizer relative to 1 mole of the above polymer component can be within the range of 0.7 mole to 1.0 mole respectively.

[0042] The content of the inorganic filler in the first polymer electrolyte layer is not particularly limited and can be appropriately adjusted within the range that can exert the performance of the present invention. In a specific example, the content of the inorganic filler relative to 100 parts by weight of the above polymer component can be within the range of 50 parts by weight to 100 parts by weight. If the content of the inorganic filler is within the above range, the first polymer electrolyte layer can have a form in which the polymer is dispersed in the inorganic filler. That is, the dispersibility of the inorganic filler can be not considered within the above range of the content of the inorganic filler.

[0043] In a specific example, in order to obtain a lithium secondary battery that exhibits appropriate ionic conductivity and structural stability, the thicknesses of the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer can be adjusted.

[0044] In a specific example, the thickness ratio (T1 / T2) of the above-mentioned first polymer solid electrolyte layer (T1) and the above-mentioned second polymer solid electrolyte layer (T2) can be in the range of 0.5 to 2. The lower limit of the above range can be 0.6, 0.7, 0.8, 0.9, or 1.0. The upper limit of the above range can be 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, or 1.0.

[0045] In a specific example, the thicknesses of the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer can also be adjusted. The thicknesses of the above-mentioned first polymer solid electrolyte layer and the above-mentioned second polymer solid electrolyte layer can each be in the range of 30 μm to 50 μm.

[0046] Hereinafter, the present invention will be described in more detail using examples. However, the following examples do not limit the scope of protection of the present invention.

[0047] [Materials]

[0048] Table 1 summarizes the types and sources of the materials used in the examples and comparative examples.

[0049] [Table 1]

[0050]

[0051] [Comparative Example 1] Polymer Solid Electrolyte

[0052] The polymer components were uniformly mixed in a solvent at 80 °C for 2 hours and dissolved. Then, 0.74 M of a lithium salt was added thereto and mixed for 1 hour. Then, PAN:GN was added thereto at a molar ratio of 1:1.8 and mixed at 70 °C for 30 minutes to produce a polymer solid electrolyte solution. The above solution was cast on a glass plate at 30 μm and then vacuum dried at 90 °C for 50 minutes to produce a polymer solid electrolyte. The content of the solvent remaining in the polymer solid electrolyte was 50 wt%.

[0053] [Comparative Example 2] Polymer Solid Electrolyte

[0054] The polymer component is uniformly mixed in the solvent at 80°C for 2 hours and dissolved. Next, 0.74M of lithium salt is added thereto and mixed for 1 hour. Then, PAN:GN is added thereto at a molar ratio of 1:1.8 and mixed at 70°C for 30 minutes. Next, 50 parts by weight of inorganic filler are added thereto relative to 100 parts by weight of the polymer component and mixed using a think mixer to prepare a polymer solid electrolyte solution. After the above solution is cast on a glass plate at 30μm, it is vacuum dried at 90°C for 50 minutes to prepare a polymer solid electrolyte. The content of the residual solvent in the polymer solid electrolyte is 50% by weight.

[0055] [Comparative Example 3] Polymer Solid Electrolyte

[0056] The same procedure as in Comparative Example 2 was repeated except that the content of the inorganic filler was changed to 75 parts by weight relative to 100 parts by weight of the polymer component.

[0057] [Comparative Example 4] Polymer Solid Electrolyte

[0058] The same procedure as in Comparative Example 2 was repeated except that the content of the inorganic filler was changed to 100 parts by weight relative to 100 parts by weight of the polymer component.

[0059] [Example 1] Polymer solid electrolyte

[0060] Example 1 is a reference Figure 1 and Figure 2 to explain.

[0061] On the glass substrate, a first polymer solid electrolyte layer with a thickness of 200 μm was prepared using the polymer solid electrolyte solution of Comparative Example 2. Here, the surface of the first polymer solid electrolyte layer in contact with the above-mentioned glass substrate ( Figure 1 and Figure 2 1) has a lower roughness than the opposite side surface of the first polymer solid electrolyte layer ( Figure 1 and Figure 2 2) Roughness.

[0062] A second polymer solid electrolyte layer with a thickness of 100 μm was prepared using the polymer solid electrolyte solution of Comparative Example 1. On the surface of the first polymer solid electrolyte layer in contact with the glass ( Figure 1 and Figure 2 The opposite side surface of 1) ( Figure 1 and Figure 2 2) The second polymer solid electrolyte layer was stacked without applying any additional pressure, thereby manufacturing a polymer solid electrolyte layer with a thickness of 300 μm.

[0063] [Example 2]

[0064] The polymer solid electrolyte solution of Comparative Example 3 was used instead of the polymer solid electrolyte solution of Comparative Example 2, and the same procedure as in Example 1 was repeated otherwise.

[0065] [Example 3]

[0066] The polymer solid electrolyte solution of Comparative Example 4 was used instead of the polymer solid electrolyte solution of Comparative Example 2, and the same procedure as in Example 1 was repeated otherwise.

[0067] [Experimental Example 1] Ion Conductivity Evaluation

[0068] The polymer electrolyte layers of the Examples and Comparative Examples were placed between lithium metals. A button battery laminated in the order of lithium metal / polymer solid electrolyte layer / lithium metal was assembled. The internal resistance of the button battery was measured to evaluate the ion conductivity of the polymer solid electrolyte. The diameters of the lithium metal electrodes were 16 mm Φ each. As the measuring device, MP1 of WonATech was used. The measuring conditions were normal temperature and a frequency range of 1 Hz to 1 MHz.

[0069] [Experimental Example 2] Capacity Retention Evaluation

[0070] The polymer electrolyte layers of the Examples and Comparative Examples were placed between NCM811 (anode) and lithium metal (cathode). A button battery laminated in the order of NCM811 / polymer solid electrolyte layer / lithium metal was assembled. The diameters of the electrodes were 16 mm Φ each. The charge-discharge capacity of the button battery was measured to evaluate the capacity retention. Charging / discharging was performed at 0.3C, and at this time, the voltage range was 3.0 V to 4.3 V. The battery life was calculated from the discharge retention rate. Specifically, based on the initial battery discharge amount, the number of charge-discharge cycles when the discharge amount reached 80% was defined as the battery life.

[0071] [Experimental Example 3] Battery Resistance Evaluation

[0072] The impedance of the button battery fabricated in Experimental Example 2 was measured to evaluate the battery resistance. As the measuring device, MP1 of WonATech was used. The measuring conditions were normal temperature and a frequency range of 1 Hz to 1 MHz. At normal temperature, after assembling the button battery, the button battery was aged for about 30 minutes, and then the above measurement was performed so that the polymer solid electrolyte layer could be well joined between the electrodes.

[0073] [Results and Discussion]

[0074] Table 2 shows the composition of the components used in the Comparative Examples and the results of the ion conductivity evaluation of the Comparative Examples.

[0075] Table 3 shows the composition of the components used in the examples and the evaluation results of the ionic conductivity of the examples.

[0076] [Table 2]

[0077]

[0078]

[0079] [Table 3]

[0080]

[0081] Comparative Examples 1 to 3 show that when an inorganic filler is introduced into a single polymer electrolyte layer, the ionic conductivity either increases or decreases even more. Examples 1 to 2 show that if a polymer electrolyte layer without an inorganic filler is laminated on top of a single polymer electrolyte layer into which an inorganic filler has been introduced, the effect of increasing the ionic conductivity brought about by the introduction of the inorganic filler can be significantly exerted.

[0082] Figure 3 is the capacity retention rate of Example 1 and Comparative Example 1. By Figure 3 it can be confirmed that the capacity of Comparative Example 1 decreased to 60% within 50 charge / discharge cycles. In addition, by Figure 3 it can be confirmed that in Example 1, the number of cycles at which the capacity decreased to 60% increased to 100 cycles. From this, it can be confirmed that if a polymer electrolyte layer without an inorganic filler is laminated on top of a single polymer electrolyte layer into which an inorganic filler has been introduced, the interfacial resistance between the electrolyte layer and the electrode is reduced, and thus the capacity retention rate of the battery is also improved.

[0083] Figure 4 is the impedance of Example 2 and Comparative Example 2. The resistance of the electrolyte is shown as a semicircle in the graph, and the larger the diameter, the greater the resistance. From this, it can be confirmed that if a polymer electrolyte layer without an inorganic filler is laminated on top of a single polymer electrolyte layer into which an inorganic filler has been introduced, the interfacial resistance of the battery is reduced.

Claims

1. A lithium secondary battery, comprising: A solid electrolyte layer; An anode layer disposed on one side of the solid electrolyte layer; and A cathode layer disposed on the opposite side of the anode layer with respect to the solid electrolyte layer, The solid electrolyte layer includes a first polymer solid electrolyte layer and a second polymer solid electrolyte layer, The first polymer solid electrolyte layer includes a first polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt, and an inorganic filler, The second polymer solid electrolyte layer includes a second polymer solid electrolyte containing a polymer component, a plasticizer, and a lithium salt but does not include an inorganic filler, The first polymer electrolyte layer includes a first surface and a second surface having different surface roughnesses in the thickness direction, The surface roughness of the first surface is greater than that of the second surface, The first surface is disposed on the side of the second polymer solid electrolyte layer, The inorganic filler is barium titanate.

2. The lithium secondary battery according to claim 1, The first polymer solid electrolyte layer is disposed on the side of the cathode layer, The second polymer solid electrolyte layer is disposed on the side of the anode layer.

3. The lithium secondary battery according to claim 1, wherein the anode layer includes a compound represented by the following Chemical Formula 1: [Chemical Formula 1] LiNi a Co b Mn c O2 Among them, 0.6 ≤ a ≤ 0.9, a + b + c = 1.

4. The lithium secondary battery according to claim 1, wherein the cathode layer contains lithium metal.

5. The lithium secondary battery according to claim 1, wherein the polymer component is polyacrylonitrile in the first polymer solid electrolyte layer and the second polymer solid electrolyte layer, respectively.

6. The lithium secondary battery according to claim 1, wherein the content of the polymer component is in the range of 18% by weight to 21% by weight in the first polymer solid electrolyte layer and the second polymer solid electrolyte layer, respectively.

7. The lithium secondary battery according to claim 1, wherein the plasticizer is valeronitrile in the first polymer solid electrolyte layer and the second polymer solid electrolyte layer, respectively.

8. The lithium secondary battery according to claim 1, wherein the content of the plasticizer is in the range of 0.7 moles to 1.0 moles with respect to 1 mole of the polymer component in the first polymer solid electrolyte layer and the second polymer solid electrolyte layer, respectively.

9. The lithium secondary battery according to claim 1, wherein the content of the inorganic filler is in the range of 50 parts by weight to 100 parts by weight with respect to 100 parts by weight of the polymer component.

10. The lithium secondary battery according to claim 1, wherein the thicknesses of the first polymer solid electrolyte layer and the second polymer solid electrolyte layer are each in the range of 30 μm to 50 μm.

11. The lithium secondary battery according to claim 1, wherein the thickness ratio of the first polymer solid electrolyte layer T1 to the second polymer solid electrolyte layer T2, i.e., T1 / T2, is in the range of 0.5 to 2.

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