CN101728509A - Lithium battery, positive electrode and forming method thereof - Google Patents

Abstract

The invention relates to a positive electrode applied to a lithium battery and a forming method thereof. Firstly, a lithium alloy oxide layer is formed on a substrate, then additional high-density low-energy plasma treatment is carried out, so that nano-crystalline grains which are mutually tightly jointed and are evenly compact are formed on the upper surface of the lithium alloy oxide layer, and the original larger crystalline grain structure is maintained in the lower inner part of the lithium alloy oxide layer. Experiments prove that the anode with the characteristics has higher capacitance and cycle life, and can further improve the performance of the lithium battery.

Description

Lithium battery, positive electrode and method for forming the same

technical field

The present invention relates to a lithium battery, more particularly to a positive electrode therein and a forming method thereof.

Background technique

Today's portable electronic products such as digital cameras, mobile phones, and notebook computers require lightweight batteries. Among all kinds of batteries, rechargeable lithium batteries can provide three times more power per unit weight than traditional batteries such as lead storage batteries, nickel-metal hydride batteries, nickel-zinc batteries, and nickel-cadmium batteries. In addition, the lithium battery can be charged quickly.

In a lithium battery, the positive electrode material is generally a lithium metal oxide, such as LiNiO ₂ , LiCoO ₂ , LiMn ₂ O ₄ , LiFePO ₄ , or LiNi _x Co _1-x O ₂ . The negative electrode material is generally lithium metal, an alloy of lithium and other metals, or a carbonaceous material such as graphite. Generally, whether the method of forming the positive electrode is to use a thin film preparation process or a powder preparation process, the lithium alloy oxide layer will form an uneven, non-dense and coarse-grained upper surface, and reduce the capacity (Capacity, mAh/g) and cycle of the positive electrode. life. To sum up, there is an urgent need for new methods to improve the unevenness of the upper surface of the positive electrode.

Contents of the invention

The purpose of the present invention is to provide a method for forming a positive electrode for a lithium battery, which can basically solve the defects of uneven upper surface of the positive electrode in the prior art.

Another object of the present invention is to provide a positive electrode whose upper surface is closely bonded to each other and is dense on average.

Another object of the present invention is to provide a lithium battery with the above-mentioned positive electrode, which can further improve the performance performance of the lithium battery because the positive electrode has a higher capacity and cycle life.

The invention provides a method for forming a positive electrode, which includes providing a substrate; forming a lithium alloy oxide layer on the substrate; and modifying the lithium alloy oxide layer with high-density and low-energy plasma, so that the upper surfaces are closely bonded to each other and dense on average. Nanocrystalline grains, the lower part maintains the original larger grain structure.

The present invention also provides a positive electrode, including a substrate; and a lithium alloy oxide layer located on the substrate; wherein the upper surface of the lithium alloy oxide layer has nano-grains that are closely bonded to each other and are evenly dense, and the lower interior maintains the original larger crystal grains structure.

The present invention further provides a lithium battery, comprising the above-mentioned positive electrode; a negative electrode; a separator positioned between the positive electrode and the negative electrode to define an accommodating area; an electrolyte solution positioned in the accommodating area; and a packaging structure covering the positive electrode and the negative electrode , separator, and electrolyte solution.

The advantage of the present invention is that: the method for forming the positive electrode of the present invention applies an additional low-energy, high-density plasma treatment to the lithium alloy oxide layer, so that the upper surface of the positive electrode lithium alloy oxide layer is closely bonded to each other, dense on average, and the crystal grains are closely spaced. The nano-crystal grains of 3nm-10nm maintain the original larger grain structure in the lower part, thereby basically overcoming the defect of the uneven upper surface of the positive electrode in the prior art. Moreover, since the positive electrode of the present invention has higher capacity and longer cycle life, the lithium battery using the electrode of the present invention also has better performance.

Description of drawings

Fig. 1 is in the embodiment of the present invention, the sectional view of lithium battery;

Fig. 2 is in the embodiment of the present invention, the sectional view of plasma source;

Fig. 3 a is in the comparative example of the present invention, LiMn ₂ O The electron diffraction spectrum of ₄ layers;

Fig. 3b is the electron diffraction pattern of LiMn ₂ O ₄ layer in the embodiment of the present invention;

Fig. 4 is in the embodiment of the present invention and comparative example, the ESCA collection of illustrative plates of LiMn ₂ O ₄ layers;

Fig. 5 is in the embodiment of the present invention and comparative example, the RAMAN collection of graphs of LiMn ₂ O ₄ layers;

Fig. 6 is in comparative example of the present invention, the SEM photo of LiMn ₂ O ₄ upper surfaces of the layer;

Figure 7a is an SEM photo of the upper surface of the LiMn ₂ O ₄ layer in an embodiment of the present invention;

Figure 7b is a TEM bright field photograph of the upper surface of the LiMn ₂ O ₄ layer in an embodiment of the present invention;

Fig. 8 is the capacitance curve of the charge-discharge cycle at room temperature in a comparative example of the present invention;

Fig. 9 is the capacitance curve of the charge-discharge cycle at high temperature in the comparative example of the present invention;

Fig. 10 is the capacitance curve of the charge-discharge cycle at room temperature in an embodiment of the present invention; and

Fig. 11 is the capacitance curve of charge and discharge cycle at high temperature in the embodiment of the present invention.

Among them, the main component symbol description:

1～positive pole; 2～housing area; 3～negative pole 3;

5～isolation film; 6～encapsulation structure; 7～upper electrode;

8～anode shielding; 9～enclosed magnetic field; 10～lower electrode.

Detailed ways

The invention provides a lithium battery positive electrode and a forming method thereof. First, a lithium metal oxide is formed on a substrate. Substrates can be stainless steel, polyamide, acrylonitrile-butadiene-styrene copolymer (ABS), mica, glass, polyethylene terephthalate (PET), polyparaphenylene benzodioxine oxazole (PBO), or epoxy resin. Lithium metal oxides are common cathode materials, such as LiMnO ₂ , LiMn ₂ O ₄ , LiCoO ₂ , Li ₂ Cr ₂ O ₇ , Li ₂ CrO4, LiNiO ₂ , LiFeO ₂ , LiNi _x Co _1-x O ₂ , LiFePO ₄ , LiMn _0.5 Ni _0.5 O ₂ , LiMn _1/3 Co _1/3 Ni _1/3 O ₂ , LiMc _0.5 Mn _1.5 O ₄ , or a combination thereof, wherein 0<x<1, and Mc is a divalent metal. There are three types of formation methods: (1) traditional powder pressing, (2) vapor deposition, such as plasma enhanced chemical vapor deposition (PECVD), laser melting, low pressure vapor deposition (LPCVD), rapid thermal chemical vapor deposition (RTCVD) ), atomic layer deposition (ALD), or physical vapor deposition (PVD), magnetron sputtering (magnetron sputtering), radio frequency sputtering (RF sputtering), radio frequency plasma sputtering (RF plasma sputtering), (3) chemical solution Synthetic deposition, such as sol-gel coating, electrochemical plating, hydrothermal synthetic deposition, etc. So far, a thin film with a thickness of about 50 nm to 2000 nm or a thick lithium alloy oxide layer of 2 to 100 μm is formed on the substrate.

In an embodiment of the present invention, it can be seen from the SEM photos of the cut surface that the lithium alloy oxide layer formed by the above preparation process is not dense, and the SEM photos of its upper surface are also uneven. If the above structure is directly used as a positive electrode material, its capacitance will be greatly reduced after several cycles of charging and discharging.

In order to improve the above problems, the key steps of the present invention are carried out after the lithium alloy oxide layer is formed. The invention applies additional plasma treatment to the lithium alloy oxide layer, which is characterized by low energy and high density plasma treatment. After this treatment, the upper surface of the lithium alloy oxide layer has nano-grains that are closely bonded to each other, averagely dense, and the grains are between 3nm and 10nm, and the lower part maintains the original larger grain structure. According to ESCA and X-Ray diffraction patterns, the surface before and after plasma treatment has the same chemical composition and crystal structure, and only the surface grain size is changed. Plasma treatment did not cause unnecessary oxidation-reduction reactions in the lithium alloy oxide layer, and RAMAN spectra showed the formation of nanocrystalline structures on the upper surface. The source of the above plasma preparation process can be radio frequency plasma or microwave plasma, the power of which depends on the size of the material to be treated, and the power density of the surface to be treated is between 0.8W/cm ² and 5W/cm ² . If the power is less than 0.8W/ ^cm2 , the treatment effect will not be produced due to the low plasma density, but if the power is greater than 4W/ ^cm2 , high-energy, high-density plasma will damage the surface of the material or cause crystallization. grain coarsening phenomenon. The atmosphere of the above plasma preparation process is preferably an inert gas such as argon to avoid unnecessary reactions. Oxygen can also be used if the positive electrode material to be treated contains metals in a high oxidation state such as 3-4 valent cobalt or manganese. In order to achieve high-density and low-energy plasma, the plasma cavity can be equipped with anode shielding, inductive coupling coils, and closed magnetic fields. The purpose is to make the plasma density between 10 ¹¹ and 10 ¹³ cm ^-3 , and the electron temperature at Between ~2eV, if the relevant parameters are too low, the treatment effect will not be produced, but if it is too high, high-energy, high-density plasma will damage the surface of the material or a large phase change will occur. The plasma preparation process takes between 5 minutes and 60 minutes. If it is less than 5 minutes, it will not be treated uniformly, but if it is longer than 60 minutes, it will affect the inside of the processed material and cause plasma damage. This plasma preparation process is a low-temperature preparation process, and the temperature of the treated material is controlled below 150°C. If it is higher than 150°C, it will damage the soft substrate that is not resistant to high temperature.

It is worth noting that although some known technologies use a machine with a plasma device to form a lithium alloy oxide layer, the plasma is used to assist preparation processes such as deposition or sputtering, and after the deposition or sputtering is completed, it is also Thereupon, the process is terminated, and the above-mentioned lithium alloy oxide layer assisted by plasma still has an uneven upper surface. Although some known techniques apply additional plasma treatment to the lithium alloy oxide layer, without the plasma source design and plasma parameters of this technique, the treatment results cannot be precisely controlled, but the overall structure of the treated material is changed, resulting in coarse grains. Changes, phase changes, destruction, etc., have no effect on improving battery performance.

In an embodiment of the present invention, the substrate is made of a relatively rigid material such as stainless steel, which can be prepared by high-temperature tempering before or after the plasma treatment. The above-mentioned high-temperature tempering preparation process is between 300 and 700°C. If it is less than 300°C, the crystallinity of the material is poor and the capacitance is low, but if it is greater than 700°C, high-temperature impurity phases will be generated.

The lithium metal oxide layer on the surface treated by plasma can be used as the positive electrode material of lithium battery. The above positive electrode material can be applied but not limited to the lithium battery shown in FIG. 1 . In FIG. 1 , there is a separator 5 between the positive electrode 1 and the negative electrode 3 to define the accommodation area 2 . An electrolyte solution is contained in the accommodation area 2 . In addition, outside the above structure is an encapsulation structure 6 for covering the positive electrode 1 , the negative electrode 3 , the separator 5 , and the electrolyte solution.

The negative electrode 3 includes carbide and lithium alloy. The carbide can be carbon powder, graphite, carbon fiber, carbon nanotube, or a mixture of the above. In an embodiment of the present invention, the carbide is carbon powder, and the particle size is approximately between 5 μm and 30 μm. Lithium alloys can be LiAl, LiZn, Li ₃ Bi, Li3Cd, Li ₃ Sb, Li ₄ Si, Li _4.4 Pb, Li _4.4 Sn, LiC ₆ , Li ₃ FeN ₂ , Li _2.6 Co _0.4 N, Li _2.6 Cu _0.4 N, or a combination of the above. In addition to the above two substances, the negative electrode 3 can further contain metal oxides such as SnO, SnO ₂ , GeO, GeO ₂ , In ₂ O, In ₂ O ₃ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Ag ₂ O, AgO, Ag ₂ O ₃ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , SiO, ZnO, CoO, NiO, FeO, or combinations thereof. In addition, the negative electrode 3 can further have a polymer binder to increase the mechanical properties of the electrode. A suitable polymer adhesive can be polyvinylidene fluoride (PVDF for short), styrene-butadiene rubber (SBR for short), polyamide (polyamide), melamine resin (melamine resin), or the above composition.

The above-mentioned isolation film 5 is an insulating material, which can be polyethylene (PE), polypropylene (PP), or the above-mentioned multi-layer structure such as PE/PP/PE.

The main components of the above electrolytic solution are organic solvent, lithium salt, and additives. The organic solvent can be γ-butyrolactone (abbreviated as GBL), ethylene carbonate (abbreviated as EC), propylene carbonate (abbreviated as PC), diethyl carbonate (diethyl carbonate, abbreviated as DEC), propyl acetate (PA for short), dimethyl carbonate (DMC for short), ethylmethyl carbonate (EMC for short), or a combination of the above. The lithium salt can be LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiAlCl ₄ , LiGaCl ₄ , LiNO ₃ , LiC(SO ₂ CF ₃ ) ₃ , LiN(SO ₂ CF ₃ ) ₂ , LiSCN, LiO ₃ SCF ₂ CF ₃ , LiC ₆ F ₅ SO ₃ , LiO ₂ CCF ₃ , LiSO ₃ F, LiB(C ₆ H ₅ ) ₄ , LiCF ₃ SO ₃ , or a combination thereof. The additives include common vinylene carbonate (VC for short), and may further contain maleimide-based compounds described in a patent (Taiwan application number: 96145902) filed earlier by the inventor.

The plasma source design for plasma surface treatment is shown in Figure 2. The upper electrode 7 is a single plate or an inductance coil or a microwave generator. An anode shield 8 is set between the upper and lower plates, coupled with the ground terminal through an adjustable capacitor, and a closed The magnetic field 9 places the material to be processed on the lower electrode plate 10 .

Since the positive electrode of the present invention has a higher capacity and a longer cycle life, the lithium battery using the above electrode also has better performance.

In order to make the above and other objects, features, and advantages of the present invention more comprehensible, examples are described as follows.

【Example】

Positive electrode preparation:

Comparative Example 1

LiMn ₂ O ₄ with a thickness of 500 nm was formed on a 304 stainless steel substrate (purchased from Yongfaxin Industrial Co., Ltd.) by magnetron sputtering, followed by a tempering step at 600°C. The above-mentioned LiMn ₂ O ₄ layer is cut off by the focused ion beam (Focused Ion Beam) and the electron diffraction spectrum is shown in Figure 3a, the ESCA spectrum is shown in Figure 4, the RAMAN spectrum is shown in Figure 5, and the SEM photo of the upper surface As shown in Figure 6. It can be seen from FIG. 6 that the upper surface of the LiMn ₂ O ₄ layer without additional plasma treatment is uneven and has coarse grains.

Example 1

Form 500nm thick LiMn ₂ O ₄ on the stainless steel substrate by magnetron sputtering method. After sputtering, a 600°C tempering step follows. Then the LiMn ₂ O ₄ layer is treated with radio frequency plasma, the power of the plasma treatment is 30 watts, the atmosphere is argon, the temperature is lower than 100° C., and the time is 30 minutes. The ESCA spectrum of the LiMn ₂ O ₄ layer after the above plasma treatment is shown in FIG. 4 , and the RAMAN spectrum is shown in FIG. 5 .

Example 2

Similar to Example 1, the difference is that the power of the plasma treatment is increased to 50 watts. The LiMn ₂ O ₄ layer after the above plasma treatment is also cut off the surface layer by the Focused Ion Beam (Focused Ion Beam) and the electron diffraction spectrum is shown in Figure 3b, the ESCA spectrum is shown in Figure 4, and the RAMAN spectrum is shown in Figure 5 , the SEM photo of the upper surface is shown in Fig. 7a. It can be seen from Figure 7a that the upper surface of the LiMn ₂ O ₄ layer treated with additional plasma is dense on average. Due to the extremely small and tightly bonded nanocrystals formed on the surface, SEM cannot effectively analyze it. Figure 7b is a TEM bright field image of the same surface layer as in Figure 3b, showing the tightly bonded nanocrystal structure.

Example 3

Similar to Example 1, the difference is that the power of the plasma treatment is increased to 80 watts. The ESCA spectrum of the LiMn ₂ O ₄ layer after the above plasma treatment is shown in FIG. 4 , and the RAMAN spectrum is shown in FIG. 5 .

From the comparison of the electron diffraction spectrum in FIG. 3 and the ESCA spectrum in FIG. 4 , it can be seen that the chemical structures of the surface molecules in Comparative Example 1 and Examples 1-3 are not changed by the plasma preparation process. However, the RAMAN spectrum in Figure 5 shows that after high-density and low-energy plasma treatment, the peak between wavenumbers 630 and 660cm ^-1 shifts to lower wavenumbers due to the nanonization of surface grains. From the SEM and TEM photos of Figures 6, 7a and 7b, it can be seen that the plasma treatment effectively improves the surface uniformity and density of the lithium alloy oxide layer, and induces the formation of surface nanocrystals.

Electrical property measurement (battery capacity and cycle life):

Comparative Example 1

The electrode of Comparative Example 1 was inserted into the electrolyte solution (1M LiPF ₆ EC/EMC), with a positive/negative current of 0.3mA/mg, a voltage of 1.5-4.5 (V vs. Li/Li ⁺ ) at room temperature (25 ℃) to conduct charge and discharge experiments, and measure its capacitance (mAh/g). As shown in Figure 8, the curve of the first charge and discharge is the rightmost curve, and its capacitance is 140mAh/g. As the number of cycles increases, the curve begins to shift to the left, and the capacity of the 20th cycle is only 115mAh/g.

The electrode of Comparative Example 1 was inserted into the electrolyte solution (1M LiPF ₆ EC/EMC), with a positive/negative current of 0.3mA/mg, a voltage of 2.0-4.5 (V vs. Li/Li ⁺ ) at a high temperature (55 ℃) to conduct charge and discharge experiments, and measure its capacitance (mAh/g). As shown in Figure 9, the curve of the first charge and discharge is the rightmost curve, and its capacitance is 400mAh/g. As the number of cycles increases, the curve begins to shift to the left, and the capacity of the 20th cycle is only 200mAh/g.

Example 2

The electrode of Example 2 was placed in the electrolyte solution (1M LiPF ₆ EC/EMC), with a positive/negative current of 0.3mA/mg and a voltage of 2.0-4.5 (V vs. Li/Li ⁺ ) at room temperature (25°C ) under charge and discharge experiments, and measure its capacitance (mAh/g). As shown in Figure 10, the charge and discharge curves from the first to the twentieth time almost overlap, and they are all maintained at 140mAh/g.

The electrode of Example 2 was placed in the electrolyte solution (1M LiPF ₆ EC/EMC), with a positive/negative current of 0.3mA/mg and a voltage of 1.5-4.5 (V vs. Li/Li ⁺ ) at high temperature (55°C ) under charge and discharge experiments, and measure its capacitance (mAh/g). The curve of the second charge and discharge is the rightmost curve, and its capacity is 500mAh/g, which is higher than the capacity of 410mAh/g for the first charge and discharge. But as the number of cycles increases, the curve begins to shift to the left, and the capacity of the 20th cycle is still 370mAh/g.

From the comparison of Figures 8-11, it can be seen that the cycle life and capacitance of the lithium alloy oxide layer on the surface treated by plasma are better regardless of the operating environment at high temperature or room temperature.

Although the present invention has been disclosed above with several embodiments, it is not intended to limit the present invention. Anyone with ordinary knowledge in the technical field can make any changes and modifications without departing from the spirit and scope of the present invention. Modification, therefore, the scope of protection of the present invention should be defined by the scope of the appended claims.

Claims (11)

1. A method for forming a positive electrode, comprising: providing a substrate; forming a lithium alloy oxide layer on the substrate; and The lithium alloy oxide layer is modified with a plasma, so that the upper surface forms closely connected and uniformly dense nano-grains, and the lower part maintains the original larger grain structure. 2. The forming method of positive electrode according to claim 1, wherein the substrate comprises stainless steel, polyamide, acrylonitrile-butadiene-styrene copolymer, mica, glass, polyethylene terephthalate, polyethylene terephthalate phenylenebenzobisoxazole or epoxy resin. 3. The method for forming the positive electrode according to claim 1, wherein the lithium metal oxide comprises LiMnO ₂ , LiMn ₂ O ₄ , LiCoO ₂ , Li ₂ Cr ₂ O ₇ , Li ₂ CrO4, LiNiO ₂ , LiFeO ₂ , LiNi _x Co _1-x O ₂ , LiFePO ₄ , LiMn _0.5 Ni _0.5 O ₂ , LiMn _1/3 Co _1/3 Ni _1/3 O ₂ , LiMc _0.5 Mn _1.5 O ₄ , or a combination of the above, where 0<x <1, and Mc is a divalent metal. 4. The method for forming the positive electrode according to claim 1, wherein the step of forming the lithium alloy oxide layer on the substrate comprises powder compaction, vapor phase deposition, or chemical solution synthesis deposition. 5. The forming method of the positive electrode according to claim 1, wherein in the step of modifying the lithium alloy oxide layer with a plasma, the plasma cavity that it adopts has an anode shield, an inductively coupled coil, and a closed magnetic field , to provide a high-density low-energy plasma. 6. The method for forming the positive electrode according to claim 1, wherein the plasma comprises radio frequency plasma or microwave plasma. 7. The method for forming the positive electrode according to claim 1, wherein the power density of the plasma applied to the upper surface of the lithium alloy oxide layer is between 0.8W/cm ² and 5W/cm ² , and the power of the plasma The density is between 10 ¹¹ and 10 ¹³ cm ^-3 , and the electron temperature of the plasma is between 1 and 2 eV. 8. A positive electrode comprising: a substrate; and a lithium alloy oxide layer located on the substrate; Wherein, the upper surface of the lithium alloy oxide layer has nano-grains that are closely bonded to each other and averagely dense, while the lower interior maintains a larger grain structure. 9. The positive electrode according to claim 8, wherein the substrate comprises stainless steel, polyamide, acrylonitrile-butadiene-styrene copolymer, mica, glass, polyethylene terephthalate, polyparaphenylene And bisoxazole or epoxy resin. 10. The positive electrode according to claim 8, wherein the lithium alloy oxide layer comprises LiMnO ₂ , LiMn ₂ O ₄ , LiCoO ₂ , Li ₂ Cr ₂ O ₇ , Li ₂ CrO4, LiNiO ₂ , LiFeO ₂ , LiNi _x Co _{1 - x} O ₂ , LiFePO ₄ , LiMn _0.5 Ni _0.5 O ₂ , LiMn _1/3 Co _1/3 Ni _1/3 O ₂ , LiMc _0.5 Mn _1.5 O ₄ , or a combination thereof, wherein 0<x<1, and Mc is a divalent metal. 11. A lithium battery, comprising: The positive electrode as claimed in claim 8; a negative pole; a separator located between the positive electrode and the negative electrode to define a containment area; an electrolyte solution located in the containment area; and An encapsulation structure covers the positive electrode, the negative electrode, the separator, and the electrolyte solution.

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