A positive electrode and its preparation method and application

Abstract

The present application discloses a positive electrode, a preparation method thereof, and an application. It includes a current collector and an active layer attached to the surface of the current collector; the active layer includes a positive electrode active material, a lithium lanthanum zirconium oxide solid electrolyte, a conductive agent, and a binder. The mixed materials are directly sprayed onto the current collector by means of electrostatic spraying, avoiding the problems of poor conductivity and poor contact in the composite part with the current collector in other methods. At the same time, by introducing a ceramic solid electrolyte, the adhesion of the positive electrode active substance is improved, thereby promoting the working efficiency of electrons to be more efficient. At the same time, by introducing a biodegradable additive, the material usage cost is reduced. At the same time, by introducing a composite solid electrolyte, the ability to apply mechanical stress to the positive electrode during the expansion process is improved. Thereby, the cycle performance stability of the battery is improved.

Description

Technical Field

[0001] The present application relates to a positive electrode and its preparation method and application, belonging to the field of lithium batteries. Background Art

[0002] Since their commercialization, lithium-ion batteries (LIBs) have conquered almost all applications where high energy and power density, as well as good cycling stability, are required. Electric mobility has consumed the largest share of the annual LIB production and is expected to boom in the short and medium term as it may help to overcome the dependence on fossil fuels and improve the air quality in cities. Due to high costs, safety issues, and the limited range of most electric vehicles (EVs), consumer acceptance remains rather low. The candidates that could at least potentially solve the safety and range issues are all-solid-state batteries (ASSBs). Due to the lack of a flammable liquid electrolyte, ASSBs inherently have a higher safety standard than conventional LIBs. In addition, they allow the use of a lithium metal anode, which is more favorable in terms of high energy density than a graphite anode. The lithium metal anode not only increases the battery voltage but also the actually achievable weight, especially the volumetric energy density, by 40 and 70%, respectively, which is important for the design of most EV manufacturers.

[0003] Commercial lithium-ion battery electrodes are manufactured by casting a slurry onto a current collector. The slurry contains active materials, conductive carbon, and a binder. The binder, most commonly polyvinylidene fluoride (PVDF), is pre-dissolved in a solvent, most commonly N-methyl-2-pyrrolidone (NMP). During the mixing process, the polymer binder flows around them and coats the active materials and carbon particles. After uniform mixing, the resulting slurry is poured onto the current collector and must be dried. Manufacturing the battery requires evaporating the solvent to form a dry porous electrode. Drying takes a long time, and some electrodes require 12 - 24 hours at 120 °C to be completely dried. In commercial applications, due to the high cost and potential pollution of NMP, an NMP recovery system must be established during the drying process to recover the evaporated NMP. Electrodes manufactured with dry particles coated on the current collector are an ideal manufacturing process, thus eliminating the solvents and the drawbacks associated with their use.

[0004] CN 115548243 A discloses a method for preparing a self-supporting electrode sheet. First, an electrode active material, a conductive agent, a solid additive, and a binder are mixed to obtain a mixture, and then the mixture is fibrillated and formed to obtain the self-supporting electrode sheet. Among them, due to the introduction of "solid additive camphene" in the dry-process electrode sheet, the electrode sheet becomes softer, and the cracking phenomenon is inhibited. This method improves the softness and performance of the electrode sheet to a certain extent, but the conductivity of its dry-process electrode sheet still cannot be improved, which limits the improvement of its performance. Therefore, we should find a solid additive that can increase the softness of the electrode sheet and improve the conductivity of the electrode sheet at the same time. CN 115377424 A discloses a method for preparing a dry-process electrode sheet containing a sulfide solid electrolyte and using a "composite binder of polytetrafluoroethylene and styrene-butadiene rubber". This technology introduces a sulfide solid electrolyte and a composite binder on the basis of the traditional dry process. Although this method plays a certain role in improving the dry-process technology, its conductivity and stability still cannot be solved, and styrene-butadiene rubber will have certain side reactions at high temperatures. Therefore, there is an urgent need to develop a new composite solid electrolyte additive that is used in combination with a PTFE binder, has low cost, and is environmentally friendly. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the object of the present invention is to provide a solvent-free preparation method for a high-energy density electrode sheet. The dry-process electrode sheet containing a solid electrolyte can provide ionic conductivity without coating an adhesive layer and without adding other additives. The obtained solid electrolyte has a high ionic conductivity and a high lithium ion transference number, thus significantly enhancing the electrochemical performance of the dry-process electrode sheet; the added biodegradable polypeptide sericin significantly reduces the problem of electrode sheet cracking during the preparation process of the electrode sheet and increases its flexibility.

[0006] According to one aspect of the present application, a positive electrode is provided. The positive electrode includes a current collector and an active layer attached to the surface of the current collector;

[0007] The active layer includes a positive electrode active material, a lithium lanthanum zirconium oxide solid electrolyte, a conductive agent, and a binder.

[0008] The positive electrode active material is selected from at least one of lithium manganate and lithium iron phosphate;

[0009] In the active layer, the content of the positive electrode active material is 80-95 wt%.

[0010] The lithium lanthanum zirconium oxide solid electrolyte contains a promoter element;

[0011] The promoter element is selected from at least one of aluminum, tantalum, and tungsten;

[0012] In the active layer, the content of the lithium lanthanum zirconium oxide solid electrolyte is 3 to 5 wt%.

[0013] The conductive agent is selected from Ketjen black;

[0014] In the active layer, the content of the conductive agent is 3 to 6 wt%.

[0015] The binder is selected from at least one of polytetrafluoroethylene and biodegradable polypeptide sericin;

[0016] In the active layer, the content of the binder is 2 to 5 wt%.

[0017] According to another aspect of the present application, there is provided a method for preparing the above positive electrode, comprising the following steps:

[0018] Mix the raw materials containing the positive electrode active material, lithium lanthanum zirconium oxide solid electrolyte, conductive agent and binder, and ball mill to obtain a powder. Spray the powder onto the surface of the current collector and hot press to obtain the positive electrode.

[0019] During the ball milling process, the mass ratio of the ball milling beads to the total mass of the positive electrode active material, lithium lanthanum zirconium oxide solid electrolyte, conductive agent and binder is 10 to 15:1 to 2.5.

[0020] The process parameters of the spraying include:

[0021] Electrostatic voltage 15 to 70 kV;

[0022] Compressed air pressure 4 to 30 kg / cm 2 ;

[0023] Electrostatic current 5 to 30 μA;

[0024] Powder flow rate pressure 0.5 to 5 MPa;

[0025] Atomization pressure 0.4 to 8 MPa;

[0026] The current collector forms a 45-degree angle with the spray gun and the distance is 5 to 50 cm.

[0027] The temperature of the hot press is 60 to 200 °C;

[0028] The pressure of the hot press is 3 to 10 MPa;

[0029] The time of the hot press is 0.5 to 3 h.

[0030] Specifically,

[0031] First, the positive electrode active material lithium manganate is pretreated by a jet mill pulverizer. After that, the positive electrode active material, conductive agent, and PTFE binder are mixed and ball-milled to prepare a powder. After ball-milling, we select a high-speed disperser for secondary mixing. On this basis, a lithium lanthanum zirconium-based oxide composite solid electrolyte is added and uniformly mixed. Then, the powder is sprayed onto the surface of the positive electrode current collector to prepare a positive electrode sheet with powder spraying. Finally, the positive electrode sheet with powder spraying is hot-pressed by a hot press to prepare the dry electrode sheet containing the solid electrolyte.

[0032] The staged stirring of the high-speed disperser for secondary mixing is divided into three stages: 500 - 1500 rpm, 2000 - 3500 rpm, and 4000 - 8000 rpm.

[0033] According to another aspect of the present application, there is provided an application of the above positive electrode for a lithium-ion battery.

[0034] The beneficial effects that the present application can produce include:

[0035] The present invention provides a dry electrode sheet containing a solid electrolyte, in which a composite solid electrolyte is incorporated into the positive electrode active layer. The composite solid electrolyte is a lithium lanthanum zirconium-based oxide solid electrolyte and a biodegradable polypeptide sericin. The present application uses an electrostatic spraying method to directly spray the mixed materials onto the current collector, avoiding the problems of poor conductivity and poor contact in the composite part with the current collector in other methods. At the same time, by introducing a ceramic solid electrolyte, the adhesion of the positive electrode active material is improved, thereby promoting more efficient working efficiency of electrons. At the same time, by introducing a biodegradable additive, the material usage cost is reduced. At the same time, by introducing a composite solid electrolyte, the ability to apply mechanical stress to the positive electrode during the swelling process is improved. Thereby, the cycle performance stability of the battery is improved. Description of the Drawings

[0036] Figure 1 The SEM photograph of the dry electrode sheet prepared in Example 1 of the present invention is shown, with a scale of 1 μm.

[0037] Figure 2 The electrochemical charge and discharge curve when the LALZO@sericin-LMO positive electrode material prepared in Example 1 is used in a lithium metal battery.

[0038] Figure 3 The long cycle performance curve when the LALZO@sericin-LMO positive electrode material prepared in Example 1 is used in a lithium metal battery.

[0039] Figure 4 The long cycle performance curve when the LMO positive electrode material without a composite solid electrolyte prepared in Comparative Example 1 is used in a lithium metal battery. Detailed implementation manners

[0040] The present application will be described in detail below in conjunction with embodiments, but the present application is not limited to these embodiments.

[0041] Unless otherwise specified, the raw materials in the embodiments of the present application are all purchased through commercial channels.

[0042] Embodiment 1

[0043] (1) Pretreatment and premixing of active substances

[0044] First, the positive active material lithium manganese oxide is pretreated by a jet mill pulverizer, and then the positive active material, conductive agent, and PTFE binder are mixed and ball-milled to prepare a powder. The mass ratio of the ball milling beads to the total mass of the positive active material, composite solid electrolyte, conductive agent, and binder is 12:1.5; after ball milling, we select a high-speed disperser for secondary mixing. The staged stirring of the high-speed disperser for secondary mixing is divided into three stages: 900 rpm, 2300 rpm, and 5000 rpm. On this basis, a lithium lanthanum zirconium-based and biodegradable polypeptide sericin sericin composite solid electrolyte is added and uniformly mixed. The material ratio is 90:5:3:2 of lithium manganese oxide, composite solid electrolyte, Ketjen black, and PTFE binder.

[0045] (2) Electrostatic spraying

[0046] Then, the powder is sprayed onto the surface of the positive current collector to prepare a positive electrode sheet with powder spraying; the process parameters of the powder spraying include: electrostatic voltage 50 kV, compressed air pressure 10 kg / cm 2 , electrostatic current 20 μA, powder flow rate pressure 3 MPa, atomization pressure 1 MPa. The angle and distance between the positive current collector and the powder: 45 degrees with the spray gun current collector, distance 20 cm. The coating surface density of the prepared positive electrode sheet with powder spraying is 80 mg / cm 2

[0047] (3) Hot pressing by a hot press

[0048] Finally, the positive electrode sheet with powder spraying is hot pressed by a hot press to prepare the dry electrode sheet containing a solid electrolyte.

[0049] The process parameters of the hot pressing of the positive electrode sheet with powder spraying include a temperature of 120 °C and a pressure of: 5 MPa. The hot pressing time is: 1 h. The positive electrode sheet after hot pressing is 89 μm ± 2 μm, and the final LALZO@sericin-LMO positive electrode sheet is obtained

[0050] As Figure 1 shown is the SEM photo of the dry electrode sheet prepared in Embodiment 1 of the present invention, with a scale of 1 μm.

[0051] From Figure 2 The electrochemical charge-discharge curve when the LALZO@sericin-LMO cathode material prepared in Example 1 is used in a lithium metal battery.

[0052] From Figure 3 The long cycle curve when the LALZO@sericin-LMO cathode material prepared in Example 1 is used in a lithium metal battery. It can be observed in the figure that the discharge specific capacity is as high as 118 mAh / g in the first cycle at 1C. As the cycle progresses, the charge amount decreases. After 100 cycles, the specific capacity still remains at 99 mAh / g. This excellent cycle stability benefits from the introduction of the LALZO@sericin composite solid electrolyte and the network structure formed by the binder.

[0053] Example 2

[0054] (1) Pretreatment and premixing of the active material

[0055] First, the cathode active material lithium manganate is pretreated by a jet mill pulverizer. Then, the cathode active material, Ketjen black, and PTFE binder are mixed and ball-milled to prepare a powder. The mass ratio of the ball milling beads to the total mass of the cathode active material and the binder is 12:1.5. After ball milling, we select a high-speed disperser for secondary mixing. The staged stirring of the high-speed disperser for secondary mixing is divided into three stages: 900 rpm, 2300 rpm, and 5000 rpm. The material ratio is 90:6:4 of lithium manganate, Ketjen black conductive agent, and PTFE binder.

[0056] (2) Electrostatic spraying

[0057] Then, the powder is sprayed onto the surface of the cathode current collector to prepare a cathode sheet with powder spraying. The process parameters of the powder spraying include: electrostatic voltage 50 kV, compressed argon pressure 10 kg / cm 2 , electrostatic current 20 μA, powder flow rate pressure 3 MPa, atomization pressure 1 MPa. The angle and distance between the cathode current collector and the powder: 45 degrees with the spray gun current collector, distance 20 cm. The coating surface density of the prepared cathode sheet with powder spraying is 230 mg / cm 2

[0058] (3) Hot pressing by a hot press

[0059] Finally, the cathode sheet with powder spraying is hot-pressed by a hot press to prepare the dry electrode sheet containing the solid electrolyte.

[0060] The process parameters for hot pressing the positive electrode sheet sprayed with the powder include a temperature of 120 °C and a pressure of 5 MPa. The hot pressing time is 1 h. The positive electrode sheet after hot pressing is 89 μm ± 2 μm, and the final LALZO@sericin-LMO positive electrode sheet is obtained.

[0061] Comparative Example 1

[0062] Other conditions are the same as in Example 1, except that the LALZO@sericin composite solid electrolyte is not used.

[0063] (1) Pretreatment and premixing of the active material

[0064] First, the positive electrode active material lithium manganate is pretreated by a jet mill, and then the positive electrode active material, Ketjenblack conductive agent, and PTFE binder are mixed and ball-milled to prepare a powder. The mass ratio of the ball milling beads to the total mass of the positive electrode active material, conductive agent, and binder is 12:1.5; after ball milling, we select a high-speed disperser for secondary mixing. The staged stirring of the high-speed disperser for secondary mixing is divided into three stages: 900 rpm, 1300 rpm, and 2000 rpm. The material ratio is 90:7:3 of lithium manganate, Ketjenblack conductive agent, and PTFE binder.

[0065] (2) Electrostatic spraying

[0066] Then, the powder is sprayed onto the surface of the positive electrode current collector to prepare a positive electrode sheet sprayed with the powder; the process parameters for the powder spraying include: an electrostatic voltage of 50 kV, a compressed argon pressure of 10 kg / cm 2 , an electrostatic current of 20 μA, a powder flow rate pressure of 0.8 MPa, an atomization pressure of 0.5 MPa. The angle and distance between the positive electrode current collector and the powder are: 45 degrees with the spray gun current collector, and the distance is 8 cm. The coating surface density of the prepared positive electrode sheet sprayed with the powder is 80 mg / cm 2

[0067] (3) Hot pressing by a hot press

[0068] Finally, the positive electrode sheet sprayed with the powder is hot pressed by a hot press to prepare the dry electrode sheet containing the solid electrolyte.

[0069] The process parameters for hot pressing the positive electrode sheet sprayed with the powder include a temperature of 120 °C and a pressure of 4 MPa. The hot pressing time is 45 min. The positive electrode sheet after hot pressing is 89 μm ± 2 μm, and the final LMO positive electrode sheet is obtained

[0070] From Figure 4The following is the long cycle curve of the LMO cathode material prepared in Comparative Example 1 of the present invention when used in a lithium metal battery. It can be observed from the figure that the discharge specific capacity in the first cycle is 112 mAh / g at 1C. As the cycle progresses, the battery capacity decreases. After 100 cycles, the specific capacity is only retained at 49 mAh / g. The reason is that without adding the LALZO@biodegradable polypeptide sericin composite solid electrolyte, the lithium ion transport is restricted and the flexibility is poor, and obvious cracking occurs during the hot pressing process, thus affecting the cycle stability. It can be seen that the composite solid electrolyte enhances the ion transport efficiency, flexibility and uniform stress during the hot pressing process.

[0071] Comparative Example 2

[0072] Other conditions are the same as those in Example 2, except that only the LALZO ceramic solid electrolyte is used and the LALZO@sericin composite solid electrolyte is not selected.

[0073] (1) Pretreatment and premixing of the active material

[0074] First, the cathode active material lithium manganate is pretreated by a jet mill, and then the cathode active material, Ketjenblack conductive agent and PTFE binder are mixed and ball-milled to prepare a powder. The mass ratio of the ball milling beads to the total mass of the cathode active material and the composite binder is 12:1.5. After ball milling, we select a high-speed disperser for secondary mixing. The staged stirring of the high-speed disperser for secondary mixing is divided into three stages: 900 rpm, 1300 rpm, and 2000 rpm. The material ratio is 90:4:3:3 of lithium manganate, solid electrolyte, Ketjenblack conductive agent and PTFE binder.

[0075] (2) Electrostatic spraying

[0076] Then, the powder is sprayed onto the surface of the cathode current collector to prepare a cathode sheet with powder spraying; the process parameters of the powder spraying include: electrostatic voltage 50 kV, compressed argon pressure 10 kg / cm 2 , electrostatic current 20 μA, powder flow rate pressure 0.8 MPa, atomization pressure 0.5 MPa. The angle and distance between the cathode current collector and the powder: 45 degrees with the spray gun current collector, distance 8 cm. The coating surface density of the prepared cathode sheet with powder spraying is 75 mg / cm 2

[0077] (3) Hot pressing by a hot press

[0078] Finally, the cathode sheet with powder spraying is hot pressed by a hot press to prepare the dry electrode sheet containing the solid electrolyte.

[0079] The process parameters for hot pressing the positive electrode sheet sprayed with powder include a temperature of 120 °C, a pressure of 4 MPa, and a hot pressing time of 45 min. The positive electrode sheet after hot pressing is 89 μm ± 2 μm, and the final LALZO-LMO positive electrode sheet is obtained.

[0080] As described above, these are only several embodiments of the present application and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art, without departing from the scope of the technical solution of the present application, making some changes or modifications using the disclosed technical content are equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

Claims

1. A method for preparing a positive electrode, characterized in that, It includes the following steps: Mix the cathode active material, conductive agent and binder, and perform ball milling to prepare a powder material; select a high-speed disperser for secondary mixing, add a composite solid electrolyte, uniformly mix, and then spray it onto the surface of the cathode current collector to prepare a cathode sheet with the powder material sprayed thereon; hot press the cathode sheet with the powder material sprayed thereon to obtain the cathode electrode; The secondary mixing is carried out in stages of stirring, and the staged stirring is divided into three stages: 500 - 1500 rpm, 2000 - 3500 rpm, 4000 - 8000 rpm; The cathode electrode includes a current collector and an active layer attached to the surface of the current collector; The active layer includes a cathode active material, a lithium lanthanum zirconium oxide solid electrolyte, a conductive agent and a binder; The binder is selected from polytetrafluoroethylene; The composite solid electrolyte is a lithium lanthanum zirconium oxide solid electrolyte and a biodegradable polypeptide sericin; The lithium lanthanum zirconium oxide solid electrolyte contains a promoter element; The promoter element is selected from at least one of aluminum and tungsten; The process parameters of the spraying include: Electrostatic voltage 15 - 70 kV; Compressed air pressure 4 - 30 kg / cm 2 ; Electrostatic current 5 - 30 μA; Powder flow rate pressure 0.5 - 5 MPa; Atomization pressure 0.4 - 8 MPa; The current collector forms a 45-degree angle with the spray gun and the distance is 5 - 50 cm.

2. The preparation method according to claim 1, characterized in that, The cathode active material is selected from at least one of lithium manganate and lithium iron phosphate; In the active layer, the content of the cathode active material is 80 - 95 wt%; 3. The preparation method according to claim 1, characterized in that, In the active layer, the content of the lithium lanthanum zirconium oxide solid electrolyte is 2 - 5 wt%; 4. The preparation method according to claim 1, characterized in that, The conductive agent is selected from Ketjen black; In the active layer, the content of the conductive agent is 3 - 6 wt%; 5. The preparation method according to claim 1, characterized in that, In the active layer, the content of the binder is 2 - 5 wt%; 6. The preparation method according to claim 1, characterized in that, During the ball milling process, the mass ratio of the ball milling beads to the total mass of the cathode active material, lithium lanthanum zirconium oxide solid electrolyte, conductive agent and binder is 10 - 15:1 - 2.5; 7. The preparation method according to claim 1, characterized in that, The temperature of the hot pressing is 60 - 200 °C; The pressure of the hot pressing is 3 - 10 MPa; The time of the hot pressing is 0.5 - 3 h.

8. An application of a positive electrode prepared by the preparation method according to any one of claims 1 to 7, characterized in that, It is used for lithium-ion batteries.

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