A method of constructing ultra-low loss vias on thick electrodes using time / frequency shaped laser

Abstract

The application relates to a method for constructing an ultra-low-loss via hole on a thick electrode by using time / frequency shaping laser, and belongs to the field of electrode material laser processing. The implementation method is as follows: a birefringent crystal, BBO and a high-power objective are used to shape a traditional Gaussian laser into a short-wavelength laser with a certain time delay and focal depth; a harmonic beam splitter is used to solve the safety problem caused by reflected light; a light-limiting diaphragm is used to ensure that laser with high single-pulse energy is used while the spot diameter is reduced, so that an ultra-low-loss low-tortuosity pore structure is prepared on the thick electrode, and the loss is controlled within 1%. The application can solve the safety problem caused by the reflected energy of the frequency-doubled laser, reduce the problem that the introduced pore loss of the thick electrode is too high and cannot be calendered, solve the problem of capacity and rate performance reduction of the thick electrode, and improve the power density, energy density and cycle life of the thick electrode.

Description

TECHNICAL FIELD

[0001] The application relates to a method for constructing an ultra-low-loss via hole on a thick electrode by time / frequency shaping laser, and belongs to the technical field of laser processing of electrode materials. BACKGROUND

[0002] In portable electronic products, electric vehicles and large energy storage systems, higher demands are put forward for the energy density and power density of the battery under the premise of safety. Thick electrodes can improve the energy density of the battery, but their high tortuosity is seriously detrimental to the penetration of electrolyte and the migration of lithium ions, reducing the rate performance and capacity. In addition, the inter-powder gap of the traditional electrode is prone to blockage due to negative reactions of materials and electrolyte in long cycles, ultimately resulting in poor cycle life. The introduction of a low-tortuosity pore structure can solve this problem. Specific methods include template method, 3D printing method and the like. These methods have the problems of complex preparation steps, incompatibility with calendering process and excessively high pore loss rate.

[0003] By utilizing the interaction between a high-energy laser beam and a substance, micro-scale grooves, holes, lines, grids and other geometric structures can be constructed to realize high-precision and pollution-free processing of thick electrode structures on the electrode surface. However, this manufacturing process will cause the loss of active materials and reduce the energy density. Taking a battery with LFP as the positive electrode and lithium metal as the negative electrode as an example: when the thickness of the LFP electrode layer increases from 60 mu m to 260 mu m, the weight energy density can increase by 45.6%. When the introduced pore loss increases to 20%, the energy density will decrease by 16.9%, which seriously weakens the energy density improvement brought by the thick electrode. Pore loss of 1% hardly reduces the energy density improvement brought by the thick electrode. Therefore, optimizing the laser parameters and process flow to prepare low-loss pore structures on the thick electrode to reduce the loss of active materials is the key to the commercial application of this technology, which can improve the power density, energy density and cycle life of the thick electrode.

[0004] Traditional punching methods include single-pulse punching, multi-pulse punching, ring-cutting punching and spiral punching. Single-pulse punching is suitable for relatively thin materials, and the thickness of the plate can allow a pulse to punch through. Ring-cutting punching and spiral punching are suitable for the construction of large-size holes. Although multi-pulse punching is suitable for the construction of small holes, the shielding effect of plasma on subsequent laser during the machining process will limit the depth of laser punching. Moreover, electrode materials are powder materials that are easy to process. It is very difficult to use conventional laser drilling methods to process ultra-low-loss holes on thick electrodes. SUMMARY

[0005] In order to solve the problems such as too high porosity loss (20%-40%) and unable to be calendered caused by traditional methods such as template method, 3D printing method and the like, and the method of machining ultra-low loss hole diameter on thick electrode by conventional laser drilling, the method for constructing ultra-low loss via hole on thick electrode by time / frequency shaping laser is adopted to prepare the ultra-low loss low tortuosity hole structure on thick electrode, the loss is controlled within 1%, the problem of too high active material loss caused by traditional hole manufacturing method on thick electrode is reduced, and the improvement of power density, energy density and cycle life of thick electrode is realized.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is:

[0007] The method for constructing ultra-low loss via hole on thick electrode by time / frequency shaping laser is disclosed, the traditional Gaussian laser time / frequency is shaped into short wavelength laser with certain delay and focal depth by adopting birefringent crystal, BBO and high-power objective lens, the common BBO combined with optical filter is improved by harmonic beam splitting to perform energy protection to curb the safety problem caused by reflected light under large energy, the approach of light limiting diaphragm is adopted to ensure the use of high single pulse energy laser while reducing the spot diameter, the ultra-low loss low tortuosity hole structure is prepared on thick electrode, and the loss is controlled within 1%. The safety problem caused by the energy of short wavelength laser is solved, the problem of too high porosity loss (20%-40%) and unable to be calendered caused by traditional methods such as template method and 3D printing method is reduced, the capacity and rate performance caused by the difficulty of electrolyte penetration and the increase of lithium ion transmission distance are improved, and the power density, energy density and cycle life of thick electrode are improved.

[0008] Further, the ultra-low loss low tortuosity hole structure with loss controlled within 1% on thick electrode is specifically constructed by via hole with a diameter of 5-6 mu m and a hole spacing of 60 mu m, and the hole is a via hole.

[0009] Further, the thickness of the thick electrode in the method is adjusted according to the demand of energy density and power density of the actual battery, and the range is 60 mu m-260 mu m.

[0010] The method for constructing ultra-low loss via hole on thick electrode by time / frequency shaping laser comprises the following steps:

[0011] Step one: the delay between pulse lasers is changed by birefringent crystal, and the laser wavelength is doubled by BBO, and the time / frequency shaping laser femtosecond laser system is built by combining high-power objective lens, harmonic beam splitting and light limiting diaphragm;

[0012] Step two: focus the time / frequency shaped femtosecond laser on the thick electrode surface to perform multi-pulse impact drilling; after optimization, super-low-loss low-tortuosity through-hole aperture structures are obtained on thick electrodes of different thicknesses;

[0013] Step three: assemble in the order of positive shell-LFP positive plate-separator-negative plate-gasket-spring plate-negative shell, add an appropriate amount of electrolyte during the process, and assemble into a button cell;

[0014] Step four: assemble the untreated electrode and the electrode with processed aperture array into a battery, and perform rate test and cycle stability test by a blue light tester.

[0015] Further, the time / frequency shaping laser femtosecond laser system comprises a femtosecond laser, an attenuator, a BBO, a harmonic beam splitter, a birefringent crystal, a mechanical switch, an ultrafast mirror group, an aperture, a dichroic mirror, a high-power objective, an electrode material, a three-dimensional precision displacement stage, a beam splitter, a white light, and a CCD dynamic imaging unit; the BBO converts the 800nm femtosecond laser into mixed laser of 800nm and 400nm, the harmonic beam splitter reflects the 400nm femtosecond laser onto the birefringent crystal to convert it into time-domain shaped femtosecond laser with a certain delay, the aperture with a limited size enters the dichroic mirror and is focused on the electrode sample on the precision displacement stage through the high-power objective; the white light source passes through the beam splitter, the dichroic mirror and the CCD dynamic imaging unit to realize real-time imaging and monitoring of laser drilling on the electrode surface.

[0016] Further, the birefringent crystal shapes the conventional pulse laser into laser pulses with a delay of less than 10ps; the harmonic beam splitter simultaneously reflects 400nm laser and transmits 800nm laser; the aperture size is controlled at 2mm-3mm.

[0017] Advantages:

[0018] 1. The method of constructing ultra-low loss via on thick electrodes using time / frequency shaped laser, the time / frequency shaped laser femtosecond laser system comprises a femtosecond laser, an attenuator, a BBO, a harmonic beam splitter, a birefringent crystal, a mechanical switch, an ultrafast mirror group, a light barrier, a dichroic mirror, a high-power objective, an electrode material, a three-dimensional precision displacement table, a beam splitter, white light, and a CCD dynamic imaging unit. The time / frequency shaped laser generated by the system has the advantages of sub-pulse sequence with ps delay and small spot diameter. The time-domain shaped femtosecond laser has sub-pulse sequence with ps delay, which minimizes the reflection of the plasma to the laser, thereby promoting more efficient energy transfer to the bottom of the hole, which is beneficial to machining high aspect ratio via; the frequency domain shaped femtosecond laser has a small spot diameter, which can prepare ultra-low loss low tortuosity via on different thick electrodes, reducing the loss of active substances; the harmonic beam splitter is used to improve the optical path of the commonly used BBO combined with the optical filter to perform energy protection to curb the safety problem caused by reflected light under high energy; the light barrier ensures the use of high single pulse energy laser while reducing the spot diameter.

[0019] 2. The method of constructing ultra-low loss via on thick electrodes using time / frequency shaped laser, the manufacturing difficulty of preparing low loss high aspect ratio via on thick electrodes in the method is relatively high. Using the method, ultra-low loss low tortuosity pore structures with loss controlled within 1% are prepared on electrodes of different thicknesses, and the pore structure is composed of via with a diameter of 5-6 μm and a hole spacing of 60 μm, and the pores are all via. The thickest electrode has a thickness of 260 μm, and the current collector has a thickness of 20 μm. Under the above conditions, the low loss pore structure prepared has a high aspect ratio of 47, which is the highest among the via structures prepared in the current opaque materials.

[0020] 3. The method of constructing ultra-low loss via on thick electrodes using time / frequency shaped laser, the electrodes with pore arrays prepared by the method are assembled into batteries together with untreated electrodes, and the power density, energy density, and cycle life of thick electrodes are improved. The straight via provides a shorter diffusion path for lithium ions, significantly reduces the internal resistance, and thus realizes faster charging and discharging rate; in terms of energy density, reducing the electrode internal resistance and enhancing ion diffusion makes it possible to use thicker electrodes, and the thick electrodes can increase the proportion of active materials in the entire battery, thereby improving the overall energy density; the powder gap of the traditional electrode is easy to be blocked due to the negative reaction of the material and the electrolyte in long-term cycling, the via structure is not easy to be blocked, thereby ensuring smooth diffusion of ions and more stable cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A schematic diagram of the method of constructing ultra-low loss via on thick electrodes using time / frequency shaped laser.

[0022] Figure 2 The relationship between the thickness of the thick electrode, the porosity loss rate, and the battery energy density.

[0023] Figure 3 This is a schematic diagram of the optical path of the time / frequency shaping femtosecond laser in this invention.

[0024] Figure 4 The pore array morphology of a low-loss thick electrode with a thickness of 280 μm is shown.

[0025] Figure 5 A comparison of the rate performance of batteries with low-loss thick electrodes of different thicknesses and untreated electrodes.

[0026] Figure 6 Comparison of cycle life between low-loss thick electrodes of different thicknesses and untreated electrodes.

[0027] Among them: 1—femtosecond laser, 2—attenuator, 3—BBO, 4—harmonic beam splitter, 5—birefringent crystal, 6—mechanical switch, 7—ultrafast mirror assembly, 8—aperture, 9—dichroic mirror, 10—high-magnification objective lens, 11—electrode material, 12—three-dimensional precision displacement stage, 13—beam splitter, 14—white light, 15—CCD dynamic imaging unit, 16—baffle. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, the embodiments of this invention are not limited to the following embodiments.

[0029] like Figure 1 As shown in this embodiment, a method for constructing ultra-low loss vias on thick electrodes using time / frequency shaped lasers is disclosed. This method employs a birefringent crystal, a BBO (Browser-Based Beam) laser, and a high-magnification objective lens to time-frequency shape a traditional Gaussian laser into a short-wavelength laser with a certain delay and depth of focus. Harmonic beam splitting is used to improve the optical path of the commonly used BBO combined with a filter to protect energy and mitigate safety issues caused by reflected light under high energy. An aperture-limiting approach is used to ensure the use of high single-pulse laser energy while simultaneously reducing the spot diameter. This achieves the fabrication of ultra-low loss, low-torsion pore structures on thick electrodes, with losses controlled to within 1%, thereby improving the power density, energy density, and cycle life of the thick electrodes.

[0030] Introducing porous structures into thick electrodes can help solve the problems of decreased capacity and rate performance caused by difficulties in electrolyte penetration and increased lithium-ion transport distance. However, the introduction of pores can also lead to the loss of active material, thus reducing energy density. Figure 2As shown, taking a battery with LFP (LiFeP04, lithium iron phosphate) as the positive electrode and lithium metal as the negative electrode as an example: when the thickness of the LFP electrode layer increases from 60 μm to 260 μm, the weight energy density can increase by 45.6%. When the introduced porosity loss increases to 20%, the energy density will decrease by 16.9%, which seriously weakens the energy density improvement brought by the thick electrode. The energy density improvement brought by the thick electrode is almost not reduced by 1% porosity loss.

[0031] The embodiment discloses a method for constructing an ultra-low-loss through hole on a thick electrode by using time / frequency shaping laser, and the specific implementation steps are as follows:

[0032] Step one: the time / frequency domain shaping femtosecond laser system (such as Figure 3 As shown) includes a femtosecond laser 1, an attenuator 2, a BBO (BaB2O4) 3, a harmonic beam splitter 4, a birefringent crystal 5, a mechanical switch 6, an ultrafast mirror group 7, an aperture 8, a dichroic mirror 9, a high-power objective lens 10, an electrode material 11, a three-dimensional precision displacement stage 12, a beam splitter 13, a white light source 14, and a CCD dynamic imaging unit 15. The 800 nm femtosecond laser emitted by the femtosecond laser 1 passes through the attenuator 2 and then converts the 800 nm femtosecond laser into mixed laser of 800 nm and 400 nm through the BBO 3; the mixed laser is separated into 800 nm transmitted laser and 400 nm reflected laser through the harmonic beam splitter 4, and the 400 nm laser is converted into time-domain shaped femtosecond laser with a 0.45 ps delay after passing through the 0.8 mm thick birefringent crystal 5; the time / frequency shaped femtosecond laser passes through the mechanical switch 6 and then enters the mirror group 7, and then enters the aperture 8 limited to a size of 2-3 mm; the laser passes through the dichroic mirror 9 and is focused on the electrode sample 11 on the precision displacement stage 12 through the high-power objective lens 10. The white light source 14, the beam splitter 13, the dichroic mirror 9, and the CCD dynamic imaging unit 15 together realize real-time imaging and monitoring of the time / frequency shaped femtosecond laser drilling on the electrode surface. The 800 nm transmitted laser is shielded by a baffle 16, so as to realize energy protection to curb the safety problem caused by the reflected light under large energy.

[0033] The time-domain shaping method is not limited to the birefringent crystal, and other methods such as the Michelson interferometer, the spatial light modulator, and the film system design can also shape the conventional laser into a sub-pulse sequence with a certain delay; the frequency domain shaping is not limited to the BBO second harmonic generation, and the third harmonic generation or the fourth harmonic generation also belongs to the frequency domain shaping.

[0034] Step two: prepare different thicknesses (130 μm, 170 μm, 260 μm; Al foil 20 μm; the corresponding surface densities are 30 mg cm -2 , 40 mg cm -2 , and 60 mg cm -2) LFP electrode. The time / frequency shaped femtosecond laser was focused on the surface of the thick electrode to perform multi-pulse impulsive drilling. The super low-loss low-tortuosity pore structure array was obtained on thick electrodes with different thicknesses after optimization, and the loss was controlled within 1%. The structure of the pore array under this loss consisted of through holes with a diameter of 5-6 μm and a hole spacing of 60 μm. The 60 mg cm -2 For example, as shown in the scanning electron microscope results in Figure 4 , the surface morphology of the through hole array prepared by the time / frequency shaped femtosecond laser is uniform and has no micro-cracks, material peeling and other problems. The results of the tomographic scan show that the introduction of the through hole greatly reduces the tortuosity of the electrode, and can realize a through hole with a depth-to-diameter ratio of 47 on the LFP, which is the highest in the currently prepared through hole structure of opaque materials and is beneficial to the penetration of the electrolyte and the migration of lithium ions.

[0035] In this embodiment, four thicknesses of LFP electrodes are selected for the manufacture of super low-loss low-tortuosity pores. The method is not limited to these four electrode thicknesses and is applicable to electrodes with a thickness in the range of 60 μm-260 μm and other thicknesses.

[0036] Step three: Assemble in the order of positive electrode shell-LFP positive electrode sheet (5x5 mm)-separator-negative electrode sheet (lithium sheet)-gasket-spring sheet-negative electrode shell, and add 30 μL of lithium hexafluorophosphate LiPF6 (ethylene carbonate EC: diethyl carbonate DEC = 1:1) electrolyte during the process to assemble a coin cell. The LFP electrode without laser treatment and the super low-loss low-tortuosity LFP electrode prepared by the time / frequency shaped femtosecond laser are assembled into coin cells, respectively.

[0037] Step four: Test the performance of the assembled battery by a blue light tester. The low-loss low-tortuosity thick electrode is higher than the untreated electrode in power density, energy density and cycle life. This is because the introduction of the pore structure on the thick electrode is beneficial to the penetration of the electrolyte, shortens the migration distance of lithium ions, reduces the loss of active material, and uniformizes the lithium ion flux. Specifically, as shown in Figure 5 , the electrodes with different thicknesses show significant differences in rate performance. The 130 μm laser treated LFP electrode obtains a specific capacity of 128, 97 and 41 mAh g -1 at 1C, 2C and 3C, respectively, while the untreated electrode only has a capacity of 113, 50 and 15 mAh g -1 . The 170 μm laser treated LFP electrode obtains a specific capacity of 130, 113 and 78 mAh g -1 at 1C, 1.5C and 2C, respectively, while the untreated electrode only has a capacity of 51, 20 and 9 mAh g -1 . The 260 μm laser treated LFP electrode has a capacity of 85 mAh g-1 While the capacity of the untreated electrode is 8 mAh g -1 The through-hole provides a shorter diffusion path for ions, significantly reducing the internal resistance, thus achieving faster charge and discharge rates. The ultra-low-loss low-tortuosity pore structure prepared by time / frequency shaping femtosecond laser can increase the power density of thick electrodes by 100-200% without affecting the specific capacity. Under the condition of 75 mAh g -1 The use of thicker electrodes is allowed under the condition of acceptable battery capacity. For example, under the condition of 2C, the thickness can be increased from 95 μm to 185 μm, i.e. the energy density is increased by 17.26%. Reducing the electrode internal resistance and enhancing ion diffusion makes it possible to use thicker electrodes, increasing the proportion of active materials in the battery, thereby increasing the energy density of the battery.

[0038] The results of the cycle stability test of LFP electrodes with different thicknesses (130 μm, 170 μm, 260 μm) are shown in Figure 6 Compared with unstructured electrodes, laser-structured LFP electrodes also show superior cycle performance. At a rate of 0.2C, all thick electrodes with unstructured electrodes can only maintain 40-60 cycles, while the cycle of laser-structured electrodes can be as high as 150. The inter-powder gaps of traditional electrodes are prone to be blocked due to the negative reaction of materials and electrolyte in long-term cycling. The low-loss low-tortuosity pore structure of laser treatment is beneficial to the uniformization of lithium ion flow and the inhibition of lithium dendrite growth, thereby significantly improving the cycle life.

[0039] The electrode material selected in this embodiment is LFP electrode material. The method of constructing ultra-low-loss through-holes on thick electrodes by time / frequency shaping laser can also be applied to the pore manufacturing of other electrode materials, such as ternary electrodes (NMC811).

[0040] The above specific description further details the purpose, technical solutions and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the invention and is not intended to limit the protection scope of the invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the invention shall be included in the protection scope of the invention.

Claims

1. A method for constructing ultra-low loss vias on thick electrodes using time / frequency shaping lasers, characterized in that, By using a birefringent crystal, BBO, and a high-magnification objective lens, the time and frequency of a traditional Gaussian laser is shaped into a short-wavelength laser with a certain delay and depth of focus. Harmonic beam splitting is used to improve the optical path of the BBO combined with a filter for energy protection. A limiting aperture is used to ensure the use of high single-pulse energy laser while reducing the spot diameter, so as to realize the fabrication of an ultra-low loss and low tortuosity pore structure on a thick electrode, with the loss controlled within 1%. In the aforementioned time / frequency shaping femtosecond laser system, the BBO converts an 800nm ​​femtosecond laser into a mixed laser of 800nm ​​and 400nm. The harmonic beam splitter reflects the 400nm femtosecond laser onto a birefringent crystal, transforming it into a time-domain shaped femtosecond laser with a certain delay. After passing through a limited-size aperture and entering a dichroic mirror, it is focused onto the electrode sample located on a precision displacement stage by a high-magnification objective lens. The white light source, through the beam splitter, dichroic mirror, and CCD dynamic imaging unit, jointly realizes real-time imaging and monitoring of laser drilling on the electrode surface. The birefringent crystal shapes the pulsed laser into a laser pulse with a delay of less than 10 ps; harmonic beam splitting simultaneously reflects 400 nm laser and transmits 800 nm laser; the aperture size is controlled between 2 mm and 3 mm.

2. The method for constructing ultra-low loss vias on thick electrodes using time / frequency shaping lasers as described in claim 1, characterized in that, The ultra-low loss, low tortuosity pore structure with loss controlled within 1% on thick electrodes is specifically constructed by through holes with a diameter of 5-6 μm and a hole spacing of 60 μm, and all pores are through holes.

3. The method for constructing ultra-low loss vias on thick electrodes using time / frequency shaping lasers as described in claim 2, characterized in that... The thickness of the thick electrode is adjusted according to the actual energy density and power density requirements of the battery, ranging from 60μm to 260μm.

4. The method according to any one of claims 1 to 3, characterized in that, Includes the following steps: Step 1: Frequency doubling of the laser wavelength is achieved by changing the delay between pulsed lasers and the BBO through a birefringent crystal, combined with a high-magnification objective lens, harmonic beam splitter, and aperture limiter to build a time / frequency shaping femtosecond laser system; Step 2: Focus the time / frequency shaped femtosecond laser onto the surface of the thick electrode and perform multi-pulse impact drilling. After optimization, obtain an ultra-low loss, low tortuosity through-hole pore structure on thick electrodes of different thicknesses. Step 3: Assemble the battery in the following order: positive electrode shell - LFP positive electrode sheet - separator - negative electrode sheet - gasket - spring sheet - negative electrode shell. Add an appropriate amount of electrolyte during the process to assemble the button cell. Step 4: Assemble the untreated electrodes and the electrodes with the processed pore array into a battery, and perform rate testing and cycle stability testing using a Blue Electricity Tester.

Images