CN114361409A

CN114361409A - Method for improving high-voltage long-cycle performance of layered positive electrode material at thick electrode

Info

Publication number: CN114361409A
Application number: CN202111473315.4A
Authority: CN
Inventors: 谢佳; 李书萍; 何仁杰; 张策; 韩智龙
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2021-12-02
Filing date: 2021-12-02
Publication date: 2022-04-15

Abstract

The invention belongs to the technical field of lithium batteries, and particularly relates to a method for improving the high-voltage long-cycle performance of a thick electrode of a layered positive electrode material. The method comprises the steps of mixing and coating, drying the solvent in the slurry, and enabling the final dried electrode component to keep the uniformity of the liquid-phase slurry and the orderliness of the solid-phase slurry through the liquid-solid-gas phase conversion process of the solvent, so as to form the thick electrode with low tortuosity and directionally arranged pore channels. On one hand, the conductive agent and the binder in the electrode with low tortuosity are distributed uniformly, and on the other hand, the construction of a lithium ion transmission pore channel is favorable for the sufficient infiltration of electrolyte to a pole piece and the rapid transmission of lithium ions, so that sufficient electron and lithium ion transmission networks are provided around each particle in the electrode, the uniformity of electrochemical reaction in a thick electrode is ensured, the cracking of the particles caused by the volume change of various special shapes is effectively avoided, and the long cycle stability is improved.

Description

Method for improving high-voltage long-cycle performance of layered positive electrode material at thick electrode

Technical Field

The invention belongs to the technical field of lithium batteries, and particularly relates to a method for improving high-voltage long-cycle performance of a thick electrode of a layered positive electrode material, in particular to a method for improving high-voltage long-cycle performance of the thick electrode of the layered positive electrode material through electrode architecture design.

Background

The lithium ion battery has the advantages of high specific energy, long cycle life, high working voltage, small self-discharge and no memory effect, and is widely applied to the fields of electric vehicles, energy storage systems and the like. Power batteries and 3C products place higher demands on battery endurance and fast charge capacity, and therefore it is necessary to construct battery systems with high energy density and high power density.

In lithium ion batteries, the positive electrode material is the most critical component, and is crucial to the electrochemical performance and cost of the battery. The layered positive electrode material has the theoretical specific capacity of 280mAh g-1 and high average discharge potential, and the energy density and the power density of the battery can be remarkably improved by constructing a thick electrode. However, in the case of a thick electrode, the overpotential conditions around the active particles are not uniform due to the difference in the electron and ion distribution states around the active material particles, resulting in high nonuniformity of the electrochemical reaction, cracking of the active material, and thus rapid degradation of electrochemical performance. Particularly under the condition of high voltage, the increase of reaction nonuniformity in the high delithiation state material is more likely to cause the generation of internal cracks, so that the cycling performance of the layered positive electrode material under the condition of thick electrode under the condition of high voltage is not ideal.

At present, modification of a layered positive electrode material is mainly focused on a material layer, and can be divided into three main categories: the interface stability of the layered material can be improved by the coating and the use of the film-forming additive; the high-pressure bulk phase stability of the layered anode material can be improved through the design of doping and material concentration gradient; the cycling stability of the layered positive electrode is obviously improved through the optimization of single crystal, polycrystal and microstructure. The improved layered cathode material can exhibit excellent electrochemical performance in the case of a thin electrode. However, in the case of a thick electrode, the electrochemical environment of the active material varies due to the disordered stacking of particles, and the problem of reaction nonuniformity becomes an important factor that causes particle cracking and performance degradation.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a method for improving the high-voltage long-circulation performance of a layered anode material under a thick voltage, which is characterized in that the structure of the thick electrode containing the layered anode material is designed, and smooth ion and electron conduction around active particles in the thick electrode is ensured, so that the reaction uniformity of the anode material in the thick electrode is improved, the long-circulation stability is improved, and the technical problems that the electrochemical performance of a thin electrode can only be improved, the high-voltage performance of the thick electrode is still poor and the like by improving the high-voltage long-circulation performance of the layered anode material at the material level in the prior art are solved.

In order to achieve the purpose, the invention provides a method for improving the high-voltage long cycle performance of a thick electrode of a layered anode material, which comprises the following steps:

(1) coating slurry containing a layered positive electrode material, a water-based binder, a conductive agent and a water-based solvent on a current collector to obtain a wet pole piece coated with the slurry;

(2) the wet pole piece coated with the slurry is frozen at low temperature, and then is gasified and dried at low temperature and low pressure, so that a water system solvent in the wet pole piece undergoes a conversion process from a liquid phase to a solid phase and then to a gas phase, the uniformity of the liquid phase slurry and the orderliness of the solid phase slurry are maintained by the components of the dried pole piece obtained after freezing and drying, a thick electrode with low tortuosity and directionally arranged pore channels is formed, the uniformity of electrochemical reaction in the electrode is improved, and the long cycle performance of the electrode under high voltage is improved.

Preferably, the chemical formula of the layered positive electrode material is lithium cobaltate or a ternary positive electrode material, and the ternary positive electrode material is LiNi_xCo_yMn_1-x-yO₂Wherein x is more than or equal to 0 and less than or equal to 1, and y is more than or equal to 0 and less than or equal to 1.

Preferably, the water-based binder is one or a mixture of more of sodium alginate, guar gum, polyacrylic acid, polyacrylonitrile, polyacrylate, styrene-butadiene rubber emulsion and carboxymethyl cellulose; further preferably one or a mixture of more of polyacrylic acid, guar gum and carboxymethyl cellulose.

Preferably, the conductive agent is selected from one or more of carbon fiber, activated carbon, carbon nanotube, graphene, and ketjen black.

Further preferably, the conductive agent contains carbon fibers, and further contains carbon nanotubes or ketjen black.

Preferably, the aqueous solvent is deionized water, ethanol or a mixture of water and ethanol, and more preferably water or a mixture of water and ethanol.

Preferably, the thickness loading of the slurry coating is 1-100mg cm^-2。

Preferably, the solid content of the slurry is 35 wt% -55 wt%, and the mass ratio of the layered positive electrode material, the water-based binder and the conductive agent in the slurry is 0.9 (0.01-0.05): (0.01-0.05).

Preferably, the low-temperature freezing is refrigerator refrigeration, liquid nitrogen refrigeration or freezer refrigeration, the freezing temperature is-120 ℃ to 0 ℃, and the freezing time is 2-24 hours; the mixture is gasified and dried at low temperature and low pressure, the vacuum degree is 0.1Pa-10Pa, and the temperature is lower than or equal to 0 ℃.

Preferably, the low-temperature freezing has the freezing temperature of-20 ℃ to-90 ℃ and the freezing time of 3 to 12 hours; the low-temperature low-pressure gasification drying is carried out, the vacuum degree is 0.1Pa-10Pa, and the temperature is lower than or equal to minus 30 ℃.

Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:

(1) the invention provides a method for improving the high-voltage long-cycle performance of a thick electrode of a layered positive electrode material, which belongs to a design preparation method of a thick electrode framework.

(2) Compared with the traditional thick electrode, the low-tortuosity thick electrode pole piece is constructed by the transformation of the liquid phase, the solid phase and the gas phase of the solvent in the slurry, on one hand, the conductive agent and the binder in the low-tortuosity electrode are distributed more uniformly, on the other hand, the construction of the lithium ion transmission pore channel is beneficial to the sufficient infiltration of the electrolyte to the pole piece and the rapid transmission of lithium ions, so that the good electrochemical environment, namely a sufficient electron and lithium ion transmission network, around each particle in the electrode can be effectively ensured, the uniformity of the electrochemical reaction in the thick electrode is ensured, the cracking of the particle caused by the volume change of each abnormal shape is effectively avoided, and the long-cycle stability is improved.

(3) According to the invention, the low-tortuosity thick electrode plate is constructed, and the directional transmission pore canal of lithium ions in the low-tortuosity thick electrode plate effectively accelerates the lithium ion transmission in the thick electrode, reduces the concentration polarization in the thick electrode, and thus the rate capability of the low-tortuosity thick electrode is obviously improved. The thick electrode has higher rate performance under the condition of high voltage, and the power performance of the thick electrode can be obviously improved, so that the design of the thick electrode can have high energy density and high power density.

(4) In the preferred embodiment of the invention, through screening the components such as the conductive agent in the slurry and matching with a freeze drying process, the transmission uniformity and stability of electrons and lithium ions around each electrode material particle in the thick electrode are further improved, the uniformity of electrochemical reaction is ensured, and the long-cycle stability under high voltage is improved. The method comprises the steps of preparation of slurry, drying process of the pole piece, analysis of the failed pole piece and the like, wherein the thick electrode with the up-and-down communicated pore channels is prepared through liquid-solid-gas phase conversion of a water system solvent in the slurry, and meanwhile, active particles in the prepared thick electrode have a good electrochemical environment through the long-range conductivity effect of carbon fibers, so that the reaction uniformity of the active particles in the thick electrode is ensured, and the long-cycle stability under high voltage is improved. The invention aims to solve the problems of electrode particle breakage and rapid cycle performance attenuation caused by the non-uniformity of electrochemical reaction of a layered positive electrode material under the conditions of a thick electrode and high voltage, and meanwhile, the lithium ions in the thick electrode with the up-and-down communicated pore channels are efficiently transmitted, so that the concentration polarization is reduced, the rate performance of the positive electrode is improved, and high power density is favorably obtained. Therefore, the method is beneficial to constructing high energy density and high power density, and simultaneously has long cycle stability. In a preferred embodiment, compared with a thick electrode prepared by a vacuum drying method in the prior art, the method disclosed by the invention has the advantages that the capacity retention rate of the thick electrode at the cut-off voltage of 4.3V for 150 circles is improved from 62.3% to 100%, and the capacity retention rate of the thick electrode at the cut-off voltage of 4.5V for 150 circles is improved from 48% to 72%.

Drawings

FIG. 1 is a schematic diagram illustrating the principle of thick electrode preparation in an embodiment of the present invention;

FIG. 2 is a scanned graph of the pole piece materials prepared in example 1 and comparative example 1 of the present invention;

FIG. 3 is a graph comparing the 4.5V cut-off voltage rate capability of thick electrodes prepared in example 1 and comparative example 1 of the present invention;

FIG. 4 is a graph comparing long cycle performance at 4.3V and 4.5V cut-off voltage for thick electrodes prepared in example 1 and comparative example 1 of the present invention;

fig. 5 is a cross-sectional comparison of the thick electrode prepared in inventive example 1 and comparative example 1 after 4.5V cycling and argon ion beam cutting.

FIG. 6 is a graph comparing the long cycle performance at 4.5V for thick electrodes prepared in example 1 and example 6;

fig. 7 is a photograph comparing the thick electrodes prepared by the method of comparative example 2 and example 1.

Fig. 8 is a graph comparing the long cycle performance at 4.5V for thick electrodes prepared in example 7 and example 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

At present, the layered positive electrode material, whether being a single crystal material or a polycrystalline material, has poor cycle performance under high voltage of a thick electrode. By observing the cross-sectional morphology of the electrode after argon ion beam cutting of the electrode after cycle failure, it was found that particle breakage, presumably grain cracking and intergranular cracks, was accompanied by an increase in electrochemical impedance of the failed cell and a loss of electronic contact of the active particles, thus leading to rapid degradation of cycle performance. In order to solve the above problems, the present invention provides a method for improving the high voltage performance of a thick electrode of a layered positive electrode material by an electrode architecture design, which comprises the following steps:

The preparation method is used for preparing the thick electrode of the layered positive electrode material, the electrode active substance in the step (1) is selected from lithium cobaltate or a ternary positive electrode material, and the ternary positive electrode material is LiNi_xCo_yMn_1-x-yO₂Wherein x is more than or equal to 0 and less than or equal to 1, and y is more than or equal to 0 and less than or equal to 1. Such as a ternary positive electrode material LiNi₅Co₃Mn₂O₂,LiNi₆Co₂Mn₂O₂,LiNi₇Co₁Mn₂O₂, LiNi₈Co₁Mn₁O₂And the like. The increase of the nickel content in the layered positive electrode material is accompanied by the increase of the specific capacity.

In some embodiments, the water-based binder in step (1) is selected from one or a mixture of several of sodium alginate, guar gum, polyacrylic acid, polyacrylonitrile, polyacrylate, styrene-butadiene rubber emulsion and carboxymethyl cellulose, and in preferred embodiments, the binder is a mixture of guar gum, carboxymethyl cellulose or polyacrylic acid, and these binders are stable at a high voltage of 4.5V.

In some embodiments, the conductive agent of step (1) is selected from one or more of carbon fiber, activated carbon, carbon nanotube, graphene, and ketjen black. In a preferred embodiment, the conductive agent contains carbon fibers and also contains carbon nanotubes or Ketjen black, wherein the carbon fibers can provide long-range conductivity, and the carbon nanotubes or Ketjen black can improve short-range conductivity, and together construct a good electron transport network in a thick electrode. The carbon fiber adopted by the invention can be selected from carbon fibers with the length of 10-200 microns, and preferably from carbon fibers with the length of 30-200 microns.

In some embodiments, the solvent in step (1) is water, ethanol or a mixture of water and ethanol, preferably water or a mixture of water and ethanol, wherein the freezing point of ethanol is too low to be harsh to the conditions of low-temperature solidification, and pure water or a mixture of water and ethanol is generally selected.

The preparation method has certain universality, and is suitable for preparing the electrode with common thickness and the thick electrode. In some embodiments, the step (2) coating thickness loading is 1-100mg cm^-2Preferably 20-40mg cm^-2. The solid content (mass percentage) of the slurry is 35-55%. In addition, in order to slow down the corrosion of the alkaline slurry of the water-based binder for the ternary material on the aluminum current collector, the mixed solvent of water and ethanol can weaken the influence of water on the ternary positive electrode. In some embodiments, the mass ratio of the layered positive electrode material, the aqueous binder and the conductive agent in the slurry is 0.9 (0.01-0.05) to (0.01-0.05).

The wet pole piece coated with the slurry is frozen at low temperature, then is gasified and dried at low temperature and low pressure, and is treated through two steps. Freezing and solidifying the pole piece in a low-temperature environment, wherein the freezing temperature is-120 ℃ to 0 ℃, the freezing time is 2-24 hours, and the low-temperature freezing can be refrigerator refrigeration, liquid nitrogen refrigeration or freezer refrigeration. In order to enable the solvent in the pole piece to be rapidly cured in an oriented manner and slow down the corrosion of the alkaline slurry of the water-based adhesive used for the ternary material to the aluminum current collector, the freezing temperature is preferably-20 ℃ to-90 ℃, more preferably-30 ℃ to-90 ℃, and the freezing time is preferably 3-12 hours; then the pole piece is gasified at low temperature and low pressure, and the lower the pressure is, the more beneficial the sublimation of the crystal water is. In some embodiments, the pressure is only 0.1Pa-10Pa, and in order to ensure sublimation of the curing solvent, the temperature is not higher than 0 deg.C, preferably not higher than-30 deg.C, and the temperature of the sample is not higher than the lowest eutectic point of the water and ethanol mixed solvent, so as to prevent the product from appearance defect and internal block defect.

After the thick electrode is prepared according to the method, the thick electrode is assembled into a battery and then is subjected to long cycle test under high voltage, and the method comprises the following steps:

(3) assembling the low-tortuosity thick electrode obtained in the step (2) into a battery, and representing the rate capability and the long cycle performance at the cut-off voltage of 4.5V; the assembled battery can be a battery for metal lithium and graphite, and the layered positive electrode material is used for firstly assembling a half battery for the metal lithium, wherein the metal lithium is far excessive and mainly represents the electrochemical performance of the prepared thick electrode; and then, matching the graphite cathode with a full battery, and characterizing the electrochemical performance of the thick electrode under the full battery. The assembled battery is subjected to electrochemical performance characterization, and the selected electrolyte additives are Vinylene Carbonate (VC), ethylene sulfate (DTD), 1, 3-propane sultone and lithium bis (oxalato) borate (LiBOB), wherein the vinylene carbonate and the lithium bis (oxalato) borate (LiBOB) are preferred.

(4) And (4) analyzing the electrode subjected to the cyclic failure in the step (3), cutting the electrode, observing the section of the electrode, and analyzing the cause of the failure of the electrode. The cutting technology of the electrode plate can be argon ion beam cutting, focused ion beam cutting, scissors shearing or brittle fracture after the electrode plate is frozen, wherein the argon ion beam cutting or the focused ion beam cutting is preferred.

The thick pole piece prepared by the invention is made into a sample, and the shape of the surface and the cross section is observed under a scanning electron microscope, so that the formation of the upper and lower communicated pore passages can be clearly seen.

As shown in fig. 1, it is a schematic diagram of the present invention for preparing a thick electrode with low tortuosity. Firstly, a layered positive electrode material, a water-based binder, a conductive agent and a water-based solvent are mixed to prepare slurry, then the slurry is coated on a current collector, and then low-temperature curing and low-temperature low-pressure drying are carried out to obtain a thick electrode with a pore passage communicated up and down, as shown in figure 2.

Generally, for nickel-cobalt-manganese ternary positive electrode materials, particularly when the nickel content is high, slurry is prepared by mixing an aqueous binder and an aqueous solvent, the positive electrode material is easy to hydrolyze into an alkaline substance, and the slurry is coated on an aluminum current collector to easily corrode the current collector. The invention adopts a freeze drying method or low-temperature low-pressure drying to prepare the electrode, and the water system binder and the solvent can be quickly frozen to avoid corrosion, so that the preparation of the electrode becomes possible. The solvent is frozen into a solid state through liquid state fast, and then is sublimated into a gaseous state, so that thick electrodes with up-and-down communicated pore channels can be formed, a uniform dispersion state is kept when the slurry is in a mixed state, a good electrochemical environment is formed around active substance particles, and the uniformity of electrochemical reaction of the particles is ensured.

The inventor speculates that the high-voltage long cycle performance of the thick electrode corresponding to the layered positive electrode material is rapidly attenuated due to the internal strain and stress of the material caused by non-uniform electrochemical reaction under high voltage through a large amount of experiments, and concludes that to improve the high-voltage long cycle performance of the thick electrode, not only a high-efficiency lithium ion transmission channel needs to be formed in the electrode material, but also a good conductive network needs to be required, and the electrode material particles are surrounded by the good electron and ion transmission network, in other words, the conductive agent, the positive electrode material particles and the electrolyte are in a uniformly dispersed state. Therefore, the invention provides a layered cathode material lithium cobaltate or a ternary cathode material adopted by the invention, which is matched with an aqueous binder and an aqueous solvent, and utilizes the characteristic of high freezing points of the aqueous binder and the aqueous solvent, the solvent is rapidly frozen into a solid state by a low-temperature freezing or low-temperature low-pressure drying method and then sublimated into a gaseous state, so that the uniformity of liquid-phase slurry and the orderliness of solid-phase slurry can be kept in the cathode material, a thick electrode with low tortuosity and directionally arranged pore channels is formed, the uniformity of electrochemical reaction in the electrode is improved, and the long cycle performance of the electrode under high voltage is improved.

Further, in the experimental process, different conductive agents are compared to find that when the conductive agent contains carbon fibers, the prepared electrode has excellent long-cycle performance under the same condition and high voltage compared with the conductive agent without the carbon fibers, and the possible reason is that the carbon fibers have long-range conductivity, the utilization rate of active substances can be improved, and higher specific capacity is exerted. The carbon fiber adopted in the following embodiment of the invention is the carbon fiber with the model number of M40JB-6000, which is produced by lion king chemical company Limited, the diameter is 7-8 microns, the length is 10-100 microns, and the electric conductivity is 27.8S/cm.

The following are specific examples:

example 1

Subjecting a single crystal LiNi₇Co₁Mn₂O₂The adhesive, the carboxymethyl cellulose, the conductive agent KB800 and the carbon fiber are dispersed in deionized water according to the mass ratio of 0.9:0.05:0.035:0.015 and are uniformly mixed in a Zhongyi dispersing machine to obtain mixed slurry, and the solid content of the slurry is 45 wt.%. Coating the prepared pole piece with the coating capacity of 20mg cm^-2. And (3) putting the wet pole piece into a low-temperature environment of-60 ℃, freezing for 3 hours, putting the pole piece into a low pressure environment of-10 ℃ and 5Pa, and drying for 12 hours to obtain the thick electrode with low tortuosity. And the rolled pole piece is used for the characterization of electrochemical properties.

2. Preparation of SEM samples

And respectively preparing the pole pieces before and after rolling into SEM samples, and respectively observing the appearances of the surfaces and the sections of the pole pieces.

3. Assembled battery and electrochemical performance testing

And (3) cutting the laminated thick electrode prepared in the step (2) into a circular sheet with the diameter of 12 mm, and assembling the circular sheet into a battery in a glove box in an argon atmosphere. The electrolyte is selected to be 0.9M LiDFOB and 0.6M LiBF4 dissolved in a mixed solvent of FEC and DEC in a volume ratio of 3:7, wherein the electrolyte additive is 2% VC. And carrying out rate performance test and long cycle test on the assembled battery.

4. Analyzing the failed battery

And (4) characterizing the electrochemically failed battery in the step (3), firstly testing the electrochemical impedance of the failed battery, then disassembling the battery in an argon-protected glove box, and observing the section of the pole piece after the anode material is subjected to argon ion beam cutting.

Example 2

The positive electrode material is polycrystalline LiNi₇Co₁Mn₂O₂The other conditions were the same as in example 1.

Example 3

When the battery was assembled using the thick electrode prepared, the electrolyte solution used was 0.9M LiDFOB and 0.6M LiBF4 dissolved in the mixed solvent of FEC and DEC at a volume ratio of 3:7, and the other conditions were the same as in example 1.

Example 4

For the analysis of the electrode sheet after failure, the cross section of the electrode sheet was observed in the manner of shearing the electrode sheet with scissors, and the other conditions were the same as in example 1.

Example 5

And (3) putting lithium cobaltate, binder carboxymethyl cellulose, KB800 and carbon fibers into a dispersion machine according to the proportion of 0.9:0.05:0.035:0.015, and uniformly mixing, wherein the solid content of the slurry is 45 wt.%. The loading of the prepared thick electrode is 20mg cm^-2. And (3) putting the wet pole piece into a low-temperature environment of-40 ℃, freezing for 3 hours, putting the pole piece into a low pressure environment of-10 ℃ and 5Pa, and drying for 12 hours to obtain the thick electrode with low tortuosity. The other conditions were the same as in example 1.

Example 6

Subjecting a single crystal LiNi₇Co₁Mn₂O₂The adhesive carboxymethyl cellulose, the conductive agent KB800 and the carbon fiber are dispersed in deionized water according to the proportion of 0.92:0.05:0.015:0.015 and are persevered in the deionized waterMixing in a dispersing machine. The rest is the same as in example 1.

Comparative example 1

Subjecting a single crystal LiNi₇Co₁Mn₂O₂The adhesive polyvinylidene fluoride and the conductive agent KB800 are dispersed in N-methyl pyrrolidone according to the proportion of 0.9:0.05:0.05 and are uniformly mixed in a Zhongyi dispersion machine. Coating the prepared pole piece with the coating capacity of 20mg cm^-2. And (3) preparing the thick electrode with high tortuosity by using a vacuum drying oven for the wet pole piece at 120 ℃. And the rolled pole piece is used for the characterization of electrochemical properties.

2. Preparation of SEM samples

3. Assembled battery and electrochemical performance testing

And (3) cutting the laminated thick electrode prepared in the step (2) into a circular sheet with the diameter of 12 mm, and assembling the circular sheet into a battery in a glove box in an argon atmosphere. The electrolyte is selected to be 0.9M LiDFOB and 0.6M LiBF4 dissolved in a mixed solvent of FEC and DEC in a volume ratio of 3:7, wherein the electrolyte additive is 2% VC. The assembled battery was tested for rate capability and long cycle, with performance in two voltage ranges, 3-4.3V and 3-4.5V, tested.

4. Analyzing the failed battery

FIG. 1 is a schematic diagram of the preparation of a thick electrode of a low tortuosity layered material in an example of the present invention, in which a solvent undergoes a liquid-solid-gas transition to prepare a thick electrode having a plurality of channels communicating from top to bottom. Due to the construction of the carbon fiber and the efficient lithium ion transmission channel, the uniformity of electrochemical reaction of the thick electrode under high pressure is ensured.

Fig. 2 is a schematic surface and cross-sectional view of two thick electrodes prepared in example 1 and comparative example 1, wherein, a and b in fig. 2 are the surface and cross-section of example 1 respectively, the a surface in fig. 2 is an alternate arrangement of active materials and lithium ion transmission channels, and the b cross-section in fig. 2 has the lithium ion transmission channels which are communicated up and down, so that lithium ions are transmitted efficiently, and uniform electrochemical reaction is obtained. The thick electrode of comparative example 1 is prepared by a common thermal drying method, and thus has the morphological characteristics of a typical porous electrode (fig. 2, content c and fig. 2, content d), the transmission of lithium ions is relatively hindered, the concentration polarization is large, and the electrochemical reaction is not uniform.

Fig. 3 is a graph comparing the rate capability of thick electrodes prepared in example 1 and comparative example 1. The thick electrode in example 1 has a highly efficient lithium ion transport channel, effectively reduces concentration polarization and electrochemical reaction overpotential, and thus exhibits excellent rate performance. The porous electrode in comparative example 1 has the characteristics of hindered lithium ion transmission, large concentration polarization, large reaction overpotential and poor rate performance.

FIG. 4 is a graph comparing long cycle performance at 4.3V and 4.5V for thick electrodes prepared in example 1 and comparative example 1, respectively. In embodiment 1, due to the long-range conductivity and the efficient lithium ion transmission channel of the carbon fiber, a good electrochemical environment is provided around the active material particles, the uniformity of the electrochemical reaction of the particles is ensured, and the particle breakage caused by the volume change of various shapes in the particles is avoided. In comparative example 1, the ionic transport or the electron transport around the active particles was hindered, resulting in non-uniformity of the electrochemical reaction, and the presence of strain and stress inside the material resulted in rapid deterioration of the electrochemical properties upon particle breakage. The embodiment 1 has better cycling stability under the voltage of 4.3V and the voltage of 4.5V, the capacity retention rate of 150 circles of the cut-off voltage of 4.3V is 100%, the capacity retention rate of 150 circles of the cut-off voltage of 4.5V is 72%, the initial capacity is higher than 4.3V under the specific capacity of 4.5V, the capacity retention rate of 150 circles of the cut-off voltage of 4.3V of the comparative example 1 is 62.3%, and the capacity retention rate of 150 circles of the cut-off voltage of 4.5V is 48%, so that the long cycling performance of the thick electrode of the embodiment 1 under the voltage of 4.3V and the voltage of 4.5V is obviously improved compared with that of the thick electrode of the comparative example 1.

The specific capacity of the general ternary cathode material of 4.5V is higher than that of the voltage range of 4.3V, but the specific capacity of the ternary cathode material is not as high as 4.3V because the comparative example 1 is a thick electrode and has the characteristics of a porous electrode, the concentration polarization is large, and the reaction is not uniform. Further, the capacity of comparative example 1 at the 4.3V cutoff voltage was slowly decreased as the long cycle proceeded, but when the cutoff voltage was 4.5V, a sharp decrease in capacity occurred at 130 cycles, probably due to crack generation.

Fig. 5 is a cross-sectional comparison diagram after cycles of preparing thick electrodes in example 1 and comparative example 1, and it can be seen that the active material of the low-tortuosity thick electrode layered cathode material in example 1 does not crack after long cycles, and maintains relatively high integrity, while the cathode particles of the high-tortuosity thick electrode in comparative example 1 crack, which causes rapid decay of electrochemical performance.

FIG. 6 is a comparison of long cycle performance between example 6 and example 1, showing that a slight increase in active material content, and a slight decrease in conductive agent content, results in a slight decrease in long cycle performance.

Comparative example 2

Subjecting a single crystal LiNi₇Co₁Mn₂O₂The adhesive carboxymethyl cellulose and the conductive agent KB800 are dispersed in deionized water according to the proportion of 0.9:0.05:0.05 and are uniformly mixed in a dispersing machine. Coating the prepared pole piece with the coating capacity of 20mg cm^-2. Other conditions were the same as in comparative example 1.

Fig. 7 is a photograph of the pole pieces prepared in example 1 and comparative example 2. The pole piece of example 1 is relatively complete, and in comparative example 2, in the process of hot drying of the pole piece, the ternary positive electrode corrodes the current collector aluminum due to the alkalinity in water, hydrogen is generated, and holes are formed. The water-based binder is not suitable for the layered positive electrode in the ordinary drying process, and can effectively inhibit the corrosion of the current collector only by low-temperature curing.

Example 7

Subjecting a single crystal LiNi₇Co₁Mn₂O₂The adhesive carboxymethyl cellulose and the conductive agent KB800 are dispersed in deionized water according to the proportion of 0.9:0.05:0.05 and are uniformly mixed in a dispersing machine. The rest is the same as in example 1.

FIG. 8 is a graph comparing the long cycle performance of thick electrodes prepared in example 7 and example 1. Example 7, in which no carbon fiber was added, exhibited a smaller specific capacity than the thick electrode of example 1, to which carbon fiber was added, and had a larger capacity fluctuation and a poor long cycle performance.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A method for improving the high-voltage long cycle performance of a thick electrode of a layered anode material is characterized by comprising the following steps:

2. The method of claim 1, wherein the layered positive electrode material has a chemical formula of lithium cobaltate or a ternary positive electrode material, and the ternary positive electrode material is LiNi_xCo_yMn_1-x-yO₂Wherein x is more than or equal to 0 and less than or equal to 1, and y is more than or equal to 0 and less than or equal to 1.

3. The method as claimed in claim 1, wherein the water-based binder is selected from one or a mixture of sodium alginate, guar gum, polyacrylic acid, polyacrylonitrile, polyacrylate, styrene-butadiene rubber emulsion and carboxymethyl cellulose; preferably one or more of polyacrylic acid, guar gum and carboxymethyl cellulose.

4. The method of claim 1, wherein the conductive agent is selected from one or more of carbon fiber, activated carbon, carbon nanotubes, graphene, and ketjen black.

5. The method of claim 1, wherein the conductive agent comprises carbon fibers, and further comprises carbon nanotubes or ketjen black.

6. The method according to claim 1, wherein the aqueous solvent is deionized water, ethanol or a mixture of water and ethanol, preferably water or a mixture of water and ethanol.

7. The method of claim 1, wherein the slurry coating has a thickness loading of 1 to 100mg cm^-2。

8. The method according to claim 1, wherein the slurry has a solid content of 35 wt% to 55 wt%, and the mass ratio of the layered positive electrode material, the aqueous binder, and the conductive agent in the slurry is 0.9 (0.01 to 0.05) to (0.01 to 0.05).

9. The method of claim 1, wherein the cryofreezing is refrigerator refrigeration, liquid nitrogen refrigeration, or freezer refrigeration, the freezing temperature is from-120 ℃ to 0 ℃, and the freezing time is from 2 to 24 hours; the mixture is gasified and dried at low temperature and low pressure, the vacuum degree is 0.1Pa-10Pa, and the temperature is lower than or equal to 0 ℃.

10. The method of claim 1, wherein said cryofreezing has a freezing temperature of-20 ℃ to-90 ℃ and a freezing time of 3 to 12 hours; the low-temperature low-pressure gasification drying is carried out, the vacuum degree is 0.1Pa-10Pa, and the temperature is lower than or equal to minus 30 ℃.