CN112117443A - Negative electrode for improving manganese deposition and lithium ion secondary battery thereof - Google Patents
Negative electrode for improving manganese deposition and lithium ion secondary battery thereof Download PDFInfo
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
- H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
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Abstract
The invention belongs to the technical field of batteries, and particularly relates to a negative electrode for improving manganese deposition and a lithium ion secondary battery thereof. The negative electrode includes: a negative current collector; an active material layer provided on the negative electrode current collector; and a functional coating layer disposed on the active material layer, the functional coating layer comprising a binder, inorganic particles, and a conductive agent. The functional coating is a porous coating with a certain porosity, so that manganese ions can be conducted and can be deposited and stored in the functional coating, preferably, the porosity of the functional coating is 15% -50%, preferably 20% -40%, the functional coating is uniform and stable, the negative electrode has good industrial production and manufacturing performance, powder falling can be avoided, the manganese deposition phenomenon is effectively avoided, and in addition, the cycle life and the dynamic performance of the lithium ion secondary battery using the negative electrode are improved.
Description
Technical Field
The invention belongs to the technical field of batteries, and particularly relates to a negative electrode for improving manganese deposition and a lithium ion secondary battery thereof.
Background
With the continuous maturity of the technology, the market demand for power batteries is increasing. At present, the types of the anode materials of the power battery are mainly ternary, lithium iron phosphate and lithium manganate by classifying the anode materials. The lithium iron phosphate material is relatively stable, does not have side reactions such as metal dissolution and the like in the using process, and is suitable for long-life design. However, when the ternary material and the lithium manganate material are used, manganese is dissolved out due to structural damage, and the manganese in the positive electrode is deposited on the negative electrode after being dissolved out, so that a side reaction occurs in the negative electrode, and the service life of the battery is rapidly reduced due to damage of the negative electrode.
At present, the improvement scheme aiming at the manganese deposition problem mainly focuses on the improvement of the positive electrode material and the electrolyte, such as coating or lattice site doping of the positive electrode material, addition of a special film-forming additive in the electrolyte and the like, and the improvement research result aiming at the negative electrode material is not found.
Disclosure of Invention
In view of the above problems in the background art, it is desirable to provide an anode for improving manganese deposition, which can not only maintain the stability of an SEI film, prevent manganese from being directly deposited on the SEI film on the surface of the anode, but also ensure good cycle life and battery performance of a lithium ion secondary battery prepared therefrom, and a lithium ion secondary battery using the same.
To achieve the above object, in a first aspect of the present invention, the inventors provide an anode comprising: a negative current collector; an active material layer provided on the negative electrode current collector; and a functional coating layer disposed on the active material layer, the functional coating layer comprising a binder, inorganic particles, and a conductive agent.
In a second aspect of the present invention, the inventors provide a lithium ion secondary battery comprising: the negative electrode according to the first aspect of the invention; an isolation film; an electrolyte; and a positive electrode containing a positive electrode active material containing a manganese element.
Different from the prior art, the technical scheme at least has the following technical effects:
the cathode pole piece with the uniform functional coating is arranged on the active material layer, so that the cathode pole piece has good industrial production and manufacturing performance, the cathode pole piece cannot fall off powder, and the phenomenon that manganese of the cathode is directly deposited on an SEI (solid electrolyte interphase) film of the cathode is effectively avoided. Effectively solves the problem of SEI film damage caused by manganese deposition on the surface of the negative pole piece, thereby effectively improving the cycle performance of the lithium ion battery.
Drawings
Fig. 1 is a schematic diagram of the structure and potential of the negative electrode active material layer and the functional coating layer according to the embodiment;
fig. 2 is a graph showing the effects of the functional coating according to the embodiment on the cycle of the experimental group and the comparative group of lithium ion secondary batteries.
Reference is made to the accompanying drawings in which:
1. a negative current collector;
2. an active material layer;
3. and (4) functional coating.
Detailed Description
The negative electrode of the first aspect of the invention and the lithium ion secondary battery of the second aspect of the invention are explained in detail below.
First, a negative electrode according to a first aspect of the present invention is explained, which includes: a negative current collector; an active material layer provided on the negative electrode current collector; and a functional coating layer disposed on the active material layer, the functional coating layer comprising a binder, inorganic particles, and a conductive agent.
The negative electrode current collector is generally a structure or part that collects current and may be any of a variety of materials suitable for use as a negative electrode current collector for an electrochemical energy storage device in the art, for example, the negative electrode current collector may include, but is not limited to, a metal foil, and more specifically, may include, but is not limited to, a copper foil. In the present invention, the negative electrode current collector may employ, but is not limited to, a copper foil commonly used for lithium ion secondary batteries.
The negative active material layer may be any of a variety of materials suitable for use in negative active materials for electrochemical energy storage devices in the art, and may be, for example, one or more combinations including, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesocarbon microbeads, silicon-based materials, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium. Wherein, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon-oxygen compound, silicon-carbon compound and silicon alloy; the tin-based material may be selected from elemental tin, tin-oxygen compounds, tin alloys, or combinations of one or more thereof.
In the present invention, the functional coating layer provided on the active material layer as a reaction site for manganese deposition comprises a binder including, but not limited to, polytetrafluoroethylene or SBR, which is an aqueous binder; the inorganic particles are selected from stable oxides such as aluminum oxide, magnesium oxide, calcium carbonate and the like and one or a mixture of more than two of carbonate and sulfate; the conductive agent is selected from one or a mixture of more than two of conductive carbon SuperP, acetylene black, conductive carbon black, carbon fiber (VGCF), Carbon Nano Tube (CNT) and Ketjen black, preferably the mixture of SuperP and acetylene black, the conductive performance of the mixture of SuperP and acetylene black as the conductive agent of the functional coating is superior to the conductive performance of a single conductive agent, because the SuperP and acetylene black are not in the same form, the acetylene black is dendritic, and the SuperP is grape-shaped, so that usually, the dendritic acetylene black can form a conductive network with a larger distribution range, but the conductive network formed by the dendritic acetylene black has gaps, the existence of the gaps enables electrons to be only linearly conducted through the conductive network, and the grape-shaped SuperP can be effectively filled in the gaps of the conductive network formed by the acetylene black, so that lithium ions are converted into planar conduction from the linear conduction of the conductive network, the conductivity of the functional coating is greatly improved, the internal resistance of the functional coating is reduced, and the dynamic performance of the negative pole piece is improved.
Further, the functional coating is a porous coating with a certain porosity, which can conduct lithium ions without affecting normal insertion and extraction of the lithium ions, and can also enable manganese ions extracted from the positive electrode to be deposited and stored in the functional coating (the manganese ions extracted from the positive electrode can damage a negative electrode SEI film, so that the functional coating mainly serves to provide a deposition site for the manganese ions), but in order to maintain normal operation of the lithium ion battery, the porosity of the functional coating needs to be ensured within a certain range, so that the lithium ions can normally pass through. Preferably, the functional coating has a porosity of 15% to 50%, preferably 20% to 40%, and it is noted that the ionic radius of manganese ions is higher than that of lithium ions, so that manganese ions can be physically blocked from passing through the functional coating to some extent by controlling the porosity, but the important influence factors for allowing manganese ions to be deposited on the functional coating are mainly the difference between the deposition potentials of manganese ions and lithium ions, the deposition potential of manganese ions is higher than that of lithium ions, and the potential of the functional coating can reach the deposition potential of manganese ions, so that manganese ions are preferentially deposited on the functional coating.
Furthermore, the thickness of the functional coating has certain influence on the performance of the lithium ion secondary battery, if the thickness of the functional coating is lower than 2 μm, the potential difference of the functional coating is small, and partial manganese is deposited on a graphite layer, so that the service life of the battery is influenced; if the thickness of the functional coating is more than 10 μm, the loss of energy density is serious, and the transmission of ions in the pole piece is influenced, and the electrical property of the battery is also influenced to a certain extent. Therefore, on one side of the negative electrode current collector, the thickness of the functional coating is 2-10 μm, preferably 2-8 μm, and for the comprehensive performance of the battery, the thickness of the functional coating needs to be controlled within a certain range, which is beneficial for the battery to have the advantages of good cycle performance and high energy density.
Further, if the inorganic content is too low (less than 80 wt%), the pores formed in the functional layer will be too low to facilitate the storage of manganese in the functional coating; if the content of the inorganic substance is too high (more than 95 wt%), the amount of the non-conductive substance is too large, which results in the breakage of the electron network of the functional coating and is not favorable for the functional coating to function. The adhesive used by the negative electrode functional coating can be an oil-based adhesive or a water-based adhesive, the content of the adhesive cannot be too low (lower than 2 wt%), and if the content of the adhesive is too low, the cohesion of the functional coating is insufficient, and the functional coating is subjected to powder removal; it should not be too high (higher than 7 wt%), and if the content of the binder is too high, the solid content of the slurry is low, and the processing manufacturability is affected. The content of the conductive agent is not too low or too high, and too low (less than 3 wt%) can affect the formation of a conductive network; if the content is too high (more than 10 wt%), the adhesive force of the functional coating is reduced, and the powder falling phenomenon is easy to occur. Thus, preferably, the weight percentage of inorganic particles is 75 wt% to 98 wt%, preferably 80 wt% to 95 wt%, based on the total weight of the functional coating; the weight percentage of the adhesive is 1 to 13 weight percent, preferably 2 to 10 weight percent; the weight percentage of the conductive agent is 1 wt% -12 wt%, and preferably 3 wt% -10 wt%.
Further, the inorganic particles are mainly used for forming pores of the functional coating, so that lithium ions are conveniently conducted among functional layers, and therefore, the inorganic particles with stable properties can meet the requirements.
Furthermore, the particle size of the inorganic particles has a certain influence on the uniformity of the functional coating, and the particle size of the inorganic particles is within a proper range, so that the influence on the performance of the lithium ion secondary battery due to more side reactions generated between the too small particle size and the electrolyte can be avoided, and the phenomenon that the too large particle size causes too large constructed gaps can be avoided, so that the transmission of manganese ions cannot be hindered, and the performance of the lithium ion secondary battery is influenced. Preferably, the inorganic particles have a particle size Dv50 of 0.01 μm to 2 μm, preferably 0.01 μm to 1.5 μm.
More preferably, the conductive agent is pure zero-dimensional, one-dimensional, two-dimensional conductive carbon or a composite of any two of the two conductive carbons, preferably a mixture of the Super P and the acetylene black, and the conductive performance of the mixture of the Super P and the acetylene black as the conductive agent of the functional coating is better than that of a single conductive agent because the Super P and the acetylene black are not in the same form, but the acetylene black is dendritic, and the Super P is grape-shaped, so that generally, the dendritic acetylene black can form a conductive network with a relatively large distribution range, but the conductive network formed by the dendritic acetylene black has more voids, and the voids exist to enable electrons to conduct only linearly through the conductive network, and the grape-shaped Super P can effectively conduct electricity to the voids of the conductive network formed by the acetylene black, so that the conductive network is changed from linear conduction to planar conduction, greatly improves the conductivity of the functional coating and is more beneficial to reducing the internal resistance of the functional coating.
Next, a lithium ion secondary battery according to a second aspect of the present invention is explained, comprising: the negative electrode according to the first aspect of the invention; an isolation film; an electrolyte; and a positive electrode containing a positive electrode active material containing a manganese element.
Further, the positive electrode active material contains one or more of lithium manganate, lithium nickel manganese oxide and lithium nickel cobalt manganese oxide.
Further, the positive electrode active material contains lithium manganate, and the mass content of the lithium manganate in the positive electrode active material is 10 wt% -100 wt%.
However, in the lithium ion deintercalation process, part of manganese can be dissolved out, the dissolved manganese can be electrically migrated to the negative electrode, and an SEI (solid electrolyte interface) film of the negative electrode is damaged, so that the cycle performance of the lithium ion secondary battery is reduced, therefore, the functional coating on the surface of the negative electrode is mainly used for solving the manganese dissolution problem of the positive electrode material, the content of the manganese contained in the positive electrode active material directly influences the manganese dissolution amount, and therefore, the content of the manganese in the positive electrode, the difficulty of manganese dissolution of different manganese-containing active materials, the influence of the functional coating on manganese deposition and other factors of the surface of a negative electrode sheet are comprehensively considered, the inventor finds that the function of the functional coating is optimal when the manganese-containing active material of the positive electrode active material is lithium manganate, and the manganese deposition speed of the functional coating is probably because the manganese dissolution speed of the lithium manganate material and the manganese deposition speed of the functional coating are higher than the manganese deposition speed of the lithium manganate material For the reason of the matching.
To explain technical contents, structural features, and objects and effects of the technical solutions in detail, the following detailed description is given with reference to the accompanying drawings in conjunction with the embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application.
The lithium ion secondary batteries of examples 1 to 18 and comparative examples 1 to 3 were each prepared as follows.
(1) Preparation of positive pole piece
Mixing a positive ternary material containing manganese, a conductive agent Super-P and KS-6 and an adhesive PVDF according to a mass ratio of 96:2:1:1, adding a solvent NMP, and stirring in a vacuum stirrer to obtain positive slurry; and uniformly coating the positive electrode slurry on two sides of the current collector aluminum foil, drying at room temperature, transferring to a 120 ℃ oven for drying, and then carrying out cold pressing and slitting to obtain the positive electrode piece.
(2) Preparation of negative pole piece
Mixing a negative electrode active material graphite, a conductive agent Super-P, a binder SBR and a thickening agent CMC according to a mass ratio of 96.4:1:1.1:1.5, adding solvent deionized water, and stirring in a vacuum stirrer to obtain negative electrode slurry; and uniformly coating the negative electrode slurry on two sides of the current collector copper foil, airing at room temperature, transferring to a 110 ℃ oven for drying, and performing a cold pressing process to obtain the negative electrode plate to be coated with the functional coating.
(3) Preparation of negative pole piece with functional coating
Mixing inorganic particles, a conductive agent and an acrylate emulsion according to the proportion shown in the table 1, adding solvent deionized water, and stirring in a vacuum stirrer to obtain functional coating slurry; and (3) uniformly coating the functional coating slurry on two sides of the negative pole piece prepared in the step (2), airing at room temperature, transferring to a 110 ℃ oven for drying, and then carrying out cold pressing and slitting to obtain the negative pole piece.
(4) Preparation of the electrolyte
Ethyl Methyl Carbonate (EMC) and Ethylene Carbonate (EC) were mixed in a volume ratio of 7:3 to obtain an organic solvent, and LiPF6, which was sufficiently dried, was dissolved in the organic solvent to prepare an electrolyte solution having a concentration of 1 mol/L.
(5) Preparation of the separator
And selecting a PE isolating film.
(6) Preparation of the Battery
Obtaining a bare cell by adopting a winding process for the positive pole piece, the isolating film and the negative pole piece with the functional coating; and (3) placing the bare cell into a shell, and obtaining the battery through the working procedures of Baking, electrolyte injection, vacuum packaging, standing, formation and the like.
In the negative electrode plate of the invention, the parameters of the functional coating can be tested according to the following method, and can also be tested according to other known methods in the field:
the porosity a of the functional coating can be obtained by calculation according to a formula (1), wherein the pore volume b of the pole piece can be obtained by testing a full-automatic true density tester AccuPyc II 1340, the apparent volume c of the pole piece can be obtained by calculation according to a formula (2), the thickness d of the pole piece and the thickness e of the base material can be obtained by measuring with a spiral ten-thousandth micrometer with the resolution ratio of 0.1 mu m, and the area f of the pole piece can be obtained by measuring with a straight ruler.
Porosity a ═ pole piece pore volume b/pole piece apparent volume c (1)
Apparent volume of pole piece c ═ (pole piece thickness d-base material thickness e) × (pole piece area f (2)
The thickness of the functional coating can be measured in two ways, one is: the thickness of the functional coating is the thickness of the negative pole piece with the functional coating-the thickness of the negative pole piece without the functional coating, and the thickness can be measured by using a spiral ten-thousandth ruler with the resolution of 0.1 mu m; and the second method is that the negative pole piece with the functional coating is cut off perpendicularly to the large surface by a plasma or liquid nitrogen brittle fracture method, the section of the pole piece is observed by a scanning electron microscope, and the thickness of the functional coating is measured by a scanning electron microscope ruler.
The particle size Dv50 of the functional coating was measured by a particle size distribution laser diffraction method (specifically, GB/T19077-2016) using a laser diffraction particle size distribution measuring instrument (Mastersizer 3000), and the average particle size was represented by a median Dv50 of volume distribution.
The lithium ion secondary battery dynamics and cycle performance tests of examples 1 to 18 and comparative examples 1 to 3 were performed in the following manner.
(1) And (3) testing the dynamic performance: the batteries prepared in examples and comparative examples were fully charged at 1C, fully activated at 1C for 5 times, and the last 1 discharge energy value was recorded, and the batteries were fully charged at 1C and then 3C discharged, and the 3C discharge energy retention rate was recorded (3C discharge energy retention rate ═ 3C discharge energy/1C discharge energy recorded for the last 1 activation) at 25 ℃.
(2) And (3) testing the cycle performance: the cells prepared in examples and comparative examples were charged at 1C, discharged at 1C, fully charged, and subjected to a cycling test at 25C until the capacity of the cell had decayed to 80% of the initial capacity and the number of cycles recorded.
The characteristics of each component of the functional coating in the negative electrodes provided in examples 1 to 18 and comparative examples 1 to 3 and the characteristics of the functional coating are shown in table 1.
Table 1 characteristics of each component of the functional coating in the negative electrodes and characteristics of the functional coating provided in examples 1 to 18 and comparative examples 1 to 3
The results of the cycle performance and kinetic performance tests of the lithium ion secondary batteries of examples 1 to 18 and comparative examples 1 to 3 are shown in Table 2.
TABLE 2 results of testing cycle characteristics and kinetic properties of lithium ion secondary batteries of examples 1 to 18 and comparative examples 1 to 3
As can be seen from tables 1 and 2 and fig. 1 and 2, in examples 1 to 18 of the present invention, the surfaces of the prepared negative electrode sheets are covered with a uniform functional coating, and lithium ions can freely pass through the functional coating during the charging and discharging processes of the full battery, and then are extracted and inserted into the active material layer, so as to achieve the charging and discharging effects. As can be seen from comparison of examples 1 and 2, as the content of the binder increases, the porosity of the inorganic solid electrolyte decreases, and the energy retention rate of the obtained lithium ion secondary battery decreases, and the amount of the binder increases, so that the slurry is liable to settle during the manufacturing process, and the processability and manufacturability are poor. Comparison of examples 16-17 shows that as the particle size of the inorganic particles increases, the porosity of the functional coating increases and the energy retention increases. Comparison between examples 13-15 and examples 16 and 18 shows that the increase of the thickness of the functional coating does not significantly change the porosity, but the increase of the thickness of the functional coating prolongs the transmission distance of lithium ions in the electrode plate, so that the energy retention rate of the battery is low, and the dynamic performance of the battery is significantly affected. From examples 10 to 11, it can be seen that the energy retention rate increases with the increase of the content of the conductive agent, i.e., the content.
In the comparative example 1, the negative pole piece is not coated with the functional coating, so that the cycle performance is poor; in the comparative example 2, the content of the conductive agent Super-P is too much, the cohesion between particles is poor after coating, the powder falling of a negative pole piece is serious, the coating procedure cannot be normally carried out, and the processing manufacturability is poor; the functional coating in the comparative example 3 does not contain a conductive agent Super-P, the conductive network of the functional layer is poor, the content of the adhesive is too much, the porosity of the coating is low, and the dynamic performance of the battery is greatly influenced.
Compared with the comparative example 1, in the examples 2 to 18, after the functional coating is coated, the cycle life of the battery is remarkably prolonged, and mainly, manganese ions are dissolved out from the positive electrode in the battery cycle process of the functional coating which is not coated provided by the comparative example 1, the manganese ions are dissociated to the negative electrode through the electrolyte and deposited on the surface of the negative electrode, the SEI film of the negative electrode is damaged, the lithium ion consumption and the electrolyte decomposition are accelerated, and meanwhile, the resistance of the negative electrode is increased, the internal resistance of the battery is increased, and the cycle life of the lithium ion secondary battery is.
In example 13, since the coating thickness is too thin, the functional coating has an obvious missing coating phenomenon, the protection effect on the negative electrode is weakened, and the cycle life is not improved as much as the thickness group provided by other examples.
It should be noted that, although the above embodiments have been described herein, the invention is not limited thereto. Therefore, based on the innovative concepts of the present invention, the technical solutions of the present invention can be directly or indirectly applied to other related technical fields by making changes and modifications to the embodiments described herein, or by using equivalent structures or equivalent processes performed in the content of the present specification and the attached drawings, which are included in the scope of the present invention.
Claims (10)
1. An anode, comprising:
a negative current collector;
an active material layer provided on the negative electrode current collector; and
a functional coating layer disposed on the active material layer, the functional coating layer comprising a binder, inorganic particles, and a conductive agent.
2. The negative electrode according to claim 1, characterized in that the functional coating is a porous coating with a porosity of 15-50%, preferably 20-40%.
3. The negative electrode according to claim 1, characterized in that the functional coating has a thickness of 2 μm to 10 μm, preferably 2 μm to 8 μm.
4. The negative electrode of claim 1, wherein the functional coating comprises, based on the total weight of the functional coating,
the weight percentage of the inorganic particles is 75-98 wt%, preferably 80-95 wt%;
the weight percentage of the adhesive is 1 to 13 weight percent, preferably 2 to 10 weight percent;
the weight percentage of the conductive agent is 1 wt% -12 wt%, and preferably 3 wt% -10 wt%.
5. The negative electrode of claim 4, wherein the inorganic particles are a mixture of one or more of oxides, carbonates, or sulfates of aluminum, magnesium, and calcium.
6. The anode according to claim 5, wherein the inorganic particles have a particle size Dv50 of 0.01 μm to 2 μm, preferably 0.01 μm to 1.5 μm.
7. The anode of claim 4, wherein the conductive agent is pure zero-dimensional, one-dimensional, two-dimensional conductive carbon, and any combination thereof.
8. A lithium ion secondary battery comprising:
the negative electrode of any one of claims 1 to 7;
an isolation film;
an electrolyte; and
a positive electrode containing a positive electrode active material, characterized in that the positive electrode active material contains a manganese element.
9. The lithium ion secondary battery of claim 8, wherein the positive electrode active material comprises one or more of lithium manganate, lithium nickel manganate, and lithium nickel cobalt manganate.
10. The lithium ion secondary battery according to claim 9, wherein the positive electrode active material contains lithium manganate, and a content of the lithium manganate in the positive electrode active material is 10 wt% to 100 wt%.
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