Disclosure of Invention
Aiming at the problem that the silicon monoxide coating process in the prior art mainly refers to a carbon coating process (adopting asphalt to soften, coat and carbonize) in artificial graphite processing, a single asphalt carbon coating layer is difficult to uniformly cover the surface of the silicon monoxide, and the interface stability of the material cannot be effectively improved; meanwhile, the introduction of the simple carbon material has no obvious improvement on the ionic conductivity of the material and the interface compatibility between the material and the electrolyte, and the invention provides a negative electrode material with a multiple coating structure, a preparation method and application thereof. The cathode material with the multiple coating structure improves the cycling stability of the silicon oxide, improves the electronic conductivity and the electrochemical reaction rate of the surface of the material, limits the volume expansion of the material, and can effectively avoid the cracking of the material. The multiple coating structure is a structure that a silicon oxide surface containing lithium is coated with three coating layers.
The invention aims to provide a negative electrode material with a multiple coating structure, which comprises lithium-containing silicon oxide, and a lithium salt coating layer, a carbon coating layer and a polymer coating layer which are sequentially coated on the surface of the silicon oxide.
According to the invention, the lithium salt coating layer is obtained by the in-situ reaction by utilizing the characteristic that the surface of the lithium-containing silicon oxide is alkaline, so that firstly, alkaline lithium on the surface of the material is reduced, the pH value of the negative electrode slurry is reduced, and the problems of binder failure and slurry bubbling caused by overhigh alkalinity of the slurry are solved; secondly, the in-situ inorganic lithium salt coating layer improves the ionic conductivity of the material surface and improves the characteristic of poor dynamics of the silicon oxide negative electrode material; thirdly, the in-situ lithium salt coating layer is similar to the reduction product of the electrolyte in the SEI film, and meanwhile, the mechanical strength of the inorganic lithium salt is high, so that the volume expansion of the material and the generation of lithium dendrites can be effectively inhibited, and the cycling stability of the electrode is improved; fourth, compared with the lithium salt aqueous solution coating method and the solid-phase lithium salt blending method in the prior art, the in-situ gas phase reaction method has the advantage that the inorganic lithium salt coating layer is more complete and compact.
According to the conductive carbon coating layer of the negative electrode material, the characteristic that the inorganic lithium salt coating layer has multiple active sites is utilized, a small molecular carbon source is easier to adsorb and decompose into a nano carbon layer, and compared with a solid-phase asphalt coating method, the conductive carbon coating layer has the characteristics that the coating layer is uniform and controllable, and the like, and simultaneously, alkaline groups on the surface of high-efficiency oxidized silica are further coated, so that the hydrolysis reaction of a compound in slurry is limited, the problem of overhigh alkalinity of the slurry is solved, and the existence of the conductive carbon layer improves the electronic conductivity of the surface of the material, so that the oxidation-reduction reaction rate.
The polymer coating layer of the cathode material disclosed by the invention utilizes the oxidation groups on the surface of the conductive carbon coating layer, so that a better coating effect can be realized, the compatibility between the silicon monoxide and the organic electrolyte is improved, the electrolyte can be used for wetting the surface of the material more quickly, and meanwhile, the polymer layer has elasticity after swelling, so that the volume expansion of the material is limited to a certain extent, the cracking of the material is avoided, and the cycling stability of the electrode is improved more effectively.
Fig. 1 is a schematic structural diagram of a negative electrode material with a multiple-coating structure provided by the present invention, wherein 1 is a lithium-containing silicon monoxide, 2 is a lithium salt coating layer, 3 is a carbon coating layer, and 4 is a polymer coating layer.
Preferably, the size of the lithium-containing silica is 1 to 8 μm, for example, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, or 7.5 μm.
Preferably, the thickness of the lithium salt coating layer is 0.1-20 nm, such as 0.5nm, 1nm, 2nm, 4nm, 5nm, 8nm, 10nm, 12nm, 14nm, 15nm, 16nm or 18 nm.
Preferably, the lithium salt in the lithium salt coating layer includes Li2CO3、Li2SO4、LiNO3Any one or a combination of at least two of LiF and LiCl.
Preferably, in the negative electrode material having a multiple coating structure, the lithium salt coating layer is contained in an amount of 0.01 wt% to 10 wt%, preferably 0.1 to 0.5 wt%, such as 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, or 8 wt%, etc.
The lithium salt coating layer of the invention has a content of 0.01 wt% -10 wt%, and the content is too low to effectively improve the ionic conductivity and the interface strength; the content is too high, and the interface resistance of the material is increased.
Preferably, the carbon coating layer has a thickness of 0.1 to 20nm, such as 0.5nm, 1nm, 2nm, 4nm, 5nm, 8nm, 10nm, 12nm, 14nm, 15nm, 16nm, 18nm, and the like.
Preferably, in the negative electrode material having a multiple coating structure, the content of the carbon coating layer is 0.01 wt% to 10 wt%, preferably 2 wt% to 4 wt%, such as 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, or 8 wt%, and the like.
The content of the carbon coating layer is 0.01 wt% -10 wt%, and the content is too low, so that the electronic conductivity of a material interface cannot be effectively improved; the content is too high, and the first coulombic efficiency of the negative electrode is reduced.
Preferably, the polymer coating layer has a thickness of 0.1 to 20nm, such as 0.5nm, 1nm, 2nm, 4nm, 5nm, 8nm, 10nm, 12nm, 14nm, 15nm, 16nm, 18nm, and the like.
Preferably, in the negative electrode material having a multiple coating structure, the content of the polymer coating layer is 0.01 wt% to 10 wt%, preferably 0.5 wt% to 2 wt%, such as 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, or 8 wt%, and the like.
The polymer coating layer of the invention has the content of 0.01-10 wt%, and the content is too low, so the wettability between the material and the electrolyte can not be effectively improved; too high a content increases the interface resistance.
The second purpose of the present invention is to provide a method for preparing the negative electrode material with the multiple coating structure according to the first purpose, which comprises the following steps:
(1) reacting lithium-containing silicon monoxide with acid gas under the heating condition to obtain silicon monoxide coated with a lithium salt coating layer on the surface;
(2) carrying out carbon coating reaction on the silicon oxide coated with the lithium salt coating layer on the surface in the step (1) and a gas carbon source to obtain a double-coated silicon oxide material;
(3) and (3) mixing the double-coated silica material obtained in the step (2) with polymer slurry to obtain the negative electrode material with the multiple-coated structure.
The invention adopts a multiple coating process to modify the surface of the lithium-containing silicon monoxide, controls the surface alkalinity of the material and improves the electrochemical performance. Firstly, utilizing the characteristic that high-first-efficiency silicon monoxide is alkaline, carrying out in-situ reaction with acid gas to obtain a lithium salt coating layer, constructing an artificial SEI (solid electrolyte interphase) film, improving the interface strength and the ionic conductivity, and consuming partial alkaline lithium; on the basis, the lithium salt coating layer is generated, so that more active sites appear on the surface of the lithium-containing high-first-efficiency oxidized sub-silicon, and when the carbon coating layer is constructed by adopting a gas phase method, the small molecular carbon source can be better attached to and decomposed on the surface of the oxidized sub-silicon and can be grown into a compact coating layer, so that the alkalinity is further reduced, and the electronic conductivity of the material is improved; and finally, the polymer layer is further coated by utilizing the oxidation group on the surface of the carbon coating layer, so that the interface compatibility between the material and the electrolyte is improved, and meanwhile, the elasticity of the polymer can accommodate the volume expansion of the silicon oxide to a certain extent, so that the improvement of the cycle stability is realized.
The inorganic lithium salt coating layer is prepared by utilizing the in-situ reaction of acid gas on the basis of the alkalinity of the surface of the lithium-containing oxidized silicon, and compared with a lithium salt aqueous solution coating method and a solid-phase lithium salt mixing method in the prior art, the inorganic lithium salt coating layer has the characteristics of uniformity, controllable thickness and the like; in the preparation process of the gas-phase conductive carbon coating, because of the existence of the in-situ lithium salt coating, a plurality of active sites are generated on the surface of the lithium-containing oxidized silicon, and small-molecule carbon source gas can be more easily adsorbed on the surface of the oxidized silicon to be decomposed to generate a nano carbon layer, so that the nano carbon layer has higher coating strength; in the preparation process of the polymer coating layer, the polymer and the carbon coating layer can be better combined by utilizing the surface oxidation group characteristic of the second conductive carbon coating layer, and in the prior art, the polymer is mostly directly contacted with the surface of the silicon oxide or inorganic lithium salt, and because the surface groups have self-difference, the interface compatibility is poor, and the strength of the polymer coating layer is poor.
Preferably, the process for preparing the lithium-containing silicon oxide of step (1) comprises: and carrying out heat treatment on the silicon monoxide precursor and a lithium-containing compound to obtain the lithium-containing silicon monoxide.
Different from the common silicon oxide material in the prior art, the lithium-containing silicon oxide has higher primary efficiency due to lithium pre-intercalation, but the aqueous slurry of the lithium-containing silicon oxide is alkaline, is more prone to side reaction and has poorer cycle performance, and better surface modification is needed to improve the stability.
Preferably, the structural formula of the silicon monoxide precursor is SiOx0.5 of<x<1.2, such as 0.6, 0.7, 0.8, 0.9, 1 or 1.1, etc.
Preferably, the lithium-containing compound includes any one of lithium metal, lithium sodium alloy, lithium magnesium alloy, lithium aluminum alloy, lithium oxide, lithium carbonate, lithium hydroxide, lithium sulfate, lithium nitrate, lithium fluoride, lithium phosphate, dilithium hydrogen phosphate, lithium chloride, lithium acetate, lithium hydride, lithium borohydride, lithium aluminum borohydride, lithium nitride, lithium amide, and lithium imide, or a combination of at least two thereof.
Preferably, the temperature of the heat treatment is 300 to 1200 ℃, for example 350 ℃, 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃, 650 ℃, 700 ℃, 750 ℃, 800 ℃, 850 ℃, 900 ℃, 950 ℃, 1000 ℃, 1050 ℃, 1100 ℃, 1150 ℃ or the like.
Preferably, the time of the heat treatment is 0.5-8 h, such as 1h, 2h, 3h, 4h, 5h, 6h or 7 h.
Preferably, the lithium-containing silica has a content of lithium element of 0.01 wt% to 20 wt%, preferably 2 wt% to 10 wt%, such as 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 18 wt%, or the like.
Preferably, the reaction is carried out under heating in the step (1) at a temperature of 200 to 800 ℃, for example, 250 ℃, 300 ℃, 350 ℃, 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃, 650 ℃, 700 ℃, or 750 ℃.
Preferably, the reaction time under the heating condition in the step (1) is 5-60 min, such as 10min, 15min, 20min, 25min, 30min, 35min, 40min, 45min, 50min or 55 min.
Preferably, the acid gas of step (1) comprises CO2、SO2、NO2、F2And Cl2Any one or a combination of at least two of them.
Preferably, the flow rate of the acid gas in step (1) is 10-100 sccm/min, such as 10sccm/min, 15sccm/min, 20sccm/min, 25sccm/min, 30sccm/min, 35sccm/min, 40sccm/min, 45sccm/min, 50sccm/min, 55sccm/min, 60sccm/min, 65sccm/min, 70sccm/min, 75sccm/min, 80sccm/min, 85sccm/min, 90sccm/min, or 95 sccm/min.
Preferably, the gaseous carbon source of step (2) comprises any one of natural gas, methane, ethane, propane, ethylene, propylene and acetylene or a combination of at least two thereof.
Preferably, the flow rate of the gaseous carbon source in step (2) is 30-100 sccm/min, such as 35sccm/min, 40sccm/min, 45sccm/min, 50sccm/min, 55sccm/min, 60sccm/min, 65sccm/min, 70sccm/min, 75sccm/min, 80sccm/min, 85sccm/min, 90sccm/min, or 95 sccm/min.
Preferably, the reaction time in step (2) is 0.5-3 h, such as 0.6h, 0.8h, 1h, 1.2h, 1.4h, 1.5h, 1.6h, 1.8h, 2h, 2.4h, 2.5h, 2.6h or 2.8 h.
Preferably, the double-coated silica material in the step (2) is prepared by a chemical vapor deposition method.
Preferably, the polymer in the polymer slurry in step (3) comprises any one of styrene-butadiene rubber, acrylonitrile multipolymer, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyacrylic acid, lithium polyacrylate and polyimide or the combination of at least two of the two.
Preferably, the mass ratio of the double-coated silica material in the step (3) to the polymer in the polymer slurry is (5-10): 1, such as 5.2:1, 5.5:1, 5.8:1, 6:1, 6.2:1, 6.5:1, 6.8:1, 7:1, 7.2:1, 7.5:1, 7.8:1, 8:1, 8.2:1, 8.4:1, or 9.8: 1.
Preferably, after the mixing in step (3), a spray drying and/or freeze drying process is further included.
As a preferred technical solution, the present invention provides a method for preparing a negative electrode material with a multiple coating structure, including the steps of:
(1) carrying out heat treatment on the silicon monoxide precursor and a lithium-containing compound at the temperature of 300-1200 ℃ for 0.5-8 h to obtain lithium-containing silicon monoxide with the lithium element content of 0.01-20 wt%;
(2) reacting the lithium-containing silicon monoxide with an acid gas at the temperature of 200-800 ℃ for 5-60 min, wherein the flow rate of the acid gas is 10-100 sccm/min, and thus obtaining silicon monoxide coated with a lithium salt coating layer on the surface;
(3) reacting the silicon monoxide coated with the lithium salt coating layer on the surface with a gas carbon source under the heating condition, wherein the flow rate of the gas carbon source is 30-100 sccm/min, the reaction time is 0.5-3 h, and preparing a double-coated silicon monoxide material by a chemical vapor deposition method;
(4) and mixing the double-coated silica material with polymer slurry, wherein the mass ratio of the double-coated silica material to the polymer in the polymer slurry is (5-10): 1, and performing spray drying to obtain the negative electrode material with the multiple-coating structure.
The invention also provides a negative pole piece, which comprises the negative pole material with the multiple coating structure.
The fourth purpose of the invention is to provide a lithium ion battery, which comprises the negative pole piece of the third purpose.
Compared with the prior art, the invention has the following beneficial effects:
(1) according to the invention, the characteristic that the surface of the lithium-containing silicon oxide is alkaline is utilized, and the lithium salt coating layer is obtained through in-situ reaction, so that the problems of binder failure and slurry bubbling caused by overhigh alkalinity of the slurry are solved; the ionic conductivity of the inorganic lithium salt is high, and the characteristic of poor dynamics of the silicon oxide negative electrode material is improved; the inorganic lithium salt coating layer is more complete and compact, the volume expansion of the material and the generation of lithium dendrites are effectively inhibited, and the cycling stability of the electrode is improved;
(2) the conductive carbon coating layer of the cathode material has the characteristics of uniformity, controllability and strong adhesive force, improves the electronic conductivity of the surface of the material, and improves the electrochemical reaction rate;
(3) the polymer coating layer improves the compatibility between the silicon monoxide and the organic electrolyte, limits the volume expansion of the material to a certain degree, avoids the cracking of the material and more effectively improves the cycling stability of the electrode;
(4) the invention adopts a multiple coating process to carry out surface modification on the lithium-containing high-first-efficiency silicon oxide, controls the surface alkalinity of the material, improves the electrochemical performance, improves the interface compatibility between the material and the electrolyte and realizes the improvement of the cycle stability.