CN115275209B - High-first-efficiency silicon cathode with stable structure, preparation method and lithium ion battery - Google Patents
High-first-efficiency silicon cathode with stable structure, preparation method and lithium ion battery Download PDFInfo
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Abstract
The invention discloses a high-first-efficiency silicon cathode with a stable structure, a preparation method and a lithium ion battery, and relates to the technical field of lithium battery cathode materials, wherein the silicon cathode sequentially comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside, the thickness of the inner undoped silicon core is 0.1nm to 3nm, the gradient double-doped layer consists of a gradient atom doped layer and a gradient oxide doped layer, the thickness of the gradient double-doped layer is 70nm to 95nm, the co-embedded buffer layer consists of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, and the thickness of the co-embedded buffer layer is 3nm to 7nm. According to the invention, the gradient oxide doping layer slows down the stress concentration of the silicon cathode from inside to outside, and the co-embedded buffer layer is constructed by utilizing the reactions of molecular levels such as esterification and carboxylation in a synergistic manner, so that the structural stability of the silicon cathode is effectively improved.
Description
Technical Field
The invention relates to the technical field of lithium battery cathode materials, in particular to a high-first-efficiency silicon cathode with a stable structure, a preparation method and a lithium ion battery.
Background
The silicon-based negative electrode (4200 mAh/g) has high specific capacity and is considered to be the most promising graphite negative electrode substitute material. However, in the practical development and application process, the silicon-based negative electrode also has the following problems: firstly, the efficiency is low for the first time, and the reason is that the surface of a silicon-based negative electrode is active and reacts violently with electrolyte, excessive lithium ions are consumed, and a thick SEI film is formed on the surface; meanwhile, the silicon-based negative electrode has a low lithium ion diffusion rate, the inner layer is limited in lithium ion extraction (the closer to the core, the more difficult the lithium ion extraction), once the silicon-based negative electrode is charged and discharged at a high rate or under a large current, the defect is amplified, and the first efficiency of the silicon-based negative electrode is further reduced. Secondly, the silicon-based negative electrode can generate obvious volume expansion during charging and discharging, and the stress release from inside to outside can cause the silicon-based negative electrode to generate microcracks, damage the body structure, greatly reduce the stability of the silicon-based negative electrode and cause the rapid reduction of the electrochemical performance; and the structural damage can generate a new surface, and can cause a series of adverse reactions such as repeated SEI film formation, excessive lithium ion consumption and the like.
In order to solve the above technical problems, the research field and the industrial field mainly improve the silicon-based negative electrode by means of carbon coating, pre-lithiation and the like. Patent application No. 201711094379.7 discloses a lithium ion battery composite negative electrode material and a preparation method thereof, wherein nano silicon with a silicon dioxide-coated surface is obtained by oxidizing the surface of nano silicon, and the nano silicon is mixed with a lithium source and is heated in an inert atmosphere to obtain a nano silicon material coated with lithium silicate. The patent application No. 201810873657.7 discloses a silicon-carbon negative electrode material, a lithium ion battery negative electrode and a lithium ion battery, wherein a multilevel silicon-carbon composite material is prepared through a silicon-carbon composite process for multiple times, so that the structural stability and the electrical conductivity of the silicon-carbon negative electrode are improved. Patent No. 201980003453.0 discloses a silicon-based negative electrode material for a secondary battery and a preparation method thereof, wherein the secondary battery comprises: the shell comprises a core, a first shell and a second shell, wherein the core is coated by the first shell, the second shell comprises a carbon film layer or a composite film layer formed by the carbon film layer and a conductive additive, and the first shell is coated by the second shell. However, the above-mentioned several structural designs for silicon-based anodes have the following two problems: (1) The technical problems that the silicon-based negative electrode is low in lithium ion diffusion rate are not solved by improving the silicon negative electrode, and only the surface is subjected to corresponding carbon coating or pre-lithiation treatment, so that the effect of improving the primary efficiency is very limited; (2) The carbon source coated with carbon and the pre-lithiated lithium source are both directly mixed with the silicon-based cathode, so that the carbon coating layer or the pre-lithiated layer is randomly distributed on the surface of the silicon-based cathode, the volume expansion of the silicon-based cathode cannot be effectively inhibited, and the structural stability cannot be effectively improved.
The patent of application number 202010345495.7 discloses a phosphorus-doped silicon-based lithium ion battery cathode material and a preparation method and application thereof, wherein the phosphorus-doped silicon-based cathode material is a powder material, and the powder conductance is 3.0S/cm-6.0S/cm; the phosphorus doped silicon-based negative electrode material comprises: 90-99.49 wt% of silicon-based powder material, 0.01-3 wt% of phosphorus-containing doping material doped in the silicon-based powder material and 0.5-7 wt% of soft carbon material. However, in the application, the conductivity of the silicon-based negative electrode can only be improved by using phosphorus doping, the diffusion rate of lithium ions in the inner layer is low, and the improvement effect cannot be achieved, and the first efficiency cannot be obviously improved; in addition, the only layer of flexible soft carbon on the surface cannot play a rigid protection role in the volume expansion of the silicon-based negative electrode. The patent of application number 202110976701.9 discloses a germanium-doped silicon-like cathode material, a preparation method and an application thereof, wherein the germanium-doped silicon-like cathode material is formed by connecting nano frameworks and has three-dimensional through pore channels, germanium is uniformly dispersed in the silicon frameworks, and particles formed by the silicon frameworks are about 1-10 μm. However, in the method, germanium oxide particles are directly mixed with silicon powder, so that the atom doping effect is poor, the distribution of doping atoms is random, the effect of improving the primary efficiency can be achieved only to a certain extent, and the improvement on the lithium ion difficult to be extracted from the inner layer is limited; the germanium oxide particles and the silicon powder are directly mixed, an oxide doped layer cannot be formed in the silicon powder, the stress release of the silicon cathode from inside to outside cannot be slowed down, and the structural stability of the silicon cathode cannot be improved.
Disclosure of Invention
The invention aims to overcome the defects of low initial efficiency, poor cycle performance and poor stability of a pole piece structure of the conventional silicon negative electrode, and provides a high-initial-efficiency silicon negative electrode with a stable structure and long cycle stability, a preparation method thereof and a lithium ion battery thereof.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the high-first-efficiency silicon cathode with a stable structure comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside in sequence, wherein the thickness of the inner undoped silicon core is 0.1nm to 3nm, the gradient double-doped layer is composed of a gradient atom doped layer and a gradient oxide doped layer, the thickness of the gradient double-doped layer is 70nm to 95nm, the co-embedded buffer layer is composed of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, and the thickness of the co-embedded buffer layer is 3nm to 7nm.
The further technical scheme is that the mass fraction of the gradient atom doping layer is 0.5-6%, the atom doping concentration of the gradient atom doping layer is in gradient distribution gradually decreasing from inside to outside, and the ratio of the atom doping concentration of the inner layer to the atom doping concentration of the outer layer is (4.5-9.5): 1.
wherein the inner layer refers to a position close to the undoped silicon core, namely a position close to the inner center of the particle; the outer layer refers to a position far away from the undoped silicon core, namely a position close to the surface of the particle.
The further technical scheme is that the mass fraction of the gradient oxide doping layer is 0.2% -4%, the oxide doping concentration of the gradient oxide doping layer is gradually and gradually distributed in a gradient manner from inside to outside, and the ratio of the outer layer oxide doping concentration to the inner layer oxide doping concentration is (8-15): 1.
wherein the inner layer refers to a position close to the undoped silicon core, namely a position close to the inner center of the particle; the outer layer refers to a position far away from the undoped silicon core, i.e., a position close to the particle surface.
The further technical scheme is that the rigid carboxylated pre-lithium layer consists of lithium silicate and lithium oxide.
The invention also provides a preparation method of the high-efficiency silicon cathode with the stable structure, which comprises the following steps:
(1) Preparing a gradient double-doped layer: dispersing a micron silicon raw material in a mixed solution of ethanol and water, adding organic acid salt, and performing ball milling to obtain nano silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding a carboxylation auxiliary agent into the product obtained in the step (1), and then performing ball milling, wherein partial carboxyl of the carboxylation auxiliary agent and the nano silicon perform esterification reaction, and partial carboxyl performs carboxylation on the nano silicon;
(3) And (3) complexing reaction: adding lithium salt into the product obtained in the step (2) and continuously ball-milling to realize complexation with lithium ions contained in the lithium salt;
(4) Preparation of a co-embedded buffer layer: and (4) baking and roasting the product obtained in the step (3) to obtain a final product.
All ball milling in the steps is carried out in plasma ball milling equipment.
The further technical scheme is that the mass ratio of the micron silicon raw material to the organic acid salt in the step (1) is (1.2 to 14): 1, ball milling time is 12h to 169h, and the organic acid salt is selected from any one or more of tin citrate, germanium tannin or tin tannin.
The citric acid tin is obtained by coprecipitation precipitation of citric acid and tin chloride or tin nitrate, the citric acid germanium is obtained by coprecipitation precipitation of citric acid and germanium chloride or germanium nitrate, the tannic germanium is obtained by coprecipitation precipitation of tannic acid and germanium chloride or germanium nitrate, and the tannic tin is obtained by coprecipitation precipitation of tannic acid and tin chloride or tin nitrate.
The further technical scheme is that the molar ratio of the carboxylation auxiliary agent to the organic acid salt in the step (2) is (2.5 to 4): 1, the carboxylation auxiliary agent is one or a mixture of citric acid and tannic acid, and the ball milling time is 5h to 7h.
The further technical scheme is that in the step (3), the ball milling time is 1h to 3h, and the molar ratio of the lithium salt to the micron silicon raw material is 1: (8-15), wherein the lithium salt is selected from one or more of lithium carbonate, lithium hydroxide or lithium chloride.
The further technical scheme is that the conditions of baking and roasting in the step (4) are that after the product obtained in the step (3) is baked in an oven at the temperature of 60-95 ℃ for 15-25h, the product is roasted at the temperature rise rate of 2-4.5 ℃/min for 3-5h in an inert atmosphere at the temperature of 680-750 ℃.
The invention relates to a negative pole piece of a lithium ion battery, which comprises a high-efficiency first-efficiency silicon negative pole with a stable structure.
A lithium ion battery comprises the negative pole piece.
The invention is further illustrated below: in the preparation process of the gradient double-doped layer, the citric acid tin, the citric acid germanium, the tannin germanium or the tannin tin have the characteristics of slightly solubility and hydrolysis in the ethanol/water mixed solution, the slightly solubility promotes the continuous ionization of metal ions and doping into the silicon cathode, and the ion concentration in the ethanol/water mixed solution is reduced due to the doping, so that the gradient atom doped layer is formed; the hydrolysis promotes the metal ions to form hydroxide and further convert the hydroxide into oxide, and because the doping amount in the silicon cathode is certain, a gradient oxide doping layer with the concentration distribution opposite to that of the gradient atom doping layer is formed. The formed gradient atom doped layer and the gradient oxide doped layer can both enlarge a lithium ion diffusion channel and improve the diffusion rate of lithium ions.
During esterification, carboxylation and complex reaction, partial carboxyl of a carboxylation auxiliary agent (citric acid contains carboxyl and tannin acid hydrolyzes to generate carboxyl) and nano silicon (a micron silicon raw material is ball-milled in an ethanol/water mixed solution, and a large amount of hydroxyl groups can be generated on the surface) are subjected to esterification reaction; carboxyl which does not undergo esterification reaction is uniformly adsorbed on the surface of the nano silicon, so that carboxylation is realized on the nano silicon; after carboxylation, the complex has certain complexing ability to lithium ions generated after lithium salt is dissolved, so that strong combination with the lithium ions is realized.
During the preparation process of the co-embedded buffer layer, moisture or carbon dioxide is generated by the carbonization of citric acid or tannin, a certain amount of mesopores are formed by the overflow from the inside, and the carbon material obtained by roasting also has an activation reaction (the carbon material reacts with the moisture or the carbon dioxide), so that the number of the mesopores is further increased; the lithium ions are converted during firing into a rigid carboxylated pre-lithium layer consisting of lithium silicate and lithium oxide.
Compared with the prior art, the invention has the following beneficial effects:
1. aiming at the technical problems that the lithium ion diffusion rate of the silicon-based negative electrode is low and the inner layer lithium ion extraction is limited (the lithium ion extraction is more difficult as the silicon-based negative electrode approaches to the inner core), the silicon-based negative electrode is improved by constructing a gradient atom doped layer, so that the lithium ion diffusion rate is greatly improved, especially the extraction capacity of the inner layer lithium ion which is difficult to extract is improved, and the primary efficiency of the silicon negative electrode is improved;
2. the method comprises the following steps of constructing a co-intercalation buffer layer uniformly coated on a gradient double-doped layer by utilizing molecular-level reactions such as esterification, carboxylation, complexation and the like, wherein a rigid carboxylation pre-lithium layer in the co-intercalation buffer layer can play a pre-lithiation role, so that the primary efficiency is further improved, and the flexible mesoporous carbon protects an interface, so that excessive lithium ions are prevented from being consumed due to the activity of the interface, and the stable exertion of the primary efficiency is further ensured; on the basis, the high first-time efficiency of the silicon cathode is realized by cooperating with the gradient atom doping layer;
3. in the process of charging and discharging the silicon cathode, stress generated by volume expansion is concentrated from inside to outside, in the gradient oxide doping layer, the concentration of the oxide doping layer closer to the outer layer is higher, the protection effect on the structural stability damaged by the volume expansion is more obvious, and therefore the concentration gradient distribution of the gradient oxide doping layer can just meet the protection of the silicon cathode structure; in addition, the co-embedded buffer layer with the functions of improving the first effect and protecting the interface is formed by molecular-level reaction, and the surface of the gradient double-doped layer is coated uniformly, so that the secondary protection effect on the structural stability of the silicon cathode can be realized.
Drawings
Fig. 1 is a first charge and discharge curve of a high first-efficiency silicon negative electrode having a stable structure according to example 1 of the present invention;
fig. 2 is a graph showing cycle performance of the high-first-efficiency silicon negative electrode having a stable structure according to example 1 of the present invention, and comparative examples 1 and 2.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in 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.
Example 1
The invention provides a high-efficiency first-effect silicon cathode with a stable structure and a preparation method thereof, wherein the silicon cathode sequentially comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside, the gradient double-doped layer consists of a gradient atom doped layer and a gradient oxide doped layer, the co-embedded buffer layer consists of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, the thickness of the inner undoped silicon core is 0.5nm to 2nm, the thickness of the gradient double-doped layer is 72nm to 88nm, and the thickness of the co-embedded buffer layer is 3nm to 4.5nm; the mass fraction of the gradient atom doping layer is 0.8-2.4%, and the ratio of the atom doping concentration of the inner layer to the atom doping concentration of the outer layer is (5.7-6.1): 1; the mass fraction of the gradient oxide doping layer is 0.3-0.75%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (9.2-11.1): 1.
the preparation method comprises the following steps:
(1) Preparing a gradient double-doped layer: dispersing 700g of micrometer silicon raw materials with the particle sizes of 1um to 3um in an ethanol/water mixed solution, adding 140g of tin citrate, and carrying out ball milling in plasma ball milling equipment for 12 hours to prepare nanometer silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding 187.9g of citric acid into the product obtained in the step (1), and then carrying out ball milling in a plasma ball milling device for 6 hours;
(3) And (3) complexing reaction: adding 117.8g of lithium chloride into the product obtained in the step (2), and continuing to perform ball milling in plasma ball milling equipment for 2 hours;
(4) Preparation of the co-embedded buffer layer: and (4) baking the product obtained in the step (3) in an oven at 70 ℃ for 17h, and then roasting at the heating rate of 3 ℃/min for 4h in an inert atmosphere at 720 ℃.
The high-efficiency first-effect silicon negative electrode with the stable structure is prepared by the embodiment, the thickness of the inner layer undoped silicon core is 0.5nm to 2nm, the thickness of the gradient double-doped layer is 72nm to 88nm, and the thickness of the co-embedded buffer layer is 3nm to 4.5nm; the mass fraction of the gradient atom doping layer is 0.8-2.4%, and the ratio of the atom doping concentration of the inner layer to the atom doping concentration of the outer layer is (5.7-6.1): 1; the mass fraction of the gradient oxide doping layer is 0.3-0.75%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (9.2-11.1): 1.
the electrochemical performance of the high-first-efficiency silicon cathode with a stable structure obtained in the embodiment is tested by using a CR2032 half-cell, and the cell assembly and test procedures are as follows: according to the active material (high-efficiency silicon cathode with stable structure): polyacrylic acid: the mass ratio of the conductive carbon black is 7.5 6 The separator is a polyethylene porous membrane. For the test, the first turn was performed at 0.1C, and the magnification condition from the second turn was 0.5C.
Fig. 1 is a first charge-discharge curve of a high-first-efficiency silicon negative electrode with a stable structure in embodiment 1 of the present invention, where the first charge-discharge specific capacity is 2699.9mAh/g, the first charge specific capacity is 2502.8mAh/g, and the first efficiency is 92.7%.
Fig. 2 is a cycle performance graph of a high-first-efficiency silicon negative electrode having a stable structure according to example 1 of the present invention and comparative examples 1 and 2, in which the capacity retention rate of 200 cycles of example 1 is 96.3%, and the capacity retention rates of comparative examples 1 and 2 are 73.3% and 74.2%, respectively.
Other performance indexes of the high-first-efficiency silicon negative electrode having a stable structure obtained in example 1 are shown in table 1.
Example 2
The invention provides a high-efficiency first-effect silicon negative electrode with a stable structure and a preparation method thereof, wherein the silicon negative electrode sequentially comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside, the gradient double-doped layer consists of a gradient atom doped layer and a gradient oxide doped layer, the co-embedded buffer layer consists of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, the thickness of the inner undoped silicon core is 0.1nm to 2nm, the thickness of the gradient double-doped layer is 70nm to 87nm, and the thickness of the co-embedded buffer layer is 4nm to 5.8nm; the mass fraction of the gradient atom doping layer is 0.5% -0.9%, and the ratio of the inner layer atom doping concentration to the outer layer atom doping concentration is (7.3% -9.5): 1; the mass fraction of the gradient oxide doping layer is 0.2% -0.4%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (10.3% -12.7): 1.
the preparation method comprises the following steps:
(1) Preparing a gradient double-doped layer: dispersing 600g of micrometer silicon raw materials with the particle sizes of 1um to 4um in an ethanol/water mixed solution, adding 42.9g of germanium citrate, and performing ball milling in plasma ball milling equipment for 13 hours to prepare nano silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding 756.3g of tannic acid into the product obtained in the step (1), and then carrying out ball milling for 5 hours in a plasma ball milling device;
(3) And (3) complexing reaction: adding 64.2g of lithium hydroxide into the product obtained in the step (2), and continuously carrying out ball milling in plasma ball milling equipment for 2.5 hours;
(4) Preparation of a co-embedded buffer layer: and (4) baking the product obtained in the step (3) in a baking oven at 65 ℃ for 15h, and then roasting at the heating rate of 3.5 ℃/min for 3.5h in an inert atmosphere at 700 ℃.
The high-efficiency first-efficiency silicon negative electrode with the stable structure is prepared by the embodiment, the thickness of the inner layer undoped silicon core is 0.1nm to 2nm, the thickness of the gradient double-doped layer is 70nm to 87nm, and the thickness of the co-embedded buffer layer is 4nm to 5.8nm; the mass fraction of the gradient atom doping layer is 0.5% -0.9%, and the ratio of the inner layer atom doping concentration to the outer layer atom doping concentration is (7.3% -9.5): 1; the mass fraction of the gradient oxide doping layer is 0.2-0.4%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (10.3-12.7): 1.
the battery assembly and test procedure of the high-first-efficiency silicon negative electrode with the stable structure obtained in example 2 is the same as that of example 1, and the performance index of the high-first-efficiency silicon negative electrode with the stable structure obtained in example 2 is shown in table 1.
Example 3
The invention provides a high-initial-efficiency silicon cathode with a stable structure and a preparation method thereof, wherein the silicon cathode sequentially comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside, the gradient double-doped layer consists of a gradient atom doped layer and a gradient oxide doped layer, the co-embedded buffer layer consists of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, the thickness of the inner undoped silicon core is 0.8nm to 3nm, the thickness of the gradient double-doped layer is 79nm to 95nm, and the thickness of the co-embedded buffer layer is 3nm to 6.5nm; the mass fraction of the gradient atom doping layer is 3.2-6%, and the ratio of the atom doping concentration of the inner layer to the atom doping concentration of the outer layer is (4.8-5.4): 1; the mass fraction of the gradient oxide doping layer is 2.7-4%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (13.2-15): 1.
the preparation method comprises the following steps:
(1) Preparing a gradient double-doped layer: dispersing 650g of micrometer silicon raw material with the particle size of 2um to 5um in an ethanol/water mixed solution, adding 541.7g of tannin germanium, and performing ball milling in plasma ball milling equipment for 16 hours to prepare nanometer silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding 140.9g of citric acid into the product obtained in the step (1), and then carrying out ball milling for 7 hours in plasma ball milling equipment;
(3) And (3) complexing reaction: adding 114.4g of lithium carbonate into the product obtained in the step (2), and continuing to perform ball milling in plasma ball milling equipment for 3 hours;
(4) Preparation of a co-embedded buffer layer: and (4) baking the product obtained in the step (3) in an oven at 60 ℃ for 18h, and then roasting the product at the temperature rise rate of 2 ℃/min for 3h in an inert atmosphere at 680 ℃.
The high-efficiency first-effect silicon negative electrode with the stable structure is prepared by the embodiment, the thickness of an inner layer undoped silicon core is 0.8nm to 3nm, the thickness of a gradient double-doped layer is 79nm to 95nm, and the thickness of a co-embedded buffer layer is 3nm to 6.5nm; the mass fraction of the gradient atom doping layer is 3.2-6%, and the ratio of the atom doping concentration of the inner layer to the atom doping concentration of the outer layer is (4.8-5.4): 1; the mass fraction of the gradient oxide doping layer is 2.7-4%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (13.2-15): 1.
the battery assembly and test procedure of the first-efficiency silicon negative electrode with a stable structure obtained in example 3 are the same as those of example 1, and the performance index of the first-efficiency silicon negative electrode with a stable structure obtained in example 3 is shown in table 1.
Example 4
The invention provides a high-first-efficiency silicon cathode with a stable structure and a preparation method thereof, wherein the silicon cathode sequentially comprises an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside, the gradient double-doped layer consists of a gradient atom doped layer and a gradient oxide doped layer, the co-embedded buffer layer consists of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, the thickness of the inner undoped silicon core is 0.3nm to 2.4nm, the thickness of the gradient double-doped layer is 68nm to 89nm, and the thickness of the co-embedded buffer layer is 3.8nm to 7nm; the mass fraction of the gradient atom doping layer is 1.2% -2.7%, and the ratio of the inner layer atom doping concentration to the outer layer atom doping concentration is (4.5% -6.2): 1; the mass fraction of the gradient oxide doping layer is 0.4% -0.9%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (8% -9.9): 1.
the preparation method comprises the following steps:
(1) Preparing a gradient double-doped layer: 680g of micron silicon raw material with the particle size of 2um to 5um is dispersed in ethanol/water mixed solution, 75.6g of tannin tin is added, and the mixture is ball-milled for 15 hours in plasma ball-milling equipment to prepare nano silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding 265.2g of tannic acid into the product obtained in the step (1), and performing ball milling for 6 hours in plasma ball milling equipment;
(3) And (3) complexing reaction: adding lithium chloride into the product obtained in the step (2) according to the molar ratio of the micron silicon raw material to the lithium chloride of 12;
(4) Preparation of a co-embedded buffer layer: and (4) baking the product obtained in the step (3) in an oven at 95 ℃ for 25h, and then roasting at the heating rate of 4.5 ℃/min for 5h under the inert atmosphere at 750 ℃.
The high-first-efficiency silicon negative electrode with the stable structure is prepared through the embodiment, the thickness of an inner layer undoped silicon core is 0.3nm to 2.4nm, the thickness of a gradient double-doped layer is 68nm to 89nm, and the thickness of a co-embedded buffer layer is 3.8nm to 7nm; the mass fraction of the gradient atom doping layer is 1.2% -2.7%, and the ratio of the inner layer atom doping concentration to the outer layer atom doping concentration is (4.5% -6.2): 1; the mass fraction of the gradient oxide doping layer is 0.4% -0.9%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (8% -9.9): 1.
the battery assembly and test procedure of the first-efficiency silicon negative electrode with a stable structure obtained in example 4 are the same as those of example 1, and the performance index of the first-efficiency silicon negative electrode with a stable structure obtained in example 4 is shown in table 1.
Comparative example 1
The preparation method of the silicon negative electrode provided by the comparative example comprises the following steps:
(1) Dispersing 700g of micron silicon raw materials with the particle sizes of 1um to 3um in an ethanol/water mixed solution, and carrying out ball milling for 12 hours in plasma ball milling equipment to prepare an inner layer undoped silicon core;
(2) Esterification and carboxylation processes: adding 187.9g of citric acid into the product obtained in the step (1), and then ball-milling for 6 hours in a plasma ball-milling device;
(3) And (3) complexing reaction: adding 117.8g of lithium chloride into the product obtained in the step (2), and continuing to perform ball milling in plasma ball milling equipment for 2 hours;
(4) Preparation of a co-embedded buffer layer: and (4) baking the product obtained in the step (3) in an oven at 70 ℃ for 17h, and then roasting at the heating rate of 3 ℃/min for 4h in an inert atmosphere at 720 ℃.
The silicon negative electrode prepared by the comparative example 1 has the thickness of the inner layer undoped silicon core ranging from 72.5nm to 90nm and the thickness of the co-embedded buffer layer ranging from 3nm to 4.5nm.
The battery assembly and test procedure of the silicon negative electrode obtained in comparative example 1 were the same as in example 1, and the performance index of the silicon negative electrode obtained in comparative example 1 is shown in table 1.
Comparative example 2
The preparation method of the silicon negative electrode provided by the comparative example comprises the following steps:
(1) Preparing a gradient double-doped layer: dispersing 700g of micrometer silicon raw materials with the particle sizes of 1um to 3um in an ethanol/water mixed solution, adding 140g of tin citrate, and carrying out ball milling in plasma ball milling equipment for 20 hours to prepare nanometer silicon with a gradient double-doped layer;
(2) And (2) baking the product obtained in the step (1) in an oven at 70 ℃ for 17 hours, and then roasting at a heating rate of 3 ℃/min for 4 hours at 720 ℃ in an inert atmosphere.
The silicon cathode prepared by the comparative example 2 has the thickness of the inner layer undoped silicon core of 0.5nm to 2nm and the thickness of the gradient double-doped layer of 72nm to 88nm; the mass fraction of the gradient atom doping layer is 0.8% -2.4%, and the ratio of the inner layer atom doping concentration to the outer layer atom doping concentration is (5.7-6.1): 1; the mass fraction of the gradient oxide doping layer is 0.3% -0.75%, and the ratio of the doping concentration of the outer layer oxide to the doping concentration of the inner layer oxide is (9.2% -11.1): 1.
the battery assembly and test procedure of the silicon negative electrode obtained in comparative example 2 were the same as in example 1, and the performance index of the silicon negative electrode obtained in comparative example 2 is shown in table 1.
TABLE 1 comparison of Performance indices of examples and comparative examples
Although the present invention has been described herein with reference to the illustrated embodiments thereof, which are intended to be preferred embodiments of the present invention, it is to be understood that the invention is not limited thereto, and that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure.
Claims (9)
1. The high-first-efficiency silicon cathode with a stable structure is characterized by comprising an inner undoped silicon core, a gradient double-doped layer and a co-embedded buffer layer from inside to outside in sequence, wherein the thickness of the inner undoped silicon core is 0.1 to 3nm, the gradient double-doped layer is composed of a gradient atom doped layer and a gradient oxide doped layer, the thickness of the gradient double-doped layer is 70 to 95nm, the co-embedded buffer layer is composed of a rigid carboxylated pre-lithium layer and flexible mesoporous carbon, the thickness of the co-embedded buffer layer is 3 to 7nm, the mass fraction of the gradient atom doped layer is 0.5 to 6 percent, the atom doping concentration of the gradient atom doped layer is gradually and gradiently distributed from inside to outside, and the ratio of the inner atom doping concentration to the outer atom doping concentration is (4.5 to 9.5): 1, the mass fraction of the gradient oxide doping layer is 0.2% -4%, the oxide doping concentration of the gradient oxide doping layer is in gradually increasing gradient distribution from inside to outside, and the ratio of the outer layer oxide doping concentration to the inner layer oxide doping concentration is (8% -15): 1.
2. a high-first-efficiency silicon negative electrode with a stable structure according to claim 1, characterized in that the rigid carboxylated pre-lithium layer consists of lithium silicate and lithium oxide.
3. A preparation method of a high-efficiency silicon negative electrode with a stable structure is characterized by comprising the following steps:
(1) Preparing a gradient double-doped layer: dispersing a micron silicon raw material in a mixed solution of ethanol and water, adding organic acid salt, and performing ball milling for a certain time to obtain nano silicon with a gradient double-doped layer;
(2) Esterification and carboxylation processes: adding a carboxylation auxiliary agent into the product obtained in the step (1), and then performing ball milling for a certain time, wherein partial carboxyl of the carboxylation auxiliary agent and nano silicon perform esterification reaction, and partial carboxyl performs carboxylation on the nano silicon;
(3) And (3) complexing reaction: adding lithium salt into the product obtained in the step (2), and continuing ball milling for a certain time to realize complexation with lithium ions contained in the lithium salt;
(4) Preparation of the co-embedded buffer layer: and (4) baking and roasting the product obtained in the step (3) to obtain a final product.
4. The method for preparing the high-first-efficiency silicon negative electrode with the stable structure according to claim 3, wherein the mass ratio of the micrometer silicon raw material to the organic acid salt in the step (1) is (1.2 to 14): 1, ball milling time is 12 to 169h, and the organic acid salt is selected from any one or more of tin citrate, germanium tannin or tin tannin.
5. The method for preparing a high-efficiency first-pass silicon negative electrode with a stable structure according to claim 3, wherein the molar ratio of the carboxylation auxiliary agent to the organic acid salt in the step (2) is (2.5 to 4): 1, the carboxylation auxiliary agent is one or a mixture of citric acid and tannic acid, and the ball milling time is 5 to 7h.
6. The preparation method of the high-first-efficiency silicon negative electrode with the stable structure according to claim 3, wherein the ball milling time in the step (3) is 1 to 3 hours, and the molar ratio of the lithium salt to the micron silicon raw material is 1: (8-15), wherein the lithium salt is selected from one or more of lithium carbonate, lithium hydroxide and lithium chloride.
7. The method for preparing a high-first-efficiency silicon negative electrode with a stable structure according to claim 3, wherein the conditions for baking and roasting in the step (4) are that after a product obtained in the step (3) is baked in an oven at 60 to 95 ℃ for 15 to 25h, the product is roasted at a heating rate of 2 to 4.5 ℃/min for 3 to 5h in an inert atmosphere at 680 to 750 ℃.
8. A negative pole piece of a lithium ion battery is characterized by comprising the high-efficiency first-efficiency silicon negative pole with a stable structure according to any one of claims 1 to 2.
9. A lithium ion battery, characterized in that the lithium ion battery comprises the negative electrode tab of claim 8.
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