Disclosure of Invention
The invention provides a preparation method of a high-strength silicon alloy composite negative electrode material of a lithium ion battery, aiming at overcoming the problem of low mechanical strength in the prior art. The method can reduce the critical dimension of silicon and simultaneously achieve the purpose of enhancing the mechanical strength of the cathode material.
In order to achieve the purpose, the invention adopts the following technical scheme:
a preparation method of a high-strength lithium ion battery silicon alloy composite negative electrode material comprises the following steps:
(1) mixing alloy powder, silicon nano powder, a carbon material, polyacrylonitrile, pullulan gum and/or gelan gum to form mixed slurry;
(2) crushing the mixed slurry in the step (1);
(3) grinding the mixed solution obtained in the step (2), and adding N, N-dimethylformamide in the grinding process;
(4) then coating the slurry obtained by grinding in the step (3) on a copper foil;
(5) vacuum drying the copper foil coated in the step (4);
(6) carrying out thermal decomposition treatment on the electrode slice obtained in the step (5);
(7) and annealing to reduce the temperature of the electrode plate to room temperature.
Preferably, the mass percentages of the components in the negative electrode material are respectively as follows: 10-20% of alloy powder, 25-45% of silicon nano powder, 15-25% of carbon material, 15-25% of polyacrylonitrile, 5-10% of pullulan and/or gellan gum and 5-15% of N, N-dimethylformamide.
The fluid crushing treatment technology is added before grinding, so that the particle sizes of the alloy powder and the silicon nano powder are reduced, all components in the slurry are more fully mixed, and the subsequent grinding effect is ensured. The addition of Polyacrylonitrile (PAN) has three main functions: firstly, after thermal decomposition treatment, micron-sized channels can be formed in the pole piece, and the channels can play a role in buffering huge volume changes of silicon powder in the charging and discharging processes; polyacrylonitrile (PAN) and a carbon conductive additive can form delocalized electrons, so that the electronic conductivity of the negative pole piece is increased; and Polyacrylonitrile (PAN) can also be used as a silicon-based negative electrode binder of the lithium ion battery, and can form a strong hydrogen bond effect with the surface of silicon particles. The N, N-Dimethylformamide (DMF) is added in the grinding process, so that the alloy powder and silicon nano particles can be prevented from agglomerating, and the powder particles are more uniformly distributed in the solution. The adhesive force between particles, between conductive agents, between a coating and a copper foil current collector and between the conductive agents can be obviously increased after the pullulan and/or the Gurley adhesive are added and the subsequent process treatment is carried out.
Preferably, the alloy powder in the step (1) is ferrosilicon alloy powder.
The ferrosilicon alloy powder is added, and because silicon and iron form an alloy state, the volume change of pure silicon powder in the actual charging and discharging process can be alleviated, and the mechanical strength of the pole piece is enhanced. The ferrosilicon alloy has good conductivity and ductility, so that the conductivity of the silicon material can be improved, the structural damage of internal stress generated by volume expansion in the lithium insertion/lithium removal process to the material can be relieved, and the electrochemical performance of the silicon negative electrode material is improved.
Preferably, the carbon material in the step (1) is one or more of graphene, carbon black, carbon nanotubes or conductive graphite.
The carbon materials are all conductive agents which are commercialized and used on a large scale in the industry at present.
Preferably, the grinding beads used for grinding in step (3) are zirconium beads.
The natural abrasion of the chemical composition of the grinding beads in the grinding process has certain influence on the slurry performance, and the zirconium beads with large mass density and excellent abrasion resistance are used for the slurry grinding treatment in the invention.
Preferably, in the step (4), the copper foil has a thickness of 6 to 15 μm and a coating surface density of 30 to 100g/m2。
The copper foil thickness and the coating areal density are closely related to the battery energy density, and are within the range of values commonly used in the industry.
Preferably, the vacuum drying temperature in the step (5) is 60-90 ℃, and the drying time is 6-12 hours.
The surface of the pole piece is cracked due to the overhigh vacuum drying temperature, and the moisture content in the pole piece is overhigh due to the overlow drying temperature, so that the performance of the battery is influenced.
Preferably, the thermal decomposition temperature in the step (6) is 400 to 600 ℃.
Too high decomposition temperature can cause the pole piece component to generate oxidation reaction to cause failure, and too low temperature can cause insufficient PAN thermal decomposition, thereby achieving the expected effect.
Preferably, the temperature rising speed for reaching the thermal decomposition temperature in the step (6) is 5-15 ℃/min, and the constant temperature treatment time in the thermal decomposition process is 50-120 min.
Preferably, the annealing speed in the step (7) is 7-10 ℃/min.
Too fast annealing speed can lead to pole piece cracks, the microstructure distribution is not uniform, and too slow annealing speed can lead to too long processing time and increase production cost.
Therefore, the invention has the following beneficial effects: (1) compared with the traditional silicon cathode electrode, the prepared electrode structure has higher mechanical strength due to the adoption of a novel fluid grinding technology; (2) meanwhile, the novel mixed binder is adopted, so that the rapid reduction of the battery performance caused by the volume change of silicon can be effectively relieved, and a new reference way is provided for the preparation of the electrode with low cost and excellent electrochemical performance; (3) the Polyacrylonitrile (PAN) and graphene conductive agents formed by pyrolysis can be uniformly distributed on the surface of the nano silicon particles, and graphene around the silicon particles has high mechanical elasticity and is beneficial to adapting to obvious volume change.
Detailed Description
The invention is further described with reference to specific embodiments.
A preparation method of a high-strength lithium ion battery silicon alloy composite negative electrode material comprises the following steps:
(1) mixing ferrosilicon alloy powder, silicon nano powder, a carbon material, polyacrylonitrile, pullulan gum or gelan gum and N, N-dimethylformamide to form mixed slurry, wherein the negative electrode material comprises the following components in percentage by mass: 10-20% of ferrosilicon alloy powder, 25-45% of silicon nano powder, 15-25% of carbon material (one or more of graphene, carbon black, carbon nano tube or conductive graphite), 15-25% of polyacrylonitrile, 5-10% of pullulan gum or gelan gum and 5-15% of N, N-dimethylformamide.
(2) Crushing the mixed slurry in the step (1);
(3) grinding the solution obtained in the step (2), wherein grinding beads are zirconium beads, and N, N-dimethylformamide is added in the grinding process;
(4) then coating the slurry obtained by grinding in the step (3) on a copper foil, wherein the thickness of the copper foil is 6-15 mu m, and the coating surface density is 30-100 g/m2。
(5) And (3) drying the copper foil coated in the step (4) in vacuum at the drying temperature of 60-90 ℃ for 6-12 hours.
(6) And (5) carrying out thermal decomposition treatment on the electrode slice obtained in the step (5), wherein the thermal decomposition temperature is 400-600 ℃, the temperature rising speed is 5-15 ℃/min, and the constant temperature treatment time in the thermal decomposition process is 50-120 min.
(7) And annealing to cool the electrode plate to room temperature, wherein the annealing speed is 7-10 ℃/min.
Example 6
Comparative example 1 (comparative example 1, lowering the thermal decomposition temperature from 450 ℃ to 300 ℃)
Comparative example 2 (comparative example 1, silicon nanopowder instead of ferrosilicon powder.)
Comparative example 3 (comparative example 2, styrene-butadiene rubber instead of polyacrylonitrile.)
Comparative example 4 (comparative example 1, sodium carboxymethylcellulose substituted pullulan gum.)
Comparative example 5 (comparative example 3, the grinding beads used in the grinding process were steel balls.)
Comparative example 6 (comparative example 1, eliminating the high-speed emulsification Dispersion grinding step.)
Conclusion analysis: the mechanical property of the negative electrode material is mainly evaluated through the peel strength and the rebound rate of the pole piece, and the higher the peel strength of the pole piece is, the lower the rebound rate of the pole piece is, which indicates that the mechanical elastic property of the material is better.
As can be seen from the data of examples 1-6 and comparative examples 1-6, only the scheme within the scope of the claims of the present invention can satisfy the above performance requirements in all aspects, and the negative electrode material can have better mechanical properties. The change of the proportion, the replacement or addition and subtraction of the raw material components, or the change of the charging sequence and the change of the processing technological process or parameters can bring corresponding negative effects.
The raw materials and equipment used in the invention are all the raw materials and equipment which are commonly used in the field if not specifically stated, and the method used in the invention is all the conventional method in the field if not specifically stated.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and all simple modifications, alterations and equivalents of the above embodiments according to the technical spirit of the present invention are still within the protection scope of the technical solution of the present invention.