Patent Application
20180108936
The present application claims the benefit of Chinese Patent Application No. 201610900168.7 filed on Oct. 14, 2016, the contents of which are hereby incorporated by reference.
The present application generally relates to batteries and, more particularly, to a lithium iron phosphate power battery and a method for preparing the same.
With continuous improvement of green environment consciousness, electric vehicles have been increasingly popular in people's daily life. Lithium iron phosphate power batteries are core energy storage devices for electric vehicles. At present, the energy density of existing lithium iron phosphate battery material is low. Even a highly automated steel shell lithium iron phosphate battery has an energy density of less than 110 Wh/kg, which affects the endurance mileage of the electric vehicles.
In addition, the lithium iron phosphate itself has poor electric conductivity and slow lithium ion diffusion rate, which may lead to poor low temperature performance. The discharge capacity of an existing lithium iron phosphate battery at −20° C. is only 40% to 70% as that of the discharge capacity at normal temperature. Existing lithium iron phosphate battery even cannot be discharged or discharge capacity is close to 0% at −40° C., which seriously affect the promotion and operation of electric vehicles in extremely cold areas.
In view of the foregoing, what is needed, therefore, is to provide a novel lithium iron phosphate power battery and a method for preparing the same, so as to overcome the disadvantages as discussed above.
One object of the present application is to provide a lithium iron phosphate power battery having high energy density and desirable low temperature performances and a method for preparing the same.
According to one embodiment of the present application, a lithium iron phosphate power battery including:
a battery case;
a battery cell received in the battery case, the battery cell including:
an electrolyte filled in the battery case.
One embodiment of the present application provides a method for preparing a lithium iron phosphate power battery, including the steps of:
1) preparing a positive active material: adding lithium iron phosphate and positive conductive agent into a dry mixer, carrying out a first stirring; adding 30-60% of a positive glue solution and carrying out a second stirring, and scraping material; and adding residual glue solution and carrying out a third stirring, wherein the positive glue solution is obtained by adding an oily binder PVDF and an oily solvent NMP into a double planetary mixer and obtaining a mixture, and stirring and dispersing the mixture uniformly for 1-2 h;
2) preparing a negative active material: adding an aqueous binder and a deionized water into a double planetary mixer and dispersing, adding graphite powder into the double planetary mixer and stirring and scraping material, and adding a negative conductive agent and continuing stirring under vacuum;
3) preparing a positive plate and a negative plate: coating the prepared positive active material on the positive current collector and coating the prepared negative active material on the negative current collector respectively, and obtaining the positive plate and the negative plate after baking; and
4) assembling a battery: winding the positive plate, a separator and the negative plate and obtaining a battery cell, setting the battery cell in a battery case and injecting an electrolyte into the battery case, and obtaining a lithium iron phosphate power battery.
Compared with the prior art, the lithium iron phosphate power battery according to the present application has high energy density and excellent low temperature performances.
In order that the objects, technical solution and technical effects of the present invention can be understood more clearly, the present invention will be described in more detail with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are illustrative only and are not intended to limit the present invention.
Referring to
a battery case 10;
a battery cell 20 received in the battery case 10, the battery cell 20 including:
an electrolyte filled in the battery case 10.
Specifically, the positive active material includes a positive glue solution. The positive glue solution includes an oily binder and an oily solvent. The oily binder is HSV900, 5130 or equivalent polyvinylidene fluoride (PVDF) binder. The oily solvent is N-methylpyrrolidone (NMP). The solid content of the positive glue solution is 5-12%.
Specifically, the positive active material includes a positive conductive agent. The positive conductive agent is carbon nanotube (CNT) and/or graphene. A mass content of the positive conductive agent in the positive active material is 1-3%. The negative active material includes a negative conductive agent. The negative conductive agent is selected from a group consisting of conductive carbon black (SP), Ketjenblack (CB), carbon nanotube (CNT), graphene and carbon nanofiber (VGCF). The mass content of the negative conductive agent in the negative active material is 1-3%.
Specifically, the negative active material includes an aqueous binder and a deionized water. The aqueous binder is selected from a group consisting of water dispersion of acrylonitrile copolymer, polyacrylic acid, carboxymethylcellulose and styrene-butadiene rubber, and a mass content of the aqueous binder in the negative active material is 2.5-4%.
Specifically, the electrolyte includes a lithium salt, a solvent and an additive. The concentration of the lithium salt is 1-1.3 mol/L. The solvent is selected from two to four of propylene carbonate (PC), ethyl acetate (EA), propyl acetate (PA) and ethyl propionate (EP). The mass content of the solvent in the electrolyte is 70-85%. The additive is selected from a group consisting of fluorinated ethylene carbonate (FEC), propane sultone (PS), bistrifluoromethanesulfonimidate lithium (LiTFSi), vinylene carbonate (VC) and ethylene sulfate (DT). The mass content of the additive in the electrolyte is 0.5-5%.
In the lithium iron phosphate power battery according to the present application, the positive active material contains lithium iron phosphate coated with ultrafine carbon having a particle diameter of no more than 200 nm. The conductivity of the lithium iron phosphate material is improved. Conventional spherical particle conductive agent (conductive carbon black, Ketjenblack, etc.) is completely or partially replaced by linear or planar carbon nanotube or graphene. The linear or planar carbon nanotube or graphene are distributed on the surface of the positive active material particles and gaps between the positive active material particles, which can play a role as a “bridge”. The active material particles can be connected desirably, to provide a desirable path for the transmission of the lithium ions, reduce the resistance in the diffusion process, and increase the conductive capability of the positive active material under low temperature.
In the lithium iron phosphate power battery according to the present application, the negative active material contains graphite having desirable layer structure, crystal spacing, electrical conductivity and ion diffusion rate, which can improve the electrical conductivity and ion diffusion rate, reduce the impedance of lithium ion migration. By selecting a low temperature co-solvent with low melting point, low viscosity, high dielectric constant and high electrochemical stability, and by adjusting the proportion of each solvent component, the conductivity of the electrolyte at low temperature can be improved. Via selecting appropriate film forming additives, the SEI film formed can keep stable even be charged and discharged at low temperature, which can improve the charge and discharge performances at low temperature.
One embodiment of the present application provides a method for preparing a lithium iron phosphate power battery, including the steps of:
1) preparing a positive active material: adding lithium iron phosphate and positive conductive agent into a dry mixer and carrying out a first stirring; adding 30-60% of a positive glue solution, carrying out a second stirring, and scraping material; and adding residual glue solution and carrying out a third stirring, wherein the positive glue solution is obtained by adding an oily binder PVDF and an oily solvent NMP into a double planetary mixer and obtaining a mixture, and stirring and dispersing the mixture uniformly for 1-2 h;
2) preparing a negative active material: adding an aqueous binder and a deionized water into a double planetary mixer and dispersing; adding graphite powder into the double planetary mixer and stirring and scraping material; and adding a negative conductive agent and continuing stirring under vacuum;
3) preparing a positive plate and a negative plate: coating the prepared positive active material on the positive current collector and coating the prepared negative active material on the negative current collector respectively, and obtaining a positive plate and a negative plate after baking; and
4) assembling a battery: winding the positive plate, a separator and the negative plate and obtaining a battery cell, setting the battery cell in a battery case and injecting an electrolyte into the battery case, and obtaining a lithium iron phosphate power battery.
Specifically, in the first stirring of step 1), the revolution frequency of the dry mixer is 35-40 Hz, the autorotation speed is 6000-8000 rpm, and the stirring time is 0.5-2 h. In the second stirring of step 1), the revolution frequency of the dry mixer is 40-45 Hz, the autorotation speed is 4000-5000 rpm, and the stirring time is 10-30 min. In the third stirring of step 1), the revolution frequency of the dry mixer is 40-45 Hz, the autorotation speed is 4000-5000 rpm, and the stirring time is 2-3 h.
Specifically, the dispersing time of the double planetary mixer in step 2) is 0.5-1.5 h, the stirring time after adding the graphite powder is 10-30 min, the stirring time under vacuum is 0.5-1.5 h, and the viscosity of the negative active material obtained in step 2) is 500-2000 mPas.
Specifically, the positive conductive agent is carbon nanotubes (CNT) and/or graphene, and the mass content of the positive conductive agent in the positive active material is 1-3%. The negative conductive agent is selected from a group consisting of conductive carbon black (SP), Ketjenblack (CB), carbon nanotubes (CNT), graphene and carbon nanofiber (VGCF), and the mass content of the negative conductive agent in the negative active material is 1-3%.
The electrolyte contains a lithium salt, a solvent and an additive. The concentration of the lithium salt is 1-1.3 mol/L. The solvent is selected from two to four of ethyl acetate (EA), propylene carbonate (PC), propyl acetate (PA) and ethyl propionate (EP). The mass content of the solvent in the electrolyte is 70-85%. The additive is selected from a group consisting of fluorinated ethylene carbonate (FEC), propane sultone (PS), bistrifluoromethanesulfonimidate lithium (LiTFSi), vinylene carbonate (VC) and ethylene sulfate (DT), and the mass content of the additive in the electrolyte is 0.5-5%.
In the method for preparing a lithium iron phosphate power battery according to the present application, the lithium iron phosphate hard to be dispersed is mixed with the carbon nanotube/graphene conductive agent powder, and then is mixed with the glue solution. Via high viscosity kneading beating technology, desirable dispersion of the nanoscale lithium iron phosphate and the conductive agent can be realized. The lithium iron phosphate power battery has the low temperature performance as that of the nanoscale lithium iron phosphate material, and the charge and discharge performances of the battery at low temperature is improved remarkably.
1) Preparing a positive active material: adding lithium iron phosphate powder and positive conductive agent powder into a dry mixer and dispersing the mixture for 0.5-2 h at a high speed, wherein the revolution frequency of the dry mixer is 35-40 Hz, the autorotation speed is 6000-8000 rpm. Adding 30-60% of the positive glue solution into the uniformly mixed powder, wherein the revolution frequency of the dry mixer is 40-45 Hz, the autorotation speed is 4000-5000 rpm, stirring for 10-30 min and scraping material; adding the residual glue solution and stirring for 2-3 h, wherein the revolution frequency of the dry mixer is 40-45 Hz, the autorotation speed is 4000-5000 rpm, the viscosity of the positive active material is 4000-10000 mPas; removing the bubbles via stirring and vacuum pumping, the rotation of the dry mixer does not open, while the revolution frequency is 15-20 Hz. The positive glue solution is obtained via adding oily binder PVDF and oily solvent NMP into the double planetary mixer, stirring and mixing uniformly for 1-2 h.
2) preparing a negative active material: adding an aqueous binder and a deionized water into a double planetary mixer and dispersing for 0.5-1.5 h, adding graphite powder into the double planetary mixer and stirring for 10-30 min, scraping material, adding negative conductive agent SP and continuing to vacuum stirring for 0.5-1.5 h, and adding deionized water to adjust the viscosity to 500-2000 mPas.
3) preparing a positive plate and a negative plate: coating the prepared positive active material on the positive current collector, coating the prepared negative active material on the negative current collector respectively, and obtaining the positive plate and the negative plate after baking; and
4) assembling a battery: winding the positive plate, a separator and the negative plate and obtaining a battery cell, setting the battery cell in a battery case and injecting an electrolyte into the battery case, and obtaining a lithium iron phosphate power battery.
According to the standard method of Chinese new national standard of “lithium ion battery for electric vehicles”, the low temperature test of the lithium iron phosphate power battery according to the embodiment of the present application is carried out, and the test results are shown as following.
Table 1 shows the capacity and energy density data of the lithium iron phosphate power battery according to one embodiment of the present application. Table 2 shows the charging data of the lithium iron phosphate power battery according to one embodiment of the present application at −20° C.;
As shown in Table 2, when the lithium iron phosphate power battery according to one embodiment of the present application is charged with 0.5C constant current at −20° C., the charging constant current ratio is more than 75%. When the battery is charged with 1C constant current at −20° C., the charging constant current ratio is more than 55%. Referred to
It should be understood that, the above examples are only used to illustrate the technical concept and feature of the present invention, and the purpose thereof is familiarize the person skilled in the art to understand the content of the present invention and carry it out, which cannot restrict the protection scope of the present invention based on above. Any equivalent transformation or modification made in the spirit of the present invention should all be included within the protection scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201610900168.7 | Oct 2016 | CN | national |
