Method for preparing diaphragm of lithium-ion battery including three layer co-extrusion followed by treatment in annealing box

A method for preparing a diaphragm of a lithium-ion battery by three layer co-extrusion, the method comprising:

• adding two different types of polypropylene resins into inlets of two different extruders respectively for melting and plasticizing at a conveying-section temperature of 60-70° C., a compression-section temperature of 150-200° C., a metering-section temperature of 200-230° C., and a rotating speed of 100-250 rpm;
• synchronously co-extruding the two different types of polypropylene resins obtained after melting and plasticizing from a three-layer composite casting die head of a three-layer co-extrusion casting machine, and
• drawing the polypropylene resins co-extruded to form a casting film with an A/B/A three-layer co-extrusion structure;
• placing the casting film with the A/B/A three-layer co-extrusion structure in an annealing box to be subjected to annealing at an annealing temperature of 130-150° C. for 1-24 hours;
• and
• carrying out a first longitudinal stretching and a second longitudinal stretching on the casting film annealed,
• wherein a stretching temperature of the second longitudinal stretching is higher than a stretching temperature of the first longitudinal stretching, thereby forming a porous film structure,
• preparing the diaphragm with a microporous structure and the A/B/A three-layer co-extrusion structure, and
• finally, thermoforming the diaphragm and then cooling the diaphragm to a room temperature;
• wherein, the annealing box comprises a box body, a motor and a sealing cover;
• wherein the box body is of a cuboid structure and provided with a cavity;
• and an inner surface of the box body is fixedly connected with heating plates which are uniformly arranged;
• wherein a driving shaft is horizontally arranged in the box body in a front-back direction, a first driven shaft is arranged in a left side of the driving shaft in the box body, and a second driven shaft is arranged in a right side of the driving shaft in the box body;
• wherein the driving shaft, the first driven shaft and the second driven shaft are rotationally connected with the box body through bearings, and front end surfaces of the first driven shaft and the second driven shaft penetrate through the box body and extend out of the box body;
• wherein a front end surface of the box body is fixedly connected with a power box cover, and a front end surface of the power box cover is fixedly connected with the motor;
• wherein an output shaft of the motor penetrates through the power box cover, extends into the power box cover, and the output shaft is fixedly connected with a front end surface of the driving shaft;
• wherein a driving gear is fixedly connected to an outer arc face of the output shaft of the motor, and the driving gear is arranged between a flange face of the motor and the front end surface of the box body;
• wherein an outer arc surface of the second driven shaft is fixedly connected with a driven gear in front of the box body, and the driving gear is engaged with the driven gear;
• wherein a driving rotary disc is fixedly connected to an outer arc face of the output shaft of the motor, and the driving rotary disc is arranged between the driving gear and the front end surface of the box body;
• wherein a driven rotary disc is fixedly connected to an outer arc face of the first driven shaft in front of the box body;
• wherein a driving belt is connected between the driving rotary disc and the driven rotary disc;
• wherein an interlayer film is in coiled connection between the driving shaft and the first driven shaft in the box body in a horizontal manner;
• wherein the diaphragm is in coiled connection between the driving shaft and the second driven shaft in the box body in an oblique manner;
• wherein an opening is formed in a right upper edge of the box body, and a sealing door is arranged at the opening of the box body;
• and
• wherein a control box is fixedly connected to a left side face of the box body.