The Preparation Method of Battery Cathode Slurry

Wet Electrode Preparation Process

A double planetary mixer was used as the cathode electrode slurry preparation equipment. First, prepare polyvinylidene fluoride (PVDF) glue. Use an ordinary mixing tank to pour a certain amount of solvent NMP (N-methylpyrrolidone) first, add the binder PVDF powder according to the designed solid content, and stir for 4 to 6 hours to obtain PVDF glue. PVDF glue is a colorless and transparent liquid with a certain viscosity, and its solid content can be controlled between 5% and 10% according to needs. The prepared glue solution generally needs to be vacuumized and left to stand for more than 12 hours in order to eliminate the bubbles generated during the stirring process. Then a certain amount is delivered to the slurry preparation mixer through a metering pump through a sealed pipeline. Add the conductive agent SP, and start the mixer to rotate and rotate at the same time. The revolution speed is set to (25±5) r/min, the autorotation speed is set to (500±50) r/min, and the NMP spray is assisted, so that the extremely light-density SP can be fully mixed into the PVDF glue. The stirring time is 1 h.

Finally, spray all the remaining NMP solvent into the mixer, increase the stirring speed, the rotation speed is (35±5) r/min, and the revolution speed is (800±50) r/min. After stirring for 10-30 min, increase the revolution speed to (1300±50) r/min and stir for 1.5 h to complete the slurry preparation. The prepared slurry needs to be transferred to a transfer tank for vacuum defoaming treatment before use.

Semi-Dry Electrode Preparation Process

First, add all the cathode electrode main materials (such as NCM or LFP, etc.) and conductive agent SP to the double planetary mixer, only turn on the equipment for revolution, set the speed at (25±5) r/min, and stir the powder for 30 min.

Secondly, according to the wetting effect of the main material, the corresponding kneading solid content is designed and controlled at 68% to 72%. According to the designed kneading solid content, add a certain amount of PVDF glue and an appropriate amount of solvent. Considering the equipment capacity and preventing the equipment from being damaged due to high-load operation, it may be appropriate to perform a one-step pre-kneading, that is, set the twist paddle to reverse, the rotation speed is (10±2) r/min, and the time is 15 min. After the equipment adapts to the stirring state of the powder and glue in the tank, turn to the most critical step of this process—kneading, set the revolution speed to (25±5) r/min, and kneading time to 1 h. At this time, the slurry presents a paste-like high-viscosity non-flowing state. Then add the remaining glue and an appropriate amount of NMP solvent, start the mixer to disperse, set the rotation speed at (1200±50) r/min, keep the revolution speed unchanged, and stir for 1 h.

Finally, according to the solid content designed by the slurry formula, add the remaining NMP solvent, further increase the dispersion speed to (1350±50) r/min, keep the revolution speed constant, and continue stirring for 1 h to complete the slurry preparation. Transfer the slurry to a transfer tank for vacuum defoaming treatment, and the vacuum degree does not exceed -85 kPa.

Dry Electrode Preparation Process

Compared with wet electrode or semi-dry electrode, this method further simplifies the pulping process and further shortens the process time. The dry electrode omits the process of glue preparation, and uses the method of pure dry mixing and pre-stirring to mix the powder before pulping. It also adopts double planetary mixer as pulping equipment.

First, add 50% of the main material of the cathode electrode (such as NCM or LFP, etc.), conductive agent SP, binder PVDF, and 50% of the main material of the cathode electrode into the mixer in sequence. Keep the revolution speed (5±1) r/min during the feeding process. After the feeding is completed, start the dry mixing pre-mixing, set the revolution speed to run at the reverse speed (7±1) r/min, and temporarily stop the rotation dispersion. After 5 minutes, switch to normal dry mixing, set the revolution speed (13±1) r/min, the rotation speed (400±2) r/min, and stir for 0.5 h. It is necessary to ensure that the dry mixing can fully mix the main material of the cathode electrode, the conductive agent SP and the binder PVDF powder, so as to avoid the inability to obtain the final stable slurry after adding the solvent in the next step.

Secondly, spray the uniformly dispersed powder with NMP solvent, add NMP solvent in the first step, design a suitable solid content according to the equipment capacity and refer to the effect of the semi-dry kneading step, and generally control it at about 70%. The wetting step of entering the powder is mainly based on revolution stirring, and the revolution speed is set at (15±5) r/min, supplemented by (150±2) r/min rotation, stirring for 1.5 h. At this time, the slurry is in a thick liquid state with high viscosity. Add the remaining NMP solvent, dilute and stir for 10-15 minutes. The revolution speed remains unchanged, and the rotation speed increases to (500±5) r/min. If necessary, conductive glue carbon nanotubes (CNTs) can be added before NMP spraying in this step.

Finally, enter the high-speed dispersion viscosity reduction step. Keep the revolution speed (15±5) r/min unchanged, and increase the rotation speed to (1200±50) r/min. After stirring for 2 hours, reduce the speed and stir slowly (revolution speed (10±1) r/min, rotation speed (200±5) r/min) to cool down and complete the slurry preparation.

Solvent Infiltration Electrode Preparation Process

Integrating dry and wet processes, the slurry is prepared in the way of "wet process without glue". Whether it is considered from the perspective of process simplification or process time, it has obvious advantages. It is important to consider the stability of the final slurry.

First, a certain amount of NMP solvent, 50% of the main material of the cathode electrode (such as NCM or LFP), conductive agent SP, and 50% of the main material of the cathode electrode are sequentially added to the double planetary mixer according to the solid content of 70%. Add conductive glue CNTs if necessary. During the feeding process, the revolution speed was maintained at (5±2) r/min. After the feeding was completed, the revolution speed was gradually increased to (15±2) r/min, and pre-stirring was carried out for 5 minutes.

Secondly, directly enter the high-speed dispersion step, the revolution speed is (25±5) r/min, the dispersion speed is (1350±50) r/min, stir for 1.5 h, and keep the vacuum degree not exceeding -80 kPa. 

Finally, put in PVDF powder, spray and add the remaining NMP solvent, and if necessary, add conductive glue CNTs. Keep the revolution speed constant, and first use the rotation speed of 900 r/min to stir for 5-10 min. After completion, enter the second step of high-speed dispersion, increase the rotation speed to (1350±50) r/min, and complete the slurry preparation after 2 hours of dispersion.