This illustration shows the entire process chain of battery cell production as it is applied in the BatteryLabFactory Braunschweig. Thereby everything from material pre-treatment to the finished cell is covered.
Pretreatment & Mixing At the beginning of the classical battery process technology the powdery components are mixed and dispersed to obtain a suspension suitable for coating. The suspension consists of various active materials, inactive components (conductive carbon black, conductive additives, binders) and a solvent. The aim of mixing is to homogeneously mix and pre-structure the powdery components. The dispersion process serves to disperse the powders within the solvent and to specifically disperse the conductive carbon black particles in order to achieve defined power and energy properties inside the cell.
Coating & Drying After the production of the suspensions for anodes and cathodes, these are applied to conductive films with layer thicknesses of 10-20 ?m using a wet coating process via an application tool. These composite films already represent an electrode which is further processed in following process steps. In the industrial series production of electrodes, a continuous slot die process is used, which is usually followed by a convection drying step. The wet film thickness ranges from 150 to 300 ?m and results in a dry film thickness of more than 100 ?m. Compared to coating, drying represents the speed-determining production process. Typically, the drying time is approx. 1-2 minutes. The duration depends in particular on the thickness of the coating, the solids content of the suspension and the solvent used. At the end of these processes, several hundred meters of so-called electrode coils are wound onto a coil core and are ready for calendering.
Calendering Continuous roller compaction is referred to as calendaring. This involves calendering the previously produced electrodes to a specific target density. Structural properties such as the porosity, adhesive strength or conductivity of the electrode and thus the electrochemical performance are significantly influenced. In addition, a homogenization of the layer thickness is achieved, which is of great importance for the subsequent stacking process.
Cutting & Drying After the electrode production, which contains the coating, drying and compaction process, the laser cutting process follows. The electrode coils were separated into a flexible electrode format by means of laser radiation in an automated process. The cutting speed, laser beam and electrode properties play a particular important role on the fabrication of the electrode components and influences the resulting cutting-edge quality and possible particle contamination of the electrode.
Packaging During package production, a cell package with the desired number of compartments is created. A compartment consists of a cathode and an anode, separated by a separator layer. There are three different technologies for this process: The winding process, the stacking process and the Z-folding process. In the winding process, the electrode and separator materials are fixed to a core and continuously wound around it. The electrode and separator webs are guided by separate web edge and tension control systems during the winding process. In the stacking process, the electrodes and the separator sheets are stacked alternately with the help of an industrial robot. The finished stack is finally fixed with an adhesive tape and transported further. In the Z-folding process, the electrode sheets are also handled by a robot. The separator is continuously fed from a roll. Web edge and tension control is necessary for precise positioning of the separator. Here, too, the stack is fixed by an adhesive and discharged for further transport.
Contacting During contacting, the individual arresters of the anodes and cathodes are connected to each other. This creates a conductive connection between the individual layers. Then two suitable current conductors are attached to the cell. There are different methods of contacting for the different cell types. The two alternative technologies for contacting are laser and ultrasonic welding.
Enclosure & Welding With enclosure, the cell stack is placed in a suitable cell housing and this is sealed. The housing serves to protect against mechanical damage and the penetration of moisture into the cell interior. Three different types of cell types are distinguished depending on their cell housings: the round cell, the prismatic cell and the pouch cell. Depending on the type of housing, the final cell closure takes place after electrolyte filling (pouch cell) or before electrolyte filling with a recess for an attachment for evacuation of the cell and electrolyte dosing (prismatic cell, round cell).
Filling The process step of filling is divided into the two sub-processes of filling and wetting. During filling, the electrolyte is dosed into the cell by means of a dosing lance. Wetting describes the penetration of the electrolyte into the fine pores of the electrodes and the separator. The wetting rate depends on various conditioning parameters such as pressure and temperature. In the functionality of the cell, filling is a technologically demanding and decisive step. The choice of technology depends very much on the cell type and the physico-chemical properties of the materials and the electrolyte.
Formation & Ageing After the cells have been filled with electrolyte and sealed gas-tight, they can be electrically charged for the first time. For this purpose, the cells are tempered in climate chambers at a defined ambient temperature and contacted with the battery test equipment. During the first charge (formation), the electrolyte on the negative electrode decomposes and forms a film layer - the so-called solid electrolyte interphase (SEI). The SEI protects the electrode from further decomposition of the electrolyte and is essential for the functional principle of the lithium-ion battery. The properties of the SEI depend on the temperature profile and the current strategy and must be adapted to the specific cell type. Typically, there are also several formation cycles to stabilize the properties of the SEI. These formation cycles can last from a few hours to several days. After the formation is completed, the electrochemical quality testing begins.
Electrochemical Quality Control The electrochemical quality test provides various tests to evaluate the cell properties. Typical test procedures are capacity and internal resistance tests. In the capacitance test, cells are first fully charged to the end-of-charge voltage and then discharged at a defined current. From this, the nominal capacity can be determined as a function of the current. In the internal resistance test, short current pulses are applied at defined states of charge in order to determine the performance of the cells. There is also the self-discharge test, in which the cell is usually monitored without current and the voltage drop is checked over several days or weeks to determine the self-discharge rate. This rest period can even lead to an improvement in the cell properties for certain cell types, as the processes in the cell homogenize (ageing).
