How can we succeed in transferring the production of solid-state batteries on a laboratory scale to mass production? Which processes are particularly well suited for series production and where is there still a need to catch up? This article provides an overview.
The transition from prototype cells to mass production is one of the challenges that must be solved to help the solid-state battery achieve a breakthrough. The key to industrialization is to reduce costs and develop production steps that can be manufactured in a continuous manufacturing process to minimize changeover times and manual operations. To manufacture the solid-state batteries, efforts are also being made to adopt as many of the manufacturing steps as possible using existing processes from the production of Li-ion batteries, as this can save investment costs in new equipment. In addition, there is already experience with the production process, so that production can be scaled up more quickly (see [1]).
Fabrication of the elementary cell
A cell essentially consists of the cathode, the anode, the solid state electrolyte and the current collectors. More information on the general structure of the solid-state battery can be found here.
The fabrication process is described in this article using a material combination of Li metal as anode, LLZO as solid state electrolyte and LCO as cathode material. This material combination is well known and there is information in the literature on how the manufacturing process works, which is why it is used here. LLZO is a Garnet-type electrolyte and thus belongs to the group of oxide electrolytes, which are considered to be particularly demanding in manufacturing because the electrolyte material is hard and brittle (More information on oxide electrolytes is available: here).
Figure 1: Process flow diagram for the production of a solid-state element cell. Figure created based on RWTH Aachen: Production of All-Solid-State-Battery Cells [2].
Process flow using the example of an LCO|LLCO|Li elementary cell
Figure 1 shows the process flow diagram for the production of an elementary cell. Here, an elementary cell is a single logic cell with the typical voltage for the cell chemistry. For pouch or prismatic cells, it is common to connect several elementary cells in a package in parallel to form a total cell in order to increase the capacity of the cell.
The manufacture of a solid-state element cell begins with the production of the cathode. The process for this is essentially no different from the production for Li-ion batteries [3]. LCO granules are used as the raw cathode material and are placed in an extruder that melts the material. Parallel to the cathode, the LLCO electrolyte is melted as granules in a second extruder.
The current collector film of the cathode is used as a carrier film in the next process step. A double extruder is now used to apply first the liquid cathode layer and then the liquid electrolyte layer to this. The coated film now runs through several rollers and is calendered (i.e. compacted) to permanently bond the layers together.
The anode is produced as a Li foil. For this purpose, lithium metal is melted and pressed into a thin foil in an extrusion and calendering process. In the final process step, this anode is then laminated onto the cathode-electrolyte connection and pressed so that they stick together well [2].
Supplementary process steps and alternative manufacturing methods
The process described in the previous section is a simplified representation of the process. In practice, additives must sometimes be added in the manufacturing process to achieve the desired properties. Depending on the material combination, it may also be necessary to apply extra protective layers, for example to prevent reactions between the anode and electrolyte. For sulfide electrolytes, for example, it is known that without a protective layer they undergo parasitic reactions with the Li metal layer [3].
There are various alternatives for the fabrication of the unit cell, which differ considerably in the state of research. The most important alternatives are listed below:
- Cathode: In addition to the extruder process, a cathode can also be produced in a wet process. These processes are used on a large scale in conventional Li-ion manufacturing processes and are characterized by their good scalability [3].
- Electrolyte: A wet process can also be used for electrolyte production. The disadvantage here is that a solvent is used, which has to be dried out of the electrolyte at the end of the production process. In another process, the electrolyte is brought into a vacuum chamber via a carrier gas, where it is then deposited on the cathode. However, this process is very slow and therefore still far from commercial application [3].
- Anode: In addition to the extruder process, there is also the approach of completely omitting the anode in the manufacturing process. This approach is pursued by QuantumScape [4], for example. Excess lithium is introduced on the cathode side, which then migrates to the anode side during initial charging, thus forming the anode [3].
Further processing of the elementary cell
Figure 2: Process diagram of the manufacturing process from the elementary cell to the finished product, figure based on RWTH Aachen: Production of All-Solid-State-Battery Cells [2].
However, the manufacturing process of the solid-state battery is not yet completed with a finished elementary cell. Figure 2 gives an overview of the remaining process until a cell ready for sale exists at the end. First, the elementary cell is cut to the respective cell size. A laser is usually used for this purpose [3]. The next step is stacking the elementary cell to form a finished cell. The elementary cells are connected in parallel so that the capacitances add up. However, there are also approaches to connect the cells in series and thus increase the voltage of a cell (so-called bipolar structure). This approach is called CellisPack (CIP) because it theoretically requires only a single cell to form a complete pack. However, it is in an early research phase and is not expected for the first generation of solid-state batteries [5]. In the next step, all positive current collectors are connected to the positive cell terminal and all negative current collectors are connected to the negative cell terminal, respectively. In the final production step, the cells are then packed into their final cell envelope (metal case or pouch foil) [3]. For conventional Li-ion cells, the packaging of the cell is accompanied by the filling of the liquid electrolyte. This process step is omitted for all-solid-state batteries [1].
Before the cell can be used, the so-called forming process is carried out. This is also required for Li-ion cells and serves to build up the necessary interface layers between the electrode and electrolyte. For forming, the cell is charged and discharged with low currents. It is expected that for solid-state batteries, one cycle is sufficient to complete the forming process [2]. In the next step the cell is monitored for several days under controlled conditions to identify damaged cells. Finally, some characterization tests are performed to prove the quality of the cell [3]. Subsequently, the cell can be incorporated into modules or launched on the market as a single cell.
Conclusion
The production path shown is only one possibility among many processes. Ultimately, a new assessment must be made for each material combination as to which production process is most suitable. In particular, the choice of electrolyte has a considerable influence on the possible process steps. While the first vehicles are already being produced in small series for polymer electrolytes (cf. Mercedes eCitaro [6]) and initial practical experience has thus already been gained with the manufacturing and scaling process, this experience is still largely lacking for sulfide- and oxide-based electrolytes.
However, it is already becoming apparent that production and scaling will be particularly successful if familiar manufacturing processes can be used. It remains to be seen which manufacturer will be the first to succeed in producing larger quantities of solid-state batteries and thus offer an alternative to today’s Li-ion batteries.
Sources
[1] InsideEV: Solid Power Will Power BMW and Ford with Its Solid-State Cells, https://insideevs.com/news/505087/solid-power-bmw-ford-ssb/
[2] Heimes, H., Kampker, A. et al.: Production of All-Solid-State Battery Cells, 2018 RWTH Aachen University.
[3] Fraunhofer Institute for Systems and Innovation Research ISI: Solid-State Battery Roadmap 2035+, Karlsruhe, 2022
[4] Quantumscape: Delivering on the promise of solid-state technology, 2023, https://www.quantumscape.com/technology
[5] ProLogium: Vinfast and Prologium Launching JV to Build SolidState EV Battery Pack in Vietnam, March 02, 2021, https://prologium.com/vinfast-and-prologium-launching-jv-to-build-solidstate-ev-battery-pack-in-vietnam/
[6] Mercedes Benz: eCitaro, Battery Technology, 2023, https://www.mercedes-benz-bus.com/de_DE/models/ecitaro/technology/battery-technology.html
