Scientific, technical and economic problems
In order to significantly increase the nominal energy density of modern battery systems and ensure operational safety, solid electrolytes are increasingly being researched in the industry. There is a wide range of such materials, such as polymer electrolytes, sulphur and oxide ceramic compounds. These solid electrolyte materials differ in their mechanical properties, ionic conductivity and chemical composition. A particular focus of science is on the use of ceramic solid electrolytes. Due to their complete oxidation, these materials do not pose a fire hazard. For more than 20 years, work has been carried out on varying the chemical composition of these materials and in recent years they have achieved a similar ionic conductivity to current liquid electrolyte materials, taking into account the ionic transfer number . The greatest advantage of ceramic solid electrolytes is the possibility of using metallic lithium as the anode material. This has a theoretical capacity of 3860 mAh/g, which is 10 times the capacity of graphite (anode in the LIB) with 372 mAh/g. At present, the construction of a cycle-resistant solid-state battery (Li(Na)MSSB, AFSSB, Fig. 1) has failed due to the insufficient contact surface between the solid electrolyte (rough ceramic, see Fig. 3) and the anode .
CoCoBatt will address this critical interface, which currently limits the long-term stability of AFSSBs (also Li(Na)MSSBs), with a dedicated interface design. There are two basic solutions for the critical interface. First: Li(Na)MSSB, the anode-side current collector can first be coated with metallic Li(Na) , but this requires processing in an inert atmosphere, an expensive process step in an industrial production line. Secondly, AFSSB, for an anode-free cell concept, the realization of a large-area interface contact to the ceramic electrolyte on the anode side is the challenge. Here, the easily brittle electrolyte ceramic and the current collector (copper), which should be applied to the surface as thinly as possible, have conflicting requirements. The higher the pressure with which the lithium-coated copper foil is pressed onto the ceramic, the lower the contact resistance. The price of this is the formation of cracks in the ceramic. Both the small number of metal-ceramic contact points and the cracks in the ceramic caused by pressing lead to the formation of metallic lithium micro- and nanostructures at the interface during subsequent cycling. This short-circuits the cell at low cycle numbers. 
Practical solutions to this dilemma are copper coatings that do not require mechanical pressure. These include ALD, CVD and PVD processes (atomic layer, chemical or physical vapor deposition), which can depict copper layer thicknesses of several μm on large surfaces, but not economically. Electrochemical (ECD) and electroless (ELD) deposition of metals, which are typically used as alternative processes for CVD and PVD processes in semiconductor metallization, also have a high material quality, but are not suitable for the cost-effective production of large-area coatings.
Thermal spray processes offer great potential for efficient large-area coatings. The economic application of thin (max. 20 μm) copper layers without destroying the solid electrolyte or having to switch to harmful nanoparticles due to thermal shock or the high kinetic energies of the particles is challenging. Cold gas spraying would be well suited for low oxidation; the layer is formed by mechanical deformation when the particles impact at high speed (500 – 1500 m/s). There is a risk of ceramic fracture due to the induced stresses. With the cold atmospheric plasma coating process, the particle speed measured in the AiF Herkules project is 0.2 m/s, which is why even thin, sensitive substrates can be metallized. The application rate is currently 0.4 kg/h, with cold gas spraying it is 10 times higher – which is a challenge for the economic implementation of the technology .
Economic significance of the intended research results
The use of the cold atmospheric plasma coating process addresses the technical, economic and ecological bottlenecks in the production of anode-free solid-state batteries. By investing in a plasma coating system, SMEs can carry out a process step with high added value without having to invest in battery production lines, cathode material laboratories or complete cell production facilities that are prohibitively expensive for them. Instead, the project concentrates on a defined process step that can be clearly separated economically and locally from the production of a full cell. After the coating process, there is still no metallic lithium or sodium in the semi-finished product (solid electrolytic ceramic plasma-coated with copper current collector). This means that the semi-finished product can be stored in large quantities and can be sold on to an end user or processor. The project deals with the anode side of a battery. After the process step, the process is cathode-independent and allows an end user to apply any cathode to the back of the coated electrolyte, thereby increasing the customer market. There is a general desire in the literature for metallic lithium or sodium to be deposited on the anode side of a solid-state battery .
The production of a battery cathode has a much larger number of variables: The cell chemistry and its exact synthesis and the production of specially shaped cathode particles, which are influenced by the selection of suitable base particles. Their shape, size distribution, sintering time and temperature steps, as well as coating with suitable coating paste, influence the properties of the cathode particles. This is followed by the selection of suitable electrolytes. These are processes that can only be mastered by large industrial groups, such as BASF in Schwarzheide in Europe in the near future for cathode material production and the subsequent cell production of automotive OEMs. The CoCoBatt project offers an SME the opportunity to work on a technological leap in battery development and production without having to invest the kind of capital that is usually only affordable for internationally operating stock corporations. Since the current collector can be produced with little plant and process engineering effort, provided the parameters and process windows – which are to be determined in this project – are known. In addition, the plasma coating system does not have to be designed for a specific ceramic material or substrate geometry, but can be adapted for large and small-area coatings or even for large and small series production by adapting the workpiece design.
For further contact details, please contact the office. See contact details
- Fraunhofer IKTS
- University Erlangen-Nürnberg
Lehrstuhl für Fertigungsautomatisierung und Produktionssystematik (FAPS)