Steinbeis research into new types of contacts on solid-state batteries
Lithium-ion batteries are attracting interest as a rechargeable type of solid-state battery for use in future applications, particularly in the international supply market for stationary and electrical energy. In a joint research project with LIOFIT, the Steinbeis Innovation Center for Materials, Surfaces and Joining Technology has been looking more closely into external battery structures and contact areas on batteries. The goal was to find new ways of making battery terminals to allow electrical and thermal energy to transfer without loss.
The particular area of focus for the project team lay in how external energy is distributed outside battery cells. “We wanted the current method of adding spot welds, which generates heat, to be replaced with flat contact points with adjacent connection plates, with good electrical and thermal conduction properties,” explains Steinbeis Entrepreneur Professor Dr.-Ing. Reinhard Rosert. The Steinbeis Innovation Center supported the project by helping with idea generation –focusing on industrial application – with the formulation of development goals, and with workflow coordination. Industrial partner LIOFIT, from the city of Kamenz in Saxony, is a market leader in the repair, replacement, and recycling of lithium-ion batteries.
Turning the spotlight on alternatives to established technology
The search for alternative forms of battery technology includes giving consideration to electronegativity differences between different elements in the periodic table, as well as combinations of those elements. The first sectors of industry to apply new development strategies and use new battery designs were civil aviation and the military aerospace industry, and a central role in this was played by the United States. The movement of electrons within such batteries requires so-called activators. Introducing nickel-cadmium technology to batteries has for the first time made it possible to fabricate portable battery packs. Using more environmentally friendly nickel-metal hydride batteries has further raised energy density by around 40%. Energy density is particularly high in lithium cobalt and lithium cobalt oxide (LiCoO2) batteries. Cobalt acts as an activator and supports electron leakage and the movement of electrons within battery cells. Lithium is not only the lightest metal in density terms but is also particularly electropositive. In combination with a cobalt activator, it can also supply so-called free electrons within atoms. Intensive internal heating is detrimental to the goal of achieving the highest possible energy densities with the accumulators. Heat weakens power output, even to the point of destroying entire battery cells and battery packs.
It was against this background that the expert researchers at Steinbeis and a research team at LIOFIT decided to focus on the connections between cells, also looking at how electrical energy is transmitted and different ways to allow heat to dissipate via the connection elements between batteries. Their design focus lay in cell connections and electricity transfer with the aim of ensuring energy is transmitted without loss between individual cells, within so-called battery packs, right to the final point of consumption. The connecting plates perform two functions: On the one hand, they conduct electric current and, on the other, they dissipate external ohmic heat and heat generated within cells. This heat is caused by varying densities of electron flows within the electrolyte, reactions (reduction-oxidation reactions/redox) on internal electrodes, as well as the reactions of lithium and associated materials.
It is important to dissipate spikes in heat, which is why the research project looked at “flat” joints on the contact points in order to improve heat dissipation. However, the quality of lithium-ion batteries is largely determined by contacts between the anodes and connecting plates. Inevitably, there is contact resistance both within the spot-welding zone and across the contact area between the connecting plate and the surface of the battery anode head. When heat is generated and, correspondingly, the electric current sinks to a minimum, this results in energy efficiency losses.
Heat on the contacts – a major challenge
The problem lies in the variations in contact resistance and the tendency for external contact points to overheat. One remedy is to dissipate heat via the connection plates, which offer high levels of thermal conductivity. However, the metallic HILUMIN strips (connection plates) currently spot-welded into place are less suitable for this purpose. The thickness of these strips is roughly 800 µm, with a thermal conductivity of around 85 W/mK. When additional heat builds up, this leads to the formation of so-called heat islands, resulting in further negative impacts on energy transfer. Another aspect that merits consideration is the mechanical properties of the connection elements that form the contacts on batteries. These cannot be reproduced and vary considerably depending on possible tensile and shear loads and the different number of spot welds.
Another important factor in qualitative terms, particularly when it comes to electrical and thermal energy transfer, is the size of contact areas and how connection strips are positioned on the body of batteries. This determines functional safety. Steinbeis’s project partner, LIOFIT, repairs, renews, and replaces connections on all kinds of batteries on the market, both individually and in packs. It processes the 18650 batteries of all manufacturers, as well as the new 21700 from TESLA. Its battery contacts are made using spot welding, with welds arranged in pairs of between two and six spot welds. The weld spots are roughly 80 to 100 µm in size, depending on how much pressure is applied and tip formation on the tungsten needles.
Diffusion welding performs excellently on all fronts
The research project involved using “cold” ultrasonic welding and a new type of aluminum connection plate. Beforehand, extensive simulations and laboratory tests had been conducted. The features of this new welding technology are:
- Largest possible surface connection between the connection plates and the surfaces of anode contacts on the batteries
- A choice of connection plate materials offering high electrical and thermal conductivity (aluminum)
- Strong mechanical stability, not only on the connection plates but also in transition areas around welded contact points
- Avoidance of localized heating in the contact area
- Use of “cold” joining processes for reproducible electrical and mechanical connection parameters
- Diffusion-based metallic transitions (at an atomic level) between the aluminum surface and the contact material on the HILUMIN battery anode (mixed compound)
- A choice of standard diffusion welding technologies based on ultrasound principles
Using mixed compounds based on so-called liquid phases was ruled out. The project team opted for an innovative method of diffusion bonding in the form of so-called “cold” welding technology without welding materials. Spikes in temperature on an atomic level (without liquid phases) combine with pressure to trigger mass transfer between the surfaces being joined. Within a very short timeframe, semi-finished products merge into a new, single component. The prerequisites for this to happen are cleaned, low-oxide surfaces and special activation of the surface electrons and atomic bonds, especially with aluminum contact strips. It was decided not to use inert gases and vacuum processes. The welding tool itself comprised a small “profile hammer” the size of the required contact area, set in motion by ultrasound and subjected to compaction pressure.
Collaboration on the project with LIOFIT brought great benefit to the Steinbeis Innovation Center. Providing lithium-ion batteries from a variety of manufacturers, as well as the equipment required to carry out diffusion bonding, was central to the success of the project, as was the technological know-how of LIOFIT. And it’s a success to be proud of. The achieved levels of mechanical strength and the electrical and thermal conductivity produced by the diffusion bonding process exceeded all expectations.
This article (in German) was co-authored at the Steinbeis Innovation Center for Materials, Surfaces and Joining Technology (SIZ-WOV)
Prof. Dr.-Ing. Reinhard Rosert
Steinbeis Innovation Center Materials, Surfaces and Joining Technology (Dresden)