A 4680 cell concept
The battery industry is targeting larger cell formats, which enable simplified module design and cell-to-pack or even cell-to-chassis solutions.
In this regard, hard case battery cells and in particular cylindrical cells will play an important role in enabling the named developments. Tesla’s announcement of a tabless 4680 cylindrical LIB has triggered a revival of cylindrical cells in the automotive industry. While the increased cell size provides several benefits, thermal management becomes more challenging. Larger cell dimensions lead to decreased surface area for heat dissipation in proportion to the cell volume and affect the rate of conductive heat transfer from the cell core to the cell housing.
While Tesla tackles this challenge with the tabless design, switching the cell housing material opens up an alternative or complementary approach to overcome the drawbacks in thermal properties of 4680 sized LIBs, while simultaneously boosting their fast-charging performance and lifetime.
A recent concept study by Speira, PEM Motion and the Chair of Production Engineering of E-Mobility Components RWTH-Aachen (available at speira.com/publications) outlines potential benefits of an aluminium cell housing based on a 4680 concept cell, comprising an aluminium cell housing compared to a reference LIB cell with commonly used nickel-plated steel housing.
For large-format cylindrical cells with high energy density, the safety design is of central importance
The safety design of the concept cell features three independent thermal runaway (A spontaneous chain reaction releasing thermal and electrical energy of cell due to improper cell thermal management, improper loads or internal cell defects) preventing measures – namely CID opening, venting and cap expulsion, which are activated in sequence. The safety measures avoid build-up of explosive gas mixtures within the cell and direct energy towards the top of the cell in case of malfunction or misuse and hence, avoid propagation of a thermal runaway event to adjacent cells.
State-of-the-art LIB cells guarantee a minimum housing burst pressure of 95 bar, while ensuring geometric stability. Therefore, wall thickness and mechanical properties of the housing material must ensure integrity up to that pressure. Based on calculations using Bar-low’s & Lamè equation and finite element simulations, these parameters were optimized to withstand 95 bar.
Thermal management is crucial for the performance of LIBs
Exceeding the optimal temperature working window will increase aging and throttle or decrease charging performance of the cell. In particular for larger cell formats, maintaining a homogenous temperature distribution within the 4680 cell at an optimum temperature level becomes even more challenging, because the cooling surface to cell volume ratio decreases, while distances between sources of heat generation and sinks for heat dissipation increase. Consequently, designing a 4680 high energy LIB for EVs requires an optimization of the cooling efficiency and temperature distribution.
Due to the significantly higher conductivity of aluminium alloys compared to NPS, improved fast-charging thermal performance can be anticipated when substituting the housing material. In order to verify this assumption, thermal simulations of our 4680 aluminium concept cell were performed and compared to our NPS reference cell.
Improvements in cooling performance
Thermal simulations reveal significant improvements in cooling performance at 3C fast-charging of the aluminium housing version compared to nickel-plated steel reference cell. The impact of the cell housing material is particularly pronounced in case of a sidewall cooling. In this case, simulation reveals differences in maximum temperature (hot spot) of 11°C after 10 minutes.
In addition, the aluminium housing cell design enables improved temperature homogeneity within the cell, as heat is transferred from the entire skin surface to the 60° contact area where the cell housing meets the active cooling system. In contrast the reference cell with low thermal conductivity housing material reveals rather inhomogeneous temperature distribution, promoting accelerated ageing.
In summary, the simulation reveals clear advantages in thermal management for 4680 cells featuring an aluminium cell housing compared to a NPS cell housing. Thereby, efficient heat transfer and a homogenous temperature distribution within the cell enable improved temperature levels and enable enhanced fast-charging performance and slower aging of the cell.
Additional details on cell design, methodology and performance indicators are available on Speira’s website: https://bit.ly/speira_whitepaper_battery
Speira is a global aluminium rolling and recycling company and consists of seven manufacturing facilities, as well as one R&D center. Locations in Germany and Norway including our Joint Venture Alunorf, the world´s largest aluminium rolling mill, Grevenbroich, the world´s largest rolled aluminium finishing mill, as well as several international sales offices. Speira employs around 5,000 people mainly in Germany and Norway.
Speira is a volume supplier to leading global battery manufactures and offers an extensive product portfolio within the brand ION that covers the entire spectrum from battery electrode foils to cell connectors, heat exchangers, and housing materials.