Understanding the Effect of Heterogeneities on Battery Pack Lifetime

Lithium-ion batteries are now a dominant force worldwide as energy storage devices for a range of applications, notably electric vehicles. In electric vehicles, individual cells with a relatively small capacity (10 - 500 Wh range) are connected in series and parallel to form a battery pack with a much higher overall capacity (20 - 100kWh). This pack is also typically at a higher voltage, commonly 400 V. These large battery packs may contain thousands of cells, such as in the Tesla Model S which carries an 85 kWh pack containing 7104 cells in a 74P96S configuration packaged as 16 74P6S modules. Within a pack of this size, any inconsistency between cells will be magnified and heterogeneities within the pack may accelerate the rate of degradation of the pack. For example, placement of the cooling system may lead to hotter cells passing higher peak currents, accelerating degradation, or tab position may influence the peak current seen within a parallel string. Therefore, a critical understanding of pack level effects, which are complexly coupled, combined with insight into the influence of these effects on single cell degradation and performance is key to creating improved battery packs in the future.

The research questions underpinning this project are as follows:
- What are the principal causes of heterogeneity within a battery pack?
- How do heterogeneities and thermal effects influence the lifetime of a battery pack?
- Can we better understand the coupling between thermal effects, cell-to-cell variability and other heterogeneities within a battery pack?
- Can we better understand the influence of physical and thermal heterogeneities on degradation modes within individual cells of a battery pack?

Additionally, the following sub-questions will be answered:
- How can we use diagnostic techniques better inform battery pack design?
- How can we use differential thermal voltammetry (DTV) and incremental capacity analysis (ICA) to better regroup cells within second life battery packs?
- Are there any benefits to creating hybrid packs using difference cell types over homogeneous packs?

Simulation and experimental validation show that cells closest to the load points of a pack experience higher currents than cells further away due to uneven overpotentials caused by the interconnects. When a cell with a four times internal impedance was placed in the location with the higher currents it helped to equalise the cell-to-cell current distribution, however if it was placed at a location furthest from the load point this would cause a 6% reduction in accessible energy at 1.5 C. The influence of thermal gradients can further affect this current heterogeneity leading to accelerated aging. Simulations showed that in all cases, cells degrade at different rates in a pack due to the uneven currents, with this being amplified in the case of a thermal gradient. Therefore, the insights from this work highlight the highly coupled nature of battery pack performance and can inform designs for higher performance and longer lasting battery packs.

Next Steps
To further investigate the effects of cell-to-cell variations on battery pack strings, a new rig has been designed and fabricated with a MEng 4th year student and is currently testing the effects of thermal gradients within parallel cell strings. This rig allows control of the surface temperature of both sides of up to 6 pouch cells (Kokam 5 Ah) and will enable investigation of the effect of temperature on current distribution within the pack as well as effect of tab position and interconnect resistances as well as allowing a deeper investigation of the effects of cell-to-cell heterogeneities.