The paper numerically explores the electrochemical and thermal behaviour of a larger format 4680 cylindrical cell recently proposed by Tesla and explains the need to go “tabless.”
An idealized spiral geometry is used for 2D simulations with the traditional tab-based current collection method and a new continuous current collection method compared.
The new design is found to mitigate the ohmic losses experienced around the “jelly-roll” current collectors which are significant for the traditional tabbed case, thus leading to higher efficiency and capacity and reduced heat production.
What exactly is a lug?
It is actually a metal conductor welded to the positive and negative current collectors, which is the contact point between the positive and negative poles of the battery during charging and discharging, and the current must flow through the tabs to connect to the outside of the battery. If we unfold the wound cell, since there is only one point where the tab is connected to the external circuit, it will require all the electrons to traverse the entire length of the cell to reach the internal welding tab, and then pass through the wire.
It can reach another level, and the whole process will generate heat due to the long distance and high resistance. The safety of the battery is the biggest bottleneck.
Tesla’s electrodeless ear is actually a full electrode ear. One end of the electrode is coated with a conductive material to connect it directly to the shell or a specially designed cover plate.
The current is directly in the electrode collector, cover plate, and shell.
Conduct conduction between them, and change the previous point contact to 100% surface contact, so that heat does not collect at one point, and the conduction area is increased and the heat accumulation is greatly reduced.
At the same time, it is precisely because the external contact of the positive and negative electrodes has changed from a point to a surface, all the electrons do not need to squeeze a single-plank bridge as before, but it is equivalent to having n bridges, thus shortening the moving distance of the electrons, reducing the resistance, and reducing the heat.
Produced, Tesla's patent claims that the non-polar ear design can reduce the resistance by 5-20 times, thereby controlling heat generation.
And reduce the waste of odorless calories, but also help to increase the mileage
Traditional Li-ion cell designs have their limitations from a thermal management perspective.
In particular, the long length of the electrode spiral or “jelly-roll” in cylindrical cells leads to
heterogeneity in current distribution and temperature when current collection is performed by tabs at the ends of the roll.
The insulative nature of the components being wrapped around themselves also
leads to internal temperature gradients between the core and surface
of the battery. Combined, these effects can lead to other non uniformities in state-of-charge, particle stress and levels of degradation.Introducing additional tabs may alleviate current
heterogeneity but can also lead to mechanical stress that can cause accelerated degradation and loss of capacity.
Tesla has recently proposed a larger format cylindrical 4680 cell, referring to 46 mm in
diameter and 80 mm in cylinder height, as measured by the external casing.
Larger format cells provide benefits for energy density and
power output but potentially exacerbate the internal current and
temperature heterogeneity by having a longer jelly-roll.
Increasing the cylinder diameter may also prove problematic for effective
thermal management due to decreased surface-area-to-volume ratio
of the cell. To address these problems,
Tesla proposes a “tabless”current collection method by using the current collector foil itself
with a contiguous array of current collectors extending from the edgeof the foil.
This should mean that the current distribution inside the cell is much more uniform with the majority of the edge of each current collector foil being held at the same potential.
In theory, this design reduces much of the ohmic loss inside the cell and with it much of the heat produced.
Simulations are presented that utilize the new cell format and
different tabbed and tabless current collection designs to demonstrate current heterogeneity and predict temperature resulting under different cooling scenarios.
The simulations provide data that is in stark contrast and demonstrates the importance of the ohmic losses to thermal management, even with a modest discharge rate of 1C.
The computational framework makes use of underlying physical models, rather than equivalent circuits and should provide valuable insights for battery design and future degradation studies.