# Thermal

Most battery cells operate happily within the temperature range that we are happy to operate in, namely 0°C to 35°C. However, in lots of applications we want them to operate below freezing and up to much higher temperatures.

Depending on the extremes of the environment the battery cell(s) might have to be heated and/or cooled.

Also, the fundamental operation of the battery cell changes with temperature. This means that temperature gradients in cells can have a significant effect on performance and lifetime. You can extend this to cells at different temperatures in the pack.

### Heat Generation in a Cell

Heat generation in a cell can be defined quite simple for the case where the cell is operating within it’s normal limits. The first expression gives the heat flow [W].

The first part of this equation is the irreversible Joule heating term, the I2R term.

The second part is the reversible entropy term or Reaction heat terms. The charge and discharge reaction can be exothermic or endothermic under certain conditions.

### Specific Heat Capacity

In lots of applications we use the heat capacity of the cell to buffer the peak heat generation during charge and discharge events. The specific heat capacity and mass of the cell can be used to give an idea as to how hot the cell would get during that event.

• vented flooded 1080J/kg.K
• VRLA-gel 900J/kg.K
• VRLA-AGM 792J/kg.K
• Lithium ion cells range in heat capacity between 800 and 1100 J/kg.K
• NCA 830 J/kg.K
• NMC 1040 J/kg.K
• LFP 1145 J/kg.K
• Nickel metal hydride
• Sodium Ion

The maximum operating temperature of the cell will limit how long the cell can be charged or discharged. Even if the cell is passively cooled it is likely that the design will mean that the heat is conducted to other components and the structure.

### Thermal Conductivity

The thermal conduction of the heat from the core of the cell to the cooling system is an important path that needs to be considered when designing a battery pack.

#### Thermal Conduction in a Cell

Whatever way we cool a battery cell we will create temperature gradients in the cell.

It is not possible to apply cooling to all of the active area of the electrodes, this would be nice, but would significantly reduce the energy density of the overall battery pack. So we have to apply cooling to the outside surface of the cell.

The active material in a cell is laminated with electrodes of copper, aluminium and a separator. The thermal conductivity changes depending on whether it is in plane or through plane. In-plane the conductivity will be dominated by the metallic electrodes and is approximately 10x to 100x greater than the through plane conductivity.

The maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.

• High temperature and the SEI layer on the anode grows faster. If the SEI layer grows fast it tends to be more porous and unstable.
• At low temperatures we see slower diffusion and intercalation with the possibility of lithium plating. Lithium plating removes lithium from the active cell, reducing cell capacity. Also, lithium plating can subsequently into lithium dendrites that can cause electrical shorts.

Thermal Conductivity versus SoC – the through plane thermal conductivity of a pouch cell was measured [Ref 3] and showed a negative parabolic dependence for fresh cells, where the highest thermal conductivity values were shown for the highest state of charge. This increase in conductivity at high SoC was shown for new and aged cells.

Thermal Conductivity versus SoH – for cells where a pressure was applied to the cell the thermal conductivity in plane decreased with age by around 4%.

Importance of Pressure – the through-plane thermal conductivity degraded by 4% when a pressure was applied to the cells [Ref 3]. When the cells were not under pressure the thermal conductivity degraded by 23% when they were aged.

This is just down to the thermal conductivity of the active layers.

##### Thermal Interface Materials

Use the right material, check compatibility, check the tolerances of the design, ensure it can be applied as per the design and ensure it will survive the lifetime of the battery pack.

### Temperature Limits

There are a number of temperature limits of a battery cell, some harder limits than others. It is worth understanding these in general before looking at a specific cell.

These temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.

Cold Temperature Charge / Discharge is limited by:

1. poor kinetics on both the interphase and the electrodes, which means larger SEI resistance and a reduction in the Li+ diffusion coefficient in the cathode and anode
2. decreased ionic and electronic conductivity, lower viscosity, and high freezing point of the electrolyte
3. lithium plating and Li dendrites on the surface of the anode, which threaten the safety and cycle life of LIBs.

### Cooling Systems

There are a number of different cooling systems / media used to extract the heat generated in a battery pack, the main options are:

• Air cooled
• Liquid cooled
• Phase change
• Active refrigerant cooling
• Passive phase change material (PCM)

For all of these cooling approaches there are a number of hybrids. eg passive cooling could also include a phase change material. There are pros and cons with all of these approaches on top of the fundamental criteria as to how much heat each method can extract.

Tesla Fast Charge Thermal Model – a look at coolant flow options and impact on cell to cell maximum temperature difference.

The test condition is fast charging.

##### Pouch Cell Cooling

The best overall option comes out as Edge Cooling and this is the most common pouch cell cooling system that you will see in battery electric vehicle applications.

As we pursue faster charging and we solve the electrical isolation and thermal conductivity in more cost effective ways the tab cooling approach is likely to become more accepted.

#### Pouch Cell Design and Edge Cooling

The design of the pouch cell and the optimisation of the cooling system have gone hand in hand. The cell has been optimised to reduce internal resistance and to improve the pathway to the cooling plate. This optimisation of the edge cooled design has also reduced cost, space requirements and mass.

Also, we need to look at the path between the cell active materials and the coolant system. This pathway has to consider electrical isolation, thermal conductivity, chemical compatability and mechanics. The behaviour of this pathway over the lifetime requirements must also be taken into account as cell resistance increases with age and hence they tend to generate more heat for the same performance.

#### Base vs Side Cooling Cylindrical Cells

For the dimensions of the 21700 cell base cooling gives ~12% greater heat flux, for the same temperature gradient, than side cooling.

##### Single Side 21700 Busbars

In the case of cylindrical cells it is possible to connect to both the positive and negative terminals of the cell on the top surface.

Thus leaving the bottom of the cell free for cooling.

The application of thermal interface materials is also an important consideration in manufacturing as this pattern can result in non-uniform or even voids in the TIM.

### Storage Temperature Range

A battery cell can withstand a wide range of temperatures in storage. However, a lithium ion cell will age in storage and that ageing will increase with temperature.

• Lithium ion cells are best stored between 5°C to 20°C is optimal with an SoC between 30% and 50%
• Nickel metal hydride cells can be stored between -20°C to 35°C. Panasonic suggest a narrower temperature range for their cells of 10°C to 30°C.

#### References

1. “Handbook for Stationary Lead-Acid Batteries, Part 1: Basics, Operation Modes and Applications,” Handbook (part1), Industrial Power, Application Engineering Edition 6, 2012