Solid-state batteries in automotive technology

Our researchers analysed the commercial viability of solid-state batteries in automotive technology and whether elevated operational temperature is a barrier to mainstream adoption. Dr Chris Vagg, Lecturer in Advanced Automotive Propulsion Systems at the University of Bath, and Ryan Hughes, Postgraduate Research Student, looked into the role operational temperature plays and whether adjusting the thermal management architecture can help overcome the current challenges and accelerate route-to-market.

The challenge

Unlike conventional batteries that are used in most electric vehicles and electronic applications and which feature a liquid electrolyte, solid-state batteries use a solid electrolyte between the anode and cathode. Due to an increase in structural and chemical stability, solid electrolytes enable the use of a lithium metal anode, which can potentially double the gravimetric and volumetric energy densities of a liquid electrolyte cell that uses a graphic anode. However, issues with unstable layer interfaces have prevented many chemistries from reaching automotive commercialisation. To bridge this gap, solid polymer electrolytes have been used as a solution.

While the benefits include low flammability, faster charging times and higher energy densities, the key barrier to using polymer electrolytes is that they require an elevated operating temperature, typically between 60-80°C. Preheating the batteries is therefore highly energy-intensive and necessitates prior planning. In their project, Dr Chris Vagg and Ryan Hughes sought to overcome this issue in the adoption of solid-state batteries and accelerate their commercial viability.

The approach

In order to avoid the issue of pre-heating, a modular battery architecture was simulated. In this architecture a number of smaller modules were thermally isolated, allowing them to be heated and function independently.

Before the first solid-state module is heated, a lithium iron prosphate (LFP) module is used to meet the initial power demand. In the first instance, the heat for the solid-state modules is provided by an electrical positive temperature co-efficient (PTC) heater, and subsequently by waste heat from the motor and other modules. Understanding the thermal energy flows within the system was key in the methodology applied here. This enabled the researchers to determine whether each module could be heated to its operational temperature fast enough to fulfil the tractive power demands of a typical passenger vehicle.

Using a lumped capacitance approach for the thermal modelling, heating power was calculated using system efficiencies. However, for increased accuracy, this was limited to rates which can be accommodated by the battery cells, with any excess heat rejected to the environment.

The outcome

For the initially used LFP pack, a capacity of 10kWh was chosen, as well as for each of the solid-state modules, providing optimal balance between space requirements and heating precision. Initial findings relating to the use of insulation showed that this greatly impacts volumetric energy density, which is only improved by approximately 25% due to the current cathode limitations. Consequently, insulation that is thicker than 10mm negates volumetric improvements achieved through the use of a solid-electrolyte. However, gravimetric energy densities are still significantly improved. Heat losses less than 50W per module could be easily covered by heat generated from the motor and their own inefficiencies, with a final insulation thickness of 15mm being chosen.

The feasibility of the system was analysed by repeating the Worldwide Harmonised Light Vehicle Test Procedure (WLTP), which replicates real world driving. It demonstrated that there is sufficient waste energy to heat the modules quickly enough to meet tractive energy demands.

Additional simulations revealed similar results during a constant 130 kph motorway drive cycle. The feasibility, however, is greatly reduced in ambient conditions below 10°C.

After the first solid-state module is heated, the PTC heater is deactivated and only waste heat is used. Waste heat from the motor is delayed, allowing it to reach 60°C, while further into the drive cycle, more of the heating energy comes from waste heat instead of otherwise useful energy from the battery.

Conclusion

The research results suggest that despite their elevated operating temperature, it should be possible to accommodate solid-state batteries by redesigning the thermal management architecture. This could significantly accelerate their commercial viability and adoption, particularly if combined with ongoing efforts to lower the operating temperature.