Introduction
Solid state batteries (SSB) have been heralded as the successors of lithium-ion batteries (LIB), with superior safety and high performance projected to alleviate EV drivers’ anxieties in terms of range, charging time and cost. The benefit of SSB technology can be expected at cell level, but also at pack level, with weight and cost reduction possible, and design simplification enabled by SSB’s unique features.
In this paper, UK battery experts Balance Batteries Ltd (Balance Batteries) modelled an hypothetical battery pack that capitalizes on the expected benefits of Ilika’s solid state battery cells.
Benchmark Vehicle
The baseline electric vehicle selected for this modelling exercise was the Hyundai Ioniq 5, a popular mass-produced SUV. The pack yields a usable energy of 84.1 kWh and weighs a class-leading 482 kg. A teardown of the Hyundai battery pack is publicly-available in this video from Munro & Associates, which, for example, describes how the cooling circuit constitutes of a cold plate: fixed at the bottom of the battery enclosure frame, the S-shape path in the cold plate, placed in contact with the open bottom of the modules disperses heat via the coolant fluid. The pack integrates 384 cells from Korean manufacturer SK On in a 6 parallel, 2 series configuration in each module.
Cells Assumptions
The SK On cell was selected for this work, due to its similarity, in terms of high-performance NMC cathode chemistry and pouch cell format, with Ilika’s cell. Of course, both cells have key differences, with Ilika’s cells incorporating an oxide-based solid electrolyte and silicon-based anode. A second important point is that Ilika’s SSB cells are currently in development, hence this modelling work makes some assumptions on the anticipated performance of the SSB cells, which will only be verified later.
The following parameters were considered in the model: gravimetric energy density, Direct Current Internal Resistance (DCIR), maximum continuous and pulse power, specific heat capacity, maximum operation temperature, cell expansion and stack pressure required. Two features explain the attraction to the SSB technology: its ability to potentially operate at higher temperatures and its high safety, even in NMC cells. See a comparison of nail penetration tests between Ilika’s SSB and a lithium-ion cell (not SK On), in this video.https://www.youtube.com/watch?v=fIP_jwKa9SU
Cell-level Benefits
Balance Batteries assumed that the SK On cell used in the Hyundai pack was the ABD0013, which yields 60 Ah, 3.65 V and an impressive gravimetric energy density of 293 Wh/kg. Lithium-ion technology still continues to improve but it is likely that it will eventually reach a performance plateau, whilst Ilika’s cells are targeted to yield 350 Wh/kg in their first product. If an Ilika cell of comparable capacity (60 Ah) was used in the pack, the energy density difference would lead to a cell-level weight reduction of 46.8 kg, i.e. the pack would be 16% lighter.
Pack-level Benefits
Pack design simplification is already taking place in the EV industry, taking advantage of cell-level safety features. BYD have been pioneers in cell to pack design with their Blade cell, thanks to the inherent safety benefit of the LFP chemistry. In this work, Balance Batteries have designed a variation on the cell to pack theme that also capitalizes on Ilika’s cell high safety. The intra-cell foam material was replaced by a thinner solid cell carrier instead, which saves weight of 26 kg. Additional components such as the modules, clamp plates, module wiring loom and in-module circuit boards could be fully or partially removed. Stack pressure is applied via a single clamp plate and air piston arrangement, this mechanism also allows for cell expansion / contraction during cycling and with life.
In some geographic areas, new regulations require not only a containment of thermal propagation within the battery pack, but also that the vehicle is able to continue driving for 5 minutes after onset of failure. This means that (a) the inside of the battery pack lid requires a thermal containment material and (b) the top of the cells/bus-bars also require this material. The non-flammable nature of Ilika’s cells implies that thermal barrier materials and cell venting parts may not be required, leading to an additional 6.6 kg weight reduction.
The above-mentioned cold plate (consisting of two layers – top and bottom) represents a significant weight of the pack enclosure. Simplifying the cooling system on the basis of Ilika’s extended safety and increased operating temperature is possible, but removing the plates altogether was judged a step too far at this stage. Similarly, thinning down the two anti-intrusion beams, which protect against side impact, 15.8 kg each, may be a more realistic possibility than removing them completely.
Fast Charging
Finally, a cell technology like SSB, which may withstand higher temperatures than LIB, could save valuable time during charging. This is because cells self-heat during charging, especially so during fast charges. Whilst the temperature of conventional LIB cells is kept below 55°C by the Battery Management System, SSB cells could perhaps withstand 15°C more. In Balance Batteries model, this would yield a 12 min fast charge for the SSB cell as opposed to 18 min for the LIB cell.
Cost
Do these weight reductions translate into financial benefits? Assuming European manufacture in volumes of 70,000 per year, Balance Batteries modelled that modification of the pack to a cell to pack design including pneumatic actuation would add £78k to the capital expenditure (tooling) cost, however this was partially offset by removing the cell thermal propagation materials, saving £17k of tooling cost and making a net increase of £61k.
Redesign of the installation, was calculated to save more than £2.5k in variable (part) costs. This saving means that the additional £61k in tooling cost would break even after only 22 vehicles produced and overall save £175M/yr at this production volume.
Why Pack Simplification Matters
Weight reduction in less complex pack designs has a real impact on the driving experience of EV owners. Higher accelerating power would be available to a lighter vehicle for the same energy. Less tyre wear could take place during the life of the lighter vehicle. Using the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) requiring a maximum of approximately 60 kW power, Balance Batteries modeled that for the cycle distance of of 23.3 km, the baseline vehicle used 185.8 Wh/km and the SSB vehicle 180.1 Wh/km, representing a 3% energy consumption reduction from the reduced weight alone.
Conclusion
Although Ilika’s SSB cells are not yet available commercially and their specifications may change with further development, Balance Batteries used realistic assumptions to model and compare the Hyundai Ioniq 5 pack with one that would hypothetically use a SSB cell. Key findings include:
- Pack weight reduction of more than 100 kg
- Bill of Materials cost reduction of more than £2.5k
- 6 min shorter charge (12 min vs 18 min, 10-80% SoC)
- 3% energy consumption reduction during drive
No endorsement or partnership between Hyundai, SK On and Ilika is in any way implied in this article. Only public information was used: from the Hyundai and SK On websites; from public websites not affiliated with either Hyundai or SK On, such as YouTube. Any error with Hyundai Ioniq 5 and SK On specifications are unintended. This work is a model based on assumptions about the performance of Ilika’s cells which may change in the future.
