High-voltage batteries are increasingly being used in the automotive field in the course of electrification as a means of reducing CO2 and pollutant emissions. This is taking form in the use of existing vehicle platforms within hybrid or plug-in hybrid concepts as well as on the basis of new, fully electric solutions.
Currently, automotive manufacturers are amending – or replacing – their vehicle portfolios with electrified applications. Furthermore, new companies are being established worldwide that develop and launch electric vehicles in various manifestations. Driven by this need for new technologies, there is a strong need for support in the development of high-voltage batteries, which FEV can provide from the first concept to serial production and, beyond that, up to recovery and recycling.
The mechanisms described are not exclusively limited to the automotive sector. For commercial vehicle, industrial, and marine applications as well, research is increasingly being conducted regarding how vehicles previously powered by combustion engines can be battery operated. Here, the focus is mainly on smaller commercial vehicles, building machines, or smaller boats.
These changes enable both established and new manufacturers to secure further market share. The resulting pressure on development, and mainly battery development, frequently presents a significant challenge to the manufacturers’ planning. In the current projects, electric drives and batteries are frequently being integrated into existing vehicle architectures (also called mixed architectures) which are build for both conventional and electrified drives. This leads to battery installation spaces with significant free-form surfaces and complex or two-tier battery structures. Such configurations significantly increase the effort and expense required for development with regard to cooling system components, high-voltage performance, low-voltage cable harnesses, understeering devices, holders, and fixing elements.
However, market pressure requires that battery development projects be carried out within the planned time frame, with no possibility for subsequent changes. Battery cells are the core of every high-voltage battery. These cells are the basis for the configuration of the modules that then determine the energy and power of the battery within the corresponding electric wiring.
The enormous increase in demand has considerably restricted the availability of the different cell types and products from different manufacturers. Smaller manufacturers in particular are faced with significant challenges with regard to ensuring cell availability for planned applications. The serial production of battery systems may also prove to be a hurdle within a given development activity. For smaller annual unit quantities in particular, an economically viable concept can be difficult to create under certain circumstances. All this can have a long-term impact on the evolution of development projects.
FEV provides support using its experience from many serial development projects, and can assess the individual situation early on and make corresponding proposals in order to create a stable basis for such a development activity. In this context, the FEV engineering portfolio covers all development activities as well as, when necessary, the identification, recommendation, and qualification of a production partner that will serially produce the battery for the client.
“FEV IS A STRONG PARTNER FOR SMALL SERIAL PRODUCTIONS OF BATTERY SYSTEMS AND HANDLES ALL THE NECESSARY PROCESS STEPS IN THIS CONTEXT”
FEV is capable of offering development services in different manifestations. The basis of the FEV battery development portfolio includes all necessary services for development, from the first battery concept up until serial production, and for providing support beyond.
If required, FEV is also a strong partner for small serial productions of battery systems and handles all the necessary process steps in this context for the preparation and subsequent serial production for batch sizes of up to 1,000 units per year.
Battery-powered electric vehicles will achieve high acceptance in the market when they are at least equal to conventionally powered vehicles in all points relevant to clients
One of the primary requirements for clients is range. Clients do not wish to give up the advantages they are used to with vehicles that are powered by combustion engines. The range of electric vehicles depends directly of the available energy charge of the battery. However, since growing capacity also leads to an increase in the weight and volume of the battery, a gravimetric and volumetric energy density as high possible is desirable in order to offer a vehicle that continues to be attractive.
To travel longer distances, the driver will be obligated to make a pit stop in order to recharge the battery. This cannot take significantly longer than with fuel-powered vehicles. Therefore, another requirement is the optimization of charging time via better quick-charging capacity. The overall capacity of the battery will increase in the future. According to current forecasts, this capacity will reach 50 to 75 kWh (mass market) or 80 to 120 kWh (premium segment). For an increase of the State of Charge (SoC) from approx. 10 to 80 percent, the charging time will also be likely reduced to 15 minutes (premium segment) or 30 minutes (mass market). This leads to charging performances of up to 350 kW, which must be provided by the infrastructure. The material compositions of the anode and the cathode are being further optimized in order to increase the energy density. Currently, a Si/C composite is used for anodes, while for cathodes, in contrast, the nickel ratio is being increased. In the long term, the solid-state battery possibly offers considerable potential. In order to optimize the quick charging capacity, the cell design (solid-state battery) can be adjusted and the thermal management can be further optimized. Furthermore, the connection and the contacting systems must also be improved with regard to current carrying capacity.
- Driving power
Previous purely electric vehicles mainly showed restrained driving performance, thereby creating a first impression among clients. Current models must do away with these prejudices and offer clients driving comfort equal or superior to that which they are used to. To this end, good acceleration values and the possibility of repeatedly demanding maximum performance, as well as long-term travel at maximum speed without any restrictions whatsoever are all important criteria.
In order to achieve such acceleration values, high maximum amperage values in the range of 1,200 to 2,000 A for 4 to 10 seconds are necessary. Strong currents for a period of 30 to 120 seconds are necessary for the repeated demand for maximum performance, as is high continuous current for travel at maximum speed. At the same time, the cells (approx. 50°C) or the lines (approx. 100 to 150°C) must be prevented from exceeding their maximum authorized temperatures.
This requires the optimization of the current path from the active material of the cell to the inverter and e-engine. This includes, among other things, internal cell connection technology, plug systems, separation systems, and safety installations. The cells must be actively cooled (e.g. with water-glycol), and the lines must be passively or actively cooled (e.g. through heat pipes) to avoid overheating.
Another current, significant challenge is the aging of Li-Ion batteries. In the past, clients have occasionally had to deal with negative experiences with regard to the longevity of Li-Ion batteries used in consumer products (for instance, laptop computers, smartphones, digital cameras, etc.). For modern Li-Ion batteries as well, the longevity depends on use, time, and temperature. If the usable energy charge in relation to the new status reaches 80 percent or less of State of Health (depending on the cell chemistry and manufacturer), use of the battery in battery electric vehicles is no longer sensible. If this is taken into account in the battery design (hardware and software), the battery can then later be used in a second life approach – e.g. as a stationary buffer battery.
Currently, a Li-Ion battery in automotive applications can be used for 8 to 10 years. The aim is to achieve a medium-term duration of 15 years and a long-term duration of 20 years. In addition to calendar aging, cyclical aging must also must also be considered. Currently, said aging is between 150,000 and 250,000 km, until the Li-Ion batteries reaches 80% of SoH.
In order to reduce calendar aging, the average temperature of the cells must be reduced with effective thermal management. Cyclical aging is equivalent to state-of-the-art cells with around 1,000 to 2,000 full cycles with 80 percent of discharge depth each. The optimal working range of the cells is between 10 and 25°C (parking) and 40°C (driving). FEV can prepare an assessment of the longevity for different stress and utilization profiles with simulations. Improvements in the stability of cell chemistry (electrolyte, coating, nanostructure of the electrodes, and other elements) are necessary for the optimization of cyclical aging in order to reduce irreversible processes (e.g. electrolyte decomposition, formation of a SEI coat).
“IN ORDER TO OPTIMIZE THE QUICK CHARGING CAPACITY, THE CELL DESIGN CAN BE ADJUSTED AND THE THERMAL MANAGEMENT CAN BE FURTHER OPTIMIZED”
Regarding the safety of high-voltage batteries, a distinction must be made between utilization safety (UtSa) and functional safety (FuSa). While safety in use is intended to guarantee that there are no safety risks when used as expected or misused, functional safety based on ISO26262 ensures that no safety risks occur in the event of electric function failure.
Through utilization safety, risks are identified, assessed, and reduced with measures. The risks notably include thermal runaway, coolant leakage, high-voltage contact protection, and crash safety. If functional measures are taken for the avoidance of these risks, said measures fall under FuSa and must be robust, in accordance with Automotive Safety Integrity Level (ASIL) integrity. To prevent thermal runaway, a utilization safety first step is protecting the cells from overcurrent and overvoltage/undervoltage (overdischarge/underdischarge). In another step, the FEV functional safety concept additionally protects the cells using appropriate hardware (sensors, actuators) and software, in accordance with ASIL integration (A-D).
“THE BASIS OF THE FEV BATTERY DEVELOPMENT PORTFOLIO INCLUDES ALL NECESSARY SERVICES FOR DEVELOPMENT, FROM THE FIRST BATTERY CONCEPT UP UNTIL SERIAL PRODUCTION, AND FOR PROVIDING SUPPORT BEYOND”
Currently, battery-powered vehicles are more expensive for clients than those that are equipped with an internal combustion engine with comparable product characteristics, mostly due to the battery. Optimistic forecasts predict that, by 2023/2024, the first electric vehicles will reach the purchase price of a comparable combustion engine model.
For this reason, it is necessary to reduce the current high costs of cell production in relation to the energy charge in kWh/kg. On the one hand, this can be achieved with a higher energy density with almost the same amount of material used. On the other hand, raw material extraction, processing, production automation, and cost-reducing measures in cell design are necessary in order to decrease the resulting costs per kWh.
A very promising measure for the reduction of the cell costs is the substitution of the relatively expensive raw material cobalt with the cheaper option – nickel – in the cathode. The higher nickel ratio also helps to increase the range, whereby the nickel ratio is gradually increased in the N:M:C ratio (Nickel-Manganese-Cobalt) from 111 to 532 to 622, and up to 811 (“High-Ni roadmap”). However, these measures represent a trade-off with stability and, accordingly, with safety and longevity, which cannot be neglected.
Increasing the nickel content within the cell enables a longer range with short charging times. On the other hand, this increase also creates a thermally unstable system, which increases the security challenges. Furthermore, calendar and cyclical aging are increased, which reduces the longevity. However, the substitution of cobalt with nickel has a positive impact on costs. Due to the changed cell design, however, there is an increased risk of lithium plating and overtemperature during rapid charging, which can lead to loss of capacity and thermal runaway. Optimized rapid charging leads to a higher thermal load due to higher currents, which creates bigger challenges for safety. Furthermore, the higher currents lead to reinforced lithium plating, which restricts longevity.
To increase driving performance, the overall system is subject a higher current load. This increases the risk of an overload of the individual components, which can lead to a thermal event or the loss of insulation protection. Furthermore, the higher currents have an influence on cyclical aging as well as on calendar aging due to the higher average temperatures; this, in turn, leads to reduced longevity of the Lithium-Ion batteries. In addition, the lines and the (plug) connectors must be designed to be more robust, which leads to additional costs due to changes in material needs.
If security is increased, there will be additional costs, since further functional measures using hardware (sensors, actuators) and software (algorithms, functions) will become necessary. Larger security reserves in the battery management system can also limit maximum performance, performance reproducibility, and range.
FEV provides consulting with a team of internationally recognized specialists at various sites, OEMs, Tier 1 suppliers, and cell manufacturers or takes over entire projects as part of general development. Initial technical concepts are created and coordinated so that they can be specified in the series development process for the start of production. In addition to the resolution of the described target conflicts in development phases, prototype batteries and small serial productions can be reated and validated on our own test benches for cells, modules, and packs.