High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (Part 1: Introduction and Battery Systems)

High Performance Polymers in Electrification: A Must-Have Or A Nice-To-Have (www.findoutaboutplastics.com)



In automotive industry, electrification gains traction and design engineers all over are working to increase the energy density of batteries, create higher voltage traction motors and increase allover system voltage. In this three part series, we will discuss the requirements of different system components, i.e. batteries, traction motor, power electronics and autonomous driving in terms of polymers. Let’s get started with batteries.

Introduction
- Plastic parts in electric vehicles (EV’s)

In 2019, we see all major OEMs and Tier-1 supplier continue their efforts toward electrification and new mobility solutions. Currently, development progress of internal combustion engines is at 1% per year [1], whereas electrification developments progress at a rate of 7 %. Industry experts predict that the connected mobility can grow according metcalfe's law with a rate of 70% (taking Facebook user rates as a baseline). The overall plastic weight per car will not change significantly with EV’s. However, there will be a slight increase in weight in total. There are currently 10 000 parts made out of plastic in an average car and these use ca. 39 different polymers. Out of the 39, 6 are used the most, i.e. polypropylene, polyurethane, polyamides, polyethylenes, acryle-butadien-sytrenes, and polyvinylchloride [2].

Check out here my infographic on “Plastics in Automotive”

Also in electrification, light weighting together with fuel economy will continue to be a megatrend. The rule of thumb says that for every 10% of weight reduction, fuel economy improves by ca. 6-8%. This additionally drives the consumption of plastics in automotive. The Chinese car market shows a lot of potential for light weighting since most cars run on old technology platforms. Furthermore, this is the largest car market today.

- What changes with EV’s?

EV’s have no longer the need for fuel tanks, AdBlue tanks, pumps and fuel connections. Here, we will see a drop in engineering polymers, together with elastomers since fuel lines and gaskets are not in demand. On the commodity side, high density polyethylene (HDPE) will also lose out since it is used for making tanks. Under the hood, combustion engine covers will not be needed either, however new engine covers for traction motor will rise.

- Which challenges are there concerning material selection?

In the past, material selection for internal combustion cars was easier since lots of experience as well as know-how were already built-up. This included the handling of specifications. For under-the-bonnet applications for example, aliphatic Nylons such as PA 6, PA 6.6., and aromatic Nylons such as polyphtalamide (PPA) are widely used. Furthermore, polyphenylene sulfide (PPS) is used where the highest heat and chemical requirements over the lifetime of the car are needed. PBT, PP, PU and POM/Acetals round up the polymers used in several exterior and interior applications.

Today, in hybrid and full EV’s, material selection tends to be much more differentiated, since applications need to fulfill specific requirements. The one-fits-all approach is no longer working in a similar sense. Standards from other industries such as electronics influence now material selection in automotive. As a result, a “wedding” between e.g. consumer electronics and automotive standards may take place.

Looking into the high performance material portfolio, semi-crystalline polymers fulfill stringent electronic requirements such as high CTI (>600), intrinsic flame retardancy, dielectric strength, creep, tracking resistance, EMI shielding, and high (140°C) Relative Temperature Index (RTI).

Lithium ion batteries – the heart of the EV

For coating the separator, fluoropolymers such as PVDF in aqueous dispersions can be used. Commodity polymers such as ultra-high-molecular-weight polyethylene (UHMWPE) can be used as separator substrate material. PVDF plays a role in making electrode binders (anode and cathode) too. In cell manufacturing, high adhesion between electrodes and separator is needed to obtain good laminates. Most common cell systems are pouch and jelly roll systems.

Battery pack and module – holding all together

Going outside the cell to the finished battery pack, several challenges need to be overcome. In general, a battery pack has between 10 to 16 modules and each module can have between 10 to 12 pouch cells.

For battery packs, three things need to be balanced:

• energy density (range),

• power (rapid charging),

• safety (non-flammable).

Every battery is connected over the pack’s housing with an electrical control unit, which protects the cells from overloading. The housing must ensure that all batteries stay in place and impact or vibration do not compromise performance. Figure 1 summarizes the potential plastic materials for cells, module, and pack.

Figure 1:  Overview of polymeric materials for battery technologies in the electrical powertrain.

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Cells are connected to one another over busbars. Stability must be ensured. Busbars are usually long copper parts (up to 30 cm) overmoulded with PPS to obtain low dimensional changes. Flame-retardant plastics become increasingly important for packaging of the cells into modules. Modules have endplates and separator plates. Polymers such as PPS are a perfect candidate to be used as endplate and separator material in such environments since it ensures a best-in class thermal and chemical stability (RTI up to 220°C), inherently flame retardancy, and high dimensional stability. Selecting a linear PPS grade is beneficial since it has lower flashing during moulding. This minimizes post-processing steps after moulding and the investment of a de-flashing station. In addition, polycarbonate and their blends offer long-term capability and low-temperature impact strength which are needed for the battery modules as well.


Generally, all different assembles can benefit from thermal conductive materials which support the removal of the heat generated by the cells. Different polymer grades can be improved in this context by means of additives.


Thermal management – keep it cool
Next to the classic thermal management systems which use water-glycol, new ways of cooling are on the rise. This is driven by water-glycol reaching its performance borders with higher voltage system (>500 V). A new way is the so called “direct liquid cooling” which uses a dielectric cooling fluid and the cooling component is directly immersed in this fluid. This can be realized with fluorinated fluids such as perfluoropolyether (PFPE). These kinds of fluids have good thermal conductivity and combine a low electrical conductivity with low viscosity. Furthermore, excellent chemical resistance is given through the fluoro properties.


Heating systems for interior and battery
In contrast to traditional combustion cars, EV’S dissipate less heat to enable sufficient heating of the passenger cabin. Therefore, additional heaters need to be installed. Most of such heaters are based on the Positive Temperature Coefficient (PTC) effect. The ceramic based PTC elements can be lined up next to each other holding together via a support frame.


Here, a stringent requirement is not having ions in your polymer formulation. This is important to prevent corrosion of overmoulded metal parts and connectors. Galvanic corrosion can reduce the electric performance down to failure. Therefore, electrical friendly, halogen free stabilized semi-aromatic Nylons or PPS might be suitable here.


During operation, the battery pack temperature should be kept at 60°C. Heaters are used to keep temperatures on a certain temperature to ensure efficient charging. Heaters can operate in the high-voltage range (500V) with an output of 7 kW. High performance polymers will ensure their proper function over lifetime.


Charging Systems – high voltage with high safety
Charging the batteries of your electric vehicle in a reasonable time requires high voltage charging systems. Temperatures should be kept between 0°C to 45°C. Additionally, interconnection systems provide the power for the electric engine. Electrical properties such as dielectric strength, volume resistivity, creep, tracking resistance need to be carefully considered by design engineers.


Color coding helps safely handle parts in the event of an accident. Above 60 V, a system is considered to be high voltage and then orange color coding of connectors and cables is required.


For covering all these needs, polymeric materials need to fulfill Comparative Tracking Index (CTI) values of over 600, dielectric strength in the range of 30 kV to 35 kV, and Relative Temperature Index (RTI) of 140°C.


Aliphatic Nylons such as PA6, PA66, PA 46 fulfill this stringent criteria. Exception are certain temperature levels. Semi-aromatic Nylons such as PPA (6T/6I; 6T/6.6; 6T/6I/6.6) fulfill the temperature and electrical requirements. High mechanical strength combined with reflow soldering durability and laser welding properties make them a good choice. Important is to select types which are halogen free and free of red phosphorous which in turn allows Nylons to reach high CTI values. Electric corrosion of assembly bins is prevented by not using ionic heat stabilizers. Most Nylons have an HB flame rating and V0 rating is possible, however it might involve inorganic heat stabilizers.



With this I will close the first part of this high performance polymers series for EV.

In the second part we will look into the traction motor.


Thank you for reading!

Till next time!

Herwig Juster


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Literature:
[1] http://www.professoren.tum.de/lienkamp-markus/ and https://www.youtube.com/watch?v=5moHQFbEsDU&t=2238s
[2] A. Patil: An overview of Polymeric Materials for Automotive Applications, Materials Today: Proceedings 4 (2017) 3807-3815

The battle for electric cars: BMW i3 vs. Tesla model S

Last weekend we visited the beautiful sand dunes of Zealand in the Netherlands. When we parked our car, I noticed the electric car BMW i3 next to us. It was loading his batteries to be ready for the travel back home.  It can clearly be seen that this car doesn’t look like a classic BMW.

From the polymer engineering point of view, the BMW i3 is quite unique, since its whole body structure is based on carbon-fibre reinforced plastics (CFRP), which was not yet used in such a dimension in a commercially available car.

Apart of the BMW i3, I noticed more and more the Tesla model S on the streets of the Netherlands. Both car types are indeed impressive. Then I wondered: What are the main strategies of those two manufactures? And why is Tesla so successful compared to the other electric car manufactures?

So, here is what I found out and would like to share with you:

• The battery system: the most important thing of an electrical car is its batteries. Tesla manufactures its own battery systems.  It uses standard battery cells, as those you can find in your notebook, and align them together to a big battery comprising about 5000 individual cells. Thus, Tesla is not dependent on a battery system supplier. Furthermore the use of standard cells (from the world market) allows Tesla to minimize associated costs due to the existence of competition. Tesla holds all the patents related to the production of such big battery systems composed of 5000 to 8000 standard cells. Conversely, BMW started a partnership with Samsung SDI to also develop its own batteries. Now, Samsung can either take advantage of this situation by increasing the battery price, or they can help BMW i3 to develop cheaper batteries, which may contribute to lower down the price of a car and, thus, make this technology accessible for more and more people. In the most critical scenario Samsung builds its own electrical car. We will see!
• Fact and rule as well is: the company which holds the patent for the batteries holds the car! The engine is not so important because it is no longer a secret, since Nicola Tesla developed the three-phase A.C.-motor. Those engines have a high energy density (69 kW) in small dimensions.
• Tesla cars can be indeed considered electricity driven cars. The Tesla model S has one engine and that’s it! A different situation you find under the hood of the BMW i3, wherein there is an electrical engine and in addition, a 34 PS Otto- combustion engine that generates additional battery power. This is in my point of view one system too much. Double systems result in more weight, space and, thus, cars become more expensive. You can also choose the BMW i3 without range extender; however, instead of including more batteries, the space stays then unused and empty.
• Regarding technology innovations: A rule of thumb says that you should only put one new innovation onto the market. Too many innovations at one time are overloading the users and probably could even turn them down. This means that in the case of Tesla model S, the central positioned computer system is the new innovation. The batteries and the engine were already used in the previous model Tesla roadster. Both cars use an aluminium car body, which is also well known. In the BMW i3, several things were new: the engine, the battery system, and the carbon-fibre reinforced plastics structure (CFRP) for the whole body. This is too much for the customer and as well for the manufacturer.
• Apply new philosophy to electric cars: they ripen at the customer. In the old industry of car manufacturing, you were used to have a face-lift after 1 to 2 years that the car entered the market. In the Tesla model S, the control computer downloads updates and you can customize several things on your own. These are the new ways of car face-lifts.
• Last but not least - the selling strategy: it is easier to sell new technology to people, who have more money than the average population. For the normal customer is the BMW i3 too expensive for just having a car for getting into the city and go shopping. The model S from Tesla has dimensions of the Mercedes S-class and looks just good. It is aimed towards the population driving upper middle class and upper class cars. It is a limousine and this is the reason that people will spend more money than usual! The BMW i3 is much too high and the BMW-logo is pressed in the front, which looks like there was no space anymore.

So what are the main conclusions to take-away from this comparison: Tesla has a fresher view on the electric car philosophy, where else the embedded car manufactures still have strong component of old industry thinking. This old thinking is based on turning a combustion driven car into an electrical car. It is like in polymer engineering when more and more metal-made products get replaced by a 1:1 looking plastic part although total different design rules apply for polymer based products.

Furthermore, there will be new car manufactures such as Google and Samsung and these have the power to seriously compete with the well-known car producers. This summer, Google tests their developed electrical car in their home town Mountain View. The car is designed for two people and will probably not even have gas and brake pedals, neither a stirring wheel because it is driven by the central computer only. The rumours around a development of an electrical car by Apple are not stopping.

Audi wants to bring a battery driven SUV by 2018 on the market. Audi wants LG Chem and Samsung SDI as battery supply companies. Here we have the same situation as with the battery supply of BMW.

One thing is clear: the way of mobility will substantially change and I think that this will be decided in the next five years.

Kind regards,

Herwig

Literature:

[1] Tesla model S: http://www.teslamotors.com/models

[2] BMW i3:  http://www.bmw.de/de/neufahrzeuge/bmw-i/i3/2015/erleben.html

[3] Tesla strategy: https://www.youtube.com/watch?v=nkfeR7-4c8E