Porous Power’s new microporous membrane will boost Li-ion battery performance

Q&A with Bernard Perry of Porous Power Technologies.

In 2006, Porous Power Technologies was formed around a new microporous membrane material designed specifically for use as a lithium-ion battery separator. At that time, most battery separator materials were originally intended for other applications and adapted when the lithium-ion market began to develop.

The company’s founders saw an opportunity to offer better performing separators and developed its proprietary polyvinylidene fluoride (PVDF) technology for commercialization over the next five years. In 2011, Porous Power formed a strategic partnership with the high-performance fiber-based materials company Ahlstrom. The billion-Euro-a-year-Finnish company, which now has a 49% stake in Porous Power, specializes in manufacturing nonwoven fabrics – one of the components in the separators.

The company’s initial products are available for early-stage evaluations, with over 30 customers around the world at different stages of testing. On January 1st, 2014, Porous Power will launch its next generation of safety-enhanced, commercially available product – a ceramic-grade PVDF separator. 

Charged caught up with Bernard Perry, Chief Business Development Officer at Porous Power, to get some details about the advantages of using the purpose-built material in EV batteries.

Charged: Give us the sales pitch. What makes Porous Power’s new products special?

 

Bernard Perry: Most of the battery separators on the market now are polyolefins, either polypropylene or polyethylene. They were manufactured with heat extrusion processes. After extrusion, they are stretched to make the film porous and to orient them. 

The problem is that the more you stretch them, the more energy is tensioned into them. When the cells get hot, this tension is released and causes shrinkage, which reduces performance and can cause shorting. Nonetheless, this is how they make state-of-the-art products that are going into the Li-ion batteries that we know and love today.

These polyolefin separators are reasonably good products for smaller electronic applications – cell phones, computers and power tools – but as you start getting into the larger format cell assemblies like electric drive vehicles, you need something more. 

Porous Power’s polymer is not polyolefin. We use an engineering grade PVDF. It’s a very flexible polymer that we solution cast. In other words, we turn it into a liquid solution and then cast it onto a substrate. We never stretch it. We actually coat it onto another film – a sacrificial film. Then we remove the high porosity PVDF. It looks like a kitchen sponge, as opposed to a rigid piece of plastic like polyolefin. Because our separator is cast, not stretched, the porosity is very high, exceptionally uniform and stable.

We also reinforce the product with a polyester nonwoven scrim. That works like a structural scaffolding to give it tensile strength and help in the battery assembly process. It also provides a skeleton inside the battery separator to prevent the battery from shorting out if the electrodes come in contact.

 

Charged: Why is high porosity important?

 

BP: You need a high-porosity separator in a Li-ion cell in order for the ions to flow back and forth between the electrodes. Higher porosity means less heat generated in the cell and more energy available to be used.

Uniform porosity is beneficial for this and for things like absorbing the electrolyte and avoiding variations in ion flow.

Polyolefin separators tend not to absorb electrolytes very easily or thoroughly. People have figured out how to do it quite efficiently for small cells – for example one inch by two inch. But for a larger-format cell – like the 10- or 12-inch square surface of a 30 Ah EV cell – getting the electrolyte evenly distributed through all the layers is a real trick. It takes a lot of time, and you have to be really good at what you’re doing, otherwise you’ll leave dry spots that won’t be detected easily. Dry spots, without electrolyte, are likely to be a point of failure at some point during the battery’s life. 

If you look closely at a lot of polyolefin separators, they’re just not that uniform from corner to corner and edge to edge. 

There are areas of high density and areas of low density because of the nature of that manufacturing process. In the areas of low density, ions will flow easily, and in the areas of high density ions will flow slowly – forcing them around to the low-density areas and causing concentrated flow. That generates different heat profiles across the surface of large-format batteries. This can cause damage to the separator, but also change the surface effect on the electrodes adjacent to the separator. As it gets heated differently, it develops different surface characteristics. The more variation you put into the separator – or the surface of the electrode – the quicker it will fail, significantly reducing the cycle life of that cell. 

Because we have exceptionally uniform porosity, you do not have as much variation in ion flow. 

The surface degradation of the electrodes is very uniform, which contributes to improved safety and longer battery life.

Charged: What is the role of ceramics in battery separators?

 

BP: Virtually all large-format cells are using battery separators that contain ceramics. It reduces shrinkage, adds thermal stability and higher temperature performance. 

When a polyolefin separator gets up to about 110 degrees C, it starts shrinking quite a bit – up to 5%. If it shrinks away from the electrode edges, there are problems. So, they put ceramics on top to reduce the shrinkage and add thermal stability. The problem is that when you overcoat a porous structure, you fill up the pores, and it’s very difficult to maintain high or effective porosity.

We don’t overcoat ceramics. We actually mix ceramic particles into the solution before we cast the separator. When the pores are formed, the ceramics are embedded into the wall system and not in the pores. So our separators with a high loading of ceramics are about the same porosity as our separators without ceramics. 

With a polyolefin separator, the maximum you can really get them to is about 55 percent porous. Overcoat them with ceramics on two sides, and you net out at about an effective 40 percent porosity. Not only do they lose a lot of effective porosity, but most start with a lower porosity to begin with. 

We start with about 65 percent porosity. After the addition of the ceramics, we only lose a couple of percent and net out at around 60 percent. That allows us to have a safer, low-shrinkage separator with increased robustness, extended cycle life, lower internal impedance and improved safety. That is the big benefit that we have over the competition.

There are a couple other companies that have ceramic products similar to ours – that are not coated – but there are also big differences between us and them.

 

Charged: How does the PVDF polymer compare in terms of safety?

 

BP: Using the solution casting technique, we’re able to make separators that shrink very little compared to polyolefins that are stretched. That is a benefit on the performance side and also on the safety side. 

It’s a flexible product. If the cell is ever damaged, as in a penetration abuse situation, the PVDF will not shrink away from the nail. Rather, it will stay in place. The PVDF will not burn or support a flame. It’s stable through 160 degrees C, at which point it starts going into a gel state. A damaged cell may no longer function, but when the PVDF gets hot it tends to stay where it is even if you put a nail through it or have an internal short in the system. 

With a polyolefin separator, the tendency is for it to shrink away from any hot spot. By stretching it during manufacturing, energy is put into the system. If you have a short in the cell, you get a super hot spot. Then the polyolefin will shrink away from that spot, exposing the electrodes. This opens up the likelihood that the electrodes will come into contact and have a potentially catastrophic failure.

 

Charged: How does the PVDF polymer affect cycle life?

 

BP: It’s a higher-grade polymer that’s intended to last for many years. Our targets are the EV market, the power battery market and the energy storage systems market, where you need the batteries to last.

By mixing our polymer into a solution, were able to add in all sorts of active and inactive ingredients to provide different functionalities – like scavenging of contaminates that can be liberated though improper cycling of a battery. 

When a battery cycles, there is a lot of electrochemical activity going on in the cell. When there is a charge put across the chemistries, you can start forming insoluble reaction products inside the battery. You can call them bad actors, because they basically work to slow or eventually kill the performance of the battery. So there’s a lot of emphasis these days on finding ways to soak up these bad actors and you can do that with certain types of additives. We have a particularly effective way of delivering these additives inside the separator, again without affecting porosity. 

Also, as a cell cycles it expands and contracts because of the heat generated. There is a micro-hammering effect on the surface of the separators because they repeatedly contact the electrodes. After a few hundred of these events, the surface of a rigid polyolefin separator will change. In other words, the porosity you start with is not the porosity you end up with because of the physical effects of the hammering. Because our material is very spongelike, it flexes with each of these charge and discharge events. It’s like a mattress. The surface porosity does not change very much even after 1000 cycles. 

Another of the more interesting aspects of our PVDF material is that we feel the battery separator of the future is going to have to operate in a higher voltage environment. 4.2 V is great for these smaller cells, but in order to improve EV batteries, you’re going to have to improve all the different components. For cells to work at high voltages – the 4.7 V, 4.9 V, 5 V range – you need electrolytes and electrodes that are specifically designed for those voltages, and a separator that is not going to oxidize. This oxidation is a change in the polymer chemistry, initially on the surface, and eventually all the way through – kind of like rust. 

Polyolefin separators oxidize very quickly at high voltages. PVDF separators exhibit much less of a change over time. If you open up a battery with a Polyolefin separator that’s been cycled 100 times at high voltage, the separator is going to be discolored. The PVDF separator is going to look brand new – it’s very stable at high voltage.

 

Charged: How does the cost of your products compare to those currently on the market?

 

BP: We’re very cost-competitive with the high-end ceramic separators that are offered by the top-of-the-line manufacturers. Our new ceramic-grade product comes out at the beginning of 2014, and it’s priced to be competitive. We’re still pre-revenue, but expect our first over-the-hump sales will be early next year. 

 

 

This article originally appeared in Charged Issue 11 – DEC 2013