How Panasonic improves lithium ion safety and energy density at the same time

Matsushita, better known by its brand name Panasonic, recently announced an advancement in lithium ion cells that the company claims will make lithium ion battery packs, commonly used in laptops, cell phones, and other digital products, safer while at the same time slightly increasing energy density, or the capacity of the battery to store power. How does the new technology work? EDN talked with the battery gurus at battery pack designer Micro-Power to understand how Panasonic can increase energy density at the same time it increases safety.

First, some background: The 18650 format has been around for about 15 years, during which the cell's capacity has increased from about 1.2 Amp-hours in the early 90s to the 2.9AH of Panasonic's new technology. Dr. Rory Pynenburg, Applications Engineering Managerat Micro Power, explains that battery manufacturers have been cramming more battery-active materials into the same cell dimensions. (The 18650 cell gets its name from its dimensions of about 188mm in diameter by 65 mm long.) In order to do that they've had to compromise the thickness of the separator layer between the anode and cathode. The separators are polyolefin which is typically a polyethylene with about 40% void volume. Pynenburg says, "The downside of using polyethylene is that at about 93 to 98 deg centigrade the separator softens and it can allow penetration by [contaminants], leading to a short. In pure polyethylene, it will start off with a pin-hole penetration and then rapidly widen out, increasing the anode-to-cathode contact area, and you end up in a thermal runaway situation." Or in layman's terms, a fire.

But the need for greater energy density in lithium ion batteries only increased during the nineties. When cell manufacturers increased the thickness of the battery-acting materials and thus the cell's energy density, the tradeoff was to make that separator thinner, going from a 25 micron to a 20 micron separator, with the corresponding increase in the probability of having a thermal runaway event.

In retrospect, it seems inevitable that Sony would be the manufacturer to experience the massive battery recalls – not because of any quality problems, but because its dominant position in the battery cell marketplace as the major supplier made the numbers catch up to it. When you manufacture millions of cells, a failure rate of 1 in 200,000 becomes significant.

Pynenburg explains Panasonic's improvement: "What Panasonic has done is introduce a ceramic material — a metal oxide — that goes between the separator and the electrodes [the anode and cathode]. It's shrouded in a bit of mystery as to whether they're  coating it onto the electrodes or onto the separator. But what the metal oxide does is greatly increase the thermal stability so that if there is a penetration through the separator it stops there and you don’t get the thermal runaway situation." The polymeric separator is still present, they’ve just added the additional heat-resistant layer (HRL).

Pynenburg thinks the technology wil be valuable to Panasonic and its customers. Again, because of the relatively low defect rates the advantages fo the new technology will probably be invisible to the vast majority of Panasonic's customers, but as Pynenburg says, "When its needed, it's going to be a nice parachute to have."