Heat inside lithium-ion batteries is like a funny cat video among feline fans: It needs to be spread around. The idea is to prevent any one battery cell from getting too hot, which can drag down performance. Similar principles also apply to cell phones more generally, beyond just their batteries. In the iPhone, for example, an ultra-thin layer of graphite known as a “heat spreader” helps distribute heat evenly throughout the device and keeps the temperature of the touch screen in a comfortable zone.
It turns out cell phones and electric cars have more in common than you might think, and technology developed for phones could help pave the way for more powerful electric cars. A prime example of this is that one of the world’s largest carbon and graphite producers, GrafTech International, has begun eying the world of electric vehicles as a new opportunity for materials designed to handle heat in the shrinking confines of gadgets.
From iPhone to EV
GrafTech International, a massive graphite electrode supplier, has been manufacturing flexible graphite materials since the 1960s. In electronics, GrafTech first saw its graphite heat spreaders used in flat panel TVs, and later in laptops and smart phones, including Apple’s iPhone. As Julian Norley, a senior corporate fellow at GrafTech, explained in an interview, “It can take heat from any component and basically spread it out.”
Today, GrafTech is in the process of turning its heat-spreading materials into a component for battery packs that could appear in retrofits of current electric vehicles as early as 2014, and in production EVs sometime after that, according to the company, which also makes carbon and graphite-based materials for applications ranging from solid state lighting and semiconductors to fuel cells and nuclear reactors.
In the electric car space, battery pack manufacturers and systems integrators are GrafTech’s target customers, although as Ian McCallum, manager of GrafTech’s market development group, noted, some automakers (notably General Motors and Tesla Motors) are taking it upon themselves to own their own battery pack technology.
Aluminum and copper were the traditional heat spreaders for electronics, said Norley. But graphite, boasting lighter weight and higher thermal conductivity than either metal, has displaced aluminum and copper on “the higher-performance end.”
Yet EV makers and their battery suppliers, “without any other obvious options,” said McCallum, are commonly using “relatively thick aluminum dividers between cells and calling that their thermal solution.” Often, he added, there is also a liquid or air cooling system integrated on top of that.
What graphite-based alternatives can do, at least in theory, is handle the same amount of heat with much less bulk than aluminum (or handle significantly more heat for the same bulk). Based on internal models, Norley said the combined weight of heat spreaders in a typical automotive battery pack could be reduced by about 75 percent when using graphite materials instead of aluminum.
Of course, heat spreaders are but a sliver of the cell. Swapping out aluminum for graphite heat spreaders in a 9-millimeter-thick cell, for example, might make room for 214 cells in a pack where previously only 200 cells would fit. “Not very impressive,” as McCallum put it. “But battery manufacturers would kill for a 7 percent increase in energy density” (packing those 14 extra cells into the space of a 200-cell pack).
Simply swapping out the aluminum for the graphite has its benefits: making it possible to build a battery with the “same cells, but less stuff in the pack,” as GrafTech research scientist Ryan Wayne put it. But what gets Norley and Wayne really excited is the possibility of designing batteries in new ways with these new materials. “Maybe you fit two packs where you could only fit one,” suggested Wayne, or use “a thicker graphite that can handle more heat” for a more powerful pack.
Cost is key
Alex Carter, an analyst with the market research firm Lux Research, agreed that thermal management materials offer a “big opportunity going forward,” since they sit at the intersection of two growth industries: electronics and energy storage.
Yet in a time when batteries still make up as much as 40-50 percent of the total cost of an electric car, said Carter, low-cost aluminum has a distinct advantage. EV makers are “in a phase right now where cost is paramount,” he said.
As the cost of other battery components comes down, Carter predicted, it will create “breathing room” for car companies and battery suppliers to consider investing in higher performance, higher cost materials like graphite heat spreaders.
Sizing up the competition
In addition to GrafTech and other suppliers of engineered graphite, companies like Leyden Energy are already using graphite foil in lithium-ion batteries. And advanced graphite materials are part of a larger trend of “carbon materials coming into their own,” said Carter. Down the road, he added, flexible graphite materials could face stiff competition from graphene (a single-atom-thick sheet of carbon), which Intel is developing for use in heat spreaders for computer chips.
Plus, aluminum producers shouldn’t be expected to stand still. “Alcoa wouldn’t want to lose a major growth market,” said Carter. So as aluminum comes under pressure from new materials, expect higher-performance aluminum alloys to come on the market. That’s what happened in the aerospace segment, said Carter, when carbon fiber began to compete with aluminum.
So far, GrafTech has tested its materials in battery packs for an electric bicycle and an electric motorcycle (the latter in partnership with Ohio State University). The University of Einhoven, another GrafTech partner, is building a 16 kWh lithium-ion pack for racing using all-graphite heat spreading materials. Beyond academia, GrafTech said multiple battery makers are testing its heat spreaders for use in electric vehicle applications.
According to Norley, incumbent technology is the biggest competitor for graphite heat spreaders in electric vehicle applications. Working with graphite would require the understanding of a new material and a new way of doing things in a field where already “everything’s uncomfortably fast,” said McCallum. But then, that’s the same challenge GrafTech faced in consumer electronics, and now we have graphite heat spreaders sandwiched into our phones.
Images courtesy of yellowcloud, GrafTech, Appfire, International Battery, and Leyden Energy.