Source: The Economist | May 30, 2015
The lithium-ion battery is steadily improving, but new research aims to turbocharge the technology.
It is hard to imagine modern life without batteries. These storehouses of power open up new vistas, whether connecting people with the world through portable devices or travelling in electric cars. Yet, like many freedoms, the price is vigilance; the constant fretting over the charge meter.
Anyone who has spent time in an airport in recent years can attest that one of the most popular places to wait for the plane is by the rare wall socket or specially built tables festooned with electrical outlets. And for all the promise of the electric car, the distance it can travel on a single charge is limited, adding a new phrase to the lexicon of motoring: “range anxiety”.
The inability of power storage to keep up with new technology frustrates many, especially entrepreneurs in Silicon Valley who bemoan the lack of a Moore’s law for batteries. This is the name given to a 1965 prediction by Gordon Moore, a co-founder of Intel, that the cost of microchips would continue to fall as the number of transistors crammed onto a given area of silicon would double every 18 months or so. For chipmakers like Intel this turned into a self-propelling prophecy that is—just about—still delivering cheaper computing power.
Batteries have improved, but nowhere near the pace of Moore’s law. Most mobile devices and electric cars are now powered by lithium-ion batteries (see box 1). They were commercialised by Sony in the early 1990s and have got steadily better. The batteries are lighter and their capacities have increased several times over the years, as witnessed by ever-thinner laptops and smartphones.
The lithium-ion battery “is almost an ideal battery,” says Vincent Battaglia, the head of the Electrochemical Technologies Group at Lawrence Berkeley National Laboratory in California. Lithium is the lightest metal on the planet and it can hold a charge extremely well compared to heavier alternatives, such as lead, zinc and nickel-cadmium. Unlike the latter it does not suffer from a “memory effect”, which means lithium batteries do not need to be run down before recharging.
There are some drawbacks. Lithium is highly reactive—overcharging and manufacturing faults can result in an internal short-circuit causing the battery to heat up and sometimes burst into flames. Millions of laptop batteries have been recalled and some of Boeing’s 787s were grounded in 2013 because of fires. Engineers are now better at managing this hazard.
An important measure of a battery’s ability is its “energy density”; the amount of energy that can be stored for a given weight or volume. Depending on construction, a lithium battery can store 100-250 watt-hours per kilogram—more than twice as much as a nickel-cadmium one. An electric car with a 24 kilowatt-hour lithium battery has a range of 175km (109 miles) or so.
Even though China is now applying its manufacturing muscle to their production, lithium batteries remain relatively expensive: typically around $500 per kilowatt-hour of capacity. Hence a battery pack for even a small electric car can cost around $10,000. Many in the car industry believe the range needs to be close to 500km and the cost around $100 per kilowatt-hour before all-electric vehicles will move into the mass market. That would also allow smartphones and laptops to run for days.
Such a battery would require a step-change in technology. Many researchers are trying, but often run into difficulties scaling up promising experiments into a product that can be mass-produced. Some scientists are not sure if the energy density of lithium-ion batteries can be improved much beyond present levels without significant changes in the materials used to create the electrodes.
Dr Battaglia’s team are working on what he describes as “transition metals”. These are combinations of manganese, nickel, cobalt and graphite which can be added to a lithium battery’s electrodes. Once the right recipe is determined, the idea is that it will increase energy density without having to develop a whole new type of battery. It would, though, be more of an incremental improvement.
Other researchers are trying for more. Yi Cui and his colleagues at Stanford University are developing thin films—some only atoms thick—to enclose the positive electrode. This would allow it to safely contain more lithium, which coupled with a sulphur negative electrode (sulphur, like lithium, also has a very high energy capacity) would enable a battery to hold about five times as much energy by weight as today’s lithium batteries do. A similarly huge increase in capacity is promised with work by Chengdu Liang and his team at Oak Ridge National Laboratory. They are developing a lithium-sulphur battery that has a solid rather than liquid or gel-like electrolyte. This would also make the battery more stable. But both projects face several more years of work even if these batteries can be made commercially.
Tiny solid-state batteries, as those with a solid electrolyte are known, are already found in small devices and sensors, often providing backup power to a microchip. They can be made by depositing materials onto a substrate, rather like the way semiconductors are made. Despite an extremely high energy density, making large solid-state batteries has been too expensive for phones and cars. Nevertheless, some companies hope to change that. Sakti3, a Michigan firm, aims to make big lithium-based ones at the $100 per kilowatt-hour scale—although it does not say when. Dyson, a British maker of vacuum cleaners, has been sufficiently impressed by the technology to invest $15m in the company recently. Volkswagen has put money into QuantumScape, a Silicon Valley company also working on solid-state batteries.
In theory, lithium-air batteries would provide the highest energy densities—air, after all, is extremely light. Researchers have been experimenting for years how to make such batteries, but no commercial breakthrough appears in sight.
In some applications, though, weight is less of a problem and here lithium will face competitors. Giant batteries are being developed to store electricity on the grid, which could transform the market for intermittent renewable sources such as wind and solar. The way utilities deal with spikes in demand is to add generating capacity—so called “peaker” stations. If surplus power could be stored fewer such stations would be needed and supplies could be balanced more easily and cheaply.
Less than 0.01% of electricity is presently stored, says Philippe Bouchard, vice-president of business development for EOS Energy Storage, a New York startup. “Every other commodity supply chain has some form of storage at the point of generation and through delivery,” he adds. Mr Bouchard is pitching his large container-sized zinc-based batteries for storage to New York and Californian utilities. These cost $160 a kilowatt-hour to store electricity, which the company says makes battery grid-storage financially worthwhile.
Other firms making big batteries include giants like GE in America, South Korea’s LG, Japan’s NEC and startups such as Aquion Energy, which was spun out of Carnegie Mellon University in Pittsburgh and is backed by Microsoft’s co-founder, Bill Gates, among others.
Such batteries could also be used by businesses and households. If it were possible to accumulate power overnight, when it is cheap, in a battery and discharge it during the day, when it is expensive, it would save users money. Solar and wind power could be stored this way too, instead of being pumped back into the grid in exchange for a discount on the bill. Such systems might allow some businesses and homes to move, at least in part, “off the grid”.
Smaller versions of grid-scale energy-storage systems would be required for domestic use. One possibility is flow batteries, which generate electricity when a charged liquid-electrolyte is pumped through them (see box 2). In theory, the capacity of a flow battery is as big as the containers in which the electrolytes are stored—which for a stationary flow battery may not be a problem. Michael Aziz at Harvard University has been working on flow batteries for the grid, but his team thinks they are close to being able to make a safe, relatively inexpensive system, about the size of a domestic heating-oil tank, to fit in the basement of a home.
Many flow batteries rely on pumping pricey solutions that contain metals, but the Harvard team says their system could have a greater commercial potential because it could be made more cheaply than those using metal-based solutions. This is because the battery relies on naturally occurring organic materials known as quinones, which bear similarities to the energy-storage molecules used by plants and animals. At present the researchers need to use bromine, a particularly toxic material, on one of the electrodes to boost performance. “It’s not something I would put in my basement,” Dr Aziz admits. But his team are optimistic they will find a way to use organic materials on both electrodes. Such a battery would be allowed in Dr Aziz’s basement.
It could take several years before flow batteries for domestic use come to the market. But lithium batteries, if they could be made cheaply enough, could compete as a lighter and more compact alternative to batteries made from heavier and cheaper materials for storing energy in buildings.
This is the thinking of Elon Musk, the billionaire founder of Tesla, a Californian producer of electric cars (such as the Model S pictured on the previous page). Tesla is building a $5 billion “gigafactory” in Nevada in partnership with Panasonic, its Japanese battery supplier. Production should start next year and by 2020 reach the equivalent of enough lithium battery-packs to power 500,000 cars.
Rather than banking on a breakthrough, Tesla reckons steady improvements and the manufacturing efficiencies of its giant factory will reduce battery costs by some 30% and help it sell more electric cars. Some of the batteries, though, will also be offered to businesses and households for energy-storage. In May, Tesla announced the Powerwall battery for this market. A 10-kilowatt-hour version will cost $3,500 (excluding controls and installation). Even if big innovations in lithium technology fail to materialise, lithium batteries seem to have a bright future.