Technology: A solid future.
During the first eight months of 2015, a row of five electric vehicles sat
parked in Indianapolis, Indiana, available for test drives. The compact
vehicles are part of the first US wave of the ambitious BlueIndy electric
car-sharing programme from the Bolloré Group, headquartered in Paris,
which is already running similar large-scale schemes in London and
the French capital.
These Bluecars are a technological milestone: with unique 30-
kilowatt-hour battery packs designed by Bolloré, they are the first
plug-in vehicles on the road to have solid-state batteries, rather than
the liquid-electrolyte batteries that many other electric vehicles use.
BlueIndy president Hervé Muller says that the plug-in vehicles'
batteries have been performing well. And the safety record for
Bolloré's batteries is good. “We haven't had an issue in Indianapolis,
nor in Paris with the 3,000 cars in service there that have driven more
than 10 million miles,” Muller says. Bolloré's BlueIndy batteries,
although groundbreaking, are an early iteration of solid-state
technology and not yet ready for the mass market. Scientists, however,
are working on more advanced cells.
Promises and challenges
The lithium-ion batteries in most electric cars consist of a negative —
usually graphite — anode, a positive cathode and a liquid electrolyte.
When these batteries release power, lithium ions move from the anode,
through the electrolyte, to the cathode. The more conductive the
electrolyte, the better the battery performs.
The promise of lithium-ion solid-state batteries is that they will
replace the heavy and sometimes dangerous liquid electrolyte — which
in car batteries can be volatile at high temperatures when the battery
is charged or discharged quickly or when packs are damaged in
accidents — with a lighter, more versatile solid alternative.
Although finding a solid electrolyte with conductivity that is
comparable with liquids has been a challenge, the advantages are
many. The batteries are safer because flammable components have
been removed. They deliver more power because solid electrolytes
mean that the carbon-based anodes can be replaced with lithium
metal, which has a higher energy density and cycle life, with less
weight and cost. And without the need to package the liquid
electrolyte safely, solid-state batteries can be made in more versatile
shapes (even thin films), reducing manufacturing costs. This could
make electric cars a more enticing proposition, with longer ranges and
a lower purchase price.
“Imagine batteries that do not catch fire and do not lose storage
capacity. That is the promise of solid-state batteries,” says Gerbrand
Ceder, a materials scientist at the Massachusetts Institute of
Technology (MIT) in Cambridge.
Solid-state batteries are not quite ready for mass-market vehicles yet
— the Bluecars' batteries need to be warmed up and can power a very
small car for 240 kilometres. Scientists are struggling to find solid
materials with conductivity similar to that of liquids, as well as
working to increase cycle life and longevity, and to improve the solid
electrolyte's ability to operate at ambient temperatures.
Yan Eric Wang, a materials scientist who worked with Ceder at MIT,
and his team may have found the ideal solid electrolyte ( Y. Wang et al.
Nature Mater. 14 , 1026–1031; 2015). “We found a framework that could
lead to identifying electrolyte materials with high ionic conductivity,”
says Wang. The research, in partnership with electronics firm
Samsung, pointed to a class of compounds of lithium, phosphorus,
germanium and sulfur called super-ionic conductors.
A solid electrolyte allows batteries to switch to lithium metal anodes,
says Jeff Chamberlain, a battery researcher at the Joint Center for
Energy Storage Research at Argonne National Laboratory in Illinois.
“With a solid-state battery you open up the field of metallic anodes,
and make possible a big jump in energy density. That would be a game
changer.”
To be commercially viable, solid-state batteries need to work reliably
for years in a tough automotive environment. And durability is an issue
for researchers. Lithium metal, although excellent at storing large
amounts of energy at low volume, is very reactive. And it is prone to
forming 'dendrites' — tiny lithium spurs that degrade battery
performance and can cause a short circuit if they reach the cathode.
With conventional lithium-ion batteries, a chemical separation layer
guards against dendrites.
Sam Jaffe, managing director of Cairn Energy Research Advisors in
Boulder, Colorado, says that solid-state researchers are working with
additives in the electrolyte carrier, as well as ceramic shields, in an
attempt to block dendrite formation.
Phones first
In the short term, as solid-state science is evolving, the durability
issue may not matter as much in applications such as mobile phones,
because consumers tend to switch to newer technology in a year or
two — whereas cars are kept for a decade or more. “People turn over
portable electronics very quickly,” Chamberlain says, “so the ability to
inject new technology is higher.”
SolidEnergy Systems, headquartered in Waltham, Massachusetts, uses
technology developed at MIT and is targeting smartphones. The firm
developed an improved electrode material for solid-state batteries that
replaces the usual graphite with a very thin film of lithium-metal foil.
The electrode's ability to store lithium seems promising
(go.nature.com/b3xkhh ), and the higher the storage capacity, the
more energy the battery can deliver.
Something of a hybrid, SolidEnergy's cell maximizes efficiency by
using both a solid and an ionic liquid electrolyte — and works at room
temperature. A high-performance car battery is promised within the
next four years.
International investors
Commercial solid-state cells for mass-market electric cars are at least
a decade away, but they are coming, researchers say. Solving the
significant technical hurdles has become a central focus for a group of
start-up companies and research labs. Most companies, even those
that have generated interest from vehicle manufacturers, are still in the
developmental stage. Cosmin Laslau at Lux Research, headquartered in
Boston, Massachusetts, says that today's best lithium-ion batteries
are approaching 250 watt-hours per kilogram (the early Nissan Leaf
had only 140 watt-hours), an impressive measure of energy density,
but they will struggle to reach 350 — a performance goal set by the US
Battery Consortium. If solid-state batteries can reach 350 “that's a
pretty good sustainable advantage over traditional lithium-ion”, he
says.
This could be good news for the electric vehicle industry. Donald
Sadoway, a materials chemist at MIT, says that achieving such high
energy densities is key to widespread adoption of electric vehicles. “If
we had batteries with 350 watt-hours per kilogram we'd have EVs with
350 miles of range, and that's the end of petroleum,” he says.
But scaling up from a cell that generates results in a bench test to a
durable, roadworthy battery pack will be a long road. Menahem
Anderman, a physical chemist and chief executive of Total Battery
Consulting in Oregon House, California, authored the California Air
Resources Board's 2000 report on battery technology. “I have not seen
any indications of a significant breakthrough or signs that they have a
technology likely to find its way into the mass-produced vehicle
inside the next ten years,” he says. “A new development often shows
improvement in one or two areas but experiences difficulties in
others.”
Chamberlain agrees, and he stretches the timeline a bit further. “To be
in showroom cars ten years from now, the solid-state cells would
essentially have to be leaving the laboratory now, and I don't think
that's happening,” he says.
Still, vehicle manufacturers are interested, whatever the timeframe, as
evidenced by a rash of investment and acquisitions over the past few
years. In 2014, Volkswagen bought a 5% stake in QuantumScape, an
electronics firm in San Jose, California, that is hoping to benefit from
developing solid-state technology that could triple the range of its
forthcoming electric cars.
Car manufacturer Toyota says that cells that it has developed with
double the energy density of today's alternatives could provide electric
vehicles with a range of 480 kilometres on one charge. It has built
prototype cells and even a small scooter powered by the batteries.
Positive results have been reported for solid super-ionic electrolytes
(N. Kamaya et al. Nature Mater. 10 , 682–686; 2011 ). But the company
says that cell-energy density is still far below the potential, and
production of the batteries is not expected to start until the early
2020s.
Battery-development firm Seeo, based in Hayward, California, which
was acquired in August by German auto supplier Bosch, is also making
solid-state cells.
Ulrik Grape, a vice-president at Seeo, says that the company's cells
represent “a very durable technology with good cycle life. They're as
good or better than current lithium-ion.” He adds that Seeo packs
could be half the weight of those in the current Nissan Leaf — the
leading battery electric car worldwide in terms of sales. The company
also says its cells achieve 350 watt-hours per kilogram in the lab, but
the real-world performance could be less.
One more player, Solid Power, which is based in the Colorado
Technology Center in Louisville and is a spin-off from the University of
Colorado at Boulder, says it has made lab-scale cells that have
reached 400 to 500 watt-hours per kilogram, and up to 500 charging–
discharging cycles of durability. Company founder Doug Campbell says
that Solid Power is targeting the automotive sector, although its first
market may be the armed forces, for use in communications equipment.
“Troops in the field can carry 60 pounds of batteries, and if we can cut
that in half, it's a strong value proposition for the US military,” he
says.
For competitive reasons, many of the
lab-stage battery companies are
keen to emphasize the positive
aspects of their research, but not all
are forthcoming with sample cells or
technical details. Sakti3, based in
Ann Arbor, Michigan, and headed by
former University of Michigan
engineer Ann Marie Sastry, is
promising greatly improved energy
density and an improved
manufacturing process, with weight
and cost savings, from its thin-film
cells that operate at ambient temperatures. Sastry says that the
company, with investments from General Motors and UK-based Dyson,
has demonstrated that it can double the density of conventional
lithium-ion at the lab scale.
But Sakti3 has released little information about its chemistry or its
results, so its claims are difficult to verify. Laslau estimates that Sakti3
has reached a fairly impressive 300 watt-hours per kilogram, but Sastry
says that the company is not disclosing energy-density results just
yet.
Cost to manufacture is another important factor. “We do think that
solid-state technology will enable much better performance, at lower
cost, than the incumbents,” Sastry says. Her company is aiming for
cells that cost just $100 per kilowatt-hour, which means around
$2,400 for an average-sized electric-vehicle pack, plus costs for
packaging, transportation and other factors. Nissan currently sells its
lithium-ion packs for $5,499 in the United States, although it may be
losing money on every sale. Asked if her cells could enable the goal of
a 450-km, $25,000 electric car, Sastry says: “At least.”
Scaling up to that level, Sastry admits, will
be challenging because of the need to
invent manufacturing processes. But
“everything we have done to date can in
principle be done at scale”.
The 30-kilowatt-hour solid-state lithium metal polymer battery packs
in the Bluecars are here now, but the 240-km range is not a huge
improvement on current technology. BlueIndy's packs require heating
above ambient temperature, and that uses some of the battery's
energy and reduces range.
Solid-state-battery researchers will surely hit some as-yet unforeseen
roadblocks on the way to a commercially durable cell. For cars,
conventional lithium-ion probably has at least a decade of dominance
remaining, although Sadoway believes that “the rush to deployment
would be very, very fast” if stable, high-density solid-state cells were
developed.
Solid-state batteries, Sadoway adds, “would be preferable, because
they're a lot safer”. The lithium-ion fires that have occurred on some
aircraft show what can happen when you rush to scale up battery
technology without doing your homework first.
