At the advent of the Integrated Circuit Age, portable electric-powered evices were few. Less than a handful of battery chemistries and
form factors served most applications. Truck and auto engines depended on lead-acid batteries. Flashlights, portable radios, and photo strobes
operated from carbon-zinc batteries in a few cylindrical- and prismat-ic-package sizes with open-circuit potentials ranging from 1.5 to 510 V.
The fancy gear of the day — electric wristwatches, hearing aids, and
calculators — ate up mercuric-oxide-zinc button cells like little toxic
Decades of innovation in integrated-circuit fabrication chemistries
and process technologies have garnered much attention from the trade
press, professional journals, conferences, and symposia for good reason:
Advances in monolithic semiconductor devices has increased electronic
functional density by many orders of magnitude and, in the process,
made portable many devices previously tethered to grid power.
However, the technology advances that have propelled product portability are not solely those of the semiconductor sector. Advances in electrochemistries,
materials processing, and manufacturing have
radically improved battery performance and
made practical a level of system functionality not practical with earlier generations of
For applications requiring appropriately
low average power, advanced lithium primary
batteries, such as Tadiran’s lithium thionyl
chloride (LiSOCL2) cells, provide exceptionally
long life with no maintenance. For example,
the company’s XOL cell exhibits self-discharge
rates as low as 0.7 percent per year, resulting
in capacity retention of 70 percent over 40
years. By contrast, more common lithium iron
disulfide (LiFeS2) cells exhibit typical charge leakage four times as great.
With self-discharge rates this low, system maintenance cycles are
likely not determined by shelf (non-operational) time as they can be
with other electrochemistries, but rather by a design’s ability to optimize capacity utilization. The XOL series offers single-cell capacities
ranging from 0.42 to 19 Ahrs. Given the XOL’s lifecycle performance,
designers previously dependent on energy harvesters and secondary
cells can consider switching to a primary cell to reduce an application’s
size and complexity while eliminating an exposure to energy-source
variability, and achieving long maintenance cycles.
A battery’s nominal capacity is expressed as a uni-dimensional
quantity in ampere hours with the end-of-cycle voltage as the limiting condition. The nominal conditions specify discharge current and
operating temperature — usually 25°C. Actual or realizable capacity is
a multi-dimensional function of real discharge current and operating
temperature. Designs making the most of a primary battery’s available
capacity should account for the application’s anticipated operating temperature. Additionally, applications that exhibit high load-current crest
factors with low duty cycles may benefit from load-leveling techniques,
which can limit the maximum battery discharge current while adding
little design complexity.
For many battery applications, the goal is to provide space-efficient
energy storage in support of portability — a means of untethering a
system from the grid. For some applications, however, the goal is energy storage to aggregate energy from dynamic sources. Although some
of these uses also require portability, like EVs and HEVs, the biggest of
these is a stationary application: source leveling for renewable generation, such as solar and wind farms.
Unlike typical battery-powered applications, grid-scale energy-stor-age systems must be capable of handling enormous currents. At present, no battery technology has demonstrated scaling to utility-genera-tion size. That said, liquid-metal battery technology — developed by
MIT materials science professor Donald Sadoway and his team — is one
of the evolving approaches showing promise.
As Dr. Sadoway observes, electricity supply on the grid must be in
constant balance with electricity demand. Traditional generating capac-
ity, be it coal, oil, gas, or nuclear powered, cannot respond fast enough
to track output dynamics from renewables,
such as wind and solar. The grid must operate
with excess base capacity and, in so doing,
fails to take full advantage of the generating
potential renewables offer.
The lack of large capacity electrical-ener-
gy storage that can mitigate the renewables’
intermittency prevents wind and solar from
contributing to grid energy in the same way
as traditional energy sources. Sadoway asserts
the key missing component is a grid-scale
battery technology that provides unusally
high power capability, long service life, and
extremely low cost. Traditional battery chem-
istries and structures do not scale with accept-
able economy for high-power applications.
For reasons of economy and scalability, Sadoway’s R&D team eschews
rare elements and, instead, focuses on those that are, as he says, as
abundant as dirt—“preferably locally sourced dirt.” Grid-scale battery
development, according to Sadoway, requires inventing to the price point
of the electricity market, rather than depending on the application of
economies of scale to otherwise uneconomic storage technologies.
The liquid-metal battery exploits three mutually immiscible materials
of differing density. The top and bottom layers are low- and high-den-
sity metals, respectively, and the electrolyte between them is molten
salt (Figure 1). Unlike existing electric-storage systems, the liquid-metal
battery naturally operates at elevated temperature and, thereby, sup-
ports high currents without thermal degradation.
During the discharge cycle, the ionization of liquid metal A supplies
electrons and the metal-A ions transport through the electrolyte to
alloy with metal B. During the charge cycle, the alloy separates and the
charging current reverses the process.
Through liquid-battery startup Ambri, Sadoway and co-founder
David Bradwell expect to test a 2-MWhr modular liquid-metal battery
prototype this year. Current estimates are that the technology can deliver reliable bulk energy storage with no moving parts for well below
$500 per k Whr. ■
Figure 1: The liquid-metal battery’s operation in its
charged state during its discharge cycle (A) and
discharged state during its recharge cycle (B).