Cell power

Thu, 08/07/2014 - 12:16pm
Joshua Israelsohn, Director, JAS Technical Media

At the advent of the Integrated-Circuit Age, portable electric-powered devices were few. Correspondingly, 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 prismatic-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 so many toxic M&Ms.

Decades of innovation in integrated-circuit fabrication chemistries and process technologies has 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.

The technology advances that have propelled product portability are not solely those of the semiconductor sector, however. 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 batteries.

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% per year resulting in capacity retention of 70% 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 life-cycle 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.

Figure 1. Discharge characteristic curves such as these for a Tadiran XOL-series TL-4903 AA-size primary battery show the influence of discharge rate on realizable capacity. This cell has a nameplate capacity of 2.4 Ah at a 1 mA discharge rate and an end-of-cycle potential of 2.0 V. (graph courtesy Tadiran)

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 (figure 1) and operating temperature. Designs making the most of a primary battery’s available capacity, therefore, 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, as is the case with 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-storage systems must be capable of handling enormous currents. At present, no battery technology has demonstrated scaling to utility-generation size. That said, liquid-metal battery technology developed by MIT materials science professor Donald Sadoway and his team is one of the evolving approaches that is showing promise.

Figure 2. The liquid-metal battery’s operation in its charged state during its discharge cycle (A) and discharged state during its recharge cycle (B).

As Dr Sadoway observes, on the grid, electricity supply must be in constant balance with electricity demand. Traditional generating capacity, be it coal, oil, gas, or nuclear powered, cannot respond fast enough to track output dynamics from renewables such as wind and solar so 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-energy storage that can mitigate the renewables’ intermittency prevents wind and solar from contributing to grid energy in the same way traditional energy sources do today. Sadoway asserts that the key missing component has been a grid-scale battery technology that provides unusally high power capability, long service life, and extremely low cost. Traditional battery chemistries and structures do not scale with acceptable 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-density metals, respectively, and the electrolyte between them is molten salt (figure 2). Unlike existing electric-storage systems, the liquid-metal battery naturally operates at elevated temperature and, thereby, supports 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 (figure 2A). During the charge cycle, the alloy separates and the charging current reverses the process (figure 2B).

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 kWhr.



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