{"id":10557,"date":"2015-11-18T14:00:06","date_gmt":"2015-11-18T21:00:06","guid":{"rendered":"http:\/\/cafe.foundation\/blog\/?p=10557"},"modified":"2015-12-13T22:19:49","modified_gmt":"2015-12-14T05:19:49","slug":"researchers-strike-battery-fools-gold-on-two-continents","status":"publish","type":"post","link":"http:\/\/cafe.foundation\/blog\/researchers-strike-battery-fools-gold-on-two-continents\/","title":{"rendered":"Researchers Strike Battery Fools Gold on Two Continents"},"content":{"rendered":"

Better, Cheaper, Faster.\u00a0 That was the mantra when your editor worked\u00a0in the semiconductor manufacturing world.\u00a0 Designs, processes and materials were all recalibrated constantly to enable the march toward those three goals.\u00a0 And to some extent, constant repetition helped us achieve the ideal of Moore\u2019s Law, the dictum that computer chips would double the number of transistors they contained every two years.\u00a0 Transistor density in computer chips determines the level of performance they can achieve, and this doubling has yet to reach its end.<\/p>\n

Unfortunately, batteries haven\u2019t doubled in performance every two years, but seem to follow an annual five-to-eight-percent increase in energy density.\u00a0 This would mean, at best, that energy densities would double every nine years.\u00a0 The Tesla Forum <\/a>notes this progress would not be continuous, but introduced in steps.<\/p>\n

Without either party sharing much information on the energy densities of their experimental cells, researchers in America and Switzerland find the \u201csuper environmentally friendly\u201d nature of fool\u2019s gold in batteries readily apparent.\u00a0 One uses nanocrystals of the material, the other quantum dots.<\/p>\n

Empa \u2013 Swiss Gold<\/strong><\/p>\n

Empa, <\/a>\u201cthe Swiss Federal Laboratories for Materials Science and Technology, is part of ETH Zurich, \u00a0one of the world\u2019s leading universities for technology and the natural sciences.\u00a0 Empa researchers, working from the premise that lithium, the most common basis for high-energy batteries, is in short supply and therefore tending toward being more costly.\u00a0 To counter scarcity and cost, they\u2019ve devised their \u201cfool\u2019s gold battery,\u201d<\/p>\n

Maksym Kovalenko, Marc Walter and their colleagues at Empa’s Laboratory for Thin Films and Photovoltaics have combined a magnesium anode with an electrolyte made of magnesium and sodium ions. The cathode is nanocrystals made of pyrite (fool’s gold).<\/p>\n

\"Empa's<\/a>

Empa’s fools gold hybrid battery with magnesium anode and iron pyrite cathode. \u00a0Illustration: Empa<\/p><\/div>\n

Because pyrite is crystalline iron sulfide, the electrolyte\u2019s sodium ions flow to the cathode during discharging. During recharging, the pyrite re-releases the sodium ions.<\/p>\n

A lab-based product so far, the battery has made it through a mere 40 charging-discharging cycles, but still has several potential advantages worth exploring.\u00a0 The magnesium anode is far safer than highly flammable lithium according to researchers (not that magnesium fires are all that easy to extinguish). The research team thinks the best feature is that \u201cIngredients for this kind of battery are easily affordable and in plentiful supply: Iron sulfide nanocrystals, for instance, can be produced by grinding dry metallic iron with sulfur in conventional ball-mills. Iron, magnesium, sodium, and sulfur\u2026 hold 4th, 6th, 7th and 15th place by the abundance in the Earth’s crust (by mass). One kilogram of magnesium costs at most four Swiss francs, which makes it 15 times cheaper than lithium. There are also savings to be made when it comes to constructing the cheap batteries: Lithium ion batteries require relatively expensive copper foil to collect and conduct away the electricity. For the fool’s gold battery, however, inexpensive aluminum foil is perfectly sufficient.\u201d<\/p>\n

Empa scientists don\u2019t see this as an EV battery \u2013 its output is too low.\u00a0 Cost and environmental friendliness make the technology a winner for large, stationary plants, though.\u00a0 The researchers\u2019 paper in the journal, Chemistry of Materials<\/a><\/em>, suggests using such a battery to temporarily store the annual production from the Swiss nuclear power station in Leibstadt, for instance.\u00a0 Perhaps a lighter, longer-lasting battery is possible.<\/p>\n

Kovalenko suggests, “The battery’s full potential has not been exhausted yet.\u00a0 If we refine the electrolytes, we’re bound to be able to increase the electric voltage of the sodium-magnesium hybrid cell even further and to extend its cycling life. We also look for investors willing to support research into such post-Li-ion technologies and bring them to the market.”<\/p>\n

Vanderbilt University<\/strong><\/p>\n

Starting with an apparent flash in the pan, Vanderbilt researchers<\/a> added quantum dots to a smartphone battery, enabling it to charge in only 30 seconds.\u00a0 Unfortunately that only lasted a few charges.\u00a0 Making iron pyrite quantum dots, nanocrystals 10,000 times smaller than the width of a human hair, and adding millions of those to the same phone battery caused it to charge quickly and last dozens of cycles \u2013 still nowhere near the longevity of commercial cells.<\/p>\n

The team headed by Assistant Professor of Mechanical Engineering Cary Pint and led by graduate student Anna Douglas reported on this in the November 11 <\/a>ACS Nano<\/a>.\u00a0 <\/em>Like the Empa reseachers, they were drawn to the abundance and inexpensive nature of the material.<\/p>\n

\"Cary<\/a>

Professor Cary Pint and graduate student Anna\u00a0Douglas size up benefits of\u00a0fools gold. \u00a0Photo: John Russell, Vanderbilt University<\/p><\/div>\n

 <\/p>\n

Sizing the particles was a major problem, though, and lack of longevity also plagues Vanderbilt\u2019s initial effort.\u00a0 Pint explains, “Researchers have demonstrated that nanoscale materials can significantly improve batteries, but there is a limit. When the particles get very small, generally meaning below 10 nanometers (40 to 50 atoms wide), the nanoparticles begin to chemically react with the electrolytes and so can only charge and discharge a few times. So this size regime is forbidden in commercial lithium-ion batteries.”<\/p>\n

Despite all their promise, researchers have had trouble getting nanoparticles to improve battery performance.\u00a0 They added millions of iron pyrite quantum dots of different sizes to standard lithium button batteries normally used in watches, automobile key remotes and LED flashlights.\u00a0 Most successful were those about 4.5 nanometers in size, \u201csubstantially\u201d improving cycling and rate capabilities.<\/p>\n

\"Distribution<\/a>

Optimum-size quantum dot, about 4.5 nanometers. \u00a0Distribution of nanoparticle sizes in Vanderbilt battery<\/p><\/div>\n

According to Douglas, iron pyrite has a unique way to changing form into an iron and a lithium-sulfur (or sodium-sulfur) compound to store energy.\u00a0 “This is a different mechanism from how commercial lithium-ion batteries store charge, where lithium inserts into a material during charging and is extracted while discharging — all the while leaving the material that stores the lithium mostly unchanged,” she explained.<\/p>\n

Pint has a culinary explanation.\u00a0 “You can think of it like vanilla cake. Storing lithium or sodium in conventional battery materials is like pushing chocolate chips into the cake and then pulling the intact chips back out. With the interesting materials we’re studying, you put chocolate chips into vanilla cake and it changes into a chocolate cake with vanilla chips.”<\/p>\n

Transformations like this don\u2019t readily explain why the size factor for the nanoparticles is so important, \u201cbig\u201d nanoparticles not working as well as \u201cvery small\u201d nanoparticles.<\/p>\n

Eschewing poetry for prose, Douglas explains further.\u00a0 “Instead of just inserting lithium or sodium ions in or out of the nanoparticles, storage in iron pyrite requires the diffusion of iron atoms as well. Unfortunately, iron diffuses slowly, requiring that the size be smaller than the iron diffusion length — something that is only possible with ultrasmall nanoparticles.”<\/p>\n

Bigger particles have a harder time fighting their way to the surface while the sodium or lithium reacts with the sulfurs in the iron pyrite.\u00a0 That inability of the iron to move through the iron pyrite materials limits their storage capability.\u00a0 Knowing just how big and small to make nanoparticles will affect how fast battery makers can trend toward Moore\u2019s Law.<\/p>\n

Pint concludes, “The batteries of tomorrow that can charge in seconds and discharge in days will not just use nanotechnology, they will benefit from the development of new tools that will allow us to design nanostructures that can stand up to tens of thousands of cycles and possess energy storage capacities rivaling that of gasoline.\u00a0 Our research is a major step in this direction.”<\/p>\n

Getting the basics down and making these batteries work as hoped is one thing.\u00a0 Getting them light and energy dense for EV use is another \u2013 one which we hope researchers are able to solve.<\/p>\n

Paper co-authors include mechanical engineering graduate students Rachel Carter and Adam Cohn and interdisciplinary materials science graduate students Keith Share and Landon Oakes. The research was funded in part by National Science Foundation grant EPS 1004083 and NSF’s graduate research fellowship program grant 1445197.<\/p>\n