The charred remains of a Li-ion battery that caught fire in the electronics bay of a Boeing 787 airplane in January 2013 highlights the potential hazards of these batteries, which use flammable liquid organic electrolyte solutions.
ENERGY STORAGE
Replacing flammable liquids with nonflammable solids would eliminate key battery hazard MITCH JACOBY, C&EN CHICAGO
L
ithium-ion batteries grabbed headlines—but not the good kind—this fall when Samsung Galaxy Note 7 devices began overheating and bursting into flames. Samsung has not publicly identified the exact cause of its smartphone woes, but early last month, the firm acknowledged that these high-end devices “can overheat and pose a safety risk.” The South Korean manufacturer issued a recall. And as of Oct. 15, the U.S. Federal Aviation Administration banned Galaxy Note 7 devices from all air transport in the U.S. Samsung’s plight is just the latest in a long list of incidents for Li-ion batteries. In the past decade, the energy storage components have been thrust into the safety spotlight over and over, with reports of laptops, “hoverboards,” and airplane electronics smoldering or catching fire. Li-ion batteries can become dangerously hot and ignite for a number of reasons (see the graphic on page 33). But they often fare worse than other types of overheated bat-
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teries because they contain flammable electrolyte solutions. For this reason, scientists have been searching for substitutes for the liquids. One option they’ve come up with is nonflammable solid electrolytes. Not only would switching to these solids lower the risk of battery fires, it could also lead to more rugged and longer lived batteries. Doing away with the liquid would also broaden the options regarding battery shape and design and would make the batteries compatible with thin-film fabrication techniques, opening the way to miniaturization and new applications. Common lead-acid car batteries and ordinary alkaline batteries use water-based electrolytes made of acids and bases to
shuttle ions around inside. To transport lithium ions, Li-ion batteries use an electrolyte made of lithium salts dissolved in flammable organic solvents such as ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. During Li-ion battery charging, lithium ions flow from the cathode to the anode, and during discharging—when the battery supplies electric power to a device—they flow from the anode to the cathode. When a Li-ion battery overheats because, for example, its overcharge protection circuitry fails and a power source continues pumping energy into the battery, driving unwanted heat-evolving electrochemical reactions, the flammable liquid electrolyte can behave badly. In principle, scientists could fabricate a solid electrolyte that replaces this liquid version with elements from anywhere on the periodic table. In practice, they have found only a limited measure of success so
“When it comes to the search for new solid electrolytes, it’s always a compromise— you need to balance properties.” —Annie Pradel, research director and ceramics specialist, University of Montpellier
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far, mainly with oxides and sulfides. Within those classes, only a small number of materials have shown real promise. A few companies, such as ST Microelectronics in Geneva and Cymbet in Minneapolis, have recently begun selling oxy nitride-based microbatteries for use in devices such as miniature sensors and smart cards. With the exception of the electrolyte used in those batteries, solid electrolytes have not yet worked their way into commercially successful solid-state Li-ion batteries. The problem is that most solids that conduct ions do so only sparingly, especially near room temperature, which is key for portable electronics. And an improvement in one property often comes at the expense of another. For example, modifying a solid to boost its ionic conductivity via chemical treatment or other means could make it more difficult to process, less stable electrochemically, or more expensive. “When it comes to the search for new solid electrolytes, it’s always a compromise—you need to balance properties,” says Annie Pradel, a research director and ceramics specialist at the University of Montpellier. Multiple examples of that give-and-take with electrolyte properties have surfaced during the past several years of research on sulfide-based Li-ion conductors. In 2011, for example, Ryoji Kanno of Tokyo Institute of Technology, Yuki Kato of Toyota’s Battery Research Division, and coworkers reacted Li2S, GeS2, and P2S5 at high temperature. The team reported that the product, Li10GeP2S12, exhibited a Li-ion conductivity value of 12 mS (millisiemens) per cm at 27 °C, the highest conductivity value measured for a To see how a solid electrolyte as pyrolysis instrument of that date (Nat. makes ceramic Mater. 2011, DOI: nanopowder that 10.1038/nmat3066). can be sculpted into Although that a solid electrolyte compound’s ionic thin film, visit cenm. conductivity value ag/solid. exceeds even that of
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some liquid organic electrolytes minum, giving the record-setused in common Li-ion batterting value (Chem. Mater. 2014, ies, its high cost—due to the DOI: 10.1021/cm5008069). presence of germanium—drove Even with those LLZO researchers to look for similar modification methods in hand, but less expensive Ge-free solid researchers still have a tough electrolytes. Scientists in Gertime making c-LLZO-based many soon found one. batteries. The problem is, to By replacing the germanium slot a solid electrolyte between with tin, a team led by the Unian anode and cathode inside versity of Marburg’s Stefanie a battery requires dense thin Dehnen and Bernhard Roling films, less than 50 µm or so came up with Li10SnP2S12. A fast flame pyrolysis method thick. But ceramics such as The team estimated that the converts organometallic c-LLZO are tough to fashion in element substitution should precursors to ceramic that form. reduce the raw material cost electrolyte nanoparticles To begin with, the starting by roughly a factor of three. (micrograph, left) that are easily converted to dense, yet flexible material is often a coarse powBut it also reduced the ionic and translucent, thin films. der, according to Eongyu Yi, a conductivity by about the graduate student working with same amount (J. Am. Chem. Soc. 2013, DOI: mechanism and boost this value. the University of Michigan’s Richard M. 10.1021/ja407393y). Not only that, but the As is the case with the sulfides, attainLaine. Simply compressing such a powder new compound turned out to be unstable in ing a record-setting conductivity value leads to a Swiss-cheese-like material with the presence of water, forming toxic, flamrequires juggling the electrolyte’s properlarge voids between the particles. That mable hydrogen sulfide. ties—especially structure. Only the cubic structure inhibits Li ions from hopping Fortunately, the sulfide electrolyte form of LLZO is a good ion conductor. But from particle to particle, so the material story doesn’t end there, thanks to the the tetragonal structure is more stable conducts ions poorly. work of Kanno, Kato, and coworkers. thermodynamically. To boost the conductivity, researchers Earlier this year, the Toyota-Tokyo InResearchers in several labs have turned often compact the particles and sinter stitute of Technology team found that to element substitution to try to stabithem, heating them to as high as 1,250 °C Li9.54Si1.74P1.44S11.7Cl0.3 can transport lize the cubic form of LLZO. Swapping a for up to 40 hours. That standard process Li ions with conductivities as high as small amount of aluminum for lithium yields dense pellets, which are suitable for 25 mS per cm, a new record for solid elecworks, but as Carlos Bernuy-Lopez of CIC measuring conductivity in the lab, but not trolytes. The researchers fashioned test Energigune, an energy research center in for making batteries. cells using that compound and found that Álava, Spain, discovered, doping with galliLaine’s group, in contrast, uses less enthose batteries are electrochemically um works better: Along with his coworkers, ergy- and time-intensive methods to make stable and amenable to ultrafast charging. Bernuy-Lopez showed that gallium doubles dense thin films. The group feeds alcohol And when subjected to 100 oC conditions, the ionic conductivity compared with alusolutions of organometallic Li, Al, La, and the test cells proved to be more stable and Zr compounds to a flame spray pyrolysis durable than reference cells filled with Substituting Ga3+ into this instrument, which converts the mixture liquid electrolytes (Nat. Energy 2016, DOI: Li-La-Zr-oxide (LLZO) compound to an aerosol and instantly combusts it, 10.1038/nenergy.2016.30). stabilizes its cubic structure. This forming nanopowders. By applying simple, Oxides are the other class of substances material features Li-ion vacancies and a inexpensive casting and sintering meththat scientists have succeeded in fashionlow energy threshold for Li ions to jump ods to the nanopowder form of c-LLZO, ing into solid electrolytes. One material in from site to site, making it a good Li-ion the team produces flexible ceramic films particular, a garnet-type compound known conductor. LaO8 polyhedra are blue; ZrO6 less than 30 µm thick that exhibit ionic as cubic Li7La3Zr2O12 or c-LLZO, draws octahedra are green; the three lattice conductivities of roughly 0.2 mS per cm most of the attention because of its combi- positions occupied by Li+ are red, orange, (J. Mater. Chem. A 2016, DOI: 10.1039/ nation of useful properties. The material is and yellow. c6ta04492a). thermally and chemically stable—unlike Now, the researchers are using their sulfides, it does not require a controlled method to dope the ceramic with gallium environment for processing and has no and boost conductivity. Then they plan to sulfur so it cannot emit toxic hydrogen make batteries with Ga-doped thin films. sulfide. And it’s electrochemically inert Li-ion batteries’ ability to store so much over a wider voltage range than common energy in a small package full of a flamliquid electrolytes. That property means mable liquid means that when things go that c-LLZO should be suitable for use in wrong, they can do so spectacularly. The high-voltage batteries. intensity of battery failure coupled with the In contrast to sulfide electrolytes, enormous number of batteries in use keeps c-LLZO tends to exhibit a maximum a steady stream of such episodes in the room temperature ionic conductivity of news. By searching for materials endowed just 1–2 mS per cm, which is relatively with just the right combination of properlow but far greater than that of other oxties, researchers around the globe are strivides. Researchers are hopeful that they’ll ing to keep these battery workhorses out of be able to elucidate the conductivity the safety spotlight. ◾
