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TechFront: Battery Breakthrough: Stable Lithium Anodes

A Stanford University (Palo Alto, CA) research team has developed a stable pure lithium metal anode that could lead to the design of smaller, more efficient and less-expensive batteries for use in a wide range of applications from consumer electronics to electric vehicles.

The researchers published their findings in a paper entitled “Interconnected hollow carbon nanospheres for stable lithium anodes” in the Nature Nanotechnology journal. It describes how lithium metal would be an optimal choice for anode material because it has the highest specific capacity (3860 mAh g–1) and the lowest anode potential of all. The team’s pure lithium anode was developed with a protective layer of interconnected carbon domes.

Current lithium batteries are made of lithium ion (Li-ion) but the lithium is in the electrolyte rather than in the anode. The researchers claim that a pure lithium anode holds the potential for a huge energy boost that could greatly extend battery life.

“Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail,” said Yi Cui, a Stanford professor of materials science and engineering and leader of the research team. “It is very lightweight and it has the highest energy density. You get more power per volume and weight, leading to lighter, smaller batteries with more power.”Professor Yi Cui said lithium has the greatest potential as an anode material. A Stanford research team has developed a stable lithium anode using hollow carbon nanospheres.

The problems in using lithium as an anode material have long vexed engineers. According to Guangyuan Zheng, a doctoral candidate in Cui’s lab and first author of the paper, “Lithium has major challenges that have made its use in anodes difficult. Many engineers had given up the search, but we found a way to protect the lithium from the problems that have plagued it for so long.”

If engineers are successful in building future batteries with the anode material, the potential exists to greatly increase the energy density and dramatically lower costs, noted research team member Steven Chu, the former US Secretary of Energy and Nobel laureate who recently returned as a Stanford professor. “In practical terms, if we can triple the energy density and simultaneously decrease the cost four-fold, that would be very exciting,” Chu said. “We would have a cell phone with triple the battery life and an electric vehicle with a 300-mile range that cost $25,000—and with better performance than an internal combustion engine car getting 40 mpg.”

In the paper, the research team discussed the engineering challenges posed by using lithium as anode material. Because lithium ions expand during charging, they haven’t been practical to use as material for anodes, which are typically made of graphite and silicon and expand less than lithium during charging cycles.

Researchers say that lithium’s expansion during charging is “virtually infinite” relative to the other materials. Its expansion is also uneven, causing pits and cracks to form in the outer surface, like paint on the exterior of a balloon that is being inflated. The resulting fissures on the surface of the anode allow the lithium ions to escape, forming hair-like or mossy growths, called dendrites. Dendrites, in turn, short-circuit the battery and shorten its life. Preventing this buildup is the first challenge of using lithium for the battery’s anode. Other problems using lithium are that lithium anodes are highly chemically reactive with the electrolyte and they also heat up upon contact. By building a protective layer of interconnected carbon domes on top of their lithium anode, the Stanford research solved these issues, using a layer that the team called nanospheres. This nanosphere layer looks like a honeycomb, with a flexible, uniform and nonreactive film that protects the unstable lithium. The carbon nanosphere wall is just 20-nm thick.

“The ideal protective layer for a lithium metal anode needs to be chemically stable to protect against the chemical reactions with the electrolyte and mechanically strong to withstand the expansion of the lithium during charge,” said Cui, who is a member of the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory (formerly the Stanford Linear Accelerator Center). For more information, see http://tinyurl.com/MetalAnodes or visit https://www6.slac.stanford.edu. ME

Solid-State Battery Design Doubles Energy Density

Battery startup Sakti3 Inc. (Ann Arbor, MI) announced Aug. 20 that it has produced a battery cell on fully scalable equipment that is said to achieve more than 1100 Watt hours per liter (Wh/l) in volumetric density.

A University of Michigan spin-off, Sakti3 has been developing solid-state lithium batteries that could find use in a variety of applications ranging from hand-held devices to electric cars. Sakti3 said the company’s latest development translates into more than double the usage time for wearable devices like smartwatches, to 3.5 hr to more than 9 hr, and with an EV like a Tesla Model S, driving distances could be extended from 265 to 480 miles (426–772 km). The Sakti3 solid-state technology used to create lithium batteries is the same thin-film process that is used to build flat-panel screens and photovoltaic solar cells.

The company’s solid-state batteries use vacuum deposition technology and its batteries feature a solid-state cathode. Sakti3 demonstrated over 1000 Wh/l over two years ago and it has since moved to a pilot tool, using scalable materials and equipment. The development was guided by mathematical simulations, starting with materials, continuing to full-scale plant layout to avoid high-cost materials, equipment or processes.

“Our target is to achieve mass production of cells at ~$100/kWh,” said Ann Marie Sastry, CEO of Sakti3. “Our first market will be consumer electronics, and after that, we’ll move to other sectors.”

In addition to being an ultra-low-cost solution, the battery cells are also the safest ever demonstrated because of the all-solid-state construction and materials in the cells. Sakti3 has released a video (http://youtu.be/ICQcj73dMgI) of an abuse test in which hot solder is dropped into the cycling cell—and it continues to operate normally. ME

Deep Deburring Knowledge

Manufacturing practitioners’ continual quest to learn and do is particularly exemplified in the topic of deburring. Deburring, or edge finishing, is accomplished by one of many techniques to remove burrs formed during material removal processes on different materials. SME Technical Papers have presented important knowledge, progress and innovations on burrs and deburring since at least the early 1970s.

SME’s “resident expert,” past president LaRoux K. Gillespie, Dr. Eng., FSME, CMfgE, PE, has shared his industry experience and exhaustive international literature research in scores of SME Tech Papers papers and several books on the subject of deburring. The series of seven bibliographies he edited from 1974 to 1984 lists thousands of literature citations from around the globe (SME paper numbers TP74PUB346, TP75PUB291, TP76PUB462, TP77PUB306, TP78PUB203, TP79PUB248 and TP84PUB344).

Gillespie and other authors have documented changing challenges in deburring. While today it is possible to relatively consistently successfully deburr edges of metal parts, even miniature and micro parts (TP76PUB467, TP10PUB83), accomplishing conventional edge quality for composites is more difficult (TP12PUB42). Deburring generally refers to removal of burrs produced by machining or shearing. Composites also exhibit undesirable conditions at edges and on adjoining surfaces including projecting fibers, burrs, smearing of resins, fusion of fibers, delamination, loose debris, roughness and voids.

Many papers provide “how to” descriptions of deburring processes using power brushes (TP75PUB238), the thermal energy method (TEM) (TP75PUB281, TP04PUB127), coated abrasives (TP76PUB394), buffing and polishing (TP76PUB283), vibratory barrel and centrifugal barrel (TP77PUB257), nonwoven abrasives (TP90PUB483), honing (TP91PUB173), robots (TP82PUB166, TP83PUB310, TP85PUB408), electrolytic processes (TP04PUB241), turbo-abrasive machining (TAM) (TP04PUB349), electrochemical finishing (TP08PUB18), rotary ultrasonic machining (TP11PUB39) and lasers (TP11PUB9).

Other offerings show the range of perpetual interest in how to deburr in all kinds of conditions: burr removal difficulties in gas turbine engine components (TP10PUB38), advances in nonwoven abrasive technology (TP14PUB15), burr formation in stacked aluminum sheets (TP08PUB96), emerging trends in abrasive finishing processes (TP08PUB113), burr formation in drilling of intersecting holes (TP99PUB106) and deburring small intersecting holes (TP80PUB205).

Cost to Industry

Sometimes still referred to as the “common cold of industry,” despite major advances in manufacturing technology, deburring is an economic fact of life for most machining plants.

The preceding statements from TP10PUB84 accompany the results of a survey on the cost of burrs conducted in March 2010 by SME. Previous cost of burrs studies by SME in 1978 and 1983 are compared to the 2010 findings. Across all industries, in 2010 the cost of burrs was roughly 5.5% of the cost of the parts made, or $5 billion annually for American and Canadian industry. Several other SME Tech Papers over the years have reported on deburring costs (TP80PUB180, TP81PUB166, TP04PUB179 and TP10PUB110).

Deburring costs were listed as a concern by 95% of respondents in the 2010 survey, compared with 72% in 1977. The tutorial knowledge and practical techniques in SME’s paper collection can be just the finishing touch to help reduce burr formation and deburr today’s materials and complex parts. ME

TechFront is edited by Senior Editors Patrick Waurzyniak, pwaurzyniak@sme.org, and Ellen Kehoe, ekehoe@sme.org.

 

All SME Technical Papers are available by entering the paper number or a keyword in the search box at http://tinyurl.com/searchsme.

This article was first published in the October 2014 edition of Manufacturing Engineering magazine. Click here for PDF.


Published Date : 10/1/2014

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