Argonne’s new electrolyte mixture stabilizes silicon anodes during cycling.
The lithium-ion battery is ubiquitous. Because of its versatility, this battery can be tailored to powering cell phones, laptops, power tools or electric vehicles.
It is now the source of a multibillion-dollar enterprise annually that continues to grow each year.
Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have developed a new electrolyte mixture and a simple additive that could have a place in the next generation of lithium-ion batteries.
For many decades, scientists have been vigorously hunting for new electrode materials and electrolytes that can produce a new generation of lithium-ion batteries offering much greater energy storage while lasting longer, costing less and being safer.
This new generation will likely make electric vehicles more widespread and accelerate the electric grid’s expansion into renewable energy through cheaper and more reliable energy storage.
For scientists developing advanced lithium-ion batteries, the silicon anode has been the preeminent candidate to replace the current graphite anode.
Silicon has a significant theoretical energy storage capacity advantage over graphite, being able to store almost ten times the lithium as does graphite. Increasing silicon’s attractiveness commercially is its low cost.
It is the second most abundant material in the Earth’s crust, and its prevalence in computing and telecommunications hardware means there exist substantial processing technologies.
“But a stumbling block has remained,” noted Jack Vaughey, a senior chemist in Argonne’s Chemical Sciences and Engineering (CSE) division. “On cycling, a silicon-based anode in a lithium-ion cell becomes very reactive with the electrolyte, and this process degrades the cell over time, causing a shortened cycle life.”
Lithium-ion battery electrolytes currently contain a solvent mixture, with a dissolved lithium salt and at least one, often more than three organic additives.
Argonne scientists have developed a unique electrolyte additive strategy – a small amount of a second salt containing any one of several doubly or triply charged metal cations (Mg2+, Ca2+, Zn2+, or Al3+).
These enhanced electrolyte mixtures, collectively named “MESA” (which stands for mixed-salt electrolytes for silicon anodes), give silicon anodes increased surface and bulk stabilities, improving long-term cycling and calendar life.
“We have thoroughly tested MESA formulations with full cells fabricated with standard commercially relevant electrodes,” said Baris Key, a chemist in the CSE division.
“The new chemistry is simple, scalable and fully compatible with existing battery technology.”
“In this project, we greatly benefited from Argonne’s Cell Analysis, Modeling and Prototyping (CAMP) facility,” added Vaughey.
“It was there that we tested our MESA formulations.”
The Argonne researchers also investigated how the MESA-containing electrolytes work. During charging, the metal cation additions in electrolyte solution migrate into the silicon-based anode along with the lithium ions to form lithium-metal-silicon phases, which are more stable than lithium-silicon.
This new cell chemistry greatly reduces the detrimental side reactions between the silicon anode and electrolyte that had plagued cells with the traditional electrolyte.
Of the four metal salts tested in cells, the added electrolyte salts with either magnesium (Mg2+) or calcium (Ca2+) cations proved to work the best over hundreds of charge-discharge cycles. The energy densities obtained with these cells surpassed those for comparable cells having graphite chemistry by up to 50%.
“Based on these test results,” said Key, “We have every reason to believe that, if silicon anodes ever replace graphite or constitute the anode in more than a few percent concentration, this invention will be part of it and could have far reaching impact.”
More information: Binghong Han et al, Using Mixed Salt Electrolytes to Stabilize Silicon Anodes for Lithium-Ion Batteries via in Situ Formation of Li–M–Si Ternaries (M = Mg, Zn, Al, Ca), ACS Applied Materials & Interfaces (2019). DOI: 10.1021/acsami.9b07270
Journal information: ACS Applied Materials and Interfaces
Provided by Argonne National Laboratory
Developing high-energy rechargeable lithium-ion batteries (LIBs) is vital to the substantial development of electric vehicles and portable electronic devices.
The limited specific capacity of the state-of-the-art cathode and anode materials is the biggest obstacle to high-energy LIBs. With regard to anode materials, Si has been regarded as one of the most promising next-generation anodes due to its substantially higher capacity (~ 4200 mA h g−1 for Li4.4Si) than traditional graphite anode (~ 372 mA h g−1), low operation potential, high abundance, and environmental friendliness. Several challenges need to be addressed, however, to make Si-based anodes commercially available, including such drawbacks as the tremendous volume variation during the discharge/charge process, unstable solid electrolyte interphase films, and poor electrical conductivity, which significantly restrict its practical application. In this review, we summarize the recent progress on Si-based anode materials from both the fundamental science point of view and the industrial perspective.
From fundamental research to industrial application, the Si-based shell-containing nanostructures (core/shell and yolk/shell) and Si/graphite-based composites (Si/carbon and SiOx/carbon) are mainly covered to illustrate how these designs could solve the challenges of Si-based anodes.
In addition, research progress on binders, electrolytes, and electrode additives towards enhanced electrochemical performance of Si-based anodes is also described. Finally, the remaining challenges and perspectives on the rational design of Si-based anode materials to realize commercialization are discussed and proposed.
The recent research progresses of Si-based anodes are summarized from two aspects. From the industrial perspective, recent progress on Si/graphite-based composites toward practical application is summarized in terms of fabrication processes and electrochemical performance. From the fundamental point of view, Si-based shell-containing nanostructures are reviewed to illustrate how these designs address the challenges of Si anodes. In addition, binders and electrolyte additives are also discussed.
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Graphite vs Silicon
Existing lithium-ion batteries rely on anodes made from graphite, a form of carbon. They generate an electric current by transferring lithium ions between two electrodes – a cathode and an anode – through a liquid electrolyte. The more efficiently the lithium ions can enter the two electrodes during charge and discharge cycles, the larger the battery’s capacity will be. Silicon-based anodes theoretically offer as much as a ten-fold capacity improvement over graphite, but silicon-based anodes have so far not been stable enough for practical use.Produced with a “bottom-up” self-assembly technique, the new high-performance anode structure developed by researchers at the Georgia Institute of Technology takes advantage of nanotechnology to fine-tune its materials properties, addressing the shortcomings of earlier silicon-based battery anodes.
Graphite anodes use particles ranging in size from 15 to 20 microns. If silicon particles of that size are simply substituted for the graphite, expansion and contraction as the lithium ions enter and leave the silicon creates cracks that quickly cause the anode to fail.
The new nanocomposite material solves that degradation problem, potentially allowing battery designers to tap the capacity advantages of silicon. That could facilitate higher power output from a given battery size – or allow a smaller battery to produce a required amount of power.
Electrical measurements of the new composite anodes in small coin cells showed they had a capacity more than five times greater than the theoretical capacity of graphite.
Building a Better Battery
Fabrication of the composite anode begins with formation of highly conductive branching structures – similar to the branches of a tree – made from carbon black nanoparticles hardened in a high-temperature tube furnace. Silicon nanospheres with diameters of less than 30 nanometers are then formed within the carbon structures using a chemical vapor deposition process. The silicon-carbon composite structures resemble “apples hanging on a tree.”Using graphitic carbon as an electrically-conductive binder, the silicon-carbon composites are then self-assembled into rigid spheres that have open, interconnected internal pore channels. The spheres, formed in sizes ranging from 10 to 30 microns, are used to form battery anodes. The relatively large composite powder size – a thousand times larger than individual silicon nanoparticles – allows easy powder processing for anode fabrication.
The internal channels in the silicon-carbon spheres serve two purposes. They admit liquid electrolyte to allow rapid entry of lithium ions for quick battery charging, and they provide space to accommodate expansion and contraction of the silicon without cracking the anode. The internal channels and nanometer-scale particles also provide short lithium diffusion paths into the anode, boosting battery power characteristics.
The size of the silicon particles is controlled by the duration of the chemical vapor deposition process and the pressure applied to the deposition system. The size of the carbon nanostructure branches and the size of the silicon spheres determine the pore size in the composite.
Easy Adoption
The simple, low-cost fabrication technique was designed to be easily scaled up and compatible with existing battery manufacturing. Once fabricated, the nanocomposite anodes would be used in batteries just like conventional graphite structures. That would allow battery manufacturers to adopt the new anode material without making dramatic changes in production processes. And because the final composite spheres are relatively large when they are fabricated into anodes, the self-assembly technique avoids the potential health risks of handling nanoscale powders.
So far, the researchers have tested the new anode through more than a hundred charge-discharge cycles. Gleb Yushin, an assistant professor in the School of Materials Science and Engineering at the Georgia Institute of Technology, believes the material would remain stable for thousands of cycles because no degradation mechanisms have become apparent.
“If this technology can offer a lower cost on a capacity basis, or lighter weight compared to current techniques, this will help advance the market for lithium batteries,” he said. “If we are able to produce less expensive batteries that last for a long time, this could also facilitate the adoption of many ‘green’ technologies, such as electric vehicles or solar cells.”
A paper detailing the new high-performance anode appears in the March 14, 2010 issue of the journal Nature Materials.
