New pomegranate-inspired design may bring silicon anodes for Li-ion batteries closer to commercialization
16 February 2014
Researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory, led by Professor Yi Cui, are proposing a new nanoscale design inspired by pomegranates for high energy capacity but large-volume-change lithium battery anodes such as those using silicon. “While a couple of challenges remain, this design brings us closer to using silicon anodes in smaller, lighter and more powerful batteries for products like cell phones, tablets and electric cars,” said Cui.
As described in a paper in Nature Nanotechnology, using the pomegranate design concept the team encapsulated single silicon nanoparticles with a conductive carbon layer that leaves enough room for expansion and contraction following lithiation and delithiation. An ensemble of these hybrid nanoparticles is then encapsulated by a thicker carbon layer in micrometer-size pouches to act as an electrolyte barrier.
In their paper, the team reported 97% capacity retention after 1,000 cycles using the a hierarchical structured silicon anode. In addition, the microstructures lower the electrode–electrolyte contact area, resulting in high Coulombic efficiency (99.87%) and volumetric capacity (1,270 mAh cm-3), and the cycling remains stable even when the areal capacity is increased to the level of commercial lithium-ion batteries (3.7 mAh cm-2).
As has been noted often, with 10x the theoretical capacity of graphite, silicon is an attractive material for anodes in energy storage devices. Silicon can be used both in traditional lithium-ion batteries or in Li–O2 and Li–S batteries as a replacement for the dendrite-forming lithium metal anodes.
However, the major well-known challenges associated with silicon anodes include (1) structural degradation and instability of the solid-electrolyte interphase caused by the large volume change (∼300%) during cycling; (2) the occurrence of side reactions with the electrolyte; and (3) the low volumetric capacity when the material size is reduced to a nanometer scale.
Here, we propose a hierarchical structured silicon anode that tackles all three problems.—Liu et al.
Over the past eight years, Cui’s team has addressed the structural degradation problem by using silicon nanowires or nanoparticles that are too small to break into even smaller bits and encasing the nanoparticles in carbon “yolk shells” that give them room to swell and shrink during charging. (Earlier post.)
The new study builds on that work. Graduate student Nian Liu and postdoctoral researcher Zhenda Lu used a microemulsion technique common in the oil, paint and cosmetic industries to gather silicon yolk shells into clusters, and coated each cluster with a second, thicker layer of carbon. These carbon rinds hold the pomegranate clusters together and provide a sturdy highway for electrical currents. Since each cluster has just one-tenth the surface area of the individual particles inside it, a much smaller area is exposed to the electrolyte.
The pomegranate design affords remarkable battery performance … its reversible capacity reached 2,350 mAh g-1 for a rate of C/20 (1C 1⁄4 charge/discharge in 1 h). If not mentioned, all reported capacities are based on the total mass of silicon and carbon in the pomegranate structure. Because silicon is only 77% of the mass of the pomegranate structure, the capacity with respect to silicon is as high as 3,050 mAh g-1.
The volumetric capacity based on electrode volume was determined to be 1,270 mAh cm-3, which is more than twice the 600 mAh cm-3 obtained for graphite anodes. From the 2nd to 1,000th cycle at a rate of C/2, the capacity retention was more than 97%. After 1,000 cycles, over 1,160 mAhg-1 capacity remained, which is more than three times the theoretical capacity of graphite. The cycle stability (0.003% decay per cycle) is among the best cycling performances of silicon anodes reported to date. Furthermore, it was achieved with a conventional polyvinylidene fluoride (PVDF) binder, which has been considered a poor binder for silicon anodes.
Under the same conditions, secondary particles without an internal void space (nanoparticle clusters directly coated by carbon) demonstrated significant decay after 200 cycles. Bare nanoparticles decayed even more quickly. The voltage profiles of silicon pomegranate electrodes … exhibited typical electrochemical features of silicon, with little change over 1,000 cycles. Coulombic efficiency is an indicator of the reversibility of the electrode reaction. SEI rupture and reformation usually results in decreased Coulombic efficiency, especially in later cycles. The average Coulombic efficiency from the 500th to 1,000th cycles of the silicon pomegranate is as high as 99.87% … At a relatively slow rate of C/2, this Coulombic efficiency is superior to most previous reports.—Liu et al.
The researchers said that two interdependent characteristics of the pomegranate design enabled the superior performance. The first is the internally accommodated volume expansion; the second is the s spatially confined SEI formation.
While these experiments show the technique works, Cui said, the team will have to solve two more problems to make it viable on a commercial scale: They need to simplify the process and find a cheaper source of silicon nanoparticles.
One possible source is rice husks: They’re unfit for human food, produced by the millions of tons and 20 percent silicon dioxide by weight. According to Liu, they could be transformed into pure silicon nanoparticles relatively easily, as his team recently described in Scientific Reports.
To me it’s very exciting to see how much progress we’ve made in the last seven or eight years, and how we have solved the problems one by one.—Prof. Cui
The research team also included Jie Zhao, Matthew T. McDowell, Hyun-Wook Lee and Wenting Zhao of Stanford. Cui is a member of the Stanford Institute for Materials and Energy Sciences, a joint SLAC/Stanford institute. The research was funded by the DOE Office of Energy Efficiency and Renewable Energy.
SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the US Department of Energy Office of Science.
The Stanford Institute for Materials and Energy Sciences (SIMES) is a joint institute of SLAC National Accelerator Laboratory and Stanford University. SIMES studies the nature, properties and synthesis of complex and novel materials in the effort to create clean, renewable energy technologies.
N. Liu, Z. Lu et al. (2014) “A pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes,” Nature Nanotechnology, 16 February 2014 doi: 10.1038/nnano.2014.6
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