The chemically inert graphene cage also forms a stable solid electrolyte interface, minimizing irreversible consumption of lithium ions and rapidly increasing the Coulombic efficiency in the early cycles.

In a paper published in Nature Energy, the team, led by Professor Yi Cui, reported that even in a full-cell electrochemical test, for which the requirements of stable cycling are stringent, stable cycling (100 cycles; 90% capacity retention) is achieved with the graphene-caged Si microparticles.

Time-lapse images from an electron microscope show a silicon microparticle expanding and cracking within its graphene cage as lithium ions rush in during battery charging. The cage is outlined in black, and the particle in red. (Y. Li et al., Nature Energy) Click to enlarge.

In testing, the graphene cages actually enhanced the electrical conductivity of the particles and provided high charge capacity, chemical stability and efficiency. The method can be applied to other electrode materials, too, making energy-dense, low-cost battery materials a realistic possibility.

—Yi Cui

Si is an attractive anode material for next-generation lithium-ion batteries, offering ten times the theoretical capacity of commercial graphite anodes. Cui and his collaborators have been working for years on different approaches to resolving the practical challenges preventing the commercialization of silicon-based anodes. These include large volume expansion of silicon (∼300%) during battery operation, which causes mechanical fracture; loss of inter-particle electrical contact; and repeated chemical side reactions with the electrolyte.

The quest first led to anodes made of silicon nanowires or nanoparticles, which are so small that they are much less likely to break apart. The team came up with a variety of ways to confine and protect silicon nanoparticles, from structures that resemble pomegranates to coatings made of self-healing polymers or conductive polymer hydrogels like those used in soft contact lenses. Despite these advances in producing Si anodes, critical challenges remained: heavy reliance on expensive nanostructured Si for stable cycling and the poor first- and later-cycle Coulombic efficiencies.

Si microparticles are a low-cost alternative but, unlike Si nanoparticles, suffer from unavoidable particle fracture during electrochemical cycling, thus making stable cycling in a real battery impractical. Here we introduce a method to encapsulate Si microparticles (∼1–3 µm).

… There are huge challenges associated with using low-cost,micrometre-sized Si source materials. Typically, Si particles larger than ∼150 nm and Si nanowires larger than ∼250 nm have been shown to fracture on lithiation. During lithiation, SiMP (1–3 µm) would be broken into small nano-Si particles, losing electrical contact and increasing the surface area to form additional SEI. Here, we introduce the conformal growth of a conductive graphene cage as an ideal encapsulation material for stabilizing the previously non-functional SiMP during battery cycling.

—Li et al.

For the graphene cages to work, they have to fit the silicon particles exactly. The scientists accomplished this in a series of steps: First they coated silicon particles with nickel, which can be applied in just the right thickness. Then they grew layers of graphene on top of the nickel; the nickel acts as a catalyst to promote graphene growth. Finally they etched the nickel away, leaving just enough space within the graphene cage for the silicon particle to expand.

This new method allows us to use much larger silicon particles that are one to three microns, or millionths of a meter, in diameter, which are cheap and widely available. In fact, the particles we used are very similar to the waste created by milling silicon ingots to make semiconductor chips; they’re like bits of sawdust of all shapes and sizes. Particles this big have never performed well in battery anodes before, so this is a very exciting new achievement, and we think it offers a practical solution.

—Yi Cui

When used in lithium-ion battery anodes, silicon microparticles swell, break apart and react with the battery’s electrolyte to form a thick coating that saps the anode's performance. To address these problems, scientists built a graphene cage around each particle, bottom. The cage gives the particle room to swell during charging, holds its pieces together when it breaks apart, controls the growth of the coating and preserves electrical conductivity and performance. (Y. Li et al., Nature Energy) Click to enlarge.

Researchers have tried a number of other coatings for silicon anodes, but they all reduced the anode’s efficiency. The form-fitting graphene cages are the first coating that maintains high efficiency, and the reactions can be carried out at relatively low temperatures.

—Stanford postdoc Kai Yan

Now the team will work on fine-tuning the process and on producing caged silicon particles in large enough quantities to build commercial-scale batteries for testing.

The research was carried out by SIMES, the Stanford Institute for Materials and Energy Sciences at SLAC, and funded by the Battery Materials Research program of the DOE’s Vehicle Technologies Office.

Resources

• Yuzhang Li, Kai Yan, Hyun-Wook Lee, Zhenda Lu, Nian Liu & Yi Cui (2016) “Growth of conformal graphene cages on micrometre-sized silicon particles as stable battery anodes” Nature Energy 1, Article number: 15029 doi: 10.1038/nenergy.2015.29