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Brown U, GM researchers calculate optimum design geometries for Si/C core-shell materials for Li-ion anodes

23 March 2014

Master.img-008
Conditions of fracture and debonding. The shaded regions demonstrate the safe regimes of operation as a function of top shell thickness and bottom core size with state of charge. Credit: ACS, Stournara et al. Click to enlarge.

A team from Brown University and General Motors Global Research and Development has calculated optimum design geometries that will avert fracture and debonding in silicon/carbon heterostructures—such as the hollow core-shell nanostructure proposed by Prof. Yi Cui (e.g., earlier post) and others—used as high-capacity anodes in advanced Li-ion batteries.

In their work, reported in a paper published in the ACS journal Nano Letters, they combined properties calculated from ab initio simulations of lithiated a-Si/a-C interface structures with linear elastic fracture mechanics to construct a continuum level diagram which outlines the safe regimes of operation in terms of the core and shell thickness and the state of charge. Among their findings, they determined that high states of charge are achieved and failure is prevented if the thickness of the core is less than 200 nm and the thickness of the shell is approximately 5 nm.

Master.img-001
Partially lithiated Si core (yellow) restricted by a rigid C core (gray). The inner radius α accommodates for the deformation of the core. The architecture allows for controlled growth of SEI formation on the carbon side.

The lithium ions that penetrate through the mechanically rigid shell react with the inner silicon wall, which softens significantly on considerable Li insertion and expands inward. Hence, the continuous SEI shedding and reforming is suppressed and as long as the electric contact between the two active materials is maintained, capacity is retained. Credit: ACS, Stournara et al.

Silicon is unanimously one of the most attractive and widely investigated candidates for anode materials due to its ultrahigh theoretical specific capacity of 4200 mAh/g, which is 10-fold higher than that of graphite. However, the large volumetric expansion it undergoes during lithium insertion (∼300%) is associated with fracture and delamination from the current collector, the active and conductive carbon phase and the surface passivation layer (for example, the solid electrolyte interface (SEI) or protective coatings) between silicon and the liquid electrolyte. Hence, it results in rapid capacity fade and loss of cycle life.

… Cui et al. and others introduced an electrode architecture that involves Si/C hollow core−shell nanostructures and allows Si electrodes to sustain high capacity over thousands of cycles with high Coulombic efficiency. This new architecture is of paramount importance for the successful design of next generation Si-enhanced anodes, as it can improve their mechanical and chemical stability simultaneously.

… Clearly, the successful design of such nanocomposite architectures lies in the structural, mechanical, and electronic properties of the a-Si/a-C interface. Hence, the open question, yet to be addressed, is whether and how lithiation changes the interfacial and fracture strength of these heterostructures.

—Stournara et al.

For the study, they constructed three different interface models.

They found that the a-Si/a-C interface retains good adhesion even at high stages of lithiation. For average lithiated structures, they predicted that the strong Si–C bonding averts fracture at the interface; instead, the structure ruptures within lithiated a-Si, with the fracture energy being more than five times lower than the energy required to separate the interface.

The results suggested that upon lithiation, the interface adhesion decreases by only ∼20%, suggesting that the two active materials are not threatened by delamination, even at high stages of lithiation.

Master.img-007
C atoms are shown as brown, Si atoms as blue, and Li atoms as green spheres The yellow highlighted regions demonstrate the locus of fracture. The structure ruptures at ε = 12% and bond break and reform occurs until ε = 18%. After ε = 12%, the structure is completely fragmented. The C-Si interface retains good adherence even at high stages of lithiation. Credit: ACS, Stournara et al. Click to enlarge.

Those calculated fracture and debonding parameters fed into the modeling to determine the optimum design conditions.

… our results suggested that nanosized particles, whose core is C < 200 nm, demonstrated higher SOC, compared to microscaled particles. The predicted allowed dimensions for the shell indicate that a thinner core in the order of C − B = 5nm would allow for SOC ≈ 0.77, which corresponds to almost fully lithiated Si (Li3.75Si), and would therefore contribute to higher capacity of the half cell.

—Stournara et al.

Resources

  • Maria E. Stournara, Yue Qi, and Vivek B. Shenoy (2014) “From Ab Initio Calculations to Multiscale Design of Si/C Core–Shell Particles for Li-Ion Anodes,” Nano Letters doi: 10.1021/nl500410g

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Comments

When I see Brown U and GM cooperating on a design like this, does this mean this has a strong chance of becoming the next vehicle battery?

Jeffgreen: This paper says it all in the title: "Ab Initio Calculations to Multiscale Design". Operating from first principles they worked through very detailed simulations and conceptual designs of the interface to arrive at some promising architectures.

I'm not willing to pony up for the pdf, but if I could get some help in understanding the work it looks cool. I can tell you for certain however: they did not actually make anything. It's a simulation and design exercise. Not the nano-est of nanostructures of this family of geometries actually exist, as best as I can tell.

So first they will refine their geometries and make a plan to build some. Then they'll build these on a small scale and see if predicted and actual performance match. From there, if the results are verifiable and repeatable will come larger scale tests on a cell level. From there... I think you can see where this is going. We are years from a working non-industrialized multicell demonstrator.

The "next vehicle battery" will be something that is already being thrashed in labs today at full-scale and running on test tracks in prototypes, with industrial processes being flyspecked for yield in pilot production. From time to time in the rumor mill you hear of a 4100mAh 18650 cell being brewed at Panasonic. If Tesla brings that into their ESS in the next five years that will be the best we'll see, improving specific energy by 25-30% by 2017 in the best case. None of this anti-gravity stuff will make it by then; the industrial and real-world proving ground take too long for that to happen.

Doesn't mean you don't do it, and I'm as happy as anyone to see GM tinkering. But this is very long-range work.

Doing lithium sulfur, then magnesium, then magnesium sulfur may make progress. There are a lot of analytical tools being applied to the decision process, they can narrow it down and then do the lab work.

Pellion and others are working on magnesium, several labs are working on sulfur, with enough qualified people working on the solutions and sharing information, we could see progress. The problem as always if funding, no one wants to fund something that may not work.

Fine tuning existing lithium batteries to gain another 30% in capacity is certainly a strong possibility but it is not enough for future extended range BEVs.

New technologies with up to 5X the current capacity (from 120 Wh/Kg to 600 Wh/Kg) is what will be required.

An intermediate 3X technology may be possible?

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