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