Alloy-type anodes (Si, Ge, Sn, Al, Sb, etc.) have much higher Li storage capacity than the intercalation-type graphite anode that is currently used in Li-ion batteries. Among all the alloy anodes, silicon has the highest specific capacity: Experiments have demonstrated an initial specific capacity of >3,500 mAh/g, which is 10 times the capacity of graphite. In addition, silicon is the second most abundant element in the earth’s crust (28% by mass), indicating its potential to be utilized in large quantities at low cost....Despite these advantages, graphite anodes still dominate the marketplace due to the fact that alloy anodes have two major challenges that have prevented their widespread use.

First, alloy anodes undergo significant volume expansion and contraction during Li insertion/extraction. This volume change (∼300% for Si) can result in pulverization of the initial particle morphology and causes the loss of electrical contact between active materials and the electrode framework. Second, due to the low electrochemical potential of Li insertion/extraction (<0.5 V vs Li+/Li), the anode surface becomes covered by a solid-electrolyte interphase (SEI) film, which forms due to the reductive decomposition of the organic electrolyte.

In graphite anodes, a thin passivating SEI forms during the first few cycles, and its further formation is terminated due to the electronically insulating nature of the SEI. In alloy anodes, however, the SEI will rupture due to the volume change during cycling, causing the electrode surface to be cyclically exposed to the electrolyte. This results in continual formation of very thick SEI films, which causes the electrolyte to be continually consumed during cycling. The formation of SEI is further complicated by particle fracture, since fracture creates new active surfaces for SEI growth. The excessive growth of SEI causes low Coulombic efficiency, higher resistance to ionic transport, and low electronic conductivity of the whole electrode, and it will eventually result in the exhaustion of the electrolyte and dry-out of the cell.

—Nian et al.

Much effort has been targeted at devising a solution for silicon anodes that would meet the requirements of commercial application, especially in the automotive sector. A significant amount of this work, especially with nanostructures, has been done by Cui and his colleagues.

The “yolk-shell” structure is intended to support a stabilized and scalable Si anode. The structure has silicon nanoparticles (∼100 nm) as the “yolk” and amorphous carbon (5−10 nm thick) as the “shell”. Each SiNP is attached to one side of the carbon shell, while there is an 80−100 nm void space on the other side. This yolk-shell structure has several advantages for LIB alloy anodes, the authors note in their paper:

• The carbon shell is a self-supporting framework, and the well-controlled void space between the SiNPs and the carbon shell allows for the SiNPs to expand upon lithiation without breaking the carbon. This in turn allows for the growth of a stable SEI on the static surface of the carbon shell and prevents the continual rupturing and reformation of the SEI.

• The carbon shell is uniform and mostly free of pinholes, which prevents the electrolyte from reaching the SiNP surface inside the shell. Lithiation of the Si occurs by Li diffusion through the carbon shell into the Si core. Even if there are some minor imperfections or pinholes in the carbon shell initially, the SEI formed on the carbon shell will fill the holes and isolate the inside of the shell from the electrolyte with cycling.

• The carbon shell is both electronically and ionically conducting, which allows for good kinetics.

• Unlike high-aspect-ratio nanotubes or nanowires, the yolk-shell nanostructure is fully compatible with current slurry coating technology.

• Unlike traditional slurry coated electrodes, the electrode has a well-defined void space around every Si particle, which allows for each particle to expand upon lithiation without deforming the electrode microstructure.

The fabrication is carried out without special equipment and mostly at room temperature.

Among the results of the testing of the material:

• Upon deep galvanostatic cycling between 0.01 and 1 V, the reversible capacity reaches 2,833 mAh/g for the first cycle at C/10 and stabilizes at ∼1500 mAh/g for later cycles at 1C. The specific capacity of the yolk-shell structure is much higher than other reported carbon coated SiNP structures because the carbon shell only comprises <30% of the total weight of the material.

• No capacity decay was observed in the first 300 cycles, and the capacity retention values after 500, 750, and 1000 cycles are 88%, 81%, and 74%, respectively.

• he shape of the voltage profile does not change from the 250^th to the 1000^th cycle, indicating stable electrochemical behavior.

• Constant capacity galvanostatic cycling with the lithiation capacity limited to 1000 mAh/g results in stable cycling for 1400 cycles.

• The average CE for the cell with alginate binder, from the 500^th to the 1000^th cycles, is as high as 99.84%, owing to the stable, thin, and smooth SEI built on the outside of the carbon shell.

• The stable and thin SEI on the material enables good rate capability. Even at a rate of 4C, the electrode can still achieve a capacity of 630 mAh/g, almost two times that of the theoretical capacity of graphite.

The yolk-shell structure also performs well with conventional PVDF binder, confirming the successful materials design and fabrication, the team said.

In addition to Si, this yolk-shell structure can also be applied to other high capacity alloy-type anode materials for next generation Li-ion batteries to improve cycle life and Coulombic efficiency.

—Nian et al.

Resources

• Nian L, H Wu, MT McDowell, Y Yao, C Wang, and Y Cui. (2012) “A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes.” Nano Letters 12(6):3315-3321. doi: 10.1021/nl3014814