While there is great interest in developing novel anode materials for high-performance Li-ion batteries for electric vehicles (EVs), the researchers note in a paper published in the Journal of the American Chemical Society, all of the high-capacity anode materials such as silicon, germanium, and tin suffer from the major problem of poor capacity retention, which prevents them from being used in large commercial applications.

The intake and removal of Lithium ions causes anode volume changes and stresses, resulting in particle cracking and pulverization, resulting in irreversibility. The volume change can also cause disconnections between the active materials and the current collector, interrupting the charge-transportation pathway. All this translates into a loss in capacity and failure of the battery.

The authors noted that different approaches to solving the problem have included:

• Tolerance enhancement;
• Accommodation;
• Buffering (which may involve making composites with carbon and/or inactive components, carbon coating, alloying, thin filming, and modifying binders and the solid electrolyte interface (SEI) layer coating); and
• Limitation, such as narrowing the voltage window and fixing the lithiation level.

Tailored nanostructures are showing some good stability in other work, the authors note. These nanostructures generally are grouped into two classes: carbon composites and thin films.

However, both classes have several disadvantages. In carbon composites (in which the added carbon constituted over half the weight of the composites in some cases), the following drawbacks were evident: (i) the presence of low-capacity carbon suppresses the overall energy density; (ii) the synthesis usually involves multiple complicated steps; (iii) the intact surface coating decreases the electrode’s kinetics; and (iv) carbon affords only limited accommodation, so the composites either must be porous to provide preformed voids or require inactive oxides to buffer the volume change, which magnifies the effects of the other three disadvantages. On the other hand, thin films are suitable only for microbatteries, and their performance must be stabilized via thick inactive substrates.

Therefore, searching for an inexpensive, high-performance material without a carbon coating has always been greatly attractive but very challenging. Here we present a simple, cheap, and easily scaled-up synthetic procedure for making GeO_x powders with a high capacity of ∼1250 mAh g⁻¹ for 600 cycles when used as anodes. Interestingly, the resulting nanostructure exhibits features of hierarchical porous agglomerates, is amorphous, and contains ∼3.7 nm primary particles that afford us an ideal model system for studying the mechanism whereby this nanostructure exhibits extended cycling in the absence of extra carbon compositing or confinement from substrates.

—Wang et al.

The team attributed the performance of the GeO_x material to the synergy of its four characteristics: small primary particles; porous structure; amorphous state; and the incorporation of oxygen. They suggested that the small size of primary particles might play a crucial role for the following reasons:

1. Avoidance of the widespread problem of pulverization of high-capacity anode materials. Other work had reported that micrometer-sized Ge particles used as anodes in Li-ion batteries were broken into 5−15 nm particles after some cycles. Because the size of the primary particles in their samples is already only ∼3.7 nm, “it is hard to envision their further reduction”, the team said.

2. Small per-particle absolute volume change—good for preserving electrical contact as well as the particle’s integrity.

3. Small particle size facilitates the charge transfer (both ionic and electronic) over shorter distances, and having more surfaces for transfer enhances the reactivity.

The small primary particle thus is the basis for efficiently maintaining the hierarchical porous structure’s stability and integrity upon high-capacity cycling. Growing the particles to 20 nm resulted in lower capacities but still good stability at this level of lithiation (cf. the 300 °C sample). Besides the very small particles, the amorphous structure is another important factor directly related to a stable capacity.

—Wang et al.

Resources

• Xiao-Liang Wang, Wei-Qiang Han, Haiyan Chen, Jianming Bai, Trevor A. Tyson, Xi-Qian Yu, Xiao-Jian Wang, and Xiao-Qing Yang (2011) Amorphous Hierarchical Porous GeO_x as High-Capacity Anodes for Li Ion Batteries with Very Long Cycling Life. Journal of the American Chemical Society doi: 10.1021/ja208880f