Silicon-based anodes for high-capacity lithium-ion batteries

Many countries have set decarbonization targets to meet the goal of limiting global warming, as outlined in the Paris Agreement. These objectives primarily focus on promoting renewable energy resources and electric vehicles as crucial steps for reducing greenhouse gas emissions. Consequently, significant efforts have been dedicated to enhancing the capacity and energy density of lithium (Li)-ion batteries, which power all modern electronics and electric vehicles.

In this regard, silicon (Si) anodes for Li-ion batteries are a promising candidate owing to their high theoretical energy capacity. Unfortunately, Si anodes suffer from significant volume expansion upon Li insertion and extraction during battery operation. This can lead to cracking and detachment from the current collector, resulting in capacity loss and a shorter lifespan. To address these issues, Si anodes are usually manufactured as thin, elongated nanowires with a high surface-to-volume ratio. These one-dimensional structures can expand both radially and axially, enabling them to accommodate large volume changes without fracturing. However, existing fabrication methods require multiple steps, limiting their scalability.

Now, a team of researchers from Japan, led by Professor Giichiro Uchida from the Faculty of Science and Technology at Meijo University, has developed a simple, one-step method for fabricating Si/tin (Sn) composite battery anodes with higher capacity than that of graphite anodes typically utilized in Li-ion batteries. Their study, published in the journal Scientific Reports on September 8, 2023, holds promise for more powerful and efficient next-generation batteries.

The team employed a one-step plasma sputtering process using helium (He) gas at a high pressure of 100–500 mTorr to deposit Si/Sn nanowires onto a substrate without heating via a vapor–liquid–solid growth mechanism. The nanowires comprised an amorphous Si core with a crystalline Sn shell, which was added during the deposition process to improve the electrical conductivity.

The team obtained fibrous Si/Sn nanowires with a spider-web-like network structure, featuring thinner nanowires extending from a main wire. They could modify the morphology of the wire by simply changing the conditions of the sputtering process. “The film morphology could be controlled from a 3D nanoporous to a 1D nanowire morphology by changing the discharge gas from argon to He. In particular, 1D nanowire films were successfully fabricated under the special plasma sputtering conditions of a Si/Sn (3–6 at%) target and high-pressure He discharge gas at 100–500 mTorr,” highlights Prof. Uchida.

Among the three nanowire structures, the spider-web-like network and nanowire-nanoparticle aggregation demonstrated stable performance. They showed a high Li-storage capacity of 1219 and 977 mAh/g, respectively, during the first 54 cycles at a slow C-rate of 0.01 and retained a high capacity of 644 and 580 mAh/g, respectively, after 135 cycles at 0.1 C-rate. The latter values were 1.5–1.7 times higher than that of current graphite anodes.

This single-step film production approach simplifies anode manufacturing, reduces the need for additional materials like metal catalysts and binders, and enables large-scale film formation. “The developed plasma sputtering process enables a continuous formation of a binder-free Si nanowire film, making it potentially applicable for the roll-to-roll fabrication of 1D nanowire electrodes,” remarks Prof. Uchida.

Nevertheless, the high energy capacity demonstrated by the Si/Sn composite nanowire anodes highlights their potential to advance our battery technology and, in turn, help us realize a carbon-neutral society.