Given the dominance of lithium-ion chemistry in the storage market, research efforts to increase capacity, performance and safety are intensifying. Now a team at Shangdong University in China have opened a new promising direction: replacing the graphite in Li-ion battery anodes with amorphous silicon.
Silicon demonstrates a theoretical capacity almost ten times higher than graphite. But silicon has not been a good material for rechargeable batteries. During cycling, its crystalline structure expands and shrinks during every charge-discharge cycle, which degrades its crystalline structure and sacrifices capacity. The team at Shangdong University in China has now prepared a porous amorphous silicon modification that compensates for the disadvantages.
The key insight: since silicon will lose crystallinity in a battery application, why not just start with amorphous silicon and improve its performance?
Amorphous silicon structures can be challenging to adapt to specific applications. The scientists eventually came up with a relatively simple process, using safe substances as the starting materials, as they write in the journal Angewandte Chemie (and summarized in R&D Magazine). For example, they used cheap and common glyme as the solvent, and liquid silicon tetrachoride as the silicon precursor. All this makes their procedure “very attractive for the mass production,” they report.
The resulting porous amorphous silicon material exhibited excellent electrochemical characteristics and better cycling stability than crystalline silicon. The stability is explained by the presence of large, solvent-filled pores in the material, and by the partial oxidation of the silicon surface in air, according to team director Jian Yang. He proposes that adding carbon to the structure would even further enhance its electrochemical performance.
Best of all, the amorphous silicon material demonstrated a capacity three times better than graphite. If this advantage holds at scale, it may give battery manufacturers new ways to balance the capacity, performance and safety of Li-ion storage applications.