“At the end of the project, the material is expected to have a capacity of at least 1,000 mAh/g with good stability for high energy cells with long cycle life (up to 2,000 cycles) but also good compatibility with other cell components and fabrication processes”

What are the key features and benefits of the silicon/carbon composites you are developing for the anode?

Silicon is considered as the most promising anode material for next-generation high-energy Li-ion batteries due to its large capacity compared to graphite. Indeed, silicon demonstrates a capacity of 3,578 mAh/g which is almost 10 times the capacity of graphite with only 370 mAh/g. To reach this capacity, silicon and lithium form the Li₁₅Si₄ alloy (meaning 3.75 Li per Si unit). This alloy contains much more lithium than lithiated graphite with the formation of LiC₆ (meaning around 0.17 Li per C unit). Thus, silicon can store 22.5 times more lithium than graphite. However, with this added quantity of lithium, the Li₁₅Si₄ alloy reaches a volume around 3 times the silicon volume which conducts different degradations during cycling (particle pulverisation, solid electrolyte interface -or SEI- instability).

To limit these degradations, Nanomakers synthesize carbon-coated silicon nanoparticles with high purity and a narrow size distribution which are then introduced into a carbon matrix to form a silicon/carbon composite. This method limits degradation during cycling in different manners:

• Nanoscale avoids silicon particle cracking
• Carbon matrix limits direct contact between silicon and electrolyte and thus constant SEI formation

What methods are you using to characterise and test the anode materials?

Physicochemical characterisations are conducted on the material itself to be aware of some of its properties such as specific surface area, aspect, homogeneity and composition. Those are key parameters which determined the composite electrochemical features.

To study its electrochemical performance, the anode material is formulated in an electrode and integrated into a coin cell prototype. An electrochemical test, called galvanostatic cycling potential limited (or GCPL), is performed on the cell to determine the capacity, cycle life and rate capability of the silicon/carbon composite. During GCPL, the cell is charged and discharged alternatively and the capacity at each cycle is measured. The objective is to obtain the longest cycle life (number of cycles before reaching 80% capacity retention) with the desired capacity.

What innovations or advancements do you foresee in the development of anode materials over the course of this project?

Nextcell’s cells are based on innovative gellified technology for next-generation batteries. This project is a good opportunity for Nanomakers to codevelop the anode material with the other components of this new battery technology. At the end of the project, the material is expected to have a capacity of at least 1,000 mAh/g with good stability for high energy cells with long cycle life (up to 2,000 cycles) but also good compatibility with other cell components and fabrication processes to show that it can integrate gellified electrodes.