Silicon anodes: where are we now?

In 2020, we wrote about some of the approaches being taken to develop viable silicon-based anodes. We highlighted the clear potential of silicon, as well as the technical challenges that stand in the way of its immediate use. Briefly, silicon leads the field in theoretical capacities, but excessive mechanical stress due to swelling and contraction upon lithiation and de-lithiation can result in cell degradation. Lithiation of a silicon anode is an alloying process – there is no pre-existing space for the lithium to occupy like there is between sheets of graphite.

Investment in anode materials was an area of particularly strong growth from 2021 to 2022, and we have seen industry ties with R&D-focused silicon anode producers strengthen and multiply over recent years. Perhaps this is recognition by the industry of the promise that silicon holds; equally it may be driven by fear that current anode technology is, or will, hold back battery performance.

Given the pace of innovation and rocketing commercial interest, now is a good time to take stock. We have taken a look into some examples of how the inherent problems with silicon as an anode material are being addressed, and how relevant business and commercial challenges such as scaling, sustainable material sourcing and revenue generation can be met.

An-ode to silicon

LeydenJar make thin-film anodes of pure silicon on copper using a plasma-enhanced chemical vapor deposition (PECVD) process, common to the world of photovoltaics and semiconductors. The anodes that result feature an array of pure silicon columns, making them porous and flexible, mitigating the detrimental effects of swelling and contraction.

The numbers are impressive: 1350 Wh/L volumetric capacity at the stack level and 80% capacity retention when cycled more than 450 times. The high capacity of silicon allows a reduction in anode width by an order of magnitude.

After receiving EU backing, LeydenJar have plans for a 100 MWh factory to become operational in 2026. Scale-up can often be a roadblock for new technology, but LeydenJar’s proprietary “roll-to-roll” PECVD production platform is a promising base for increasing output.

Ionic Mineral Technologies own the world’s largest high-purity deposits of halloysite, a naturally nanotubular aluminosilicate mineral. Fascinating in itself, but more so is what Ionic do with it. Their product, Ionisil™, is a drop-in solution that can be added to existing graphite anodes, replacing anything up to 100% of the graphite. The structure of Ionisil™ allows the silicon to swell into free space, including the space within the halloysite nanotubes.

Ionisil™ can be produced simply and at scale in a clean and safe way, producing little waste and useful by-products. The mining process also has lower impact than is typical in mineral mining. The sustainable credentials of batteries is a debated topic that we have commented on before. Here, vertical integration of the supply chain allows for more control over the sustainability of the end-product.

Nexeon produce two silicon anode solutions: NSP1™ and NSP2™. NSP1™ is a drop-in solution that can replace up to 10 wt% of the graphite in traditional anodes. NSP2™ can better account for the expansion and contraction of silicon, allowing for greater replacement of graphite.

In 2022, Nexeon licensed their NSP1™ technology exclusively to Korean advanced materials company, SKC. And they recently followed this up with a significant deal to supply Panasonic with their innovative anode material for use in EVs. For R&D intensive companies like Nexeon, who have an extensive patent portfolio, securing revenue streams through commercial partnerships is a vital means for long-term sustainability and growth of the business, and patents are an important tool for achieving this.

Finally, zooming out slightly, Enovix remind us that, when developing a multi-component technology like a battery, a holistic approach is needed. Yes, the cell chemistry needs to be right, but so does the cell architecture. Enovix have developed a cell platform which can address a number of related issues arising from the chemistry of silicon anodes.

When a silicon anode is first charged, around 50-60% of the lithium gets trapped inside, reducing the battery capacity accordingly. Enovix employ a pre-lithiation step to counteract this, whereby a lithium source is coupled to the electrodes, creating a reservoir that diffusively restores lithium supplies, keeping capacity and voltage at useful levels.

Additionally, Enovix have found that, by orienting components to face the small side of the battery and by integrating a mechanical constraint into their cells, first charge expansion, cycle swelling and poor cycle life can all be addressed.

Round-up

Silicon anode technology has the potential to encourage a leap forward in battery performance. For OEMs and consumer-facing companies alike looking to catch this wave, there are going to be a number of technologies available. For innovators, a robust patent portfolio, and the market exclusivity that it confers, allows commercial decisions and partnerships to be made with confidence.