Silicon-dominant anodes pave the way for future Li-ion EV batteries

By DANIEL ARONOV, PhD, CTO

From laptops and cell phones to grid storage and electric cars, lithium-ion batteries enrich the lives of millions of people every day, and with the world pivoting to clean energy, the automotive sector is playing an increasingly important role in the Lithium-ion battery (LiB) market.?

Electric vehicle (EV) performance relies heavily on batteries, and companies involved in R&D and manufacturing are making significant investments to meet market requirements in terms of balancing the performance, weight, cost, and safety of these energy storage systems.

Batteries equipped with silicon-dominant anodes achieve a balance between performance, weight, cost, and safety | Image Source: StoreDot

According to startup data platform PitchBook, EV battery technology investments jumped more than sixfold to $9.4 billion in 2021 from $1.5 billion in 2020 as carmakers ratchet up their efforts to electrify the global fleet.

Because the energy density in the LiB is largely determined by the specific capacity of the electrodes and the potential differential between the anode and cathode, it comes as no surprise that the development of electrode technologies would take center stage.

So, whilst research continues into cathodes - such as Lithium-Nickel-Cobalt-Aluminum Oxide, Lithium-Nickel-Manganese-Cobalt (NMC), and Lithium Iron-Phosphate (LFP) – several companies are focusing their attention on the anode to improve energy density and performance. A shift in the materials used in the anode - namely from graphite to Silicon (Si) or Lithium-metal (LiM) - is of particular interest in providing a stepwise improvement in energy density.

Number of start-ups in technology area

Start-ups developing silicon, Li-metal, and solid-state technology feature prominently | Image Source: IDTechEx

Whilst LiM shows promise this nascent technology still has several challenges to resolve before it can be commercialized. Most important amongst these are dendrite formation, poor cycling performance, and safety concerns.

Si-anode-based LiBs, on the other hand, offer higher electrochemical performances than those utilizing graphite electrodes without many of the debilitating limitations of LiM electrodes. Thus, where it takes six carbon (graphite) atoms to bind to a single Li-ion, a single Si atom can bind to four Li ions. This means that the Si anode could theoretically store over 10 times the energy of graphite. What is more, Si is an environmentally friendly and low-cost material.

Replacing graphite with lithiated silicon in the anode increases the energy density | Image Source: StoreDot

According to IdTechEx the cumulative funding for Si anode start-ups reached $1.9B in 2021, with the global Si battery market estimated to grow from $38M in 2020 to $177M by 2025 at a CAGR of 36.2% according to research by MarketsandMarkets.

Replacing graphite with silicon anodes in EV Li-ion batteries

With an energy density of ~372 mAh/g, low-cost and safe graphite electrodes have long been the anode of choice in the LiB. However, as manufacturers search for higher energy densities, alloying materials, such as Si, with good thermodynamic lithiation potential, increased average voltage, and higher gravimetric and volumetric energy capacities could be key to higher capacity batteries. Si offers a theoretical specific capacity up to ~4,200 mAh/g, which is about 10 times higher than conventional graphite anodes.

This could theoretically see cell level energy density almost doubling, bringing obvious benefits to EVs. Beyond this, Si anodes could also improve the appeal of lower energy cells, such as LFP, narrowing the gap to NMC-based cells and minimizing the core disadvantage of LFP – limited energy density - especially in EV applications. Estimates show that incorporating 20 percent Si into an anode could improve an LFP cell’s energy density by 17 percent – although the additional cost of the Si anode may be prohibitive.

Si is also environmentally friendly, non-toxic, and found in abundance in the Earth’s crust. Consequently, Si is reasonably priced (Although after processing, a slightly higher per-kWh cost than graphite) without the strategic threats faced by many other battery materials such as cobalt.

Nevertheless, Si is a challenging anode material to work with because it expands volumetrically up to 400 percent upon full lithium insertion while shrinking upon lithium extraction. Due to the significant expansion and contraction in electrodes with high percentages of Si, huge stresses are placed on the anode during charge/discharge cycles leading to the mechanical fracture of the electrodes. As a result, pure Si anodes are not practical, and, to maintain good cycle life, only 3 to 10 percent of Si is typically added to the anode of a graphite-dominant battery.

Silicon anodes expand volumetrically up to 400 percent upon full lithium insertion | Image Source: StoreDot

Other challenges to the adoption of Si-based anodes include their low ionic and electronic conductivity and the need to improve the cycle and electrochemical performance. At the same time manufacturers are tasked with increasing production yields, reducing costs, and minimizing the environmental impact.

Trends in the development of silicon-dominant anodes

As the industry moves to incorporate higher levels of Si, the success of the Si-dominant battery will depend on achieving a balance between performance (energy density, range, and time-to-charge), weight, cost, and safety. This will primarily be accomplished through the development of Si-based materials (nanostructured, composite, and highly porous) and the development of unique electrolytes and binders to match these new materials.

The key challenge to increasing the percentage of Si lies in dealing with the volume change that the anode undergoes during cycling. This not only increases the mechanical stresses within the electrode but also the consumption of electrolyte and Li which ultimately results in the loss of electrical and ionic conductivity.?

To overcome these limitations manufacturers are exploring a wide range of solutions, including ways of increasing electrode porosity, electrolyte additives, and conductive and binding materials.?

StoreDot is able to overcome the challenges of introducing a high content of Si to the anode by using a proprietary and optimizied cell design that is also compatible with traditional manufacturing processes.??

Other approaches to overcome Si limitations are available. For instance: using wet ball milling it is possible to produce Si nanoparticles and flakes with less than 150 nm diameter, thereby significantly reducing the electrode’s predisposition to fracture. What is more, different Si nanostructures and Si bulk properties (amorphous vs. crystalline phase) can be produced by applying different mill processes, ball-to-batch ratios, and ball materials (e.g. ZrO2 with varying sizes). Etching of Si can also be used to produce porous Si architectures that reduce volume expansion during Si lithiation.

A similar result to etching can be achieved by using laser manufacturing techniques to ‘drill holes’ into the graphite anodes thereby essentially creating channels that allow Li ions to enter thicker electrodes.?

However, the above approaches are costly and rely heavily on effective and robust processing.

StoreDot’s XFC mass production process is simple, scalable and fully compatible with existing lithium-ion battery technology | Image Source: StoreDot

Prelithiation of anode materials is another important strategy to compensate for Li loss resulting from the formation of the solid electrolyte interphase (SEI) on the surface of the anode. Prelithiation, sometimes referred to as “pre-doping of lithium ions,” involves the addition of lithium to the active lithium content of a LiB before battery cell operation. Whilst an effective solution, due to the additional processing costs it is currently prohibitively expensive.

These improvements, coupled with Si’s inherently low risk of dendrite formation, will also increase the LiB’s fast-charge capability and operation at low temperatures, as well as extend the cycle life of the battery.?

StoreDot has overcome the challenges of silicon

Overall, Si-dominant anodes represent a highly promising proposition with companies such as StoreDot, the leader in extremely fast charging (XFC) battery technology, already capable of charging 100 miles of range in 5 minutes. This compared to the best in class EV, that with a nominal battery capacity of 60 kWh, is capable of adding 100 miles of range in a little as 15 minutes of high-powered DC fast-charging.

Despite the above mentioned challenges and tradeoffs, StoreDot Si-dominant XFC battery has already achieved over 1,200 continuous and consecutive extreme fast-charging cycles at a charge rate more than three times higher than most current LiBs while still retaining a state of health of 80 percent. Even after 1,700 cycles, well beyond the accepted industry norm, batteries maintain 70 percent of the original capacity.

Moreover, StoreDot’s XFC battery already achieves an energy density of ~300Wh/Kg - proof that many of the challenges facing Si-dominant anode technologies are being resolved, which opens an avenue to faster EV adoption.

Conclusion

The future of energy storage, advanced Li-ion batteries, and electric vehicles is incredibly bright. There are tremendous opportunities for innovation in the chemistry and materials space to improve Li-ion battery components that can drop into existing factories and the many factories being built around the world in the next decade. The reality of a Li-ion battery capable of simultaneously delivering lower costs, fast charging, higher cyclability, and safety, all while being made with abundant raw materials found all around the world and recycled is within reach in the next 10 years. The demand for such a battery would reach unprecedented levels, as high as 30,000 GWh annually by mid-century, as the world transforms from a fossil fuel-based economy to an entirely solar and wind-powered one.