Efforts to increase the energy density of BEV batteries are being driven by the desire for more driving range, lower cost, and faster charging. While much has already been achieved in improved cathode technology, the next frontier is the anode.
Currently, BEV batteries overwhelmingly use graphite anodes, but other materials are being introduced. Most of these offer a significant energy density benefit, but they also present difficult technical challenges. Some have the potential to alleviate expected graphite supply deficits, and some of them will unlock battery-electric technology for the automotive industry and applications beyond.
Silicon can store significantly more lithium ions than graphite, hence a more energy-dense battery. The main technical challenge to overcome with the use of silicon-dominant anodes is volume expansion during charge/discharge cycles, which can crack the silicon particles after a few cycles and lead to a drop in capacity. Designing containment for that level of expansion – up to a factor of 3 – in a cell is a significant engineering challenge (with significant progress being made).
For this reason, the early use of silicon in BEV batteries has been limited to silicon-graphite blends.
Here are some solutions that have received commercial attention:
Silicon oxide (SiOx): this has been the first usage for silicon, blended with graphite, and, so far, has not gone beyond a 7% silicon mix in BEV batteries. This percentage will gradually be increased, perhaps to a maximum of 15%, but even at 7%, there are claims that it improves anode-specific energy capacity by 60%. The focus, however, has been on fast charging capability rather than specific energy capacity.
Nano-silicon: this approach can involve blended or pure silicon applications, but the trend is towards the latter. By reducing the size of the particles and manipulating their shape at the nano-level, volume expansion can be minimised and there is more surface area with a shorter diffusion path for lithium ions, in turn increasing the reaction rate at the anode and delivering more power. Several sub-paths are being explored, including nano-porous silicon, silicon nanowires, and silicon nanoparticles.
Silicon-Carbon Powder Composite: the most common composite material is carbon due to its good electric conductivity. There are multiple ways to prepare this silicon-carbon material. Examples are carbon coating or positioning silicon in a porous carbon structure. Both of these methods are beneficial in limiting the volume expansion of silicon. Also, the carbon layers avoid direct contact between silicon and electrolyte, preventing the repeated growth of an SEI (Solid Electrolyte Interphase) membrane, which consumes the active lithium in the battery over time.
We currently expect the first commercial applications of nano-silicon-dominant cells in BEV batteries to be around 2024, and for silicon-carbon-dominant cells around 2023.
Lithium metal anode
Lithium metal is regarded as the ‘holy grail’ anode material. It contains all the electrochemical properties desired for an anode, such as low lithiation/delithiation potential and high specific capacity. However, lithium metal is very reactive with conventional liquid electrolytes. As a result, lithium metal consumes electrolytes and creates dendrites. A branch-like dendrite can penetrate the separator and cause a short circuit by touching the cathode.
Solid-state electrolytes (SSE) are envisaged as a solution to this. On the other hand, some companies are not waiting for the commercialisation of SSE and instead developing a stable liquid electrolyte that can inhibit the growth of dendrites in lithium metal anode cells. This method allows existing manufacturing methods to be used and adds more flexibility in the format of the cells, which is a shortfall of current solid-state cells.
We currently expect the first commercial applications of lithium metal anode cells in BEVs to be around 2027 (with solid-state and stable next-generation liquid electrolytes).
Niobium-based anodes are a relatively new type and exhibit similar characteristics to current Lithium Titanate anodes, such as high C-rate (a high rate of power delivery and charging).
In Nyobolt’s patent, their Niobium tungsten oxide anode material shows excellent lithium-ion diffusivity. Simply speaking, the battery can charge and discharge quickly without forming an SEI and dendrites. However, this material has a low specific energy density compared to graphite. Even so, we can foresee these battery types having significant potential in hybrid vehicles since the battery can withstand continuous high-power charge-discharge cycles.
Hard carbon is envisaged as an anode for sodium-ion batteries, which are not conducive to a graphite anode.
One sustainable option is based on lignin derived from trees. However, there are currently no technology demonstrators to indicate the relative performance and suitability of lignin anodes in lithium-ion batteries. Theoretically, hard carbon anodes can offer high C-rates and may be attractive in the long term.