The race to develop and sell new electric vehicles (EVs) is driving huge investments in new battery materials; new cell manufacturing process technologies; and new factories in key markets. In fact, The International Energy Agency predicts that by 2040, 55% of all new car sales and 33% of the global fleet will be electric.
The battery advances we read about today focus on cathode chemistries; solid electrolytes; and the vertical integration and localization of EV battery supply chains by EV makers to increase and diversify production capacity while decreasing production costs.
As consumers continue to demand better, more affordable EVs, one manufacturing process is gaining significant traction – adding more silicon onto the battery. To better understand the end-user benefits, it’s important to review the near- and long-term impact of silicon on developing better EV batteries.
The role of silicon in developing better EV batteries
At least two leading EVs already incorporate silicon additives mixed with graphite in the anode, to offer longer range and faster charging: the Tesla Y and the Porsche Taycan. Industry experts predict that in the next few years, the amount of silicon in the EV anodes and the number of EVs with silicon-graphite anodes will both increase. It is the only practical way to increase the anode energy density and charging speed while also decreasing anode costs and carbon footprint. In a few years, EVs without silicon in the battery will become increasingly obsolete.
Today, EV companies have yet to settle on the best way to add more silicon. As car manufacturers and cell makers race to a solution, it’s important to examine the hurdles that to date, have prevented scalable solutions.
Challenges with adding silicon to anode
The propensity of silicon particles to break during cycling.
It is well established that particles greater than 200 nm easily fracture when silicon alloys with lithium, which causes large volume changes.
The current solution – already used today in some existing EVs – is to add oxygen atoms and use micron-sized silicon oxide particles: the oxygen atoms form bonds that hold together the silicon particles. Unfortunately, this solution is expensive and limited. Silicon oxide additives increase the first cycle loss (and thus the cathode cost) and decrease the silicon reversible capacity and they cannot be used to further increase current silicon loading.
The silicon surface consumes too much electrolyte to form a SEI (solid electrolyte interface).
This is most apparent in silicon powders with particles sizes less than 200nm, which exhibit a very high surface area (BET). Various solutions have been proposed, from carbon-coatings to amorphous carbon shells and conductive polymer matrices. All these solutions have three common drawbacks: increased inactive materials; increased costs; and lack of scalability. The lack of scalability can be traced to the difficulty producing and mixing large quantities of these new silicon additives to EV-grade graphite powders produced by multiple suppliers and achieving uniform anode coatings in existing large EV cell factories.
Silicon fragments become electrically isolated during long cycling, thus trapping lithium ions and decreasing battery life, especially at higher silicon loadings.
Various solutions have been tested, from adding carbon nanotubes and specialized polymers to mixing silicon nanoparticles within graphene flakes and forming secondary composite particles. These solutions are highly dependent on the specific slurry composition, preparation and coating techniques of each EV cell maker, which are typically well-guarded trade secrets, and are not compatible with the broad range of synthetic and natural graphite powders available from the leading suppliers in key markets.
So what is the solution?
The industry needs a way to marry more silicon with many EV-grade commercial graphite powders, which leads virtually no changes in the anode active material surface area (BET); in the anode slurry preparation and coating; in the active to inactive materials anode ratio; in the EV cell formation protocol and in the EV cell cycle life. Large scale production that reduces costs and carbon footprint is also a must. The way to answer these demands in the next five years is through perfectly shaped and sized silicon nanowires – materials that can triple the energy density of the anode while decreasing costs and carbon footprint.
It has become apparent that competitive forces are driving the need to innovate faster, to reduce risks and to optimize new cell designs that can be produced in large quantities by leveraging multiple suppliers – growing silicon nanowires directly onto the particles of EV-grade graphite powders, fusing the silicon mechanically and electrically to the graphite without adding any inactive materials and without increasing the BET surface area. This ensures that existing EV factories can benefit from silicon-enhanced graphite from their already qualified leading suppliers and can use the same slurry preparation and anode electrode coating techniques that have already been perfected.
About the author
Vincent Pluvinage, PhD is the CEO of OneD Battery Sciences that has developed a technology to add more energy-dense silicon into the anodes of EV batteries. It fuses silicon nanowires onto commercial graphite powders, claiming to triple the energy density of the anode while halving its cost per kWh.
The views presented in the article are attributed to the author.
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