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On September 22, 2020 Tesla held its annual battery day event, where the company rolled out a slew of new design innovations for its electric vehicle battery cells. Prior to the event, and in typical Elon Musk fashion, Musk released multiple announcements via Twitter hinting at major breakthroughs for Tesla, and news outlets began to buzz. A CNN article titled “Tesla's 'Battery Day' is here. This is what to watch out for” speculated that this might entail the currently elusive solid state battery (SSB), a technology that Madhav has overviewed on Automotive Future previously. The article goes on to quote Richard Laine, a professor of Materials Science and Engineering at the University of Michigan and one of my research mentors over the last year. Professor Laine puts it simply by stating that a SSB would be the “holy grail of batteries”. While the SSB announcement never panned out for Tesla’s 2020 Battery Day, interest in this area has roared to life in recent years, particularly in the automotive sector, which stands to benefit from achieving such a massive leap in battery technology. But if you’re not familiar with SSBs, you're well justified in asking the questions: what's wrong with our current batteries now? and why don’t we have SSB’s yet? That’s what I’ll be diving into below.
Current lithium-ion batteries use organic liquid electrolytes as a medium to transfer lithium ions in between electrodes. From a safety perspective, we immediately run into some pretty significant challenges here: if a high enough voltage is applied to the battery, the temperature of the battery can be raised past operational safety limits, and a process called thermal runaway initiates. Thermal runaway is a feedback cycle that traps heat in the cell, causing degradation that can allow the liquid electrolyte to produce flammable vapors, which in some cases can lead to battery fires.
The next problem posed by liquid electrolytes is the formation of solid electrolyte interphase (SEI) at the interface between the electrolyte and electrode. SEI occurs when the electrolyte reacts with the moving lithium ions to form a thin film on the surface of the anode. The formation of this layer is inevitable, but not necessarily bad on its own. SEI can inhibit electron movement into the electrode, facilitate lithium ion movement to the anode side, and provide a mechanical barrier that decreases the probability of side reactions between the electrode and electrolyte--all of these are positive properties. However, if the SEI layer grows, drawing more transportable lithium away from the internal circuit, the battery will see an increased impedance and loss of capacity which will inhibit its performance.
Lastly, the formation and growth of lithium dendrites poses a risk to the stability of the liquid electrolyte and therefore the entire battery cell. Lithium dendrites are tendril-like aggregates that form as a result of lithium-ion deposition on the surface of the anode during charging of the battery, particularly during the application of high current drops that would be used in fast-charging. These dendrites also have the effect of siphoning capacity, but what’s more is that if the dendrites successfully pierce the battery separator and reach across the electrolyte to connect electrodes, an effect called short circuiting takes place. Short circuiting lowers resistance, allowing high current densities that the battery cannot sustain. Short circuit failure is catastrophic, damaging the circuit and in many cases leading to fires and explosions.
The above reasons are several motivations for replacing the liquid electrolyte with a solid state, but if these reasons are so apparently detrimental to lithium-ion battery performance, why haven’t we made the switch yet? It turns out that once again, the interface between the solid electrolyte and electrode is to blame. Transitioning to a solid material is inherently worse for the transport of lithium ions across the battery, which is pretty intuitive since particles move easier through liquids than solids . Therefore, the interfacial contact must be extremely well engineered to allow for optimal levels of lithium diffusivity into the electrode. Without this prerequisite, we cannot access all of the energy advantages that solid state technology has over current generation cells. This explains why we haven’t realized widespread usage of SSBs in the commercial space, because solutions to this issue are still in the research-to-industry pipeline. As a member of Dr. Richard Laine’s research team, one novel method I am investigating to optimize the electrode interface is the coating of electrode materials with solid electrolyte nanoparticles. With this approach, we can enhance the mechanical and electrochemical performance of the battery cell and suppress the aforementioned issues with the liquid electrolyte. In the future, I hope to integrate this method into a fully solid system, but this too is an intensive optimization process that is reflective of the overall difficulty in determining the timeline of SSBs.
It’s probably safe to conclude that SSBs will not be in tomorrow’s newest model of EV, even as the industry continues to heat up with new competitors and battery chemistries. However, understanding SSBs is a worthwhile investment, and can provide important insights into the direction of the EV industry as the world electrifies.
Please feel free to reach out with any questions, happy reading!
Charlie Cappelletti ctcapp@umich.edu
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