Can Solid-State Electrolytes Improve Li-Ion Battery Charging Speed and Safety?
Conventional lithium-ion batteries in electric vehicles rely on liquid electrolytes to transport lithium ions during charging and discharging. Researchers have found a high-performing solid-state electrolyte that could surpass the performance and safety offered by the liquid electrolytes currently used in Li-ions.
Switching from combustion engine cars to electric vehicles (EVs) is hardly a simple swap. Engineers have been constantly improving EVs to offer the same reliable performance, safety, and convenience achieved by their gasoline-powered rivals.
Conventional Li-ion battery used in commercial EVs. Image used courtesy of Transport & Environment
The core focus of innovation has been on the battery at the center of the EV, as researchers tinker with cathode materials, electrolyte formulas, and other various components to keep advancing battery performance. Storage capacity, long-lasting life cycles, and safety are three primary concerns when testing new Li-ion battery configurations, and researchers have been studying a solid-state electrolyte (SSE) formula that delivers on all three of these metrics. One recent study on an SSE, a vacancy-rich Li9N2Cl3, demonstrates how it offers dry air stability and higher ionic conductivity.
Li-Ion Safety and Charging Challenges
As the shift toward EVs continues, perceptions of EV ownership remain a substantial hurdle. People are worried about charging at home, finding charging stations as they commute, and some occasional but frightening mishaps with Li-ions catching fire are also cause for concern.
While some headlines have sensationalized the danger of EVs as rolling fire hazards, these worries are rooted in real concerns about how Li-ions perform, and engineers keep pushing to improve their performance. Currently, a Level 1 charging system, which refers to drivers plugging their EVs into conventional home outlets, may offer convenience but does not offer a quick charging experience. A full eight hours of charging will barely allow a driver to make a 40-mile round-trip commute to work, so forgetful drivers can easily find themselves stranded if they did not plug in overnight.
Level 1 charging at home. Image used courtesy of Nissan
At the battery material level, the liquid electrolyte is in part responsible for these slower charging times. And there is a degree of complexity with how the liquid electrolyte becomes rate-limiting. In the separator electrolyte, ionic conductivity is not inhibited and so does not limit fast-charging capabilities. However, ionic transport in the liquid electrolyte at the anode side of the battery does suppress charging speeds.
Charging speed is a universal concern when it comes to EV adoption because it is a basic functionality issue for the consumer, but safety concerns also still linger even though EV fires are not necessarily common. When lithium ions build up on the anode surface, lithium dendrites form, and they can lead to short circuits and fires. The high conductivity of liquid electrolytes makes them ideal from a performance perspective, but high flammability correlates with high conductivity. The most common commercial Li-ions use cyclic and linear carbonate solvents that exhibit significant flammability behavior.
Frequency of waste and recycling facility fires. Image used courtesy of Waste360
And while EV fires are relatively rare, with the first of half 2023 seeing about 500 worldwide, there are major concerns about flammability behavior in Li-ions that have been discarded in waste facilities. Just because a battery has reached the end of its life cycle does not mean it has lost its flammable properties, and fires have been increasing at waste and recycling facilities that experts tie to the increased disposal of Li-ions.
Performance and Safety Benefits of SSEs
A recent study published in the journal Science Advances explores how one SSE can improve charging speeds while also reducing flammability.
The researchers conducted experiments using a lithium-based material (Li9N2Cl3) for the electrolyte. They found that this SSE offers the same conductivity associated with liquid electrolytes with increased potential for even faster charging speeds. In addition, the SSE material suppresses dendrite growth, ultimately reducing the short-circuit potential that creates fires.
The impact of an SSE that can simultaneously improve safety and charging performance is substantial, as it would further facilitate adoption and reduce the pressure on EV charging infrastructure. Experts are currently concerned about the ratio of EVs hitting the road and available charging stations, and they suspect there may be significant congestion at charging stations as EV ownership increases. But an SSE that amps up the charging potential of the battery itself will ease stress on charging stations. some solid-state batteries can reach an 80% charge in less than 15 minutes.
As the EV market grows, engineers are searching for SSE candidates that can replace the Li-ion’s dependence on liquid electrolytes, and studies like this one show that there are viable SSE options, and it is only a matter of time before one transforms Li-ion functionality.