Solid State Batteries - the Answer to Li-Ion Batteries' Shortcomings?
Lithium-ion is the best battery technology the world has ever seen. According to a July 2016 report by IDTechX, energy density has increased by 5 % per year whilst cost has reduced by 8 % per year. However, it is still not a perfect fit: Firstly, it is unlikely to achieve the medium-term transformation which requires improvements in cost and performance of the order of five and, secondly, there are safety concerns over the inherently unstable technology.
Current Lithium-ion batteries rely on the flow of ions through a liquid electrolyte between two electrodes; however, cells incorporating a liquid electrolyte are prone to problems, including low charge retention, reduced efficiency when operating at high and low temperature and susceptibility to thermal runaway.
Consequently, the question is often asked whether there’s an alternative electrical energy storage system that is cost efficient and safe with a similar or superior power density and performance.
Solid state electrolytes promise a safe alternative
Scientists have spent decades searching for a safe alternative to the flammable liquid electrolytes used in lithium-ion batteries. Now Stanford University researchers have identified nearly two-dozen solid electrolytes that could someday replace the volatile liquids employed in EV batteries, smartphones, laptops and other electronic devices.21 solid materials could replace the flammable liquid electrolytes in lithium-ion batteries and in so doing improve safety.
According to the study’s lead author Austin Sendek, a doctoral candidate in applied physics: “The main advantage of solid electrolytes is stability. Solids are far less likely to blow up or vaporize than organic solvents. They’re also much more rigid and would make the battery structurally stronger.”
Despite years of laboratory trial and error, researchers have yet to find an inexpensive solid material that performs as well as liquid electrolytes at room temperature, so instead of randomly testing individual compounds, the team turned to AI and machine learning to build predictive models from experimental data. They trained a computer algorithm to get to identify good and bad compounds based on existing data, much like a facial-recognition algorithm learns to recognize faces after seeing several examples.
After screening more than 12,000 lithium-containing compounds, a list of 21 promising solid electrolytes was compiled, which could radically speed up the development of a safe, efficient and cost-effective solid state battery.
However, due to their higher electrolyte-resistivity than conventional liquid electrolyte, the low power characteristics of solid-state batteries will require the development of 'superionic' materials which allow ions to move quickly and freely through their crystalline structure. So far the best results have been obtained using the costly element germanium.
Even so, with their sights set on the magical $ 100 / kWh researchers have continued looking for alternative superionic conductors that could provide the basis for all solid state batteries.
One such study, conducted by Yuki Kato and Ryoji Kanno in collaboration with colleagues from Toyota Motor Corporation, Tokyo Institute of Technology and High Energy Accelerator Research Organization-Japan (KEK), investigated synthesizing two crystal materials to produce a promising new solid electrolyte.
The development of these two new lithium-based superionic conductor materials (structures: Li9.54Si1.74P1.44S11.7Cl0.3 and Li9.6P3S12 ) is seen as a significant step forward in the creation of usable solid state batteries.
Two test cells based on the novel solid electrolytes performed very well in trials when compared to lithium-ion batteries. The cells remained stable and operated reliably across a temperature range of -30 to 100 °C. Under these conditions they exhibited high energy and power densities with negligible internal resistance; being solid state, there are no safety concerns.
Furthermore, the cells displayed ultrafast charging, retained their charge for lengthy periods, and appeared to have a long lifespan with excellent cycling ability: After over 500 cycles, the cells retained around 75 % of their initial discharge capacity.
Reducing the risks of runaway thermal events in Li-ion batteries
Li-ion battery thermal events where a damaged cell, a short, or even overcharging can cause the battery to catch fire are well documented. At about 150 degrees Celsius, the electrolyte gel carrying particles between the two electrodes of the standard lithium-ion battery can ignite, resulting in uncontrolled combustion or even an explosion.
To control these runaway thermal occurrences, a group of researchers at Stanford University have developed a nickel-filled plastic overlay to prevent batteries from bursting into flame. The overlay, produced by coating nickel particles with graphene and then embedding them in elastic polyethylene, will shut the battery down when it overheats, preventing combustion.
As Zhenan Bao, professor of chemical engineering and one of the study researchers, explained: “To conduct electricity, the spiky particles [of nickel] have to physically touch one another. But during thermal expansion, polyethylene stretches. That causes the particles to spread apart, making the film non-conductive so that electricity can no longer flow through the battery.”
To demonstrate the efficacy of the system the researchers used a hot-air gun to raise the temperature, and each time the cells responded as predicted – once the battery hit about 70 °C, the film expanded and shut down the battery. When the battery cooled, the particles came back into contact, and the battery began to work again.
“We can even tune the temperature higher or lower depending on how many particles we put in or what type of polymer materials we choose,” Bao said. “We might want the battery to shut down at 50 °C or even 100 °C; with this technology the setpoints are infinitely variable.”
As Toyota’s chief engineer for the Prius, Koji Toyoshima said to Reuters, “It’s a tall order to develop a lithium-ion car battery which can perform reliably and safely for 10 years, or over hundreds of thousands of kilometers. We have double braced and triple braced our battery packs to make sure they’re fail-safe: It’s all about safety, safety, safety.”
Because of these safety concerns, Toyota used nickel-metal hydride batteries in previous Prius models except for one very early version. The newly announced Prius Prime, however, uses a newly designed lithium-ion battery, which gives the Prius Prime hybrids an electric-power-only range of about 40 km, whilst ensuring protection from thermal runaway.
These new lithium-ion battery packs have 95 cells with a control technology that tracks the temperature and condition of each cell.
Toyota senior engineer Hiroaki Takeuchi claims, “Our control system can identify even slight signs of a potential short-circuit in individual cells, and will either prevent it from spreading or shut down the entire battery.”
According to Takeuchi, Toyota’s battery production facilities aren’t exactly built to semiconductor clean room standards, “but are very close.” Lithium-ion batteries can short-circuit, overheat, and even explode if microscopic metal particles or other impurities are introduced during manufacture, but with this new “management” system thermal events will be identified and controlled before doing any damage.
So with no apparent shortage of technologies to improve current Li-ion battery performance and safety, one can only speculate as to which one will eventually proliferate, although the mystical $100 / kWh may have the final say.