Two groups of scientists are reporting breakthroughs in battery technology this week. Both say their discoveries will lead to longer range for battery-operated vehicles. Is there reason to get excited about either? Let’s take a closer look at what is happening in the lab these days.
Overstuffed Battery Cathodes
You may have missed the report published by Nature Communications recently with this mind numbing title: “Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.” If you have trouble wrapping your head around such arcane studies, the folks at Science Daily have decoded it for you.
Researchers at the SLAC National Accelerator Lab in conjunction with others say that overstuffing a cathode with lithium results in a battery with 30 to 50 percent greater range, but the “fat” cathode (my term, not theirs) quickly deteriorates. Michael Toney, one of the authors of the study, says, “It is a huge deal if you can get these lithium-rich electrodes to work because they would be one of the enablers for electric cars with a much longer range. There is enormous interest in the automotive community in developing ways to implement these, and understanding what the technological barriers are may help us solve the problems that are holding them back.”
To figure out what was happening to the cathodes required examining them at the atomic level, work that was made possible by collaborating with the Stanford Synchrotron Radiation Lightsource, Lawrence Berkeley National Laboratory’s Advanced Light Source, and Berkeley Lab’s Molecular Foundry. The theoretical results were then used by people at the Samsung Advanced Institute of Technology, who used commercially relevant techniques to assemble batteries similar to those used in electric vehicles. No sense going to all that work if the results are not commercially viable.
“The cathode in today’s lithium-ion batteries operates at only about half of its theoretical capacity, which means it should be able to last twice as long per charge,” says Professor William Chueh at SLAC. He continues, “But you can’t charge it all the way full. It’s like a bucket you fill with water, but then you can only pour half of the water out. This is one of big challenges in the field right now — how do you get these cathode materials to behave up to their theoretical capacity? That’s why people have been so excited about the prospect of storing a lot more energy in lithium-rich cathodes.”
Cathodes in use today are composed of layers of lithium sandwiched between layers of transition metal oxides — elements like nickel, manganese or cobalt combined with oxygen. Adding lithium to the oxide layer increases the cathode’s capacity by 30 to 50 percent. What the researchers were able to do is to examine the cathode material in real time as charging and discharging took place. What they found is that over time, some of the lithium ions did not return to their original location after a charge/discharge cycle, leading to a loss of capacity.
“We knew all these phenomena were probably related, but not how,” Chueh said. “Now this suite of experiments at SSRL and ALS shows the mechanism that connects them and how to control it. This is a significant technological discovery that people have not holistically understood.” Knowing how to control what happens inside the cathode is expected to lead to batteries with the combination of longer range and longer life necessary to move the electric car revolution forward.
Creating A Protective Coating For Lithium
Researchers at the University of Waterloo in the UK have published a new study in the journal Joule with this attention grabbing title: “An In Vivo Formed Solid Electrolyte Surface Layer Enables Stable Plating of Li Metal.” They claim they have found a way to double or triple the potential range of electric cars by altering the electrolyte that transports ions between the cathode and anode of a battery. The researchers found that adding phosphorous and sulfur to the electrolyte caused it to spontaneously coat the lithium metal in electrodes with “an extremely thin protective layer.”
This discovery is related to the SLAC study, in that it may help increase the useful life of those cathodes overstuffed with lithium the SLAC researchers are working on. Protecting the lithium in electrodes could increase storage capacity without the risks of rapid degradation.
On balance, the Stanford work may be closer to commercial use, thanks to the involvement of Samsung and a focus on using existing production techniques. But the University of Waterloo research could be vital to future batteries with greater range and long life. Such advanced batteries hold great promise for electric cars, but their real value may be in hastening the demise of the diesel engine for heavy duty trucks, buses and ships. The implications for grid scale storage batteries are also exciting to think about.