New research from SLAC aims to understand degradation in lithium-rich cathodes, with the hope of designing new, higher-capacity EV batteries
Researchers from universities, government laboratories and battery maker Samsung have been investigating how to pack more lithium into battery cathodes, with a view to creating more energy-dense cells.
While enriching positive electrodes with extra lithium has long been feasible, they quickly lose voltage after use, for reasons that until now haven’t been clear. However, in a new paper, researchers from Stanford University, Lawrence Berkeley National Laboratory and Samsung can offer new insight into how the same chemical processes that can contribute to higher capacities are also linked to changes in the atomic structure that lessen performance.
“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,” commented Stanford Professor William Chueh, of the Stanford Institute for Materials and Energy Sciences (SIMES) at the Stanford National Accelerator Laboratory (SLAC).
“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?”
In this case, the benefit is being able to match the energy density of today’s batteries in a smaller space. In automotive applications this can either mean reducing vehicle weight or getting extra mileage from the same amount of cells.
Lithium-rich cathodes are made of layers of lithium slotted between layers of metal oxides – elements like nickel, manganese or cobalt. Adding additional lithium to the oxide layer therefore increases the amount of ions that can be transferred.
However, when lithium ions move out, some of the transition metal ions move in, while oxygen atoms release electrons to make an electric current. Yet when these li-ions return upon recharging, not all of the transition metals return to their previous spots, meaning that with each cycle the cathode’s atomic structure is changed and less energy is available.
“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.”
The researchers studied this phenomenon at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL) and Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS), using cathodes made by Samsung and assembled into batteries similar to those in EVs.
“This is good news,” said William E. Gent, a Stanford University graduate student who led the study. “It gives us a promising new pathway for optimising the voltage performance of lithium-rich cathodes by controlling the way their atomic structure evolves as a battery charges and discharges.”
Michael Toney distinguished staff scientist at SLAC and a co-author of the paper added that: “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.”
As the team state in their paper, published in Nature: “Our results suggest that it may be possible to tune the stability and voltage of anion redox through control of the TM migration pathways. Thus, we suggest a new strategy for designing Li-rich layered oxides with improved cycling performance whereby the oxygen redox chemistry is tuned through structural modifications rather than the more common covalency modifications.”
According to SLAC, the group is now working toward using the understanding gained to design li-ion battery materials that can reach the full theoretical capacity without a loss of voltage over time.