A team of researchers led by chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory has learned that an electrolyte additive allows stable high-voltage cycling of nickel-rich layered cathodes. Their work could lead to improvements in the energy density of lithium batteries that power electric vehicles.
Sha Tan, a co-first author and Ph.D. candidate at Stony Brook University conducting research with the Electrochemical Energy Storage group at Brookhaven Lab, was originally studying how an additive, lithium difluorophosphate (LiPO2F2), could be used to improve low-temperature performance of batteries. Out of curiosity, she tried using the additive for high voltage cycling at room temperature.
“I found if I pushed the voltage up to 4.8 V, this additive really gives great protection over the cathode and the battery achieved excellent cycling performance,” Tan said.
Batteries consist of two electrical terminals — electrodes called the cathode and the anode — that are separated by another battery component, the electrolyte. Electrons go through an external circuit connecting the two electrodes and ions go through the electrolyte. Both shuttle back and forth between the electrodes during charge-discharge cycles.
Nickel-rich layered cathode materials promise high energy density for next-generation batteries when paired with lithium metal anodes. But those materials are prone to capacity loss. One of the main issues is particle cracking during high-voltage charge-discharge cycles. High-voltage operation is important because the total energy stored in a battery, important for vehicle range, goes up as the useful operating voltage increases.
Another issue is transition metal dissolution from the cathode and its subsequent deposition on the anode. This is known as “crosstalk” in the battery community. During high-voltage charging, small amounts of transition metals in the cathode crystal lattice dissolve, and then journey through the electrolyte, and deposit on the anode side. When this happens, both cathode and anode degrade. The result is poor battery capacity retention.
Researchers found that introducing a small amount of additive to the electrolyte stifles that crosstalk. As the additive decomposes, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF) to form a highly protective cathode-electrolyte-interphase — a solid thin layer that forms on the battery’s cathode during cycling.
“By forming a very stable interphase on the cathode, this protective layer significantly suppresses the transition metal loss on the cathode surface,” Hu said. “Reduced transition metal loss helps to decrease the deposition of those transition metals on the anode. In that sense, the anode is protected to a certain extent as well. We believe suppression of transition metal dissolution is one of the key contributors that lead to significantly improved cycling performance.”
The electrolyte additive enables a nickel-rich layered cathode to be cycled at high voltages to increase the energy density and still retain 97 percent of its initial capacity after 200 cycles, the researchers found.
Looking ahead, the researchers want to test the additive under more challenging conditions to explore whether the cathode materials can withstand even more cycles for practical battery use.