Additive Engineering for Ultra-High Voltage Battery Cycling
Researchers found an electrolyte additive to stabilize electrodes at an ultra-high voltage of 4.8 V.
Lithium-ion batteries (LIBs) are transforming the automotive sector by electrifying vehicles. Vehicle applications demand design considerations beyond LIBs suitable for consumer electronics. LIBs with high energy and power density are vital components in EVs. The progress of LIBs has been significant over the decades. However, there are several technical challenges for LIBs to meet the future needs of the automotive application. Increasing energy density is one of the barriers to the widespread application of lithium-ion batteries in electric vehicles.
Eliminating Battery Instability
High energy density can be achieved by increasing the specific capacity of electrodes and by enhancing the operation voltage of batteries. However, increasing the operating voltage can lead to unstable behavior in batteries. To eliminate the instability, a careful selection of compatible electrolytes is necessary. An electrolyte plays a dominant role in the electrochemical performance of both lithium-metal anode and high-voltage cathode. Researchers at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have identified that an electrolyte additive allows stable high-voltage cycling of nickel-rich layered cathodes.
Co-first author Sha Tan, left, and Brookhaven chemist Enyuan Hu. Image used courtesy of Brookhaven National Laboratory
Electrolyte Additive for Protecting Battery Electrodes
A lithium-ion battery consists of two electrodes - an anode and a cathode - separated by an electrolyte. Electrons go back and forth through the electrodes to an external circuit, and ions flow through the electrolyte.
The researchers suggest that nickel-rich layered cathode materials offer high energy density when paired with lithium metal anodes. However, these materials are prone to capacity loss. The cathode cracks during high-voltage charge and discharge cycles. But as mentioned earlier, high voltage cycling is necessary for enhanced energy density. Another issue with high voltage operation is what the researchers call "crosstalk". During high voltage charging, a small amount of transition metal in the cathode dissolves and deposits on the anode electrode, degrading both the anode and cathode electrodes.
The researchers found that using an appropriate amount of lithium difluorophosphate as an additive in the electrolyte can allow for stable cycling with an ultra-high cut-off voltage of 4.8 V. As the additive decomposes, it produces lithium phosphate and lithium fluoride to form a highly protective layer between the cathode and the electrolyte, which suppresses the transition metal loss on the cathode surface.
The researchers believe that suppressing transition metal dissolution can lead to significantly improved cycling performance. The researchers cycled a nickel-rich cathode battery at the voltage of 4.8V and found that the battery retains 97% of its initial capacity after 200 cycles.
Stabilizing Polycrystalline Cathode
The most common nickel-rich cathode is in the form of polycrystals, where many nanoparticles are lumped together to form another particle. Polycrystals are easy to manufacture but are prone to causing particle cracks and capacity fading.
Recent studies have shown that single-crystal cathodes perform better than polycrystals in suppressing particle cracks. But they are very costly to manufacture. Researchers at Brookhaven National Laboratory found that the additive helps in the uniform distribution of lithium within the cathode to mitigate the crack formation.
Sha Tan, a co-first author and PhD candidate at Stony Brook University conducting research with the Electrochemical Energy Storage group at Brookhaven Lab, says, "Our strategy uses a very small amount of additive to achieve such great improvement of the electrochemical performance. Practically speaking, this could be a low-cost and easy-to-adopt solution."
The researchers now plan to test the additive under more challenging conditions to explore if the cathode material can withstand even more cycles comparable to commercial batteries.