Despite the ubiquity of lithium-ion batteries in portable electronic devices and electric vehicles, they still have inefficiencies. One problem is that oxygen from the metal-oxide cathode leaks during the chemical cycling process by which lithium ions transport charge. More than 100 recharging cycles can degrade a battery’s energy capacity by up to 15%. Now, using a combination of tools to investigate the electrode itself, Stanford University PhD student Peter Csernica and his colleagues have inferred the rate at which oxygen atoms leak and shown how those lost atoms change the electrode’s structure and chemistry. Better understanding the oxygen-loss process could lead to new ways of mitigating battery degradation.
The researchers charged and discharged Samsung lithium-ion batteries up to 500 times. Then they dismantled the batteries and extricated different-sized clusters of the metal-oxide nanoparticles that together make up the cathode. Using a synchrotron light source to take high-resolution x-ray microscope images of those clusters, they probed the chemical and structural makeup at different length scales. The images indicated that, initially, a burst of oxygen atoms escaped the nanoparticles’ surface. Subsequently, that leakage transitioned into a slow trickle from the nanoparticles’ interior. Large clusters of nanoparticles lost less oxygen from their centers than from their surfaces. After 500 cycles of battery charging and discharging, 6% of the oxygen in the cathode had escaped.
Those observations prompted Csernica and his colleagues to question whether the electrode structure collapsed around the gaps left behind by lost oxygen atoms. Surprisingly, a comparison of their data with theoretical models of different possible escape mechanisms showed that the gaps remained. However, the presence of those gaps caused the nickel, cobalt, and manganese metal atoms to rearrange. That rearrangement, along with chemical changes caused by the missing oxygen, ultimately caused the electrode to degrade over time.
The findings counter the common perception that oxygen leaks only from a nanoparticle’s surface. Whereas most mitigation efforts focus on surface coatings, researchers could explore different ways of stacking the layered electrode materials to inhibit structural changes that accommodate the oxygen gaps. Alternatively, they could design larger nanoparticle clusters to reduce oxygen release and preserve electrochemical capacity. (P. M. Csernica et al., Nat. Energy 6, 642, 2021.)
