Investigation of NMC-811 Surface Degradation in Pure CO2 and Humid Air

Nickel-rich NMC-811 is a benchmark cathode material for high-energy density lithium-ion batteries due to its high specific capacity (>200 mAh g−1) and operating voltage (~3.8 V). However, its strong surface reactivity toward atmospheric species, particularly moisture and CO2, poses significant challenges during storage and processing, leading to the formation of LiOH- and Li2CO3-rich surface layers. Although the effects of humid air have been widely investigated, a direct comparison between high relative humidity and pure CO2 exposure remains limited. Here, we systematically examine the morphological, structural, chemical, and electrochemical evolution of commercial NMC-811 electrodes after 5 h exposure to 80% relative humidity or CO2-saturated atmosphere. Moisture treatment induces substantial surface reconstruction, lattice shrinkage, and increased cation disorder, accompanied by extensive hydroxide and carbonate formation. In contrast, CO2 exposure mainly modifies the outermost surface layer without significant bulk structural changes. Electrochemical testing reveals that CO2-treated electrodes display higher initial polarization but quickly recover near-pristine performance, whereas humidity-treated electrodes exhibit persistent kinetic limitations, accelerated capacity fading, and earlier end-of-life. Overall, degradation severity follows the trend: pristine < CO2 < RH 80%, highlighting the dominant role of moisture in irreversible structural deterioration.