(Invited) Probing the in-Situ Surface Chemistry of Ni-Rich LiNi _x Co _y Mn _z O₂ (x+y+z=1) Cathode Materials Using Synchrotron-Based Ambient-Pressure X-Ray Photoelectron Spectroscopy

Ni-rich layered cathode materials are important candidates for lithium-ion batteries used for applications requiring high energy density, such as electric vehicles and mobile electronic devices. However, comparing with low Ni content NMC, Ni-rich NMCs frequently exhibit fast capacity decay and relatively shorter lifetime due to the formation of more reactive surface with high Ni content. Previous reports demonstrated that Ni-rich NMCs are sensitive to the CO2 and water in the air during ambient storage, because of these oxidizing gases can lead to the formation of surface impurities such as Li2CO3 and LiOH. [1] these surface impurities can react with electrolyte during the electrochemical cycling process and form a nonuniform cathode/electrolyte interface (CEI) layer. Surface instability of Ni-rich NMCs can also cause problems during the large-scale electrode preparation, and material storage. Research results reported in the literature have suggested that the degradation of Ni-rich NMCs could be attributed to the reduction of Ni and evolution of active oxygen species as well as the side reaction between the surface impurities and the electrolyte. [2-3] However, only limited experimental results have been reported about the formation mechanism of surface impurities and side reaction during the battery cycling for Ni-rich NMCs. Therefore, it is very important to develop an in-situ characterization method to reveal the CO2 and moisture effects on the surface chemistry of Ni-rich NMCs. Synchrotron-based Ambient-Pressure XPS (AP-XPS) is a powerful technique with controllable gas environments and tunable energy. These enable us to probe the surface chemistry evolution of the samples in-situ in atmospheric gas environments with different depth profile. Taking these advantages, we have systematically investigated the CO2 and moisture effect on the LiNi0.94Co0.06O2 cathode material with and without Al doping using AP-XPS technique. Preliminary results showed that only Ni2+ can be observed in NC sample surface after exposed to CO2 environment even measured at 2000 eV with probing depth of ~9 nm. In contrast, characteristic peak of Ni3+ still exists in NCA sample, even measured at 1200 and corresponding to probing depth of ~5.5 nm. These results show that better surface stability (less Ni2+ at surface) can be achieved by Al doping. We also comprehensively investigated the depth profile of CEI layer of NMC cathode materials with different Ni content and found that CEI composition really depends on Ni content, dopant as well as the cut off voltage for charging. These results will be presented at the meeting. [1] You et al. Angewandte Chemie, 572018 (2018) 6480-6485. [2] Shizuka et al. Journal of Power Sources, 166 (2007) 233-238. [3] Jung et al. Journal of The Electrochemical Society, 165 (2) (2018) A132-A141. Acknowledgement The work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract No. DE-SC0012704.