Lithium (Li) metal batteries attracted intensive research attention for their high energy density (up to 500 Wh/Kg), enabled by high-capacity Li metal anodes paired with high-voltage, high-capacity Ni-based cathodes (LiNi_x(MnCo)_1-xO₂, x ≥ 0.8; NMC).[1] However, there are many obstacles to the commercial deployment of Li metal batteries, such as Li-dendrite growth,[2] transition metal (TM) dissolution,[3] among many others. Additional issues may also arise from the cross-talk, a well-known phenomenon associated with TM dissolution and re-deposition onto graphite anodes, consequently causing degradation of Li-ion cells.[4] So far, studies on the cross-talk in Li metal/high-Ni NMC batteries are largely missing, and it is unclear how the cross-talk could downgrade cell performance.[5]

In this study, a comprehensive investigation was conducted on Ni dissolution and the related cross-talk phenomena in Li metal/high-Ni NMC batteries through multimodal X-ray characterization. In situ and ex situ submicron-resolution X-ray spectroscopy (SRX) was applied to track and quantify the Ni dissolution and re-deposition on the Li metal surface (Figure 1). The impact of Ni re-deposition on the Li dendrites was obtained by examining the morphology evolution of the cycled Li metal electrodes retrieved from Li/high-Ni NMC cells, in comparison to those cycled Li metal electrodes from Li/Li reference cells under the same cycling conditions. Further analysis was made to the compositional evolution of solid electrolyte interphase (SEI) and build-up of interfacial impedance for direct correlation between Ni dissolution and the interfacial instability of Li metal when subjected to elongated cycling. The implications of the observations to designing commercially viable Li metal/high-Ni NMC batteries will be discussed.

[1] Liu, Jun, et al. "Pathways for practical high-energy long-cycling lithium metal batteries." Nat. Energy 4 (2019) 180.

[2] Cheng, Xin-Bing, et al. "Toward safe lithium metal anode in rechargeable batteries: a review." Chem. Rev. 117 (2017) 10403.

[3] Gilbert, James A., et al. "Transition metal dissolution, ion migration, electrocatalytic reduction and capacity loss in lithium-ion full cells." J. Electrochem. Soc. 164 (2017) A389.

[4] Harris, Oliver C., et al. "mechanisms and consequences of chemical cross-talk in advanced Li-ion batteries." J. Phys. Energy 2 (2020) 032002.

[5] Betz, Johannes, et al. "Cross talk between transition metal cathode and Li metal anode: unraveling its influence on the deposition/dissolution behavior and morphology of lithium." Adv. Energy Mater. 9 (2019) 1900574.

[6] Li, Li, et al. "PyXRF: Python-based X-ray fluorescence analysis package", Proc. SPIE 10389, X-Ray Nanoimaging: Instruments and Methods III, (2017), 103890U.