Julia Hestenes1,Richard May1,Jerzy Sadowski2,Naiara Munich3,Lauren Marbella1
Columbia University1,Brookhaven National Laboratory2,Barnard College, Columbia University3
Julia Hestenes1,Richard May1,Jerzy Sadowski2,Naiara Munich3,Lauren Marbella1
Columbia University1,Brookhaven National Laboratory2,Barnard College, Columbia University3
The high specific capacities of Ni-rich transition metal oxides have garnered immense interest for improving the energy density of Li-ion batteries (LIBs). Despite the potential of these materials, Ni-rich cathodes suffer from interfacial instabilities that lead to crystallographic rearrangement of the active material surface as well as the formation of a cathode electrolyte interphase (CEI) layer on the composite during electrochemical cycling. While changes in crystallographic structure can be detected with diffraction-based methods, probing the chemistry of the disordered, heterogeneous CEI layer is challenging. In this work, we use a combination of ex situ solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information on the CEI deposited on LiNi<sub>0.8</sub>Mn<sub>0.1</sub>Co<sub>0.1</sub>O<sub>2</sub> (NMC811) composite cathode films. Specifically, XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI. Separately, paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in <sup>13</sup>C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, we find that LiF, Li<sub>2</sub>CO<sub>3</sub>, and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ <sup>1</sup>H and <sup>19</sup>F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows for spatially-resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films.