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    Mechanistic insights into cathode-driven capacity degradation of NMC111/ graphite pouch cells under long-term cycling
    (Pergamon-Elsevier Science Ltd, 2025) Ates, Mehmet Nurullah; Zengin, Feyza; Whba, Rawdah; Tunaboylu, Bahadir; Aydemir, Umut; Peighambardoust, Naeimeh Sadat; Karslioglu, Nergiz Gurbuz
    To investigate long-term degradation, 2000 mAh NMC111/graphite (Gr) pouch cells were cycled 5500 times at a 1C rate. After cycling, the resulting degradation mechanisms were systematically analyzed. Structurally, X-ray diffraction (XRD) peak shifts (003, 108, 110) revealed Jahn-Teller (JT) distortion, evidenced by an increase in the c-lattice parameter. This led to the rise in internal resistance, consistent with scanning electron microscopy (SEM) images that revealed pronounced grain deformation on the cathode. Chemically, ex-situ X-ray absorption near-edge structure (XANES) spectroscopy revealed an increase in the valence states of Mn, Ni, and Co ions, indicating significant bulk changes that could potentially destabilize the oxygen lattice. X-ray absorption fine structure (XAFS) analysis further underscored the key role of weakening transition metal-oxygen (TM-O) bonds in driving this structural deformation. At the surface, X-ray photoelectron spectroscopy (XPS) confirmed the formation of a cathode-electrolyte interphase (CEI) comprising lithium fluoride (LiF), LixPFy, and organic carbonates. The progression of these surface reactions is a key contributor to impedance growth and capacity fade over long-term cycling.