Understanding the Capacity Fade in Polyacrylonitrile Binder‐based LiNi _0.5 Mn _1.5 O ₄ Cells**

Abstract Binders are electrochemically inactive components that have a crucial impact in battery ageing although being present in only small amounts, typically 1–3 % w/w in commercial products. The electrochemical performance of a battery can be tailored via these inactive materials by optimizing the electrode integrity and surface chemistry. Polyacrylonitrile (PAN) for LiNi _0.5 Mn _1.5 O ₄ (LNMO) half‐cells is here investigated as a binder material to enable a stable electrode‐electrolyte interface. Despite being previously described in literature as an oxidatively stable polymer, it is shown that PAN degrades and develops resistive layers within the LNMO cathode. We demonstrate continuous internal resistance increase in LNMO‐based cells during battery operation using intermittent current interruption (ICI) technique. Through a combination of on‐line electrochemical mass spectrometry (OEMS) and X‐ray photoelectron spectroscopy (XPS) characterization techniques, the degradation products can be identified as solid on the LNMO electrode surface, and no excessive gas formation seen. The increased resistance and parasitic processes are correlated to side‐reactions of the PAN, possibly intramolecular cyclization, which can be identified as the main cause of the comparatively fast capacity fade.