In situ ec-AFM Investigation of a High-Capacity Aluminum-organic Battery: Visualizing Anion-Induced Swelling in Redox-Active Polymer Electrodes

Rechargeable aluminum batteries (RABs) are promising post-lithium energy storage systems due to the high abundance and volumetric capacity of aluminum, yet stable positive electrodes development remains a bottleneck. Cross-linked poly(3-vinyl-N-methylphenothiazine) (X-PVMPT) as a p-type redox polymer shows reversible two-electron redox chemistry at relatively high potentials and excellent cycling stability in RABs when paired with chloroaluminate-based ionic liquid electrolytes. Here, we present the investigation of volume expansion and morphological evolution of X-PVMPT composite electrodes during cycling using electrochemical (ec-) atomic force microscopy (AFM) and cyclic voltammetry. ec-AFM data reveals the dynamics of reversible expansion during anion insertion and (ir)reversible changes associated with prolonged cycling. In situ linescan profiling and 2D nanomechanical imaging allows visualization of pronounced and reversible volume change during cycling, associated with the insertion of AlCl$_4$$^−$ / Al$_2$Cl$_7$$^−$ anions. Complementary electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurements validate these observations at the macroscopic scale, confirming that volume expansion and electrochemical stiffening are homogeneous, bulk phenomena of the X-PVMPT framework. ... mehrWe determined a reversible swelling amplitude of 1.1 ± 0.1 µm during anion insertion for a 9.6 µm thick film, correlated with a 4-fold increase in stiffness. While initial cycles involve large-scale structural changes, the network rapidly reaches a mechanically stabilized state after the initial cycle, where the polymer matrix undergoes irreversible reorganization. After the steady-state phase (from cycle 120 onwards), the swelling amplitude drops by 60-75% to 0.3 – 0.4 µm with sustained faradaic performance, highlighting the ability of the cross-linked matrix to establish ion-conduction pathways during prolonged cycling.