Anomalous Capacity Enhancement in Bi 2 Sn 2 O 7 Sheet Anodes Driven by Dynamic Conversion‐Alloying Processes

Pyrochlore with a sheet‐like morphology is demonstrated as a conversion‐alloying anode for high‐performance lithium‐ion storage. Cyclic voltammetry reveals well‐resolved Bi–O/Sn–O conversion (2.0 V), stepwise Bi and Sn alloying, and a dominant full‐alloying feature (0.019 V), with reversible Sn (0.196 V) and Bi (0.918 V) dealloying. Galvanostatic cycling reveals an anomalous three‐stage capacity evolution, initial fading, prolonged activation, and gradual postpeak decline, reaching peak discharge capacities of 239.26 (0.5 C) and 267.69 (1 C), sustained by 99% Coulombic efficiency. The sheet‐like morphology provides short diffusion pathways and mechanical flexibility that accommodate the alloying‐induced volumetric strain, suppressing pulverisation over extended cycling. Galvanostatic intermittent titration technique measurements show the diffusion coefficient spanning seven orders of magnitude ( to ), with minima at 0.7–0.9 V reflecting overlapping Bi/Sn alloying that mutually hinders transport. In‐situ electrochemical impedance spectroscopy corroborates this, revealing diffusion‐controlled transport during lithiation that transitions to a more resistive, geometrically constrained state upon delithiation. Ex‐situ time‐of‐flight secondary ion mass spectrometry confirms the conversion mechanism through a spatially concentrated hotspot colocalised with , consistent with formation, alongside spatially heterogeneous inorganic SEI species overlying uniformly distributed organic fragments.