The Reaction Mechanism and Capacity Degradation Model in Lithium Insertion Organic Cathodes, Li_2C_6O_6, Using Combined Experimental and First Principle Studies

Herein, we explore the capacity degradation of dilithium rhodizonate salt (Li_2C_6O_6) in lithium rechargeable batteries based on detailed investigations of the lithium de/insertion mechanism in Li_2C_6O_6 using both electrochemical and structural ex situ analyses combined with first-principles calculations. The experimental observations indicate that the Li_xC_6O_6 electrode undergoes multiple two-phase reactions in the composition range of 2 ≤ x ≤ 6; however, the transformations in the range 2 ≤ x ≤ 4 involve a major morphological change that eventually leads to particle exfoliation and the isolation of active material. Through first-principles analysis of Li_xC_6O_6 during de/lithiation, it was revealed that particle exfoliation is closely related to the crystal structural changes with lithium deinsertion from C_6O_6 interlayers of the Li_xC_6O_6. Among the lithium ions found at various sites, the extraction of lithium from C_6O_6 interlayers at 2 ≤ x ≤ 4 decreases the binding force between the C_6O_6 layers, promoting the exfoliation of C_6O_6 layers and pulverization at the electrode, which degrades capacity retention.