High-energy composite cathode for solid-state lithium-oxygen battery boosted by ultrafine carbon nanotube catalysts and amorphous lithium peroxide

Abstract

Rechargeable solid-state lithium-oxygen batteries (SSLOBs) promise high energy density and safety but suffer from the severe interface impedance in composite cathodes and the failure to detect discharge products by spectroscopy and microscopy originating from low actual discharge capacity. Here, an ultrafine carbon nanotube (CNT) is developed as a cathode catalyst for SSLOBs, which unexpectedly improves the solid-solid contact issue with Li1·5Al0.5Ge1.5(PO4)3 (LAGP) solid electrolyte and provides a superior specific capacity of 5803 mAh/g (100 mA/g, 2.0–5.0 V vs. Li/Li+) in oxygen atmosphere that is about 4–5 times higher than that of SSLOBs using CNTs with large diameters. This phenomenon is attributed to the optimization of the impedance of CNTs@LAGP induced by the enhanced catalytic activity and enlarged specific surface area of ultrafine CNTs. The amorphous discharge products formed on the CNT surface and between the CNTs and LAGP particles are first found to significantly reduce the cell impedance and increase new reactive sites. Moreover, the irreversible disruption and passivation of CNTs and the accumulation of residual discharge products, which are the fundamental sources of capacity degradation, are further discussed. Importantly, Li2O2 growth morphology as a function of discharge depth and the possible conduction paths of amorphous (Li2O2)n cluster with p-type semiconductor characteristics are also determined by computations based on molecular dynamics and density functional theory for the first time. Our results innovatively reveal the controversial association between impedance and microstructure of composite cathodes, and thus, benefit the development of high-efficiency catalysts and the design of high-energy cathode structures for SSLOBs and other potential solid-state batteries.