The limitation mechanisms on the discharge behavior of Li-O2 batteries

A comprehensive model for lithium-oxygen (Li-O2) batteries is proposed and the discharge performance across varying cathode thickness, porosity, and pore structural evolution modes is thoroughly investigated. The results reveal that the cathode surface passivation restricts the discharge capacity but promotes the uniform distribution of lithium peroxide (Li2O2), especially near the O2 side. The electrode thickness of 255 μm serves as the boundary point between the two limiting mechanisms, with thicker electrodes primarily governed by O2 diffusion and thinner ones by surface passivation. In addition, a novel hybrid model is proposed with the morphological change of the cathode pore structure, which can predict the discharge performance made of different carbon materials, proving its superiority over the commonly used Single model. The maximum discharge capacities of carbon nanoparticle (CNP)- and carbon nanotube (CNT)-based cathodes are obtained at the radius of CNP and CNT of 30 nm and 20 nm, respectively. The superiority of discharge performance of the CNP-based cathode over the CNT-based cathodes becomes more pronounced as the carbon material radius increases. The evolution of the cathode pore structure morphology needs to be considered for the cathode made of small-sized CNP or CNT, such as rCNP = 10 nm and rCNT = 20 nm.