Thick film formation on Li-O2 cathodes - breaking the true capacity barrier.

The theoretical energy density of Li-O2 batteries is more than an order of magnitude larger than the current generation of Li-ion batteries. However, their practical performance is far less, in part due to their low true capacities, that is, capacity including the mass of the typically porous active material, electrolyte in the pores and the Li2O2 discharge product. This is further limited by the particulate nature of the product, yielding a theoretical maximum packing density of 74%, impacting both gravimetric and volumetric capacities. Here we introduce carbon nanochains (CNCs), a material synthesized from biomass that is structurally similar to a commonly studied cathode material, multiwalled carbon nanotubes (MWCNTs). CNCs form densely packed agglomerates with mesoporosity that is similar to that of MWCNTs but far less microporosity, resulting in nearly twice the volumetric capacity and significantly larger true capacity despite obtaining essentially identical gravimetric capacity. Finally, while the initial discharge product is found to be toroidal Li2O2 particles typical of Li-O2 cathodes, it proceeds with the formation of thick films covering the surface of the cathode. This is, to our knowledge, the first report of thick film formation in Li-O2 cells, overcoming the 74% density limit of particulate formation.