▎ 摘　　要

Sodium-metal batteries (SMBs) employing transition-metal-free cathodes are of great importance for energy storage applications that require low cost and high energy density. A strategy to enhance the energy density of transition-metal-free-cathode SMBs by transforming the electrolyte from a dead mass to an energy-storage contributor is reported. NaI is used for the partial substitution of NaClO4 in the electrolyte and thus provides the additional electrochemistry of I-3(-)/I- redox couple to enhance battery performance. Graphene oxide (GO) encapsulated mesoporous (10 nm) carbon spheres (N-MCS@GO) that are nitrogen-doped (15.71 at%) are fabricated as the cathode for the I-3(-)/I- redox enhanced SMBs. It is experimentally demonstrated that: the mesoporous structure increases the capacitive energy storage by providing a substantial interface that enhances the electrochemistry of I-3(-)/I- redox couples; and encapsulation of the mesoporous carbon spheres with GO suppresses self-discharge and increases Coulombic efficiencies from 70.4% to 91.9%. In full-cell configuration, N-MCS@GO working with the NaI-activated electrolyte can deliver a capacity of 279.6 mAh g(-1) with an energy density of 459.2 Wh kg(-1) in 0.5-3 V at 200 mA g(-1). I-3(-)/I- redox in the full cell maintains its activity without obvious decay after 1000 cycles at 1 A g(-1), highlighting the practical application of the I-3(-)/I- redox enhanced SMBs.