While lithium resources are scarce for high energy-dense lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), serving as an alternative, inherently suffer from low capacity and the high-cost use of non-graphite anodes. Combining Li- and Na-ions within a single battery system is expected to mitigate the shortcomings of both systems while leveraging their respective advantages. In this study, we developed and assembled a nanodiamonds (NDs)-assisted co-Li/Na-ion battery (ND–LSIB). This innovative battery system comprised a commercial graphite anode, an ND-modified polypropylene (DPP) separator, a hybrid lithium/sodium-based electrolyte, and a cathode. It is theoretically and experimentally demonstrated that the ND/Li co-insertion can serve as an ion-drill opening graphite layers and reconstructing graphite anodes into few-layered graphene with expanding interlayer space, achieving highly efficient Li/Na storage and the theoretical maximum of LiC₆ for Li storage in graphite. In addition, ND is helpful for creating a LiF-/NaF-rich hybrid solid electrolyte interface with improved ionic mobility, mechanical strength, and reversibility. Consequently, ND–LSIBs have higher specific capacities ~1.4 times the theoretical value of LIBs and show long-term cycling stability. This study proposes and realizes the concept of Li/Na co-storage in one ion battery with compatible high-performance, cost-effectiveness, and industrial prospects.