Real-time imaging of Na⁺ reversible intercalation in "Janus" graphene stacks for battery applications

Sodium, in contrast to other metals, cannot intercalate in graphite, hindering the use of this cheap, abundant element in rechargeable batteries. Here, we report a nanometric graphite-like anode for Na⁺ storage, formed by stacked graphene sheets functionalized only on one side, termed Janus graphene. The asymmetric functionalization allows reversible intercalation of Na⁺, as monitored by operando Raman spectroelectrochemistry and visualized by imaging ellipsometry. Our Janus graphene has uniform pore size, controllable functionalization density, and few edges; it can store Na⁺ differently from graphite and stacked graphene. Density functional theory calculations demonstrate that Na⁺ preferably rests close to -NH₂ group forming synergic ionic bonds to graphene, making the interaction process energetically favorable. The estimated sodium storage up to C_6.9Na is comparable to graphite for standard lithium ion batteries. Given such encouraging Na⁺ reversible intercalation behavior, our approach provides a way to design carbon-based materials for sodium ion batteries.