We created a unique sodium ion battery (NIB, SIB) cathode based on selenium in cellulose-derived carbon nanosheets (CCN), termed Se-CCN. The elastically compliant two-dimensional CCN host incorporates a high mass loading of amorphous Se (53wt.%), which is primarily impregnated into the 1 cm³g^-1 of the nanopores. This results in facile sodiation kinetics due to short solid-state diffusion distances and large charge transfer area of the nanosheets. The architecture also leads to an intrinsic resistance to polyselenide shuttle and to disintegration/coarsening. As a Na half-cell, the Se-CCN cathode delivers a reversible capacity of 613 mAh g^-1 with 88% retention over 500 cycles. The exceptional stability is achieved employing a standard electrolyte (1M NaClO₄ EC-DMC), without secondary additives or high salt concentrations. The rate capability is also superb, achieving 300 mAhg^-1 at 10C. Compared to recent state-of-the-art literature, the Se-CCN is the most cyclically stable and offers the highest rate performance. As a Se-Na battery, the system achieves 992 Wh/kg at 68 W/kg and 384 Wh/kg at 10144 W/kg (by active mass in cathode). We are the first to fabricate and test a Se-based full NIB, which is based on Se-CCN coupled to a Na intercalating pseudographitic carbon anode (PGC). It is demonstrated that the PGC anode increases its structural order in addition to dilating as a result of Na intercalation at voltages below 0.2 V vs. Na/Na⁺. The {110} Na reflections are distinctly absent from the XRD patterns of PGC sodiated down to 0.001 V, indicating that Na metal pore filling is not significant for pseudographitic carbons. The battery delivers highly promising Ragone chart characteristics, for example yielding 203 and 50 Wh kg^-1 at 70 and 14000 W kg^-1 (by total material mass in anode and cathode).