Ultra-micropores of hard carbons for ultrafast Na-ion storage

The plateau capacity of a hard carbon anode plays a crucial role in achieving the high energy density of sodium-ion batteries. However, the loss of plateau capacity due to polarization at high current densities imposes significant limitations on hard carbon applications. Ultra-micropores could maintain plateau capacity at high current densities but are generally prepared accompanied by micropores and mesopores that deteriorate the Na-ion storage performance. Herein, a hard carbon with ultra-micropores is prepared by a protonation-mediated strategy using an N, P co-doped biomass precursor. The P dopants favor reducing the interaction between N and C so that the confined volatiles of NH3 could be utilized to create ultra-micropores concentrated at 0.4–0.8 nm. These ultra-micropores enable the hard carbon to deliver a reversible capacity of 386 mA h g−1 at 20 mA g−1, a high plateau capacity of 173 mA h g−1, and an excellent rate capability of 106 mA h g−1 at 2 A g−1. The plateau capacity and rate capability are superior to those of the reported hard carbons. This work provides a new approach for ultra-micropore construction within hard carbons and a new perspective for sodium-ion batteries toward high energy and power densities.