Manipulating free-standing, flexible and scalable microfiber carbon papers unlocking ultra-high initial Coulombic efficiency and storage sodium behavior

Abstract Hard carbon (HC) is praised as the state-of-the-art anode material for sodium ion batteries (SIBs). However, developing HC anode with both ultra-high initial Coulombic efficiency (ICE) and superb rate performance is still one of the most critical challenges in practical application. Herein, the renewable filter papers were directly converted into a series of free-standing and flexible microfiber carbon papers (MFCPs) as practical additive-free anode for SIBs, adopting a simple graphite plate-assisted pyrolysis tactics. In ether electrolyte, the obtained MFCPs (at pyrolysis temperature > 1100 ℃) achieve an ICE>95%, one of the top values in the HC ranking list. Excitingly, the practical powers of MFCPs also reflect in the greater superior rate capability (251.2 mAh g−1 at 1 A g−1) compared to traditional materials and excellent cycle life (217.3 mAh g−1 after 500 cycles at 1 A g−1). Moreover, when assorted with an O3-Na(NiFeMn)1/3O2 cathode, the pouch full cell delivers a splendid energy density of 246 Wh kg−1 with superb cycle life. Importantly, mechanism analysis reveals that three-stage storage sodium behavior of MFCPs: adsorption, intercalation and filling. Interestingly, sodium ions in ether electrolyte insert the interlayer as naked Na+ form instead of solvated Na+, revealing high ICE and rate-capability depends on pseudo-graphitic structures with high-level ordered degree. This work provides an extensible practical HC anode and insights in the sodium storage mechanism of HC in ether electrolyte..