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

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 degrees C) 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)(1/3)O-2 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.

Automated summary

This study successfully converted renewable filter papers into free-standing and flexible microfiber carbon papers as anodes for sodium ion batteries, achieving ultra-high initial Coulombic efficiency and superb rate performance. Sodium ions insert as naked Na+ form instead of solvated Na+, highlighting the dependence of high initial Coulombic efficiency and rate capability on pseudo-graphitic structures with high-level ordered degree.