The structural stability and capacity increase of a phosphorus-doped hard carbon produced by zinc oxide templating used in sodium-ion batteries:

Na-ion batteries (NIBs) are attracting increasing attention as next-generation batteries because they do not rely on rare metals. Hard carbon (HC) is considered the most promising anode material for NIBs. By tailoring the pore structure through templating methods, HC materials with high energy density have been developed for NIBs. Further improvements are required to enhance the desired properties without compromising the excellent characteristics of the materials already achieved. Accordingly, this study attempts to further increase the battery capacity of ZnO-templated HCs by employing a simple phosphorus doping method. We investigated the effects of phosphorus introduced via soaking in phosphoric acid and subsequent heat treatment on the carbon morphology and electrochemical properties. The results showed that phosphorus doping increased battery capacity without altering the morphology of the ZnO-templated HC. Notably, both the sloping and plateau regions of the capacity increased, suggesting that phosphorus doping promotes not only Na adsorption on the carbon surface but also Na storage within the interlayer spacing and pores. Furthermore, the types of phosphorus functional groups were dependent on synthesis conditions, which in turn influenced battery performance. These findings highlight that surface modification with specific phosphorus functional groups can effectively enhance the Na storage capability of HCs.