Green synthesis of N,P-codoped porous biomass carbon for high-performance Li-S batteries

Rechargeable lithium-sulfur battery (Li-S) has attracted extensive attention, due to its abundant resource, low cost, environmental friendliness, and high theoretical specific capacity. However, the main challenge for Li-S battery industrialization is to solve the shuttle effect of lithium polysulfide and the low conductivity of sulfur-related species. A large number of previous studies reported that combining sulfur with porous carbon can effectively alleviate the problem. Unfortunately, most carbon materials are non-polar and mainly restrain the dissolution of polysulfide by limited physical adsorption, resulting in low capacity and poor cycle stabilities. Alternatively, heteroatom doping can serve as electro-catalysts to accelerate polysulfide redox kinetics, and thereby improve electrochemical performance. At present, the preparation of heteroatom-doped porous carbon relies mainly on the use of highly corrosion activations, such as KOH, and foreign heteroatom-containing compounds for tailoring the specific structure, accompanied by low yield and costly. Therefore, it is very urgent to explore a simple, feasible, and environmentally friendly approach to synthesize the heteroatom-doped porous carbon. Herein, a cross-link and in-situ doping strategy was first proposed for the green synthesis of high yield, abundant functional groups and N, P-codoped porous biomass carbon (NPOC). The methodology involved simultaneous cross-linked pore-forming, carbon modification, and in-situ P, N codoping to generate high yield and high conductivity biochar. In this system, phytic acid (a plant extract and contains six phosphate groups) was chosen as the reaction medium, whereas, tobacco stalk (a type of economical waste biomass rich in nitrogen element and oxygen-containing functional groups) was employed as the carbon precursor. Due to its unusual coordination ability, phytic acid can cross-link with the oxygen-containing functional groups of biomass (tobacco stalk) to produce a three-dimensional porous network structure. Because of its abundant carbon and phosphorus content, the decomposition of phytic acid during heat treatment can provide carbon modification and P-doped for rich-nitrogen tobacco stems biochar, leading to high yield and conductivity. A physical melting was used to obtain the NOPC/S composites. The N and P dopants were beneficial to the immobilization and catalytic conversion of polysulfide, thus boosting the cyclic stability and rate capacity. The electrochemical performance indicated that when the amount of added phytic acid was set as 20 mL, the synthesized NOPC-2 material delivered a high discharge capacity of 1 211 mAh/g at 0.1C. More importantly, there was a capacity retention value of 885 mAh/g after 300 cycles at 1C, with only 0.029% capacity attenuation rate of each cycle. These findings can illuminate a promising excellent S storage porous carbon material. A novel and green pore-forming strategy can be expected to fabricate the high-performance sulfur host for Li-S batteries, particularly with the waste biomass in sustainable agriculture.