Preparation And Electrochemical Properties Of Pinecone-based Porous Carbon As Anode Materials For Potassium Ion Batteries

Energy storage batteries can overcome the shortage of nonuniform space-time distribution of renewable energy,such as wind power,photovoltaic,etc.Therefore,energy storage batteries are particularly important for realizing large-scale,sustainable,and stable utilization of renewable energy.Potassium ion batteries（PIBs）have become promising energy storage batteries because of their abundant resources and low cost.Graphite is a mature anode material for lithium-ion batteries.However,it is difficult for K⁺to insert/extract in graphite due to its ion radius（1.38（?））,meaning that graphite cannot be used as the anode material for PIBs.Therefore,it is urgent to develop low-cost and high-performance anode materials for PIBs.Among the investigated anode materials for PIBs,biomass-derived carbon is an ideal candidate due to its stable structure,low cost,non-toxicity,safety,and rich resources.Pine is one of the most widely distributed tree species in the world.Pinecone is the fruit of pine.However,only a small part of the pinecone is used to produce pine nuts and extract active components.There are no uses for the remaining pinecone biomass.Therefore,large amounts of pinecone biomass are discarded or burned,causing environmental pollution and waste.In this paper,simple carbonization in molten Zn Cl₂was used to prepare pinecone-based carbon anode material for PIBs.XRD,Raman,XPS,SEM/TEM,and electrochemical methods were performed to investigate the effects of the carbonization process,non-metallic elements doping,and transition metal oxide composite on the structure,morphology,and electrochemical performance of the obtained materials.In the carbonization process,Zn Cl₂molten salt acts as both a catalyst and a template.The pinecone-based carbon prepared in Zn Cl₂molten salt is amorphous carbon with a hierarchical porous structure.The hierarchical porous pinecone-based carbon can be prepared by calcinating the mixture of pinecones and Zn Cl₂（4:1 mass ratio）at 600℃for 5 h.After 100 cycles at 50 and 1000 m A·g^-1,the obtained pinecone-based carbon delivers a discharge capacity of 109 and 32.1 m Ah·g^-1,respectively.To investigate the effects of nonmetallic element doping,N-doped,B-doped,N-P co-doped,and N-S co-doped pineal carbon were prepared,respectively.N-doped pinecone-based carbon is a pie-like hierarchical porous structure,B-doped pinecone-based carbon has a foam-like 3D interconnected hierarchical porous structure,and N-P co-doped and N-S co-doped pinecone-based carbons possess honeycomb-like 3D interconnected hierarchical porous structure.N-doped and B-doped samples give a discharge capacity of 158.8 and 117.1 m Ah·g^-1after 1000cycles at 1000 m A·g^-1,respectively,which are better than the undoped sample.The enhanced performance of the doped pinecone-based carbons can be ascribed to the nonmetallic element doping and the unique porous structure.This unique structure can expand interlayer distances(d₀₀₂)and offer more defect sites,which can facilitate the insertion/adsorption and diffusion of K⁺and alleviate the volume changes during the cycles.After1000 cycles of 1000 m A·g^-1,the discharge capacity of N-P co-doped pinecone-based carbon is 118.7 m Ah·g^-1,which is 57 and 30 m Ah·g^-1higher than that of N-doped and P-doped samples,and the discharge capacity of N-S co-doped pinecone-based carbon is 98.6 m Ah·g^-1,which is 48 and 26 m Ah·g^-1higher than that of N-doped and S-doped samples,respectively,indicating that the synergistic effect between nonmetallic atoms can effectively improve the electrochemical performance of pinecone-based carbon materials.A one-step hydrothermal method was developed to prepare Molybdenum oxide/N-doped pinecone-based carbon composite（Mo O_x/NC）and iron oxide/N-doped pinecone-based carbon composite（Fe₂O₃/NC）,in which thin-flake Mo O_xand Fe₂O₃nanoparticles were uniformly distributed on the surface of the carbon substrate,respectively.After 100cycles at 50 m A·g^-1,the reversible capacity of Mo O_x/NC(210.4 m Ah·g^-1)and Fe₂O₃/NC(178.7 m Ah·g^-1)is higher than that of pure Mo O₂(122.7 m Ah·g^-1)and pure Fe₂O₃(79.8 m Ah·g^-1).These results suggest that compositing with nitrogen-doped pinecone-based porous carbon can effectively enhance the conductivity,promote the penetration and diffusion of electrolytes,and tolerate the volume change of molybdenum（iron）oxides during charge/discharge,thus improving their performance.The results provided here not only offer a new cheap raw material for biomass carbon anode materials for PIBs but also can promote the utilization of pinecones.The findings in this paper can provide insights into the development of pinecone-based carbon and are beneficial to the development of other biomass-based carbon anodes for PIBs.