Fabrication And Storage Mechanism Of Heteroatoms Doped Carbon Materials For Potassium-ion Batteries

The rapid development of human society has increased the demand for large-scale energy storage systems.Although lithium-ion batteries possess high energy density,due to the gradual scarcity of lithium resources and the increasing in cost,it is urgent to develop next-generation energy storage batteries to achieve the application of large-scale energy storage system.Potassium-ion battery is a favorable candidate for large-scale energy storage equipment in the future due to its abundant reserves,low price and similar working principle with lithium-ion battery.However,the size of K+is much lagger than Li+and Na+,which inevitably causes severe volume expansion and sluggish diffusion kinetics,leading lower energy density.All of these problems have seriously hindered its commercialization process.In recent years,the performance of potassium-ion battery has been improved by adjusting electrode materials.Carbon materials are considered as one of the most promising anode candidates due to their favorable properties such as excellent chemical stability,good electrical conductivity,diverse structures,large specific surface area,and unique porosity.Researchers have explored better electrochemical potassium ion storage properties by chemical modifying carbon materials.Among them,introducing heteroatoms and defects into materials is a common and effective modification method.Although the doping mechanism of nitrogen atoms has been thoroughly analyzed by researchers,the doping mechanism and effect of other heteroatoms,or even the co-doping atoms remain unclear.Therefore,reasonable design of electrode materials and optimization of physical and chemical properties are very important to improve the electrochemical properties of potassium-ion batteries.In view of the above problems,this article will focuse on the design,regulation and optimization of anode materials for potassium-ion batteriey,promoting ion transport,electron diffusion and improvement electrochemical kinetics via introduceing heteroatoms to create defects and active sites.A series of instrumental characterizations and electrochemical tests were employed to investigate the mechanism of heteroatoms for potassium storage.The specific research contents are as follows:(1)B-O co-doped hard carbon particles(BO-CPs)were prepared for potassium-ion battery anodes by plasma enhanced chemical vapor deposition(PECVD)route using biomass materials as templates at low temperature.Three-dimensional(3D)carbon framework and heteroatom dual-doped configuration promote ion transport and diffusion,improve the electrochemical inertia of the material,and endow with the material unique electrochemical properties.Accordingly,our BO-CPs anode harvests a high reversible specific capacity(426.5 mAh g-1 at 0.1 A g-1)and an excellent rate capability(166.0 mAh g-1 at 5.0 A g-1).The storage mechanism toward potassium-ion can be explored through detailed electrochemical tests,in-situ instrumental characterization and theoretical calculation analysis.It was found that boron dopant improves the conductivity of carbon framework and the oxygen dopant affords ample active sites and thus harvests additional specific capacity.(2)Heteroatom dual-doped hard carbon crumples were designed by pro tic-salt synthetic route for PIBs anode.Benefiting from the versatility of protic base and acid materials,sulfur-nitrogen,phosphorus-nitrogen and boron-nitrogen co-doped carbon(SNC,PNC,and BNC)architectures can be readily obtained by varying the contents and types of precursors.BNC enables the highest electrical conductivity and largest adsorption energy of K-among the investigated dual-doped carbons because of the presence of B-N moiety.In response to K-ion storage,BNC displays an impressive reversible specific capacity(360.5 mAh g-1 at 0.1 A g-1)and outstanding cyclic stability(125.5 mAh g-1 after 3000 cycles at 1.0 A g-1).Consequently,it is identified that the synergy of B and N atoms in the form of B-N moiety greatly enhances the pseudocapacitive K-storage.(3)A soft-hard carbon composite material combining soft carbon and hard carbon was achieved by MOF synthesis method,making up for their shortcomings and combining the good rate capability of soft carbon and the excellent cycle stability of hard carbon.More importantly,the material has the characteristics of self-doping,which provides more active sites for the storage of potassium ions,and realizes the effective regulation of carbon materials.Through systematic instrumental characterization and dynamic and electrochemical analysis,it is revealed that the soft carbon prepared by this method is pseudocapacitance and insertion hybrid storage mechanism,and the hard carbon is insertion storage mechanism.Therefore,thus-derived composite with soft carbon as core and hard carbon as shell could achieve stable cycling capability(1.0 A g-1 current density with capacity decay rate of only 0.017%per cycle over 2000 cycles)and good rate performance(86.0 mAh g-1 at 5.0 A g-1 current density).