| Carbon-based materials are regarded as one of the most valuable anode materials for potassium ion batteries owing to their advantages of low production cost,abundant sources and stable physical and chemical properties.Graphite has been commercialized in the field of lithium ion batteries anode,but it shows low potassium storage capacity when applied in potassium ion batteries.Amorphous carbon with natural defects can accommodate the intercalation of large-sized potassium ions for reversible potassium storage.However,carbon-based materials face many challenges on account of their low theoretical capacity and unclear potassium storage mechanism.Therefore,it is necessary to modify the carbon-based anode materials to improve their electrochemical performance,such as enhanced rate performance,extended cycle life and increased specific capacity.Sulfur atoms are important elements in the preparation of sulfide or sulfur-modified electrode materials.Metal sulfide has the advantages of high redox activity,facile preparation process and high theoretical capacity.It is a potential and effective measure to modify carbon-based materials.In addition,the doping of sulfur atoms can not only change the electronic structure of carbon-based material,but also increase the electronic conductivity,defect number and active site number of carbon-based material,which is an important means to achieve excellent potassium storage performance.Based on the above considerations,this project successively took carbon-based materials as the main research objects,modified them such as metal sulfide modification or heteroatom doping,and explored their potassium storage properties.The main research contents are as follows:（1）In view of the low potassium storage capacity of carbon-based materials,the design idea of this work is to modify commercial graphene oxide with sulfides with high theoretical capacity in order to improve the potassium storage capacity.In particular,flower-like spherical Fe Co S2 coated with reduced graphene oxide（Fe Co S2@r GO）was produced via a one-step hydrothermal synthesis method employing commercial graphene oxide.The potassium storage capacity of Fe Co S2@r GO has been improved obviously through the compound metal sulfide.This flower-like spherical structure with extensive mesopores is conducive to the rapid penetration of electrolyte,thereby accelerating the transmission of potassium ions.Meanwhile,the encapsulation of reduced graphene oxide not only effectively reduces the structural deformation during the reaction,but also limits the aggregation of Fe Co S2,which provides favorable conditions for improving the reversibility of the electrode material.As a result,Fe Co S2@r GO exhibits promising electrochemical stability,presenting a reversible capacity of 371.0 m Ah/g after 150 cycles at 100 m A/g.The Fe Co S2@r GO possess better rate performance:the average reversible capacity is 428.0 m Ah/g at 50 m A/g and 248.0 m Ah/g at 1000 m A/g.（2）The previous work found that a single surface coating is difficult to maintain the structural stability of electrode materials.In order to further enhance the electrochemical reversible properties of carbon-based materials,more appropriate structural regulation was carried out.Ni S2/carbon nanotubes（Ni S2/CNTs）hybrids with a reinforced concrete structure are prepared based on the solvent thermal method based on the good mechanical strength and flexibility of commercial carbon nanotubes（CNTs）.In this hybrid,one part of CNTs is covered in the interior of the Ni S2 microspheres,while the other part is distributed on the surface of the microspheres.Among them,C NTs with a flexible skeleton embedded in Ni S2 microspheres act as rebar,and Ni S2 with high theoretical capacity acts as concrete.This design effectively buffers the structural strain of Ni S2 and ensures the close interconnection between each nano-building blocks,which not only improves the structural stability,but also facilitates the full utilization of active materials.Meanwhile,the CNTs distributed on the surface of the Ni S2 microspheres are interconnected into a network structure,which provides a highly conductive carbon support for the active material,accelerates the electron transfer on the interface and promotes the effective mass transfer of potassium ions.As anode for potassium ion batteries,Ni S2/CNTs hybrids show attractive rate performance（the average reversible capacity is 469.4 m Ah/g at 50 m A/g and 250.8 m Ah/g at 2000 m A/g）.The Ni S2/CNTs possesses fascinating cycling life,keeping a reversible capacity of 353.3 m Ah/g after 200 cycles at 100 m A/g（capacity retention:81.2%）.（3）The study found that the improvement of potassium storage performance with commercial carbon-based materials as the modification target is still limited.To fundamentally modify carbon-based materials to maximize the potassium storage capacity and reversible performance of carbon-based materials,this work uses sulfur atoms to modify the self-synthesized carbon-based materials and explore their potassium storage performance.Specifically,polystyrene was prepared by a one-pot method,and a high sulfur-doped hard carbon material（SHC-3）was further prepared.Structure and kinetic studies demonstrate that the larger interlayer spacing（0.382 nm）of the SHC-3accelerates the diffusion of potassium ions and effectively alleviates the volume expansion,and thus maintains the structure stability during the process of potassization/de-potassization.Meanwhile,the density functional theory calculation shows that the doped sulfur atoms provide abundant active sites for the adsorption of potassium ions,thereby increasing the reversible capacity of potassium ion batteries.As an anode for potassium ion batteries,SHC-3 has wonderful reversibility and rate performance:at a low current density of 100m A/g,the reversible capacity of SHC-3 can be obtained 298.1 m Ah/g after 1000cycles（capacity retention:95.2%）;after being upgraded to 500 m A/g,the reversible capacity holds 220.2 m Ah/g after 5200 cycles.（4）The above study found that the reversible properties of the self-synthesized sulfur-doped carbon-based materials were greatly improved.In order to further enhance the reversible capacity and cyclic stability of carbon-based materials,S,N and O tri-doped carbon（SNOC-700）nanospheres are prepared by polymerization reaction and in-situ vulcanization process.Among them,sulfur powder is sulfur source,and polybenzoxazine is carbon source containing N and O.Based on the large carbon interlayer spacing,the structural stability of SNOC-700 is enhanced during the potassization/depotassization process,thus enabling PIBs to achieve ultra-long cycle life.Meanwhile,the high specific surface area of SNOC-700 nanospheres is beneficial to increase the contact area with the electrolyte,which not only boosts the transport of ions and electrons,but also motivates the sufficient employment of SNOC-700,thus obtaining high electron conductivity and reversible capacity.Moreover,S/N/O triple doping provide rich electrochemical active sites for potassium ion adsorption,which plays an important role in improving the capacity of PIBs.As an anode for potassium ion batteries,SNOC-700 presents attractive rate performance with a reversible capacity of 174.5 m Ah/g at 2000 m A/g.In the meantime,SNOC-700 possesses unexceptionable electrochemical stability:the reversible capacity is 397.4 m Ah/g at 100 m A/g after 700 cycles and 218.9m Ah/g at 2000 m A/g after 7300 cycles.Even at a high current density of 3000m A/g,an ultra-long cycle of 16,500 cycles can be achieved and a reversible capacity of 123.1 m Ah/g is obtained.Density functional theory（DFT）calculations and in situ FT-IR tests are used to evaluate the potassium storage mechanism of SNOC-700.The successful assembly of the full cell demonstrates the potential of the SNOC-700 electrode for practical applications. |
PDF Full Text Request