| Potassium ion batteries（PIBs）,with abundant resources and low cost potassium resources,show great potential in the large-scale energy storage field and become a hotspot.Carbon material is the most perspective candidate for PIBs’anodes due to low cost,simple preparation and controllable structure.However,the large radius of potassium ion makes it difficult to intercalate/extract between carbon layers,and the serious volume changes during the charge/discharge process,resulting in the poor rate and cycling performance.This paper aims at obtaining high-performance PIBs’anode materials and studies the preparation and modification method of graphite and coal-based carbon material.The structure stability of bulk graphite during the process of potassium storage is improved by coating modification.Using flake graphite as the anode of PIBs,the graphite based anode material with excellent performance was obtained through densifying flake graphite with graphene oxide.Through regulating the microstructure of carbon material by oxidation treatment and heterostructure construction,the coal based carbon materials with high specific capacity,excellent cycling and rate performance were prepared using low-cost lignite as a carbon precursor.The main research contents and results are as follows:（1）Hard carbon coated bulk graphite composite was prepared with dopamine hydrochloride and 3,4,9,10-perylene tetracarboxylic anhydride（PTCDA）as carbon precursors and then pyrolysis.The effect of carbon precursors and carbon coating content on the structure and electrochemical performance of the composite was studied.Using dopamine hydrochloride as a carbon precursor,the composite with a carbon coating content of 8.9%shows the best potassium storage performance with a capacity retention of77.8%after 150 cycles at 0.5 C,much higher than that of the bulk graphite（31.4%）.Moreover,the composite displays improved rate performance.Compared with hard carbon,soft carbon has less defects and high conductivity,which is more suitable to be a carbon coating precursor.Adopting PTCDA as a soft carbon precursor and concentrated H2SO4 as a solvent.When the pyrolysis temperature is 900°C,the composite has a capacity retention of 76.1%after 150 cycles at 0.5 C,and the reversible specific capacity is 131.3 m Ah g-1 at 2 C.The excellent potassium storage performance is attributed to that the carbon coating can effectively buffer the volume changes of graphite during the charge/discharge,and the large layer distance can significantly improve the diffusion kinetics of potassium ions in the bulk phase of the composite.（2）To further improve the volumetric capacity of flake graphite as an anode,hydrothermal method was adopted to prepare the flake graphite/reduced graphene oxide gel.During the drying process,the evaporation of water molecules produces a"tensile force"on the reduced graphene oxide sheets,which makes the contraction of the three-dimensional structure of the gel,thereby preparing a high density flake graphite/reduced graphene oxide composite.When the mass ratio of flake graphite to graphene oxide is 8:2,the composite（FG/r GO-82）has a high tap density of of 0.81 g cm-3,much higher than flake graphite(0.26 g cm-3).Because of the increased tap density,the FG/r GO-82 electrode displays a reversible volumetric specific capacity of 218.9 m Ah cm-3 at 0.1 C,which is much higher than that of the flake graphite electrode(106.0 m Ah cm-3).Moreover,the FG/r GO-82electrode also has excellent rate performance with 152.3 and 56.9 m Ah cm-3 at2 and 5 C,respectively.The results show that this is an effective way to prepare PIBs’anode materials with both high volumetric specific capacity and excellent cycling and rate performance.（3）As a low-cost carbon precursor,lignite was first oxidized with concentrated sulfuric acid and potassium permanganate to own rich oxygen functional groups,and then pyrolysis to obtain coal-based carbon materials with large layer distance and rich oxygen functional groups.The proportion of oxygen functional groups in coal-based carbon materials can be controlled by adjusting the oxidation degree of lignite and the pyrolysis temperature.The coal-based carbon material shows the best potassium storage performance when the mass ratio of lignite to potassium permanganate is 1:2 and the pyrolysis temperature is 900°C.Because the carbon material has a high oxygen content of 18.5 at%with mainly-C=O and-COOR,which can provide pseudo-capacitance,and the carbon layer distance increases to 0.385 nm,helpful for the rapid intercalation/extraction of potassium ions.The carbon material exhibits a reversible specific capacity of 283.2 m Ah g-1 at 0.1 C,and the reversible specific capacity is 142.9 m Ah g-1 at 5 C with great cyclic stability for 1 200 cycles.Through oxidation and carbonization,carbon material with lignite as a precursor shows excellent potassium storage performance,which provides a new method for the preparation of electrode materials for other energy storage systems.（4）Hard/hard heterogeneous carbon materials were prepared by pre-oxidation and carbonization with lignite and starch as precursors.In the process of pre-oxidation,the functional groups of lignite and starch react through dehydration and cross-linking,which makes the carbon material has large layer distance and less defects,showing improved capacity,initial coulombic efficiency and cycling performance.After optimizing the ratio of lignite to starch（1:1）and pyrolysis temperature（1300°C）,the heterogeneous carbon material shows a high specific capacity(284.9 m Ah g-1 at 0.1 C with an initial coulombic efficiency of（74.2%）,excellent cycle stability（92.2%after 500 cycles at 1 C）and rate performance(102.5 m Ah g-1 at 5 C).The excellent cycling and rate performance is due to the large carbon layer distance（0.376 nm）and reduced defects.The lignite/starch heterogeneous material with simple preparation method,low cost and excellent potassium storage performance,is a promising anode material for PIBs.In addition,the successful construction of heterostructures also sheds a light on the design and preparation of anode materials in other battery systems. |
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