Hard Carbon Anode Materials And Electrodes For Sodium Ion Batteries

Efficient,low cost large-scale energy storage technology is indispensable to the development of renewable energy,such as wind and solar.Due to the wide abundance and low cost of sodium resources,sodium ion batteries(SIBs)are considered as a new secondary rechargeable battery for large-scale energy storage,and have recently attracted growing attention.Hard carbons(HCs)are the most promising anode materials because of their high specific capacity,stable structure and good cycling performance,accompanying with the simple and environment friendly preparation process.The goal of this thesis is to obtain high performance hard carbon anode materials and electrodes for SIBs.Hard carbon(HC)materials with high specific capacity and high cycle stability were prepared by adjusting layer spacing and graphite crystalline microstructure.Moreover,the sodium storage mechanism of HCs was studied and an extended "adsorption-insertion" model was proposed.Furthermore,focusing on the electrode structure for practical application,a novel strategy was proposed for the preparation of flexible and free-standing HC electrode using graphene and MXene as multifunctional conductive binder.(1)Using biological waste-shaddock peel as precursor,amorphous HCs were prepared by one-step pyrolysis method.The obtained pyrolytic carbons inherit the loose porous structure of the precursor and have large interlayer distance,which is beneficial for the insertion/extraction of Na ion.The porosity is favorable for the infiltration of electrolyte and shortens the migration path of ions,therefore,the obtained biomass-derived HCs show high sodium storage capacity and excellent cycling stability.The Na-storage capacity of biomass-derived HCs show a "volcano"-shaped tendency with increasing pyrolysis temperature.Due to its large interlayer spacing(0.38 nm)and relative developed porosity,the HC pyrolyzed at 1200? shows very high reversible sodium storage capacities up to 430.5 mA h g-1 at a current density of 30 mA g-1 and superior cycling stability with a high capacity retention ratio of 97.5%over 200 charge-discharge cycles.The excellent Na-storage performance,combined with the facile synthesis procedure make the biomass-derived HC be a promising anode material for practical SIBs.(2)The sodium-ion storage mechanism of HC remains unclear,with no consensus in the literature.Here,a series of HCs are synthesized by pyrolyzing fallen ginkgo leaves over a wide temperature range of 600?2500?.Based on the correlation between the microstructure and Na storage behaviour of HCs,an extended "adsorption-insertion" sodium-ion storage mechanism is proposed.According to the degree of order in the crystallite,which is characterized by the interlayer distance(d-spacing),the microstructure of HCs can be divided into three types:a highly disordered state(d-spacing above 0.4 nm),a pseudo-graphitic state(d-spacing in 0.36?0.4 nm)and a graphite-like state(d-spacing below 0.36 nm).The highly disordered carbon,with a d002 large enough to allow sodium ions to freely transfer in,has a "pseudo-adsorption" sodium storage mechanism,contributing to the capacity in the sloping region above 0.1 V,together with other "defects"(pores,edges,heteroatoms,etc.).The pseudo-graphitic carbon contributes to the low-potential(<0.1 V)plateau region capacity,according to the "interlayer insertion" model.The graphite-like carbon is inaccessible for sodium storage because its d002(below 0.36 nm)is too small for sodium ions to insert.Different sodium storage mechanisms co-exist due to the presence of inhomogeneous structures with the gradual crystalline evolution in the HCs.The extended "adsorption-insertion" model can well explain the dependence of the sodium storage behaviour of HCs with different microstructures on the pyrolysis temperature and provides new insight into the design of HC anodes for SIBs.During pyrolysis,the microstructure of HCs evolves from highly disordered state to graphite-like state,accompanied with the evolution of sodium storage behaviours and performances.The "interlayer insertion"behaviour offers a high theoretical capacity,while the "pseudo-adsorption"behaviour is superior in cycle stability and rate capability.Thus,the microstructure of HCs can be adjusted by the pyrolysis temperature for different applications.(3)Aiming to improve the performance from the electrode structure,the use of reduced graphene oxide(rGO)as an environmentally friendly,multi-functional conductive binder is proposed as a novel strategy for manufacturing free-standing,flexible,high-performance HC anodes for SIBs.The rGO-bonded HC film was fabricated via a facile process,which includes the co-filtration of aqueous mixture of HC and GO,and followed thermal treatment to convert GO to rGO.The rGO not only acts as a binder and flexible backbone in the film,but also as an active material for Na ion storage and a conductive additive.Replacing the conventional insulating poly(vinylidene difluoride)(PVDF)and conductive additive by rGO,the rGO-bonded free-standing,flexible HC film delivers a high reversible capacity of 372.4 mAh g-1 with excellent cycle performance.Over 200 charging/discharging cycles,the capacity retention is still as high as 90%.Benefiting from the unique three-dimensional(3D)conductive network constructed by the graphene nanosheets,the rGO-bonded HC films also show superior rate capabilities to conventional PVDF-bonded electrodes.Moreover,the whole fabrication process is facile and environmentally friendly.Therefore,the free-standing,flexible rGO-bonded HC film can be viewed as ideal anode candidates for flexible SIBs.(4)Aiming to solve the problem of electrode material pulverization and detachment from the current collector,giving rise to the deteriorated cycle performance during charge/discharge process of traditional HC electrode,a new strategy for electrode fabrication is proposed by using Ti3C2Tx MXene as a multifunctional conductive binder.Different from the conventional insulating PVDF binder,the 3D conductive network constructed by MXene sheets can effectively stabilize the electrode structure and buffer the volume expansion of HC during the charge/discharge process.Moreover,the obtained MXene-bonded HC electrode is free-standing and can be directly used as anode without any current collector.Due to the synergetic effect of MXene and HC particles,the HC-M-2:1 film can deliver a high Na-storage capacity of 368.5 mAh g-1 at 30 mA g-1 and show impressive cycling performances with no capacity decay over 1500 cycles.Moreover,benefiting from the 3D conductive framework constructed by Ti3C2Tx MXene sheets,which has metal-like conductivity,the MXene-bonded HC film electrodes also present improved rate capability,the Na-storage capacity of HC-M-2:1 film can retain 66.7 mAh g-1 at the current of 10 A g-1.The idea of using MXene as a multifunctional conductive binder to prepare flexible electrode and buffer volume expansion also provides a new sight for the research of other energy storage devices.