Heteroatom-Doped Porous Carbons:Preparation By Deep Eutectic Solvents As Activators And Their Application In Lithium-Ion Capacitors

As a new type of energy storage equipment with high power density and high energy density,Lithium-ion capacitors（LICs）are received much more attention in the very recent years.However,LICs are still troubled by two problems:low energy density and the mismatch of the kinetics properties between the cathode and anode materials.Therefore,the preparation and optimization of high-performance electrode materials are considered as one of the most key strategies to solve these problems.Heteroatom doped porous carbons（HDPCs）are regarded as one of the ideal electrode materials for LICs,due to their stable chemical property,high conductivity,adjustable structurally and rich electrochemical active sites.Nevertheless,the traditional chemical activation route for the preparation of HDPCs is suffered from a series of problems such as complex preparation procedures,long time consuming,inhomogeneous mixing of activator in precursor and the resultant toxic waste.It is thus very meaningful to develop a method for the preparation of high-performance HDPCs with high efficiency and environmental friendless.In view of the above-mentioned problems,the research works on the preparation of HDPCs using novel low eutectic solvents（DES）as the activators and investigation the electrochemical performances of the as-prepared HDPCs for LICs are carried out in this thesis.These works involve the related mechanism of activation of carbon-based precursors using the DES activator,the influence of activation conditions on the properties of the as-achieved HDPCs,as well as the effect of different electrolytes on the electrochemical performance of HDPCs.Also,the possible reasons for the influence of electrolyte on the electrochemical properties of HDPCs are discussed.The detail contents of the thesis are listed as follows:1.The development history and energy storage mechanism of LICs,the characteristics of various electrode materials for LICs are briefly introduced.Subsequently,the preparation routes and the relative properties of HDPCs are summarized.In view of the existing disadvantages of the preparation of HDPCs and the challenges facing the performance improvement of LICs,the research purpose and a new preparation route for HDPCs using DES as new activators are thus proposed.2.The merits and disadvantages of the traditional impregnation method and a novel route using ZnCl₂-urea DES as an activator for the preparation of nitrogen-doped porous carbons（NDPCs）are compared.Then,the effect of KCl additive on the physical and electrochemical properties of the NDPCs prepared by the new route with the ZnCl₂-urea DES activator are explored.Finally,the influence of electrolyte on the electrochemical performances of the as-prepared NDPCs is investigated.The results show that the new route using the DES as an activator for the NDPCs not only reduces the preparation time,but also greatly increases the yields（up to 41.7 wt%）,compared with the traditional impregnation method.Specifically,the NDPC-0.5 displays the best electrochemical performance as both anode and cathode materials for a LIC in a binary LiPF₆-based electrolyte（1.0 mol LiPF₆ was dissolved in 1 L of EC and DEC mixed solvent,and the volume ratio of EC and DEC is 1:1）.The LIC assembled with the NDPC-0.5 sample as the anode and cathode materials delivers a high energy density of 116.9Wh kg^-1 at 500 W kg^-1,with a capacity retention of 81%after 8000 cycles at 2 A g^-1.It is noted that the NDPC-0.5 sample shows better electrochemical performance in a ternary LiBF₄electrolyte（1.0 mol LiBF₄ was dissolved in 1 L of EC,DEC and EMC mixed solvent,and the volume ratio of EC,DEC and EMC is 1:1:1）,that is,a high specific capacity of 92.4 m Ah g^-1at 0.25A g^-1 is achieved.In contrast,a specific capacity of 82.9 m Ah g^-1 is achieved in a binary LiPF₆-based electrolyte.3.In view of the intrinsic disadvantages of the ZnCl₂-urea DES,such as the high preparation temperature and water sensitivity,a water-stable DES（MgCl₂·6H₂O-urea）prepared at a lower temperature is selected as another activator for the preparation of NDPCs.Several NDPC-based samples are then prepared by activating bagasse or sugar with the MgCl₂·6H₂O-urea DES containing Fe Cl₃ additive.After that,the electrochemical performances of the as-achieved samples are studied in detail.The results show that the NDPC sample prepared from the sucrose as the carbon source displays better electrochemical performance due to its unique laminar structure.By optimizing the ratio of DES,Fe Cl₃ and sucrose,the as-obtained NPCS-1sample exhibits highly dispersed lamellar structure,which is beneficial for the improvement of its electrochemical performances.And thus,the LIC assembled with the NPCS-1 sample as both the anode and cathode materials shows excellent electrochemical performances in the ternary LiBF₄-based electrolyte.It delivers a high energy density of 135.6 W h kg^-1 at 500 W kg^-1,with a capacity retention of 82%after 10000 cycles at 2 A g^-1.4.A new DES activator is prepared by mixing of the MgCl₂·6H₂O-ZnCl₂-urea and H₃BO₃.Several B,N co-doped porous carbon（BNPC）materials are prepared by activating bagasse or sugar as the carbon source using the new DES,respectively.Subsequently,the electrochemical performances are further studied.The results show that the BNPC sample prepared from the bagasse as the carbon source exhibits better electrochemical performances due to its rich active groups and suitable lamellar structure.Among them,the BNPC-8 sample,which is prepared from bagasse with appropriate ratio of H₃BO₃,has rich heteroatoms’content,surface electrochemically active groups and suitable microstructure,showing a high specific capacity of 101.6 m Ah g^-1 at 0.25 A g^-1 in the ternary LiBF₄-based electrolyte.Although it shows the best rate performance among the BNPC-based samples,it is still needs to be improved as compared with the rate performance of NDPC reported in the second charter.5.Based on the discussion on the great influence of electrolyte on the electrochemical performance of NDPC,the influence of different electrolytes on the electrochemical performances of the BNPC-based samples is then explored,to further improve the rate performance of the BNPC-8 sample.To us surprised,the results show that the electrochemical performances of the BNPC-8 sample is significantly improved in the binary LiPF₆-based electrolyte,which is contrary to the results discussed on the NDPC-0.5 sample in Chapter 2.Specifically,the specific capacity of the BNPC-8 sample as a cathode material for a LIC reaches121.2 m Ah g^-1 at 0.25 A g^-1.Furthermore,the BNPC-8 sample can work stably in an operating voltage window of 2.0-4.8 V（vs.Li/Li⁺）with an ultra-high specific capacity of 159.0 m Ah g^-1,which may be attributed to a stable cathode electrolyte interface（CEI）film is formed on the on the cathode surface.The LIC fabricated by the NDPC-0.5 material as the anode and the BNPC-8 material as the cathode displays high energy densities of 179.9 Wh kg^-1(562.5W kg^-1)and 224.6 Wh kg^-1(600 W kg^-1)when it is worked a voltage window of 0～4.5 V and 0～4.8 V,respectively.Meanwhile,the LIC shows good cycle stability,with a capacity retention ratio of 80.8%after 1000 cycles at 2 A g^-1 in the work voltage window of 0～4.5 V.