Researches On Synthesis And Modification Of Hard Carbon Anode Material For Lithium Ion Batteries

Hard carbon is characterized by a large fraction of graphene units but being highly disordered.Such structures exhibit considerable meso-/micro-pores and larger average interplanar distances than graphite.When hard carbon was used as an anode material for lithium batteries,it has many advantages compared to graphite.The advantages include high reversible capacity,good rate capabilities and low cost of production.Also,hard carbon has different lithiation and delithiation behavior from graphite.However,hard carbon suffers from a fatal deficiency:the high irreversible capacity or low initial coulombic efficiency.Besides,the disordered structure leads to relatively poor conductivity resulting that the rate capability is far from satisfying.In this article,hard carbon pyrolyzed from phenolic epoxy resin is studied in detail in terms of the initial coulombic efficiency,reversible capacity,rate capability and cycling performance.Herein,we demonstrate herein a simple mass production routine to prepare hard carbon by pyrolysing resin precursors.The effects of main parameters in the pre-carbonization process on structure and electrochemical performance is studied.Pre-carbonization before high temperature pyrolysis(1000 ?)by controlled process conditions,such as temperature and heating rates,have an important effect on the structure and performance of hard carbon as anode material.Hard carbon material was characterized by X-ray diffraction(XRD),Raman spectroscopy,Brunauer-Emmett-Teller(BET)surface area and scanning electron microscope(SEM).The hard carbon treated by pre-carbonization process resulted in decreasing Raman ID/IG ratio,increasing BET specific surface area and micro pore volume,which can serve as high reversible capacity anode material for Li-ion batteries.The sample treated at precarbonization temperature of 500 ? performed a first discharge capacity and reversible capacity of 540 and 390 mAh g-1 respectively,which was much higher than that of NP(without pre-carbonization)sample(290,210 mAh g-1 respectively).Hard carbon enveloped with graphene networks is fabricated through thermal reduction of the mixture of graphene oxide(GO)and hard carbon powder.The effects of GO addition and thermal reduction temperature on the structure and electrochemical performance are discussed carefully.In the constructed architecture,hard carbon offers large lithium storage and flexible graphene layers can provide a highly conductive matrix for enabling good contact between particles and facilitate the diffusion and transport of electrons and ions.GO is reduced by thermal treatment in inert atmosphere at a temperature in the range of 800-1100 ?.In this way,hard carbon/graphene composites with different degree of reduction on graphene oxide are obtained.Increasing heat treat temperature improves the reduction of graphene oxide and diminishes the specific surface area and pore volume slightly.Furthermore,these materials behave as high-performance lithium ion battery electrodes and exhibit large reversible capacity(in the range of 300-480 mAh g-1)at a current density of 20 mA g-1,and superb stability over long-term cycling(78-82%of its initial specific capacity after 500 cycles).Superior results are obtained with the electrode fabricated from the material treated at 1000 ?,which delivers 480 mAh g-1 and 170 mAh g-1 at current density of 20 and 2000 mA g-1,respectively.In addition,interesting results are obtained that electrodes prepared by two types of copper foils display a significant difference on galvanostatic charge/discharge processes.Furthermore,we develop a facile prelithiation technique for hard carbon/graphene(HC/G)anodes based on a spontaneous electrochemical reaction with lithium metal foil.The initial coulombic efficiency(ICE)can reach a desirable level by easily tuning the prelithiation time.Importantly,the accurate amount of lithium preloaded into HC/G is determined by an atomic adsorption spectrum(AAS)method.Besides,a similar presodiation process is employed to demonstrate the versatility of this strategy.The surface characterization of prelithiated and presodiated HC/G confirms that generated solid electrolyte interface(SEI)layers have similar compositions as those formed during the conventional electrochemical charge-discharge cycles.Moreover,the prelithiated HC/G paring with a commercial high capacity cathode,LiNi0.5Co0.2Mn0.3O2(NCM),enables the full-cell a comparable galvanostatic capacity and rate capability to NCM half-cell(vs.Li),and a superior cycling performance.These encouraging results indicate an accessible solution to solve problems related to low ICEs of hard carbons.The coal tar pitch modified hard carbon(PHC)is prepared via thermal treatment of the mixture of coal tar pitch and hard carbon powders.The effects of pitch addition on the structure and electrochemical performance of hard carbon is studied.PHC exhibits enhanced electronic conductivity as well as electrochemical performance.Electrochemical performance of hard carbon(HC)and PHC electrodes are investigated over a wide temperature range(from-10 ? to 50 ?).Due to improved electronic conductivity,PHC eletrodes exhibit much larger reversible specific capacities,i.e.568 mAh g-1(50 ?),413 mAh g-1(25 ?),366 mAh g-1(0 ?),298 mAh g-1(-10 ?),than HC electrodes.In addition,PHC eletrodes show superior rate capability at the selected temperature range.In addition,electrochemical impedance spectroscopy(EIS)is used to study the electrochemical behavior of electrodes at different temperatures,and an equivalent circuit is established to fit the EIS data.The EIS results confirm that the substantially high resistances of solid electrolyte interface and charge transfer,giving rise to high polarization or over-potential,lead to poor low temperature performance of the electrodes.We report that vinylene carbonate(VC)is a desirable electrolyte additive for HC electrodes especially at elevated temperatures.The influence of VC on electrochemical performance and surface chemistry of HC electrodes at both 25 ? and 50 ? is investigated in detail by using Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and SEM.In the presence of VC additive,a stable SEI film is formed upon cycling,which contains increased amounts of Li2CO3 and decreased F species contents.It demonstrates that the SEI formed in the presence of VC helps suppress salt anion(PF6-)decomposition as well as SEI transformation occurred at 50 ?.