The Study On High-Performance Electrochemical Energy Storage Devices By Electrochemical Tuning Methods

With the development of electric vehicles,smart grids and portable intelligent electronic devices,improving the electrochemical performances of existing electrochemical energy storage technologies and designing new devices,have become hot research topics.In order to achieve the above objectives,lots of work have focused on the preparation of advanced nanomaterials.However,high cost and severe side reaction prevent the practical application of nanomaterials.Therefore,based on the energy-storage mechanisms and existing problems of electric double-layer carbon materials and transition metal sulfide materials,this dissertation explores practical electrochemical tuning method for above materials,and obtains a high-energy,highpower,high-efficiency and long-life lithium ion capacitor and a novel symmetrical rechargeable battery.The main results achieved include:(1)Widening the stable potential window of carbon materials by electrochemical coating methods.Energy density of electrochemical capacitor significantly depends on the stable potential windows of carbon electrodes.In the carbonates-based ester electrolyte,the stable potential window of the carbon electrode is decided by the the oxidation potential(4.5 V(vs.Li/Li+))and the reduction potential(1.5 V)of the electrolyte.A dramatic electrolyte decomposition will happen when working potential of carbon electrodes exceed the upper/lower potential limits,leading to fast decay of capacitance.Here,to restrict the electrolyte decomposition and widen the stable potential window of carbon material electrodes,the solid/solid interfaces were constructed,which were composed of electrode and solid electrolyte interfacial layer by electrochemical coating methods.The electrochemical coating methods are based on the oxidative decomposition of ester electrolyte and the reductive decomposition of lithium(difluorooxalate)borate,respectively.By the former,a stable high-potential solid/solid interface was constructed on graphene with an average pore size of 2.6 nm.This coated graphene can be stably cycled in the range of 3?5 V with a coulomb efficiency of 99.8%.By the latter,the low-potential solid/solid interface can be constructed,which extends the lower limit of stable windows of varied carbon electrodes to 1.1 V.(2)Proposing the new solid/solid electric-double-layer model,exploring its electrochemical behaviour inside the solid/solid interface and constructing a graphene lithium ion capacitor based on this interface.After characterizations of solid/solid interface constructed by the reductive decomposition of lithium(difluorooxalate)borate,a novel electric double layer model was proposed,in which the desolvated ions were adsorbed on the surface to form an electric double layer.Compared with the solid/liquid interfacial electric double layer,the solid/solid interfacial one has a smaller Helmholtz layer thickness,resulting in the increase of capacitance.At the same time,a strong interaction between the electrode surface and the ions inside the solid/solid interface reduces the self-discharge rate of carbon materials.Based on these considerations,a high-performance graphene lithium ion capacitor was constructed.This device was able to cycle stably at 0?4.3 V with a battery-level energy density and a high power density.In addition,the self-discharge rate was reduced by half and the energy efficiency is doubled compared to the solid/liquid interfacial device.(3)Proposing an electrochemical activation method and making the transition metal sulfide electrode being used as both of cathode and anode.Binary transition metal sulfides are intrinsically comprised of electroanode(S)and electrocathode(metal)elements.These elements have been widely reported as promising candidates for high-energy anodes or cathodes.However,there are few reports to activate redox of electroanode and electrocathode parts in one compound synchronously up to now.Here,an electrochemical activity modulation(EAM)strategy is proposed to activate redox of electroanode and electrocathode parts in metalsulfide synchronously.After EAM,a Mo-redox MoS2 anode was obtained,which has a capacity of 243.7 mAh g-1 and can cycle stably between 0?1.4 V with a high colombic efficiency of 99.8%.At the same time,after EAM,a S-redox MoS2 cathode could be obtained and the capacity was increased by an order of magnitude compared with the pristine MoS2 cathode.Besides,this method is universal and suitable for various metal sulfides.(4)Proposing a new MoS2 symmetrical rechargeable battery.According to the types of the cathode and anode materials,electrochemical energy storage devices can be divided into asymmetrical and symmetrical cells.The symmetrical configuration with the same active material for cathode and anode shows great attraction and promise from commercial standpoint as it greatly reduces the manufacturing costs and simplifies the fabrication process.However,the symmetrical configuration is restricted to electrochemical capacitors,especially electric double layer capacitors.Here,a symmetrical MoS2 cell involving metal sulfides as both cathode and anode materials was obtained by EAM.This symmetrical MoS2 cell shows the higher theoretical energy density compared with lithium ion batteries.Besides,the research provides a new guidance for the development of high-energy,high-efficiency new electrochemical energy storage devices.