Interface Regulation And Electrochemical Investigation Of High Capacity Anodes In Secondary Batteries

Metallic lithium and sodium have theoretical specific capacities of 3860and 1166 mAh g^-1,respectively,and very low electrode potentials,while Li-insertable silicon anode material has the theoretical capacity up to 4200 mAh g^-1 and moderate lithiation/delithiation potential.They are all the most promising high-capacity anode materials for second lithium batteries.When high capacity cathode is matched with Li（Na）or Si anode,the obtained full cell has an extremely high energy density and application propect.However,metal Li（Na）is very reactive and easily reacts with liquid electrolytes,which increases the interface impedance and leads to low Coulombic efficiency during charge and discharge.Moreover,the uneven Li deposition on the Li（Na）electrode surface could induce the dendrite formation,resulting in shorter battery life,or even serious safety hazard.In view of the above problems,this dissertation designs a new type of electrolyte and an artificial SEI film for Li（Na）electrodes to regulate the structure and composition of electrode interface,inhibit the dendrite formation and improve the Coulombic efficiency and cycle stability.On the other hand,Si is one of the most potential high-capacity anode materials for industrial applications.Li alloying could completely solve the dendrite problem and improve the safety performance.Nevertheless,there are also serious problems for Si materials,such as the strong volume effect during Li insertion and removal and poor conductivity.Although great progress has been made in solving these problems in recent years,most of researches focus on the preparations of nano-or porous-structured Si materials,which will significantly increase the cost and difficulty of production.Herein,the effective electrode binder and electrolyte additives with the practical value are proposed to regulate and optimize the interface stability between the Si-based electrode and electrolyte for greatly improving the cell cycle life.The specific contents are as follows:1.A high-performance carbonate electrolyte（1 M LiODFB/EC-DMC-FEC）was designed and prepared.It has high ionic conductivity(7.2 mS cm^-1),wide electrochemical window（5.5 V vs.Li/Li⁺）,high thermal stability（60℃）,and compatible with Li anode.Using this electrolyte,Li|Li cells can cycle stably for above 2000 hours at 0.28 mA cm^-22 and the average Coulumbic efficiency of Li deposition-stripping in Li|Cu cells reaches 98.8%.Moreover,the Li-S@pPAN battery delivers a capacity retention of 89%for 1100 cycles and superior rate performance.2.A facile but effective methodology is proposed to form a multistructural protective layer containing LiF-rich solid electrolyte interphase（SEI）,and Li₃Sb alloy on Li electrode surface.This protective layer could greatly improve the physical and chemical properties of Li electrode.Besides,this method is also applicable to Na metal electrodes.The protected Na metal electrode in symmetric Na|Na cell is stable and dendrite-free during the plating/stripping cycling processes in both ether and carbonate electrolytes.Especially in ether electrolyte,the cycle life of protected Na metal electrode reaches more than2400 h at 2 mA cm^-2.The modified metal anode（Li or Na）is compatible with different cathodes,leading to much better cycle performance for their full cells.3.A new water-soluble and self-healing binder PAA-P（HEA-co-DMA）is prepared for Si electrode.It can endure 400%tension,and has highly reversible elasticity.The combination and crosslinking of rigid-soft chains and bonds lead to the 3D network flexible structure with special self-healing capability.Additionally,the abundant carboxyl and catechol groups on the side chains create the strong interactions between the binder and functional groups on the surface of active material particles,which can enhance the wet/dry adhesion force and electrolyte absorption.Hence,it can endure strong volume changes of Si anodes during charge and discharge,and depress the pulverization of sub-micron Si particles,thus the electrode performance is significantly improved.The electrode could cycle stably for 200 times even with high areal capacities up to 3.2 mAh cm^-2.Moreover,it demonstrates the durable high rate cyclability for 200 cycles at 5 A g^-1 with capacity retention of 90%.The good stability of Si particles also effectively improved the interface stability,and thus the initial Coulombic efficiency was increased from 81.2%to 89.3%.4.Based on the above binder research,new additives are further proposed for Si electrode.After introducing the bicomponent additives（1%LiPO₂F₂+2%DMTFA）to traditional electrolyte（1 M LiPF₆/EC-DMC）,the resulting electrolyte shows high ionic conductivity(10.9 mS cm^-1),good anti-oxidation capability（5 V vs.Li/Li⁺）and admirable thermal stability（60 ^oC）.Moreover,it is also compatible for Si-based anode with excellent electrochemical performance,which can completely replace the common additive FEC.At room temperature,the electrochemical performance of Si-based cells using these additives is not much different from that using FEC.However,the cell containing our proposed electrolyte can be cycled stably for more than 100times at 60 ^oC,while it suffers obvious capacity decline by using FEC-containing electrolyte.