Investigation On Lithium Metal And Lithium Silicon Alloy Anodes For High Energy Secondary Batteries

Lithium metal as the anode of lithium batteries has theoretical specific capacity of 3860 mAh g^-1 and very low electrode potential,which is called the"Holy Grail"in lithium battery anode materials.Silicon as an alloy-type lithium storage material has a theoretical specific capacity of 3590 mAh g^-1 at room temperature,and its electrochemical potential is moderate,which is also a promising anode material.Using lithium metal or silicon materials as the anode can greatly increase the energy density of lithium secondary batteries.Therefore,research on lithium metal and silicon materials has become a recent hot spot.However,the chemical nature of lithium metal is very active,which is very easy to react with the liquid electrolytes,causing the irreversible consumption of lithium,low Coulomb efficiency and high interface impedance.In addition,due to the poor stability of the solid electrolyte interphase(SEI),lithium deposition is often uneven,resulting in severe dendrite growth.The accumulation of lithium dendrites can cause serious safety issues.On the other hand,both the lithium metal and silicon are accompanied by huge volume changes during the charge and discharge process,which brings challenges to the design of the battery.Compared with lithium metal,silicon anode theoretically avoids the problem of lithium dendrite growth.However,the matching with Li-free cathodes(such as sulfur,oxygen)also brings certain difficulties to battery design because both the anode and cathode do not contain lithium.In view of the above problems,this dissertation designs a functional separator for lithium metal anode.The active material coated on the separator can react with lithium metal,forming a protective layer on the surface of lithium metal,thereby inhibiting the formation of lithium dendrites and reducing side reactions between lithium metal and liquid electrolyte.On the other hand,we designed a 3D structured composite lithium metal electrode.The lithium metal is embedded in a 3D framework with dual ionic/electronic conductivity,which has a significant effect on inhibiting lithium dendrites and alleviating the volume change.In addition,the preparation of lithium-silicon alloy and the in-situ lithiation method of silicon electrode were explored.The lithium-silicon alloy was used to replace lithium metal to assemble a full battery with sulfur or oxygen as cathode and achieved excellent cycle stability.The specific research contents of this dissertation are as follows:1.A 5?m thick composite protective layer containing nano-silicon,polyacrylic acid(PAA)and conductive carbon black is coated on the surface of a commercial polypropylene(PP)separator.The coating layer not only greatly improves the wettability between the separator and liquid electrolyte,but also improves thermal stability of the separator.The coating layer will not fall off after winding and folding,which is suitable for practical production.The coating layer will react with lithium metal by short-circuiting contact in the presence of electrolytes,transforming into a protective layer containing lithium-silicon alloy and Li PAA,which tightly adhere to the surface of lithium metal to prevent the corrosion of electrolyte.The Li?Li symmetric cell with PP@PS separator in carbonate electrolyte can stably cycle for more than 500hours(1 mA cm^-2,5 mAh cm^-2).Li?LiFePO₄ and Li?S@pPAN cells can stably cycle for 300 and 1,000 times with capacity retention of 86%and 87.7%,respectively.2.A method for preparing 3D composite lithium metal electrode has been proposed.Li₃N with high ionic conductivity and lithiophilic lithium-aluminum alloy were generated on the surface of lithium metal by the reaction of lithium metal with AlN at high temperature.The capacity of the composite electrode is?1900 mAh g^-1.Li₃N,Li-Al alloy and CNT ensure the ion and electron transport in the electrode.The Li-Al alloy provides abundant nucleation sites for the lithium deposition.The 3D structure of the electrode not only reduces the real current density,but also provides space for lithium platting.LAN?LAN symmetrical cell can stably cycle for more than 600 h in carbonate electrolyte(2 mA cm^-2,4 mAh cm^-2).Full cells with LiFePO₄or S@pPAN as cathode can cycle for 500 or 600 times with capacity retention of 94%and 97%,respectively.3.A method of preparing lithium-silicon alloy electrodes has been proposed.Lithium powder,silicon powder and carbon nanofibers are mixed in certain proportions and pressed to form a pellet.Lithium metal reacts with silicon to form lithium-silicon alloy at high temperature.The carbon nanofibers provide a 3D framework and electron transport channels.The capacity and cycle stability of electrodes with different lithium-silicon alloy content have been investigated.With the lithium-silicon alloy content of 60 wt%,the cycle of electrode is stable and its specific capacity is 940 mAh g^-1.The lithium-silicon alloy electrode and the S@pPAN are prepared to assemble the full cell.Since both the silicon anode and S@pPAN have good stability in the carbonate electrolyte containing fluoroethylene carbonate(FEC),the Li-Si?S@pPAN full cell can stably cycle for 3000 times under 3C with capacity retention of85%.4.The in-situ electrochemical lithiation method of silicon electrodes has been explored.The self-supporting Si/C film electrode containing nano-silicon has been prepared by the electrospinning.A thin lithium metal foil was used to react with Si/C film by short-circuit inside the cell,converting the Si/C film electrode into a lithium-silicon alloy electrode.The cycle stability of S@pPAN and oxygen cathodes are greatly improved by using the lithium-silicon alloy anode produced by the in-situ reaction.A Li-ion-oxygen cell with Li₃N loaded on oxygen cathode and the silicon as anode is proposed.Li₃N decomposes to release nitrogen when the cell is charged for the first time,and the silicon anode undergoes electrochemical lithiation to form lithium-silicon alloy.Subsequently,the cell operates via reversible oxygen reduction/evolution and Side-lithiation/lithiation.The as-prepared Li-Si?O₂cell can stably cycle for more than 120 times(500 mA g^-1,500 mAh g^-1).