Surface Modification And Electrochemical Performance Optimization Of Alkali Metal Anodes

The traditional lithium-ion battery has occupied the first place in the field of secondary batteries.However,with the continuous development of technology,the exploitation of lithium-ion batteries is close to its theoretical capacity,so people need to find electrode materials to further improve the energy density of batteries.As the ultimate anode of next generation of high-energy-density rechargeable batteries,alkali metals have ultra-high theoretical specific capacity,extremely low redox potential and low mass density.However,dendrite growth of anode and serious potential safety hazard limit the commercialization of AMBs.This paper mainly researches from the perspective of alkali metal anode protection:In the first work,we use the principle of lithium metal reacting with nitrogen to form lithium nitride,and design a simple and easy-to-operate closed experimental device filled with nitrogen,so that the lithium nitride protective layer is formed in situ on the surface of the lithium metal placed in the device.The presence of the protective layer avoids direct contact between lithium metal and the electrolyte,promotes the formation of a stable SEI film,and effectively inhibits the growth of dendrites.Symmetrical battery test results show that the electrode covered with lithium nitride can stably cycle for at least 900 cycles under the current density of 1 mA cm-2,and at least 2500 cycles under the higher current density of 5 mA cm-2.In addition,the anode is paired with Li4Ti5O16(LTO)for a full cell,which exhibits good cycle stability and rate performance.In the second work,we use the concept of surface alloying to design a lithium metal anode with a silicon-lithium alloy layer on the surface.The anode electrode can realize a stable reversible cycle under the conditions of ultra-high current density and ultra-large area capacity test.Just add the silicon membranes with a certain thickness of 20-30 ?m through a chemical etching method to the lithium metal surface,and then LixSi alloy layer with certain mechanical strength and high lithium ion conductivity can be obtained on its surface by simple electrochemical activation.According to the test results of the symmetrical battery,the presence of the alloy layer can effectively reduce the overpotential of the anode electrode and improve its cycle stability.Especially under the limit test conditions where the current density is increased to 25 mA cm-2 and the area capacity is increased to 100 mAh cm-2,the surface alloyed lithium metal anode can still cycle stably.And then the lithium anode is also paired with the MoS3 and achieve excellent full-cell performance.In the third work,we use two methods which let sodium as the reactant to form a bismuth-sodium alloy layer on the surface of sodium metal to stabilize the sodium anode.The alloy layer can effectively inhibit the growth of sodium dendrites and prolong the working life of the battery.The first method which based on the principle that sodium will alloy with bismuth is to use the magnetron sputtering technique to plate a bismuth layer on one side of the battery separator.When the thickness of the bismuth layer is 2?m,the sodium anode shows the best electrochemical performance,which can work 200 cycles at 5 mA cm-2.The second method which takes advantage of the spontaneous substitution reaction and alloying reaction between sodium and BiF3 is to drop the 0.01 M BiF3-DME solution on the surface of sodium to obtain the surface bismuth-sodium alloyed sodium anode electrode.When the volume is 100 ?L,the anode can cycle at least 600 cycles at 1 mA cm-2.According to the data of the symmetrical battery,we initially prove that the above two methods can play a role in stabilizing the sodium anode.All in all,we build a lithium nitride protective layer or a silicon-lithium alloy protective layer on the surface of lithium metal,which not only achieves a stable and reversible cycling stability,but also significantly improves the performance of the full cell and a bismuth sodium alloy layer on the surface of sodium metal to extend its cycle life.These strategies provide ideas with application value for lithium or sodium metal anode protection.