Lithium-rich Li-Zn Alloy As A Novel Anode Material For Lithium Secondary Batteries

Since the environmental pollution and greenhouse effect have been induced by the excessive exploitation and utilization of fossil energy,a variety of renewable energy are emerged gradually.Among them,lithium(Li)battery with its small size,high capacity,rechargeable and other advantages is the most promising energy technology.As the demand for the miniaturization of portable electronic equipment and long driving-distance of electric vehicles is encouraged,the specific capacity of the anode(graphite,372 mAh/g)is eagerly to be promoted for building high energy battery.Thus,Li metal anode is considered as the first choice for the next generation of Li battery due to its extremely high theoretical specific capacity(3860 mAh/g)and the lowest redox potential(-3.045V vs.SHE).Unfortunately,the Li metal anode suffers from few disadvantages.Firstly,Li metal is reactive toward organic liquid electrolyte,resulting in an unstable SEI film on the surface of the Li metal anode.Secondly,the uneven Li deposition is caused by uneven distribution of highly active sites,leading to Li dendrites and"dead Li".Therefore,many efforts have been devoted to modify the Li metal anode.Among these methods,the bulk modification of Li metal is undoubtedly the most effective and promising methodology.In this thesis,a Li-rich dual-phase Li alloy is fabricated by a facile,easy-to-scale-up,low-cost and environmentally friendly high-temperature melting method,and the corresponding electrochemical properties are investigated in detail.(1)Li10Zn alloy anode was synthesized by using a high-temperature melting method.The LiZn phase and Li phase are formed due to the phase separation in the cooling process.LiZn only serves as a supporting skeleton during the cycling,which can accommodate Li deposition and reduce the actual current density.In addition,LiZn alloys is a mixed electron-/ion-conductor due to their high electronic conductivity and high Li~+ion diffusivity,which can not only show superior high rate performance,but also regulate Li deposition behavior.Combining the above advantages,the Li10Zn alloy anode exhibits excellent performance at a high current density of 5 mA/cm~2 with the lifespan over 10000cycles,and the full cell pairing with LTO cathode exhibits the capacity retention of 80%after 4000 cycles.(2)The Li10Zn@Foam Ni compound anode was formed by cooling of the molten nickel foam and Li10Zn alloy,where the foam is designed to strengthen the supporting skeleton.The nickel foam skeleton that runs through the entire anode not only further enhances the ability of the compound anode to transmit electrons,but also greatly improves the stability of the compound anode,and greatly prevents the structure degradation caused by excessive Li removal.This is because the LiZn alloy phase in the Li-rich dual-phase Li-Zn alloy is more easily adsorbed on the nickel foam skeleton,so even after the LiZn alloy in the compound anode participates in the discharge to become the Zn skeleton,it can still be strong with the foam nickel stably,and will undergo alloying reaction with Li at the initial stage of the Li insertion process to reform LiZn alloy.The symmetrical battery can be stably cycled for more than 1000 cycles(2000 h)under a large current density of 10 mA/cm~2 and a high discharge capacity of 10 mAh/cm~2.(3)Li2Zn,a Li-rich dual-phase Li-Zn alloy with high Zn ratio,was obtained by increasing the proportion of Zn element in the synthesis process.Consequently,the amount of LiZn intermetallic compound phase was increased.Since the melting points of LiZn and Li are quite different,Li2Zn alloy shows double-layer structure after a rapid cooling treatment,in which the top-surface is dominated with LiZn and the other is a mixture of LiZn and active Li.This unique layered structure can protect the Li metal phase from the corrosion in the air.The Li2Zn alloy after being exposed in the air for 6 h behaves the same as that of as-prepared Li2Zn alloy anode.That is,both anode can circulate for 500 cycles under the current density of 1 mA/cm~2 and the discharge capacity of 1 mAh/cm~2,proving that the Li2Zn alloy anode has good air stability.