Research On The Liquid Anode Of Lithium Compound For Li-air Batteries

In the high-energy-density lithium batteries,the energy density of Li-air battery is much higher than that of conventional Li-ion battery as delivering capacity by reducing O₂ from air.For the Li metal as anode in Li-air batteries,two main challenges of Li metal anode also exist in this system.One is the growth of Li dendrites.It arises intrinsically from the heterogeneous diffusion of Li ion during charge and discharge processes.The continued growth of Li dendrites could lead to safety issues.Another is the poor interface stability.As the low negative electrochemical potential of Li metal,the unstable solid electrolyte interface?SEI?film could be formed through the redox reaction between Li and common nonaqueous electrolytes.Besides,the unstable SEI film experiences repeated breakdown by the Li dendrites during Li depositing/stripping.It cause the rapid consumption of Li metal and electrolytes,resulting in a short life span.We improved the performance of the Li-air batteries by the modification of the solid Li metal anode and introduction of the liquid lithium anode respectively in this paper.As to the modification of the solid Li metal anode,a facile and low-cost method of a stable artificial SEI film was provided through directly recuding Graphene fluoride?CF_x?using Li metal in this paper.This uniform artificial SEI film has higher inorganic species Li F and the components of this film are homogenous on the Li surface?r-CF_xLi?.As the homogenous diffusion of Li ion through the uniform r-CF_x artificial SEI film during Li depositing/stripping,the growth of Li dendrites was restrained and the cycling perfomace of batteries has been improved with r-CF_x Li.The symmetrical r-CF_x Li|r-GF_x Li cell runs stably and has a long life span at different current density.The stability of the r-CF_x Li|Li Fe PO₄ and r-CF_x Li|S cells have also been imporoved by using r-CF_x Li as anode.The r-CF_x Li|Li Fe PO₄ cell has a stable specific capacity of 151.2m Ah g^-1 and excellent capacity retention of 94.3%for 200 cycles.The specific capacity of r-CF_x Li|S cell is also relatively high(558.5 m Ah g^-1)after 450 cycles.Moreover,r-CF_x Li is stable in corrosive air and solution as the compact r-CF_x film on Li metal.So,the r-CF_x Li can be expected to be applied in Li-air batteries as its stability in the air.Although the modification of the Li metal anode can improve the electrochemical performance,the dendrite growth and high reactivity of Li metal anode also lead to severe safety concerns in the long cycling.The liquid lithium anode was introduced to further improve the safety performance of the high-energy-density lithium batteries as its free dendrite growth and stability in this paper.In Li-air batteries,a hybrid electrolyte based Li-air battery is by uniting aqueous and organic electrolyte with a water-stable lithium super ionic conductor ceramic?LISICON?plate.This smart design was proposed to circumvent the drawbacks of nonaqueous Li-air batteries,such as insoluble discharge products clogging air electrode,corrosion of metallic Li from humidity and decomposition of organic solvents.Unfortunately,the brittle ceramic plate endures fragile risks under harsh operational and abuse conditions including serious bump and crashes,leading to safety issues of a violent reaction between Li metal and water from the catholyte.To improve the safety of the hybrid electrolyte based Li-air battery,we designed a liquid anode of lithium biphenyl?Li BP?replacing the lithium metal.The liquid Li BP anode with a low redox potential is about 0.30 V?vs.Li/Li⁺?.To our surprise,the liquid Li BP anode presented a very mild reaction even though mixed with a large amount of water.In this system,the cell exhibited a relatively high discharge voltage of 2.81 V at the current density of 0.5 m A cm^-2,as well as a specific power of1638 W kg^-1 at the current density of 6 m A cm^-2by using this liquid anode.The ex-situ Raman spectrometry and in-situ GC-MS analyses reveal that the redox reaction of BP^-/BP and OH^-/O₂ in anolyte and catholyte respectively.Furthermore,as side reactions between the solid electrolyte Li_1.5Al_0.5Ge_1.5?PO₄?₃?LAGP?and liquid Li BP anode,the hybrid electrolyte based Li-air battery with Li BP anode has a poor cycling performance.The reactions were investigated through XPS and Raman spectroscopy.For the low redox potential of liquid Li BP anode,Ge⁴⁺was reduced to the low oxidation states Ge²⁺,Ge⁰in LAGP and BP^-was oxidized to BP in Li BP anode.To further improve the cycling life of this cell,300 nm Ti layer was sputtered onto the anodic side of the ceramic LAGP plate via the DC magnetron sputtering system.The protective Ti layer can alleviate side-reaction rate between LAGP and liquid anode and the cell with Ti layer can operate for over 120 cycles at the current density of 0.5 m A cm^-2.