Stabilizing Metal Lithium Anode By Electrolyte Additives For High-Performance Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have been considered as one of the most promising high-energy battery systems because of the extremely high specific energy of～2600Wh kg^-1.In recent years,great progresses have been made to deal with the problems of sulfur cathodes by improving the electronic conductivity and suppressing the shuttle effect.However,the issues related to metal Li anode are raraly mentioned and have become one of the biggest obstacles for further high performance lithium-sulfur batteries.Firstly,the nonuniform deposition of Li induces the disordered growth of Li dendrites,which would further pierce the separator,causing short circuit or even explosion of the battery system.Secondly,the unstable solid electrolyte interface（SEI）on the surface of Li anode causes the continuous consumption of active Li and the electrolyte,thereby leading to low Coulombic efficiency（CE）.Finally,lithium polysulfides,an intermediate product of lithium-sulfur battery dissolution,are dissolved in the electrolyte,thereby triggering the shuttle effect.This not only leads to the irreversible loss of sulfur,but also reacts with lithium metal that shuttles to the anode,further reducing the stability of the lithium anode.Therefore,how to form a strong SEI film on the surface of lithium,prevent lithium polysulfide from corroding lithium metal,and then improve the stability of lithium anode is crucial to the development of lithium-sulfur batteries and other secondary battery systems based on lithium metal anodes.From the perspective of electrolyte,this thesis explores the performance and mechanism of different types of electrolyte additives in stabilizing lithium metal anodes,and further studies the performance of lithium-sulfur batteries based on modified electrolytes.Therefore,we believe that this thesis can provide references for the development of functional electrolyte additives in high-performance Li-S batteries and other Li metal batteries.In the first part,2-fluoropyridine（2-FP）was successfully used as an electrolyte additive to stabilize Li anode and improve the electrochemical performance of Li-S batteries.2-FP can form a LiF-stable SEI protective layer on the surface of lithium metal,while the lithiophilic N functional group reduces the nucleation overpotential of lithium,reduces dendrite growth,and improves the morphology of lithium deposition.The tests of Li||Cu cells showed that the coulombic efficiency remained stable at a current density of 1 mA cm^-2,up to～98.3%.By contrast,the cells without 2-FP additive show a rapid decay of CE to～70%within 20 cycles.The nucleation overpotential is low as 180 mV of the cell with 2-FP additive in the first voltage distribution plot,which is much lower than that of the cell without 2-FP（318 mV）.Finally,the Li-S batteries assembled with the modified electrolytes containing 3%2-FP show excellent rate performance,and with a high capacity of 540.3 mAh g^-1 at 4 C.And the capacity can also be maintained at 777.6 mAh g^-1 when the current returns to 0.5 C.It is proved that2-FP is an effective additive for lithium sulfur electrolyte,which plays an positive role in improving electrochemical performance and stabilizing lithium metal.In the second part,copper phthalocyanine（CuPc）was applied as an additive to stabilize both Li anode and sulfur cathode.Copper phthalocyanine has more lithiophilic N functional groups,so that it can form a uniform and stable protective layer on the surface of lithium metal.Meanwhile,Cu²⁺ions can react with Li PSs to generate stable copper sulfides.Such a protective layer can effectively restrict the direct contact of the solvent molecules and metal Li,decrease the consumption of the electrolyte,stabilize the electrolyte/anode interface,and realize a stable and compact surface of Li anode.Under the condition of low electrolyte（E/S=9）with sulfur load of 3 mg cm^-2,the lithium-sulfur battery with copper phthalocyanine can still maintain the discharge capacity of 698 mAh g^-1 after 50 cycles at 0.2 C.Even at a high rate of 1 C,the batteries with CuPc additive show much better cycling performance than the control cells.This indicates that Cu₂S/Cu S formed by CuPc can effectively passivation surface,stabilize SEI and reduce the loss of sulfur.In the final part,potassium perfluorobutane sulfonate（KPBS）is proposed to form a stable SEI layer on the surface of Li anode to improve the electrochemical performance of Li-S batteries.On one hand,K⁺ions exhibit a lower effective reduction potential than Li⁺ions at a specific concentration,and accumulate on the surface of Li protrusions,forming an electrostatic shield layer that pushes lithium ions to the edge region,thus greatly modulating the uniform deposition of Li⁺ions and reducing the possibility of dendrite growth.It can be calculated by Nernst equation that the effective reduction potential of K⁺is lower than that of Li⁺when electrolyte containing 0.01M KPBS.On the other hand,a LiF-rich SEI layer can be formed by the reduction of fluorine-rich anions（PBS^-）on the surface of Li anode,which is conducive to protect the morphology of Li anode.With the modified electrolyte with 0.01 M KPBS,the Li||Li cells show excellent cycling stability with steady overpotentials within 2000 h at the current density of 1 mA cm^-2.The CEs of Li||Cu cells are significantly improved and maintain as～99%.As a result,Li-S batteries shows excellent cycling performance with a high capacity of 711.8 mAh g^-1 after 100 cycles at 0.2 C.Even at 1 C,the batteries with KPBS additive also remains a higher capacity of 514 mAh g^-1 after 300cycles,implying a lower capacity decay rate of 0.086%per cycle.However,the batteries without KPBS additive show a lower capacity of 111 mAh g^-1 after 300 cycles and a rapid capacity decay of 0.25%per cycle.