| Lithium metal is among one of the most promising anode materials for high-energy batteries due to its high theoretical capacity and lowest electrochemical potential,but the growth of lithium dendrites is still a major challenge.And nickel-rich lithium nickel cobalt manganese oxides have received considerable attention as a promising cathode material.However,they have suffered from poor interfacial stability.The development of suitable electrolyte additives is one of the most economical and effective methods to solve the above problems.The sulfone additives that have great polarity and good thermal stability,are excellent film-forming additives.Herein,the effects of diphenyl sulfone(DPS)and bis(4-florophenyl)sulfone(BFS)as bifunctional electrolyte additives for nickel-rich lithium metal batteries are studied.Firstly,in the NMC811/Li batteries,with 0.5 wt%DPS and BFS,both of the initial discharge capacities are around 160 mAh g-1,and the capacity retention per cycle are 99.93%and 99.91%at 1 C over 500 cycles,respectively.From the long-term high rate tests at 5C,a specific capacity of 89 mAh g-1 at the 500th cycle and the capacity retention of 99.95%per cycle are reached by 0.5 wt%DPS,much higher than that with 0.5 wt%BFS(70 mAh g-1,99.92%)and no additives(25 mAh g-1,99.84%).The cyclability and the capacity retention of NMC811/Li battery with 0.5 wt%of DPS show much better than that without any additive,when the mass loading of the active material is 20 mg/cm2(like commercial electrode)or the temperature reaches 60?.In addition,from the LSV and CV tests,DPS and BFS are apt to be oxidized and decomposed prior to the carbonate solvents,thus forming a uniform cathode electrolyte interphase(CEI)thin film,which well protects NMC811 cathode from being eroded.And thin CEI film resulted from the additives can reduce the interfacial resistance of the batteries,revealed by the EIS tests and TEM.Proper contents of LiF,Li2SO3 and ROSO2 Li caused by the reaction between additives and electrolyte or electrode are beneficial to form a stable solid electrolyte interphase film for the full batteries,which facilitates lithium ion transport,and inhibits electron transfer.Therefore,NMC/Li batteries with additives exhibit longer cycle life,higher capacity retention and better fast charging performance and DPS works best.In the Li/Li symmetrical half-batteries,with 0.5 wt%DPS and BFS,high cycle stability for 360 h and 240h,respectively at the current density of 1 mA cm-2 is realized by forming a dense and smooth LiF-rich solid electrolyte interphase(SEI)that can suppress the growth of lithium dendrites.This can prevent the further decomposition of the electrolyte and improve the cycle performance of the battery.In addition,it is found that DPS without F is more conducive to the formation of stable and dense SEI layer.Therefore,DPS is better than BFS in improving the cycling performance of symmetrical batteries and inhibiting the growth of lithium dendrites... |
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