Study On Interface Between Lithium-based Anode And Electrolyte For Secondary Lithium Batteries

Lithium is one of the best anode candidates for secondary lithiumbatteries because of its extremely high theoretical specific capacity and mostnegative potential. Nowadays, lithium-sulfur and lithium-air batteries haveattracted increasing attention and become important directions for secondarylithium batteries. However, lithium anode is not used in practical applicationsin a large scale owing to its following problems:（1） side reactions betweenlithium metal anode and electrolyte will give rise to interface resistanceincrease, electrolyte consumption and coulomb efficiency reduction duringcycling;（2） formation of dendrites and “dead lithium” during cycling leads toefficiency reduction, and if dendrite punctures the separator reaching thecounter electrode, it will cause safety hazards, such as shortcircuit and explosion.Aiming at the two above-mentioned problems of lithium electrode, weundertake the research work from two sides of both electrolyte andlithium-based anode structure: on one hand, different kinds of additives wereused for both liquid and solid polymer electrolytes. The effects of differentadditives on the lithium anode/electrolyte interface property, electrochemicalperformance, and the relationship between dendrite onset time （t0） and currentdensity （J） for solid polymer electrolyte were studied. On the other hand, alithium-rich multiphase alloy Li_2.6BMg_0.05was synthesized as a potentialanode to replace lithium in the rechargeable lithium batteries. We comparedthe interface property, electrochemical performance and dendrite formationbetween the above alloy and lithium metal anode. The main results weresummarized as follows.1. Thionyl chloride (SOC_l2), sulfuryl chloride (SO₂C_l2), dimethylsulfoxide （DMSO）, benzenesulfonyl chloride （BSC） and phenyl vinylsulfoxide （PVSO） were used as additives in1M LiPF6-EC:DMC （1:1in vol）for the first time. The results showed that SO₂C_l2and BSC remarkablyimproved the cycling efficiency and life. The improvements can be ascribedto the formation of reductive products （Li2S2O5, Li2SO3,（C6H5）2SO and Li2S）during the first cycle, which are the effective components in SEI film thatreduces the side reaction at lithium/electrolyte interface. 2. Different composite solid polymer electrolytes PEO18LiTFSI-X, whereX refers to nano-SiO₂, H2SO4modified nano SiO₂（m-SiO₂） and ionic liquidPP13TFSI, were synthesized by casting method. The results showed that:（1）The addition of X improved the ionic conductivity of PEO18LiTFSI,especially when X was SiO₂-PP13TFSI, the ionic conductivity was increasedfrom5.1×10^-6 to3.7×10^-5 S cm-1at room temperature.（2） Addition of Xstabilized interface property of electrolyte/lithium anode to a certain extent.The interfacial resistance for symmetric cells Li/PEO18LiTFSI-SiO₂/Li,Li/PEO18LiTFSI-m-SiO₂/Li, Li/PEO18LiTFSI-PP13TFSI/Li andLi/PEO18LiTFSI-SiO₂-PP13TFSI/Li maintained the value of110,100,100and60cm2respectively during24days storage. In contrast, the interfacialresistance of Li/PEO18LiTFSI/Li increased from250to450cm2.（3） Theaddition of X has effectively suppressed the dendrites formation. The lithiumdendrite onset time （t0） was25,32,35and46h when X was SiO₂, m-SiO₂,PP13TFSI and SiO₂-PP13TFSI respectively, compared to15h for the case ofno additive. Hence the estimated specific capacity before dendrite appearedwas, in turn,548（no additive）,914,1170,1277and2154mAh g-1. Therelationship between the logarithms of dendrite onset time （t0） and currentdensity （J） was linear for all solid polymer electrolytes.3. A lithium-rich ternary Li_2.6BMg_0.05alloy foil containing three phasesof Li, Li5B4and trace amount of Li3Mg7was synthesized by fusing method. The calculated specific capacity of Li_2.6BMg_0.05for the reaction of lithiumphase was1181.6mAh g^-1. The resistances of Li_2.6BMg_0.05/electrolyteinterface during both storage and cycling were more stable and lower thanthose of Li counterparts. In addition, lithium dendrite formation can besuppressed by use of Li_2.6BMg_0.05alloy anode during cycling and long-termdeposition （100C）. These results indicated that lithium-rich multiphase alloyLi_2.6BMg_0.05can be promising anode candidate for high capacity rechargeablelithium batteries.