Study Of Novel Electrolyte And Compatibility To Lithium Electrode For Lithium-sulfur Batteries

Owing to the increasing performance demands of mobile devices, clean energy, electric and hybrid vehicles, the conventional lithium ion batteries are not enough for the key applications of the future. Thus, large efforts are devoted to develop rechargeable battery systems based on lithium metal with higher energy density. Among the most promising candidates for next generation of battery systems, lithium-sulfur batteries have received worldwide attention due to their high theoretical energy density（about 2600 Wh kg-1）, natural abundance of key elements and environmental friendliness.In the previous reports, considerable advances have been achieved in building the cathode matrix and preventing the diffusion of polysulfides to improve the electrochemical performance of lithium-sulfur batteries. However, the behavior of the lithium anode has received little attention. In this work, we present a systematic study on the role of components of electrolyte on the formation and stability of SEI on lithium anode, these components are polysulfides, additive（LiNO₃）, lithium salt（Li TFSI） and solvent in the conventional electrolyte for lithium-sulfur batteries. To overcome the drawbacks of conventional electrolyte, novel electrolytes based on ionic liquid and solvate ionic liquid were developed for rechargeable Li metal batteries. Our work focused on the impact of the components and local structure of those electrolytes on the composition and properties of the SEI on lithium electrode. Furthermore, we introduced other electrolytes, like organic solvent-ionic liquid mixture electrolyte, fumed silica-ionic liquid gel electrolyte and conductive particles-ionic liquid hybrid electrolyte, to optimize their performance and compatibility to lithium anode.In chapter 3, we have investigated the effect of the main components of conventional electrolyte, such as polysulfides, LiTFSI, LiNO₃ and solvent, on the composition and stability of SEI on lithium anode for lithium-sulfur batteries. The formation and the cycling behavior of SEI were analyzed using electrochemical measurement, morphology techniques and electron spectroscopy for surface analysis. The results showed that the SEI film formed with polysulfides and LiNO₃ consisted of two sub layers, the top layer composed of oxidized products from polysulfides（such as Li₂SO₄ and Li₂S₂O₃） and the bottom layer composed of the reduced products of polysulfides and LiNO₃（such as Li₃N, Li₂N₂O₂, LiN_xO_y and Li₂S₂）. The SEI with the specific structure can prevent the contact of lithium with polysulfides and LiNO₃ to stop further reaction on anode. Thus the continuous grown of SEI and the shuttle in lithium-sulfur batteries were suppressed at the same time. We found that the contribution of LiNO₃ and polysulfides were equally important to form a stable and compact SEI film on lithium anode. The cycling behavior and impedance spectra of the symmetrical cells showed that the stability and property of those SEI films depended on the selection of the solvent. The SEI film formed in the electrolyte with higher ether/alkyl ratio showed a corresponding higher resistance for lithium ion migration. The main chemical composition of this SEI film came from the decomposition of solvent（RCH₂OLi）. Nevertheless, the SEI film formed with lower ether/alkyl ratio mainly composed of the species with C-S group, which was produced by the decomposition of lithium salt.In chapter 4, we have investigated the formation of the SEI in the ionic liquid electrolytes composed of N-Methyl-（n-butyl）pyrrolidinium bis（trifluoromethanesulfonyl）-imide（PYR14-TFSI） and Li TFSI, with and without the addition of polysulfides. The cycling behavior of the symmetrical cells showed that the SEI films were relatively stable in the ionic liquid electrolyte, also in the presence of polysulfides. However, the presence of polysulfides results in a higher SEI layer resistance（RSEI） and a higher activation energy. Four ionic liquid electrolytes were introduced to inquire the impact of cations and anions on the formation and structure of the SEI. These ionic liquids are N-Methyl-（n-butyl）pyrrolidinium bis（trifluoromethanesulfonyl）imide（PYR14-TFSI）, N-Methyl-（n-butyl）pyrrolidinium bis（fluorosulfonyl）imide（PYR14-FSI）, N-Methyl-（n-butyl）piperidinium bis（fluorosulfonyl）imide（PP14-FSI） and N-Methyl-（n-butyl）imidazolium bis（fluorosulfonyl）imide（BMIM-FSI）. From X-ray photoelectron spectroscopy spectra（XPS）, with argon-ion sputtering for depth profiling, we found that the species and distribution of the compositions in SEI depended on the selection of cation and anion. Lithium sulfide was observed in the SEI films formed with FSI- anion but barely seen in the SEI film formed in TFSI- based electrolyte. IR and Raman spectra showed that the local structure of ionic liquid electrolytes were affected by combination of cations and anions. For instance, the wavenumbers for the bending of FSI- anion were 650 cm-1, 640 cm-1 and 641 cm-1 in BMIM-FSI, PP14-FSI and PYR14-FSI, respectively.In chapter 5, we present a systematic study on the physicochemical properties and local structures of the mixtures composed of Li TFSI and oligoethers（Tetraethylene glycol dimethyl ether, TEGDME, G4） using vibrational spectroscopy and simulated calculation. Furthermore, we tried to figure out the correlation between the interaction of molecules and the decomposition mechanism of TFSI- anions. The results showed that the structure difference of mixtures with various ratio of G4 to Li TFSI was the distribution of two association states, CIP（contact ion pair） and SSIP（solvent-separated ion pair）. The number of CIP in solvate ionic liquid was much larger than that in conventional electrolyte solutions with lower concentration of lithium salt. Solvate ionic liquid composed of G4 and lithium polysulfides was introduced to improve the compatibility to lithium anode for lithium-sulfur batteries. The results of vibrational spectroscopy indicated the presence of sulfur radicals and a series of polysulfides anoins in the mixture of polysulfides and G4. Compared to the solvate ionic liquid based on Li TFSI, the electrolyte based lithium polysulfides showed more SSIP and less CIP.In chapter 6, we have investigated organic solvent-ionic liquid mixture electrolyte, fumed silica-ionic liquid gel electrolyte and conductive particles-ionic liquid hybrid electrolyte to overcome the issues of neat ionic liquid electrolytes, like high viscosity and grown of dendrite on lithium electrode. A mixed electrolyte, formed by adding 1,3-dioxolane（DIOX） to an ionic liquid, showed an enhanced ion transport evidenced by an increased ionic conductivity and higher limiting current density. The capacity of a Li-S cell using this electrolyte was more than doubled, the rate capability and the cycling performance considerably improved, compared to a cell utilizing a neat ionic liquid electrolyte. The fumed silica-ionic liquid gel electrolyte revealed considerable mechanical strength and a lower SEI layer resistance in lithium symmetrical cells. To build a SEI with uniform conductivity and lower resistance, we prepared a hybrid electrolyte composed of ionic liquid and conductive particles, which were nanosized grains from the bulk solid electrolyte(Li_1.5Al_0.5Ge_1.5（PO₄）₃). The hybrid electrolyte showed a conductivity of 10-3 S cm-1 at 30 °C. Raman spectra and DSC traces demonstrated that the presence of conductive particles barely brought any influence to the interaction between Li+ ion and ions from ionic liquid. The electrochemical performance and SEM images indicated that the hybrid electrolyte suppressed the grown of lithium dendrite and lowered the SEI resistance.