High Energy Density Lithium-Sulfur Batteries: Effect of Low Electrolyte/Sulfur Ratios

高能量密度鋰硫電池: 低電解液/硫比例的影響

Student thesis: Doctoral Thesis

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Award date20 Nov 2018

Abstract

Sulfur is a low cost and high capacity cathode material to build future high energy density lithium batteries. Lithium-sulfur batteries (LSBs) are considered to be a candidate technology for electric vehicles and renewable energy storage applications. However, there are still some challenges in LSBs, like insufficient cycle life and poor rate performance. In the past decades, researches mainly focused on high sulfur utilization (specific capacity) and cycling performance. In recent years, the importance of electrolyte/sulfur (E/S) ratio is gradually recognized. How to obtain energy density on a low E/S ratio is a new challenge and have been discussed in this thesis.

Firstly, Li-S cells with E/S ratios from 40 to 4 μL/mg were prepared. As the sulfur loading increased and electrolyte volume kept constant, the value of E/S ratio was reduced. Physical vapor deposited sulfur was found to have better solubility than that made by melt diffusion method. At a ratio of 4 μl/mg, cathode showed a high overpotential at the end period of the S8 to process Li2S4. Based on the classical electrochemical model, it was low limiting current that causes the poor sulfur utilization. By decreasing lithium salt (LiTFSI) concentration electrolyte (from 1 M to 0.5 M) and coating TiO2 on cathode substrate surface, the ion diffusivity, and diffusion layer thickness could be modified, leading to less polarization and higher capacity. The cell resistance was analyzed by electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). A high charge transfer resistance was responsible for the high overpotential. Potentiostatic intermittent titration technique (PITT) experiments further demonstrated the improved kinetic on Li2S deposition process. The energy density of low E/S ratio LSBs was estimated and compared with Li-ion batteries.

Then, the internal resistance and its influence on energy density were systematically studied. Full cell resistance could be separated into three parts: the cathode (electrolyte, contact, porous and charge transfer resistance), the anode (solid electrolyte interphase resistance) and the separator (electrolyte resistance). In order to obtain impedance information from different parts of the cell, four-electrode symmetric setup was proposed. A systematic study proved this setup was more accurate than three-electrode setup. EIS results indicated the resistance of each components changes with the state of charge and discharge. A cell with E/S ratio of 8 μL/mg was fabricated as an example for flooded electrolyte condition. Analysis of internal resistance showed the highest charge transfer resistance at the end of discharge because the surface of carbon paper was passivated by Li2S. A maximum local value on electrolyte and charge transfer resistance before the Li2S deposition was observed and high polysulfide concentration should be attributed to this peak.

Four-electrode symmetric setup was further applied to study the internal resistance of LSBs on low E/S ratios condition. In this situation, the solid electrolyte interphase (SEI) resistance was the main reason for high overpotential and low sulfur utilization. The internal resistance, produced by the electrolyte, porous structure, SEI and charge transfer significantly reduced the energy density in low E/S ratio condition when cells were discharged or charged at a high rate. The SEI contributed most to the overall internal resistance and needed to be optimized. Based on these resistance results, further reduced the E/S ratio to one was needed to achieve high energy density for competitive Li-S cells.