Encapsulating sulfur into Magnéli phase Ti₄O₇ nanotube array for lithium sulfur battery cathode

Abstract

Lithium sulfur (𝐿𝑖-𝑆) battery offers high volumetric and gravimetric energy densities about four to five times higher than those of the state-of-the-art lithium ion batteries (LIBs) based on intercalation compounds. The low cost and environmental benignity of sulfur have justified the application of 𝐿𝑖-𝑆 system in the storage of clean renewable energy. Despite these attractive features of sulfur as an active material, some of its properties and the characteristics of its redox reaction pose design difficulties in an 𝐿𝑖-𝑆 battery, and hinder its large-scale commercialization. Elemental sulfur and lithium sulfide (𝐿𝑖₂𝑆) are electronically and ionically insulating, which renders incomplete utilization of sulfur and limits the rate capability of the cell. The redox reaction of sulfur, from 𝑆⁰ to 𝑆²⁻ or vice versa, is multi-step in nature, and the reaction intermediates, lithium polysulfides (𝐿𝑖𝑃𝑆), are soluble in commonly used ether-based liquid electrolyte. The dissolved polysulfide anions may diffuse across the electrolyte and react with lithium anode, covering it with a layer of insulating 𝐿𝑖₂𝑆 and therefore increasing the internal resistance of the 𝐿𝑖-𝑆 cell. This active materials loss results in fast capacity fading upon cycling, and the shuttling of dissolved 𝐿𝑖𝑃𝑆 during cell operation lowers the energy efficiency of an 𝐿𝑖-𝑆 cell. Furthermore, non-uniform deposition of solid charged/discharged products from dissolved 𝐿𝑖𝑃𝑆, as well as the 80% volume change between sulfur and 𝐿𝑖₂𝑆, may lead to collapse of electrode structure and detachment of active materials from the electronic-conducting pathway. Magnéli phase 𝑇𝑖₄𝑂₇ has been proved to improve the performance of 𝐿𝑖-𝑆 cells, because of its metallic grade electronic conductivity of 1035𝑆/𝑐𝑚 and its capability of strong chemisorption to bind 𝐿𝑖𝑃𝑆. In this study, Magnéli phase 𝑇𝑖₄𝑂₇ nanotube array (𝑇𝑖₄𝑂₇-NTA) was synthesized on a substrate of titanium mesh through anodization and high temperature reduction under hydrogen. Sulfur was then introduced into the nanotubes by electrodeposition followed by melt-infusion to form a composite material 𝑆-𝑇𝑖₄𝑂₇ NTA. Successful synthesis of 𝑇𝑖₄𝑂₇ NTA was confirmed by XRD spectra, SEM images, and the Ti2p XPS spectrum. The prepared S-𝑇𝑖₄𝑂₇ NTA mesh composites, when made into coin 𝐿𝑖-𝑆 cells, showed much improved capacity retention upon cycling compared to sulfur-carbon composite. When 𝑆-𝑇𝑖₄𝑂₇ NTA was optimized by carbon coating with acetylene black (𝐴𝐵), labeled 𝐴𝐵-𝑆-𝑇𝑖₄𝑂₇ NTA, the cycling stability was further improved with an ultra-low capacity decay rate of 0.0322% per cycle for more than 1800 cycles. Excellent rate capability was also observed in 𝐴𝐵-𝑆-𝑇𝑖₄𝑂₇ NTA mesh composite cells, delivering high values of specific capacity of 1604mAh/g at 0.05C, 1220mAh/g at 0.1C, 1060mAh/g at 0.2C, 830mAh/g at 0.5C, 750mAh/g at 1C, 660mAh/g at 2C, 500mAh/g at 4C, and 270mAh/g at 6C. The sulfur loading for some of the best performing cells is, reported in areal loading, in the range of 1.3~2.8mg/cm² which is comparable to other publications. 𝐴𝐵-𝑆-𝑇𝑖₄𝑂₇ NTA mesh cells of higher loadings of 3.5~4.8mg/cm² were also tested, and their performance under moderate currents demonstrated good capacity retention ability. The high sulfur loading was achieved by a two-step sulfur incorporation method, i.e., electrodeposition followed by melt-infusion. The chemisorption between 𝑇𝑖₄𝑂₇ and polysulfides was confirmed by XPS, and it was found that this binding force was a redox interaction in nature. It is proposed that this interaction may be attributed to the partial donation of the lone-pair electrons from 𝐿𝑖𝑃𝑆 to the under-coordinated titanium atoms on the surface of 𝑇𝑖₄𝑂₇. The effect of acetylene black carbon coating was investigated with galvanostatic voltage vs. capacity curve and cyclic voltammetry. It was found that the carbon coating significantly lowered the polarization of 𝐿𝑖-𝑆 cells upon cycling by better confining the redox species of sulfur within the nanotubes. Electrochemical impedance spectra showed lower values of charge-transfer resistance in 𝐴𝐵-𝑆-𝑇𝑖₄𝑂₇ NTA mesh cells than those in 𝑆-𝐴𝐵 cells, consolidating the positive effects of 𝑇𝑖₄𝑂₇ in improving the performance of 𝐿𝑖-𝑆 cells. The synergetic effects of 𝑇𝑖₄𝑂₇ and carbon-coating, combined with the synthesis of an ordered structure of nanotube array on a mesh of titanium nitride as a free-standing electrode, have contributed to the excellent performance of the 𝐿𝑖-𝑆 cells in this study.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology