Metal Sulfide-Blended Sulfur Cathodes in High Energy Lithium-Sulfur Cells

Abstract

Lithium-sulfur cells exhibit poor cycle life, due to the well-known 'polysulfide shuttle' enabled by the dissolution of the sulfur reduction products in organic electrolyte. Different strategies have been implemented to reduce the shuttle effects, with limited success, especially through use of low sulfur loadings (1-2 mg/cm 2 ). Dense electrodes with high sulfur loadings are essential for high energy cells, however, such electrodes experience more serious polysulfide effects. In this paper, we describe the benefits of blending sulfur with a transition metal sulfide (here, TiS 2 and MoS 2 ) to form dense composite cathodes with enhanced conductivity. There is an improvement in both the initial capacity from sulfur utilization (∼800 mAh/g based on sulfur content), the coulombic efficiency (>96%) and also in cycle life upon blending with the metal sulfide. High sulfur loadings (>12 mg/cm 2 or ∼6 mAh/cm 2 per side) were demonstrated to display high sulfur utilization in Li-S cells containing the metal sulfide blends either with or without coatings over the sulfur cathode. XRD studies were carried out to understand the redox behavior of the metal sulfide additive during charge/discharge cycling of the sulfur cathodes. DC polarization and Potentiometric Intermittent Titration Technique (PITT) measurements were made on sulfur cathodes with and without metal sulfide blends to determine the charge transfer and diffusional kinetics. Since their inception in 1991, Li-ion batteries 1 have progressed at a rapid pace, with a three-fold enhancement in their performance achieved through many advances in both electrode materials and electrolytes.2-9 Current Li-ion cells from commercial vendors (e.g., Panasonic, LG and Samsung) provide an impressive specific energy of >250 Wh/kg and energy density of >600 Wh/l which are benefiting a wide range of applications including portable electronic devices, electric vehicles and aerospace needs. Yet, many of the emerging markets such as electric vehicles and renewable energy technologies place even higher demands on the battery technologies, both in terms of performance and cost. It is believed that the performance of lithium-ion technologies has reached a plateau, and any future improvements will only be marginal. Replacement of graphite anodes with Si, and conventional 4 V cathodes with the high voltage Li-rich layered-layered composite cathodes, 10,11 has not yet successfully been applied to commercial batteries. Accordingly, any gains in specific energy and energy density have been modest so far after a decade of development. 12,13 There is, thus, a pursuit for more energetic battery technologies beyond Li-ion batteries. This has led to a renewed interest in the lithiumsulfur system, which has the highest theoretical specific energy of all the known rechargeable systems (due to the high capacity of sulfur, 1672 mAh/g, ∼ 6-10x of Li-ion cathodes), with the notable exception of Li-O 2 which itself has several serious fundamental hurdles that are not close to being overcome.14-16 The spe...

Enhancing Catalytic Activity of Titanium Oxide in Lithium–Sulfur Batteries by Band Engineering

“…XPS was applied to interpret the subtle local interaction between CNT@TiO 2−x and Li 2 S 6 . [33] The presence of TiS bonds was also verified by the lower Ti 2p bands of CNT@TiO 2−x -Li 2 S 6 than CNT@TiO 2−x ( Figure S14A, Supporting Information). The peak located at 161.7 and 163.3 eV can be attributed to terminal (S T −1 ) and bridging sulfur (S B 0 ), respectively.…”

Defect Control in the Synthesis of 2 D MoS2 Nanosheets: Polysulfide Trapping in Composite Sulfur Cathodes for Li–S Batteries

“…Although it was demonstrated that the addition of commercial, bulk MoS 2 showed improvements in rate capability and capacity retention, the morphology and physical properties of the MoS 2 additive were not modified or controlled. Another report on using a related method of adding either commercially available MoS 2 or TiS 2 to electrode slurries also showed significant improvements during electrochemical testing, including increased sulfur utilization and Coulombic efficiency . These prior reports achieved significant gains with the incorporation of micron‐scale commercially available MoS 2 materials.…”

“…Another report on using ar elatedm ethodo f adding either commercially available MoS 2 or TiS 2 to electrode slurriesa lso showed significant improvements during electrochemicaltesting, including increased sulfur utilization and Coulombic efficiency. [45] These prior reports achieved significant gains with the incorporation of micron-scale commercially availableM oS 2 materials. Here we complementt hese prior reports with as ystematic study of synthesized nanosheet materials with controlled defect structures.…”

Polysulfide Binding to Several Nanoscale Magnéli Phases Synthesized in Carbon for Long‐Life Lithium–Sulfur Battery Cathodes

“…These approaches have issues affectingt heir viability.F irst, they involvet he use of ap ure hydrogen reduction environment and requireh igh reduction temperatures that promote the sintering of nanoparticles (NPs). [42] These limitations can be alleviated by integrating metal oxide NPs into lightweight conductive carbon matrices and rationally controlling nanoscale MagnØli phases concurrently. [41] Pure metal oxide cathodes are prone to pulverization,a nd cells assembled from these cathodes also require ah igh concentration of electrolyte to realize good performance, which is not appropriate for high gravimetric capacities.…”

“…[41] Pure metal oxide cathodes are prone to pulverization,a nd cells assembled from these cathodes also require ah igh concentration of electrolyte to realize good performance, which is not appropriate for high gravimetric capacities. [42] These limitations can be alleviated by integrating metal oxide NPs into lightweight conductive carbon matrices and rationally controlling nanoscale MagnØli phases concurrently.…”