Scalable Composites Benefiting from Transition‐Metal Oxides as Cathode Materials for Efficient Lithium‐Sulfur Batteries

Abstract

Composite materials achieved by including transition-metal oxides with different structures and morphologies in sulfur are suggested as scalable cathodes for high-energy lithium-sulfur (LiÀ S) batteries. The composites contain 80 wt.% sulfur and 20 wt.% of either MnO 2 or TiO 2 , leading to a sulfur content in the electrode of 64 wt.% and revealing a reversible, fast, and lowly polarized conversion process in the cell with limited interphase resistance. The SÀ TiO 2 composite exhibits an excellent rate capability between C/10 and 2C, and a cycle life extended over 400 cycles at 2C, owing to the effects of the nanometric TiO 2 additive in boosting the reaction kinetics.Instead, the micrometric sized particles of MnO 2 partially limit the electrochemical activity of SÀ MnO 2 to the current rate of 1C. Nevertheless, both SÀ MnO 2 and SÀ TiO 2 withstand a sulfur loading up to values approaching 6 mg cm À 2 , and deliver an areal capacity ranging from about 4.5 to 5.5 mAh cm À 2 at C/5. The excellent performances of the metal oxide-sulfur electrodes, even at high active material loading, and the possible scalability of the synthetic pathway adopted in the work suggest that the composites are viable cathodes for next-generation LiÀ S batteries with high energy density and efficient electrochemical process.

Diffusional Features of a Lithium‐Sulfur Battery Exploiting Highly Microporous Activated Carbon

“…The sulfur content in the composites is evaluated by thermogravimetry under N 2 atmosphere, and the corresponding weight losses together with that of bare S are plotted in Figure 3b. The figure likely indicates the correspondence between the initially projected S to C mass ratios (i.e., 70 : 30, 80 : 20 and 90 : 10) and the ones achieved for the composites after synthesis, thus indicating the almost complete absence of sulfur evaporation during the process [76] . The TGA profiles of Figure 3b, and the corresponding differential curves (DTG) reported in Figure S5 (Supporting Information) reveal significant differences.…”

“…The figure likely indicates the correspondence between the initially projected S to C mass ratios (i.e., 70 : 30, 80 : 20 and 90 : 10) and the ones achieved for the composites after synthesis, thus indicating the almost complete absence of sulfur evaporation during the process. [76] The TGA profiles of Figure 3b, and the corresponding differential curves (DTG) reported in Figure S5 (Supporting Information) reveal significant differences. Hence, S evaporates from the composites within temperature ranging from 200 to 450 °C, instead the pure S shows an interval restricted between 200 and 330 °C.…”

A Lithium–Sulfur Battery Using Binder-Free Graphene-Coated Aluminum Current Collector

“…In addition , sulfur holds various bonuses including the large abundance on the earth’s crust, low price, and environmental compatibility . Sulfur (S 8 ) can reversibly operate in a lithium cell through a multi-step electrochemical process, leading to the formation of various polysulfide intermediates, which can be highly soluble (Li 2 S x , 6 ≤ x ≤ 8) or almost insoluble (Li 2 S 2 and Li 2 S) into the electrolyte media according to the overall reaction: S 8 + 16Li + + 16e – ⇄ 8Li 2 S. , Unfortunately, soluble polysulfides can migrate and directly react with the lithium anode or shuttle between the anode and cathode throughout a continuous process without any charge accumulation. , This leads to an efficiency decrease, active material loss, or even to short circuits and cell failure, while insoluble polysulfides can precipitate into the cell and cause resistance increase and capacity fading. , It is worth mentioning that the low electronic and ionic conductivity of elemental sulfur triggered its use as composite mainly with carbons, − metals, − metal oxides, − and conductive polymers. , Furthermore, the characteristic electrochemical process involving the electro-deposition/dissolution of soluble species at the cathode side focused the attention on the nature of the current collector. − Aluminum is typically used as the cathode support in lithium batteries for either insertion or sulfur-based electrodes due to its relevant oxidative stability, promoted by the presence of an Al 2 O 3 nanometric passivating layer which remarkably protects the metal surface from further reactions and enhances the safety content of the system. , However, flat and thin metal supports ( e.g ., bare Al current collector) may lead to poor performances due to high overall impedance of the cell and modest ability in allowing the complex multi-step reaction pathway, while thicker porous supports ( e.g ., gas diffusion layer, GDL) can enhance the cell response, reduce the impedance, and actually boost the kinetics of the Li/S process. − In fact, the rough microporous surface of the GDL (micropore area of 0.3 m 2 g –1 with pore volume of 0.04 cm 3 g –1 ) can host the active material, allow a continuous contact of sulfur with the current collector, and facilitate the electrochemical reaction of dissolved intermediates . As a result, GDL shows a higher specific capacity compared to bare Al (1060 vs 770 mAh g –1 at C/5 using a graphene-based sulfur composite), which is typically used as support in LIBs.…”

Artificially Layered CoSe2 Nanosheets by a Dual-Templating Strategy for High-Performance Lithium–Sulfur Batteries

“…15,16 (ii) The soluble lithium polysulfides (LiPSs) in the electrolyte induce an undesirable phenomenon of "shuttle effect", resulting in active material loss and specific capacity degeneration. 17,18 (iii) Not only the starting active material (S) but also the discharging product (Li 2 S) has low electronic/ionic conductivities, leading to poor rate performance. 19,20 (iv) The inert Li 2 S requires an activation potential in the charging process.…”

“…One the other hand, lithium–sulfur batteries (LSBs) possessing a high theoretical specific capacity (1675 mAh g –1 ) are promising as the next-generation high-energy-density rechargeable batteries. − Nevertheless, the development of LSBs still encounters several critical issues and obstacles. (i) The slow reaction kinetics of S 8 -Li 2 S x -Li 2 S in the electrochemical process leads to low capacity and poor rate capability. , (ii) The soluble lithium polysulfides (LiPSs) in the electrolyte induce an undesirable phenomenon of “shuttle effect”, resulting in active material loss and specific capacity degeneration. , (iii) Not only the starting active material (S) but also the discharging product (Li 2 S) has low electronic/ionic conductivities, leading to poor rate performance. , (iv) The inert Li 2 S requires an activation potential in the charging process . To overcome these issues, incorporation of active MX 2 electrocatalysts featuring strong chemisorption and high catalytic activity is proposed as a powerful route to enhance the discharge/charge performance. − L-CoSe 2 has the merits of a layered structure for fast Li + diffusion, a metallic conductivity for fast electron transport, and a large surface area providing sufficient catalytic sites for strong immobilization and high catalytic activity.…”