Biopolymer separators from polydopamine-functionalized bacterial cellulose for lithium-sulfur batteries

The advancement of the lithium-sulfur (Li-S) batteries is immensely impeded by two main challenges: polysulfide shuttling between the electrodes and Li dendrite formation associated with the Li-metal anode. To tackle these challenges, we synthesized a polydopamine coated bacterial cellulose (PDA@BC) separator in a way to create physical and chemical traps for the shuttling polysulfides and to control the Li+ flux. While nanocellulose offers its dense network as a physical trap, the presence of polydopamine in the separator offers polar functional groups which not only has a high binding energy towards the polysulfides but also helps in redistribution of the Li+ ions across it. The electrochemical and physiochemical results suggest that the synthesized separator can have practical applicability owing to its superior performance compared to a commercial separator. The Li-S batteries assembled with this separator showed a specific discharge capacity of 1449 mAh/g at 0.1C and 877 mAh/g at 1C, and a capacity fade of 0.03 % per cycle over 650 cycles at 1C. Using a PDA@BC separator, a practical Li-S battery cell with S loading of 7.5 mg cm-2 (and E/S ratio of 10 µLmg-1, 82 % S ratio) was also tested at 1C, which delivered a capacity of ∼ 6 mAh cm-2 for 500 cycles.

Boosting polysulfides immobilization and conversion through CoS2 catalytic sites loaded carbon fiber for robust lithium sulfur batteries

The practical applications of lithium sulfur battery is impeded by the lithium polysulfide shuttling and sluggish redox kinetics. To address the issues, herein, a multifunctional host is developed by the combination of nitrogen, phosphorus co-doped carbon fiber (NPCF) and CoS₂ towards boost the soluble polysulfides adsorption and transformation. Benefiting from the NPCF originated from biomass cattail fibers, a high conductive network is provided, and shuttle effect is reduced due to the strong chemical interaction between abundant heteroatom polar sites and lithium polysulfides. Moreover, the electrocatalytic CoS₂ on the carbon skeleton facilitate lithium polysulfides conversion and lithium sulfide deposition based on the density functional theory calculations and experiments. The efficient lithium polysulfides entrapment and subsequent electrocatalytic conversion improve dynamic stability during cycling, especially for rate capability. With these advantageous features, the electrode with NPCF/CoS₂ host can deliver a good rate capability (903 and 782 mAh g^-1 at 1C and 2C, respectively) and stable cycling performance with an ultra-low capacity decay of 0.014% per cycle at 1C. Notably, the cell can achieve a high areal capacity of 4.96 mA h cm^-2 under an elevated sulfur loading of 5.0 mg cm^-2. Overall, the improvement on the electrochemical performance ascertains the validity of the design strategy based on synergy engineering, which is a highly suitable approach for energy storage and conversion application.

Flexible S@C-CNTs cathodes with robust mechanical strength via blade-coating for lithium-sulfur batteries

Lithium sulfur batteries (LSBs) with high energy density hold some promising applications in the wearable and flexible devices. However, it has been still challenging to develop a simple and feasible approach to prepare flexible LSB cathodes with both robust mechanical strength. Herein, flexible S@C-CNTs cathodes with controllable thicknesses are successfully fabricated via a facile blade-coating method. Due to the strong cohesion among CNTs bundles and the well-designed structure, the flexible S@C-CNTs cathodes are demonstrated to be with a combination of impressive mechanical strength and enhanced electrochemical performance. For the flexible S@C-CNTs cathodes with the sulfur mass loading of 4 mg cm^-2, the areal capacity is close to 3.0 mA h cm^-2, and the breaking stress is up to 5.59 MPa with 7.77% strain. Meanwhile, the pouch cell exhibits excellent cyclic stability at both flat/bent conditions. All demonstrate that the flexible S@C-CNTs cathodes may satisfy the demands of practical application. Moreover, this methodology is suitable for designing other flexible battery electrodes, such as flexible Si@C-CNTs anodes for lithium ion batteries, flexible P@C-CNTs anodes for sodium/potassium ion batteries, etc.

An air-stable and waterproof lithium metal anode enabled by wax composite packaging

The reviving use of lithium metal anode (LMA) is one of the most promising ways to upgrade the energy density of lithium ion batteries. In the roadmap towards the real use, besides the formation of the dendrite, various adverse reactions due to the high activity of LMA when exposed to air or the electrolyte limit its practical applications. Learning from the packaging technology in electronic industry, we propose a wax-based coating compositing with the ion conducting poly (ethylene oxide) by a simple dip-coating technology and the prepared LMA is featured with an air-stable and waterproof surface. The LMA thus remains stable for 24 h in ambient air even with the relative humidity of 70% while retaining about 85% its electrochemical capacity. More importantly, the LMA is accessible to water and when dipping in water, no obvious adverse reactions or capacity decay is observed. With the composite coating, a steady cycling performance for 500 h in symmetrical cells and a low capacity decay rate of 0.075% per cycle after 300 cycles in lithium-sulfur batteries assembled with the packaged anode have been achieved. This work demonstrates a very simple and effective LMA package technology which is easily scalable and is very promising for speeding up the industrialization of lithium-sulfur batteries and shows potentials for the large-scale production of air-sensitive electrode materials not limited to LMAs.

A Collaboratively Polar Conductive Interface for Accelerating Polysulfide Redox Conversion

In order to alleviate the inferior cycle stability of the sulfur cathode, a self-assembled SnO₂-doped manganese silicate nanobubble (SMN) is designed as a sulfur/polysulfide host to immobilize the intermediate Li₂S _x, and nitrogen-doped carbon (N-C) is coated on SMN (SMN@C). The exquisite N-C conductive network not only provides sufficient free space for the volume expansion during the phase transition of solid sulfur into lithium sulfide but also reduces R_ct of SMN. During cycling, the soluble polysulfide could be fastened by the silicate with an oxygen-rich functional group and heteronitrogen atoms through chemical bonding, enabling a confined shuttle effect. The synergistic effect between N-C and SMN could also effectively facilitate the interconversion between lithium polysulfides and Li₂S, reducing the potential barrier and accelerating the redox kinetics. With an areal sulfur loading of 2 mg/cm², the S-SMN@C cathodes demonstrate a high initial capacity of 1204 mA·h/g at 0.1 C, and an outstanding cycle stability with a capacity fading rate of 0.0277%, ranging from the 2nd cycle to the 1000th cycle at 2 C.

Greatly Suppressed Shuttle Effect for Improved Lithium Sulfur Battery Performance through Short Chain Intermediates

The high solubility of long-chain lithium polysulfides and their infamous shuttle effect in lithium sulfur battery lead to rapid capacity fading along with low Coulombic efficiency. To address above issues, we propose a new strategy to suppress the shuttle effect for greatly enhanced lithium sulfur battery performance mainly through the formation of short-chain intermediates during discharging, which allows significant improvements including high capacity retention of 1022 mAh/g with 87% retention for 450 cycles. Without LiNO₃-containing electrolytes, the excellent Coulombic efficiency of ∼99.5% for more than 500 cycles is obtained, suggesting the greatly suppressed shuttle effect. In situ UV/vis analysis of electrolyte during cycling reveals that the short-chain Li₂S₂ and Li₂S₃ polysulfides are detected as main intermediates, which are theoretically verified by density functional theory (DFT) calculations. Our strategy may open up a new avenue for practical application of lithium sulfur battery.