Lithium Polysulphides Research Articles

Effect of oxygen-substituted lattice carbon in Ti3C2 MXene as anchoring material for Li-S battery: DFT calculations and experiments

Electrochemical processes involving the adsorption/desorption of polysulfides are usually related to the composition of adsorbed-sulfur electrode material. Here, we investigate the moderate influence of non-metallic O and C elements on anchoring property towards lithium polysulphides (LiPSs), whereby the adsorption behavior of 2D-Ti3C2 MXene with continuous O-substitution into C site is analyzed via density functional theory (DFT) calculations in concerning of reported inevitable oxygen in the lattice. The introduction of O into 2D-Ti3C2 lattice reduces the density of states of Ti elements near the Fermi level, thus weakening the Ti-S binding interaction. Ti3C2 MXene with C/O ratio of 3:1 (Ti3C2-C12O4 model) exhibits moderate adsorption superior to other compositions, where the calculated binding energies of S8 and Li2Sn (n = 8, 6, 4) are −2.88, −2.57, −2.31 and −2.19 eV atom−1, respectively. The substitution of O atom theoretically hinders the transfer of Li+ and reduces the energy barrier of S-migration, thus promoting the anchoring and transformation of LiPSs. Notably, consistent result is obtained from the comparative study on electrochemical performance of titanium oxycarbide with precisely different C/O ratios as alternative, considering the unpractical controlling of O-content in 2D-Ti3C2 and the homologous crystal structure. Titanium oxycarbide with C/O ratio of 3:1 (Ti(C,O)-C12O4) as anchoring material in Li-S battery exhibits a superior initial specific capacity of 841 mAh/g at 1C and fasten ion diffusion coefficients (1.92 cm2 s−1 and 2.94 cm2 s−1). This study suggest attention on regulatory effect of non-metallic elements in lattice instead of surface towards performance optimization of titanium-carbide-based material as host for sulfur cathode utilized in Li-S battery.

Solvation-property relationship of lithium-sulphur battery electrolytes

The Li-S battery is a promising next-generation battery chemistry that offers high energy density and low cost. The Li-S battery has a unique chemistry with intermediate sulphur species readily solvated in electrolytes, and understanding their implications is important from both practical and fundamental perspectives. In this study, we utilise the solvation free energy of electrolytes as a metric to formulate solvation-property relationships in various electrolytes and investigate their impact on the solvated lithium polysulphides. We find that solvation free energy influences Li-S battery voltage profile, lithium polysulphide solubility, Li-S battery cyclability and the Li metal anode; weaker solvation leads to lower 1st plateau voltage, higher 2nd plateau voltage, lower lithium polysulphide solubility, and superior cyclability of Li-S full cells and Li metal anodes. We believe that relationships delineated in this study can guide the design of high-performance electrolytes for Li-S batteries.

Electropolymerisation Technologies for Next-Generation Lithium-Sulphur Batteries.

Lithium-sulphur batteries (LiSBs) have garnered significant attention as the next-generation energy storage device because of their high theoretical energy density, low cost, and environmental friendliness. However, the undesirable "shuttle effect" by lithium polysulphides (LPSs) severely inhibits their practical application. To alleviate the shuttle effect, conductive polymers have been used to fabricate LiSBs owing to their improved electrically conducting pathways, flexible mechanical properties, and high affinity to LPSs, which allow the shuttle effect to be controlled. In this study, the applications of various conductive polymers prepared via the simple yet sophisticated electropolymerisation (EP) technology are systematically investigated based on the main components of LiSBs (cathodes, anodes, separators, and electrolytes). Finally, the potential application of EP technology in next-generation batteries is comprehensively discussed.

Nanoscale transition metal catalysts anchored on perovskite oxide enabling enhanced kinetics of lithium polysulfide redox in lithium-sulfur batteries

To obtain high-performance lithium-sulfur (Li-S) batteries, it is necessary to rationally design electrocatalytic materials that can promote efficient sulfur electrochemical reactions. Herein, the robust heterostructured material of nanoscale transition metal anchored on perovskite oxide was designed for efficient catalytic kinetics of the oxidation and reduction reactions of lithium polysulphide (LiPSs), and verified by density functional theory (DFT) calculations and experimental characterizations. Due to the strong interaction of nanoscale transition metals with LiPSs through chemical coupling, heterostructured materials (STO@M) (M = Fe, Ni, Cu) exhibit excellent catalytic activity for redox reactions of LiPSs. The bifunctional heterostructure material STO@Fe exhibits good rate performance and cycling stability as the cathode host, realizing a high-performance Li-S battery that can maintain stable cycling under rapid charge–discharge cycling. This study presents a novel approach to designing electrocatalytic materials for redox reactions of LiPSs, which promotes the development of fast charge–discharge Li-S batteries.

Ultrathin Ti3C2Tx nanosheets modified separators for lithium–sulphur batteries

AbstractCurrently, among the various emerging energy storage systems, the lithium–sulphur (Li‐S) battery is expected to be one of the next‐generation lithium secondary batteries with high efficiency. However, the practical application of Li‐S batteries still faces many obstacles. To solve the shuttle effect of lithium polysulphides, ultrathin Ti3C2Tx nanosheets were prepared through the in‐situ acid etching method and applied to separator modification to suppress the shuttle effect of lithium polysulphides. Ultrathin Ti3C2Tx nanosheets with enlarged interlayer spacing accelerated the migration of Li+. The abundant termination groups on the surface of Ti3C2Tx played the role of the lithium polysulphide capture centre. When the mass loading of separator modification materials was set as 0.025 mg cm−2, the as‐prepared battery exhibited a reversible specific capacity as high as 780 mAh g−1 after 200 cycles at 0.2 C, and the single‐cycle capacity decay rate was only 0.09%.

DFT Simulation-Based Design of 1T-MoS2 Cathode Hosts for Li-S Batteries and Experimental Evaluation.

The main challenge in lithium sulphur (Li-S) batteries is the shuttling of lithium polysulphides (LiPSs) caused by the rapid LiPSs migration to the anode and the slow reaction kinetics in the chain of LiPSs conversion. In this study, we explore 1T-MoS2 as a cathode host for Li-S batteries by examining the affinity of 1T-MoS2 substrates (pristine 1T-MoS2, defected 1T-MoS2 with one and two S vacancies) toward LiPSs and their electrocatalytic effects. Density functional theory (DFT) simulations are used to determine the adsorption energy of LiPSs to these substrates, the Gibbs free energy profiles for the reaction chain, and the preferred pathways and activation energies for the slow reaction stage from Li2S4 to Li2S. The obtained information highlights the potential benefit of a combination of 1T-MoS2 regions, without or with one and two sulphur vacancies, for an improved Li-S battery performance. The recommendation is implemented in a Li-S battery with areas of pristine 1T-MoS2 and some proportion of one and two S vacancies, exhibiting a capacity of 1190 mAh/g at 0.1C, with 97% capacity retention after 60 cycles in a schedule of different C-rates from 0.1C to 2C and back to 0.1C.

A multifunctional nano filler for solid polymer electrolyte toward stable cycling for lithium-metal anodes in lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries are considered as very promising energy storage systems due to their higher theoretical performance and lower active material cost. However, the notorious shuttle effect and lithium dendrite growth have severely restricted the industrial production of Li–S batteries. In this study, we used polyethylene oxide (PEO) as the matrix and added multifunctional nanofillers (nano In2O3) to effectively inhibit the shuttle effect caused by the dissolution of lithium polysulphides (LiPSs) in Li–S batteries. As a multifunctional filler, nano-In2O3 not only improves the ionic conductivity of the PEO electrolyte, but also forms a Li–In alloy layer in situ at the polymer/anode interface, which can cause the rapid diffusion of lithium ions and prevent the side reactions between lithium metal anode and the electrolyte. Achieving the effect of stabilizing the lithium metal anode and suppressing the shuttle effect. Multiple problems in the system are solved by introducing a single component with multiple functions. The symmetrical lithium battery based on the PEO/LiTFSI/In2O3 solid polymer electrolyte (PEO/LiTFSI/In2O3 SPE) has higher cycle stability (over 1200 h) because it is more stable to the lithium metal anode. The Li–S full battery also exhibits excellent electrochemical performance. After 500 cycles at 1C, its capacity can be maintained at 570 mAh g−1, the capacity decay rate is 0.038%. The coulombic efficiency (CE) during the cycle is also close 100%.

Synergistic Effect of Fe2O3/NC-Gdl Interlayer and Partially Exfoliated Carbon Nanotubes/Sulfur Cathode for Efficient Polysulphide Confinement in High Performance Lithium–Sulfur Batteries

Lithium sulfur batteries have gained considerable research interest for electric vehicles and portable electronic devices due to their high theoretical energy density (2600 Wh/Kg) compared to existing metal ion batteries. In addition, the low cost, high abundance and high theoretical specific capacity (1675 mAh/g) of sulfur have made Li-S batteries to be the potential replacement for lithium ion batteries. However, the low electrical conductivity of sulfur (5 × 10−30 S/cm), Li2S, low sulfur loading (<70%) at the composite cathode, large volume expansion during S to Li2S conversion and multistep reduction of sulfur to lithium polysulphides (LPS) act as the prohibitor for the commercialization. Elemental sulfur undergoes a various structural variation during the charge–discharge and forms soluble higher order lithium polysulfides Li2S n (8 ≤ n ≤ 3) and lower order insoluble sulfides Li2S2/Li2S in liquid. The poor conductivity of the intermediates formed and their structural changes result in high polarization. Also, the consecutive shuttling of the soluble polysulfides between the anode and cathode results in reoxidation at the cathode and further leads to capacity fading of the Li-S battery. To mitigate these, researchers have developed several types of conducting carbon matrix i.e., acetylene black, super P, carbon nanotubes, graphene, carbon paper etc. However, due to poor interaction of polar polysulfides with non-polar carbon matrix, polysulfide migrates into the electrolyte and increases the shuttle effect. Metal oxides (TiO2, Fe2O3 , MgO, Co3O4, SiO2) based interlayer though offer abundant polar active sites for polysulfides adsorption, but suffers from low electrical conductivity and high binding energy between oxides and sulphides, which results degradation in the electrochemical performance. Hence optimization of oxide interlayers are necessary to increase the performance of Li-S batteries.So, in our work, further to minimize the LPS confinement and conversion, we have proposed an efficient cathode where partially exfoliated carbon nanotubes are used as sulfur host. Due to the high surface area, and high electrical conductivity, it serves as an efficient cathode material with high sulfur loading (>75%) and results in high sulfur utilization. In addition, we have also considered the effect of polar sites and designed Fe2O3/NC nanoparticles coated on the polytetrafluoroethylene ethylene coated carbon paper (GDL paper) as interlayer between separator and cathode, which can efficiently adsorb the polysulphides and effectively mitigates the shuttling effect due to the active polar sites and oxygen vacancy. From our electrochemical studies, we have observed with PECNT/S cathode without interlayer initial discharge capacity of 900 mAh/g can be obtained at 0.1 C rate. However, with only GDL interlayer this capacity increases to ~1400 mAh/g. Further, on coating of Fe2O3/NC nanoparticles on GDL paper the capacity can further be enhanced and reached to 1610 mAh/g. For GDL-Fe2O3/NC interlayer better reversibility and faster charge transfer kinetics are confirmed from the CV curves. Cyclic stability of 500 cycles is carried out for PECNT/S, PECNT/S/GDL and PECNT/S/GDL- Fe2O3/NC at 0.5 C rate. ~80% of initial capacity retention can be obtained even after 500 cycles. This substantiate that with PECNT/S and GDL-Fe2O3/NC as cathode and interlayer respectively can synergistically result in a high capacity and stable lithium-sulfur batteries.

Exploring anchoring performance of defective MgO nanotubes for lithium–sulphur batteries: A density functional theory (DFT) study

AbstractIn recent years, lithium–sulphur batteries have received great attention due to their high theoretical specific capacity. In the exploration of improving battery performance, developing suitable anchoring materials is one of the ways to suppress the shuttle effect which is one of the main problems of lithium–sulphur batteries. In this work, we investigated the anchoring ability of MgO nanotubes (MgONT) and defective MgO nanotubes (MgONTv) to lithium polysulphides (LiPSs) by density functional theory (DFT). The defect formation energy, the HOMO and LUMO of the defective MgONT, the energy difference (ΔE), and the adsorption energy were calculated. The optimized structures of LiPSs adsorbed on the MgONT and MgONTv were also obtained. The calculation results show that MgONTv1 has a strong adsorption effect on LiPSs, and its adsorption energy ranges from −1.78 to −4.51 eV. The adsorption of LiPSs narrows the bandgap of MgONTv. In other words, the conductivity of MgONTv is better than the pristine one. Our study demonstrates that MgONTv has more robust adsorption performance for LiPSs, which is an effective addictive material for the cathode of lithium–sulphur batteries. It can provide a theoretical basis for exploring the application of new one‐dimensional anchoring materials in the cathode of lithium–sulphur batteries.

Perspective on ultramicroporous carbon as sulphur host for Li–S batteries

This article provides a comprehensive insight into the state of sulphur infused in ultramicroporous carbon (UMC), its electrochemical reaction mechanism with Li, and clarifies the role of SEI on the reactivity of UMC-S composites. Lithium–sulphur (Li–S) batteries are currently considered as next-generation battery technology. Sulphur is an attractive positive electrode for lithium metal batteries, mainly due to its high capacity (1675 mAh g −1 ) and high specific energy (2600 Wh kg −1 ). The electrochemical reaction of lithium with sulphur in non-aqueous electrolytes results in the formation of electrolyte soluble intermediate lithium–polysulphides. The dissolved polysulphides shuttle to the anode and get reduced at the anode resulting in Li metal corrosion. The solubility of polysulphide gradually reduces the amount of sulphur in the cathode, thereby limiting the cycle life of Li–S batteries. Several strategies have been proposed to improve the cycling stability of Li–S batteries. A unique approach to eliminate the polysulphide shuttle is to use ultramicroporous carbon (UMC) as a host for sulphur. The pore size of UMC which is below 7 Å, is the bottleneck for carbonate solvents to access sulphur/polysulphides confined in the pores, thereby preventing the polysulphide dissolution. This perspective article will emphasise the role of UMC host in directing the lithiation mechanism of sulphur and in inhibiting polysulphide dissolution, including the resulting parasitic reaction on the lithium anode. Further, the challenges that need to be addressed by UMC-S based Li–S batteries, and the strategies to realise high power density, high Coulombic efficiency, and resilient Li–S batteries will be discussed.

VN/S Nanoclusters Encapsulated with Graphene via Zeta Potential Control: A Pomegranate‐Like Cathode for Lithium‐Sulfur Batteries with Enhanced Rate Performance

AbstractA pomegranate‐like cathode, VN/S@G, is synthesized according to a simple principle of electrostatic attraction through a controllable Zeta potential method, establishing a hierarchical‐structured VN/S nanoclusters encapsuled with graphene nanosheets. Internal VN nanoparticles trap lithium polysulphides (LiPSs) and catalyse them transforming from long‐chain to short‐chain species; whereas the external cladding layers of graphene nanosheets confine the transformations in a nanoscale‐catalysis reactors. VN/S@G cathode exhibits excellent long‐cycling life at 2 C rate during the 2000 cycles, corresponding to 0.038 percent of capacity fade per cycle. According to in‐situ Raman and electrochemical impedance spectroscopies, VN catalyst accelerates chemical transformations of liquid‐state LiPSs to solid‐state Li2S2/Li2S and graphene intensifies Li+ diffusion behaviour. Improvement of electrochemical performance of the VN/S@G cathode depends on a coefficient of physical and chemical interactions between VN catalyst and LiPSs species.

An efficient bi-functional permselective separator coated with cubic type-Li7La3Zr2O12and activated carbon for lithium–sulfur batteries

Controlling the shuttling of lithium polysulphides (LiPS) to enhance the utilization of elemental sulfur has been considered as an important issue to improve the energy density and coulombic efficiency of lithium–sulfur batteries.

Nitrogen-doped hierarchical porous carbon derived from low-cost biomass pomegranate residues for high performance lithium‑sulfur batteries

A low-cost hierarchical porous carbon (HPC) derived from pomegranate residues has been prepared via facile carbonization (at 600 °C, 700 °C and 800 °C) and activation, and applied as a sulfur host for lithium‑sulfur batteries. The rich protein in pomegranate residues provides nitrogen sources for the N-doped HPC. The influence of carbonization temperatures on the microstructure and performance of HPC has been investigated. In comparison to the HPC-700 and HPC-800, the HPC-600 sample has the highest BET surface area of 2064.32 m2 g−1 and pore volume of 1.12 cm3 g−1. When tested in LiS batteries, the as-prepared HPC/S-600 cathode with loading sulfur content of 71.54 wt% shows a high initial discharge capacity of 1010 mAh g−1 at 0.1C，and the reversible capacity retains 550 mAh g−1 after 500 cycles. The high cycling stability of HPC/S electrode is attributed to the synergistic effect of hierarchical porous structure and N-doping, which can provide strong physical and chemical confinement to sulfur and the soluble lithium polysulphides.

Enhanced electrochemical performance of lithium–sulphur battery by negating polysulphide shuttling and interfacial resistance through aluminium nanolayer deposition on a polypropylene separator

Shuttling of lithium polysulphides and the formation of lithium dendrites are the key challenges that impede the commercialization of high-capacity lithium–sulphur batteries. Here, we report on a binder-free novel aluminium nanolayer–coated polypropylene separator prepared by radio frequency magnetron sputtering technique for shuttle-free lithium metal battery applications. A symmetric cell with a modified separator exhibits smooth lithium striping and plating cycles at higher current density under room temperature conditions for an extended time period of 1550 h, suggesting better safety and protection of lithium anode from dendrite formation. Coin cells assembled with sulphurized cathode and Li metal as anode in a liquid electrolyte medium using a surface-modified separator display significantly improved specific initial discharge capacity with reasonable reversible discharge capacity and high Coulombic efficiency after 100 galvanostatic cycles at a cycling rate of 0.1 C. This result suggests that the aluminium nanolayer–coated separator can effectively help in reduction of polysulphide crossover during the electrochemical process.

Concentrated Electrolytes for High-Energy Metallic Li Batteries

Liquid electrolyte plays a key role in commercial lithium batteries to allow conduction of lithium ion between cathode and anode. Traditionally, taking into account the ionic conductivity, viscosity and dissolubility of lithium salt, the salt concentration in most of liquid electrolytes is typically less than 1.2 mol/L. We propose a new class of “Solvent-in-Salt” (SIS) electrolytes with high salt concentration, in which salt holds a dominant position in the lithium ion transport system. It remarkably enhances cyclic and safety performance of the next-generation high-energy rechargeable lithium batteries via an effective suppression of lithium dendrite and shape change of metallic lithium anode. First, when used in lithium-sulfur battery, the power of this electrolyte is further demonstrated that lithium polysulphide dissolution is inhibited, thus overcoming one of the most challengeable technological hurdles: “polysulphide shuttle phenomenon”. Consequently, a Coulombic efficiency nearing 100% and long cycling stability are achieved. Second, when used in lithium-NCA pouch cell, the ca. 3Ah cell exhibits over 150 cycles.

Performance enhancement of lithium-polysulphide batteries by atomic layer deposition of lithium tantalate on sulphide solid electrolytes

Lithium sulphur technology for energy storage is considered to be one of the most promising of emerging technologies due to its high theoretical capacity, but the performance of these batteries is limited by problems such as dissolution of polysulphide species in electrolyte, lithium dendrite formation. Previously we demonstrated the capability of Li6PS5Cl, Li6PS6Br and Li10GeP2S12 anode-protecting solid electrolyte membranes to mitigate these problems. To further enhance the stability of these solid electrolyte membranes, here we demonstrate that atomic layer deposition of a thin conformal coating of lithium tantalate leads to lithium polysulphide semi-flow batteries with superior performance.

Ultrathin Sheets of MoS2/g-C3N4 composite As a Hosting Material of Sulphur for Lithium-Sulphur Batteries with Long Cycle Life and High Rate Capability

High energy density rechargeable batteries are the future of energy storage systems. Lithium ion batteries have brought revolutionary changes in the portable electronic devices for more than thirty years. However, the presently available lithium-ion batteries (LIBs) that follow intercalation-deintercalation mechanism cannot fulfil the energy demand of emerging electric vehicles (EVs), hybrid EVs, and next generation portable electronic devices. Therefore, alternative chemistry of lithium-ion batteries has drawn tremendous attention to overcome this bottleneck of high energy demand. Lithium-sulphur batteries holds enormous potential to meet the future energy demands because of its high theoretical specific capacity (1675mAh/g) and cost effectiveness [1]. The specific energy density of lithium-sulphur battery has a theoretical value of 2567Wh/kg, which is significantly higher than the commercially available lithium-ion batteries (387Wh/kg for LiCoO2 with graphite anode) [1]. In spite of the tremendous potential, lithium-sulphur battery systems have some challenging issues hindering its practical usage. The major challenge is the “shuttle effect” of higher order lithium polysulphides (Li2Sn, 4<n<8) between cathode and anode. This is derived from the dissolution of polysulphides in the organic electrolyte. The shuttle effect results in severe capacity loss, poor rate performance and low coulombic efficiency. Another challenge is the high electrical resistivity of elemental sulphur (1015 Ω-m) that prevents the direct utilization of sulphur as an electrode material. In the present study, MoS2/g-C3N4 nanosheets are utilized to chemically confine the lithium polysulphides. Both MoS2 and graphitic carbon nitride (g-C3N4) will provide anchoring sites for lithium polysulphides leading to superior cycling and rate performance. X-Ray photon spectroscopy (XPS) measurements are performed to investigate the chemical composition and nitrogen content of MoS2/g-C3N4 nanosheets. The N1s spectrum shows three nitrogen components, with the most prominent one being pyridinic-N (398.9 eV) and the other two pyrrolic-N (400.1 eV) and graphitic-N (401.3 eV), respectively [2-4]. The pyridinic-N is the major one accounting for more than half of the overall nitrogen content (42%). The C1s spectra show three deconvoluted peaks that could be assigned to graphite like sp2 carbon (284.8 eV), C-N (285.1 eV) and O-C=O (290.2 eV), respectively [3-4]. The Mo3d spectrum shows the expected Mo3d5/2 (229.8 eV), Mo3d3/2 (232.9 eV) and Mo+6 species (236.1 eV) as commonly observed in partially oxidized MoS2 complexes [5-6]. The S2p spectrum splits into four peaks located at 162.3, 163.4, 163.9, 169.0 eV, which are assigned to S2p3/2, S2p1/2, S-2, and S+4, respectively [5-6]. To our knowledge, this is the first example of utilizing MoS2/g-C3N4 composite as a sulphur host for high performance lithium sulphur batteries. Previously, Ghazi and co-workers[7] had utilized MoS2/Celgard separator as a polysulphide barrier which delivered a reversible capacity of 401mAh/g at 0.5C after 600 cycles with 0.083% capacity decay/cycle and an initial discharge capacity of 550mAh/g at 1C rate. Recently Yu and co-workers [8] reported the utilization of ultrathin mesoporous graphitic carbon frameworks as a sulphur host which showed improved cycling performance with 0.044% capacity decay/ cycle after 400 cycles at 2C and good rate capability by delivering initial discharge capacity of 430mAh/g at 6C. Electrochemical testing of MoS2/g-C3N4 composite showed remarkable cycling and rate performance by delivering specific discharge capacity of around 674, 612 and 510mAh/g at 2, 4 and 8C rate with capacity retention of 80% (0.049% decay/cycle), 77%(0.057% decay/cycle) and 88%(0.03% decay/cycle)respectively, after 400 cycles as shown in the figure. This work demonstrates the development of flexible 2D cathode materials beyond graphene for superior performance of lithium-sulphur batteries.

Two‐dimensional boron as an impressive lithium‐sulphur battery cathode material

β12-borophene is a recently fabricated two-dimensional (2D) allotrope of boron that has many unique electrical, mechanical and chemical properties that make it an excellent candidate as a cathode material in lithium sulphur (Li-S) batteries. Ab initio calculations were performed to determine β12-borophene's applicability as a Li-S battery cathode material. Binding energies of sulphur (S8) and various lithium polysulphides (Li2Sx, x=2,3,4,6,8) on the β12-borophene surface were calculated, along with the diffusion barriers for Li2S6 and Li2S4. Our calculations suggest a moderate adsorption of sulphur and a strong adsorption of all lithium polysulphides, implying β12-borophene would sufficiently inhibit the dissolution of lithium polysulphides into the electrolyte, preventing the “shuttle effect”. Through an exhaustive adsorption site sampling the binding energies were determined to be −1.19eV for S8, −2.98eV for Li2S8, −2.77eV for Li2S6, −3.08eV for Li2S4, −3.48eV for Li2S3, −3.79eV for Li2S2, and −4.57eV for Li2S. The diffusion barriers for Li2S6 and Li2S4 were determined to be moderate, ensuring efficient diffusion, with the lowest diffusion energy barriers being 0.61eV and 0.99eV for Li2S6 and Li2S4, respectively. The strong binding energies and moderate diffusion barriers suggest β12-borophene would be an excellent lithium polysulphide immobilizer in Li-S cathodes.

A carbon nanofiber@mesoporous δ-MnO2 nanosheet-coated separator for high-performance lithium-sulfur batteries

Hollow carbon nanofiber@mesoporous δ-MnO2 nanosheets (HCNF@pδ-MnO2) with large surface area composite are prepared by a facile hydrothermal method and decorated on the celgard separator as lithium polysulphides reservoir for lithium-sulfur (Li-S) batteries. In this structure, the bifunctional HCNF provides excellent electrical conductivity and physical confinement of polysulfides. The porous δ-MnO2 coated on the wall HCNF provides more adsorption sites of polysulfides due to the large surface area. Due to the synergistic effect between HCNF and mesoporous δ-MnO2, the Li-S batteries with simple cathode (sulfur content reach 70wt%) structure and HCNF@δ-MnO2 (coating thickness ≈ 2 μm) modified separator delivers an initial capacity of 1156mAhg−1 at 0.2C and a reversible capacity of 856.1mAhg−1 after 200 cycles at 0.5C. This result provides a simple and scalable way to improve the electrochemical property of Li-S batteries.

A Series of Unique Carbon Materials for Lithium-Sulphur Batteries

Lithium-sulphur (Li-S) cells are attractive electrochemical energy storage systems because of the high theoretical capacity of 1675 mAh g-1of sulphur. But low active material utilization, poor cycle life and low rate performance hinder their practicality, which arising from the highly insulating nature of sulphur, the high solubility/diffusivity of lithium polysulphides in the organic electrolyte and volumetric expansion of sulphur during lithiation. Carbon materials with high specific surface area and the porous structure are beneficial for uniform loading and distribution of sulphur in the structure to improve the reaction of sulphur in the cathode. It is important to maintain the electrical conductivity network of carbon providing envelope for lithium sulphide reduction and oxidation within a liquid solvent electrolyte without loss of fluidity, while minimising shuttling away of sulphur and avoiding or controlling the deposition of insoluble sulphides which can block the conducting paths. In addition, porous space could accommodate the volume variation of sulphur. Therefore, sulphur-carbon composites, using porous carbon have become a key research topic in Li-S batteries. Herein, the design and preparation of a series of unique carbon materials, based on metal organic framework (MOF)-derived hierarchical porous carbon, graphene-wrapped microporous carbon composite, few-layer graphene foam and carbon nanotube forest, have been demonstrated for sulphur loading to fabricate cathode structures for Li-S batteries. The improved electrochemical performance provides novel approaches to designing and fabricating long cycle life cathodes in Li-S cells. Reference 1. Nano Energy, 2015, 12, 538-546. 2. Nanoscale, 2014, 6, 5746-5753. 3. APL Materials, 2014, 2(12), 124109. 4. Chemical Communications , 2013, 49, 2192-2194.