Rational Design of Low Cost and High Energy Lithium Batteries through Tailored Fluorine-free Electrolyte and Nanostructured S/C Composite

Polysulfide Speciation and Migration in Catholyte Lithium-Sulfur Cells

Semi-liquid catholyte Lithium-Sulfur (Li-S) cells have shown to be a promising path to realise high energy density energy storage devices. In general, Li-S cells relies on the conversion of elemental sulfur to soluable polysulfide species. In the case of catholyte cells, the active material is added through polysulfide species dissolved in the electrolyte. Herein, we use operando Raman spectroscopy to track the speciation and migration of polysulfides in the catholyte to shed light on the processes taking place. Combined with ex-situ surface and electrochemical analysis we show that the migration of polysulfides is central in order to maximise the performance in terms of capacity (active material utilization) as well as interphase stability on the Li-metal anode during cycling. More specifically we show that using a catholyte where the polysulfides have the dual roles of active material and conducting species, e.g. no traditional Li-salt (such as LiTFSI) is present, results in a higher mobility and faster migration of polysulfides. We also reveal how the formation of long chain polysulfides in the catholyte is delayed during charge as a result of rapid formation and migration of shorter chain species, beneficial for reaching higher capacities. However, the depleation of ionic species during the last stage of charge, due to the conversion to and perciptaion of elemental sulfur on the cathode support,  results in polarization of the cell before full conversion can be achieved.

Sandwiching Sulfur into the Dents Between N, O Co-Doped Graphene Layered Blocks with Strong Physicochemical Confinements for Stable and High-Rate Li-S Batteries

The development of lithium-sulfur batteries (LSBs) is restricted by their poor cycle stability and rate performance due to the low conductivity of sulfur and severe shuttle effect. Herein, an N, O co-doped graphene layered block (NOGB) with many dents on the graphene sheets is designed as effective sulfur host for high-performance LSBs. The sulfur platelets are physically confined into the dents and closely contacted with the graphene scaffold, ensuring structural stability and high conductivity. The highly doped N and O atoms can prevent the shuttle effect of sulfur species by strong chemical adsorption. Moreover, the micropores on the graphene sheets enable fast Li⁺ transport through the blocks. As a result, the obtained NOGB/S composite with 76 wt% sulfur content shows a high capacity of 1413 mAh g^-1 at 0.1 C, good rate performance of 433 mAh g^-1 at 10 C, and remarkable stability with 526 mAh g^-1 at after 1000 cycles at 1 C (average decay rate: 0.038% per cycle). Our design provides a comprehensive route for simultaneously improving the conductivity, ion transport kinetics, and preventing the shuttle effect in LSBs.

A Highly Conductive MOF of Graphene Analogue Ni3 (HITP)2 as a Sulfur Host for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been considered as one of the most promising energy storage systems owing to their high theoretical capacity and energy density. However, their commercial applications are obstructed by sluggish reaction kinetics and rapid capacity degradation mainly caused by polysulfide shuttling. Herein, the first attempt to utilize a highly conductive metal-organic framework (MOF) of Ni₃ (HITP)₂ graphene analogue as the sulfur host material to trap and transform polysulfides for high-performance Li-S batteries is made. Besides, the traditional conductive additive acetylene black is replaced by carbon nanotubes to construct matrix conduction networks for triggering the rate and cycling performance of the active cathode. As a result, the S@Ni₃ (HITP)₂ with sulfur content of 65.5 wt% shows excellent sulfur utilization, rate performance, and cyclic durability. It delivers a high initial capacity of 1302.9 mAh g^-1 and good capacity retention of 848.9 mAh g^-1 after 100 cycles at 0.2 C. Highly reversible discharge capacities of 807.4 and 629.6 mAh g^-1 are obtained at 0.5 and 1 C for 150 and 300 cycles, respectively. Such kinds of pristine MOFs with high conductivity and abundant polar sites reveal broad promising prospect for application in the field of high-performance Li-S batteries.

Designing a Safe Electrolyte Enabling Long-Life Li/S Batteries

Lithium-sulfur (Li/S) batteries suffer from "shuttle" reactions in which soluble polysulfide species continuously migrate to and from the Li metal anode. As a consequence, the loss of active material and reactions at the surface of Li limit the practical applications of Li/S batteries. LiNO₃ has been proposed as an electrolyte additive to reduce the shuttle reactions by aiding the formation of a stable solid electrolyte interphase (SEI) at the Li metal, limiting polysulfide shuttling. However, LiNO₃ is continuously consumed during cycling, especially at low current rates. Therefore, the Li/S battery cycle life is limited by the LiNO₃ concentration in the electrolyte. In this work, an ionic liquid (IL) [N-methyl-(n-butyl)pyrrolidinium bis(trifluoromethylsulfonyl)imide] was used as an additive to enable longer cycle life of Li/S batteries. By tuning the IL concentration, an enhanced stability of the SEI and lower flammability of the solutions were demonstrated, that is, higher safety of the battery. The Li/S cell built with a high sulfur mass loading (4 mg cm^-2 ) and containing the IL-based electrolyte demonstrated a stable capacity of 600 mAh g^-1 for more than double the number of cycles of a cell containing LiNO₃ additive.

Polysulfide Trapping in Carbon Nanofiber Cloth/S Cathode with a Bifunctional Separator for High-Performance Li-S Batteries

The development of carbon nanofiber (CNF) cloth-based cathodes is essential for the fabrication of high energy density and flexible Li-S batteries. Surface modification is generally used to improve the electrochemical performance of CNF cloth/S cathodes. However, this strategy creates some problems such as structure collapse, complex fabrication steps, and poor consistency. Herein, a β-MnO₂ nanowire/graphene-modified separator is used to improve the performance of CNF cloth/S cathodes without changing their structure. β-MnO₂ can facilitate chemical bonding with polysulfides, whereas graphene can decrease the inner resistance and trap polysulfide by physical shielding. The cathode with the bifunctional separator displayed a high discharge capacity of 529.9 mAh g^-1 with a low capacity decay of 0.051 % per cycle after 500 cycles at 1 C, which is 3 times higher compared with a bare separator. Even with a high sulfur loading of 9.0 mg cm^-2 , a high areal capacity of 3.8 mAh cm^-2 was delivered over 100 cycles.

Fluorine-Enriched Graphdiyne as an Efficient Anode in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with low energy density or power density owing to the lack of active storage sites and ion diffusion limitation. In this study, fluorine-enriched graphdiyne (F-GDY) is prepared by a solvothermal reaction. Owing to the 42-C hexagonal porous structure, abundant sp and sp² hybrid carbon atoms, and even distribution of fluorine, F-GDY has enormous potential as an anode for lithium-ion storage. The outstanding rate performance (1825.9 mAh g^-1 at 0.1 A g^-1 , 979.2 mAh g^-1 at 5 A g^-1 ) and stable cycling stability of F-GDY in the lithium-ion battery inspire the assembly of a LIC with F-GDY as an anode and activated carbon (AC) as a cathode. When the AC/F-GDY mass ratio is 7:1, the LIC gives the largest energy density of 200.2 Wh kg^-1 , corresponding to a power density of 131.17 W kg^-1 . This LIC also shows excellent long-term cycling stability with a retention of approximately 80 % after 5000 cycles at 2 A g^-1 and a retention of more than 80 % after 6000 cycles at 5 A g^-1 .