Study On The Reversible Conversion Reaction Of Iron-based Fluoride And Sulfide Cathode Materials

As low-cost and environmentally benign conversion-type cathodes,iron-based fluorides/sulfides offer extremely high theoretical specific capacities and energy/power densities,which are considered as promising alternatives to intercalation-type cathodes for the next-generation rechargeable battery.However,their reversible conversion reaction is hindered by sluggish reaction kinetic involving multiphase interfacial transport and difficult spatial confinement for active species.For pyrite（Fe S₂）cathode,excess conductive framework decoration and loose nanostructure configuration not only compromise the energy storage efficiency at the cell level,but also do not seem to facilitate satisfactory performance,especially for rate capability.Spatial confinement and catalytic activation of S–S moieties are critical factors for sustainable conversion of chalcogenide cathodes.For iron fluoride cathodes,the highest electronegativity of fluorine endows them with higher operational voltages than oxide and sulfide counterparts,but at the cost of intrinsic low electronic conductivity as well as the sluggish kinetics involving repeated splitting and re-bonding of metal-fluorine moieties during the multiphase conversion reaction.Herein,we propose cathode structure modulation and electrolyte design strategies to enable highly reversible conversion reaction of iron-based sulfide and（oxy）fluoride.（1）Thermally sulfurating ionic liquid wrapped open-framework fluorides can obtain pyrite cathodes characterized by compact grain stacking and surface fluorination,which are two crucial factors for their high-rate and long-life conversion reaction.Conductive pyrite accompanied by the in-situ conversion of Fe nanodomain as electron wires enables an intergrain propagation of self-built conductive network via intimately cemented grains.The surface fluorination is potentially promoting the acceleration of Li⁺/Na⁺interfacial transport and consequent conversion propagation between adjacent grain,as well as the reinforcement of interface stability.Benefiting from the reaction kinetic upgrade,Pyrochlore-derivative Fe S₂with more intimate grain interconnection achieves stable long-term cycling with highly reversible capacities of 425 m Ah/g after1000 cycles at 1 C for Li storage and 450 m Ah/g after 1200 cycles at 2 C for Na storage.Its reversible capacities could be preserved at 488 and 249 m Ah/g for Li and Na storage under endurance up to 10 C,respectively.Such a high-rate performance endows the energy densities of Fe S₂ reaching to 800 and 350 Wh/kg for Li and Na cells,respectively,under a power density as high as 10000 W/kg.The attenuation of carbon layer does not compromise electrochemical performance,but is even beneficial for the compact linkage manner of grains.（2）For the high-efficiency utilization of S–S moieties,we propose tightly bonded pyrite and sulfur as a dual active-phase cathode（Fe S₂@S）,which is enabled by thermal sulfuration of polydopamine（PDA）coated iron difluoride.The in-situ derived carbon coating with fluorination decoration contributes to the preservation of the sulfur interlayer sandwiched between Fe S₂ core and carbon shell.The tight bonding of S–S moieties and sulfiphilic Fe-S structured core reinforces the built-in absorption-catalysis effect.The sulfiphobic fluorinated surface act as outer protective layer for further retarding flowing-out of dissoluble polysulfides across the carbon shell into the electrolyte.In addition,the interconnected Fe S₂ grains wrapped by cross-linked carbon network provide continuous charge/mass transfer pathways to promote reaction kinetics.Such modulation of cathode architecture promotes the synergistic conversion of the dual active-phase,which endows the Fe S₂@S cathode a high reversible capacity of1000 m Ah/g for Li-storage and 600 m Ah/g for Na-storage.Fe S₂@S cathode performs stable long-term cycling at 1 C with preserved capacities of 800 m Ah/g after 700 cycles and 370 m Ah/g after 1000 cycles for Li and Na cells,corresponding to cathode energy densities as high as 1200 and 480 Wh/kg,respectively.The rational configuration design with sulfiphilic/sulfiphobic components guarantees the spatial confinement and high-efficiency utilization of S–S moieties.（3）In view of the crucial role of Li F activation on reversible phase transformation of fluoride cathode,we design a solid-liquid“fluorine-channel”enabled by an anion receptor of tris（pentafluorophenyl）borane（TPFPB）additive in ether-based electrolyte to achieve highly reversible conversion reaction of iron oxyfluorides.TPFPB molecule as F^-receptor is able to dissociate inactive Li F and form a solvated F^--intermediate[TPFPB-F]^-at multiphase interface,which could provide a facile F-transport channel between Li F and Fe-species by bypassing the tough solid-solid conversion.The in-situ formation of conformal and fluorinated cathode electrolyte interface（CEI）layer is favorable for cathode chemo-mechanical stability and consequently sustaining electroactive behaviors.Two kinds of iron（oxy）fluoride composites with different oxygen contents(denoted as FeO_0.3F_1.7 and FeO_0.7F_1.3)were synthesized by thermal-induced self-oxygen penetration of the hydrated iron fluoride.The O-doping in fluoride regulates the evolution pathway by introducing a stable second-generation parent phase of rocksalt and extruded phases within a confined voltage region.Benefiting from facile round-trip F-transport along with reversible Li-insertion/extraction in oxyfluoride cathodes,both FeO_0.3F_1.7 and FeO_0.7F_1.3 cathodes achieve sustaining conversion reactions with energy efficiency approaching 80%,high capacity retention of 472 and484 m Ah/g after 100 cycles under 100 m A/g and superior rate capability with reversible capacities of 271 and 320 m Ah/g at the endurance up to 2 A/g,respectively.They perform the superior energy densities of 1100 Wh/kg for FeO_0.3F_1.7 and 700 Wh/kg for FeO_0.7F_1.3 under the power densities of 220 and 4300 W/kg,respectively.（4）For the pursuit of energy storage technology with superior electrochemical performance and high safety,we design a non-flammable electrolyte with the function of stabilizing the highly reversible phase conversion of iron fluoride cathode.The flame-retarding solvents of phosphate TEP and highly-fluorinated ether TTE exhibit highly oxidative stability（>4.5 V）,which ensures the prevention of uncontrollable decomposition for them when against high-voltage fluoride cathode.Non-polar linear TTE molecules are mainly distributed over the outer sheath of solvation structure with Li⁺-TEP coordination,which is beneficial for interfacial wetting and potentially takes a steric hindrance effect on the mitigation of undesired interaction between TEP and lithium metal.The electron-affinity Li DFOB additive and highly-fluorinated TTE solvent promote the derivation of a robust B/F-rich CEI layer on fluoride cathode.The introduction of functional moieties in CEI layer further reinforces the interface stability and potentially reduce the surface defects by Fe/F dissolution or side reaction.Benefiting from the rational manipulation of the nonflammable electrolyte,the iron fluoride（Fe F₃）cathode performs a sustainable and reversible phase transformation with a well-maintained two-stage lithiation plateaus（corresponding to intercalation and conversion process）over 100 cycles.The stable conversion reaction endows Fe F₃cathode with a high capacity retention of 300 m Ah/g after 200 cycles with only 0.016%decay per cycle.