Surface-coated AlF3 nanolayers enable polysulfide confinement within biomass-derived nitrogen-doped hierarchical porous carbon microspheres for improved lithium-sulfur batteries.

The electrically insulating and volumetric deformation of sulfur and the shuttle effect of the intermediate lithium polysulfide (LiPSs) have severely hindered the development of lithium-sulfur batteries (LSBs). Herein, a synergistic strategy of hierarchical porous nitrogen-doped carbon microspheres (PNCM) derived from low-cost biomass with surface-coated AlF3 nanolayer as a multifunctional sulfur host (denoted as PNCM@S@AlF3) was developed. The PNCM not only possesses an abundant pore structure, large surface area, and high electrical conductivity but also features an intrinsic N-doped and fluorinated framework, which effectively enhances the physical adsorption and chemical anchoring to LiPSs. In addition, the AlF3 nanolayer protects the open surface of the porous carbon to isolate sulfur species from the electrolyte to reduce irreversible losses while accelerating the redox kinetics of LiPSs through strong polar adsorption and bonding. Hence, the PNCM@S@AlF3 cathode exhibits an initial capacity as high as 1176.2 mAh/g at 0.2C, and the cycling stability and rate capability are superior to that of PNCM@S without AlF3 coating. Impressively, the PNCM@S@AlF3 cathode delivers stable long-term cycling performance at a high rate of 2C, with 95.6% capacity retention after 500 cycles. This work presents a facile, sustainable, and efficient synergistic strategy for developing advanced LSBs.

Revealing the Mechano-Electrochemical Coupling Behavior and Discharge Mechanism of Fluorinated Carbon Cathodes toward High-Power Lithium Primary Batteries.

Unclear reaction mechanisms and unsatisfactory power performance hinder the further development of advanced lithium/fluorinated carbon (Li/CFx ) batteries. Herein, the mechano-electrochemical coupling behavior of a CFx cathode is investigated by in situ monitoring strain/stress using digital image correlation (DIC) techniques, electrochemical methods, and theoretical equations. The DIC monitoring results present the distribution and dynamic evolution of the plane strain and indicate strong dependence toward the material structure and discharge rate. The average plane principal strain of fully discharged 2D fluorinated graphene nanosheets (FGNSs) at 0.5 C is 0.50%, which is only 38.5% that of conventional bulk-structure CFx . Furthermore, the superior structural stability of the FGNSs is demonstrated by the microstructure and component characterization before and after discharge. The plane stress evolution is calculated based on theoretical equations, and the contributions of electrochemical and mechanical factors are examined and discussed. Subsequently, a structure-dependent three-region discharge mechanism for CFx electrodes is proposed from a mechanical perspective. Additionally, the surface deformation of Li/FGNSs pouch cells formed during the discharge process is monitored using in situ DIC. This study reveals the discharge mechanism of Li/CFx batteries and facilitates the design of advanced CFx materials.

Surface Engineering of Fluorinated Graphene Nanosheets Enables Ultrafast Lithium/Sodium/Potassium Primary Batteries.

Fluorinated carbon (CFx ) is considered as a promising cathode material for lithium/sodium/potassium primary batteries with superior theoretical energy density. However, achieving high energy and power densities simultaneously remains a considerable challenge due to the strong covalency of the C-F bond in the highly fluorinated CFx . Herein, an efficient surface engineering strategy combining surface defluorination and nitrogen doping enables fluorinated graphene nanosheets (DFG-N) to possess controllable conductive nanolayers and reasonably regulated C-F bonds. The DFG-N delivers an unprecedented dual performance for lithium primary batteries with a power density of 77456 W kg-1 and an energy density of 1067 Wh kg-1 at an ultrafast rate of 50 C, which is the highest level reported to date. The DFG-N also achieves a record power density of 15 256 and 17 881 W kg-1 at 10 C for sodium and potassium primary batteries, respectively. The characterization results and density functional theory calculations demonstrate that the excellent performance of DFG-N is attributed to surface engineering strategies that remarkably improve electronic and ionic conductivity without sacrificing the high fluorine content. This work provides a compelling strategy for developing advanced ultrafast primary batteries that combine ultrahigh energy density and power density.

Ultrafast Li/Fluorinated Graphene Primary Batteries with High Energy Density and Power Density.

Lithium/fluorinated carbon (Li/CFx) primary batteries have essential applications in consumer electronics and medical and high-power military devices. However, their application is limited due to the difficulty in achieving simultaneous high power density and high energy density in the CFx cathode. The tradeoff between conductivity and fluorine content is the decisive factor. Herein, by rational design, 3D porous fluorinated graphene microspheres (FGS-x) with both high conductivity and a high F/C ratio are successfully synthesized for the first time. FGS-x possesses an F/C ratio as high as 1.03, a nanosheet structure with hierarchical pores, abundant C═C bonds, few inactive C-F2 bonds, and electrochemically active C-F bonds. The beneficial features that can increase discharge capacity, shorten the diffusion length for both ions and electrons, enhance the Li+ intercalation kinetics, and accommodate the volume change are demonstrated. The Li/FGS-1.03 coin cell delivers an unprecedented power density of 71,180.9 W/kg at an ultrahigh rate of 50 C (43.25 A/g), coupled with a high energy density of 830.7 Wh/kg. Remarkably, the Li/FGS-1.03 pouch cell exhibits a record cell-level power density of 12,451.2 W/kg at 20 C. The in-depth investigation by the ex situ method on structural evolution at different discharge depths reveals that the excellent performance benefits from the structural stability and the uniform formation of LiF. The FGS-1.03 cathode also has excellent performance in extreme operating temperatures (0 to 100 °C) and high active material mass loading (4.3 mg/cm2). These results indicate that the engineered fluorinated graphene developed here has great potential in applications requiring both high power density and high energy density.

Accordion-Like Fluorinated Graphite Nanosheets with High Power and Energy Densities for Wide-Temperature, Long Shelf-Life Sodium/Potassium Primary Batteries.

Sodium and potassium are considered to be the most promising anode candidates due to their easy availability, low-cost and similar chemical properties to lithium. Here, novel 3D accordion-like fluorinated graphite nanosheets (FGNSs) are reported as cathodes for sodium primary batteries (SPBs) and potassium primary batteries (PPBs). The FGNSs-x cathode exhibits unprecedented power and energy density due to the impressive 3D structure, high F/C ratio (1.0), and more surface CC bonds (7.14%). The FGNSs-1.0 exhibits very high specific capacities of 831.3 and 834.1 mAh g-1 for SPBs and PPBs, respectively, close to the theoretical capacity. Besides, the maximum energy density of FGNSs-1.0 in SPBs and PPBs are 1960.5 and 2144.6 Wh kg-1 , respectively. The maximum power density for Na/CFx and K/CFx batteries could reach up to 7076.8 and 6227.4 W kg-1 , respectively. The electrochemical performance of FGNSs-1.0 at extreme temperatures (-30 to 100 °C), long storage time (60 days), high mass loading (3.6 mg cm-2 ), and pouch-type cell is also evaluated for the first time. Surprisingly, FGNSs-1.0 has outstanding performance in these projects. Therefore, the new-type Na/CFx and K/CFx primary battery systems developed here have broad application prospects in high-energy applications that require high-power, low-cost, and normal use under extreme conditions.

Stable Water Oxidation in Acid Using Manganese-Modified TiO2 Protective Coatings.

Accomplishing acid-stable water oxidation is a critical matter for achieving both long-lasting water-splitting devices and other fuel-forming electro- and photocatalytic processes. Because water oxidation releases protons into the local electrolytic environment, it becomes increasingly acidic during device operation, which leads to corrosion of the photoactive component and hence loss in device performance and lifetime. In this work, we show that thin films of manganese-modified titania, (Ti,Mn)O x, topped with an iridium catalyst, can be used in a coating stabilization scheme for acid-stable water oxidation. We achieved a device lifetime of more than 100 h in pH = 0 acid. We successfully grew (Ti,Mn)O x coatings with uniform elemental distributions over a wide range of manganese compositions using atomic layer deposition (ALD), and using X-ray photoelectron spectroscopy, we show that (Ti,Mn)O x films grown in this manner give rise to closer-to-valence-band Fermi levels, which can be further tuned with annealing. In contrast to the normally n-type or intrinsic TiO2 coatings, annealed (Ti,Mn)O x films can make direct charge transfer to a Fe(CN)63-/4- redox couple dissolved in aqueous electrolytes. Using the Fe(CN)63-/4- redox, we further demonstrated anodic charge transfer through the (Ti,Mn)O x films to high work function metals, such as iridium and gold, which is not previously possible with ALD-grown TiO2. We correlated changes in the crystallinity (amorphous to rutile TiO2) and oxidation state (2+ to 3+) of the annealed (Ti,Mn)O x films to their hole conductivity and electrochemical stability in acid. Finally, by combining (Ti,Mn)O x coatings with iridium, an acid-stable water-oxidation anode, using acid-sensitive conductive fluorine-doped tin oxides, was achieved.