Identification and Catalysis of the Potential-Limiting Step in Lithium-Sulfur Batteries.

The Li-S chemistry is thermodynamically promising for high-density energy storage but kinetically challenging. Over the past few years, many catalyst materials have been developed to improve the performance of Li-S batteries and their catalytic role has been increasingly accepted. However, the classic catalytic behavior, i.e., reduction of reaction barrier, has not been clearly observed. Crucial mechanistic questions, including what specific step is limiting the reaction rate, whether/how it can be catalyzed, and how the catalysis is sustained after the catalyst surface is covered by solid products, remain unanswered. Herein, we report the first identification of the potential-limiting step of Li-S batteries operating under lean electrolyte conditions and its catalysis that conforms to classic catalysis principles, where the catalyst lowers the kinetic barrier of the potential-limiting step and accelerates the reaction without affecting the product composition. After carefully examining the electrochemistry under lean electrolyte conditions, we update the pathway of the Li-S battery reaction: S8 solid is first reduced to Li2S8 and Li2S4 molecular species sequentially; the following reduction of Li2S4 to a Li2S2-Li2S solid with an almost constant ratio of 1:4 is the potential-limiting step; the previously believed Li2S2-to-Li2S solid-solid conversion does not occur; and the recharging reaction is relatively fast. We further demonstrate that supported cobalt phthalocyanine molecules can effectively catalyze the potential-limiting step. After Li2S2/Li2S buries the active sites, it can self-catalyze the reaction and continue driving the discharging process.

Solar-Driven CO2 Conversion via Optimized Photothermal Catalysis in a Lotus Pod Structure.

Photothermal CO2 reduction is one of the most promising routes to efficiently utilize solar energy for fuel production at high rates. However, this reaction is currently limited by underdeveloped catalysts with low photothermal conversion efficiency, insufficient exposure of active sites, low active material loading, and high material cost. Herein, we report a potassium-modified carbon-supported cobalt (K+ -Co-C) catalyst mimicking the structure of a lotus pod that addresses these challenges. As a result of the designed lotus-pod structure which features an efficient photothermal C substrate with hierarchical pores, an intimate Co/C interface with covalent bonding, and exposed Co catalytic sites with optimized CO binding strength, the K+ -Co-C catalyst shows a record-high photothermal CO2 hydrogenation rate of 758 mmol gcat -1 h-1 (2871 mmol gCo -1 h-1 ) with a 99.8 % selectivity for CO, three orders of magnitude higher than typical photochemical CO2 reduction reactions. We further demonstrate with this catalyst effective CO2 conversion under natural sunlight one hour before sunset during the winter season, putting forward an important step towards practical solar fuel production.

Mechanistic Insights into Fast Charging and Discharging of the Sodium Metal Battery Anode: A Comparison with Lithium.

Na metal anode receives increasing attention as a low-cost alternative to Li metal anode for the application in high energy batteries. Despite extensive research efforts to improve the reversibility and cycle life of Na metal electrodes, their rate performance, i.e. electrochemical plating and stripping of Na metal at high current, is underexplored. Herein, we report that Na metal electrodes, unlike the more widely studied Li metal electrodes which survive high current density up to 20 mA/cm2, cannot be fast charged or discharged in common ether electrolyte. The fast charging, namely metal plating, is comprised by severe side reactions that decompose electrolyte into electrochemically inactive Na(I) solid species. The fast discharging, namely metal stripping, is disabled by local Na removal that deteriorates the electrical contact with the current collector. While the fast charging failure is permanent, the capacity loss from fast discharging can be recovered through a restructuring process at a low discharging current which rebuilds the electrical connection. We further reveal that the unsatisfactory rate performance of Na metal electrodes is associated with intrinsic physicochemical properties of Na. This study delineates the mechanistic origins of Na's limitation in fast plating and stripping, and demonstrates the necessity of improving the charging and discharging rate performance of Na metal electrodes.

High-capacity rechargeable batteries based on deeply cyclable lithium metal anodes.

Discovering new chemistry and materials to enable rechargeable batteries with higher capacity and energy density is of paramount importance. While Li metal is the ultimate choice of a battery anode, its low efficiency is still yet to be overcome. Many strategies have been developed to improve the reversibility and cycle life of Li metal electrodes. However, almost all of the results are limited to shallow cycling conditions (e.g., 1 mAh cm-2) and thus inefficient utilization (<1%). Here we achieve Li metal electrodes that can be deeply cycled at high capacities of 10 and 20 mAh cm-2 with average Coulombic efficiency >98% in a commercial LiPF6/carbonate electrolyte. The high performance is enabled by slow release of LiNO3 into the electrolyte and its subsequent decomposition to form a Li3N and lithium oxynitrides (LiN x Oy)-containing protective layer which renders reversible, dendrite-free, and highly dense Li metal deposition. Using the developed Li metal electrodes, we construct a Li-MoS3 full cell with the anode and cathode materials in a close-to-stoichiometric amount ratio. In terms of both capacity and energy, normalized to either the electrode area or the total mass of the electrode materials, our cell significantly outperforms other laboratory-scale battery cells as well as the state-of-the-art Li ion batteries on the market.

Ultrathin dendrimer-graphene oxide composite film for stable cycling lithium-sulfur batteries.

Lithium-sulfur batteries (Li-S batteries) have attracted intense interest because of their high specific capacity and low cost, although they are still hindered by severe capacity loss upon cycling caused by the soluble lithium polysulfide intermediates. Although many structure innovations at the material and device levels have been explored for the ultimate goal of realizing long cycle life of Li-S batteries, it remains a major challenge to achieve stable cycling while avoiding energy and power density compromises caused by the introduction of significant dead weight/volume and increased electrochemical resistance. Here we introduce an ultrathin composite film consisting of naphthalimide-functionalized poly(amidoamine) dendrimers and graphene oxide nanosheets as a cycling stabilizer. Combining the dendrimer structure that can confine polysulfide intermediates chemically and physically together with the graphene oxide that renders the film robust and thin (<1% of the thickness of the active sulfur layer), the composite film is designed to enable stable cycling of sulfur cathodes without compromising the energy and power densities. Our sulfur electrodes coated with the composite film exhibit very good cycling stability, together with high sulfur content, large areal capacity, and improved power rate.

Solvent Molecule Cooperation Enhancing Lithium Metal Battery Performance at Both Electrodes.

Developing electrolytes compatible with efficient and reversible cycling of electrodes is critical to the success of rechargeable Li metal batteries (LMBs). The Coulombic efficiencies and cycle lives of LMBs with ethylene carbonate (EC), dimethyl carbonate, ethylene sulfite (ES), and their combinations as electrolyte solvents show that in a binary-solvent electrolyte the extent of electrolyte decomposition on the electrode surface is dependent on the solvent component that dominates the solvation sheath of Li+ . This knowledge led to the development of an EC-ES electrolyte exhibiting high performance for Li||LiFePO4 batteries. Carbonate molecules occupy the solvation sheath and improve the Coulombic efficiencies of both the anode and cathode. Sulfite molecules lead to desirable morphology and composition of the solid electrolyte interphase and extend the cycle life of the Li metal anode. The cooperation between these components provides a new example of electrolyte optimization for improved LMBs.

A Highly Efficient All-Solid-State Lithium/Electrolyte Interface Induced by an Energetic Reaction.

The energetic chemical reaction between Zn(NO3 )2 and Li is used to create a solid-state interface between Li metal and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) electrolyte. This interlayer, composed of Zn, ZnLix alloy, Li3 N, Li2 O, and other species, possesses strong affinities with both Li metal and LLZTO and affords highly efficient conductive pathways for Li+ transport through the interface. The unique structure and properties of the interlayer lead to Li metal anodes with longer cycle life, higher efficiency, and better safety compared to the current best Li metal electrodes operating in liquid electrolytes while retaining comparable capacity, rate, and overpotential. All-solid-state Li||Li cells can operate at very demanding current-capacity conditions of 4 mA cm-2 -8 mAh cm-2 . Thousands of hours of continuous cycling are achieved at Coulombic efficiency >99.5 % without dendrite formation or side reactions with the electrolyte.

Surface Chemistry in Cobalt Phosphide-Stabilized Lithium-Sulfur Batteries.

Chemistry at the cathode/electrolyte interface plays an important role for lithium-sulfur batteries in which stable cycling of the sulfur cathode requires confinement of the lithium polysulfide intermediates and their fast electrochemical conversion on the electrode surface. While many materials have been found to be effective for confining polysulfides, the underlying chemical interactions remain poorly understood. We report a new and general lithium polysulfide-binding mechanism enabled by surface oxidation layers of transition-metal phosphide and chalcogenide materials. We for the first time find that CoP nanoparticles strongly adsorb polysulfides because their natural oxidation (forming Co-O-P-like species) activates the surface Co sites for binding polysulfides via strong Co-S bonding. With a surface oxidation layer capable of confining polysulfides and an inner core suitable for conducting electrons, the CoP nanoparticles are thus a desirable candidate for stabilizing and improving the performance of sulfur cathodes in lithium-sulfur batteries. We demonstrate that sulfur electrodes that hold a high mass loading of 7 mg cm-2 and a high areal capacity of 5.6 mAh cm-2 can be stably cycled for 200 cycles. We further reveal that this new surface oxidation-induced polysulfide-binding scheme applies to a series of transition-metal phosphide and chalcogenide materials and can explain their stabilizing effects for lithium-sulfur batteries.

Strong Metal-Phosphide Interactions in Core-Shell Geometry for Enhanced Electrocatalysis.

Rational design of multicomponent material structures with strong interfacial interactions enabling enhanced electrocatalysis represents an attractive but underdeveloped paradigm for creating better catalysts for important electrochemical energy conversion reactions. In this work, we report metal-phosphide core-shell nanostructures as a new model electrocatalyst material system where the surface electronic states of the shell phosphide and its interactions with reaction intermediates can be effectively influenced by the core metal to achieve higher catalytic activity. The strategy is demonstrated by the design and synthesis of iron-iron phosphide (Fe@FeP) core-shell nanoparticles on carbon nanotubes (CNTs) where we find that the electronic interactions between the metal and the phosphide components increase the binding strength of hydrogen adatoms toward the optimum. As a consequence, the Fe@FeP/CNT material exhibits exceptional catalytic activity for the hydrogen evolution reaction, only requiring overpotentials of 53-110 mV to reach catalytic current densities of 10-100 mA cm-2.

High-Performance Sodium Metal Anodes Enabled by a Bifunctional Potassium Salt.

Developing Na metal anodes that can be deeply cycled with high efficiency for a long time is a prerequisite for rechargeable Na metal batteries to be practically useful despite their notable advantages in theoretical energy density and potential low cost. Their high chemical reactivity with the electrolyte and tendency for dendrite formation are two major issues limiting the reversibility of Na metal electrodes. In this work, we introduce for the first time potassium bis(trifluoromethylsulfonyl)imide (KTFSI) as a bifunctional electrolyte additive to stabilize Na metal electrodes, in which the TFSI- anions decompose into lithium nitride and oxynitrides to render a desirable solid electrolyte interphase layer while the K+ cations preferentially adsorb onto Na protrusions and provide electrostatic shielding to suppress dendritic deposition. Through the cooperation of the cations and anions, we have realized Na metal electrodes that can be deeply cycled at a capacity of 10 mAh cm-2 for hundreds of hours.

Electrocatalysis in Lithium Sulfur Batteries under Lean Electrolyte Conditions.

The presence of electrocatalysis in lithium-sulfur batteries has been proposed but not yet sufficiently verified. In this study, molybdenum phosphide (MoP) nanoparticles are shown to play a definitive electrocatalytic role for the sulfur cathode working under lean electrolyte conditions featuring a low electrolyte/active material ratio: the overpotentials for the charging and discharging reactions are greatly decreased. As a result, sulfur electrodes containing MoP nanoparticles show faster kinetics and more reversible conversion of sulfur species, leading to improvements in charging/discharging voltage profiles, capacity, rate performance, and cycling stability. Taking advantage of the electrocatalytic properties of MoP, high-performance sulfur electrodes were successfully realized that are steadily cyclable at a high areal capacity of 5.0 mAh cm-2 with a challenging electrolyte/sulfur (E/S) ratio of 4 µLE mg-1 S .

Ferrocene-Promoted Long-Cycle Lithium-Sulfur Batteries.

Confining lithium polysulfide intermediates is one of the most effective ways to alleviate the capacity fade of sulfur-cathode materials in lithium-sulfur (Li-S) batteries. To develop long-cycle Li-S batteries, there is an urgent need for material structures with effective polysulfide binding capability and well-defined surface sites; thereby improving cycling stability and allowing study of molecular-level interactions. This challenge was addressed by introducing an organometallic molecular compound, ferrocene, as a new polysulfide-confining agent. With ferrocene molecules covalently anchored on graphene oxide, sulfur electrode materials with capacity decay as low as 0.014 % per cycle were realized, among the best of cycling stabilities reported to date. With combined spectroscopic studies and theoretical calculations, it was determined that effective polysulfide binding originates from favorable cation-π interactions between Li+ of lithium polysulfides and the negatively charged cyclopentadienyl ligands of ferrocene.

Rational design of Ni nanoparticles on N-rich ultrathin carbon nanosheets for high-performance supercapacitor materials: embedded- versus anchored-type dispersion.

Highly dispersed Ni nanoparticles (NPs) and abundant functional N-species were integrated into ultrathin carbon nanosheets by using a facile and economical sol-gel route. Embedded- and anchored-type configurations were achieved for the dispersion of Ni NPs in/on N-rich carbon nanosheets. The anchored-type composite exhibited outstanding pseudocapacitance of 2200 F g(-1) at 5 A g(-1) with unusual rate capability and extraordinary cyclic stability over 20 000 cycles with little capacitance decay. Aqueous asymmetric supercapacitors fabricated with this composite cathode demonstrated a high energy density of 51.3 Wh kg(-1) at a relatively large power density of 421.6 W kg(-1) , along with outstanding cyclic stability. This approach opens an attractive direction for enhancing the electrochemical performances of metal-based supercapacitors and can be generalized to design high-performance energy-storage devices.

High-Mass-Loading Li-S Batteries Catalytically Activated by Cerium Oxide: Performance and Failure Analysis under Lean Electrolyte Conditions.

Increasing sulfur mass loading and minimizing electrolyte amount remains a major challenge for the development of high-energy-density Li-S batteries, which needs to be tackled with combined efforts of materials development and mechanistic analysis. This work, following the same team's most recent identification of the potential-limiting step of Li-S batteries under lean electrolyte conditions, seeks to advance the understanding by extending it to a new catalyst and into the high-sulfur-mass-loading region. CeOx nanostructures are integrated into cotton-derived carbon to develop a multifunctional 3D network that can host a large amount of active material, facilitate electron transport, and catalyze the sulfur lithiation reaction. The resulting S/CeOx /C electrode can deliver a stable areal capacity of 9 mAh cm-2 with a high sulfur loading of 14 mg cm-2 at a low electrolyte/sulfur ratio of 5 µL mg-1 . This study discovers that Li||S/CeOx /C cells usually fail during charging at high current density, as a consequence of local short circuiting caused by electrochemically deposited Li dendrites penetrating through the separator, a previously overlooked failure pattern distinctive to cells operating under lean electrolyte conditions. This work highlights the importance of developing new material structures and analyzing failure mechanisms in the advancement of Li-S batteries.

Interface Modulation of Metal Sulfide Anodes for Long-Cycle-Life Sodium-Ion Batteries.

Although studies of transition metal sulfides (TMS) as anode materials for sodium-ion batteries are extensively reported, the short cycle life is still a thorny problem that impedes their practical application. In this work, a new capacity fading mechanism of the TMS electrodes is demonstrated; that is, the parasitic reaction between electrolyte anions (i.e., ClO4 - ) and metal sulfides yields non-conductive and unstable solid-electrolyte interphase (SEI) and meanwhile, corrosively turns metal sulfides into less-active oxides. This knowledge guides the development of an electrochemical strategy to manipulate the anion decomposition and construct a stable interface that prevents extensive parasitic reactions. It is shown that introducing sodium nitrate to the electrolyte radically changes the Na+ solvation structure by populating nitrate ions in the first solvation sheath, generating a stable and conductive SEI layer containing both Na3 N and NaF. The optimized interface enables an iron sulfide anode to stably cycle for over 2000 cycles with negligible capacity loss, and a similar enhancement in cycle performance is demonstrated on a number of other metal sulfides. This work discloses metal sulfides' cycling failure mechanism from a unique perspective and highlights the critical importance of manipulating the interface chemistry in sodium-ion batteries.

A bio-inspired O2-tolerant catalytic CO2 reduction electrode.

The electrochemical reduction of CO2 to give CO in the presence of O2 would allow the direct valorization of flue gases from fossil fuel combustion and of CO2 captured from air. However, it is a challenging task because O2 reduction is thermodynamically favored over that of CO2. 5% O2 in CO2 near catalyst surface is sufficient to completely inhibit the CO2 reduction reaction. Here we report an O2-tolerant catalytic CO2 reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with >95% Faradaic efficiency (FE) for CO2 reduction to CO coated with a polymer of intrinsic microporosity that works as a CO2-selective layer with a CO2/O2 selectivity of ∼20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO2 feed gas containing 5% O2 exhibits a FECO of 75.9% with a total current density of 27.3 mA/cm2 at a cell voltage of 3.1 V. A FECO of 49.7% can be retained when the O2 fraction increases to 20%. Stable operation for 18 h is demonstrated. The electrochemical performance and O2 tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.

Functional metal-organic framework boosting lithium metal anode performance via chemical interactions.

Dendrite growth and low coulombic efficiency are two major factors that limit the utilization of Li metal electrodes in future generations of high-energy-density rechargeable batteries. This article reports the first study on metal-organic framework (MOF) materials for boosting the electrochemical performance of Li metal electrodes and demonstrates the power of molecular-structure functionalization for realizing desirable ion transport and Li metal nucleation and growth. We show that dendrite-free dense Li deposition and stable Li plating/stripping cycling with high coulombic efficiency are enabled by modifying a commercial polypropylene separator with a titanium-based MOF (NH2-MIL-125(Ti)) material. The NH2-MIL-125(Ti)-coated-separator renders Li|Cu cells that can run for over 200 cycles at 1 mA cm-2-1 mA h cm-2 with average coulombic efficiency of 98.5% and Li|Li symmetric cells that can be cycled at 1 mA cm-2-1 mA h cm-2 for more than 1200 h without short circuiting. The superior cycling stability is attributed to the amine substituents in the NH2-MIL-125(Ti) structure which induce increased Li+ transference numbers and uniform and dense early-stage Li deposition.

Coupled Metal/Oxide Catalysts with Tunable Product Selectivity for Electrocatalytic CO2 Reduction.

One major challenge to the electrochemical conversion of CO2 to useful fuels and chemical products is the lack of efficient catalysts that can selectively direct the reaction to one desirable product and avoid the other possible side products. Making use of strong metal/oxide interactions has recently been demonstrated to be effective in enhancing electrocatalysis in the liquid phase. Here, we report one of the first systematic studies on composition-dependent influences of metal/oxide interactions on electrocatalytic CO2 reduction, utilizing Cu/SnOx heterostructured nanoparticles supported on carbon nanotubes (CNTs) as a model catalyst system. By adjusting the Cu/Sn ratio in the catalyst material structure, we can tune the products of the CO2 electrocatalytic reduction reaction from hydrocarbon-favorable to CO-selective to formic acid-dominant. In the Cu-rich regime, SnOx dramatically alters the catalytic behavior of Cu. The Cu/SnOx-CNT catalyst containing 6.2% of SnOx converts CO2 to CO with a high faradaic efficiency (FE) of 89% and a jCO of 11.3 mA·cm-2 at -0.99 V versus reversible hydrogen electrode, in stark contrast to the Cu-CNT catalyst on which ethylene and methane are the main products for CO2 reduction. In the Sn-rich regime, Cu modifies the catalytic properties of SnOx. The Cu/SnOx-CNT catalyst containing 30.2% of SnOx reduces CO2 to formic acid with an FE of 77% and a jHCOOH of 4.0 mA·cm-2 at -0.99 V, outperforming the SnOx-CNT catalyst which only converts CO2 to formic acid in an FE of 48%.