Reducing Gases Triggered Cathode Surface Reconstruction for Stable Cathode-Electrolyte Interface in Practical All-Solid-State Lithium Batteries.

The interfacial compatibility between cathodes and sulfide solid-electrolytes (SEs) is a critical limiting factor of electrochemical performance in all-solid-state lithium-ion batteries (ASSLBs). This work presents a gas-solid interface reduction reaction (GSIRR), aiming to mitigate the reactivity of surface oxygen by inducing a surface reconstruction layer (SRL) . The application of a SRL, CoO/Li2 CO3 , onto LiCoO2 (LCO) cathode results in impressive outcomes, including high capacity (149.7 mAh g-1 ), remarkable cyclability (retention of 84.63% over 400 cycles at 0.2 C), outstanding rate capability (86.1 mAh g-1 at 2 C), and exceptional stability in high-loading cathode (28.97 and 23.45 mg cm-2 ) within ASSLBs. Furthermore, the SRL CoO/Li2 CO3 enhances the interfacial stability between LCO and Li10 GeP2 S12 as well as Li3 PS4 SEs. Significantly, the experiments suggest that the GSIRR mechanism can be broadly applied, not only to LCO cathodes but also to LiNi0.8 Co0.1 Mn0.1 O2 cathodes and other reducing gases such as H2 S and CO, indicating its practical universality. This study highlights the significant influence of the surface chemistry of the oxide cathode on interfacial compatibility, and introduces a surface reconstruction strategy based on the GSIRR process as a promising avenue for designing enhanced ASSLBs.

Regulating the Spin State Configuration in Bimetallic Phosphorus Trisulfides for Promoting Sulfur Redox Kinetics.

Lithium-sulfur (Li-S) batteries suffer from sluggish kinetics due to the poor conductivity of sulfur cathodes and polysulfide shutting. Current studies on sulfur redox catalysis mainly focus on the adsorption and catalytic conversion of lithium polysulfides but ignore the modulation of the electronic structure of the catalysts which involves spin-related charge transfer and orbital interactions. In this work, bimetallic phosphorus trisulfides embedded in Prussian blue analogue-derived nitrogen-doped hollow carbon nanocubes (FeCoPS3/NCs) were elaborately synthesized as a host to reveal the relationship between the catalytic activity and the spin state configuration for Li-S batteries. Orbital spin splitting in FeCoPS3 drives the electronic structure transition from low-spin to high-spin states, generating more unpaired electrons on the 3d orbit. Specifically, the nondegenerate orbitals involved in the high-spin configuration of FeCoPS3 result in the upshift of energy levels, generating more active electronic states. Such tailored electronic structure increases the charge transfer, influences the d-band center, and further modifies the adsorption energy with lithium polysulfides and the potential reaction pathways. Consequently, the cell with FeCoPS3/NC host exhibits an ultralow capacity decay of 0.037% per cycle over 1000 cycles. This study proposed a general strategy for sculpting geometric configurations to enable spin and orbital topology regulation in Li-S battery catalysts.

Motif-Based Exploration of Halide Classes of Li5M10.5M20.5X8 Conductors Using the DFT Method: Toward High Li-Ion Conductivity and Improved Stability.

The development of all-solid-state lithium-ion batteries (ASSLIBs) is highly dependent on solid-state electrolyte (SSEs) performance. However, current SSEs cannot satisfactorily meet the requirements for high interfacial stability and Li-ion conductivity, especially under high-voltage cycling conditions. To overcome the intractable problems, we theoretically develop the chemistry of structural units to build a series of MX6-unit mixed framework Li5M10.5M20.5X8 (total 184 halides) for use as SSEs and recommend six halide candidates that combine the (electro)chemical stability with a low Li-ion migration barrier. Among them, three Li5M10.5M20.5F8 compounds (M1 = Ca and Mg; M2 = Ti and Zr) exhibit expansive electrochemical windows with a high cathodic limit (6.3 V vs µLi) and three-dimensional Li diffusion associated with moderate Li-migration barriers. To discuss their stability and compatibility (and in turn as a reference for experiments), the energy above the convex hull, the electrochemical stability window, the predicted (electro)reaction products, and the calculated reaction energies of Li5M10.5M20.5X8 in combination with Li-metal and several cathodes are tabulated. We stress that the importance of the cation-mixed effect and specific moieties for the halide anion leads to a design principle for a halide class of Li-ion SSEs. We provide insight into selecting the optimal halide anion and cations and open a new avenue of broad compositional spaces for stable Li-ion SSEs.

LiMgPO4 -Coating-Induced Phosphate Shell and Bulk Mg-Doping Enables Stable Ultra-High-Voltage Cycling of LiCoO2 Cathode.

Stable cycling of LiCoO2 (LCO) cathode at high voltage is extremely challenging due to the notable structural instability in deeply delithiated states. Here, using the sol-gel coating method, LCO materials (LMP-LCO) are obtained with bulk Mg-doping and surface LiMgPO4 /Li3 PO4 (LMP/LPO) coating. The experimental results suggest that the simultaneous modification in the bulk and at the surface is demonstrated to be highly effective in improving the high-voltage performance of LCO. LMP-LCO cathodes deliver 149.8 mAh g-1 @4.60 V and 146.1 mAh g-1 @4.65 V after 200 cycles at 1 C. For higher cut-off voltages, 4.70 and 4.80 V, LMP-LCO cathodes still achieve 144.9 mAh g-1 after 150 cycles and 136.8 mAh g-1 after 100 cycles at 1 C, respectively. Bulk Mg-dopants enhance the ionicity of CoO bond by tailoring the band centers of Co 3d and O 2p, promoting stable redox on O2- , and thus enhancing stable cycling at high cut-off voltages. Meanwhile, LMP/LPO surface coating suppresses detrimental surface side reactions while allowing facile Li-ion diffusion. The mechanism of high-voltage cycling stability is investigated by combining experimental characterizations and theoretical calculations. This study proposes a strategy of surface-to-bulk simultaneous modification to achieve superior structural stability at high voltages.

Layered Li-Co-B as a Low-Potential Anode for Lithium-Ion Batteries.

An anode material is one of the key factors affecting the capacity, cycle, and rate (fast charge) performance of lithium-ion batteries. Using the adaptive genetic algorithm, we found a new ground-state Li2CoB and two metastable states LiCoB and LiCo2B2 in the Li-Co-B system. The Li2CoB phase is a lithium-rich layered structure, and it has an equivalent lithium-ion migration barrier (0.32 eV) in addition to the lower voltage platform (0.05 V) than graphite, which is the most important commercial anode material at present. Moreover, we analyzed the mechanism of delithiation for Li2CoB and found that it maintained metallicity in the process of delithiation, indicating its good conductivity as an electrode material. Therefore, it is an excellent potential anode material for lithium-ion batteries. Our work provides a promising theoretical basis for the experimental synthesis of Li-Co-B and similar new materials.

Targeting Hippo pathway: A novel strategy for Helicobacter pylori-induced gastric cancer treatment.

The Hippo pathway plays an important role in cell proliferation, apoptosis, and differentiation; it is a crucial regulatory pathway in organ development and tumor growth. Infection with Helicobacter pylori (H. pylori) increases the risk of developing gastric cancer. In recent years, significant progress has been made in understanding the mechanisms by which H. pylori infection promotes the development and progression of gastric cancer via the Hippo pathway. Exploring the Hippo pathway molecules may yield new diagnostic and therapeutic targets for H. pylori-induced gastric cancer. The current article reviews the composition and regulatory mechanism of the Hippo pathway, as well as the research progress of the Hippo pathway in the occurrence and development of H. pylori-related gastric cancer, in order to provide a broader perspective for the study and prevention of gastric cancer.

Insights Into the Interfacial Degradation of High-Voltage All-Solid-State Lithium Batteries.

Poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE) is considered as a promising solid-state electrolyte for all-solid-state lithium batteries (ASSLBs). Nevertheless, the poor interfacial stability with high-voltage cathode materials (e.g., LiCoO2) restricts its application in high energy density solid-state batteries. Herein, high-voltage stable Li3AlF6 protective layer is coated on the surface of LiCoO2 particle to improve the performance and investigate the failure mechanism of PEO-based ASSLBs. The phase transition unveils that chemical redox reaction occurs between the highly reactive LiCoO2 surface and PEO-based SPE, resulting in structure collapse of LiCoO2, hence the poor cycle performance of PEO-based ASSLBs with LiCoO2 at charging voltage of 4.2 V vs Li/Li+. By sharp contrast, no obvious structure change can be found at the surface of Li3AlF6-coated LiCoO2, and the original layered phase was well retained. When the charging voltage reaches up to 4.5 V vs Li/Li+, the intensive electrochemical decomposition of PEO-based SPE occurs, leading to the constant increase of cell impedance and directly causing the poor performance. This work not only provides important supplement to the failure mechanism of PEO-based batteries with LiCoO2, but also presents a universal strategy to retain structure stability of cathode-electrolyte interface in high-voltage ASSLBs.

Polyiodide Confinement by Starch Enables Shuttle-Free Zn-Iodine Batteries.

Aqueous Zn-iodine (Zn-I2 ) batteries have been regarded as a promising energy-storage system owing to their high energy/power density, safety, and cost-effectiveness. However, the polyiodide shuttling results in serious active mass loss and Zn corrosion, which limits the cycling life of Zn-I2 batteries. Inspired by the chromogenic reaction between starch and iodine, a structure confinement strategy is proposed to suppress polyiodide shuttling in Zn-I2 batteries by hiring starch, due to its unique double-helix structure. In situ Raman spectroscopy demonstrates an I5 - -dominated I- /I2 conversion mechanism when using starch. The I5 - presents a much stronger bonding with starch than I3 - , inhibiting the polyiodide shuttling in Zn-I2 batteries, which is confirmed by in situ ultraviolet-visible spectra. Consequently, a highly reversible Zn-I2 battery with high Coulombic efficiency (≈100% at 0.2 A g-1 ) and ultralong cycling stability (>50 000 cycles) is realized. Simultaneously, the Zn corrosion triggered by polyiodide is effectively inhibited owing to the desirable shuttling-suppression by the starch, as evidenced by X-ray photoelectron spectroscopy analysis. This work provides a new understanding of the failure mechanism of Zn-I2 batteries and proposes a cheap but effective strategy to realize high-cyclability Zn-I2 batteries.

Structural Engineering of Cobalt-Free Perovskite Enables Efficient and Durable Oxygen Reduction in Solid Oxide Fuel Cells.

Developing low-cost, efficient, and durable cobalt-free perovskite oxides for oxygen reduction reaction at intermediate-to-low temperatures is crucial to enhance the viability of solid oxide fuel cells (SOFCs), a promising ingredient for establishing a more sustainable future. Herein, a highly active and robust cobalt-free perovskite Ba0.75 Sr0.25 Fe0.95 P0.05 O3-δ (BSFP) oxygen electrode via a facile co-doping strategy for intermediate-to-low temperature SOFCs (ILT-SOFCs) is reported by a combined experimental and theoretical approach. Attributed to stable and oxygen defect-rich structure, and remarkable intrinsic oxygen transport kinetics, the BSFP cathode shows exceptional catalytic performance, including record-level power output among iron-based perovskite cathodes (1464 mW cm-2 at 600 °C), low area-specific resistance (≈0.1 Ω cm2 at 600 °C), robust stability both in symmetrical and single cell configurations, and outstanding CO2 tolerance/reversibility. The first-principle calculations validate the role of co-doping of strontium and phosphorus for the high activity and durability. Central to this work is the combined experiment-calculation approach to point to an effective strategy in the development of highly active and stable perovskite-type cathodes for ILT-SOFCs and related applications.

Thiotetrelates Li2ZnXS4 (X = Si, Ge, and Sn) As Potential Li-Ion Solid-State Electrolytes.

A novel inorganic solid-state electrolyte (ISSE) with high ionic conductivity is a crucial part of all-solid-state lithium-ion (Li-ion) batteries (ASSLBs). Herein, we first report on Li2ZnXS4 (LZXS, X = Si, Ge, and Sn) semiconductor-based ISSEs, crystallizing in the corner-sharing tetrahedron orthorhombic space group, to provide valuable insights into the structure, defect chemistry, phase stability, electrochemical stability, H2O/CO2 chemical stability, and Li-ion conduction mechanisms. A key feature for the Li-ion transport and low migration barrier is the interconnected and corner-shared [LiS4] units along the a-axis, which allows Li-ion transport via empty or occupied tetrahedron sites. A major finding is the first indication that Li-ion migration in Li2ZnSiS4 (LZSiS) has lower energy barriers (∼0.24 eV) compared to Li2ZnGeS4 (LZGS) and Li2ZnSnS4 (LZSnS), whether through vacancy migration or interstitial migration. However, LZGS and LZSnS exhibit greater H2O/CO2 stability compared to LZSiS. The novel framework of LZXS with relatively low Li-ion migration barriers and moderate electrochemical stability could benefit the ASSLB communities.

Co4 N-Decorated 3D Wood-Derived Carbon Host Enables Enhanced Cathodic Electrocatalysis and Homogeneous Lithium Deposition for Lithium-Sulfur Full Cells.

The sluggish kinetics of sulfur conversion in the cathode and the nonuniform deposition of lithium metal at the anode result in severe capacity decay and poor cycle life for lithium-sulfur (Li-S) batteries. Resolving these deficiencies is the most direct route toward achieving practical cells of this chemistry. Herein, a vertically aligned wood-derived carbon plate decorated with Co4 N nanoparticles host (Co4 N/WCP) is proposed that can serve as a host for both the sulfur cathode and the metallic lithium anode. This Co4 N/WCP electrode host drastically enhances the reaction kinetics in the sulfur cathode and homogenizes the electric field at the anode for the uniform lithium plating. Density functional theory calculations confirm the experimental observations that Co4 N/WCP provides a lower energy barrier for the polysulfide redox reaction in the cathode and a low adsorption energy for lithium deposition at the anode. Employing the Co4 N/WCP host at both electrodes in a S@Co4 N/WCP||Li@Co4 N/WCP full cell delivers a specific capacity of 807.9 mAh g-1 after 500 cycles at a 1 C rate. Additional experiments are performed with high areal sulfur loading of 4 mg cm-2 to demonstrate the viability of this strategy for producing practical Li-S cells.

Cyclized Polyacrylonitrile as a Promising Support for Single Atom Metal Catalyst with Synergistic Active Site.

Metal single atom catalysts (SAC) have been successfully used in heterogeneous catalysis but developing a scalable and economic support for SAC is still a great challenge. Here, cyclized polyacrylonitrile (CPAN) is proposed as a promising support for single atom metal catalysts. CPAN can be easily prepared from cheap industrial product polyacrylonitrile (PAN), which has excellent processability. A series of SAC on CPAN (M/CPAN, M = Ag, Cu, Ru) are designed and the catalytic activities of the as synthesized M/CPAN are investigated by the model reduction reaction of p-nitrophenol (4-NP). M/CPAN presents excellent catalytic performance with high stability and theoretical calculations elucidate that Ag/CPAN synergistically catalyze 4-NP reduction following the Langmuir-Hinshelwood (L-H) mechanism with 4-NP preferentially adsorbing at the Ag sites and H adsorbing at the bridge C sites. These results, for the first time, reveal that the single atom on CPAN can catalyze 4-NP reduction efficiently. This methodology provides a convenient route for the preparation of a variety of SAC, and this strategy is readily scalable and holds great potential in catalytic applications.

Potential Solid-State Electrolytes with Good Balance between Ionic Conductivity and Electrochemical Stability: Li5-xM1-xMx'O4 (M = Al and Ga and M' = Si and Ge).

Exploring new solid-state electrolyte (SSE) materials with good electrochemical stability and high Li-ion conductivity for all-solid-state Li-ion batteries is vital for the development of technologies. Herein, we employ two lithium aluminates, α- and ß-Li5AlO4 (α- and ß-LAO), as the model framework, which have an orthorhombic crystal structure and isolated AlO4 tetrahedron units connected in lithium atoms, exhibiting large band gaps, low migration barriers (0.30-0.40 eV), fast Li-ion conductivity (LIC, in a magnitude of 10-4 S/cm), and a good electrochemical stability window (ESW, [0.01-3.20 V] vs Li+/Li). We tabulate the expected decomposition products at the interface, while considering cathodes in combination with the LAO electrolyte to discuss their compatibility. We also examine the electrochemical stability, H2O/CO2 stability, and Li-ion mobility of Li4.6Al0.6Si0.4O4 (LASO), Li5GaO4 (LGaO), and Li4.6Ga0.6Ge0.4O4 (LGaGeO) compounds. In general, there is usually a trade-off between the LIC and the ESW; however, LAO features a good balance between an outstanding LIC and a wide ESW, making the compound a promising candidate for next-generation SSE materials.

Aqueous Supramolecular Binder for a Lithium-Sulfur Battery with Flame-Retardant Property.

A lithium-sulfur (Li-S) battery based on multielectron chemical reactions is considered as a next-generation energy-storage device because of its ultrahigh energy density. However, practical application of a Li-S battery is limited by the large volume changes, insufficient ion conductivity, and undesired shuttle effect of its sulfur cathode. To address these issues, an aqueous supramolecular binder with multifunctions is developed by cross-linking sericin protein (SP) and phytic acid (PA). The combination of SP and PA allows one to control the volume change of the sulfur cathode, benefit soluble polysulfides absorbing, and facilitate transportation of Li+. Attributed to the above merits, a Li-S battery with the SP-PA binder exhibits a remarkable cycle performance improvement of 200% and 120% after 100 cycles at 0.2 C compared with Li-S batteries with PVDF and SP binders. In particular, the SP-PA binder in the electrode displays admirable flame-retardant performance due to formation of an isolating layer and the release of radicals.

Tuning Site Energy by XO6 Units in LiX2(PO4)3 Enables High Li Ion Conductivity and Improved Stability.

Solid-state electrolytes (SSEs) with high ion conductivity are necessary for all-solid-sate lithium ion batteries. Here, a less studied NASICON-type LiZr2(PO4)3 (LZP) is screened out from seven LXP compounds (LiX2(PO4)3, X = Si, Ge, Sn, Ti, Zr, Hf, and Mo), which combines the electrochemical stability with high Li conductivity. The bond valence site energy (BVSE), climbing image nudged elastic band (Cl-NEB) method, and electrochemical phase diagram prove LZP has a lower Li migration barrier and the largest electrochemical stability window. The underlying reason for high Li conductivity is analyzed from the structural features to the electronic structures. Furthermore, the XO6 unit mixed frameworks Li1.667Ca0.333Zr1.667(PO4)3 (LCZP) and Li1.667Mg0.333Zr1.667(PO4)3 (LMZP) exhibit high Li ion conductivity associated with a very low Li migration barrier (∼0.20 eV). This work opens a new avenue of broad compositional spaces in LXP for SSEs.

Water-Soluble Trifunctional Binder for Sulfur Cathodes for Lithium-Sulfur Battery.

Conventional polymer binder in a lithium-sulfur (Li-S) battery, poly(vinylidene fluoride) (PVDF), suffers from insufficient ion conductivity, poor polysulfide-trapping ability, weak mechanical property, and requirement of organic solvents, which significantly encumber the industrial application of Li-S battery. Herein, a water-soluble binder with trifunctions, covalently cross-linked quaternary ammonium cationic starch (c-QACS), is developed to confront these issues. Similar to the poly(ethylene oxide) solid electrolytes, the c-QACS binder remarkably improves Li+ ion transfer capacity. The abundant O actives endow the c-QACS binder with admirable lithium polysulfide-trapping capability to retard the shuttle effect. In addition, the formed 3D network effectively maintains the electrode integrity during cycling. Benefiting from the above merits, the sulfur cathode with the c-QACS binder demonstrates a performance improvement of 300 and 150% compared with sulfur cathode with PVDF and bulk QACS binder after 100 cycles at 0.2C.

Li-Ion Cooperative Migration and Oxy-Sulfide Synergistic Effect in Li14 P2 Ge2 S16-6 x Ox Solid-State-Electrolyte Enables Extraordinary Conductivity and High Stability.

Critical to the development of all-solid-state lithium-ion batteries technology are novel solid-state electrolytes with high ionic conductivity and robust stability under inorganic solid-electrolyte operating conditions. Herein, by using density functional theory and molecular dynamics, a mixed oxygen-sulfur-based Li-superionic conductor is screened out from the local chemical structure of ß-Li3 PS4 to discover novel Li14 P2 Ge2 S8 O8 (LPGSO) with high ionic conductivity and high stability under thermal, moist, and electrochemical conditions, which causes oxygenation at specific sites to improve the stability and selective sulfuration to provide an O-S mixed path by Li-S/O structure units with coordination number between 3 and 4 for fast Li-cooperative conduction. Furthermore, LPGSO exhibits a quasi-isotropic 3D Li-ion cooperative diffusion with a lesser migration barrier (≈0.19 eV) compared to its sulfide-analog Li14 P2 Ge2 S16 . The theoretical ionic conductivity of this conductor at room temperature is as high as ≈30.0 mS cm-1 , which is among the best in current solid-state electrolytes. Such an oxy-sulfide synergistic effect and Li-ion cooperative migration mechanism would enable the engineering of next-generation electrolyte materials with desirable safety and high ionic conductivity, for possible application in the near future.

Achieving Both High Ionic Conductivity and High Interfacial Stability with the Li2+xC1-xBxO3 Solid-State Electrolyte: Design from Theoretical Calculations.

A crystalline solid electrolyte interphase Li2CO3 material with a large band gap shows promise toward next-generation all-solid-state lithium batteries (ASSLBs). However, the inferior ionic diffusivity restricts such structures to a real battery setup. Herein, based on density functional theory calculation and Python materials genomics, we theoretically develop the chemistry and local structural motifs to build a mixed boron-carbon framework Li2+xC1-xBxO3 (LCBO). We examine the electrochemical and chemical stabilities of LCBO-electrode interfaces by analyzing the thermodynamics of formation of interfacial phases. Interestingly, the LCBO material is automatically protected from further decomposition through the self-generated resistive interphase (Li2CO3 and Li3BO3), which gives a wide range of operating potential. LCBO shows high interfacial stability with LiCoO2, LiMnO2, and LiMn2O4. More importantly, the theoretical Li-ion migration barrier of LCBO (x = 0.375) is approximately 0.23 ± 0.02 eV through a cooperative migration mechanism. Therefore, the LCBO material combines high Li-ion diffusivity with good interfacial stability, which makes it a promising solid-state electrolyte material for ASSLBs.

Integrating Conductivity, Immobility, and Catalytic Ability into High-N Carbon/Graphene Sheets as an Effective Sulfur Host.

Lithium-sulfur (Li-S) batteries are considered to be one of the most promising candidate systems for next-generation electrochemical energy storage. The major challenge of this system is the polysulfide shuttle, which results in poor cycling efficiency. In this work, a highly N-doped carbon/graphene (NC/G) sheet is designed as a sulfur host, which combines the merits of abundant N active sites and high electrical conductivity to achieve in situ anchoring-conversion of lithium polysulfides (LiPSs). Such a host not only has strong binding with LiPSs but also promotes redox kinetics, which are revealed by both experimental investigations and theoretical studies. The sulfur cathode based on the NC/G host exhibits a high initial capacity of 1380 mA h g-1 and a superior cycle stability with a low capacity decay of 0.037% per cycle within 500 cycles at 2 C. Steady areal capacity with a high sulfur loading (5.6 mg cm-2 ) is also attained even without the addition of LiNO3 in the electrolyte. This work proposes and illustrates the importance of in situ anchoring-conversion of LiPSs, offering a new strategy to design multifunctional sulfur hosts for high-performance Li-S batteries.

Rational design of a mesoporous silica-based cathode for efficient trapping of polysulfides in Li-S batteries.

Lithium-sulfur batteries are one of the most promising candidates for next-generation energy storage systems. The major challenge hindering their commercialization is the polysulfide shuttle effect, which causes a series of problems including the loss of active materials, corrosion of the lithium anode, low coulombic efficiency, and poor cycling performance. In this work, we develop a mesoporous silica-based cathode for efficient trapping of lithium polysulfides (LiPSs). This cathode material consists of mesoporous silica (HMS), highly dispersed NiO nanoparticles embedded in the silica structure, and a conductive polymer (polypyrrole-ppy) prepared by in situ polymerization. We employ the concept of both the physical and chemical entrapment of LiPSs, i.e., physically trapping LiPSs by spatial confinement of LiPSs in the silica porous structure and physical adsorption of LiPSs by the silica surface, and chemically binding LiPSs by highly dispersed NiO nanoparticles in the silica structure. The NiO/silica/ppy/S cathode exhibits good cycling stability and maintains over 700 mA h g-1 after 300 cycles. As far as we know, this is the first time that mesoporous silica has been directly employed as a sulfur host material, rather than an additive. The present study opens up a window for nanoporous silica to be employed as the sulfur host.