Energy band modulation of Li2O-rGO core-shell as cathode sacrificial additive enables capacity enhancement of hard carbon anode in Li-ion batteries.

Lithium oxides (Li2O) possess a considerable theoretical capacity, rendering them highly promising as cathodic pre-lithiation additives. However, its decomposition voltage exceeds the charging cut-off voltage of most cathode materials, hindering its direct use as a cathode sacrificial additive. Herein, we design a facile and safe method to reduce the decomposition energy of Li2O at room temperature to offset the irreversible capacity loss by using a core-shell structured Li2O-reduced graphene oxide (rGO)-polyethylene glycol (PEG) composite (denoted as Li2O-rGO-PEG). The graphene oxide (GO) was heat-treated to remove oxygen functional groups to synthesize rGO, and then reacted with Li2O to form a Li2O-rGO composite. According to the DFT calculations, the density of states at the Fermi level of Li2O-rGO becomes continuous and features a metallic nature, which significantly improves the electrical conductivity of Li2O and facilitates electron conduction that modify the delithiation potential of Li2O. PEG was used to enhance the cohesive force between rGO and Li2O and to protect Li2O from atmospheric contamination. Moreover, in order to demonstrate the excellent pre-lithiation ability of Li2O-rGO-PEG composite, hard carbon (HC) with low initial coulombic efficiency (ICE) was used as the anode. In the application of LFP (Li2O)/HC full cell, Li2O was decomposed to Li+ to effectively improve the initial charge capacity from 149.7 to 200 mAh/g and discharge capacity from 104.2 to 147.5 mAh/g, which are 33.6 % and 41.6 % higher than those of the pristine LFP/HC full cell, respectively. The cathode pre-lithiation method proposed in this work is simple and environmentally friendly. The successful utilization of Li2O as a pre-lithiation additive effectively addressed the issue of low initial coulombic efficiency of the HC, indicating excellent prospects for practical applications.

PEO-Based Solid Composite Polymer Electrolyte for High Capacity Retention All-Solid-State Lithium Metal Battery.

The limited ionic conductivity at room temperature and the constrained electrochemical window of poly(ethylene oxide) (PEO) pose significant obstacles that hinder its broader utilization in high-energy-density lithium metal batteries. The garnet-type material Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) is recognized as a highly promising active filler for enhancing the performance of PEO-based solid polymer electrolytes (SPEs). However, its performance is still limited by its high interfacial resistance. In this study, a novel hybrid filler-designed SPE is employed to achieve excellent electrochemical performance for both the lithium metal anode and the LiFePO4 cathode. The solid composite membrane containing hybrid fillers achieves a maximum ionic conductivity of 1.9 × 10-4 S cm-1 and a Li+ transference number of 0.67 at 40 °C, respectively. Additionally, the Li/Li symmetric cells demonstrate a smooth and stable process for 2000 h at a current density of 0.1 mA cm-2 . Furthermore, the LiFePO4 /Li battery delivers a high-rate capacity of 159.2 mAh g-1 at 1 C, along with a capacity retention of 95.2% after 400 cycles. These results validate that employing a composite of both active and inactive fillers is an effective strategy for achieving superior performance in all-solid-state lithium metal batteries (ASSLMBs).

Interfacial Plasticization Strategy Enabling a Long-Cycle-Life Solid-State Lithium Metal Battery.

The limited ionic conductivity and unstable interface due to poor solid-solid interface pose significant challenges to the stable cycling of solid-state batteries (SSBs). Herein, an interfacial plasticization strategy is proposed by introducing a succinonitrile (SN)-based plastic curing agent into the polyacrylonitrile (PAN)-based composite polymer electrolytes (CPE) interface. The SN at the interface strongly plasticizes the PAN in the CPE, which reduces the crystallinity of the PAN drastically and enables the CPE to obtain a low modulus surface, but it still maintains a high modulus internally. The reduced crystallinity of PAN provides more amorphous regions, which are favorable for Li+ transport. The gradient modulus structure not only ensures intimate interfacial contact but also favors the suppression of Li dendrites growth. Consequently, the interfacial plasticized CPE (SF-CPE) obtains a high ionic conductivity of 4.8 × 10-4 S cm-1 as well as a high Li+ transference number of 0.61. The Li-Li symmetric cell with SF-CPE can cycle for 1000 h at 0.1 mA cm-2, the LiFeO4 (LFP)-Li full-cell demonstrates a high capacity retention of 86.1% after 1000 cycles at 1 C, and the LiCoO2 (LCO)-Li system also exhibits an excellent cycling performance. This work provides a novel strategy for long-life solid-state batteries.

Surface modification of garnet fillers with a polymeric sacrificial agent enables compatible interfaces of composite solid-state electrolytes.

The poly (vinylidene fluoride) (PVDF)-based composite solid-state electrolyte (CSE) has garnered attention due to its excellent comprehensive performance. However, challenges persist in the structural design and preparation process of the ceramic-filled CSE, as the PVDF-based matrix is susceptible to alkaline conditions and dehydrofluorination, leading to its incompatibility with ceramic fillers and hindering the preparation of solid-state electrolytes. In this study, the mechanism of dehydrofluorination failure of a PVDF-based polymer in the presence of Li2CO3 on the surface of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is analyzed, and an effective strategy is proposed to inhibit the dehydrofluorination failure on the basis of density functional theory (DFT). We introduce a molecule with a small LUMO-HOMO gap as a sacrificial agent, which is able to remove the Li2CO3 impurities. Therefore, the approach of polyacrylic acid (PAA) as a sacrificial agent reduces the degree of dehydrofluorination in the PVDF-based polymer and ensures slurry fluidity, promoting the homogeneous distribution of ceramic fillers in the electrolyte membrane and enhancing compatibility with the polymer. Consequently, the prepared electrolyte membranes exhibit good electrochemical and mechanical properties. The assembled Li-symmetric cell can cycle at 0.1 mA cm-2 for 3500 h. The LiFePO4‖Li cell maintains 91.45% of its initial capacity after 650 cycles at 1C, and the LiCoO2‖Li cell maintains 84.9% of its initial capacity after 160 cycles, demonstrating promising high-voltage performance. This facile modification strategy can effectively improve compatibility issues between the polymer and fillers, which paves the way for the mass production of solid-state electrolytes.

Gradient fluorinated and hierarchical selective adsorption coating for Zn-Based aqueous battery.

Rechargeable aqueous zinc-ion batteries (ZIBs) are considered as one of the most promising large-scale energy storage system due to their high energy density, low cost and inherent safety. However, the notorious dendrite growth and severe side reactions, impede their practical application. Herein, we constructed a multifunctional gradient composite fluorinated coating with insulating ZnF2 outside and Zn/Sn alloy inside. ZnF2 outside and Zn/Sn alloy inside perform their own functions and solve the dendrites and side reactions jointly. Density functional theory (DFT) calculations and Molecular dynamics (MD) simulations demonstrate that the electronically insulating ZnF2 layer on the surface can regulate the transport of Zn2+ cations, limit the free H2O molecules and improve the dissolution of Zn2+, at the same time, the zincophilicity Zn/Sn alloy inside work as the favorable nucleation sites for Zn atoms and lowers the Zn2+ diffusion energy barrier. As a result, the ZnF2-Sn@Zn electrode in a symmetrical cell exhibits a long cycle life of about 1400 h, as well as 91 % capacity retention after 1400 cycles at 1 A/g in the ZnF2-Sn@Zn//MnO2@CNT full batteries. This work provides a practically promising strategy and new insights for the electrolyte and anode interface design.

Low-temperature electrolytes based on linear carboxylic ester co-solvents for SiO x /graphite composite anodes.

Silicon-based anode materials have been applied in lithium-ion batteries with high energy density. However, developing electrolytes that can meet the specific requirements of these batteries at low temperatures still remains a challenge. Herein, we report the effect of linear carboxylic ester ethyl propionate (EP), as the co-solvent in a carbonate-based electrolyte, on SiO x /graphite (SiOC) composite anodes. Using electrolytes with EP, the anode provides better electrochemical performance at both low temperatures and ambient temperature, showing a capacity of 680.31 mA h g-1 at -50 °C and 0.1C (63.66% retention relative to that at 25 °C), and a capacity retention of 97.02% after 100 cycles at 25 °C and 0.5C. Within the EP-containing electrolyte, SiOC‖LiCoO2 full cells also exhibit superior cycling stability at -20 °C for 200 cycles. These substantial improvements of the EP co-solvent at low temperatures are probably due to its involvement to form a solid electrolyte interphase with high integrity and facile transport kinetics in electrochemical processes.

Garnet/polymer solid electrolytes for high-performance solid-state lithium metal batteries: The role of amorphous Li2O2.

Solid-state electrolytes have been widely investigated for lithium batteries since they provide a high degree of safety. However, their low ionic conductivity and substantial growth of lithium dendrites hamper their commercial applications. Garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is one of the most promising active fillers to advance the performance of the solid polymer electrolyte. Nevertheless, their performance is still limited due to their large interfacial resistance. Herein, we embedded the amorphous Li2O2 (LO) into LLZTO particles via the quenching process and successfully achieved an interfacial layer of Li2O2 around LLZTO particles (LLZTO@LO). Amorphous Li2O2 acts as a binder and showed an excellent affinity for Li+ ions which promotes their fast transference. Moreover, the stable and dense interfacial Li2O2 layer enhances interfacial contact and suppresses the lithium dendrite growth during the long operation cycling process. The PEO/10LLZTO@2LO solid composite polymer electrolyte (SCPE) showed the highest ionic conductivity of 3.2 × 10-4 S cm-1 at 40 °C as compared to pristine LLZTO-based SCPE. Moreover, the Li│(PEO/10LLZTO@2LO) │Li symmetric cell showed a stable and smooth long lifespan up to 1100 h at 40 °C. Furthermore, the LiFePO4//Li full battery with PEO/10LLZTO@2LO SCPE demonstrated stable cycling performance for 400 cycles. These results constitute a significant step toward the practical application of solid-state lithium metal batteries (SS-LMBs).

Nanoscale control and tri-element co-doping of 4.6 V LiCoO2 with excellent rate capability and long-cycling stability for lithium-ion batteries.

Structural instability at high voltage severely restricts the reversible capacity of the LiCoO2 cathode. Moreover, the main difficulties in achieving high-rate performance of LiCoO2 are the long Li+ diffusion distance and slow Li+ intercalation/extraction during the cycle. Thus, we designed a modification strategy of nanosizing and tri-element co-doping to synergistically enhance the electrochemical performance of LiCoO2 at high voltage (4.6 V). Mg, Al, and Ti co-doping maintains the structural stability and phase transition reversibility, which promotes the cycling performance of LiCoO2. After 100 cycles at 1 C, the capacity retention of the modified LiCoO2 reached 94.3%. In addition, the tri-elemental co-doping increases Li+ interlayer spacing and enhances Li+ diffusivity by tens of times. Simultaneously, nanosize modification decreases Li+ diffusion distance, leading to a significantly enhanced rate capacity of 132 mA h g-1 at 10 C, much better than that of the unmodified LiCoO2 (2 mA h g-1). After 600 cycles at 5 C, the specific capacity remains at 135 mA h g-1 with a capacity retention of 91%. The nanosizing co-doping strategy synchronously enhanced the rate capability and cycling performance of LiCoO2.

Melting lithium alloying to improve the affinity of Cu foil for ultra-thin lithium metal anode.

Before lithium (Li) metal can be formally used as the anode material of Li-ion battery, the key technical defects of Li metal electrode, such as low active Li metal proportion and low mechanical strength, must be solved. Herein, the surface affinity of molten lithium-copper (LiCu) alloy with Cu foil is improved by alloying Cu and Li in a molten state. The surface of Cu foil naturally adsorbs an ultra-thin (∼30 µm) composite Li metal layer. The ultra-thin composite Li metal layer can greatly reduce the amount of inactive Li, and the Cu foil improves the mechanical strength and engineering workability of Li metal anode. In addition, the enhanced Young's modulus facilitates the uniform Li plating/stripping process. As a result, the stable cycle stability of up to 600 h and the average overpotential of 13 mV (area specific capacity is 1 mAh cm-2 and current density is 1 mA cm-2) are achieved. The cycle life is higher than 150 h even though the maximum utilization rate of Li is greater than 50%. The Li metal full battery assembled with the commercial NCM811 cathode shows more stable cycle performance and Coulombic efficiency. Such strategy can effectively pave the way for the practical application of Li metal anode.

Organic-inorganic hybrid ferrocene/AC as cathodes for wide temperature range aqueous Zn-ion supercapacitors.

Organic and inorganic materials have their own advantages and limitations, and new properties can be displayed in organic-inorganic hybrid materials by uniformly combining the two categories of materials at small scale. The objective of this study is to hybridize activated carbon (AC) with ferrocene to obtain a new material, ferrocene/AC, as the cathode for Zn-ion hybrid supercapacitors (ZHSCs). The optimized ferrocene/AC material owns fast charge transfer kinetics and can obtain pseudo-capacitance through redox reaction. Due to the introduction of ferrocene/AC, the ZHSCs exhibit remarkable electrochemical performances relative to that using ferrocene cathode, including high discharge specific capacity of 125.1 F g-1, high energy density (up to 44.8 Wh kg-1 at 0.1 A g-1) and large power density (up to 1839 W kg-1 at 5 A g-1). Meanwhile, the capacity retention rate remains 73.8% after 10 000 charge and discharge cycles. In particular, this cathode material can be used at low temperatures (up to -30 °C) with 60% capacity remained, which enlarges the application temperature range of ZHSCs. These results of this study can help understand new properties of organic-inorganic hybrid materials.

Morphologically and chemically regulated 3D carbon for Dendrite-free lithium metal anodes by a plasma processing.

For the high theoretical specific capacity and low redox potential, lithium (Li) metal is considered as one of the most promising anode materials for the next generation of rechargeable batteries. In this work, we have developed an effective and accurate plasma strategy to regulate the surface morphology and functional groups of three-dimensional nitrogen-containing carbon foam (CF) to control the Li nucleation and growth. Besides the rougher surface induced by oxygen (O2) plasma, the conversion of carbon-nitrogen chemical bond (CN), namely, change from the quaternary N to pyrrolic/pyridinic N was realized by the nitrogen (N2) plasma. This chemical regulation of nitrogen boosts the lithiophilicity of carbon foam, which is evidenced by lower overpotential obtained from the experiment and higher binding energy for Li ions (Li+) calculated by density functional theory (-1.43, -1.85, -2.41 and -2.45 eV for the amorphous C-, C-quaternary N-, C-pyrrolic N- and C-pyridinic N-, respectively). The electrochemical performance of the half cells and full cells based on this plasma regulated carbon foam collectors also proved the prominent effectiveness of this plasma strategy on guiding the uniform dispersion of Li+ and thus inducing the homogeneous Li nucleation, as well as suppressing the growth of Li dendrites.

Enhancing ionic conductivity in solid electrolyte by relocating diffusion ions to under-coordination sites.

Solid electrolytes are highly important materials for improving safety, energy density, and reversibility of electrochemical energy storage batteries. However, it is a challenge to modulate the coordination structure of conducting ions, which limits the improvement of ionic conductivity and hampers further development of practical solid electrolytes. Here, we present a skeleton-retained cationic exchange approach to produce a high-performance solid electrolyte of Li3Zr2Si2PO12 stemming from the NASICON-type superionic conductor of Na3Zr2Si2PO12. The introduced lithium ions stabilized in under-coordination structures are facilitated to pass through relatively large conduction bottlenecks inherited from the Na3Zr2Si2PO12 precursor. The synthesized Li3Zr2Si2PO12 achieves a low activation energy of 0.21 eV and a high ionic conductivity of 3.59 mS cm-1 at room temperature. Li3Zr2Si2PO12 not only inherits the satisfactory air survivability from Na3Zr2Si2PO12 but also exhibits excellent cyclic stability and rate capability when applied to solid-state batteries. The present study opens an innovative avenue to regulate cationic occupancy and make new materials.

Zn-doping Effects of Na-rich Na3+xV2-xZnx(PO4)3/C cathodes for Na-Ion Batteries: Lattice distortion induced by doping site and enhanced electrochemical performance.

To tackle the intrinsic inferior conductivity of the sodium ion batteries (SIBs) cathode Na3V2(PO4)3, transitional metal cation doping, and carbon frame design are employed for NASICON structural modification. Herein, a hard carbon skeleton Na3+xV2-xZnx(PO4)3 NASICON structure is proposed resorting to the combination of flimsy hard carbon slices coating and Zn2+ doping along with the introduction of spare Na+. The structural distortion caused by the insertion of Zn2+ and Na+ broadens the transfer channels and increases diffusion routes for Na+. At the same time, the anchoring effect for Na3+xV2-xZnx(PO4)3 nanoparticles brought by external hard carbon layers and pillar effect aroused by Zn2+ provide a stable and firm skeleton, which is conducive to structural stability and reversibility at high current density. Among various doping concentrations, Na3.03V1.97Zn0.03(PO4)3 performs a significantly enhanced rate performance with a reversible capacity up to 60 mAh·g-1 (40C) and ultra-long cycle life of 1000 cycles with a capacity retention of 92.6% at 5C.

Understanding of the Mechanism Enables Controllable Chemical Prelithiation of Anode Materials for Lithium-Ion Batteries.

By compensating the irreversible loss of lithium ions during the first cycle, prelithiations can solve the issue of insufficient initial Coulombic efficiency for various anodes. Recently, the chemical prelithiation using organolithium compounds has attracted increasing attention because of its uniform and fast reaction, safety, and easily adjustable degree of prelithiation. However, the nature and activity of organolithium involved in chemical prelithiations have not been deeply explored yet. Here, by monitoring the electrical conductivity change in the lithiation solution in the duration of its formation, we have demonstrated the essential role of lithium radical anions for chemical prelithiation and compared the prelithiation activity of dissociated species and aggregates of lithium radical anions. The mechanistic understanding of the nature of the lithiation solution leads to controllable chemical prelithiation, as demonstrated in full cells of prelithiated hard carbon and LiFePO4.

Bimetallic composite induced ultra-stable solid electrolyte interphase for dendrite-free lithium metal anode.

Lithium metal is the most promising anode materials for the next generation lithium ion battery. However, the electrode polarization leads to the formation of dendrites and "dead lithium", which degrades the performance of lithium metal batteries and induce a variety of security risk. The electrode polarization and lithium dendrites can be suppressed by lithium metal composite electrode. Herein, a simple and effective strategy is adopted to construct nickel and lithium bimetallic composite (NiLi-BC) electrode by a double roll process. The Ni framework inside the electrode can optimize the electric field and Li+ distribution at the electrode/electrolyte interface and induce the uniform lithium deposition. As a result, the NiLi-BC exhibits a lithium dendrite-free feature and stable cycling performance under a low overpotential (<15 mV throughout 2180 h at 1 mA cm-2 with a deposition capacity of 1 mAh cm-2). Moreover, the assembled NiLi-BC||LiFePO4 coin cell and pouch cell exhibit improved capability and stable cycling performance. Finally, the in-situ optical microscopy and in-situ Raman spectroscopy are employed to obtain a better understanding of the interfacial structure and chemical component during the Li plating and striping processes. The scheme of this study of the NiLi electrode has great practical application value.

Intelligent phase-transition MnO2 single-crystal shell enabling a high-capacity Li-rich layered cathode in Li-ion batteries.

Layered, Li-rich Mn-based oxides (LLMOs) are the most promising next-generation, high-energy batteries due to their relatively high specific capacities and high voltages. However, the practical application of LLMO cathodes is limited by low initial coulombic efficiencies (CEs) and poor cycling performance. Herein, we used the reaction of KMnO4 and MnSO4 under hydrothermal conditions to grow a nano-SCMO shell on the LLMO material surface (SCMO@LLMO). The unique particle/sheet compound structure of the SCMO shell is beneficial to the electrochemical reaction. SCMO has good Li storage characteristics and excellent surface structure stability in the single-crystal phase which further improves the reversible capacity, CE, and cyclic stability of the LLMO cathode. Therefore, the optimal coated sample (feedstock: 2 M KMnO4, SCMO@LLMO-2.0) exhibits a good initial discharge capacity (238.2 mA h g-1 at 1C and 173.8 mA h g-1 at 5C), initial CE (89.6% at 1C and 86.5% at 5C), and cycling performance (capacity retention of 84.67% at 1C and 62.72% at 5C after 200 cycles). This work adopts a hydrothermal method to synthesize a nano-single crystal composite material, laying a foundation for the preparation of the SCMO@LLMO cathodes for LLMO primary battery cathodes with high electrochemical performance.

Pure-phase ß-Mn2V2O7 interconnected nanospheres as a high-performance lithium ion battery anode.

This work primarily exhibits a systematic study of the large-scale hydrothermal synthesis of ß-Mn2V2O7 interconnected nanospheres without templates. An optimal combination of hydrothermal/annealing/atmosphere parameters is identified for the pure phase, which exhibits an excellent cycling performance of 760 mA h g-1 at 0.5 A g-1 over 120 cycles and a rate capability of 470 mA h g-1 at 2 A g-1 as an anode for a lithium ion battery. Guidelines have been provided for the first time for the synthesis of ß-Mn2V2O7, which opens broad opportunities for this earth-abundant chemical in electrochemical devices.

Capacity Contribution Induced by Pseudo-Capacitance Adsorption Mechanism of Anode Carbonaceous Materials Applied in Potassium-ion Battery.

The intrinsic bottleneck of graphite intercalation compound mechanism in potassium-ion batteries necessitates the exploitation of novel potassium storage strategies. Hence, utmost efforts have been made to efficiently utilize the extrinsic pseudo-capacitance, which offers facile routes by employing low-cost carbonaceous anodes to improve the performance of electrochemical kinetics, notably facilitating the rate and power characteristics for batteries. This mini-review investigates the methods to maximize the pseudo-capacitance contribution based on the size control and surface activation in recent papers. These methods employ the use of cyclic voltammetry for kinetics analysis, which allows the quantitative determination on the proportion of diffusion-dominated vs. pseudo-capacitance by verifying a representative pseudo-capacitive material of single-walled carbon nanotubes. Synergistically, additional schemes such as establishing matched binder-electrolyte systems are in favor of the ultimate purpose of high-performance industrialized potassium-ion batteries.

Direct Structure-Performance Comparison of All-Carbon Potassium and Sodium Ion Capacitors.

A hybrid ion capacitor (HIC) based on potassium ions (K+) is a new high-power intermediate energy device that may occupy a unique position on the Ragone chart space. Here, a direct performance comparison of a potassium ion capacitor (KIC) versus the better-known sodium ion capacitor is provided. Tests are performed with an asymmetric architecture based on bulk ion insertion, partially ordered, dense carbon anode (hard carbon, HC) opposing N- and O-rich ion adsorption, high surface area, cathode (activated carbon, AC). A classical symmetric "supercapacitor-like" configuration AC-AC is analyzed in parallel. For asymmetric K-based HC-AC devices, there are significant high-rate limitations associated with ion insertion into the anode, making it much inferior to Na-based HC-AC devices. A much larger charge-discharge hysteresis (overpotential), more than an order of magnitude higher impedance R SEI, and much worse cyclability are observed. However, K-based AC-AC devices obtained on-par energy, power, and cyclability with their Na counterpart. Therefore, while KICs are extremely scientifically interesting, more work is needed to tailor the structure of "Na-inherited" dense carbon anodes and electrolytes for satisfactory K ion insertion. Conversely, it should be possible to utilize many existing high surface area adsorption carbons for fast rate K application.

MOF-derived manganese monoxide nanosheet-assembled microflowers for enhanced lithium-ion storage.

Achieving high energy density, power density and cycling performance is a great challenge for lithium-ion battery (LIB) anodes. To obtain favorable electrochemical properties, an effective approach for designing composite nanomaterials with good stability and large specific surface area has been reported here. In this work, metal-organic framework (MOF)-derived manganese monoxides with a stable macromolecular framework were synthesized by utilizing the template agent 1,2,3,4-butanetetracarboxylic acid (BTCA) and the organic salt manganese acetylacetone, which possess a compact microflower structure assembled by nanosheets. As a synergistic effect, not only the amorphous carbon derived from MOFs enhances the specific capacity and stability, but also the unique nanosheet exhibits a significant nano-effect and high areal capacity, which is in favour of an electrochemical reaction. For further enhancement of the electrochemical performance, reduced graphene oxide (rGO) was introduced. When tested as a LIB anode, the MnO@rGO composite displays superior reversible capacities (1716 mA h g-1 at 0.1 A g-1 and 930 mA h g-1 at 2 A g-1) and remarkable rate performances. The research results of the composite nanomaterials lay a foundation for the fabrication of high-capacity and stable anode materials in LIBs.