Reconstructing Helmholtz Plane Enables Robust F-Rich Interface for Long-Life and High-Safe Sodium-Ion Batteries.

Hard carbon (HC) is the most commonly used anode material in sodium-ion batteries. However, the solid-electrolyte-interface (SEI) layer formed in carbonate ester-based electrolytes has an imperceptible dissolution tendency and a sluggish Na+ diffusion kinetics, resulting in unsatisfactory performance of the HC anode. Given that electrode/electrolyte interface property is highly dependent on the configuration of Helmholtz plane, we filtrate proper solvents by PFBE (PF6- anion binding energy) and CAE (carbon absorption energy) and disclose the function of chosen TFEP to reconstruct the Helmholtz plane and regulate the SEI film on HC anode. Benefiting from the preferential adsorption tendency on HC surface and strong anion-dragging interaction of TFEP, a robust and thin anion-derived F-rich SEI film is established, which greatly enhances the mechanical stability and the Na+ ion diffusion kinetics of the electrode/electrolyte interface. The rationally designed TFEP-based electrolyte endows Na||HC half-cell and 2.8 Ah HC||Na4Fe3(PO4)2P2O7 pouch cell with excellent rate capability, long cycle life, high safety and low-temperature adaptability. It is believed that this insightful recognition of tuning interface properties will pave a new avenue on the design of compatible electrolyte for low-cost, long-life, and high-safe sodium-ion batteries.

Revisit of Polyaniline as a High-Capacity Organic Cathode Material for Li-Ion Batteries.

Polyaniline (PANI) has long been explored as a promising organic cathode for Li-ion batteries. However, its poor electrochemical utilization and cycling instability cast doubt on its potential for practical applications. In this work, we revisit the electrochemical performance of PANI in nonaqueous electrolytes, and reveal an unprecedented reversible capacity of 197.2 mAh g-1 (244.8 F g-1) when cycled in a wide potential range of 1.5 to 4.4 V vs. Li+/Li. This ultra-high capacity derives from 70% -NH- transformed to =NH+- during deep charging/discharging process. This material also demonstrates a high average coulombic efficiency of 98%, an excellent rate performance with 73.5 mAh g-1 at 1800 mA g-1, and retains 76% of initial value after 100 cycles, which are among the best reported values for PANI electrodes in battery applications.

Scalable Synthesis of Bilayer Graphene at Ambient Temperature.

In this work, we develop for the first time a facile chemical lithiation-assisted exfoliation approach to the controllable and scalable preparation of bilayer graphene. Biphenyl lithium (Bp-Li), a strong reducing reagent, is selected to realize the spontaneous Li-intercalation into graphite at ambient temperature, forming lithium graphite intercalation compounds (Li-GICs). The potential of Bp-Li (0.11 V vs Li/Li+), which is just lower than the potential of stage-2 lithium intercalation (0.125 V), enables the precise lithiation of graphite to stage-2 Li-GICs (LiC12). Intriguingly, the exfoliation of LiC12 leads to the bilayer-favored production of graphene, giving a high selectivity of 78%. Furthermore, the mild intercalation-exfoliation procedure yields high-quality graphene with negligible structural deterioration. The obtained graphene exhibits ultralow defect density (ID/IG ∼ 0.14) and a considerably high C/O ratio (∼29.7), superior to most current state-of-the-art techniques. This simple and scalable strategy promotes the understanding of chemical Li-intercalation methods for preparing high-quality graphene and shows great potential for layer-controlled engineering.

LiNO3 -Based Electrolytes via Electron-Donation Modulation for Sustainable Nonaqueous Lithium Rechargeable Batteries.

LiPF6 as a dominant lithium salt of electrolyte is widely used in commercial rechargeable lithium-ion batteries due to its well-balanced properties, including high solubility in organic solvents, good electrochemical stability, and high ionic conductivity. However, it suffers from several undesirable properties, such as high moisture sensitivity, thermal instability, and high cost. To address these issues, herein, we propose an electron-donation modulation (EDM) rule for the development of low-cost, sustainable, and electrochemically compatible LiNO3 -based electrolytes. We employ high donor-number solvents (HDNSs) with strong electron-donation ability to dissolve LiNO3 , while low donor-number solvents (LDNSs) with weak electron-donation ability are used to regulate the solvation structure to stabilize the electrolytes. As an example, we design the LiNO3 -DMSO@PC electrolyte, where DMSO acts as an HDNS and PC serves as an LDNS. This electrolyte exhibits excellent electrochemical compatibility with graphite anodes, as well as the LiFePO4 and LiCoO2 cathodes, leading to stable cycling over 200 cycles. Through spectroscopy analyses and theoretical calculation, we uncover the underlying mechanism responsible for the stabilization of these electrolytes. Our findings provide valuable insights into the preparation of LiNO3 -based electrolytes using the EDM rule, opening new avenues for the development of advanced electrolytes with versatile functions for sustainable rechargeable batteries.

Controllable synthesis of a Na-enriched Na4V2(PO4)3 cathode for high-energy sodium-ion batteries: a redox-potential-matched chemical sodiation approach.

Exploring a sodium-enriched cathode (i.e. Na4V2(PO4)3, which differs from its traditional stoichiometric counterpart Na3V2(PO4)3 that can provide extra endogenous sodium reserves to mitigate the irreversible capacity loss of the anode material (i.e. hard carbon), is an intriguing presodiation method for the development of high energy sodium-ion batteries. To meet this challenge, herein, we first propose a redox-potential-matched chemical sodiation approach, utilizing phenazine-sodium (PNZ-Na) as the optimal reagent to sodiate the Na3V2(PO4)3 precursor into Na-enriched Na4V2(PO4)3. The spontaneous sodiation reaction enables a fast reduction of one-half V ions from V3+ to V2+, followed by the insertion of one Na+ ion into the NASICON framework, which only takes 90 s to obtain the phase-pure Na4V2(PO4)3 product. When paired with a hard carbon anode, the resulting Na4VP‖HC full cell exhibits a high energy density of 251 W h kg-1, which is 58% higher than that of 159 W h kg-1 for the Na3VP‖HC control cell. Our chemical sodiation methodology provides an innovative approach for designing sodium-rich cathode materials and could serve as an impetus to the development of advanced sodium-ion batteries.

Routes to Electrochemically Stable Sulfur Cathodes for Practical Li-S Batteries.

Lithium-sulfur (Li-S) batteries have been investigated intensively as a post-Li-ion technology in the past decade; however, their realizable energy density and cycling performance are still far from satisfaction for commercial development. Although many extremely high-capacity and cycle-stable S cathodes and Li anodes are reported in literature, their use for practical Li-S batteries remains challenging due to the huge gap between the laboratory research and industrial applications. The laboratory research is usually conducted by use of a thin-film electrode with a low sulfur loading and high electrolyte/sulfur (E/S) ratios, while the practical batteries require a thick/high sulfur loading cathode and a low E/S ratio to achieve a desired energy density. To make this clear, the inherent problems of dissolution/deposition mechanism of conventional sulfur cathodes are analyzed from the viewpoint of polarization theory of porous electrode after a brief overview of the recent research progress on sulfur cathodes of Li-S batteries, and the possible strategies for building an electrochemically stable sulfur cathode are discussed for practically viable Li-S batteries from the authors' own understandings.

Revealing the structural chemistry in Na6-2xFex(SO4)3 (1.5 ≤ x ≤ 2.0) for low-cost and high-performance sodium-ion batteries.

Fe-based polyanionic sulfate materials are one of the most promising candidates for large-scale applications in sodium-ion batteries due to their low cost and excellent electrochemical performance. Although great achievements have been gained on a series of Na6-2xFex(SO4)3 (NFSO-x, 1.5 ≤ x ≤ 2.0) materials such as Na2Fe2(SO4)3, Na2Fe1.5(SO4)3, and Na2.4Fe1.8(SO4)3 for sodium storage, the phase and structure characteristics on these NFSO-x are still controversial, making it difficult to achieve phase-pure materials with optimal electrochemical properties. Herein, six NFSO-x samples with varied x are investigated via both experimental methods and density functional theory calculations to analyze the phase and structure properties. It reveals that a pure phase exists in the 1.6 ≤ x ≤ 1.7 region of the NFSO-x, and part of Na ions tend to occupy Fe sites to form more stable frameworks. The NFSO-1.7 exhibits the best electrochemical performance among the NFSO-x samples, delivering a high discharge capacity (104.5 mAh g-1 at 0.1 C, close to its theoretical capacity of 105 mAh g-1), excellent rate performance (81.5 mAh g-1 at 30 C), and remarkable cycle stability over 10,000 cycles with high-capacity retention of 72.4%. We believe that the results are useful to clarify the phase and structure characteristics of polyanionic materials to promote their application for large-scale energy storage.

High-Voltage and Intrinsically Safe Sodium Metal Batteries Enabled by Nonflammable Fluorinated Phosphate Electrolytes.

Sodium metal is a promising anode for high-energy-density sodium rechargeable batteries (RSBs). However, the low Coulombic efficiency (CE) of the Na plating/stripping process and the problem of safety hinder their practical application. Herein, we report a facile strategy for employing the fluorinated phosphate solvents to realize highly reversible Na plating/stripping and improve the safety performance. The fluorinated phosphate molecules reduce the polarity of the solvent and lower the coordination number to Na+, which makes it possible to form the anion-induced ion-solvent-coordinated (AI-ISC) structures with high reduction tolerance. Moreover, the fluorination treatment enhances the oxidation resistance of the phosphate solvent, enabling compatibility with the high-voltage Na3V2(PO4)2F3 (NVPF) cathode. As expected, the Na@Al//NVPF full cell with the as-prepared 0.9 M NaFSI/tris(2,2,2-trifluoroethyl) phosphate (TFEP) demonstrates a capacity retention of 83.4% after 200 cycles with an average CE of 99.6%. This work opens a new avenue for designing high-energy-density RSBs with improved safety performance.

Ion-Migration Mechanism: An Overall Understanding of Anionic Redox Activity in Metal Oxide Cathodes of Li/Na-Ion Batteries.

The anionic redox reaction (ARR) has attracted extensive attention due to its potential to enhance the reversible capacity of cathode materials in Li/Na-ion batteries (LIBs/SIBs). However, the understanding of its activation mechanism is still limited by the insufficient mastering of the underlying thermodynamics and kinetics. Herein, a series of Mg/Li/Zn-substituted Nax MnO2 and Lix MnO2 cathode materials are designed to investigate their ARR behaviors. It is found that the ARR can be activated in only Li-substituted Lix MnO2 and not for Mg- and Zn-substituted ones, while all Mg/Li/Zn-substituted Nax MnO2 cathode materials exhibit ARR activities. Combining theoretical calculations with experimental results, such a huge difference between Li and Na cathodes is closely related to the migration of substitution ions from the transition metal layer to the alkali metal layer in a kinetic aspect, which generates unique Li(Na)-O-□TM and/or □Li/ Na -O-□TM configurations and reducing reaction activation energy to trigger the ARR. Based on these findings, an ion-migration mechanism is proposed to explain the different ARR behaviors between the Nax MnO2 and Lix MnO2 , which can not only reveal the origin of ARR in the kinetic aspect, but also provide a new insight for the development of high-capacity metal oxide cathode materials for LIBs/SIBs.

Exfoliation of MoS2 Nanosheets Enabled by a Redox-Potential-Matched Chemical Lithiation Reaction.

Ion intercalation assisted exfoliation is the oldest and most popular method for the scalable synthesis of molybdenum disulfide (MoS2) nanosheets. The commonly used organolithium reagents for Li+ intercalation are n-butyllithium (n-BuLi) and naphthalenide lithium (Nap-Li); however, the highly pyrophoric nature of n-BuLi and the overly reducing power of Nap-Li hinder their extensive application. Here, a novel organolithium reagent, pyrene lithium (Py-Li), which has intrinsic safe properties and a well-matched redox potential, is reported for the intercalation and exfoliation of MoS2. The redox potential of Py-Li (0.86 V vs Li+/Li) is located just between the intercalation (1.13 V) and decomposition (0.55 V) potentials of bulk MoS2, thus allowing precise Li+ intercalation to form a lamellar LiMoS2 compound without undesirable structural damage. The lithiation reaction can be accomplished within 1 h at room temperature and the exfoliated nanosheets are almost single layer. This method also offers the advantages of low cost, high repeatability, and ease in realizing large-scale production.

A Solid-Phase Conversion Sulfur Cathode with Full Capacity Utilization and Superior Cycle Stability for Lithium-Sulfur Batteries.

Solid phase conversion sulfur cathode is an effective strategy for eliminating soluble polysulfide intermediates (LiPSs) and improving cyclability of Li-S batteries. However, realizing such a sulfur cathode with high sulfur loading and high capacity utilization is very challenging due to the insulating nature of sulfur. In this work, a strategy is proposed for fabricating solid phase conversion sulfur cathode by encapsulating sulfur in the mesoporous channels of CMK-3 carbon to form a coaxially assembled sulfur/carbon (CA-S/C) composite. Vinyl carbonate (VC) is simultaneously utilized as the electrolyte cosolvent to in-situ form a dense solid electrolyte interface (SEI) on the CA-S/C composite surface through its nucleophilic reaction with the freshly generated polysulfides at the beginning of initial discharge, thus separating the direct contact of interior sulfur with the outer electrolyte. As expected, such a CA-S/C cathode can operate in a solid phase conversion manner in the VC-ether cosolvent electrolyte to exhibit a full capacity utilization (1667 mA h g-1 , ≈100%), a high rate capability of 2.0 A g-1 and excellent long-term cyclability over 500 cycles even at a high sulfur loading (75%, based on the weight of CA-S/C composite), demonstrating great promise for constructing high-energy-density and cycle-stable Li-S batteries.

The underlying mechanism for reduction stability of organic electrolytes in lithium secondary batteries.

Many organic solvents have very desirable solution properties, such as wide temperature range, high solubility of Li salts and nonflammability, and should be able but fail in reality to serve as electrolyte solvents for Li-ion or -metal batteries due to their reduction instability. The origin of this interfacial instability remains unsolved and disputed so far. Here, we reveal for the first time the origin of the reduction stability of organic carbonate electrolytes by combining ab initio molecular dynamics (AIMD) simulations, density functional theory (DFT) calculations and electrochemical stability experiments. It is found that with the increase of the molar ratio (MR) of salt to solvent, the anion progressively enters into the solvation shell of Li+ to form an anion-induced ion-solvent-coordinated (AI-ISC) structure, leading to a "V-shaped" change of the LUMO energy level of coordinated solvent molecules, whose interfacial stability first decreases and then increases with the increased MRs of salt to solvent. This mechanism perfectly explains the long-standing puzzle about the interfacial compatibility of organic electrolytes with Li or similar low potential anodes and provides a basic understanding and new insights into the rational design of the advanced electrolytes for next generation lithium secondary batteries.

Microstructure-Dependent Charge/Discharge Behaviors of Hollow Carbon Spheres and its Implication for Sodium Storage Mechanism on Hard Carbon Anodes.

Hard carbons are actively developed as a promising anode material for sodium ion batteries (SIBs). However, their sodium storage mechanism is poorly understood, leading to difficulties in design and development of high-performance hard carbon anode materials. In this work, hollow carbon spheres (HCSs) with different shell thickness as a model material to investigate the correlation between the microstructural change and resulting Na+ storage behavior during charge/discharge cycles are designed and synthesized. Ex situ X-ray diffraction and Raman evidences reveal that an interlayer spacing change of the graphitic nanodomains occurs in HCS electrode, leading to a shift of the reversible capacity from the high-potential sloping (HPS) region to the low-potential plateau (LPP) region. This unusual capacity shift suggests a microstructure-dependent Na+ storage reaction on the HCS electrode and can be well explained by "adsorption-intercalation" mechanism for these HCS materials. This work strengthens the understanding of the sodium storage behavior and provides a new perspective for the morphological and structural design of hard carbon anode materials for high-performance SIBs.

In Situ-Formed Artificial Solid Electrolyte Interphase for Boosting the Cycle Stability of Si-Based Anodes for Li-Ion Batteries.

Si is being actively developed as one of the most promising high-capacity anodes for next-generation lithium-ion batteries (LIBs). However, low cycling coulombic efficiency (CE) due to the repetitive growth of the solid electrolyte interphase (SEI) film is still an issue for its application in full batteries. Here, we propose a strategy to in situ form an artificial solid electrolyte interphase (ASEI) on the ferrosilicon/carbon (FeSi/C) anode surface by a purposely designed nucleophilic reaction of polysulfides with vinylene carbonate (VC) and fluoroethylene carbonate (FEC) molecules. The as-formed ASEI layer is mechanically dense and ionically conducting and therefore can effectively prevent the electrolyte infiltration and decomposition while allowing Li+ transport across, thus stabilizing the interface of the FeSi/C anode. As a result, the ASEI-modified FeSi/C anode exhibits a large reversible capacity of 1409.4 mA h g-1, an excellent cycling stability over 650 cycles, and a greatly elevated cycling CE of 99.8%, possibly serving as a high-capacity anode of LIBs.

Electrochemical Insight into the Sodium-Ion Storage Mechanism on a Hard Carbon Anode.

Hard carbon (HC) has been actively investigated as a high-capacity and low-cost anode material for sodium-ion batteries (SIBs); however, its sodium-storage mechanism has remained controversial, which imposes great difficulties in the design and construction of better microstructured HC materials. To obtain a deeper understanding of the Na-storage mechanism, we comparatively investigated electrochemical behaviors of HC and graphite for Na- and Li-storage reactions. The experimental results reveal that the Na-storage reaction on HC at a low-potential plateau proceeds in a manner similar to the Li+-insertion reaction on graphite but very differently from the Li+-storage process on HC, suggesting that the Na-storage mechanism of HC at a low-voltage plateau operates through the Na+ intercalation into the graphitic layers for the formation of sodium-graphite intercalation compounds (Na-GICs) and is consistent with the "adsorption-intercalation" mechanism. Our work might provide new insight for designing better HC materials of high-energy density SIBs.

Tunable Electrocatalytic Behavior of Sodiated MoS2 Active Sites toward Efficient Sulfur Redox Reactions in Room-Temperature Na-S Batteries.

Room-temperature (RT) sodium-sulfur (Na-S) batteries hold great promise for large-scale energy storage due to the advantages of high energy density, low cost, and resource abundance. The research progress on RT Na-S batteries, however, has been greatly hindered by the sluggish kinetics of the sulfur redox reactions. Herein, an elaborate multifunctional architecture, consisting of N-doped carbon skeletons and tunable MoS2 sulfiphilic sites, is fabricated via a simple one-pot reaction followed by in situ sulfurization. Beyond the physical confinement and chemical binding of polarized N-doped carbonaceous microflowers, the MoS2 active sites play a key role in catalyzing polysulfide redox reactions, especially the conversion from long-chain Na2 Sn (4 ≤ n ≤ 8) to short-chain Na2 S2 and Na2 S. Significantly, the electrocatalytic activity of MoS2 can be tunable via adjusting the discharge depth. It is remarkable that the sodiated MoS2 exhibits much stronger binding energy and electrocatalytic behavior compared to MoS2 sites, effectively enhancing the formation of the final Na2 S product. Consequently, the S cathode achieves superior electrochemical performance in RT Na-S batteries, delivering a high capacity of 774.2 mAh g-1 after 800 cycles at 0.2 A g-1 , and an ultrahigh capacity retention with a capacity decay rate of only 0.0055% per cycle over 2800 cycles.

Efficient and Facile Electrochemical Process for the Production of High-Quality Lithium Hexafluorophosphate Electrolyte.

The global consumption for lithium hexafluorophosphate (LiPF6) has increased dramatically with the rapid growth of Li-ion batteries (LIBs) for large-scale electric energy storage applications. Conventional LiPF6 production has a high cost and high energy consumption due to complicated separation and purification processes. Here, based on the electrode materials of LiMn2O4 and polyaniline (PANI), we propose a facile electrochemical extraction/release process for LiPF6 electrolyte production. This new technology consists of two independent steps: a PF6-- and Li+-extracting step using a PANI/LixMn2O4 cell in aqueous solution (an ion extraction step) and a LiPF6 electrolyte production step from the charged LiMn2O4/PANI+PF6- cell in an organic electrolyte (an ion release step). This new process can effectively avoid the contamination of HF residue in the final product, providing a great possibility to create a facile, energy-efficient, and low-cost LiPF6 electrolyte production.

Building a Cycle-Stable Fe-Si Alloy/Carbon Nanocomposite Anode for Li-Ion Batteries through a Covalent-Bonding Method.

Si is being intensively developed as a safe and high-performance anode for next-generation Li-ion batteries (LIBs); however, its battery application still remains challenging because of its low cycling Coulombic efficiency. To address this issue, we chose a conjugated polymer, polynaphthalene, as a carbon precursor and a low-cost commercial ferrosilicon (Fe-Si) alloy as the active phase to prepare a Fe-Si/C nanocomposite with a core-shell-like architecture through sand milling-assisted covalent-bonding method, followed by a carbonization reaction, thus forming a covalently bonded carbon coating on the surfaces of Fe-Si alloy nanoparticles. Benefitting from the greatly reduced volumetric expansion of Fe-Si alloy cores in the lithiation process and the stable interface provided by the outer carbon shell, the thus-prepared Fe-Si/C nanocomposite exhibits a high structural stability in repeated charge/discharge cycles. The experimental results reveal that the Fe-Si/C composite anode can demonstrate a high reversible capacity of 1316.2 mA h g-1 with an active mass utilization of 82.6%, a long-term cycle stability of more than 1000 cycles even at a considerably high current rate of 2.0 A g-1, and, in particular, a high cycling Coulombic efficiency of 99.7%, showing great prospect for application in practical LIBs.

Enabling electrochemical compatibility of non-flammable phosphate electrolytes for lithium-ion batteries by tuning their molar ratios of salt to solvent.

We develop a new type of electrolyte with a high molar ratio (MR) of salt to solvent but a low molar concentration by adjusting the molar mass of the solvent. The present 1 : 2 LiFSI-triamyl phosphate electrolyte exhibits a low molar concentration of only 1.35 M along with excellent electrochemical stability against the graphite anode.

Surface Modification of Fe7 S8 /C Anode via Ultrathin Amorphous TiO2 Layer for Enhanced Sodium Storage Performance.

Iron sulfides with high theoretical capacity and low cost have attracted extensive attention as anode materials for sodium ion batteries. However, the inferior electrical conductivity and devastating volume change and interface instability have largely hindered their practical electrochemical properties. Here, ultrathin amorphous TiO2 layer is constructed on the surface of a metal-organic framework derived porous Fe7 S8 /C electrode via a facile atomic layer deposition strategy. By virtue of the porous structure and enhanced conductivity of the Fe7 S8 /C, the electroactive TiO2 layer is expected to effectively improve the electrode interface stability and structure integrity of the electrode. As a result, the TiO2 -modified Fe7 S8 /C anode exhibits significant performance improvement for sodium-ion batteries. The optimal TiO2 -modified Fe7 S8 /C electrode delivers reversible capacity of 423.3 mA h g-1 after 200 cycles with high capacity retention of 75.3% at 0.2 C. Meanwhile, the TiO2 coating is conducive to construct favorable solid electrolyte interphase, leading to much enhanced initial Coulombic efficiency from 66.9% to 72.3%. The remarkable improvement suggests that the interphase modification holds great promise for high-performance metal sulfide-based anode materials for sodium-ion batteries.