Boosting Molecular Cross-Linking in a Phenolic Resin for Spherical Hard Carbon with Enriched Closed Pores toward Enhanced Sodium Storage Ability.

Phenolic resin (PF) is considered a promising precursor of hard carbon (HC) for advanced-performance anodes in sodium-ion batteries (SIBs) because of its facile designability and high residual carbon yield. However, understanding how the structure of PF precursors influences sodium storage in their derived HC remains a significant challenge. Herein, the microstructure of HC is controlled by the degree of cross-linking of resorcinol-benzaldehyde (RB) resin. We reveal that robust molecular cross-linking in RB resin induced by hydrothermal treatment promotes closed-pore formation in the derived HC. The mechanism is devised for the decomposition of a highly cross-linked RB three-dimensional network into randomly stacked short-range graphitic microcrystals during high-temperature carbonization, contributing to the abundant closed pores in the derived HC. In addition, the high cross-linking degree of RB resin endows its derived HC with a small-sized spherical morphology and large interlayer spacing, which improves the rate performance of HC. Consequently, the optimized hydrothermal treatment HC anode shows a higher specific capacity of 372.7 mAh g-1 and better rate performance than the HC anode without hydrothermal treatment (276.0 mAh g-1). This strategy can provide feasible molecular cross-linking engineering for the development of closed pores in PF-based HC toward enhanced sodium storage.

One-Step Construction of Closed Pores Enabling High Plateau Capacity Hard Carbon Anodes for Sodium-Ion Batteries: Closed-Pore Formation and Energy Storage Mechanisms.

Closed pores play a crucial role in improving the low-voltage (<0.1 V) plateau capacity of hard carbon anodes for sodium-ion batteries (SIBs). However, the lack of simple and effective closed-pore construction strategies, as well as the unclear closed-pore formation mechanism, has severely hindered the development of high plateau capacity hard carbon anodes. Herein, we present an effective closed-pore construction strategy by one-step pyrolysis of zinc gluconate (ZG) and elucidate the corresponding mechanism of closed-pore formation. The closed-pore formation mechanism during the pyrolysis of ZG mainly involves (i) the precipitation of ZnO nanoparticles and the ZnO etching on carbon under 1100 °C to generate open pores of 0.45-4 nm and (ii) the development of graphitic domains and the shrinkage of the partial open pores at 1100-1500 °C to convert the open pores to closed pores. Benefiting from the considerable closed-pore content and suitable microstructure, the optimized hard carbon achieves an ultrahigh reversible specific capacity of 481.5 mA h g-1 and an extraordinary plateau capacity of 389 mA h g-1 for use as the anode of SIBs. Additionally, some in situ and ex situ characterizations demonstrate that the high-voltage slope capacity and the low-voltage plateau capacity stem from the adsorption of Na+ at the defect sites and Na-cluster formation in closed pores, respectively.

A Review of the Relationship between Gel Polymer Electrolytes and Solid Electrolyte Interfaces in Lithium Metal Batteries.

Lithium metal batteries (LMBs) are a dazzling star in electrochemical energy storage thanks to their high energy density and low redox potential. However, LMBs have a deadly lithium dendrite problem. Among the various methods for inhibiting lithium dendrites, gel polymer electrolytes (GPEs) possess the advantages of good interfacial compatibility, similar ionic conductivity to liquid electrolytes, and better interfacial tension. In recent years, there have been many reviews of GPEs, but few papers discussed the relationship between GPEs and solid electrolyte interfaces (SEIs). In this review, the mechanisms and advantages of GPEs in inhibiting lithium dendrites are first reviewed. Then, the relationship between GPEs and SEIs is examined. In addition, the effects of GPE preparation methods, plasticizer selections, polymer substrates, and additives on the SEI layer are summarized. Finally, the challenges of using GPEs and SEIs in dendrite suppression are listed and a perspective on GPEs and SEIs is considered.

Utilizing Ultra-homogeneous SiOx and Defects to Achieve Interlayer Protection for Lithium Metal Anodes.

The uncontrollable growth and uneven nucleation of lithium metal can be addressed by utilizing spatial confinement structures in conjunction with lithiophilic sites. However, their complex fabrication technique and the inhomogeneous dispersion of lithiophilic sites make the application ineffective. In this work, ultra-uniformly dispersed SiOx seeds and defects are produced in situ to achieve the spatially restricted protection within the reduced graphene oxide (rGO) layer. The in situ formed SiOx and defects during annealing double constrain lithium nucleation and growth behaviors thanks to the superlithiophilic characteristic, while both provide the fast Li+ transport channel to utilize the interlayer protection of rGO in limiting lithium dendrite growth. Furthermore, XANES and XPS analyze the SiOx seeds that are dominated by various valence states, and theoretical calculations further verify the control on the nucleation of lithium atoms. Benefiting from the optimum average valence of three for the "control site", the host realizes steady circulation. In asymmetric cells, the host demonstrates excellent coulombic efficiency of 99.1% and stable lifespans over 1250 h at 1 mA cm-2 . When assembled in LiFePO4 full cells, it retains a favorable capacity of 116.2 mA h g-1 after 170 cycles.

Revealing the Self-Doping Defects in Carbon Materials for the Compact Capacitive Energy Storage of Zn-Ion Capacitors.

Zn-ion capacitors are attracting great attention owing to the abundant and relatively stable Zn anodes but are impeded by the low capacitance of porous carbon cathodes with insufficient energy storage sites. Herein, using ball-milled graphene with different defect densities as the models, we reveal that the self-doping defects of carbon show a capacitive energy storage behavior with robust charge-transfer kinetics, providing a capacitance contribution of ca. 90 F g-1 per unit of defect density (AD/AG value from Raman spectra) in both aqueous and organic electrolytes. Furthermore, a simple NaCl-assisted ball-milling method is developed to prepare novel graphene blocks (BSG) with abundant self-doping defect density, enriched pores, balanced electric conductivity, and high compact density (0.83 g cm-3). The optimized ion and electron transfer paths promote efficient utilization of the self-doping defects in BSG, contributing to improved gravimetric and volumetric capacitance (224 F g-1/186 F cm-3 at 0.5 A g-1) and remarkable rate performance (52.2% capacitance retention at 20 A g-1). The defect engineering strategy may open up a new avenue to improve the capacitive performance of dense carbons for Zn-ion capacitors.

Simple Construction of Multistage Stable Silicon-Graphite Hybrid Granules for Lithium-Ion Batteries.

Because of its high specific capacity, the silicon-graphite composite (SGC) is regarded as a promising anode for new-generation lithium-ion batteries. However, the frequently employed two-section preparation process, including the modification of silicon seed and followed mixture with graphite, cannot ensure the uniform dispersion of silicon in the graphite matrix, resulting in a stress concentration of aggregated silicon domains and cracks in composite electrodes during cycling. Herein, inspired by powder engineering, the two independent sections are integrated to construct multistage stable silicon-graphite hybrid granules (SGHGs) through wet granulation and carbonization. This method assembles silicon nanoparticles (Si NPs) and graphite and improves compatibility between them, addressing the issue of severe stress concentration caused by uncombined residue of Si NPs. The optimal SGHG prepared with 20% pitch content exhibits a highly reversible specific capacity of 560.0 mAh g-1 at a current density of 200 mA g-1 and a considerable stability retention of 86.1% after 1000 cycles at 1 A g-1 . Moreover, as a practical application, the full cell delivers an outstanding capacity retention of 85.7% after 400 cycles at 2 C. The multistage stable structure constructed by simple wet granulation and carbonization provides theoretical guidance for the preparation of commercial SGC anodes.

Lithiophilic onion-like carbon spheres as lithium metal uniform deposition host.

Lithium metal is considered as a promising anode material for next-generation secondary batteries, owing to its high theoretical specific capacity (3860 mA h g-1). Nevertheless, the practical application of Li in lithium metal batteries (LMBs) is hampered by inhomogeneous Li deposition and irreversible "dead Li", which lead to low coulombic efficiency (CE) and safe hazards. Designing unique lithiophilic structure is an efficient strategy to control Li uniformly plating /stripping. Here, we report the silver (Ag) nanoparticles coated with nitrogen-doped onion-like carbon microspheres (Ag@NCS) as a host to reduce the nucleation overpotential of Li for dendrite-free LBMs. The Ag@NCS were prepared by a simple one-step injection pyrolysis. The lithiophilic Ag is demonstrated to be priority selective deposition of Li in the carbon cage. Meanwhile, the onion-like structure benefits to uniform lithium nucleation and dendrite-free lithium during cycling. Impressively, we successfully captured lithium metal on different hosts at atomic scale, further proving that Ag@NCS can effectively and uniformly deposit Li. Besides, Ag@NCS show a superiorly electrochemical performance with a low nucleation overpotential (∼1 mV), high CE and stable cycling performance (over 400 cycles at 0.5 mA cm-2) compared to the Ag-free onion-like carbon in LMBs. Even under harsh conditions (1 mA cm-2, 4 mA h cm-2), Ag@NCS still present superior cycling stability for more than 150 cycles. Furthermore, a full cell composed of LiFePO4 cathode exhibits significantly improved voltage hysteresis with low voltage polarization. This work provides a new choice and route for the design and preparation of lithiophilic host materials.

Improving the surface area of metal organic framework-derived porous carbon through constructing inner support by compatible graphene quantum dots.

Metal-organic frameworks (MOFs) have emerged as promising precursors to prepare porous carbons due to their unique coordination structure with abundant pores and various chemical compositions. However, the structural collapse and pore shrinkage during pyrolysis severely decrease the surface area of the prepared porous carbons. Herein, we propose an inner support strategy to prepare MOF-derived carbons with improved surface area using graphene quantum dots (GQDs) as the compatible frameworks. GQDs with abundant carboxyl groups (-COOH) and rigid structure can uniformly distribute in MOF-5 precursor by coordinating with [Zn4O]6+ clusters and effectively reinforce the carbon skeleton during pyrolysis. Therefore, the rational GQDs embedded MOF-5 derived porous carbon (GMPC-0.35) shows greatly improved specific surface area (1841 m2 g-1) and mesopore volume (1.62 cm3 g-1) than pure MOF-5 derived carbon (1358 m2 g-1, 0.59 cm3 g-1). As an application exemplification, GMPC-0.35 performs high specific capacitance of 200 F g-1 at 1 A g-1 and good capacitance retention of 53% at 100 A g-1 as the electrode material for supercapacitors, which are higher than most of the reported MOF-5 derived carbons. Therefore, the compatible GQDs support is promising for preparing functional MOF-derived carbon materials.

Efficient Utilization of the Active Sites in Defective Graphene Blocks through Functionalization Synergy for Compact Capacitive Energy Storage.

Designing dense carbon materials with both high capacitance and good rate performance is crucial for future development of minimized and light-weight supercapacitors but remains challenging because sluggish ion transport inhibits the efficient utilization of the energy storage sites. Herein, we report a defective and functionalized graphene block (DFGB) prepared through ball milling using controllably reduced graphene oxide (RGO) as the precursor. Rational oxygen configuration enables good electrolyte wettability and improves ion migration kinetics, facilitating high utilization of the "self-doping" defects as active sites. Benefiting from this synergistic effect, the optimized DFGB with a high compact density of 0.92 g cm-3 shows high capacitances of 302 F g-1 and 278 F cm-3 at 1 A g-1 and good rate performance with a capacitance retention of 42% at 100 A g-1, which are among the best of the reported carbons. Moreover, the symmetric device at the commercial mass loading still shows a high energy density and remarkable cycle stability, demonstrating the importance of functionalization synergy in fully realizing the compact energy storage ability of carbon materials.