A Slightly Expanded Graphite Anode with High Capacity Enabled By Stable Lithium-Ion/Metal Hybrid Storage.

Integrating lithium-ion and metal storage mechanisms to improve the capacity of graphite anode holds the potential to boost the energy density of lithium-ion batteries. However, this approach, typically plating lithium metal onto traditional graphite anodes, faces challenges of safety risks of severe lithium dendrite growth and short circuits due to restricted lithium metal accommodation space and unstable lithium plating in commercial carbonate electrolytes. Herein, a slightly expanded spherical graphite anode is developed with a precisely adjustable expanded structure to accommodate metallic lithium, achieving a well-balanced state of high capacity and stable lithium-ion/metal storage in commercial carbonate electrolytes. This structure also enables fast kinetics of both Li intercalation/de-intercalation and plating/stripping. With a total anode capacity of 1.5 times higher (558 mAh g-1) than graphite, the full cell coupled with a high-loading LiNi0.8Co0.1Mn0.1O2 cathode (13 mg cm-2) under a low N/P ratio (≈1.15) achieves long-term cycling stability (75% of capacity after 200 cycles, in contrast to the fast battery failure after 50 cycles with spherical graphite anode). Furthermore, the capacity of the full cell also reaches a low capacity decay rate of 0.05% per cycle at 0.2 C under the low temperature of -20 °C.

Free-Standing Stable Silicon-Based Anode with Exceptional Flexibility Realized by a Multifunctional Structure Design in Multiple Dimensions.

Cyclized polyacrylonitrile (cPAN) with decently flexible, elastic, and conductive properties is a promising substrate or binder material for flexible devices. However, it is infeasible to accommodate the large volume expansion and contribute the exceptional rate capability of silicon anodes in lithium-ion batteries only counting on the limited elasticity and conductivity of cPAN. Herein, we report a robust silicon/carbon-cPAN-graphene (SC-CP-G) composite membrane with excellent flexibility based on a multifunctional structure design in multiple dimensions, which can be used as a free-standing integrated anode for lithium ion batteries. In this integrated electrode, silicon nanoparticles are encapsulated in porous carbon with in situ formed confined space, and the silicon/carbon particles are further embedded in cPAN nanofibers, which are inextricably interwoven with a reduced graphene oxide film, forming an interpenetrating network architecture. The unique hierarchical and functional structure design greatly improves the mechanical performance, cycling stability, and capacity accessibility of silicon electrodes, delivering a specific capacity of 1847 mA h g-1 at 2 A g-1 and a capacity retention of 87% after 150 cycles.

Commercially Viable Hybrid Li-Ion/Metal Batteries with High Energy Density Realized by Symbiotic Anode and Prelithiated Cathode.

The energy density of commercial lithium (Li) ion batteries with graphite anode is reaching the limit. It is believed that directly utilizing Li metal as anode without a host could enhance the battery's energy density to the maximum extent. However, the poor reversibility and infinite volume change of Li metal hinder the realistic implementation of Li metal in battery community. Herein, a commercially viable hybrid Li-ion/metal battery is realized by a coordinated strategy of symbiotic anode and prelithiated cathode. To be specific, a scalable template-removal method is developed to fabricate the porous graphite layer (PGL), which acts as a symbiotic host for Li ion intercalation and subsequent Li metal deposition due to the enhanced lithiophilicity and sufficient ion-conducting pathways. A continuous dissolution-deintercalation mechanism during delithiation process further ensures the elimination of dead Li. As a result, when the excess plating Li reaches 30%, the PGL could deliver an ultrahigh average Coulombic efficiency of 99.5% for 180 cycles with a capacity of 2.48 mAh cm-2 in traditional carbonate electrolyte. Meanwhile, an air-stable recrystallized lithium oxalate with high specific capacity (514.3 mAh g-1) and moderate operating potential (4.7-5.0 V) is introduced as a sacrificial cathode to compensate the initial loss and provide Li source for subsequent cycles. Based on the prelithiated cathode and initial Li-free symbiotic anode, under a practical-level 3 mAh capacity, the assembled hybrid Li-ion/metal full cell with a P/N ratio (capacity ratio of LiNi0.8Co0.1Mn0.1O2 to graphite) of 1.3 exhibits significantly improved capacity retention after 300 cycles, indicating its great potential for high-energy-density Li batteries.

Bidirectional Lithiophilic Gradients Modification of Ultralight 3D Carbon Nanofiber Host for Stable Lithium Metal Anode.

Using 3D host is an effective way to solve the dendrite growth problem and accommodate volume changes of lithium (Li) metal anode. However, the preferred Li deposition on the top surface leads to the Li metal agglomeration at the surface. In addition, the large weight of the 3D host also greatly decreases the capacity based on the whole anode. Herein, a bidirectional lithiophilic gradient modification, including a top-down ZnO gradient and a bottom-up Sn gradient, is applied to an ultralight 3D carbon nanofiber host (density: 0.1 g cm-3 ) and ensures the evenly filling lithium deposition in the 3D host. ZnO transforms into highly ionic conductive Li-Zn alloy and Li2 O during cycling, enhancing the Li-ion transportation from top to bottom. The metallic Sn also lowers the Li nucleation potential, guiding the preferential Li deposition from the bottom. With such a host, a stable CE of 97.5% over 100 cycles at 1 mA cm-2 and 3 mAh cm-2 is achieved, and the full battery also delivers good cycling stability over 300 cycles with a high CE of 99.8% coupled with high loading LiFePO4 cathode (10 mg cm-2 ) and low N/P ratio (≈3).

Highly porous carbon nanofiber electrodes for vanadium redox flow batteries.

The electrochemical performance of carbon nanofiber (CNF) electrodes in vanadium redox flow batteries (VRFBs) is enhanced by optimizing the morphological and physical properties of low-cost electrospun CNFs. The surface area, porosity and electrical conductivity of CNFs are tailored by modifying the precursor composition, especially the sacrificing agent, Fe(acac)3, in the polymer precursor and carbonization temperature. A highly porous structure with a large surface area is generated by the catalytic growth of graphitic carbon spheres surrounding the iron nanoparticles which are removed by an acid etching process. The graphitic carbon layers formed at a high carbonization temperature improve the electrical conductivity of CNFs. The large surface area of 349 m2 g-1 together with the abundant mesopore-dominant structure leads to high wettability and high activity for redox reactions of the electrode, giving rise to enhanced electrochemical performance in VRFBs. It delivers an energy efficiency (EE) of 91.4% at a current density of 20 mA cm-2 and 79.3% at 100 mA cm-2, and maintains an average EE of 72.5% after 500 charge/discharge cycles at 100 mA cm-2.

Revisiting the Roles of Natural Graphite in Ongoing Lithium-Ion Batteries.

Graphite, commonly including artificial graphite and natural graphite (NG), possesses a relatively high theoretical capacity of 372 mA h g-1 and appropriate lithiation/de-lithiation potential, and has been extensively used as the anode of lithium-ion batteries (LIBs). With the requirements of reducing CO2 emission to achieve carbon neutral, the market share of NG anode will continue to grow due to its excellent processability and low production energy consumption. NG, which is abundant in China, can be divided into flake graphite (FG) and microcrystalline graphite (MG). In the past 30 years, many researchers have focused on developing modified NG and its derivatives with superior electrochemical performance, promoting their wide applications in LIBs. Here, a comprehensive overview of the origin, roles, and research progress of NG-based materials in ongoing LIBs is provided, including their structure, properties, electrochemical performance, modification methods, derivatives, composites, and applications, especially the strategies to improve their high-rate and low-temperature charging performance. Prospects regarding the development orientation as well as future applications of NG-based materials are also considered, which will provide significant guidance for the current and future research of high-energy-density LIBs.

Gradient Structure Design of a Floatable Host for Preferential Lithium Deposition.

Herein, a novel sandwichlike host with expandable accommodation and gradient characteristics of lithiophilicity and conductivity is prepared by constructing a reduced graphene oxide (rGO)/SiO2/rGO intercalated structure on the basis of electrospraying and coating an additional PVDF-HFP layer on the top surface. This gradient host electrode enables preferential, ordered, and uniform Li deposition in the SiO2-embedded interlayer space. The dendrite growth and isolated Li are suppressed by the combined rGO/PVDF-HFP layer with robust, flexible, and floatable features, which could function as an artificial solid-electrolyte interphase to impede reckless electrolyte infiltration, homogenize the Li ion flux distribution, and build a stable electrochemical interface. The designed electrodes could be stably cycled with a high capacity of 5 mAh cm-2 and give rise to a high average Coulombic efficiency (CE) of 99.14%. Furthermore, the derived full cells can deliver an average CE of 99.87% in 300 cycles with a capacity retention of 90.22% and successfully operate under lean electrolyte conditions.

In Situ Constructed Ionic-Electronic Dual-Conducting Scaffold with Reinforced Interface for High-Performance Sodium Metal Anodes.

The formation of severe dendritic sodium (Na) microstructure reduces the reversibility of anode and further hinders its practical implementation. In this work, an ionic-electronic dual-conducting (IEDC) scaffold composed of Na3 P and carbon nanotubes is in situ developed by a scalable strategy with subsequent alloying reaction, for realizing dendrite-free Na deposition under high current density and large areal capacity. The in situ formed Na3 P with high sodiophilicity not only sets up a hierarchically efficient ionic conducting network, but also participates in the construction of reinforced solid electrolyte interphase, while carbon nanotubes can assemble an electronic conducting framework. As a result, the multifunctional IEDC scaffold contributes to smooth Na plating and exceptionally reversible Na stripping. High average Coulombic efficiency of 99.8% after prolonged 1200 cycles at 3 mA cm-2 and small overpotential of 20 mV over 250 h (equals to 530 cycles) at high rate of 5 mA cm-2 are obtained. The high availability of Na in IEDC scaffold enables the impressive performance of full cell with limited Na, using Na3 V2 (PO4 )3 (NVP) cathode at practical level. More importantly, the as-developed anode-free full cell with IEDC||NVP configuration delivers a high capacity retention with long lifetime, indicating its great potential for practical Na metal batteries.

Heterogeneous Degradation in Thick Nickel-Rich Cathodes During High-Temperature Storage and Mitigation of Thermal Instability by Regulating Cationic Disordering.

The thermal instability is a major problem in high-energy nickel-rich layered cathode materials for large-scale battery application. Due to the scarce investigation of thick electrodes at the practical full-cell level, the understanding of thermal failure mechanism is still insufficient. Herein, an intrinsic origin of thermal instability in fully charged industrial pouch cells during high-temperature storage is discovered. Through the investigation from crystals to particles, and from electrodes to cells, it is shown that serious top-down heterogeneous degradation occurs along the depth direction of the thick electrode, including phase transition, cationic disordering, intergranular/intragranular cracks, and side reactions. Such degradation originates from the abundant oxygen vacancies and reduced catalytic Ni2+ at cathode surface, causing microstructural defects and directly leading to the thermal instability. Nonmagnetic elements doping and surface modification are suggested to be effective in mitigating the thermal instability through modulating cationic disordering.

Hierarchical Porous Graphene Bubbles as Host Materials for Advanced Lithium Sulfur Battery Cathode.

The serious shuttle effect, low conductivity, and large volume expansion have been regarded as persistent obstacles for lithium sulfur (Li-S) batteries in its practical application. Carbon materials, such as graphene, are considered as promising cathode hosts to alleviate those critical defects and be possibly coupled with other reinforcement methods to further improve the battery performance. However, the open structure of graphene and the weak interaction with sulfur species restrict its further development for hosting sulfur. Herein, a rational geometrical design of hierarchical porous graphene-like bubbles (PGBs) as a cathode host of the Li-S system was prepared by employing magnesium oxide (MgO) nanoparticles as templates for carbonization, potassium hydroxide (KOH) as activation agent, and car tal pitch as a carbon source. The synthesized PGBs owns a very thin carbon layer around 5 nm that can be comparable to graphite nanosheets. Its high content of mesoporous and interconnected curved structure can effectively entrap sulfur species and impose restrictions on their diffusion and shuttle effect, leading to a much stable electrochemical performance. The reversible capacity of PGBs@S 0.3 C still can be maintained at 831 mAh g-1 after 100 cycles and 512 mAh g-1 after 500 cycles.

Facile Synthesis of Ant-Nest-Like Porous Duplex Copper as Deeply Cycling Host for Lithium Metal Anodes.

Suppressing the dendrite formation and managing the volume change of lithium (Li) metal anode have been global challenges in the lithium batteries community. Herein, a duplex copper (Cu) foil with an ant-nest-like network and a dense substrate is reported for an ultrastable Li metal anode. The duplex Cu is fabricated by sulfurization of thick Cu foil with a subsequent skeleton self-welding procedure. Uniform Li deposition is achieved by the 3D interconnected architecture and lithiophilic surface of self-welded Cu skeleton. The sufficient space in the porous layer enables a large areal capacity for Li and significantly improves the electrode-electrolyte interface. Simulations reveal that the structure allows proper electric field penetration into the connected tunnels. The assembled Li anodes exhibit high coulombic efficiency (97.3% over 300 cycles) and long lifespan (>880 h) at a current density of 1 mA cm-2 with a capacity of 1 mAh cm-2 . Stable and deep cycling can be maintained up to 50 times at a high capacity of 10 mAh cm-2 .

Electrosprayed Robust Graphene Layer Constructing Ultrastable Electrode Interface for High-Voltage Lithium-Ion Batteries.

Aluminum (Al) foil serving as the most widely used cathode current collector for lithium-ion batteries (LIBs) is still not flawless to fulfill the increasing demand of rechargeable energy storage systems. The limited contact area and weak adhesion to cathode material as well as local corrosion during long-term operations could deteriorate the performance of LIBs with a higher working voltage. Herein, a reduced graphene oxide (RGO)-modified Al foil (RGO/Al) is developed via electrospraying to increase interfacial adhesion and inhibit anodic corrosion as a functional current collector. Valid corrosion resistance to electrolyte and strengthened adhesion of electrode particles to current collectors are beneficial to improve the interfacial reaction dynamics. The RGO/Al-based LiNi0.5Mn1.5O4 cells (LNMO-RGO/Al) exhibit better electrochemical performances in terms of long-term cycling discharge capacity retention (90% after 840 cycles at 1 C), rate capability (101.8 mAh g-1 at 5 C), and interfacial resistance, prominently superior to bare Al-based cells (LNMO-Al). This work not only contributes to long-term stable operation of high-voltage LIBs but also brings new opportunities for the development of next-generation 5 V LIBs.

Simultaneously Homogenized Electric Field and Ionic Flux for Reversible Ultrahigh-Areal-Capacity Li Deposition.

High areal capacity and stable Coulombic efficiency (CE) in lithium metal anodes (LMAs) play pivotal roles in developing high-energy-density rechargeable batteries. However, few reported LMAs delivering high and stable CE (>50 cycles) under ultrahigh areal capacity (>10 mA h cm-2). We demonstrated that the simultaneous homogenization of electric field and Li ion flux by using self-supported and surface-oxidized 3D hollow porous copper fibers (3D-HPCFs) can greatly increase both the areal capacity and reversibility of Li deposition. Li can be easily confined inside the hollow porous fibers and within the interspaces among fibers without uncontrollable Li dendrites. The 3D-HPCF-based anode can be deeply cycled at high capacity of 15 mA h cm-2 with average CE of 98.87% for 53 cycles, enabling a practical cell to realize high capacity retentions at a surplus Li of 10%. This work provides a novel Li deposition-regulation technology in LMAs targeting for next generation high-energy-density batteries.

Advanced Matrixes for Binder-Free Nanostructured Electrodes in Lithium-Ion Batteries.

Commercial lithium-ion batteries (LIBs), limited by their insufficient reversible capacity, short cyclability, and high cost, are facing ever-growing requirements for further increases in power capability, energy density, lifespan, and flexibility. The presence of insulating and electrochemically inactive binders in commercial LIB electrodes causes uneven active material distribution and poor contact of these materials with substrates, reducing battery performance. Thus, nanostructured electrodes with binder-free designs are developed and have numerous advantages including large surface area, robust adhesion to substrates, high areal/specific capacity, fast electron/ion transfer, and free space for alleviating volume expansion, leading to superior battery performance. Herein, recent progress on different kinds of supporting matrixes including metals, carbonaceous materials, and polymers as well as other substrates for binder-free nanostructured electrodes in LIBs are summarized systematically. Furthermore, the potential applications of these binder-free nanostructured electrodes in practical full-cell-configuration LIBs, in particular fully flexible/stretchable LIBs, are outlined in detail. Finally, the future opportunities and challenges for such full-cell LIBs based on binder-free nanostructured electrodes are discussed.

Basal Nanosuit of Graphite for High-Energy Hybrid Li Batteries.

Lithium (Li) metal anode has attracted tremendous attention for its highest capacity (3860 mAh g-1). Herein, we report that the formation of dead Li can be effectively suppressed through Li plating on porous lithiated graphite lamina (PLGL). A lithiophilic carbon layer was decorated on the lithiophobic basal plane of porous graphite lamina (PGL) with an industry-scalable slurry-coating strategy. Moreover, the higher delithiation potential of PLGL will ensure the complete stripping of the plated Li before its delithiation, thus dramatically enhancing the average Coulombic efficiency (ACE) of Li plating/stripping to 98.5% at a high Li plating/stripping capacity of 2 mAh cm-2 (∼1100 mAh g-1) at 2 mA cm-2. Even at an ultrahigh current density of 4 mA cm-2 (with Li capacity of 4 mAh cm-2 (∼1900 mAh g-1)), the ACE could still be maintained at 96.2% in an ordinary carbonate electrolyte.

In-Plane Highly Dispersed Cu2O Nanoparticles for Seeded Lithium Deposition.

Uncontrollable dendrite growth is one of the major problems that hinders the application of lithium (Li) metal anode in rechargeable Li batteries. Achieving uniform Li deposition is the key to tackle this intractable problem. Herein, we report the highly dispersed Cu2O nanoparticles (NPs) in situ anchored on partially reduced graphene oxide via a low-temperature pyrolysis process could serve as seeds for the Li metal deposition. The lithiophilic nature of Cu2O NPs reduces the overpotential of Li nucleation and relieves the electrode polarization, enabling uniform Li nucleation and smooth plating, thus effectively eliminating dendritic and dead Li. As a result, the resulted Li metal electrodes deliver a high Coulombic efficiency of 95.6% after 140 cycles at a current density of 2 mA cm-2 and a prolonged lifespan (800 h at 1 mA cm-2) for the symmetrical cell.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research.

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

A room-temperature sodium-sulfur battery with high capacity and stable cycling performance.

High-temperature sodium-sulfur batteries operating at 300-350 °C have been commercially applied for large-scale energy storage and conversion. However, the safety concerns greatly inhibit their widespread adoption. Herein, we report a room-temperature sodium-sulfur battery with high electrochemical performances and enhanced safety by employing a "cocktail optimized" electrolyte system, containing propylene carbonate and fluoroethylene carbonate as co-solvents, highly concentrated sodium salt, and indium triiodide as an additive. As verified by first-principle calculation and experimental characterization, the fluoroethylene carbonate solvent and high salt concentration not only dramatically reduce the solubility of sodium polysulfides, but also construct a robust solid-electrolyte interface on the sodium anode upon cycling. Indium triiodide as redox mediator simultaneously increases the kinetic transformation of sodium sulfide on the cathode and forms a passivating indium layer on the anode to prevent it from polysulfide corrosion. The as-developed sodium-sulfur batteries deliver high capacity and long cycling stability.

Fe3O4-Decorated Porous Graphene Interlayer for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries are seriously restrained by the shuttling effect of intermediary products and their further reduction on the anode surface. Considerable researches have been devoted to overcoming these issues by introducing carbon-based materials as the sulfur host or interlayer in the Li-S systems. Herein, we constructed a multifunctional interlayer on a separator by inserting Fe3O4 nanoparticles (NPs) in a porous graphene (PG) film to immobilize polysulfides effectively. The porous structure of graphene was optimized by controlling the oxidation conditions for facilitating ion transfer. The polar Fe3O4 NPs were employed to trap sulfur species via strong chemical interaction. By exploiting the PG-Fe3O4 interlayer with optimal porous structure and component, the Li-S battery delivered a superior cycling performance and rate capability. The reversible discharge capacity could be maintained at 732 mAh g-1 after 500 cycles and 356 mAh g-1 after total 2000 cycles at 1 C with a final capacity retention of 49%. Moreover, a capacity of 589 mAh g-1 could also be maintained even at 2 C rate.

Electrospun N-Doped Hierarchical Porous Carbon Nanofiber with Improved Degree of Graphitization for High-Performance Lithium Ion Capacitor.

The lithium-ion capacitor (LIC) has been regarded as a promising device that combines the merits of lithium-ion batteries and supercapacitors, and that meets the requirements for both high energy and high power density. The development of advanced electrode materials is the key requirement. Herein, we report the bottom-up synthesis of activated carbon nanofiber (a-PANF) with a hierarchical porous structure and a high degree of graphitization. Electrospinning has been employed to prepare an interconnected fiber network with macropores, and ferric acetylacetonate has been introduced as both a mesopore-creating agent and a graphitic catalyst to increase the degree of graphitization. Furthermore, chemical activation enlarges the specific surface area by producing abundant micropores. Half-cell evaluation of the as-prepared a-PANF gave a discharge capacity of 80 mA h g-1 at 0.1 A g-1 within 2-4.5 V and no capacity fading after 1000 cycles at 2 A g-1 , which represents a significant improvement compared to conventional activated carbon (AC). Furthermore, an as-assembled LIC with a-PANF cathode and Fe3 O4 anode showed a superior energy density of 124.6 W h kg-1 at a specific power of 93.8 W kg-1 , which remained at 103.7 W h kg-1 at 4687.5 W kg-1 . This indicates promising application potential of a-PANF as an electrode material for efficient energy storage systems.