A 3D Hierarchical Host with Enhanced Sodiophilicity Enabling Anode-Free Sodium-Metal Batteries

The Enormous Potential of Sodium/Potassium-Ion Batteries as the Mainstream Energy Storage Technology for Large-Scale Commercial Applications

Cost-effectiveness plays a decisive role in sustainable operating of rechargeable batteries. As such, the low cost-consumption of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) provides a promising direction for "how do SIBs/PIBs replace Li-ion batteries (LIBs) counterparts" based on their resource abundance and advanced electrochemical performance. To rationalize the SIBs/PIBs technologies as alternatives to LIBs from the unit energy cost perspective, this review gives the specific criteria for their energy density at possible electrode-price grades and various battery-longevity levels. The cost ($ kWh-1 cycle-1) advantage of SIBs/PIBs is ascertained by the cheap raw-material compensation for the cycle performance deficiency and the energy density gap with LIBs. Furthermore, the cost comparison between SIBs and PIBs, especially on cost per kWh and per cycle, is also involved. This review explicitly manifests the practicability and cost-effectiveness toward SIBs are superior to PIBs whose commercialization has so far been hindered by low energy density. Even so, the huge potential on sustainability of PIBs, to outperform SIBs, as the mainstream energy storage technology is revealed as long as PIBs achieve long cycle life or enhanced energy density, the related outlook of which is proceeded as the next development directions for commercial applications.

A "Liquid-In-Solid" Electrolyte for High-Voltage Anode-Free Rechargeable Sodium Batteries

Developing anode-free batteries is the ultimate goal in pursuit of high energy density and safety. It is more urgent for sodium (Na)-based batteries due to its inherently low energy density and safety hazards induced by highly reactive Na metal anodes. However, there is no electrolyte that can meet the demanding Na plating-stripping Coulomb efficiency (CE) while resisting oxidative decomposition at high voltages for building stable anode-free Na batteries. Here, a "liquid-in-solid" electrolyte design strategy is proposed to integrate target performances of liquid and solid-state electrolytes. Breaking through the Na+ transport channel of Na-containing zeolite molecular sieve by ion-exchange and confining aggregated liquid ether electrolytes in the nanopore and void of zeolites, it achieves excellent high-voltage stability enabled by solid-state zeolite electrolytes, while inheriting the ultra-high CE (99.84%) from liquid ether electrolytes. When applied in a 4.25 V-class anode-free Na battery, an ultra-high energy density of 412 W h kg-1 (based on the active material of both cathodes and anodes) can be reached, which is comparable to the state-of-the-art graphite||LiNi0.8Co0.1Mn0.1O2 lithium-ion batteries. Furthermore, the assembled anode-free pouch cell exhibits excellent cycling stability, and a high capacity retention of 89.2% can be preserved after 370 cycles.

A Critical Review on Room-Temperature Sodium-Sulfur Batteries: From Research Advances to Practical Perspectives

Room-temperature sodium-sulfur (RT-Na/S) batteries are promising alternatives for next-generation energy storage systems with high energy density and high power density. However, some notorious issues are hampering the practical application of RT-Na/S batteries. Besides, the working mechanism of RT-Na/S batteries under practical conditions such as high sulfur loading, lean electrolyte, and low capacity ratio between the negative and positive electrode (N/P ratio), is of essential importance for practical applications, yet the significance of these parameters has long been disregarded. Herein, it is comprehensively reviewed recent advances on Na metal anode, S cathode, electrolyte, and separator engineering for RT-Na/S batteries. The discrepancies between laboratory research and practical conditions are elaborately discussed, endeavors toward practical applications are highlighted, and suggestions for the practical values of the crucial parameters are rationally proposed. Furthermore, an empirical equation to estimate the actual energy density of RT-Na/S pouch cells under practical conditions is rationally proposed for the first time, making it possible to evaluate the gravimetric energy density of the cells under practical conditions. This review aims to reemphasize the vital importance of the crucial parameters for RT-Na/S batteries to bridge the gaps between laboratory research and practical applications.

Intercalation-Induced Localized Conversion Reaction in h-CuSe for Ultrafast-Rechargeable and Long-Cycling Sodium Metal Battery

Cathode materials of sodium-based batteries with high specific capacity and fast charge-discharge mode, as well as ultralong reversible cycles at wide applied temperatures, are essential for future development of advanced energy storage system. Developing transition metal selenides with intercalation features provides a new strategy for realizing the above cathode materials. Herein, this work reports a storage mechanism of sodium ion in hexagonal CuSe (h-CuSe) based on the density functional theory (DFT) guidance. This work reveals that the two-dimensional ion intercalation triggers localized redox reaction in the h-CuSe bulk phase, termed intercalation-induced localized conversion (ILC) mechanism, to stabilize the sodium storage structure by forming localized Cu7Se4 transition phase and adjusting the near-edge coordination state of the Cu sites to achieve high reversible capacity and ultra-long cycling life, while allowing rapid charge-discharge cycling over a wide temperature range.

Toward a Metal Anode-Free Zinc-Air Battery for Next-Generation Energy Storage

Rechargeable aqueous zinc-air batteries (ZABs) promise high energy density and safety. However, the use of conventional zinc anodes affects the energy output from the battery, so that the theoretical energy density is not achievable under operation conditions. A large portion of the zinc is shielded by anode passivation during the discharge process and remains electrochemically unused, making the operation of rechargeable ZABs inefficient up to date. In a metal anode-free ZAB, there is no unnecessary excess zinc if the zinc reservoir can be precisely adjusted by electrodeposition of zinc from the electrolyte. In this respect, an anode-free battery uses the electrolyte offering a dual-mode functionality not only providing ionic conductivity but also being the source of zinc. In addition, it is shown that a defined porous anode architecture is crucial for high rechargeability in this new type of ZAB. 3D-spatially arranged carbon nanotubes as geometrically defined host structures allow a homogeneous zinc deposition from the electrolyte. Together with carbon nanohorns as an active 2e- catalyst on the cathode side, the rechargeability of this new concept reaches up to 92%.

Role of Interface Strain and Chemo-Mechanical Effects during Electroplating in Sodium Metal Battery Anodes

In this study, we demonstrate that elastic strain applied to a current collector can influence the overall thermodynamic and kinetic picture of sodium metal electrodeposition and hence the performance of a sodium metal battery. To controllably study the role of strain in electrochemical performance, we utilize NiTi foil as a stable current collector, nucleation interface, and superelastic material. Our findings demonstrate that a locked-in elastic tensile strain near 8% results in 40 mV lower onset potential for sodium electrodeposition, 19% decrease in charge transfer resistance, and 20% lower cumulative sodium loss, among other effects. These performance improvements are correlated primarily to the control of the irreversible behavior in the first few minutes of electroplating. Given the prevalence of strain buildup in commercial battery cell configurations, our work highlights that strained current collector interfaces can result in significant long-term chemo-mechanical performance outcomes broadly relevant to sodium and other metal battery design considerations.

Anode-Free Sodium Metal Pouch Cell Using Cu3P Nanowires In Situ Grown on Current Collector

Anode-free sodium metal battery (AFSMB) promises high energy density but suffers from the difficulty of maintaining high cycling stability. Nonuniform sodium (Na) deposition on the current collector is largely responsible for capacity decay in the cycling process of AFSMB. Here, a unique copper phosphide (Cu3P) nanowire is constructed on copper (Cu3P@Cu) as a sodium deposition substrate by an in situ growth method. Superior electrochemical performance of Cu3P@Cu anode is delivered in asymmetric cells with an average Coulombic efficiency of 99.8% for over 800 cycles at 1 mA cm-2 with 1 mA h cm-2. The symmetric cell of Cu3P@Cu displayed a cycling lifespan of over 2000 h at 2 mA cm-2 with 1 mA h cm-2. Cryo-transmission electron microscope characterization and first principles calculation revealed that the low Na+ absorption energy and low Na+ diffusion energy barrier on Na3P promoted uniform Na nucleation and deposition, thus enhancing the Na surface stability. Moreover, anode-free Na3V2(PO4)3//Cu3P@Cu full pouch cell delivered a considerable cycling capacity of ≈15 mA h in 170 cycles, demonstrating its practical feasibility.

Building Stable Anodes for High-Rate Na-Metal Batteries

Due to low cost and high energy density, sodium metal batteries (SMBs) have attracted growing interest, with great potential to power future electric vehicles (EVs) and mobile electronics, which require rapid charge/discharge capability. However, the development of high-rate SMBs has been impeded by the sluggish Na+ ion kinetics, particularly at the sodium metal anode (SMA). The high-rate operation severely threatens the SMA stability, due to the unstable solid-electrolyte interface (SEI), the Na dendrite growth, and large volume changes during Na plating-stripping cycles, leading to rapid electrochemical performance degradations. This review surveys key challenges faced by high-rate SMAs, and highlights representative stabilization strategies, including the general modification of SMB components (including the host, Na metal surface, electrolyte, separator, and cathode), and emerging solutions with the development of solid-state SMBs and liquid metal anodes; the working principle, performance, and application of these strategies are elaborated, to reduce the Na nucleation energy barriers and promote Na+ ion transfer kinetics for stable high-rate Na metal anodes. This review will inspire further efforts to stabilize SMAs and other metal (e.g., Li, K, Mg, Zn) anodes, promoting high-rate applications of high-energy metal batteries towards a more sustainable society.

Restructuring Electrolyte Solvation by a Versatile Diluent Toward Beyond 99.9% Coulombic Efficiency of Sodium Plating/Stripping at Ultralow Temperatures

The reversible and durable operation of sodium metal batteries at low temperatures (LT) is essential for cold-climate applications but is plagued by dendritic Na plating and unstable solid-electrolyte interphase (SEI). Current Coulombic efficiencies of sodium plating/stripping at LT fall far below 99.9%, representing a significant performance gap yet to be filled. Here, the solvation structure of the conventional 1 m NaPF6 in diglyme electrolyte by facile cyclic ether (1,3-dioxolane, DOL) dilution is efficiently reconfigured. DOL diluents help shield the Na+-PF6 - Coulombic interaction and intermolecular forces of diglyme, leading to anomalously high Na+-ion conductivity. Besides, DOL participates in the solvation sheath and weakens the chelation of Na+ by diglyme for facilitated desolvation. More importantly, it promotes concentrated electron cloud distribution around PF6 - in the solvates and promotes their preferential decomposition. A desired inorganic-rich SEI is generated with compositional uniformity, high ionic conductivity, and high Young's modulus. Consequently, a record-high Coulombic efficiency over 99.9% is achieved at an ultralow temperature of -55 °C, and a 1 Ah capacity pouch cell of initial anode-free sodium metal battery retains 95% of the first discharge capacity over 100 cycles at -25 °C. This study thus provides new insights for formulating electrolytes toward increased Na reversibility at LT.

3D mixed ion/electron-conducting scaffolds for stable sodium metal anodes

Sodium (Na) metal batteries represent an optimal choice for the forthcoming generation of large-scale, cost-effective energy storage systems. However, Na metal anodes encounter several formidable challenges during the Na plating and stripping processes, which encompass the formation of an unstable solid electrolyte interface, uncontrollable dendrite growth, and infinite volume expansion. These issues result in a reduced Coulombic efficiency, shortened battery lifespan, and potential safety hazards, thereby constraining their commercial development. Therefore, addressing these challenges to ensure the cycling stability of Na metal anodes stands as a paramount requirement for practical applications. Among the reported strategies, three-dimensional conductive scaffolds possessing a high surface area and porous structure are acknowledged for their significant potential in stabilizing Na metal anodes. Compared with conventional electron-conducting scaffolds, emerging mixed ion/electron-conductive (MIEC) scaffolds provide rapid ion/electron transport pathways, which enable uniform Na+ flux and promote dendrite-free Na deposition, thus improving the cycle life of Na metal anodes, even at high current densities and large areal capacities. Therefore, this review primarily emphasizes the recent progress in applying MIEC scaffolds to Na metal anodes. It introduces diverse design methods, examines the electrochemical performance of MIEC scaffolds, and delves into their regulation mechanisms over Na deposition behaviour. Finally, the development prospects and research strategies for MIEC scaffolds from both fundamental research and practical application perspectives are discussed, suggesting directions for further designing high-performance Na metal batteries.

Entropy-Driven Enhancement of the Conductivity and Phase Purity of Na4Fe3(PO4)2P2O7 as the Superior Cathode in Sodium-Ion Batteries

Na4Fe3(PO4)2(P2O7) (NFPP) is regarded as a promising cathode material for sodium-ion batteries (SIBs) owing to its low cost, easy manufacture, environmental purity, high structural stability, unique three-dimensional Na-ion diffusion channels, and appropriate working voltage. However, for NFPP, the low conductivity of electrons and ions limits their capacity and power density. The generation of NaFeP2O7 and NaFePO4 inhibits the diffusion of sodium ions and reduces reversible capacity and rate performance during the manufacturing process in synthesis methods. Herein, we report an entropy-driven approach to enhance the electronic conductivity and, concurrently, phase purity of NFPP as the superior cathode in sodium-ion batteries. This approach was realized via Ti ions substituting different ratios of Fe-occupied sites in the NFPP lattice (denoted as NTFPP-X, T is the Ti in the lattice, X is the ratio of Ti-substitution) with the configurational entropic increment of the lattice structures from 0.68 R to 0.79 R. Specifically, 5% Ti-substituted lattice (NTFPP-0.05) inducing entropic augmentation not only improves the electronic conductivity from 7.1 × 10-2 S/m to 8.6 × 10-2 S/m but also generates the pure-phase of NFPP (suppressing the impure phases of the NaFeP2O7 and NaFePO4) of the lattice structure, which is validated by a series of characterizations, including powder X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Benefiting from the Ti replacement in the lattice, the optimal NTFPP-0.05 composite shows a high first discharge capacity (118.5 mAh g-1 at 0.1 C), superior rate performance (70.5 mAh g-1 at 10 C), and excellent long cycling life (1200 cycles at 10 C with capacity retention of 86.9%). This research proposes a new entropy-driven approach to improve the electrochemical performance of NFPP and reports a low-cost, ultrastable, and high-rate cathode material of NTFPP-0.05 for SIBs.

Multifunctional Separator Enables High-Performance Sodium Metal Batteries in Carbonate-Based Electrolytes

Sodium metal has become one of the most promising anodes for next-generation cheap and high-energy-density metal batteries; however, challenges caused by the uncontrollable sodium dendrite growth and fragile solid electrolyte interphase (SEI) restrict their large-scale practical applications in low-cost and wide-voltage-window carbonate electrolytes. Herein, a novel multifunctional separator with lightweight and high thinness is proposed, assembled by the cobalt-based metal-organic framework nanowires (Co-NWS), to replace the widely applied thick and heavy glass fiber separator. Benefitting from its abundant sodiophilic functional groups and densely stacked nanowires, Co-NWS not only exhibits outstanding electrolyte wettability and effectively induces uniform Na+ ion flux as a strong ion redistributor but also favors constructing the robust N,F-rich SEI layer. Satisfactorily, with 10 µL carbonate electrolyte, a Na|Co-NWS|Cu half-cell delivers stable cycling (over 260 cycles) with a high average Coulombic efficiency of 98%, and the symmetric cell shows a long cycle life of more than 500 h. Remarkably, the full cell shows a long-term life span (over 1500 cycles with 92% capacity retention) at high current density in the carbonate electrolyte. This work opens up a strategy for developing dendrite-free, low-cost, and long-life-span sodium metal batteries in carbonate-based electrolytes.

Impact of channel nanostructures of porous carbon particles on their catalytic performance

Mesoporous carbon particles have great potential due to their unique structural properties as support materials for catalytic applications. Particle shapes and channel nanostructures of mesoporous carbon particles can determine the reactant/product transport efficiency. However, the role of the channel nanostructure in the catalytic reaction has not been much explored. Herein, we introduce a facile method to fabricate a series of porous carbon particles (PCPs) with controlled channel exposure on the carbon surface and investigate the impact of the channel nanostructure of the PCPs on the catalytic activity. By employing a membrane emulsification method with a controlled solvent evaporation rate, we fabricate block copolymer (BCP) particles with uniform size and regulated degrees of cylindrical channel exposed to the particle surface. Followed by the carbonization of the BCP particles, a low amount (1.3 wt%) of Pt is incorporated into the PCP series to investigate the impact of channel nanostructures on the catalytic oxidation reaction of o-phenylenediamine (OPD). Specifically, PCP featuring highly open channel nanostructures shows a high reaction rate constant of 0.154 mM-1 s-1 for OPD oxidation, showing 5.5 times higher catalytic activity than those of closed channel nanostructures (0.028 mM-1 s-1). This study provides a deeper understanding of the impact of channel nanostructure within mesoporous carbon particles on catalytic activity.

Unraveling the Solvent Effect on Solid-Electrolyte Interphase Formation for Sodium Metal Batteries

Ether-based electrolytes are considered as an ideal electrolyte system for sodium metal batteries (SMBs) due to their superior compatibility with the sodium metal anode (SMA). However, the selection principle of ether solvents and the impact on solid electrolyte interphase formation are still unclear. Herein, we systematically compare the chain ether-based electrolyte and understand the relationship between the solvation structure and the interphasial properties. The linear ether solvent molecules with different terminal group lengths demonstrate remarkably distinct solvation effects, thus leading to different electrochemical performance as well as deposition morphologies for SMBs. Computational calculations and comprehensive characterizations indicate that the terminal group length significantly regulates the electrolyte solvation structure and consequently influences the interfacial reaction mechanism of electrolytes on SMA. Cryogenic electron microscopy clearly reveals the difference in solid electrolyte interphase in various ether-based electrolytes. As a result, the 1,2-diethoxyethane-based electrolyte enables a high Coulombic efficiency of 99.9 %, which also realizes the stable cycling of Na||Na3 V2 (PO4 )3 full cell with a mass loading of ≈9 mg cm-2 over 500 cycles.

Ultrahigh areal capacity and long cycling stability of sodium metal anodes boosted using a 3D-printed sodiophilic MXene/rGO microlattice aerogel

Sodium metal has emerged as a highly promising anode material for sodium-based batteries, owing to its intrinsic advantages, including its high theoretical capacity, low working plateau and low cost. However, the uncontrolled formation of sodium dendrites accompanied by unrestricted volume expansion severely limits its application. To tackle these issues, we propose an approach to address these issues by adopting a three-dimensional (3D) structure of Ti3C2Tx/reduced graphene oxide (Ti3C2Tx/rGO) constructed by a direct-ink writing (DIW) 3D printing technique as the Na metal anode host electrode. The combination of the 3D-printed rGO skeleton offering artificial porous structures and the incorporation of sodiophilic Ti3C2Tx nanosheets provides abundant nucleation sites and promotes uniform sodium metal deposition. This specially designed architecture significantly enhances the Na metal cycling stability by effectively inhibiting dendrite formation. The experimental results show that the designed Ti3C2Tx/rGO electrode can achieve a high coulombic efficiency (CE) of 99.91% after 1800 cycles (3600 h) at 2 mA cm-2 with 2 mA h cm-2. Notably, the adopted electrodes exhibit a long life span of more than 1400 h with a high CE over 99.93% when measured with an ultra-high capacity of 50 mA h cm-2 at 5 mA cm-2. Furthermore, a 3D-printed full cell consisting of a Na@Ti3C2Tx/rGO anode and a 3D-printed Na3V2(PO4)3C-rGO (NVP@C-rGO) cathode was successfully demonstrated. This 3D-printed cell could provide a notable capacity of 85.3 mA h g-1 at 100 mA g-1 after 500 cycles. The exceptional electrochemical performance achieved by the 3D-printed full cell paves the way for the development of practical sodium metal anodes.

Stabilize Sodium Metal Anode by Integrated Patterning of Laser-Induced Graphene with Regulated Na Deposition Behavior

Metallic sodium is regarded as the most potential anode for sodium-ion batteries due to its high capacity and earth-abundancy. Nevertheless, uncontrolled Na dendrite growth and infinite volume change remain great challenges for developing high-performance sodium metal batteries. This work provides a simple and general approach to stabilize sodium metal anode (SMA) by constructing Sn nanoparticles-anchored laser-induced graphene on copper foil (Sn@LIG@Cu) consisting of Sn@LIG composite, polyimide (PI) columns, and Cu current collector. The Sn-based sodiophilic species effectively reduce the Na nucleation overpotential and regulate the dendrite Na-free deposition. While the flexible PI columns act as binder and buffer the volume variation of Na during cycling. Besides, the unique patterned structure provides continuous and rapid channels for ion transportation, promoting the Na+ transport kinetics. Therefore, the as-fabricated Sn@LIG@Cu electrode exhibits outstanding rate performance to 40 mA cm-2 and excellent cycling stability without dendrite growth, which is confirmed by in-situ optical microscopy observation. Moreover, the practical full cell based on such an anode displays a favorable rate capability of up to 10 C and cycling performance at 5 C for 600 cycles. This work thus demonstrates a facile, highly-efficient, and scalable approach to stabilize SMAs and can be extended to other battery systems.

Fluorinated porous frameworks enable robust anode-less sodium metal batteries

Sodium metal batteries hold great promise for energy-dense and low-cost energy storage technology but are severely impeded by catastrophic dendrite issue. State-of-the-art strategies including sodiophilic seeding/hosting interphase design manifest great success on dendrite suppression, while neglecting unavoidable interphase-depleted Na+ before plating, which poses excessive Na use, sacrificed output voltage and ultimately reduced energy density. We here demonstrate that elaborate-designed fluorinated porous framework could simultaneously realize superior sodiophilicity yet negligible interphase-consumed Na+ for dendrite-free and durable Na batteries. As elucidated by physicochemical and theoretical characterizations, well-defined fluorinated edges on porous channels are responsible for both high affinities ensuring uniform deposition and low reactivity rendering superior Na+ utilization for plating. Accordingly, synergistic performance enhancement is achieved with stable 400 cycles and superior plateau to sloping capacity ratio in anode-free batteries. Proof-of-concept pouch cells deliver an energy density of 325 Watt-hours per kilogram and robust 300 cycles under anode-less condition, opening an avenue with great extendibility for the practical deployment of metal batteries.

Reversible Na Plating/Stripping with High Areal Capacity Using an Electroconductive Liquid Electrolyte System

Anode-free sodium-metal batteries (AFSMBs) are promising candidates for maximizing energy density and minimizing cost and safety hazards in the absence of metallic sodium during cell assembly. The practical implementation of AFSMBs is hindered by the low cycling stability of Na-metal plating and stripping, particularly under high areal capacities, due to unstable solid electrolyte interphase (SEI) layer formation with electrolyte decomposition and inactive dead Na formation. Here, we proposed an electroconductive electrolyte system consisting of liquid electrolytes that accept electrons at a certain energy level and form electronically conductive and solid electrolytes that prevent internal short circuit through low electronic conductivity. The electron acceptability and high electronic conductivity of the liquid electrolyte can suppress the irreversible electron transfer with electrolyte decomposition and reutilize the inactive dead metal, respectively. The functions of the system were demonstrated using a sodium biphenyl liquid electrolyte-NASICON solid electrolyte in a seawater battery (SWB) system, which features an infinite sodium source. The anode-free SWB cells achieved a high Coulombic efficiency of ≥99.9% for over 60 cycles at a high areal capacity of ∼24 mAh/cm2. This study provides insight into the Na plating/stripping properties in anode-free systems and proposes a significant strategy for improving the reversibility of metal anodes for various battery systems with solid electrolytes.

Structure-Controlled Carbon Hosts for Dendrite-Free Aqueous Zinc Batteries

The surging demand for environmental-friendly and safe electrochemical energy storage systems has driven the development of aqueous zinc (Zn)-ion batteries (ZIBs). However, metallic Zn anodes suffer from severe dendrite growth and large volume change, resulting in a limited lifetime for aqueous ZIB applications. Here, it is shown that 3D mesoporous carbon (MC) with controlled carbon and defect configurations can function as a highly reversible and dendrite-free Zn host, enabling the stable operation of aqueous ZIBs. The MC host has a structure-controlled architecture that contains optimal sp2 -carbon and defect sites, which results in an improved initial nucleation energy barrier and promotes uniform Zn deposition. As a consequence, the MC host shows outstanding Zn plating/stripping performance over 1000 cycles at 2 mA cm-2 and over 250 cycles at 6 mA cm-2 in asymmetric cells. Density functional theory calculations further reveal the role of the defective sp2 -carbon surface in Zn adsorption energy. Moreover, a full cell based on Zn@MC900 anode and V2 O5 cathode exhibits remarkable rate performance and cycling stability over 3500 cycles. These results establish a structure-mechanism-performance relationship of the carbon host as a highly reversible Zn anode for the reliable operation of ZIBs.

Space-Confined Guest Synthesis to Fabricate Sn-Monodispersed N-Doped Mesoporous Host toward Anode-Free Na Batteries

Severe issues including volume change and dendrite growth on sodium metal anodes hinder the pursuit of applicable high-energy-density sodium metal batteries. Herein, an in situ reaction approach is developed that takes metal-organic frameworks as nano-reactor and pore-former to produce a mesoporous host comprised of nitrogen-doped carbon fibers embedded with monodispersed Sn clusters (SnNCNFs). The hybrid host shows outstanding sodiophilicity that enables rapid Na infusion and ultralow Na nucleation overpotential of 2 mV. Its porous structure holds a high Na content and guides uniform Na deposition. Such host provides favorable Na plating/stripping with an average Coulombic efficiency of 99.96% over 2000 cycles (at 3 mA cm-2 and 3 mA h cm-2 ). The Na-infused SnNCNF anode delivers extreme Na utilization of 86% in symmetric cells (at 10 mA cm-2 and 10 mA h cm-2 ), outstanding rate capability and cycle life in Na-SnNCNF||Na3 V2 (PO4 )3 full cells (at 1 A g-1 for over 1000 cycles with capacity retention of 92.1%). Furthermore, high-energy/power-density anode-less and anode-free Na cells are achieved. This work presents an effective heteroatom-doping approach for fabricating multifunctional porous carbon materials and developing high-performance metal batteries.

Toward High Performance Anodes for Sodium-Ion Batteries: From Hard Carbons to Anode-Free Systems

Sodium-ion batteries (SIBs) have been deemed to be a promising energy storage technology in terms of cost-effectiveness and sustainability. However, the electrodes often operate at potentials beyond their thermodynamic equilibrium, thus requiring the formation of interphases for kinetic stabilization. The interfaces of the anode such as typical hard carbons and sodium metals are particularly unstable because of its much lower chemical potential than the electrolyte. This creates more severe challenges for both anode and cathode interfaces when building anode-free cells to achieve higher energy densities. Manipulating the desolvation process through the nanoconfining strategy has been emphasized as an effective strategy to stabilize the interface and has attracted widespread attention. This Outlook provides a comprehensive understanding about the nanopore-based solvation structure regulation strategy and its role in building practical SIBs and anode-free batteries. Finally, guidelines for the design of better electrolytes and suggestions for constructing stable interphases are proposed from the perspective of desolvation or predesolvation.

Dual Mechanism for Sodium based Energy Storage

A dual-mechanism energy storage strategy is proposed, involving the electrochemical process of sodium ion battery (SIB) and sodium metal battery (SMB). This strategy is expected to achieve a higher capacity than SIB, and obtain dendrite-free growth of SMB with a well-designed anode. Here, self-constructed bismuth with "sodiophilic" framework and rapid ion transmission characteristics is employed as the sodium host (anode) integrating alloy/de-alloy and plating/stripping process that suppresses the dendrite growth and overcomes the limited capacity of traditional anode. Benefited from this, the capacity (capacity contributed by alloy and plating of sodium in total) of 2000 mAh g^-1 can be reached, which can retain up to 800 h at 1 A g^-1 . Also, the capacity of 3100 mAh g^-1 can be achieved that is ≈7.7 times than that of alloyed-bismuth (Bi). This work proposes a dual-mechanism strategy to tackle the dilemma of high-performance sodium (Na) storage devices, which opens a new avenue for the development of next-generation energy storage device.

Optimizing the Fermi Level of a 3D Current Collector with Ni3 S2 /Ni3 P Heterostructure for Dendrite-Free Sodium-Metal Batteries

Rechargeable sodium-metal batteries (RSMBs) with high energy density and low cost are attracting extensive attention as promising energy-storage technologies. However, the poor cyclability and safety issues caused by unstable solid electrolyte interphase (SEI) structure and dendrite issues limit their practical application. Herein, it is theoretically predicted that constructing the Ni₃ S₂ /Ni₃ P heterostructure with high work function can lower the Fermi energy level, and therefore effectively suppressing continuous electrolyte decomposition derived from the electron-tunneling effect after long-term sodiation process. Furthermore, the Ni₃ S₂ /Ni₃ P heterostructure on 3D porous nickel foam (Ni₃ S₂ /Ni₃ P@NF) is experimentally fabricated as an advanced Na-anode current collector. The seamless Ni₃ S₂ /Ni₃ P heterostructure not only offers abundant active sites to induce uniform Na⁺ deposition and enhance ion-transport kinetics, but also facilitates the formation of stable SEI for dendrite-free sodium anode, which are confirmed by cryogenic components transmission electron microscopy tests and in situ spectroscopy characterization. As a result, the Na-composite anode (Ni₃ S₂ /Ni₃ P@NF@Na) delivers stable plating/stripping process of 5000 h and high average Coulombic efficiency of 99.7% over 2500 cycles. More impressively, the assembled sodium-ion full cell displays ultralong cycle life of 10 000 cycles at 20 C. The strategy of stabilizing the sodium-metal anode gives fundamental insight into the potential construction of advanced RSMBs.

Superfast Mass Transport of Na/K Via Mesochannels for Dendrite-Free Metal Batteries

Fast ion diffusion in anode hosts enabling uniform distribution of Li/Na/K is essential for achieving dendrite-free alkali-metal batteries. Common strategies, e.g. expanding the interlayer spacing of anode materials, can enhance bulk diffusion of Li but are less efficient for Na and K due to their larger ionic radius. Herein, a universal strategy to drastically improve the mass-transport efficiency of Na/K by introducing open mesochannels in carbon hosts is proposed. Such pore engineering can increase the accessible surface area by one order of magnitude, thus remarkably accelerating surface diffusion, as visualized by in situ transmission electron microscopy. In particular, once the mesochannels are filled by the Na/K metals, they become the superfast channels for mass transport via the mechanism of interfacial diffusion. Thus-modified carbon hosts enable Na/K filling in their inner cavities and uniform deposition across the whole electrodes with fast kinetics. The resulting Na-metal anodes can exhibit stable dendrite-free cycling with outstanding rate performance at a high current density of up to 30 mA cm^-2 . This work presents an inspiring attempt to address the sluggish transport issue of Na/K, as well as valuable insights into the mass-transport mechanism in porous anodes for high-performance alkali-metal storage.

Single-Atom Metallophilic Sites for Liquid NaK Alloy Confinement toward Stable Alkali-Metal Anodes

Room temperature liquid NaK alloy is a promising candidate for high performance metal batteries, due to its dendrite-free property and high energy density. However, its practical application is hindered by the high surface tension of liquid NaK, which causes difficulties in maintaining a stable contact with a current collector. Here, the authors demonstrate the extraordinary stable confinement of NaK alloy at room temperature by constructing a super-wetting substrate, which is based on highly dispersed cobalt-single-atom carbon nanoarrays. The developed liquid anode electrode prevented successfully the leakage of NaK alloy even in harsh stress (>5 MPa) or sharp shock conditions. The symmetric cells achieved ultra-long reversible plating/stripping cycling life in both Na-ion (>1010 hrs) and K-ion electrolytes (>4000 hrs) at 10 mA cm^-2 /10 mAh cm^-2 . Upon fitting with Na₃ V₂ (PO₄ )₃ , the NaK assembled full battery provided high energy density (332.6 kWh kg^-1 ) and power density (11.05 kW kg^-1 ) with excellent stability after >21600 cycles, which is the best value reported so far. The prepared pouch cell was able to drive a four-axis aircraft, demonstrating a great prospect in practical application. This work offers a new approach in the preparation of advanced dendrite-free liquid metal anodes with promising applications in electrochemical energy storage.

Designing Solid Electrolyte Interfaces towards Homogeneous Na Deposition: Theoretical Guidelines for Electrolyte Additives and Superior High-Rate Cycling Stability

Metallic Na is a promising metal anode for large-scale energy storage. Nevertheless, unstable solid electrolyte interphase (SEI) and uncontrollable Na dendrite growth lead to disastrous short circuit and poor cycle life. Through phase field and ab initio molecular dynamics simulation, we first predict that the sodium bromide (NaBr) with the lowest Na ion diffusion energy barrier among sodium halogen compounds (NaX, X=F, Cl, Br, I) is the ideal SEI composition to induce the spherical Na deposition for suppressing dendrite growth. Then, 1,2-dibromobenzene (1,2-DBB) additive is introduced into the common fluoroethylene carbonate-based carbonate electrolyte (the corresponding SEI has high mechanical stability) to construct a desirable NaBr-rich stable SEI layer. When the Na||Na₃ V₂ (PO₄ )₃ cell utilizes the electrolyte with 1,2-DBB additive, an extraordinary capacity retention of 94 % is achieved after 2000 cycles at a high rate of 10 C. This study provides a design philosophy for dendrite-free Na metal anode and can be expanded to other metal anodes.

Recent Progress in Rechargeable Sodium Metal Batteries: A Review

Sodium metal batteries (SMBs) have been widely studied owing to their relatively high energy density and abundant resources. However, they still need systematic improvement to fulfill the harsh operating conditions for their commercialization. In this review, we summarize the recent progress in SMBs in terms of sodium anode modification, electrolyte exploration, and cathode design. Firstly, we give an overview of the current challenges facing Na metal anodes and the corresponding solutions. Then, the traditional liquid electrolytes and the prospective solid electrolytes for SMBs are summarized. In addition, insertion- and conversion-type cathode materials are introduced. Finally, an outlook for the future of practical SMBs is provided.

Stable Sodium-Metal Batteries in Carbonate Electrolytes Achieved by Bifunctional, Sustainable Separators with Tailored Alignment

Sodium (Na) is the most appealing alternative to lithium as an anode material for cost-effective, high-energy-density energy-storage systems by virtue of its high theoretical capacity and abundance as a resource. However, the uncontrolled growth of Na dendrites and the limited cell cycle life impede the large-scale practical implementation of Na-metal batteries (SMBs) in commonly used and low-cost carbonate electrolytes. Herein, the employment of a novel bifunctional electrospun nanofibrous separator comprising well-ordered, uniaxially aligned arrays, and abundant sodiophilic functional groups is presented for SMBs. By tailoring the alignment degree, this unique separator integrates with the merits of serving as highly aligned ion-redistributors to self-orientate/homogenize the flux of Na-ions from a chemical molecule level and physically suppressing Na dendrite puncture at a mechanical structure level. Remarkably, unprecedented long-term cycling performances at high current densities (≥1000 h at 1 and 3 mA cm^-2 , ≥700 h at 5 mA cm^-2 ) of symmetric cells are achieved in additive-free carbonate electrolytes. Moreover, the corresponding sodium-organic battery demonstrates a high energy density and prolonged cyclability over 1000 cycles. This work opens up a new and facile avenue for the development of stable, low-cost, and safe-credible SMBs, which could be readily extended to other alkali-metal batteries.

Codoped porous carbon nanofibres as a potassium metal host for nonaqueous K-ion batteries

Potassium metal is an appealing alternative to lithium as an alkali metal anode for future electrochemical energy storage systems. However, the use of potassium metal is hindered by the growth of unfavourable deposition (e.g., dendrites) and volume changes upon cycling. To circumvent these issues, we propose the synthesis and application of nitrogen and zinc codoped porous carbon nanofibres that act as potassium metal hosts. This carbonaceous porous material enables rapid potassium infusion (e.g., < 1 s cm−2) with a high potassium content (e.g., 97 wt. %) and low potassium nucleation overpotential (e.g., 15 mV at 0.5 mA cm−2). Experimental and theoretical measurements and analyses demonstrate that the carbon nanofibres induce uniform potassium deposition within its porous network and facilitate a dendrite-free morphology during asymmetric and symmetric cell cycling. Interestingly, when the potassium-infused carbon material is tested as an active negative electrode material in combination with a sulfur-based positive electrode and a nonaqueous electrolyte solution in the coin cell configuration, an average discharge voltage of approximately 1.6 V and a discharge capacity of approximately 470 mA h g−1 after 600 cycles at 500 mA g−1 and 25 °C are achieved.

A sodiophilic VN interlayer stabilizing a Na metal anode

Sodium (Na) metal is a very encouraging anode material for next-generation rechargeable batteries owing to its high specific capacity, earth-abundance and low-cost. However, the application of Na metal anodes (SMAs) is hampered by dendrite growth and "dead" Na formation caused by the uncontrollable Na deposition, leading to poor cycle life and even safety concerns. Herein, a high-performance Na anode is designed by introducing an artificial VN interlayer on the Na metal surface (Na/VN) by a simple mechanical rolling process to regulate Na nucleation/deposition behaviors. The density functional theory (DFT) and experiment results uncover that the VN possesses high "sodiophilicity", which can facilitate the initially homogeneous Na nucleation and cause Na to distribute evenly on the VN interlayer. Therefore, uniform Na deposition with dendrite-free morphology and prolonged cycling lifespan (over 1060 h at 0.5 mA cm^-2/1 mA h cm^-2) can be realized. Moreover, the full cell assembled by coupling a Na₃V₂(PO₄)₃ (NVP) cathode and Na/VN anode presents superior cycling performance (e.g., 96% capacity retention even after 800 cycles at 5C). This work provides a promising direction for regulating Na nucleation and deposition to achieve dendrite-free metal anodes.