The chemical evolution of solid electrolyte interface in sodium metal batteries

Multiple Na+ transport pathways and interfacial compatibility enable high-capacity, room-temperature quasi-solid sodium batteries

Sodium-metal batteries (SMBs) are ideal for large-scale energy storage due to their stable operation and high capacity. However, they have safety issues caused by severe dendrite growth and side reactions, particularly when using liquid electrolytes. Therefore, it is critically important to develop electrolytes with high ionic conductivity and improved safety that are non-flammable and resistant to dendrites. Here, we developed polymerized polyethylene glycol diacrylate (PEGDA)-modified poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes (PPEs) with highly conductive sodium bis(trifluoromethanesulfonyl)imide and corrosion-inhibitive sodium bis(oxalato)borate salts for SMBs. Well-complexed PEGDA not only increases the amorphicity of the PVDF matrix, but also offers numerous Lewis basic sites through the polar groups of carbonyl and ether groups (i.e., electron donors). The presence of the Lewis basic sites facilitates the dissociation of sodium salt and transportation of Na+ within the PVDF matrix. This results in the generation of additional Na+ transport pathways, which can enhance the performance of the battery. Among PPEs, the optimized PPE-50 exhibits a high ionic conductivity of 3.42 × 10-4 S cm-1 and a mechanical strength of 14.0 MPa. A Na||Na symmetric cell with PPE-50 displays high stability at 0.2 mA cm-2 for 800 h. PPE-50 further displays high capacity, e.g., a Na3V2(PO4)3|PPE-50|Na battery delivers a decent discharge capacity of 101.5 mAh g-1 at 1.0C after 650 cycles. Our work demonstrates the development of high-performance quasi-solid polymer electrolytes with multiple transport pathways suitable for room-temperature SMBs.

NaBix/NaVyOz Hybrid Interfacial Layer Enables Stable and Dendrite-Free Sodium Anodes

The challenges of sodium metal anodes, including formation of an unstable solid-electrolyte interphase (SEI) and uncontrolled growth of sodium dendrites during charge-discharge cycles, impact the stability and safety of sodium metal batteries. Motivated by the promising commercialization potential of sodium metal batteries, it becomes imperative to systematically explore innovative protective interlayers specifically tailored for sodium metal anodes. In this work, a NaBix/NaVyOz hybrid and porous interfacial layer on sodium anode is successfully fabricated via pretreating sodium with bismuth vanadate. The hybrid interlayer effectively combines the advantages of sodium vanadates and alloys, raising a synergistic effect in facilitating sodium deposition kinetics and inhibiting the growth of sodium dendrites. As a result, the modified sodium electrodes (BVO-Na) can stably cycle for 2000 h at 0.5 mA cm-2 with a fixed capacity of 1 mAh cm-2, and the BVO-Na||Na3V2(PO4)3 full cell sustains a high capacity of 94 mAh g-1 after 600 cycles at 5 C. This work demonstrates that constructing an artificial hybrid interlayer is a practical solution to obtain high performance anodes in sodium metal batteries.

Synergistic interface and structural engineering for high initial coulombic efficiency and stable sodium storage in metal sulfides

Transition metal sulfides (TMS) have gained significant attention as potential anode materials for sodium ion batteries (SIBs) due to their high theoretical capacity and abundance in nature. Nevertheless, their practical use has been impeded by challenges such as large volume changes, unstable solid electrolyte interphase (SEI), and low initial coulombic efficiency (ICE). To address these issues and achieve both long-term cycling stability and high ICE simultaneously, we present a novel approach involving surface engineering, termed as the "dual-polar confinement" strategy, combined with interface engineering to enhance the electrochemical performance of TMS. In this approach, CoS crystals are meticulously coated with polar TiO2 and embedded within a polar S-doped carbon matrix, forming a composite electrode denoted as CoS/TiO2-SC. Significantly, an ether-based electrolyte with chemical stability and optimized solvation properties synergistically interacts with the Co-S-C bonds to create a stable, ultra-thin SEI. This concerted effect results in a notably high ICE, reaching approximately 96%. Advanced characterization and theoretical simulations confirm that the uniform surface modification effectively facilitates sodium ion transport kinetics, restrains electrode pulverization, and concurrently enhances interaction with the ether-based electrolyte to establish a robust SEI. Consequently, the CoS/TiO2-SC electrode exhibits high reversible capacity, superior rate capability, and outstanding cycling stability.

Visualizing the Structure-Property Nexus of Wide-Bandgap Perovskite Solar Cells under Thermal Stress

Wide-bandgap perovskite solar cells (PSCs) toward tandem photovoltaic applications are confronted with the challenge of device thermal stability, which motivates to figure out a thorough cognition of wide-bandgap PSCs under thermal stress, using in situ atomic-resolved transmission electron microscopy (TEM) tools combing with photovoltaic performance characterizations of these devices. The in situ dynamic process of morphology-dependent defects formation at initial thermal stage and their proliferations in perovskites as the temperature increased are captured. Meanwhile, considerable iodine enables to diffuse into the hole-transport-layer along the damaged perovskite surface, which significantly degrade device performance and stability. With more intense thermal treatment, atomistic phase transition reveals the perovskite transform to PbI2 along the topo-coherent interface of PbI2/perovskite. In conjunction with density functional theory calculations, a mutual inducement mechanism of perovskite surface damage and iodide diffusion is proposed to account for the structure-property nexus of wide-bandgap PSCs under thermal stress. The entire interpretation also guided to develop a thermal-stable monolithic perovskite/silicon tandem solar cell.

Tuning the Electrolyte and Interphasial Chemistry for All-Climate Sodium-ion Batteries

Sodium-ion batteries (SIBs) present a promising avenue for next-generation grid-scale energy storage. However, realizing all-climate SIBs operating across a wide temperature range remains a challenge due to the poor electrolyte conductivity and instable electrode interphases at extreme temperatures. Here, we propose a comprehensively balanced electrolyte by pairing carbonates with a low-freezing-point and low-polarity ethyl propionate solvent which enhances ion diffusion and Na+-desolvation kinetics at sub-zero temperatures. Furthermore, the electrolyte leverages a combinatorial borate- and nitrile-based additive strategy to facilitate uniform and inorganic-rich electrode interphases, ensuring excellent rate performance and cycle stability over a wide temperature range from -45 °C to 60 °C. Notably, the Na||sodium vanadyl phosphate cell delivers a remarkable capacity of 105 mAh g-1 with a high rate of 2 C at -25 °C. In addition, the cells exhibit excellent cycling stability over a wide temperature range, maintaining a high capacity retention of 84.7 % over 3,000 cycles at 60 °C and of 95.1 % at -25 °C over 500 cycles. The full cell also exhibits impressive cycling performance over a wide temperature range. This study highlights the critical role of electrolyte and interphase engineering for enabling SIBs that function optimally under diverse and extreme climatic environments.

Nonflammable Succinonitrile-Based Deep Eutectic Electrolyte for Intrinsically Safe High-Voltage Sodium-Ion Batteries

Intrinsically safe sodium-ion batteries are considered as a promising candidate for large-scale energy storage systems. However, the high flammability of conventional electrolytes may pose serious safety threats and even explosions. Herein, a strategy of constructing a deep eutectic electrolyte is proposed to boost the safety and electrochemical performance of succinonitrile (SN)-based electrolyte. The strong hydrogen bond between S═O of 1,3,2-dioxathiolane-2,2-dioxide (DTD) and the α-H of SN endows the enhanced safety and compatibility of SN with Lewis bases. Meanwhile, the DTD participates in the inner Na+ sheath and weakens the coordination number of SN. The unique solvation configuration promotes the formation of robust gradient inorganic-rich electrode-electrolyte interphase, and merits stable cycling of half-cells in a wide temperature range, with a capacity retention of 82.8% after 800 cycles (25 °C) and 86.3% after 100 cycles (60 °C). Correspondingly, the full cells deliver tremendous improvement in cycling stability and rate performance.

Molecular Engineering toward Robust Solid Electrolyte Interphase for Lithium Metal Batteries

Lithium-metal batteries (LMBs) with high energy density are becoming increasingly important in global sustainability initiatives. However, uncontrollable dendrite seeds, inscrutable interfacial chemistry, and repetitively formed solid electrolyte interphase (SEI) have severely hindered the advancement of LMBs. Organic molecules have been ingeniously engineered to construct targeted SEI and effectively minimize the above issues. In this review, multiple organic molecules, including polymer, fluorinated molecules, and organosulfur, are comprehensively summarized and insights into how to construct the corresponding elastic, fluorine-rich, and organosulfur-containing SEIs are provided. A variety of meticulously selected cases are analyzed in depth to support the arguments of molecular design in SEI. Specifically, the evolution of organic molecules-derived SEI is discussed and corresponding design principles are proposed, which are beneficial in guiding researchers to understand and architect SEI based on organic molecules. This review provides a design guideline for constructing organic molecule-derived SEI and will inspire more researchers to concentrate on the exploitation of LMBs.

Mechanically Interlocked Interphase with Energy Dissipation and Fast Li-Ion Transport for High-Capacity Lithium Metal Batteries

Constructing an artificial solid electrolyte interphase (ASEI) on Li metal anodes (LMAs) is a potential strategy for addressing the dendrite issues. However, the mechanical fatigue of the ASEI caused by stress accumulation under the repeated deformation from the Li plating/stripping is not taken seriously. Herein, this work introduces a mechanically interlocked [an]daisy chain network (DC MIN) into the ASEI to stabilize the Li metal/ASEI interface by combining the functions of energy dissipation and fast Li-ion transport. The DC MIN featured by large-range molecular motions is cross-linked via efficient thiol-ene click chemistry; thus, the DC MIN has flexibility and excellent mechanical properties. As an ASEI, the crown ether units in DC MIN not only interact with the dialkylammonium of a flexible chain, forming the energy dissipation behavior but also coordinate with Li ion to support the fast Li-ion transport in DC MIN. Therefore, a stable 2800 h-symmetrical cycling (1 mA cm-2 ) and an excellent 5 C-rate (full cell with LiFePO4 ) performance are achieved by DC MIN-based ASEI. Furthermore, the 1-Ah pouch cell (LiNi0.88 Co0.09 Mn0.03 O2 cathode) with DC MIN-coated LMA exhibits improved capacity retention (88%) relative to the Control. The molecular design of DC MIN provides new insights into the optimization of an ASEI for high-energy LMAs.

Non-aqueous Liquid Electrolyte Additives for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have been recognized as one of the most promising new energy storage devices for their rich sodium resources, low cost and high safety. The electrolyte, as a bridge connecting the cathode and anode electrodes, plays a vital role in determining the performance of SIBs, such as coulombic efficiency, energy density and cycle life. Therefore, the overall performance of SIBs could be significantly improved by adjusting the electrolyte composition or adding a small number of functional additives. In this review, the fundamentals of SIB electrolytes including electrode-electrolyte interface and solvation structure are introduced. Then, the mechanisms of electrolyte additive action on SIBs are discussed, with a focus on film-forming additives, flame-retardant additives and overcharge protection additives. Finally, the future research of electrolytes is prospected from the perspective of scientific concepts and practical applications.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.

Molecule Isolation Protective Interface Formed by Planar Additive for Stable Sodium Metal Anodes

Sodium (Na) metal is an ideal anode for Na-based batteries because of its high specific capacity and low potential. However, interface issues such as side reactions with the electrolyte and uneven deposition severely hinder its practical application. Here, we report a zinc phthalocyanine (ZnPc) electrolyte additive with a planar molecular structure that can form a dense molecular layer when tightly adsorbed on the Na metal anode surface. Such a planar molecular layer can suppress side reactions between the anode and the electrolyte as well as homogenize Na+ flux to reduce dendrite growth. As a result, the molecular isolation interface formed by ZnPc adsorption on the surface of the Na metal anode enhances the interface stability and the cycling performance of the Na metal anode, with the average Coulombic efficiency of the half-cell of 99.95% after 350 stable cycles at 1 mA cm-2 for 1 mAh cm-2. Moreover, the assembled Na||Na3V2(PO4)3 full-cell with this additive delivers excellent stability over 120 cycles, proving the effectiveness of the ZnPc additive in practical application.

A nanowire-assembled Co3S4/Cu2S@carbon binary metal sulfide hybrid as a sodium-ion battery anode displaying high capacity and recoverable rate-performance

A binary metal sulfide hybrid consisting of nanowire-assembled and polypyrrole-coated Co3S4/Cu2S spheres after nitrogen-doped carbon coating (Co3S4/Cu2S@NC) is developed as an anode, which displays a capacity exceeding 412.3 mA h g-1 after 550 cycles under 1.0 A g-1. Recoverable rate-performance and good temperature tolerance under 50 °C and -10 °C are achievable; a full cell delivers 339.5 mA h g-1, indicating promising potential for applications in various conditions.

Research progress of organic liquid electrolyte for sodium ion battery

Electrochemical energy storage technology has attracted widespread attention due to its low cost and high energy efficiency in recent years. Among the electrochemical energy storage technologies, sodium ion batteries have been widely focused due to the advantages of abundant sodium resources, low price and similar properties to lithium. In the basic structure of sodium ion battery, the electrolyte determines the electrochemical window and electrochemical performance of the battery, controls the properties of the electrode/electrolyte interface, and affects the safety of sodium ion batteries. Organic liquid electrolytes are widely used because of their low viscosity, high dielectric constant, and compatibility with common cathodes and anodes. However, there are problems such as low oxidation potential, high flammability and safety hazards. Therefore, the development of novel, low-cost, high-performance organic liquid electrolytes is essential for the commercial application of sodium ion batteries. In this paper, the basic requirements and main classifications of organic liquid electrolytes for sodium ion batteries have been introduced. The current research status of organic liquid electrolytes for sodium ion batteries has been highlighted, including compatibility with various types of electrodes and electrochemical properties such as multiplicative performance and cycling performance of electrode materials in electrolytes. The composition, formation mechanism and regulation strategies of interfacial films have been explained. Finally, the development trends of sodium ion battery electrolytes in terms of compatibility with materials, safety and stable interfacial film formation are pointed out in the future.

The applications of solid-state NMR and MRI techniques in the study of rechargeable sodium-ion batteries

In order to develop new electrode and electrolyte materials for advanced sodium-ion batteries (SIBs), it is crucial to understand a number of fundamental issues. These include the compositions of the bulk and interface, the structures of the materials used, and the electrochemical reactions in the batteries. Solid-state NMR (SS-NMR) has unique advantages in characterizing the local or microstructure of solid electrode/electrolyte materials and their interfaces-one such advantage is that these are determined in a noninvasive and nondestructive manner at the atomic level. In this review, we provide a survey of the recent advances in the understanding of the fundamental issues of SIBs using advanced NMR techniques. First, we summarize the applications of SS-NMR in characterizing electrode material structures and solid electrolyte interfaces (SEI). In particular, we elucidate the key role of in-situ NMR/MRI in revealing the complex reactions and degradation mechanisms of SIBs. Next, the characteristics and shortcomings of SS-NMR and MRI techniques in SIBs are also discussed in comparison to similar Li-ion batteries. Finally, an overview of SS-NMR and MRI techniques for sodium batteries are briefly discussed and presented.

Structure and Interface Engineering of Ultrahigh-Rate 3D Bismuth Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attention as promising low-cost energy storage devices in future grid-scale energy management applications. Bismuth is a promising anode for SIBs due to its high theoretical capacity (386 mAh g-1 ). Nevertheless, the huge volume variation of Bi anode during (de)sodiation processes can cause the pulverization of Bi particulates and rupture of solid electrolyte interphase (SEI), resulting in quick capacity decay. It is demonstrated that rigid carbon framework and robust SEI are two essentials for stable Bi anodes. A lignin-derived carbonlayer wrapped tightly around the bismuth nanospheres provides a stable conductive pathway, while the delicate selection of linear and cyclic ether-based electrolytes enable robust and stable SEI films. These two merits enable the long-term cycling process of the LC-Bi anode. The LC-Bi composite delivers outstanding sodium-ion storage performance with an ultra-long cycle life of 10 000 cycles at a high current density of 5 A g-1 and an excellent rate capability of 94% capacity retention at an ultrahigh current density of 100 A g-1 . Herein, the underlying origins of performance improvement of Bi anode are elucidated, which provides a rational design strategy for Bi anodes in practical SIBs.

Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

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.

Few-layer Graphene Lithiophilic and Sodiophilic Diffusion Layer on Porous Stainless Steel as Lithium and Sodium Metal Anodes

In order to subdue the obvious problem of uneven electric field distribution on regularly used copper/aluminum current collectors for alkali metal batteries, graphene on porous stainless steel (pSS_Gr) was fabricated using the ion etching technique that is employed as an effective host for lithium and sodium metal anodes. The binder-free pSS_Gr demonstrated stable Li plating and stripping at areal current and capacity of 6 mA cm^-2 and 2.54 mAh cm^-2 , respectively, for over 1000 cycles with 98% coulombic efficiency (C.E.). Also, in the case of Na metal anode, the host has shown stable performance at 4 mA cm^-2 and 1 mAh cm^-2 over 1000 cycles with ∼100% C.E.. Further, a full cell composed of Li-plated pSS_Gr as an anode and LiFePO₄ as a cathode is electrochemically tested at 50 mA g^-1 current density with stable 100 cycles.

3D Sodiophilic Ti3C2 MXene@g-C3N4 Hetero-Interphase Raises the Stability of Sodium Metal Anodes

Owing to several advantages of metallic sodium (Na), such as a relatively high theoretical capacity, low redox potential, wide availability, and low cost, Na metal batteries are being extensively studied, which are expected to play a major role in the fields of electric vehicles and grid-scale energy storage. Although considerable efforts have been devoted to utilizing MXene-based materials for suppressing Na dendrites, achieving a stable cycling of Na metal anodes remains extremely challenging due to, for example, the low Coulombic efficiency (CE) caused by the severe side reactions. Herein, a g-C₃N₄ layer was attached in situ on the Ti₃C₂ MXene surface, inducing a surface state reconstruction and thus forming a stable hetero-interphase with excellent sodiophilicity between the MXene and g-C₃N₄ to inhibit side reactions and guide uniform Na ion flux. The 3D construction can not only lower the local current density to facilitate uniform Na plating/stripping but also mitigate volume change to stabilize the electrolyte/electrode interphase. Thus, the 3D Ti₃C₂ MXene@g-C₃N₄ nanocomposite enables much enhanced average CEs (99.9% at 1 mA h cm^-2, 0.5 mA cm^-2) in asymmetric half cells, long-term stability (up to 700 h) for symmetric cells, and stable cycling (up to 800 cycles at 2 C), together with outstanding rate capability (up to 20 C), of full cells. The present study demonstrates an approach in developing practically high performance for Na metal anodes.

Exploring ionic liquid-laden metal-organic framework composite materials as hybrid electrolytes in metal (ion) batteries

This review focuses on the combination of metal-organic frameworks (MOFs) and ionic liquids (ILs) to obtain composite materials to be used as solid electrolytes in metal-ion battery applications. Benefiting from the controllable chemical composition, tunable pore structure and surface functionality, MOFs offer great opportunities for synthesizing high-performance electrolytes. Moreover, the encapsulation of ILs into porous materials can provide environmentally benign solid-state electrolytes for electrochemical devices. Due to the versatility of MOF-based materials, in this review we also explore their use as anodes and cathodes in Li- and Na-ion batteries. Finally, solid IL@MOF electrolytes and their implementation into Li and Na batteries have been analyzed, as well as the design and advanced manufacturing of solid IL@MOF electrolytes embedded on polymeric matrices.