A Hybrid Poly(ethylene oxide)/ Poly(vinylidene fluoride)/TiO2 Nanoparticle Solid‐State Redox Electrolyte for Dye‐Sensitized Nanocrystalline Solar Cells

High‐efficiency all‐solid‐state dye‐sensitized nanocrystalline solar cells have been fabricated using a poly(ethylene oxide)/poly(vinylidene fluoride) (PEO/PVDF)/TiO₂‐nanoparticle polymer redox electrolyte, which yields an overall energy‐conversion efficiency of about 4.8 % under irradiation by white light (65.2 mW cm^–2). The introduction of PVDF (which contains the highly electronegative element fluorine) and TiO₂ nanoparticles into the PEO electrolyte increases the ionic conductivity (by about two orders of magnitude) and effectively reduces the recombination rate at the interface of the TiO₂ and the solid‐state electrolyte, thus enhancing the performance of the solar cell.     

Tailoring Sodium Anodes for Stable Sodium–Oxygen Batteries

Sodium–oxygen batteries based on abundant sodium resource have caused increasing interest due to their high energy density and low cost. However, the reactive sodium anode always induces unpredictable side reactions especially in oxygen atmosphere and battery short circuit due to the uncontrolled formation of dendrites. Here, an accessible method to grow a durable protective passivation film on the surface of sodium anodes is reported. The sodium fluoride‐rich film efficiently suppresses O₂ crossover and electrolyte decay related side reactions on sodium anodes. Significantly, no dendrites form during long‐term stripping/plating cycles. Benefiting from these superiorities, sodium–oxygen batteries achieve impressive improvement of discharge–charge ability and reliable safety. These results enrich the approaches for stabilizing sodium anodes and allow researchers to focus on the investigations on the air cathode and the overall chemistry of sodium–oxygen batteries.     

Phase-Transition Interlayer Enables High-Performance Solid-State Sodium Batteries with Sulfide Solid Electrolyte

All-solid-state sodium (Na) batteries (ASSSBs) using sulfide-based solid electrolytes (SEs) have attracted intensive attention due to their superior safety, high energy density, and low cost. However, the interfacial issue is one of the biggest challenges to achieve high-performance sulfide-based ASSSBs due to the serious reactions between active Na metal and sulfide SEs at the interface. To address the interfacial challenges, a simple and efficient approach by introducing a phase transition polymer electrolyte as an interlayer to stabilize the interface is proposed. Na₃SbS₄ as a model sulfide SE is used to demonstrate the interlayer strategy to stabilize the interface by preventing the detrimental reactions and inhibiting Na dendrites. As a result, stable Na plating/stripping is observed in Na symmetric cells under the current density of 0.1 mA cm⁻². Moreover, ASSSBs with Na metal and TiS₂ electrode deliver long-term stability over 300 cycles remaining a specific capacity above 100 mAh g⁻¹, and FeS₂||Na cells exhibit an impressive specific capacity of up to 200 mAh g⁻¹ after the 20th cycles. This study demonstrates an efficient strategy to address interfacial challenges between sulfide SEs and Na metal, which contributes to the development of ASSSBs in next-generation energy storage systems.     

A Rooted Multifunctional Heterogeneous Interphase Layer Enabled by Surface-Reconstruction for Highly Durable Sodium Metal Anodes

Sodium plating–stripping with high reversibility is still an intractable challenge for sodium metal-based batteries due to the fragile natural solid-electrolyte interphase (SEI) film and severe Na dendrites growth. Herein, a surface reconstruction strategy is proposed and a rooted heterogeneous interlayer derived from in situ reactions between tin selenide and Na metal (abbr. Na/SnSe) is produced to regulate Na⁺ deposition behavior and impede dendrite growth. The high sodiophilic Na₁₅Sn₄ component demonstrates the robust combination and dendrite suppression capability, inhibiting fracture and delamination problems during volume variation. Meanwhile, the superionic Na₂Se ingredient contributes to the optimized Na⁺ conduction efficiency and low nucleation overpotential, enabling uniform distribution of electrical fields and ultimately eliminating Na dendrites. Consequently, the reconfigured multifunctional Na/SnSe interphase realizes a long-term lifespan over 2400 h at 0.5 mA cm⁻²/1 mAh cm⁻² in symmetric cell with an extremely low voltage hysteresis. Moreover, the assembled Na/SnSe||NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cell achieves exceptional cycling stability and capacity retention (90.4 mAh g⁻¹ after 1800 cycles at a high current density of 2 A g⁻¹), exploiting an avenue for designing durable SEI layer and high-quality sodium metal batteries.     

Pillared MXene with Ultralarge Interlayer Spacing as a Stable Matrix for High Performance Sodium Metal Anodes

Sodium (Na) metal is a promising alternative to lithium metal as an anode material for the next‐generation energy storage systems due to its high theoretical capacity, low cost, and natural abundance. However, dendritic/mossy Na growth caused by uncontrollable plating/stripping results in serious safe concerns and rapid electrode degradation. This study presents Sn²⁺ pillared Ti₃C₂ MXene serving as a stable matrix for high‐performance dendrite‐free Na metal anode. The intercalated Sn²⁺ between Ti₃C₂ layers not only induces Na to nucleate and grow within Ti₃C₂ interlayers, but also endows the Ti₃C₂ with larger interlayer space to accommodate the deposited Na by taking advantage of the “pillar effect,” contributing to uniform Na deposition. As a result, the pillar‐structured MXene‐based Na metal electrode could enable high current density (up to 10 mA cm^?2) along with high areal capacity (up to 5 mAh cm^?2) over long‐term cycling (up to 500 cycles). The full cell using MXene‐based Na metal anode exhibits superior electrochemical performance than that using host‐less commercial Na. It is believed that the well‐controlled MXene‐based Na anode not only extends the application scope of MXene, but also provides guidance in designing high‐performance Na metal batteries.     

Reaction Mechanism Optimization of Solid-State Li–S Batteries with a PEO-Based Electrolyte

The shuttle effect of long-chain polysulfides (Li₂S_n, n = 4–8) from the multistep reactions reduces the cycling life of solid-state lithium–sulfur (Li–S) batteries with a poly(ethylene oxide) (PEO)-based solid polymer electrolyte (SPE). Moreover, the ambiguous reaction mechanism of polysulfides in an SPE also limits the development of high-performance solid-state Li–S batteries. Here, a solid-state Li–S cell with a much-improved cycling performance is reported by coating the sulfur cathode with a layer of polyvinylidene fluoride (PVDF), which not only suppresses the formation of soluble polysulfides, but also changes the reaction mechanism of the sulfur from a multistep “solid–liquid–solid” reaction to a single-step “solid–solid” reaction. These results show that long-chain polysulfides are insoluble and unstable in PVDF polymers with a low solvent property, which facilitates the direct transformation of elemental sulfur to solid Li₂S₂/Li₂S without the formation of intermediary products. However, the strong PEO–Li₂S_n (n = 4–8) attraction causes a dissolution of polysulfides in PEO. The introduction of a polymer with a low solvent property in the sulfur cathode would be promising for the development of solid-state Li–S batteries with a long cycling life.     

Excellent Cycling Stability of Sodium Anode Enabled by a Stable Solid Electrolyte Interphase Formed in Ether‐Based Electrolytes

Sodium‐ion batteries have been considered one of the most promising power sources beyond Li‐ion batteries. Although the Na metal anode exhibits a high theoretical capacity of 1165 mAh g^?1, its application in Na batteries is largely hindered by dendrite growth and low coulombic efficiency. Herein, it is demonstrated that an electrolyte consisting of 1 m sodium tetrafluoroborate in tetraglyme can enable excellent cycling efficiency (99.9%) of a Na metal anode for more than 1000 cycles. This high reversibility of a Na anode can be attributed to a stable solid electrolyte interphase formed on the Na surface, as revealed by cryogenic transmission electron microscopy and X‐ray photoelectron spectroscopy (XPS). These electrolytes also enable excellent cycling stability of Na||hard‐carbon cells and Na||Na_2/3Co_1/3Mn_2/3O₂ cells at high rates with very high coulombic efficiencies.     

Hierarchical Co3O4 Nanofiber–Carbon Sheet Skeleton with Superior Na/Li‐Philic Property Enabling Highly Stable Alkali Metal Batteries

Uncontrollable dendritic behavior and infinite volume expansion in alkali metal anode results in the severe safety hazards and short lifespan for high‐energy batteries. Constructing a stable host with superior Na/Li‐philic properties is a prerequisite for commercialization. Here, it is demonstrated that the small Gibbs free energy change in the reaction between metal oxide (Co₃O₄, SnO₂, and CuO) and alkali metal is key for metal infusion. The as‐prepared hierarchical Co₃O₄ nanofiber–carbon sheet (CS) skeleton shows improved wettability toward molten Li/Na. The 3D carbon sheet serves as a primary framework, offering adequate lithium nucleation sites and sufficient electrolyte/electrode contact for fast charge transfer. The secondary framework of Co/Li₂O nanofibers provides physical confinement of deposited Li and further redistributes the Li⁺ flux on each carbon fiber, which is verified by COMSOL Multiphysics simulations. Due to the uniform deposition behavior and near‐zero volume change, modified symmetrical Li/Li cells can operate under an ultrahigh current density of 20 mA cm^?2 for more than 120 cycles. When paired with LiFePO₄ cathodes, the Li/Co–CS cell shows low polarization and 88.4% capacity retention after 200 cycles under 2 C. Convincing improvement can also be observed in Na/Co–CS symmetrical cells applying NaClO₄‐based electrolyte. These results illustrate a significant improvement in developing safe and stable alkali metal batteries.     

Vertically Aligned Nanocomposite Thin Films as a Cathode/Electrolyte Interface Layer for Thin‐Film Solid Oxide Fuel Cells

A thin layer of a vertically aligned nanocomposite (VAN) structure is deposited between the electrolyte, Ce_0.9Gd_0.1O_1.95 (CGO), and the thin‐film cathode layer, La_0.5Sr_0.5CoO₃ (LSCO), of a thin‐film solid‐oxide fuel cell (TFSOFC). The self‐assembled VAN nanostructure contains highly ordered alternating vertical columns of CGO and LSCO formed through a one‐step thin‐film deposition process that uses pulsed laser deposition. The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode. Low cathode polarization resistances of 9 × 10^?4 and 2.39 Ω were measured for the cells with the VAN interlayer at 600 and 400 °C, respectively. Furthermore, anode‐supported single cells with LSCO/CGO VAN interlayer demonstrate maximum power densities of 329, 546, 718, and 812 mW cm^?2 at 550, 600, 650, and 700 °C, respectively, with an open‐circuit voltage (OCV) of 1.13 V at 550 °C. The cells with the interlayer triple the overall power output at 650 °C compared to that achieved with the cells without an interlayer. The binary VAN interlayer could also act as a transition layer that improves adhesion and relieves both thermal stress and lattice strain between the cathode and the electrolyte.     

Sodium Storage Behavior in Natural Graphite using Ether‐based Electrolyte Systems

This work reports that natural graphite is capable of Na insertion and extraction with a remarkable reversibility using ether‐based electrolytes. Natural graphite (the most well‐known anode material for Li–ion batteries) has been barely studied as a suitable anode for Na rechargeable batteries due to the lack of Na intercalation capability. Herein, graphite is not only capable of Na intercalation but also exhibits outstanding performance as an anode for Na ion batteries. The graphite anode delivers a reversible capacity of ≈150 mAh g^?1 with a cycle stability for 2500 cycles, and more than 75 mAh g^?1 at 10 A g^?1 despite its micrometer‐size (≈100 μm). An Na storage mechanism in graphite, where Na⁺‐solvent co‐intercalation occurs combined with partial pseudocapacitive behaviors, is revealed in detail. It is demonstrated that the electrolyte solvent species significantly affect the electrochemical properties, not only rate capability but also redox potential. The feasibility of graphite in a Na full cell is also confirmed in conjunction with the Na_1.5VPO_4.8F_0.7 cathode, delivering an energy of ≈120 Wh kg^?1 while maintaining ≈70% of the initial capacity after 250 cycles. This exceptional behavior of natural graphite promises new avenues for the development of cost‐effective and reliable Na ion batteries.     

Sodium‐Ion Batteries: Building Effective Layered Cathode Materials with Long‐Term Cycling by Modifying the Surface via Sodium Phosphate

Surface stabilization of cathode materials is urgent for guaranteeing long‐term cyclability, and is important in Na cells where a corrosive Na‐based electrolyte is used. The surface of P2‐type layered Na_2/3[Ni_1/3Mn_2/3]O₂ is modified with ionic, conducting sodium phosphate (NaPO₃) nanolayers, ≈10 nm in thickness, via melt‐impregnation at 300 °C; the nanolayers are autogenously formed from the reaction of NH₄H₂PO₄ with surface sodium residues. Although the material suffers from a large anisotropic change in the c‐axis due to transformation from the P2 to O2 phase above 4 V versus Na⁺/Na, the NaPO₃‐coated Na_2/3[Ni_1/3Mn_2/3]O₂/hard carbon full cell exhibits excellent capacity retention for 300 cycles, with 73% retention. The surface NaPO₃ nanolayers positively impact the cell performance by scavenging HF and H₂O in the electrolyte, leading to less formation of byproducts on the surface of the cathodes, which lowers the cell resistance, as evidenced by X‐ray photoelectron spectroscopy and time‐of‐flight secondary‐ion mass spectroscopy. Time‐resolved in situ high‐temperature X‐ray diffraction study reveals that the NaPO₃ coating layer is delayed for decomposition to Mn₃O₄, thereby suppressing oxygen release in the highly desodiated state, enabling delay of exothermic decomposition. The findings presented herein are applicable to the development of high‐voltage cathode materials for sodium batteries.     

A Quasi-Solid-State Polyether Electrolyte for Low-Temperature Sodium Metal Batteries

Taken the unlimited Na reservoir worldwide, battery technology based on Na-ion chemistry poses as an ideal candidate for large-scale energy storage systems. Especially, with metallic Na replacing traditional carbon anodes, it's able to maximize the energy density inexpensively. Nevertheless, sodium metal batteries (SMBs) face intrinsically poor stability due to their highly-reactive nature, where low Coulombic efficiency and short lifetime are often witnessed. The situation can be further aggravated at low temperatures due to insurmountable kinetic barriers. Herein, a 1,3-dioxolane-based quasi-solid-state electrolyte (PDGE) is proposed with a high ionic conductivity of 3.68 mS cm⁻¹ even at −20 ^◦C for SMBs. Moreover, a weak solvation environment is tailored by PDGE, which possesses a high Na⁺ transference number of 0.7. Concurrently, the solid electrolyte interphase induced from PDGE presents inorganic Na₂O, NaF as the major components, which offers accelerated Na⁺ diffusion and superior stability upon long-term cycling. With such a quasi-solid-state electrolyte, the Na/Na₃V₂(PO₄)₃ full cell exhibits great stability over 1000 cycles at −20 ^◦C. This study has significant implications to the development for SMBs under low-temperature conditions.     

Visualizing the Oxidation Mechanism and Morphological Evolution of the Cubic‐Shaped Superoxide Discharge Product in Na–Air Batteries

Sodium–air (Na–O₂) batteries have recently developed as a high theoretical energy density energy storage and conversion system. In particular, Na–O₂ batteries with superoxide as the discharge product have a very high round‐trip energy efficiency over lithium–air batteries due to their significantly reduced charging overpotential. However, Na–O₂ batteries yet suffer from limited cycling lives because of the formation and incomplete removal of side products during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, while the mechanism of these processes is still not fully understood. Herein, a detailed investigation on tracking the decomposition pathway of cubic‐shaped micrometer‐sized NaO₂ discharge products in Na–O₂ batteries with carbon‐based air electrodes is reported. A detailed electrochemical charging mechanism is revealed during the charging process. The evolution of the chemical compositions of the discharge/side products in air electrode during charging is also verified by synchrotron‐based X‐ray absorption spectroscopy experiments. The formation of these intermediate phases other than NaO₂ during the charging process results in high overpotentials. These new findings can contribute to a better understanding and the rational design of future Na–O₂ batteries.     

Gradiently Sodiated Alucone as an Interfacial Stabilizing Strategy for Solid‐State Na Metal Batteries

All‐solid‐state metal batteries (ASSMBs) are attracting much attention due to their cost effectiveness, enhanced safety, room‐temperature performance and high theoretical specific capacity. However, the alkali metal anodes (such as Li and Na) are active enough to react with most solid‐state electrolytes (SSEs), leading to detrimental reactions at the metal–SSE interface. In this work, a molecular layer deposition (MLD) alucone film is employed to stabilize the active Na anode/electrolyte interface in the ASSMBs, limiting the decomposition of the sulfide‐based electrolytes (Na₃SbS₄ and Na₃PS₄) and Na dendrite growth. Such a strategy effectively improves the room‐temperature full battery performance as well as cycling stability for over 475 h in Na–Na symmetric cells. The modified interface is further characterized by X‐ray photoelectron spectroscopy (XPS) depth profiling, which provides spatially resolved evidence of the synergistic effect between the dendrite‐suppressed sodiated alucone and the insulating unsodiated alucone. The coupled layers reinforce the protection of the Na metal/electrolyte interface. Therefore, alucone is identified as an effective and bifunctional coating material for the enhancement of the metal/electrolyte interfacial stability, paving the way for rapid development and wide application of high‐energy ASSMBs.     

Surface Defects Reinforced Polymer-Ceramic Interfacial Anchoring for High-Rate Flexible Solid-State Batteries

High Li⁺ conductivity, good interfacial compatibility and high mechanical strength are desirable for practical utilization of all-solid-state electrolytes. In this study, by introducing Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) with surface defects into poly(ethylene oxide) (PEO), a composite solid electrolyte (OV-LLZTO/PEO) is prepared. The surface defects serve as anchoring points for oxygen atoms of PEO chains, forming a firmly bonded polymer-ceramic interface. This bonding effect effectively prevents the agglomeration of LLZTO particles and crystallization of PEO domains, forming a homogeneous electrolyte membrane exhibiting high mechanical strength, reduced interfacial resistance with electrodes as well as improved Li⁺ conductivity. Owing to these favorable properties, OV-LLZTO/PEO can be operated under a high current density (0.7 mA cm⁻²) in a Li–Li symmetric cell without short circuit. Above all, solid-state full-cells employing OV-LLZTO/PEO deliver state-of-the-art rate capability (8 C), power density and capacity retention. As a final proof of concept study, flexible pouch cells are assembled and tested, exhibiting high cycle stability under 5 C and excellent safety feature under abusive working conditions. Through manipulating the interfacial interactions between polymer and inorganic electrolytes, this study points out a new direction to optimizing the performance of all-solid-state batteries.     

Low Resistance–Integrated All‐Solid‐State Battery Achieved by Li7La3Zr2O12 Nanowire Upgrading Polyethylene Oxide (PEO) Composite Electrolyte and PEO Cathode Binder

All‐solid‐state lithium metal battery is the most promising next‐generation energy storage device. However, the low ionic conductivity of solid electrolytes and high interfacial impedance with electrode are the main factors to limit the development of all‐solid‐state batteries. In this work, a low resistance–integrated all‐solid‐state battery is designed with excellent electrochemical performance that applies the polyethylene oxide (PEO) with lithium bis(trifluoromethylsulphonyl)imide as both binder of cathode and matrix of composite electrolyte embedded with Li₇La₃Zr₂O₁₂ (LLZO) nanowires (PLLN). The PEO in cathode and PLLN are fused at high temperature to form an integrated all‐solid‐state battery structure, which effectively strengthens the interface compatibility and stability between cathode and PLLN to guarantee high efficient ion transportation during long cycling. The LLZO nanowires uniformly distributed in PLLN can increase the ionic conductivity and mechanical strength of composite electrolyte efficiently, which induces the uniform deposition of lithium metal, thereby suppressing the lithium dendrite growth. The Li symmetric cells using PLLN can stably cycle for 1000 h without short circuit at 60 °C. The integrated LiFePO₄/PLLN/Li batteries show excellent cycling stability at both 60 and 45 °C. The study proposed a novel and robust battery structure with outstanding electrochemical properties.     

Electrochemical Mechanism Investigation of Cu2MoS4 Hollow Nanospheres for Fast and Stable Sodium Ion Storage

Sodium ion batteries (SIBs) are promising alternatives to lithium ion batteries with advantages of cost effectiveness. Metal sulfides as emerging SIB anodes have relatively high electronic conductivity and high theoretical capacity, however, large volume change during electrochemical testing often leads to unsatisfactory electrochemical performance. Herein bimetallic sulfide Cu₂MoS₄ (CMS) with layered crystal structures are prepared with glucose addition (CMS1), resulting in the formation of hollow nanospheres that endow large interlayer spacing, benefitting the rate performance and cycling stability. The electrochemical mechanisms of CMS1 are investigated using ex situ X‐ray photoelectron spectroscopy and in situ X‐ray absorption spectroscopy, revealing the conversion‐based mechanism in carbonate electrolyte and intercalation‐based mechanism in ether‐electrolyte, thus allowing fast and reversible Na⁺ storage. With further introduction of reduced graphene oxide (rGO), CMS1–rGO composites are obtained, maintaining the hollow structure of CMS1. CMS1–rGO delivers excellent rate performance (258 mAh g^?1 at 50 mA g^?1 and 131.9 mAh g^?1 at 5000 mA g^?1) and notably enhanced cycling stability (95.6% after 2000 cycles). A full cell SIB is assembled by coupling CMS1–rGO with Na₃V₂(PO₄)₃‐based cathode, delivering excellent cycling stability (75.5% after 500 cycles). The excellent rate performance and cycling stability emphasize the advantage of CMS1–rGO toward advanced SIB full cells assembly.     

Micrometer‐Sized RuO2 Catalysts Contributing to Formation of Amorphous Na‐Deficient Sodium Peroxide in Na–O2 Batteries

The results obtained herein demonstrate that the oxygen electrode plays a critical role in determining the morphology and chemical composition of discharge products in Na–O₂ batteries. Micrometer‐sized cubic NaO₂, film‐like NaO₂, and nano‐sized amorphous spherical Na_2‐xO₂ are characterized as the main discharge products on the surface of reduced graphite oxide (rGO), boron‐doped rGO (B‐rGO), and micrometer‐sized RuO₂ catalyst‐coated B‐rGO (m‐RuO₂‐B‐rGO) cathodes, respectively. The Na–O₂ battery with m‐RuO₂‐B‐rGO as the cathode exhibits a much longer cycle life than those with the other cathodes, maintaining an unchanged capacity (0.5 mAh cm^‐2) after 100 cycles at a current density of 0.05 mA cm^‐2. A good rate capability and deep discharge–charge energy efficiency are also obtained. The excellent electrochemical performance of the battery is attributed to the effect of the micrometer‐sized RuO₂ catalyst. Owing to the high affinity of RuO₂ for oxygen, the amorphous phase Na_2‐xO₂ discharge product, which has good electrical contact with the RuO₂ particles, can decompose completely under 3.1 V without a sudden voltage jump. Meanwhile, the micrometer‐sized RuO₂ catalysts also provide enough active sites and space for the reactions, and effectively minimize the occurrence of side reactions between discharge products and carbon defects.     

Electrocatalytic Interlayer with Fast Lithium–Polysulfides Diffusion for Lithium–Sulfur Batteries to Enhance Electrochemical Kinetics under Lean Electrolyte Conditions

Lithium–sulfur batteries are promising energy‐storage devices because of their high theoretical energy densities. For practical Li–S batteries, reducing the amount of electrolyte used is essential for achieving the high energy densities. However, reducing the electrolyte amount leads to severe performance degradation, mainly because of sluggish deposition of discharge products (Li₂S) and the accompanying passivation issue that arise from the insulating nature of Li₂S. In this study, a lightweight, robust interlayer, with a 3D open structure and a low surface area is designed and fabricated. The structure facilitates electrolyte infiltration without trapping too much electrolyte. Moreover, the electrocatalytic Co nanoparticles embedded in the skeleton surface within the interlayer effectively promote Li ion diffusion, polysulfides conversion, and Li₂S deposition, and therefore enhance the electrochemical kinetics under lean electrolyte conditions. The mechanisms involved in the interlayer effects are investigated by microstructural characterizations, electrochemical performance tests, density functional theory calculations, and in situ X‐ray diffraction characterization. These results show the feasibility of using an interlayer strategy to improve the electrochemical performances of Li–S batteries under lean electrolyte conditions to potentially increase the practical energy densities of Li–S batteries.     

A Water Stable,Near‐Zero‐Strain O3‐Layered Titanium‐Based Anode for Long Cycle Sodium‐Ion Battery

Layered transition metal (TM) oxides of the stoichiometry Na_xMO₂ (M = TM) have shown great promise in sodium‐ion batteries (SIBs); however, they are extremely sensitive to moisture. To date, most reported titanium‐based layered anodes exhibit a P2‐type structure. In contrast, O3‐type compounds are rarely investigated and their synthesis is challenging due to their higher percentage of unstable Ti³⁺ than the P2 type. Here, a pure phase and highly crystalline O3‐type Na_0.73Li_0.36Ti_0.73O₂ with high performance is successfully proposed in SIBs. This material delivers a reversible capacity of 108 mAh g^?1 with a stable and safe potential of 0.75 V versus Na/Na⁺. In situ X‐ray diffraction reveals that this material does not undergo any phase transitions and exhibits a near‐zero volume change upon Na⁺ insertion/de‐insertion, which ensures exceptional long cycle life over 6000 cycles. Importantly, it is found that this O3‐Na_0.73Li_0.36Ti_0.73O₂ shows superior moisture stability, even when immersed into water, which are both elusive for conventional layered TM oxides in SIBs. It is believed that the small interlayer distance and high occupation of interlayer vacancy promise such unprecedented water stability.