Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes

Emerging high voltage V4+/V5+ redox reactions in Na3V2(PO4)3-based cathodes for sodium-ion batteries

Na3V2(PO4)3 (NVP) cathode materials with the advantages of long cycle life and superior thermal stability have been considered promising cathode candidates for SIBs. However, the unsatisfactory energy density derived from low theoretical capacity and operating voltage (3.35 V vs. Na+/Na, based on the V3+/V4+ redox couple) inevitably limits their practical application. Therefore, the activation of the V4+/V5+ redox couple (∼4.0 V vs. Na+/Na) in NVP-based cathode materials to boost the energy density of SIBs has attracted extensive attention. Herein, we first analyze the challenges of activation of the V4+/V5+ redox couple in NVP-based cathode materials. Subsequently, the recent achievement of NVP-based cathode materials with activated V4+/V5+ redox reactions for SIBs is overviewed. Finally, further research directions of high voltage V4+/V5+ redox reactions in NVP-based cathodes are proposed. This review provides valuable guidance for developing high energy density NVP-based cathode materials for SIBs.

Design Strategies toward High-Performance Zn Metal Anode

Rechargeable aqueous Zn-ion batteries (AZIBs) are one of the most promising alternatives for traditional energy-storage devices because of their low cost, abundant resources, environmental friendliness, and inherent safety. However, several detrimental issues with Zn metal anodes including Zn dendrite formation, hydrogen evolution, corrosion and passivation, should be considered when designing advanced AZIBs. Moreover, these thorny issues are not independent but mutually reinforcing, covering many technical and processing parameters. Therefore, it is necessary to comprehensively summarize the issues facing Zn anodes and the corresponding strategies to develop roadmaps for the development of high-performance Zn anodes. Herein, the failure mechanisms of Zn anodes and their corresponding impacts are outlined. Recent progress on improving the stability of Zn anode is summarized, including structurally designed Zn anodes, Zn alloy anodes, surface modification, electrolyte optimization, and separator design. Finally, this review provides brilliant and insightful perspectives for stable Zn metal anodes and promotes the large-scale application of AZIBs in power grid systems.

Interfacial Spinel Local Interlocking Strategy Toward Structural Integrity in P3 Oxide Cathodes

P3-layered transition oxide cathodes have garnered considerable attention owing to their high initial capacity, rapid Na+ kinetics, and less energy consumption during the synthesis process. Despite these merits, their practical application is hindered by the substantial capacity degradation resulting from unfavorable structural transformations, Mn dissolution and migration. In this study, we systematically investigated the failure mechanisms of P3 cathodes, encompassing Mn dissolution, migration, and the irreversible P3-O3' phase transition, culminating in severe structural collapse. To address these challenges, we proposed an interfacial spinel local interlocking strategy utilizing P3/spinel intergrowth oxide as a proof-of-concept material. As a result, P3/spinel intergrowth oxide cathodes demonstrated enhanced cycling performance. The effectiveness of suppressing Mn migration and maintaining local structure of interfacial spinel local interlocking strategy was validated through depth-etching X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and in situ synchrotron-based X-ray diffraction. This interfacial spinel local interlocking engineering strategy presents a promising avenue for the development of advanced cathode materials for sodium-ion batteries.

Rational Regulation of High-Voltage Stability in Potassium Layered Oxide Cathodes

Layered oxide cathode materials may undergo irreversible oxygen loss and severe phase transitions during high voltage cycling and may be susceptible to transition metal dissolution, adversely affecting their electrochemical performance. Here, to address these challenges, we propose synergistic doping of nonmetallic elements and in situ electrochemical diffusion as potential solution strategies. Among them, the distribution of the nonmetallic element fluorine within the material can be regulated by doping boron, thereby suppressing manganese dissolution through surface enrichment of fluorine. Furthermore, in situ electrochemical diffusion of fluorine from the surface into the bulk of the materials after charging reduces the energy barrier of potassium ion diffusion while effectively inhibiting irreversible oxygen loss under high voltage. The modified K0.5Mn0.83Mg0.1Ti0.05B0.02F0.1O1.9 layered oxide cathode exhibits a high capacity of 147 mAh g-1 at 50 mA g-1 and a long cycle life of 2200 cycles at 500 mA g-1. This work demonstrates the efficacy of synergistic doping and in situ electrochemical diffusion of nonmetallic elements and provides valuable insights for optimizing rechargeable battery materials.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

P2-type Na0.67Ni0.33Mn0.67O2 has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the Na0.67Ni0.33Mn0.67O2 cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (Na0.67Ni0.23Cu0.1Mn0.67O2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal-oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode maintains the P2 phase within the voltage window of 2.5-4.3 V, whereas the P2-Na0.67Ni0.33Mn0.67O2 cathode transforms entirely into O2-type Na0.67Ni0.33Mn0.67O2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode occurred at 4.6 V, indicating that Cu2+ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na0.67Ni0.23Cu0.1Mn0.67O2 cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g-1) and retaining more than 78 % of its capacity after 500 cycles.

Facilitating Layered Oxide Cathodes Based on Orbital Hybridization for Sodium-Ion Batteries: Marvelous Air Stability, Controllable High Voltage, and Anion Redox Chemistry

Layered oxides have become the research focus of cathode materials for sodium-ion batteries (SIBs) due to the low cost, simple synthesis process, and high specific capacity. However, the poor air stability, unstable phase structure under high voltage, and slow anionic redox kinetics hinder their commercial application. In recent years, the concept of manipulating orbital hybridization has been proposed to simultaneously regulate the microelectronic structure and modify the surface chemistry environment intrinsically. In this review, the hybridization modes between atoms in 3d/4d transition metal (TM) orbitals and O 2p orbitals near the region of the Fermi energy level (EF) are summarized based on orbital hybridization theory and first-principles calculations as well as various sophisticated characterizations. Furthermore, the underlying mechanisms are explored from macro-scale to micro-scale, including enhancing air stability, modulating high working voltage, and stabilizing anionic redox chemistry. Meanwhile, the origin, formation conditions, and different types of orbital hybridization, as well as its application in layered oxide cathodes are presented, which provide insights into the design and preparation of cathode materials. Ultimately, the main challenges in the development of orbital hybridization and its potential for the production application are also discussed, pointing out the route for high-performance practical sodium layered oxide cathodes.

Accelerating the Phase Formation Kinetics of Alluaudite Sodium Iron Sulfate Cathodes via Ultrafast Thermal Shock

Alluaudite sodium iron sulfate (NFS) exhibits great potential for use in sodium-ion battery cathodes due to its elevated operating potential and abundant element reserves. However, conventional solid-state methods demonstrate a low heating/cooling rate and sluggish reaction kinetics, requiring a long thermal treatment to effectively fabricate NFS cathodes. Herein, we propose a thermal shock (TS) strategy to synthesize alluaudite sodium iron sulfate cathodes using either hydrous or anhydrous raw materials. The analysis of the phase formation process reveals that TS treatment can significantly facilitate the removal of crystal water and decomposition of the intermediate phase Na2Fe(SO4)2 in the hydrous precursor. In the case of the anhydrous precursor, the kinetics of the combination reaction between Na2SO4 and FeSO4 can be also accelerated by TS treatment. Consequently, pure NFS phase formation can be completed after a substantially shorter time of post-sintering, thereby saving significant time and energy. The TS-treated NFS cathode derived from hydrous precursor exhibits higher retention after 200 cycles at 1C and better rate capability than the counterpart prepared by conventional long-term tube furnace sintering, demonstrating the great potential of this novel strategy.

A thermodynamically stable O2-type cathode with reversible O2-P2 phase transition for advanced sodium-ion batteries

Low-cost sodium-ion batteries (SIBs) have shown very promise in the applications of renewable energy and low-speed electric vehicles. The development of a new O2-type cathode in SIBs is very challenging in that this compound is only stable as an intermediate product of P2-type oxides during redox reactions. Here, we report a thermodynamically stable O2-type cathode obtained by Na/Li ion exchange from P2-type oxide in a binary molten salt system. It is demonstrated that the as-prepared O2-type cathode exhibits a highly reversible O2-P2 phase transition during Na+ de-intercalation. The unusual O2-P2 transition has a low volume change of ∼11%, much lower than that of 23.2% for P2-O2 transformation in the P2-type cathode. The lowered lattice volume change of this O2-type cathode gives rise to superior structural stability upon cycling. Therefore, the O2-type cathode possesses a reversible capacity of about 100 mAh/g with a good capacity retention of 87.3% even after 300 cycles at 1C, indicating outstanding long-term cycling stability. These achievements will promote the development new class of cathode materials with high capacity and structural stability for advanced SIBs.

Li, Na, K, Mg, Zn, Al, and Ca Anode Interface Chemistries Developed by Solid-State Electrolytes

Solid-state batteries (SSBs) have received significant attention due to their high energy density, reversible cycle life, and safe operations relative to commercial Li-ion batteries using flammable liquid electrolytes. This review presents the fundamentals, structures, thermodynamics, chemistries, and electrochemical kinetics of desirable solid electrolyte interphase (SEI) required to meet the practical requirements of reversible anodes. Theoretical and experimental insights for metal nucleation, deposition, and stripping for the reversible cycling of metal anodes are provided. Ion transport mechanisms and state-of-the-art solid-state electrolytes (SEs) are discussed for realizing high-performance cells. The interface challenges and strategies are also concerned with the integration of SEs, anodes, and cathodes for large-scale SSBs in terms of physical/chemical contacts, space-charge layer, interdiffusion, lattice-mismatch, dendritic growth, chemical reactivity of SEI, current collectors, and thermal instability. The recent innovations for anode interface chemistries developed by SEs are highlighted with monovalent (lithium (Li+ ), sodium (Na+ ), potassium (K+ )) and multivalent (magnesium (Mg2+ ), zinc (Zn2+ ), aluminum (Al3+ ), calcium (Ca2+ )) cation carriers (i.e., lithium-metal, lithium-sulfur, sodium-metal, potassium-ion, magnesium-ion, zinc-metal, aluminum-ion, and calcium-ion batteries) compared to those of liquid counterparts.

Poly(p-phenylenediamine)-Coated Metal-Organic Frameworks for High-Performance Sodium-Ion Batteries: The Balance of Capacity and Stability

Metal-organic frameworks (MOFs) with well-defined porous structures and highly active frameworks are considered as promising electrode materials for sodium-ion batteries (SIBs). However, the structure pulverization upon sodiation/desodiation impacts on their practical application in SIBs. To address this issue, poly(p-phenylenediamine) (PPA) was uniformly coated onto the surface of MIL-88A, a typical Fe-based MOF through in situ polymerization initiated by the metal ions (Fe3+) of MIL-88A. Used as an anode material for SIBs, the PPA-coated MIL-88A, denoted as PPA@MIL-88A, showed significantly improved electrochemical performance. A reversible capacity as high as 230 mAh g-1 was achieved at 0.2 A g-1 even after 500 cycles. MIL-88A constructed with electrochemically active Fe3+ and fumaric acid ligands guarantees the high specific capacity, while the PPA polymer coating effectively inhibits the pulverization of MIL-88A. This work provides an efficient strategy for improving the structure and cycling stability of MOFs-based electrode materials.

P2-Type Moisture-Stable and High-Voltage-Tolerable Cathodes for High-Energy and Long-Life Sodium-Ion Batteries

P2-Na_0.67Ni_0.33Mn_0.67O₂ represents a promising cathode for Na-ion batteries, but it suffers from severe structural degradation upon storing in a humid atmosphere and cycling at a high cutoff voltage. Here we propose an in situ construction to achieve simultaneous material synthesis and Mg/Sn cosubstitution of Na_0.67Ni_0.33Mn_0.67O₂ via one-pot solid-state sintering. The materials exhibit superior structural reversibility and moisture insensitivity. In-operando XRD reveals an essential correlation between cycling stability and phase reversibility, whereas Mg substitution suppressed the P2-O2 phase transition by forming a new Z phase, and Mg/Sn cosubstitution enhanced the P2-Z transition reversibility benefiting from strong Sn-O bonds. DFT calculations disclosed high chemical tolerance to moisture, as the adsorption energy to H₂O was lower than that of the pure Na_0.67Ni_0.33Mn_0.67O₂. A representative Na_0.67Ni_0.23Mg_0.1Mn_0.65Sn_0.02O₂ cathode exhibits high reversible capacities of 123 mAh g^-1 (10 mA g^-1), 110 mAh g^-1 (200 mA g^-1), and 100 mAh g^-1 (500 mA g^-1) and a high capacity retention of 80% (500 mA g^-1, 500 cycles).

Transition Metal Vacancy in Layered Cathode Materials for Sodium-Ion Batteries

Anionic redox has been considered as a promising strategy to break the capacity limitation of cathode materials that solely relies on the intrinsic cationic redox in secondary batteries. Vacancy, as a kind of defect, can be introduced into transition metal layer to trigger oxygen redox, thus enhancing the energy density of layer-structured cathode materials for sodium-ion batteries. Herein, the formation process, recent progress in working mechanisms of triggering oxygen redox, as well as advanced characterization techniques for transition metal (TM) vacancy were overviewed and discussed. Strategies applied to stabilize the vacancy contained structures and harness the reversible oxygen redox were summarized. Furthermore, the challenges and prospects for further understanding TM vacancy were particularly emphasized.

Optimizing Vanadium Redox Reaction in Na3V2(PO4)3 Cathodes for Sodium-Ion Batteries by the Synergistic Effect of Additional Electrons from Heteroatoms

Na3V2(PO4)3 (NVP) is one of the most potential cathode materials for sodium-ion batteries (SIBs), but its actual electrochemical performance is limited by the defects of large electron and ion transfer resistance. Multicomponent design is considered an effective method to optimize the conductivity of NVP electrodes. Therefore, Cr and Si are added in NVP to form a multielement component of Na3V1.9Cr0.1(PO4)2.9(SiO4)0.1 (NVP-CS). It is confirmed that 3d electrons of Cr are beneficial for improving the conductivity and increasing the average potential by activating V4+/V5+. Theoretical calculations show that the introduction of Si changes the electronic structure of V and O, thus promoting the electrochemical reaction of V3+/V4+ to exert higher capacity. Due to the coordination of the two elements, a lower migration barrier is obtained in NVP-CS. Specifically, NVP-CS retains the advantages of single-doped electrodes very well (capacity retention of 90% after 300 cycles at 1 C and a high capacity of 94.1 mA h g-1 at 5 C, compared to NVP with only 82.6% capacity retention at 1 C and 59.4 mA h g-1 at 5 C). The excellent electrochemical performance results show that NVP can be successfully optimized by the introduction of Cr and Si. This work can provide some inspiration for multicomponent material research of cathode materials.

Altering the Alkaline Metal Ions in Lepidocrocite-Type Layered Titanate for Sodium-Ion Batteries

The relatively large ionic radius of the Na ion is one of the primary reasons for the slow diffusion of Na ions compared to that of Li ions in de/intercalation processes in sodium-ion batteries (SIBs). Interlayer expansion of intercalation hosts is one of the effective techniques for facilitating Na-ion diffusion. For most ionic layered compounds, interlayer expansion relies on intercalation of guest ions. It is important to investigate the role of these ions for material development of SIBs. In this study, alkali-metal ions (Li⁺, Na⁺, K⁺, and Cs⁺) with different sizes were intercalated into lepidocrocite-type layered titanate by a simple ion-exchange technique to achieve interlayer modulation and those were then evaluated as anode materials for SIBs. By controlling the intercalated alkaline ion species, basal spacings of layered titanates (LTs) in the range of 0.68 to 0.85 nm were obtained. Interestingly, the largest interlayer spacing induced by the large size of Cs did not yield the best performance, while the Na intercalated layered titanate (Na-ILT) demonstrated a superior performance with a specific capacity of 153 mAh g^-1 at a current density of 0.1 A g^-1. We found that the phenomena can be explained by the high alkaline metal ion concentration and the efficient utilization of the active sites in Na-ILT. The detailed analysis indicates that large intercalating ions like Cs can hamper sodium-ion diffusion although the interlayer spacing is large. Our work suggests that adopting an appropriate interlayer ion species is key to developing highly efficient layered electrode materials for SIBs.

Suppressing the Dynamic Oxygen Evolution of Sodium Layered Cathodes through Synergistic Surface Dielectric Polarization and Bulk Site-Selective Co-Doping

Utilizing anionic redox activity within layered oxide cathode materials represents a transformational avenue for enabling high-energy-density rechargeable batteries. However, the anionic oxygen redox reaction is often accompanied with irreversible dynamic oxygen evolution, leading to unfavorable structural distortion and thus severe voltage decay and rapid capacity fading. Herein, it is proposed and validated that the dynamic oxygen evolution can be effectively suppressed through the synergistic surface CaTiO₃ dielectric coating and bulk site-selective Ca/Ti co-doping for layered Na_2/3 Ni_1/3 Mn_2/3 O₂ . The surface dielectric coating layer not only suppresses the surface oxygen release but more importantly inhibits the bulk oxygen migration by creating a reverse electric field through dielectric polarization. Meanwhile, the site-selective doping of oxygen-affinity Ca into Na layers and Ti into transition metal layers effectively stabilizes the bulk oxygen through modulating the O 2p band center and the oxygen migration barrier. Such a strategy also leads to a reversible structural evolution with a low volume change because of the enhanced structural integrality and improved oxygen rigidity. Because of these synergistic advantages, the designed electrode exhibits greatly suppressed voltage decay and capacity fading upon long-term cycling. This study affords a promising strategy for regulating the dynamic oxygen evolution to achieve high-capacity layered cathode materials.

Multifunctional Carbon Modification Enhancement for Vanadium-Based Phosphates as an Advanced Cathode of Zinc-Ion Batteries

In recent years, rechargeable aqueous zinc-ion batteries (ZIBs) have shown extraordinary potential due to their safety, nontoxicity, sustainable zinc resources, and low price. However, the lack of suitable cathode materials hinders the development of ZIBs. Recently, layered phosphates have been widely used as cathode materials. As one typical phosphate cathode, vanadium oxyphosphate (VOPO₄) has inherently low electronic conductivity and structural dissolution in electrochemical reactions, limiting its development. To solve these problems, VOPO₄/C is prepared by combining multifunctional carbon material with a VOPO₄ interlayer and an external surface, which not only improves the electronic conductivity of the composite material but also effectively inhibits the dissolution of VOPO₄ in the electrolyte. As a result, the prepared VOPO₄/C could deliver a reversible capacity of 140 mA h g^-1 at a current density of 100 mA g^-1. Furthermore, the rate performance of the VOPO₄/C composite has also been improved significantly. In the process of charging and discharging, zinc ions in the composite show perfect intercalate and deintercalate performance.

Dynamic Biomolecular "Mask" Stabilizes Zn Anode

Zn anode is confronted with serious Zn dendrite growth and water-induced parasitic reactions, which severely hinders the rapid development and practical application of aqueous zinc metal batteries (AZMBs). Herein, inspired by sodium hyaluronate (SH) biomolecules in living organisms featured with the functions of water retention, ion-transport regulation, and film-formation, the SH working as a dynamic and self-adaptive "mask" is proposed to stabilize Zn anode. Benefiting from the abundant functional groups with high hydrophilicity and zincophilicity, SH molecule can constrain active water molecules on the Zn-electrolyte interface and participate in Zn²⁺ solvation structure to suppress parasitic reactions. Furthermore, the dynamical adsorption of SH with high-density negative charge on the Zn surface could serve as Zn²⁺ reservoirs to guide uniform Zn deposition. Consequently, stable Zn plating and an ultrahigh cumulative plating capacity (CPC) of 4.8 Ah cm^-2 are achieved even at 20 mA cm^-2 (20 mAh cm^-2 ) in a Zn||Zn symmetric battery, reaching a record level in AZMBs. In addition, the Zn||β-MnO₂ full battery exhibits a substantially improved cycle stability. This work presents a route to realize a highly reversible and stable Zn metal anode by learning from nature.

Issues and Opportunities Facing Aqueous Mn2+ /MnO2 -based Batteries

Aqueous Mn²⁺ /MnO₂ -based batteries have attracted enormous attentions in aqueous energy storage fields, owing to their high working voltage and theoretical capacity (616 mAh g^-1 ) brought by the two-electron reaction (Mn²⁺ /Mn⁴⁺ ). However, there are currently several tricky challenges facing Mn²⁺ /MnO₂ -based batteries: their complicated working mechanisms, existing issues, and optimization strategies. This Perspective aims to provide a mechanistic understanding and an overview of the insufficiency, optimization, and future development for Mn²⁺ /MnO₂ -based batteries. The existing issues and deficiency in Mn²⁺ /MnO₂ -based batteries have been systematically analyzed, and optimization strategies have also been rationally summarized and discussed with deep insights. Also, the often-overlooked optimized objects and aspects have been highlighted with unique perspectives. The proposals of testing methods and performance assessment are presented, containing different degradation mechanisms. Based on the above points, this Perspective will provide guidance and contribute to the further development of aqueous Mn²⁺ /MnO₂ -based batteries.

Se/N co-doped carbon nanorods for potassium ion storage

Herein, we make use of the large-sized Se atom to regulate the local structure of the graphitic lattice to optimize N-doping species to strengthen the adsorption of K+, which can greatly boost the capacitive capacity for PIBs.

Cumulative effects of doping on Sn3O4 structure and electrode performance for rechargeable sodium-ion batteries

Interconnected nanoflakes of modified Sn3O4 are capable of providing fast carrier transmission dynamics and outstanding structural integrity, suggesting the feasibility of the current framework for sodium-ion storage.

The phosphate cathodes for aqueous zinc-ion batteries

We categorize phosphate-based cathodes in zinc-ion battery and highlight the relationship between structural properties and energy storage mechanisms. The major problems faced by each kind of materials and rational optimization strategies are summarized.