Probing the Structural Transition Kinetics and Charge Compensation of the P2-Na0.78Al0.05Ni0.33Mn0.60O2 Cathode for Sodium Ion Batteries

A review of Ni-based layered oxide cathode materials for alkali-ion batteries

Compared with the costly and toxic LiCoO2 cathode in lithium-ion batteries (LIBs), nickel-based layered oxide (NLO) cathode materials exhibit the advantages of high capacity, natural abundance, environment-friendliness, and low cost, displaying tremendous application potentials in power batteries for automobiles and aircrafts. This review comprehensively introduces the challenges faced by NLO cathode materials in all alkali-ion batteries (AIBs) in their material synthesis, cation mixing, particle cracking, phase changes, cation dissolution of Mn, and oxygen loss Various strategies, including heteroatom doping, surface coating, and concentration gradient, are applied to tackle these problems by developing layered LiNi1-xMxO2 (M: metal; 0 < x < 1) and LiNixCoyMnzO2 (x + y + z = 1) materials. The successful commercial application of NLO cathode materials in LIBs has further driven their developments in sodium/potassium-ion batteries via the synthesis of (Na/K)Ni1-xMxO2. Moreover, many sophisticated techniques, including in situ X-ray diffraction, scanning/transmission electron microscopy, operando neutron diffraction, and elemental analysis, are used to simultaneously monitor real-time phase changes, lattice variations, structural distortions, and elemental dissolutions of NLO-based materials. Furthermore, density functional theory (DFT) calculations are discussed as a powerful tool for predicting structural evolution, energy band structures, optimal doping concentrations, and ion diffusion pathways, thereby guiding the reasonable design of these materials. Finally, this review provides perspectives on future research directions and modification strategies for NLO cathode materials in AIBs, aiming to accelerate their deployment in electric vehicles and other energy storage devices. These efforts are expected to contribute significantly to the advancement of sustainable energy technologies and the global pursuit for carbon neutrality.

In intercalation and an Si-containing protective layer enhance the electrochemical performance of NaNi0.5Mn0.5O2 for sodium-ion batteries

To enhance the electrochemical performance of NaNi0.5Mn0.5O2 (NM) cathode material for sodium-ion batteries, a novel indium-intercalated NaNi0.5-xMn0.5InxO2 cathode material enriched with oxygen vacancies was first synthesized via a high-shear co-precipitation method. Subsequently, a thin Si-containing protective layer was produced on the surface through the interfacial reaction between oxygen vacancies and tetraethyl orthosilicate. Results of performance evaluation and characterization tests, including in situ XRD, Ar+ sputtering XPS, and HAADF-STEM, indicated that the optimal sample Siy@NaNi0.497Mn0.5In0.003O2-y exhibited superior initial discharge capacity of 125.0 mA h g-1 (0.1 C) and excellent capacity retention of 98.4% after 100 cycles at 1 C. In particular, the Si@In-doped sample offered larger lattice spacing, higher Na+ diffusion rate and better conductivity than the In-intercalated sample and pristine NM. DFT calculations illustrated that In preferentially substituted the Mn site, while Si preferred to substitute the Ni site closer to the In-intercalating location. The Na+ diffusion energy barrier was greatly reduced with In intercalation and Si doping. Such a facile strategy to augment the lattice spacing of the O3-layer cathode while producing a thin protective layer using the oxygen vacancies on the surface has promising applications to explore new cathode materials with high electrochemical performance for sodium-ion batteries.

Anion engineering in lithium cobalt oxide for application in high-performance supercapacitors

This study has focused on enhancing the effectiveness of supercapacitors, which are crucial for energy storage applications. Traditionally, supercapacitors have faced challenges in achieving higher energy density than batteries. This study hypothesizes that modifying the anionic structure of lithium cobalt oxide can significantly improve supercapacitors' energy density and charge storage capability. Lithium cobalt oxide was synthesized by sol-gel method, and LiCoO2-x(F0.8Cl0.2)x with x = 0.1, 0.2, and 0.4 (F = 0.8x, Cl = 0.2x), was obtained by anion-exchange method. The structure and crystalline nature of the synthesized samples were analyzed using Fourier-transform infrared spectroscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy. To further confirm the correctness of the structures, microstructural and morphological studies were conducted using Field emission scanning electron microscopy and Transmission electron microscopy. The charge-discharge investigations showed that the electrode made of LiCoO1.6(F0.8Cl0.2)0.4 had a high specific capacitance (522.16 F g-1 at a current density of 1 A g-1) compared to the Lithium Cobalt Oxide electrode. In addition, it showed a fabulous cycle life stability with 92.04% coulombic efficiency after 4000 charge-discharge cycles.

Revealing the microstructure and mechanism of layered oxide cathodes for sodium-ion batteries by advanced TEM techniques

Sodium-ion batteries (SIBs) stand as promising alternatives to current lithium-ion batteries in various energy storage fields. Despite their potential, challenges arise due to the intricate nature of large-size Na+ charge carriers, impacting the cycling stability and rate performance, which currently fall short of commercialization requirements. Therefore, it is crucial to gain a deeper understanding of the structural changes and chemical evolution of battery components. The advancement of transmission electron microscopy (TEM) technology enables multi-dimensional characterization and analysis of SIB cathode materials. This review offers an in-depth overview and comparison of the utilization of advanced TEM techniques for studying layered oxide cathode materials. It covers various aspects, including the common analysis of atomic structures, structural phase transitions, elemental valence tracing, and anion redox, and provides insights from current in situ TEM experiments. The presented review aims to provide valuable insights to inform the rational design of high-performance SIB cathodes.

Ca/Li Synergetic-Doped Na0.67Ni0.33Mn0.67O2 to Realize P2-O2 Phase Transition Suppression for High-Performance Sodium-Ion Batteries

P2-type layered transition metal oxide Na0.67Ni0.33Mn0.67O2 is considered as a promising cathode for advanced sodium-ion batteries due to its high theoretical specific capacity. However, the P2-type cathode suffers severe P2-O2 phase transition during cycling process, resulting unsatisfactory cyclic stability and rate capability. Herein, a Ca/Li co-doped P2-type Na0.62Ca0.05Ni0.33Mn0.57Li0.10O2 (NCNMLO) cathode material was synthesized through a simple sol-gel method. With the synergistic effect of Ca-doping at Na sites and Li substitution at transition metal (TM) sites, the cathode achieves an excellent electrochemical performance due to the inhibited P2-O2 phase transition and improved ion diffusion with Na+/vacancy disordering arrangement. The NCNMLO cathode exhibits a good cyclic stability with 70.8 % of capacity retention at 1 C after 200 cycles and excellent rate capability with 40.1 mAh g-1 at 20 C. The dual sites doping strategy provides an effective and simple approach for designing high-performance layered oxide cathode materials for sodium-ion batteries.

Enabling Rapid and Stable Sodium Storage via a P2-Type Layered Cathode with High-Voltage Zero-Phase Transition Behavior

Currently, a major target in the development of Na-ion batteries is the concurrent attainment of high-rate capacity and long cycling stability. Herein, an advanced Na-ion battery with high-rate capability and long cycle stability based on Li/Ti co-doped P2-type Na0.67Mn0.67Ni0.33O2, a host material with high-voltage zero-phase transition behavior and fast Na+ migration/conductivity during dynamic de-embedding process, is constructed. Experimental results and theoretical calculations reveal that the two-element doping strategy promotes a mutually reinforcing effect, which greatly facilitates the transfer capability of Na+. The cation Ti4+ doping is a dominant high voltage, significantly elevating the operation voltage to 4.4 V. Meanwhile, doping Li+ shows the function in charge transfer, improving the rate performance and prolonging cycling lifespan. Consequently, the designed P2-Na0.75Mn0.54Ni0.27Li0.14Ti0.05O2 cathode material exhibits discharge capacities of 129, 104, and 85 mAh g- 1 under high voltage of 4.4 V at 1, 10, and 20 C, respectively. More importantly, the full-cell delivers a high initial capacity of 198 mAh g-1 at 0.1 C (17.3 mA g-1) and a capacity retention of 73% at 5 C (865 mA g-1) after 1000 cycles, which is seldom witnessed in previous reports, emphasizing their potential applications in advanced energy storage.

Understanding the Correlation between Electrochemical Performance and Operating Mechanism of a Co-free Layered-Spinel Composite Cathode for Na-Ion Batteries

Compositing different crystal structures of layered transition metal oxides (LTMOs) is an emerging strategy to improve the electrochemical performance of LTMOs in sodium-ion batteries. Herein, a cobalt-free P2/P3-layered spinel composite, P2/P3-LS-Na1/2Mn2/3Ni1/6Fe1/6O2 (LS-NMNF), is synthesized, and the synergistic effects from the P2/P3 and spinel phases were investigated. The material delivers an initial discharge capacity of 143 mAh g-1 in the voltage range of 1.5-4.0 V and displays a capacity retention of 73% at the 50th cycle. The material shows a discharge capacity of 72 mAh g-1 at 5C. This superior rate performance by the material could be by virtue of the increased electronic conductivity contribution of the incorporated spinel phase. The charge compensation mechanism of the material is investigated by in operando X-ray absorption spectroscopy (in a voltage range of 1.5-4.5 V vs Na+/Na), which revealed the contribution of all transition metals toward the generated capacity. The crystal structure evolution of each phase during electrochemical cycling was analyzed by in operando X-ray diffraction. Unlike in the case of many reported P2/P3 composite cathode materials and spinel-incorporated cobalt-containing P2/P3 composites, the formation of a P'2 phase at the end of discharge is absent here.

Enhancing Kinetics in Sodium Super Ion Conductor Na3MnTi(PO4)3 through Microbe-Assisted and Structural Optimization

Sodium (Na) super ion conductor (NASICON) structure Na3MnTi(PO4)3 (NMTP) is considered a promising cathode for sodium-ion batteries due to its reversible three-electron reaction. However, the inferior electronic conductivity and sluggish reaction kinetics limit its practical applications. Herein, we successfully constructed a three-dimensional cross-linked porous architecture NMTP material (AsN@NMTP/C) by a natural microbe of Aspergillus niger (AsN), and the structure of different NMTP cathodes was optimized by adjusting different transition metal Mn/Ti ratios. Both approaches effectively altered the three-dimensional NMTP structure, not only improving electronic conductivity and controlling Na+ diffusion pathways but also enhancing the electrochemical kinetics of the material. The resultant AsN@NMTP/C-650, sintered at 650 °C, exhibits better electrochemical performance with higher reversible three-electron reactions corresponding to the voltage platforms of Ti4+/3+, Mn3+/2+, and Mn4+/3+ around 2.1, 3.6, and 4.1 V (vs Na+/Na), respectively. The capacity retention rate is up to 89.3% after 1000 cycles at a 2C rate. Moreover, a series of results confirms that the Na3.4Mn1.2Ti0.8(PO4)3 cathode has the most excellent electrochemical performance when the Mn/Ti ratio is 1.2/0.8, with a high capacity of 96.59 mAh g-1 and 97.1% capacity retention after 500 cycles.

Sustainable Anionic Redox by Inhibiting Li Cross-Layer Migration in Na-Based Layered Oxide Cathodes

The irrational utilization of an anionic electron often accompanies structural degradation with an irreversible cation migration process upon cycling in sodium-layered oxide cathodes. Moreover, the insufficient understanding of the anionic redox involved cation migration makes the design strategies of high energy density electrodes even less effective. Herein, a P3-Na0.67Li0.2Fe0.2Mn0.6O2 (P3-NLFM) cathode is proposed with the in-plane disordered Li distribution after an in-depth remolding of the Li ribbon-ordered P3-Na0.6Li0.2Mn0.8O2 (P3-NLM) layered oxide. The disordered Li sublattice in the transition metal slab of P3-NLFM leads to the dispersed |O2p orbitals, the lowered charge transfer gap, and the suppressed phase transition at high voltages. Then the enhanced Mn-O interaction and electronic stability are disclosed by the crystal orbital Hamilton population (COHP) analysis at high voltage in P3-NLFM. Furthermore, ab initio molecular dynamics (AIMD) simulation suggests the order/disorder of the transition metal layer is highly correlated with the stability of the Li sublattice. The cross-layer migration and loss of Li in P3-NLM are suppressed in P3-NLFM to enable the high reversibility upon cycling. As a result, the P3-NLFM delivers a high capacity of 163 mAh g-1 without oxygen release and an enhanced capacity retention of 81.9% (vs 42.9% in P3-NLM) after 200 cycles, which constitutes a promising approach for sustainable oxygen redox in rechargeable batteries.

High Voltage Ga-Doped P2-Type Na2/3 Ni0.2 Mn0.8 O2 Cathode for Sodium-Ion Batteries

Ni/Mn-based oxide cathode materials have drawn great attention due to their high discharge voltage and large capacity, but structural instability at high potential causes rapid capacity decay. How to moderate the capacity loss while maintaining the advantages of high discharge voltage remains challenging. Herein, the replacement of Mn ions by Ga ions is proposed in the P2-Na2/3 Ni0.2 Mn0.8 O2 cathode for improving their cycling performances without sacrificing the high discharge voltage. With the introduction of Ga ions, the relative movement between the transition metal ions is restricted and more Na ions are retained in the lattice at high voltage, leading to an enhanced redox activity of Ni ions, validated by ex situ synchrotron X-ray absorption spectrum and X-ray photoelectron spectroscopy. Additionally, the P2-O2 phase transition is replaced by a P2-OP4 phase transition with a smaller volume change, reducing the lattice strain in the c-axis direction, as detected by operando/ex situ X-ray diffraction. Consequently, the Na2/3 Ni0.21 Mn0.74 Ga0.05 O2 electrode exhibits a high discharge voltage close to that of the undoped materials, while increasing voltage retention from 79% to 93% after 50 cycles. This work offers a new avenue for designing high-energy density Ni/Mn-based oxide cathodes for sodium-ion batteries.

Tuning oxygen release of sodium-ion layered oxide cathode through synergistic surface coating and doping

Layered transition metal oxides have the greatest potential for commercial application as cathode materials for sodium-ion batteries. However, transition metal oxides inevitably undergo an irreversible oxygen loss process during cycling, which leads to structural changes in the material and ultimately to severe capacity degradation. In this work, using density function theory (DFT) calculations, the Ni-O bond is revealed to be the weakest of the M-O bonds, which may lead to structural failure. Herein, the synergistic surface CeO2 modification and the trace doping of Ce elements stimulate oxygen redox and improve its reversibility, thus improving the structural stability and electrochemical performance of the material. Theoretical calculations prove that Na0.67Mn0.7Ni0.2Co0.1O2 (MNC) obtains electrons from CeO2, avoiding destruction of the Ni-O bond by over-energy released during the charging process and inhibiting oxygen loss. The capacity retention was 77.37% for 200 cycles at 500 mA g-1, compared to 33.84% for the unmodified Na0.67Mn0.7Ni0.2Co0.1O2. Overall, the present work demonstrates that the synergistic effect of surface coating and doping is an effective strategy for realizing tuning oxygen release and high electrochemical performance.

Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g^-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g^-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF₆ electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg^-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na⁺ kinetics facilitating Na⁺ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na⁺ kinetics, boosting the overall Na storage.

Rational design of intergrowth P2/O3 biphasic layered structure with reversible anionic redox chemistry and structural evolution for Na-ions batteries

Layered oxides have attracted unprecedented attention for their outstanding performance in sodium-ion battery cathodes. Among them, the two typical candidates P2 and O3 type materials generally demonstrate large diversities in specific capacity and cycling endurance with their advantages. Thus, composite materials that contain both P2 and O3 have been widely designed and constructed. Nevertheless, the anionic/cationic ions' behavior and structural evolution in such complex structures remain unclear. In this study, a deep analysis of an advanced Na_0.732Ni_0.273Mg_0.096Mn_0.63O₂ material that contains 78.39 wt% P2 phase and 21.61 wt% O3 phase is performed based on two typical cathodes P2 Na_0.67Ni_0.33Mn_0.67O2 and O3 NaNi_0.5Mn_0.5O₂ that have the same elemental constitution but different crystal structures. Structural analysis and density functional theory (DFT) calculations suggest that the composite is preferred to form a symbiotic structure at the atomic level, and the complex lattice texture of the biphase structure can block unfavorable ion and oxygen migration in the electrode process. Consequently, the biphase structure has significantly improved the electrochemical performance and kept preferable anionic oxygen redox reversibility. Furthermore, the hetero-epitaxy-like structure of the intergrowth of P2 and O3 structures share multi-phase boundaries, where the inconsistency in electrochemical behavior between P2 and O3 phases leads to an interlocking effect to prevent severe structural collapse and relieves the lattice strain from Na⁺ de/intercalation. Hence, the symbiotic P2/O3 composite materials exhibited a preferable capacity and cyclability (∼130 mAh g^-1 at 0.1 C, 73.1% capacity retention after 200 cycles at 1 C), as well as reversible structural evolution. These findings confirmed the advantages of using the bi/multi-phase cathode for high-energy Na-ion batteries.

In-Plane BO3 Configuration in P2 Layered Oxide Enables Outstanding Long Cycle Performance for Sodium Ion Batteries

P2-phase layered cathode materials with distinguished electrochemical performance for sodium-ion batteries have attracted extensive attention, but they face critical challenges of transition metal layer sliding and unfavorable formation of hydration phase upon cycling, thus showing inferior long cycle life. Herein, a new approach is reported to modulate the local structure of P2 material by constructing a state-of-the-art in-plane BO₃ triangle configuration ((Na_0.67 Ni_0.3 Co_0.1 Mn_0.6 O_1.94 (BO₃ )_0.02 ). Both are unveiled experimentally and theoretically that such a structure can serve as a robust pillar to hold up the entire structure, by inhibiting the H₂ O insertion upon Na (de)intercalation and preventing the structure from deformation, which significantly boost the long cycle capability of P2-materials. Meanwhile, more Na ions in the architecture are enabled to site on the edge sharing octahedrons (Na_e ), thus benefiting the Na⁺ transportation. Consequently, the as produced material demonstrates an ultralow volume variation (1.8%), and an outstanding capacity retention of 80.1% after 1000 cycles at 2 C. This work sheds light on efficient architecture modulation of layered oxides through proper nonmetallic element doping.

Slight compositional variation-induced structural disorder-to-order transition enables fast Na+ storage in layered transition metal oxides

The omnipresent Na⁺/vacancy orderings change substantially with the composition that inevitably actuate the ionic diffusion in rechargeable batteries. Therefore, it may hold the key to the electrode design with high rate capability. Herein, the influence of Na⁺/vacancy ordering on Na⁺ mobility is demonstrated firstly through a comparative investigation in P2-Na_2/3Ni_1/3Mn_2/3O₂ and P2-Na_2/3Ni_0.3Mn_0.7O₂. The large zigzag Na⁺/vacancy intralayer ordering is found to accelerate Na⁺ migration in P2-type Na_2/3Ni_1/3Mn_2/3O₂. By theoretical simulations, it is revealed that the Na⁺ ordering enables the P2-type Na_2/3Ni_1/3Mn_2/3O₂ with higher diffusivities and lower activation energies of 200 meV with respect to the P3 one. The quantifying diffusional analysis further prove that the higher probability of the concerted Na⁺ ionic diffusion occurs in P2-type Na_2/3Ni_1/3Mn_2/3O₂ due to the appropriate ratio of high energy ordered Na ions (Na_f) occupation. As a result, the interplay between the Na⁺/vacancy ordering and Na⁺ kinetic is well understood in P2-type layered cathodes.

Transition metal oxides as a cathode for indispensable Na-ion batteries

The essential requirement to harness well-known renewable energy sources like wind energy, solar energy, etc. as a component of an overall plan to guarantee global power sustainability will require highly efficient, high power and energy density batteries to collect the derived electrical power and balance out variations in both supply and demand. Owing to the continuous exhaustion of fossil fuels, and ever increasing ecological problems associated with global warming, there is a critical requirement for searching for an alternative energy storage technology for a better and sustainable future. Electrochemical energy storage technology could be a solution for a sustainable source of clean energy. Sodium-ion battery (SIB) technology having a complementary energy storage mechanism to the lithium-ion battery (LIB) has been attracting significant attention from the scientific community due to its abundant resources, low cost, and high energy densities. Layered transition metal oxide (TMO) based materials for SIBs could be a potential candidate for SIBs among all other cathode materials. In this paper, we discussed the latest improvement in the various structures of the layered oxide materials for SIBs. Moreover, their synthesis, overall electrochemical performance, and several challenges associated with SIBs are comprehensively discussed with a stance on future possibilities. Many articles discussed the improvement of cathode materials for SIBs, and most of them have pondered the use of Na x MO2 (a class of TMOs) as a possible positive electrode material for SIBs. The different phases of layered TMOs (Na x MO2; TM = Co, Mn, Ti, Ni, Fe, Cr, Al, V, and a combination of multiple elements) show good cycling capacity, structural stability, and Na+ ion conductivity, which make them promising cathode material for SIBs. This review discusses and summarizes the electrochemical redox reaction, structural transformations, significant challenges, and future prospects to improve for Na x MO2. Moreover, this review highlights the recent advancement of several layered TMO cathode materials for SIBs. It is expected that this review will encourage further development of layered TMOs for SIBs.

koLayered Oxide Cathode-Electrolyte Interface towards Na-Ion Batteries: Advances and Perspectives

With the ever increasing demand for low-cost and economic sustainable energy storage, Na-ion batteries have received much attention for the application on large-scale energy storage for electric grids because of the worldwide distribution and natural abundance of sodium element, low solvation energy of Na⁺ ion in the electrolyte and the low cost of Al as current collectors. Starting from a brief comparison with Li-ion batteries, this review summarizes the current understanding of layered oxide cathode/electrolyte interphase in NIBs, and discusses the related degradation mechanisms, such as surface reconstruction and transition metal dissolution. Recent advances in constructing stable cathode electrolyte interface (CEI) on layered oxide cathode are systematically summarized, including surface modification of layered oxide cathode materials and formulation of electrolyte. Urgent challenges are detailed in order to provide insight into the imminent developments of NIBs.

Anionic Redox Activities Boosted by Aluminum Doping in Layered Sodium-Ion Battery Electrode

Sodium-ion batteries (SIBs) have attracted widespread attention for large-scale energy storage, but one major drawback, i.e., the limited capacity of cathode materials, impedes their practical applications. Oxygen redox reactions in layered oxide cathodes are proven to contribute additionally high specific capacity, while such cathodes often suffer from irreversible structural transitions, causing serious capacity fading and voltage decay upon cycling, and the formation process of the oxidized oxygen species remains elusive. Herein, a series of Al-doped P2-type Na_0.6 Ni_0.3 Mn_0.7 O₂ cathode materials for SIBs are reported and the corresponding charge compensation mechanisms are investigated qualitatively and quantitatively. The combined analyses reveal that Al doping boosts the reversible oxygen redox reactions through the reductive coupling reactions between orphaned O 2p states in NaOAl local configurations and Ni⁴⁺ ions, as directly evidenced by X-ray absorption fine structure results. Additionally, Al doping also induces an increased interlayer spacing and inhibits the unfavorable P2 to O2 phase transition upon desodiation/sodiation, which is common in P2-type Mn-based cathode materials, leading to the great improvement in capacity retention and rate capability. This work provides deeper insights into the development of structurally stable and high-capacity layered cathode materials for SIBs with anion-cation synergetic contributions.

Unraveling the Synergistic Effect of Mg and Ti Codoping to Realize an Ordered Structure and Excellent Performance for Sodium-Ion Batteries

Layered cathodes have been recognized as potential advanced candidates for sodium-ion batteries (SIBs), but the poor electrochemical performance has seriously hindered their further development. Herein, an ordered Na_2/3[Ni_2/9Mg_1/9Mn_5/9Ti_1/9]O₂ (NMMT) is designed and investigated as a high-performance cathode for SIBs through the synergistic effect of Mg and Ti codoping. Compared to the single Mg- or Ti-doped materials, NMMT clearly exhibits superstructure ordering diffraction peaks, and neutron diffraction further confirms that the diffraction peaks can be well indexed by a larger supercell P6₃, rather than the common unit cell P6₃/mmc by X-ray diffraction (XRD). High-resolution transmission electron microscopy also approves the ordering arrangement. This material shows an obvious capacity activation process during the first cycles, thus delivering 113 mA h g^-1 specific capacity at 0.1 C (close to the theoretical value). Excellent rate capability even at 15 C and cycling stability after 500 cycles between 2.0 and 4.3 V can also be achieved, indicating that an ordered cathode is still promising. Besides, a single-phase reaction mechanism is revealed by ex situ/in situ XRD experiments. This study offers some insights into the material design and characterization of layered oxide cathodes for high-performance SIBs in the future.

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

The conventional P2-type cathode material Na0.67Ni0.33Mn0.67O2 suffers from an irreversible P2-O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na0.67Ni0.33Mn0.67O2. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg2+ and Ca2+ make high-voltage reactions (oxygen redox and Ni4+/Ni3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg2+ and Ca2+ ions, the adverse effects on high-voltage reactions involving Ni-O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2 exhibits a satisfactory initial energy density of 468.2 W h kg-1 and good capacity retention of 83% after 100 cycles at 50 mA g-1 within the voltage range of 2.2-4.35 V. This work deepens our understanding of the specific effects of Mg2+ and Ca2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

Yeast Template-Derived Multielectron Reaction NASICON Structure Na3MnTi(PO4)3 for High-Performance Sodium-Ion Batteries

The sodium super ion conductor (NASICON) structure materials are essential for sodium-ion batteries (SIBs) due to their robust crystal structure, excellent ionic conductivity, and flexibility to regulate element and valence. However, the poor electronic conductivity and inferior energy density caused by the nature of these materials have always been obstacles to commercialization. Herein, using yeast as a template to derive NASICON structure Na₃MnTi(PO₄)₃ (NMTP) materials (noted as Yeast@NMTP/C) is presented. The Yeast@NMTP/C material retains the microsphere morphology of the yeast template and not only controls the particle size (around 2 μm) to shorten the Na⁺ diffusion pathways but also improves the electronic conductivity to optimize the electrochemical kinetics. The Yeast@NMTP/C cathode delivers reversible multielectron redox reactions including Ti^4+/3+, Mn^3+/2+, and Mn^4+/3+ and exhibits a high capacity of 108.5 mAh g^-1 with a 79.2% capacity retention after 1000 cycles at a 2C rate. The sodium storage mechanism of Yeast@NMTP/C reveals that the addition of Ti^4+/3+ redox plays a key role in improving the Na⁺ diffusion kinetics, and both solid-solution and two-phase reactions take place during the desodiation and sodiation process. Additionally, the high-rate and long-span cycle performance of Yeast@NMTP/C at 10C is ascribed to contribute to pseudocapacitance.

Long-lifespan Polyanionic Organic Cathodes for Highly Efficient Organic Sodium-ion Batteries

An organic Na-ion battery is reported with a polyanionic 9,10-anthraquinone-2,6-disulfonate (Na₂ AQ26DS, 130 mAh g^-1 ) cathode and the Na-intercalated state (Na₄ TP) of sodium terephthalate (Na₂ TP, 255 mAh g^-1 ) as the anode. The resulting full cells deliver the maximum discharge capacity of 131 mAh g^-1 _cathode in 0.5-3.2 V, simultaneously maintaining the average value of ≈62 mAh g^-1 _cathode during 1200 cycles (0.5 A g^-1 , ≈4 C). These results are among the best performing organic sodium-ion full cells reported to date.