Na+-Conductive Na2Ti3O7-Modified P2-type Na2/3Ni1/3Mn2/3O2 via a Smart in Situ Coating Approach: Suppressing Na+/Vacancy Ordering and P2-O2 Phase Transition

Medium-entropy P2-type layered metal oxide cathode demonstrating complete solid-solution behavior for improved Na-ion battery performance

P2-type layered metal oxides are promising cathode materials for Na-ion batteries (NIBs), but they suffer from Na+/vacancy ordering and P2-O2 phase transitions during charge-discharge processes, resulting in rapid capacity decay and poor rate performance. In this study, a non-equimolar five-component medium-entropy strategy was employed to design a Na0.67Ni0.19Cu0.07Zn0.07Mn0.60Ti0.07O2 (NNCZMT) material. Unlike equimolar high-entropy designs, Ni and Mn remain as the main components, with Ni providing high capacity through the Ni2+/Ni4+ redox couple and Mn stabilizing the P2 structure. The introduction of Cu2+, Zn2+, and Ti4+, which have slightly larger ionic radii and the same valence states as Ni2+ and Mn4+, respectively, allows for mutual substitution, synergistically promoting a solid-solution reaction and excellent Na+ diffusion kinetics. The NNCZMT electrode delivers a high reversible capacity of 110 mAh g-1 at 10 mA g-1, an initial Coulombic efficiency of 96.8 %, remarkable cycling stability (94.7 % capacity retention after 100 cycles at 200 mA g-1), and excellent rate performance (57.2mAh g-1 at 2000 mA g-1).

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.

Layered Cathode with Ultralow Strain Empowers Rapid-Charging and Slow-Discharging Capability in Sodium Ion Battery

The development of the electric vehicle industry has spurred demand for secondary batteries capable of rapid-charging and slow-discharging. Among them, sodium-ion batteries (SIBs) with layered oxide as the cathode exhibit competitive advantages due to their comprehensive electrochemical performance. However, to meet the requirements of rapid-charging and slow-discharging scenarios, it is necessary to further enhance the rate performance of the cathode material to achieve symmetrical capacity at different rates. Simultaneously, minimizing lattice strain during asymmetric electrochemical processes is also significant in alleviating strain accumulation. In this study, the ordered distribution of transition metal layers and the diffusion pathway of sodium ions are optimized through targeted K-doping of sodium layers, leading to a reduction of the diffusion barrier and endowment of prominent rate performance. At a 20C rate, the capacity of the cathode can reach 94% of that at a 0.1C rate. Additionally, the rivet effect of the sodium layers resulted in a global volume strain of only 0.03% for the modified cathode during charging at a 10C rate and discharging at a 1C rate. In summary, high-performance SIBs, with promising prospects for rapid-charging and slow-discharging capability, are obtained through the regulation of sodium layers, opening up new avenues for commercial applications.

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.

Structural and electrochemical progress of O3-type layered oxide cathodes for Na-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention being the most promising sustainable energy technology owing to their competitive energy density, great safety and considerable low-cost merits. Nevertheless, the commercialization process of SIBs is still sluggish because of the difficulty in developing high-performance battery materials, especially the cathode materials. The discovery of layered transition metal oxides as the cathode materials of SIBs brings infinite possibilities for practical battery production. Thereinto, the O3-type layered transition metal oxides exhibit attractive advantages in terms of energy density benefiting from their higher sodium content compared to other kinds of layered transition metal oxides. Enormous research studies have largely put forward their progress and explored a wide range of performance improvement approaches from the morphology, coating, doping, phase structure and redox aspects. However, the progress is scattered and has not logically evolved, which is not beneficial for the further development of more advanced cathode materials. Therefore, our work aims to comprehensively review, classify and highlight the most recent advances in O3-type layered transition metal oxides for SIBs, so as to scientifically cognize their progress and remaining challenges and provide reasonable improvement ideas and routes for next-generation high-performance cathode materials.

Ion Substitution Strategy of Manganese-Based Layered Oxide Cathodes for Advanced and Low-Cost Sodium Ion Batteries

Sodium ion batteries (SIBs) have recently been promising in the large-scale electric energy storage system, due to the low cost, abundant sodium resources. Mn-based layered oxide cathode materials have been widely investigated, because of the high theoretical specific capacity, low cost, and abundant reserves. However, their development is limited by the problems of Jahn-Teller distortion, Na⁺ /vacancy ordering, complex phase transitions, and irreversible anionic redox during cycling. Ion substitution strategy is one simple and effective way to regulate the crystal structure and boost sodium-storage performances of Mn-based cathode materials. In this review, we summarize the progress and mechanism of ion-substituted Mn-based oxides, establish a composition-crystal structure-electrochemical performance relationship, and also offer perspectives for guiding the design of high-performance Mn-based oxides for SIBs.

Heteroatom-Substituted P2-Na2/3Ni1/4Mg1/12Mn2/3O2 Cathode with {010} Exposing Facets Boost Anionic Activity and High-Rate Performance for Na-Ion Batteries

As an attractive cathode candidate for sodium-ion batteries, P2-type Na_2/3Ni_1/3Mn_2/3O₂ is famous for its high stability in humid air, attractive capacity, and high operating voltage. However, the low Na⁺ transport kinetics, oxygen-redox reactions, and irreversible structural evolution at high-voltage areas hinder its practical application. Herein, a comprehensive study of a microbar P2-type Ni_2/3Ni_1/4Mg_1/12Mn_2/3O₂ material with {010} facets is presented, which exhibits high reversibility of structural evolution and anionic redox activity, leading to outstanding rate capability and cyclability. The notable rate performance (53 mA h g^-1 at 20 C, 2.0-4.3 V) contributed to the high exposure of {010} facets via controlling the growth orientation of the precursor, which is certified by density functional theory calculation and lattice structural analysis. Mg substitution strengthens the reversibility of anionic oxygen redox and structural evolution in high-voltage areas that was confirmed by the in situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy tests, leading to outstanding cyclic reversibility (68.9% after 1000 cycles at 5 C) and slowing down the voltage fading. This work provides new insights into constructing electrochemically active planes combined with heteroatom substitution to improve the Na⁺ transport kinetics and structural stability of layered oxide cathodes for sodium storage.

Unusual Site-Selective Doping in Layered Cathode Strengthens Electrostatic Cohesion of Alkali-Metal Layer for Practicable Sodium-Ion Full Cell

P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ is a dominant cathode material for sodium-ion batteries due to its high theoretical capacity and energy density. However, charging P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ to voltages higher than 4.2 V (vs. Na⁺ /Na) can induce detrimental structural transformation and severe capacity fading. Herein, stable cycling and moisture resistancy of Na_0.67 Ni_0.33 Mn_0.67 O₂ at 4.35 V (vs. Na⁺ /Na) are achieved through dual-site doping with Cu ion at transition metal site (2a) and unusual Zn ion at Na site (2d) for the first time. The Cu ion doping in 2a site stabilizes the metal layer, while more importantly, the unusual alkali-metal site doping by Zn ion serves as O^2- Zn²⁺ O^2- "pillar" for enhancing electrostatic cohesion between two adjacent transition metal layers, preventing the crack of active material along the a-b-plane and restraining the generation of O2 phase upon deep desodiation. This unique dual-site-doped [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ cathode exhibits a prominent cyclability with 80.6% capacity retention over 2000 cycles at an ultrahigh rate of 10C, demonstrating its great potential for practical applications. Impressively, the full cell devices with [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ and commercial hard carbon as cathode and anode, respectively, can deliver a high energy density of 217.9 Wh kg^-1 and excellent cycle life over 1000 cycles.

Suppressing the P2 - O2 phase transformation and Na+/vacancy ordering of high-voltage manganese-based P2-type cathode by cationic codoping

High-voltage and low-cost manganese-based P2-type oxides show real promise as promising cathode for sodium-ion batteries (SIBs). But the P2 - O2 phase transformation and Na⁺/vacancy ordering results in the inferior structural stability and Na⁺ diffusion coefficient, which further leads to rapid decay of capacity and poor rate capability. Herein, in consideration of the synergetic effects of dual cationic doping, electrochemically inactive Li⁺ and active Co³⁺ codoping are proposed to solve the above issues. The novel two-step doping strategy, Co doping during synthesis of precursors via coprecipitation reaction followed by Li doping during solid-state reaction, are rationally developed. As anticipated, the Li/Co codoped P2-type oxide exhibits the absence of P2 - O2 phase transformation and Na⁺/vacancy disordering, which gives rise to an outstanding cycling stability (86.7% capacity retention within 100 cycles at 0.1C) and high-rate capability (reversible capacity of 109 mAh g^-1 even at 10C). In addition, the full-cells composed of the codoped P2-type positive and hard carbon negative show high energy-density, good lifespan and high-rate property. This proposed cationic codoping provides an effective and scalable tactics for modulating the structural properties of high-voltage P2-type cathodes for advanced SIBs.

Stabilizing P2-Type Ni-Mn Oxides as High-Voltage Cathodes by a Doping-Integrated Coating Strategy Based on Zinc for Sodium-Ion Batteries

The key to development of high-voltage P2-type Na_0.66Ni_0.33Mn_0.67O₂ is the modification methods that can effectively improve its electrochemical reversibility. Herein, a doping-integrated coating strategy based on zinc element is proposed to modify P2-type Na_0.66Ni_0.33Mn_0.67O₂, which can be achieved by a facile one-step solid-state reaction. The formation mechanism of Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂@0.06ZnO (NNZM@0.06ZnO) is investigated, revealing that the spinel and P3 intermediate phases appear prior to the formation of the P2 phase. Ni²⁺ can be preferentially incorporated into the P2 structure in competition with Zn²⁺ at high temperature, resulting in a uniform enrichment of ZnO on the surface. A small amount of Zn²⁺ doping significantly suppresses the Na⁺/vacancy ordering effect and improves the structural reversibility. Furthermore, the electrolyte decomposition is effectively reduced because of the presence of the ZnO coating layer, leading to the formation of a thin cathode electrolyte interphase film that is favorable to fast Na⁺ diffusion. In virtue of the Zn²⁺ doping and in situ formed ZnO coating, NNZM@0.06ZnO exhibits excellent cycling stability with a capacity retention of 83.7% after 100 cycles at 100 mA g^-1 and rate performance with a discharge capacity of 56.4 mAh g^-1 at 2000 mA g^-1, which significantly outperforms the uncoated Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ and the Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂/0.06ZnO with the coating layer introduced by mechanical milling. This work provides a new strategy to design high-performance cathode materials for sodium-ion batteries.

Improved Cycling Performance of P2-Na0.67Ni0.33Mn0.67O2 Based on Sn Substitution Combined with Polypyrrole Coating

P2-Na_0.67Ni_0.33Mn_0.67O₂ presents high working voltage with a theoretical capacity of 173 mAh g^-1. However, the lattice oxygen on the particle surface participates in the redox reactions when the material is charged over 4.22 V. The resulting oxidized oxygen aggravates the electrolyte decomposition and transition metal dissolution, which cause severe capacity decay. The commonly reported cation substitution methods enhance the cycle stability by suppressing the high voltage plateau but lead to lower average working voltage and reduced capacity. Herein, we stabilized the lattice oxygen by a small amount of Sn substitution based on the strong Sn-O bond without sacrificing the high voltage performance and further protected the particle surface by polypyrrole (PPy) coating. The obtained Na_0.67Ni_0.33Mn_0.63Sn_0.04O₂@PPy (3.3 wt %) composite showed excellent cycling stability with a reversible capacity of 137.6 (10) and 120.0 mAh g^-1 (100 mA g^-1) with a capacity retention of 95% (10 mA g^-1, 50 cycles) and 82.5% (100 mA g^-1, 100 cycles), respectively. The present work indicates that slight Sn substitution combined with PPy coating could be an effective approach to achieving superior cycling stability for high-voltage layered transition metal oxides.