O2 Phase Transition Research Articles

Slope-structure design towards high-stability P2-Na0.67MnO2 cathode

Layered Mn-based oxide P2-Na0.67MnO2 (NMO) arouses enormous interest for its high specific capacity, easy preparation and low cost. Meanwhile, the P2 → O2 phase transition derived from the slippage of transition metal (TM) layer leads to the inferior performance. Herein, we design a co-doping into Na+ (102 pm) lattice site with smaller-sized Li+ (76 pm) and larger-sized K+ (138 pm) to construct a slope-structured P2-Na0.63K0.02Li0.02MnO2 (NKLMO). The anchored Li+ and K+ can serve as pillar effects and form a geometrically stable resembling triangular structure in the Na-interlayer. Moreover, such a slope-structure is clearly verified by the HRTEM images. In-situ XRD and in-situ EIS tests efficaciously clarify the improved structural stability and even lower impedance during cycling within 2.0–4.2 V, respectively. DFT calculations further confirm the stronger metallicity of NKLMO electrode than the bare NMO originated from the more mobile free electrons surrounding the vicinity of Fermi level. By grace of the more stable slope-structure and faster reaction kinetics, NKLMO is capable of displaying high initial capacity (206.5 mA h/g) and energy density (540 W h kg−1), with a competitive cycling stability at both low and high current rates (96 %@100 cycles@0.1C; 88.4 %@500 cycles@5C). This slope-design is above rubies for tuning chemistry environment to exploit high-stability P2-phase cathodes for sodium-ion batteries.

Research on Sodium Storage Performance of Cu and Mg Doped P2 Type Layered Oxide Cathode Materials

The P2-type Ni–Mn-based oxide cathode materials have drawbacks such as the Jahn-Teller effect and crystal phase transition under high pressure. In this study, Cu and Mg modifications were introduced to inhibit the P2–O2 phase transition and increase the lattice spacing, thereby reducing the resistance of sodium ion de-embedding and intercalation to improve the overall electrochemical performance of the battery. A series of P2-type Na0.67Ni0.33−xMn0.67CuxO2 and Na0.67Ni0.33−xMn0.67MgxO2 cathode materials were synthesized through solid-state reaction. Cu substitution significantly alters the structural stability and electrochemical properties. In the voltage range from 1.5 V to 4.2 V and at 0.1 C, the initial discharge specific capacity of Na0.67Ni0.18Mn0.67Cu0.15O2 was 167 mAh·g−1, with 86 mAh·g−1 remaining after 100 cycles at 1 C. Additionally, a high discharge specific capacity of 207 mAh·g−1 was achieved with 0.075 Mg doping, suggesting that part of O2− participated in the internal electrochemical reaction. Furthermore, the rate performance of Na0.67Ni0.28Mn0.67Mg0.05O2 was found to be the best. This phenomenon was attributed to the inactive Mg2+ retaining more Na+ in the interlayer, inhibiting crystal structure transformation and Jahn-Teller distortion. The larger radius of Mg2+ increased the layer spacing, widened the Na+ de-embedding channels, and increased the diffusion coefficient.

Promoting Na-ion storage in P2-Na0.67Ni0.33Mn0.67O2 cathode via synergetic Ti and Zn Co-incorporation

P2-Na0.67Ni0.33Mn0.67O2 (NNMO) is regarded as a promising cathode candidate for sodium-ion batteries due to its high energy density. However, the electrochemical performance is hindered by Na+/vacancy order, irreversible P2–O2 phase transition at high voltage (>4.2 V), and harmful oxygen evolution. Herein, a synergetic Zn and Ti co-incorporation tactic is proposed for designing a Na0.67Ni0.29Zn0.04Mn0.63Ti0.04O2 (NNZMTO) cathode to overcome the above-mentioned challenges. First, the incorporated Ti heteroatom could break down Na+/vacancy order of NNMO by taking advantage of a similar ionic radius and substantially different Fermi levels with host Mn atom. Subsequently, the introduced Zn heteroatom could induce local Na–O–Zn configurations, buffer interlayer O2−–O2− electrostatic repulsion, as well as inhibit unfavorable phase transition. Moreover, the d10 band of Zn is lower than the oxygen states, and the Zn behaves like an s/p metal with oxygen, thus avoiding O2 release. Notably, in comparison with highly oxidized (Ni4+/Mn4+O6)δ− octahedron, the partial Na+ for charge neutrality in alkali metal layers could be well maintained in the as-designed (Zn2+/Ti4+O6)δ′−, which could be served as “pillars” to avoid layer gliding and structural collapse in the c-direction. As a result, an excellent electrochemical performance with high specific capacity of 90.9 mA h g−1 at 7 C could be retained for NNZMTO thanks to the synergetic effect from Ti and Zn incorporation. This study provides deep insights for designing superior layered cathode via conducting a rational cations co-incorporation strategy.

Competing Mechanisms Determine Oxygen Redox in Doped Ni-Mn Based Layered Oxides for Na-Ion Batteries.

Cation doping is an effective strategy for improving the cyclability of layered oxide cathode materials through suppression of phase transitions in the high voltage region. In this study, Mg and Sc are chosen as dopants in P2-Na0.67Ni0.33Mn0.67O2, and both have found topositively impact the cycling stability, but influence the high voltage regime in different ways. Through a combination of synchrotron-based methods and theoretical calculations it is shown that it is more than just suppression of the P2 to O2 phase transition that is critical for promoting the favorable properties, and that the interplay between Ni and O activity is also a critical aspect that dictates the performance. With Mg doping, the Ni activity can be enhanced while simultaneously suppressing the O activity. This is surprising because it is in contrast to what has been reported in other Mn-based layered oxides where Mg is known to trigger oxygen redox. This contradiction is addressed by proposing a competing mechanism between Ni and Mg that impacts differences in O activity in Na0.67MgxNi0.33- xMn0.67O2 (x<0<0.33). These findings provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxides.

Suppressing the P2–O2 phase transition of P2-type Ni/Mn-based layered oxide by synergistic effect of Zn/Ti co-doping for advanced sodium-ion batteries

P2-type Ni/Mn-based layered oxides are regarded to be promising cathode materials for advanced sodium-ion batteries (SIBs) owing to their rapid sodium ion diffusion kinetics, high working voltage, and high theoretical capacity. However, P2-type Ni/Mn-based layered oxides are prone to phase transition during charging and discharging processes, resulting in serious capacity fading. Here, introduction of Zn and Ti into transition-metal layers of P2-type layered Na0.66Ni0.33Mn0.67O2 can effectively inhibit P2–O2 phase transition at high voltage, leading to the improved cycling endurance. When being used as cathode materials for SIBs, Na0.66Ni0.27Zn0.06Mn0.61Ti0.06O2 (NNZMT) can deliver a discharge specific capacity of 106.6 mAh g−1, with 95.17% capacity retention after the 100th cycle at 100 mA g−1 within 2.1–4.3 V, which are much higher than those (58.4 mAh g−1 and 45.2%) of Zn/Ti-undoped Na0.66Ni0.33Mn0.67O2 (NNM). Besides, NNZMT displays a much better rate capability compared to the NNM sample. The full cell, based on P2-type layered NNZMT as cathode material and hard carbon as anode material, can provide an initial discharge specific capacity of 94.1 mAh g–1, with a capacity retention of 75.6% after 100 cycles at 100 mA g–1 within 1.0–4.2 V. This research work confirms that low-lost Zn/Ti co-doping strategy is an effective approach for designing and preparing cathode materials for advanced SIBs.

Inhibiting Oxygen Redox to Boost Ni Activity Enables Highly Stable Properties in Na0.67Ni0.33Mn0.67O2

Cation doping is a conventional and effective strategy for promoting favorable properties in layered oxides. Here, Mg and Sc were used as dopants in Na0.67Ni0.33Mn0.67O2 to significantly improve the cycling stability in the high voltage range. It was found that both dopants could successfully suppress the P2 to O2 phase transition, but induced a sloping curve as well as a short high voltage plateau above 4.0 V, respectively. We demonstrate that these different electrochemical behaviors are indicative of the suppression of oxygen redox by means of resonant inelastic X-ray scattering (RIXS) measurements. Furthermore, both X-ray absorption spectroscopy (XAS) and density functional theory show that Mg doping could enhance Ni activity while simultaneously suppressing O activity. However, these results are in contrast to other Mn-based layered oxides where Mg is known to effectively trigger the oxygen redox. We address this contradiction by proposing a competing mechanism between Ni and Mg that impacts differences in oxygen activity in Na0.67MgxNi0.33-xMn0.67O2 (x≤0≤0.33). These findings will provide a new direction in understanding the effects of cation doping on the electrochemical behavior of layered oxide materials. Figure 1

Mitigating the P2–O2 phase transition of Ni–Co–Mn based layered oxide for improved sodium-ion batteries via interlayered structural modulation

Mn-based layered oxides are promising cathode materials for sodium-ion batteries owing to their high reversible capacities. However, P2–O2 phase transition and Jahn–Teller distortion of Mn3+ frequently occur at the end of the discharging process, resulting in low cycling stability and severe capacity degradation during the cycling processes. In this study, we proposed an effective strategy to inhibit the harmful phase transition and suppress the Jahn–Teller distortion using Al substitution. The proposed method was validated via in situ and ex situ X-ray diffraction. Furthermore, the Al-substituted P2-Na0.7Ni0.2Co0.2Mn0.58Al0.02O2(NCMA2) electrode exhibited only a topotactic phase transition from P2 to OP4 and excellent cycling stability (80% retention of the initial capacity at 170 mAh/g after 50 cycles at 0.1 C within 1.50–4.50 V). X-ray photoelectron spectroscopy analysis showed that the Mn4+ content of the Al-substituted electrode was higher than the bare NCM (Na0.7Ni0.2Co0.2Mn0.6O2) electrode in both the charge and discharge states. This confirms that Al substitution can improve the electrochemical performance by mitigating the irreversible P2–O2 phase transition and suppressing Jahn–Teller distortion. This study provides a new perspective for effectively mitigating the P2–O2 phase transition in Mn-based layered oxide cathodes via interlayered structural modulation.

Quasi‐Zero Volume Strain Cathode Materials for Sodium Ion Battery through Synergetic Substitution Effect of Li and Mg

AbstractP2‐type layered oxide material Na2/3Ni1/3Mn2/3O2 is a competitive candidate for sodium‐ion batteries (SIBs). Nevertheless, it suffers from the strong P2–O2 phase transition during charging to the high voltage regime, rendering drastic volume variations and poor cycling performance. Here, a Quasi‐zero strain P2‐Na0.75Li0.15Mg0.05Ni0.1Mn0.7O2 cathode is synthesized, which reflects the vanishing P2–O2 transition with a volume change as low as 0.49%, thus resulting in the material an excellent cycling performance (83.9% capacity retention after 500 cycles at 5 C). The low‐volume strain can be attributed to two aspects: (1) the Mg2+ riveted in the Na layer can act as a “pillar” to stabilize the crystal structure under the condition of sodium removal, thus restricting the structural changes under high voltage. (2) The entry of Li+ into the transition metal (TM) layer can mitigate the electron localization in the highly desodiation state and can effectively immobilize the coordination oxygen atoms, thus suppressing the slip of P2–O2 transition. This study not only provides a new insight of Li and Mg synergetic substitution effect on the structural stability of P2‐type cathode, but also an efficient avenue for developing cathode materials of SIBs with ultralow bulk strain.

Dual Site-Selective Substitution Strategy Achieving Anionic Redox Activity and Solid-Solution Reaction in P2-Type Cathode Materials

P2-type Na0.67Ni0.33Mn0.67O2 has been considered as the potential cathode for sodium-ion batteries. However, its practical application is plagued by Na+/vacancy ordering, harmful phase transition, and lattice oxygen loss. Herein, we develop a dual site-selective substitution strategy to fabricate a P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode. The substitution of Li+ for Ni2+ introduces lone pair oxygen via forming a Li–O–Li configuration and make O 2p close to its Fermi level due to the weakened TM–O (TM: transition metal) bond, which triggers the anionic redox for charge compensation, while the introduction of Ca2+ in a Na layer enhances the electrostatic cohesion of neighboring TM layers by forming a strengthened O–Ca–O configuration, which suppresses the glide of adjacent TM layers and reduces the excessive lattice oxygen loss. Therefore, with a dual site-selective substitution strategy, the P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode can suppress the Na+/vacancy ordering, P2–O2 phase transition, and lattice oxygen loss even at a potential of 4.35 V, achieving a reversible anionic redox and solid-solution reaction. The P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode exhibits high discharge capacity (142.7 mA h g–1 at 20 mA g–1), excellent rate capability (57.1 mA h g–1 at 2 A g–1), and cyclic stability (a capacity retention of 83.2% after 700 cycles).

Structure, Electrochemical, and Transport Properties of Li- and F-Modified P2-Na2/3Ni1/3Mn2/3O2 Cathode Materials for Na-Ion Batteries

The development of cobalt-free P2-Na2/3Ni1/3Mn2/3O2 cathodes is hampered by poor electrochemical performance, resulting from structural instability during high-voltage cycling. Herein, Li+ and F− ions are introduced simultaneously via a simple sol–gel method. The F not only enters the lattice but forms chemically stable NaF on the surface. The modified electrode delivered significantly better electrochemical performance than the pristine one, including much-enhanced capacity retention (64% vs. 36%, 100 cycles) at 0.5 C and a four-time higher capacity output at 10 C. The ex situ XRD and in situ Raman analysis revealed cyclability enhancement mechanisms in terms of inhibiting the P2–O2 phase transition and Na+/vacancy ordering. The conductivity measurements (based on AC impedance and DC polarization) and GITT analysis proved, on both bulk material and electrode levels, that Na+ conduction and, thus, rate performance is notably promoted by doping. The individual contribution of Li and F to the overall performance improvement was also discussed. Furthermore, a solid-state sodium-metal battery was successfully demonstrated with the modified cathode. The above results verify that Li+/F− incorporation can enable enhancements in both cyclability and rate capability of the P2-Na2/3Ni1/3Mn2/3O2 cathodes and are expected to provide a new perspective for the rational design of high-performance layered oxide cathode materials for progressive sodium-ion batteries.

Dual‐strategy modification on P2‐Na0.67Ni0.33Mn0.67O2 realizes stable high‐voltage cathode and high energy density full cell for sodium‐ion batteries

AbstractP2‐type Na0.67Ni0.33Mn0.67O2 is considered as a potential cathode material for sodium‐ion batteries due to the merits of high voltage, low cost, and air stability. However, the unsatisfied cycling stability and rate performance caused by the destructive phase transition and side reactions hinder its practical application. Herein, we present a feasible dual strategy of Mg2+ doping integrated with ZrO2 surface modification for P2‐Na0.67Ni0.33Mn0.67O2, which can well address the issues of phase transition and side reactions benefitting from the enhanced structural and interfacial stabilities. Specifically, it exhibits a decent cycling stability with a capacity retention of 81.5% at 1 C and promising rate performance with a discharge capacity of 76.6 mA h g−1 at 5 C. The in situ X‐ray diffraction measurement confirms that the damaged P2–O2 phase transition is suppressed with better reversibility in high‐voltage region, whereas the side reactions are inhibited due to the protective ZrO2 surface modification. Commendably, the full cell achieves an outstanding operating voltage of 3.57 V and a fabulous energy density of 238.91 W h kg−1 at 36.73 W kg−1, demonstrating great practicability. This work is expected to provide a new insight for designing stable high‐voltage cathode materials and high energy density full cells for sodium ion batteries.

Nano-Crystallites of P2-Type Layered Transition Metal Oxide High Voltage Cathode for Sodium-Ion Battery

In this era of transitioning from conventional sources of energy to non-conventional, sodium-ion battery research has been burgeoning as an indigenous solution to energy storage applications, considering the sustainability, cost effectiveness, high availability and a familiar redox chemistry 1. P2-type Na2/3Ni1/3Mn2/3O2 is one of the preeminent cathode for sodium-ion batteries because of their environmental friendliness, open framework, superior specific capacity, higher operating voltage and air-moisture stability. However, rapid capacity decay on charging it to a higher voltage because of P2 to O2 phase transition and a large volume change leading to exfoliation of the layers have impeded the practicability of this as an electrode material for Na-ion battery 2,3,4.Here in this work, we report the preparation of hexagonal nanocrystals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, which is titanium doped in pristine Na2/3Ni1/3Mn2/3O2 via a novel and quick microwave synthesis technique. This provides sharp and clear facets that allows accelerated sodium-ion migration within the crystal during extraction and insertion of Na-ions, making this material a highly efficient cathode. Unlike conventional heating, which requires around 12-20 hours of synthesis time and high energy consumption, microwave radiation induces rapid solid-state reaction that heats the material on molecular level leading to uniform heating, thus retaining the nanocrystallinity of the structure 5. The distinctive hierarchical nanostructure having large surface area could efficiently facilitate the transportation of Na+ ions, fast utilization of active materials, overcome the effect of internal strain generated inside and reduces the pulverization of active materials, thereby restraining the P2 to O2 phase transition at higher voltage 6. Aiding from the combined effect of titanium doping at manganese site and designing hierarchical nanocrystrals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, we obtained a rate capability of 145 mAh g-1 at 0.1 C and a prolonged cycling life (87.3% capacity retention after 500 cycles at 1C) within a voltage range of 2.5 – 4.2 V, restraining the P2 to O2 type phase transition at higher potential. The combined analysis of X-ray diffraction, scanning electron microscopy and transmission electron microscopy along with density functional theory (DFT) calculations demonstrated the optimization of the structure and the physical properties of pristine Na2/3Ni1/3Mn2/3O2 and Ti doped structure along with their Bader charge analysis and electronic properties. Further the mechanical integrity of the nano Na2/3Ni1/3Mn1/2Ti1/6O2 and micro Na2/3Ni1/3Mn1/2Ti1/6O2 were analyzed through micro-compression test of the as prepared pellets. The underlying mechanism for the suppression of phase transition in Na2/3Ni1/3Mn1/2Ti1/6O2 was elucidated by ex-situ X-ray diffraction (XRD) and ex-situ Transmission electron microscopy (TEM). The electrochemical kinetics regarding Na+ diffusion coefficient was further studied through galvanostatic intermittent titration technique (GITT). An analytical model was established to probe deeper into the reason for exfoliation and thus, support our hypothesis. In addition, a sodium-ion full cell was constructed by pairing the as-prepared P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 with a hard carbon anode. This modification of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 nanocrystallites with comprehensive electrochemical performance can be a path breaking, highly efficient cathode material for large-scale energy storage applications.

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 Na0.6 Ni0.3 Mn0.7 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.

Improving Structural and Moisture Stability of P2-Layered Cathode Materials for Sodium-Ion Batteries

P2-type Ni/Mn-based layered oxides are promising cathode materials for sodium-ion batteries (SIBs). However, ground challenges, e.g., irreversible phase transition during cycling, moisture instability, and inferior electrochemical performance, greatly impede their practical applications. Herein, a series of Cu-substituted P2–Na0.6Ni0.3–xMn0.7CuxO2 (0 ≤ x ≤ 0.2) cathode materials for SIBs are fabricated and the mechanisms responsible for their improved electrochemical performances are comprehensively investigated. It is discovered that Cu dopants with strong electronegativity could stabilize the crystal structure by inhibiting the common P2–O2 phase transition, leading to improved cycling stability. The expanded interlayer spacing after Cu doping is facilitated for the charge transfer kinetics, which ensures excellent rate performance. In addition, all Ni, Mn, Cu, and O participate in the charge compensation upon sodiation and desodiation through reversible redox reactions. More importantly, Cu substitution improves the moisture stability of the cathode materials because the Cu2+/Cu3+ redox couple increases the initial charging potential. This work provides a promising guidance for the design of low-cost, high-performance, and air-stable cathode materials with both cationic and anionic redox activities for SIBs.

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.

A dual-modification strategy for P2-type layered oxide via bulk Mg/Ti co-substitution and MgO surface coating for sodium ion batteries

P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 → O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na+. Moreover, the as-prepared MgO-coated Na0.67Ni0.17Co0.17Mn0.66Mg0.1O2 exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g−1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g−1 within 2–4.5 V, which has been obviously enhanced than that of Na0.67Ni0.17Co0.17Mn0.66O2. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.

Boosting the Redox Kinetics of High‐Voltage P2‐Type Cathode by Radially Oriented {010} Exposed Nanoplates for High‐Power Sodium‐Ion Batteries

High‐voltage P2‐type cathode with large capacity, high air stability, and low cost has attracted extensive attention. However, large ionic radius of Na+ and Na+/vacancy ordering result in low reaction kinetics and poor high‐rate capability. Herein, cobalt‐doped hierarchical structure assembled by radially oriented {010} exposed nanoplates is developed. With radially oriented grains infiltrating from surface to interior, all the {010} exposed surfaces are electrochemically active planes, and Na+ ions diffuse directly into electrolyte without passing through grain boundaries, building up 3D transfer channels. The trivalent cobalt substitution suppresses unwanted Na+/vacancy ordering and increases energy barrier of the phase transition from P2‐ to O2‐type oxide. These synergetic effects remarkably boost the redox kinetics of high‐voltage P2‐type cathode. Consequently, a large reversible capacity of 112 mAh g−1 is achieved even at 20 C, indicating excellent high‐rate capability. The absence of P2–O2 phase transition also gives rise to superior cycling stability, maintaining a capacity retention of 85% after 200 cycles. In addition, full cells composed of this hierarchical P2‐type cathode and hard carbon anode deliver high energy‐ and power‐densities. These achievements offer a new insight into boosting redox kinetics of P2‐ and O3‐type layered cathodes for high‐power sodium‐ion batteries.

Reversible OP4 phase in P2–Na2/3Ni1/3Mn2/3O2 sodium ion cathode

P2–Na2/3Ni1/3Mn2/3O2 as a promising cathode material for sodium ion batteries (SIBs) has attracted much attention owing to high capacity and wide operation voltage. However, it suffers from inferior rate and cycling performances due to P2–O2 phase transition and Na+/vacancy ordering during charge/discharge process. Herein, a Fe/Ti co-substitution strategy is successfully utilized to mitigate these issues. Notably, as probed by In-situ X-ray diffraction, the substitution of Ni with Fe significantly stabilizes the interlayer framework by impeding slab glide during deep Na+ extraction, resulting in reversible P2-OP4 phase transition with smaller volume change of host structure compared to P2–O2 transition. Meanwhile, synchrotron X-ray diffraction shows that, the Na+/vacancy ordering is greatly suppressed by further replacing Mn with Ti, which breaks down the long-range ordering of Ni/Mn distribution, promoting Na ion transport. In addition, the charge compensation mechanism of Fe/Ti co-substituted P2–Na0.67Ni0.23Fe0.1Mn0.57Ti0.1O2 (NFMT) revealed by Ex-situ X-ray absorption spectroscopy demonstrates that the incorporation of Fe3+/4+ redox couple largely improve the working voltage. Consequently, the obtained NFMT cathode delivers a high energy density over 420 Wh/kg within 4.3–2.6 V, accompanied with remarkably enhanced kinetics and capacity retention. Such co-substitution strategy provides a promising approach to develop high-voltage and cycle-stable cathode materials for SIBs.

High-Operating Voltage, Long-Life Layered Oxides for Sodium Ion Batteries Enabled by Cosubstitution of Titanium and Magnesium

P2-type layered oxides are considered as promising cathode materials for rechargeable sodium ion batteries, but preparing P2-type cathodes with high-operating voltage and long-life is still a big challenge. Herein, spherical P2-type cathode Na0.67Ni0.17Co0.17Mn0.66Ti0.05Mg0.05O2 has been designed, and the critical roles of Ti and Mg on electrochemical performance of the cathodes are systematically investigated. The high-temperature XRD clearly exhibits the forming process of the pure phase material and suggests that the most suitable calcination temperature is 850 °C. The Ti/Mg cosubstitution does not break the long-range P2 structure and the spherical morphology of the material. In the electrochemical processes, the Na0.67Ni0.17Co0.17Mn0.56Ti0.05Mg0.05O2 electrode exhibits better electrochemical performance than that of the undoped Na0.67Ni0.17Co0.17Mn0.66O2. It delivers an initial reversible capacity of 151 mAh g–1 (2–4.5 V) with an average voltage of 3.8 V and exhibits a high capacity retention of 87.7% after 300 cycles at 100 mA g–1. The improved electrochemical performance benefits from the Ti/Mg cosubstitution; Ti improves the average voltage while Mg and Ti significantly mitigate the undesired P2 → O2 phase transition of the cathode, and these two elements jointly promote the development of the electrochemical performance. This strategy is also applicable to the optimization design of layered transition oxides and provides a new approach to prepare high-voltage, long-life cathodes for sodium ion batteries.

A high-stability biphasic layered cathode for sodium-ion batteries.

A novel biphasic layered Na0.62Ni0.33Mn0.62Sb0.05O2 cathode is introduced which can be easily synthesized via a solid-state reaction method. Compared with its monophasic counterpart Na0.67Ni0.33Mn0.67O2, biphasic Na0.62Ni0.33Mn0.62Sb0.05O2 displays a smoother (dis)charge profile, superior rate capability along with splendid cycling performance. Structural and kinetic studies respectively demonstrate extremely small volume strain and rapid charge transfer during the electrochemical process.