P2-type Layered Oxide Research Articles

Modulation of Local Charge Distribution Stabilized the Anionic Redox Process in Mn-Based P2-Type Layered Oxides.

An anionic redox reaction is an extraordinary method for obtaining high-energy-density cathode materials for sodium-ion batteries (SIBs). The commonly used inactive-element-doped strategies can effectively trigger the O redox activity in several layered cathode materials. However, the anionic redox reaction process is usually accompanied by unfavorable structural changes, large voltage hysteresis, and irreversible O2 loss, which hinders its practical application to a large extent. In the present work, we take the doping of Li elements into Mn-based oxide as an example and reveal the local charge trap around the Li dopant will severely impede O charge transfer upon cycling. To overcome this obstacle, additional Zn2+ codoping is introduced into the system. Theoretical and experimental studies show that Zn2+ doping can effectively release the charge around Li+ and homogeneously distribute it on Mn and O atoms, thus reducing the overoxidation of O and improving the stability of the structure. Furthermore, this change in the microstructure makes the phase transition more reversible. This study aimed to provide a theoretical framework for further improve the electrochemical performance of similar anionic redox systems and provide insights into the activation mechanism of the anionic redox reaction.

Realizing High-Performance Cathodes with Cationic and Anionic Redox Reactions in High-Sodium-Content P2-Type Oxides for Sodium-Ion Batteries.

P2-type layered transition-metal oxides with anionic redox reactions are promising cathodes for sodium-ion batteries. In this work, a high-sodium-content P2-type Na7/9Li1/9Mg1/9Cu1/9Mn2/3O2 (NLMC) cathode material is prepared by substituting Li/Mg/Cu for Mn sites in Na2/3MnO2. The Li/Mg ions trigger the anionic redox reaction, while the Cu ions enhance the structure stability during electrochemical cycling. As a result, the oxide has a high reversible capacity of 225 mAh g-1 originating from both cationic and anionic redox activities with a capacity retention of 77% after 100 cycles. The migration energy barrier and Na ion diffusion kinetics are studied using density functional theory (DFT) calculations and the galvanostatic intermittent titration technique. Furthermore, X-ray diffraction, DFT, scanning electron microscopy, and transmission electron microscopy are applied to reveal the structural evolution and charge compensation of NLMC, providing a thorough understanding of the structural and morphology evolution of Na-deficient oxides during cycling. The results are inspiring for the design of a high-Na content P2-type layered oxide cathode for sodium-ion batteries.

Highly stabilized single-crystal P2-type layered oxides obtained via rational crystal orientation modulation for sodium-ion batteries

The development of P2-type Na–Ni–Mn oxides as high-voltage cathode materials for sodium-ion batteries is being extensively researched. However, achieving good electrochemical reversibility for such oxides at high voltage remains challenging. Herein, highly dispersed hexagonal-prism-like single-crystal P2-type Na0.66Ni0.26Zn0.07Mn0.67O2 (MC-NNZM) with a high proportion of {001} planes is synthesized through a combined coprecipitation and molten-salt method. The presence of molten Na2SO4 and the low surface energy of {001} planes are critical in forming the anisotropic single-crystal structure. In the 2.0–4.4 V voltage window, MC-NNZM exhibits a reversible capacity of 122.1 mAhg−1 with a median discharge voltage of 3.5 V at 10 mA g−1. Moreover, the capacity retentions of MC-NNZM reach 95.8% and 98.3% in the 2.0–4.4 V and 2.0–4.3 V voltage ranges after 100 cycles at 100 mA g−1, respectively. Electrochemical in situ X-ray diffraction patterns reveal that, because of the {001}-dominated single-crystal structure, the gliding of transition metal oxide slabs in the high-voltage region is effectively restrained; subsequently, this significantly mitigates the volume change of MC-NNZM during cycling. Consequently, unlike layered oxides with random crystal orientation, MC-NNZM is effectively resistant to mechanical fracture without the irreversible formation of intragranular cracks and dislocations after extensive cycling, thus avoiding continuous electrolyte decomposition on the material surface and stabilizing efficient ionic/electronic transport paths in the cathode. This work paves a new avenue for improving the cycling stability of P2-type layered oxides with a high upper-cutoff voltage based on a single-crystal structure formed via rational crystal orientation modulation.

Functional surface modification of P2-type layered Mn-based oxide cathode by thin layer of NASICON for sodium-ion batteries

P2-type layered manganese-based oxides, as promising high-energy-density and low-cost cathodes, have been triggered considerable attentions in sodium ion batteries (SIBs). Nevertheless, P2-type manganese-based oxides still suffer from the sluggish kinetics and unfavorable Jahn-Teller effect. Herein, motivated by the high ionic conductivity and rigid three-dimension framework of NASICON-type materials, the well-known Na3Zr2Si2PO12 thin layer was coated on P2-type Na0.612K0.056MnO2 surface to modify electrochemical sodium storage and structural stability. The optimal Na0.612K0.056MnO2@Na3Zr2Si2PO12 exhibited a high discharge capacity of 203 mA h g−1 within 1.8 − 4.3 V at a low current density of 10 mA g−1. When the current density enhances into 500 mA g−1, it delivers a high initial discharge capacity of 145.7 mA h g−1 within 1.8 − 4.3 V at 500 mA g−1 and maintains 69% capacity retention after 100 cycles. Besides, at low temperature of −20 °C, it offers 100.1 mA h g−1 after 100 cycles within 1.8 − 4.3 V at 100 mA g−1, which is much higher than that of NKMO (65.2 mA h g−1). The Na3Zr2Si2PO12 can decrease Mn3+ content and avoid the Jahn-Teller effect of Na0.612K0.056MnO2 in structural level, enabling the enhanced Na+ diffusion coefficient of Na0.612K0.056MnO2. Such strategy NASICON coating on cathode would pave a new pathway on rational design of high-performance cathode for SIBs.

(Invited, Digital Presentation) Design of New Cathodes for Sodium-Ion Batteries Harnessing Anionic Redox Activity: Two Case Studies

Sodium-ion batteries (SIBs) are widely pursued as pragmatic alternative to the Li-ion battery technology having close operational similarity along with natural resource abundance. In this sector, apart from classical cationic redox-based cathodes, various new cathodes have been unveiled based on anionic (oxygen) redox activity. In this work, we will showcase two such case studies of novel cathode composition for SIBs exploiting anionic redox activity.(1) P2-type manganese-rich layered oxides offers high capacity along with fast rate kinetics. To improve their electrochemical properties, reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations can be explored. We have tested a unique Mn-rich layer-spinel composite, Na0.7(Li1/18Mn11/18Ni3/18Fe2/181 /18)O2–xNa2MoO4, leading to a synergistic effect of layered P2 and spinel phases. This layer/spinel biphasic composite was stabilised through Mo doping and its electrochemical activity was studied at different voltage windows. Upon cycling between 1.5–4.5 V, this Mo-based composite exhibited a high specific capacity of 183 mAh.g-1 involving both cationic and anionic (O2-/O2 n-) redox activity. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2´´ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis. The structural and electrochemical properties of this new composite cathode will be described (Fig. a) (manuscript submitted).(2) The second part will dwell on anionic redox activity of Na-based mixed-metal oxide cathode [Na0.7Mn0.6Ni0.3Co0.1O2]. Combining cationic and anionic redox activity, it exhibited a reversible capacity of 120 mAh/g with anionic redox activity centered around 4.3 V (Fig. b). The underlying redox mechanism has been investigated using in-situ XRD and Raman spectroscopy showing peroxo bond formation (Fig. c). The structural and electrochemical properties of this new cathode composition will be explained (manuscript submitted). Figure 1

Cu-substitution P2-Na0.66Mn1-xCuxO2 sodium-ion cathode with enhanced interlayer stability

P2-type Mn-based layered oxides are viewed as promising cathode materials for sodium ion battery by virtue of their high theoretical capacity. Considering that pure Na2/3MnO2 suffers from poor cycling performances, Cu-substitution strategy is proposed to effectively alleviate this issue. However, the structural evolution mechanism of the Cu-containing samples still remains unclear. Herein, we propose that Cu-Substitution P2-type Na0.66Mn1-xCuxO2 with enlarged lattice parameters are conducive to improving the interlayer structure stability through mitigating TMO2 slabs distortion. Proved by synchrotron XAS spectra and ex/in situ XRD, the expansion/contraction of MnO6 octahedron is dramatically reduced with the increased Cu content, showing the facilitated Na ion diffusion. Furthermore, when the ratio of Cu to Mn reaches 1:4, the phase transition from P2 to P'2 type at the end of discharge can be suppressed, resulting in the improved interlayer skeleton stability. The Cu-containing samples with stable interlayer structure exhibit high capacity retention and outstanding rate performances (a capacity of 79.9 mAh g−1 at 5 C). This Cu-substitution strategy provides a promising approach to designing highly stable cathodes.

P2-type layered high-entropy oxides as sodium-ion cathode materials

P2-type layered oxides with the general Na-deficient composition Na x TMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2 and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.

A molybdenum doped layer-spinel composite cathode material for sodium-ion battery

Sodium-ion batteries are pursued as pragmatic alternative to the Li-ion battery technology having operational similarity along with natural resource abundance. P2-type manganese-rich layered oxides are widely explored exhibiting high capacity along with fast rate kinetics. To improve their electrochemical performance and reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations have been proposed. Here, we report a unique Mn-rich layer-spinel composite, Na0.7(Li1/18Mn11/18Ni3/18Fe2/18χ1/18)O2–xNa2MoO4, leading to a synergistic effect of layered P2 and spinel phases. This stable layer/spinel biphasic composite was stabilized through Mo doping and its electrochemical activity was studied at different voltage windows. When cycled between 1.5–4.5 V, this composite delivered a high specific capacity of 183 mAh.g–1 involving both cationic and anionic (O2–/O2n–) redox. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2′′ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis.

Copper Substitution in P2-Type Sodium Layered Oxide To Mitigate Phase Transition and Enhance Cyclability of Sodium-Ion Batteries.

Development of high-performance cathode materials is one of the key challenges in the practical application of sodium-ion batteries. Among all the cathode materials, layered sodium transition-metal oxides are particularly attractive. However, undesired phase transitions are often reported and have detrimental effects on the structure stability and electrochemical performance. Cu substitution of zinc in the P2-type Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 (x = 0, 0.075, and 0.15) composites was investigated in this study for mitigating the biphase transition and enhancing the electrochemical performance of sodium-ion batteries. The coupling effect of Zn and Cu enables an excellent capacity retention of 96.4% of the initial discharge capacity after 150 cycles at 0.1 C in the Na/Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 cell. The biphase transition that occurred in the high voltage range has been significantly suppressed after the incorporation of Cu in Na0.6Mn0.7Ni0.15Zn0.15O2, which was confirmed by in situ X-ray diffraction studies. Moreover, the substitution of the inert element Zn with electrochemically active Cu leads to the suppression of anionic redox and the occurrence of Cu2+/3+ redox reaction, and the electrolyte decomposition is impeded after the introduction of electrochemically active Cu in the Na0.6Mn0.7Ni0.15Zn0.15-xCuxO2 composite cathode. The enhanced electrochemical performance in the Na0.6Mn0.7Ni0.15Zn0.075Cu0.075O2 electrode can be ascribed to the coexistence of Zn and Cu and alleviated volumetric change as well as suppressed electrode/electrolyte side reaction after Cu substitution.

Tailoring Anionic Redox Activity in a P2-Type Sodium Layered Oxide Cathode via Cu Substitution.

Na-ion cathode materials cycling at high voltages with long cycling life and high capacity are of imminent need for developing future high-energy Na-ion batteries. However, the irreversible anionic redox activity of Na-ion layered cathode materials results in structural distortion and poor capacity retention upon cycling. Herein, we develop a facile doping strategy by incorporating copper into the layered cathode material lattice to relieve the irreversible oxygen oxidation at high voltages. On the basis of a comprehensive comparison with the Cu-free material, both the over-oxidation of O2- to trapped molecular O2 and Mn-related Jahn-Teller distortion have been effectively inhibited by restraining both the oxygen activity and participation of Mn4+/Mn3+ redox activity. Not limited to discovering stable cycling behavior at high voltages after Cu substitution, our findings also highlight an effective strategy to stabilize the anionic redox activity and elucidate the stabilization mechanism of Cu substitution, thus paving the way for further improvement of layered oxide cathode materials for high-energy Na-ion batteries.

Enhanced rate performance and specific capacity in Ti-substituted P2-type layered oxide enabled by crystal structure and particle morphology modifications

In recent years, P2-type layered oxides have received considerable attention as potential cathodes for Na-ion batteries owing to their compositional diversity, good specific capacity, and cyclability. However, poor rate performance and low-capacity retention at high discharge rates have limited their use in commercial battery applications. This study aims to mitigate this issue by synthesizing a series of structurally engineered P2-type cathode materials, through Ti substitution, with high cyclability and improved rate performance. P2-type Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 were prepared through a sol–gel route and were characterized for their structural, electrical, and electrochemical properties In the Ti-substituted samples, the Rietveld refinement of XRD data revealed an increased size of the bottleneck area of the Na-O6 prism planes, while the SEM images showed a decrease in the aspect ratio of hexagon-type morphology of particles which facilitates faster Na-ion conduction through the material. These changes in the crystal structure and particle morphology induced by Ti substitution have significantly improved electrical and electrochemical performance compared to the parent material. The sample with x = 7.5% exhibited a specific capacity of 126 mAh/g at a discharge rate of 0.1C in the 2.00–4.25 V window, which was about 25% more than that of the undoped material. At a discharge rate of 1C, the specific capacity of Na0.70Ni0.20Cu0.15Mn0.575Ti0.075O2 was 97 mAh/g compared to 74 mAh/g for Na0.70Ni0.20Cu0.15Mn0.65O2 sample. Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 samples also exhibited excellent cyclability, with over 95% of original capacity retained after 300 cycles. Complex impedance measurements corroborated the improved Na-ion conductivity in Ti-doped samples and an associated increase in Na-ion transference number from 0.86 for the sample with x = 0 to 0.97 for the sample with x = 0.075.

Niobium-doped layered cathode material for high-power and low-temperature sodium-ion batteries

The application of sodium-based batteries in grid-scale energy storage requires electrode materials that facilitate fast and stable charge storage at various temperatures. However, this goal is not entirely achievable in the case of P2-type layered transition-metal oxides because of the sluggish kinetics and unfavorable electrode|electrolyte interphase formation. To circumvent these issues, we propose a P2-type Na0.78Ni0.31Mn0.67Nb0.02O2 (P2-NaMNNb) cathode active material where the niobium doping enables reduction in the electronic band gap and ionic diffusion energy barrier while favoring the Na-ion mobility. Via physicochemical characterizations and theoretical calculations, we demonstrate that the niobium induces atomic scale surface reorganization, hindering metal dissolution from the cathode into the electrolyte. We also report the testing of the cathode material in coin cell configuration using Na metal or hard carbon as anode active materials and ether-based electrolyte solutions. Interestingly, the Na||P2-NaMNNb cell can be cycled up to 9.2 A g−1 (50 C), showing a discharge capacity of approximately 65 mAh g−1 at 25 °C. Furthermore, the Na||P2-NaMNNb cell can also be charged/discharged for 1800 cycles at 368 mA g−1 and −40 °C, demonstrating a capacity retention of approximately 76% and a final discharge capacity of approximately 70 mAh g−1.

Achieving reversible Mn2+/Mn4+ double redox couple through anionic substitution in a P2-type layered oxide cathode

P2-type Mn-based layered oxides have emerged as promising cathode materials for sodium-ion batteries (SIBs) due to their high theoretical capacity. However, the incorporation of Mn3+ cations induce severe Jahn-Teller (J-T) effect, which seriously distorts the localized lattice structure and results in insufficient battery performance. Herein, we successfully introduce reversible Mn2+/Mn4+ double redox couple into P2-type Na0.6Mg0.3Mn0.7O2 cathode with suppressed J-T effect through F anion doping strategy. The comprehensive results clearly suggest that F-substitution with an appropriate amount enlarges the interlayer spacing and reduces the band gap, resulting in accelerated charge transfer kinetics. In addition, the strong interaction between Mn and F suppresses the Mn2+ dissolution at discharged states and promotes the reversible conversion from Mn2+ to Mn4+ at charged states, leading to ameliorated J-T effect and enhanced electrochemical performance. Moreover, the oxygen redox reaction is also enhanced through F anion doping strategy. Our present study provides an effective avenue to address the J-T effect by introducing reversible Mn2+/Mn4+ double redox couple in Mn-based oxides, which is expected to revive this cathode family toward practical applications of SIBs.

Effect of lithium doping in P2-Type layered oxide cathodes on the electrochemical performances of Sodium-Ion batteries

Anionic redox reaction offers an effective way to design high-performance cathode materials for sodium-ion batteries. Here to trigger the lattice oxygen redox reaction, the effect of Li doping on the electrochemical performances of P2-type layered oxide cathodes are systematically investigated. Results show that the introduction of Li can not only induce oxygen anions to participate in the charge compensation under high operating voltage, but also suppress the unfavorable P2-O2 transition and facilitate the diffusion of Na+ and electrons during the sodiation/desodiation process. Therefore, Li-doped layered oxide cathodes show improved electrochemical performances with higher reversible capacity, better rate capability, and longer cycling stability. This study may provide a reference point for designing advanced cathode materials through activating the anionic redox reaction.

Designing a P2-type cathode material with Li in both Na and transition metal layers for Na-ion batteries

P2-type layered oxides have been considered as promising cathode materials for Na-ion batteries, but the capacity decay resulting from the Na+/vacancy ordering and phase transformation limits their future large-scale applications. Herein, the impact of Li-doping in different layers on the structure and electrochemical performance of P2-type Na0.7Ni0.35Mn0.65O2 is investigated. It can be found that Li ions successfully enter both the Na and transition metal layers. The strategy of Li-doping can improve the cycling stability and rate capability of P2-type layered oxides, which promotes the development of high-performance Na-ion batteries.

Structural and transport properties of P2-Type Na0.70Ni0.20Cu0.15Mn0.65O2 layered oxide

P2-type Layered oxides have attracted increasing attention recently as the cathode materials for Na-ion batteries with promising cyclability and good specific capacity. In this work, structural, electrical, and electrochemical properties of P2-type Na0.70Ni0.20Cu0.15Mn0.65O2 (NNCM) ceramic fabricated via a sol-gel method were investigated. The Rietveld refinement of the room temperature XRD diffraction data confirmed the formation of a single P2-type phase with space group P63/mmc for the powder calcined at 850 °C. Complex impedance spectroscopy was used to deconvolute the contributions of grains and grain boundaries to the overall conduction inside the sample. The room temperature conductivity of the grains and grain boundaries calculated for the NNCM ceramic sintered at 950 °C were estimated to be (5.25 ± 0.03) × 10−5 Scm−1 and (4.70 ± 0.05) × 10−6 Scm−1, respectively. The respective activation energies for the grain and grain boundary conduction were 0.189 ± 0.008 eV and 0.22 ± 0.01 eV, respectively. Moreover, NNCM exhibited a sodium-ion transference number of ≈0.86, suggesting that the conduction in this material is dominated by the Na-ions. The conduction mechanisms and related relaxations were also investigated using the dielectric and ac conductivity formalisms. NNCM showed specific capacities of 99 mAh/g and 74 mAh/g at 0.1C and 1C discharge rates, respectively, between 2 V and 4.25 V (vs. Na/Na+) with 95% capacity retained after 300 cycles at 1C.

Achieving High Stability and Performance in P2-Type Mn-Based Layered Oxides with Tetravalent Cations for Sodium-Ion Batteries.

P2-type sodium-manganese-based layered cathodes, owing to their high capacity from both cationic and anionic redox, are a potential candidate for Na-ion batteries (NIBs) to replace Li-ion technology in certain applications. Still, the structure instability originating from irreversible oxygen redox at high voltage remains a challenge. Here, a high sustainability cobalt-free P2-Na0.72 Mn0.75 Li0.24 X0.01 O2 (X = Ti/Si) cathode is developed. The outstanding capacity retention and voltage retention after 150 cycles are obtained in half-cells. The finding shows that Ti localizes on the surface while Si diffuses to the bulk of the particles. Thus, Ti can act as a protective layer that alleviates side reactions in carbonate-based electrolyte. Meanwhile, Si can regulate the local electronic structure and suppress oxygen redox activities. Notably, full-cells with hard carbon (≈300-335Whkg-1 based on the cathode mass) deliver the capacity retention of 83% for P2-Na0.72 Mn0.75 Li0.24 Si0.01 O2 and 66% for P2-Na0.72 Mn0.75 Li0.24 Ti0.01 O2 after 500 cycles; this electrochemical stability is the best compared to other reported cathodes based on oxygen redox at present. The superior cycle performance also stems from the ability to inhibit microcracking and planar gliding within the particles. Altogether, this finding offers a new composition for developing high-performance low-cost cathodes for NIBs and highlights the unique role of Ti/Si ions.

New insights to build Na+/vacancy disordering for high-performance P2-type layered oxide cathodes

P2-type Na-based layered oxides are potential cathode materials for high power Na-ion batteries (NIBs). However, complex Na+/vacancy ordering rearrangement evidenced by obvious voltage plateaus in the electrochemical profiles severely affects the Na storage capacities and cycling stability, hindering the commercialization of P2 materials. Herein, we show that the unfavorable rearrangement can be avoided by pre-forming lower Na content P2 materials with a delocalized electronic structure and modulated site energy difference between Naf and Nae, as demonstrated by DFT calculations. The disordered framework and enlarged interlayer spacing can be sustained throughout the whole electrochemical process, ensuring both a wider solid-solution region and smaller volume change during de-/sodiation. As a consequence, an excellent electrochemical performance, namely a high reversible capacity of 165.1 mAh g−1, a superior rate performance of 76.7% of capacity retention at 1000 mA g−1, and 91.7% capacity retention after 150 cycles, is harvested. Moreover, this strategy is universal and can be used to synthesize various disordering high-capacity P2 materials. This work provides a long-neglected and unexpected idea for improving the comprehensive performance of P2-type materials for facilitating their practical applications in NIBs.

Phase tuning of P2/O3-type layered oxide cathode for sodium ion batteries via a simple Li/F co-doping route

• P2-LNMTCOF has higher sodium content due to the basis material with the component of O3-type cathode material. • The LiF additive can promote the formation of P2-type cathode material. • The LiF additive can realize the co-doping of anions and cations at the same time. • The voltage drop during cycling has been suppressed due to the more stable TM-O (F) octahedron. • The TOF-SIMS techniques provide novel and powerful tools for exploring the binding state between elements. Element doping is a common and effective method for improving the electrochemical performance of cathode materials, and typically it can be divided into cation ion and anion ion doping. Among the cathode materials of sodium-ion batteries, the important layered oxide cathodes mainly include those of O3 and P2 types, according to the sites of Na ions and the number of oxide layer packings. Hereon, the successful transformation of O3-type cathode NaNi 0.45 Mn 0.4 Ti 0.1 Co 0.05 O 2 (O3-NMTCO) into P2-type cathode of NaNi 0.45 Mn 0.4 Ti 0.1 Co 0.05 O 2 -0.08LiF (P2-LNMTCOF) is achieved via a facile Li/F co-doping route. The modified P2-type cathode (P2-LNMTCOF) have main three advantages over the traditional P2-type cathode Na 0.7 Ni 0.45 Mn 0.4 Ti 0.1 Co 0.05 O 2 (P2-NMTCO): higher Na content, solid solution reaction in high-voltage range, and stronger TM-F bond. As a result, the current P2-LNMTCOF cathode shows the best cycling performance, with the initial capacity of about 160 mA h g −1 and the capacity retention of 61% after 200 cycles. In addition, because of the more stable TM-O (F) octahedron, the voltage drop during cycling has been effectively suppressed. This study provides a new idea and reference for synthesizing high performance P2-type layered oxide cathode materials for sodium-ion batteries.

Revisiting the capacity-fading mechanism of P2-type sodium layered oxide cathode materials during high-voltage cycling

P2-type sodium layered oxide cathode (Na2/3Ni1/3Mn2/3O2, P2-NNMO) has attracted great attention as a promising cathode material for sodium ion batteries because of its high specific capacity. However, this material suffers from a rapid capacity fade during high-voltage cycling. Several mechanisms have been proposed to explain the capacity fade, including intragranular fracture caused by the P2-O2 phase transion, surface structural change, and irreversible lattice oxygen release. Here we systematically investigated the morphological, structural, and chemical changes of P2-NNMO during high-voltage cycling using a variety of characterization techniques. It was found that the lattice distortion and crystal-plane buckling induced by the P2-O2 phase transition slowed down the Na-ion transport in the bulk and hindered the extraction of the Na ions. The sluggish kinetics was the main reason in reducing the accessible capacity while other interfacial degradation mechanisms played minor roles. Our results not only enabled a more complete understanding of the capacity-fading mechanism of P2-NNMO but also revealed the underlying correlations between lattice doping and the moderately improved cycle performance.