Layered Transition Metal Research Articles

Mg/Fe site-specific dual-doping to boost the performance of cobalt-free nickle-rich layered oxide cathode for high-energy lithium-ion batteries

Layer-type LiNi0.9Mn0.1O2 is promising to be the primary cathode material for lithium-ion batteries (LIBs) due to its excellent electrochemical performance. Unfortunately, the cathode with high nickel content suffers from severely detrimental structural transformation that causes rapid capacity attenuation. Herein, site-specific dual-doping with Fe and Mg ions is proposed to enhance the structural stability of LiNi0.9Mn0.1O2. The Fe3+ dopants are inserted into transition metal sites (3b) and can favorably provide additional redox potential to compensate for charge and enhance the reversibility of anionic redox. The Mg ions are doped into the Li sites (3a) and serve as O2−-Mg2+-O2− pillar to reinforce the electrostatic cohesion between the two adjacent transition-metal layers, which further suppress the cracking and the generation of harmful phase transitions, ultimately improving the cyclability. The theoretical calculations, including Bader charge and crystal orbital Hamilton populations (COHP) analyses, confirm that the doped Fe and Mg can form stable bonds with oxygen and the electrostatic repulsion of O2−-O2−can be effectively suppressed, which effectively mitigates oxygen anion loss at the high delithiation state. This dual-site doping strategy offers new avenues for understanding and regulating the crystalline oxygen redox and demonstrates significant potential for designing high-performance cobalt-free nickel-rich cathodes.

Low-Electronegativity Cationic High-Entropy Doping to Trigger Stable Anion Redox Activity for High-Ni Co-Free Layered Cathodes in Li-Ion Batteries.

LiNi0.8 Co0.1 Mn0.1 O2 (NCM-811) exhibits the highest capacity in commercial lithium-ion batteries (LIBs), and the high Ni content (80 %) provides the only route for high energy density. However, the cationic structure instability arisen from the increase of Ni content (>80 %) limits the further increase of the capacity, and inevitable O2 release related to anionic structure instability hinders the utilization of anion redox activity. Here, by comparing various combinations of high-entropy dopants substituted Co element, we propose a low-electronegativity cationic high-entropy doping strategy to fabricate the high-Ni Co-free layered cathode (LiNi0.8 Mn0.12 Al0.02 Ti0.02 Cr0.02 Fe0.02 O2 ) that exhibits much higher capacity and cycling stability. Configurational disorder originated from cationic high-entropy doping in transition metal (TM) layer, anchors the oxidized lattice oxygen ((O2 )n- ) to preserve high (O2 )n- content, triggering the anion redox activity. Electron transfer induced by applying TM dopants with lower electronegativity than that of Co element, increases the electron density of O in TM-O octahedron (TM-O6 ) configuration to reach higher (O2 )n- content, resulting in the higher anion redox activity. With exploring the stabilization effect on both cations and anions of high-entropy doping and low-electronegativity cationic modified anion redox activity, we propose an innovative and variable pathway for rationally tuning the properties of commercial cathodes.

Low‐Electronegativity Cationic High‐Entropy Doping to Trigger Stable Anion Redox Activity for High‐Ni Co‐Free Layered Cathodes in Li‐Ion Batteries

AbstractLiNi0.8Co0.1Mn0.1O2 (NCM‐811) exhibits the highest capacity in commercial lithium‐ion batteries (LIBs), and the high Ni content (80 %) provides the only route for high energy density. However, the cationic structure instability arisen from the increase of Ni content (>80 %) limits the further increase of the capacity, and inevitable O2 release related to anionic structure instability hinders the utilization of anion redox activity. Here, by comparing various combinations of high‐entropy dopants substituted Co element, we propose a low‐electronegativity cationic high‐entropy doping strategy to fabricate the high‐Ni Co‐free layered cathode (LiNi0.8Mn0.12Al0.02Ti0.02Cr0.02Fe0.02O2) that exhibits much higher capacity and cycling stability. Configurational disorder originated from cationic high‐entropy doping in transition metal (TM) layer, anchors the oxidized lattice oxygen ((O2)n−) to preserve high (O2)n− content, triggering the anion redox activity. Electron transfer induced by applying TM dopants with lower electronegativity than that of Co element, increases the electron density of O in TM‐O octahedron (TM‐O6) configuration to reach higher (O2)n− content, resulting in the higher anion redox activity. With exploring the stabilization effect on both cations and anions of high‐entropy doping and low‐electronegativity cationic modified anion redox activity, we propose an innovative and variable pathway for rationally tuning the properties of commercial cathodes.

Enhanced reversibility of single crystal Ni-rich cathode materials via Ti doping

Single crystal nickel-rich cathodes have been widely studied in the field of lithium-ion batteries (LIBs) due to their high volumetric energy density and good structural/electrochemical stability. However, a larger particle size (∼ 5 µm) prolongs the diffusion path of Li+ and leads to poor rate performance. In this work, a modification strategy of Ti4+ doping was used to prepare single-crystal Ni-rich cathodes LiNi0.88Co0.06Mn0.06O2 with good rate performance. Compared to the polycrystalline Ni-rich cathode, the modified cathodes will have wider lithium channels to strip and embed lithium ions more easily. Concurrently, the secure Ti-O bonds can not only maintain the transition metal layer but also inhibit the “air sensitive effect”. After 100 cycles, the electrochemical performance shows that the modified cathodes with 1 at.% Ti4+ doping have a comparatively high original capacity of 193.8 mAh g − 1 and an upgraded capacity retention of ∼ 93.6 % (2.8–4.3 V) at 1 C. A better rate performance (118.6 mAh g − 1) can be achieved at 10 C. This discovery will provide a good research direction for improving the insufficiency of single crystal nickel-rich cathodes.

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.

Thermodynamically stable low-Na O3 cathode materials driven by intrinsically high ionic potential discrepancy

The difference in cationic potentials of the various elements in and between the transition metal layers is another essential factor to be taken into account to discriminate between the P/O types of layered materials.

Realizing long-term cycling stability of O3-type layered oxide cathodes for sodium-ion batteries.

O3-type layered oxide cathodes are promising for practical sodium-ion batteries (SIBs) owing to their high theoretical capacity, facile synthesis, and sufficient Na+ storage. However, they face challenges such as rapid capacity loss and poor cycling stability, mainly attributed to irreversible phase transitions. To address these challenges, a novel cathode material, Li/Sn co-substituted O3-Na0.95Li0.07Sn0.01Ni0.22Fe0.2Mn0.5O2 (LSNFM), has been designed by regulating the electronic structure, in which Li+ activates more redox reactions of Ni2+/3+ and Fe3+/4+ above 2.5 V and suppresses the redox reactivity of Mn3+/4+ below 2.5 V, while Sn4+ can prevent the charge delocalization in the transition metal layer, contributing to structural stability. Due to this synergistic effect, the as-prepared LSNFM electrode with high structural reversibility displays a 27.2% capacity increase contributed by the high-voltage transition metal ion redox activity and exhibits excellent long-term cycling stability, an 84.0% capacity retention after 500 cycles at 1 C and an 84.7% capacity retention after 2000 cycles at 5 C. The fundamental mechanism is fully investigated using systematic in situ/ex situ characterization techniques and density functional theory computations. This work provides a paradigm for designing long-term cycle life cathode materials by synergistically regulating the electronic structure in practical SIBs.

Layer-structured P3-K0.5Mn0.95W0.05O2 for enhanced potassium-ion batteries by mitigating phase transformation

The movement of transition metal layers and the transformation from the P3-type to the O3-type structure were successfully suppressed in the designed P3-type K0.5Mn0.95W0.05O2 cathode for potassium-ion batteries.

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.

Enhanced conduction and phase transition reversibility of O3-type NaNi1/3Fe1/3Mn1/3O2 cathode via Ta doping for high-performance sodium-ion battery

The complex phase transition that occurs during the charge/discharge process leads to poor rate and cycling performance of O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM). In this paper, we propose a novel modification strategy for NFM that involves doping its transition metal layer with Ta5+ ions of higher valence and stronger binding energy to oxygen than the transition metal. The effects of Ta doping on the crystal structure, transition metal valence states, activation barrier energy, and bandgap of NFM were investigated, and the modification mechanisms were explored. The capacity retention of NFM-Ta0.01 (80.15 %) was higher than that of NFM (35.44 %) after 200 cycles. This improvement is attributable to the strong bonding energy effect of TaO, which stabilized the crystal structure of NFM, delayed the transition of the material from the hexagonal O3 phase to the hexagonal P3 phase beyond 3.2 V, and enhanced the phase transition reversibility of NFM. Moreover, Ta introduction increased the sodium layer spacing and reduced the Na+ activation barrier energy of the material from 0.773 to 0.692 eV, and the band gap from 1.7416 to 1.5951 eV, leading to a significant improvement in electronic conductivity. Owing to the synergistic effect of these factors, the discharge capacity of NFM-Ta0.01 at a 5C rate (100.6 mAh g−1) was considerably higher than that of NFM (79.3 mAh g−1). This work presents a proven modification strategy for achieving structurally stable and improved rate performance for NFM.

Increasing the Sodium Content in P2-Type Layered Oxides As Cathode Materials for Sodium-Ion Batteries

P2-type layered oxides exhibit superior Na ion conductivity and structural stability compared to their O3 analogues, making them promising candidate materials for next generation batteries. However, their lower initial Na contents (Na/transition metals < 1) restrict their capacity during the first charge process, and the transitions between P- and O2-type structures during cycling further deteriorate their stability.[1,2] Our work aims to increase the Na inventory in pristine cathode compositions, especially relevant for full cell systems, while maintaining their cycling stability. We started with the well-studied P2 material Na2/3Ni1/3Mn2/3O2 and have worked on incorporating Li in the transition metal layers to stabilize the structure, while at the same time increasing the Na content.[3,4] In this respect, a great challenge is to control the phase formation during the synthesis. When the Na content exceeds a certain limit, an O3 structure is more likely to form than a P2 structure.[1] On the other hand, at too low temperatures, a P3 polymorph is preferred.[5] By regulating the synthesis conditions, Na contents have been successfully increased in a series of materials between two end compositions, Na2/3Ni1/3Mn2/3O2 and Na5/6Li5/18Mn13/18O2.In this presentation, we report on unpublished in situ XRD characterizationsof these compositions during their solid-state synthesis, as well as on their crystal structures and electrochemical behavior, which involves varying amount of Ni redox. These time-resolved studies of structural evolutions provide guidance for further synthesis of desired phases and compositions. Along with the electrochemical results, we also aim to answer the questions on the charge compensation mechanisms that sustain the electrochemical performance as the amount of Li increases and Ni decreases. Finally, we also explore which amount of Li substitution can best balance the structural stability and capacity retention of the cathode material.

Optimizing Dopant for Enhanced Oxygen-Redox Performance in P2-Na Cathode Materials for Sodium-Ion Batteries

Recently, there has been a lot of interest in using oxygen-redox-based cathode materials for sodium-ion batteries (SIBs) due to their potential for providing additional capacity in the high-voltage range. However, they have some matter, including fast capacity fading and poor rate capability. Herein, P2-Na0.7[Li0.1Ni0.1Mg0.1Mn0.7]O2 is introduced, an layered oxide cathode material for Na-ion batteries based on oxygen-redox. The effect of Ni doping is improved electrochemical performance. Mg2+ in transition metal layer delivered relatively higher capacity via oxygen redox in high voltage region by comparison with Mg-free P2-Na0.75[Li0.15Ni0.15Mn0.7]O2, showing a specific discharge capacity of 175mAh g-1 with 83% of capacity retention after 100 cycles. This study show how the co-doping of Li, Ni and Mg can impact P2-type layered materials and proposes a novel approach to utilizing Mn-rich cathode materials through oxygen redox, with a focus on optimizing the dopant for sodium-ion batteries.

Improved Cycle Life and Determination of Li Ion Transport Parameters in Mixed Cation Doped Ni-Rich NMC

Ni-rich Li-ion cathode materials like NMC-9.5.5 (LiNi0.9Mn0.05Co0.05O2) are of interest due to their higher energy density and limited cobalt content, aiding the transition away from critical raw materials, when compared to materials like NMC111 - 622. This has the advantage of reducing production costs while maintaining a high working voltage and capacity. However, the high nickel content of these materials can be detrimental due to disorder brought by Li+/Ni2+ cation mixing, lithium residuals, and irreversible phase changes during operation resulting in significant capacity fade during cycling. Similarly, the different synthesis methods and treatments drastically affect the morphological, structural, and electrochemical propertiesWe present a structural and electrochemical study of NMC-9.5.5 prepared at scale via a hydroxide coprecipitation synthesis in a 5 L continuous stirred reactor tank (CSTR). In this work, bulk doping and co-doping with two metal cations are investigated with a simple and scalable method to achieve mixed metal and mixed valence doped NMC-9.5.5 with the aim to further improve cycle life and stability. Additionally, the effect of bulk doping on the thermodynamic and kinetic properties of the cathode material are investigated in both half-cells and full-cells using galvanostatic intermittent titration technique (GITT) and electrical impedance spectroscopy (EIS), where a 3-electrode configuration is used to elucidate the individual electrode potentials and stoichiometries.The bulk doping of the transition metal layer was found to significantly reduce the capacity fade in the NMC-9.5.5, increasing the capacity retention after 100 cycles from 78% in the undoped material to 92% and 89% in the doped and co-doped NMC-9.5.5 half-cells. The mixed metal co-doped material was also effective in delivering a higher specific capacity than the baseline throughout the full cycling range. The improved cycling performance can be understood by the inspection of the differential capacity plots, which show a suppression of the H2-H3 irreversible phase transition in the doped material and stabilisation of the peak to a higher voltage compared to the baseline. The effect of doping on the structural stabilisation is similarly reflected in the XRD characterisation, in which the introduction of the metal cation acts to constrain the detrimental Li+/Ni2+ cation mixing due to a widening of the transition metal channel. This is also supported by the EXAFS characterisation that indicate the dopants may act to stabilise the local Ni environment. These structural changes appear to be further beneficial in facilitating improved Li ion transport properties in the doped material as evidenced by the lower charge transfer resistances during cycling and increased diffusion coefficient.This research illustrates the high capacity performance of low cobalt cathode materials thus reducing the reliance on critical materials, as well as the ability to produce Ni-rich cathode material at large scale and further stabilise its cycle life by simple structural engineering. Improved materials manufacturing processes will enable a greater understanding of the synthesis process and enable more sustainable methods of synthesis. There are also benefits for cell makers with improvement in processability and additional stabilisation knowledge for battery manufacturers, as well as the consumers that benefit from longer driving ranges and battery life in their EV’s. Figure 1

A Dual-Approach for Extending the Lifespan of Cobalt-Free Na[Ni0.55Mn0.35Fe0.1]O2 cathode Via Bi5+-Doping and Bi2O3 Coating in Sodium-Ion Batteries

A cobalt-free layered Na[Ni0.55Mn0.35Fe0.1]O2(NFM) cathode material has been synthesized and dual surface and structural modifications have been performed. Bi5+ is doped into pure NFM in order to tune the structure, whereas, a thin layer of Bi2O3 is coated on surface of the Bi5+-coated NFM to modify the surface. The structure, morphology, and electrochemical performance of prepared samples are analyzed and compared by various characterization techniques. The pristine NFM cathode exhibits the specific discharge capacity of 170 mAh g-1, while Bi-doped cathode exhibits 181 mAh g-1, and Bi2O3 coated cathode renders 180 mAh g-1. It is observed that, pristine NFM cathode suffers rapid capacity degradation and nearly ~80% capacity loss within first 250 cycles. After 1000 cycles, BNFM shows 47% capacity retention, while, BNMF@Bi2O3 shows 73% capacity retention of initial capacity. This improvement in the rate capability is obtained due to the effect of Bi-doping and Bi2O3 coating, where, former enlarges interlayer spacing and latter provides the ionic conducting channel as well as protects the particle from the contact of the electrolyte. The combined effect of Bi-doping and Bi2O3 coating facilitates fast diffusion of Na-ions within the transition metal layers resulting in superior rate capability. Fig: Cyclability of pristine, Bi-doped and Bi2O3 coated NFM cathode materials Figure 1

High-Capacity Nickel-Based Layered Oxides Containing Niobium Ions for Lithium-Ion Batteries

Lithium-ion batteries have been widely used in recent years, from portable electronic devices to electric vehicles. Although LiNiO2, is expected to be ideal electrode materials for electric vehicle applications due to its high reversible capacity, Ni ions migrate from octahedral sites of transition metal layers to tetrahedral sites of adjacent Li layers during charging. This process is partly reversible process but Ni ions are left the tetrahedral sites of the Li layer after the continuous cycle test, which prevents the re-insertion of Li ions, resulting in capacity degradation. [1]To suppress the capacity degradation LiNiO2, the effect on the phase transition behavior by Nb substitution for Ni ions in LiNiO2 is examined.The samples with different Nb contents LixNi1–xNbxO2, are synthesized and tested as electrode materials. The Nb-substituted sample shows highly improved cycle characteristics compared with non-substituted LiNiO2. SEM observation of the samples reveals that the primary particles become smaller, suggesting that the Nb-substituted influences the crystallization process for LiNiO2. By comparing the results of electrochemical and structural characterizations for the samples with or without Nb substitution, the origin of the improvement of electrode reversibility is discussed in details.Reference [1] N. Ikeda et al., and N. Yabuuchi, Journal of Materials Chemistry A, 9, 15963 (2021).

Engineering Transition Metal Layers for Long Lasting Anionic Redox in Layered Sodium Manganese Oxide

Oxygen-redox-based layered cathode materials are of great importance in realizing high-energy-density sodium-ion batteries (SIBs) that can satisfy the demands of next-generation energy storage technologies. However, Mn-based layered materials (P2-type Na-poor Nay[AxMn1−x]O2, where A=alkali ions) still suffer from poor reversibility during oxygen-redox reactions and low conductivity. Herein, we introduce P2-Na0.75[Li0.15Ni0.15Mn0.7]O2 and P2-Na0.6[Li0.15Co0.15Mn0.7]O2, an oxygen-redox-based layered oxide cathode materials for SIBs. The effect of Ni and Co doping on the electrochemical performance was investigated by comparison with Ni-free and Co-free P2-Na0.67[Li0.22Mn0.78]O2 material. Combined experiments (galvanostatic cycling, neutron powder diffraction (NPD), X-ray absorption spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (7Li NMR)) and theoretical studies (density functional theory, DFT calculations) confirmed that Ni substitution not only increases the operating voltage and decreases voltage hysteresis but also improves the cycling stability by reducing Li migration from transition metal (TM) to Na layers. It is also anticipated that having Na–O–Li configuration in a Mn4+-based layered material is important to active oxygen redox and that Co doping is crucial for improving the electrical conductivity. This research demonstrates the effect of Ni and Co doping in P2-type layered materials and suggests a new strategy of using Mn-rich cathode materials via oxygen redox with optimization of doping elements for SIBs.

Mn-Rich P′2-Na0.67[Ni0.1Fe0.1Mn0.8]O2 As High-Energy-Densityand Long-Life Cathode Material for Sodium-Ion Batteries

P′2-type Na0.67[Ni0.1Fe0.1Mn0.8]O2 is introduced as a promising new cathode material for sodium-ion batteries (SIBs) that exhibits remarkable structural stability during repetitive Na+ de/intercalation. The O-Ni-O-Mn-O-Fe-O bond in the octahedra of transition-metal layers is used to suppress the elongation of the Mn-O bond and to improve the electrochemical activity, leading to the highly reversible Na storage mechanism. A high discharge capacity of ≈220 mAh g−1 (≈605 Wh kg−1) is delivered at 0.05 C (13 mAg−1) with a high reversible capacity of ≈140 mAh g−1 at 3 C and excellent capacity retention of 80% over 200 cycles. This performance is associated with the reversible P′2–OP4 phase transition and small volume change upon charge and discharge (≈3%). The nature of the sodium storage mechanism in a full cell paired with a hard carbon anode reveals an unexpectedly high energy density of ≈542 Wh kg−1 at 0.2 C and good capacity retention of ≈81% for 500 cycles at 1 C (260 mAg−1).

Li/Ni Intermixing: The Real Origin of Lattice Oxygen Stability in Co‐Free Ni‐Rich Cathode Materials

AbstractThe Co‐free Ni‐rich layered cathode materials with excellent structural stability and low‐cost in Ni‐rich family are emerging as the promising candidates for the next generation of high‐energy density cathodes. Previously, Mn is generally regarded as the structural stabilizer in Co‐free Ni‐rich cathodes, while the role of inevitable Li/Ni intermixing is underestimated. Herein, the study reveals that the real origin of the lattice oxygen/structure stability of Co‐free Ni‐rich cathodes is dominated more by Li/Ni intermixing than the widely accepted Mn. In the Li/Ni intermixing configuration, the intermixed Ni ions can suppress the oxidation of lattice oxygen by increasing the formation energy of oxygen vacancy and reducing charge compensation. Besides, the Li vacancy formed via the delithiation of intermixed Li in transition metal layer can serve as O2 capture sites. This dual stabilization mechanism is proposed and proved to be effectively in enhancing the reversibility of lattice oxygen and the stability of material structural. This work sheds light on the mechanism of stabilizing lattice oxygen through Li/Ni intermixing, which provides new insights for designing better batteries.

A Ni/Co-free high-entropy layered cathode with suppressed phase transition and near-zero strain for high-voltage sodium-ion batteries

High-entropy layered oxides have emerged as a new class of cathode materials for sodium-ion batteries by providing infinite possibilities to tailor energy storage capabilities. However, owing to the lack of advanced high-entropy layered material, the influence of configurational entropy/compositional disorder on the high-voltage electrochemical performance of sodium-based layered cathode has seldom been explored. In this work, a Ni/Co-free high-entropy layered oxide P2-Na0.65Mn0.65Cu0.2Li0.06Mg0.015Ti0.015Al0.015Zr0.015Y0.015La0.015O2 (Mn-Cu-HEO) was prepared by stirring hydrothermal method, and investigated as a new cathode material for sodium-ion batteries. It is found that high configurational entropy and strong M−O configurations (M = Ti, Al, Zr, Y, and La) greatly stabilize the layered framework structure and MnO6 octahedral local structure, restraining the deleterious phase transition and large volume change during high-voltage cycling, thus resulting in high reversible cationic/anionic redox. In the meantime, disordered atomic arrangement in transition metal layer efficaciously mitigates Na+/vacancy ordering during de-/sodiation, enhancing the Na+ transport kinetics. Benefiting from the high-entropy stabilization effect, complex atomic arrangement, and multielemental composition, Mn-Cu-HEO displays splendid cyclic stability (87.2 % capacity retention after 500 cycles, very small change in cell volume (0.53 %) after 100 cycles), excellent rate capability (55.5 mAh g−1 at 10C), and a usable reversible capacity of 85.1 mAh g−1 at 1C in high-voltage range of 2.0–4.5 V. This work expands the horizons of high-entropy layered materials, providing new insight in the design and construction of highly stable and high-voltage sodium ion host.

Trace Ti/Mg co-doped O3-type layered oxide cathodes with enhanced kinetics and stability for sodium-ion batteries

O3-type layered oxide cathodes are identified as a promising cathode material for sodium-ion batteries (SIBs) due to their high specific capacity and moderate operating voltage, yet it usually suffers from sluggish Na-ion diffusion and structure instability. Herein, we demonstrate the synthesis of a trace Ti/Mg co-doped NaNi1/3Fe1/3Mn1/3O2 (TiMg-NFM) cathode, in which Ti-ion is distributed in transition metal layers and Mg-ion is doped into sodium layers. The co-doping can enlarge the Na-layer spacing inside the layered structure to reduce Na-ion diffusion barrier. Meanwhile, the structural stability is effectively reinforced via the stronger Ti-O bond and the pillar effect of Mg-ions, further reducing surface side reaction with electrolyte. These merits endow a high reversible capacity of the TiMg-NFM cathode with 144.5 mAh g−1 at 0.1C, and much higher capacity retention of 111.4 mAh g−1 at 5C versus the pristine NFM (62.1 mAh g−1). More impressively, 80.1% of initial capacity can be maintained after 300 cycles at 2C. This work presents a feasible strategy to enhance the reaction kinetics and structure stability for developing advanced O3-type layered oxide cathodes.