Recent Progress of P2-Type Layered Transition-Metal Oxide Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted much attention due to their abundance, easy accessibility, and low cost. All of these advantages make them potential candidates for large-scale energy storage. The P2-type layered transition-metal oxides (Na_xTMO₂; TM=Mn, Co, Ni, Ti, Fe, V, Cr, and a mixture of multiple elements) exhibit good Na⁺ ion conductivity and structural stability, which make them an excellent choice for the cathode materials of SIBs. Herein, the structural evolution, anionic redox reaction, some challenges, and recent progress of Na_xTMO₂ cathodes for SIBs are reviewed and summarized. Moreover, a detailed understanding of the relationship of chemical components, structures, phase compositions, and electrochemical performance is presented. This Review aims to provide a reference for the development of P2-type layered transition-metal oxide cathode materials for SIBs.     

Suppressing the P2–O2 Phase Transition of Na0.67Mn0.67Ni0.33O2 by Magnesium Substitution for Improved Sodium‐Ion Batteries

Room‐temperature sodium‐ion batteries (SIBs) have shown great promise in grid‐scale energy storage, portable electronics, and electric vehicles because of the abundance of low‐cost sodium. Sodium‐based layered oxides with a P2‐type layered framework have been considered as one of the most promising cathode materials for SIBs. However, they suffer from the undesired P2–O2 phase transition, which leads to rapid capacity decay and limited reversible capacities. Herein, we show that this problem can be significantly mitigated by substituting some of the nickel ions with magnesium to obtain Na_0.67Mn_0.67Ni_0.33?xMg_xO₂ (0≤x≤0.33). Both the reversible capacity and the capacity retention of the P2‐type cathode material were remarkably improved as the P2–O2 phase transition was thus suppressed during cycling. This strategy might also be applicable to the modulation of the physical and chemical properties of layered oxides and provides new insight into the rational design of high‐capacity and highly stable cathode materials for SIBs.     

Copper sulfides and their composites for high-performance rechargeable batteries

Copper sulfides (Cu_xS) are widely used as the promising electrode materials for secondary batteries because of the rich abundance, low cost, excellent capacity (～337/560 mA h/g for Cu₂S/CuS) as well as favorable electrical conductivity (10^?3 S/cm). Moreover, nanostructure designing and compounding with other conductive materials can enhance the electrochemical performance of Cu_xS. In this review, the up-to-date progress in the synthesis method as well as the application for secondary batteries (lithium-ion batteries, lithium-sulfur batteries, sodium-ion batteries, magnesium-ion batteries, and so on) of Cu_xS and their relevant composites have been discussed detailly. In the end, the challenges, feasible strategies, and application prospects for the Cu_xS are also summarized.     

A High-Energy NASICON-Type Na3.2MnTi0.8V0.2(PO4)3 Cathode Material with Reversible 3.2-Electron Redox Reaction for Sodium-Ion Batteries

Na superionic conductor (NASICON) structured cathode materials with robust structural stability and large Na⁺ diffusion channels have aroused great interest in sodium-ion batteries (SIBs). However, most of NASICON-type cathode materials exhibit redox reaction of no more than three electrons per formula, which strictly limits capacity and energy density. Herein, a series of NASICON-type Na_3+xMnTi_1−xV_x(PO₄)₃ cathode materials are designed, which demonstrate not only a multi-electron reaction but also high voltage platform. With five redox couples from V^5+/4+ (≈4.1 V), Mn^4+/3+ (≈4.0 V), Mn^3+/2+ (≈3.6 V), V^4+/3+ (≈3.4 V), and Ti^4+/3+ (≈2.1 V), the optimized material, Na_3.2MnTi_0.8V_0.2(PO₄)₃, realizes a reversible 3.2-electron redox reaction, enabling a high discharge capacity (172.5 mAh g⁻¹) and an ultrahigh energy density (527.2 Wh kg⁻¹). This work sheds light on the rational construction of NASICON-type cathode materials with multi-electron redox reaction for high-energy SIBs.     

High‐Entropy Layered Oxide Cathodes for Sodium‐Ion Batteries

Material innovation on high‐performance Na‐ion cathodes and the corresponding understanding of structural chemistry still remain a challenge. Herein, we report a new concept of high‐entropy strategy to design layered oxide cathodes for Na‐ion batteries. An example of layered O3‐type NaNi_0.12Cu_0.12Mg_0.12Fe_0.15Co_0.15Mn_0.1Ti_0.1Sn_0.1Sb_0.04O₂ has been demonstrated, which exhibits the longer cycling stability (ca. 83 % of capacity retention after 500 cycles) and the outstanding rate capability (ca. 80 % of capacity retention at the rate of 5.0 C). A highly reversible phase‐transition behavior between O3 and P3 structures occurs during the charge‐discharge process, and importantly, this behavior is delayed with more than 60 % of the total capacity being stored in O3‐type region. Possible mechanism can be attributed to the multiple transition‐metal components in this high‐entropy material which can accommodate the changes of local interactions during Na⁺ (de)intercalation. This strategy opens new insights into the development of advanced cathode materials.     

Facile Synthesis of Ultra-Small Few-Layer Nanostructured MoSe2 Embedded on N,P Co-Doped Bio-Carbon for High-Performance Half/Full Sodium-Ion and Potassium-Ion Batteries

Sodium/potassium-ion batteries (SIBs/PIBs) arouse intensive interest on account of the natural abundance of sodium/potassium resources, the competitive cost and appropriate redox potential. Nevertheless, the huge challenge for SIBs/PIBs lies in the scarcity of an anode material with high capacity and stable structure, which are capable of accommodating large-size ions during cycling. Furthermore, using sustainable natural biomass to fabricate electrodes for energy storage applications is a hot topic. Herein, an ultra-small few-layer nanostructured MoSe₂ embedded on N, P co-doped bio-carbon is reported, which is synthesized by using chlorella as the adsorbent and precursor. As a consequence, the MoSe₂/NP-C-2 composite represents exceedingly impressive electrochemical performance for both sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). It displays a promising reversible capacity (523 mAh g⁻¹ at 100 mA g⁻¹ after 100 cycles) and impressive long-term cycling performance (192 mAh g⁻¹ at 5 A g⁻¹ even after 1000 cycles) in SIBs, which are some of the best properties of MoSe₂-based anode materials for SIBs to date. To further probe the great potential applications, full SIBs pairing the MoSe₂/NP-C-2 composite anode with a Na₃V₂(PO₄)₃ cathode also exhibits a satisfactory capacity of 215 mAh g⁻¹ at 500 mA g⁻¹ after 100 cycles. Moreover, it also delivers a decent reversible capacity of 131 mAh g⁻¹ at 1 A g⁻¹ even after 250 cycles for PIBs.     

Synthesis and electrochemical characterization of xLi(Ni0.8Co0.15Mg0.05)O2–(1−x)Li[Li1/3Mn2/3]O2 (0.0 ≤ x ≤ 1.0) cathodes for Li rechargeable batteries

Using solution based processing route, we have successfully synthesized xLi(Ni_0.8Co_0.15Mg_0.05)O₂–(1?x)Li[Li_1/3Mn_2/3]O₂ (0.0 ≤ x ≤ 1.0) cathode materials for lithium rechargeable batteries. The phase formation behavior of these cathode materials is characterized by X-ray diffraction measurements. The Galvanostatic charge–discharge characteristic of these cathodes is reported in various cut-off voltage limits. When these composite cathodes are charged to 4.8 V, electrochemical extraction of lithium takes place from active (Li[Ni_0.8Co_0.15Mg_0.05]O₂) as well as inactive (Li[Li_1/3Mn_2/3]O₂) components. Good cycleability of these cathodes is obtained when cycled in the cut-off voltage limits of 4.6–3.0 V. The cycleability of these cathodes are deteriorated when charged above 4.8 V and deep discharged up to 1.2 V followed by repeated cycling in these voltage limits. Based on the analyses of impedance spectra at various charge and discharge states, the probable reasons for such findings are discussed.     

Progress on multiphase layered transition metal oxide cathodes of sodium ion batteries

As one of the most promising secondary batteries in large-scale energy storage, sodium ion batteries (SIBs) have attracted wide attention due to the abundant raw materials and low cost. Layered transition metal oxides are one kind of popular cathode material candidates because of its easy synthesis and large theoretical specific capacity. Yet, the most common P2 and O3 phases show distinct structural characteristics respectively. O3 phase can serve as a sodium reservoir, but it usually suffers from serious phase transition and sluggish kinetics. For the P2 phase, it allows the fast sodium ion migration in the bulk and the structure can maintain stable, but it is lack of sodium, showing a great negative effect on Coulombic efficiency in full cell. Thus, single phase structure almost cannot achieve satisfied comprehensive sodium storage performances. Under these circumstances, exploiting novel multiphase cathodes showing synergetic effect may give solution to these problems. In this review, we summarize the recent development of multiphase layered transition metal oxide cathodes of SIBs, analyze the mechanism and prospect the future potential research directions.     

Electron-poor SrAuxIn4−x (0.5?x?1.2) and SrAuxSn4−x (1.3?x?2.2) phases with the BaAl4-type structure

Solid solutions SrAu_xIn_4−x (0.5?x?1.2) and SrAu_xSn_4−x (1.3?x?2.2) have been prepared at 700 °C and their structures characterized by powder and single-crystal X-ray diffraction. They adopt the tetragonal BaAl₄-type structure (space group I4/mmm, Z=2; SrAu_1.1(1)In_2.9(1), a=4.5841(2) Å, c=12.3725(5) Å; SrAu_1.4(1)Sn_2.6(1), a=4.6447(7) Å, c=11.403(2) Å), with Au atoms preferentially substituting into the apical over basal sites within the anionic network. The phase width inherent in these solid solutions implies that the BaAl₄-type structure can be stabilized over a range of valence electron counts (vec), 13.0-11.6 for SrAu_xIn_4−x and 14.1-11.4 for SrAu_xSn_4−x. They represent new examples of electron-poor BaAl₄-type compounds, which generally have a vec of 14. Band structure calculations confirm that substitution of Au, with its smaller size and fewer number of valence electrons, for In or Sn atoms enables the BaAl₄-type structure to be stabilized in the parent binaries SrIn₄ and SrSn₄, which adopt different structure types.     

Preparation of Bi0.4Sb1.6Se3xTe3(1−x) hexagonal rods and effect of Se on structure and electrical property

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ raw materials and dispersed with x amounts of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3xTe_3(1−x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field emission scanning electron microscopy (FESEM). The crystalline of the powders were examined by X-ray diffraction (XRD), which showed that the powders of 0.0 ≤ x ≤ 0.8 samples can be indexed as the rhombohedral phase, whereas the sample with x = 1.0 has an orthorhombic phase structure. The influence of variations in Se content on thermoelectric properties was studied in the temperature range of 300–523 K. The alloying of Se in Bi_0.4Sb_1.6Te₃ effectively caused a decrease in hole concentration and, thus, a decrease in electrical conductivity and an increase in Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/m K² at 373 K for the x = 0.8 sample.     

Lithium-doped,organic ionic plastic crystal electrolytes exhibiting high ambient-temperature conductivities

Film-forming, ionic plastic crystal—N,N-diethyl-N-methyl-N-(n-propyl)ammonium trifluoromethyltrifluoroborate (N₁₂₂₃[CF₃BF₃])—has been doped with Li[CF₃BF₃] salt to prepare stable Li-ion conducting plastic crystal electrolytes. Solid-state conductivities as high as 10⁻⁴–10⁻³ S cm⁻¹ have been obtained within the ambient-temperature plastic crystalline phase of the Li-doped materials. Solid-state Li deposition/dissolution based on these materials has been realized at 25 °C. The outstanding properties of these materials make them attractive candidates for all-solid-state Li batteries.     

A new high voltage alluaudite sodium battery insertion material

Large-scale stationary storage forms a key sector that can be economically served by sodium-ion batteries. In realizing practical sodium-ion batteries, discovery and development of novel cathodes is essential. In this spirit, alluaudite-type Na₂Fe₂(SO₄)₃ was reported in 2014 to have the highest Fe³⁺/Fe²⁺ redox potential (～3.8 V vs. Na). This finding led to reports on various PO₄^3? and SO₄^2? based alluaudite compounds exhibiting high energy densities. In 2017, MoO₄^2? based alluaudite, Na_2.67Mn_1.67(MoO₄)₃, was found as a 3.45 V cathode material. Exploring molybdenum chemistry further, this work reports alluaudite type Na_3.36Co_1.32(MoO₄)₃ (NCMo) as a novel versatile electroactive cathode for Li-ion and Na-ion batteries. It was synthesized by a wet solution-combustion route with a restricted annealing duration of 1 min at 600 °C. Calorimetric study revealed the formation enthalpy from component oxides (ΔH°_f,ox = ?575.49 ± 7.75 kJ/mol) to be highly exothermic. Unlike the sulfate class of alluaudites, this material is highly stable in air and moisture (ΔH_ds = 537.42 ± 0.78 kJ/mol). Having an ionic conductivity of 6.065 × 10^?8 S/cm (at 50 °C), it offers a pseudo two-dimensional Na⁺ migration pathway. Without any material optimization, NCMo was found to work as a high-voltage insertion cathode (ca. 4.0 V vs. Na/Na⁺ and 4.1 V vs. Li/Li⁺) in sync with theoretically predicted potential of 3.98 V (vs. Na/Na⁺). Ex-situ X-ray diffraction and photoelectron spectroscopy studies revealed the occurrence of solid-solution redox mechanism solely involving Co³⁺/Co²⁺ redox centre. It benchmarks Na_3.36Co_1.32(MoO₄)₃ as a novel electrochemically active Mo-based alluaudite-type polyanionic cathode insertion material.     

Rapid synthesis of layered KxMnO2 cathodes from metal–organic frameworks for potassium-ion batteries

Layered transition metal oxides (LTMOs) are a kind of promising cathode materials for potassium-ion batteries because of their abundant raw materials and high theoretical capacities. However, their synthesis always involves long time calcination at a high temperature, leading to low synthesis efficiency and high energy consumption. Herein, an ultra-fast synthesis strategy of Mn-based LTMOs in minutes is developed directly from alkali-transition metal based-metal–organic frameworks (MOFs). The phase transformation from the MOF to LTMO is systematically investigated by thermogravimetric analysis, variable temperature optical microscopy and X-ray diffraction, and the results reveal that the uniform distribution of K and Mn ions in MOFs promotes fast phase transformation. As a cathode in potassium-ion batteries, the fast-synthesized Mn-based LTMO demonstrates an excellent electrochemical performance with 119 mA h g⁻¹ and good cycling stability, highlighting the high production efficiency of LTMOs for future large-scale manufacturing and application of potassium-ion batteries.

An ultra-fast synthesis method for layered transition metal oxide cathodes (K_xMnO₂) was developed via minute calcination of metal–organic frameworks for potassium-ion batteries.     

P2‐NaCo0.5Mn0.5O2 as a Positive Electrode Material for Sodium‐Ion Batteries

As a promising positive electrode material for sodium‐ion batteries (SIBs), layered sodium oxides have attracted considerable attention in recent years. In this work, stoichiometric P2‐phase NaCo_0.5Mn_0.5O₂ was prepared through the conventional solid‐state reaction, and its structural and physical properties were studied in terms of XRD, XPS, and magnetic susceptibility. Furthermore, the P2‐NaCo_0.5Mn_0.5O₂ electrode delivered a discharge capacity of 124.3 mA h g^?1 and almost 100 % initial coulombic efficiency over the potential window of 1.5–4.15 V. It also showed good cycle stability, with a reversible capacity and capacity retention reaching approximately 85 mA h g^?1 and 99 %, respectively, at the 5 C rate after 100 cycles. Additionally, cyclic voltammetry and ex situ XRD were employed to explain the electrochemical behavior at the different electrochemical stages. Owing to the applicable performances, P2‐NaCo_0.5Mn_0.5O₂ can be considered as a potential positive electrode material for SIBs.     

Atomistic simulation for disordered TbCu7-type compounds SmCo7 and Sm(Co,T)7 (T = Ti,Ga, Si,Cu, Hf,Zr)

The computational evolution from ordered SmCo₅ (CaCu₅-type) compound to disordered SmCo₇ (TbCu₇-type) compound has been implemented in this work. Based on statistical simulation, both the phase stabilities and the lattice constants of the calculated SmCo₇ series are in good agreement with experiments and a sort of 3g layer twisting deformation is observed. The calculated site preferences and lattice constants of the disordered ternary SmCo_7?xT_x (T = Ti, Ga, Si, Cu, Hf, Zr) are also in consonance with experimental data. SmCo₇'s natural structure selection has also been investigated and understood on the basis of phase stability and cohesive energy. This work lays a foundation for further first principles studies on the magnetic properties of disordered TbCu₇-type high-temperature permanent magnets.     

Research progress of tunnel-type sodium manganese oxide cathodes for SIBs

Among the large energy storage batteries, the sodium ion batteries（SIBs） are attracted huge interest due to the fact of its abundant raw materials and low cost, and has become the most promising secondary battery. Tunnel-type sodium manganese oxides（TMOs） are industrialized cathode materials because of their simple synthesis method and proficient electrochemical performance. Na_0.44MnO₂（NMO） is considered the best candidate material for all tunnel-type structural materials. ...     

An Abnormal 3.7 Volt O3‐Type Sodium‐Ion Battery Cathode

Layered O3‐type sodium oxides (NaMO₂, M=transition metal) commonly exhibit an O3–P3 phase transition, which occurs at a low redox voltage of about 3 V (vs. Na⁺/Na) during sodium extraction and insertion, with the result that almost 50 % of their total capacity lies at this low voltage region, and they possess insufficient energy density as cathode materials for sodium‐ion batteries (NIBs). Therefore, development of high‐voltage O3‐type cathodes remains challenging because it is difficult to raise the phase‐transition voltage by reasonable structure modulation. A new example of O3‐type sodium insertion materials is presented for use in NIBs. The designed O3‐type Na_0.7Ni_0.35Sn_0.65O₂ material displays a highest redox potential of 3.7 V (vs. Na⁺/Na) among the reported O3‐type materials based on the Ni²⁺/Ni³⁺ couple, by virtue of its increased Ni?O bond ionicity through reduced orbital overlap between transition metals and oxygen within the MO₂ slabs. This study provides an orbital‐level understanding of the operating potentials of the nominal redox couples for O3‐NaMO₂ cathodes. The strategy described could be used to tailor electrodes for improved performance.     

Structural and spectroscopic characterization of Mo1−xWxO3−δ mixed oxides

Mo_1−xW_xO₃ oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO₃ and WO₃ were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo_1−xW_xO₃ in oxidizing atmosphere to Mo_1−xW_xO_3−δ in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperature are peculiar of each mixed oxide.     

Structural and optical absorption studies of barium substituted strontium ferrite powder

Attempt has been made to synthesize Ba_xSr_1−xFeO_3−ξ (x = 0–1.0) ferrite powder by decomposition of sol–gel derived oxalate at 800–1000 °C for 5–10 h to study the effect of barium insertion with regard to phase(s), stability, optical behavior, oxidation states of iron, and oxygen deficiency. It is shown that these ferrites possess a perovskite-type cubic phase (a = 3.877–4.020Å, Z = 1, space group Pm3m) for 0.1 ≤ x ≤ 0.94, a mixture of 82% rhombohedral (a_R = 5.666Å and α_R = 59.761°, Z = 2, space group R3c) and 18% hexagonal phases for x = 0.96 and a pure hexagonal (a = 5.689Å, c = 13.944Å, Z = 6, space group P6₃/mmc) phase for x = 1. Barium substitution in SrFeO_3−ξ system leads to lattice expansion, weakening of the metal-oxygen bond, reduction of tetravalent iron ions (as evident from Mossbauer analysis), and decrease of oxygen content. The optical absorption peaks observed in the range 3.17–4.11 eV are attributed to charge transfer transitions from O²⁻ (2p) to Fe (3d) band. The values of optical energy band gap of Ba_xSr_1−xFeO_3−ξ are found to be ∼5.48 and ∼4.04 for x = 0.1 and 1.0, respectively. A stable perovskite-type cubic phase in Ba_xSr_1−xFeO_3−ξ system with significant anion deficiency (ξ = 0.26–0.32) may possibly act as an oxygen permeable membrane.     

Research progress on Na3V2(PO4)2F3-based cathode materials for sodium-ion batteries

Sodium-ion batteries (SIBs) have received significant attention in large-scale energy storage due to their low cost and abundant resources. To obtain high-performance SIBs, many intensive studies about electrode materials have been carried out, especially the cathode material. As various types of cathode material for SIBs, a 3D open framework structural Na₃V₂(PO₄)₂F₃ (NVPF) with Na superionic conductor (NASICON) structure is a promising cathode material owing to its high operating potential and high energy density. However, its electrochemical properties are severely limited by the poor electronic conductivity due to the insulated [PO₄] tetrahedral unit. In this review, the challenges and strategies for NVPF are presented, and the synthetic strategy for NVPF is also analyzed in detail. Furthermore, recent developments of modification research to enhance their electrochemical performance are discussed, including designing the crystal structure, adjusting the electrode structure, and optimizing the electrolyte components. Finally, further research and application for future development of NVPF are prospected.