Realizing Complete Solid‐Solution Reaction in High Sodium Content P2‐Type Cathode for High‐Performance Sodium‐Ion Batteries

P2‐type layered oxides suffer from an ordered Na⁺/vacancy arrangement and P2→O2/OP4 phase transitions, leading them to exhibit multiple voltage plateaus upon Na⁺ extraction/insertion. The deficient sodium in the P2‐type cathode easily induces the bad structural stability at deep desodiation states and limited reversible capacity during Na⁺ de/insertion. These drawbacks cause poor rate capability and fast capacity decay in most P2‐type layered oxides. To address these challenges, a novel high sodium content (0.85) and plateau‐free P2‐type cathode‐Na_0.85Li_0.12Ni_0.22Mn_0.66O₂ (P2‐NLNMO) was developed. The complete solid‐solution reaction over a wide voltage range ensures both fast Na⁺ mobility (10^?11 to 10^?10 cm² s^?1) and small volume variation (1.7 %). The high sodium content P2‐NLNMO exhibits a higher reversible capacity of 123.4 mA h g^?1, superior rate capability of 79.3 mA h g^?1 at 20 C, and 85.4 % capacity retention after 500 cycles at 5 C. The sufficient Na and complete solid‐solution reaction are critical to realizing high‐performance P2‐type cathodes for sodium‐ion batteries.     

Electrochemical and Structural Study of Layered P2‐Type Na2/3Ni1/3Mn2/3O2 as Cathode Material for Sodium‐Ion Battery

P2‐type Na_2/3Ni_1/3Mn_2/3O₂ was synthesized by a controlled co‐precipitation method followed by a high‐temperature solid‐state reaction and was used as a cathode material for a sodium‐ion battery (SIB). The electrochemical behavior of this layered material was studied and an initial discharge capacity of 151.8 mA h g^?1 was achieved in the voltage range of 1.5–3.75 V versus Na⁺/Na. The retained discharge capacity was found to be 123.5 mA h g^?1 after charging/discharging 50 cycles, approximately 81.4 % of the initial discharge capacity. In situ X‐ray diffraction analysis was used to investigate the sodium insertion and extraction mechanism and clearly revealed the reversible structural changes of the P2‐Na_2/3Ni_1/3Mn_2/3O₂ and no emergence of the O2‐Ni_1/3Mn_2/3O₂ phase during the cycling test, which is important for designing stable and high‐performance SIB cathode materials.     

Layered LiNi<Subscript>0.80</Subscript>Co<Subscript>0.15</Subscript>Al<Subscript>0.05</Subscript>O<Subscript>2</Subscript> as cathode material for hybrid Li<Superscript>+</Superscript>/Na<Superscript>+</Superscript> batteries

LiNi_0.80Co_0.15Al_0.05O₂ (NCA) is explored to be applied in a hybrid Li⁺/Na⁺ battery for the first time. The cell is constructed with NCA as the positive electrode, sodium metal as the negative electrode, and 1 M NaClO₄ solution as the electrolyte. It is found that during electrochemical cycling both Na⁺ and Li⁺ ions are reversibly intercalated into/de-intercalated from NCA crystal lattice. The detailed electrochemical process is systematically investigated by inductively coupled plasma-optical emission spectrometry, ex situ X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The NCA cathode can deliver initially a high capacity up to 174 mAh g^?1 and 95% coulombic efficiency under 0.1 C (1 C?=?120 mA g^?1) current rate between 1.5–4.1 V. It also shows excellent rate capability that reaches 92 mAh g^?1 at 10 C. Furthermore, this hybrid battery displays superior long-term cycle life with a capacity retention of 81% after 300 cycles in the voltage range from 2.0 to 4.0 V, offering a promising application in energy storage.     

A High‐Voltage and Ultralong‐Life Sodium Full Cell for Stationary Energy Storage

Recently, there has been great interest in developing advanced sodium‐ion batteries for large‐scale application. Most efforts have concentrated on the search for high‐performance electrode materials only in sodium half‐cells. Research on sodium full cells for practical application has encountered many problems, such as insufficient cycles with rapid capacity decay, low safety, and low operating voltage. Herein, we present a layered P2‐Na_0.66Ni_0.17Co_0.17Ti_0.66O₂, as both an anode (ca. 0.69 V versus Na⁺/Na) and as a high‐voltage cathode (ca. 3.74 V versus Na⁺/Na). The full cell based on this bipolar electrode exhibits well‐defined voltage plateaus near 3.10 V, which is the highest average voltage in the symmetric cells. It also shows the longest cycle life (75.9 % capacity retention after 1000 cycles) in all sodium full cells, a usable capacity of 92 mAh g^?1, and superior rate capability (65 mAh g^?1 at a high rate of 2C).     

Zinc‐Reduced Mesoporous TiOx Li‐Ion Battery Anodes with Exceptional Rate Capability and Cycling Stability

We demonstrate a unique synthetic route for oxygen‐deficient mesoporous TiO_x by a redox–transmetalation process by using Zn metal as the reducing agent. The as‐obtained materials have significantly enhanced electronic conductivity; 20 times higher than that of as‐synthesized TiO₂ material. Moreover, electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) measurements are performed to validate the low charge carrier resistance of the oxygen‐deficient TiO_x. The resulting oxygen‐deficient TiO_x battery anode exhibits a high reversible capacity (～180 mA h g^?1 at a discharge/charge rate of 1 C/1 C after 400 cycles) and an excellent rate capability (～90 mA h g^?1 even at a rate of 10 C). Also, the full cell, which is coupled with a LiCoO₂ cathode material, exhibits an outstanding rate capability (>75 mA h g^?1 at a rate of 3.0 C) and maintains a reversible capacity of over 100 mA h g^?1 at a discharge/charge of 1 C/1 C for 300 cycles.     

Li+/Mg2+ Hybrid‐Ion Batteries with Long Cycle Life and High Rate Capability Employing MoS2 Nano Flowers as the Cathode Material

The demand for large‐scale and safe energy storage is increasing rapidly due to the strong push for smartphones and electric vehicles. As a result, Li⁺/Mg²⁺ hybrid‐ion batteries (LMIBs) combining a dendrite‐free deposition of Mg anode and Li⁺ intercalation cathode have attracted considerable attention. Here, a LMIB with hydrothermal‐prepared MoS₂ nano flowers as cathode material was prepared. The battery showed remarkable electrochemical properties with a large discharge capacity (243 mAh g^?1 at the 0.1 C rate), excellent rate capability (108 mAh g^?1 at the 5 C rate), and long cycle life (87.2 % capacity retention after 2300 cycles). Electrochemical analysis showed that the reactions occurring in the battery cell involved Mg stripping/plating at the anode side and Li⁺ intercalation at the cathode side with a small contribution from Mg²⁺ adsorption. The excellent electrochemical performance and extremely safe cell system show promise for its use in practical applications.     

Towards Highly Stable Storage of Sodium Ions: A Porous Na3V2(PO4)3/C Cathode Material for Sodium‐Ion Batteries

A porous Na₃V₂(PO₄)₃ cathode material coated uniformly with a layer of approximately 6 nm carbon has been synthesized by the sol–gel method combined with a freeze‐drying process. The special porous morphology and structure significantly increases the specific surface area of the material, which greatly enlarges the contact area between the electrode and electrolyte, and consequently supplies more active sites for sodium ions. When employed as a cathode material of sodium‐ion batteries, this porous Na₃V₂(PO₄)₃/C exhibits excellent rate performance and cycling stability; for instance, it shows quite a flat potential plateau at 3.4 V in the potential window of 2.7–4.0 V versus Na⁺/Na and delivers an initial capacity as high as 118.9 and 98.0 mA h g^?1 at current rates of 0.05 and 0.5 C, respectively, and after 50 cycles, a good capacity retention of 92.7 and 93.6 % are maintained. Moreover, even when the discharge current density is increased to 5 C (590 mA g^?1), an initial capacity of 97.6 mA h g^?1 can still be achieved, and an exciting capacity retention of 88.6 % is obtained after 100 cycles. The good cycle performance, excellent rate capability, and moreover, the low cost of Na₃V₂(PO₄)₃/C suggest that this material is a promising cathode for large‐scale sodium‐ion rechargeable batteries.     

Single Crystalline Na0.7MnO2 Nanoplates as Cathode Materials for Sodium‐Ion Batteries with Enhanced Performance

Single crystalline rhombus‐shaped Na_0.7MnO₂ nanoplates have been synthesized by a hydrothermal method. TEM and HRTEM analyses revealed that the Na_0.7MnO₂ single crystals predominantly exposed their (100) crystal plane, which is active for Na⁺‐ion insertion and extraction. When applied as cathode materials for sodium‐ion batteries, Na_0.7MnO₂ nanoplates exhibited a high reversible capacity of 163 mA h g^?1, a satisfactory cyclability, and a high rate performance. The enhanced electrochemical performance could be ascribed to the predominantly exposed active (100) facet, which could facilitate fast Na⁺‐ion insertion/extraction during the discharge and charge process.     

Electrochemical Properties and Sodium‐Storage Mechanism of Ag2Mo2O7 as the Anode Material for Sodium‐Ion Batteries

Silver molybdate, Ag₂Mo₂O₇, has been prepared by a conventional solid‐state reaction. Its electrochemical properties as an anode material for sodium‐ion batteries (SIBs) have been comprehensively examined by means of galvanostatic charge–discharge cycling, cyclic voltammetry, and rate performance measurements. At operating voltages between 3.0 and 0.01 V, the electrode delivered a reversible capacity of nearly 190 mA h g^?1 at a current density of 20 mA g^?1 after 70 cycles. Ag₂Mo₂O₇ also demonstrated a good rate capability and long‐term cycle stability, the capacity reaching almost 100 mA h g^?1 at a current density of 500 mA g^?1, with a capacity retention of 55 % over 1000 cycles. Moreover, the sodium storage process of Ag₂Mo₂O₇ has been investigated by means of ex situ XRD, Raman spectroscopy, and HRTEM. Interestingly, the anode decomposes into Ag metal and Na₂MoO₄ during the initial discharge process, and then Na⁺ ions are considered to be inserted into/extracted from the Na₂MoO₄ lattice in the subsequent cycles governed by an intercalation/deintercalation mechanism. Ex situ HRTEM images revealed that Ag metal not only remains unchanged during the sodiation/desodiation processes, but is well dispersed throughout the amorphous matrix, thereby greatly improving the electronic conductivity of the working electrode. The “in situ” decomposition behavior of Ag₂Mo₂O₇ is distinct from that of chemically synthesized, metal‐nanoparticle‐coated electrode materials, and provides strong supplementary insight into the mechanism of such new anode materials for SIBs and may set a precedent for the design of further materials.     

An Ultrastable Anode for Long‐Life Room‐Temperature Sodium‐Ion Batteries

Sodium‐ion batteries are important alternative energy storage devices that have recently come again into focus for the development of large‐scale energy storage devices because sodium is an abundant and low‐cost material. However, the development of electrode materials with long‐term stability has remained a great challenge. A novel negative‐electrode material, a P2‐type layered oxide with the chemical composition Na_2/3Co_1/3Ti_2/3O₂, exhibits outstanding cycle stability (ca. 84.84 % capacity retention for 3000 cycles, very small decrease in the volume (0.046 %) after 500 cycles), good rate capability (ca. 41 % capacity retention at a discharge/charge rate of 10 C), and a usable reversible capacity of about 90 mAh g^?1 with a safe average storage voltage of approximately 0.7 V in the sodium half‐cell. This P2‐type layered oxide is a promising anode material for sodium‐ion batteries with a long cycle life and should greatly promote the development of room‐temperature sodium‐ion batteries.     

Ruthenium‐Oxide‐Coated Sodium Vanadium Fluorophosphate Nanowires as High‐Power Cathode Materials for Sodium‐Ion Batteries

Sodium‐ion batteries are a very promising alternative to lithium‐ion batteries because of their reliance on an abundant supply of sodium salts, environmental benignity, and low cost. However, the low rate capability and poor long‐term stability still hinder their practical application. A cathode material, formed of RuO₂‐coated Na₃V₂O₂(PO₄)₂F nanowires, has a 50 nm diameter with the space group of I4/mmm. When used as a cathode material for Na‐ion batteries, a reversible capacity of 120 mAh g^?1 at 1 C and 95 mAh g^?1 at 20 C can be achieved after 1000 charge–discharge cycles. The ultrahigh rate capability and enhanced cycling stability are comparable with high performance lithium cathodes. Combining first principles computational investigation with experimental observations, the excellent performance can be attributed to the uniform and highly conductive RuO₂ coating and the preferred growth of the (002) plane in the Na₃V₂O₂(PO₄)₂F nanowires.     

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.     

Realizing Complete Solid-Solution Reaction in High Sodium Content P2-Type Cathode for High-Performance Sodium-Ion Batteries

P2-type layered oxides suffer from an ordered Na⁺/vacancy arrangement and P2→O2/OP4 phase transitions, leading them to exhibit multiple voltage plateaus upon Na⁺ extraction/insertion. The deficient sodium in the P2-type cathode easily induces the bad structural stability at deep desodiation states and limited reversible capacity during Na⁺ de/insertion. These drawbacks cause poor rate capability and fast capacity decay in most P2-type layered oxides. To address these challenges, a novel high sodium content (0.85) and plateau-free P2-type cathode-Na_0.85Li_0.12Ni_0.22Mn_0.66O₂ (P2-NLNMO) was developed. The complete solid-solution reaction over a wide voltage range ensures both fast Na⁺ mobility (10⁻¹¹ to 10⁻¹⁰ cm² s⁻¹) and small volume variation (1.7 %). The high sodium content P2-NLNMO exhibits a higher reversible capacity of 123.4 mA h g⁻¹, superior rate capability of 79.3 mA h g⁻¹ at 20 C, and 85.4 % capacity retention after 500 cycles at 5 C. The sufficient Na and complete solid-solution reaction are critical to realizing high-performance P2-type cathodes for sodium-ion batteries.     

Enhancing Capacity Performance by Utilizing the Redox Chemistry of the Electrolyte in a Dual‐Electrolyte Sodium‐Ion Battery

A strategy is described to increase charge storage in a dual electrolyte Na‐ion battery (DESIB) by combining the redox chemistry of the electrolyte with a Na⁺ ion de‐insertion/insertion cathode. Conventional electrolytes do not contribute to charge storage in battery systems, but redox‐active electrolytes augment this property via charge transfer reactions at the electrode–electrolyte interface. The capacity of the cathode combined with that provided by the electrolyte redox reaction thus increases overall charge storage. An aqueous sodium hexacyanoferrate (Na₄Fe(CN)₆) solution is employed as the redox‐active electrolyte (Na‐FC) and sodium nickel Prussian blue (Na_x‐NiBP) as the Na⁺ ion insertion/de‐insertion cathode. The capacity of DESIB with Na‐FC electrolyte is twice that of a battery using a conventional (Na₂SO₄) electrolyte. The use of redox‐active electrolytes in batteries of any kind is an efficient and scalable approach to develop advanced high‐energy‐density storage systems.     

Cu3V2O8 Nanoparticles as Intercalation‐Type Anode Material for Lithium‐Ion Batteries

Cu₃V₂O₈ nanoparticles with particle sizes of 40–50 nm have been prepared by the co‐precipitation method. The Cu₃V₂O₈ electrode delivers a discharge capacity of 462 mA h g^?1 for the first 10 cycles and then the specific capacity, surprisingly, increases to 773 mA h g^?1 after 50 cycles, possibly as a result of extra lithium interfacial storage through the reversible formation/decomposition of a solid electrolyte interface (SEI) film. In addition, the electrode shows good rate capability with discharge capacities of 218 mA h g^?1 under current densities of 1000 mA g^?1. Moreover, the lithium storage mechanism for Cu₃V₂O₈ nanoparticles is explained on the basis of ex situ X‐ray diffraction data and high‐resolution transmission electron microscopy analyses at different charge/discharge depths. It was evidenced that Cu₃V₂O₈ decomposes into copper metal and Li₃VO₄ on being initially discharged to 0.01 V, and the Li₃VO₄ is then likely to act as the host for lithium ions in subsequent cycles by means of the intercalation mechanism. Such an “in situ” compositing phenomenon during the electrochemical processes is novel and provides a very useful insight into the design of new anode materials for application in lithium‐ion batteries.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

P2‐Na0.67AlxMn1−xO2: Cost‐Effective,Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na_0.67MnO₂ materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn⁴⁺/Mn³⁺ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn³⁺, we obtain the low cost pure P2‐type Na_0.67Al_xMn_1?xO₂ (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na⁺ layer thus producing a remarkable rate capability of 96 mAh g^‐1 at 1200 mA g^‐1.     

High Crystalline Prussian White Nanocubes as a Promising Cathode for Sodium‐ion Batteries

Prussian blue and its analogues (PBAs) have been recognized as one of the most promising cathode materials for room‐temperature sodium‐ion batteries (SIBs). Herein, we report high crystalline and Na‐rich Prussian white Na₂CoFe(CN)₆ nanocubes synthesized by an optimized and facile co‐precipitation method. The influence of crystallinity and sodium content on the electrochemical properties was systematically investigated. The optimized Na₂CoFe(CN)₆ nanocubes exhibited an initial capacity of 151 mA h g^?1, which is close to its theoretical capacity (170 mA h g^?1). Meanwhile, the Na₂CoFe(CN)₆ cathode demonstrated an outstanding long‐term cycle performance, retaining 78 % of its initial capacity after 500 cycles. Furthermore, the Na₂CoFe(CN)₆ Prussian white nanocubes also achieved a superior rate capability (115 mA h g^?1 at 400 mA g^?1, 92 mA h g^?1 at 800 mA g^?1). The enhanced performances could be attributed to the robust crystal structure and rapid transport of Na ions through large channels in the open‐framework. Most noteworthy, the as‐prepared Na₂CoFe(CN)₆ nanocubes are not only low‐cost in raw materials but also contain a rich sodium content (1.87 Na ions per lattice unit cell), which will be favorable for full cell fabrication and large‐scale electric storage applications.     

Mitigation of Jahn–Teller distortion and Na+/vacancy ordering in a distorted manganese oxide cathode material by Li substitution

Layered manganese-based oxides are promising candidates as cathode materials for sodium-ion batteries (SIBs) due to their low cost and high specific capacity. However, the Jahn–Teller distortion from high-spin Mn³⁺ induces detrimental lattice strain and severe structural degradation during sodiation and desodiation. Herein, lithium is introduced to partially substitute manganese ions to form distorted P′2-Na_0.67Li_0.05Mn_0.95O₂, which leads to restrained anisotropic change of Mn–O bond lengths and reinforced bond strength in the [MnO₆] octahedra by mitigation of Jahn–Teller distortion and contraction of MnO₂ layers. This ensures the structural stability during charge and discharge of P′2-Na_0.67Li_0.05Mn_0.95O₂ and Na⁺/vacancy disordering for facile Na⁺ diffusion in the Na layers with a low activation energy barrier of ∼0.53 eV. It exhibits a high specific capacity of 192.2 mA h g⁻¹, good cycling stability (90.3% capacity retention after 100 cycles) and superior rate capability (118.5 mA h g⁻¹ at 1.0 A g⁻¹), as well as smooth charge/discharge profiles. This strategy is effective to tune the crystal structure of layered oxide cathodes for SIBs with high performance.

Li-Substitution in P′2-Na_0.67MnO₂ mitigates the anisotropic change of Mn–O bonds and Na/vacancy ordering, and hence significantly promotes its cycling stability and rate capability as a cathode material for sodium-ion batteries.     

Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries

We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na_0.7MnO_2?+?z and Na_0.7Co_0.11Mn_0.89O_2?+?z via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5–3.8 V vs. Na | Na⁺ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA?h?g^?1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA?h?g^?1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na_0.7MnO_2?+?z as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material.