Large-Scale Synthesis of the Stable Co-Free Layered Oxide Cathode by the Synergetic Contribution of Multielement Chemical Substitution for Practical Sodium-Ion Battery

A Universal Strategy Based on Bridging Microstructure Engineering and Local Electronic Structure Manipulation for High-Performance Sodium Layered Oxide Cathodes

Due to their high capacity and sufficient Na+ storage, O3-NaNi0.5Mn0.5O2 has attracted much attention as a viable cathode material for sodium-ion batteries (SIBs). However, the challenges of complicated irreversible multiphase transitions, poor structural stability, low operating voltage, and an unstable oxygen redox reaction still limit its practical application. Herein, using O3-NaNi0.5Mn0.5-xSnxO2 cathode materials as the research model, a universal strategy based on bridging microstructure engineering and local electronic structure manipulation is proposed. The strategy can modulate the physical and chemical properties of electrode materials, so as to restrain the unfavorable and irreversible multiphase transformation, improve structural stability, manipulate redox potential, and stabilize the anion redox reaction. The effect of Sn substitution on the intrinsic local electronic structure of the material is articulated by density functional theory calculations. Meanwhile, the universal strategy is also validated by Ti substitution, which could be further extrapolated to other systems and guide the design of cathode materials in the field of SIBs.

High-Voltage Cyclic Ether-Based Electrolytes for Low-Temperature Sodium-Ion Batteries

Cyclic ethers are promising solvents for low-temperature electrolytes, but they still suffer from intrinsic poor antioxidant abilities. Until now, ether-based electrolytes have been rarely reported for high-voltage sodium-ion batteries (SIBs) operated under a low-temperature range. Herein, a novel ether-based electrolyte consisting of tetrahydrofuran as the main solvent is proposed and it could be utilized for a high-voltage Na2/3Mn2/3Ni1/3O2 (MN) cathode in a wide-temperature range from -40 to 25 °C. Meanwhile, a thin and robust inorganic component-rich cathode electrolyte interface layer is elaborately introduced on the MN cathode by this tailored electrolyte, resulting in excellent cycle life of MN cathode. Specifically, a capacity retention of 97.2% after 140 cycles could be delivered by MN at 0.3 C at room temperature (RT). Especially at an ultra-low temperature of -40 °C, the initial discharge capacity of MN could still approach 89.3% of that at RT, and the capacity retention is 94.1% at 0.2 C after 100 cycles. This work provides a new insight into the rational design of ether-based electrolytes for high-voltage and stable SIBs operated in a wide-temperature range.

Intercalation Reaction in Amorphous Layer-Wrapped Ni0.2Mo0.8N/Ni3N Heterostructure Toward Efficient Lithium-Ion Storage

Transition metal nitrides (TMNs) with high specific capacity and electric conductivity have drawn considerable attention as electrode materials of lithium-ion batteries (LIBs). However, the cycling stability of most TMNs is not satisfactory, which was caused by the large volume variation during cycles due to their intrinsic conversion reaction mechanism. Herein, by rational design, a much stable tremella-like Ni_0.2Mo_0.8N/Ni₃N heterostructure with amorphous Ni_0.2Mo_0.8N wrapped layer has been fabricated. The Ni₃N particles worked as pillars to support the Ni_0.2Mo_0.8N material as well as conductive medium to facilitate ionic and electronic transport. The amorphous layer can relieve the structural stress of Ni_0.2Mo_0.8N during cycles. Moreover, an exotic intercalation-type reaction mechanism in the ternary nitride Ni_0.2Mo_0.8N was revealed by a series ex situ and in situ characterization. Profiting from these advantages, the Ni_0.2Mo_0.8N/Ni₃N heterostructure anode displays an outstanding electrochemical performance with a high initial reversible discharge capacity of 1001.6 mA h g^-1 at 0.1 A g^-1, excellent cycle stability of 695.5 mA h g^-1 at 2 A g^-1 after 600 cycles, and superior rate capability of 595.3 mA h g^-1 at a high current density of 5 A g^-1. This work provides a new insight for designing high efficiency LIBs based on intercalation reaction for practical applications.

Formulating High-Rate and Long-Cycle Heterostructured Layered Oxide Cathodes by Local Chemistry and Orbital Hybridization Modulation for Sodium-Ion Batteries

It is still very urgent and challenging to simultaneously develop high-rate and long-cycle oxide cathodes for sodium-ion batteries (SIBs) because of the sluggish kinetics and complex multiphase evolution during cycling. Here, the concept of accurately manipulating structural evolution and formulating high-performance heterostructured biphasic layered oxide cathodes by local chemistry and orbital hybridization modulation is reported. The P2-structure stoichiometric composition of the cathode material shows a layered P2- and O3-type heterostructure that is explicitly evidenced by various macroscale and atomic-scale techniques. Surprisingly, the heterostructured cathode displays excellent rate performance, remarkable cycling stability (capacity retention of 82.16% after 600 cycles at 2 C), and outstanding compatibility with hard carbon anode because of the integrated advantages of intergrowth structure and local environment regulation. Meanwhile, the formation process from precursors during calcination and the highly reversible dynamic structural evolution during the Na⁺ intercalation/deintercalation process are clearly articulated by a series of in situ characterization techniques. Also, the intrinsic structural properties and corresponding electrochemical behavior are further elucidated by the density of states and electron localization function of density functional theory calculations. Overall, this strategy, which finely tunes the local chemistry and orbitals hybridization for high-performance SIBs, will open up a new field for other materials.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.

Expanding the ReS2 Interlayer Promises High-Performance Potassium-Ion Storage

Improving the electrochemical kinetics and the intrinsic poor conductivity of transition metal dichalcogenide (TMD) electrodes is meaningful for developing next-generation energy storage systems. As one of the most promising TMD anode materials, ReS₂ shows attractive performance in potassium-ion batteries (PIBs). To overcome the poor kinetic ion diffusion and limited cycling stability of the ReS₂-based electrode, herein, the interlayer distance expanding strategy was employed, and reduced graphene oxide (rGO) was introduced into ReS₂. Few-layered ReS₂ nanosheets were grown on the surface of the rGO with expanded interlayer distance. The prepared ReS₂ nanosheets show an expanded distance (∼0.77 nm). The synthesized EI-ReS₂@rGO composites were used in PIBs as anode materials. The K-ion storage mechanism of the ReS₂-based anode was investigated by in situ X-ray diffraction (XRD) technology, which shows the intercalation and conversion types. The EI-ReS₂@rGO nanocomposites show high specific capacities of 432.5, 316.5, and 241 mAh g^-1 under 0.05, 0.2, and 1.0 A g^-1 current densities and exhibit excellent reversibility at 1.0 A g^-1. Overall, this strategy, which finely tunes the local chemistry and orbital hybridization for high-performance PIBs, will open up a new field for other materials.

Validating the Structural (In)stability of P3- and P2-Na0.67Mg0.1Mn0.9O2-Layered Cathodes for Sodium-Ion Batteries: A Time-Decisive Approach

In this study, magnesium-ion-substituted, sodium-deficient, P3- and P2-layered manganese oxide cathodes (Na_0.67Mg_0.1Mn_0.9O₂) were synthesized through a facile polyol-assisted combustion technique for applications in sodium-ion batteries. The electrochemical reaction pathways, structural integrity, and long cycling ability at low current rates of the P3- and P2-phases of the Na_0.67Mg_0.1Mn_0.9O₂ cathodes were investigated using time-consuming techniques, such as galvanostatic titration and series cyclic voltammetry. The results obtained from these techniques were supported by those obtained from operando X-ray diffraction (XRD) analysis. Particularly, the P2-phase provided excellent structural stability owing to its intrinsic crystal structure, thereby exhibiting a reversible capacity retention of 82.6% after 262 cycles at a low rate of 0.1 C; in contrast, the P3-phase exhibited a capacity retention of 38.7% after 241 cycles at a similar current rate. The air stability of these as-prepared powders, which were stored under ambient conditions, was progressively analyzed over a period of 6 months through XRD without conducting any special experiments. The results suggest that in the P3-phase, the formation of NaHCO₃ and hydrated phase impurities, resulting from Na⁺/H⁺ exchange and hydration reactions, respectively, was likely to occur more quickly, that is, within a few days, compared to that in the P2-phase.

Synergistic Effect of Microstructure Engineering and Local Crystal Structure Tuning to Improve the Cycling Stability of Ni-Rich Cathodes

Ultrahigh Ni-rich layered oxides have been regarded as one of the most promising cathode candidates. However, cycling instability induced by interfacial reactions and irreversible H2-H3 lattice distortion is yet to be demonstrated by an effective strategy that could construct a stable grain interface and microstructure. Here, Ni-rich cathode LiNi_0.92Co_0.05Mn_0.03O₂ is modified by B and Ti to realize the synchronous regulation of a microstructure and the oxygen framework robustness. Compared with the large equiaxed crystalline grains for the pristine cathode, highly elongated grains with a strong radially oriented crystallographic texture in which the (003) facet is maximized are produced for Ti and B-modified LiNi_0.92Co_0.05Mn_0.03O₂. With the suppressed H2-H3 phase transition and cation mixing provided by radially oriented grains and turned local crystal oxygen framework robustness during cycling, the co-modified cathode exhibits enhanced Li⁺ diffusion kinetics and a capacity retention of 78.3% after 100 cycles, which outperformed the 38.5% for the pristine cathode. The improved cycling performance suggests the significance of the turned microstructure and local crystal structure in suppressing internal strain and crystal structure degradation. The synchronous realization of microstructure engineering and local crystal structure turning by optimal element combination would provide a heuristic solution for the construction of high perform Ni-rich cathodes.

Copper and Zirconium Codoped O3-Type Sodium Iron and Manganese Oxide as the Cobalt/Nickel-Free High-Capacity and Air-Stable Cathode for Sodium-Ion Batteries

Considering the abundance of iron and manganese within the Earth's crust, the cathode O3-NaFe_0.5Mn_0.5O₂ has shown great potential for large-scale energy storage. Following the strategy of introducing specific heteroelements to optimize the structural stability for energy storage, the work has obtained an O3-type NaFe_0.4Mn_0.49Cu_0.1Zr_0.01O₂ that exhibits enhanced electrochemical performance and air stability. It displays an initial reversible capacity of 147.5 mAh g^-1 at 0.1C between 2 and 4.1 V, a capacity retention ratio exceeding 69.6% after 100 cycles at 0.2C, and a discharge capacity of 70.8 mAh g^-1 at a high rate of 5C, which is superior to that of O3-NaFe_0.5Mn_0.5O₂. The codoping of Cu/Zr reserves the layered O3 structure and enlarges the interlayer spacing, promoting the diffusion of Na⁺. In addition, the structural stability and air stability observed by Cu-doping is well maintained via the incorporation of extra Zr favoring a highly reversible phase conversion process. Thus, this work has demonstrated an efficient strategy for developing cobalt/nickel-free high-capacity and air-stable cathodes for sodium-ion batteries.

Tailoring a Phenolic Resin Precursor by Facile Pre-oxidation Tactics to Realize a High-Initial-Coulombic-Efficiency Hard Carbon Anode for Sodium-Ion Batteries

As the leading anode material for sodium-ion batteries (SIBs), hard carbon (HC) still faces the puzzle of low initial Coulombic efficiency (ICE) in achieving commercialization. From the perspective of precursors, the low ICE has been attributed to the large specific surface area and porosity produced by the rapid decomposition of polymers during the carbonization. Therefore, increasing the cross-linking degree of precursors will be an effective shortcut to improve the ICE. Herein, a facile pre-oxidation tactic was successfully employed to tailor the cross-linking degree of phenolic resin precursors to precisely control the specific surface area of the obtained HC. As the pre-oxidation time is increased, the optimal HC with the lowest specific surface area shows an ICE elevated by 22.2% (from 62.5 to 84.7%) compared to the original pre-oxidation HC and delivers a high reversible capacity of 334.3 mAh g^-1 at 20 mA g^-1. Besides, the pre-oxidation also introduces abundant carbonyl groups, which increase the disorder degree of HC and supply abundant adsorption sites of Na⁺, thus enhancing the rate performance. When matched with a layered O3-NaNi_1/3Fe_1/3Mn_1/3O₂ cathode, the full cell achieves an energy density of ca. 256.2 Wh kg^-1 with superior rate performance. This work sheds light on the positive effect of pre-oxidation in elevating the ICE of HC and provides effective guidance to achieve a high ICE for other HC materials.

A P3-Type K1/2Mn5/6Mg1/12Ni1/12O2 Cathode Material for Potassium-Ion Batteries with High Structural Reversibility Secured by the Mg-Ni Pinning Effect

Mn-based layered oxides are very attractive as cathodes for potassium-ion batteries (PIBs) due to their low-cost and environmentally friendly precursors. Their transfer to practical application, however, is inhibited by some issues including consecutive phase transitions, sluggish K⁺ deintercalation/intercalation, and serious capacity loss. Herein, Mg-Ni co-substituted K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ is designed as a promising cathode material for PIBs, with suppressed phase transitions that occurred in K_1/2MnO₂ and improved K⁺ storage performance. Part of Mg²⁺ and Ni²⁺ occupies the K⁺ layer, playing the role of a "nailed pillar", which restrains metal oxide layer gliding during the K⁺ (de)intercalation. The "Mg-Ni pinning effect" not only suppresses the phase transitions but also reduces the cell volume variation, leading to the improved cycle performance. Moreover, K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ has low activation barrier energy for K⁺ diffusion and high electron conductivity as demonstrated by first-principles calculations, resulting in better rate capability. In addition, K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ also delivers a higher reversible capacity owing to the participation of the Ni element in electrochemical reactions and the pseudocapacitive contribution. This study provides a basic understanding of structural evolution in layered Mn-based oxides and broadens the strategic design of cathode materials for PIBs.

Micro/Nano Na3V2(PO4)3/N-Doped Carbon Composites with a Hierarchical Porous Structure for High-Rate Pouch-Type Sodium-Ion Full-Cell Performance

Polyanion-type Na₃V₂(PO₄)₃ (NVP) is an overwhelmingly attractive cathode material for sodium-ion batteries (SIBs) because of its high structural stability and fast Na⁺ mobility. However, its practical application is strongly plagued by either nanoscale particle size or poor rate performance. Herein, a micro/nanocomposite NVP cathode with a hierarchical porous structure is proposed to solve the problem. The microscale NVP material assembled by interconnected nanoflakes with N-doped carbon coating that is capable of simultaneously providing fast carrier transmission dynamics and outstanding structural integrity exhibits precedent sodium-storage behavior. It delivers a superior rate capability (79.1 mAh g^-1 at 200C) and excellent long-life cycling (capacity retention of 73.4% after 10 000 cycles at 100C). Remarkably, a pouch-type sodium-ion full cell consisting of the as-obtained NVP cathode and a hard carbon anode demonstrates the gravimetric energy density as high as 212 Wh kg^-1 and an exceptional rate performance (71.8 mAh g^-1 at 10C). Such structural design of fabricating micro/nanocomposite electrode materials is expected to accelerate the practical applications of SIBs for large-scale energy storage.