Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g^-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g^-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF₆ electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg^-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na⁺ kinetics facilitating Na⁺ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na⁺ kinetics, boosting the overall Na storage.

Layered-Oxide Cathode Materials for Fast-Charging Lithium-Ion Batteries: A Review

Layered oxides are considered prospective state-of-the-art cathode materials for fast-charging lithium-ion batteries (LIBs) owning to their economic effectiveness, high energy density, and environmentally friendly nature. Nonetheless, layered oxides experience thermal runaway, capacity decay, and voltage decay during fast charging. This article summarizes various modifications recently implemented in the fast charging of LIB cathode materials, including component improvement, morphology control, ion doping, surface coating, and composite structure. The development direction of layered-oxide cathodes is summarized based on research progress. Further, possible strategies and future development directions of layered-oxide cathodes to improve fast-charging performance are proposed.

Exploiting Cation Intercalating Chemistry to Catalyze Conversion-Type Reactions in Batteries

Effective harvest of electrochemical energy from insulating compounds serves as the key to unlocking the potential capacity from many materials that otherwise could not be exploited for energy storage. Herein, an effective strategy is proposed by employing LiCoO₂, a widely commercialized positive electrode material in Li-ion batteries, as an efficient redox mediator to catalyze the decomposition of Na₂CO₃ via an intercalating mechanism. Differing from traditional redox mediation processes where reactions occur on the limited surface sites of catalysts, the electrochemically delithiated Li_1-xCoO₂ forms Na_yLi_1-xCoO₂ crystals, which act as a cation intercalating catalyzer that directs Na⁺ insertion-extraction and activates the reaction of Na₂CO₃ with carbon. Through altering the route of the mass transport process, such redox centers are delocalized throughout the bulk of LiCoO₂, which ensures maximum active reaction sites. The decomposition of Na₂CO₃ thus accelerated significantly reduces the charging overpotential in Na-CO₂ batteries; meanwhile, Na compensation can also be achieved for various Na-deficient cathode materials. Such a surface-induced catalyzing mechanism for conversion-type reactions, realized via cation intercalation chemistry, expands the boundary for material discovery and makes those conventionally unfeasible a rich source to explore for efficient utilization of chemical energy.

Facile Synthesis of O3-Type NaNi0.5Mn0.5O2 Single Crystals with Improved Performance in Sodium-Ion Batteries

Sodium-ion batteries can be a practical alternative to lithium-ion batteries due to the relatively high abundance of sodium and the projected scarcity of lithium. Both of these factors are critical considerations for grid-scale energy storage, but the central challenge to implementing sodium layered oxides in sodium-ion batteries is their relatively poor cycle life. Single-crystal particles with micrometer size can mitigate several failure mechanisms related to sodium layered oxides and can improve performance when compared to the commonly used polycrystalline particles. This work demonstrates a novel two-step molten-salt synthesis method using sodium chloride and metal oxides to form "single crystals" of a mixed-phase, spinel/rock-salt intermediate that crystallizes as micron-sized truncated octahedra. The mixed-phase spinel/rock-salt material is effectively used as a precursor to form O3-type NaNi_0.5Mn_0.5O₂ with large primary particles and substantially improved cycle life. This synthesis route offers the added benefit of using simple metal oxides instead of hydroxide precursors, eliminating the need for coprecipitation. Particle morphology is found to be a critical factor in mitigating the structural damages incurred during phase transformations and maintaining the electrochemical performance.

Ba1/3CoO2: A Thermoelectric Oxide Showing a Reliable ZT of ∼0.55 at 600 °C in Air

Thermoelectric energy conversion technology has attracted attention as an energy harvesting technology that converts waste heat into electricity by means of the Seebeck effect. Oxide-based thermoelectric materials that show a high figure of merit are promising because of their good chemical and thermal stability as well as their harmless nature compared to chalcogenide-based state-of-the-art thermoelectric materials. Although several high-ZT thermoelectric oxides (ZT > 1) have been reported thus far, the reliability is low due to a lack of careful observation of their stability at elevated temperatures. Here, we show a reliable high-ZT thermoelectric oxide, Ba1/3CoO2. We fabricated Ba1/3CoO2 epitaxial films by the reactive solid-phase epitaxy method (Na3/4CoO2) followed by ion exchange (Na+ → Ba2+) treatment and performed thermal annealing of the film at high temperatures and structural and electrical measurements. The crystal structure and electrical resistivity of the Ba1/3CoO2 epitaxial films were found to be maintained up to 600 °C. The power factor gradually increased to ∼1.2 mW m-1 K-2 and the thermal conductivity gradually decreased to ∼1.9 W m-1 K-1 with increasing temperature up to 600 °C. Consequently, the ZT reached ∼0.55 at 600 °C in air.

Stable Sodium-Based Batteries with Advanced Electrolytes and Layered-Oxide Cathodes

Sodium-ion batteries offer a promising alternative to the more expensive, resource-limited lithium-ion batteries, in particular to accommodate the growing demand for large-scale energy storage. One of the main challenges for sodium-ion batteries, however, is the poor electrolyte stability, which leads to rapid capacity fade during cycling. Recent advances in the lithium-ion-battery field have expanded our understanding of electrolyte compositions and stability, paving the way for better sodium-ion-battery electrolytes. Two of the most promising new classes of electrolytes are evaluated herein with a sodium layered-oxide cathode, for the first time: a localized high-concentration electrolyte (LHCE) composed of sodium bis(fluorosulfonyl)imide, dimethyl ether, and tetrafluoropropyl ether and a "highly fluorinated" electrolyte (HFE) composed of 20% fluoroethylene carbonate with a lithium difluorophosphate additive. With a combination of electrochemical and post-mortem characterization techniques, the stability of each electrolyte is assessed with the O3-type Na(Ni_0.3Fe_0.4Mn_0.3)O₂ cathode and sodium metal anode. Both electrolytes significantly improve the surface and bulk stability of the cathode, but only the LHCE has a meaningful improvement on sodium metal stability. For the purpose of developing a long-lasting, sodium-ion full cell, both classes of electrolyte show great promise.

Reactivity at the Electrode-Electrolyte Interfaces in Li-Ion and Gel Electrolyte Lithium Batteries for LiNi0.6Mn0.2Co0.2O2 with Different Particle Sizes

The layered oxide LiNi_0.6Mn_0.2Co_0.2O₂ is a very attractive positive electrode material, as shown by the good reversible capacity, chemical stability, and cyclability upon long-range cycling in Li-ion batteries and, hopefully, in the near future, in all-solid-state batteries. Three samples with variable primary particle sizes of 240 nm, 810 nm, and 2.1 μm on average and very similar structures close to the ideal 2D layered structure (less than 2% Ni²⁺ ions in Li⁺ sites) were obtained by coprecipitation followed by a solid-state reaction at high temperatures. The electrochemical performances of the materials were evaluated in a conventional organic liquid electrolyte in Li-ion batteries and in a gel electrolyte in all-solid-state batteries. The positive electrode/electrolyte interface was analyzed by X-ray photoelectron spectroscopy to determine its composition and the extent of degradation of the lithium salt and the carbonate solvents after cycling, taking into account the changes in particle size of the positive electrode material and the nature of the electrolyte.

Effect of Cationic (Na+) and Anionic (F-) Co-Doping on the Structural and Electrochemical Properties of LiNi1/3Mn1/3Co1/3O2 Cathode Material for Lithium-Ion Batteries

Elemental doping for substituting lithium or oxygen sites has become a simple and effective technique to improve the electrochemical performance of layered cathode materials. Compared with single-element doping, this work presents an unprecedented contribution to the study of the effect of Na⁺/F⁻ co-doping on the structure and electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂. The co-doped Li_1-zNa_zNi_1/3Mn_1/3Co_1/3O_2-zF_z (z = 0.025) and pristine LiNi_1/3Co_1/3Mn_1/3O₂ materials were synthesized via the sol–gel method using EDTA as a chelating agent. Structural analyses, carried out by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy, revealed that the Na⁺ and F⁻ dopants were successfully incorporated into the Li and O sites, respectively. The co-doping resulted in larger Li-slab spacing, a lower degree of cation mixing, and the stabilization of the surface structure, which substantially enhanced the cycling stability and rate capability of the cathode material. The Na/F co-doped LiNi_1/3Mn_1/3Co_1/3O₂ electrode delivered an initial specific capacity of 142 mAh g⁻¹ at a 1C rate (178 mAh g⁻¹ at 0.1C), and it maintained 50% of its initial capacity after 1000 charge–discharge cycles at a 1C rate.

Accelerated Degradation in a Quasi-Single-Crystalline Layered Oxide Cathode for Lithium-Ion Batteries Caused by Residual Grain Boundaries

The rapidly growing demand of electrical vehicles (EVs) requires high-energy-density lithium-ion batteries (LIBs) with excellent cycling stability and safety performance. However, conventional polycrystalline high-Ni cathodes severely suffer from intrinsic chemomechanical degradation and fast capacity fade. The emerging single-crystallization strategy offers a promising pathway to improve the cathode's chemomechanical stability; however, the single-crystallinity of the cathode is not always guaranteed, and residual grain boundaries (GBs) could persist in nonideal synthesis conditions, leading to the formation of "quasi-single-crystalline" (QSC) cathodes. So far, there has been a lack of understanding of the influence of these residual GBs on the electrochemical performance and structural stability. Herein, we investigate the degradation pathway of a QSC high-Ni cathode through transmission electron microscopy and X-ray techniques. The residual GBs caused by insufficient calcination time dramatically exacerbate the cathode's chemomechanical instability and cycling performance. Our work offers important guidance for next-generation cathodes for long-life LIBs.

Unusual Site-Selective Doping in Layered Cathode Strengthens Electrostatic Cohesion of Alkali-Metal Layer for Practicable Sodium-Ion Full Cell

P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ is a dominant cathode material for sodium-ion batteries due to its high theoretical capacity and energy density. However, charging P2-type Na_0.67 Ni_0.33 Mn_0.67 O₂ to voltages higher than 4.2 V (vs. Na⁺ /Na) can induce detrimental structural transformation and severe capacity fading. Herein, stable cycling and moisture resistancy of Na_0.67 Ni_0.33 Mn_0.67 O₂ at 4.35 V (vs. Na⁺ /Na) are achieved through dual-site doping with Cu ion at transition metal site (2a) and unusual Zn ion at Na site (2d) for the first time. The Cu ion doping in 2a site stabilizes the metal layer, while more importantly, the unusual alkali-metal site doping by Zn ion serves as O^2- Zn²⁺ O^2- "pillar" for enhancing electrostatic cohesion between two adjacent transition metal layers, preventing the crack of active material along the a-b-plane and restraining the generation of O2 phase upon deep desodiation. This unique dual-site-doped [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ cathode exhibits a prominent cyclability with 80.6% capacity retention over 2000 cycles at an ultrahigh rate of 10C, demonstrating its great potential for practical applications. Impressively, the full cell devices with [Na_0.67 Zn_0.05 ]Ni_0.18 Cu_0.1 Mn_0.67 O₂ and commercial hard carbon as cathode and anode, respectively, can deliver a high energy density of 217.9 Wh kg^-1 and excellent cycle life over 1000 cycles.

Interface-Stabilized Layered Lithium Ni-Rich Oxide Cathode via Surface Functionalization with Titanium Silicate

Nickel-rich lithium metal oxide cathode materials have recently be en highlighted as next-generation cathodes for lithium-ion batteries. Nevertheless, their relatively high surface reactivity must be controlled, as fading of the cycling retention occurs rapidly in the cells. This paper proposes functionalized nickel-rich lithium metal oxide cathode materials by a multipurpose nanosized inorganic material-titanium silicon oxide-via a simple thermal treatment process. We examined the topologies of the nano-titanium silicate-functionalized nickel-rich lithium metal oxide cathodes with scanning electron microscopy and quantitatively analyzed their improved mechanical properties using microindentation. The cell containing nickel-rich lithium metal oxide cathodes suffered from poor cycling behavior as the electrolytes persistently decomposed; however, this behavior was effectively inhibited in the cell by nano-titanium silicate-functionalized nickel-rich lithium metal oxide cathodes. Further ex situ analyses indicated that the particle hardness of the nano-titanium silicate-functionalized nickel-rich lithium metal oxide cathode materials was maintained, and decomposition of the electrolyte by the dissolution of transition metals was thoroughly inhibited even after 100 cycles. Based on these results, we concluded that the use of nano-titanium silicate as a coating material for nickel-rich lithium metal oxide cathode materials is an effective way to enhance the cycling performance of lithium-ion batteries.

Sulfate-Containing Composite Based on Ni-Rich Layered Oxide LiNi0.8Mn0.1Co0.1O2 as High-Performance Cathode Material for Li-ion Batteries

Composite positive electrode materials (1−x) LiNi_0.8Mn_0.1Co_0.1O₂∙xLi₂SO₄ (x = 0.002–0.005) for Li-ion batteries have been synthesized via conventional hydroxide or carbonate coprecipitation routes with subsequent high-temperature lithiation in either air or oxygen atmosphere. A comparative study of the materials prepared from transition metal sulfates (i.e., containing sulfur) and acetates (i.e., sulfur-free) with powder X-ray diffraction, electron diffraction, high angle annular dark field transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy revealed that the sulfur-containing species occur as amorphous Li₂SO₄ at the grain boundaries and intergranular contacts of the primary NMC811 crystallites. This results in a noticeable enhancement of rate capability and capacity retention over prolonged charge/discharge cycling compared to their sulfur-free analogs. The improvement is attributed to suppressing the high voltage phase transition and the associated accumulation of anti-site disorder upon cycling and improving the secondary agglomerates’ mechanical integrity by increasing interfacial fracture toughness through linking primary NMC811 particles with soft Li₂SO₄ binder, as demonstrated with nanoindentation experiments. As the synthesis of the (1−x) LiNi_0.8Mn_0.1Co_0.1O₂∙xLi₂SO₄ composites do not require additional operational steps to introduce sulfur, these electrode materials might demonstrate high potential for commercialization.

Elucidating Anionic Redox Chemistry in P3 Layered Cathode for Na-Ion Batteries

The emergence of anionic redox has recently garnered intense interest for lithium/sodium-ion batteries because of the increasing specific capacities of cathodes, which is considered as a transformative approach for designing cathode materials. Nevertheless, the widespread use of such oxygen-related anionic redox is still precluded because of the oxygen release and the correlated irreversible structural transformations and voltage fade. To fundamentally unravel the related mechanism, we have investigated the corresponding anionic redox process based on a new P3-type layered material Na_0.5Mg_0.15Al_0.2Mn_0.65O₂. Here, we prove an excellent structural stability via the operando/ex situ structural evolution within this cathode and further elucidate the complete anionic/cationic redox activity via both surface-sensitive (X-ray photoelectron spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) spectroscopies. Moreover, based on the characterization of the ex situ state to the operando evolution for the whole anionic redox process by Raman and differential electrochemical mass spectrometry, the nature of the reversible oxygen redox chemistry is clarified. Meanwhile, the origin of a small portion irreversible oxygen release generated upon the first charging and its resulting impact on subsequent processes are also fully illuminated. These insights provide guidelines for future designing of anionic redox-based high-energy-density cathodes in lithium/sodium-ion batteries.

Structural and Electrochemical Properties of Low-Cobalt-Content LiNi0.6+xCo0.2-xMn0.2O2 (0.0 ≤ x ≤ 0.1) Cathodes for Lithium-Ion Batteries

The layered oxides LiNi_0.6+xCo_0.2-xMn_0.2O₂ are promising cathode materials for Li-ion batteries (LIBs) owing to their moderate energy densities and structure stabilities. In this study, we systematically investigate the effects of substitution of Co by Ni on the structures, morphologies, and electrochemical properties of LiNi_0.6+xCo_0.2-xMn_0.2O₂ (0.0 ≤ x ≤ 0.1). The physical characteristics of these materials are studied by particle size analysis, scanning electron microscopy, inductively coupled plasma-atomic emission spectroscopy, Rietveld refinement of X-ray diffraction data, and X-ray photoelectron spectroscopy. The electrochemical properties are investigated by charge-discharge cycling, galvanostatic intermittent titration, and electrochemical impedance spectroscopy. As the Co content decreases and the Ni content increases, the discharge capacity and voltage platform are slightly improved, while the initial efficiency, cycling performance, rate capability, and thermal stability gradually decrease. The decreased kinetic performance is attributed to the increased degree of cation mixing and resistance, which decreases the Li⁺ diffusivity. Moreover, the activation energy gradually increases with the decrease in the Co content, which decreases the low-temperature performance. Considering its cost, energy density, cycling lifetime, kinetic performance, and safety properties, LiNi_0.65Co_0.15Mn_0.2O₂ is a promising cathode candidate for use in LIBs.

Chalcopyrite-Derived NaxMO2 (M = Cu, Fe, Mn) Cathode: Tuning Impurities for Self-Doping

Discovering cathode materials composed of earth-abundant elements has become the current priority for developing sodium-ion batteries (SIBs) to meet the ever-increasing demand of large-scale energy storage. Herein, for the first time, layered Na_xMO₂ (M = Cu, Fe, Mn) cathodes are successfully prepared by directly using concentrated chalcopyrite ores as precursors. Greatly, impurity elements like Si and Ca are found to be crucial to tailoring the phase structure of as-obtained layered oxides as a P2 or O3 type, which removes the traditional concern that the impurities may restrict the utilization of natural ores. More interestingly, a certain amount of the Ca elements remaining in the Na sites through a self-doping process endows the P2-type products with enhanced structural stability. In half-cells, P2-type Na_xMO₂ with self-doped Ca elements shows superior rate capability and cycling stability (56 mAh g^-1 at 5 C and 90% capacity retention after 100 cycles at 1 C). In contrast, less impurity elements are favorable for O3-type oxides to achieve a high capacity of 107 mAh g^-1 at 0.1 C and 84% capacity retention after 200 cycles at 2 C. This new strategy would efficiently shorten the process for preparing electrode materials and open a feasible route to construct cheap and durable SIBs.

Doped Nanoscale NMC333 as Cathode Materials for Li-Ion Batteries

A series of Li(Ni_1/3Mn_1/3Co_1/3)_1−xM_xO₂ (M = Al, Mg, Zn, and Fe, x = 0.06) was prepared via sol-gel method assisted by ethylene diamine tetra acetic acid as a chelating agent. A typical hexagonal α-NaFeO₂ structure (R-3m space group) was observed for parent and doped samples as revealed by X-ray diffraction patterns. For all samples, hexagonally shaped nanoparticles were observed by scanning electron microscopy and transmission electron microscopy. The local structure was characterized by infrared, Raman, and Mössbauer spectroscopy and ⁷Li nuclear magnetic resonance (Li-NMR). Cyclic voltammetry and galvanostatic charge-discharge tests showed that Mg and Al doping improved the electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂ in terms of specific capacities and cyclability. In addition, while Al doping increases the initial capacity, Mg doping is the best choice as it improves cyclability for reasons discussed in this work.

Design and Comparative Study of O3/P2 Hybrid Structures for Room Temperature Sodium-Ion Batteries

Rechargeable sodium-ion batteries have drawn increasing attention as candidates for the post lithium-ion batteries in large-scale energy storage systems. Layered oxides are the most promising cathode materials and their pure phases (e.g., P2, O3) have been widely investigated. Here we report a series of cathode materials with O3/P2 hybrid phase for sodium-ion batteries, which possesses advantages of both P2 and O3 structures. The designed material, Na_0.78Ni_0.2Fe_0.38Mn_0.42O₂, can deliver a capacity of 86 mAh g^-1 with great rate capability and cycling performance. 66% capacity is still maintained when the current rate reaches as high as 10C, and the capacity retention is 90% after 1500 cycles. Moreover, in situ XRD was performed to examine the structure change during electrochemical testing in different voltage ranges, and the results demonstrate 4 V as the optimized upper voltage limit, with which smaller polarization, better structural stability, and better cycling performance are achieved. The results obtained here provide new insights in designing cathode materials with optimal structure and improved performance for sodium-ion batteries.

Tunable Electrochemistry via Controlling Lattice Water in Layered Oxides of Sodium-Ion Batteries

Layered oxides based on abundant elements have been extensively studied as cathodes of sodium-ion batteries. Among them, birnessite-type sodium manganese oxide containing lattice water meets the low-cost and high-performance requirement for stationary batteries. Herein, we for the first time present the controllable states of lattice water via adjusting the cutoff voltages, effectively enhancing the reversible capacity, cycling stability, and rate ability of the materials. The current investigation not only highlights the significance of intercalated lattice water for reversible Na (de)insertion of birnessite as well as other similar compounds, but also opens up new opportunities for advanced cathode materials for sodium storage.

2D Electronic Transport with Strong Spin-Orbit Coupling in Bi(2-) Square Net of Y2O2Bi Thin Film Grown by Multilayer Solid-Phase Epitaxy

Highly crystalline Y2O2Bi epitaxial thin film with monatomic Bi(2-) square net layer was grown by newly developed multilayer solid phase epitaxy. High reactivity of the nanometer-scale multilayered precursor enabled efficient formation of single crystalline Y2O2Bi phase with one-step heating. The reductive state of Bi(2-) square net was observed by X-ray photoemission spectroscopy. The electrical resistivity was one order lower than that of polycrystalline powder in previous study. The magnetotransport showed weak antilocalization effect well fitted by the Hikami-Larkin-Nagaoka model, exhibiting two-dimensional electronic nature with strong spin-orbit coupling in the Bi(2-) square net.

A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-x Eu x Ta2O7 nanosheets

La-Eu solid solution nanosheets La_2/3-x Eu _x Ta₂O₇ have been synthesized, and their photoluminescence properties have been investigated. La_2/3-x Eu _x Ta₂O₇ nanosheets were prepared from layered perovskite compounds Li₂La_2/3-x Eu _x Ta₂O₇ as the precursors by soft chemical exfoliation reactions. Both the precursors and the exfoliated nanosheets exhibit a decrease in intralayer lattice parameters as the Eu contents increase. However, there is a discontinuity in this trend between the nominal Eu content ranges x≤ 0.3 and x ≥ 0.4. This discontinuity is attributed to the difference in degree of TaO₆ octahedra tilting for the La- and Eu-rich phases. La_2/3-x Eu _x Ta₂O₇ nanosheets exhibit red emission, characteristic of the f-f transitions in Eu³⁺ photoactivators. The photoluminescence emission can be obtained from both host and direct photoactivator excitation. However, photoluminescence emission through host excitation is much more dominant than that through direct photoactivator excitation, and this behavior is consistent with that of all the other rare-earth photoactivated nanosheets reported previously. The absolute photoluminescence quantum efficiency of the La_2/3-x Eu _x Ta₂O₇ nanosheets increases as the experimentally determined Eu contents increase up to x=0.45 and decrease above it. This result is in good agreement with the optimum photoactivator concentration expected from the percolation theory. These solid solution La_2/3-x Eu _x Ta₂O₇ nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.