Synergistic Effects of Ni²⁺ and Mn³⁺ on the Electrochemical Activation of Li₂MnO₃ in Co-Free and Ni-Poor Li-Rich Layered Cathodes

Although a very high energy density can be stored in Co-free Li-rich layered oxide cathodes, it is difficult to fully exploit the reactivity of the dominant Li2MnO3 component. In this study, regulating the Ni2+ content and introducing trivalent Mn3+ are comprehensively investigated for developing a higher available capacity from the Li2MnO3 component. As the content of Ni2+ increases, the capacity property and the electrochemical stability are improved due to the decreased Li2MnO3-like domain and the enhanced layered structure. Under a proper oxygen partial pressure, the valence of partial manganese can be adjusted to trivalency without generating any impurity phase. Further analyzed by X-ray absorption fine spectroscopy and simulated by Monte Carlo calculation, we find that the local structure of the Li2MnO3-like domain is modulated to be more dispersed and uniform with the presence of Mn3+. With the assistance of Ni2+ ions, Mn3+ exhibits a greater effect on optimizing the local structure. As a consequence, under the synergy of Ni2+ and Mn3+, the optimized sample exhibits the available discharge capacity of over 280 mAh g–1 after several cycles

Tuning Local Structural Configurations to Improve Oxygen-Redox Reversibility of Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are regarded as one of the most desirable cathode materials due to their high specific capacity. Nevertheless, the irreversible oxygen release associated with low oxygen stability prevents their widespread application. Herein, an improved oxygen redox reversibility was achieved by constructing Ni2+–O2––Ni2+ configurations. Superconducting Quantum Interference Device (SQUID) magnetometry measurements are used to track the evolution of the Ni2+–O2––Ni2+ configuration during the electrochemical process. The strongest 180° superexchange interaction in the Ni2+–O2––Ni2+ configuration, derived from the inevitable Li/Ni mixing in LLOs, regulates the local structure to form the ferrimagnetic (FiM) structural units. Consequently, the FiM structural units prevent the irreversible oxygen release and endow LLOs with high initial Coulombic efficiency (ICE). This work emphasizes the importance of the Ni2+–O2––Ni2+ configuration for LLOs with high reversible capacity and proposes a synthesis approach to modulate the amount of FiM structural units

Simultaneous Thermal Enhancement of Upconversion and Downshifting Luminescence by Negative Thermal Expansion in Nonhygroscopic ZrSc(WO₄)₂PO₄:Yb/Er Phosphors

Thermal quenching (TQ) is still a critical challenge for lanthanide (Ln3+)-doped luminescent materials. Herein, we report the novel negative thermal expansion nonhygroscopic phosphor ZrSc(WO4)2PO4:Yb3+/Er3+. Upon excitation with a 980 nm laser, a simultaneous thermal enhancement is realized on upconversion (UC) and downshifting (DS) emissions from room temperature to 573 K. In situ temperature-dependent X-ray diffraction and photoluminescence dynamics are used to reveal the luminescence mechanism in detail. The coexistence of the high energy transfer efficiency and the promoted radiative transition probability can be responsible for the thermally enhanced luminescence. On the basis of the luminescence intensity ratio of thermally coupled energy levels 2H11/2 and 4S3/2 at different temperatures, the relative and absolute sensitivities of the targeted samples reach 1.10% K–1 and 1.21% K–1, respectively, and the low-temperature uncertainty is approximately 0.1–0.4 K on the whole temperature with a high repeatability (98%). Our findings highlight a general approach for designing a hygro-stable, thermostable, and highly efficient Ln3+-doped phosphor with UC and DS luminescence

Low-Cost Orthorhombic Na_<i>x</i>[FeTi]O₄ (<i>x</i> = 1 and 4/3) Compounds as Anode Materials for Sodium-Ion Batteries

Abundant and low-cost sodium, iron, and titanium have great potentials to act as raw materials for large-scale power sources. Here we report the synthesis of novel orthorhombic Na_<i>x</i>[FeTi]­O₄ (<i>x</i> = 1 and 4/3) anode materials by a solid-state reaction method and their electrochemical behaviors in sodium-ion batteries. These materials are able to reversibly insert additional Na⁺ ions and show very good cycling stabilities. In particular, the Na_4/3[FeTi]­O₄ material can deliver a high reversible capacity of 120 mA h g^–1 at 0.1 C, and cyclic voltammetry (CV) investigation proves that there is no phase transformation during testing cycles. The Na­[FeTi]­O₄ material exhibits an even higher initial charge capacity of 181 mA h g^–1 at 0.1 C, and <i>in situ</i> X-ray diffraction (XRD) results indicate that Na⁺ ions behave in topotactic insertion and extraction manners inside this material. Meanwhile, gas evolutions during the initial redox process are analyzed by an <i>operando</i> mass spectrometry technique. The result suggests that the Na­[FeTi]­O₄ material exhibits an enhanced safety

Morphological Evolution of High-Voltage Spinel LiNi_0.5Mn_1.5O₄ Cathode Materials for Lithium-Ion Batteries: The Critical Effects of Surface Orientations and Particle Size

An evolution panorama of morphology and surface orientation of high-voltage spinel LiNi_0.5Mn_1.5O₄ cathode materials synthesized by the combination of the microwave-assisted hydrothermal technique and a postcalcination process is presented. Nanoparticles, octahedral and truncated octahedral particles with different preferential growth of surface orientations are obtained. The structures of different materials are studied by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption near edge spectroscopy (XANES), and transmission electron microscopy (TEM). The influence of various morphologies (including surface orientations and particle size) on kinetic parameters, such as electronic conductivity and Li⁺ diffusion coefficients, are investigated as well. Moreover, electrochemical measurements indicate that the morphological differences result in divergent rate capabilities and cycling performances. They reveal that appropriate surface-tailoring can satisfy simultaneously the compatibility of power capability and long cycle life. The morphology design for optimizing Li⁺ transport and interfacial stability is very important for high-voltage spinel material. Overall, the crystal chemistry, kinetics and electrochemical performance of the present study on various morphologies of LiNi_0.5Mn_1.5O₄ spinel materials have implications for understanding the complex impacts of electrode interface and electrolyte and rational design of rechargeable electrode materials for lithium-ion batteries. The outstanding performance of our truncated octahedral LiNi_0.5Mn_1.5O₄ materials makes them promising as cathode materials to develop long-life, high energy and high power lithium-ion batteries

Locally Saturated Ether-Based Electrolytes With Oxidative Stability For Li Metal Batteries Based on Li-Rich Cathodes

Li metal batteries applying Li-rich, Mn-rich (LMR) layered oxide cathodes present an opportunity to achieve high-energy density at reduced cell cost. However, the intense oxidizing and reducing potentials associated with LMR cathodes and Li anodes present considerable design challenges for prospective electrolytes. Herein, we demonstrate that, somewhat surprisingly, a properly designed localized-high-concentration electrolyte (LHCE) based on ether solvents is capable of providing reversible performance for Li||LMR cells. Specifically, the oxidative stability of the LHCE was found to heavily rely on the ratio between salt and solvating solvent, where local-saturation was necessary to stabilize performance. Through molecular dynamics (MD) simulations, this behavior was found to be a result of aggregated solvation structures of Li+/anion pairs. This LHCE system was found to produce significantly improved LMR cycling (95.8% capacity retention after 100 cycles) relative to a carbonate control as a result of improved cathode-electrolyte interphase (CEI) chemistry from X-ray photoelectron spectroscopy (XPS), and cryogenic transmission electron microscopy (cryo-TEM). Leveraging this stability, 4 mAh cm–2 LMR||2× Li full cells were demonstrated, retaining 87% capacity after 80 cycles in LHCE, whereas the control electrolyte produced rapid failure. This work uncovers the benefits, design requirements, and performance origins of LHCE electrolytes for high-voltage Li||LMR batteries

Green Facile Scalable Synthesis of Titania/Carbon Nanocomposites: New Use of Old Dental Resins

A green facile scalable method inspired by polymeric dental restorative composite is developed to synthesize TiO₂/carbon nanocomposites for manipulation of the intercalation potential of TiO₂ as lithium-ion battery anode. Poorly crystallized TiO₂ nanoparticles with average sizes of 4−6 nm are homogeneously embedded in carbon matrix with the TiO₂ mass content varied between 28 and 65%. Characteristic discharge/charge plateaus of TiO₂ are significantly diminished and voltage continues to change along with proceeding discharge/charge process. The tap density, gravimetric and volumetric capacities, and cyclic and rate performance of the TiO₂/C composites are effectively improved