Multi-Functional Surface Engineering for Li-Excess Layered Cathode Material Targeting Excellent Electrochemical and Thermal Safety Properties

Surface LiMn1.4Ni0.5Mo0.1O4 Coating and Bulk Mo Doping of Li-Rich Mn-Based Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with Enhanced Electrochemical Performance for Lithium-Ion Batteries

To improve the initial Coulombic efficiency, cycling stability, and rate performance of the Li-rich Mn-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode, the combination of LiMn_1.4Ni_0.5Mo_0.1O₄ coating with Mo doping has been successfully carried out by the sol-gel method and subsequent dip-dry process. This strategy buffers the electrodes from the corrosion of electrolyte and enhances the lattice parameter, which could inhibit the oxygen release and maintain the structural stability, thus improving the cycle stability and rate capability. After LiMn_1.4Ni_0.5Mo_0.1O₄ modification, the initial discharge capacity reaches 272.4 mAh g^-1 with a corresponding initial Coulombic efficiency (ICE) of 84.2% at 0.1C (1C = 250 mAh g^-1), far higher than those (221.5 mAh g^-1 and 68.9%) of the pristine sample. Besides, the capacity retention of the coated sample is enhanced by up to 66.8% after 200 cycles at 0.1C. Especially, the rate capability of the coated sample is 95.2 mAh g^-1 at 5C. XRD, SEM, TEM, XPS, and Raman spectroscopy are adopted to characterize the morphologies and structures of the samples. This coating strategy has been demonstrated to be an effective approach to construct high-performance energy storage devices.

Insights into the Enhanced Structural and Thermal Stabilities of Nb-Substituted Lithium-Rich Layered Oxide Cathodes

Lithium-rich manganese-based layered oxides (LLOs) are considered to be the most promising cathode materials for next-generation lithium-ion batteries (LIBs) for their higher reversible capacity, higher operating voltage, and lower cost compared with those of other commercially available cathode materials. However, irreversible lattice oxygen release and associated severe structural degradation that exacerbate under high temperature and deep delithiation hinder the large-scale application of LLOs. Herein, we propose a strategy to stabilize the layered lattice framework and improve the thermal stability of cobalt-free Li_1.2Mn_0.53Ni_0.27O₂ by doping with 4d transition metal niobium (Nb). Detailed atomic-scale imaging, in situ characterization, and DFT simulations confirm that the induced strong Nb-O bonds stabilize the oxygen lattice framework and restrains the fracture of TM-O bonds, thereby inhibiting the release of lattice oxygen and the continuous migration of TM ions to the lithium layer during the cycle. Furthermore, Nb doping also promotes the surface rearrangement to form a Ni-enrichment layered/rocksalt heterogeneous interface to enhance surface structural stability. As a result, the Nb-doped material delivers a capacity of 181.7 mAh g^-1 with retention of 85.5% after 200 cycles at 1C, extraordinary thermal stability with a capacity retention of 80.7% after 200 cycles at 50 °C, and superior rate capability.

Nonstoichiometry of Li-rich cathode material with improved cycling ability for lithium-ion batteries

Lithium-rich layered oxides are considered as promising cathode materials for lithium-ion batteries due to its high capacity, but the rapid decay of capacity and operating voltage are great challenges to achieve its commercial application. In this work, the nonstoichiometry of Li-rich layered oxide Li_1.2Mn_0.6Ni_0.2O₂ was designed by directly declining the Mn amounts in the form of Li_1.2Mn_xNi_0.2O₂ (x = 0.59, 0.57, 0.55). The nonstoichiometric sample Li_1.2Mn_0.55Ni_0.2O₂ exhibits a capacity of 170.73 mAh g^-1 at 0.5 C, a little lower than 187.29 mAh g^-1 of Li_1.2Mn_0.6Ni_0.2O₂, however, better cycling stability of operating voltage and capacity is attained with the reduction of Mn amounts, compared to that of Li_1.2Mn_0.6Ni_0.2O₂. The capacity retention of Li_1.2Mn_0.55Ni_0.2O₂ is enhanced to 88.7% via 74.7% of Li_1.2Mn_0.6Ni_0.2O₂ after 100 cycles at 0.5 C. The declining value of operating voltage for Li_1.2Mn_0.55Ni_0.2O₂ is 0.200 V as compared to 0.559 V for Li_1.2Mn_0.6Ni_0.2O₂. X-ray photoelectron spectra (XPS) was employed to confirm the existence of Ni³⁺ in the nonstoichiometric samples, and the amounts of Ni³⁺ increase along the Mn contents decrease. The improvement of electrochemical properties for nonstoichiometric samples is attributed to the presence of Ni³⁺ due to Ni³⁺ can defer the transition of layered-to-spinel structure through decreasing the Li/Ni mixing.

Thermal Stability Enhancement through Structure Modification on the Microsized Crystalline Grain Surface of Lithium-Rich Layered Oxides

Lithium-rich layered oxides have been considered as the most promising candidate for offering a high specific capacity and energy density for lithium-ion batteries. However, their practical applications are still suffered by the cycle instability and also closely related thermal stability. Here, microsized crystalline grains with good dispersion of lithium-rich layered oxides are prepared by a molten-salt method, while a spinel structure is also introduced on a grain surface by following chemical oxidation and annealing process, and their thermal performance with different cutoff voltages during the charge process is systematically studied using differential scanning calorimetry method. Results have shown that thermal stability of microsized crystalline grains is better than that of spherical secondary agglomerates, the spinel structure introduction on the grain surface of microsized crystalline grains can contribute obviously to their thermal stability, in which the onset temperature of the exothermic peak has been increased by 103 °C, and the thermal release value can be reduced as much as about 40% when the battery was charged to 4.8 V. Furthermore, the electrochemical performance, especially cycle stability under a high temperature, has also been enhanced for spinel-modified microsized crystalline grains. This work not only develops the microsized crystalline grains with good dispersion of lithium-rich layered oxides, confirming the advantages of these materials compared to spherical secondary agglomerates, but also reveals the method to improve their thermal stability by grain surface structure modification, opening the way to optimize the comprehensive performance of electrode materials for batteries.

Transition metal (Fe, Co, Ni) fluoride-based materials for electrochemical energy storage

The improvement of advanced battery performance has always been a key issue in energy research. Therefore, it is necessary to explore the applications of excellent materials in advanced batteries. Transition-metal (Fe, Co, Ni) fluoride-based materials exhibit excellent chemical tailorability due to their different functional groups, and they have attracted wide research interest for use in next-generation electrochemical energy storage. This review introduces methods to synthesize transition metal (Fe, Co, Ni) fluoride materials and their applications in batteries and supercapacitors. We also present the current challenges and future opportunities of iron fluoride in electrochemistry, including processing techniques, composite properties, and prospective applications. It is believed that in the future, the research and influence of iron fluoride and its composites will be more far-reaching and lasting.

Aegis of Lithium-Rich Cathode Materials via Heterostructured LiAlF4 Coating for High-Performance Lithium-Ion Batteries

Lithium-rich oxides have been regarded as one of the most competitive cathode materials for next-generation lithium-ion batteries due to their high theoretical specific capacity and high discharge voltage. However, they are still far from being commercialized due to low rate capability and poor cycling stability. In this study, we propose a heterostructured LiAlF₄ coating strategy to overcome those obstacles. The as-developed lithium-rich cathode material shows outstanding performance including a high reversible capacity (246 mA h g^-1 at 0.1C), excellent rate capability (133 mA h g^-1 at 5C), and ultralong cycling stability (3000 cycles). Comparing with those of pristine and AlF₃-coated lithium-rich cathode materials, the enhanced performances can be attributed to the introduction of the lithium-ion-conductive nanolayer and the generation of nonbonding O ^n- species in the active material lattice, which enable rapid and effective lithium ion transport and diffusion. Our work provides a new strategy to develop high-performance lithium-rich cathode materials for high-energy-density lithium-ion batteries.

Characterization and Control of Irreversible Reaction in Li-Rich Cathode during the Initial Charge Process

Li-rich layered oxide has been known to possess high specific capacity beyond the theoretical value from both charge compensation in transition metal and oxygen in the redox reaction. Although it could achieve higher reversible capacity due to the oxygen anion participating in electrochemical reaction, however, its use in energy storage systems has been limited. The reason is the irreversible oxygen reaction that occurs during the initial charge cycle, resulting in structural instability due to oxygen evolution and phase transition. To suppress the initial irreversible oxygen reaction, we introduced the surface-modified Li[Li_0.2Ni_0.16Mn_0.56Co_0.08]O₂ prepared by carbon coating (carbonization process), which was verified to have reduced oxygen reaction during the initial charge cycle. The electrochemical performance is improved by the synergic effects of the oxygen-deficient layer and carbon coating layer formed on the surface of particles. The sample with suitable carbon coating exhibited the highest structural stability, resulting in reduced capacity fading and voltage decay, which are attributed to the mitigated layered-to-spinel-like phase transition during prolonged cycling. The control over the oxygen reaction of Li₂MnO₃ by surface modification affects the activation reaction above 4.4 V in the initial charge cycle and structure changes during prolonged cycling. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy analyses as well as electrochemical performance measurement were used to identify the correlation between reduced oxygen activity and structural changes.

Spinel/Layered Heterostructured Lithium-Rich Oxide Nanowires as Cathode Material for High-Energy Lithium-Ion Batteries

Lithium-rich oxide material has been considered as an attractive candidate for high-energy cathode for lithium-ion batteries (LIBs). However, the practical applications are still hindered due to its low initial reversible capacity, severe voltage decaying, and unsatisfactory rate capability. Among all, the voltage decaying is a serious barrier that results in a large decrease of energy density during long-term cycling. To overcome these issues, herein, an efficient strategy of fabricating lithium-rich oxide nanowires with spinel/layered heterostructure is proposed. Structural characterizations verify that the spinel/layered heterostructured nanowires are a self-assembly of a lot of nanoparticles, and the Li₄Mn₅O₁₂ spinel phase is embedded inside the layered structure. When the material is used as cathode of LIBs, the spinel/layered heterostructured nanowires can display an extremely high invertible capacity of 290.1 mA h g^-1 at 0.1 C and suppressive voltage fading. Moreover, it exhibits a favorable cycling stability with capacity retention of 94.4% after charging/discharging at 0.5 C for 200 cycles and it shows an extraordinary rate capability (183.9 mA h g^-1, 10 C). The remarkable electrochemical properties can be connected with the spinel/layered heterostructure, which is in favor of Li⁺ transport kinetics and enhancing structural stability during the cyclic process.

In Situ Encapsulation of the Nanoscale Er2O3 Phase To Drastically Suppress Voltage Fading and Capacity Degradation of a Li- and Mn-Rich Layered Oxide Cathode for Lithium Ion Batteries

A novel strategy of in situ precipitation and encapsulation of the Er₂O₃ phase on the Li(Li_0.2Ni_0.13Co_0.13Mn_0.54)O₂ (LNCMO) cathode material for lithium ion batteries is proposed for the first time. The Er₂O₃ phase is precipitated from the bulk of the LNCMO material and encapsulated onto its entire surface during the calcining process. Electrochemicial performance is investigated by a galvanostatic charge and discharge test. The structure and morphology are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The results show that an about 10 nm Er₂O₃ layer is successfully encapsulated onto the entire surface of the LNCMO matrix material. This unique nanoscale Er₂O₃ encapsulation can significantly prevent the LNCMO cathode material from being corroded by electrolytes and stabilize the crystal structure of the LNCMO cathode during cycling. Therefore, the prepared Er₂O₃-coated LNCMO composite exhibits excellent cycling performace and a high initial Coulombic efficiency.

The effect of cation mixing controlled by thermal treatment duration on the electrochemical stability of lithium transition-metal oxides

An optimal degree of Ni²⁺occupancy in the lithium layer as “pillaring” that enhances the electrochemical performance of NMC materials.