Study on the Electrochemical Reaction Mechanism of NiFe₂O₄ as a High-Performance Anode for Li-Ion Batteries

Nickel ferrite (NiFe₂O₄) has been previously shown to have a promising electrochemical performance for lithium-ion batteries (LIBs) as an anode material. However, associated electrochemical processes, along with structural changes, during conversion reactions are hardly studied. Nanocrystalline NiFe₂O₄ was synthesized with the aid of a simple citric acid assisted sol–gel method and tested as a negative electrode for LIBs. After 100 cycles at a constant current density of 0.5 A g^–1 (about a 0.5 C-rate), the synthesized NiFe₂O₄ electrode provided a stable reversible capacity of 786 mAh g^–1 with a capacity retention greater than 85%. The NiFe₂O₄ electrode achieved a specific capacity of 365 mAh g^–1 when cycled at a current density of 10 A g^–1 (about a 10 C-rate). At such a high current density, this is an outstanding capacity for NiFe₂O₄ nanoparticles as an anode. Ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were employed at different potential states during the cell operation to elucidate the conversion process of a NiFe₂O₄ anode and the capacity contribution from either Ni or Fe. Investigation reveals that the lithium extraction reaction does not fully agree with the previously reported one and is found to be a hindered oxidation of metallic nickel to nickel oxide in the applied potential window. Our findings suggest that iron is participating in an electrochemical reaction with full reversibility and forms iron oxide in the fully charged state, while nickel is found to be the cause of partial irreversible capacity where it exists in both metallic nickel and nickel oxide phases

Structural Evolution of Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_1‑y‑zO₂ Cathode Materials during High-Rate Charge and Discharge

Ni-rich lithium transition metal oxides have received significant attention due to their high capacities and rate capabilities determined via theoretical calculations. Although the structural properties of these materials are strongly correlated with the electrochemical performance, their structural stability during the high-rate electrochemical reactions has not been fully evaluated yet. In this work, transmission electron microscopy is used to investigate the crystallographic and electronic structural modifications of Ni-based cathode materials at a high charge/discharge rate of 10 C. It is found that the high-rate electrochemical reactions induce structural inhomogeneity near the surface of Ni-rich cathode materials, which limits Li transport and reduces their capacities. This study establishes a correlation between the high-rate electrochemical performance of the Ni-based materials and their structural evolution, which can provide profound insights for designing novel cathode materials having both high energy and power densities

Investigation of Thermal Stability of P2–Na_<i>x</i>CoO₂ Cathode Materials for Sodium Ion Batteries Using Real-Time Electron Microscopy

Here, we take advantage of <i>in situ</i> transmission electron microscopy (TEM) to investigate the thermal stability of P2-type Na_<i>x</i>CoO₂ cathode materials for sodium ion batteries, which are promising candidates for next-generation lithium ion batteries. A double-tilt TEM heating holder was used to directly characterize the changes in the morphology and the crystallographic and electronic structures of the materials with increase in temperature. The electron diffraction patterns and the electron energy loss spectra demonstrated the presence of cobalt oxides (Co₃O₄, CoO) and even metallic cobalt (Co) at higher temperatures as a result of reduction of Co ions and loss of oxygen. The bright-field TEM images revealed that the surface of Na_<i>x</i>CoO₂ becomes porous at high temperatures. Higher cutoff voltages result in degrading thermal stability of Na_<i>x</i>CoO₂. The observations herein provide a valuable insight that thermal stability is one of the important factors to be considered in addition to the electrochemical properties when developing new electrode materials for novel battery systems

Nanoscale Zirconium-Abundant Surface Layers on Lithium- and Manganese-Rich Layered Oxides for High-Rate Lithium-Ion Batteries

Battery performance, such as the rate capability and cycle stability of lithium transition metal oxides, is strongly correlated with the surface properties of active particles. For lithium-rich layered oxides, transition metal segregation in the initial state and migration upon cycling leads to a significant structural rearrangement, which eventually degrades the electrode performance. Here, we show that a fine-tuning of surface chemistry on the particular crystal facet can facilitate ionic diffusion and thus improve the rate capability dramatically, delivering a specific capacity of ∼110 mAh g^–1 at 30C. This high rate performance is realized by creating a nanoscale zirconium-abundant rock-salt-like surface phase epitaxially grown on the layered bulk. This surface layer is spontaneously formed on the Li⁺-diffusive crystallographic facets during the synthesis and is also durable upon electrochemical cycling. As a result, Li-ions can move rapidly through this nanoscale surface layer over hundreds of cycles. This study provides a promising new strategy for designing and preparing a high-performance lithium-rich layered oxide cathode material

P2 Orthorhombic Na_0.7[Mn_1–<i>x</i>Li_<i>x</i>]O_2+<i>y</i> as Cathode Materials for Na-Ion Batteries

P2-type manganese-based oxide materials have received attention as promising cathode materials for sodium ion batteries because of their low cost and high capacity, but their reaction and failure mechanisms are not yet fully understood. In this study, the reaction and failure mechanisms of β-Na_0.7[Mn_1–<i>x</i>Li_<i>x</i>]­O_2+<i>y</i> (<i>x</i> = 0.02, 0.04, 0.07, and 0.25), α-Na_0.7MnO_2+<i>y</i>, and β-Na_0.7MnO_2+<i>z</i> are compared to clarify the dominant factors influencing their electrochemical performances. Using a quenching process with various amounts of a Li dopant, the Mn oxidation state in β-Na_0.7[Mn_1–<i>x</i>Li_<i>x</i>]­O_2+<i>y</i> is carefully controlled without the inclusion of impurities. Through various in situ and ex situ analyses including X-ray diffraction, X-ray absorption near-edge structure spectroscopy, and inductively coupled plasma mass spectrometry, we clarify the dependence of (i) reaction mechanisms on disordered Li distribution in the Mn layer, (ii) reversible capacities on the initial Mn oxidation state, (iii) redox potentials on the Jahn–Teller distortion, (iv) capacity fading on phase transitions during charging and discharging, and (v) electrochemical performance on Li dopant vs Mn vacancy. Finally, we demonstrate that the optimized β-Na_0.7[Mn_1–<i>x</i>Li_<i>x</i>]­O_2+<i>y</i> (<i>x</i> = 0.07) exhibits excellent electrochemical performance including a high reversible capacity of ∼183 mA h g^–1 and stable cycle performance over 120 cycles

Anatase Titania Nanorods as an Intercalation Anode Material for Rechargeable Sodium Batteries

For the first time, we report the electrochemical activity of anatase TiO₂ nanorods in a Na cell. The anatase TiO₂ nanorods were synthesized by a hydrothermal method, and their surfaces were coated by carbon to improve the electric conductivity through carbonization of pitch at 700 °C for 2 h in Ar flow. The resulting structure does not change before and after the carbon coating, as confirmed by X-ray diffraction (XRD). Transmission electron microscopic images confirm the presence of a carbon coating on the anatase TiO₂ nanorods. In cell tests, anodes of bare and carbon-coated anatase TiO₂ nanorods exhibit stable cycling performance and attain a capacity of about 172 and 193 mAh g^–1 on the first charge, respectively, in the voltage range of 3–0 V. With the help of the conductive carbon layers, the carbon-coated anatase TiO₂ delivers more capacity at high rates, 104 mAh g^–1 at the 10 C-rate (3.3 A g^–1), 82 mAh g^–1 at the 30 C-rate (10 A g^–1), and 53 mAh g^–1 at the 100 C-rate (33 A g^–1). By contrast, the anode of bare anatase TiO₂ nanorods delivers only about 38 mAh g^–1 at the 10 C-rate (3.3 A g^–1). The excellent cyclability and high-rate capability are the result of a Na⁺ insertion and extraction reaction into the host structure coupled with Ti^4+/3+ redox reaction, as revealed by X-ray absorption spectroscopy

Lattice Water for the Enhanced Performance of Amorphous Iron Phosphate in Sodium-Ion Batteries

We report amorphous iron phosphate with lattice water, namely FePO₄·<i>x</i>H₂O (<i>x</i> ∼ 2.39), as a promising sodium-ion battery (SIB) cathode. After carbon coating, micrometer-sized FePO₄·<i>x</i>H₂O exhibits a reversible capacity that is higher than that of its counterpart without lattice water (130.0 vs 50.6 mAh g^–1 at 0.15<i>C</i> rate) along with clearly enhanced rate capability and cyclability. The superior electrochemical performance of FePO₄·<i>x</i>H₂O is attributed to the lattice water that facilitates sodium-ion diffusion via enlarged channel dimensions and the screening of the electrostatic interactions between sodium ions and host anions. The amorphous phase is also advantageous in accommodating the stress created in the host framework during sodium-ion (de)­intercalation. The presence of lattice water also protects the oxidation state of Fe from reductive surface carbon coating and slightly lowers the operation voltage via reduced inductive effect. The current study provides a useful insight into how to design SIB electrode materials particularly focusing on facile sodium-ion diffusion

Investigation of Changes in the Surface Structure of Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ Cathode Materials Induced by the Initial Charge

We use transmission electron microscopy (TEM) to investigate the evolution of the surface structure of Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ cathode materials (NCA) as a function of the extent of first charge at room temperature using a combination of high-resolution electron microscopy (HREM) imaging, selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). It was found that the surface changes from the layered structure (space group <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and eventually to the rock-salt structure (<i>Fm</i>3̅<i>m</i>), and that these changes are more substantial as the extent of charge increases. EELS indicates that these crystal structure changes are also accompanied by significant changes in the electronic structure, which are consistent with delithiation leading to both a reduction of the Ni and an increase in the effective electron density of oxygen. This leads to a charge imbalance, which results in the formation of oxygen vacancies and the development of surface porosity. The degree of local surface structure change differs among particles, likely due to kinetic factors that are manifested with changes in particle size. These results demonstrate that TEM, when coupled with EELS, can provide detailed information about the crystallographic and electronic structure changes that occur at the surface of these materials during delithiation. This information is of critical importance for obtaining a complete understanding of the mechanisms by which both degradation and thermal runaway initiate in these electrode materials

Correlating Structural Changes and Gas Evolution during the Thermal Decomposition of Charged Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ Cathode Materials

In this work, we present results from the application of a new in situ technique that combines time-resolved synchrotron X-ray diffraction and mass spectroscopy. We exploit this approach to provide direct correlation between structural changes and the evolution of gas that occurs during the thermal decomposition of (over)­charged cathode materials used in lithium-ion batteries. Results from charged Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ cathode materials indicate that the evolution of both O₂ and CO₂ gases are strongly related to phase transitions that occur during thermal decomposition, specifically from the layered structure (space group <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and finally to the rock-salt structure (<i>Fm</i>3̅<i>m</i>). The state of charge also significantly affects both the structural changes and the evolution of oxygen as the temperature increases: the more extensive the charge, the lower the temperature of the phase transitions and the larger the oxygen release. Ex situ X-ray absorption spectroscopy (XAS) and in situ transmission electron microscopy (TEM) are also utilized to investigate the local structural and valence state changes in Ni and Co ions, and to characterize microscopic morphology changes. The combination of these advanced tools provides a unique approach to study fundamental aspects of the dynamic physical and chemical changes that occur during thermal decomposition of charged cathode materials in a systematic way

Controlling the Intercalation Chemistry to Design High-Performance Dual-Salt Hybrid Rechargeable Batteries

We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg²⁺/Li⁺ rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg²⁺ and Li⁺ co-insertion into the Mo₆S₈ cathode host using density functional theory calculations, we show that there is a threshold Li⁺ activity for the pristine Mo₆S₈ cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature