Electrochemical kinetics and cycling performance of nano Li[Li0.23Co0.3Mn0.47]O2 cathode material for lithium ion batteries

Studies on the Kinetic Behaviors of Na Ions Insertion/Extraction in Na2FeSiO4/C Cathode Material at Various Desodiation States

Na₂FeSiO₄, as one of the promising cathode materials in sodium-ion batteries, has attracted great interests. However, studies on the kinetic behaviors of Na ions insertion/extraction in Na₂FeSiO₄ composite electrode have been barely considered, until now. Importantly, the specific capacity and rate capability of Na₂FeSiO₄ cathode materials are greatly correlated with the kinetics of Na⁺ transfer in the host material. Herein, on the basis of the characterizations of microstructure and morphologies (i.e., Rietveld refinement, FESEM, HRTEM, etc.), the electrochemical kinetics of Na ions extraction in Na₂FeSiO₄/C electrode are first studied in detail via two electrochemical techniques (EIS and GITT), establishing the rate-controlling steps of Na⁺ transport in Na₂FeSiO₄/C, evaluating series of kinetic parameters, as well as calculating the Na⁺ diffusion coefficient at various state-of-desodiation. Changes of impedance response of Na₂FeSiO₄/C electrode depending on the different levels of desodiation show that a serial features of electrode process for Na ions migration have tremendous discrepancies, indicating that the kinetics of Na⁺ extraction from Na₂FeSiO₄/C electrode are largely influenced by different electrode reaction processes. These results provide useful insight into the inner properties of Na₂FeSiO₄/C electrode, and it is significant to optimize the electrochemical performance of Na₂FeSiO₄/C. Moreover, two models of equivalent circuits are also constructed to simulate the electrode processes and describe the behaviors of Na ions transfer in Na₂FeSiO₄/C.

Rapid Self-Assembly Spherical Li1.2Mn0.56Ni0.16Co0.08O2 with Improved Performances by Microwave Hydrothermal Method as Cathode for Lithium-Ion Batteries

Spherical Li-rich Li1.2Mn0.56Ni0.16Co0.08O2 compound is rapidly synthesized through a facile microwave hydrothermal method followed by a high-temperature solid-state reaction. Homogenous spherical precursor can be precipitated through the microwave hydrothermal (MH) method within 30 min without rigorous coprecipitation condition. The as-prepared Li-rich compound exhibits a hierarchical structure composed of spherical secondary particles (2-3 μm) and small primary particles (150-250 nm) with pores. X-ray diffractometry (XRD) and Brunauer-Emmett-Teller (BET) tests prove that a well-formed layered structure and a large specific surface area containing pores are obtained through the MH method. Such structure is a benefit for the thorough contact between active materials and electrolyte to increase the reactive points. Thus, the as-prepared Li-rich compound exhibits perfect electrochemical performances with a high discharge capacity of 235.6 mAh g(-1) at a current density of 200 mA g(-1). Even at higher current densities of 1000 and 2000 mA g(-1), discharge capacities of 168.6 and 131.2 mAh g(-1) are still maintained, respectively. Furthermore, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) are carried out to study the material prepared by microwave hydrothermal method. It is considered as an efficient way to synthesize Li-rich compound as cathode material for applications.

Recent progress in Li-rich layered oxides as cathode materials for Li-ion batteries

This review systematically summarized Li-rich layered oxides and states the strategies to enhance such materials when used in Li-ion batteries.

The impact of upper cut-off voltages on the electrochemical behaviors of composite electrode 0.3Li2MnO3·0.7LiMn1/3Ni1/3Co1/3O2

This work has initiated an investigation on the electrochemical behaviors and the structure changes of the composite electrode 0.3Li(2)MnO(3)·0.7LiMn(1/3)Ni(1/3)Co(1/3)O(2) when charged with different cut-off voltages. It is found that the charge cut-off voltages could not only affect the capacity property and coulombic efficiency, but also alter the electrode kinetics of the composite. As a consequence, the electrochemical activation of the composite electrode is highly dependent on the charge cut-off voltages: when the charge cut-off voltage is higher than 4.5 V, the inert component Li(2)MnO(3) in the composite electrode is completely activated. At the meanwhile, there occurred an irreversible oxygen loss during the initial charge process, which yielded a hollow sphere in the electrode. Regardless of charge voltages, Mn ions in the composite electrode were presented in an oxidation state of +4, while Co(2+) ions were detected at the surface of the electrode when cycled at low voltages. Ni ions in the composite could react with organic or inorganic species and then cover the surface of the cycled electrode.

Size-dependent lattice expansion in nanoparticles: reality or anomaly?

Size-dependent lattice expansion of nanoparticles is observed for many ionic compounds, including metal oxides, while lattice contraction prevails for pure metals. However, the physical origin of this effect, which is of importance for the thermodynamic, chemical and electronic properties of nanoparticles, is discussed controversially. After a survey of the experimental literature, revealing a wide variety of materials with size-dependent lattice expansion, we show that the negative surface stress is the key reason for lattice expansion, while the excess of lattice sums or point defects of various charge states can be excluded as general explanations. Ab initio calculations of surface stresses for various surface structures of metal oxides confirm the model of a surface-induced lattice expansion.