Mg doped Li2FeSiO4/C nanocomposites synthesized by the solvothermal method for lithium ion batteries

Hollow Hemispherical Lithium Iron Silicate Synthesized by an Ascorbic Acid-Assisted Hydrothermal Method as a Cathode Material for Li Ion Batteries

High-capacity and high-voltage cathode materials are required to meet the increasing demand for energy density in Li ion batteries. Lithium iron silicate (Li2FeSiO4) is a cathode material with a high theoretical capacity of 331 mAh·g-1. However, its poor conductivity and low Li ion diffusion coefficient result in poor capability, hindering practical applications. Morphology has an important influence on the properties of materials, and nanomaterials with hollow structures are widely used in electrochemical devices. Herein, we report a novel hollow hemispherical Li2FeSiO4 synthesized by a template-free hydrothermal method with the addition of ascorbic acid. The hollow hemispherical Li2FeSiO4 consisted of finer particles with a shell thickness of about 80 nm. After carbon coating, the composite was applied as the cathode in Li ion batteries. As a result, the hollow hemispherical Li2FeSiO4/C exhibited a discharge capacity as high as 192 mAh·g-1 at 0.2 C, and the average capacities were 134.5, 115.5 and 93.4 mAh·g-1 at 0.5, 1 and 2 C, respectively. In addition, the capacity increased in the first few cycles and then decayed with further cycling, showing a warm-up like behavior, and after 160 cycles the capacities maintained 114.2, 101.6 and 79.3 mAh·g-1 at 0.5, 1 and 2 C, respectively. Such a method of adding ascorbic acid in the hydrothermal reaction can effectively synthesize hollow hemispherical Li2FeSiO4 with the enhanced electrochemical performance.

Intrinsic Defects, Diffusion and Dopants in AVSi2O6 (A = Li and Na) Electrode Materials

The alkali metal pyroxenes of the AVSi2O6 (A = Li and Na) family have attracted considerable interest as cathode materials for the application in Li and Na batteries. Computer modelling was carried out to determine the dominant intrinsic defects, Li and Na ion diffusion pathways and promising dopants for experimental verification. The results show that the lowest energy intrinsic defect is the V–Si anti-site in both LiVSi2O6 and NaVSi2O6. Li or Na ion migration is slow, with activation energies of 3.31 eV and 3.95 eV, respectively, indicating the necessity of tailoring these materials before application. Here, we suggest that Al on the Si site can increase the amount of Li and Na in LiVSi2O6 and NaVSi2O6, respectively. This strategy can also be applied to create oxygen vacancies in both materials. The most favourable isovalent dopants on the V and Si sites are Ga and Ge, respectively.

Superior Na-Storage Properties of Nickel-Substituted Na2FeSiO4@C Microspheres Encapsulated with the In Situ-Synthesized Alveolation-like Carbon Matrix

The poor electronic conductivity of Na₂FeSiO₄ always limits its electrochemical reactivities and no effective solution has been found to date. Herein, the novel Ni-substituted Na₂Fe_1-xNi_xSiO₄@C nanospheres (50-100 nm) encapsulated with a 3D hierarchical porous skeleton (named as alveolation-like configuration) constructed using in situ carbon are first synthesized via a facile sol-gel method, and the effects of Ni substitution combined with the design of a unique carbon network on Na-storage properties are assessed systematically, focusing on alleviating the inherent defects of the Na₂FeSiO₄ cathode material. A series of characterization technologies such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and so forth, coupled with the electrochemical measurements and first-principles calculations, are used to explore the structure, morphology and electrochemical behaviors of the as-prepared materials. The results show that the synergism between Ni substitution and the special alveolation-like configuration enables fast Na ions mobility (from 10^-14 to 10^-12 cm² s^-1), reduces band gap energy (from 2.82 to 1.79 eV) and lowers Na-ion diffusion barriers, finally reciprocating the vigorous electrochemical kinetics of the electrode. Especially, the elaborately designed material-Na₂Fe_0.97Ni_0.03SiO₄@C-displays superior Na-storage properties of around 197.51 mA h g^-1 (corresponding to 1.43 Na⁺ intercalation) at 0.1 C within 1.5-4.5 V along with desirable capacity retention (84.44% after 100 cycles), and the rate capability is also markedly enhanced (a capacity of 133.62 mA h g^-1 at 2 C). Such the effective methodology employed in this work opens a potential pathway to synthesize the silicate cathode material with excellent electrochemical properties.

Carbon Loaded Nano-Designed Spherically High Symmetric Lithium Iron Orthosilicate Cathode Materials for Lithium Secondary Batteries

In the present study, Li2FeSiO4 (LFS) cathode material has been prepared via a modified polyol method. The stabilizing nature of polyol solvent was greatly influenced to reduce the particle size (~50 nm) and for coating the carbon on the surface of the as-mentioned materials (~10 nm). As-prepared nano-sized Li2FeSiO4 material deliver initial discharge capacity of 186 mAh·g-1 at 1C with the coulombic efficiency of 99% and sustain up to 100 cycles with only 7 mAh·g-1 is the difference of discharge capacity from its 1st cycle to 100th cycle. The rate performance illustrates the discharge capacity 280 mAh·g-1 for lower C-rate (C/20) and 95 mAh·g-1 for higher C-rate (2C).