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[en] Highlights: • The LiNiPO4@C powders were synthesized by microwave-assisted solvothermal process. • A diversified special type of microwave synthesis ‘low-level and long-time microwave synthesis’ was applied. • The formation of a perfect core–shell morphology about 6 nm thickness was achieved. • LiNiPO4 synthesized using isopropanol as the reaction medium shows the best electrochemical performance. • LiNiPO4@C (isopropanol) electrode reaches a discharge capacity of 157 mAh g−1 at 0.1 C-rate. - Abstract: Nanoscale, LiNiPO4-core and carbon-shell high voltage (>5 V) LiNiPO4@C cathode materials have been synthesized using a microwave & solvothermal methodology using different solvents, ethylene glycol, isopropanol, isobutanol and water as the reaction medium. The effects of these solvents on the crystal-quality, morphology and electrochemical qualification of the produced materials have been evaluated in terms of the heating efficiency in the microwave field by using various opto-analytical techniques and electrochemical measurements. The heating characteristics in terms of the absorption of the energy as part of the microwave-material interaction phenomenon is also discussed. X-ray diffraction analyses demonstrate that it is possible to synthesize substantially pure LiNiPO4 crystal using isopropanol as the reaction medium. High-resolution transmission electron microscopy analysis reveals that this combined methodology can provide core-shell morphology with a 5–6 nm coating thickness. The LiNiPO4@C cathode material produced in an isopropanol environment exhibits the best electrochemical properties, achieving a discharge capacity of 157 mAh g−1 at a 0.l C-rate, and shows almost 81% capacity retention at the end of the 80th cycle. Thus, this paper offers a perspective for solving the difficulties encountered in modifying a high voltage LiNiPO4 cathode, especially the deficiency in terms of cycle life behavior, and the further benefits are highlighted.
[en] Graphical abstract: Display Omitted - Highlights: • Structural, morphological and electrochemical effects of Mg doped LiFe_1_−_xMg_xPO_4-C are investigated. • LiFe_1_−_xMg_xPO_4-C (x = 0.00–0.08) cathode materials are compared pure LiFePO_4 and LiFePO_4-C. • Cheaply and uncomplicated sol-gel assisted carbothermal reduction method is used. • End of the 300 cycles, capacity retention is almost 97.1% for LiFe_0_._9_6Mg_0_._0_4PO_4-C electrode. - ABSTRACT: Pure LiFePO_4 and the nano-sized LiFe_1_−_xMg_xPO_4-C (x = 0.00, 0.02, 0.04, 0.06 and 0.08) cathode materials have been prepared and investigated for Li-ion batteries. Samples were synthesized by handy and cheap sol–gel-assisted carbothermal reduction method. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and inductively coupled plasma (ICP) have been used to study the crystal structure, morphology and chemical composition of all produced materials. XRD findings reveal the slight decrease in crystal lattice of LiFePO_4 after Mg"2"+ doping. Phase pure samples crystallize in the olivine-type structure with a linear relationship between lattice parameters (a, b and c) and chemical composition. The FE-SEM images have proved that Mg-doped particles are not agglomerated and the particle sizes (40–60 nm) are able to compete with the literature. The synthesized small particles of the sol–gel-assisted carbothermal reduction process lead to a superior capacity in comparison to a common solid-state synthesis. For a Mg content of 0.04% the capacity is reached to a higher level (167 mA h g"−"1) and good capacity retention of 97.0% over 300 cycles is observed. Although doping with Mg has a remarkable effect on improving its electronic or ionic mobility, but serious electrochemical degradation will occur when its doping density is beyond 0.04 mol. The cycling voltammogram (CV) shows that Mg-doped LiFe_0_._9_6Mg_0_._0_4PO_4-C electrode has improved electrical conductivity and diffusion coefficient of Li"+ ions, in which Mg"2"+ is related to effectively act as a pillar in crystal lattice structure to prevent the collapse during lithium intercalation process
[en] The carbon-free LiNiPO_4 and cobalt doped LiNi_1_−_xCo_xPO_4/C (x = 0.0–1.0) were synthesized and investigated for high voltage applications (> 4 V) for Li-ion batteries. Nano-scale composites were prepared by handy sol–gel approach using citric acid under slightly reductive gas atmosphere (Ar-H_2, 85:15%). Structural and morphological characteristics of the powders were revealed by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and inductively coupled plasma (ICP). Except for a small impurity phase (Ni_3P), phase pure samples crystallized in the olivine-lattice structure with a linear relationship between lattice parameters (a, b and c) and chemical composition. The FE-SEM images proved that LiNiPO_4/C particles (50–80 nm) did not agglomerate, and showed that as the cobalt content was higher agglomeration had increased. The electrochemical properties of all electrodes were investigated by galvanostatic charge–discharge measurements. Substitution of Ni"2 "+ by Co"2 "+ caused higher electronic conductivities and showed more effective Li"+ ion mobility. When the cobalt content is 100%, the capacity reached to a higher level (146.2 mA h g"− "1) and good capacity retention of 85.1% at the end of the 60 cycles was observed. The cycling voltammogram (CV) revealed that LiCoPO_4/C electrode improved the electrochemical properties. The Ni"3 "+–Ni"2 "+ redox couple was not observed for carbon free LiNiPO_4. Nevertheless, it was observed that carbon coated LiNiPO_4 sample exhibits a significant oxidation (5.26 V)–reduction (5.08 V) peaks. With this study, characteristics of the LiNi_1_−_xCo_xPO_4/C series were deeply evaluated and discussed. - Highlights: • Structural, morphological and electrochemical effects of Co doped LiNi_1_− _xCo_xPO_4/C electrodes are investigated. • Cheap, effective and simple sol–gel approach is used. • After the 60th cycle, capacity retention is almost 87% for LiCoPO_4/C electrode. • LiNiPO_4/C sample exhibits a distinctive oxidation (5.26 V)–reduction (5.08 V) peaks
[en] Nanostructured LiCo_1_−_xMn_xPO_4/C (x = 0 and 0.05) materials were successfully produced as superior quality cathodes by combined sol-gel and carbothermal reduction methods. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive spectroscopy (EDS), fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), cyclic voltammetry (CV) and galvanostatic measurements were applied to determine the phase purity, morphology and electrochemical qualifications. HR-TEM analysis reveals that the thickness of the surface carbon layer of 5 to 10 nm range with the uniform distribution. LiCo_0_·_9_5Mn_0_·_0_5PO_4/C particles were between 40 and 80 nm and the same material exhibits a higher and stable reversible capacity (140 mA h g"−"1) with the long voltage plateau (4.76 V). Substitution of Co"2"+ with Mn"2"+ in LiCoPO_4/C has an influence on the initial discharge capacity and excellent cycling behaviour. The obtained results have attributed that production dynamics in nano-synthesis, the coating process with proper carbon source and an effective doping represent three parameters to prepare favorable cathode materials. - Highlights: • Structural, morphological and electrochemical effects of Mn doped LiCo_1_−_xMn_xPO_4–C electrodes are investigated. • Cheap, effective and simple sol-gel assisted carbothermal reduction approach is used. • After 60th cycle, capacity retention is almost 92% for LiCo_0_·_9_5Mn_0_._0_5PO_4–C electrode. • Mn-doped sample exhibits distinctive oxidation (4.76 V and 4.12 V) peaks.