Synthesis of high-performance Fe–Mg-co-doped LiMnPO<Subscript>4</Subscript>/C via a mechano-chemical liquid-phase activation technique

Carbon-coated Fe–Mg-homogeneously dispersed Li(Mn_0.9Fe_0.10)_1???x Mg _x PO₄/C (x?=?0.00, 0.01, 0.03, 0.05, and 0.07) powders are synthesized via a mechano-chemical liquid-phase activation technique. Fine-sized and Fe²⁺ and Mg²⁺ evenly distributed precursors are formed using this efficient approach successfully. The synthesis temperature and the Mg²⁺ doping ratio are investigated and optimized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and electrochemical measurements. Mg doping decreases the lattice parameters of LiMn_0.9Fe_0.1PO₄/C, which will ease the expansion/shrinking effect during the insertion/de-insertion processes. Li(Mn_0.9Fe_0.1)_0.95Mg_0.05PO₄/C synthesised at 700 °C with ~3 wt% of carbon additive presents the best comprehensive electrochemical properties, and it displays good rate capability with specific discharge capacity of 153 mAh g^?1 at 0.1C, 140 mAh g^?1 at 1C, and 132 mAh g^?1 at 2C rate. The results suggest that the electrochemical performance of the LiMnPO₄-based cathode is improved as (Mn_0.9Fe_0.1) is partially substituted by Mg.     

Structural and electrochemical properties of LiFe1 − 3x/2Bi x PO4/C synthesized by sol–gel

In this study, LiFe_1???3x/2Bi _x PO₄/C cathode material was synthesized by sol–gel method. From XRD patterns, it was found that the Bi-doped LiFePO₄/C cathode material had the same olivine structure with LiFePO₄/C. SEM studies revealed that Bi doping can effectively decrease the particle sizes. It shortened Li⁺ diffusion distance between LiFePO₄ phase and FePO₄ phase. The LiFe_0.94Bi_0.04PO₄/C powder exhibited a specific initial discharge capacity of about 149.6 mAh g^?1 at 0.1 rate as compared to 123.5 mAh g^?1 of LiFePO₄/C. EIS results indicated that the charge-transfer resistance of LiFePO₄/C decreased greatly after Bi doping.     

Electrochemical comparison of LiFe<Subscript>0.4</Subscript>Mn<Subscript>0.595</Subscript>Cr<Subscript>0.005</Subscript>PO<Subscript>4</Subscript>/C and LiMnPO<Subscript>4</Subscript>/C cathode materials

A comparison of electrochemical performance between LiFe_0.4Mn_0.595Cr_0.005PO₄/C and LiMnPO₄/C cathode materials was conducted in this paper. The cathode samples were synthesized by a nano-milling-assisted solid-state process using caramel as carbon sources. The prepared samples were investigated by XRD, SEM, TEM, energy-dispersive X-ray spectroscopy (EDAX), powder conductivity test (PCT), carbon-sulfur analysis, electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge cycling. The results showed that LiFe_0.4Mn_0.595Cr_0.005PO₄/C exhibited high specific capacity and high energy density. The initial discharge capacity of LiFe_0.4Mn_0.595Cr_0.005PO₄/C was 163.6 mAh g^?1 at 0.1C (1C = 160 mA g^?1), compared to 112.3 mAh g^?1 for LiMnPO₄/C. Moreover, the Fe/Cr-substituted sample showed good cycle stability and rate performance. The capacity retention of LiFe_0.4Mn_0.595Cr_0.005PO₄/C was 98.84 % over 100 charge-discharge cycles, while it was only 86.64 % for the pristine LiMnPO₄/C. These results indicated that Fe/Cr substitution enhanced the electronic conductivity for the prepared sample and facilitated the Li⁺ diffusion in the structure. Furthermore, LiFe_0.4Mn_0.595Cr_0.005PO₄/C composite presented high energy density (606 Wh kg^?1) and high power density (574 W kg^?1), thus suggested great potential application in lithium ion batteries (LIBs).     

Enhancement of capacity at high charge/discharge rate and cyclic stability of LiFePO4/C by nickel doping

By introducing nickel chemical into the precursor sol of LiFePO₄, a series of Ni-doped LiFePO₄ composite cathode materials, denoted as LiFe_1???x Ni _x PO₄/C (x?=?0, 0.01, 0.03, 0.05 and 0.10) were prepared by a spray drying–carbothermal approach. The materials were characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and electrochemical impedance spectrum etc. It is found that the doping of nickel with appropriate amount caused a slight shift of diffraction peaks towards higher angles and enhanced the dispersion of nanoprimary particles, which could be observed from their XRD patterns and SEM images. For the sample with 3 mol% Ni doing, the charge transfer resistance reduced from 52.4?Ω of LiFePO₄ to 18.7?Ω of LiFe_0.97Ni_0.3PO₄/C, and the potential interval of the redox peaks reduced from 0.51 to 0.40 V, indicating the better reversible of Ni-doped materials. For the sample LiFe_0.97Ni_0.03PO₄/C, its initial discharge capacities at various rates are 169.2 (0.2 C), 156.2 (1.0 C), 147.9 (2.0 C), 135.5 (5.0 C), and 94.0 (10.0 C)?mAh g^?1, respectively, enhanced by 55.2 % (at 5.0 C) and 82.1 % (at 10.0 C) compared with LiFePO₄. Furthermore, after 200 cycles of charge/discharge at 0.5 C, the capacity of LiFe_0.97Ni_0.03PO₄/C only decreased 8.8 %, but over 25 % decrease was observed for LiFePO₄/C.     

Simple co-precipitation synthesis of high-voltage spinel cathodes with different Ni/Mn ratios for lithium-ion batteries

The high-voltage spinel is a promising cathode material in next generation of lithium-ion batteries. Samples LiNi_0.5???xMn_1.5?+?xO₄ (x?=?0, 0.05, 0.1) are synthesized by a simple co-precipitation method, in which pH value and temperature conditions do not need control. In the simple co-precipitation method, NaHCO₃ solution is poured into transition metal solution to produce precursor. Ni and Mn are distributed uniformly in the products. The as-prepared samples are composed of ~?200 nm primary particles. Samples LiNi_0.5???xMn_1.5?+?xO₄ (x?=?0, 0.05, 0.1) are also tested to study the effects of different Ni/Mn ratios. Sample LiNi_0.5Mn_1.5O₄ delivers discharge capacities of 130 mAh g^?1 at 0.2 C. The decreasing of Ni/Mn ratio in samples reduces specific capacity. With the decreasing of Ni/Mn ratios in spinel, amount of Mn³⁺ are increased. Attributed to its high Mn³⁺ contents, sample LiNi_0.4Mn_1.6O₄ delivers the highest discharge capacity of 106 mAh g^?1 at a large current density of 15 C, keeping 84.5% of that at 0.2 C rate. With the increasing of Ni/Mn ratios in spinel, cycling performance is improved. Sample LiNi_0.5Mn_1.5O₄ shows the best cycling stability, keeping 94.4% and 90.4% of the highest discharge capacities after 500 cycles at 1 C and 1000 cycles at 5 C.     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

LiFexMn1-xPO4固溶体的密度泛函理论与电化学研究

本文用DFT计算方法研究了LiFe_xMn_1-xPO₄的热力学稳定性和嵌/脱锂电位. 结果表明,LiFe_xMn_1-xPO₄固溶体的自由能比相分离的LiFePO₄/LiMnPO₄混合物略高,这两种形式可能在实际LiFe_xMn_1-xPO₄材料中共存. 计算表明,LiFe_xMn_1-xPO₄固溶体的嵌/脱锂电位随锰/铁比以及过渡金属离子的空间排列而变化,并用计算结果解释了放电曲线的形状. 采用固相反应法合成了LiFe_xMn_1-xPO₄材料并研究了其电化学性质,实验中观察到附加的放电平台,其出现可能与LiFe_xMn_1-xPO₄固溶体的存在有关.     

Magnetization and Mössbauer study of double layer perovskites La1.5Ca1.5Mn2 − x Fe x O7 prepared by sol–gel method

Magnetization and Mössbauer studies have been made for understanding magnetic behavior of three double perovskite systems La_1.5Ca_1.5Mn_2???x Fe _x O₇ corresponding to x = 0.05, 0.10 and 0.50. These have been prepared following sol–gel route. Substitution of Fe does not lead to any major change in the tetragonal cell but increased iron leads to greater distortion in octahedral site. The three samples undergo paramagnetic–ferromagnetic transition. Curie temperature (T _c) for the system with 0.05 Fe is ～150 K which is lower than (190 K) for the system without iron; with 0.50 Fe T _c goes below 50 K. Iron goes as Fe^3?+? and replaces Mn in ab plane. With increasing Fe the valence states of Mn get re-distributed in a way that number of the Jahn–Teller ions Mn^3?+? increases and that of the pairs of Mn^3?+?–O–Mn^4?+? experiencing double exchange decreases.     

Insertion properties of LiFe<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>PO<Subscript>4</Subscript> electrode materials for Li-ion batteries

This paper addresses the synthesis structural and electrochemical properties of LiFe_0.5Mn_0.5PO₄ electrode materials for Li-ion batteries. The charge–discharge reaction of Li/LiPF₆-EC–DEC/LiFe_0.5Mn_0.5PO₄ cell carried out at the 1-C rate shows a capacity retention of 128 mAh/g. The local structure of the delithiated Li _x Fe_0.5Mn_0.5PO₄ phases have been studied by Fourier transform infrared spectroscopy and magnetometry. Spectral features indicate that the structure of the delithiated phase remains in the orthorhombic system. The compositional dependence of the magnetic moment is found to be in quantitative agreement with the theoretical value predicted for oxidation of M ²⁺ ions in the high spin state. Paper presented at the 11th Euro-Conference on Science and Technology of Ionics, Batz-sur-Mer, France, 9–15 Sept. 2007     

Effects of yttrium ion doping on electrochemical performance of LiFePO<Subscript>4</Subscript>/C cathodes for lithium-ion battery

The olivine-type LiFe_1-x Y _x PO₄/C (x?=?0, 0.01, 0.02, 0.03, 0.04, 0.05) products were prepared through liquid-phase precipitation reaction combined with the high-temperature solid-state method. The structure, morphology, and electrochemical performance of the samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS), galvanostatic charge-discharge, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). We found that the small amount of Y³⁺ ion-doped can keep the microstructure of LiFePO₄, modify the particle morphology, decrease charge transfer resistance, and enhance exchange current density, thus enhance the electrochemical performance of the LiFePO₄/C. However, the large doping content of Y³⁺ ion cannot be completely doped into LiFePO₄ lattice, but existing partly in the form of YPO₄. The electrochemical performance of LiFePO₄/C was restricted owing to YPO₄. Among all the doped samples, LiFe_0.98Y_0.02PO₄/C showed the best electrochemical performance. The LiFe_0.98Y_0.02PO₄/C sample exhibited the initial discharge capacity of 166.7, 155.8, 148.2, 139.8, and 121.1 mAh g^?1 at a rate of 0.2, 0.5, 1, 2, and 5 C, respectively. And, the discharge capacity of the material was 119.6 mAh g^?1 after 100 cycles at 5 C rates.     

Effects of Al doping for Li[Li0.09Mn0.65*0.91Ni0.35*0.91]O2 cathode material

Li-rich Mn-based Li[Li_0.09Mn_{0.65*(0.91???x)} Ni_{0.35*(0.91???x)} Al _x ]O₂ cathode materials have been prepared by traditional solid-state reaction. The lattice parameters a, c, and V have decreased, but c/a increased with the increase of Al doping. All the samples show analogy morphology of a quasi-spherical shape. Li[Li_0.09Mn_0.591Ni_0.319]O₂ sample shows a higher initial discharge capacity of 239.4 mAh?g^?1 at 20 mA?g^?1, while Li[Li_0.09Mn_0.582Ni_0.314Al_0.015]O₂ sample presents a higher discharge capacity of 170.1 mAh?g^?1 and ratio of 72.0 % with 200 vs. 20 mA?g^?1. The solid electrolyte interface resistance (R _SEI) and charge transfer process resistance (R _ct ) values are relatively smaller for Al-doped samples than those of non-doped samples. Almost no reduction is observed after 24-time cycles in different discharge rates for the samples prepared.     

Electrochemical performance of Mo-doped LiFePO4/C composites prepared by two-step solid-state reaction

To improve the performance of LiFePO₄, LiFe_1?x Mo _x PO₄/C (x?=?0, 0.005, 0.010, 0.015, 0.020, 0.025) cathode materials were synthesized via two-step ball milling solid-state reaction. The prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectra, and galvanostatic charge–discharge test. It is apparent from XRD analysis that Mo doping enlarges the interplanar distance of crystal plane parallel to [010] direction in LiFePO₄. In other words, it widens one-dimensional diffusion channels of Li⁺ along the [010] direction. The results of electrochemical test indicate that the LiFe_0.99Mo_0.01PO₄/C composite exhibits a discharge capacity of 144.8 mAh g^?1 at 1 C rate, a decreased charge transfer resistance of 162.4 Ω and better reversibility of electrode reactions. The present synthesis route is promising and practical for the preparation of LiFePO₄ materials.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

Study of electrochemical performances of multi-doped spinel Li1.1Mn1.85Co0.075Ni0.075O4 at 4.3 and 5 V

Li_1.1Mn_2???2x Co _x Ni _x O₄ (x?=?0, 0.075) spinel powders were successfully synthesized using a liquid stirred tank reactor method. The electrochemical performances of the undoped and doped spinels at 4.3 and 5 V were investigated by X-ray diffraction, field-emission scanning electron microscopy, and electrochemical impedance spectroscopy. The capacity of Li_1.1Mn_2???2x Co _x Ni _x O₄ could be divided into two parts, with 4.3 V as the dividing line in the 3–5 V charge–discharge range. Low capacity and good cyclic performance were obtained when cycled in the 3–4.3 V range for the multi-doped Li_1.1Mn_2???2x Co _x Ni _x O₄ spinel. In comparison with multi-doped spinel at 4.3 V, the results of the cyclic performance worsened at 5 V because the structure underwent further shrinkage, the charge transfer resistance rose and the electrolyte decomposed.     

Synergistic effect of magnesium and fluorine doping on the electrochemical performance of lithium-manganese rich (LMR)-based Ni-Mn-Co-oxide (NMC) cathodes for lithium-ion batteries

Mg-doped-LMR-NMC (Li_1.2Ni_0.15-xMg_xMn_0.55Co_0.1 O₂) is synthesized by combustion method followed by fluorine doping by solid-state synthesis. In this approach, we substituted the Ni²⁺ by Mg²⁺ in varying mole percentages (x = 0.02, 0.05, 0.08) and partly oxygen by fluorine (LiF: LMR-NMC = 1:50 wt%). The synergistic effect of both magnesium and fluorine substitution on electrochemical performance of LMR-NMC is studied by electrochemical impedance spectroscopy and galvanostatic-charge-discharge cycling. Mg-F-doped LMR-NMC (Mg 0.02 mol) composite cathodes shows excellent discharge capacity of ~300 mAh g^?1 at C/20 rate whereas pristine LMR-NMC shows the initial capacity around 250 mAh g^?1 in the voltage range between 2.5 and 4.7 V. Mg-F-doped LMR-NMC shows lesser Ohmic and charge transfer resistance, cycles well, and delivers a stable high capacity of ~280 mAh g^?1 at C/10 rate. The voltage decay which is the major issue of LMR-NMC is minimized in Mg-F-doped LMR-NMC compared to pristine LMR-NMC.     

Coatings and weathering rinds at Gusev crater, Mars, investigated by depth selective Mössbauer spectroscopy

Powders of Al substituted Ni-ferrites (NiAl _x Fe_2???x O₄ x?=?0, 0.5, 1.0 and 1.5) were prepared using the calcination method. Mössbauer spectroscopy results of all samples show two well defined magnetic sextets corresponding to Fe^3?+? ions in the A (tetrahedral) and B (octahedral) sites of the spinel structure. In addition, a quadrupole doublet with quadrupole splitting of (~0.53 mm/s) starts evolving upon Al substitution. This doublet is associated with a new Fe–Al-oxide phase. The X-ray diffraction patterns show, in addition to the original peaks, new peaks evolve and increase in intensity as the Al concentration increases, confirming the evolution of this new phase.     

Study on LiFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript>/C used as cathode materials for lithium-ion batteries with low Sm component

LiFe_1???x Sm _x PO₄/C cathode materials were synthesized though a facile hydrothermal method. Compared with high-temperature solid-phase sintering, the method can allow for the fabrication of low Sm content (2 %), a scarce and expensive rare earth element, while the presence of an optimized carbon coating with large amount of sp²-type carbon sharply increases the material’s electrochemical performance. The high-rate dischargeability at 5 C, as well as the exchange current density, can be increased by 21 and 86 %, respectively, which were attributed to the fine size and the large cell parameter a/c as much. It should be pointed out that the a/c value will be increased for the LiFePO₄ Sm-doped papered by both of the two methods, while the mechanism is different: The value c is increased for the front and the value a is decreased for the latter, respectively.     

Influence of differential stress on the galvanic interaction of pyrite–chalcopyrite

The effect of stress action on pyrite–chalcopyrite galvanic corrosion was investigated using polarization curves and electrochemical impedance spectroscopy (EIS) measurements. When stress increased from 0 to 4?×?10⁵ Pa, the corrosion current density of pyrite–chalcopyrite increased from 5.678 to 6.719 μA cm^?2, and the corrosion potential decreased from 281.634 to 270.187 mV, accompanied by a decrease in polarization resistance from 25.09 to 23.79 Ω·cm². EIS results show there have three time constants in the Nyquist diagrams, which indicated the presence of different steps during the corrosion process. Stress dramatically enhanced pyrite–chalcopyrite galvanic corrosion by affecting the Cu_1???x Fe_1???y S₂ film and the double layer, whereas had little impact on the adsorption species. When the stress changed from 0 to 4?×?10⁵ Pa, the pore resistance and capacitance of the Cu_1???x Fe_1???y S₂ film, R _p and Q _p, changed by 25.72 and 72.28 %, respectively. The adsorption species resistance, R _sl, and capacitance, Q _sl, only changed by 9.77 and 2.31 %, respectively.     

Synthesis and pseudocapacitive investigation of LiCr x Mn2-x O4 cathode material for aqueous hybrid supercapacitor

A series of Cr-substituted LiMn₂O₄ samples (LiCr _x Mn_2-x O₄, 0?≤?x?≤?0.3) were synthesized by a urea-assisted combustion method to enhance pseudocapacitive properties of LiMn₂O₄ material in aqueous electrolyte. Their structure and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The LiCr _x Mn_2-x O₄ and activated carbon (AC) electrode were used as the cathode and anode in hybrid supercapacitors, respectively, which capacitive properties were determined by cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectroscopy (EIS) in Li₂SO₄ solution. The results revealed that the partial substitution of Mn³⁺ by Cr³⁺ decreased initial capacity, but it prevented capacity fading. In the working voltage of 0–1.4 V, the AC/LiCr_0.1Mn_1.9O₄ capacitor delivered an initial specific capacitance of 41.6 F g^?1 (based on the total active mass of two electrodes) at a current density of 100 mA g^?1 in 1 M Li₂SO₄ solution. After 1,000 cycles, its capacity loss was only 1.7 %.     

Synthesis and characterization of LiMn1-x Fe x PO4/carbon nanotubes composites as cathodes for Li-ion batteries

Carbon nanotubes (CNT) coated with LiMn_1-x Fe _x PO₄ (0.2?≤?x?≤?0.8), as possible cathode materials, was synthesized by using a sol–gel process (Polyol method), after annealing under flowing nitrogen. X-ray diffraction (XRD) patterns of the composites confirmed the formation of the olivine structured LiMn_1-x Fe _x PO₄ phase and no secondary phases were detected. The morphological investigation revealed the formation of agglomerates with particles size ranging between 300 and 700 nm. XRD investigation of composites shows difference of the morphology by doping CNT and carbon black in the composites. Transmission electron microscopy shows the growth of nano-sized particles on CNT (20–70 nm) and the agglomeration of primary particles to form secondary particles. The X-ray photoelectron spectroscopy showed that the Fe and Mn ions are in divalent states in the LiMn_1-x Fe _x PO₄ composites. The cyclic voltamograms showed the oxidation peaks of iron and manganese ions at 3.53–3.63 and 4.05–4.33 V, respectively, while the reduction peaks were found at 3.21–3.42 V (iron reduction) and 3.85–3.93 V (manganese reduction) depending on the iron content in the composition. The LiMn_0.6Fe_0.4PO₄/CNT composite (x?=?0.4) (with 20 %?wt CNT) delivered a specific capacity of 120 mAhg^?1 (at a discharge rate of C/20 and RT).