Enhanced electrochemical performance of carbon-LiMn1−xFexPO4 nanocomposite cathode for lithium-ion batteries

4 V-class olivine C-LiMn_1−xFe_xPO₄ (x = 0 and 0.15) are synthesized by ultrasonic pyrolysis followed by ball milling with AB carbon to evaluate the doping effect of iron. The C-LiMn_0.85Fe_0.15PO₄ shows excellent rate capability having discharge capacity of 150 mAh g⁻¹ at 0.5C-rate and 121 mAh g⁻¹ at 2C-rate. The capacity retention of the C-LiMn_0.85Fe_0.15PO₄ is 91% after 50 cycles at 55 °C whereas C-LiMnPO₄ is limited to 87%. The improved electrochemical performance of the C-LiMn_0.85Fe_0.15PO₄ electrode is attributed to the enhanced electrical conductivity caused by tighter binding on the carbon particles with the LiMn_0.85Fe_0.15PO₄ primary particles as well as by the surface coating of carbon on the primary particles.     

Electrochemical performances of LiFe1−xMnxPO4 with high Mn content

A series of LiFe_1−xMn_xPO₄/C materials with high Mn content (0.7 ≤ x ≤ 0.9) are synthesized by solid state reaction. The samples have mesoporous structure with an average pore size of 25 nm, particle size around 200-300 nm, crystalline size around 30 nm and specific areas around 50 m² g⁻¹. Their electrochemical performances are studied and the reversible capacity and rate performance decrease with the increase of Mn content. The redox potential of the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox couple also shift accordingly. The overpotential value of the Mn²⁺/Mn³⁺ redox couple (80 mV) is close to that of the Fe²⁺/Fe³⁺ couple (60 mV) in all three compositions and shows a maximum (∼300 mV) in the regions of voltage transition.     

Synthesis and performance of Li3(V1−xMgx)2(PO4)3 cathode materials

In order to search for cathode materials with better performance, Li₃(V_1−xMg_x)₂(PO₄)₃ (0, 0.04, 0.07, 0.10 and 0.13) is prepared via a carbothermal reduction (CTR) process with LiOH·H₂O, V₂O₅, Mg(CH₃COO)₂·4H₂O, NH₄H₂PO₄, and sucrose as raw materials and investigated by X-ray diffraction (XRD), scanning electron microscopic (SEM) and electrochemical impedance spectrum (EIS). XRD shows that Li₃(V_1−xMg_x)₂(PO₄)₃ (x = 0.04, 0.07, 0.10 and 0.13) has the same monoclinic structure as undoped Li₃V₂(PO₄)₃ while the particle size of Li₃(V_1−xMg_x)₂(PO₄)₃ is smaller than that of Li₃V₂(PO₄)₃ according to SEM images. EIS reveals that the charge transfer resistance of as-prepared materials is reduced and its reversibility is enhanced proved by the cyclic votammograms. The Mg²⁺-doped Li₃V₂(PO₄)₃ has a better high rate discharge performance. At a discharge rate of 20 C, the discharge capacity of Li₃(V_0.9Mg_0.1)₂(PO₄)₃ is 107 mAh g⁻¹ and the capacity retention is 98% after 80 cycles. Li₃(V_0.9Mg_0.1)₂(PO₄)₃//graphite full cells (085580-type) have good discharge performance and the modified cathode material has very good compatibility with graphite.     

High-throughput studies of Li1−xMgx/2FePO4 and LiFe1−yMgyPO4 and the effect of carbon coating

A two-dimensional sample array synthesis has been used to screen carbon-coated Li_(1−x)Mg_x/2FePO₄ and LiFe_(1−y)Mg_yPO₄ powders as potential positive electrode materials in lithium ion batteries with respect to x, y and carbon content. The synthesis route, using sucrose as a carbon source as well as a viscosity-enhancing additive, allowed introduction of the Mg dopant from solution into the sol–gel pyrolysis precursor. High-throughput XRD and cyclic voltammetry confirmed the formation of the olivine phase and percolation of the electronic conduction path at sucrose to phosphate ratios between 0.15 and 0.20. Measurements of the charge passed per discharge cycle showed that the capacity deteriorated on increasing magnesium in Li_(1−x)Mg_x/2FePO₄, but improved with increasing magnesium in LiFe_(1−y) Mg_yPO₄, especially at high scan rates. Rietveld-refined XRD results on samples of LiFe_(1−y)Mg_yPO₄ prepared by a solid-state route showed a single phase up to y = 0.1 according to progressive increases in unit cell volume with increases in y. Carbon-free samples of the same materials showed conductivity increases from 10⁻¹⁰ to 10⁻⁸ S cm⁻¹ and a decrease of activation energy from 0.62 to 0.51 eV. Galvanostatic cycling showed near theoretical capacity for y = 0.1 compared with only 80% capacity for undoped material under the same conditions.     

High-power LiFePO4 cathode materials with a continuous nano carbon network for lithium-ion batteries

To meet the requirements of high-power products (ex. electric scooters, hybrid electric vehicles, pure electric vehicles and robots), high-energy safe lithium-ion batteries need to be developed in the future. This research will focus on the microstructures and electrochemical properties of olivine-type LiFePO₄ cathode materials. The morphologies of LiFePO₄/C composite materials show spherical-type particles and have good carbon conductive networks. From the TEM bright field image and EELS mapping, the LiFePO₄/C powder shows continuous, dispersive nano-carbon network. These structures will improve electron transfer and lithium-ion diffusion for LiFePO₄ cathode materials, and increase their conductivity from 10⁻⁹ S cm⁻¹ to 10⁻³ S cm⁻¹. The electrochemical properties of LiFePO₄/C cathode material in this work demonstrated high rate capability (≥12 C) and long cycle life (≥700 cycles at a 3 C discharge rate).     

Effect of carbon coating on LiNi1/3Mn1/3Co1/3O2 cathode material for lithium secondary batteries

Amorphous carbon is coated on LiNi_1/3Mn_1/3Co_1/3O₂ cathode material for lithium batteries. The carbon-coated material shows improved thermal stability and electrochemical performance compared with bare material.     

Shift of redox potential and kinetics in Lix(MnyFe1−y)PO4

The relatively high redox potential in the olivine-type LiMPO₄ (M = Mn, Fe, Co, Ni) materials has largely been explained by the M–O–P inductive effect which increases the ionic character of transition metal atoms. Here, we identify the additional perturbative effect that slightly but systematically shifts the redox potential. The substitution of iron by manganese in the olivine LiMPO₄ framework raises both of the Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox potentials by ∼0.1 V. The overall volume expansion upon Mn substitution in the whole Li_x(Mn_yFe_1−y)PO₄ system possibly increases the average metal-oxide bond length and hence the ionicity of each transition metal. The voltage shift in a single cell is small but should be significant in a larger-scale battery where there exist a large number of series connections. The kinetic shift for each of the Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox reactions is also investigated.     

The electrochemical property of ZrFx-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode material

Electrochemical and thermal properties of pristine and ZrF_x-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are compared. The hydrothermal method is introduced for the fabrication of a uniform coating layer. The formation of a compact coating layer on the surface of pristine powder is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From TEM-EDS and XPS analysis, it is inferred that the coating layer is ZrO_xF_y (zirconium oxyfluoride) form. The coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrodes have better rate capability and cyclic performance at high temperature compared with the pristine electrode. The thermal stability of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode is also enhanced by the ZrF_x coating. Such enhancements are due to the presence of a stable coating layer, which effectively suppresses the chemical instability ascribed to surface reaction between electrode and electrolyte.     

Thermal stability of Li1−yNixMn(1−x)/2Co(1−x)/2O2 layer-structured cathode materials used in Li-Ion batteries

In order to develop safe lithium-ion batteries using Ni-based cathode active materials, such as LiNi_xMn_(1−x)/2Co_(1−x)/2O₂, thermal stability is one of the most important requirements. We used XRD and TDS-MS in the first step of our study to elucidate the thermal stability and to improve it under anomalous high temperature conditions. We investigated the relationship between the thermal stability and cathode composition, especially for that of the nickel and lithium content. The XRD indicated that the crystal structure of electrochemically delithiated materials changed from a layered into a spinel structure followed by a rock-salt structure as the temperature rose. The TDS-MS indicated that these changes coincided with the release of oxygen from the cathode materials. We found that decreasing the lithium content and increasing the nickel content made the temperature of the crystal structure change and oxygen release lower, and thus, influenced the cathode composition.     

Enhanced electrochemical properties of Li(Ni0.4Co0.3Mn0.3)O2 cathode by surface modification using Li3PO4-based materials

The surface of a commercial Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode is modified using Li₃PO₄-based coating materials. The electrochemical properties of the coated materials are investigated as a function of the pH value of the coating solution and the composition of coating materials. The Li₃PO₄ coating solution with pH 2 is found to be favorable for the formation of stable coating layers having enhanced electrochemical properties. The Li₃PO₄, Li_1.5PO₄, and PO₄ coating layers are formed as amorphous phases. However, the Li_3−xNi_x/2PO₄ coating layers are composed of small particles with a crystalline phase covered with an amorphous phase. Li₃PO₄ and Li_1.5PO₄ coatings considerably enhance the rate capability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode. In contrast, the Li_3−xNi_x/2PO₄ coating material, which contained Ni, has an inferior rate capability compared to the Li_xPO₄ series (x = 1.5 and 3), although the LiNiPO₄-coated electrode shows a better rate capability than a pristine one. Li₃PO₄-based coating materials are effective at enhancing the cyclic performance of the electrode in the voltage range of 3.0-4.8 V. DSC analysis also confirms the improved thermal stability attained by coating the cathode with Li₃PO₄-based materials.     

Effects of fluorine-substitution on the electrochemical behavior of LiFePO4/C cathode materials

The effects of fluorine substitution on the electrochemical properties of LiFePO₄/C cathode materials were studied. Samples with stoichiometric proportion of LiFe(PO₄)_1−xF_3x/C (x = 0.025, 0.05, 0.1) were prepared by adding LiF in the starting materials of LiFePO₄/C. XRD and XPS analyses indicate that LiF was completely introduced into bulk LiFePO₄ structure in LiFe(PO₄)_1−xF_3x/C (x = 0.025, 0.05) samples, while there was still some excess of LiF in LiFe(PO₄)_0.9F_0.3/C sample. The results of electrochemical measurement show that F-substitution can improve the rate capability of these cathode materials. The LiFe(PO₄)_0.9F_0.3/C sample showed the best high rate performance. Its discharge capacity at 10 C rate was 110 mAh g⁻¹ with a discharge voltage plateau of 3.31–3.0 V versus Li/Li⁺. The LiFe(PO₄)_0.9F_0.3/C sample also showed obviously better cycling life at high temperature than the other samples.     

Preparation and characterization of Ti-doped LiFePO4 cathode materials for lithium-ion batteries

Olivine structured LiFePO₄ (lithium iron phosphate) and Ti⁴⁺-doped LiFe_1−xTi_xPO₄ (0.01 ≤ x ≤ 0.09) powders were synthesized via a solution route followed by heat-treatment at 700 °C for 8 h under N₂ flowing atmosphere. The compositions, crystalline structure, morphology, carbon content, and specific surface area of the prepared powders were investigated with ICP-OES, XRD, TEM, SEM, EA, and BET. Capacity retention study was used to investigate the effects of Ti⁴⁺ partial substitution on the intercalation/de-intercalation of Li⁺ ions in the olivine structured cathode materials. Among the prepared powders, LiFe_0.97Ti_0.03PO₄ manifests the most promising cycling performance as it was cycled with C/10, C/5, C/2, 1C, 2C, and 3C rate. It showed initial discharge capacity of 135 mAh g⁻¹ at 30 °C with C/10 rate. From the results of GSAS refinement for the prepared samples, the doped-Ti⁴⁺ ions did not occupy the Fe²⁺ sites as expected. However, the occupancy of the doped Ti⁴⁺ ions are still not clear, and theoretical calculations are needed for further studies. From the variation of lattice parameters calculated by the least square method without refinement, it suggested that Ti⁴⁺-doped LiFePO₄ samples formed solid solutions at low doping levels while TiO₂ was also observed with TEM in samples prepared with doping level higher than 5 mol%.     

Electrochemical and structural studies of LiCo1/3Mn1/3Fe1/3PO4 as a cathode material for lithium ion batteries

A solid-state reaction to synthesize a lithium multi-transition metal phosphate LiCo_1/3Mn_1/3Fe_1/3PO₄ is used in this work, which has a high voltage of 3.72 V and capacity of 140 mAh g⁻¹ at a 0.05 C rate. From the in-situ XRD analysis, LiCo_1/3Mn_1/3Fe_1/3PO₄ has shown a high stability during cell charge/discharge, even operating at 5 V, which is due to the stable olivine structure. Although all the transition metals Co²⁺, Mn²⁺ and Fe²⁺ are at the same 4c site of the LiCo_1/3Mn_1/3Fe_1/3PO₄ structure, they seem to have different chemical activities and reflect on the electrochemical performance. The capacity contributed by the Co²⁺/Co³⁺ redox couple is only 20 mAh g⁻¹, which is less than that of the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox couples. This is because of the fact that the diffusivity of lithium ion for the Co²⁺/Co³⁺ redox couple is 10⁻¹⁶ cm² s⁻¹ which is one order less than that of the Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox couples in LiCo_1/3Mn_1/3Fe_1/3PO₄.     

Influence of lithium content on high rate cycleability of layered Li1+xNi0.30Co0.30Mn0.40O2 cathodes for high power lithium-ion batteries

Layered Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) materials have been synthesized using citric acid assisted sol-gel method. The materials with excess lithium showed distinct differences in the structure and the charge and discharge characteristics. The rate capability tests were performed and compared on Li_1+xNi_0.30Co_0.30Mn_0.40O₂ (x = 0, 0.05, 0.10, 0.15) cathode materials. Among these materials, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode demonstrated higher discharge capacity than that of the other cathodes. Upon extended cycling at 1C and 8C, Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ showed better capacity retention when compared to other materials with different lithium content. Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ exhibited 93 and 90% capacity retention where as Li_1.05Ni_0.30Co_0.30Mn_0.40O₂, Li_1.15Ni_0.30Co_0.30Mn_0.40O₂, and Li_1.00Ni_0.30Co_0.30Mn_0.40O₂ exhibited only 84, 71, and 63% (at 1C), and 79, 66 and 40% (at 10C) capacity retention, respectively, after 40 cycles. The enhanced high rate cycleability of Li_1.10Ni_0.30Co_0.30Mn_0.40O₂ cathode is attributed to the improved structural stability due to the formation of appropriate amount of Li₂MnO₃-like domains in the transition metal layer and decreased Li/Ni disorder (i.e., Ni content in the Li layer).     

Synthesis and electrochemical properties of Li[Ni0.45Co0.1Mn0.45−xZrx]O2 (x = 0, 0.02) via co-precipitation method

Li[Ni_0.45Co_0.1Mn_0.45−xZr_x]O₂ (x = 0, 0.02) was synthesized via co-precipitation method. Partial Zr doping on the host structure of Li[Ni_0.45Co_0.1Mn_0.45]O₂ was carried out to improve the electrochemical properties. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.43Zr_0.02]O₂ was evaluated in terms of specific discharge capacity, cycling performance and thermal stability. The Zr-doped Li[Ni_0.45Co_0.1Mn_0.45−xZr_0.02]O₂ shows the improved cycling performance and stable thermal stability. The major exothermic reaction was delayed from 252.1 °C to 289.4 °C.     

Synthesis and characterization of LiNi0.6Mn0.4−xCoxO2 as cathode materials for Li-ion batteries

LiNi_0.6Co_xMn_0.4−xO₂ (x = 0.05, 0.10, 0.15, 0.2) cathode materials are prepared, and their structural and electrochemical properties are investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetric (DSC) and charge–discharge test. The results show that well-ordering layered LiNi_0.6Co_xMn_0.4−xO₂ (x = 0.05, 0.10, 0.15, 0.2) cathode materials are successfully prepared in air at 850 °C. The increase of the Co content in LiNi_0.6Mn_0.4−xCo_xO₂ leads to the acceleration of the grain growth, the increase of the initial discharge capacity and the deterioration of the cycling performance of LiNi_0.6Mn_0.4−xCo_xO₂. It also leads to the enhancement of the ratio Ni³⁺/Ni²⁺ in LiNi_0.6Co_xMn_0.4−xO₂, which is approved by the XPS analysis, resulting in the increase of the phase transition during cycling. This is speculated to be main reason for the deteriotion of the cycling performance. All synthesized LiNi_0.6Co_xMn_0.4−xO₂ samples charged at 4.3 V show exothermic peaks with an onset temperature of larger than 255 °C, and give out less than 400 J g⁻¹ of total heat flow associated with the peaks in DSC analysis profile, exhibiting better thermal stability. LiNi_0.6Co_0.05Mn_0.35O₂ with low Co content and good thermal stability presents a capacity of 156.6 mAh g⁻¹ and 98.5% of initial capacity retention after 50 cycles, showing to be a promising cathode materials for Li-ion batteries.     

Nano-sized LiMn2O4 spinel cathode materials exhibiting high rate discharge capability for lithium-ion batteries

Nano-sized LiMn₂O₄ spinel with well crystallized homogeneous particles (60 nm) is synthesized by a resorcinol-formaldehyde route. Micro-sized LiMn₂O₄ spinel with micrometric particles (1 μm) is prepared by a conventional solid-state reaction. These two samples are characterized by XRD, SEM, TEM, BET, and electrochemical methods. At current rate of 0.2C (1C = 148 mA g⁻¹), a discharge capacity of 136 mAh g⁻¹ is obtained on the nano-sized LiMn₂O₄, which is higher than that of micro-sized one (103 mAh g⁻¹). Furthermore, compared to the micro-sized sample, nano-sized LiMn₂O₄ shows much better rate capability, i.e. a capacity of 85 mAh g⁻¹, 63% of that at 0.2C, is realized at 60C. The excellent high rate performance of nano-sized LiMn₂O₄ spinel may be attributed to its impurity-free nano-sized particles, higher surface area and well crystalline. The outstanding electrochemical performances demonstrate that the nano-sized LiMn₂O₄ spinel will be the promising cathode materials for high power lithium-ion batteries used in hybrid and electric vehicles.     

xLi2MnO3·(1 − x)LiMO2 blended with LiFePO4 to achieve high energy density and pulse power capability

A lithium-ion positive electrode is proposed that contains both high energy density and efficient pulse power capability, even at low state-of-charge (SOC). The pulse power capability at low SOC is attractive for applications, such as plug-in hybrid electric vehicles (PHEVs), which require pulse power operation over the entire useable SOC window. A lithium- and manganese-rich transition-metal layered-oxide (LMR-NMC), also classified as a layered-layered oxide material, is blended with a lithium iron phosphate (LFP) to achieve a potentially low-cost, high-performance electrode. The LMR-NMC material provides high energy by delivering cathode material gravimetric energy densities greater than 890 Wh kg⁻¹. The pulse power capability of this material at low SOC is greatly improved by incorporating a modest quantity of LFP. The LFP serves as an internal redox couple to charge and discharge the more rate-limited LMR-NMC material at moderate to low SOCs.     

One-pot, glycine-assisted combustion synthesis and characterization of nanoporous LiFePO4/C composite cathodes for lithium-ion batteries

Nanoporous LiFePO₄/C composite cathodes have been synthesized by a novel one-pot, glycine-assisted combustion (GAC) method in presence of 2 wt.% Super P carbon in both Ar and 90% Ar-10% H₂ atmospheres at 750 °C for a short time of 6 h. While the Ar atmosphere offers phase pure samples, the Ar-H₂ atmosphere leads to the formation of impurity phases as indicated by X-ray diffraction data. The combustion-initiated expulsion of gases aids the formation of a nanoporous LiFePO₄/C composite structure as evident from electron microscopic analysis, which could allow easy penetration of the electrolyte and realization of an electronic-ionic 3D network. The nanoporous LiFePO₄/C sample synthesized in Ar atmosphere exhibits a high discharge capacity of 160 mAh g⁻¹ with 3% capacity fade in 50 cycles and high rate capability. With a short reaction time, the GAC method offers an energy efficient approach to synthesize high performance olivine LiFePO₄/C composite cathodes.