Effect of synthetic conditions on the electrochemical properties of LiMn0.4Fe0.6PO4/C synthesized by sol–gel technique

Carbon-coated LiMn_0.4Fe_0.6PO₄ (LMFP) was synthesized by sol–gel technique using citric acid as foaming agent and carbon precursor. To evaluate the effect of synthetic conditions on the electrochemical properties of LMFP for use as cathode active material, the carbon-coated olivines were synthesized by a two-step thermal treatment at different temperatures. The composites were characterized by elemental analysis, XRD, SEM, TEM, Raman microprobe spectroscopy and their electrochemical properties were also studied. The composite that shows the better electrochemical performance has more porous structure, lower D/G band ratio in Raman spectra, and charge and discharge capacities of same 155 mAh g⁻¹ with higher material utilization of 97% at 0.1 C-rate (0.05 mA cm⁻²). The material exhibiting the better performance was also incorporated in a polymer electrolyte hosted in an electrospun P(VdF-HFP) membrane. The lithium polymer battery composed of LiMn_0.4Fe_0.6PO₄ cathode and polymer electrolyte showed a good cycling performance with the initial discharge capacity of 146 mAh g⁻¹.     

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.     

Fabrication and characterization of a co-fired La0.6Sr0.4Co0.2Fe0.8O3−δ cathode-supported Ce0.9Gd0.1O1.95 thin-film for IT-SOFCs

A dense membrane of Ce_0.9Gd_0.1O_1.95 on a porous cathode based on a mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was fabricated via a slurry coating/co-firing process. With the purpose of matching of shrinkage between the support cathode and the supported membrane, nano-Ce_0.9Gd_0.1O_1.95 powder with specific surface area of 30 m² g⁻¹ was synthesized by a newly devised coprecipitation to make the low-temperature sinterable electrolyte, whereas 39 m² g⁻¹ nano-Ce_0.9Gd_0.1O_1.95 prepared from citrate method was added to the cathode to favor the shrinkage for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ. Bi-layers consisting of <20 μm dense ceria film on 2 mm thick porous cathode were successfully fabricated at 1200 °C. This was followed by co-firing with NiO–Ce_0.9Gd_0.1O_1.95 at 1100 °C to form a thin, porous, and well-adherent anode. The laboratory-sized cathode-supported cell was shown to operate below 600 °C, and the maximum power density obtained was 35 mW cm⁻² at 550 °C, 60 mW cm⁻² at 600 °C.     

Synthesis and electrochemical performance of the high voltage cathode material Li[Li0.2Mn0.56Ni0.16Co0.08]O2 with improved rate capability

The high voltage layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode material, which is a solid solution between Li₂MnO₃ and LiMn_0.4Ni_0.4Co_0.2O₂, has been synthesized by co-precipitation method followed by high temperature annealing at 900 °C. XRD and SEM characterizations proved that the as prepared powder is constituted of small and homogenous particles (100-300 nm), which are seen to enhance the material rate capability. After the initial decay, no obvious capacity fading was observed when cycling the material at different rates. Steady-state reversible capacities of 220 mAh g⁻¹ at 0.2C, 190 mAh g⁻¹ at 1C, 155 mAh g⁻¹ at 5C and 110 mAh g⁻¹ at 20C were achieved in long-term cycle tests within the voltage cutoff limits of 2.5 and 4.8 V at 20 °C.     

Preparation of manganese oxide with high density by decomposition of MnCO3 and its application to synthesis of LiMn2O4

Manganese oxide with high tap density was prepared by decomposition of spherical manganese carbonate, and then LiMn₂O₄ cathode materials were synthesized by solid-state reaction between the manganese oxide and lithium carbonate. Structure and properties of the samples were determined by X-ray diffraction, Brunauer–Emmer–Teller surface area analysis, scanning electron microscope and electrochemical measurements. With increase of the decomposition temperature from 350 °C to 900 °C, the tap density of the manganese oxide rises from 0.91 g cm⁻³ to 2.06 g cm⁻³. Compared with the LiMn₂O₄ cathode made from chemical manganese dioxide or electrolytic manganese dioxide, the LiMn₂O₄ made from manganese oxide of this work has a larger tap density (2.53 g cm⁻³), and better electrochemical performances with an initial discharge capacity of 117 mAh g⁻¹, a capacity retention of 93.5% at the 15th cycle and an irreversible capacity loss of 2.24% after storage at room temperature for 28 days.     

Lithium difluoro(oxalato)borate/ethylene carbonate + propylene carbonate + ethyl(methyl) carbonate electrolyte for LiMn2O4 cathode

Lithium difluoro(oxalato)borate (LiODFB) was investigated as a lithium salt for non-aqueous electrolytes for LiMn₂O₄ cathode in lithium-ion batteries. Linear sweep voltammetry (LSV) tests were used to examine the electrochemical stability and the compatibility between the electrolytes and LiMn₂O₄ cathode. Through inductively coupled plasma (ICP) analysis, we compared the amount of Mn²⁺ dissolved from the spinel cathode in 1 mol L⁻¹ LiPF₆/EC + PC + EMC (1:1:3 wt.%) and 1 mol L⁻¹ LiODFB/EC + PC + EMC (1:1:3 wt.%). AC impedance measurements and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ cathode. These results demonstrate that ODFB anion can capture the dissolution manganese ions and form a denser and more compact surface film on the cathode surface to prevent the continued Mn²⁺ dissolution, especially at high temperature. It is found that LiODFB, instead of LiPF₆, can improve the capacity retention significantly after 100 cycles at 25 °C and 60 °C, respectively. LiODFB is a very promising lithium salt for LiMn₂O₄ cathode in lithium-ion batteries.     

Electrical conductivity and reaction with lithium of LiFe1−yMnyPO4 olivine-type cathode materials

Structural, electrical and electrochemical properties of Mn-substituted phospho-olivines LiFe_1−yMn_yPO₄ were investigated and compared to those of LiFePO₄. Rietvield refined XRD patterns taken in the course of delithiation process showed apparent difference between phase compositions of these cathode materials upon lithium extraction. Contrary to the LiFePO₄ and LiMnPO₄ compositions for which a two-phase mechanism of electrochemical lithium extraction/insertion is observed, in case of Mn-substituted LiFe_1−yMn_yPO₄ samples a single-phase mechanism of deintercalation was observed in the studied range of lithium concentration. Electrochemical characterization of the cathode materials were performed in Li/Li⁺/Li_xFe_1−yMn_yPO₄-type cells for y = 0.0, 0.25, 0.55, 0.75 and 1.0 compositions. Voltammery studies showed low reversibility of the lithium extraction process in the high-voltage “manganese” range, while in the “iron” range the reversibility of lithium extraction is high. Impedance measurements of the LiFe_1−yMn_yPO₄ cathode materials, which enabled separation of the ionic and electronic components of their entire electrical conductivity, showed distinct influence of Mn content on the electronic part of conductivity. EIS measurements performed at different states of cell charge revealed that the charge-transfer impedance in Li_xFe_1−yMn_yPO₄ is much lower than that of Li_xFePO₄.     

Effects of roasting temperature and modification on properties of Li2FeSiO4/C cathode

Li₂FeSiO₄/C cathodes were synthesized by combination of wet-process method and solid-state reaction at high temperature, and effects of roasting temperature and modification on properties of the Li₂FeSiO₄/C cathode were investigated. The XRD patterns of the Li₂FeSiO₄/C samples indicate that all the samples are of good crystallinity, and a little Fe₃O₄ impurity was observed in them. The primary particle size rises as the roasting temperature increases from 600 to 750 °C. The Li₂FeSiO₄/C sample synthesized at 650 °C has good electrochemical performances with an initial discharge capacity of 144.9 mAh g⁻¹ and the discharge capacity remains 136.5 mAh g⁻¹ after 10 cycles. The performance of Li₂FeSiO₄/C cathode is further improved by modification of Ni substitution. The Li₂Fe_0.9Ni_0.1SiO₄/C composite cathode has an initial discharge capacity of 160.1 mAh g⁻¹, and the discharge capacity remains 153.9 mAh g⁻¹ after 10 cycles. The diffusion coefficient of lithium in Li₂FeSiO₄/C is 1.38 × 10⁻¹² cm² s⁻¹ while that in Li₂Fe_0.9Ni_0.1SiO₄/C reaches 3.34 × 10⁻¹² cm² s⁻¹.     

The influence of compositional change of 0.3Li2MnO3·0.7LiMn1−xNiyCo0.1O2 (0.2 ≤ x ≤ 0.5, y = x − 0.1) cathode materials prepared by co-precipitation

Cathode materials prepared by a co-precipitation are 0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (0.2 ≤ x ≤ 0.4) cathode materials with a layered-spinel structure. In the voltage range of 2.0-4.6 V, the cathodes show more than one redox reaction peak during its cyclic voltammogram. The Li/0.3Li₂MnO₃·0.7LiMn_1−xNi_yCo_0.1O₂ (x = 0.3, y = 0.2) cell shows the initial discharge capacity of about 200 mAh g⁻¹. However, when x = 0.2 and y = 0.1, the cell exhibits a rapid decrease in discharge capacity and poor cycle life.     

Structural and electrochemical studies of Ba0.6Sr0.4Co1−yTiyO3−δ as a new cathode material for IT-SOFCs

The influence of titanium doping level in Ba_0.6Sr_0.4Co_1−yTi_yO_3−δ (BSCT) oxides on their phase structure, electrical conductivity, thermal expansion coefficient (TEC), and single-cell performance with BSCT cathodes has been investigated. The incorporation of Ti can lead to the phase transition of Ba_0.6Sr_0.4CoO_3−δ from hexagonal to cubic structure. The solid solution limitation of Ti in Ba_0.6Sr_0.4Co_1−yTi_yO_3−δ is 0.15–0.3 under 1100 °C. BSCT shows a small polaron conduction behavior and the electrical conductivity increases steadily in the testing temperature range (300–900 °C), leading to a relatively high conductivity at high temperatures. The electrical conductivity decreases with increasing Ti content. The addition of Ti deteriorates the cathode performance of BSCT slightly but decreases the TEC significantly. The TEC of BSCT is about 14 × 10⁻⁶ K⁻¹, which results in a good physical compatibility of BSCT with Gd_0.2Ce_0.8O_2−δ (GDC) electrolyte. BSCT also shows excellent thermal cyclic stability of electrical conductivity and good chemical stability with GDC. These properties make BSCT a promising cathode candidate for intermediate temperature solid oxide fuel cells (IT-SOFCs).     

Li3V2(PO4)3 cathode material synthesized by chemical reduction and lithiation method

The monoclinic-type Li₃V₂(PO₄)₃ cathode material was synthesized via calcining amorphous Li₃V₂(PO₄)₃ obtained by chemical reduction and lithiation of V₂O₅ using oxalic acid as reducer and lithium carbonate as lithium source in alcohol solution. The amorphous Li₃V₂(PO₄)₃ precursor was characterized by using TG–DSC and XPS. The results showed that the V⁵⁺ was reduced to V³⁺ by oxalic acid at ambient temperature and pressure. The prepared Li₃V₂(PO₄)₃ was characterized by XRD and SEM. The results indicated the Li₃V₂(PO₄)₃ powder had good crystallinity and mesoporous morphology with an average diameter of about 30 nm. The pure Li₃V₂(PO₄)₃ exhibits a stable discharge capacity of 130.08 mAh g⁻¹ at 0.1 C (14 mA g⁻¹).     

Electrochemical behavior of Li3−xM′xV2−yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries

Composites of monoclinic Li_3−xM′_xV_2−yM″_2y(PO₄)₃ (M′ = K, M″ = Sc, Mg + Ti) with carbon were synthesized by solid-state reaction using oxalic acid or 6% H₂/Ar gas mixture as reducing agents at sintering temperature of 850 °C. The samples were characterized by X-ray diffraction (XRD), voltammetry and electrochemical galvanostatic cycling. The capacity of Li₃V₂(PO₄)₃ synthesized using hydrogen as the reducing agent was 127 mA h g⁻¹ and decreased to 120 mA h g⁻¹ after 20 charge-discharge cycles. The substitution of lithium and vanadium for other ions did not result in the improvement of the electrochemical characteristics of the samples.     

Electrochemical insertion/deinsertion of sodium on NaV6O15 nanorods as cathode material of rechargeable sodium-based batteries

A well defined nano-structured material, NaV₆O₁₅ nanorods, was synthesized by a facile low temperature hydrothermal method. It can perform well as the cathode material of rechargeable sodium batteries. It was found that the NaV₆O₁₅ nanorods exhibited stable sodium-ion insertion/deinsertion reversibility and delivered 142 mAh g⁻¹ sodium ions when worked at a current density of 0.02 A g⁻¹. In galvanostatic cycling test, a specific discharge capacity of around 75 mAh g⁻¹ could be obtained after 30 cycles under 0.05 A g⁻¹ current density. Concerned to its good electrochemical performance for reversible delivery of sodium ions, it is thus expected that NaV₆O₁₅ may be used as cathode material for rechargeable sodium batteries with highly environmental friendship and low cost.     

Electrochemical properties of LiFe0.9Mn0.1PO4/Fe2P cathode material by mechanical alloying

LiFePO₄, olivine-type LiFe_0.9Mn_0.1PO₄/Fe₂P composite was synthesized by mechanical alloying of carbon (acetylene back), M₂O₃ (M = Fe, Mn) and LiOH·H₂O for 2 h followed by a short-time firing at 900 °C for only 30 min. By varying the carbon excess different amounts of Fe₂P second phase was achieved. The short firing time prevented grain growth, improving the high-rate charge/discharge capacity. The electrochemical performance was tested at various C/x-rate. The discharge capacity at 1C rate was increased up to 120 mAh g⁻¹ for the LiFe_0.9Mn_0.1PO₄/Fe₂P composite, while that of the unsubstituted LiFePO₄/Fe₂P and LiFePO₄ showed only 110 and 60 mAh g⁻¹, respectively. Electronic conductivity and ionic diffusion constant were measured. The LiFe_0.9Mn_0.1PO₄/Fe₂P composite showed higher conductivity and the highest diffusion coefficient (3.90 × 10⁻¹⁴ cm² s⁻¹). Thus the improvement of the electrochemical performance can be attributed to (1) higher electronic conductivity by the formation of conductive Fe₂P together with (2) an increase of Li⁺ ion mobility obtained by the substitution of Mn²⁺ for Fe²⁺.     

Synthesis and electrochemical characterization of carbon-coated nanocrystalline LiFePO4 prepared by polyacrylates-pyrolysis route

A carbon-coated nanocrystalline LiFePO₄ cathode material was synthesized by pyrolysis of polyacrylate precursor containing Li⁺, Fe³⁺ and PO₄⁻. The powder X-ray diffraction (XRD) and high-resolution TEM micrographs revealed that the LiFePO₄/C composite as prepared has a core-shell structure with pure olivine LiFePO₄ crystallites as cores and intimate carbon coating as a shell layer. Between the composite particulates, there exists a carbon matrix binding the nanocrystallites together into micrometer particles. The electrochemical measurements demonstrated that the LiFePO₄/C composite with an appropriate carbon content can deliver a very high discharge capacity of 157 mAh g⁻¹ (>92% of the theoretical capacity of LiFePO₄) with 95% of its initial capacity after 30 cycles. Since this preparation method uses less costly materials and operates in mild synthetic conditions, it may provide a feasible way for industrial production of the LiFePO₄/C cathode materials for the lithium-ion batteries.     

High voltage spinel cathode materials for high energy density and high rate capability Li ion rechargeable batteries

We have synthesized LiMn_1.5Ni_0.4Cr_0.1O₄ cathode material for high energy density Li ion rechargeable batteries using sol-gel method. The synthesized materials were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, cyclic voltammetry and charge-discharge characteristics. It was found that phase pure materials were obtained an annealing temperature of 875 °C for 15 h. The maximum discharge capacity at a constant charge-discharge current rate 1C, 0.5C, and 0.2C were found to be about 99 mAh g⁻¹, 110 mAh g⁻¹, and 131 mAh g⁻¹, respectively. The capacity retentions after 50 charge-discharge cycles were found to be about 99%, 97%, and 97.3% at discharge current rates of 0.2C, 0.5C, and 1C. The stable electrochemical behavior of the above cathode material even at high C rate, showed that it could be used for high energy density and high rate capability Li ion rechargeable batteries.     

In-situ XRD investigations on structure changes of ZrO2-coated LiMn0.5Ni0.5O2 cathode materials during charge

The structural changes of pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂ cathode materials were investigated by using in situ X-ray diffraction (XRD) during charging process. An obviously solid solution phase transition from a hexagonal structure (H1) to another hexagonal structure (H2) was observed during the charging process at a constant current of 0.3 mA in the potential range of 2.5–5.7 V. The second hexagonal structure has a shorter a-axis and a longer c-axis before the crystal collapse. Before the structure collapses the c-axis length increases to maximum and then significantly decreases to 14.1 Å. The c-axis length of the pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂ increases to the maximum at the charge capacity of 119.2 and 180.9 mAh g⁻¹, respectively. It can be concluded that the ZrO₂ coating can strongly stabilize the crystal structure of the LiMn_0.5Ni_0.5O₂ compound from the comparison of the lattice parameter variations between the pristine and the ZrO₂-coated LiMn_0.5Ni_0.5O₂ compounds upon charge. The potential fluctuation resulting from the decomposition of electrolytes starts at the charge capacity of around 200 and 260 mAh g⁻¹ for the pristine and ZrO₂-coated LiMn_0.5Ni_0.5O₂, respectively. It suggests that the ZrO₂ coating layer can impede the reaction between the cathode material and electrolyte.     

Structural and electrochemical characterization of xLi[Li1/3Mn2/3]O2·(1 − x)Li[Ni1/3Mn1/3Co1/3]O2 (0 ≤ x ≤ 0.9) as cathode materials for lithium ion batteries

A series of cathode materials with molecular notation of xLi[Li_1/3Mn_2/3]O₂·(1 − x)Li[Ni_1/3Mn_1/3Co_1/3]O₂ (0 ≤ x ≤ 0.9) were synthesized by combination of co-precipitation and solid state calcination method. The prepared materials were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, and their electrochemical performances were investigated. The results showed that sample 0.6Li[Li_1/3Mn_2/3]O₂·0.4Li[Ni_1/3Mn_1/3Co_1/3]O₂ (x = 0.6) delivers the highest capacity and shows good capacity-retention, which delivers a capacity ∼250 mAh g⁻¹ between 2.0 and 4.8 V at 18 mA g⁻¹.     

Synthesis and characterization of Li3V(2 − 2x/3)Mgx(PO4)3/C cathode material for lithium-ion batteries

Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C (x = 0, 0.15, 0.30, 0.45) composites have been synthesized by the sol-gel assisted solid state method, using adipic acid C₆H₁₀O₄ (hexanedioic acid) as carbon source. The particle size of the composites is ∼1 μm. During the pyrolysis process, Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C network structure is formed. The effect of Mg²⁺ doped on the electrochemical properties of Li₃V₂(PO₄)₃/C positive materials has been studied. Li₃V_1.8Mg_0.30(PO₄)₃/C as the cathode materials of Li-ion batteries, the retention rate of discharge capacity is 91.4% (1 C) after 100 cycles. Compared with Li₃V₂(PO₄)₃/C, Li₃V_{(2 − 2x/3)}Mg_x(PO₄)₃/C composites have shown enhanced capacity and retention rate capability. The long-term cycles and ex situ XRD tests disclose that Li₃V_1.8Mg_0.30(PO₄)₃ exhibits higher structural stability than the undoped system.     

Electrochemical properties of LiFePO4 prepared via hydrothermal route

LiFePO₄ as a cathode material for rechargeable lithium batteries was prepared by hydrothermal process at 170 °C under inert atmosphere. The starting materials were LiOH, FeSO₄, and (NH₄)₂HPO₄. The particle size of the obtained LiFePO₄ was 0.5 μm. The electrochemical properties of LiFePO₄ were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate (1:1 in volume) containing 1.0 mol dm⁻³ LiClO₄. The hydrothermally synthesized LiFePO₄ exhibited a discharge capacity of 130 mA h g⁻¹, which was smaller than theoretical capacity (170 mA h g⁻¹). The annealing of LiFePO₄ at 400 °C in argon atmosphere was effective in increasing the discharge capacity. The discharge capacity of the annealed LiFePO₄ was 150 mA h g⁻¹.