Daniela Kovacheva

Comparing the Behavior of Nano- and Microsized Particles of LiMn1.5Ni0.5O4 Spinel as Cathode Materials for Li-Ion Batteries

We report on a rigorous comparative study of nano- and microparticles of Limn 1.5 Ni 0.5 O 4 spinel as cathode materials for Li-ion batteries. The stability of these materials in LiPF 6 /alkyl carbonate solutions in temperatures up to 70°C was explored. Capacity, cycling, rate capabilities, and impedance behavior were also studied. The methods included X-ray diffraction, Raman, X-ray photelectron, Fourier transform infrared, and electron paramagnetic resonance spectroscopies, and electron microscopy, in conjunction with standard electrochemical techniques: voltammetry, chronopotentiometry, and impedance spectroscopy. These materials show an impressive stability in solutions at elevated temperature. The use of nanomaterials was advantageous for obtaining a better rate capability of LiMn 1.5 Ni 0.5 O 4 electrodes. LiMn 1.5 Ni 0.5 O 4 particles develop a unique surface chemistry in solutions that passivates and protects them from detrimental interactions with solution species at elevated temperatures.

Synthesizing nanocrystalline LiMn2O4 by a combustion route

Nanocrystalline samples of lithium manganese oxide with cubic spinel structure have been prepared by combustion of reaction mixtures containing Li(I) and Mn(II) nitrates that operate as oxidisers, and sucrose that acts as fuel. The samples were characterised by X-ray diffraction, transmission electron microscopy, thermal analysis, and impedance and electrochemical measurements. The effect of the fuel content on the purity and morphology of the products was analysed. The samples as prepared showed small amounts of Mn2O3 and Mn3O4 as impurities, depending on the amount of sucrose used in the synthesis. Annealing at 700 °C led to single-phase cubic spinels. In these phases, the smallest average particle size (ca. 30 nm) corresponded to the sample obtained with a hyperstoichiometric amount of fuel. This sample showed the Li1.05Mn1.95O4 composition as deduced from the thermal and electrochemical data. No variation in conductivity associated with the cubic⇔orthorhombic phase transition was observed. The electrochemical behaviour as positive electrode showed good cyclability at high current densities (reversible capacity of 73 mAh g−1 at 2.46 mA cm−2).

Nanosize LiNiyMn2 −yO4(0 < y≤ 0.5) spinels synthesized by a sucrose-aided combustion method. Characterization and electrochemical performance

Nanosize crystalline cathode materials of LiNiyMn2 − yO4 (0 < y ≤ 0.5) composition and spinel-type structure have been obtained by a single-step sucrose-aided self-combustion method. The as-prepared samples contained some amorphous organic impurities that were removed after a short period of heating at 500 °C. The pure single-phase spinels have been characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and nitrogen sorption isotherms. The samples consist of particles (ca. 24 nm size) that are aggregated in clusters (ca. 1 µm size) in which mesopores (10–80 nm size) appear among the particles. Additional heating at 800° and 1000 °C produces a slight increase in the cubic lattice parameter and a pronounced increase in particle size (>100 nm). Electrical conductivity decreases as the Ni content increases in accordance with an electron hopping mechanism between Mn3+ and Mn4+ ions. The 500 °C- and 800 °C-heated LiNi0.5Mn1.5O4 samples show good electrochemical behaviour at 4.7 V as cathode materials. The capacity (132.7 mA h g−1) found is close to the nominal capacity (146.7 mA h g−1) and remains constant for current densities in the range C/24–2C (where C = 2.6 mA cm−2). At higher current densities (2C–10C) the capacity decreases progressively. The cyclability at the C current density is ca. 99.7% for both samples.

On the Performance of LiNi1 / 3Mn1 / 3Co1 / 3O2 Nanoparticles as a Cathode Material for Lithium-Ion Batteries

We report on the behavior of nanometric LiMn 1/3 Ni 1/3 CO 1/3 O 2 (LiMNC) as a cathode material for Li-ion batteries in comparison with the same material with submicrometric particles. The LiMNC material was produced by a self-combustion reaction, and the particle size was controlled by the temperature and duration of the follow-up calcination step. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, Raman spectroscopy, electron paramagnetic resonance, inductively coupled plasma, and atomic force microscopy were used in conjunction with standard electrochemical techniques (cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy) for characterizing the electrode materials. The effect of cycling and aging at 60°C was also explored. Nanomaterials are much more reactive in standard electrolyte solutions than LiMNC with a submicrometric particle. They develop surface films that impede their electrochemical response, while their bulk structure remains stable during aging and cycling at elevated temperatures. The use of nanomaterials in Li-ion batteries is discussed.

Stabilizing nickel-rich layered cathode materials by a high-charge cation doping strategy: zirconium-doped LiNi0.6Co0.2Mn0.2O2

Ni-rich layered lithiated transition metal oxides Li[NixCoyMnz]O2 (x + y + z = 1) are the most promising materials for positive electrodes for advanced Li-ion batteries. However, one of the drawbacks of these materials is their low intrinsic stability during prolonged cycling. In this work, we present lattice doping as a strategy to improve the structural stability and voltage fade on prolonged cycling of LiNi0.6Co0.2Mn0.2O2 (NCM-622) doped with zirconium (+4). It was found that LiNi0.56Zr0.04Co0.2Mn0.2O2 is stable upon galvanostatic cycling, in contrast to the undoped material, which undergoes partial structural layered-to-spinel transformation during cycling. The current study provides sub-nanoscale insight into the role of Zr4+ doping on such a transformation in Ni-rich Li[NixCoyMnz]O2 materials by adopting a combined experimental and first-principles theory approach. A possible mechanism for a Ni-mediated layered-to-spinel transformation in Ni-rich NCMs is also proposed.

Studies of nanosized LiNi0.5Mn0.5O2-layered compounds produced by self-combustion reaction as cathodes for lithium-ion batteries

We report here on the synthesis of layered nanosized LiNi 0.5 Mn 0.5 O 2 compound by a modified self-combustion reaction and the studies of its electrochemical behavior as a cathode material for Li cells. Capacities up to 180-200 mAh/g could be reached during cycling of these electrodes even at 60°C in LiPF 6 /alkyl-carbonate solutions. The nano-LiNi 0.5 Mn 0.5 O 2 material exhibits remarkable stability during prolonged aging and cycling at elevated temperatures. Surface films developed on the nano-LiNi 0.5 Mn 0.5 O 2 particles in the course of cycling/aging contain polycarbonates, LiF, nickel and manganese oxides, and fluorides, and lead to efficient passivation and stabilization of the electrodes.

Crystalline and Amorphous Electroless Co-W-P Coatings

Electroless deposition onto polycrystalline (Cu, Au) and amorphous (Ni-P) substrates was applied to prepare Co-W-P coatings of two types: crystalline (hexagonal close packed, hcp), with low phosphorus content about 2.4-2.7 atom %, and amorphous, with P concentration within 7.4-8.3 atom %. Tungsten content varied typically in the narrow range of 2.9-3.7 atom % in both types of coatings. Atomic force microscopy revealed substantial difference in their morphology. Polycrystalline Co-W-P coatings consist of grains of stacked plates (lamellas), confirmed by transmission electron microscopy also. Amorphous films are smoother and uniform. The crystalline structure promotes the surface oxidation to a higher extent than the amorphous structure, as shown by the X-ray photoelectron spectroscopy (XPS). Auger electron spectroscopy depth profiles display oxidation, smoothly diminishing toward the inside of the crystalline films. Amorphous coatings are oxidized only at the surface. Inside both types of coatings, however, all alloy components are in nonoxidized form, according to XPS data. Differential scanning calorimetry (DSC) studies of amorphous coatings revealed three transformation peaks, ascribed to a crystallization of hypoeutectic alloy and a transition of Co-based hcp phase into face-centered cubic. Magnetic properties variation with temperature is in agreement with DSC results.

Calorimetric study of amorphous Sb-Se thin films

The crystallization of amorphous SbxSe100−x (40≤x≤70 at.%) alloy thin films prepared by vacuum deposition was studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The crystalline phases were identified as Sb2Se3 and Sb. The compositional dependence of the glass transition and crystallization temperatures as well as the heat of crystallization were determined all of them being with maximal values at composition close to the eutectics of the Sb–Se system. These thermodynamic quantities decrease with further increasing of the antimony concentration in thin films. The crystallization kinetics was measured using non-isothermal treatment mainly. The effective activation energies of crystallization were estimated to be in the 2.48–2.66 eV range and the frequency factors were between 1026–1028 s−1.

Effect of the Thermal Treatment on the Particle Size and Electrochemical Response of LiCr0.2Mn1.8 O 4 Spinel

Spinel of composition LiCr 0 . 2 Mn 1 . 8 O 4 has been synthesized by a combustion-aided procedure. The particles of the sample are single crystals of ca. 10 nm size as deduced from X-ray diffraction and transmission electron microscopy. Thermal treatment of the as-prepared sample at increasing temperature (400, 600, 700, 750, 900, 1000, and 1100°C) for different times (1, 3, or 24 h) leads to an increase of the particle size, which attains a value as high as 1560 nm after heating at 1100°C for 1 h. Electrochemical response of the heated samples as positive electrodes in a lithium cell has been studied at three rates (C/24, C, and 3C; where C = 2.9 mA). At C/24 the discharge capacity measured at 4 and 4.8 V is not appreciably affected by the particle size. At C and 3C the discharge capacity measured at those voltages decreases as the particle size increases; a linear dependence has been found for particle sizes above 50 nm. On cycling we have found that the capacity fade also depends on the particle size; the larger the particles the less the capacity fade at the mentioned voltages.

Comparison of the Structure and Chemical Composition of Crystalline and Amorphous Electroless Ni-W-P Coatings

Electroless deposition was applied to prepare Ni-W-P coatings of two types: crystalline, with low P and W, and amorphous with high W content. Their morphology was studied by atomic force microscopy. Polycrystalline Ni-W-P alloys consist of grains of stacked plates (lamellas), as it was revealed by transmission electron microscopy. The coatings exhibit a (100) texture. X-ray diffraction and electron diffraction analysis demonstrated the unit cell parameter of the crystalline phase in Ni-W-P is practically equal to that of pure Ni. This implies W and P are localized along the grain boundaries. The Warren/Averbach method was applied to determine microstrain and size of coherent scattering domains. The crystalline structure promotes the surface oxidation of Ni-W-P to higher extent in comparison with the amorphous structure. X-ray photoelectron spectroscopy analysis demonstrated the presence of oxygen and carbon in the bulk of the crystalline Ni-W-P coatings in larger quantities than in the amorphous. In the crystalline coatings in addition to oxygen and carbon, scanning Auger electron spectroscopy showed the presence of nitrogen. It is supposed that all these elements come from ligand residues adsorbed at the grain boundaries during coating growth. Nanoindentation tests indicated that amorphous Ni-W-P samples display more uniform surface mechanical properties at the nanometer scale than the crystalline ones.