Nanoparticles have become widely utilized due to their enhanced and unique properties relative to bulk materials. The preparation of nanoparticles can be classified into physical and chemical methods. The physical methods include mechanical milling [1,2], laser ablation [3,4,5], and other aerosol processes with energy sources to provide a high temperature. Among these methods, attractive material processing with thermal plasmas have been proposed for the nanoparticles production. This is because thermal plasmas offer unique advantages; high enthalpy to enhance reaction kinetics, high chemical reactivity, rapid quenching rate in the range of 103–6 K/s, and selectivity of atmosphere in accordance with the required chemical reactions. Thermal plasmas are capable of evaporating large amount of raw materials, even with high melting and boiling temperatures [6,7,8,9]. Furthermore, high-purity nanoparticles can be synthesized in an induction thermal plasma because thermal plasma can be generated in a plasma torch without internal electrodes [10,11]. These advantages of thermal plasmas have brought about advances in plasma chemistry and plasma processing [12,13,14,15].
Lithium metal oxides have attracted many researchers because of their unique properties as cathode for lithium-ion batteries [16,17,18], CO2 sorption material , and other magnetic, electrochemical materials. Layer structured LiCoO2 is widely employed as cathodes in commercial battery applications, in spite of its toxicity and high cost. To solve the economic and environmental problems, alternatives of LiCoO2 have been intensively explored. One of the alternatives is spinel-structured LiMn2O4, which provides a promising high-voltage cathode material for lithium-ion batteries owing to their high theoretical energy density, low cost, and eco-friendliness [18,20,21,22]. LiNiO2 is also a candidate due to its excellent cycle life, with negligible capacity fading etc. [17,23]. The liquid phase method is generally used in the synthesis of lithium metal oxide nanoparticles; however, productivity of the nanoparticles in the liquid phase method is insufficient for industrial application. Therefore, the synthesis method of lithium oxide nanoparticles with a high productivity is strongly demanded.
Thermal plasmas are expected to be promising energy sources to fabricate nanoparticles at high productivity from micron-sized powder as raw materials. Here, one-step synthesis of lithium-metal oxide nanoparticles with induction thermal plasma is studied. The purpose of the present study is to synthesize the lithium metal oxide nanoparticles via the induction thermal plasma and to investigate the formation mechanism of lithium-metal oxide nanoparticles. Different lithium metal systems—Li–Mn, Li–Cr, Li–Co, and Li–Ni—were compared to understand the formation mechanism.
2. Results and Discussion
2.1. Experimental Results
Figure 1 shows X-ray diffraction spectra of the prepared nanoparticles by the induction thermal plasma in different systems—Li–Mn, Li–Co, Li–Cr, and Li–Ni. In the case of the Li–Mn system, main diffraction peaks correspond to spinel-structured LiMn2O4, while diffraction peaks of Mn3O4 are also found. In cases of Li–Co and Li–Cr, layer-structured LiCoO2 and LiCrO2 were found as well as their oxides. In the Li–Ni case, Li0.4Ni1.6O2 and unreacted Li2CO3 were confirmed. These results clearly show the strong influence of the constituent metals on the formation of lithium oxide nanoparticles. Moreover, Figure 2 indicates composition of the prepared nanoparticles analyzed from X-ray diffractometry (XRD) spectra. These compositions of the prepared nanoparticles were used to evaluate the ratio of metal that reacted with Li. Evaluated reaction ratio will be discussed in the following section.
Morphologies and particle sizes of the prepared nanoparticles were observed via transmission electron microscopy (TEM). Figure 3 shows the representative TEM images and the particle size distributions of the nanoparticles in different systems. Many spherical particles with 60 nm in mean diameter are observed in the Li–Ni system. In the cases of Li–Mn, Li–Co, and Li–Cr, most of the particles with 50–80 nm in mean diameters have polyhedral shapes including quadrangular, pentagonal, and hexagonal shapes. In particular, many particles with a hexagonal shape can be found as shown in Figure 3a. Hence, the spinel-structured LiMn2O4 is considered to have a hexagonal shape because LiMn2O4 is a major product in the Li–Mn system, according to XRD results.
Scanning electron microscopy (SEM) observation was carried out to clarify morphology of LiMn2O4 more specifically than TEM observation. Figure 4 shows the representative SEM image of the prepared nanoparticles in the Li–Mn system. Many particles have truncated an octahedral shape as shown in Figure 4. Previous research on morphology of ferrite nanoparticles synthesized by the induction thermal plasma reported that the spinel-structured nanoparticles had a truncated octahedral shape . The spinel-structured nanoparticles synthesized in thermal plasma have a truncated octahedral shape, although the stable structure of the spinel-structured particles are generally considered to be of an octahedral shape. The reason for this truncation is currently under investigation.
2.2. Nanoparticle Formation Mechanism
The relationship between the calculated nucleation temperature and the boiling and melting points is summarized in Figure 5. Because of the unknown properties of metal oxides, only melting point oxides are plotted for metal oxides. These temperatures indicate that the melting points of the oxides are higher than the nucleation temperatures of pure metals in each Li–Me system. Therefore, nucleation of metal oxides is considered to occur at first. Li oxide and metal vapors co-condense onto the nuclei just after the nucleation starts.
The above mechanism can be proposed as a common mechanism for all considered Li–Me systems because the relationship between the nucleation temperature and the melting and boiling temperature of their oxides shows the same trend. However, experimental results show a different ratio of metal reactive with Li to total metal. In the cases of Li–Cr and Li–Mn, high ratios of the reactive metals were obtained, while a low ratio was obtained in Li–Ni system. Then, melting points of each oxide in different system were focused because the reaction rate of metal oxide particles and the condensed lithium would drastically decrease after complete solidification of the growing particles.
Figure 6 shows the relationship between the lowest melting temperature of metal oxides for each Li–Me system and the reaction ratios. The results suggest a lower melting point of oxide leads to a higher reaction ratio. This can be explained by the different reaction time during the nanoparticle formation process. Figure 7 summarizes the formation mechanism of Li–Me oxide nanoparticles. A lower melting point of oxide leads to a longer residence time of growing particles in a liquid-like state, resulting in the longer reaction time with condensed lithium oxide. Consequently, a higher reaction ratio can be obtained in the Li–Me system, as in that of Li–Mn and Li–Cr. On the other hand, a shorter residence time of the growing particles in a liquid-like state leads to a shorter reaction time, resulting in a lower reaction ratio. These results suggest that the melting point of metal oxide has a strong influence on the reaction with lithium oxide.
The above mechanism can explain the obtained results well, although this is still only a hypothesis. Further experimental and theoretical investigation will be required to understand the formation mechanism of complicated oxide nanoparticles in thermal plasmas.
3. Experimental Section
3.1. Experimental Setup and Conditions
A schematic illustration of the experimental setup for Li–Me oxide nanoparticle fabrication is presented in Figure 8a. This equipment is composed of the plasma torch, the reaction chamber where the nanoparticles are synthesized, and the filter unit. Figure 8b shows the enlarged illustration of the plasma torch. High temperature plasma of more than 10,000 K was generated in the plasma torch by induction heating at 4 MHz. The input power was controlled at 20 kW. Ar was used for the carrier gas of the raw powder at 3 L/min and inner gas at 5 L/min. A mixture of argon and oxygen were used for the plasma forming gas at 60 L/min. Mixture of Li2CO3 with 3.5 μm in diameter and metal or metal oxide with 3–10 μm in mean diameter were injected from the powder feeder by Ar carrier gas. Different metals including Mn, Cr, Co, and Ni were compared to investigate the formation mechanism of Li–Me (Mn, Cr, Co, and Ni) nanoparticles. The composition ratio of Li2CO3 to metal was 0.5. The powder feed rate was fixed at 400 mg/min. These experimental conditions are listed in Table 1.
Prepared nanoparticles were collected from the filter and the inner wall of the reaction chamber. Unevaporated raw materials were not confirmed according to SEM observation of the collected particles. This fact implies that the fed raw materials were completely evaporated in the high temperature region of the thermal plasma and converted into nanoparticles during the quenching process. Therefore, the mole fractions of the Li–Me oxides indicated in Figure 2 correspond to the yields of the Li–Me oxide in this nanoparticle fabrication process by the induction thermal plasma.
3.2. Characterization of Prepared Nanoparticles
The phase identification of the prepared nanoparticles was determined by X-ray diffractometry (XRD, Multiflex, Rigaku Co., Tokyo, Japan), operated with Cu Kα source (λ = 0.1541 nm). The diffraction data was collected using a continuous scan mode with a speed of 2 degree/min in the region of 10–90 degrees with a step size of 0.04 degrees. The accelerating voltage and applied current was 40 kV and 50 mA, respectively. The quantitative analysis of the composition of the prepared nanoparticles was conducted based on the whole-powder-pattern-decomposition (WPPD) method with the assumption that no amorphous particles were included in the prepared nanoparticles.
The particle morphology and size distribution of the prepared nanoparticles were observed by TEM (JEM-2100HCKM, JEOL Ltd., Tokyo, Japan), operated at an accelerating voltage of 200 kV. The TEM specimens were prepared by dispersing the as-prepared nanoparticles in ethanol and placing a few drops of the dispersion on a carbon-grid. Furthermore, the 3D particle morphology was observed by field emission (FE)-SEM (SII TES+ Zeiss ULTRA55, Carl Zeiss, Oberkochen, Germany).
As the transition metal entering the composition of the active element of the cathode is oxidized and reduced during the cell charge and discharge, respectively (Equation (1)), the cathode is primarily involved in the cathode process and then in the electrochemical performance of the cell, i.e., potential, specific capacity, energy density, rate capability, etc. [2,3].
Conventional rechargeable Li batteries exhibit rather poor rate performance, even compared with old technologies such as lead-acid . Achieving high rate rechargeable Li-ion batteries depends ultimately on the dimension of the active particles for both negative and positive electrodes . One of the prospective solutions for the preparation of electrodes with high power density is the choice of nanocomposite materials because the geometric design of the insertion compound is a crucial intrinsic property from the viewpoint of structural stability and low kinetics of ions in oxide.
Among the materials capable of delivering high reversible capacity, the layered rhombohedral structures ( space group) that are part of the solid-solution series Li(NiyMnzCo1-y-z)O2 (called NMC hereafter) were first introduced by Liu et al. . The symmetric LiNi1/3Mn1/3Co1/3O2 compound was proposed by Ohzuku’s group in 2001 . These materials are now widely studied as alternative 4-volt cathode materials to replace LiCoO2, exhibiting much higher voltage, great structural stability, and enhanced safety even at elevated temperature and higher reversible charge capacity [10,11,12]. However, the electrochemical performance of NMCs, i.e., capacity retention and long life cycling, strongly depend on the composition, the particle morphology, and the deviation from the ideal rock-salt structure . The reversible specific capacity of NMC was measured to be 160 mAh·g−1 in the cut-off potential range of 2.5–4.4 V and 200 mAh·g−1 in that of 2.8–4.6 V . In the Li1−xNi1/3Mn1/3Co1/3O2 cathode, the charge/discharge process occurs with different oxidation states: the Ni2+/Ni4+ in the range 0 ≤ x ≤ 2/3 and the couple Co3+/Co4+ is activated in the range 2/3 ≤ x ≤ 1, while the electrochemically inactive Mn4+ ions play an important role by stabilizing the electrode structure . The schematic representation of the energy diagram vs. density of states for LixNi1/3Mn1/3Co1/3O2 is shown in Figure 2 for three states of charge (SOC). During the charge process, the Fermi level of the host material, EF, is lowered and for a high degree of delithiation EF is pinned at the top of the O(2p) band, which provides an intrinsic voltage limit for the cathode material .
In this work, nanostructured NMC cathode materials with different chemical configurations (see ternary phase diagram, Figure 3 were synthesized by wet-chemical methods. As the electrochemical performance of NMC materials is extremely dependent on the synthesis method and parameters, we report the influence of the recipe, the particle size and morphology, and the sample composition on electrochemical properties of NMC electrode materials. In addition to the cation mixing (structural defect), the crystallinity, phase purity, particle morphology, grain size, and surface area depend on the synthesis method, and they all play an important role in ionic and electronic transport . A series of mixed transition-metal oxides LiNiwMnyCozO2 samples is investigated for which the following parameters affecting their structural and electrochemical properties are considered: (i) effect of particle size; (ii) effect of the cation mixing; (iii) adjustment of the transition-metal/lithium ratio of the precursor; (iv) effect of chelating agent used in the synthesis; (v) effect of the synthesis recipe; (vi) slight deviation of the cobalt content in the symmetric NMC compound; and (vii) Li-rich integration in layered powders.
2.1. Synthesis Procedures
Layered compounds were synthesized by either solid-state reaction or wet chemistry (“chimie douce”). These solution methods consist of acidification of aqueous solution of the starting compounds was used to prepare the layered cathode materials. The acidification is generally realized by using carboxylic acids. Three classes can be considered according the synthetic process: sol-gel, co-precipitation, and combustion [18,19,20]. The synthesis routes for the different samples are as follows: (1) LiNi0.55Co0.45O2 and NMC powders were prepared by solid-state reaction at 850 °C in air as shown elsewhere ; (2) LiNi0.55Co0.45O2 nano-powders were synthesized by hydrothermal method using acetate raw materials ; (3) a series of mixed transition-metal oxides LiNiwMnyCozO2 (w + y + z = 1) were synthesized by the co-precipitation method. This consists of a hydroxide route using transition-metal hydroxide and lithium carbonate as starting materials, as reported elsewhere . (4) Li1+x(Ni1/3Mn1/3Co1/3)1-xO2 powders were prepared by a hydroxide route using an aqueous solution in which the pH was controlled with care using simultaneously NaOH and NH4OH fed into the reactor; (5) a series of symmetric LiNi1/3Mn1/3Co1/3O2 powders was prepared by the precipitation technique using metal acetates as raw materials and succinic acid as a chelating agent. Different acid to metal-ion molar ratios R were used to study the effect of this parameter on the structural properties of the final product; (6) A series of NMC powders were synthesized by co-precipitation assisted by single dicarboxyl (oxalic) and complexed dicarboxyl (tartaric) acid; (7) a series of LiNi0.33+δMn0.33+δCo0.33-2δO2 with different values of δ was obtained by the sol-gel route assisted by citric acid (tricarboxyl) in keeping the pH of the solution in the range 5–6; (8) Li-rich Li(Li1/3-2x/3NixMn2/3-x/3)O2 (0 ≤ x ≤ 0.5) powders were synthesized by a citrate–gel method using acetate salts. Citric acid was dropped wisely to the solution under continuous stirring for 6 h with molar ratio 1:1 of (Li + Ni + Mn):C6H8O7 to adjust the pH value to 7–8 with ammonium hydroxide. All final products were obtained by sintering the powders at an optimum sintering temperature of T = 900 °C in air for a few hours.
The structure of the samples was investigated using X-ray diffractometer (XRD) (PANalytical X’Pert, Lelyweg, The Netherlands) using nickel-filtered Cu-Kα radiation. The diffractograms were taken at room temperature in the 2θ range 10°–80°. Thermogravimetry (TG) analysis was performed using a Pyris1 instrument analyser (Perkin-Elmer, Sheffield, UK) to monitor the weight loss/gain and heat treatment processes under a flow of dry air with a 10 °C/min heating rate. The specific surface area was analysed by the Brunauer–Emmett–Teller (BET) method using Micromeritics ASAP 2010 in which the N2 gas adsorption was employed. The morphology and composition of the samples were investigated by scanning electron microscopy ZEISS model ULTRA 55 (Jena, Germany), equipped with an energy-dispersive X-ray spectrometer (EDX). HRTEM images were obtained using an electronic microscope JEOL model JEM-2010 (Pleasanton, CA, USA). The magnetic measurements (susceptibility and magnetization) were performed with two fully automated SQUID magnetometers (Quantum Design MPMS-5S, San Diego, CA, USA) in the temperature range 4–300 K, as described elsewhere .
2.3. Electrochemical Tests
Electrochemical tests were conducted on CR2025-type coin cells. The positive electrodes were constituted of 80 wt % active material, 10 wt % carbon black as conductive material and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl pyrrolidinone (NMP) solvent, mixed and ground to form a homogeneous slurry. The slurry was then spread onto an aluminium foil current collector and dried at 80 °C for 2 h to remove the solvent before being pressed. The cathode loading was in the range 5–7 mg·cm−2. The cells were assembled in a glove box (moisture and oxygen content ≤5 ppm) under argon atmosphere using lithium sheet as the counter electrode, Celgard 2300 film (MTI, Richmond, CA, USA) as the separator, and 1 mol·L−1 LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1) solution as electrolyte (LP30, Merck, Darmstadt, Germany). The galvanostatic charge–discharge curves were performed using a potentiostat/galvanostat (VMP3 Bio-Logic, Claix, France) in the potential range 2.0–4.8 V.
3. Results and Discussion
3.1. The Effect of Particle Size
The main drawback of layered of TMOs is their poor discharge rate capability due to low intrinsic electronic and ionic conductivity. Thus, at high current densities, i.e., J > 1C rate (the rate is denoted C/n, where C is the theoretical cathode capacity and a full discharge occurs in n hours), the poor electrochemical performance is attributed to the low electron transport of the material and the slow Li-ion kinetics within the grains. The currently adopted approach to get high rate capability is to reduce the diffusion path length of charge species by minimizing the particle size of the active phase . For instance, Okubo et al.  have observed an excellent high-rate capability, i.e., 65% of the 1C rate capability at 100C, in nanocrystalline LiCoO2 with an appropriate particle size of 17 nm. In this context, based on the Li-ion diffusion coefficient D* ≈ 2.5 × 10−12 cm2·s−1, the discharge process of 100 s requires particle size L = 100 nm against 1 h for L = 2 µm. Thus, 100-nm sized particles can be fully charged/discharged even at 10C rate (1.4 A·g−1). It was also demonstrated that control of the particle size is obtained via synthetic methods such as sol-gel , hydrothermal process , etc. As an example of the particle size effect, LiNi0.55Co0.45O2 (NCO) compounds were investigated. Figure 4 shows the HRTEM images of NCO particles and the discharge capacity curves of the corresponding Li//LiNi0.55Co0.45O2 coin-type cells as a function of C-rate. The cathode material (a) prepared by hydrothermal method has nanometric particles, 100–150 nm average size, while the micron-sized material, 1.5–2.0 µm particle size, was prepared by a two-step co-precipitation technique. An obvious difference in the electrochemical performance is observed. The Li cell with nano-sized particles allows a specific capacity 150 mAh·g−1 for 1C discharge rate, which is twice the capacity of the cell with micro-sized particles. The excellent rate capability makes nano-LiNi0.55Co0.45O2 suitable for high-power LIBs. Note that battery cycle life is favored by the architecture of the cathode material at the sub-micron scale, which allows the accommodation of volume changes caused by Li+ ions insertion/extraction into/from the single particle due to faster stress relaxation.
3.2. The Effect of Cationic Mixing
In this section, we report the properties of mixed transition-metal oxides LiNiwMnyCozO2 as 4-volt cathode materials for Li-ion batteries, for which the electrochemical features are correlated with structure and morphology of active particles. Special attention is given to the influence of the cation mixing between Li+ and Ni2+ ions on the crystallographic (3b) sites of the layered lattice due to the fact that the ionic radius of Ni2+ (0.69 Å) close to that of Li+ (0.76 Å) in an octahedral environment. A partial occupation of Li(3b) sites generates a disorder, so-called “cation mixing”, in the structure that damages the electrochemical performance [29,30,31]. Figure 5