Effect of synthesis condition on the structural and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by the metal acetates decomposition method

In order to get homogeneous layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ as a lithium insertion positive electrode material, we applied the metal acetates decomposition method. The oxide compounds were calcined at various temperatures, which results in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_1/3Mn_1/3Co_1/3]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry and SEM. XRD experiment revealed that the layered Li[Ni_1/3Mn_1/3Co_1/3]O₂ material can be best synthesized at temperature of 800 °C. In that synthesized temperature, the sample showed high discharge capacity of 190 mAh g⁻¹ as well as stable cycling performance at a current density of 0.2 mA cm⁻² in the voltage range 2.3-4.6 V. The reversible capacity after 100 cycles is more than 190 mAh g⁻¹ at room temperature.     

Synthetic optimization of spherical Li[Ni1/3Mn1/3Co1/3]O2 prepared by a carbonate co-precipitation method

A carbonate co-precipitation method was employed to prepare spherical Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material. The precursor, [Ni_1/3Co_1/3Mn_1/3]CO₃, was prepared using ammonia as chelating agent under CO₂ atmosphere. The spherical Li[Ni_1/3Co_1/3Mn_1/3]O₂ was prepared by mixing the precalcined [Ni_1/3Co_1/3Mn_1/3]CO₃ with LiOH followed by high temperature calcination. The preparation conditions such as ammonia concentration, co-precipitation temperature, calcination temperature and Li/[Ni_1/3Co_1/3Mn_1/3] ratio were varied to optimize the physical and electrochemical properties of the prepared Li[Ni_1/3Co_1/3Mn_1/3]O₂. The structural, morphological, and electrochemical properties of the prepared LiNi_1/3Co_1/3Mn_1/3O₂ were characterized by XRD, SEM, and galvanostatic charge-discharge cycling. The optimized material has a spherical particle shape and a well ordered layered structure, and it also has an initial discharge capacity of 162.7 mAh g^− 1 in a voltage range of 2.8-4.3 V and a capacity retention of 94.8% after a hundred cycles. The optimized ammonia concentration, co-precipitation temperature, calcination temperature, and Li/[Ni_1/3Co_1/3Mn_1/3] ratio are 0.3 mol L^− 1, 60 °C, 850 °C, and 1.10, respectively.     

Characterization of high tap density Li[Ni1/3Co1/3Mn1/3]O2 cathode material synthesized via hydroxide co-precipitation

Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material for lithium ion batteries was prepared by mixing metal hydroxide, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, with 6% excess LiOH followed by calcinations. The (Ni_1/3Co_1/3Mn_1/3)(OH)₂ with secondary particle of about 12 μm was prepared by hydroxide co-precipitation. The tap density of the obtained Li[Ni_1/3Co_1/3Mn_1/3]O₂ powder was 2.56 ± 0.21 g cm⁻³. The powder was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), particle size distribution (PSD) and galvanostatic charge-discharge cycling. The XRD pattern of Li[Ni_1/3Co_1/3Mn_1/3]O₂ revealed a well ordered hexagonal layered structure with low cation mixing. Secondary particles with size of 13-14 μm and primary particles with size of about 1 μm can be identified from the SEM observations. In the voltage range of 2.8-4.3 V, the initial discharge capacity of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode was 166.6 mAh g⁻¹, and 96.5% of the initial capacity was retained after 50 charge-discharge cycling.     

High-rate, high capacity ZrO2 coated Li[Li1/6Mn1/2Co1/6Ni1/6]O2 for lithium secondary batteries

Recently, there have been many reports on efforts to improve the rate capability and discharge capacity of lithium secondary batteries in order to facilitate their use for hybrid electric vehicles and electric power tools. In the present work, we present a ZrO₂-coated Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂. The bare Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂ shows a high initial discharge capacity of 224 mAh g⁻¹ at a 0.2 C rate. Owing to the stability of ZrO₂, it was possible to enhance the rate capability and cyclability. After 1 wt% ZrO₂ coating, the ZrO₂-coated Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂ showed a high discharge capacity of 115 mAh g⁻¹ after 50 cycles under a 6 C rate, whereas the bare Li[Li_1/6Mn_1/2Co_1/6Ni_1/6]O₂ showed a discharge capacity of only 40 mAh g⁻¹ and very poor cyclability under the same conditions. Based on results of XRD and EIS measurements, it was found that the ZrO₂ suppressed impedance growth at the interface between the electrodes and electrolyte and prevented collapse of the layered hexagonal structure.     

X-ray photoelectron spectroscopy and electrochemical behaviour of 4 V cathode, Li(Ni1/2Mn1/2)O2

Layered Li(Ni_1/2Mn_1/2)O₂ was prepared by the solution and mixed hydroxide methods, characterised by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and studied by cyclic voltammetry (CV) and charge discharge cycling in CC and CCCV modes at room temperature (r.t.) and at 50 °C. The XPS studies show about 8% of Ni³⁺ and Mn³⁺ ions are present in Li(Ni²⁺_1/2Mn_1/2⁴⁺)O₂ due to valency-degeneracy. The compound prepared at 950 °C, 12 h, solution method gives a second cycle discharge capacity of 150 mA h g⁻¹ (2.5-4.4 V) at a specific current of 30 mA g⁻¹ and retains 137 mA h g⁻¹ at the end of 40 cycles. CV shows that the redox process at 3.7-4.0 V corresponds to Ni²⁺↔Ni⁴⁺ and clear indication of Mn^3+/4+ couple was noted at 4.2-4.5 V. The observed capacity-fading (2.5-4.4 V) is shown to be contributed by the polarisation at the end of charging. The cathodic capacity is stable up to 40 cycles in the voltage window, 2.5-4.2 V both at room temperature and 50 °C.     

The first cycle characteristics of Li[Ni1/3Co1/3Mn1/3]O2 charged up to 4.7 V

In this research, we studied the first cycle characteristics of Li[Ni_1/3Co_1/3Mn_1/3]O₂ charged up to 4.7 V. Properties, such as valence state of the transition metals and crystallographic features, were analyzed by X-ray absorption spectroscopy and X-ray and neutron diffractions. Especially, two plateaus observed around 3.75 and 4.54 V were investigated by ex situ X-ray absorption spectroscopy. XANES studies showed that the oxidation states of transition metals in Li[Ni_1/3Co_1/3Mn_1/3]O₂ are mostly Ni²⁺, Co³⁺ and Mn⁴⁺. Based on neutron diffraction Rietveld analysis, there is about 6% of all nickel divalent (Ni²⁺) ions mixed with lithium ions (cation mixing). Meanwhile, it was found that the oxidation reaction of Ni²⁺/Ni⁴⁺ is related to the lower plateau around 3.75 V, but that of Co³⁺/Co⁴⁺ seems to occur entire range of x in Li_1−x[Ni_1/3Co_1/3Mn_1/3]O₂. Small volume change during cycling was attributed to the opposite variation of lattice parameter “c” and “a” with charging-discharging.     

Effect of synthesis condition on the structure and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by hydroxide co-precipitation method

The layered Li[Ni_1/3Co_1/3Mn_1/3]O₂ powder with good crystalline and spherical shape was prepared by hydroxide co-precipitation method. The effects of pH value, NH₄OH amount, calcination temperature and extra Li amount on the morphology, structure and electrochemical properties of the cathode material were investigated in detail. SEM results indicate that pH value affected both the morphology and the property of the cathode material, and the highest discharge capacity in the first cycle of 163 mAh g⁻¹ (2.8-4.3 V) was obtained at pH value was 12. On the contrary, the NH₄OH amount, which was used as a chelating agent, only affected the particle size distribution of the material. The calcination temperatures caused great difference in the structure and property of layered Li[Ni_1/3Co_1/3Mn_1/3]O₂, and the best electrochemical properties were obtained at the calcination temperature of 800 °C. Extra Li amount not only caused difference in the material structure, but also affected their electrochemical properties. With increasing Li amount, the lattice parameters (a and c) increased monotonously, and the highest first cycle coulombic efficiency (the ratio of discharge capacity to charge capacity in the first cycle) was obtained with the Li/M of 1.10. Therefore, the optimum synthetic conditions for the hydroxide co-precipitation reaction were: pH value was 12, NH₄OH amount was 0.36 mol L⁻¹, calcination temperature was 800 °C and the Li/M molar ratio was 1.10.     

High-voltage performance of concentration-gradient Li[Ni0.67Co0.15Mn0.18]O2 cathode material for lithium-ion batteries

A novel Li[Ni_0.67Co_0.15Mn_0.18]O₂ cathode material encapsulated completely within a concentration-gradient shell was successfully synthesized via co-precipitation. The Li[Ni_0.67Co_0.15Mn_0.18]O₂ has a core of Li[Ni_0.8Co_0.15Mn_0.05]O₂ that is rich in Ni, a concentration-gradient shell having decreasing Ni concentration and increasing Mn concentration toward the particle surface, and a stable outer-layer of Li[Ni_0.57Co_0.15Mn_0.28]O₂. The electrochemical and thermal properties of the material were investigated and compared to those of the core Li[Ni_0.8Co_0.15Mn_0.05]O₂ material alone. The discharge capacity of the concentration-gradient Li[Ni_0.67Co_0.15Mn_0.18]O₂ electrode increased with increasing upper cutoff voltage to 4.5 V, and cells with this cathode material delivered a very high capacity, 213 mAh/g, with excellent cycling stability even at 55 °C. The enhanced thermal and lithium intercalation stability of the Li[Ni_0.67Co_0.15Mn_0.18]O₂ was attributed to the gradual increase in tetravalent Mn concentration and decrease in Ni concentration in the concentration-gradient shell layer.     

Synthesis of spherical Li[Ni(1/3−z)Co(1/3−z)Mn(1/3−z)Mgz]O2 as positive electrode material for lithium-ion battery

Li[Ni_(1/3−z)Co_(1/3−z)Mn_(1/3−z)Mg_z]O₂ (z = 0, 0.04) positive electrode materials were synthesized via a co-precipitation method. These materials have α-NaFeO₂ () structure, as confirmed by X-ray diffraction (XRD) studies. Cation mixing in Li layer seemed to be decreased by Mg substitution as examined by Rietveld refinements of XRD data. Spherical morphologies were observed for the as-synthesized final products by scanning electron microscopy. Their electrochemical properties during charge and discharge were discussed. When magnesium ions are substituted, the initial reversible capacity reduced. However, the substitution for Mn sites in Li[Ni_1/3Co_1/3Mn_1/3]O₂ did not decrease the capacity because Mn sites substitution did not result in loss of electroactive elements in the compound. Differential scanning calorimetric studies showed the exothermic peaks of the charged electrode Li[Ni_(1/3−z)Co_(1/3−z)Mn_(1/3−z)Mg_z]O₂ (z = 0.04) were significantly smaller than that of Li[Ni_1/3Co_1/3Mn_1/3]O₂, which means that thermal stability was greatly improved by Mg substitution even at highly delithiated state.     

Synthesis and characterization of high-density non-spherical Li(Ni1/3Co1/3Mn1/3)O2 cathode material for lithium ion batteries by two-step drying method

Non-spherical Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders have been synthesized using a two-step drying method with 5% excess LiOH at 800 °C for 20 h. The tap-density of the powder obtained is 2.95 g cm⁻³. This value is remarkably higher than that of the Li(Ni_1/3Co_1/3Mn_1/3)O₂ powders obtained by other methods, which range from 1.50 g cm⁻³ to 2.40 g cm⁻³. The precursor and Li(Ni_1/3Co_1/3Mn_1/3)O₂ are characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM). XPS studies show that the predominant oxidation states of Ni, Co and Mn in the precursor are 2⁺, 3⁺ and 4⁺, respectively. XRD results show that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ material obtained by the two-step drying method has a well-layered structure with a small amount of cation mixing. SEM confirms that the Li(Ni_1/3Co_1/3Mn_1/3)O₂ particles obtained by this method are uniform. The initial discharge capacity of 167 mAh g⁻¹ is obtained between 3 V and 4.3 V at a current of 0.2 C rate. The capacity of 159 mAh g⁻¹ is retained at the end of 30 charge-discharge cycle with a capacity retention of 95%.     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

Effect of fluorine in the preparation of Li(Ni1/3Co1/3Mn1/3)O2 via hydroxide co-precipitation

Uniform and spherical Li(Ni_1/3Co_1/3Mn_1/3)O_(2−δ)F_δ powders were synthesized via NH₃ and F⁻ coordination hydroxide co-precipitation. The effect of F⁻ coordination agent on the morphology, structure and electrochemical properties of the Li(Ni_1/3Co_1/3Mn_1/3)O_(2−δ)F_δ were studied. The morphology, size, and distribution of (Ni_1/3Co_1/3Mn_1/3)(OH)_(2−δ)F_δ particle diameter were improved in a shorter reaction time through the addition of F⁻. The study suggested that the added F improves the layered characteristics of the lattice and the cyclic performance of Li(Ni_1/3Co_1/3Mn_1/3)O₂ in the voltage range of 2.8-4.6 V. The initial capacity of the Li(Ni_1/3Co_1/3Mn_1/3)O_1.96F_0.04 was 178 mAh g⁻¹, the maximum capacity was 186 mAh g⁻¹ and the capacity after 50 cycles was 179 mAh g⁻¹ in the voltage range of 2.8-4.6 V.     

On the LiCo2/3Ni1/6Mn1/6O2 positive electrode material

LiCo_2/3Ni_1/6Mn_1/6O₂ layered oxide was synthesized by the combustion method that led to a crystalline phase with good homogeneity and low particles size. The structural properties of the prepared positive electrode material were investigated by performing XRD Rietveld refinement. Practically no Li/Ni mixing was detected evidencing that the studied compound adopts almost an ideal α-NaFeO₂ type structure. The Li||LiCo_2/3Ni_1/6Mn_1/6O₂ cell showed a discharge capacity of 199 mAh g⁻¹ when cycled in the 2.7–4.6 V potential range while the best cycling performances were recorded when the upper cut off is fixed at 4.5 V. Structural changes in Li_xCo_2/3Ni_1/6Mn_1/6O₂ with lithium electrochemical de-intercalation were studied using X-ray diffraction. This study clearly shows the existence of a solid solution domain in the 0.1 < x < 1.0 composition range while for x = 0.1, a new phase appears explaining the decrease of the electrochemical performance when the cell is cycled at high upper cut off voltage.     

Particle size effect of Li[Ni0.5Mn0.5]O2 prepared by co-precipitation

Spherical (Ni_0.5Mn_0.5)(OH)₂ with different secondary particle size (3 μm, 10 μm in diameter) was synthesized by co-precipitation method. Mixture of the prepared hydroxide and lithium hydroxide was calcined at 950 °C for 20 h in air. X-ray diffraction patterns revealed that the prepared material had a typical layered structure with space group. Spherical morphologies with mono-dispersed powders were observed by scanning electron microscopy. It was found that the layered Li[Ni_0.5Mn_0.5]O₂ delivered an initial discharge capacity of 148 mAh g⁻¹ (3.0-4.3 V) though the particle sizes were different. Li[Ni_0.5Mn_0.5]O₂ having smaller particle size (3 μm) showed improved area specific impedance due to the reduced Li⁺ diffusion path, compared with that of Li[Ni_0.5Mn_0.5]O₂ possessing larger particle size (10 μm). Although the Li[Ni_0.5Mn_0.5]O₂ (3 μm) was electrochemically delithiated to Li_0.39[Ni_0.5Mn_0.5]O₂, the resulting exothermic onset temperature was around 295 °C, of which the value is significantly higher than that of highly delithiated Li_1−δCoO₂ (∼180 °C).     

Structural and electrochemical behaviors of metastable Li2/3[Ni1/3Mn2/3]O2 modified by metal element substitution

Layered metastable lithium manganese oxides, Li_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na_2/3[Ni_1/3−xMn_2/3−yM_x+y]O₂ precursors. The Al and Co doping produced the T^#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni_1/3−xMn_2/3−yM_x+yO₂ slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T^#2-Li_2/3[Ni_1/3Mn_2/3]O₂ were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g⁻¹ in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g⁻¹ along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T^#2 into the O2 structure was observed during the initial lithium deintercalation.     

One-step synthesis and improved electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2 by a modified radiated polymer gel method

We present the mechanism for the synthesis of a layered Li(Ni_1/3Co_1/3Mn_1/3)O₂ compound by a modified radiated gel method. Pure-phase Li(Ni_1/3Co_1/3Mn_1/3)O₂ material was achieved when the polymer gel was calcined at 900 °C between 15 and 30 h. The unit cell parameter c decreased, and a varied slightly with increased sintering time. Electrochemical characterization revealed that the optimized sample (25 h) had a high initial discharge capacity of 188 mAh/g (2.8-4.5 V, 20 mA/g), an excellent capacity retention of 90.1% after 30 cycles and a good rate performance.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Electrochemical properties and structural characterization of layered Li[Ni0.5Mn0.5]O2 cathode materials

Layered Li[Ni_0.5Mn_0.5]O₂ materials with high homogeneity and crystallinity were prepared using high speed ball milling. The Li[Ni_0.5Mn_0.5]O₂ electrode delivered a high discharge capacity of 152 mA h g⁻¹ between 2.8 and 4.3 V with excellent cycleability. The TEM analysis showed that the Li[Ni_0.5Mn_0.5]O₂ electrode went through a considerable morphological change without altering its initial layered structure while the electrode retained its initial discharge capacity even after 50 cycles.     

Hydrothermal synthesis of Li(Ni1/3Co1/3Mn1/3)O2 for lithium rechargeable batteries

Ultrafine powders of Li(Ni_1/3Co_1/3Mn_1/3)O₂ cathode materials for lithium-ion secondary batteries were prepared under mild hydrothermal conditions. The influence of the molar ratio of Li/(Ni + Co + Mn) was studied. The products were investigated by XRD, TEM and EDS. The final products were found to be well crystallized Li(Ni_1/3Co_1/3Mn_1/3)O₂ with an average particle size of about 10 nm.     

A comparison of preparation method on the electrochemical performance of cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 for lithium ion battery

Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ as a cathode material for Li-ion battery has been successfully prepared by co-precipitation (CP), sol–gel (SG) and sucrose combustion (SC) methods. The prepared materials were characterized by XRD, SEM, BET and electrochemical measurements. The XRD result shows that the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ materials prepared by different methods all form a pure phase with good crystallinity. SEM images and BET data present that the SC-material exhibited the smallest particle size (ca. 0.1 μm) and the highest surface area (7.4635 m² g⁻¹). The tap density of SC-material is lower than that of CP- and SG-materials. The result of rate performance tests indicates that the SC-material showed the best rate capability with the highest discharge capacity of 178 mAh g⁻¹ at 5.0 C, followed by SG-material and then CP-material. However, the cycling stability of SC-material tested at 0.1 and 0.5 C is relatively poor as compared to that of SG-material and CP-material. The result of EIS measurements reveals that large surface area and small particle size of the SC-electrode result in more SEI layer formation because of the increased side reactions with the electrolyte during cycling, which deteriorates the electrode/electrolyte interface and thus leads to the faster capacity fading of the SC-material.