Electrochemical properties of Li[Ni1/3Mn1/3Al1/3−xCox]O2 as a cathode material for lithium ion battery

Layered Li[Ni_1/3Mn_1/3Al_1/3−xCo_x]O₂ (0 ≤ x ≤ 1/3) cathode materials are synthesized by a solvent evaporation method. Although XRD shows that Li[Ni_1/3Mn_1/3Al_1/3]O₂ has no obvious impurity phase, it has poor electrochemical properties. To improve its capability, part of Al in Li[Ni_1/3Mn_1/3Al_1/3]O₂ compound is replaced by Co in this study. The samples are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and charge–discharge test. The results indicate that the introduction of Co has a large influence on the morphology, structure and electrochemical performances of the samples, which become more excellent with an increase of Co content in compounds. Meanwhile, the high-temperature behavior of the samples is also investigated.     

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.     

Synthesis and improved electrochemical performance of Al (OH)3-coated Li[Ni1/3Mn1/3Co1/3]O2 cathode materials at elevated temperature

Spherical Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders were synthesized from LiOH·H₂O and coprecipitated spherical metal hydroxide, (Ni_1/3Mn_1/3Co_1/3)(OH)₂ and coated with Al(OH)₃. The Al(OH)₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ showed a capacity retention of 80% at 320 mA g⁻¹ (2 C-rate) based on 20 mA g⁻¹ (0.1 C-rate), while the pristine Li[Ni_1/3Co_1/3Mn_1/3]O₂ delivered only 45% at the same current density. Also, unlike pristine Li[Ni_1/3Co_1/3Mn_1/3]O₂, the Al(OH)₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode exhibits excellent rate capability and good thermal stability.     

Improvement of electrochemical and thermal properties of Li[Ni0.8Co0.1Mn0.1]O2 positive electrode materials by multiple metal (Al, Mg) substitution

Al and/or Mg-substituted Li[Ni_0.8Co_0.1Mn_0.1−x−yAl_xMg_y]O₂ were prepared by a co-precipitation method and characterized by X-ray diffraction with Rietveld refinement, thermogravimetric analysis, differential scanning calorimetry (DSC), and electrochemical measurements. The Rietveld refinement results show that cation mixing of Al and/or Mg-substituted Li[Ni_0.8Co_0.1Mn_0.1−x−yAl_xMg_y]O₂ was reduced with increased doping amounts of Al and Mg. The Al and/or Mg substitution in Li[Ni_0.8Co_0.1Mn_0.1]O₂ also resulted in improved electrochemical cycling behavior, structural stability, and thermal stability compared to pristine Li[Ni_0.8Co_0.1Mn_0.1]O₂. The improvements of electrochemical and thermal properties resulted from the stabilized host structure by Al and/or Mg incorporation into Li[Ni_0.8Co_0.1Mn_0.1]O₂.     

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.     

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.     

Insignificant impact of designed oxygen release from high capacity Li[(Ni1/2Mn1/2)xCoy(Li1/3Mn2/3)1/3]O2 (x + y = 2/3) positive electrodes during the cycling of Li-ion cells

The properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) Li-ion cells are reported. There is an extended plateau near 4.5 V during the first charging of the cells that corresponds to the simultaneous removal of Li and oxygen from the Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3, y = 1/12 and 1/6) electrodes. The release of this oxygen directly within a Li-ion cell has been a cause for concern. However, it was found that subsequent to O₂ release, Li-ion cells delivered a high reversible positive electrode specific capacity near 250 mAh/g at C/30 between 2.5 and 4.8 V, the cells did not display increased irreversible capacity relative to counterparts having Li metal negative electrodes and the cells retained 85% of their initial capacity after 70 cycles at C/6 between 2.5 and 4.6 V. Therefore, the O₂ released during the first charge does not significantly impact the electrochemical properties of graphite/Li[(Ni_0.5Mn_0.5)_xCo_y(Li_1/3Mn_2/3)_1/3]O₂ (x + y = 2/3) lithium-ion cells.     

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.     

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).     

Stabilizing the structure and suppressing the voltage decay of Li[Li0.2Mn0.54Co0.13Ni0.13]O2 cathode materials for Li-ion batteries via multifunctional Pr oxide surface modification

The development of Li-rich layer cathode materials has been limited by poor cycle, rate performance, phase transformation and voltage decay. To improve these properties, a facile and low-cost wet method is employed to fabricate Pr₆O₁₁ coating layer on Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ nanoparticles. The 3–6 nm Pr₆O₁₁ coating layer is observed on the surface of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ by HRTEM. Interestingly, HAADF-STEM and EDS analyses show that the transition metal ions and the praseodymium ions mutually infiltrate in the Pr₆O₁₁ coating layer and Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ nanoparticles during calcination. A combination of HAADF-STEM with EDS and XPS studies reveals that Pr₆O₁₁ coating layer is bridged to Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ nanoparticles by the chemical bonds of transition phase Li_1.2M_XPr_1−xO₂. XRD patterns show that all samples are indexed to the layered structure α-NaFeO₂, but the lattice parameters are influenced lightly after Pr₆O₁₁ coating. HRTEM and SAED analyses elucidate that the super large Pr ions surface-doping and the Pr₆O₁₁ coating are verified to suppress the transformation of layer to spinel structure in the bulk nanoparticles after cycles. The sample coated with 3 wt% Pr₆O₁₁ exhibits wonderful electrochemical performance with the first coulomb efficiency of 85.6%, the capacity retention ratio of 97.9% after 50 cycles and the discharge capacity of 162.2 mAh g⁻¹ at 5 C. The resistant of charge transfer and the electrodes polarization are reduced by Pr₆O₁₁ coating according to EIS. Therefore, Pr₆O₁₁, which contains the super large Pr ions, plays two roles: the first one, it is coated on the Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ nanoparticles to optimize the environment of the interface reaction between electrodes and electrolyte; the other one, its Pr ions surface-doping stabilizes the structure in the superficial region of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ nanoparticles and suppresses the voltage decay. The multifunctional Pr₆O₁₁ can play a significant role in accelerating development of new materials with excellent stabilization and high capacity.     

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.     

Improvement of electrochemical properties of Li[Ni0.4Co0.2Mn(0.4−x)Mgx]O2−yFy cathode materials at high voltage region

Spherical Li[Ni_0.4Co_0.2Mn_(0.4−x)Mg_x]O_2−yF_y (x = 0, 0.04, y = 0, 0.08) with phase-pure and well-ordered layered structure have been synthesized by heat-treatment of spherical [Ni_0.4Co_0.2Mn_0.4−xMg_x]₃O₄ precursors with LiOH·H₂O and LiF salts. The average particle size of the powders was about 10-15 μm and the size distribution was quite narrow due to the homogeneity of the metal carbonate, [Ni_0.4Co_0.2Mn_(0.4−x)Mg_x]CO₃ (x = 0, 0.04) precursors. Although the Li[Ni_0.4Co_0.2Mn_0.36Mg_0.04]O_1.92F_0.08 delivered somewhat slightly lower initial discharge capacity, however, the capacity retention, interfacial resistance, and thermal stability were greatly enhanced comparing to the Li[Ni_0.4Co_0.2Mn_0.4]O₂ and Li[Ni_0.4Co_0.2Mn_0.36Mg_0.04]O₂.     

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.     

Synthesis and electrochemical properties of Li[Li0.07Ni0.1Co0.6Mn0.23]O2 as a possible cathode material for lithium-ion batteries

The layered Li[Li_0.07Ni_0.1Co_0.6Mn_0.23]O₂ materials were synthesized by sol-gel method with glycine or citric acid as chelating agent. The prepared materials were characterized by means of XRD, SEM and Raman spectroscopy. Li/Li[Li_0.07Ni_0.1Co_0.6Mn_0.23]O₂ cells were assembled and subjected to charge-discharge studies at different C rates, viz 0.2, 1, 2 and 4 C. Although the samples showed less discharge capacity at 4 C rate the fade in capacity per cycle is lesser than that of capacity fade at 0.2 C rate. The citric acid assisted sample is found to be superior in terms of discharge capacity, capacity retention rate and also in thermal stability to that of sample prepared with glycine as chelating agent.     

The effects of Ni doping on the performance of O3-lithium manganese oxide material

Li_0.7[Li_1/6Mn_5/6]O₂ and Li_0.7[Li_1/12Ni_1/12Mn_5/6]O₂ powders were synthesized by a sol-gel method. The powders had a typically rhombohedral layered O₃ structure. Both the samples were nanometer-sized powders and the size of Li_0.7[Li_1/12Ni_1/12Mn_5/6]O₂ was smaller than that of Li_0.7[Li_1/6Mn_5/6]O₂. The discharge curve shape of both the sample electrodes was almost equal to that of the layered structure. However, the electrode materials were transferred from layered to spinel structures with increasing the cycle number. Li/Li_0.7[Li_1/6Mn_5/6]O₂ and Li_0.7[Li_1/12Ni_1/12Mn_5/6]O₂ cells initially delivered a discharge capacity of 261 and 238 mAh/g, respectively. The capacities of Li/Li_0.7[Li_1/6Mn_5/6]O₂ and Li_0.7 [Li_1/12Ni_1/12Mn_5/6]O₂ after the 45th cycle were 174 and 221 mAh/g, respectively, corresponding to the retentions of 67% and 93%. The nanostructure of the synthesized powders seems to result in high initial discharge capacity as well as in the suppression of the discharge capacity fading by providing high surface area needed for Li ion reaction. In Ni doped-Li_0.7[Li_1/12Ni_1/12Mn_5/6]O₂, the capacity fading was reduced by suppressing the oxidation state of Mn from 4⁺ to 3⁺ due to the role of Ni ion doped.     

Comparative study of Li[Co1−zAlz]O2 prepared by solid-state and co-precipitation methods

Li[Co_1−zAl_z]O₂ (0 ≤ z ≤ 0.5) samples were prepared by co-precipitation and solid-state methods. The lattice constants varied smoothly with z for the co-precipitated samples but deviated for the solid-state samples above z = 0.2. The solid-state method may not produce materials with a uniform cation distribution when the aluminum content is large or when the duration of heating is too brief. Non-stoichiometric Li_x[Co_0.9Al_0.1]O₂ samples were synthesized by the co-precipitation method at various nominal compositions x = Li/(Co + Al) = 0.95, 1.0, 1.1, 1.2, 1.3. XRD patterns of the Li_x[Co_0.9Al_0.1]O₂ samples suggest the solid solution limit is between Li/(Co + Al) = 1.1 and 1.2. Electrochemical studies of the Li[Co_1−zAl_z]O₂ samples were used to measure the rate of capacity reduction with Al content, found to be about −250 ± 30 (mAh/g)/(z = 1). Literature work on Li[Ni_1/3Mn_1/3Co_1/3−zAl_z]O₂, Li[Ni_1−zAl_z]O₂ and Li[Mn_2−yAl_y]O₄ demonstrates the same rate of capacity reduction with Al/(Al + M) ratio. These studies serve as baseline characterization of samples to be used to determine the impact of Al content on the thermal stability of delithiated Li[Co_1−zAl_z]O₂ in electrolyte.     

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.     

Synthesis optimization of Li1 + x[Mn0.45Co0.40Ni0.15]O2 with different spherical sizes via co-precipitation

Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂ spherical cathode materials with different sizes (about 2 and 5 μm) were fabricated by calcining uniform spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃ with lithium hydroxide at high temperature. The precursor of spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃, was obtained via co-precipitation method at room temperature, which was significantly dependent on synthetic conditions, such as the reaction temperature, the concentration of NH₄HCO₃, and stirring speed, etc. The optimized condition resulted in [Mn_0.45Co_0.40Ni_0.15]CO₃, of which the particle size distribution was uniform and the particle shape was spherical. The final products, Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂, had a well-ordered layered structure and uniform homogeneity. Raman spectroscopy analysis showed the Raman-active species E_g and A_1g modes were observed at 488, 473 cm^− 1 and 597, 590 cm^− 1, respectively, for the obtained spherical cathode materials.     

Enhanced electrochemical and storage properties of La2/3−XLi3XTiO3-coated Li[Ni0.4Co0.3Mn0.3]O2

A Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode was modified by applying a La_2/3−XLi_3XTiO₃ (LLT) coating. Transmission electron microscope (TEM) images reveal that the coating layer consists of nanoparticles. The coated cathode demonstrated an enhanced rate capability, discharge capacity, and cyclic performance than the uncoated cathode. However, the influence of the coating upon these electrochemical properties is highly dependent upon the composition of the LLT coating layer. Coating layers having high La and low Li contents, such as La_0.67TiO₃, effectively improved the rate capability of the cathode. However, coating layers with a low La and high Li content greatly enhanced the discharge capacity of the cathode under high cut-off voltage (4.8 V) conditions. Overall, the thermal stability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode was improved by the LLT coating. Storage tests confirmed that the La_2/3−XLi_3XTiO₃ coating dramatically suppressed the dissolution of transition metals into the electrolyte.     

Synthetic optimization of Li[Ni1/3Co1/3Mn1/3]O2 via co-precipitation

Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders were synthesized from co-precipitated spherical metal hydroxide, (Ni_1/3Co_1/3Mn_1/3)(OH)₂. The preparation of metal hydroxide was significantly dependent on synthetic conditions, such as pH, amount of chelating agent, stirring speed, etc. The optimized condition resulted in (Ni_1/3Co_1/3Mn_1/3)(OH)₂, of which the particle size distribution was uniform and the particle shape was spherical, as observed by scanning electron microscopy. Calcination of the uniform metal hydroxide with LiOH at higher temperature led to a well-ordered layer-structured Li[Ni_1/3Co_1/3Mn_1/3]O₂, as confirmed by Rietveld refinement of X-ray diffraction pattern. Due to the homogeneity of the metal hydroxide, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, the final product, Li[Ni_1/3Co_1/3Mn_1/3]O₂, was also significantly uniform, i.e., the average particle size was of about 10 μm in diameter and the distribution was relatively narrow. As a result, the corresponding tap-density was also high approximately 2.39 g cm−3, of which the value is comparable to that of commercialized LiCoO₂. In the voltage range of 2.8-4.3, 2.8-4.4, and 2.8-4.5 V, the discharge capacities of Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode were 159, 168, and 177 mAh g⁻¹, respectively. For elevated temperature operation (55 °C), the resulted capacity was of about 168 mAh g⁻¹ with an excellent cyclability.