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.     

Morphology and electrochemical performance of Li[Ni1/3Co1/3Mn1/3]O2 cathode material by a slurry spray drying method

The spherical Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders with appropriate porosity, small particle size and good particle size distribution were successfully prepared by a slurry spray drying method. The Li[Ni_1/3Co_1/3Mn_1/3]O₂ powders were characterized by XRD, SEM, ICP, BET, EIS and galvanostatic charge/discharge testing. The material calcined at 950 °C had the best electrochemical performance. Its initial discharge capacity was 188.9 mAh g⁻¹ at the discharge rate of 0.2 C (32 mA g⁻¹), and retained 91.4% of the capacity on going from 0.2 to 4 C rate. From the EIS result, it was found that the favorable electrochemical performance of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material was primarily attributed to the particular morphology formed by the spray drying process which was favorable for the charge transfer during the deintercalation and intercalation cycling.     

Synthesis and electrochemical properties of lithium non-stoichiometric Li1+x(Ni1/3Co1/3Mn1/3)O2+δ prepared by a spray drying method

Lithium non-stoichiometric Li[Li_x(Ni_1/3Co_1/3Mn_1/3)_1−x]O₂ materials (0 ≤ x ≤ 0.17) were synthesized using a spray drying method. The electrochemical properties and structural stabilities of the synthesized materials were investigated. The synthesized materials exhibited a hexagonal structure in all the x-value and the lattice parameters of the materials were gradually decreased with increasing x-value due to an increasing amount of Ni³⁺ ions for charge compensation. The capacity retention ability and rate capability of the stoichiometric Li(Ni_1/3Co_1/3Mn_1/3)O₂ material were improved by increasing x-value, the so-called overlithiation. We found that the overlithiated materials could keep more structural integrity than the stoichiometric one during electrochemical cyclings, which could be one of reasons for a better electrochemical properties of the overlithiated materials.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

A novel method for synthesis of layered LiNi1/3Mn1/3Co1/3O2 as cathode material for lithium-ion battery

The layered LiNi_1/3Mn_1/3Co_1/3O₂ materials with good crystalline are synthesized by a novel method of hydrothermal method followed by a short calcination process. The crystalline structure and morphology of the synthesized materials are characterized by XRD, SEM. Their electrochemical performances are evaluated by CV, EIS and galvonostatic charge/discharge tests. The material synthesized at 850 °C for 6 h shows the highest initial discharge capacity of 187.7 mAh g⁻¹ at 20 mA g⁻¹. And the capacity retention of 97.9% is maintained at the end of 40 cycles at 1.0 C. CV test reveals almost no shift of anodic and cathodic peaks after first cycle, which indicates good reversible deintercalation and intercalation of Li⁺ ions.     

Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material

Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M = Ni + Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined α-NaFeO₂ structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li_1−yNi_y]_3b[Li_x+yNi_{(1−x)/3−y}Mn_(1−x)/3Co_(1−x)/3]_3aO₂ (x = 0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni²⁺-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M = 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ reflected the high degree of sample optimization. An initial discharge capacity of 150 mAh g⁻¹ was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at 1 C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂.     

Structural and transport properties of layered Li1+x(Mn1/3Co1/3Ni1/3)1−xO2 oxides prepared by a soft chemistry method

In this work structural and transport properties of layered Li_1+x(Mn_1/3Co_1/3Ni_1/3)_1−xO₂ oxides (x = 0; 0.03; 0.06) prepared by a “soft chemistry” method are presented. The excessive lithium was found to significantly improve transport properties of the materials, a corresponding linear decrease of the unit cell parameters was observed. The electrical conductivity of Li_1.03(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ composition was high enough to use this material in a form of a pellet, without any additives, in lithium batteries and characterize structural and transport properties of deintercalated Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ compounds. For deintercalated samples a linear increase of the lattice parameter c together with a linear decrease of the parameter a with the increasing deintercalation degree occurred, but only up to 0.4-0.5 mol of extracted lithium. Further deintercalation showed a reversal of the trend. Electrical conductivity measurements performed of Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ samples (y = 0.1; 0.3; 0.5; 0.6) showed an ongoing improvement, almost two orders of magnitude, in relation to the starting composition. Additionally, OCV measurements, discharge characteristics and lithium diffusion coefficient measurements were performed for Li/Li⁺/Li_1.03−y(Mn_1/3Co_1/3Ni_1/3)_0.97O₂ cells.     

An amorphous LiCo1/3Mn1/3Ni1/3O2 thin film deposited on NASICON-type electrolyte for all-solid-state Li-ion batteries

Amorphous LiCo_1/3Mn_1/3Ni_1/3O₂ thin films were deposited on the NASICON-type Li-ion conducting glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 130 °C. The amorphous films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiCo_1/3Mn_1/3Ni_1/3O₂/Au all-solid-state cells were fabricated to investigate the electrochemical performance of the amorphous films. It was found that the low-temperature deposited amorphous cathode film shows a high discharge voltage and a high discharge capacity of around 130 mAh g⁻¹.     

Synthesis and characterization of submicron-sized LiNi1/3Co1/3Mn1/3O2 by a simple self-propagating solid-state metathesis method

Submicron-sized LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized using a simple self-propagating solid-state metathesis method with the help of ball milling and the following calcination. A mixture of Li(ac)·2H₂O, Ni(ac)₂·4H₂O, Co(ac)₂·4H₂O, Mn(ac)₂·4H₂O (ac = acetate) and excess H₂C₂O₄·2H₂O was used as starting material without any solvent. XRD analyses indicate that the LiNi_1/3Co_1/3Mn_1/3O₂ materials were formed with typical hexagonal structure. The FESEM images show that the primary particle size of the LiNi_1/3Co_1/3Mn_1/3O₂ materials gradually increases from about 100 nm at 700 °C to 200–500 nm at 950 °C with increasing calcination temperature. Among the synthesized materials, the LiNi_1/3Co_1/3Mn_1/3O₂ material calcined at 900 °C exhibits excellent electrochemical performance. The steady discharge capacities of the material cycled at 1 C (160 mA g⁻¹) rate are at about 140 mAh g⁻¹ after 100 cycles in the voltage range 3–4.5 V (versus Li⁺/Li) and the capacity retention is about 87% at the 350th cycle.     

Electrochemical performance of SrF2-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for Li-ion batteries

SrF₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials with improved cycling performance over 2.5–4.6 V were investigated. The structural and electrochemical properties of the materials were studied using X-ray diffraction (XRD), scanning electron microscope (SEM), charge–discharge tests and electrochemical impedance spectra (EIS). The results showed that the crystalline SrF₂ with about 10–50 nm particle size is uniformly coated on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ particles. As the coating amount increased from 0.0 to 2.0 mol%, the initial capacity and rate capability of the coated LiNi_1/3Co_1/3Mn_1/3O₂ decreased slightly owing to the increase of the charge-transfer resistance; however, the cycling stability was improved by suppressing the increase of the resistance during cycling. 4.0 mol% SrF₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ showed remarkable decrease of the initial capacity. 2.0 mol% coated sample exhibited the best electrochemical performance. It presented an initial discharge capacity of 165.7 mAh g⁻¹, and a capacity retention of 86.9% after 50 cycles at 4.6 V cut-off cycling.     

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.     

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.     

A modified ZrO2-coating process to improve electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2

A modified Zr-coating process was introduced to improve the electrochemical performance of Li(Ni_1/3Co_1/3Mn_1/3)O₂. The ZrO₂-coating was carried out on an intermediate, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, rather than on Li(Ni_1/3Co_1/3Mn_1/3)O₂. After a heat treatment process, one part of the Zr covered the surface of Li(Ni_1/3Co_1/3Mn_1/3)O₂ in the form of a Li₂ZrO₃ coating layer, and the other part diffused into the crystal lattice of Li(Ni_1/3Co_1/3Mn_1/3)O₂. A decreasing gradient distribution in the concentration of Zr was detected from the surface to the bulk of Li(Ni_1/3Co_1/3Mn_1/3)O₂ by X-ray photoelectron spectra (XPS). Electrochemical tests indicated that the 1% (Zr/Ni + Co + Mn) ZrO₂-modified Li(Ni_1/3Co_1/3Mn_1/3)O₂ prepared by this process showed better cyclability and rate capability than bare Li(Ni_1/3Co_1/3Mn_1/3)O₂. The result can be ascribed to the special effect of Zr in ZrO₂-modified Li(Ni_1/3Co_1/3Mn_1/3)O₂. The surface coating layer of Li₂ZrO₃ improved the cycle performance, while the incorporation of Zr in the crystal lattice of Li(Ni_1/3Co_1/3Mn_1/3)O₂ modified the rate capability by increasing the lattice parameters. Electrochemical impedance spectra (EIS) results showed that the increase of charge transfer resistance during cycling was suppressed significantly by ZrO₂ modification.     

AlF3-coated LiCoO2 and Li[Ni1/3Co1/3Mn1/3]O2 blend composite cathode for lithium ion batteries

Surface modifications of electrode materials can improve the electrochemical and thermal properties of cathodes for use in lithium batteries. In this study, AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are blended, as both have the same crystal structure and exhibit similar electrochemical properties. The composite electrodes exhibit high discharge capacities of 180-188 mAh g⁻¹ in a voltage range of 3.0-4.5 V at room temperature. The capacity retention of the composite electrode is greater than 95% of the initial capacity after 50 cycles. The thermal stability of these composite electrodes is greatly improved because of the superior thermal stability of AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂. The blended AlF₃-coated LiCoO₂ and AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode shows two exothermic peaks, one at 227 °C from AlF₃-coated LiCoO₂ and another at 277 °C from AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂, accompanied by significantly reduced exothermic heat generation.     

In situ X-ray diffraction studies of mixed LiMn2O4-LiNi1/3Co1/3Mn1/3O2 composite cathode in Li-ion cells during charge-discharge cycling

The structural changes of the composite cathode made by mixing spinel LiMn₂O₄ and layered LiNi_1/3Co_1/3Mn_1/3O₂ in 1:1 wt% in both Li-half and Li-ion cells during charge/discharge are studied by in situ XRD. During the first charge up to ∼5.2 V vs. Li/Li⁺, the in situ XRD spectra for the composite cathode in the Li-half cell track the structural changes of each component. At the early stage of charge, the lithium extraction takes place in the LiNi_1/3Co_1/3Mn_1/3O₂ component only. When the cell voltage reaches at ∼4.0 V vs. Li/Li⁺, lithium extraction from the spinel LiMn₂O₄ component starts and becomes the major contributor for the cell capacity due to the higher rate capability of LiMn₂O₄. When the voltage passed 4.3 V, the major structural changes are from the LiNi_1/3Co_1/3Mn_1/3O₂ component, while the LiMn₂O₄ component is almost unchanged. In the Li-ion cell using a MCMB anode and a composite cathode cycled between 2.5 V and 4.2 V, the structural changes are dominated by the spinel LiMn₂O₄ component, with much less changes in the layered LiNi_1/3Co_1/3Mn_1/3O₂ component, comparing with the Li-half cell results. These results give us valuable information about the structural changes relating to the contributions of each individual component to the cell capacity at certain charge/discharge state, which are helpful in designing and optimizing the composite cathode using spinel- and layered-type materials for Li-ion battery research.     

Synthesis and electrochemical properties of Li1−xFe0.8Ni0.2O2–LixMnO2 (Mn/(Fe + Ni + Mn) = 0.8) material

A new type of Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ (Mn/(Fe + Ni + Mn) = 0.8) material was synthesized at 350 °C in air atmosphere using a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−xFeO₂–Li_xMnO₂ (Mn/(Fe + Mn) = 0.8) with much reduced impurity peaks. The Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell showed a high initial discharge capacity above 192 mAh g⁻¹, which was higher than that of the parent Li/Li_1−xFeO₂–Li_xMnO₂ (186 mAh g⁻¹). We expected that the increase of initial discharge capacity and the change of shape of discharge curve for the Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell is the result from the redox reaction from Ni²⁺ to Ni³⁺ during charge/discharge process. This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles.     

Structural changes and thermal stability of charged LiNi1/3Co1/3Mn1/3O2 cathode material for Li-ion batteries studied by time-resolved XRD

Structural changes and their relationship with thermal stability of charged Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode samples have been studied using time-resolved X-ray diffraction (TR-XRD) in a wide temperature from 25 to 600 °C with and without the presence of electrolyte in comparison with Li_0.27Ni_0.8Co_0.15Al_0.05O₂ cathodes. Unique phase transition behavior during heating is found for the Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode samples: when no electrolyte is present, the initial layered structure changes first to a LiM₂O₄-type spinel, and then to a M₃O₄-type spinel and remains in this structure up to 600 °C. For the Li_0.33Ni_1/3Co_1/3Mn_1/3O₂ cathode sample with electrolyte, additional phase transition from the M₃O₄-type spinel to the MO-type rock salt phase takes place from about 400 to 441 °C together with the formation of metallic phase at about 460 °C. The major difference between this type of phase transitions and that for Li_0.27Ni_0.8Co_0.15Al_0.05O₂ in the presence of electrolyte is the delayed phase transition from the spinel-type to the rock salt-type phase by stretching the temperature range of spinel phases from about 20 to 140 °C. This unique behavior is considered as the key factor of the better thermal stability of the Li_1−xNi_1/3Co_1/3Mn_1/3O₂ cathode materials.     

Preparation and characterization of 18650 Li(Ni1/3Co1/3Mn1/3)O2/graphite high power batteries

The commercial 18650 Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite high power batteries were prepared and their electrochemical performance at temperatures of 25 and 50 °C was extensively investigated. The results showed that the charge-transfer resistance (R_ct) and solid electrolyte interface resistance (R_sei) of the high power batteries at 25 °C decreased as states of charge (SOC) increased from 0 to 60%, whereas R_ct and R_sei increased as SOC increased from 60 to 100%. The discharge plateau voltage of batteries reduced greatly with the increase in discharge rate at both 25 and 50 °C. The high power batteries could be discharged at a very wide current range to deliver most of their capacity and also showed excellent power cycling performance with discharge rate of as high as 10 C at 25 °C. The elevated working temperature did not influence the battery discharge capacity and cycling performance at lower discharge rates (e.g. 0.5, 1, and 5 C), while it resulted in lower discharge capacity at higher discharge rates (e.g. 10 and 15 C) and bad cycling performance at discharge rate of 10 C. The batteries also exhibited excellent cycle performance at charge rate of as high as 8 C and discharge rate of 10 C.     

Studies on the low-heating solid-state reaction method to synthesize LiNi1/3Co1/3Mn1/3O2 cathode materials

The low-heating solid-state method has been adopted to synthesize LiNi_1/3Co_1/3Mn_1/3O₂ materials. The final product, with homogeneous phase and smooth crystals indicated by XRD and SEM results, can be synthesized at 700 °C, much lower than the synthesis temperatures of co-precipitation method. The reaction process and microstructure of precursor has been investigated by IR spectrum. By comparative studies with the mixture of CH₃COOLi and (Ni, Co, Mn)(C₂O₄), it is testified that the precursor is homogeneous, rather than a mixture. The decomposition process and the reaction energy have been studied to investigate the reaction mechanism of the precursor when heated at high temperature. The as-synthesized LiNi_1/3Co_1/3Mn_1/3O₂ exhibits excellent electrochemical properties, exhibiting initial specific capacity of 167 mAh g⁻¹ with stable cyclic performance.     

Effect of CO2 on layered Li1+zNi1−x−yCoxMyO2 (M = Al,Mn) cathode materials for lithium ion batteries

We investigated the effect of CO₂ on layered Li_1+zNi_1−x−yCo_xM_yO₂ (M = Al, Mn) cathode materials for lithium ion batteries which were prepared by solid-state reactions. Li_1+zNi_(1−x)/2Co_xMn_(1−x)/2O₂ (Ni/Mn mole ratio = 1) singularly exhibited high storage stability. On the other hand, Li_1+zNi_0.80Co_0.15Al_0.05O₂ samples were very unstable due to CO₂ absorption. XPS and XRD measurements showed the reduction of Ni³⁺ to Ni²⁺ and the formation of Li₂CO₃ for Li_1+zNi_0.80Co_0.15Al_0.05O₂ samples after CO₂ exposure. SEM images also indicated that the surfaces of CO₂-treated samples were covered with passivation films, which may contain Li₂CO₃. The relationship between CO₂-exposure time and CO₃²⁻ content suggests that there are two steps in the carbonation reactions; the first step occurs with the excess Li components, Li₂O for example, and the second with LiNi_0.80Co_0.15Al_0.05O₂ itself. It is well consistent with the fact that the discharge capacity was not decreased and the capacity retention was improved until the excess lithium is consumed and then fast deterioration occurred.