Study of the surface modification of LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion battery

The surface of LiNi_1/3Co_1/3Mn_1/3O₂ (LNMCO) particles has been studied for material synthesized at 900 °C by a two-step process from a mixture of LiOH·H₂O and metal oxalate [(Ni_1/3Co_1/3Mn_1/3)C₂O₄] obtained by co-precipitation. Samples have been characterized by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), Raman scattering (RS) spectroscopy, and magnetic measurements. We have investigated the effect of the heat treatment of particles at 600 °C with organic substances such as sucrose and starch. HRTEM images and RS spectra indicate that the surface of particles has been modified. The annealing does not lead to any carbon coating but it leads to the crystallization of the thin disordered layer on the surface of LiNi_1/3Co_1/3Mn_1/3O₂. The beneficial effect has been tested on the electrochemical properties of the LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials. The capacity at 10C-rate is enhanced by 20% for post-treated LNMCO particles at 600 °C for half-an-hour.     

Synthesis of LiNi1/3Co1/3Mn1/3O2 as a cathode material for lithium battery by the rheological phase method

LiNi_1/3Co_1/3Mn_1/3O₂ is prepared by a rheological phase method. Homogeneous precursor derived from this method is calcined at 800 °C for 20 h in air, which results in the impressive differences in the morphology properties and electrochemical behaviors of LiNi_1/3Co_1/3Mn_1/3O₂ in contrast to that obtained by a solid-state method. The microscopic structural features of LiNi_1/3Co_1/3Mn_1/3O₂ are investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD). The electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ are carried out by charge–discharge cycling test. All experiments show that the microscopic structural features and the morphology properties are deeply related with the electrochemical performances. The obtained results suggest that the rheological phase method may become an effective route to prepare LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials for lithium battery.     

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.     

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.     

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.     

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.     

Cycle life improvement of ZrO2-coated spherical LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion batteries

A modified synthesis process was developed based on co-precipitation method followed by spray drying process. In this process, a spherical shaped (Co_1/3Ni_1/3Mn_1/3)(OH)₂ precursor was synthesized by co-precipitation and pre-heated at 500 °C to form a high structural stability spinel (CoNiMn)O₄ to maintain its shape for further processing. The spherical LiNi_1/3Co_1/3Mn_1/3O₂ was then prepared by spray drying process using spherical spinel (CoNiMn)O₄. LiNi_1/3Co_1/3Mn_1/3O₂ powders were then modified by coating their surface with a uniform and nano-sized layer of ZrO₂. The ZrO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ material exhibited an improved rate capability and cycling stability under a high cut-off voltage of 4.5 V. X-ray diffraction (XRD) measurements revealed that the material had a well-ordered layered structure and Zr was not doped into the LiNi_1/3Co_1/3Mn_1/3O₂. Electrochemical impedance spectroscopy measurements showed that the coated material has stable cell resistance regardless of cycle number. The interrupt charging/discharging test indicated that the ZrO₂ coating can suppress the polarization effects during the charging and discharging process. From these results, it is believed that the improved cycling performance of ZrO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ is attributed to the ability of ZrO₂ layer in preventing direct contact of the active material with the electrolyte resulting in a decrease of electrolyte decomposition reactions.     

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.     

Thermal and electrochemical characterization of MCMB/LiNi1/3Co1/3Mn1/3O2 using LiBoB as an electrolyte additive

The gas generation associated with the use of the lithium bis(oxalate)borate—(LiBoB) based electrolyte at the elevated temperature were detected in the pouch cell (MCMB/LiNi_1/3Co_1/3Mn_1/3O₂ with 10% excess Li), which might prevent the LiBoB usage as a salt. However, the cell capacity retention was improved significantly, from 87 to 96% at elevated temperature, when using LiBoB as an electrolyte additive. The capacity fade during cycling is discussed using dQ/dE, area specific impedance, and frequency response analysis results. Most of the capacity loss in the cell is associated with the rise in the cell impedance. Moreover, results from the differential scanning calorimetry indicate that the thermal stability of the negative electrode with the solid electrolyte interface (SEI) formed by the reduction of the LiBoB additive was greatly improved compared with that obtained from the reduction of LiPF₆-based electrolyte without additive. In this case, the onset temperature of the breakdown of the LiBoB-based SEI is 150 °C which is higher than that of the conventional electrolyte without additive. Furthermore, the total heat generated between 60 and 170 °C is reduced from 213 to 70 J g⁻¹ when using LiBoB as electrolyte additive compared to the one without additive. In addition, the thermal stability of the charged LiNi_1/3Co_1/3Mn_1/3O₂ with 10% excess Li was not affected when using LiBoB as an electrolyte additive.     

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.     

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.     

Effect of Ag additive on the performance of LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion battery

The LiNi_1/3Co_1/3Mn_1/3O₂/Ag composite used for cathode material of lithium ion battery was prepared by thermal decomposition of AgNO₃ added to commercial LiNi_1/3Co_1/3Mn_1/3O₂ powders to improve the electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂. Structure and morphology analysis showed that Ag particles were dispersed on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ instead of entering the crystal structure. The results of charge–discharge tests showed that Ag additive could improve the cycle performance and high-rate discharge capability of LiNi_1/3Mn_1/3Co_1/3O₂. Extended analysis indicated that Ag was unstable in the commercial electrolyte at high potential. The improved electrochemical performance caused by Ag additive was associated not only with the enhancement of electrical conductivity of the material and the lower polarization of the cell, but also with the increased “c” parameter of LiNi_1/3Mn_1/3Co_1/3O₂ after repeated charge/discharge cycles and the compact and protective SEI layer formed on the surface of LiNi_1/3Mn_1/3Co_1/3O₂.     

Performance of LiNi0.5Mn1.5O4 prepared by solid-state reaction

LiNi_0.5Mn_1.5O₄ was prepared through a solid-state reaction using various Ni precursors. The effect of precursors on the electrochemical performance of LiNi_0.5Mn_1.5O₄ was investigated. LiNi_0.5Mn_1.5O₄ made from Ni(NO₃)₂·6H₂O shows the best charge–discharge performance. The reversible capacity of LiNi_0.5Mn_1.5O₄ is about 145 mAh g⁻¹ and remained 143 mAh g⁻¹ after 10 cycles at 3.0–5.0 V. The XRD results showed that the precursors and the dispersion methods had significant effect on their phase purity. Pure spinel phase can be obtained with high energy ball-milling method and Ni(NO₃)₂·6H₂O as precursor. Trace amount of NiO and Li₂MnO₃ phase were detected in LiNi_0.5Mn_1.5O₄ with manual-mixture method and using Ni(CH₃COO)₂·6H₂O, NiO and Ni₂O₃ as precursors.     

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.     

Microemulsion preparation and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 powders

Layer-structured LiNi_1/3Co_1/3Mn_1/3O₂ was successfully synthesized via a reverse microemulsion (RμE) route. Well-crystallized nanosized (around 45 nm) powders were obtained with calcination at 800 °C. The Rietveld refinement data revealed low degree of cationic displacement in the obtained powders. Within the voltage range of 2.5–4.5 V, the microemulsion-derived LiNi_1/3Co_1/3Mn_1/3O₂ delivered 187.2 and 195.5 mAh g⁻¹ at room temperature and 55 °C, respectively. The prepared powders were found to exhibit low irreversible capacity and good capacity retention. Microemulsion-derived LiNi_1/3Co_1/3Mn_1/3O₂ demonstrated better rate capability than the solid-state derived samples, owing to the reduced particle size and increased surface area. Once the upper cut-off voltage reached 4.6 V, the capacity faded more rapidly than in other operation potential ranges. In this study, the microemulsion process effectively improved the electrochemical characteristics of LiNi_1/3Co_1/3Mn_1/3O₂. This soft chemical route possesses a great potential for synthesizing other types of cathode materials with multiple cations.     

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.     

Aging of LiNi1/3Mn1/3Co1/3O2 cathode material upon exposure to H2O

LiNi_1/3Mn_1/3Co_1/3O₂ compound was successfully synthesized by the co-precipitation method. The effect of H₂O on LiNi_1/3Mn_1/3Co_1/3O₂ in humid atmosphere was investigated by structural, magnetic and electrochemical analysis, and Raman spectroscopy. The consequence is that immersion of LiNi_1/3Mn_1/3Co_1/3O₂ to H₂O and exposure of LiNi_1/3Mn_1/3Co_1/3O₂ to humid atmosphere (ambient atmosphere, 20 °C, 50% relative humidity) led to a rapid attack that manifests itself by the delithiation of the surface layer of the particles and the concomitant formation of LiOH and Li₂ CO₃ at the surface. This aging process occurred during the first few minutes, then it is saturated, and the thickness of the surface layer at saturation is 10 nm. After aging, an initial discharge capacity of 139 mAh/g was delivered at 1C-rate in the cut-off voltage of 3.0-4.3 V. About 95% of its initial capacity was retained after 30 cycles.     

Preparation and electrochemical characterization of single-crystalline spherical LiNi1/3Co1/3Mn1/3O2 powders cathode material for Li-ion batteries

LiNi_1/3Co_1/3Mn_1/3O₂ has aroused much interest as a new generation of cathode material for Li-ion batteries, due to its great advantages in capacity, stability, low cost and low toxicity, etc. Here we report a novel single-crystalline spherical LiNi_1/3Co_1/3Mn_1/3O₂ material that is prepared by a convenient rheological phase reaction route. The X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy indicate that the particles are highly dispersed with spherical morphologies and diameters of about 1-4 μm, and more interestingly, they show a perfect single-crystalline nature, which is not usual according to the crystal growth theories and may bring extra benefits to applications. Electrochemical tests show good performance of the material in both the capacity and cycling stability as cathode material in a model cell.     

Preparation and electrochemical properties of Li[Ni1/3Co1/3Mn1−x/3Zrx/3]O2 cathode materials for Li-ion batteries

Layer-structured Zr doped Li[Ni_1/3Co_1/3Mn_1−x/3Zr_x/3]O₂ (0 ≤ x ≤ 0.05) were synthesized via slurry spray drying method. The powders were characterized by XRD, SEM and galvanostatic charge/discharge tests. The products remained single-phase within the range of 0 ≤ x ≤ 0.03. The charge and discharge cycling of the cells showed that Zr doping enhanced cycle life compared to the bare one, while did not cause the reduction of the discharge capacity of Li[Ni_1/3Co_1/3Mn_1/3]O₂. The unchanged peak shape in the differential capacity versus voltage curve suggested that the Zr had the effect to stabilize the structure during cycling. More interestingly, the rate capability was greatly improved. The sample with x = 0.01 presented a capacity of 160.2 mAh g⁻¹ at current density of 640 mA g⁻¹(4 C), corresponding to 92.4% of its capacity at 32 mA g⁻¹(0.2 C). The favorable performance of the doped sample could be attributed to its increased lattice parameter.     

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.