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.     

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.     

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.     

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.     

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.     

Surface structure investigation of LiNi0.8Co0.2O2 by AlPO4 coating and using functional electrolyte

Surface structures of the bare and AlPO₄-coated LiNi_0.8Co_0.2O₂ particles in two electrolytes after 90 °C for 4 h storage were investigated using transmission electron microscope (TEM). The structure of bare LiNi_0.8Co_0.2O₂ particles in common electrolyte has been destructed from the layered structure with space group R-3m at interior region to a rock-salt phase (Fm-3m) at edge of the surface layer of the cycled particles, while AlPO₄-coated LiNi_0.8Co_0.2O₂ particles in common electrolyte has been transformed into a spinel phase (Fd-3m) on the surfaces of the cycled particles. However, the surface structure of bare LiNi_0.8Co_0.2O₂ particles in functional electrolyte has not been changed. The results showed that functional electrolyte can more effectively improve thermal stability of LiNi_0.8Co_0.2O₂ cathode cells than the AlPO₄ coating.     

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.     

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.     

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.     

Electrochemical effects of ALD surface modification on combustion synthesized LiNi1/3Mn1/3Co1/3O2 as a layered-cathode material

Combustion synthesized Li(Ni_1/3Mn_1/3Co_1/3)O₂ particles are coated with thin, conformal layers of Al₂O₃ by atomic layer deposition (ALD). XRD, Raman, and FTIR are used to confirm that no change to the bulk, local structure occurs after coating. Electrochemical impedance spectroscopy (EIS) results indicate that the surface of the Li(Ni_1/3Mn_1/3Co_1/3)O₂ are protected from dissolution and HF attack after only 4-layers, or ∼8.8 Å of alumina. Electrochemical performance at an upper cutoff of 4.5 V is greatly enhanced after the growth of Al₂O₃ surface film. Capacity retention is increased from 65% to 91% after 100 cycles at a rate of C/2 with the addition of only two atomic layers. Due to the conformal coating, the effects on Li(Ni_1/3Mn_1/3Co_1/3)O₂ overpotential and capacity are negligible below six ALD-layers. We propose that the use of ALD for coating on Li(Ni_1/3Mn_1/3Co_1/3)O₂ particles makes the material a stronger replacement candidate for LiCoO₂ as a positive electrode in lithium 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.     

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.     

Surface modification of LiNi1/2Mn3/2O4 thin-films by zirconium alkoxide/PMMA composites and their effects on electrochemical properties

Three kinds of surface modifications were carried out on LiNi_1/2Mn_3/2O₄ thin-films to improve the charge and discharge characteristics of LiNi_1/2Mn_3/2O₄ positive electrodes. Among them, Zr(OBu)₄/poly(methyl methacrylate) (PMMA)-treated LiNi_1/2Mn_3/2O₄ thin-film electrodes showed charge and discharge efficiency of 80–84% in the first cycle, which was much higher than that for an untreated LiNi_1/2Mn_3/2O₄ thin-film electrode (73%). The values of the charge and discharge efficiency were still higher than that for an untreated electrode after the 30th cycle. The charge and discharge curves gave two plateaus at around 4.72 and 4.76 V, which were very similar to those for the untreated electrode. Ac impedance spectroscopy revealed that the surface film resistance should not increase by Zr(OBu)₄/PMMA treatment. XPS measurements suggest that a composite layer should be formed on a LiNi_1/2Mn_3/2O₄ thin-film electrode from PMMA and Zr(OBu)₄-derived compounds introducing an electrolyte. This composite layer was lithium-ion conductive, and was sustainable enough to suppress subsequent decomposition of an electrolyte at potentials as high as 4.7 V.     

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.     

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.     

Improvement of structural and electrochemical properties of AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode materials on high voltage region

The particle surface of Li[Ni_1/3Co_1/3Mn_1/3]O₂ was modified by AlF₃ as a new coating material to improve the electrochemical properties in the high cutoff voltage of 4.5 V. The AlF₃-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ showed no difference in the bulk structure compared with the pristine one and the uniform AlF₃ coating layers whose thickness is of about 10 nm covered Li[Ni_1/3Co_1/3Mn_1/3]O₂ particles, as confirmed by a transmission electron microscopy. The AlF₃ coating on Li[Ni_1/3Co_1/3Mn_1/3]O₂ particles improved the overall electrochemical properties such as the cyclability, rate capability and thermal stability compared with those of the pristine Li[Ni_1/3Co_1/3Mn_1/3]O₂. Such enhancements were attributed to the presence of the stable AlF₃ layer which acts as the interfacial stabilizer on the surface of Li[Ni_1/3Co_1/3Mn_1/3]O₂.     

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.     

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 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.