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.     

The electrochemical property of ZrFx-coated Li[Ni1/3Co1/3Mn1/3]O2 cathode material

Electrochemical and thermal properties of pristine and ZrF_x-coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials are compared. The hydrothermal method is introduced for the fabrication of a uniform coating layer. The formation of a compact coating layer on the surface of pristine powder is observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From TEM-EDS and XPS analysis, it is inferred that the coating layer is ZrO_xF_y (zirconium oxyfluoride) form. The coated Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrodes have better rate capability and cyclic performance at high temperature compared with the pristine electrode. The thermal stability of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode is also enhanced by the ZrF_x coating. Such enhancements are due to the presence of a stable coating layer, which effectively suppresses the chemical instability ascribed to surface reaction between electrode and electrolyte.     

Electrochemical and thermal characterization of AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 cathode in lithium-ion cells

Li[Ni_0.8Co_0.15Al_0.05]O₂ particles are modified with AlF₃ as a new coating material. Even though the initial discharge capacity of the coated Li[Ni_0.8Co_0.15Al_0.05]O₂ is almost the same as that of the pristine material, the capacity retention and the thermal stability, in a highly oxidized state are both significantly improved. This effect is attributed to the thin AlF₃ coating layer protecting the oxidized cathode particles from attack by hydrogen fluoride in the electrolyte.     

Surface modification of Li[Ni0.3Co0.4Mn0.3]O2 cathode by Li-La-Ti-O coating

A Li[Ni_0.3Co_0.4Mn_0.3]O₂ cathode is modified by applying a Li-La-Ti-O coating using the hydrothermal method. The coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ is characterized by X-ray diffraction analysis, scanning electron microscopy, energy-dispersive spectrometry, transmission electron microscopy, and differential scanning calorimetry. The Li-La-Ti-O coating layer is formed as crystalline (perovskite structure) or amorphous phase depending on the heating temperature. The Li-La-Ti-O coated Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode has better rate capability than the pristine electrode. In particular, the rate capability is significantly associated with heating temperature; this is probably due to the phase of the coating layer. It appears that the Li-La-Ti-O coating of amorphous phase is superior to that of crystalline phase for obtaining enhanced rate capability of the coated samples. The thermal stability and cyclic performance of the Li[Ni_0.3Co_0.4Mn_0.3]O₂ electrode are also improved by Li-La-Ti-O coating. These improvements indicate that the Li-La-Ti-O coating is effective in suppressing the chemical and structural instabilities of Li[Ni_0.3Co_0.4Mn_0.3]O₂.     

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.     

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.     

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.     

Improvement of electrochemical properties of Li1.1Al0.05Mn1.85O4 achieved by an AlF3 coating

The electrochemical performance of AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ spinel was investigated. The morphology of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was observed by SEM and TEM, and the thickness of the coating layer was approximately 10 nm. Capacity retention and rate capability were substantially improved by the AlF₃-coating, as compared to pristine Li_1.1Al_0.05Mn_1.85O₄. Manganese dissolution was also dramatically reduced for the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄, which may reflect lower impedance for the coated spinel. The thermal stability of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was improved, exhibiting an exothermic reaction at higher temperature with reduced heat generation, compared to the pristine Li_1.1Al_0.05Mn_1.85O₄.     

Synthesis and electrochemical performances of core-shell structured Li[(Ni1/3Co1/3Mn1/3)0.8(Ni1/2Mn1/2)0.2]O2 cathode material for lithium ion batteries

Micro-scale core-shell structured Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ powders for use as cathode material are synthesized by a co-precipitation method. To protect the core material Li[Ni_1/3Co_1/3Mn_1/3]O₂ from structural instability at high voltage, a Li[Ni_1/2Mn_1/2]O₂ shell, which provides structural and thermal stability, is used to encapsulate the core. A mixture of the prepared core-shell precursor and lithium hydroxide is calcined at 770 °C for 12 h in air. X-ray diffraction studies reveal that the prepared material has a typical layered structure with an space group. Spherical morphologies with mono-dispersed powders are observed in the cross-sectional images obtained by scanning electron microscopy. The core-shell Li[(Ni_1/3Co_1/3Mn_1/3)_0.8(Ni_1/2Mn_1/2)_0.2]O₂ electrode has an excellent capacity retention at 30 °C, maintaining 99% of its initial discharge capacity after 100 cycles in the voltage range of 3-4.5 V. Furthermore, the thermal stability of the core-shell material in the highly delithiated state is improved compared to that of the core material. The resulting exothermic onset temperature appear at approximately 272  °C, which is higher than that of the highly delithiated Li[Ni_1/3Co_1/3Mn_1/3]O₂ (261 °C).     

Development of high power lithium-ion batteries: Layer Li[Ni0.4Co0.2Mn0.4]O2 and spinel Li[Li0.1Al0.05Mn1.85]O4

Layer Li[Ni_0.4Co_0.2Mn_0.4]O₂ and lithium excess spinel Li[Li_0.1Al_0.05Mn_1.85]O₄ were compared as positive electrode materials for high power lithium-ion batteries. Physical properties were examined by Rietveld refinement of X-ray diffraction pattern and scanning electron microscopic studies. From continuous charge and discharge tests at higher currents and different temperature environments using 3Ah class lithium-ion batteries, it was found that both materials presented plausible battery performances such as rate capability, cyclability at 60 °C at elevated temperature, and low temperature properties as well.     

Cyclic deterioration and its improvement for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2

Transmission electron microscopy (TEM) studies were carried out to elucidate cyclic deterioration phenomena for Li-rich layered cathode material Li[Ni_0.17Li_0.2Co_0.07Mn_0.56]O₂. The results obtained show that the deterioration starts during the initial charge/discharge to higher potential over 4.5 V, and leads to the formation of micro-cracks at the crystal surface and the distortion of crystal periodicity. These two kinds of defects lead to further non-crystallization of the crystal surface and the emergence of a very small amount of another possible phase. Our stepwise pre-cycling treatment effectively depressed the formation of the former two kinds of defects, and could significantly improve cyclic durability. The observation of non-crystallization at the cathode crystal surface, which would diminish the battery performance, is consistent with our preliminary ac impedance results.     

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.     

Effect of Co3(PO4)2 coating on Li[Co0.1Ni0.15Li0.2Mn0.55]O2 cathode material for lithium rechargeable batteries

To prepare a high-capacity cathode material with improved electrochemical performance for lithium rechargeable batteries, Co₃(PO₄)₂ nanoparticles are coated on the surface of powdered Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂, which is synthesized by a simple combustion method. The coated powder prepared under proper conditions for Co₃(PO₄)₂ content and annealing temperature shows an optimum coating layer that consists of a Li_xCoPO₄ phase formed by reaction with lithium impurities during heat treatment. A sample coated with 3 wt.% Co₃(PO₄)₂ and annealed at 800 °C proves to be the best in terms of specific capacity, cycle performance and rate capability. Thermal stability is also enhanced by the coating, as demonstrated a decrease in the onset temperature and/or the heat generated during thermal runaway.     

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.     

In situ X-ray absorption spectroscopic study of Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2

Although Li-rich solid-solution layered materials Li₂MnO₃-LiMO₂ (M = Co, Ni, etc.) are expected as large capacity lithium insertion cathodes, the fundamental charge-discharge reaction mechanism of these materials is not clear. Therefore the change in valence states of Ni, Co and Mn of Li[Ni_0.17Li_0.2Co_0.07Mn_0.56]O₂ during charge-discharge was examined in detail using in situ X-ray absorption spectroscopy (XAS), which includes both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Since the Mn K edge shift during charge-discharge was not clear to determine the valence change of Mn, the Mn K pre-edge shift was examined during charge-discharge. In our measurements, only a small shift of the Mn K pre-edge toward lower energy was observed on discharge from 4.8 to 2.0 V. This corresponds to a decrease of the Mn valence from 4+ to approximately 3.6+. However, this shift cannot explain the large reversible capacity of this material and thus strongly suggests the participation of oxygen in the reversible charge-discharge reaction of this material.     

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.     

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.     

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.     

Characterization of LiCoO2 coated Li1.05Ni0.35Co0.25Mn0.4O2 cathode material for lithium secondary cells

In this study, nano-crystalline LiCoO₂ was coated onto the surface of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ powders via sol–gel method. The influence of the coating on the electrochemical behavior of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ is discussed. The surface morphology was characterized by transmission electron microscopy (TEM). Nano-crystallized LiCoO₂ was clearly observed on the surfaces of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂. The phase and structural changes of the cathode materials before and after coating were revealed by X-ray diffraction spectroscopy (XRD). It was found that LiCoO₂ coated Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ cathode material exhibited distinct surface morphology and lattice constants. Cyclic voltammetry (2.8–4.6 V versus Li/Li⁺) shows that the characteristic voltage transitions on cycling exhibited by the uncoated material are suppressed by the 7 wt.% LiCoO₂ coating. This behavior implies that LiCoO₂ inhibits structural change of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ or reaction with the electrolyte on cycling. In addition, the LiCoO₂ coating on Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ significantly improves the rate capability over the range 0.1–4.0C. Comparative data for the coated and uncoated materials are presented and discussed.     

Enhanced electrochemical properties of Li(Ni0.4Co0.3Mn0.3)O2 cathode by surface modification using Li3PO4-based materials

The surface of a commercial Li[Ni_0.4Co_0.3Mn_0.3]O₂ cathode is modified using Li₃PO₄-based coating materials. The electrochemical properties of the coated materials are investigated as a function of the pH value of the coating solution and the composition of coating materials. The Li₃PO₄ coating solution with pH 2 is found to be favorable for the formation of stable coating layers having enhanced electrochemical properties. The Li₃PO₄, Li_1.5PO₄, and PO₄ coating layers are formed as amorphous phases. However, the Li_3−xNi_x/2PO₄ coating layers are composed of small particles with a crystalline phase covered with an amorphous phase. Li₃PO₄ and Li_1.5PO₄ coatings considerably enhance the rate capability of the Li[Ni_0.4Co_0.3Mn_0.3]O₂ electrode. In contrast, the Li_3−xNi_x/2PO₄ coating material, which contained Ni, has an inferior rate capability compared to the Li_xPO₄ series (x = 1.5 and 3), although the LiNiPO₄-coated electrode shows a better rate capability than a pristine one. Li₃PO₄-based coating materials are effective at enhancing the cyclic performance of the electrode in the voltage range of 3.0-4.8 V. DSC analysis also confirms the improved thermal stability attained by coating the cathode with Li₃PO₄-based materials.