High-voltage Electrochemical Performance Research Articles

Improvement of High-Voltage Electrochemical Performance of Surface Modified LiNi0.6Co0.2Mn0.2O2 Cathode by La2O3 Coating

Improvement of High-Voltage Electrochemical Performance of Surface Modified LiNi0.6Co0.2Mn0.2O2 Cathode by La2O3 Coating

Enhanced high-voltage cycling stability and rate capability of magnesium and titanium co-doped lithium cobalt oxides for lithium-ion batteries

To improve the high-voltage cycling stability and rate capability, the Mg2+ and Ti4+ co-doping strategy is firstly proposed to modify the LiCoO2 cathode material. The synergistic effect of co-doping with Mg2+ and Ti4+ ions on the structure, morphology and high-voltage electrochemical performance of LiCoO2 is investigated. For the co-doped sample, the introduction of Mg2+ and Ti4+ ions can efficiently optimize the particle size distribution and reduce the aggregation behavior. Compared with the undoped and single-doped samples, the Mg2+ and Ti4+ co-doped LiCoO2 sample presents better high-voltage cycling stability and rate capability due to the fact that the Mg2+ and Ti4+ ions co-doping can make full use of the respective advantages of Mg2+-doping and Ti4+-doping. When cycled at 1.0 C, the co-doped sample exhibits an initial discharge capacity of 179.6 mAh g−1 in the voltage range of 2.75–4.5 V. After 100 cycles, the capacity retention of this sample can reach up to 82.6%. Moreover, the co-doped sample shows better rate performance with high discharge capacity of 151.4 mAh g−1 at 5.0 C. These outstanding results may be attributed to the suppressed phase transition, decreased charge transfer resistance, improved thermal stability, enhanced electrical conductivity and uniform particle size distribution of the Mg2+ and Ti4+ co-doped LiCoO2 sample.

High-voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material via the synergetic modification of the Zr/Ti elements

The stabilities of the host and interfacial structure for layered LiNixCoyMn1-x-yO2 cathode materials are critical for their electrochemical properties, especially at high voltage. This study reports the synergetic effect of the Zr and Ti elements on the LiNi0.5Co0.2Mn0.3O2 cathode material (NCM523). The as-obtained Zr/Ti modified LiNi0.5Co0.2Mn0.3O2 cathode material shows the excellent electrochemical performance at high voltage. It delivers 167.3 mAh·g−1 at 1 C over 3.0–4.4 V corresponding 94.20% capacity retention after 200 cycles. Moreover, the rate capability of Zr/Ti co-modified NCM523 at 16 C (3.0–4.4 V) rises to 139.4 mAh·g−1 and corresponding capacity retention (vs. 0.5 C) is 79.57%. In addition, its capacity retention reaches to 91.71% after 100 cycles at 1 C rate even at 3.0–4.6 V. Such superior electrochemical performances are ascribed to the Zr doping and Ti modification. The synergetic modifications of the Zr/Ti elements not only stabilize the crystal structure, but also absorb the lithium residues to form mixed Li4Ti5O12/Li2TiO3 coating layers, which strengthen the stability of the interfacial structure and the kinetic characteristics of the cathode materials.

Effect of Mo doping on the structure and electrochemical performances of LiNi0.6Co0.2Mn0.2O2 cathode material at high cut-off voltage

Mo-doped nickel-rich LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode material is prepared by a solid-state method to enhance the high voltage electrochemical performances of the NCM622 material for lithium-ion batteries. The molybdenum element is proved to be homogeneously distributed inside the particles of the NCM622 material according to the result from the EDS mapping of the particle cross-section. The effects of Mo doping on the crystal structure, morphology, electrochemical properties and high-temperature performances of the NCM622 material are investigated in detail. The Mo-doped sample delivers a discharge capacity of 203.1 mAh/g and a capacity retention of 82.96% after 100 cycles at 1C under a high cut-off voltage of 4.6 V. Besides, the Mo-modified material exhibits a better rate performance of 159.9 mAh/g at 8C and lower voltage fading compared with the unmodified material. The significant improvements in electrochemical properties are ascribed to the fact that trace Mo ions doped into the transition-metal layer help to enlarge the lithium slab spacing, stabilize the crystal structure and suppress the electrode polarization. It is also demonstrated that Mo doping effectively suppresses the pulverization of particles and reduces the charge transfer resistance according to the SEM and EIS analyses.

High-voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials via Al concentration gradient modification

Al2O3-modified LiNi0.5Co0.2Mn0.3O2 cathode material is successfully synthesized via a facile carboxymethyl cellulose (CMC)-assisted wet method followed by a high-temperature calcination process. Al concentration gradient doping and accompanying formation of Al-coating are simultaneously accomplished in the modified samples. XRD and EDS analysis demonstrate that Al element is successfully doped into the crystal lattice with concentration gradient distribution inside the particles, reducing the Li/Ni cation mixing and stabilizing the layered structure. The compact distribution of Al on the surface forms a protective layer between the electrodes and the electrolyte, prohibiting the harmful side reactions and phase transition on the interphase. Compared with the pristine, the modified material with 2000 ppm Al2O3 (Al-2000) shows the best high-voltage performance with the capacity retention increased by ~13.3% from 138.3 to 163.0 mAh g−1 at 1 C in 3.0–4.6 V after 100 cycles. Even under the high current rate of 8 C (1240 mAh g−1) after 200 cycles, the Al-2000 still exhibits a capacity retention of 88.6%, greater than 80.3% for the pristine. Furthermore, results from the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) measurements confirm the roles of the Al2O3 modification in decreasing polarization and electrochemical resistances, contributing to the kinetic process of electrodes.

Enhancing the high-voltage electrochemical performance of the LiNi0.5Co0.2Mn0.3O2 cathode materials via hydrothermal lithiation

The chemical lithiated transition metal oxide precursor has been prepared via a hydrothermal process and successfully used for preparing the LiNi0.5Co0.2Mn0.3O2 cathode materials by the post-heat treatment. The results indicate that the lithiated transition metal oxide precursor inherits the morphology of the Ni0.5Co0.2Mn0.3(OH)2 precursor but has a typical α-NaFeO2-type (space group: R-3 m) layered structure with an imperfect crystallinity, and the Li is homogenously distributed in the particles. It is further confirmed that the obtained LiNi0.5Co0.2Mn0.3O2 cathode material has a suppressed cation mixing resulting in an excellent electrochemical performance. It delivers the high initial capacity of 187.3 mAhg−1 at 1 C over the high cutoff voltage range of 3.0–4.6 V and the excellent capacity retention of 81.90% after 100 cycles as well as the rate capability of 152.3 mAhg−1 at 8 C, which are attributed to the low polarization, fast Li+ diffusion and small charge–discharge resistance of the as-prepared material upon cycling.

Stable, fast and high-energy-density LiCoO2 cathode at high operation voltage enabled by glassy B2O3 modification

In this work, commercial LiCoO2 is modified with a glassy B2O3 by solution mixing with H3BO3 followed by post-calcination in order to enhance its high-voltage electrochemical performance. The glassy B2O3 coating/additive is believed to serve as an effective physiochemical buffer and protection between LiCoO2 and liquid electrolyte, which can suppress the high-voltage induced electrolyte decomposition and active material dissolution. During the early cycling and due to the electrochemical force, the as-coated B2O3 glasses which have 3D open frameworks tend to accommodate some mobile Li+ and form a more chemically-resistant and ion-conductive lithium boron oxide (LBO) interphase as a major component of the solid electrolyte interphase (SEI), which consequently enables much easier Li+ diffusion/transfer at the solid-liquid interfaces upon further cycling. Due to the synergetic effects of B2O3 coating/modification, the high-voltage capacity and energy density of the B2O3-modified LiCoO2 cathode are promisingly improved by 35% and 30% after 100 cycles at 1 C within 3.0–4.5 V vs. Li/Li+. Meanwhile, the high-rate performance of the B2O3-modified electrode is even more greatly improved, showing a capacity of 105 mAh g−1 at 10 C while the bare electrode has dropped to no more than 30 mAh g−1 under this rate condition.

Enhanced Interfacial Kinetics and High-Voltage/High-Rate Performance of LiCoO2 Cathode by Controlled Sputter-Coating with a Nanoscale Li4Ti5O12 Ionic Conductor.

The selection and optimization of coating material/approach for electrode materials have been under intensive pursuit to address the high-voltage induced degradation of lithium ion batteries. Herein, we demonstrate an efficient way to enhance the high-voltage electrochemical performance of LiCoO2 cathode by postcoating of its composite electrode with Li4Ti5O12 (LTO) via magnetron sputtering. With a nanoscale (∼25 nm) LTO coating, the reversible capacity of LiCoO2 after 60 cycles is significantly increased by 40% (to 170 mAh g-1) at room temperature and by 118% (to 139 mAh g-1) at 55 °C. Meanwhile, the electrode's rate capability is also greatly improved, which should be associated with the high Li+ diffusivity of the LTO surface layer, while the bulk electronic conductivity of the electrode is unaffected. At 12 C, the capacity of the coated electrode reaches 113 mAh g-1, being 70% larger than that of the uncoated one. The surface interaction between LTO and LiCoO2 is supposed to reduce the space-charge layer at the LiCoO2-electrolyte interface, which makes the Li+ diffusion much easier as evidenced by the largely enhanced diffusion coefficient of the coated electrode (an order of magnitude improvement). In addition, the LTO coating layer, which is electrochemically and structurally stable in the applied potential range, plays the role of a passivation layer or an artificial and friendly solid electrolyte interface (SEI) layer on the electrode surface. Such protection is able to impede propagation of the in situ formed irreversible SEI and thus guarantee a high initial columbic efficiency and superior cycling stability at high voltage.

The enhanced high cut-off voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 by the CeO2 modification

The CeO2 nano-particles dispersed on the surface of commercial LiNi0.5Co0.2Mn0.3O2, have been prepared by a co-precipitation method, in which ammonium carbonate or ammonium oxalate is served as a precipitant to generate the different precursor of CeO2, respectively. CeO2-modified LiNi0.5Co0.2Mn0.3O2 composites with ammonium oxalate as the precipitant exhibit the best electrochemical properties with a reversible lithium storage capacity of 157.7mAhg−1 with a retention of 86.36% at 25°C at the current density 1C after 100 cycles under high cut-off voltage 4.6V. At higher temperature 55°C, its discharge capacity changes from 198.2mAhg−1 to 136.6mAhg−1 after 100 cycles, with a retention of 68.29%. Besides, it also displays an excellent rate performance with a capacity of 114.0mAhg−1 at 5C in the 100th cycle. All enhanced performances could be due to the dispersion of CeO2 on the surface of commercial LiNi0.5Co0.2Mn0.3O2 and the synergistic effects between the CeO2 and LiNi0.5Co0.2Mn0.3O2, which leads to the enhanced structural stability and electric conductivity.

Effect of surface fluorine substitution on high voltage electrochemical performances of layered LiNi0.5Co0.2Mn0.3O2 cathode materials

A strategy of surface fluorine substitution to enhance the high voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 material has been proposed. The inter-slab spacing distance is broaden by fluorine doping, which is deduced from the lattice parameters calculated by Rietveld refinement method. Scanning electron microscopy indicates the fluorine substitution stimulates the growth of the primary particles. Though the initial discharge capacities of LiNi0.5Co0.2Mn0.3O2-zFz (z=0.02, 0.04, 0.06) is somewhat reduced, the capacity retention under high oxidation state were improved compared to that of bare one. For the optimal composition of LiNi0.5Co0.2Mn0.3O1.98F0.02, it exhibits a capacity retention of 81.1% at 1C after 100 cycles and delivers a discharge capacity of 123.5mAhg−1at 10C, and that of bare sample are just 70.1% and 109.6mAhg−1, respectively. Cyclic voltammetry and electron impedance spectroscopy measurements demonstrate that the fluorine doping could significantly lower the cell polarization and retard the impedance rise. Transmission electron microscope analysis of cycled electrodes is also performed and it implies that fluorine substitution can effectively safeguard the electrode from HF erosion to maintain the bulk structural stable permanently.

Co-modification of LiNi0.5Co0.2Mn0.3O2 cathode materials with zirconium substitution and surface polypyrrole coating: towards superior high voltage electrochemical performances for lithium ion batteries

The combination of zirconium doping and surface conductive polypyrrole coating is successfully demonstrated to enhance the high voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 (NCM) cathode materials. The lattice parameters calculated from the X-ray diffraction (XRD) patterns by Rietveld refinement reveal that the cation mixing degree is restrained and the lithium slabs spacing is broadened after zirconium substitution (NCMZ). A compact and continuous polypyrrole thin film is satisfactorily coated onto the surface of zirconium-doped (NCMZ/PPy) sample, which is confirmed by fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Though the initial discharge capacity of NCMZ/PPy sample is slightly lower than pristine one, the cycling stability is dramatically improved. Notably, NCMZ/PPy sample shows capacity retention of 89.83% at 1C after 100 repeated cycles, while that of pristine one remains only 69.36%. When the electrodes cycled at 2C for 200 cycles, the capacity retention of NCMZ/PPy still reaches up to 78.83%, and that of pristine one is only 56.79%. The presence of polypyrrole network acts as a capsule shell, which can protect the active substance from erosion caused by the HF attacking. As a result, the cycling stability and rate performance of co-modified sample are both better than those of bare LiNi0.5Co0.2Mn0.3O2 sample and zirconium doped materials.

Flakelike LiCoO2 with Exposed {010} Facets As a Stable Cathode Material for Highly Reversible Lithium Storage

A thick and dense flakelike LiCoO2 with exposed {010} active facets is synthesized using Co(OH)2 nanoflake as a self-sacrificial template obtained from a simple coprecipitation method, and served as a cathode material for lithium ion batteries. When operated at a high cutoff voltage up to 4.5 V, the resultant LiCoO2 exhibits an outstanding rate capability, delivering a reversible discharge capacity as high as 179, 176, 168, 116, and 96 mA h g(-1) at 25 °C under the current rate of 0.1, 0.5, 1, 5, and 10 C, respectively. When charge/discharge cycling at 55 °C, a high specific capacity of 148 mA h g(-1) (∼88% retention) can be retained after 100 cycles under 1 C, demonstrating excellent cycling and thermal stability. Besides, the flakelike LiCoO2 also shows an impressive low-temperature electrochemical activity with specific capacities of 175 (0.1 C) and 154 mA h g(-1) (1 C) at -10 °C, being the highest ever reported for a subzero-temperature lithium storage capability, as well as 52% capacity retention even after 80 cycles under 1 C. Such superior high-voltage electrochemical performances of the flakelike LiCoO2 operated at a wide temperature range are mainly attributed to its unique hierarchical structure with specifically exposed facets. The exposed {010} active facets provide a preferential crystallographic orientation for Li-ion migration, while the micrometer-sized secondary particles agglomerated by submicron primary LiCoO2 flakes endow the electrode with better structural integrity, both of which ensure the LiCoO2 cathode to manifest remarkably enhanced reversible lithium storage properties.

Effects of Li2MnO3coating on the high-voltage electrochemical performance and stability of Ni-rich layer cathode materials for lithium-ion batteries

The Li2MnO3-coated LiNi0.8Co0.1Mn0.1O2shows a higher discharge capacity and a better capacity retention. The coating layer can protect the NCM active materials from CO2, suppressing the formation of Li2CO3on the surface of NCM materials.

Role of zirconium dopant on the structure and high voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries

The severe capacity fading and poor rate performance under high voltage of LiNi0.5Co0.2Mn0.3O2 are the vital barriers for their large scale application. In order to solve above issues, zirconium doped LiNi0.5Co0.2Mn0.3O2 materials were prepared by solid state method. The effects of the zirconium content on the structure and electrochemical properties of Li(Ni0.5Co0.2Mn0.3)1-xZrxO2 were extensively studied. Though the zirconium doped samples delivered lower initial capacities, the cycling stability are dramatically enhanced. Notably, Li(Ni0.5Co0.2Mn0.3)0.99Zr0.01O2 showed a capacity retention of 83.78% at 1C after 100 repeated cycle in a high voltage of 4.6V, while that of bare sample remains only 69.35%. Moreover, the rate capability of Li(Ni0.5Co0.2Mn0.3)0.99Zr0.01O2 is also greatly reinforced especially at high current density. The improved electrochemical performances could be ascribed to the less cation mixing degree, better lithium transportation kinetic and lower impedance, which were confirmed by Rietveld refinement, X-ray photoelectron spectroscopy and electron impedance spectroscopy.

Effect of heat-treatment on Li2ZrO3-coated LiNi1/3Co1/3Mn1/3O2 and its high voltage electrochemical performance

The surface of LiNi1/3Co1/3Mn1/3O2 particles was modified by Li2ZrO3. Influences of heat-treatment were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The results show that Li2ZrO3 is well-crystallized and successfully coated on the surface of LiNi1/3Co1/3Mn1/3O2 after heated at 700 °C. Electrochemical tests demonstrate that the coated sample exhibits excellent cycling performances with 89.0% (25 °C) and 84.9% (55 °C) capacity retention at 1C after 100 cycles under the high cut-off voltage of 4.6 V, which are much higher than 82.7% (25 °C) and 62.5% (55 °C) for the pristine one. Meanwhile, the rate capability of the coated sample is dramatically improved even under the high current densities, and the capacities are delivered as high as 168.7 mAh g−1 and 155.2 mAh g−1 at 5C and 10C respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses reveal that coating with Li2ZrO3 could decrease electrode polarization and diminish the charge transfer impedance. Moreover, XRD and SEM characterizations of cycled electrodes evidence that the coating layer of Li2ZrO3 plays an important role in alleviating structural damage of LiNi1/3Co1/3Mn1/3O2 and protecting LiNi1/3Co1/3Mn1/3O2 from electrolyte during the charge/discharge processes.

Multifunctional Li2O-2B2O3 coating for enhancing high voltage electrochemical performances and thermal stability of layered structured LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries

The surface modification with Li2O-2B2O3 glass is introduced into LiNi0.5Co0.2Mn0.3O2 cathode materials in a bid to improve the high voltage electrochemical and thermal properties for the first time. X-ray diffraction, scanning electron microscopy, transmission electron microscope and X-ray photoelectron spectroscopy results show that the Li2O-2B2O3 glass is successfully coated on the surface of LiNi0.5Co0.2Mn0.3O2. It has been found that the electrochemical performances and thermal stability of LiNi0.5Co0.2Mn0.3O2 are obviously improved after Li2O-2B2O3 glass modification. Notably, the 2 wt.% Li2O-2B2O3 coated sample provides a high capacity retention of 90.1% after 100 repeated cycles, compared with 72.3% for the bare one. The outstanding high voltage electrochemical performance and thermal stability of the coated materials are related to the fact that the multifunctional Li2O-2B2O3 coating layer, which can not only isolate the bulk LiNi0.5Co0.2Mn0.3O2 from direct contact with the electrolyte, but also facilitate the diffusion of lithium ions as well as diminish charge transfer impedance of the cell.

Improved high voltage electrochemical performance of Li2ZrO3-coated LiNi0.5Co0.2Mn0.3O2 cathode material

The use of Li2ZrO3 as an effective coating to enhance the cycling performance and rate capability of LiNi0.5Co0.2Mn0.3O2 cathode material under high cut-off voltage of 4.6 V has been demonstrated. The structural and morphological characteristics of the bare and coated samples were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Electrochemical studies on bare and Li2ZrO3-coated LiNi0.5Co0.2Mn0.3O2 samples were conducted using galvanostatic charge–discharge, cyclic voltammetry and electrochemical impedance spectroscopy by constructing a lithium half-cell. It has been found that the high cut-off voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cathode material are obviously improved by Li2ZrO3 surface modification. Notably, the 1 wt.% Li2ZrO3-coated sample exhibits a capacity retention of 87.1% at 1C after 100 cycles, while that of bare one is 72.8%. The cyclic voltammograms and impedance spectra results shown that the Li2ZrO3 coating reduced the polarization and improved the electrochemical activity of cathode. DSC tests reveal that the Li2ZrO3 coating layer also helps in enhancing the thermal stability of LiNi0.5Co0.2Mn0.3O2 cathode. Thus the Li2ZrO3-coated LiNi0.5Co0.2Mn0.3O2 indicates potential lithium ion batteries for high power applications even at high operating voltage.

Improvement of high voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials via Li2ZrO3 coating

LiNi0.5Co0.2Mn0.3O2 is successfully coated with Li2ZrO3 through a simple sol–gel method. The formation of a uniform Li2ZrO3 coating layer on the surface of the bare sample is confirmed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectrometer (EDS). Similar discharge capacities are delivered by both bare and Li2ZrO3-coated samples. However, the capacity retention of Li2ZrO3-coated sample is much higher than that of the bare one at 1C after 100 charge–discharge cycles under a high cutoff voltage of 4.6V. Meanwhile, the rate performances of Li2ZrO3-coated sample are dramatically enhanced especially at high rates. Furthermore, the increase of impedance during charge–discharge process is suppressed by Li2ZrO3 coating layer.

Synthesis of Li2MnO3-stabilized LiCoO2 cathode material by spray-drying method and its high-voltage performance

xLi2MnO3⋅(1−x)LiCoO2 (x=0, 0.02, 0.05, 0.1) as a cathode material for lithium ion batteries has been prepared by a spray-drying assisted solid-state method. The effects of Li2MnO3 content on crystal structure, morphology, and high-voltage electrochemical performance of LiCoO2 have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and galvanostatic charge–discharge test. XRD results reveal that all samples have a well-ordered layered structure. SEM and EDS analyses confirm that homogeneous powders with a primary particle size of about 2μm are observed and the elementals distribute uniformly in the particles. Electrochemical tests demonstrate that the modified samples exhibit obviously enhanced cycling stability in the voltage ranges of 3.0–4.5V and 3.0–4.6V, although they deliver somewhat lower discharge capacity. Specifically, 0.02Li2MnO3⋅0.98LiCoO2 delivers the initial discharge capacity of 189.0, 216.8mAhg−1 at 0.1C in the voltage range of 3.0–4.5V and 3.0–4.6V, respectively, and excellent cycling behaviors at 1C are achieved.

Enhanced high-voltage electrochemical performance of LiCoO2 coated with ZrOxFy

LiCoO2 is successfully coated with oxyfluoride ZrOxFy by a chemical deposition method for the first time. The formation of a uniform coating layer on the surface of the cathode particles is confirmed by SEM, TEM, EDX and XPS. The coated sample maintains the pristine layered structure according to the XRD result. Compared with pristine LiCoO2, the ZrOxFy-coated sample exhibits enhanced electrochemical performance especially at high cutoff voltage. Similar discharge capacities are delivered by both pristine and coated samples in various voltage ranges. However, the coated sample shows improved cycling performance, maintaining 56.4%, 32.8% capacity retention after 200 cycles, which is much more than 30.3%, 10.5% that is shown by the pristine sample in the voltage range of 3.0–4.5V and 3.0–4.6V, respectively.