Li-rich Layered Oxide Cathode Research Articles

Defects in Li-rich manganese-based layered oxide: A first-principles study

Lithium-rich manganese-based layered oxides are of great interest as cathode materials for lithium ion batteries due to their high energy density. The voltage decay and capacity fading during prolonged charge/discharge cycling are the key obstacles for their practical usage. In this work, using density functional theory, we investigated the origin of the Ni surface segregation by calculating the defect formation energies of antisite defects, including Ni cation substituting a Li cation [Formula: see text] and pairs of Ni cation swapping with Li cation ([Formula: see text]–[Formula: see text]) in [Formula: see text], [Formula: see text] and [Formula: see text] to represent the Mn-rich, Mn–Ni equivalence and Ni-rich conditions, respectively. Results show that [Formula: see text]–[Formula: see text] defect is of energy favorable in Li-rich and Mn-rich layered oxide cathode. Al-doping is beneficial for improving the structure stability and preventing the surface segregation, thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes. This work provides deep insights for the design of layered cathode materials with both high capacity and voltage stability during cycling.

Synthesis, microstructure, and electrochemical performance of Li-rich layered oxide cathode materials for Li-ion batteries

Li-rich layered oxides Li1.2Mn0.54Ni0.13Co0.13O2 were synthesized by modified Pechini method using various compositions of the reaction mixture. Difference in the electrochemical performance of cathodes on their basis is explained by different morphology and microstructure of the materials. The porous hierarchical structure favors a better electrochemical performance. The presence of defects, including crystal twins, in the samples is considered to be a major reason that leads to their poor cyclability and rate capability.

Stable heteroepitaxial interface of Li-rich layered oxide cathodes with enhanced lithium storage

Lithium and oxygen activities can have substantial influences on the kinetics of ion and electron transport and the structural integrity of Li-rich layered oxide (LLO) cathodes, since reversible oxygen redox is ascribed to the extra capacity beyond the theoretical capacity from transition metal redox at high voltages. Herein, we demonstrate a liquid-solid interfacial reaction to generate a heteroepitaxial interface with tunable Li/O activities on LLOs using molten boric acid. The experimental and theoretical analyses indicate that the atomic scale interface is comprised of a disordered rock salt structure containing substantial Li/O vacancies along the layered structure, associated with a segregation tendency of Ni and Co. The formation of this heteroepitaxial interface with Li/O vacancies improves the ionic/electronic conduction and electrochemical/structural stability, leading to a high discharge capacity of 283 mA h g-1 with initial Coulombic efficiency of 91.7% (0.1 C, 2.0–4.7 V vs. Li+/Li), excellent rate performance (246 and 159.7 mAh g-1 at 1 C and 10 C, respectively) and enhanced cyclic performance with a capacity retention of 92% after 100 cycles. The findings highlight the importance of a well-engineered interface for the design of high performance layered cathode materials for Li storage.

Enhanced electrochemical performance of layered Li-rich cathode materials for lithium ion batteries via aluminum and boron dual-doping

Lithium-rich layer oxides can possess satisfactory specific capacity but suffer from severe voltage attenuation and poor cycle stability. In this work, Al-B dual-doping technique is introduced to modify Li-rich layered oxide cathode materials. Cross-section scanning electron microscopy, Energy Disperse Spectroscopy and X-ray photoelectron spectroscopy results confirm that Al and B successfully doped into the interior of the bulk Li1.2Ni0.2MnO2 particles, and the High-resolution transmission electron microscopy and X-ray diffraction Rietveld refinement results reveal that the c-axis distance of LMR-AB increases. The Al-B co-doped sample shows greatly enhanced electrochemical performance. Specifically, it exhibits of a discharge capacity of 120 mAh g−1 at 5 C and a capacity retention of 89.12% after 100 cycles at 1 C. The voltage decay is also greatly alleviated. The enhanced electrochemical performance of LMR-AB is due to the synergistic effects bought by the Al-B dual-doping, where increase of c-axis distance decreases Li+ intercalation/deintercalation barrier. B3+ doping into the tetrahedral site block the migration of TM ions and Al3+ act as pillars in the octahedral site, stabilizing the structure and suppressing the phase transition during cycling.

Synchronous phase transition and carbon coating on the surface of Li-rich layered oxide cathode materials for rechargeable Li-ion batteries

We describe a novel synchronous surface tailoring to form thin and uniform carbon coating layers and simultaneously induce an in-situ phase transition from a layered to spinel-like phase on the surfaces of Li-rich layered oxide (LLOs) particles by the thermal decomposition of 2,3-dihydroxynaphthalene (DN). The Li2MnO3 component in pristine LLO is not affected during carbon coating, but the coordination numbers of the Co ions and the CoO bonds in the LiTMO2 phase are diminished during carbon coating, implying that the formation of a spinel-like phase in LLO after carbon coating begins at the LiTMO2 with little variation in the Li2MnO3. As a result, the optimized DN (0.2 wt%)-coated LLO cathode material shows superb electrochemical properties owing to the formation of uniform carbon coating layers and the layered-to spinel-phase transition on the surfaces of the LLO particles, which improves the surface stability, electronic conductivity, and Li-ion kinetics in the LLO.

A multifunctional silicotungstic acid-modified Li-rich manganese-based cathode material with excellent electrochemical properties

Li-rich layered oxide (LrLO) cathode has attracted much attention for Li-ion batteries in recent years due to its superior capacity of exceeding 250 mA h g−1. However, these materials still have some inherent drawbacks such as poor rate stability and cycle performance. In this paper, Li-rich cathode material Li1.2Mn0.54Ni0.13Co0.13O2 was modified by silicotungstic acid (HSW) with high electronic and ionic conductivity via a facile approach. The material was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and electrochemical tests. The results showed that the thickness of HSW coating was about 5 nm. HSW coating could supply a transfer pathway for Li ions and electrons, resulting in the superior discharge capacity and rate capability. The HSW-LrLO could deliver 158.38 mA h g−1 even at high current density of 500 mA g−1, which was 26.9% higher than that of pristine LrLO. In addition, HSW-LrLO exhibited excellent cycling performance with the capacity retention over 90% at 1 and 5 C. These results were useful to develop effective surface modification for LrLO materials.

Template-determined microstructure and electrochemical performances of Li-rich layered metal oxide cathode

This report unravels the dependence of the microstructure and electrochemical performances of Li-rich layered transition metal oxide (LLMO) on the molecular structure of polymer templates for the formation of oxalate precursor. A representative LLMO, Li1.2Mn0.54Ni0.13Co0.13, is synthesized by co-precipitation method, and three samples, LLMO-PVP, LLMO-PEG and LLMO-PVA, are obtained with polymer templates, polyvinyl pyrrolidone (PVP), polyethyleneglycol (PEG) and polyvinyl alcohol (PVA), respectively. The physical and chemical properties of the resulting products are analyzed with SEM, TEM, XRD, BET, ICP, and XPS, and their electrochemical performances as cathode of Li-ion battery are evaluated with EIS, GITT and charge/discharge tests. It is found that LLMO-PVP exhibits the best performances, followed by LLMO-PEG, while LLMO-PVA behaves poorest. This difference is ascribed to the various microstructures of the resulting products, which are determined by the molecular structure of polymer templates.

Improving the rate capability and decelerating the voltage decay of Li-rich layered oxide cathodes by constructing a surface-modified microrod structure

In this work, Li-rich layered oxide materials of Li1.2Mn0.54Co0.13Ni0.13O2 nano/micro hierarchical microrods are easily synthesized using a solvothermal method. Li2MnO3, a Li-rich layered oxide with a framework that is consistent with that of the host, is coated onto the surface of Li1.2Mn0.54Co0.13Ni0.13O2 microrods by a facile sol–gel method. Investigations with X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM) and galvanostatic charge/discharge reactions reveal that surface-modified Li1.2Mn0.54Co0.13Ni0.13O2 microrods present better rate performance and cycling stability than other samples. The best electrochemical performance of the surface-modified Li1.2Mn0.54Co0.13Ni0.13O2 microrods is obtained with a layer composed of 5 wt% Li2MnO3 (∼10 nm thick), which provides very high discharge capacities of 244 and 159.5 mAh g−1 at C-rates of 1C and 10C, respectively (1C = 200 mAg−1), and excellent cycling stability (after 80 cycles, capacity retention of 96.0% at 1C). The composite's remarkably improved cyclability and rate capability are attributed to the combination of the nano/micro hierarchical microrod structure and the Li2MnO3-modified surface, which reduce structural degradation during electrochemical cycling. Results also show that the Li2MnO3 surface-modified Li1.2Mn0.54Co0.13Ni0.13O2 microrods significantly decelerate voltage decay and improve the initial coulombic efficiency (76.4%).

Improving cycling stability and suppressing voltage fade of layered lithium-rich cathode materials via SiO2 shell coating

Li-rich layered metal oxide (Li1.2Mn0.56Ni0.16Co0.08O2, denoted as LLMO) cathode materials with SiO2 coating have been successfully synthesized by using a wet chemical method combined with high temperature annealing. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses revealed that an amorphous SiO2 coating layer has formed on the surface of the LLMO electrode. Electrochemical analysis demonstrated that an optimal SiO2 coating amount on the surface of LLMO particles can effectively improve the cycling stability, suppress the voltage and capacity decay, and enhance the thermal stability of these electrodes. An optimal SiO2 coating amount of 0.5 wt% on an LLMO electrode (S-LLMO-0.5 wt%) improved the capacity retention to 82.6% after 200 cycles at 25 °C (67.0% for the pristine LLMO electrode) and 68.8% after 100 cycles at 60 °C (56.3% for the pristine LLMO electrode), respectively. Besides, the voltage drop of the S-LLMO-0.5 wt% sample (46.8 mV/53.6 mV) was lower than that of the pristine LLMO sample (366.1 mV/60.3 mV) after 200 cycles at 3.2 V/3.8 V, confirming that 0.5 wt% SiO2 coating can reduce the capacity loss and slower the voltage degradation rate of LLMO electrodes. In addition, the exothermic reaction of the pristine LLMO (denoted as P-LLMO from now on) electrode can be delayed by SiO2 coating while the decomposition temperature of the S-LLMO-0.5 wt% sample is significantly higher than that of the P-LLMO. These results are attributed to a uniform amorphous SiO2 coating layer that not only blocks the highly active oxygen release from the bulk material but also prevents the direct contact of the electrolyte to the positive electrodes, suppressing structural transition from a layered phase to a spinel-like one.

Selective doping of Li-rich layered oxide cathode materials for high-stability rechargeable Li-ion batteries

We report the discovery of Li-rich Li1+x[(Ni0.225Co0.15Mn0.625)1−yVy]O2 as a cathode material for rechargeable lithium-ion batteries in which a small amount of tetravalent vanadium (V4+) is selectively and completely incorporated into the manganese sites in the lattice structure. The unwanted oxidation of vanadium to form a V2O5-like secondary phase during high-temperature crystallization is prevented by uniformly dispersing the vanadium ions in coprecipitated [(Ni0.225Co0.15Mn0.625)1−yVy](OH)2 particles. Upon doping with V4+ ions, the initial discharge capacity (>275mAhg−1), capacity retention, and voltage decay characteristics of the Li-rich layered oxides are improved significantly in comparison with those of the conventional undoped counterpart.

Elucidation of key factors of water-resistance of Li-rich solid-solution layered oxide cathode materials applicable to a water-based cathode preparation process for Li-ion battery

In order to show the best composition which gives water-resistance to Li-rich solid-solution layered oxide cathode materials for a water-based cathode preparation process, the water resistance of LLOs composed of xLi2MnO3―yLiNi1/2Mn1/2O2—(1-x-y)LiNi1/3Co1/3Mn1/3O2 was examined by comparing the charge/discharge cycle results obtained with the cathodes prepared with organic solvent-based and water-based binders. The composition area where the percentage content of Li2MnO3 is 70–75% was found to show both high charge/discharge capability and water resistance. In addition, by analyzing why this composition of Li2MnO3 exhibits high water resistance, it has become apparent that the oxidation states of Mn ions and existence of Co ions on the surface of LLOs are the key factors in inhibiting the dissolution of Mn and Ni ions from the LLO surfaces, and finally leading to their high water resistance.

Effect of the Cooling Process on the Structure and Charge/Discharge Cycling Performance in Li[Li0.20Mn0.58Ni0.18Co0.04]O2 Li-Rich Solid-Solution Layered Oxide Cathode Materials for Li-Ion Battery

In this study, the cooling conditions for calcinated lithium-rich solid-solution layered oxide materials (LLOs) of Li[Li0.20Mn0.58Ni0.18Co0.04]O2 were examined to determine the cooling conditions and crystal structure that produce a high and stable charging/discharging cathode capacity for LLOs. The crystal structures of LLO samples which were quenched in liquid nitrogen, cooled in air at room-temperature and at a controlled cooling rate of 100°C h-1 in a furnace were evaluated via powder X-ray diffractometry (pXRD) experiments and transmission electron microscopy (TEM). The evaluated crystal structures of the LLOs were related to the charging/discharging cycle cathode performance.

Improving rate capability and decelerating voltage decay of Li-rich layered oxide cathodes by chromium doping

In order to improve the rate capability and mitigate the voltage decay of lithium rich layered oxides, chromium doped cathode materials Li1.2[Mn0.54Ni0.13Co0.13]1−xCrxO2 (x = 0,0.003,0.005,0.007) is synthesized via co-precipitation method followed by calcination. As a result, Cr-doped samples present shortened voltage platform at 4.5 V, indicating that the escape of lattice oxygen is suppressed by Cr doping. Among all samples, Li1.2[Mn0.54Ni0.13Co0.13]1−xCrxO2 (x = 0.005) sample shows the greatest rate capability (119.3 mAh g−1 at 10 C) and the minimum voltage drop (0.6167 V) after 200 cycles at 1.0 C. The reasons for the improved electrochemical performance are the enhanced structural stability, the stronger CrO bond than MnO band and the decreased metal–oxygen covalency induced by Cr doping.

Core-Shell Nanocomposites for Improving the Structural Stability of Li-Rich Layered Oxide Cathode Materials for Li-Ion Batteries.

The structural stability of Li-rich layered oxide cathode materials is the ultimate frontier to allow the full development of these family of electrode materials. Here, first-principles calculations coupled with cluster expansion are presented to investigate the electrochemical activity of phase-separation, core-shell-structured xLi2MnO3·(1 - x)LiNiCoMnO2 nanocomposites. The detrimental surface effects of the core region can be countered by the Li2MnO3 shell, which stabilizes the nanocomposites. The operational voltage windows are accurately determined to avoid the electrochemical activation of the shell and the subsequent structural evolution. In particular, the dependence of the activation voltage with the shell thickness shows that relatively high voltages can still be obtained to meet the energy density needs of Li-ion battery applications. Finally, activation energies of Li migration at the core-shell interface must also be analyzed carefully to avoid the outbreak of a phase transformation, thus making the nanocomposites suitable from a structural viewpoint.

Structure and Voltage Recovery Driven by Defects Elimination in Li-Rich Layered Oxide Cathode Materials

Lithium-ion batteries (LIBs) are ubiquitous in a wide range of technologies, including cell phones, electric vehicles, and energy grid, and are indispensable in the transition toward sustainable energy technology. The battery performance is inherently connected to the capability of lithium ions to reversibly intercalate with the electrodes. The capacity of classical layered transition metal (TM) oxides, the primary commercial cathode materials, is limited to cationic redox activity. Anionic redox has thus emerged as a new paradigm for designing novel cathodes for next generation LIBs. It is recently confirmed oxygen redox in lithium-rich layered oxides (LRLO) with composition xLi2MnO3∙(1-x)LiTMO2, which enables this group of materials exhibit reversible capacities exceeding 280 mAh g-1. Unfortunately, this material undergoes voltage fading and instability that is rooted in irreversible structural transformation in terms of defects formation including lithium and oxygen vacancies, lithium tetrahedron, TM migration, edge dislocations, stacking fault, and local strain. Modification methods including morphology control, composition gradient, surface treatment, foreign elements doping, etc., have been proposed in the literatures to mitigate the defects formation and maintain the structure reversibility. Despite their improvement, the intrinsic voltage fading is still inevitable and remains a challenge that will require considerable effort to overcome. In this work, we demonstrate a path to recover the layer structure and working voltage through high-temperature annealing. The treatment at high temperature recovers the local Li-excess environments around oxygen, oxygen stacking sequence, and eliminates microstrain associated with different defects. As a result, the voltage profile once more shows plateau region during charge and the average discharge voltage is restored of the cycled cathode after heat treatment. Both theoretical calculations and experimental characterizations illustrate the mechanisms underlying the heating induced recovery. The structure metastability and defects removal are decisive in structure and voltage recovery of LRLO cathode material.

Tuning Electrochemical Properties of Li-Rich Layered Oxide Cathodes by Adjusting Co/Ni Ratios and Mechanism Investigation Using in situ X-ray Diffraction and Online Continuous Flow Differential Electrochemical Mass Spectrometry.

Owing to high specific capacity of ∼250 mA h g-1, lithium-rich layered oxide cathode materials (Li1+ xNi yCo zMn(3- x-2 y-3 z)/4O2) have been considered as one of the most promising candidates for the next-generation cathode materials of lithium ion batteries. However, the commercialization of this kind of cathode materials seriously restricted by voltage decay upon cycling though Li-rich materials with high cobalt content have been widely studied and show good capacity. This research successfully suppresses voltage decay upon cycling while maintaining high specific capacity with low Co/Ni ratio in Li-rich cathode materials. Online continuous flow differential electrochemical mass spectrometry (OEMS) and in situ X-ray diffraction (XRD) techniques have been applied to investigate the structure transformation of Li-rich layered oxide materials during charge-discharge process. The results of OEMS revealed that low Co/Ni ratio lithium-rich layered oxide cathode materials released no lattice oxygen at the first charge process, which will lead to the suppression of the voltage decay upon cycling. The in situ XRD results displayed the structure transition of lithium-rich layered oxide cathode materials during the charge-discharge process. The Li1.13Ni0.275Mn0.580O2 cathode material exhibited a high initial medium discharge voltage of 3.710 and a 3.586 V medium discharge voltage with the lower voltage decay of 0.124 V after 100 cycles.

Optimization of calcination temperature in preparation of a high capacity Li-rich solid-solution Li[Li0.2Ni0.18Co0.03Mn0.58]O2 material and its cathode performance in lithium ion battery

In our previous paper [12], it has been reported that Li[Ni0.208Li0.183Co0.033Mn0.575]O2 Li-rich solid-solution layered oxide (LLO) cathode (Li2MnO3 (55%) - LiNi0.5Mn0.5O2 (35%) - LiNi1/3Co1/3Mn1/3O2 (10%)) possesses the best performance as cathode material among the examined LLO cathode materials of Li2MnO3-LiCo1/3Ni1/3Mn1/3O2-LiNi0.5Mn0.5O2 for lithium ion battery. In order to further improve its cathode performance, the calcination temperature was optimized in the range of 800–1100 °C. The cathode performance such as discharge capacity, discharge capacity retention and rate capability was improved with increasing the calcination temperature from 800 to 1000 °C, the best performance was obtained at 1000 °C and at more than 1000 °C, the cathode performance was degraded. The structural analysis with transmission electron microscope (TEM) and Rietveld analysis of XRD patterns indicated that the cation mixing between Li+ ions on the 3a site and Ni2+ ions on the 3b site of the layered oxide structure (R-3m) is reduced significantly in the case of the LLO calcinated at 1000 °C, resulting in the improvement of cathode performance. In addition, the other LLOs having different compositions were also prepared at different calcination temperatures and their cathode performance was tested. The discharge capacity of Li[Ni0.208Li0.183Co0.033Mn0.575]O2 was found to be the highest among all the LLOs examined in this study.

Novel MOF shell-derived surface modification of Li-rich layered oxide cathode for enhanced lithium storage

Li-rich layered oxide materials have attracted increasing attention because of their high specific capacity (>250 mAh g−1). However, these materials typically suffer from poor cycling stability and low rate performance. Herein, we propose a facile and novel metal-organic-framework (MOF) shell-derived surface modification strategy to construct NiCo nanodots decorated (∼5 nm in diameter) carbon-confined Li1.2Mn0.54Ni0.13Co0.13O2 nanoparticles (LLO@C&NiCo). The MOF shell is firstly formed on the surface of as-prepared Li1.2Mn0.54Ni0.13Co0.13O2 nanoparticles via low-pressure vapor superassembly and then is in situ converted to the NiCo nanodots decorated carbon shell after subsequent controlled pyrolysis. The obtained LLO@C&NiCo cathode exhibits enhanced cycling and rate capability with a capacity retention of 95% after 100 cycles at 0.4 C and a high capacity of 159 mAh g−1 at 5 C, respectively, compared with those of LLO (75% and 105 mAh g−1). The electrochemical impedance spectroscopy and selected area electron diffraction analyses after cycling demonstrate that the thin C&NiCo shell can endow LLO with high electronic conductivity and structural stability, indicating the undesired formation of the spinel phase initiated from the particle surface is efficiently suppressed. Therefore, this presented strategy may open a new avenue on the design of high-performance electrode materials for energy storage.

Improving the electrochemical performances of Li-rich Li1.20Ni0.13Co0.13Mn0.54O2 through a cooperative doping of Na+ and PO43− with Na3PO4

Li-rich layered oxide cathodes suffer from poor rate capability, voltage decay and inferior cycling stability. Herein, we propose a novel synergistic strategy to improve the electrochemical performances of Li-rich Li1.20Ni0.13Co0.13Mn0.54O2 by the co-doping of Na+ and PO43−. The co-doping of Na+ for Li and PO43− for Mn is simultaneously achieved using Na3PO4 as a dopant. The co-doping of Na+ and PO43− not only enhances the high-rate performance (106.4 mAhg−1@10C) and capacity retention (93.8%@1C@100 cycles) but also mitigates the voltage decay owing to the synergistic effect of Na+ and PO43− co-doping. The synergistic mechanism is unraveled based on neutron diffraction, aberration-corrected scanning transmission electron microscope, X-ray photoelectron spectroscopy, ex-situ X-ray absorption spectra, ex-situ X-ray diffraction, electrochemical impedance spectroscopy and electrochemical measurements. The co-doping of Na+ and PO43− enlarges the interlayer spacing and suppresses Li/Ni mixing which increases Li+ diffusivity and enhances the rate capability. Meanwhile, the co-doping of Na+ and PO43− shrinks the thickness of the slabs, weakens the TM-O covalency and alleviates the volume change in the charge/discharge process which improves the layered structure stability and the cycling performances. This study presents some new insights into designing high performance cathode materials through a cooperative modulation of different crystal sites doping.

Stabilizing High-Voltage and High-Capacity Cathode-Electrolyte Interface Using Functional Electrolyte Component

Li- and Mn-rich xLi2MnO3 ∙(1−x)LiMO2 (M = Mn, Ni, Co) layered oxides cathode materials are known to exhibit high specific capacities ≥ 250 mAhg−1 on the high-voltage operation above 4.6 V vs. Li/Li+,1,2 which are considered as promising cathode materials for achieving advanced high-energy density lithium-ion batteries. The capacity of these cathode materials can increase by increasing the charge cut-off voltage. However, the limited anodic instability of the conventional carbonate-based organic electrolytes above 4.2 V vs. Li/Li+ makes the attainment of high capacity from Li-rich layered oxide cathode material difficult. In order to mitigate the electrolyte issue, we have been developing new functional electrolyte components with high anodic stability above 4.6 V to stabilize high-voltage interface between Li-rich layered oxide cathode and electrolyte.1,2 High-voltage electrochemical and cathode-electrolyte interfacial studies and their correlation to high-voltage cycling performance would be presented in the meeting. Acknowledgements This research was supported by Ministry of Trade, Industry & Energy (R0004645) and Creative Human Resource Development Consortium for Fusion Technology of Functional Chemical/ Bio Materials of BK Plus program by Ministry of Education of Korea.