Layered Li-rich Cathode Research Articles

Enabling High-Performance Layered Li-Rich Oxide Cathodes by Regulating the Formation of Integrated Cation-Disordered Domains.

Layered Li-rich oxide cathode materials are capable of offering high energy density due to their cumulative cationic and anionic redox mechanism during (de)lithiation process. However, the structural instability of the layered Li-rich oxide cathode materials, especially in the deeply delitiated state, results in severe capacity and voltage degradation. Considering the minimal isotropic structural evolution of disordered rock salt oxide cathode during cycling, cation-disordered nano-domains have been controllably introduced into layered Li-rich oxides by co-doping of d0-TM and alkali ions. Combining electrochemical and synchrotron-based advanced characterizations, the incorporation of the phase-compatible cation-disordered domains can not only hinder the oxygen framework collapse along the c axis of layered Li-rich cathode under high operation voltage but also promote the Mn and anionic activities as well as Li+ (de)intercalation kinetics, leading to remarkable improvement in rate capability and mitigation of capacity and voltage decay. With this unique layered/rocksalt intergrown structure, the intergrown cathode yields an ultrahigh capacity of 288.4mAhg-1 at 0.1C, and outstanding capacity retention of ≈90.0% with obviously suppressed voltage decay after 100 cycles at 0.5, 1, and 2C rate. This work provides a new direction toward advanced cathode materials for next-generation Li-ion batteries.

Regulating the Potential of Anion Redox to Reduce the Voltage Hysteresis of Li-Rich Cathode Materials.

Layered Li-rich oxides (LROs) that exhibit anionic and cationic redox are extensively studied due to their high energy storage capacities. However, voltage hysteresis, which reduces the energy conversion efficiency of the battery, is a critical limitation in the commercial application of LROs. Herein, using two Li2RuO3 (LRO) model materials with C2/c and P21/m symmetries, we explored the relationship between voltage hysteresis and the electronic structure of Li2RuO3 by neutron diffraction, in situ X-ray powder diffraction, X-ray absorption spectroscopy, macro magnetic study, and electron paramagnetic resonance (EPR) spectroscopy. The charge-transfer band gap of the LRO cathode material with isolated eg electron filling decreases, reducing the oxidation potential of anion redox and thus displaying a reduced voltage hysteresis. We further synthesized Mn-based Li-rich cathode materials with practical significance and different electron spin states. Low-spin Li1.15Ni0.377Mn0.473O2 with isolated eg electron filling exhibited a reduced voltage hysteresis and high energy conversion efficiency. We rationalized this finding via density functional theory calculations. This discovery should provide critical guidance in designing and preparing high-energy layered Li-rich cathode materials for use in next-generation high-energy-density Li-ion batteries based on anion redox activity.

Entropy Stabilization Strategy for Enhancing the Local Structural Adaptability of Li-Rich Cathode Materials.

Layered Li-rich cathode materials with high reversible energy densities are becoming prevalent. However, owing to the activation of low-potential redox couples and the progressively irreversible structural transformation caused by the local adjustment of transition-metal ions in the intra/interlayer driven by anionic redox, continuous capacity degradation, and voltage decay emerge, thus greatly reducing the energy density and increasing the difficulty of battery system management. Herein, layered Li-rich cathode materials with higher intralayer configuration entropy have more local structural diversity and higher distortion energy, resulting in superior local structural adaptability with no drastic redox couple evolution, major local structural adjustment, or obvious layered-to-spinel phase transition. Consequently, the energy retention of the entropy-stabilization-strategy-enhanced Li-rich cathode materials is almost twice that of a typical Li-rich cathode material (Li1.20 Mn0.54 Ni0.13 Co0.13 O2 , T-LRM) after 3 months of cyclic testing. Moreover, when cycled at 1C, the voltage degradation per cycle is less than 0.02%, that is, it results in a voltage loss of only 0.8mV per cycle, which is excellent performance. This study paves the way for the development of Li-rich cathode materials with stabilized intralayer atomic arrangements and high local structural adaptability.

Modulating anionic activities in layered Li-rich cathode materials with inverse spinel MnFe2O4 coating

Layered Li-rich cathode oxides can provide high specific capacity due to oxygen anion redox involving in charge compensation process during cycling, but there is a severe structural transition from layered to spinel accompanying with irreversible oxygen loss during cycling, which lead to electrochemical degradation. The current researches show that the irreversible oxygen evolution reaction of layered lithiumrich materials in the first cycle mainly comes from the surface lattice oxygen, so the surface modification by the materials with more stable structure is one of the effective ways to improve the electrochemical performance of layered lithium-rich materials. In this paper, we report a modified layered lithium-rich cathode material by surface coating of inverse spinel MnFe2O4. The inverse spinel has strong polarization effect on anion migration due to its different atoms occupying octahedral sites from layered structure and it can also modulate the Fermi level and stretching the O-O bond, thereby increasing the energy barrier for surface oxygen oxidization. Furthermore, the three-dimensional connected tunnel structure of the inverse spinel also makes the surface layer of the material have a faster lithium ion transferring rate, and a large number of lithium storable vacancies inside of it improved the Li+ intercalation efficiency, initial coulombic efficiency and rate performance. Compared with the unmodified sample, the modified material coated with 2% MnFe2O4 has superior cycle stability and outstanding rate performance. It is hoped our work would provide the knowledge for the future development of high-performance cathode materials for Li-ion batteries.

Surface modification with oxygen vacancy in Li-rich layered oxide Li1.2Mn0.54Ni0.13Co0.13O2 for lithium-ion batteries

A couple of layered Li-rich cathode materials Li1.2Mn0.54Ni0.13Co0.13O2 without any carbon modification are successfully synthesized by solvothermal and hydrothermal methods followed by a calcination process. The sample synthesized by the solvothermal method (S-NCM) possesses more homogenous microstructure, lower cation mixing degree and more oxygen vacancies on the surface, compared to the sample prepared by the hydrothermal method (H-NCM). The S-NCM sample exhibits much better cycling performance, higher discharge capacity and more excellent rate performance than H-NCM. At 0.2 C rate, the S-NCM sample delivers a much higher initial discharge capacity of 292.3 mAh g−1 and the capacity maintains 235 mAh g−1 after 150 cycles (80.4% retention), whereas the corresponding capacity values are only 269.2 and 108.5 mAh g−1 (40.3% retention) for the H-NCM sample. The S-NCM sample also shows the higher rate performance with discharge capacity of 118.3 mAh g−1 even at a high rate of 10 C, superior to that (46.5 mAh g−1) of the H-NCM sample. The superior electrochemical performance of the S-NCM sample can be ascribed to its well-ordered structure, much larger specific surface area and much more oxygen vacancies located on the surface.

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.

Synthesis and Performance of Fusiform and Sphere Hierarchical Layered Li-Rich Materials Li1.2Mn0.54Ni0.13Co0.13O2 for Lithium-Ion Batteries

Layered Li-rich cathode materials have a series of issues such as sharp voltage fading and bad cycling stability. To cure the problem, we developed a urea-based hydrothermal method followed a heat treatment to synthesize microscale fusiform and sphere hierarchical Li1.2Mn0.54Ni0.13Co0.13O2 assembled with nanoparticle. The hierarchical fusiform Li1.2Mn0.54Ni0.13Co0.13O2 present high capacity of 259mAhg−1 at 0.1C and 270mAhg−1 at 0.05C and capacity retention of fusiform sample is up to 96% after 30 cycles at 0.1C, showing an excellent cycle performance.

Li1.1Na0.1Mn0.534Ni0.133Co0.133O2 as cathode with ameliorated electrochemical performance based on dual Li+/Na+ electrolyte

Layered Li-rich cathode materials Li1.2Mn0.534Ni0.133Co0.133O2 (LNCMN-0) and Na doping Li1.1Na0.1Mn0.534Ni0.133Co0.133O2 (LNCMN-0.1) are prepared successfully by a co-precipitation method and several consecutive calcination treatments. Besides, the phase structure, morphology, and electrochemical properties of the four samples are studied in detail using X-ray diffraction (XRD), scanning electron microscope (SEM), galvanostatic charge-discharge test, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). Although the discharge capacity of spherical LNCMN-0.1 decreases slightly at 0.1 C (1 C = 250 mA g−1), compared to the pristine LNCMN-0, it is noteworthy that the LNCMN-0.1 matched with dual Li+/Na+ electrolyte exhibit superior stability performance at 1 C, as well as enhanced rate capability. The LNCMN-0.1 (Li+/Na+) delivers an initial discharge specific capacity of 267.61 mAh g−1 at 0.1 C between 2.0 and 4.8 V at room temperature and initial coulombic efficiency of 83.51%, which is higher than the LNCMN-0 samples (76.42 and 81.54%). The experimental results verify that Na doping combined with dual Li+/Na+ electrolyte can generate a synergistic effect, which is a promising idea to ameliorate the electrochemical performance for this material.

Sol–gel preparation of Li-rich layered cathode material for lithium ion battery with polymer polyacrylic acid + citric acid chelators

The sol–gel method was improved to synthesize the layered Li-rich cathode material Li1.2Ni0.13Co0.13Mn0.54O2 by employing the mixture of polymer polyacrylic acid (PAA) and citric acid as the mixed chelator. The crystal structure, particle morphology and electrochemical performance of the prepared layered Li-rich cathode material are extensively studied by X-ray diffraction spectrometry, scanning electron microscopy, charge–discharge test, cyclic voltammetry and electrochemical impedance spectroscopy techniques. The results show that the Li-rich cathode materials prepared with the new method present explicit layer structure, low level of Li+/Ni2+ disordering, large specific surface area and accurate chemical composition. When measured with electrochemical test system, the prepared material exhibits a high capacity of 274.8 mAh g−1. The excellent performances of the prepared material could be ascribed to the fact that the novel approach combines the advantages of the two chelating agents of PAA and citric acid.

Understanding the effect of an in situ generated and integrated spinel phase on a layered Li-rich cathode material using a non-stoichiometric strategy.

Recently, spinel-layered integrated Li-rich cathode materials have attracted great interest due to the large enhancement of their electrochemical performances. However, the modification mechanism and the effect of the integrated spinel phase on Li-rich layered cathode materials are still not very clear. Herein, we have successfully synthesized the spinel-layered integrated Li-rich cathode material using a facile non-stoichiometric strategy (NS-LNCMO). The rate capability (84 mA h g-1vs. 28 mA h g-1, 10 C), cycling stability (92.4% vs. 80.5%, 0.2 C), low temperature electrochemical capability (96.5 mA h g-1vs. 59 mA h g-1, -20 °C), initial coulomb efficiency (92% vs. 79%) and voltage fading (2.77 V vs. 3.02 V, 200 cycles@1 C) of spinel-layered integrated Li-rich cathode materials have been significantly improved compared with a pure Li-rich phase cathode. Some new insights into the effect of the integrated spinel phase on a layered Li-rich cathode have been proposed through a comparison of the structure evolution of the integrated and Li-rich only materials before and after cycling. The Li-ion diffusion coefficient of NS-LNCMO has been enlarged by about 3 times and almost does not change even after 100 cycles indicating an enhanced structure stability. The integration of the spinel phase not only enhances the structure stability of the layered Li-rich phase during charging-discharging but also expands the interslab spacing of the Li-ion diffusion layer, and elongates TM-O covalent bond lengths, which lowers the activation barrier of Li+-transportation, and alleviates the structure strain during the cycling procedure.

Synthesis of layered xLi2MnO3·(1 − x)LiMnO2 nanoplates and its electrochemical performance as Li-rich cathode materials for Li-ion battery

Layered xLi2MnO3·(1−x)LiMnO2 (x=0.57, 0.48, and 0.44) nanoplates are firstly prepared by pyrolysis reducing the electrochemically inactive monoclinic Li2MnO3 nanoplates, which is synthesized via a solid-state reaction by using home-made MnO2 nanoplates as self-template. The obtained xLi2MnO3·(1−x)LiMnO2 nanoplates have a diameter of ∼200nm and thickness of ∼60nm with high crystallinity and the transition metal layers parallel to the plate’s radial direction. Although these xLi2MnO3·(1−x)LiMnO2 nanoplates cause a longer Li+ diffusion distance, and then a lower reversible capacity as compared to the nanoparticle-like counterpart, the nanoplate-like morphology of these xLi2MnO3·(1−x)LiMnO2 (especially with x<0.5) are beneficial for retarding the layer-to-spinel phase transformation during the charge/discharge processes, and resulting in significant improvement of the cyclic stability. Moreover, the reduced reversible capacity caused by the longer Li+ diffusion distance of the nanoplate-like morphology can be offset by decreasing the x value, and the xLi2MnO3·(1−x)LiMnO2 nanoplates with x=0.44 performs a maximum reversible capacity as high as 270mAhg−1 with a well cyclic performance. The present findings indicate that the cyclic performance and reversible capacity of xLi2MnO3·(1−x)LiMnO2 can be improved by tailoring the particle’s morphology and composition, and demonstrate a simple and effective strategy for the development of the Co/Ni-free Mn-based layered Li-rich cathode materials with good cyclic and high reversible capacity.

Improved Rate Capability of Layered Li-Rich Cathode for Lithium Ion Battery by Electrochemical Treatment

Markedly enhanced rate performance of Li-rich (Li1.2Ni0.4/3Co0.4/3Mn1.6/3O2) cathode material is observed after a simple electrochemical pretreatment. The cathode material can deliver discharge capacities of 196, 124 and 86 mAh g−1, under current densities of 2, 20 and 40 C, respectively. The capacity retention reaches 60% after 1000 cycles even at 20 C. Analysis of dQ/dV plot and impedance spectroscopy prove that more migration of Ni3+ to Li+ vacancies as well as the higher charge-transfer resistance by a partially insulating Li2O layer, which are created when a slower first charge/discharge rate is applied, result in the capacity loss upon prolonged cycling. © 2013 The Electrochemical Society. [DOI: 10.1149/2.006308eel] All rights reserved.