Li-rich Oxide Cathode Materials 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.

Extended Interfacial Stability through Simple Acid Rinsing in a Li-Rich Oxide Cathode Material.

Layered Li-rich Ni, Mn, Co (NMC) oxide cathodes in Li-ion batteries provide high specific capacities (>250 mAh/g) via O-redox at high voltages. However, associated high-voltage interfacial degradation processes require strategies for effective electrode surface passivation. Here, we show that an acidic surface treatment of a Li-rich NMC layered oxide cathode material leads to a substantial suppression of CO2 and O2 evolution, ∼90% and ∼100% respectively, during the first charge up to 4.8 V vs Li+/0. CO2 suppression is related to Li2CO3 removal as well as effective surface passivation against electrolyte degradation. This treatment does not result in any loss of discharge capacity and provides superior long-term cycling and rate performance in comparison to as-received, untreated materials. We also quantify the extent of lattice oxygen participation in charge compensation ("O-redox") during Li+ removal by a novel ex situ acid titration. Our results indicate that the peroxo-like species resulting from O-redox originate on the surface at least 300 mV earlier than the activation plateau region at around 4.5 V. X-ray photoelectron spectra and Mn L-edge X-ray absorption spectra of the cathode powders reveal a Li+ deficiency and a partial reduction of Mn ions on the surface of the acid-treated material. More interestingly, although the irreversible oxygen evolution is greatly suppressed through the surface treatment, O K-edge resonant inelastic X-ray scattering shows that the lattice O-redox behavior is largely sustained. The acidic treatment, therefore, only optimizes the surface of the Li-rich material and almost eliminates the irreversible gas evolution, leading to improved cycling and rate performance. This work therefore presents a simple yet effective approach to passivate cathode surfaces against interfacial instabilities during high-voltage battery operation.

Impact of Nickel Substitution into Model Li-Rich Oxide Cathode Materials for Li-Ion Batteries

Developments in lithium-ion batteries for energy storage are currently focused on improving energy density, increase cycle life, and reducing cost to match targets set by the automotive industry. A...

4-Aminobenzoic acid as a novel electrolyte additive for improved electrochemical performance of Li1.2Ni0.2Mn0.6O2 cathodes via in situ electrochemical polymerization

As Li-rich oxide cathode materials with high capacity suffer from critical-capacity fading during cycling, the proper design of the material interface structure is a very efficient approach for solving such a problem. In this study, 4-aminobenzoic acid is employed as an electrolyte additive to alter the interface properties of Li-rich oxide cathode materials. The cycling stability of Li1.2Ni0.2Mn0.6O2 cathodes is increased enormously by adding 0.25-wt% 4-aminobenzoic acid (4-ABA) in the electrolyte and the capacity retention increases from 61% to 94.4% after 100 cycles. It is demonstrated that 4-ABA is inclined to adsorb onto Li1.2Ni0.2Mn0.6O2 cathode surfaces and then oxidized preferentially at 4.36 V (vs. Li/Li+), leading to the creation of a polymer film on Li1.2Ni0.2Mn0.6O2 surfaces via in situ electrochemical polymerization. The adsorption and polymerization of 4-ABA generates a protective layer at the interface between the cathode material and electrolyte. As a result, electrolyte decomposition is efficiently suppressed, which reduces the thickness of the solid electrolyte interphase film. It is expected that such an approach involving in situ polymerization of an electrolyte additive to form a protective film could be applicable to various electrode systems for lithium ion batteries.

Enhanced electrochemical performances of Li2MnO3 cathode materials via adjusting oxygen vacancies content for lithium-ion batteries

In this study, oxygen vacancies are successfully introduced into Li2MnO3 by low-temperature reduction. Electron paramagnetic resonance is used to detect oxygen vacancy and the results state clearly that oxygen vacancies indeed exist in the reduced samples, and R-LMO-40 shows the highest content. The relationships between oxygen vacancies content and the electrochemical performance of Li2MnO3 as cathodes for lithium ion batteries are also investigated. The oxygen-deficient samples show much better electrochemical performance compared to the pristine Li2MnO3, especially R-LMO-40 with an optimized content of oxygen vacancies. At the rates of 0.5 C, 1 C, 2 C and 5 C, R-LMO-40 delivers discharge capacities of 143.1, 134.0, 126.1 and 90.9 mAh g−1, respectively. After cycling for 50 cycles, there is nearly no capacity decay. These better electrochemical performances are believed to be related to the characteristics of the oxygen vacancies. Electrochemical impedance spectroscopy analysis clearly indicates that oxygen vacancies can suppress the charge transfer resistance and increase lithium ions diffusion capability. These findings reveal that oxygen vacancies can be utilized to enhance the electrochemical performances of Li-rich oxide cathode materials.

Optimizing the Structural Evolution of Li-Rich Oxide Cathode Materials via Microwave-Assisted Pre-Activation

Preactivation can play a promising role in suppressing oxygen loss during the first charging process in Li-rich oxide cathode materials, relieving a series of problems such as large initial irreversible capacity loss, structure transformation due to ion rearrangement, and oxidation/decomposition of the electrolyte. However, the strategies previous adopted are mainly chemical delithiation, which has violent effect on the crystal structure and deteriorates the cycle performance. Here, we report a facile and effective microwave-assisted treatment method to preactivate Li-rich layered oxides without structural distortion. The microwave-treated sample shows a high discharge capacity of about 281 mAh g–1 with a Coulombic efficiency of 87% at 0.1 C and exhibits unnormal continuous increase of discharge capacity from 203 mAh g–1 to 218 mAh g–1 during 110 cycles at 1 C as well as presents distinctly improved cycling stability. EIS, GITT and XRD studies reveal that the kinetics of electrochemical reaction on electr...

Disordered Rocksalt Type Li-Rich Manganese Oxyfluoride As High-Capacity Cathode Material for Rechargeable Lithium Ion Batteries

For the coming full-fledged xEV era, rechargeable lithium ion batteries with higher capacities are strongly desired. However, the current commercial cathode materials, such as layered-rocksalt oxide, are reaching the limit of their theoretical capacities. Li-rich oxide cathode materials, which can access not only to transition metal redox reactions but also to redox reactions during the Li insertion/de-insertion, are expected as candidates for the next generation high capacity cathode materials as they can store much more Li ions than the current commercial cathode materials. However, one of the main drawback of these materials are voltage-fading with structural transition upon electrochemical cycling. It is considered due to oxygen loss at surface and transition metal rearrangement [1][2]. To suppress the oxygen loss without decreasing capacity, we focused on the oxyfluoride cathode material. Especially, we confirmed that disordered Rocksalt type-Li2MnO2F synthesized by mechanochemical process exhibit a discharge capacity of 330mAh/g with both transition metal and oxygen redox reaction, which is much higher than conventional cathode materials (200mAh/g) [3]. Recently, Bruce et al, disclosed that the Li1.9Mn0.95O2.05F0.95 does not exhibit oxygen loss from the lattice. This indicates that the Li-rich oxyfluoride is one of the noteworthy material for the high capacity cathode materials. In this study, we will discuss the possibility of the oxyfluoride materials for the next-generation high capacity cathode material from the view point of structures and compositions by comparing with some other oxide cathode materials without fluorine doping. [1] N. Yabuuchi et al, J. Am. Chem. Soc. 133 (2011) 4404 [2] Y. S. Meng et al, Energy Environ. Sci., 4 (2011) 2223 [3] R. Natsui, K Nakura, POSITIVE ELECTRODE ACTIVE MATERIAL AND BATTERY, WO2017/013848 [4] P. G. Bruce et al, Energy Environ. Sci., (2018) Advanced Article

Layered Li-Rich Oxide Cathode with High Rate Capability By Reductive Treatment

Layered Li-rich oxide cathode materials (denoted as OLO), such as Li1+xMn1-x-y-zNiyCozO2 (x > 0), have attracted recent research interest because these OLO materials offer high capacity performance (> 250 mAh g–1). However, it is still difficult to use these materials in commercial cathodes because their rate capability and capacity decrease with repetitive cycle life as a result of low electrical conductivity and phase transformations. Accordingly, there have been many studies focusing on structure stabilization and enhancement of their electrical conductivity, such as through substitution with Mg, Zn, Mo and Cr ions and surface coating with Al2O3, TiO2 and Li4Ti5O12. We previously reported on the stabilization of these structures and enhancing the electrochemical performance of Li-rich cathode materials via substitution and coating. There have also been recent attempts to find new approaches for surface modification such as through the use of heterostructures. S. Lee et al. reported spinel cathode material coating on layered cathode materials, which enhanced their electrochemical performance due to the spinel structure which functions as a conductive agent and prevents side reactions. J. Cho et al. also reported heterostructured cathode materials such as spinel-layered core-shell materials and Ni-rich/Li-rich core-shell materials. Y-K. Sun et al. demonstrated a concentration-gradient shell (CGS) cathode material, featuring different concentrations of transition metals (Ni, Co and Mn) at different layers in the material. While these studies showed outstanding enhancement for stabilization of structure and electrical conductivity with good electrochemical performances, they all required complex syntheses to realize their heterostructured cathode materials. Here, we report a facile approach to realize surface modification of the OLO cathode materials with heterostructures; our approach only includes gas change for conventional synthesis without additional surface modification steps. We obtained Li1.167Ni0.18Mn0.548Co0.105O2, layered Li-rich oxide cathode material with enhanced electrochemical performance by reductive treatment with Ar gas. Figure 1

Controllable synthesis of hierarchical ball-in-ball hollow microspheres for a high performance layered Li-rich oxide cathode material

Novel hierarchical ball-in-ball hollow Li-rich microspheres with a multi-elemental composition are reported as a high performance cathode material for Li-ion batteries with excellent rate capability and superior cycle stability.

An Li-rich oxide cathode material with mosaic spinel grain and a surface coating for high performance Li-ion batteries

Lithium-rich layered oxides are considered to be one of the most promising cathode materials for lithium ion batteries due to their extremely high reversible capacity. Here, we report the design of a novel heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material with mosaic spinel nanograins and a surface coating, which is synthesized by a facile and green one-step solid-phase surface-modification process. We propose that the chemical Li leaching from Li2MnO3 simultaneously induces the formation of a fluorite coating and the layer-to-spinel phase transformation at high temperatures. The fluorite coating protects the lithium-rich oxides from direct exposure to the highly active electrolyte. The spinel phase provides an efficient path for Li+ mobility and also facilitates the suppression of the initial irreversible capacity loss. This unique heterostructured Li1.2Ni0.13Co0.13Mn0.54O2 material thus exhibits an outstanding initial Coulombic efficiency, superior rate capability and excellent cyclability. The design concept and facile synthetic strategy can be applied to both advanced lithium ion batteries and other high-performance energy storage devices.

First Principles and Experimental Study of Phase Transformation Mechanism of Li-Rich Oxide Cathode Material in Li-Ion Battery

Li-rich oxide cathode is one of the most promising high capacity cathode materials for the next commercialized Li-ion battery application, and its composition can be described as a mixture of Li2MnO3 and Li(TM)O2(TM: Transition metal). Li-rich cathode material has large capacity based on relatively cheap Mn oxide, and it is easy to synthesize. However, the Li-rich cathode material has critical material challenges such as capacity degradation and voltage drop. It has been reported that the main source of the material challenge is related to phase transformation inside the bulk cathode material during charge/discharge processes.In this work, detailed phase transformation mechanism of Li2MnO3 is investigated by combining the experimental and computational approaches to develop fundamental understanding on the atomic scale processes. The structure of Li-rich oxide is a composite of Li2MnO3 and layered Li(TM)O2, and the monoclinic layered structure of Li2MnO3 is known to be the source of the degradation problems. We have observed the evidence of phase transformation in Li2MnO3experimentally, and a theoretical analysis based on density functional theory calculations is combined with experimental data for a systematic comparative study.For experimental study, we have synthesized Li2MnO3 powder by solid state method at low temperature following the literature procedures. Using this powder, we made a coin cell with standard Li reference electrode, and the CV measurement shows the phase transformation during charge/discharge processes. First, basic powder characterization was conducted through XRD, and SEM analyses. Second, coin cell was assembled for cyclic performance test, and charge/discharge profile and cyclic voltammogram were obtained. From the experimental investigation, synthesized Li2MnO3is identified as monoclinic C2/m space group structure and their particle size is around 200~300 nm. The evidences of phase transformation are found from cyclic charge/discharge profile as well as cyclic voltammogram. As reaction cycle is progressed, charge/discharge capacity operated under 4.6 V has a steady increase indicating that, the initial active material is getting transformed to another phase with lower reaction voltage than 4.6 V. As described in Fig. 1, we could observe the changes of charge/discharge profile and reaction voltage fundamentally caused by phase transformation consistent with similar previous experiment studies. As observed before, the first charge voltage is around 4.6 V, but the charge voltage changes to around 3.2 V as cycle goes on. Remarkably, discharge voltage reveals at three different values around 2.8 V, 3.3 V, and 4.0 V, suggesting that there are three different transformed phases inside the active material as the cycle goes on.To understand this and similar experimental observations of phase transformation, we have examined how and why it could happen in terms of thermodynamics and kinetics based on density functional theory investigation. In case of thermodynamic study, phase stability, intercalation voltage, electronic charge, and electronic structures are studied. From phase stability and intercalation voltage analyses, we would estimate when initial structure could be transformed. The electronic charge and partial density of state for both initial and transformed structure (whose Mn ion is migrated) are investigated for structure stability and physical/chemical characters. We found that thermodynamic stability of structures and the changes in bonding characters between Mn and O ions are to the main cause of phase transformation. For kinetic analysis, we investigated the migration barriers of Li and Mn ions in the Li2MnO3framework with controlled delithiation. Based on the kinetic calculation results, we could show the possibility of Li and Mn ion migrations with different Li contents in active material. As shown in Fig 2, not only the possibility and the delithiation effect of phase transformation, but also the detailed Mn migration mechanism could be predicted providing atomic scale explanation of phase transformation. As a result, detailed phase transformation mechanism could be quantitative understood, and it would be possible to suppress such phase transformation based on theoretical studies on material design such as doping on effect the electronic structure analyses.By understanding detailed phase transformation mechanism of Li2MnO3, we are developing material modification strategy to solve capacity degradation and voltage drop problems. The developed strategy will be critically validated by experimental implementation of the designed material modification. Such combined material design and experimental validation approaches will accelerated the high capacity cathode material development based on atomic scale understanding.This work was supported by the Industrial Strategic technology development program(10041589) funded by the Ministry of Knowledge Economy(MKE, Korea)