Li-rich Layered Oxide Materials Research Articles

Tailoring solvation chemistry in carbonate electrolytes for all-climate, high-voltage lithium-rich batteries

High-voltage Li-rich layered oxide materials (LLOs) are considered as the promising next-generation cathode materials because of their high energy density and low cost. However, their electrochemical performance continues to deteriorate under high voltages when using traditional lithium hexafluorophosphate (LiPF6)-based carbonate electrolytes, mostly due to the detrimental reactions with free anions, unstable electrode interface and limited Li+ kinetic process. Here, we demonstrate a highly oxidation-resistant carbonate electrolyte by incorporating multifunctional lithium difluoro(oxalato)borate (LiDFOB) and Tris(trimethylsilyl) Phosphite (TMSPi) additives. It is discovered that the introduction of LiDFOB and TMSPi can manipulate the electrolyte solvation structure by reducing the number of coordinated solvent molecules and free PF6−anions at the electrolyte-electrode interface, thus promoting the desolvation process of Li+, suppressing the generation of harmful HF species and conducive to forming a thin and robust cathode electrolyte interphase (CEI) layer. As a result, the Li||LLO cells with dual additives show excellent cycle stability and improved rate capability in a wide temperature ranged from -20 °C to 55 °C, and the retention rate of LLO||graphite pouch cell (2.5 Ah) using EEDB-TMSPi electrolyte is up to 91.1% after 200 cycles. It is anticipated that the work provides a promising strategy for realizing long-cycle stability of layered cathode materials under high-voltage and wide-temperature conditions.

Unravelling structural changes of the Li1.2Mn0.54Ni0.13Co0.13O2 lattice upon cycling in lithium cell

Li-rich layered oxide materials (LRLOs) are the most promising positive electrodes materials for the next-generation Li-ion batteries. Nevertheless, cycle life stability and capacity retention are still inadequate and hinder their use in the commercial Li-ion devices. In fact, LRLOs undergo structural rearrangements and phase transitions upon cycling that reduce their electrochemical performances. Li1.2Mn0.54Ni0.13Co0.13O2 is one of the most studied LRLO materials, thanks to its high initial specific capacity, exceeding 300 mAh/g in the first cycle, that fades monotonically upon cycling. In this work, after a complete characterization of the pristine material, we demonstrate the electrochemical performances both in half and full cells and we shed new light on the structural degradations occurring upon cycling. By combining ex-situ synchrotron diffraction and Raman spectroscopy, we outline the structural evolution of the LRLO phase occurring in the first 10 cycles of lithium de-intercalation/intercalation, thus extending the understanding of the origin of capacity fading beyond the first cycle.

Direct Observation of Li-Ion Transport Heterogeneity Induced by Nanoscale Phase Separation in Li-rich Cathodes of Solid-State Batteries.

Li-rich layered oxide (LLO) cathode materials with high specific capacities could significantly enhance the energy density of all-solid-state lithium batteries (ASSLBs). However, the specific practical capacities of LLO materials in ASSLBs are extremely low due to poor initial activation. Here, scanning transmission electron microscopy with in situ differential phase contrast imaging was first used to study the initial activation mechanism of Li1.2 Ni0.13 Co0.13 Mn0.54 O2 . Li-ion transport heterogeneity was observed in LLO grains and across the LLO/Li6 PS5 Cl interface, due to the coexistence of the nanoscale Li2 MnO3 and LiNi1/3 Co1/3 Mn1/3 O2 phases. Consequently, the severely constrained activation of Li2 MnO3 during the first charging could be attributed to a nanoscale phase separation in LLO, hindering Li-ion transport through its particles, and causing high impedance in the Li2 MnO3 domain/Li6 PS5 Cl interface. This study could facilitate interface design of high-performance LLO-based ASSLBs.

Impact of Overlithiation and Al doping on the battery performance of Li-rich layered oxide materials

Lithium rich layered oxides (LRLOs) are one of the best alternatives for the next generation positive electrodes materials for Li-ion batteries. However, LRLOs suffer a remarkable voltage decay upon cycling that prevents stable and prolonged electrochemical performances and contains large quantities of cobalt in the transition metal blend. Here, we demonstrate the performance in batteries of a series of innovative materials with general formula Li1.2+xMn0.54Ni0.13Co0.13-x-yAlyO2 (where 0.03≤x ≤ 0.08 and 0.03≤y ≤ 0.05) capable to supply large reversible specific capacities (around 200 mAh g−1), stable cycling performance with reduced voltage decay. This communication sheds light on the interplay between structural and electronic disorder induced by Al/Li co-doping in LRLO and the corresponding functional properties in batteries. The impact of substitution of cobalt by Li and Al co-doping on the structural and morphological properties has been examined by Synchrotron X-ray diffraction (XRD), microwave plasma atomic emission spectrometer (MP-AES) and scanning electron microscopy (SEM). Furthermore, to better understand the structure-function interplay of these over-lithiated materials, ex situ analyses are here reported coupling Synchrotron X-Ray diffraction and Raman Spectroscopy. Based on the obtained results, we proved that coupling the over-lithiation with the aluminum doping, is an effective strategy to reduce the cobalt content into the LRLOs structure maintaining high electrochemical performance. Beside the values of specific capacities obtained in lithium cells, these materials exhibit excellent capacity retention and voltage retention, in particular for the material with the smallest content of cobalt, i.e. Li1.28Mn0.54Ni0.13Co0.02Al0.03O2. Furthermore, thanks to the ex-situ analysis of the latter, we explained the structural evolution of the sample upon cycling that showed the formation of a secondary trigonal phase and the occurrence of limited local distortions of layered structure.

Synchronous construction of oxygen vacancies with suitable concentrations and carbon coating on the surface of Li-rich layered oxide cathode materials by spray drying for Li-ion batteries

Lithium-rich layered oxide materials with high reversible capacities (over 250 mAh g−1) are considered as one of the most promising cathode materials for the next generation of lithium-ion batteries. However, some challenges including low initial Coulombic efficiency, voltage decay and poor cycling restrict their commercial application. Herrin, Li-rich layered oxide materials Li1.2Ni0.2Co0.08Mn0.52O2 co-modified by constructing oxygen vacancies with suitable concentration and C coating layer have been synthesized via a facile spray drying process. The oxygen vacancies with suitable concentration can not only suppress the release of O2 and improve the stability of the layered structure, but also facilitate Li diffusion. Moreover, C coating layer acts as a physical barrier to protect the materials from electrolyte, and reduce the interface impedance. The co-modified material shows excellent electrochemical properties: a high initial Coulombic efficiency of 83.1%, a small voltage decay rate of 0.306 mV, and a capacity retention of 78.3% after 1000 cycles at 5 C. And it provides a simple and feasible way to improve the properties of lithium-rich layered oxide materials.

Critical Role of Ti4+ in Stabilizing High-Voltage Redox Reactions in Li-Rich Layered Material.

Li-rich layered oxide materials are considered promising candidates for high-capacity cathodes for battery applications and improving the reversibility of the anionic redox reaction is the key to exploiting the full capacity of these materials. However, permanent structural change of the electrode occurring upon electrochemical cycling results in capacity and voltage decay. In view of these factors, Ti4+ -substituted Li2 IrO3 (Li2 Ir0.75 Ti0.25 O3 ) is synthesized, which undergoes an oxygen redox reaction with suppressed voltage decay, yielding improved electrochemical performance and good capacity retention. It is shown that the increased bond covalency upon Ti4+ substitution results in structural stability, tuning the phase stability from O3 to O1' upon de-lithiation during charging compared with O3 to T3 and O1 for pristine Li2 IrO3 , thereby facilitating the oxidation of oxygen. This work unravels the role of Ti4+ in stabilizing the cathode framework, providing insight for a fundamental design approach for advanced Li-rich layered oxide battery materials.

A composite surface configuration towards improving cycling stability of Li-rich layered oxide materials

Scalable 345 W h kg−1 full pouch cells with guaranteed cycling life are achieved by employing the surface-modified materials.

A review on the electrochemical reaction of Li-rich layered oxide materials

Lithium-rich layered oxide materials xLi2MnO3·(1 − x)LiMO2 (M = Mn, Co, Ni, Fe, Cr, etc.) as promising cathode candidates for high energy Li-ion batteries have been summarized in this review.

Correction: Enhanced structural stability and overall conductivity of Li-rich layered oxide materials achieved by a dual electron/lithium-conducting coating strategy for high-performance lithium-ion batteries

Correction for ‘Enhanced structural stability and overall conductivity of Li-rich layered oxide materials achieved by a dual electron/lithium-conducting coating strategy for high-performance lithium-ion batteries’ by Dan Gao et al., J. Mater. Chem. A, 2019, 7, 23964–23972, DOI: 10.1039/C9TA04551A.

Regeneration of degraded Li-rich layered oxide materials through heat treatment-induced transition metal reordering

Lithium-rich layered oxides (LRLO) have drawn great attention recently as a high energy density cathode material. However, the practical deployment of this materials is hindered by voltage and capacity decay during electrochemical cycling. Applying mild thermal energy to cycled LRLO materials allows to recover the decayed voltage. In this work, a detailed mechanism study of the voltage and structure recovery is conducted through various analytical tools. Heat treatment provides the necessary energy for the transition metal to overcome the kinetic barrier to move from honeycomb center to honeycomb vertex and recover the honeycomb ordering in the transition metal layer. An ambient-air relithiation using Li+ molten salt was also conducted along with the heat treatment to the cycled LRLO cathode and lead to a material regeneration in both the voltage and capacity. This work identifies the transition metal reordering as the key factor under the structure recovery of degraded LRLO materials. It opens a door for the promising strategies to mitigate the voltage and capacity degradation problem in LRLO and provides the potential routes to recycle degraded LRLO materials.

Enhanced structural stability and overall conductivity of Li-rich layered oxide materials achieved by a dual electron/lithium-conducting coating strategy for high-performance lithium-ion batteries

A dual coating of LATP and CNTs accelerates the transportation of Li+ and electrons, resulting in improved rate capability.

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.

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%).

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.

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.

The Effect of Boron Doping on Structure and Electrochemical Performance of Lithium-Rich Layered Oxide Materials.

Polyanion doping shows great potential to improve electrochemical performance of Li-rich layered oxide (LLO) materials. Here, by optimizing the doping content and annealing temperature, we obtained boron-doped LLO materials Li1.2Mn0.54Ni0.13Co0.13BxO2 (x = 0.04 and 0.06) with comprehensively improved performance (94% capacity retention after 100 cycles at 60 mA/g current density and a rate capability much higher compared to that of the pristine sample) at annealing temperatures of 750 and 650 °C, respectively, which are much lower than the traditional annealing temperature of similar material systems without boron. The scenario of the complex crystallization process was captured using Cs-corrected high-angle annular dark field scanning transmission electron microscopic (HAADF-STEM) imaging techniques. The existence of layered, NiO-type, and spinel-like structures in a single particle induced by boron doping and optimization of annealing temperature is believed to contribute to the remarkable improvement of cycling stability and rate capability.

Hydrothermal-Assisted Synthesis of Li-Rich Layered Oxide Microspheres with High Capacity and Superior Rate-capability as a Cathode for Lithium-ion Batteries

Li-rich layered oxide materials possess high voltage and high specific capacity, which makes them attractive cathode candidates for lithium-ion batteries. However, they still suffer from a poor rate capability, which seriously blocks their widespread practical applications. In this work, Li(Li0.167Mn0.5Co0.167Ni0.167)O2 microspheres were synthesized by a hydrothermal-assisted method, in which Ni-Co-Mn-based microspherical precursors obtained by a hydrothermal process with polyethylene glycol-600 (PEG-600) as a surfactant were mixed with lithium sources and then sintered to yield the final products. It is found that the as-prepared Li-rich layered oxide microspheres exhibit high discharge capacity and superior rate performance: delivering an initial discharge capacity of 292mAhg−1 at a current density of 20mAg−1, 189mAhg−1 at a current density of 600mAg−1 and 142mAhg−1 at a current density of 2000mAg−1 (10C), which are better than that of the sample as-prepared by co-precipitation method. The high discharge capacity and improved rate-capability were beneficial from the microspheres assembled by uniform primary particles around 250nm, more reversible redox and better electrode kinetics comparing to that of the co-precipitation sample. The preparation strategy reported here may offer hints for achieving various advanced Li-rich layered composite materials that would be used in high-performance energy storage.

Electrochemical properties of nanosized Li-rich layered oxide as positive electrode materials for Li-Ion batteries

A nanosized Li-rich layered oxide/carbon composite material is successfully prepared by simple ball milling pulverization of microsphere-shaped Li-rich layered oxide materials with conductive carbon. The nanosized Li-rich layered oxide/carbon composite electrode exhibits a high 1st discharge capacity of 250 mAh g−1 with an excellent rate capability at high current density. The composite also reduces the internal resistance from oxygen release during the electrochemical activation of Li2MnO3. The improvement in the electrochemical performance of nanosized Li-rich layered oxide/carbon composite materials primarily occurs because the nanosized particles facilitate the diffusion of Li within the structure and provide innumerable reaction sites with lithium. Furthermore, the electronic conductivity of the active material is effectively enhanced by the carbon coating on the particles. In addition, unique effects of ball milling on the electrochemical properties of the Li-rich layered oxides are observed: (i) pre-activation of the Li2MnO3 component and (ii) gradual electrochemical activation under 4.3 V during cycling. Adverse effects on the electrochemical stability of the nanosized Li-rich layered oxide are also discussed, and these adverse effects mainly arise due to (i) the structural deformation of hexagonal ordering, (ii) the growth of the spinel component and (iii) the insufficient formation of a protective NiF2 layer on the surface of the active material.