Layer-structured Cathode Materials Research Articles

A Feasible Dual Modification Strategy of Internal Anion Redox Chemistry and Surface Engineering on P2 Layer-Structured Cathodes in Sodium-Ion Batteries.

Boosting the anion redox reaction opens up a possibility of further capacity enhancement on transition-metal-ion redox-only layer-structured cathodes for sodium-ion batteries. To mitigate the deteriorating impact on the internal and surface structure of the cathode caused by the inevitable increase in the operation voltage, probing a solution to promote the bulk-phase crystal structure stability and surface chemistry environment to further facilitate the electrochemical performance enhancement is a key issue. A dual modification strategy of establishing an anion redox hybrid activation trigger agent inside the crystal structure in combination with surface oxide coating is successfully developed. P2-type layer structure cathode materials with Zn/Li (Na-O-Zn@Na-O-Li) anion redox hybrid triggers and a ZnO coating layer possess superior capacity and cycle performance, along with outstanding structural stability, decreased Mn-ion dissolution effect, and less crystal particle cracking during the cycling process. This study represents a facile modification solution to perform structure optimization and property enhancement toward high-performance layered structure cathode materials with anion redox features in sodium-ion batteries.

Reversible Mn2+/Mn4+ double-electron redox in P3-type layer-structured sodium-ion cathode

The balance between cationic redox and oxygen redox in layer-structured cathode materials is an important issue for sodium batteries to obtain high energy density and considerable cycle stability. Oxygen redox can contribute extra capacity to increase energy density, but results in lattice instability and capacity fading caused by lattice oxygen gliding and oxygen release. In this work, reversible Mn2+/Mn4+ redox is realized in a P3-Na0.65Li0.2Co0.05Mn0.75O2 cathode material with high specific capacity and structure stability via Co substitution. The contribution of oxygen redox is suppressed significantly by reversible Mn2+/Mn4+ redox without sacrificing capacity, thus reducing lattice oxygen release and improving the structure stability. Synchrotron X-ray techniques reveal that P3 phase is well maintained in a wide voltage window of 1.5–4.5 V vs. Na+/Na even at 10 C and after long-term cycling. It is disclosed that charge compensation from Co/Mn-ions contributes to the voltage region below 4.2 V and O-ions contribute to the whole voltage range. The synergistic contributions of Mn2+/Mn4+, Co2+/Co3+, and O2−/(On)2− redox in P3-Na0.65Li0.2Co0.05Mn0.75O2 lead to a high reversible capacity of 215.0 mA h g−1 at 0.1 C with considerable cycle stability. The strategy opens up new opportunities for the design of high capacity cathode materials for rechargeable batteries.

Stacking Order Induced Anion Redox Regulation for Layer-Structured Na0.75 Li0.2 Mn0.7 Cu0.1 O2 Cathode Materials.

Stacking order plays a key role in defining the electrochemical behavior and structural stability of layer-structured cathode materials. However, the detailed effects of stacking order on anionic redox in layer-structured cathode materials have not been investigated specifically and are still unrevealed. Herein, two layered cathodes with the same chemical formula but different stacking orders: P2-Na0.75 Li0.2 Mn0.7 Cu0.1 O2 (P2-LMC) and P3-Na0.75 Li0.2 Mn0.7 Cu0.1 O2 (P3-LMC) are compared. It is found that P3 stacking order is beneficial to improve the oxygen redox reversibility compared with P2 stacking order. By using synchrotron hard and soft X-ray absorption spectroscopies, three redox couples of Cu2+ /Cu3+ , Mn3.5+ /Mn4+ , and O2- /O- are revealed to contribute charge compensation in P3 structure simultaneously, and two redox couples of Cu2+ /Cu3+ and O2- /O- are more reversible than those in P2-LMC due to the higher electronic densities in Cu 3d and O 2p orbitals in P3-LMC. In situ X-ray diffraction reveals that P3-LMC exhibits higher structural reversibility during charge and discharge than P2-LMC, even at 5C rate. As a result, P3-LMC delivers a high reversible capacity of 190.3 mAh g-1 and capacity retention of 125.7 mAh g-1 over 100 cycles. These findings provide new insight into oxygen-redox-involved layered cathode materials for SIBs.

Transition Metal Vacancy in Layered Cathode Materials for Sodium-Ion Batteries.

Anionic redox has been considered as a promising strategy to break the capacity limitation of cathode materials that solely relies on the intrinsic cationic redox in secondary batteries. Vacancy, as a kind of defect, can be introduced into transition metal layer to trigger oxygen redox, thus enhancing the energy density of layer-structured cathode materials for sodium-ion batteries. Herein, the formation process, recent progress in working mechanisms of triggering oxygen redox, as well as advanced characterization techniques for transition metal (TM) vacancy were overviewed and discussed. Strategies applied to stabilize the vacancy contained structures and harness the reversible oxygen redox were summarized. Furthermore, the challenges and prospects for further understanding TM vacancy were particularly emphasized.

A distinctive strategy of Sb doped quaternary oxide cathodes materials toward energy storage of electric equipment for sodium-ion batteries

The poor cycle stability and rate capability for sodium-ion batteries is largely due to electrochemical irreversibility and sluggish mobility of Na+ in the cathode materials. Herein, a new strategy of introducing Sb ions into the ternary oxide NiMnTi-based layer-structured cathode material is designed. Reversible phase transition in this cathode NMTSb0.03 restraint the rapid capacity decay issued from intense volume variation, exhibiting a reversible capacity of 147.8 mAh g−1 and retaining 103.5 mAh g−1 over 200 cycles at 1C. The enough space after Sb substitution allows the effective accommodation of Na+ intercalation and deintercalation, affording a specific capacity of 90 mAh g−1 at 8C. The improvement is attributed to the enhanced Na+ diffusion kinetics with the meliorative equilibrium conductance and the higher cohesive energy after Sb doping. Considering its simple preparation and scale-up capability, the O3-type NaNi0.5Mn0.3Ti0.2O2 with Sb5+may find extensive applications in designing electrode for sodium-ion batteries.

A layered lithium nickel manganese oxide as environmentally friendly cathode material for secondary batteries

<p>Cobalt-free cathode material development is considered necessity to assure the sustainability of Li-ion batteries. Cobalt is always considered expensive and unsafe for both human and environment. LiNi<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>2</sub> (LNMO) is a layer structured cathode material that has similar feature to LiCoO<sub>2</sub> (LCO). A simple and fast processing of LNMO is proposed. A precipitation of nickel manganese oxalate was obtained in a batch reactor under atmospheric condition. The as-obtained homogenous oxalate precursor was converted to LNMO via high temperature lithiation. Based on the XRD result, a crystalline product with layer structure is successfully obtained. The presence of impurities such as residual Li can be detected from the FTIR spectra. SEM Images confirmed quasi-spherical particles with grain size of less than 10 micrometer. The charge-discharge analysis of LNMO containing cell deliver a capacity of 42 mAh/g. In spite of its promising report, continuous improvement is necessary to obtain cell with better electrochemical performance.</p>

Modification of Li-Rich Layer-Structured Cathode Materials by Supercritical Co2 Fluid

Modification of Li-Rich Layer-Structured Cathode Materials by Supercritical Co2 Fluid

Atomistic Observation of Desodiation-Induced Phase Transition in Sodium Tungsten Bronze.

The phase instability in layered-structure Na0.5WO3.25 induced by the extraction of Na ions was investigated by applying transmission electron microscopy. Real-time atomic-scale observation reveals the phase transition pathway: Na0.5WO3.25 (triclinic) → NaxWO3 (cubic) → WO3 (monoclinic) with specific orientation relationships. The dynamic evolution of Na0.5WO3.25/NaxWO3 phase boundaries shows that Na0.5WO3.25 will cleave along the (100)T and (010)T and recrystallize as (101)C and (010)C of NaxWO3, respectively. The phase transition pathway can be well-explained according to the structural characteristics in the three phases. By better understanding of the phase instability induced by the extraction of Na ions in possible layered-structure cathode materials, this work provides a reference for the design of sophisticated strategies toward high-performance Na-ion batteries.

Improvement in the electrochemical stability of Li[Ni0.5Co0.2Mn0.3]O2 as a lithium-ion battery cathode electrode with the surface coating of branched oligomer

Although layered Li[Ni0.5Co0.2Mn0.3]O2 (LNCM 523) is one of the most promising cathode materials for lithium-ion batteries, large capacity losses during charging and discharging, due to poor surface stability, have limited its application. Surface modifications can be effective at improving the electrochemical performance of cathode materials. In this study, an oligomer polymer, named LIVING, was synthesized from bisphenol A diglycidyl ether diacrylate and barbituric acid and then used it as a coating material for LNCM 523 particles. Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis, and electrochemical methods were employed to investigate the structure, surface properties, thermal stabilities, and electrochemical properties of the electrodes. Both coated (LIVING@LNCM 523) and uncoated (LNCM 523) electrodes were tested for their battery performance. A cathode material coated with 0.5 wt.% of LIVING (0.5% LIVING@LNCM 523) at a layer thickness of approximately 10 nm showed enhanced cycle stability, with over 80.7% of the capacity retained after 100 cycles at a rate of 0.2C; in comparison, the LNCM 523 electrode retained no more than 66.2% of its capacity under the same conditions. The coated electrode (0.5% LIVING@LNCM 523) also exhibited a better rate capability, retaining about 91% of its initial capacity after applying various current loads, whereas LNCM 523 retained only 79% after applying the same current loads under the same conditions. The improved electrochemical performance of the modified electrode is attributed to the chemical interactions between LNCM 523 and LIVING providing a uniform and stable oligomer layer on the LNCM 523 surface, thereby preventing the LNCM 523 surface from deeper electrolyte ion intercalation and suppressing the growth of interfacial resistance. The coating material (LIVING) appears to have great potential for improving the electrochemical performance of layered structure cathode materials, with promising applications in highly energy demanding lithium-ion batteries.

Effect of metal composition on the structure of layer-structured cathode materials for Li-ion batteries

Layer-structured LiMO2 (M = Ni, Co, Mn, and dopants) is widely used as a cathode material for Li-ion secondary batteries. Ni-based LiMO2 is advantageous with respect to energy capacity and material cost compared to conventional LiCoO2, but its structural and thermal stabilities decrease with increases in Ni content. As a countermeasure for these decreases, the three-component design of M comprising Ni, Co, and Mn is used universally, and small amounts of foreign elements are often added to obtain doping or surface coating effects. The relation between the composition of M and structural characteristics of LiMO2 is investigated in this study. The features of defect generation, dopability of foreign elements, and formation of segregated secondary phases are analyzed by simulating phase diagrams based on density functional theory calculations. On this basis, a design strategy for the LiMO2 composition is provided, with a discussion on the proper selection of doping or coating materials to improve the cathodic properties.

Growth mechanisms for spherical Ni0.815Co0.15Al0.035(OH)2 precursors prepared via the ammonia complexation precipitation method

The crystal habit is essential to the Li-storage property for Ni-rich cathode materials. In this study, the growth behaviour of Ni 0.815 Co 0.15 Al 0.035 (OH) 2 prepared via the ammonia complexation precipitation method in the CSTR is studied in detail. The microstructures of precursors strongly affect the electrochemical performance of Ni-rich layer-structured cathode materials. In this study, the growth behaviour of Ni 0.815 Co 0.15 Al 0.035 (OH) 2 (NCA) prepared via the ammonia complexation precipitation method in a 50-L-volume continuously stirred tank reactor (CSTR) is studied in detail. The growth of Ni(OH) 2 -based hydroxide can be divided into a nucleation process, an agglomeration growth process, a process in which multiple growth mechanisms coexist, and an interface growth process over time, while the inner structure of the CSTR can be divided into a nucleation zone, a complex dissolution zone, a growth zone, and a maturation zone. The concentration of ammonium ions affects the growth habit of the primary crystal significantly due to its specific adsorption on the electronegative crystal plane. When the ammonia concentration is <1.5 mol L −1 at 60 °C at pH = 11.5, the precursors grow preferentially along the (1 0 1) crystal plane, whereas they grow preferentially along the (0 0 1) crystal plane when the concentration is >2.0 mol L −1 . The LiNi 0.815 Co 0.15 Al 0.035 O 2 materials inherit the grain structure of the precursor. Materials prepared from precursors with (1 0 1) preferential primary particles show a higher specific capacity and better rate performance than those that were prepared from (0 0 1) preferential primary particles, but the latter realize a better cycling performance than the former.

Minimum Co content limit in layer-structured cathode materials for Li-ion batteries

Research to decrease the Co content in layer-structured cathode materials for Li-ion batteries is ongoing to increase the battery capacity and decrease the cost of material by increasing the relative Ni content. However, no clear guidance has yet been provided on the minimum possible Co content. In this study, energetically stable structures of delithiated cathodes are investigated, and a complete crystal structure is modeled as a combination of these structures. By the statistical processing of Co distribution within the model crystal, as well as considering the interfacial energy between differently structured domains induced by stacking faults of the O layers, the lower limit for the Co content is calculated. The result indicates that a minimum of 10.2% Co is necessary to prevent degradation of the crystal structure. For further reduction of the Co content, the Co distribution must be concentrated in certain regions of the crystal, which can be achieved by doping with a small amount of a third element. In this manner, the Co content could be ideally reduced to 4.17%.

Stable prismatic layer structured cathode material via Cation mixing for sodium ion battery

Recently, P2-type (prismatic site) and O2 (octahedral site)/P2-type materials like Na0.66Fe0.33Mn0.5O2 exhibit favorable capacities more than its theoretical value (173 mAh g−1). However, their low operating potential window ≤ 3 V makes them not desirable for practical applications. Among the sodium (Na) cathodes, the P2-Na-Ni-Mn-O is a significant material because of its high theoretical capacity of ≤ 250 mAh g−1 even if it undergoes severe voltage decay with capacity fade due to stacking faults upon cycling above the cut-off voltage (≥ 4.2 V). En route to evade this problem, we substitute zinc (Zn) cation into the P2-Na-Ni-Mn-O system as structure stabilizer, and this material delivers high initial discharge capacity of 205 mAh g−1 while cycling in the range 1.5 to 4.5 V. The X-ray diffraction pattern, energy dispersive spectroscopy (EDS) with mapping, and XPS results show that Zn2+ befit in to the P2-layer structure of Na-Ni-Mn-O2 without changing its origin.

Suppression of irreversible capacity for Li1.16Ni0.37Mn0.47O2 due to the chemical treatment with (NH4)2SO4 in lithium ion batteries

A lithium-rich layer-structured cathode material (Li1.2Ni0.2Mn0.6O2) is promising for lithium ion batteries due to its high practical capacity; however, it has several issues such as low rate performance, potential shift during cycling, and large irreversible capacity. Previously, our group reported that the change in the metal composition from Li1.2Ni0.2Mn0.6O2 to Li1.16Ni0.37Mn0.47O2 was effective in improving rate performance and suppressing potential shift; however, the large irreversible capacity could not be addressed by the composition change. In this study, Li2O extraction, which occurs during initial charging and causes irreversible capacity, progressed using (NH4)2SO4 before charging to reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2. The results obtained in the composition, morphology, specific surface area, and crystal structure analyses indicate that the (NH4)2SO4 treatment can extract Li2O from the particle surface without changing the crystal structure. The results obtained in the electrochemical measurements indicate that the irreversible capacity was reduced due to the chemical treatment. Moreover, the high rate and cycling performance for the Li1.16Ni0.37Mn0.47O2 was maintained after the treatment. The treatment with (NH4)2SO4 can reduce the irreversible capacity for Li1.16Ni0.37Mn0.47O2 without harming this material’s advantages such as high rate and cycling performance.

Stabilizing Structure and Performances of Lithium Rich Layer-Structured Oxide Cathode Materials

Stabilizing Structure and Performances of Lithium Rich Layer-Structured Oxide Cathode Materials

Triggered reversible phase transformation between layered and spinel structure in manganese-based layered compounds

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (NaxMnO2·yH2O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries.

Surface Stabilization of O3-type Layered Oxide Cathode to Protect the Anode of Sodium Ion Batteries for Superior Lifespan.

SummaryEven though the energy density of O3-type layer-structured metal oxide cathode can fully reach the requirement for large-scale energy storage systems, the cycling lifespan still cannot meet the demand for practical application once it is coupled with a non-sodium-metal anode in full-cell system. Transition metal dissolution into the electrolyte occurs along with continuous phase transformation and accelerates deterioration of the crystal structure, followed by migration and finally deposition on the anode to form a vicious circle. Surface engineering techniques are employed to modify the interface between active materials and the electrolyte by coating them with a thin layer of AlPO4 ion conductor. This stable thin layer can stabilize the surface crystal structure of the cathode material by avoiding element dissolution. Meanwhile, it can protect the anode from increased resistance by suppressing the dissolution-migration-deposition process. This technique is a promising method to improve the lifetime for the future commercialization.

Efficient Synthesis of O2 Layered Structure for High Performance Li-Rich Cathode

Nowadays, the growing grievous environment problems together with the exhaustion of energy resources put urgent demands for developing high energy density. Considering the factors including capacity, resource and environment, Manganese-based lithium-rich layer-structured cathode materials xLi2MnO3⋅(1-x)LiMO2 (M = Ni, Co, Mn, and other metals) are drawing increasing attention due to their high reversible capacities, high discharge potentials, and low cost. They are expected to be one type of the most promising cathode materials for the next-generation Li-ion batteries (LIBs) with higher energy densities. Unfortunately, their commercial applications are hindered with crucial drawbacks such as poor rate performance, limited cycle life and continuous falling of the discharge potential. With decades of extensive studies, significant achievements have been obtained in improving their cyclability and rate performances, but the potential decay remains a severe challenge. Fundamental investigations have clarified that the continuous migration of the transition metal (TM) ions into the lithium layer and the resultant irreversible structural transformation are responsible for the potential decay. Recent study reports that the P2-NaxMO2 (M = Ni, Com Mn) shows a phase transformation from P2 to O2 with the disodium while the formation energy of O2 phase is around 25meV f.u.-1 lower than P2 structure. Such an energy difference can significantly affect the migration of the transition metal ions. Our work aims to improve the properties of Li-rich cathode by preparation of Mn based hybrid Li-Na materials and O2-typed layered structure materials for Li-rich cathode. The precursor was prepared by a facile coprecipitation method. The as-prepared precursor was then thoroughly mixed with Na2CO3 and Li2CO3 and calcined in air at 500℃ for 5h and 900℃ for 12h to get NaxLi1.2-x[Ni0.2Mn0.6]O2. The lithiation was carried out in three different methods, that is, the solid phase calcination (LRLO-C) and the electrochemical reaction (LRLO-E). The formula of the as-prepared material was determined to be Li1.2Ni0.2Mn0.6O2. The cathode electrodes were prepared by mixing 80wt% LNMO, 15wt% acetylene black and 5% Polyvinylidene fluoride onto an Al foil. The SEM of the as-prepared samples through different lithiation methods are shown in Figure 1. Obviously, we can see that the samples maintain spherical topography, but the details are not the same. The secondary particles of LRLO-C clustered and formed a solid sphere (Figure 1d), while the sample of LRLO-E keeps many hollows (Figure 2b). The result of electrochemical measurement of LRLO-E (Figure 1a) indicates such a structure has an enhanced cycle performance but relatively low capacity, while LRLO-C (Figure 1c) indicates that the O2 layered structure electrode from solid calcination delivered a discharge capacity of 238 mAh g-1 at 30mA g-1 and the retention after 50 cycles was 93%, but a long activation process. In summary, the work described here shows a potential of O2 layered structure of Li-rich materials as promising materials to deal with voltage decay and long cycle life for Li-rich materials making it possible for commercial application. Figure 1

Modification of Electrochemical Behavior of Li-Rich Layered Oxide Cathode through Defect Consideration

Cathode materials formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) with layered structure received much attention due to their surprisingly high reversible capacity for Li-ion battery applications.[1,2] For better understanding of such high-capacity layered cathode, a systematic study on powder morphology, crystal structures and reaction mechanisms of Li1.2Mn0.54Ni0.13Co0.13O2 during electrochemical charge/discharge cycles were conducted. First, the capacity of Li-rich layered cathode powder was strongly affected by its particle size. To obtain various particle sizes of cathode powder, different powder synthesis routes have been adopted. By usng sol-gel process, particle size of Li1.2Mn0.54Ni0.13Co0.13O2 powder obtained is around 200 nm. On the other hand, Li1.2Mn0.54Ni0.13Co0.13O2 powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place. Although Li-rich layer-structured cathode material show a reversible capacity as high as 250 mAh/g at low rate, its electrochemical properties such as capacity loss at first cycle, rate capability and capacity fading still need to be examined and characterized. In this study, Li1.2Mn0.54Ni0.13Co0.13O2 were investigated. The TEM structural analysis shows that the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.

Synthesis and Electrochemical Performances of the High Nickel Content Layer-Structured Cathode Materials for High-Energy Libs

Among the various cathode candidate, layer structured Li(Ni,Co,Mn)O2 (NCM) has been regarded as the most attractive alternative to LIBs owing to relatively modest volume change (LiCoO2), high specific capacity (LiNiO2) and good thermal stability (LiMn2O4). Although Li(Ni1/3Co1/3Mn1/3)O2 has been successfully commercialized as a battery cathode, researches on Ni-rich NCM (LiNixCoyMn1−x−yO2, x > 0.5) have been spotlighted due to its superior capacity (> 200 mAh/g, at 4.6 V vs. Li/Li+) [10]. Therefore, the future of NCM for high energy LIBs strongly depends on Ni-rich NCM materials. It is reported that crystallinity and morphology, influenced by the sintering temperatures, all play important roles in the electrochemical performances. In this paper, we report the synthesis of LiNi0.91Co0.06Mn0.03O2 cathode materials with different sintering temperatures. Also the relationship between the sintering temperatures and the electrochemical performances is investigated. The sintering temperatures affect a crystallinity and morphology, which play an important role in electrochemical performances of LiNi0.91Co0.06Mn0.03O2. The electrochemical performances have improved with increasing sintering temperatures. The LiNi0.91Co0.06Mn0.03O2 sintered at 720 oC shows remarkably excellent performance: initial discharge capacity: 100 mAh/g, cycle performance: 98 % retention after 100 cycles (at 0.5 C) and rate capability: 88 % (at 2 C). These results indicate that LiNi0.91Co0.06Mn0.03O2 can be regarded as a next generation cathode for high-performance lithium ion batteries.