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Highly crystalline alumina surface coating from hydrolysis of aluminum isopropoxide on lithium-rich layered oxide
Abstract
Lithium-rich layered oxides, xLi2MnO3$(1�x)LiMO2(M 1⁄4 Ni, Mn, Co), have been under intense investi- gation as high-performance cathode materials for lithium ion batteries due to their high discharge ca- pacity, low cost and environmental benignity. Unfortunately, the commercialized application of these cathode materials have so far been hindered by their severe capacity and voltage fading during high voltage cycling (>4.5 V vs. Li/Liþ). In an attempt to overcome these problems, herein, highly crystalline Al2O3 layer from the hydrolysis of aluminum isopropoxide are coated on 0.5Li2M- nO3$0.5LiNi0.5Co0.2Mn0.3O2 with controlling the growth of Al2O3 crystals. The coin cell with bare cathode material delivers a high discharge capacity over 268.2 mAh g�1 between 2.0 V and 4.8 V, while the Al2O3 coated cathode material shows the excellent cycling stability corresponding to 98% capacity retention after 100 cycles at 1C. More importantly, the highly crystalline Al2O3 coated cathode materials exhibit a significantly lower discharge voltage decay compared to the bare cathode materials, which could be ascribed to the suppression of the layered-to-spinel transformation by compact and highly crystalline Al2O3 layer. The results here will shed light on developing cathode materials with special structures and superior electrochemical properties for high-performance lithium ion batteries.
... Mn-and Li-rich transition-metal-layered oxides Li-(Li x Mn 1−x−y−z Ni y Co z )O 2 (e.g., Li 1.2 Mn 0.55 Ni 0.15 Co 0.1 O 2 as used in this study) are a very promising class of cathode materials for Li-ion batteries due to their high specific discharge capacity and energy. Nevertheless, a large capacity and voltage fade occur for these cathode materials during electrochemical cycling. 1 According to Xu et al., 2 Wen et al., 3 Park et al., 4 and Zhang et al., 5 thin inorganic coatings on the cathode particles made of aluminum oxide are supposed to prevent the capacity fading of Li-Rich NCM. Yet the underlying stabilization mechanism is not fully understood. ...
... A dense coating is said to be crucial to avoid reactions between electrolyte and cathode material as well as to improve the capacity retention. 2 The investigations of the coating influence on the electrochemical degradation process require informa-tion about the coverage, morphology, and atomic structure. Promising alumina coating processes already described in the literature 2−5 were chosen and applied on the cathode active material Li 1.2 Mn 0.55 Ni 0.15 Co 0.1 O 2 . ...
... The particles were processed with coating amounts of 1, 2, and 4 wt % following the procedure reported by Xu et al., 2 because these concentrations lead to improved capacity retention. 2−4 The previous studies showed that only the concentration of 1 wt % had the tendency to stabilize the capacity of the price of a small loss of initial capacity, similar to the results reported by Xu et al. 2 To increase the sensitivity of surface-dependent degradation processes by a higher specific surface area (14 m 2 /g), a longer duration of ball-milling was applied according to the description given above. Following Han et al., 6 a target coating amount of 0.5 wt % alumina was used. ...
Thin alumina coatings on Li-rich nickel cobalt manganese oxide (Li-rich NCM) particles used as cathode material in Li-ion batteries can improve the capacity retention during cycling. However, the underlying mechanisms are still not fully understood. It is crucial to determine the degree of coverage of the particle’s coating on various length scales from micrometer to nanometer and to link it to the electrochemical properties. Alumina coatings applied on Li-rich NCM by atomic layer deposition (ALD) or by chemical solution deposition were examined. The degree of coverage and the morphology of the particle coatings were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning electron microscopy (SEM), elemental analysis using inductively coupled plasma optical emission spectrometry (ICP-OES), and scanning / transmission electron microscopy (S/TEM). ToF-SIMS allows to investigate the coverage of a coating on large length scales with high lateral resolution and a surface sensitivity of a few nanometers. Regardless of the chosen coating route analytical investigations revealed that the powder particles were not covered by a fully closed and homogeneous alumina film. This study shows that a fully dense coating layer is not necessary to achieve an improvement in capacity retention. The results indicate that rather the coating process itself likely causes the improvement of the capacity retention and increases the initial capacity.
... [9,12,16,17] While comprehensive mechanisms for how these complex parameters mutually interact and govern overall performance degradation, surface coatings of nickel-rich layered particles such MgO and Al 2 O 3 have already been applied as effective solutions that enhance cyclic performance. [17][18][19] The improved capacity retention created by these coatings is widely believed to be enabled by suppressing both the undesired surface reactions and the structural and chemical transitions within the cathode material. [17,[19][20][21] Nonetheless, there are as yet few reports of the underlying mechanisms through which these coating layers interact with the active cathode particles. ...
... [17][18][19] The improved capacity retention created by these coatings is widely believed to be enabled by suppressing both the undesired surface reactions and the structural and chemical transitions within the cathode material. [17,[19][20][21] Nonetheless, there are as yet few reports of the underlying mechanisms through which these coating layers interact with the active cathode particles. [17,20] Along with forming an oxide protection layer (Al 2 O 3 ) and substitution of electrochemically-active elements (e. g. ...
Invited for this month's cover picture is the group of Dr B. Layla Mehdi from the University of Liverpool. The cover picture demonstrates the impact of Al impurities on the nickel‐rich NMC cathodes. This observation of increased Al content can bring new insights into the ongoing discussions concerning the capacity fading phenomenon of nickel‐rich layered oxide materials in lithium‐ion batteries. Read the full text of the Article at 10.1002/batt.202100110.
... 38,39 Coating of the particles is mostly used to stabilize the surface against side reactions with the electrolyte, to improve the conductivity of the surface layer, and reduce the oxygen release during cycling. 40,41 Besides the deposition of phosphate, [42][43][44][45] oxide, 40,[46][47][48] and fluoride coatings, 43,49 different approaches to introduce carbon-containing coatings have been reported, using precursors like urea, C-polymorphs and different polymers. [50][51][52][53][54][55][56] A second low temperature treatment is typically employed to convert the carbon precursor into carbon, which partially reduces the TM in the near-surface region, resulting in a combination of surface coating and a modified spinel-like, oxygen-vacant surface region with improved electrochemical properties. ...
The electrochemical activation of Li 2 MnO 3 domains in Li- and Mn-rich layered oxides (LRLO) is highly important, and can be tuned by surface modification of the active materials to improve their cycling performance. In this study, citric acid was employed as a combined organic acid, reducing agent, and carbon precursor in order to remove surface residues from the calcination process, implement an oxygen deficient layer on the surface of the primary LRLO particles, and cover their surface with a carbon-containing coating after a final annealing step. A broad selection of bulk and surface sensitive characterization methods was used to characterize the post-treated spherical particles, providing the evidence for successful creation of an oxygen deficient near-surface region, covered by carbon-containing deposits. Post-treated materials show enhanced electrochemical discharge capacities after progressive Li 2 MnO 3 activation, reaching maximum capacities of 247 mAh g ⁻¹ . Gassing measurements reveal the suppression of oxygen release during the first cycle, concomitant with an increased CO 2 formation for the carbon-coated materials. The voltage profile analysis in combination with post-mortem characterization after 300 cycles provide insights into the aging of the treated materials, which underlines the importance of the relationship between structural changes during scalable post-treatment and the electrochemical performance of the powders.
... Compared with the pristine, the FCG-LLOs and FCG-LLOs-500 showed weaker Li 2 MnO 3 diffraction peak due to the reduced content of Mn element in the surface region. The distinct splitting of (0 0 6)/(0 1 2) and (0 1 8)/(1 1 0) diffraction peaks indicated the well-formed layered structure for all cathodes [41]. It's worth noting that the corresponding spinel phase diffraction peaks were not detected in the FCG-LLOs-500, due to the trace content of surface spinel phase. ...
Lithium-rich layered oxides (LLOs) are considered as the most promising candidate for the cathode of high energy density lithium-ion batteries. However, the poor cycle stability especially under high temperature is hindering its practical applications. Herein, a full concentration gradient LLO with spinel modification is designed and prepared. This synergistic strategy not only makes full use of high Ni content that improving the discharge voltage but also mitigates the detrimental influence of surface residual alkalis. The surface spinel modified cathode exhibits a higher initial coulombic efficiency of 87.52% with enhanced cycle stability at 55℃ (191.5mAh/g after 200 cycles at 1C), the average discharge voltage drop is also alleviated to 3.17 mV per cycle (at 55℃). Furthermore, it also shows enhanced thermal stability, in which the exothermic onset temperature rises from 265.380 to 295.221℃, and the thermal release decreases from 211.525 to 181.181J/g. This work proposes an integrated strategy to enhance the comprehensive performance of LLOs, thus shed a light on the way for its practical application.
... Obvious cracks can be seen on the NCM523 particle in Figure 7a, cracked particles may finally fragmented and no longer available for reversible insertion/ extraction of lithium ions, which results in capacity degradation. [39] In contrast, thanks to the graphene interlayer, it can be seen in Figure 7b that the NCM523 particles are still intact and no cracks exist after cycling, which demonstrates that the graphene interlayer can protect the cathode from structural failure during the cycling process. Figure 7c and 7d are the cyclic voltammetry (CV) of c-NCM and G@c-NCM for the first three cycles. ...
Interfacial side reaction mechanism between poly(propylene carbonate) solid polymer electrolyte (PPC‐SPE) and LiNi0.5Co0.2Mn0.3O2 cathode (c‐NCM) is investigated. Ni³⁺ and Co⁴⁺ species generated by electrochemical oxidization process can decompose poly(propylene carbonate) to aldehyde. To address this interface issue, a graphene interlayer is introduced to the LiNi0.5Co0.2Mn0.3O2 cathode surface via a facile method to improve cycle stability, rate capability and interfacial resistance. After 50 cycles at 0.3 C, the capacity retention of G@c‐NCM is 97.9 % and the resistance is less than 20 Ω, the improved electrochemical properties can be attributed to the graphene interlayer slows the side reaction, facilitates interfacial charge‐transfer process and stabilizes the cathode structure. These results demonstrate that modifying LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode surface with graphene interlayer is conducive to enhance the electrochemical performance of all‐solid‐state lithium batteries.
... Many studies have shown that ion doping [19][20][21] and surface coating [22][23][24] can suppress voltage decay of lithium-rich layered oxides. In addition, increasing the nickel content in the lithium-rich layered oxide can also significantly suppress the voltage decay, and the energy density is also improved [25][26][27]. ...
Lithium-rich layered oxide is one of the most promising candidates for the next-generation cathode materials of high-energy-density lithium ion batteries because of its high discharge capacity. However, it has the disadvantages of uneven composition, voltage decay, and poor rate capacity, which are closely related to the preparation method. Here, 0.5Li2MnO3·0.5LiMn0.8Ni0.1Co0.1O2 was successfully prepared by sol–gel and oxalate co-precipitation methods. A systematic analysis of the materials shows that the 0.5Li2MnO3·0.5LiMn0.8Ni0.1Co0.1O2 prepared by the oxalic acid co-precipitation method had the most stable layered structure and the best electrochemical performance. The initial discharge specific capacity was 261.6 mAh·g−1 at 0.05 C, and the discharge specific capacity was 138 mAh·g−1 at 5 C. The voltage decay was only 210 mV, and the capacity retention was 94.2% after 100 cycles at 1 C. The suppression of voltage decay can be attributed to the high nickel content and uniform element distribution. In addition, tightly packed porous spheres help to reduce lithium ion diffusion energy and improve the stability of the layered structure, thereby improving cycle stability and rate capacity. This conclusion provides a reference for designing high-energy-density lithium-ion batteries.
... Many have shown that ion doping [17][18][19] and surface coating [20,21] can suppress voltage decay of lithium-rich layered oxides. In addition, increasing the nickel content in the lithiumrich layered oxide can also significantly suppress the voltage decay, and the energy density is also improved [22][23][24]. ...
Lithium-rich layered oxides is one of the most perspective candidates for cathode materials of lithium ion battery, because of its high discharge capacity. However, there are some disadvantages of uneven composition, voltage decay, and poor rate capacity, which are closely related to the preparation method. Here, 0.5Li2MnO3·0.5LiMn0.8Ni0.1Co0.1O2 were successfully prepared by sol-gel and oxalate co-precipitation methods. A systematic analysis of the materials shows that the 0.5Li2MnO3·0.5LiMn0.8Ni0.1Co0.1O2 prepared by the oxalic acid co-precipitation method has the most stable layered structure and the best electrochemical performance. The initial discharge specific capacity is 261.6 mAh·g-1 at 0.05 C, and the discharge specific capacity is 138 mAh·g-1 at 5 C. The voltage decay is only 210 mV, and the capacity retention is 94.2% after 100 cycles at 1 C. The suppression of voltage decay can be attributed to the high nickel content and uniform element distribution. In addition, tightly packed porous spheres help to reduce lithium ion diffusion energy and improve the stability of the layered structure, thereby improving cycle stability and rate capacity. This conclusion provides a reference for designing high energy density lithium-ion batteries.
... Rechargeable lithium ion batteries (LIBs) ( Zheng et al., 2018;Zhang et al., 2018a,b) have been applied successfully to electric vehicles (EVs) ( Xu et al., 2015) and hybrid electric vehicles (HEVs) (Goodenough and Park, 2013;Choi and Aurbach, 2016;Chen et al., 2017), which brings a great convenience and reduces exhaust emissions. However, the unsatisfied performance of cathode materials in LIBs, limits its extensive application in EVs and HEVs in the future, including low energy density, poor cycling performance et al. ( Thackeray et al., 2012;Goodenough and Kim, 2014) Therefore, it is expected that the cycling performance and rate capability would be greatly improved. ...
Nickel-rich ternary layered oxide (LiNi0.80Co0.15Al0.05O2, LNCA) cathodes are favored in many fields such as electric vehicles due to its high specific capacity, low cost, and stable structure. However, LNCA cathode material still has the disadvantages of low initial coulombic efficiency, rate capability and poor cycle performance, which greatly restricts its commercial application. To overcome this barrier, a polypyrrole (PPy) layer with high electrical conductivity is designed to coat on the surface of LNCA cathode material. PPy coating layer on the surface of LNCA successfully is realized by means of liquid-phase chemical oxidation polymerization method, and which has been verified by the scanning electron microscopy (SEM), transmission electron microscope (TEM) and fourier transform infrared spectroscopy (FTIR). PPy-coated LNCA (PL-2) exhibits satisfactory electrochemical performances including high reversible capacity and excellent rate capability. Furthermore, the capability is superior to pristine LNCA. So, it provides a new structure of conductive polymer modified cathode materials with good property through a mild modification method.
... Nowadays, the vigorous development of lithium-ion batteries (LiBs) (Chen et al., 2017;Zhang et al., 2018a) has accelerated the production of energy storage devices ( Zhang et al., 2018b;Zheng et al., 2018), electric vehicles (EVs), and hybrid electric vehicles (HEVs) ( Terada et al., 2001;Goodenough and Park, 2013;Xiong et al., 2013Xiong et al., , 2014bXu et al., 2015b;Choi and Aurbach, 2016;Liu et al., 2018b;Su et al., 2018). However, unsuitable performance limits the application of LiBs cathode materials, such as low energy density of LiCO 2 and LiFeO 4 , and lithium- rich layered oxide (LRLO) cathode materials with low coulombic efficiency and voltage attenuation. ...
The high energy density lithium ion batteries are being pursued because of their extensive application in electric vehicles with a large mileage and storage energy station with a long life. So, increasing the charge voltage becomes a strategy to improve the energy density. But it brings some harmful to the structural stability. In order to find the equilibrium between capacity and structure stability, the K and Cl co-doped LiNi0.5Co0.2Mn0.3O2 (NCM) cathode materials are designed based on defect theory, and prepared by solid state reaction. The structure is investigated by means of X-ray diffraction (XRD), rietveld refinements, scanning electron microscope (SEM), XPS, EDS mapping and transmission electron microscope (TEM). Electrochemical properties are measured through electrochemical impedance spectroscopy (EIS), cyclic voltammogram curves (CV), charge/discharge tests. The results of XRD, EDS mapping, and XPS show that K and Cl are successfully incorporated into the lattice of NCM cathode materials. Rietveld refinements along with TEM analysis manifest K and Cl co-doping can effectively reduce cation mixing and make the layered structure more complete. After 100 cycles at 1 C, the K and Cl co-doped NCM retains a more integrated layered structure compared to the pristine NCM. It indicates the co-doping can effectively strengthen the layer structure and suppress the phase transition to some degree during repeated charge and discharge process. Through CV curves, it can be found that K and Cl co-doping can weaken the electrode polarization and improve the electrochemical performance. Electrochemical tests show that the discharge capacity of Li0.99K0.01(Ni0.5Co0.3Mn0.2)O1.99Cl0.01 (KCl-NCM) are far higher than NCM at 5 C, and capacity retention reaches 78.1% after 100 cycles at 1 C. EIS measurement indicates that doping K and Cl contributes to the better lithium ion diffusion and the lower charge transfer resistance.
... Among them, surface coating is an effective and simple way to improve the electrochemical performance and stabilize the interface between electrode materials and electrolyte [20,21]. A large number of compounds have been investigated as the coating materials such as fluorides [22,23], phosphates [24,25], and metal oxides [26,27]. However, these surface protection layers are largely semiconductive/insulating materials or have inferior Li + conductivity, leading to reduced reversible capacity or rate stability of Li-rich layer oxides. ...
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.
Li‐rich layered oxides (LLOs) have been considered as the most promising cathode materials for achieving high energy density Li‐ion batteries. However, they suffer from continuous voltage decay during cycling, which seriously shortens the lifespan of the battery in practical applications. This review comprehensively elaborates and summarizes the state‐of‐the‐art of the research in this field. It is started from the proposed mechanism of voltage decay that refers to the phase transition, microscopic defects, and oxygen redox or release. Furthermore, several strategies to mitigate the voltage decay of LLOs from different scales, such as surface modification, elemental doping, regulation of components, control of defect, and morphology design are summarized. Finally, a systematic outlook on the real root of voltage decay is provided, and more importantly, a potential solution to voltage recovery from electrochemistry. Based on this progress, some effective strategies with multiple scales will be feasible to create the conditions for their commercialization in the future.
Due to their high specific capacities beyond 250 mA h g⁻¹, lithium-rich oxides have been considered as promising cathodes for the next generation power batteries, bridging the capacity gap between traditional layered-oxide based lithium-ion batteries and future lithium metal batteries such as lithium sulfur and lithium air batteries. However, the practical application of Li-rich oxides has been hindered by formidable challenges. To address these challenges, the understanding of their electrochemical behaviors becomes critical and is expected to offer effective guidance for both materials and cell development. This review aims to provide fundamental insights into the reaction mechanisms, electrochemical challenges and modification strategies of lithium-rich oxides. We first summarize the research history, the pristine structures, and the classification of lithium-rich oxides. Then we review the critical reaction mechanisms that are closely related to their electrochemical features and performances, such as lattice oxygen oxidation, oxygen vacancy formation, transition-metal migration, layered to spinel transitions, ‘two-phase mechanism’, and lattice evolution. These discussions are coupled with state-of-the-art characterization techniques. As a comparison, the anionic redox reactions of layered sodium transition metal oxides are also discussed. Finally, after a brief overview of the correlation among the aforementioned mechanisms, we provide perspectives on the rational design of lithium-rich oxides with high energy densities and long-term cycling stability.
Background
Selective catalytic reduction of nitrogen oxides with ammonia (NH3‐SCR) is an effective method for the nitrogen oxides (NOx) removal. This study aimed to develop a highly efficient NH3‐SCR catalyst using the naturally available Montmonrillonite (Mt) clay as the catalyst support.
Results
Mt was processed into PILC by Al/Zr intercalations, impregnated with MnCe mixed active components and loaded with transition metals to achieve X‐AlZr‐PILC‐MnCe (X = V, W, Co or Mo) catalysts. The optimized Mo‐AlZr‐PILC‐MnCe showed the best NH3‐SCR performance with a NO conversion rate of nearly 100% in the temperature range of 240–300 °C, and N2 selectivity of above 85% in the range of 140–300 °C.
Conclusions
The catalysts were characterized by SEM, BET, XRD, XPS, H2‐TPR, NH3‐TPD to define the relationships between structure, chemical properties and catalytic activity. The enhanced catalytic performance of Mo‐AlZr‐PILC‐MnCe was mainly owing to the increased acidity and better redox ability. In situ DRIFT studies indicated the deposition of Mo changed the reaction from the E‐R to the L‐H mechanism. © 2020 Society of Chemical Industry
Voltage fade significantly hinders the practical use of Li-rich Mn-based layered oxides (LLOs) as cathode materials for next-generation high-energy-density Li-ion batteries. Therefore, an in-depth understanding of the factors influencing the LLO voltage fade during cycling is fundamentally important for tailoring the structure and thus improving the electrochemical performance of the corresponding electrodes. Herein, we compare the electrochemical performances of LLOs with different particle size and conduct in situ high-pressure response measurements to determine the effects of particle size on voltage fade, demonstrating that small particles can undergo a reversible layer-to-spinel phase transition that results in improved voltage stability during cycling. The above finding provides a novel paradigm for the development of high-capacity LLO electrodes and thus contributes to the establishment of a more energy-efficient and green society.
The typical co-precipitation method was adopted to synthesized the Li-excess Li1.20[Mn0.52-xZrxNi0.20Co0.08]O2(x = 0, 0.01, 0.02, 0.03) series cathode materials. The influences of Zr4+doping modification on the microstructure and micromorphology of Li1.20[Mn0.52Ni0.20Co0.08]O2cathode materials were studied intensively by the combinations of XRD, SEM, LPS and XPS. Besides, after the doping modification with zirconium ions, Li1.20[Mn0.52Ni0.20Co0.08]O2cathode demonstrated the lower cation mixing, superior cycling performance and higher rate capacities. Among the four cathode materials, the Li1.20[Mn0.50Zr0.02Ni0.20Co0.08]O2exhibited the prime electrochemical properties with a capacity retention of 88.7% (201.0 mAh g-1) after 100 cycles at 45 °C and a discharge capacity of 114.7 mAh g-1at 2 C rate. The EIS results showed that the Zr4+doping modification can relieve the thickening of SEI films on the surface of cathode and accelerate the Li+diffusion rate during the charge and discharge process.
In the quest to tackle the issue of surface degradation and voltage decay associated with Li-rich phases, Li-ion conductive Li2ZrO3 (LZO) is coated on Li1.2Ni0.13Mn0.54Co0.13O2 (LNMC) by a simple wet chemical process. The LZO phase coated on LNMC, with a thickness of about 10 nm, provides a structural integrity and facilitates the ion pathways throughout the charge–discharge process, which results in significant improvement of the electrochemical performances. The surface-modified cathode material exhibits a reversible capacity of 225 mA h g–1 (at C/5 rate) and retains 85% of the initial capacity after 100 cycles. Whereas, the uncoated pristine sample shows a capacity of 234 mA h g–1 and retains only 57% of the initial capacity under identical conditions. Electrochemical impedance spectroscopy reveals that the LZO coating plays a vital role in stabilizing the interface between the electrode and electrolyte during cycling; thus, it alleviates material degradation and voltage fading and ameliorates the electrochemical performance.
Different amounts of (NH4)3AlF6 (1, 3, and 6 wt%) are successfully coated on the surface of the layered lithium-rich cathode Li[Li0.2Ni0.2Mn0.6]O2 using a wet coating method. The morphology and structure of the as-prepared materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). It is confirmed that the (NH4)3AlF6 was uniformly coated onto the surface of the Li[Li0.2Ni0.2Mn0.6]O2. The electrochemical performance of the coated materials at room temperature and 50 °C is investigated systematically. The material coated with 3 wt% (NH4)3AlF6 exhibits the highest reversible capacity of 220.3 mA h g⁻¹ (0.2C, 50 cycles) as well as the best cycling performance with a capacity retention of 83.4% (0.2C, 50 cycles), attributed to the suppression of unexpected surface side reactions by the protective layer of (NH4)3AlF6. Electrochemical impedance spectroscopy (EIS) analysis reveals that the lower charge transfer resistance of the coated sample may contribute to its excellent rate capability. Furthermore, the coated sample also shows enhanced cycling performance at elevated temperature owing to an improved thermal stability, confirmed by differential scanning calorimetry (DSC).
Li- and Mn-rich transition-metal oxides of layered structure are promising cathodes for Li-ion batteries because of their high capacity values, ?250 mAh g(-1). These cathodes suffer from capacity fading and discharge voltage decay upon prolonged cycling to potential higher than 4.5 V. Most of these Li- and Mn-rich cathodes contain Ni in a 2+ oxidation state. The fine details of the composition of these materials may be critically important in determining their performance. In the present study, we used Li1.2Ni0.13Mn0.54Co0.13O2 as the reference cathode composition in which Mn ions are substituted by Ni ions so that their average oxidation state in Li1.2Ni0.27Mn0.4Co0.13O2 could change from 2+ to 3+. Upon substitution of Mn with Ni, the specific capacity decreases but, in turn, an impressive stability was gained, about 95% capacity retention after 150 cycles, compared to 77% capacity retention for Li1.2Ni0.13Mn0.54Co0.13O2 cathodes when cycled at a C/5 rate. Also, a higher average discharge voltage of 3.7 V is obtained for Li1.2Ni0.27Mn0.4Co0.13O2 cathodes, which decreases to 3.5 V after 150 cycles, while the voltage fading of cathodes comprising the reference material is more pronounced. The Li1.2Ni0.27Mn0.4Co0.13O2 cathodes also demonstrate higher rate capability compared to the reference Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. These results clearly indicate the importance of the fine composition of cathode materials containing the five elements Li, Mn, Ni, Co, and O. The present study should encourage rigorous optimization efforts related to the fine composition of these cathode materials, before external means such as doping and coating are applied.
This chapter gives an overview over preparation, properties, and applications of alumina thin films. The preparation of polymeric and colloidal alumina sols and their deposition on diverse substrate materials with different geometries are described first. Consideration of properties (surface tension, particle size, viscosity, aging of the sols, and their optimization by using certain additives) is included, too. A description of posttreatments of the coatings, which initiate phase transformations and also determine their microstructure, completes the part on coating preparation. Data on adhesion, thickness, surface roughness, and mechanical, textural as well as optical properties of alumina thin films are provided in a further paragraph. In a third main part, experience concerning possible applications, such as (i) membranes for catalysis, separation, and filtration, (ii) barrier layers against electrochemical and high-temperature corrosion or interdiffusion of ions between different phases, (iii) mechanical protection, (iv) sensor construction, (v) improvement of optical properties, as well as (vi) achieving water repellency, is summarized.
The surface of as-prepared LiMn2O4 was modified with Al2O3 by a melting impregnation method. X-ray diffraction, field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) studies indicated that Al2O3 nano-particles are distributed around the spinel. X ray absorption fine structure analysis (XAFS) further demonstrated that Al atoms were also doped to the surface of LiMn2O4 particles. The nano-Al2O3 particle coating improves the capacity retention of spinel LiMn2O4 efficiently at both room temperature and 55°C. The mechanism of improvement for surface modified LiMn2O4 can be attributed to the inhibition of a surface Jahn-Teller distortion and the decrease of manganese dissolution, leading to good electric contact among particles.
Li(y)(Ni(0.425)Mn(0.425)CO(0.15))(0.88)O(2) materials were synthesized by a slow rate electrochemical deintercalation from Li(1.12)(Ni(0.425)Mn(0.425)CO(0.15))(0.88)O(2) during the first charge and the first discharge in order to study the structural modifications occurring during the first cycle and especially during the irreversible "plateau" observed in charge at 4.5 V vs Li(+)/Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the "plateau". Redox titrations revealed that the average transition metal oxidation state is almost constant during the "plateau", despite further lithium ion deintercalation. (1)H MAS NMR data showed that no Li(+)/H(+) exchange was associated to the "plateau" itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the "plateau", as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNi(x)Li((1/3-2x/3)) Mn((2/3-x/3))O(2) materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the "plateau" due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.
The results of the Japanese national project of R&D on large-size lithium rechargeable batteries by Lithium Battery Energy Storage Technology Research Association (LIBES), as of fiscal year (FY) 2000 are reviewed. Based on the results of 10 Wh-class cell development in Phase I, the program of Phase II aims at further improvement of the performance of large-size cells and battery modules, and the formulation of roadmaps toward worldwide dissemination of large-size lithium secondary batteries. In addition to the above R&D programs, a new target was presented particularly for the near-term practical application of several kWh-class battery modules in FY 1998.For the large-size battery modules, two types of 2 and 3 kWh-class battery modules have been developed for stationary device and electric vehicle applications, respectively. The battery modules for both types have achieved most of the targets other than cycle life. Currently, further improvements in the cycle life of the cells themselves are being pursued. For this purpose, the materials for cathodes and anodes, the shapes and structures for batteries and the methods for cell connection are being re-investigated.The development of middle-size battery systems for mini-size electric vehicles (EVs), as well as for demand-side stationary device applications is under way. These battery systems have been fabricated and their fundamental performance confirmed. They are now being subjected to field tests.
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