Lithium-rich Layered Materials Research Articles

Concentration-gradient of Li-rich Mn-based cathode materials with enhanced cycling retention

Lithium-rich layered Mn-based materials have attracted much attention because of their high specific capacity and high energy density, but they have the characteristics of irreversible capacity loss, low initial coulombic efficiency, and poor cycling performance. In this study, a core-shell structure, including Mn0.75Ni0.25C2O4 precursor as core and Li-rich Mn-based oxides (LRMO, Li1.2Mn0.54Ni0.13Co0.13O2) as shell layer respectively, was synthesized by co-precipitation and sol-gel method. After sintering at high temperatures, the element concentration-gradient oxides material (LRMO-G) was formed. The material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) and its electrochemical performance was investigated by various electrochemical methods. The LRMO-G material exhibits a high initial discharge specific capacity of 268 mAh/g at 0.1 C, and extremely superior capacity retention (95.4 %) after 100 cycles at 1 C. The initial coulomb efficiency of the LRMO-G material increases to 85 %, which is 12 % higher than that of LRMO.

Layered O6/O3 Multi-Phase Composite Derived from the P2/O3 Nanocrystalline Composite for High-Performance Li-Rich Manganese-Based Cathode Material

As a result of the redox reaction of lattice oxygen occurring at 4.5 V, the Li-rich manganese cathode material with an O3 configuration has a theoretically high discharge capacity (>250 mA h/g). However, it also suffers a structural transition from layered structure to spinel structure in the cycling process, leading to severe voltage decay and capacity fading. On contrary, the O6 type material obtained via ion exchange of P2 type material can effectively inhibit the phase transition and stabilize the layered structure. Hence, in this work, we design an O6/O3 multi-phase composite material after ion exchange of the P2/O3 nanocrystalline composite. Compared with the O3 type cathode, the as-obtained cathode composite exhibits a higher initial discharge capacity (≈265 mA h/g at 20 mA/g) and an excellent capacity retention (≈87% after 100 cycles). Moreover, the first cycle Coulombic efficiency increases by nearly 10%, and voltage attenuation is effectively inhibited. Our findings demonstrate that modulating the O6/O3 configuration is a practical and simple methodology to promote the electrochemical performance of lithium-rich layered materials.

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

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

Cationic disordering modulated electrochemical performances of layer-structured Li2MoO3

Due to the intervolved oxygen loss and cation disordering upon lithium extraction at high potentials, it is difficult to recognize the reason for the performance decay in the conventional lithium-rich layered oxides (LLOs). Herein, we chose Li2MoO3 as a prototype lithium-rich layered material to understand the effect of cationic disordering upon delithiation. On the basis of the density functional theory (DFT) calculations and performance characterization, we clarify that the local disorder structures of Li2MoO3 modulate the sequence of Li extraction from different sites, which influences the properties of the material, such as capacity, charge compensation and electric conduction. These findings provide profound views in understanding the performance decay and structural design of LLOs for the Li-ion batteries.

1,4-Dicyanobenzene as electrolyte additive for improve electrochemical performance of Li1.2Ni0.2Mn0.6O2 cathode materials in lithium metal rechargeable batteries

The cycling stability of Li1.2Ni0.2Mn0.6O2 cathodes is increased enormously by adding 0.1 %wt 1,4-dicyanobenzene (1,4-DCB) additive in the electrolyte, and the capacity retention increases from 20.3% to 63.8% after 350 cycles. It is found that 1,4-DCB eliminates the trace water in the electrolyte via hydrolysis under acidic conditions. A coating layer on the electrode surface is formed by preferentially oxidized of 1,4-DCB, which reduce the contact between electrolyte and cathode, inhibiting the decomposition of electrolyte solvent. N group in the electrooxidation product of 1,4-DCB can combine with PF5 to improve the stability of LiPF6, then reducing the decomposition of LiPF6. The benzene ring and conjugated bond of 1,4-DCB are favorable for the transfer of interfacial electron and ion, and then the electrochemical performance of lithium-rich layered materials is improved.

Improving the Diffusion of Lithium and Enhancing the Electrochemical Performance of Layered Li1.2Mn0.54Ni0.13Co0.13O2 Cathode by YPO4 Coating

Because of its high energy density, the manganese-rich lithium layered material is a potential cathode material for power batteries. In order to effectively strengthen the comprehensive performance of lithium-rich layered material, different amounts of YPO4 compounds were used to wet chemically coat the cathode materials. The LMNCO, 0.5-YPO4, 1.0-YO4 and 3.0-YPO4 cathode materials are tested by XRD, SEM, TEM, XPS, constant current charge and discharge test, and EIS. Facts show that the positive electrode material coated with YPO4 has better cycle performance and rate performance. The 1.0-YPO4 sample material has the best performance. After 50 cycles of 0.5 C, its discharge capacity can still be as high as 207.4 mAhg-1.The comprehensive performance of the cathode material is improved after being coated with YPO4, which is owing to the coating of YPO4. The YPO4 layer weakens the electron transfer resistance, and suppress the phase transformation of the Electrode material and the formation of the SEI film.

Li1.2Mn0.6Ni0.2O2 Cathode Material Prepared by the Ultrasonic Dispersionassisted Method

Background: Lithium-rich layered materials with high discharge capacity are regarded as one of the most promising cathodes for lithium-ion batteries (LIBs). However, they have been suffering from rapid voltage fading and poor rate performance, which impede their practical application. Methods: Herein, Li1.2Mn0.6Ni0.2O2 with layered structure was successfully prepared by the ultrasonic dispersion-assisted chemical reduction. X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical measurements were used to characterize its microstructure and electrochemical properties. Results: The secondary particles of an as-prepared micro/nanostructured sample consist of irregular and sheet-like rectangular blocks. Electrochemical results show that the initial charge and discharge capacity within 2.0~4.8 V is 337.5 mA h g-1 and 236.9 mA h g-1 at 0.2C (1C = 200 mA g-1). The subsequent discharge capacity is stabilized at about 210 mA h g-1 for more than 100 cycles. When the current density is increased to 2C, the cycling columbic efficiency is maintained at 99.3% after 100 cycles. Conclusion: Thus, the Li1.2Mn0.6Ni0.2O2 cathode material prepared by ultrasonic dispersion-assisted chemical reduction has a promising application in LIBs with high energy density and long cycle life.

(Invited) Elucidation of Electrochemical Reactions in Li2MnO3 Using Thin-Film Solid-State Battery

All-solid-state is an ideal form of batteries. The all-solid-state batteries are also suitable system for elucidating battery reaction using spectroscopic methods. The present study focuses on the reaction mechanism study of lithium excess cathode materials using all-solid-state batteries. The lithium manganese oxide, Li2MnO3, is a model system of the lithium excess layered rock-salt structure. In the present study, the epitaxial and polycrystalline films with 30 nm-thick with various compositions and morphologies were fabricated using pulsed laser deposition and sputtering methods. These electrodes showed charge-discharge capacity ranging from 150 to 300 mAh g-1 with different degradation characteristics depending on the compositions and morphologies. The reaction mechanisms were studied by in-situ and operando methods using surface X-ray diffraction and operando HAXPES analyses. The Li2MnO3 shows a transformation to a high-capacity phase after activation process by the first charge and the degradation during subsequent cycling depends on the sample stoichiometry and morphology. The transition to the high capacity phase is related the phase change from O3 to O1 type, together with interlayer cation disordering on the Li/Mn layer. The activation process is related to the oxidation of oxygen and a formation new oxygen species. The activation process and the reversible reactions of the lithium-rich layered rock-salt materials are elucidated.

Li1.2Ni0.25Mn0.55O2: A high-capacity cathode material with a homogeneous monoclinic Li2MnO3-like superstructure

High-energy lithium-rich and manganese-based layered materials are one of the most promising candidates for the cathode materials of lithium-ion batteries. These cathode materials can deliver anomalously high capacity due to multiple electron transfers involving both cationic and anionic redox processes. An unprecedented homogeneous monoclinic Li2MnO3-like structured material Li1.2Ni0.25Mn0.55O2, which with the highest nickel content among the Li-rich materials, is first disclosed in this work. Our proposed Li1.2Ni0.25Mn0.55O2 material can deliver a reversible capacity exceeding 295 mAh·g−1 with a high and stable discharge voltage. It is especially encouraging that the as-prepared Li1.2Ni0.25Mn0.55O2 cathode material displays high charge-discharge efficiency and good capacity retention during cycling. In this work, we applied in-situ EIS and ex-situ XPS methods to systematically investigate the reversible structural evolution of Li1.2Ni0.25Mn0.55O2 cathode material and cathode-electrolyte interfaces (CEI). These findings will shed light on the design of high-performance cathode materials for lithium-ion batteries.

Voltage decay and redox asymmetry mitigation by reversible cation migration in lithium-rich layered oxide electrodes.

Despite the high energy density of lithium-rich layered-oxide electrodes, their real-world implementation in batteries is hindered by the substantial voltage decay on cycling. This voltage decay is widely accepted to mainly originate from progressive structural rearrangements involving irreversible transition-metal migration. As prevention of this spontaneous cation migration has proven difficult, a paradigm shift toward management of its reversibility is needed. Herein, we demonstrate that the reversibility of the cation migration of lithium-rich nickel manganese oxides can be remarkably improved by altering the oxygen stacking sequences in the layered structure and thereby dramatically reducing the voltage decay. The preeminent intra-cycle reversibility of the cation migration is experimentally visualized, and first-principles calculations reveal that an O2-type structure restricts the movements of transition metals within the Li layer, which effectively streamlines the returning migration path of the transition metals. Furthermore, we propose that the enhanced reversibility mitigates the asymmetry of the anionic redox in conventional lithium-rich electrodes, promoting the high-potential anionic reduction, thereby reducing the subsequent voltage hysteresis. Our findings demonstrate that regulating the reversibility of the cation migration is a practical strategy to reduce voltage decay and hysteresis in lithium-rich layered materials.

(Invited) Reaction Mechanism of Lithium Excess Layered Manganese Oxides Studied By Thin-Film Model Electrodes and Operando Measurements

The reaction mechanism of lithium excess manganese oxide, Li2MnO3, with layered rock-salt structure was examined using model thin film electrodes with liquid or solid electrolytes. The epitaxial or polycrystalline films with 30 nm-thick with various compositions and morphologies were fabricated using pulsed laser deposition and sputtering methods. These electrodes showed charge-discharge capacity ranging from 150 to 300 mAh g-1with different degradation characteristics depending on the compositions and morphologies. The reaction mechanisms were studied by in-situ and operando methods using surface X-ray diffraction and HAXPES analyses. The Li2MnO3shows a transformation to a high-capacity phase after the activation process by the first charge and the degradation during the subsequent cycling depends on the sample stoichiometry and morphology. The transition to the high capacity phase is related the phase change from the O3 to O2 type, together with the interlayer cation disordering on the Li/Mn layer. The activation process is related to the oxidation of oxygen and a formation new oxygen species together with the reduction of the manganese from tetra-valent state. The activation process and the reversible reactions of the lithium-rich layered rock-salt materials are elucidated.

Li-Rich Layered/Spinel Cathode Composite 3/4[Li2MnO3·LiCxO2]·1/4[LiCxMnO4] (Cx = Cr1-yCoy) for Li-Ion Batteries

A new series of Li-rich layered/spinel Li-Cr-Co-Mn-O composites are designed and investigated in this article. The high voltage spinel LiCxMnO4 (Cx = Cr1-yCoy) materials are embedded into lithium-rich layered materials and these Li-rich layered/spinel integrated structures exhibit both superior reversible capacities and high rate capabilities as cathode materials for lithium-ion batteries. The Li-rich layered/spinel 3/4[Li2MnO3·LiCxO2]·1/4[LiCxMnO4] (Cx = Cr1-yCoy) composites are synthesized with sol-gel method. XRD, HRTEM and fast Fourier transform (FFT) analysis demonstrate the integrated structure of Li-rich layered component and spinel phase. The electrochemical investigations performed within a wide potential window show an excellent result of the materials with the proper ratio between Co and Cr. All the composite materials exhibit initial discharge capacities over 200 mAh g−1. In particular, y = 0.4 and 0.6 samples show preferable cycling stability among the investigated materials, which is primarily attributed to the high reversible capacity of Li-rich layered component and 3D Li-ion diffusion channels of spinel phase, as well as low charge transfer resistance and Warburg diffusion impedance owing to the synergistic effect of the proper ratio between Co and Cr elements in these composites.

Mitigating voltage decay in high-capacity Li1.2Ni0.2Mn0.6O2 cathode material by surface K+ doping

The lithium-rich layered materials have received much attention as high-energy-density cathodes to cater for future lithium-ion batteries. However, the growth of detrimental spinel configuration upon cycling can destroy the stability of structures and become the significant challenge for their commercialization. In view of previous research of alkali metal doping, we have successfully prepared the surface K+ doped Li1.2Ni0.2Mn0.6O2, which delivers an outstanding cycling durability with 99.96% of fully activated capacity (260 mAh g−1) even after 100 cycles. Moreover, the K+ doped material displays less voltage fading, demonstrating the alleviation of structure transformation from layered toward spinel phase. The effects of K+ doping were further investigated by the soft X-ray absorption spectroscopy. The Mn L-edges and the O K-edge prove that the doped K+ may play an indispensable role in stabilizing the surface O22− and facilitating the reduction of Mn3+ ion with undesirable Jahn-Teller effect towards lower valence Mn2+.

Trimethylsilyl (trimethylsiloxy) acetate as a novel electrolyte additive for improvement of electrochemical performance of lithium-rich Li1.2Ni0.2Mn0.6O2 cathode in lithium-ion batteries

Lithium-rich layered metal oxide electrode materials suffer from severe capacity fading and poor rate performance in lithium-ion batteries. Herein, we demonstrate that trimethylsilyl (trimethylsiloxy) acetate (bis-TMSA) can be used as a novel electrolyte additive to improve the electrochemical performance of Li1.2Ni0.2Mn0.6O2 cathode (LLC). The addition of 1% bis-TMSA in the electrolyte results in a large increase in the utilization efficiency of the electrode active material. Moreover, LLC in the bis-TMSA containing electrolyte can still deliver a discharge capacity of 136.97 mAh g−1 with capacity retention of 80.1% after 374 cycles at 1C, showing a superior cyclability. An enhancement in the rate capability of the material is also observed in the presence of bis-TMSA in the electrolyte. The effect of trace amount of water in the electrolyte on has been critical to the battery cycling performance. Removal of water in the electrolyte via hydrolysis of bis-TMSA leads to suppressed decomposition of electrolyte and less corrosion of electrode active material and current collector. Thinner SEI film with less F-containing species forms on the electrode surface and better contact between electrode material and current collector are responsible for the improved cyclability and rate capability for LLC in the bis-TMSA containing electrolyte.

A facile strategy to enhance the stability of Li-rich cathode: Electrochemical performance improvement and mechanism exploration

Lithium-rich layered materials are promising candidates for next generation Li-ion batteries (LIBs) due to high energy densities, which still, however, suffer low initial Coulombic efficiency (i.e. irreversible initial capacity loss), poor rate capability and capacity retention during cycling. In this work, Li+ conductive Li2SnO3 (LSO) is selected to modify the surface of Li1.2Mn0.56Ni0.17Co0.07O2 (LMNCO) without influencing its bulk structure. Electrochemical characterizations indicate that the obtained LSO@LMNCO electrode shows greatly improved cycling stability and rate capability compared with those of the pristine LMNCO. Intensive aging experiment reveals that the LSO modification layer plays a critical role in performance enhancement, which not only stabilizes the bulk structure of LMNCO in long-term storage/cycling via suppression of metal oxide dissolution, but also benefits the surface kinetics by growth inhibition of solid electrolyte interface (SEI) layer on the particle surface.

Development of Long Life and Low Cost Li-Rich Layered Oxide Positive Active Material for xEVs

Lithium-rich layered oxides have drawn much attention as a positive active material in the future automotive applications because of a high gravimetric capacity of more than 250 mAh g-1,1 which make the high energy density LiBs. However, this positive active material suffer from several problems such as low initial coulombic efficiency, poor capacity retention and voltage fading upon charge/discharge cycling.2 While a lot of efforts have been made to mitigate these problems, the problems have not sufficiently improved. This situation makes the industrial application difficult. According to previous works, a transition metal composition (Ni/Co/Mn ratio) and a Li/Metal ratio have much influence on electrochemical properties of this positive active material.3-5 However the specific chemical composition of this positive active material has not been appeared to show the high discharge capacity, the high initial coulombic efficiency and good charge/discharge cycle life performance. In this work, the various compositions of lithium-rich layered oxide positive active materials were prepared with a co-precipitation process and a calcination process. The electrochemical properties of these materials were investigated. The relation between compositions and electrochemical properties of these positive active materials were discussed. The twenty different kinds of transition metal compositions carbonate precursors were prepared by a co-precipitation method. The lithium-rich layered oxide positive active materials were made from these carbonate precursors with the different Li/Me ratio and the different calcination temperature. The materials were characterized by means of X-ray diffraction and inductively coupled atomic plasma emission spectroscopy measurements. The materials showing the high discharge capacity were chosen in each transition metal composition. The relation between the composition and electrochemical properties of these materials was organized. Then several compositions were picked up to evaluate the charge/discharge cycle life performance by means of the half-cell with a metallic lithium counter electrode. The material showing the high energy density and good charge/discharge cycle life performance was evaluated the long-term charge/discharge cycle life performance for the cell with graphite anode. Various compositions of lithium-rich layered oxide positive active materials successfully synthesized. The chemicla compositions showed the specific relation with the electrochemical properties. Discharge capacities and energy densities showed the local maximum value in the relation to the x Li2MnO3 - (1-x) LiTMO2 (TM = transition metal). On the other hand, the average potentials showed gradually decreased with the increasing the x in x Li2MnO3 - (1-x) LiTMO2. There was the threshold value of x that discharge capacity retentions suddenly decreased in the relation to the x Li2MnO3 - (1-x) LiTMO2. The average potentials retention showed gradually decreased with the increasing the x in x Li2MnO3 - (1-x) LiTMO2. These results suggest that the chemical composition is extremely important for electrochemical properties of this positive active material. The critical chemical composition has been also found out to show the high discharge capacity (> 260 mAh g-1), the high initial coulombic efficiency (> 90 %) and excellent charge/discharge cycle life performance as well as high energy density (Figure 1). Energy density retention is 78 % after 1000 cycles with 4.6-2.0 V at 1 I tA. This critical composition is based on the hypothesis that valences of Ni and Mn should be only 2+ and 4+, and the ratio of Li2MnO3 and LiTMO2 is predetermined value in order to show stable crystal structure. Moreover, this is composed of less cobalt, so that this has to be one of the promising candidates for xEV-use LiBs. Reference M. S. Whittingham, Chemical Reviews, 104, 4271-4301 (2004).J. R. Croy et al., J. Phys. Chem. C, 117, 6525–6536 (2013).Z. Lu et al., J. Electrochem. Soc., 149, A778-A791 (2002).G. Y. Kim et al., Materials Research Bulletin, 43, 3543-3552 (2008).Q. C. Zhang et al., J. Power Sources, 250, 40-49 (2014). Caption (a) The initial charge/discharge profile of the critical chemical composition lithium-rich layered oxide positive active material for the cell with metallic lithium counter electrode. Charge: CC-CV 0.1 I tA 4.7 V (vs. Li/Li+), 0.02 I tA cut-off at 25 °C. Discharge: CC 0.1 I tA 2.0 V (vs. Li/Li+) at 25 °C. (b) The cycle performance of the critical chemical composition lithium-rich layered oxide positive active material for the cell with graphite anode. The energy density means the value that calculated the cell energy in a gram of positive active material. Cycling condition Charge: CC-CV 0.5 I tA 4.6 V, 0.1 I tA cut-off at 25 °C. Discharge: CC 1 I tA 2.0 V at 25 °C. Capacity check every 50 cycles Charge: CC-CV 0.1 I tA 4.6 V, 0.05 I tA cut-off at 25 °C. Discharge: CC 0.1 I tA 2.0 V at 25 °C. Figure 1

Conformal spinel/layered heterostructures of Co3O4 shells grown on single-crystal Li-rich nanoplates for high-performance lithium-ion batteries

Lithium-rich layered materials have received much attention because of their high specific capacity and high energy density. Unfortunately, they suffer from irreversible capacity loss, low initial Coulombic efficiency and poor cyclability. Here we report a facile co-precipitation method to synthesize uniform single-crystal Li-rich Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoplates without using any template. Subsequently, a Co3O4 shell is in situ grown on the Li-rich nanoplates through a hydrothermal method, leading to spinel/layered heterostructures. The electrode made of conformal heterostructured Li-rich/Co3O4 nanoplates delivers a high discharge capacity of 296 mA h g−1 at 0.1 C with an initial Coulombic efficiency of 84%. The capacity retention reaches 83.2% with a discharge capacity of 223 mA h g−1 after 160 cycles at 0.2 C during the potential window ranging from 2.0 to 4.8 V. The enhanced electrochemical performance of the resulting Li-rich/Co3O4 nanoplates benefits from the unique conformal heterostructure as well as the electrochemically active LixCoOy generated between the reaction of Co3O4 shells and the extracted Li2O during charging/discharging processes.

Surface modification of cathode material 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 by alumina for lithium-ion batteries

In an attempt to overcome the irreversible capacity loss occurred during the first cycle and stabilize the surface structure, an alumina coating layer has been triumphantly prepared on the surface of 0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2 cathode material with different amounts (1, 2, and 3 wt%) through a simple hydrolysis reaction, followed by an annealing process. The results reveal that the coated materials have a higher crystallinity and the particles are evenly distributed. As a cathode material for lithium-ion batteries, the 2-wt% coated sample delivers initial discharge specific capacity of 211.7 mAh g−1 at a rate of 1 C between 2.0 and 4.8 V with an initial columbic efficiency of 73.2%. Meanwhile, it exhibits the highest discharge specific capacity of 206.2 mAh g−1 with 97.4% capacity retention after 100 cycles at and much elevated rate capability compared to uncoated material. The excellent cycling stability and more superior rate property can be ascribed to alumina coating layer, which has a surface stabilization effect on these cathode materials, lessening the dissolution of metal ions. The electrochemical impedance and cyclic voltammetry studies indicate that coated by alumina improved the kinetic performance for lithium-rich layered materials, showing a prospect for practical lithium battery application.

Suppressing the Voltage Fading of Li[Li0.2Ni0.13Co0.13Mn0.54]O2 Cathode Material via Al2O3 Coating for Li-Ion Batteries

Lithium-rich layered material with the structure xLi2MnO3∙(1–x)LiMO2 (M = MnyNizCo1-y-z, 0≤y≤1, 0≤z≤1, 0≤y+z≤1) is a highly potential cathode candidate for lithium-ion batteries. However, the fading voltage during cycling has seriously hindered the further development of this material. In this study, Li[Li0.2Ni0.13Co0.13Mn0.54]O2 is synthesized through a co-precipitation method and modified by surface coating with Al2O3 layer via sol-gel method. Discharge voltage fading is observed and investigated. Results show that the discharge voltage fading of the pristine material is 0.33 V after 30 cycles, compared to 0.15 V of the Al2O3-coated sample. The average discharge voltage of the Al2O3-coated material is improved, but its deterioration rate is suppressed effectively during the charge/discharge cycling. The intrinsic mechanism is investigated by ex situ XRD, XPS, TEM, CV, and EIS analysis. Results indicate that the lower discharge voltage fading should be ascribed to less structural change, lighter grain coarsening, and smaller impedance variation of the Li-rich layered material after Al2O3 coating.

Hierarchical Li1.2Mn0.54Ni0.13Co0.13O2 hollow spherical as cathode material for Li-ion battery

Lithium-rich manganese-based layered materials have been considered as the most promising cathode materials for future high-energy-density lithium-ion batteries. However, a great loss of irreversible capacity at the initial cycle, poor cycle stability, and rate performance severely restrict its application. Herein, we develop a new strategy to synthesize hierarchical hollow Li1.2Mn0.54Ni0.13Co0.13O2 microspheres using sucrose and cetyltrimethylammonium bromide as a soft template combined with hydrothermal assisted homogeneous precipitation method. The hollow microspheres are assembled by the primary particles with the size of 50 nm. As a result, the as-prepared material exhibits high reversible capacity, good cycling stability, and excellent rate property. It delivers a high initial discharge capacity of 305.9 mAh g−1 at 28 mA g−1 with coulombic efficiency of 80%. Even at high current density of 560 mA g−1, the sample also shows a stable discharge capacity of 215 mAh g−1. The enhanced electrochemical properties are attributed to the stable hierarchical hollow sphere structure and the appropriate contact area between electrode and electrolyte, thus effectively improve the lithium-ion intercalation and deintercalation kinetics.