Anionic Redox

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Revealing the electrochemical mechanism of cationic/anionic redox on Lirich layered oxides via controlling the distribution of primary particles size Li Lu, Yanjie Hu, Hao Jiang, Chengxian Zhu, Jinyu Chen, and Chunzhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03905 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 26, 2019

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Revealing

the

Electrochemical

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Mechanism

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Cationic/Anionic Redox on Li-rich Layered Oxides via Controlling the Distribution of Primary Particles Size Li Lu, Yanjie Hu,* Hao Jiang, Chengxian Zhu, Jinyu Chen and Chunzhong Li*

Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China. E-mail: [email protected] (Prof. C. Li) and [email protected] (Prof. Y. Hu);

Submitting to: ACS Applied Material & Interfaces May 15, 2019


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ABSTRACT: Lithium-manganese-rich layered oxides have sparked intense interest on high-energy density Lithium-ion batteries due to their unique anionic redox. While making efforts to adjust the primary particle size for the improved electrochemical kinetics, current research is insufficient to explain how anionic/cationic interplay governs the electrochemical behavior except for ion diffusion. Here, fully ordered Li1.2Mn0.54Ni0.13Co0.13O2 spheres of shorten primary particle size was synthesized via co-precipitation method for use as cathodes. A high discharge capacity of 303.2 mAh g-1 was achieved for the first cycle. Optimized nanostructures reduce the lithium ion diffusion length and increase electronic conductivity unsurprisingly, contributing to excellent electrochemical activity and rate capability. Furthermore, decreased primary particle size accelerated the redox reactivity of cation and the reversibility of anionic redox on path dependence. This work clarifies the electrochemical mechanism of cationic/anionic redox of these Li-rich layered oxides and provides a new vision of unique design of high-energy cathode materials with better application. KEYWORDS: anionic redox, ion diffusion, primary particles, Lithium-manganese-rich layered oxides, cathode material


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1. INTRODUCTION Next-generation energy storage technologies for electric and hybrid electric vehicles have been requiring high power density lithium-ion batteries (LIBs), which are urgently needed in practical applications. But the further commercialization of lithium-ion batteries is limited by the lack of cathode materials with high rate performance. Among various candidates, Li-rich layered oxides (LLOs) have emerged due to their high capacity, abundant resource and unique anionic mechanism. 1-2 The consensus has been reached that reversible oxygen redox provides such high capacities up to 300 mAh g-1. 3 Nevertheless, inherently low conductivity and tardy lithiation kinetics constraints the choice of long-life cycling and rate capacity, which often impede further development in LIBs. The latter is more attractive when intelligent fast rechargeable batteries are considered as one of the future directions of development. 4-5 To overcome these drawbacks, many strategies for structural design6-8, such as carbon composite, doping, surface coating and nanocrystallization, have been proposed and optimized. Zhang et al. constructed a spinel Li4Mn5O12 coating to stabilize oxygen framework and promote kinetics 9. Despite improvement achieved in adventive components like depolarization and rate performance, problems of stability and structure rearrange have been inevitable.

10-11

The

interface between carbon and bulk or exotic matter and bulk is prone to introduce layered-tospinel phase transition, which contributes to voltage fade. Subsequently, intentional adventive cations seems to be more stable in structure and voltage decay12-13. The above modification methods have more or less merits and drawbacks. Among them, decreasing the primary particle size to nanoscale is a feasible and effective solution considering the nature of LLOs, especially to promote the rate performance of cathode electrode. The nano structural design was previously attempted utilizing calcination temperature

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lithium content 15-16and phase composite 17. Zheng et al. prepared high performance of layered material with the optimized size of its primary particles (100-300 nm) via discussing the effect ACS Paragon Plus Environment

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of calcination temperature (around 750 oC). 18. Gao et al. synthesized Li1+x (Ni1/6Co1/6Mn4/6)1xO2

materials with different morphologies via altering a series of lithium content. 19. Although

enormous progress has been made in altering primary particle size, many strategies like changing calcination temperature or material composition inevitably have brought about other damages to crystal chemistry. Thus, it is necessary to seek micro/nano morphology to achieve nano-particles and avoid its defects at the same time. Besides, anionic redox is kinetically sluggish and show large voltage hysteresis for its practical obstacles. Despite the fact that role of poor electrochemical kinetics of anionic redox in layered-oxides has been assessed,20 the kinetics improvement after structural design lacks intuitive display. Hu et al. shows that the average valence state of cations is reduced from individual to ensembles of particles, and exposed pores bring unexpected oxygen loss21. Such microstructure is closely related to electrochemical performance. Based on path dependence, it is necessary to establish relationship between the distributions of primary particles with the obstacles in application from the perspective of anionic redox. From the above analysis, fully ordered Li1.2Mn0.54Ni0.13Co0.13O2 with primary nanoparticles of extreme narrow size distribution was rationally constructed by frame at the nanoscale to provide fast transport pathway. Surfactant restricts the reaction domain for co-precipitation reaction where primary particles uniformly generate. The highly ordered structure significantly accelerates electrochemical kinetics by providing sufficient electrolyte infiltration and shortening the lithium ion diffusion pathways. What’s more, the optimized size distribution of primary particles promotes the redox reaction of cations and indeed enhances the reversibility of anions in bulk. These findings give insight to electrochemical mechanism of cationic/anionic redox within the bulk, while holding the key to practical employments of LLOs-based materials. 2.

EXPERIMENTAL SECTION

2.1 Materials preparation: The carbonate precursors were synthesized by co-precipitation method. All the chemicals used were of analytical grade. Specifically, 0.16 g hexadecyl ACS Paragon Plus Environment

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trimethyl ammonium bromide (CTAB) was dissolved in 100 ml deionized water and stirred for 1h in the tank. 2M MSO4 (M=Mn, Ni, Co) solution yielding the composition of target material was pumped into the tank. To enable precipitation, 2M Na2CO3 and fixed concentration of ammonia were used. The reaction was controlled in holding at 50 oC for 20 h with intermediate stirring of 800 rpm, until carbonate precursor was formed. Nitrogen atmosphere was employerd to prevent unnecessary oxidation. Afterwards, carbonate precursor was filtrated and washed for several times, then dried in an oven for 12 h. An appropriate amount of carbonate precursor was taken and mixed with Li2CO3 to the stoichiometric ratio (5% excess for calcination loss), then put into muffle matained at 450 oC for 5 h and 850 oC for 10 h in air. After cooling down, the sample was named as fully ordered Li-rich layered oxides (FOS), partly ordered Li-rich layered oxides (POS) and pristine Li-rich layered oxides (PLLOs) according to the accessed surfactant. 2.2 Material Characterizations. All samples were investigated by X-ray diffraction (XRD, Bruker) with Cu Kα. And the morphology of powders was measured by scanning electron microscopy (SEM, JEOL-S4800) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F-20). The surface oxidation state of all samples was observed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The content of carbon residue was measured by thermogravimetric analysis (TG, Mettler Toledo 1600) under air atmosphere. The Brunauer-Emmett-Teller (BET) specific surface area and pore distribution were performed to examine the N2 adsorption/desorption isotherms. 2.3 Electrochemical Measurements. The LLOs powder (PLLOs) was mixed with carbon black (Super P), polyvinylidene fluoride (PVDF) and moderate solvent (methyl-2pyrrolidinone, NMP) in a weight ratio of 8:1:1 to form a slurry in 6h stirring. Then, the slurry was pasted onto Al foil and dried at 120 oC overnight in vacuum oven. The cut-out electrodes were assembled to CR2016-type coin cells inside an Ar-filled glovebox to maintain dryness and oxygen content. Lithium metal and polypropylene membrane were used as the counter ACS Paragon Plus Environment

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electrode and the separator. The electrolyte was the soliton of 1M LiPF6 in ethylene carbonate/dimethyl carbonate (1/1, v/v). Subsequently, galvanostatic charge/discharge process was performed in the potential range of 2.0-4.8 V (versus Li+/Li) using LAND-2001A instrument at 25 oC. Cyclic voltammetry curves (CV) were conducted on an electrochemical workstation (Autolab PGSTAT302N) at various scan rates. After several activation cycles, the opening window process for charging starts from 2.0 V in each cycle and stepwise ends with different cut-off voltage (3.5 V, 3.7 V, 3.9 V, 4.1 V, 4.3 V, 4.5 V, 4.7 V). Simultaneously, the start voltage of discharge open widow is 4.8 V and the cut-off voltages are 4.2V, 4.0 V, 3.8 V, 3.6 V, 3.4 V, 3.2 V, 3.0 V and 2.0 V. 3. RESULTS AND DISCUSSION The Li-rich layered material was fabricated via co-precipitation method. The detailed assembly processes of fully ordered Li-rich layered oxides (FOS), partly ordered Li-rich layered oxides (POS) and pristine Li-rich layered oxides (PLLOs) are depicted in Figure 1. In the first step of the precipitation process, an adequate amount of hexadecyl trimethyl ammonium bromide (CTAB) molecules formed micelles as nanoreactor with stable interior components, in which similar coprecipitation occurred as expected. Then, the primary particles were driven to hierarchized spheres by oriented attachment mechanism for the second step.22. The fully ordered primary particles agglomerated into secondary spherical particles.23 After central crystallization, smooth spherical particles were made undergoing maturation step by an Ostwald ripening mechanism. Through such a calcination process, the final material remained morphology of carbonate precursor, implying that secondary particles are composed of ordered primary particle size with narrow distribution (deduced in path a). In the preparation (path b), there were lots of smaller domains in the precipitation system by colloid formation mechanism, due to the insufficient CTAB molecules were incapable of micelles formation, provided by nanoreactors with uneven volume correspondingly. After undergoing the same steps of precipitation and calcination, the POS powder consists of partly ordered primary particles, yet ACS Paragon Plus Environment

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their size distribution was slightly broadened. In contrast, the PLLOs was prepared in the same process without the involvement of surfactant (path c). The precipitation reaction area of PLLOs was not confined, so the distribution of primary particles was wider. Moreover, its size exceeded 100 nm, which is higher than that of FOS and POS. With the ICP-OES technique, the chemical formulas of three samples are in Table S1. FOS almost has no difference in composition with the pristine PLLOs, and the Li content in POS changed from 1.22 to 1.24. The small regions of micelles provide special aging environment during the precipitation process. In particular, uneven nanoreactors provides varies interior component, due to insufficient CTAB molecules. Therefore, the POS is different from the others. 3.1 Crystal structure and morphology In Figure 2a, overall XRD patterns of three as-prepared samples obtained from the lithiation of the carbonate precursors shows an α-NaFeO2 structure with a space group of R-3m (ICCD# 01023-1399). The signals at 18.8, 37.0, 44.7, 48.9 and 58.9o belong to (003), (101), (104), (015) and (107) planes. All the samples have clear split doublets of (006)/(102) and (108)/(110) which stand for ordered layered structure. Moreover, the weak peaks between 20 and 25o are attributed to the arrangement of LiMn6 cation in the layered Li2MnO3-like monoclinic structure with a space group of C2/m. For fully ordered Li-rich layered oxides (FOS), the intensities of diffraction peaks (003) were enhanced, indicating that narrow primary particle size distribution shows main (003) facet, thereby contributing to the structure stability. The Rietveld refinement patterns24-25 were performed to further confirm the effects of controlling the primary ordering on phase and structure in Figure 2b-d, and refined structure parameters were displayed in Table S2. The FOS and POS both exhibit higher I(003)/I(104), indicating lower cation disordering. The corresponding c/a appear no significant changes during particle ordering. The slab thickness (S(MO2)) and the interslab thickness (I(LiO2)) were calculated to establish the relationship between electrochemical performance and crystal structure. 26 The S(MO2) of FOS and POS are valued to be 2.5852 and 2.5696Å. At the same time, the pristine ACS Paragon Plus Environment

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PLLOs was calculated to be 2.5943 Å. The reduced S(MO2) determines the stability of the layered structure. Compared with pristine one, the I(LiO2) values of FOS and POS have substantial increase, which are 2.1604 and 2.1644 Å respectively. The enlarged I(LiO2) will lessen the Li activation energy, which is attribute to enhance the rate of Li+ diffusion. With the detailed site occupation in Table S3-5, the Li/Ni mixing of FOS, POS and PLLOs are 0.83%, 0.004% and 0.36%. Base on the analysis of phase, the R-3m space group of the FOS and POS are 69.71% and 64.85%, when compared to 68.92% in PLLOs, suggesting that partly ordered primary particles has different content of phase. The differences of R-3m and C2/m symmetry are likely to be attributed to the phenomenon that different regions provide special aging environment during the precipitation process. Rietveld refinement results suggest the as-prepared FOS and POS samples fit the mixture structural model of layered LMO2 (R-3m) and monoclinic Li2MnO3.27 As seen in Figure S1, a small amount of mass loss is obtained by thermogravimetric analysis. Compared with pristine PPLOs, two samples with reduced primary particles have similar thermal behaviors. It shows that a loss of water and impurity occurred below 200 oC. In the temperature range of 200-550 oC, the FOS and POS shows same tendency of the second loss stage, implying crystal formation and growth. The morphologies of both FOS, POS and PLLOs powders were observed by SEM and HRTEM in Figure 3. The spherical carbonate precursor sample formed by the co-precipitation process consists of prime particles, as displayed in Figure S2. After calcination, the oxides of all samples remain the hierarchical structure of precursor as shown in Figure 3a-c. And the SEM imagers of a wider range of secondary particles are provided in Figure S3. In enlarged SEM images displays in Figure 3d-f, the primary particles is clearly identified. The FOS primary particles possess narrow size distribution (up to 96%) with ranging from 50-100 nm (Figure 3d), while those of POS have 85% in the same range (Figure 3e). Respectively, PLLOs powder has much larger size of the primary particles (major range of 100-250 nm) compared ACS Paragon Plus Environment

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with the former regularized samples (Figure 3f). The primary particle size distribution was counted from several SEM images to eliminate the measurement errors (Figure S4-6). Analyzing the grain growth process through BET (Figure S7) and SEM results, the significant differences among the three occurred since the precipitation stage. With constructed by plenty of surfactants, small primary particles determined into lithiation morphology are defined a definite confine. Thus, the distribution is narrow and primary particles is highly ordered in FOS. Besides, an inadequate amount of surfactant is unable to supply a hundred-nanometre reaction regions in POS. The smaller particles generated in smaller region mutually fill pile pores showing smaller porosity. When the pristine PLLOs is not limited by the reaction zone, the primary particles size is significantly larger and has a wide distribution as shown in Figure 3f. As expected in HR-TEM images, the FOS (Figure 3g), POS (Figure 3h) and PLLOs (Figure 3i) are composed of dense primary particles which mainly show well defined R-3m layered structure. In ordered LLOs (FOS and POS), the interplanar spacing of 0.242 nm well consists of (200) plane of the layered structure. The details are revealed in the enlarged selection area and corresponding fast Fourier transformation (FFT) patterns (Figure 3j-l). From Figure 3j, tilt boundary is observed with the tiny chaotic state of the lattice binding two grains in FOS (arrows). And the typical image appears two sets of lattice fringes that can be indexed to layered hexagonal structure and monoclinic Li2MnO3 phase. The C2/m phases are confirmed surrounded in R-3m species of POS (dash line in Figure 3k), which are observed specific phase interface. X-ray photoelectron spectroscopy (XPS) further verified the surface chemical state of the transition metal of prepared LLOs. In the spectrum of Co 2p in Figure 4a, two main peaks at 781.2 and 797.4 are attributed to Co 2p3/2 and Co 2p1/2. The energy separations between two main species are all ascribed to 14.9 eV, revealing the unchanged valence state of Co. In Figure 4b, the main peak of Ni 2p centered at 855.1 eV implies that the oxidation state of Ni remains untouched before and after ordering. Figure 4c demonstrates the mixed valence of Mn in FOS. ACS Paragon Plus Environment

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Meanwhile, Figure S8-9 represents those valences of POS and PLLOs. In addition, two peaks at 653.8 eV and 642.1 eV corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. Full spectrum (Figure S10-12) clearly confirms the existence of Mn, Ni, Co and O in the target product, as affirmed in XRD results. In general, primary particle size distribution shortened by the introduction of surfactants has no impact on the valence state of transition metal cations. 3.2 Electrochemical characterization The galvanostatic charge/discharge measurements of all samples at a 0.1C are shown in Figure S13. All plots show a smooth sloping curve below 4.5 V and a long plateau curve above 4.5 V. The FOS exhibits a charge capacity of 463.1 mAh g-1 and a discharge capacity of 303.2 mAh g-1 in the initial cycle, with a corresponding coulombic efficiency of 65.5%. Compared with ordered one, a distinct discharge plateau (around 2.6 V) belonging to spinel phase is measured in pristine one. Figure 5a-b shows the rate capability and cycling performances of the asprepared samples. At low and medium parts of current rate, the FOS electrode always delivers a higher discharge capacity than that of others. In particular, the discharge capacities of FOS at 0.1C, 0.2C, 0.5C, 1C are 303.2 mAh g-1, 277.6. mAh g-1, 239.2 mAh g-1 and 195.9 mAh g-1, respectively. When speaking to the high current density, the POS electrode displays 148.9 mAh g-1, 97.0 mAh g-1and 44.1 mAh g-1 at 2C, 5C and 10C, respectively. Highly ordered primary particle size will benefits for capacity improvement in the early stage, while partly ordered one performs better under high current density. The size distribution of primary particles can directly affect the rate performance of the material, and the relationship between them needs further analysis. As stated initially, CV curves at scan rate ranging from 0.1 mv/s to 0.5 mv s-1 were employed to further investigate the dynamical properties of Li-rich materials adjusted at primary particle scale (Figure 5c-f). In the potential window of 2.0-4.8 V, two well distinguished redox peaks displayed the transition metal redox pairs and typical anionic redox pairs of the FOS electrode (Figure 5c). With the increase in scanning rate, the characteristic oxidation peaks shifted ACS Paragon Plus Environment

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towards higher voltage, while reduction peaks shifted to the opposite direction. Simultaneously, the POS and PLLOs electrodes were measured under the same process, showing multi-CV curves in Figure 5d-e. It’s worth noting that oxygen anion redox pairs. Employing of RandleSevcik equation, the linear relationship between the peak current values and sweeping rates was analyzed to demonstrate the apparent chemical diffusion coefficient of mobile lithium ions (DLi+) in diffusion process (Figure 5f), which is an important factor of electrode kinetics. DLi+ of FOS and POS are measured to be 1.19×10-11 and 1.21×10-11 cm2 s-1, compared with 2.8×1012

cm2 s-1 of PLLOs (Figure S14). This result proves the design that the reduced primary

particles size and regular distribution are conductive to Li+ transportation, resulting in a significant increase in the rate performance.28 In addition to promoting the kinetics of cationic redox29, further electroanalytical efforts are needed to explore whether smaller size of primary particles will avail the intergranular electron conductive path when considering complex anionic mechanism. 3.3 Analysis of voltage hysteresis In view of the above performances, a full understanding of ionic diffusion path and charge transfer based on the size distribution necessary to solve these problems was dependent on the real-world Li-rich cathodes. Towards this, it was focused on optimization of voltage hysteresis for anionic/cationic redox. Toward different behavior of initial and beyond cycles, electrochemistry of as-prepared samples stabilized a sloped S-sharp after 10 activation cycles from 2.0-4.8 V. In the wide potential window, charge-discharge profiles display full-area path dependence upon the progressive opening of voltage window in Figure 6a-d. From the beginning, the charge voltage window was opened starting from 2.0V in each cycle and gradually ending with different cut-off voltage (Figure 6a for FOS and Figure 6c for PLLOs). Based on the contribution of different redox pair, voltage windows were divided into three parts.30-31 Firstly, the area below 3.5 V is probably attributed to a small amount of ACS Paragon Plus Environment

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Mn3+/Mn4+ pair (blue curves). Secondly, the transition metal cationic oxidation is happened below 4.1 V (grey curves), whereas it’s remaining reduction reaction in the part of sharp edges. Lastly, the range of 4.1-4.8 V represents the oxidation capacity which mainly recovered on reduction at high potential (about 4.0 V region). It is noteworthy that the reduction reaction of formed on splits into high (above 4.0 V) and low (below 3.6 V) potential parts. In early charging process, there is little difference between FOS and pristine one. Subsequently charging, the FOS shows much area improvement of mid- and high- voltage regions compared to the bare Li-rich cathode. Alternatively, the similar voltage window of discharging is gradually opened starting in the discharge direction from 4.8 V (Figure 6b for FOS and Figure 6d for PLLOs). In the corresponding three regions, the orange curves (2.0-3.4 V) show the obvious hysteresis that anionic reduction takes main contribution to and just a small part of Mn3+/Mn4+ redox. The reduction capacity below 3.4 V is oxidized throughout the charging window. Thus the anionic oxidation peak of FOS is more inclined to the low potential region as a whole. Overall, it can be found that voltage hysteresis in pristine one is much bigger than sample of altered primary particle size which is associated with anionic redox on either charge or discharge process. The corresponding dQ/dV curves suggest similar trend as the opened charge/discharge window in Figure 6e-f. In general, the electrodes are detailly revealed the role of modified primary size in electrochemical kinetic and hysteresis performance (Figure 7), and summarized as follows: (1) The decrease in particles size is beneficial to reversible anionic redox reaction, but results in more unstable redox of Mn4+/Mn3+. (2) Diminished primary particles size accelerates the transport of lithium-ion, thus promoting the effective cations oxidation/reduction. (3) When the charge window is opened to high potential part (third area), the charge is compensated by anions and thus caused hysteresis phenomenon, which appears to two splitting ACS Paragon Plus Environment

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oxygen anionic redox regions. Combining the above findings, the alteration of primary particle size has accelerated the redox reactivity of cation and enhanced the reversibility of anion in bulk. Though the specific capacity and rate performance are improved, some irreversible oxygen release and structural rearrangement inevitably occur because of anionic reactivity, which also challenges the stability of Li-rich materials in practical application. Therefore, based on advantages of optimized primary particle size, the efforts to improve the stability of as-prepared material should be cooperated further, such as an electrochemical inert layer

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stable crystal chemistry

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and

occupation of surface vacancies 34. 4. CONCLUSIONS The effect of primary particle size distribution on the structure, morphology and electrochemical performance of Li-rich layered materials is systematically investigated. The result demonstrates that the electrochemical performance is sensitively associated with the size distribution of primary particles. The fully ordered Lithium-rich layered oxides were achieved with a highly centralized size below 100 nm, and it is capable of delivering 303.2 mAh g-1 at 0.1C. Minished primary particle size of LLOs material brings better rate performance because of significant promotion for kinetics (1.19×10-11 cm2 s-1 for FOS and 1.21×10-11 cm2 s-1 for POS) that is closely elated to electrolyte infiltration and Li+ diffusion way between bulk and electrolyte. Towards to anionic redox, hysteresis behavior of oxygen redox has enhanced for reaction reversibility, which exhibits two redox parts in the full window. The findings highlight the importance of optimizing the direct effect on the electrochemical performance of Li-rich materials, which need to be considered for the synthesis of high-energy LLO-series to maximize their performance in practical LIBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ACS Paragon Plus Environment

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xxxxxx SEM images, BET results, XPS analysis, electrochemical performance, fitting data of kinetic and lattice parameters (PDF) AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Prof. Y. J. Hu), Fax: +86-21-64250624, Tel: +86-2164252055; *E-mail: [email protected] (Prof. C. Z. Li), Fax: +86-21-64250624; Tel: +86-21-64250949. ORCID Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21838003, 91834301), Shanghai Pujiang Program (18PJ1402100), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Shanghai Scientific and Technological Innovation Project (18JC1410500, 18DZ2252400), the Innovation Program of Shanghai Municipal Education Commission and the Fundamental Research Funds for the Central Universities (222201718002).


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(12) Shi, J. L.; Zhang, J. N.; He, M.; Zhang, X. D.; Yin, Y. X.; Li, H., Wan, L. J. Mitigating Voltage Decay of Li-Rich Cathode Material via Increasing Ni Content for Lithium-Ion Batteries. ACS Appl. Mater. Inter. 2016, 8, 20138-20146. (13) Shi, J.-L.; Xiao, D.-D.; Zhang, X.-D.; Yin, Y.-X.; Guo, Y.-G.; Gu, L.; Wan, L.-J. Improving the Structural Stability of Li-rich Cathode Materials via Reservation of Cations in the Li-slab for Li-Ion Batteries. Nano Res. 2017, 10, 4201-4209. (14) Liu, J.; Chen, H.; Xie, J.; Sun, Z.; Wu, N.; Wu, B. Electrochemical Performance Studies of Li-Rich Cathode Materials with Different Primary Particle Sizes. J. Power Sources 2014, 251, 208-214. (15) Wang, J.; Yuan, G.; Zhang, M.; Qiu, B.; Xia, Y.; Liu, Z. The Structure, Morphology, and Electrochemical Properties of Li1+xNi1/6Co1/6Mn4/6O2.25+x/2 (0.1 ≤ x ≤ 0.7) Cathode Materials. Electrochim. Acta 2012, 66, 61-66. (16) Cao, K.; Shen, T.; Wang, K.; Chen, D.; Wang, W. Influence of Different Lithium Sources on the Morphology, Structure and Electrochemical Performances of Lithium-Rich Layered Oxides. Ceram. Int. 2017, 43, 8694-8702. (17) Pan, L.; Xia, Y.; Qiu, B.; Zhao, H.; Guo, H.; Jia, K., Liu, Z. Synthesis and Electrochemical Performance of Micro-sized Li-rich Layered Cathode Material for Lithium-Ion Batteries. Electrochim. Acta 2016, 211, 507-514. (18) Zheng, J.; Yan, P.; Estevez, L.; Wang, C.; Zhang, J.-G. Effect of Calcination Temperature on the Electrochemical Properties of Nickel-Rich LiNi0.76Mn0.14Co0.10O2 Cathodes for Lithiumion Batteries. Nano Energy 2018, 49, 538-548. (19) Gao, S.; Zhang, Y.; Zhang, H.; Song, D.; Shi, X.; Zhang, L. The Effect of Lithium Content on the Structure, Morphology and Electrochemical Performance of Li-Rich Cathode Materials Li1+x(Ni1/6Co1/6Mn4/6)1−xO2. New J. Chem. 2017, 41, 10048-10053. (20) Assat, G.; Delacourt, C.; Corte, D. A. D.; Tarascon, J.-M. Practical Assessment of Anionic Redox in Li-Rich Layered Oxide Cathodes: A Mixed Blessing for High Energy Li-Ion Batteries. J. Electrochem. Soc. 2016, 163, A2965-A2976. (21) Hu, E.; Yu, X.; Lin, R.; Bi, X.; Lu, J.; Bak, S., Yang, X.-Q. Evolution of Redox Couples in Li- and Mn-rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release. Nat. Energy 2018, 3, 690-698. (22). Li, D.; Nielsen, M. H.; Lee, J. R. I.; Frandsen, C.; Banfield, J. F.; De Yoreo, J. J. DirectionSpecific Interactions Control Crystal Growth by Oriented Attachment. Science 2012, 336, 1014-1018.


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Figure 1 Schematic diagram of assembly (a) fully-ordered LLOs, (b) partly-ordered LLOs and (c) pristine LLOs via co-precipitation method. 146x88mm (220 x 220 DPI)



Figure 2 (a) Overall XRD patterns of FOS, POS and PLLOs; Rietveld refinement patterns of FOS(b), POS(c) and PLLOs(d). Rwp values are 5.49%, 5.56% and 5.74%, respectively.


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Figure 3 (a-c) SEM images, (d-f) enlarged SEM images and insert distribution record, (g-l) TEM images, enlarged selection area and corresponding FFT patterns for FOS (left), POS (middle) and for PLLO (right). 152x177mm (150 x 150 DPI)



Figure 4 (a) Co 2p and (b) Ni 2p of XPS spectra of FOS, POS and PLLOs; (c) Curve fitting provides clear distribution of Mn valence of FOS.


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Figure 5 (a) Rate performance 0.1-10 C and (b) Cycling performance in 1C; CV curves of FOS (c), POS (d) and PLLOs (e) in the voltage range from 2.0-4.8 V at various scan rates. (f) And corresponding plots of the reductive peak current density vs. the square root of sweep rate.



Figure 6 Voltage hysteresis and path dependence in FOS (a-b, e) and PLLO (c-d, f) studied by voltage window opening. (a) and (c) are the voltage curves as charging window is opened stepwise from 2.0-4.8 V. (b) and (d) are the voltage curves as discharging window is opened stepwise from 4.8-2.0 V. (e) and (f) are corresponding dQ/dV curves in discharging process. Details of steps is seen in the method section. 146x167mm (220 x 220 DPI)


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Figure 7 A schematic illustration of Li+ transfer and electron conductive behaviors of the three different materials


Anionic Redox

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