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Understanding the Discrepancy of Defect Kinetics on Anionic Redox in Lithium-Rich Cathode Oxides Wei Jiang, Chong Yin, Yonggao Xia, Bao Qiu, Haocheng Guo, Hongfu Cui, Fang Hu, and Zhaoping Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Applied Materials & Interfaces

Understanding the Discrepancy of Defect Kinetics on Anionic Redox in Lithium-Rich Cathode Oxides Wei Jiang†,║, Chong Yin†,‡, Yonggao Xia†, Bao Qiu†,*, Haocheng Guo†, Hongfu Cui†, Fang Hu║, Zhaoping Liu†,‡,* †

Ningbo Institute of Materials Technology and Engineering Chinese Academy of Sciences, Ningbo 315201, P. R. China

‡

University of Chinese Academy of Sciences, Beijing 10049, P. R. China

║

School of Materials Science and Engineering, Shenyang University of Technology,

Shenyang 110870, P. R. China

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ABSTRACT: Reversible anionic (oxygen) redox in lithium-rich cathode oxides has been becoming a blooming research topic to further boost the energy density in lithiumion batteries. There are numerous experimental observations and theoretical calculations to illustrate the importance of defects on anionic redox activity, but how the defects on the surface and bulk control the kinetics of anionic redox is not well understood. Here, we uncover this intriguing ambiguity on the correlation among defects states, Li ion diffusion and oxygen redox reaction. It is found that the surface defective microstructure behaves fast Li ion diffusion to achieve superior cationic redox activity/kinetics, while the bulk defective microstructure corresponds to a slow Li ion diffusion to result in poor cationic redox activity/kinetics. By contrast, both surface and bulk defects can be of benefit to the enhancement of oxygen redox activity/kinetics. Moreover, a positive correlation is also established between charge transfer resistance and interface reaction charge-transfer activation energy and oxygen redox activity in these electrode materials. This study on defect-anionic activity provides a new insight for controlling anionic redox reaction in lithium-rich cathode materials to real-world application.

KEYWORDS: Li ions batteries; Lithium-rich cathode materials; Anionic redox; Defects kinetics; Electrochemical kinetics

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INTRODUCTION Solid-state reactions in mixed-conducting electrodes are highly associated with the kinetics in solid materials.1-4 The kinetics properties play a crucial role on the electrochemical behaviors and performance of Li-intercalation compounds. To this end, the understanding on the kinetics characteristics in lithium-contained electrode materials is of significance for the development of new materials in batteries.5,6 Lithium-rich layered oxides (LR-NCM) with the composition of xLi2MnO3∙(1x)LiMO2 (M= Ni, Co, Mn) can deliver high discharge specific capacity value more than 250 mAh g-1, which hopefully have access to the highest practical energy density.7-9 In comparison with classical layered cathode materials such as LiCoO2 and LiNi1/3Co1/3Mn1/3O2, they present a unique two-step charge curve with the charge specific capacity value over 300 mAh g-1, in which sole charge compensation based on transition metal (TM) redox is obviously insufficient.10,11 Recently, oxygen redox activity in lithium-rich layered cathode oxides is proposed to contribute to the extra capacity,12-15 but also causes several issues, such as voltage hysteresis, voltage decay and low energy efficiency.16-18 To achieve their practical applications in high-energydensity Li-ion batteries, some approaches have been explored to overcome these problems, such as the rational utilization of oxygen redox activity,19-21 however, the efforts are limited due to the complexity of electrochemical mechanism. Numerous efforts have been made to illuminate the oxygen redox mechanism in lithium-rich cathode materials over the past several years.12-15,22-26 Tarascon and coworkers firstly proved (O2)n- species during charging process by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), and then visualized O-O dimers with the aid of scanning transmission electron microscopy (STEM) and neutron diffraction (ND).14,27 Bruce and coworkers experimentally confirmed the reversible oxygen redox activity based on 18O-labelled operando mass spectrometry and O K-edge soft x-ray absorption spectroscopy (XAS).23 Ceder and coworkers identified specific Li-O-Li configurations to construct the chemical and structural origin of 3



anionic redox activity using ab initio calculations.25 Gent and coworkers established a strong coupling between oxygen redox and cation migration though combining scanning transmission X-ray microscopy and nanoscale XAS with resonant inelastic Xray scattering.26 Consequently, the fundamental chemical and structural features of anionic redox activity in lithium-rich materials is clear.18 However, redox reactions kinetics and microstructure characteristics for anionic redox activity in lithium-rich oxides cathode are not yet systematically investigated.24,28-30 In classical layered TM oxides, high crystallinity and good layered microstructure benefit full utilization of cationic redox activity to realize the high capacity output.31-33 Moreover, fast electrochemical kinetics contribute to superior redox reaction kinetics. The structural failures in any cases would undoubtedly cause capacity fade.31,34,35 Conversely, Li2MnO3-based cathode oxides with high crystallinity deliver almost no capacity even activated at high voltage. However, recent works manifest a key point that the introduction of some structural defects into these cathode materials can largely boost the electrochemical activation of Li2MnO3 component at relatively low voltage.8,24,30,36-45 As a result, it is well acquired a highly reversible oxygen redox activity in these cathode materials to further improve their discharge capacity.46-49 A similar trend with some TM vacancies is also observed to realize highly reversible oxygen activity in layered sodium-ion battery system.50 On the basis of these results, defective microstructures have a significant impact on anionic redox in oxygen-redoxbased cathode materials. Here, we intend to improve the fundamental understanding of anionic chemistry by elucidating the anionic redox reaction in lithium-rich oxides materials. In this work, the pristine, surface-defective and bulk-defective lithium-rich electrode materials were applied to reveal this discrepancy. They are listed as P-LRNCM, SD-LR-NCM and BD-LR-NCM, respectively. Scheme. 1 shows the schematic diagram of microstructures with these materials, all of them show a secondary spherelike morphology composed of many polyhedral primary particles. Figure S2 presents the detailed microstructures of these three samples. It has a complete layered 4


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microstructure with high crystallinity of P-LR-NCM sample. For SD-LR-NCM, there is a similar layered microstructure in the bulk lattice except for defective thin layer on the surface. In contrast, abundant defects, such as spinel-like domains, stacking faults and dislocations, are incorporated into the bulk lattice of BD-LR-NCM, which also contains the spinel-like thin layer on the surface. These three lithium-rich electrode materials all possess an output of stable capacity and unique electrochemical behavior. From electrochemical results, it is found both cationic redox and anionic redox can be altered greatly by both surface and bulk defects. Furthermore, the kinetics properties of anionic redox in these electrodes depend on the state of these defects. EXPERIMENTAL SECTION Synthesis of Li1.144Mn0.544Ni0.136Co0.136O2 powders. The pristine lithium-rich layered materials Li1.144Mn0.544Ni0.136Co0.136O2 are prepared by high temperature solid-reaction as our previous works.51-53 Precursor (Ni1/6Co1/6Mn4/6)CO3 was prepared by chemical bath method. The precursor solution of 2 mol-1 with certain molar ratio of Ni2+, Co2+ and Mn2+ ions was continuously stirred. Meanwhile, the Na2CO3 solution and NH4OH solution served as coprecipitator were added separately into the above solution system. Then, the resulting powders were washed with distilled water for several times to remove residual Na+ ions. The (Ni1/6Co1/6Mn4/6)CO3 powders were mixed with 5 % excess of Li2CO3. The resultant mixed powder was heated at 500 oC for 5 hours and then calcined at 850 oC for 12 hours. The heating processes were all conducted in air with a heating rate of 5 oC. These products were cooled naturally to room temperature in the furnace. Acid treatment process. The acid treatment process was carried out as follows24: In acid treatment process, Sulphuric acid were served as chemical de-lithiation medium, and high content NiSO4, CoSO4 as co-solutes (200 g L-1, 100 g L-1) were also used to avoid excessive Ni, Co dissolutions. For a typical procedure, 5 g of P-LR-NCM powders were dispersed into 100 mL acidic solution, and continuously stirred for one hour at room temperature for ion exchange process. Then the powders were filtrated and washed with deionized water for several times. The obtained products were dried 5



in oven at 80 oC for 12 h and then annealed at 300 oC for 10 hours. When the molar ratios of H+/Li+ were 0.1 and 0.5, SD-LR-NCM and BD-LR-NCM samples were obtained, respectively. Materials characterization. The X-ray diffraction patterns of three materials were measured using X-ray Diffractometer (D8 Advance, Bruker AXS) with Cu-Kα radiation source (λ=1.5406 Å); the system was operated at 40 kV and 40 mA. The data were collected in the 2θ values range of 10o-90o. Morphology observations were conducted by Field emission scanning-electron microscope (FESEM, Hitachi S-4800). High resolution transmission-electron microscope (HRTEM) images were collected via transmission-electron microscope (TEM, FEI Talos F200x, 200 kV, and FEI Tecnai F20, 200 kV). The TEM specimens of as-obtained P-LR-NCM and SD-LR-NCM were prepared by ultrasonically dispersing active materials powders in alcohol, then dripping the fragments onto a holey carbon coated grid. TEM specimens of BD-LR-NCM were prepared via focused ion beam milling (FIB, Carl Zeiss, Auriga). The X-ray photoelectron spectroscopy tests were conducted at 1.487 keV photo energy (XPS, kratos, Axis ultra DLD). The etching processes were carried out to reduce surface deposits and achieve information in the bulk lattice. The etching depth is about 20 nm (about 8nm / min). The Ar+ ion beam at 4 keV were applied. Electrochemical tests. All the electrochemical tests are conducted in CR-2032-type coin cell with lithium metal, lithium-rich electrode materials, 1 M LiPF6 in the ethylene carbonate and diethyl methyl carbonate (EC: DMC 3/7 v/v) served as anode, cathode, electrolyte, respectively. The cathodes were fabricated by casting the slurry containing active materials (80 wt.%), acetylene black (10 wt.%) and poly (vinylidene fluoride) binder (10 wt.%) on an aluminum foil. Electrode discs of 14 mm diameter were punched from the electrode after dried at 80 oC for 12 hours. The coin cells were assembled in an argon gas glove box (H2O 1). Chem. Mater. 2006, 18, 1901-1910.

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Understanding the Discrepancy of Defect Kinetics ... - ACS Publications

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