K+ Exchange toward Double-Gradient Modification of

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Surfaces, Interfaces, and Applications

Surface Li+/K+ exchange towards a double gradient modification of layered Li-rich cathode materials Xiang Ding, Yi-Xuan Li, Xiao-Dong He, Jiaying Liao, Qiao Hu, Fang Chen, Xiao-Qiang Zhang, Yu Zhao, and Chunhua Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07659 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Applied Materials & Interfaces

Surface Li+/K+ exchange towards a double gradient modification of layered Li-rich cathode materials Xiang Dinga, Yi-Xuan Lia, Xiao-Dong Hea, Jia-Ying Liaoa, Qiao Hua, Fang Chena, Xiao-Qiang Zhangb, Yu Zhaob and Chun-Hua Chena,*

aCAS

Key Laboratory of Materials for Energy Conversions, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Anhui Hefei 230026, China bInstitute

of Electronic Engineering, CAEP, Mianyang 621900, Sichuan, China

ABSTRACT Surface coating and lattice doping are widely used to enhance the interfacial and structural stabilities of Li1.2Ni0.13Co0.13Mn0.54O2 (LNCM). In this paper, KF is used to modify LNCM for the first time. A Li+/K+ exchange in the Li-slabs is realized via a high-temperature treatment. Consequently, subsurface K+ gradient doping and surface K1-xLixF gradient coating are obtained simultaneously on LNCM. Such Li+/K+ exchange mechanism and double gradient modification are clarified by XRD, EDS line scans and HR-TEM analyses. As a result, the optimal 0.5 wt% KF modified LNCM material shows markedly alleviated voltage degradation (0.0031 V @1 cycle), improved cycling stability (88%@100 [email protected] C) and rate capability (108 mA h g1@10

C), revealing large application potential in high energy materials.

KEYWORDS: Li-rich cathode; potassium fluoride; Li+/K+ exchange; gradient coating; gradient doping.

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1. INTRODUCTION Lithium ion batteries (LIBs) are considered to be vital energy storage devices for their long cycle-life and high energy density.1 However, further improvement of energy density for LIBs is believed to be largely limited by the capacity of cathode materials.2 Thus, Li-rich materials (αLi2MnO3·(1-α)LiMO2 (M = Ni, Mn, Co etc.)), are attracting wide attention for their rather high capacity ( >250 mA h g-1), high operating voltage and relatively low cost.3,4 Nevertheless, Li-rich materials also face with challenges such as severe voltage decay and inferior cycling stability.5 The voltage decay is definitely caused by the poor structural stability, which is attributed to the phase transition.6-8 At present, the strategy of lattice doping is widely adopted to mitigate this issue, such as using transition metal (TM) ions (Ti4+, V4+, Cr3+, Cu2+),912

alkalies (Na+, K+),13,14 alkaline-earth metal ions (e.g. Mg2+),15 polyanions (PO43-,

SiO44-, BO45-),16-18 and simple anions (F-, Cl-, S2-).19-21 Here, these metal ions are generally doped to the TM sites and alkalis are doped to Li sites, while the polyanions or simple anions substitute for O sites. Generally, the lattice doping is believed to increase the layer spacing, improve the conductivity and (or) inhibit the TM ions migration.10,13,20 Besides, the poor cycling reversibility is usually ascribed to the poor interface stability,22 which can be effectively restrained by surface modifications with materials like olivines (e.g. LiFePO4),23 phosphates (Li3PO4, AlPO4, FePO4),24-26 metallic oxides (TiO2, Al2O3),27,28 vanadates (e.g. Li3VO4),29 fluorides (AlF3, CaF2, CoF2, LiF),30-33 silicides (e.g. SiO2),34 spinels (e.g. Li4Mn5O12)35 and other materials (e.g. Li0.75La0.42TiO3).36 However, only the lattice doping or surface modification alone cannot simultaneously acquire both structural and interfacial stabilities. Therefore, the realization of both lattice doping and surface coating is proposed recently by several groups. For example, Liu et al. studied Li4Ti5O12 coating and Cr doping, as well as Cr doping and LiAlO2 coating,37,38 and Li et al. investigated CaF2 coating and La doping.39 The results show that the combination strategy is more effective in enhancing the electrochemical properties than a single modification.


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In this work, we design an integrated surface gradient K+ doping in Li-rich particles and gradient K1-xLixF coating on LNCM via a facile KF coating process and subsequent Li+/K+ exchange reaction at high-temperature. Such a Li+/K+ exchange mechanism is shed light on by XRD, EDS line scans and HR-TEM analyses. Meanwhile, benefiting from the double-gradient surface modification, the modified LNCM shows improved cyclic and rate capabilities, exhibiting a strong application potential in high energy materials.

2. EXPERIMENTAL SECTION 2.1. Materials synthesis. Li1.2Ni0.13Co0.13Mn0.54O2 (LNCM) was prepared by an auto-combustion method.15 citric acid, Mn(CH3COO)2•4H2O, Ni(NO3)2•6H2O, Co(NO3)2•6H2O and LiNO3 (5% excess) with 1.7: 0.54: 0.13: 0.13: 1.2 stoichiometric ratio were dissolved with deionized water. Next, ammonium hydroxide was used to make pH=7. Then, it was heated in an electric oven to cause an auto-combustion under 250 °C for 6 h. Finally, the powder was sintered in air under 950 °C for 10 h to obtain LNCM. To prepare the KF modified LNCM materials, 0.5, 1, 3 and 5 wt% KF were mixed and grinded for 30 min, respectively, with LNCM powder in a mortar with moderate amount of ethanol as the dispersant. Then, they were heated under 400 °C for 5 h to induce a Li+/K+ exchange reaction at the interface of KF/LNCM and, finally, four KF modified LNCM samples were prepared. For comparison, the bare LNCM after heated under 400 °C for 5 h was also prepared. 2.2. Material Characterization. The structures of these bare and modified LNCM powders were characterized by X-ray diffractometer (Rigaku TTR-III). A pure



Si wafer was used to calibrate the XRD patterns. Their compositions were characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES, Optima 7300 DV). The morphologies were achieved by high-resolution transmission electron microscopy (JEM-2100F) and field emission scanning electron microscopy (FEI Apreo). The element distributions were obtained by field emission scanning electron microscopy energy-dispersive X-ray spectrometer (EDS, Bruker Quantax, XFlash 6130) and high-resolution transmission electron microscopy energy-dispersive X-ray spectrometer (EDX, JEM-2100F). Besides, the ex situ SEM and ex situ XRD were obtained by disassembling some cycled cells in an argon filled glovebox. A tablet composed of KF salt and LNCM was heated at 400 °C for 5 h (denoted as KF/LNCM tablet) to test the EDS line scans. 2.3. Electrochemical Measurements. The working electrodes including active materials (i.e. LNCM or KF modified LNCM), binder (PVDF) and carbon black (8:1:1). Next, CR2032-type coin-cells were assembled with electrodes, electrolyte (1 M LiPF6 (EC: DMC =1:1, v/v)), separator (Celgard 2400 PP membrane) and counter electrode (lithium foil) in a glovebox (MBraun Labmaster 130; argon filled). Then, they were tested on a testing system (Neware BTS-2300 battery cycler) between 2.0-4.8 V. It has to be mentioned that, for the KF modified LNCM electrodes, the charge and discharge capacities were calculated based on the total weight of KF and LNCM. Besides, the rate capacities were evaluated from 0.1 C to 10 C and the charge current density was always 1 C (1 C=250 mA g−1). The CV and EIS were conducted on an electrochemical workstation (CHI 604B).


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3. RESULTS AND DISCUSSION

Fig. 1 X-ray diffraction patterns of the bare, 0.5, 1, 3, 5 wt% KF coated LNCM materials (a) and the magnifying (003) and (104) peaks (b).

To confirm the crystal structure of these samples, XRD was performed (Fig. 1). The peaks of bare LNCM are indexed to layered α-NaFeO2 (R3m; Fig. 1a).26 Hereinto, (003) and (104) peaks (18.8° and 44.75°) are assigned to rhombohedral phase (LiMO2) and (020/110) peaks situated 20°-23° belong to monoclinic phase (Li2MnO3, C2/m).27 For the bare LNCM after heated at 400 °C for 5 h, no obvious change can be observed, suggesting that the structure of LNCM is stable under 400 °C (Fig. S1). For 0.5, 1, 3 and 5 wt% KF coated LNCM samples, no additional diffraction peaks can be detected probably because KF (PDF#36-1458) exists in an amorphous form on LNCM particles, which is clearly seen in the HR-TEM images (see below). To explore the evidence of Li+/K+ exchange, the magnifying (003) and (104) peaks are demonstrated (Fig. 1b). It can be observed that both peaks shift to smaller 2θ angles, revealing a possible K+



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doping to form Li1.2-xKxNi0.13Co0.13Mn0.54O2.14 Also, the calculated lattice parameters show the slightly increased c parameter (Table S1), which likewise suggests the existence of K+ lattice doping. The pillar effect of K+ in Li-slabs is good for the improvement of rate performance.14 Such a K+ doping can be realized via a Li+/K+ exchange mechanism: KF + Li1.2Ni0.13Co0.13Mn0.54O2→K1-xLixF + Li1.2-xKxNi0.13CO0.13Mn0.54O2

(1)

The values of the c/a ratio (4.9968, 4.9967 and 4.9969) are almost the same for these three KF modified samples. This result suggests that nearly same K+ doping extent is achieved for them. The SEM images are exhibited in Fig. S2. The bare LNCM shows a granular morphology with a 200-500 nm particle size (Fig. S2a). After KF coating, the modified samples display similar morphologies as the bare (Fig. S2b-e). Besides, EDS of 0.5 wt% KF coated LNCM sample is given in Fig. S2f-l. The result demonstrates Mn, Co, Ni, O, K and F are distributed uniformly, indicating an excellent coating effect of KF on the surface of LNCM. It guarantees a good protective effect on the LNCM particles, which can suppress the side reactions between the electrolyte and the electrode material, so as to improve the cycling performance.


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Fig. 2 TEM images of (a, b) 0.5 wt%, (c, d) 1 wt%, (e, f) 3 wt% KF modified LNCM samples and (g-j) EDX mapping images of 5 wt% KF modified LNCM sample.

To clearly observe the coating layer on LNCM particles, TEM images were demonstrated in Fig. 2 and Fig. S3. It is confirmed the bare LNCM displays a granular morphology (Fig. S3a). Besides, the legible lattice fringes (0.47 nm) is assigned to (003)



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plane (Fig. S3b).27 Moreover, we can find that the coating layers display different thicknesses for these KF coated LNCM samples (e.g. 2-3 nm for 0.5 wt%, 4 nm for 1 wt% and 5-6 nm for 3 wt%) (Fig. 2a-f). To confirm that the coating layers are mainly composed of KF, the EDX mapping images for the 5 wt% KF-coated LNCM sample were obtained (Fig. 2g-j). It is clear that the coating layer mainly contains KF by comparing the distributions of K (Fig. 2h), F (Fig. 2i) and Mn (Fig. 2j) elements. By the way, the Li element cannot be detected by the EDX because it is too light. Besides, the ICP-AES results (Table S2) show that the compositions of bare and KF modified LNCM

samples

agree

well

with

the

composition

we

designed,

i.e.

Li1.2Ni0.13Co0.13Mn0.54O2. Based on the analyses of ICP-AES and EDX, it can be inferred that all the residual KF is coated on the surface of LNCM. Meanwhile, it is clear that these coating layers are amorphous (Fig. 2a, c, f). Inspiringly, fluorides (e.g. LiF, NaF, KF) with an amorphous structure are believed to have a much better ionic conductivity than those with crystallized structures.40 Therefore, such an amorphous K1-xLixF coating layer can function both as a protective layer and a good medium for Li+ transport.


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Fig. 3 The EDS line scans of KF/LNCM tablet: (a-f) before heat-treatment and (g-l) after heat-treatment.

In order to prove the existence of Li+/K+ exchange reaction at high-temperature, the EDS line scans of KF/LNCM tablet were performed (Fig. 3). Before the heat treatment, we can find that the signals of K and F elements display almost the same both in the regions of bulk KF and LNCM in Fig. 3a-f (see the orange and yellow lines marked by green arrows). However, after the heat treatment, the signal of K shows higher intensity than that of F element at the border of LNCM region in Fig. 3g-l (see the green and red



lines marked by red arrows). This phenomenon illuminates the migration of K+ from KF side to LNCM side at the boundary, which suggests a huge possibility of Li+/K+ exchange at the interface of KF/LNCM.

Fig. 4 HR-TEM images of (a-d) 0.5 wt% KF modified LNCM sample and (e) the proposed schematic diagram of Li+/K+ exchange process.

To explore more direct evidence of Li+/K+ exchange at the interface of KF/LNCM,


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HR-TEM images of 0.5 wt% KF modified LNCM sample were performed (Fig. 4a-d). Many straight ridges can be observed with depth from 2 nm to 17 nm are situated in the Li-slabs in (003) lattice planes (Fig. 4a-d). This K+ gradient doping strongly verifies the Li+/K+ exchange in the Li-slabs rather than TM-layers, which is easy to be understood because Li+ in the Li-slabs has a less Li-O binding energy than that in the TM-layers.13 Moreover, as a result of Li+/K+ exchange, a gradient K1-xLixF coating layer can be naturally obtained on LNCM. Based on the above XRD, EDS line scans and HR-TEM analyses, a Li+/K+ exchange mechanism and a surface double-gradient modification are proposed as shown in Fig. 3e.

Fig. 5 The electrochemical properties of the bare and KF modified samples. (a) the first cycle curves at 0.1 C; (b) the cyclic test at 0.5 C; (c) the rate performance and (d) the mean voltage during 100 cycles.



The electrochemical properties are illustrated in Fig. 5. They all show typical initial cycle curves of layered LNCM material (Fig. 5a). In the initial charge process, the sloping line (

K+ Exchange toward Double-Gradient Modification of

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