Preparation and performances of the heterostructured material with a

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Preparation and performances of the heterostructured material with a Ni-rich layered oxide core and a LiNi0.5Mn1.5O4-like spinel shell Yan Huang, Xiaohui Zhang, Ruizhi Yu, Sidra Jamil, Shuang Cao, Susu Fang, Yu Wang, Ke Tang, Gairong Chen, Zhigao Luo, Xiukang Yang, and Xianyou Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01957 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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ACS Applied Materials & Interfaces

Preparation and Performances of the Heterostructured Material with a Ni-rich Layered Oxide Core and a LiNi0.5Mn1.5O4-like Spinel Shell Yan Huang,†,§,‖ Xiaohui Zhang,†,§,‖ Ruizhi Yu,†,§,‖ Sidra Jamil,†,§,‖ Shuang Cao,†,§,‖ Susu Fang,†,§,‖ Yu Wang,†,§,‖ Ke Tang,†,§,‖ Gairong Chen,┴ Zhigao Luo†,§,‖ and Xiukang Yang,†,§,‖ Xianyou Wang*,†,§,‖ †

National Base for International Science & Technology Cooperation, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China

§

National Local Joint Engineering Laboratory for Key Materials of New Energy

Storage Battery, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China ‖

Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, Hunan, China ┴

Chemistry & Chemical Engineering School, Xinxiang College, Xinxiang 453003, Henan， China

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The LiNi1-x-yCoxAlyO2 (NCA) layered materials are regard as a research focus of power LIBs due to their high capacity. However, NCA materials are still up against the defects of cation mixing and surface erosion of electrolyte. Herein, a novel design strategy is proposed to obtain a heterostructured cathode material with high capacity LiNi0.88Co0.09Al0.03O2 layer ( R3m ) core and stable LiNi0.5Mn1.5O4-like spinel ( Fd3m ) shell, which is prepared through spontaneous redox reaction of the precursor with KMnO4 in an alkaline solution and subsequent calcination procedure. The structure, morphology, element distribution and electrochemical performances of the as-prepared NCA are studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques. The results show that the LiNi0.5Mn1.5O4-like spinel ( Fd3m ) shell layer with robust cubic close-packed crystal structure is uniformly adhered to the surface of the NCA and can availably suppress the side reactions with electrolyte and surface phase transformation, which will facilitate insertion/extraction of Li+ ions during cycling. Benefiting from the enhanced structural stability and improved kinetics, the heterostructured NCA delivers a better cycling performance. The discharge specific capacity is as high as 153.7 mAh g-1 at 10 C, and even at high charge voltage of 4.5V the capacity retention can still increase 11% at 1 C (200 mA g-1) after 100 cycles. Besides, the material exhibits a prominent thermal stability of 248℃ at 4.3V. Therefore, this novel structure design strategy can contribute to the development and commercialization of high-performance cathode material for power LIBs. 2


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KEYWORDS: Ni-rich layered oxides, stable spinel phase, heterostructure, Ni2+/ Li+ mixing, surface erosion

1. INTRODUCTION Lithium-ion batteries (LIBs) as new energy storage equipment are increasingly being applied in electric vehicles (EVs), smart grids as well as renewable power stations.1-3 The high nickel layered LiMO2 cathodes (M=Ni, Mn, Co: NMC; M=Ni, Co, Al: NCA) become the most promising candidate to LIBs for well satisfying the requests of high energy density, low cost and environmental friendliness in automation industry.4 Especially they own the higher specific capacity (180-230 mAh g-1) compared with the commercially available cathodes, such as LiCoO2 (140 mAh g-1), LiFeO4 (170 mAh g-1), and LiNi1/3Co1/3Mn1/3O2 (160 mAh g-1).5, 6 However, the layered LiMO2 cathode materials are still confronted with some hurdles like poor cycling and thermal stability,7, 8 which always succumb to the result of high cation mixing9,

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and structural transformation on the particle surface.11

Firstly, only Ni2+ ions can occupy the 3b sites in Li+ layers due to the similar ionic radii of Ni2+ (0.69 Å) and Li+ (0.76 Å) ions, namely Li+/Ni2+ mixing. During charging, Ni2+ ions in Li sites are oxidized to smaller Ni3+ (0.56 Å), leading to the structural instability and capacity fading.12 In addition, the unstable and highly oxidized Ni4+ ions at charge process are apt to react with organic electrolyte and yield undesired side products to increase the block of Li+ diffusion, which can result in poor 3



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rate capacity and increased resistance.13 Furthermore, a subsequent structure transformation from the layered phase ( R3m ) to the defective spinel phase ( Fd3m ) and rock-salt phase ( Fm3m ), starts to take place in the particle surface with increasing cycles, which causes the further structural destruction and poor thermal stability.14-16 All those adverse reactions will be worse with the increase of Ni content and cut-off voltage. In this regard, the surface stabilization of nickel-rich layered oxides plays a significant role in enhancing the structural stability and cycling life of cathode materials.9, 17 To promote the electrochemical properties of nickel-rich cathode materials, abundant efforts have been made a contribution through diverse approaches, such as structural design,18 element doping9 and surface modification.19 It is common-known that

spatial

partitioning/grading

(Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O220),

of

compositions,

core/compositionally

like graded

core/shell shell

(the

Li[Ni0.8Co0.1Mn0.1]O2 cathode with a gradient shell18), full concentration gradient (the NCM cathode with a FCG21), are effective to optimize the trade-off between capacity and battery safety, but the synthesis complexity is still at a loss for production. In recent years, a large number of studies on the surface coating have been reported, such as fluoride (AlF3),19,

22

oxides(ZrO2,23 Al2O324) and lithium salts (Li3PO4,25

LiFePO426), which are regarded as a strong protective layer to separate the inner bulk from the electrolyte, decreasing the surface reactivity of electrodes with acidic species like HF.27,

28

Meanwhile, the coating layers have a certain suppression for the

non-conducting SEI formation29 and the disadvantageous phase transformation which 4


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priorly occurs on the particle surface.30 Nevertheless, most of the components of coating layers are incompatible with the lattice structure of nickel-rich layered oxides, which brings about some difficulties to offer a uniform protective effect for cathode materials. In terms of structural compatibility, the spinel oxides are a considerable choice to encapsulate layered oxides for the identical cubic close-packed arrays. Meanwhile, the spinel structure with efficient 3D Li-ions diffused paths meets the indispensable requests for the diffused kinetics of Li+ ions.31 Relevant work in lithium-rich layered oxides has been reported, for example, the cubic spinel structure on the surface of Li-rich layer-structured cathodes acquired a high rate capacity via super P treatment,32 and the spinel Li1+xMn2O4 membrane-encapsulation had an complete protection though nanocoating strategy.33 It is worthy noting that the high-voltage LiNi0.5Mn1.5O4 with high-efficient 3D Li+ ion diffusion paths gives strong backing to good cycling and rate ability.31 Therefore, simultaneously combining the advantages of structural design and spinel layer coating, it is signified that a heterostructured Ni-rich cathode with high-capacity layered core ( R3m ) and highly stable LiNi0.5Mn1.5O4 spinel phase shell ( Fd3m ) can become a potential candidate for advanced LIBs. To achieve a special heterostructure of LiNi0.88Co0.09Al0.03O2 layered core and LiNi0.5Mn1.5O4-like spinel shell, we introduce a simple spontaneous oxidation technology for hydroxide precursor, forming Ni0.88Co0.09Al0.03OOH and MnO2 layers in the precursor surface. After that Ni0.88Co0.09Al0.03OOH is converted to a ordered NCA bulk in calcination process,34 and the nanoscale MnO2 layer is turned into a 5



stable spinel LiNi0.5Mn1.5O4-like structure though high temperature solid state. Besides, the relevant characterizations of pristine and heterostructured NCA cathode materials have been thoroughly conducted via physicochemical and electro-chemical measurements.

2. EXPERIMENTAL SECTION 2.1 Material Preparation The pristine Ni0.88Co0.09Al0.03(OH)2 precursor (C-precursor) was prepared by a common co-precipitation method, then the surface layers of Ni0.88Co0.09Al0.03OOH and MnO2 were obtained by a simple spontaneous oxidation technology. At beginning, 0.05 mol KMnO4 and 0.05 mol NaOH were dissolved in 100 mL deionized water under stirring for 40 minute. Then, 3.0 g Ni0.88Co0.09Al0.03(OH)2 precursor powders were slowly added into this uniform mixed solution and stirred for 30 min at a uniform rate. After reaction, the as-obtained precursor (M-precursor) was washed with deionized water until the filtrate became clear and then dried at 75 °C for 12h. To prepare the heterostructured cathode material (M-NCA), the precursor with the surface layers of Ni0.88Co0.09Al0.03OOH and MnO2 (M-precursor) and LiOH was uniformly mixed by grinding at a stoichiometric ratio of 1:1.03. After that, the mixture was calcined for 12h at 750 °C in O2 atmosphere. In order to compare, pristine C-precursor was also mixed with LiOH and calcinated in the similar way to prepare the pristine LiNi0.88Co0.09Al0.03O2 cathode material (C-NCA).

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2.2 Materials Characterizations The morphology of the C-precursor and M-precursor, C-NCA and M-NCA samples was expressed by scanning electron microscopy (SEM) (Quanta 650 ESEM, FEI). The value states of chemical compositions were explored by X-ray photoelectron spectroscopy (XPS) with a K-Alpha 1063 using mono Al Kα (Thermo Scientific ESCALAB 250Xi). The crystalline phases of four samples were analyzed using the X-ray diffractometer (MiniFlex 600, Rigaku) with Cu Kα radiation (λ=0.154178 nm） at 40 kV and 40 mA, and the X-ray diffraction (XRD) information was obtained in a scattering angle (2θ) scope of 10-90° with a uniform scan rate of 0.2° s-1. To further observe the precise structural varieties and elemental contribution, transmission electron microscopy (TEM, JEOL JEM-2100F) and energy dispersive X-ray spectroscopy (EDS, JSM-6360LV scanning electron microscope, JEOL, Japan) were used and the TEM samples were investigated by focused ion beam (FIB). For thermal stability measurement, the 2025 coin-type half-cells were charged to 4.3 V and disassembled in an Ar-fulled glovebox. After removing the residual electrolyte from the electrodes surface, the active materials were regained from the electrodes. The 3 mg of regained materials were measured in differential scanning calorimetry (DSC) (Netzsch, Germany) at an operating step of 5 ℃ and per step of 1 min.

2.3 Electrochemical Measurements

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The cell electrode was made by a way of mixing cathode materials, with poly-vinylidene fluoride (PVDF), Super-P as well as electrode materials at a weight rate of 10 : 10 : 80 (wt%) in N-methylpyrrolidone (NMP), which was homogenized for 4 h at 400 rpm. Then the obtained slurries were covered on smooth Al foils, and dried in vacuum lasting 15 h at 80℃. The dried electrodes were under pressure of 20 Mpa to be trimmed to a diameter of 10 mm and the loading mass of active materials was 5–6 mg cm-2. The CR2025 coin cell was fabricated in an Ar-fulled glovebox with Li flake as the counter electrode, LiPF6 (1 mol L-1) in a mixture solution (ethylene carbonate (EC) and dimethylcarbonate (DMC) with a ratio of 1:1) as electrolyte, and a polyporous poly-propylene as separator. Eventually, at various current densities, the battery test system (CT-3008, Neware Co., China) was utilized to test the relevant electrochemical properties of the prepared coin cells in the range of 2.75-4.3 V (Li+ vs. Li) and 2.75-4.5 V. Moreover, GITT was dedicated to determining diffused coefficient of lithium ions, through the charge and discharge of cells under the condition of 0.1C (1C = 200 mA g-1) between 2.75 and 4.3 V for 10 minutes, and maintaining in open-circuit voltage (OCV) for 1 hour.

3. RESULT AND DISCUSSION Figure 1 displays the synthesis schematic of the heterostructured NCA cathode material.

When

the

alkaline

KMnO4

solution

contacts

with

the

solid

Ni0.88Co0.09Al0.03(OH)2 precursor, a simple spontaneous redox reaction will occur as following equation (1). 8


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MnO42- + 2Ni0.88Co0.09Al0.03(OH)2 →2Ni0.88Co0.09Al0.03OOH + MnO2 + 2OH- (1) The hydroxide precursor surface can be oxidized to Ni0.88Co0.09Al0.03OOH, which is more stable in the solution of high pH value, while Ni0.88Co0.09Al0.03OOH is apt to form an ordered LiNi0.88Co0.09Al0.03O2 bulk via calcination.34 Synchronously, the formed MnO2 can be well coated on the precursor surface. By calcinating at 750 °C, LiOH with low melting point can cause the cushy migration of Mn4+ into the particles.

Figure 1. Synthesis illustrative diagram of the heterostructured M-NCA particles.

The morphology characterizations of the as-prepared samples are displayed in Figure 2. All of samples are like-spherical shape with an average diameter of approximately 10 μm. As shown in Figure 2a and 2b, the color of M-precursor obviously changes from green (the inset of Figure 2a) to black (the inset of Figure 2b), but no clear differences can be observed on the morphology of precursor particles and this may be concerned that the Ni0.88Co0.09Al0.03OOH and MnO2 coating layers 9



are too thin to observe. To analyze the element distribution of two hydroxide precursors, the M-precursor and C-precursor samples were characterized by EDX. Comparing Figure 2e with 2f, it can be clearly found that only M-precursor possesses the distribution of Mn element, indicating the Mn element has existed in M-precursor. As indicated in Figure 2c and 2d, each secondary particle of two NCA cathode materials is the aggregation of many primary grains,35-37 and the average size of M-NCA primary grains (~500 nm) is smaller than that of C-NCA (~1 μm). Simultaneously, a compact aggregate structure of primary grains is seen on the outer surface of the M-NCA particle, unlike some mesopores generating in the C-NCA particle surface.38, 39 The obvious differences between the two samples demonstrate that the participation of Mn element may affect the surface structure of NCA particles, and offer a compact surface structure to avoid the permeation of electrolyte. Moreover, the Mn element may present a gradient distribution in the surface layer of the M-NCA particles, because high lithiation temperature can lead to the directional migration of the metal elements during calcination. In order to attest the inference, the line-scanning mapping of Mn and Ni element along the red arrow in M-NCA particle is shown in Figure 2g. The signal intensity of Mn element is greater in comparison with that of Ni element, especially close to the particle outer surface. That result makes it clear that a Mn-rich structure layer with concentration gradient is possible to grow on the M-NCA particle exterior.

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Figure 2. Morphology characterization: SEM images of (a) C-precursor; (b) M-precursor; (c) C-NCA; (d) M-NCA; the two insets indicate the color transformation of the precursor treated before and after; EDX spectrum of (e) C-precursor and (f) M-precursor; (g) Line-scanning profile for the intensity ratio of Mn element/Ni element in M-NCA particle, the inset is TEM image of one M-NCA particle.

For the sake of investigating the value state of metal elements in C-NCA as well as M-NCA cathode materials, the two samples are analyzed via XPS. As presented in Figure 3a and 3b, for M-NCA sample, the obvious Mn 2p1 peak is detected at 643.08 eV, which corresponds to the Mn-O bond, and the binding energy of Mn 2p3/2 exactly indicates the existence of Mn4+.40, 41 In addition, the Ni 2p spectra of C-NCA and M-NCA samples shown in Figure 3c and 3d reveal that Ni2+ and Ni3+ are coexisted in the samples, and the peaks are located at 855.58 and 857.08 eV, respectively. Through a further analysis, it is noted that the content ratio of Ni2+/Ni3+ is obviously decreased from 1.14 (C-NCA) to 0.91 (M-NCA) after oxidation. Thereby more Ni3+ can be presented near the M-NCA surface due to the oxidation of Ni2+, which is beneficial to form an order NCA bulk with low degree of Ni2+/Li+ disorder. To identify the crystal structure and phase composition of the established samples, as shown in Figure 3e, the C-NCA and M-NCA samples are all well indexed to a hexagonal a-NaFeO2 type structure with the space group R3m . Meanwhile, the adjoining peaks of (006)/(102) and (018)/(110) of two samples take on an obvious split, revealing a well-organized layered structure.42 Besides, it is widely known that with cation mixing generated, the transitional metal ions displace the lithium sites. 12


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The disorder causes a fractional destructive interference of the (003) plane’s constructive interference at the Bragg angle of θd(003) and a reduction in the (003) peak intensity. On the contrary, the (104) peak intensity increases because the transitional metal ions are also on the (104) plane in the lithium layer, resulting in enhancive constructive interference of the peaks for (104) planes. Thus the peak intensity ratio of I(003)/I(104) reduces while the degree of cation disorder increases.43, 44 In the M-NCA cathode material, the I(003)/I(104) ratio of 1. 652 is obviously higher than that of 1.501 in the pristine C-NCA, indicating a low mixing degree of Ni2+/Li+ for M-NCA. This can be attributed to the formation of an ordered NCA bulk by sintering the oxidized precursor 34. Although it is difficult to detect the existence of the spinel phase since the primary diffractive peaks of spinel phase are in superimposition with those of layered phase, the XRD pattern of M-NCA shows that the most of peaks become widening. The widening peaks may be relevant to the spinel-phase formation,33 which will result in an uneven strain distribution on account of two completely diverse phase structures. For the qualitative analysis of the local strain factor, the Δk45 (full width at half maximum) is fitted for every peak and the peak width of heterostructured M-NCA (Δ k = 0.0076 ± 0.002 nm-1) is increased compared to the pristine value for C-NCA sample (Δ k = 0.0065 ± 0.002 nm-1).

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Figure 3. XPS spectra of (a) M-NCA, (b) Mn 2p of M-NCA, (c) Ni 2p of C-NCA and (d) Ni 2p of M-NCA. XRD pattern of (e) C-NCA and M-NCA, left side displays the magnification part of XRD pattern at 17.4°-22°.

The TEM characterization is applied to further analyze the detailed structural feature of particle surface. It can be seen from Figure 4b that the particle surface of as-obtained M-NCA is a distinguishable heterostructure with uniform thickness about 14


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15nm. The lattice spacings in the core are 0.472 nm, 0.144 nm and 0.138 nm, respectively, in response to (003), (110) and (113) planes of the conventional layered structure. In the shell, three lattice distances 0.236 nm, 0.246 nm, 0.157 nm can be well indexed to ( 222 ), (311) and ( 511 ) planes of the LiNi0.5Mn1.5O4 spinel structure, which is further affirmed through the related FFT result (the inset of Figure 4b). Obviously, as observed in Figure 4a, the core and surface of pristine C-NCA particle is the preponderant fringes of (003) planes with an interplanar distance of 0.472 nm that is consistent with the layered phase. The apparent distinction demonstrates the formation of a heterostructure cathode material which is consisted of the LiNi0.88Co0.09Al0.03O2 layered core and the LiNi0.5Mn1.5O4 -like spinel phase shell.

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Figure 4. TEM images of (a) C-NCA and (b) M-NCA; the insets represent the lattice spacing of C -NCA and the corresponding FFT transform of C-NCA and M-NCA.

The electrochemical properties of the as-obtained C-NCA and M-NCA cathode materials are investigated by half cells. In Figure 5a and 5b, the voltages in ~3.8 V, ~4.0 V, and ~4.2 V for the CVs profiles of C-NCA and M-NCA electrodes are respectively corresponded to three pairs of redox peaks. The first is hexagonal phase to the monoclinic phase (H1 to M), M to the second hexagonal phase (M to H2), and 16


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H2 to the third hexagonal phase (H2 to H3) in the charge−discharge process.46, 47 It can be clearly seen that only two peaks appear at the first oxidation curves of C-NCA electrode, but three oxidation peaks are observed in M-NCA electrode at beginning, which can exactly respond to three reduction peaks. The suppression of this dissymmetry phenomenon in initial charge can effectively enhance the initial coulombic efficiency of the nickel-based cathodes.48, 49 In addition, an undesired peak is shown at 3.56 V for C-NCA electrode in the following reduction curves, which isn’t observed in M-NCA electrode, indicating that the hetetrostructured M-NCA controls a later disordered phase formation. Furthermore, it is important to note that the M-NCA electrode has a high overlap ratio of CV curves, and this is generally indicated to a stable cycling performance. Figure 5c displays the first charge and discharge profiles of the as-established electrodes. The C-NCA and M-NCA cells all deliver a high reversible capacities (>210 mAh g-1) under 0.1C in the voltage range of 2.75-4.3V (1 C = 200 mA g-1), and the M-NCA electrode displays a higher capacity of 219.5 mAh g-1 in comparison with that of 214.6 mAh g-1 for the C-NCA electrode, which are corresponded to a higher initial coulombic efficiency of 89.6% for M-NCA than one of 84.5% for C-NCA.2, 11 This could be because the stable core/shell structure can restain an irreversible phase transition in initial charge process. As shown in Figure 5d, 5e and 5f, with the increasing of cycling number at 0.5C, the shape of the charging and discharging profiles for M-NCA electrode is almost coincident, which proves a great electro-chemical reversible property and a small polarization phenomenon. In 17



particular, the M-NCA electrode delivers high capacity retention of 89.7% at a rate of 0.5C after 100 cycles whilst the 100th discharging capacity is still at 180.6 mAh g-1. The C-NCA electrode only sustains 76% and about 147.5 mAh g-1 after 100 cycles. Besides, in Figure 5g, the M-NCA electrode has good capacity retention of 81.9% 100 cycles later at 1C between 2.75V to 4.3V, while the C-NCA electrode only keeps 70.3% and exhibits a rapid capacity fading at high current rate. To enhance the discharge capacity of NCA cathode materials, a large voltage window is frequently used as a feasible method. However, up to 4.4V charged stage, a severe migration of transition-metal ions from TM slab to Li slab can lead to the structural instability and the anabatic crystal defects, like the NiO-rock-salt phase formation, will block the shift of Li ions on the surface,11, 43 which results in a fast capacity loss with increasing cycling number.11, 50 As shown in Figure 5h, the M-NCA electrode can express a significant improvement in cycling stability even at high voltage of 4.5V, still retaining the capacity retention of 78.7% compared with 67.5% for C-NCA electrode.11, 50, 51 Therefore, the high reversible capacity and the enhanced cycling stability for M-NCA electrode are primarily attributed to its beneficial structure design with a low level of Ni2+/ Li+ disorder, while the stable LiNi0.5Mn1.5O4-like spinel surface ensures the structural stability of the Ni-rich cathode materials thanks to avoiding the surface erosion of electrolyte.

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Figure 5. Electrochemical performances of C-NCA and M-NCA. (a) (b) Cyclic voltammograms of the first three cycles at scanning rate of 0.1 mV s

-1

for C-NCA and M-NCA; (c) initial

charging−discharging profiles of C-NCA and M-NCA at 0.1C, the inset is the LED logo lit by lithium-ion battery based on M-NCA electrode; (d) cycling properties at 0.5C between 2.75 and

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4.3 V and selective charging−discharging profiles at selected cycles (e) (f) of C-NCA and M-NCA; (g) (h) cycling performance at a rate of 1C from 2.75 to 4.3 V and in a range of 2.75 -4.5 V for C-NCA and M-NCA , respectively.

To further testify the priority of a stable LiNi0.5Mn1.5O4-like spinel surface in supporting the structural stability of M-NCA cathode materials, the particle morphology of C-NCA and M-NCA electrodes after 100 cycles at 4.5V is characterized by SEM. As shown in Figure 6a and 6b, the C-NCA and M-NCA electrodes are made up by spherical cathode material as well as other leftover that is possibly PVDF and Super-P. It can be seen that the particles of C-NCA electrode are sectionally disintegrated and the M-NCA electrode still well maintains the integrity of particle structure. As to further observation (in the insets of Figure 6a and 6b), the C-NCA particles occur a severe surface corrosion due to the undesired side reactions with electrolyte,52 but the particle morphology of M-NCA electrode can be well maintained after cycling. Above suggests that the LiNi0.5Mn1.5O4-like spinel shell can effectively mitigate the surface degradation and remarkably enhances the holistic structural stability during cycling. HRTEM was conducted to explore the crystal structural changes in pristine C-NCA and heterostructure M-NCA electrodes during cycling, the cycled electrodes were recovered from the cells after 50 cycles at 4.5V. It can be found in Figure 6c that an uneven etched region is exposed on the surface of pristine C-NCA. And the exterior zone is prone to form the spinel-like and NiO-like rocksalt structure due to the surface reconstruction in Ni-rich cathode materials within repeated cycles, which is in good 20


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accordance with recent reports.53,

54

In comparison with the severe structural

decomposition of pristine C-NCA electrode, as displayed in Figure 6d, the heterostructured M-NCA electrode is well retained, except that the spinel-like shell is slightly thicker. The observation immediately indicates that the LiNi0.5Mn1.5O4-like spinel shell can well achieve the surface and bulk crystal structure stability, thus providing continuous protection against the electrolyte erosion and contributing to the advanced capacity retention.

Figure 6. SEM images of (a) C-NCA and (b) M-NCA electrodes after 100 cycles between 2.75 V and 4.5 V at 1C; the insets are SEM images of one C-NCA and M-NCA particle 100 cycles later at 4.5V. HRTEM patterns and FFTs after 50 cycles betwixt 2.75 V and 4.5 V at 1C condition. (c) The lattice image of C-NCA where ①,②,③ refer to the layered, spinel-like and NiO-like

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rocksalt structure, corresponding to the FFTs of ①,②,③, respectively; (d) the lattice image of M-NCA and corresponding FFTs of the core and shell regions.

Figure 7a, 7b and 7c exhibits the rate capabilities of the C-NCA and M-NCA electrodes from 2.75 to 4.3 V, with rate augment from 0.1 to 0.2, 0.5, 1, 2, 5, 10 C, sustaining five cycles at each rate. The shapes of the discharging curves for both electrodes are similar at relatively low current densities. However, M-NCA can achieve rather high discharge capacities at relatively large current densities. It can be further found that the M-NCA sample delivers a reversible capacity of 153.7 mAh g-1 even at 10 C while the capacity of pristine C-NCA electrode is only 141.8 mAh g-1, which is mainly because of the surface corrosion of electrolyte in C-NCA electrode, and subsequent disordered phase transition impede the Li-ion diffusion.55 In addition, when the rate is brought back to 0.1C, there is an explicit decrease in the capacity of C-NCA electrode compared with its initial value, but the M-NCA electrode can almost restore its initial discharge capacity. The improved reversibility of the heterostructured M-NCA is attributed to its strong structural endurance for rapid Li-ion migration. Electrochemical impedance spectroscopy (EIS) experiments are conducted to research the dynamic behavior of C-NCA and M-NCA electrode materials. The corresponding Nyquist plots are shown in Figure 7d. It is well-known that Rs refers to the ohmic resistance from the electrolyte, current collectors, and cell connections. The semicircle situated in high-frequency range is on behalf of the surface film resistance Rsf as well as the charge-transfer resistance Rct, which is concerned with the electronic 22


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exchange between the electrode interface and the surface SEI. The straight line at low frequencies represents a semi-infinite Warburg impedance Wa, which is related to Li+ diffusion in a solid-state electrode.56 According to Figure 7d, the Rct value of the M-NCA electrode is only 27.9  after 3 cycles and that of the C-NCA electrode is 50.2 . Meanwhile, it can be obviously observed that the Rct values of M-NCA electrode are slowly increased from 27.9  to 89.4  after 100 cycles, whereas the Rct values of C-NCA electrode increase from 50.2  to 176.8 . The increase of impedance for the C-NCA electrode is because of an aggravated surface destruction, such as the thicker SEI formation and the disordered phase structure generation, which leads to the barrier of Li-ion diffusion and electron mobility. However, the stable LiNi0.5Mn1.5O4-like spinel shell on the surface of M-NCA can effectively restrict the corrosion of inner core, and this will be more conducive to the intercalation/deintercalation of Li ions.

23



Figure 7. Discharge capacity at various current rates and the corresponding discharge curves of (a) C-NCA and (b) M-NCA, the insets in (a) (b) represent the corresponding voltage at different current rates; (c) rate performance of C-NCA and M-NCA electrodes. Equivalent circuit for the Nyquist plots of (d) C-NCA and M-NCA in the 3rd cycles and in the 100th cycles.

Galvanostatic intermittent titration technique (GITT) is generally regarded as an accessible approach to appraise the lithium ion diffused coefficient ( DLi + ) of electrode materials,15, 57 especially being widely used to reflect the rate capability of electrodes. As revealed in Figure 8a and 8b, the typical GITT titration steps around 3.85V of the C-NCA and M-NCA electrodes are presented with marked parameters. It

24


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can be seen in Figure 8c and 8d that the voltage has good linear relation with τ 1/2 for one titration of both. Thus the DLi + can be calculated by following equation (2).58 2

DLi+

2

4  V   dE dx  L2 t  =  I0 m    DLi+ π  FS   dE / dt1 / 2  ,

(2)

where Vm (cm3 mol-1) is the molar volume of samples and inferred from crystallographic data, F (C mol-1) stands for the Faraday constant, and L (cm) represents the diffusion length and S (cm2) refers to the surface area of the sample-electrolyte interface. In Figure 8e, there is a rising trend of DLi + for both electrodes when electric voltage rises from 3.64 to 3.78V, which is related to the removal of O2--Li+-O2- bond, resulting in the expansion of the Li-slab space to favor Li-ion diffusion.15, 59 The value of DLi + is declined on end part as Li-slab contraction along the c-axis hinders the Li-ions diffusion. It is clearly seen that a high value of DLi + for M-NCA electrode is presented, which is devoted to the reduced cation mixing effect in the NCA cathode materials.58 Moreover, in three local voltage ranges (3.82~3.9 V, 4.04~4.1 V, and 4.14~4.2 V) of the M-NCA electrode, the DLi + undulations show a w-type variation, which is exactly corresponded to three anodic peaks of M-NCA at ~3.8 V (H1 and M), ~4.0 V (M and H2), and ~4.2 V (H2 and H3) of the CV profiles, being ascribed to the interaction with Li ions.60 In addition, each DLi+ profile of the C- NCA and M-NCA electrodes shows a n-type shape, and the DLi+ value of M-NCA can sustain at average range of ~1.38×10-10 cm2 s-1 and the average DLi+ value of C-NCA is only ~1.03×10-10 cm2 s-1. Accordingly, the superior holistic trend of hetetrostructured

25



M-NCA further proves the improved rate capacity and offers a highly efficient channel for lithium ion intercalation and deintercalation.

Figure 8. The GITT tests of C-NCA and M-NCA electrodes. (a) (b) Scheme for the voltage response of a charge pulse at around 3.85 V in the GITT experiment with labeling parameters; (c) (d) Plot of voltage against τ 1/2 to show the linear fit; (e) DLi+ calculated from GITT test during charging process. DSC profiles of (f) C-NCA and M-NCA electrodes at charged state of 4.3 V.

A great concern with the Ni-rich layer oxides is the thermal instability problem which is caused by a serious exothermic reaction with electrolyte, and the delithiated 26


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electrode can lead to the increased oxygen releasing from the electrode structure. Thereby, it affects the safety of lithium-ion batteries.61,

62

To explore the

thermodynamic characteristics of the C-NCA and M-NCA electrodes, DSC is carried out after charging to 4.3 V (vs Li/Li+), and the results are shown in Figure 8f. It can be found that the maximum exothermic peak of C-NCA electrode is 239 ℃, which stands for the release extent of lattice oxygen, while one for the heterostructured M-NCA is 248℃, delaying the thermal runway reaction. Meanwhile, a total heat generation of the C-NCA electrode is 556.4 J g−1, which is obviously decreased to 502.7 J g−1 for the M-NCA electrode. This can be summed up that the heterostructured M-NCA with good thermal stability can maintain the stability of lattice structure through restraining the thermal activity of electrolyte.

4. CONCLUSIONS In summary, a heterostructured NCA cathode material that is composed of a Ni-rich layered ( R3m ) core and a LiNi0.5Mn1.5O4-like spinel ( Fd3m ) shell, has been synthesized by a facile spontaneous redox reaction and subsequent calcination process. Through full examinations of structure characterization and battery performances, the remarkable advantages of the specially designed sample have attracted an attention. (1) It is relatively significant to design and prepare the heterostructured NCA cathode materials since the structural compatibility and the stable spinel structure can

27



effectively suppress the corrosion of cathode materials and surface side reactions of with electrolyte. (2) The heterostructure with a low Ni2+/ Li+ disordering degree can afford a stable structure framework and high-efficiency paths for lithium ions diffusion within extraction/insertion process, reciprocating to high Li+ ion diffusion coefficient of M-NCA of ~1.38×10-10 cm2 s-1; (3) The heterostructure can well maintain a stable lattice structure, which will enhance safety performance and suppress phase transformation, especially for guaranteeing a high capacity during current rate. The novel designed NCA cathode material with improved structural stability delivers a higher initial coulombic efficiency (89.6% at 0.1C) and an improved cyclic stability (a high retention of 89.7% after 100 cycles at 0.5C). Therefore, the new structure design with prominent electrochemical properties can assist in developing a high energy density and high safety cathode materials for the application of LIBs.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (X.W.)

ORCID Xianyou Wang: 0000-0001-8888-6405

Notes 28


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (No. 21703191) and Key Project of Strategic New Industry of Hunan Province (No. 2016GK4030 and 2016GK4005).

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Preparation and performances of the heterostructured material with a

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