Multishell Precursors Facilitated Synthesis of Concentration-Gradient

Jun 26, 2018 - ... and safety because of the mitigated side-reactions and fast Li+ kinetics. ... applications of CG cathodes for state-of-the-art LIBs...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

Multishell Precursors Facilitated Synthesis of ConcentrationGradient Nickel-Rich Cathodes for Long-Life and High-Rate LithiumIon Batteries Peiyu Hou,† Feng Li,‡ Yanyun Sun,‡ Huiqiao Li,*,§ Xijin Xu,*,† and Tianyou Zhai§ †

School of Physics and Technology, University of Jinan, Jinan 250022, China School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, Tianjin 300350, China § State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China ACS Appl. Mater. Interfaces 2018.10:24508-24515. Downloaded from pubs.acs.org by DURHAM UNIV on 08/30/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The rational design of concentration-gradient (CG) structure is demonstrated as an available approach to improve the electrochemical performances of high-energy nickel-rich cathodes for lithium-ion batteries (LIBs). However, the complicated preparing processes, especially the CG-precursors, generally result in the lessthan-ideal repeatability and consistency that is regarded as an extreme challenge for the widespread commercialization. Thus, the innovative strategy with facile steps and the feasibility of large-scale preparation for commercialized applications should be urgently developed. Herein, through the temperature-tunable cation diffusion, the feasibility of controllable preparation of nickel-rich CG-LiNi0.7Co0.15Mn0.15O2 (NCM) from multishell precursors is first demonstrated. As expected, the Li/CGNCM half cells show much enhanced cycle-life, rate property, and safety because of the mitigated side-reactions and fast Li+ kinetics. Besides, the Li4Ti5O12/CG-NCM full cells also exhibit long-term lifespan, 95% capacity retention even after 2000 cycles, and high-rate behaviors. Importantly, by contrast with the conventional techniques that prepare CG cathodes from CG precursors, the proposed new synthesis strategy from multishell precursors is suitable for large-scale preparation. Overall, this multishell precursor-facilitated synthesis probably promotes the practical applications of CG cathodes for state-of-the-art LIBs and also can be easily expanded to controllably preparing spinel- and olive-type CG cathodes. KEYWORDS: lithium-ion batteries, nickel-rich cathodes, multishell precursors, cation diffusion, electrochemical properties the Ni2+ dissolution and electrolyte decomposition;16−18 (2) the R3̅m-layered phase readily shifts to an Fm3̅m rock-salt phase in that the Ni2+ (3a sites) tends to occupy the neighboring Li+ vacancy (3b sites);19−21 and (3) active oxygen easily separate from host structure of the highly charged cathode, which further gives rise to fearful safety problem.22,23 Consequently, Ni-rich-layered cathodes show rapid capacity fading during cycling caused by the active mass loss along with sluggish Li+ intercalation/deintercalation kinetics and poor safety owing to the oxygen loss from the host structure. Additionally, state-of-the-art LIBs as energy-storage devices for EVs need to shorten the charge time by improving the rate capability to perfect their practicality. Overall, developing Nirich-layered cathodes with better electrochemical behaviors and safety are urgent for their practical applications in LIBs.

1. INTRODUCTION In the past decades, the development of desired cathodes with high capacity, long lifespan, good rate property, and safety to replace the conventional-layered LiCoO2 has been the chief driving force for lithium-ion batteries (LIBs).1−6 Three groups of cathodes, layered Li[Ni−Co−Mn]O2 and Li1+x[Mn− M]1−xO2 (M = Ni, Co, Ru, Ti, etc.), spinel LiNi0.5Mn1.5O4, and olive LiMPO 4 (M = Fe and Mn), are broadly investigated.1,7−9 Among these candidates, nickel-rich (Nirich)-layered Li[Ni1−xMx]O2 (M = Co, Mn, etc.), the solid solutions of LiNiO2−LiMO2, deliver specific capacity exceeding 200 mA h g−1, low cost, and reasonable rate capability, which are regarded as a class of promising cathodes for advanced LIBs, especially as the durable power sources applied into the electric vehicles (EVs).10−17 As for Ni-rich Li[Ni1−xMx]O2 electrode, the Ni2+/3+/4+ redox couples offer the majority of specific capacity.2,5,16 However, it usually shows severe structural and thermal instability at the charged state: (1) the high oxidizing Ni4+ ions easily react with electrolyte on the interface of electrode/electrolyte, resulting in © 2018 American Chemical Society

Received: April 18, 2018 Accepted: June 26, 2018 Published: June 26, 2018 24508

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of preparation for (a) CG and (b) multishell hydroxide precursors via coprecipitation reaction. (c) Triangular phase diagram of LiNiO2−LiCoO2−LiMnO2 and the corresponding composition points of LiNi1/3Co1/3Mn1/3O2, LiNi0.7Co0.15Mn0.15O2, and LiNi0.8Co0.1Mn0.1O2. (d) Schematic diagram of controllable preparation of Ni-rich CG cathodes from multishell precursors.

To solve the foregoing drawbacks, Sun et al.24 have proposed core−shell-structured Ni-rich cathode Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2, in which the structurally/thermally stable LiNi0.5Mn0.5O2 (NM55) shell is utilized to encapsulate the high-capacity Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) core. The core−shell configuration possesses two significant effects: (1) the shell can protect the sensitive core from directly infiltrating into electrolyte, restraining the unwanted structural transitions and the loss of Ni-rich active components; (2) the shell has stable electrode/electrolyte interface and offers available Li+/electron migration path during redox. As expected, the core−shell electrode exhibits much enhanced cycling/thermal stability because of the protection of micron-sized shell when compared with the core component. Unfortunately, after in-depth studies on the core−shell Ni-rich cathodes within repeated cycling, a structural mismatch, that is, void layer of tens of nanometers between core and shell, is detected.25 The formed void layer cuts off the available paths of Li+ migration and electron transfer, limiting the subsequent exertion of electrochemical behaviors for core−shell cathodes. Then, the concentrationgradient (CG) Ni-rich LiNi0.64Co0.18Mn0.18O2, where the core LiNi0.8Co0.1Mn0.1O2 is surrounded by an outer CG layer is reported by Sun and co-workers to solve the said shortage of core−shell cathodes.26 Recently, (full) CG Ni-rich-layered oxides are further reported and show exciting performances as high-capacity, long-life, and safe cathode for advanced LIBs.27−37 Note that previous studies have focused on preparing (full) CG cathode materials from the aimed (full) CG precursors.26−37 However, the preparation of CGprecursors must be accurately managed, as shown in Figure 1a, that is, the solution 2 should be continuously dropped into solution 1 to form CG solution. Meanwhile, the formed CG solution also should be simultaneously added into the continuously stirred tank reactor (CSTR), obtaining the aimed CG precursors. The sophisticated preparation parameters make the less-than-ideal repeatability and consistency of the CG precursors. Actually, the degree of repeatability and consistency is an extremely significant factor for the widespread commercialization of cathode/anode materials.18,20,38

Thus, innovative strategy with facile steps and the feasibility of large-scale preparation for practical applications should be urgently developed for the CG cathodes. In this article, to solve the aforesaid problem confronted in these traditional CG cathodes, the feasibility of controllable preparation of CG Ni-rich cathodes from micron-sized multishell spherical precursors is demonstrated for the first time. Importantly, by contrast with the conventional techniques that prepare CG cathodes from CG precursors, the multishell precursors can be facilely and massively synthesized only by replacing the various shell solution during coprecipitation reactions, as depicted in Figure 1b. Therefore, the preparation of CG cathodes from these target multishell precursors suggests much improved repeatability and consistency, which is suitable for large-scale production. C o n s e q u e n t l y , t h e a s - p r e p a r e d C G Ni - r i c h L i Ni0.7Co0.15Mn0.15O2 from multishell precursors delivers high specific capacity, long cycle lifespan, good rate property, and safety. This proposed ideal provides a new insight into controllably preparing compositionally graded cathode materials including layered, spinel, and olive structures and also probably promotes the widespread commercialization of CG Ni-rich cathodes for state-of-the-art LIBs.

2. RESULTS AND DISCUSSION 2.1. Design of Concentration-Gradient Ni-Rich Cathodes from Multishell Precursors. The triangular phase diagram of LiNiO2−LiCoO2−LiMnO2, as shown in Figure 1c, is utilized to design the aimed core−shell Ni-rich cathodes. Given that the composition points of three-layered oxides LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.7 Co 0.15 Mn 0.15 O 2 , and LiNi0.8Co0.1Mn0.1O2 located at the same straight line, meaning the LiNi 0.7Co0.15 Mn0.15 O 2 can be decomposed as the compositions of LiNi1/3Co1/3Mn1/3O2 and LiNi0.8Co0.1Mn0.1O2 by adjusting its molar ratio. In other words, the LiNi0.7Co0.15Mn0.15O2 can be designed as the core−shell Li[(Ni0.8Co0.1Mn0.1)0.786(Ni1/3Co1/3Mn1/3)0.214]O2, wherein high-capacity LiNi0.8Co0.1Mn0.1O2 as inner core, whereas the stable LiNi1/3Co1/3Mn1/3O2 as outer shell are rationally selected. Moreover, the multishell structure is able to 24509

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a,b) SEM images of multishell spherical precursors and (c) variation of atomic ratio on the cross section of a single secondary particle by EPMA. (d−f) SEM images of the CG-NCM cathode materials from multishell precursors, (g) SEM of the cross section prepared by FIB, (h) variation of the atomic ratio on the cross section by EPMA, and (i) XRD Rietveld refinement of CG-NCM and N-NCM.

coprecipitation reactions in the CSTR. The morphology of the as-prepared precursors are measured by scanning electron microscopy (SEM), as shown in Figures 2a,b and S1. Both precursors show monodisperse spherical particles that are assembled by nanosized primary grains. The normal precursors have an average particle size (D50) of 10.77 μm, which is slightly larger than 9.94 μm of the multishell sample in Figure S2. The D50 of multishell precursors show a growth function as follow: D = 1.3367 + 2.7488T1/3 (D is D50, T is reaction time). From Figure S4, X-ray diffractometry (XRD) patterns demonstrate the layered β-Ni(OH)2-type phase and good crystallinity of both precursors.43 ICP−AES reveals that the actual atomic ratio of Ni−Co−Mn is close to the target 0.7/ 0.15/0.15, as depicted in Table S1. To measure Ni, Co, Mn element contents inside the spherical secondary particles, the cross section of a single powder is prepared, as seen in the inset of Figure 2c. The element fraction on this cross section is analyzed by electron microprobe analysis (EMPA), as shown in Figure 2c. The inner section from 0 to 4 μm shows nearly a constant atomic ratio of Ni−Co−Mn (0.8/0.1/0.1), acting as a core component but from which the atomic ratio of Ni−Co− Mn varied. The noncontinuous changes of Ni−Co−Mn on the outer surface confirm the target multishell structure. The mixture of Li2CO3 and multishell precursors are sintered at 800 °C for 12 h to induce moderate diffusions of Ni, Co, and Mn ions on the interface of core−shell and shell/ shell and then form the designed CG structure at the outer surface after lithiation. SEM images confirm that the spherical morphologies are still retained after calcination in Figures 2d−f and S5. The close stacking of monodisperse micron-sized spherical powders render high tap density of 2.42 and 2.38 g

be further designed based on this original core−shell structure, that is, part of the core is extracted to form the outer multishells with the said single shell by controlling their molar ratio. Here, multishell Li{[(Ni 0 . 8 Co 0 . 1 Mn 0 . 1 ) 0 . 6 ] c o r e [(Ni0.706Co0.147Mn0.147)0.089]shell1[(Ni0.612Co0.194 Mn0.194)0.084]shell2[(Ni0.52Co0.24Mn0.24)0.08]shell3 [(Ni0.426Co0.287Mn0.287)0.076]shell4[(Ni1/3Co1/3Mn1/3)0.071]shell5}O2 is correspondingly fabricated. From previous reports, high-temperature solid-state reaction causes cation diffusion among core and shells, and the diffusion rate is direct to diversity of element content and reaction temperature.18,39−42 Generally, tiny diffusion is observed under 800 °C, and moderate diffusion occurs from 800 to 850 °C, whereas the high temperature of above 850 °C makes severe cation diffusion.36 Therefore, as to the foregoing multishell samples, diffusions of transition-metal ions will first occur on the interface of core−shell and shell/shell during calcination. As a result, the concentration interval variation for core−shell and shell/shell in the multishell structure will tend to develop CG structure under moderate diffusion effect from 800 to 850 °C. Then, an innovative strategy was rationally proposed to achieve the CG Ni-rich cathode materials from multishell precursors for the first time on account of the above analysis, as described in Figure 1d. The Ni-rich LiNi0.7Co0.15Mn0.15O2 is taken as an example to demonstrate the feasibility of controllable preparation of CG structure from multishell spherical precursors under moderate diffusion effect at 800 °C in this work. 2.2. Synthesis of Concentration-Gradient Ni-Rich Cathodes from Multishell Precursors. The designed multishell and the normal precursors are synthesized by 24510

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

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Figure 3. (a) Initial charge/discharge curves of both NCM at 25 °C and (b) stability of reversible capacity and median potential of both NCM at 25 °C. The continuous charge/discharge curves of (c) CG-NCM and (d) N-NCM from 5th to 200th cycle. (e) Cycling stability of both NCM at 55 °C. The (f,g) rate discharge capacity and (h) energy density for both NCM electrodes.

cm−3 for N-NCM and CG-NCM, respectively, which is beneficial for improving the volumetric-energy-density of LIBs.44−46 FIB is utilized to make the cross section of a single secondary powder, as seen in Figure 2g, and the magnified SEM image is also shown in Figure S6, wherein the primary grains stack closely. Note that the outer region (∼3 μm thickness) is made by radially aligned primary grains in a spokelike pattern, which has been proven to facilitate the Li+ migration during redox. To demonstrate the formation of CG structure at the outer surface, the atomic ratio of Ni−Co−Mn from the cross section is measured by EPMA, as shown in Figure 2h. Obviously, the atomic ratio of Ni−Co−Mn keeps relatively constant from the inner region of 0−3.5 μm, which acts as the core composition. After which, it is found that the Ni concentration presents a decreased tendency from interior (∼75%) to surface (∼50%), and the contents of Co and Mn increase continuously from interior (∼12%) to surface (∼25%), which confirms indeed the formation of the target CG structure at the outer surface after lithiation. The XRD diffraction lines (Figure 2i) confirm an αNaFeO2-type-layered phase for both NCM.47 XRD Rietveld refinement is employed to calculate lattice parameters, which are listed as follows: a = 2.8692(2) Å and c = 14.2021(4) Å for N-NCM, and a = 2.8673(3) Å and c = 14.2239(1) Å for CGNCM. Note that the CG-NCM shows a reduced Li+/Ni2+ mixing, only 2.5%. 2.3. Electrochemical Performances. Figure S7a,b shows the CVs of both NCM electrodes, in which three reduction− oxidation couples are emerged, caused by phase variations between hexagonal (H) and monoclinic phase (M) involving H1/M, M/H2, and H2/H3.48,49 In the initial cycle, both NCM electrodes exhibit reversible capacities of ∼200 mA h g−1 with the Coulombic efficiencies of around 90% in Figure 3a. The CG-NCM presents an enhanced cycle lifespan, 94.6% capacity retention within 200 cycles at 25 °C, whereas the N-NCM

electrode shows a gradual fading of capacity, only 77.5% capacity retention after the same cycling period from Figure 3b. Besides, the N-NCM electrode reveals an increasingly electrochemical polarization, resulting in higher charge potential and lower discharge potential with increasing cycles than the CG-NCM in Figure 3c,d. As a result, the gradually declined median-potential is found for the N-NCM electrode, whereas the CG-NCM electrode presents a relatively smooth median-potential variation during continuous cycles, as seen in Figure 3b. The CVs of both NCM electrodes after 200 cycles in Figure S6c indicate the reduced-phase transitions of M/H2 and H2/H3. The stable discharge capacity and well-maintained median-potential further promise superior stability of energy density for the CG-NCM electrode and 93.7% energy retention after the same cycles. The structural stability of the CG-NCM is further evaluated at a high operating temperature of 55 °C in Figure 3e. Similarly, higher capacity retention is also achieved for the CG-NCM electrode than the N-NCM sample. These foregoing results clearly indicate the remarkably enhanced structural stability during repeated Li+ intercalation/ deintercalation for this CG-NCM electrode. The rate property that determines the power density of LIBs is a significant factor when those electrodes are used in PHEVs and EVs.50−53 Then, the rate capability is evaluated when the rates improve from 0.1 to 20C in Figure 3f−h. Although both electrodes show similar discharged capacities of ∼200 mA h g−1 at 0.1C, the CG-NCM electrode shows higher capacity than that of N-NCM as the increasing rates from Figure 3f,g. Notably, even at 20C, high capacity of around 130 mA h g−1, ∼65% capacity retention of 0.1C, is obtained for the CGNCM. From the inset of Figure 3h, the CG-NCM electrode also exhibits higher median-potential, especially above 2C, than the N-NCM. Higher capacity and median potential endow higher power density for the CG-NCM in Figure 3h. Surprisingly, the CG-NCM shows an energy density of ∼450 24511

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

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ACS Applied Materials & Interfaces

Figure 4. XPS of the F 1s for the both cycled (a) N-NCM and (c) CG-NCM after 200 cycles at 0.25C. (b) Schematic illustration of the compositions of the formed SEI film after 200 cycles. CV curves of (d) CG-NCM and (e) N-NCM in half cells from 0.1 to 1.0 mV s−1 (the insets are the fitting equations of peak 1−4) (f) DSC analysis of the delithiated NCM at 4.4 V (vs Li/Li+) cutoff voltage.

W h kg−1 even at 20C, which is increased by 50% compared with the N-NCM. These results demonstrated that the rationally designed CG-NCM electrode favors improved Li+ kinetics and thus guarantees the higher rate capability. 2.4. Structural Stability and Li+ Kinetics. SEM images of the cycled electrodes confirm the solid electrolyte interface (SEI) formed on the surface, as seen in Figure S8. To evaluate the electrode surface/interface evolutions caused by the unwanted side reactions, the compositions of the SEI formed on the cycled electrodes within the depth of 50 nm was measured by X-ray photoelectron spectroscopy (XPS) in Figures 4a,c and S9. From the XPS results of the F1s, both of the cycled NCM electrodes consist of five species of fluorinecontaining compounds, LiF, MFx (M = Ni, Co, and Mn), LixPOyFz, PVdF, and LiPxFy, in which the amounts of MFx and LiF increase gradually, whereas the contents of LixPOyFz and LiPxFy decrease continuously as the increasingly analyzed depth between 0 and 50 nm. However, it is noted that, apart from the binder of PVdF, the 50 nm depth surface of the NNCM electrodes are mostly composed of LiF and MFx, whereas the LiF, MFx, and LixPOyFz largely coexist on the cycled CG-NCM electrodes. Besides, the XPS of the C 1s shows a certain amount of Li2CO3 on the outer surface (0−10 nm) of the N-NCM electrodes in Figure S8. Overall, as depicted in Figure 4b, it demonstrates the remarkably reduced contents of LiF and Li2CO3 that are Li+, and the electron insulator on the surface of CG-NCM electrodes, indicating the mitigated side reactions occurring on the electrode/electrolyte interface. Besides, comparing the absolute XPS intensity at 50 nm depth, CG-NCM is still strong, whereas N-NCM is very noisy, indicating a thicker SEI layer on the surface of CGNCM. Overall, apart from the reduced side reactions, the

formed thick and stable SEI layer can enhance cycling stability of CG-NCM electrodes. To study the improved rate properties, the Li+ migration constant (D) is calculated based on the following Randles− Sevcik equation54 i p = 2.686 × 105n3/2AD1/2Cν1/2

CV curves of CG-NCM and N-NCM electrodes in half cells between 0.1 and 1.0 mV s−1 are exhibited in Figure 4d,e. The normalized peak current (ip) has a linear relation with the square root of the 0scan rate (ν1/2), and the fitting slope values are shown in the insets of Figure 4d,e. The first reduction− oxidation peaks (H1 to M) are the main redox couple that provides the majority of the capacity. Therefore, in this work, we have calculated the Li+ diffusion coefficients from hexagonal phase to monoclinic phase (peak 1−4). The calculated Li+ diffusion coefficient is 2.61 × 10−10 and 1.19 × 10−10 cm2 s−1 for cathodic/anodic reactions (peak 1 and peak 3) of the CGNCM electrode, which are around 2 times that of the N-NCM electrode (1.44 × 10−10 and 6.26 × 10−11 cm2 s−1 for peak 2 and peak 4, respectively). Thus, the enhanced Li+ intercalation/deintercalation kinetics ensures the higher rate capability for this CG-NCM cathode. 2.5. Safety Property. Thermal stability of delithiated cathodes acts a significant role in battery safety.55,56 But as discussed in the Introduction, the oxygen loss in the Ni-rich electrodes generally occur at the highly delithiated state, which would result in poor thermal stability. Then, thermal behaviors of the delithiated CG-NCM and N-NCM are measured using differential scanning calorimetry (DSC) analysis in Figure 4f. The delithiated N-NCM electrode exhibits a sharp exothermic 24512

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

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ACS Applied Materials & Interfaces

Figure 5. Electrochemical performances of LTO/CG-NCM full cells: (a) initial charge/discharge curves at 0.1C, (b) long-term cycle life at 10C, and (c,d) rate capability from 0.1 to 20C.

because of the moderate diffusion between core−shell and shell/shell. The formed CG-NCM cathode presents mitigated side reactions and enhanced Li+ kinetics. Thus, the Li/CGNCM half cells show a remarkably enhanced cycling/thermal stability than the Li/N-NCM half cells. Besides, the LTO/CGNCM full cells also exhibit long-term cycle-life and high rate capability. Note that, by contrast with the conventional techniques that prepare CG cathodes from CG precursors, these proposed multishell precursors facilitated the synthesis approach that is suitable for large-scale preparation, which probably promotes the practical applications of the high-energy Ni-rich-layered cathodes. This strategy also can be easily expanded to controllably prepare other-types of CG cathodes including spinel and olive structures for advanced LIBs.

peak; nevertheless the delithiated CG-NCM electrode presents a broadened exothermic peak. Note that the broadened exothermic pattern indicates a slow process of heat generation, but the sharp exothermic profile corresponds to fast heat generation. Excitingly, the delithiated CG-NCM shows a higher peak temperature of 245.6 °C than that of the delithiated N-NCM (236.2 °C), suggesting the deferred thermal runaway in cells. Besides, the delithiated CG-NCM also shows the reduced heat generation (802.7 J g−1) compared with the N-NCM sample (955.3 J g−1). Overall, improved thermal stability is obtained for the delithiated CGNCM electrode, probably owing to the stable Mn-increased gradient surface. 2.6. Electrochemical Properties of Li4Ti5O12/NCM Full Cells. To assemble a long-life and high-rate lithium-ion full cell, the prepared CG-NCM acts as the cathode, whereas the spinel Li4Ti5O12 (LTO) is used as the anode. Because ∼10% excess capacity is designed for Ni-rich cathode in full cells, the capacity and energy density of full cells is evaluated based on CG-NCM cathode. The electrochemical performances of LTO/CG-NCM full cells are presented in Figure 5. The initial cycling profiles for the full cells between 1.5 and 2.9 V are presented in Figure 5a. Note that the employed voltage range (1.5−2.9 V) of full cells corresponds to 3.05−4.45 V (vs Li/Li+) of half cells and then the higher specific capacity can be achieved. The full cells exhibit high initial charge/discharge capacities, 237.6 and 216.5 mA h g−1, respectively, with initial Coulombic efficiencies of ∼90%. The full cells have the median voltage of around 2.24 V, offering an energy density of ∼490 W h kg−1. The cycling stability of the full cells at 10C is exhibited in Figure 5b. The full cells show the long cycling lifespan, ∼95% capacity retention even after 2000 cycles. Besides, high-rate behaviors is demonstrated in Figure 5c,d, large reversible capacities of 168 and 138 mA h g−1 are retained even at 10 and 20C, respectively, which correspond to ∼78 and ∼64% capacity at 0.1C.

4. EXPERIMENTAL SECTION 4.1. Preparation of Multishell Precursors and Concentration-Gradient Cathodes. To synthesize multishell precursors, stoichiometric NiSO4·6H2O (16.822 kg), CoSO4·7H2O (2.249 kg), and MnSO4·H2O (1.208 kg) (Ni−Co−Mn = 0.8:0.1:0.1) were dissolved into 40 L of deionized water to form 2.0 M core solution, whereas the stoichiometric NiSO4·6H2O (1.752 kg), CoSO4·7H2O (1.874 kg), and MnSO4·H2O (1.007 kg) (Ni−Co−Mn = 1/3:1/3:1/ 3) were dissolved into 10 L of deionized water to obtain 2.0 M shell solution. First, 30.5 L of the core solution was dropped into a CSTR (170 L) at 50 °C and 600 rpm. Meanwhile, mixed solution with 10 M NaOH solution as the precipitant and 1.5 M NH4OH solution as the ligand were also automatically dumped into CSTR to control a constant pH value (11.6). The core [Ni 0.8 Co 0.1 Mn0.1 ](OH) 2 precursors were synthesized when the whole 30.5 L of the core solution were completely consumed. Second, a mixed solution of 3.8 L of the core solution and 0.67 L of the shell solution as shell 1, a mixed solution of 2.85 L of the core solution and 1.33 L of the shell solution as shell 2, a mixed solution of 1.9 L of the core solution and 2 L of the shell solution as shell 3, a mixed solution of 0.95 L of the core solution and 2.67 L of the shell solution as shell 4, and 3.33 L of the pure shell solution as shell 5 were also added into CSTR one by one to encapsulate the core component and to form multishell precursors {[(Ni 0 . 8 Co 0 . 1 Mn 0 . 1 ) 0 . 6 ] c o r e [(Ni 0 . 7 0 6 Co 0 . 1 4 7 Mn 0 . 1 4 7 ) 0 . 0 8 9 ] s h e l l 1 [(Ni 0.612 Co 0.194 Mn 0.194 ) 0.084 ] shell2 [(Ni 0.52 Co 0.24 Mn 0.24 ) 0.08 ] shell3 [(Ni0.426Co0.287Mn0.287)0.076]shell4[(Ni1/3Co1/3Mn1/3)0.071]shell5}(OH)2. The bulk precursors [Ni0.7Co0.15Mn0.15](OH)2 were also synthesized

3. CONCLUSIONS The feasibility of preparing CG nickel-rich-layered cathodes from multishell spherical precursors is first demonstrated 24513

DOI: 10.1021/acsami.8b06286 ACS Appl. Mater. Interfaces 2018, 10, 24508−24515

Research Article

ACS Applied Materials & Interfaces

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via coprecipitation reactions, after centrifuging, washing, and drying to achieve the finally aimed precursors. The mixture of normal precursors and Li2CO3 (Li/M = 1.05) were calcined at 820 °C for 12 h under oxygen atmosphere to form layered LiNi0.7Co0.15Mn0.15O2. Moreover, the mixture of multishell precursors and Li2CO3 (Li/M = 1.05) were calcined at 800 °C for 12 h under oxygen atmosphere to achieve the CG cathode LiNi0.7Co0.15Mn0.15O2. 4.2. Materials Characterization. A particle size analyzer, OMEC, LS-POP(6), was employed to measure the particle size distribution. The structure, morphology, and chemical compositions were studied by XRD (Rigaku D/MAX-2500) and SEM (JMS-6700F, JEOL), respectively. Surface valence is measured by XPS (Thermo Escalab250). EMPA (Shimadzu EPMA-1600) was employed to analyze the element content on the cross section. For the analysis of DSC, Li/NCM half cells were initially charged to 4.4 V (vs Li/Li+) at 0.1C (1C = 200 mA g−1), and the cells were opened in an Ar-filled glovebox. After which, these delithiated cathodes were recovered from Al foil and enclosed into high-pressure crucible. Last, the thermal data were measured by a NETZSCH 204F1 instrument at a rate of 5 °C min−1 from 50 to 300 °C. 4.3. Electrochemical Measurements. The detail processes of fabricating positive electrode can refer to the literature.36 After that, cathode disks with a similar cathode loading of 2.5−3.0 mg cm−2 were punched out to assemble the CR2032 coin-type cells. 1 M LiPF6 dissolved in EC and DMC solvent (3:7 by volume) was utilized as the electrolyte. The median-potential of the cathode is defined as the potential at which the discharge capacity reaches a half of the total reversible capacity. The CR2032 coin-type half cells were cycled between 3.0 and 4.4 V (vs Li/Li+) at 0.1C (1C = 200 mA g−1). The CR2032 coin-type full cells using the LTO anode and the CG-NCM cathode were cycled between 1.5 and 2.9 V at a rate of 10C and 25 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06286. SEM, ICP-OES, XPS, XRD, and CVs of the normal and multishell hydroxide precursors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.X.). *E-mail: [email protected] (H.L.). ORCID

Peiyu Hou: 0000-0003-0476-5812 Huiqiao Li: 0000-0001-8114-2542 Xijin Xu: 0000-0002-3877-6483 Tianyou Zhai: 0000-0003-0985-4806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSF of Shandong Province (ZR2017BEM010, ZR2016JL015) and the NSFC (51672109).

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REFERENCES

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