Multishell Precursors Facilitated Synthesis of Concentration-Gradient

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Multi-Shell Precursors Facilitated Synthesis of Concentration-Gradient Nickel-Rich Cathodes for Long-Life and High-Rate Lithium-Ion Batteries Peiyu Hou, Feng Li, Yanyun Sun, Huiqiao Li, Xijin Xu, and Tianyou Zhai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06286 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Applied Materials & Interfaces

Multi-Shell Precursors Facilitated Synthesis of Concentration-Gradient Nickel-Rich Cathodes for Long-Life and High-Rate Lithium-Ion 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

KEYWORDS: lithium-ion batteries, nickel-rich cathodes, multi-shell precursors, cation diffusion, electrochemical properties

ACS Paragon Plus Environment

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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 less-than-ideal repeatability and consistency that is regarded as an extremely 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 temperaturetunable cations diffusion, the feasibility of controllable preparation of nickel-rich CGLiNi0.7Co0.15Mn0.15O2 (NCM) from multi-shell precursors is first demonstrated. As expected, the Li/CG-NCM half-cells show much enhanced cycle-life, rate property and safety due to 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 multi-shell precursors is suitable for large-scale preparation. Overall, this multi-shell 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.


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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 LiMPO4 (M=Fe and Mn), are broadly investigated.1,7-9 Among these candidates, nickel-rich (Ni-rich) layered Li[Ni1xMx]O2

(M=Co, Mn, etc.), the solid solutions of LiNiO2-LiMO2, deliver specific capacity

exceeding 200 mAh 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 show severely 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 the Ni2+ dissolution and electrolyte decomposition;16-18 (2) the R-3m layered phase readily shifts to a Fm-3m rock-salt phase in that the Ni2+ (3a sites) tend to occupy to the neighboring Li+ vacancy (3b sites);19-21 (3) active oxygen easily separate from host structure of the highly charged cathode, which further give 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 host structure. Additionally, state-of-the-art LIBs as energy storage devices for EVs need to shorten the charge time by improving rate capability to perfect their practicality. Overall, developing Ni-rich layered cathodes with better electrochemical behaviors and safety are urgent for their practical applications in LIBs.


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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 due to the protection of micron-sized shell when compared with the core component. Unfortunately, after in-depth studies on core−shell Ni-rich cathodes within repeated cycling, a structural mismatch, i.e. void layer of tens of nanometres 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 concentration-gradient Ni-rich LiNi0.64Co0.18Mn0.18O2, where the core LiNi0.8Co0.1Mn0.1O2 is surrounded by an outer concentration-gradient layer is reported by Sun and coworkers to solve the said shortage of core−shell cathodes.26 Recently, (full) concentration-gradient (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) concentration-gradient cathode materials from the aimed (full) concentration-gradient precursors.26-37 But the preparation of CG-precursors must be accurately managed, as shown in Figure 1(a), i.e., the solution 2 should be continuously dropped into solution 1 to form CGsolution. 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 CGprecursors. Actually, the degree of repeatability and consistency is an extremely significant


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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 concentrationgradient cathodes, the feasibility of controllable preparation of concentration-gradient Ni-rich cathodes from micron-sized multi-shell spherical precursors is demonstrated for the first time. Importantly, by contrast with the conventional techniques that prepare concentration-gradient cathodes from concentration-gradient precursors, the multi-shell precursors can be facilely and massively synthesized only by replacing the various shell solution during co-precipitation reactions, as depicted in Figure 1(b). Therefore, the preparation of concentration-gradient cathodes from these target multi-shell precursors suggests much improved repeatability and consistency, which is suitable for large-scale production. Consequently, the as-prepared concentration-gradient Ni-rich LiNi0.7Co0.15Mn0.15O2 from multi-shell 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 concentration-gradient Ni-rich cathodes for state-of-the-art LIBs.


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Figure 1 Schematic illustration of preparation for (a) concentration-gradient and (b) multi-shell hydroxide precursors via co-precipitation 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 multi-shell precursors. 2. RESULTS AND DISCUSSION


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Figure 2 (a,b) SEM images of multi-shell spherical precursors, and (c) the variation of atomic ratio on the cross-section of a single secondary particle by EPMA. (d-f) SEM images of the CGNCM cathode materials from multi-shell precursors, (g) SEM of the cross-section prepared by FIB, (h) the variation of atomic ratio on the cross-section by EPMA, and (i) XRD Rietveld refinement of CG-NCM and N-NCM. 2.1 Design of concentration-gradient Ni-rich cathodes from multi-shell precursors


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The triangular phase diagram of LiNiO2−LiCoO2−LiMnO2, as shown in Figure 1(c), is utilized to design the aimed core−shell Ni-rich cathodes. Given that the composition points of three layered oxides LiNi1/3Co1/3Mn1/3O2, LiNi0.7Co0.15Mn0.15O2 and LiNi0.8Co0.1Mn0.1O2 located at the same straight line, meaning the LiNi0.7Co0.15Mn0.15O2 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 while the stable LiNi1/3Co1/3Mn1/3O2 as outer shell are rationally selected. Moreover, multi-shell structure is able to be further designed based on this original core−shell structure, i.e., part of core is extracted to form the outer multi shells with the said single shell by controlling their

molar

ratio.

Here,

multi-shell

Li{[(Ni0.8Co0.1Mn0.1)0.6]core[(Ni0.706Co0.147Mn0.147)0.089]shell1[(Ni0.612Co0.194Mn0.194)0.084]shell2[(Ni0.52 Co0.24Mn0.24)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, while the high temperature of above 850 °C makes severe cation diffusion.36 Therefore, as to the foregoing multi-shell samples, diffusions of transition-metal ions will firstly 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 multi-shell structure will tend to develop concentration-gradient structure under moderate diffusion effect from 800 to 850 °C. Then, an innovative strategy was rationally proposed to achieve the concentration-gradient Ni-


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rich cathode materials from multi-shell precursors for the first time on account of above analysis, as described in Figure 1(d). The Ni-rich LiNi0.7Co0.15Mn0.15O2 is taken as an example to demonstrate the feasibility of controllable preparation of concentration-gradient structure from multi-shell spherical precursors under moderate diffusion effect at 800 °C in this work. 2.2 Synthesis of concentration-gradient Ni-rich cathodes from multi-shell precursors The designed multi-shell and the normal precursors are synthesized by co-precipitation reactions in the CSTR. The morphology of as-prepared precursors are measured by SEM, as shown in Figure 2(a,b), and Figure S1. Both precursors show monodisperse spherical particles that are assembled by nano-sized 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 multi-shell sample in Figure S2. The D50 of multi-shell precursors show a growth function as follow: D=1.3367+2.7488T1/3 (D is D50, T is reaction time). From Figure S4, XRD patterns demonstrate the layered β-Ni(OH)2type 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 insert of Figure 2(c). The element fraction on this cross-section is analyzed by electron microprobe analysis (EPMA), as shown in Figure 2(c). The inner section from 0 µm to 4 µm shows nearly constant atomic ratio of Ni−Co−Mn (0.8/0.1/0.1), acting as core component, but from which the atomic ratio of Ni−Co−Mn varied. The non-continuous changes of Ni−Co−Mn an outer surface confirms the target multi-shell structure. The mixture of Li2CO3 and multi-shell 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


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then form the designed concentration-gradient structure at outer surface after lithiation. SEM images confirm that the spherical morphologies are still retained after calcination in Figure 2(d-f) and Figure S5. The close stacking of mono-disperse micron-sized spherical powders render hightap density of 2.42 and 2.38 g cm−3 for N-NCM and CG-NCM, respectively, which is beneficial for improving volumetric-energy-density of LIBs.44-46 FIB is utilized to make the cross-section of a single secondary powder, as seen in Figure 2(g), 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) are made by radially aligned primary grains in a spoke-like pattern, which has been proven to facilitate the Li+ migration during redox. To demonstrate the formation of CG-structure at outer surface, the atomic ratio of Ni−Co−Mn from on the cross-section is measured by EPMA, as shown in Figure 2(h). Obviously, atomic ratio of Ni−Co−Mn keeps relativly 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 continously from interior (~12%) to surface (~25%), which confirms the indeed formation of the target concentration-gradient structure at outer surface after lithiation. The XRD diffraction lines (Figure 2(i)) confirm a α-NaFeO2-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 CG-NCM. Noted that the CG-NCM shows a reduced Li+/Ni2+ mixing, only 2.5%. 2.3 Electrochemical performances


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Figure S7(a,b) show 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 mAh g−1 with the Coulombic efficiencies of around 90% in Figure 3(a). The CG-NCM presents enhanced cycle lifespan, 94.6% capacity retention within 200 cycles at 25 °C, while the N-NCM electrode show gradual fading of capacity, only 77.5% capacity retention after the same cycling period from Figure 3(b). Besides, the N-NCM electrode reveals increasingly electrochemical polarization, resulting in higher charge potential and lower discharge potential with increasing cycles than the CG-NCM in Figure 3(c,d). As a result, the gradually declined median-potential is found for the N-NCM electrode, whereas the CG-NCM electrode presents relatively smooth media-potential variation during continuous cycles, as seen in Figure 3(b). The CVs of both NCM electrodes after 200 cycles in Figure S6(c) 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, 93.7% energy retention after the same cycles. The structural stability of the CG-NCM is further evaluated at high operating temperature of 55 °C in Figure 3(e). 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 used in PHEVs and EVs.50-53 Then rate capability is evaluated when the rates improve from 0.1 to 20C in Figure 3(f-h). Although both electrodes show similar discharged capacities of ~200 mAh g−1 at 0.1C, the CG-NCM electrode shows higher capacity than that of N-NCM as the


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increasing rates from Figure 3(f,g). Notably, even at 20C, high capacity of around 130 mAh g−1, ~65% capacity retention of 0.1C, is obtained for the CG−NCM. From the insert of Figure 3(h), the CG-NCM electrode also exhibits higher median potential, especially above 2C, than the NNCM. Higher capacity and median potential endow higher power density for the CG-NCM in Figure 3(h). Surprisingly, the CG-NCM shows an energy density of ~450 Wh 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, thus guarantees the higher rate capability.

Figure 3 (a) The initial charge/discharge curves of both NCM at 25 °C, (b) Stability of reversible capacity and median potential of both NCM at 25 °C. The continuous charge/discharge curves of


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(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, (h) energy density for both NCM electrodes.

Figure 4 XPS of the F1s for the both cycled (a) N-NCM and (c) CG-NCM after 200 cycles at 0.25C. (b) Schematic illustration of the compostions 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 inserts are the fitting equations of peak 1-4). (f) DSC analysis of the delithiated NCM at 4.4 V (vs. Li/Li+) cutoff voltage. 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


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within the depth of 50 nm was measured by XPS in Figure 4(a,c) and Figure S9. From the XPS results of the F1s, both cycled NCM electrodes consist of five species of fluorine-containing compounds, LiF, MFx (M=Ni, Co, Mn), LixPOyFz, PVdF, and LiPxFy, in which the amounts of MFx and LiF increase gradually while the contents of LixPOyFz, and LiPxFy decrease continuously as the increasingly analyzed depth between 0 to 50 nm. But noted that, apart from binder of PVdF, the 50 nm depth surface of the N-NCM 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 C1s 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 4(b), it demonstrates the remarkably reduced contents of LiF and Li2CO3 that are Li+ and electron insulator on the surface of CG-NCM electrodes, indicating the mitigated side reactions occurred on electrode/electrolyte interface. Besides, comparing the absolute XPS intensity at 50 nm depth, CG-NCM is still strong while N-NCM is very noisy, indicating a thicker SEI layer on the surface of CG-NCM. 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 ip = 2.686×105 n3/2AD1/2Cv1/2 CV curves of CG-NCM and N-NCM electrodes in half-cells between 0.1 and 1.0 mv s−1 are exhbited in Figure 4(d,e). The normalized peak current (ip) has a linear relation with the square root of scan rate (υ1/2), and the fitting slope values are shown in the inserts of Figure 4(d,e). The first reduction-oxidation peaks (H1 to M) are the main redox couple that provides the majority of


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capacity. Therefore, in this work we have calculated the Li+ diffusion coefficients from hexagonal phase to monoclinic phase (Peak1-4). The calculated Li+ diffusion coeffcient is 2.61 × 10−10 and 1.19 × 10−10 cm2 s−1 for cathodic/anodic reactions (Peak 1 and Peak 3) of the CG-NCM electrode, which are around two times 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 Ni-rich electrodes generally occur the oxygen loss 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 DSC analysis in Figure 4(f). The delithiated NNCM electrode exhibits a sharp exothermic 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 CG-NCM electrode, probably owing to the stable Mnincreased gradient surface.


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Figure 5 Electrochemical performances of LTO/CG-NCM full cells: (a) the initial charge/discharge curves at 0.1C, (b) long-term cycle life at 10C, and (c,d) rate capability from 0.1C to 20C. 2.6 Electrochemical properties of LTO/NCM full cells To assemble a long-life and high-rate lithium-ion full cell, the prepared CG-NCM is acted as cathode while the spinel LTO is used as anode. Since ~10% excess capacity is designed for Nirich cathode in full cells, the capacity and energy density of full cells is evaluated based on CGNCM cathode. The electrochemical performances of LTO/CG-NCM full cells are presented in Figure 5. The initial cycling profiles for the full cells between1.5 and 2.9 V are presented in Figure 5(a). 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, then higher specific capacity can be achieved. The full cells exhibit high initial charge/discharge capacities, 237.6 and 216.5 mAh g−1, respectively, with


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initial Coulombic efficiencies of ~90%. The full cells have the median voltage of around 2.24 V, offering an energy density of ~490 Wh kg−1. The cycling stability of the full cells at 10C is exhibited in Figure 5(b). The full cells show the long cycling lifespan, ~95% capacity retention even after 2000 cycles. Besides, high-rate behaviors is demonstrated in Figure 5(c,d), large reversible capacities of 168 and 138 mAh g−1 are retained even at 10C and 20C, respectively, which correspond to ~78 and ~64% capacity at 0.1C. 3. CONCLUSIONS The feasibility of preparing concentration-gradient nickel-rich layered cathodes from multishell spherical precursors is first demonstrated, due to 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/CG-NCM half-cells show remarkably enhanced cycling/thermal stability than the Li/N-NCM half-cells. Besides, the Li4Ti5O12/CG-NCM 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, this proposed multi-shell precursors facilitated synthesis approach 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 preparing other-type CG-cathodes including spinel and olive structures for advanced LIBs. 4. EXPERIMENTAL SECTION Preparation of multi-shell precursors and concentration-gradient cathodes: To synthesize multi-shell 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 deionized water to


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form 2.0 M core solution while 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 deionized water to obtain 2.0M shell solution. First, 30.5 L core solution was dropped into a continuously stirred tank reactor (CSTR, 170 L) at 50 °C and 600 rpm. Meanwhile, mixed solution with 10M NaOH solution as the precipitant and 1.5M NH4OH solution as the ligand were also automatically dumped into CSTR to control a constant pH value (11.6). The core [Ni0.8Co0.1Mn0.1](OH)2 precursors were synthesized when the whole 30.5 L core solution were completely consumed. Secondly, a mixed solution of 3.8 L core solution and 0.67 L shell solution as shell 1, a mixed solution of 2.85 L core solution and 1.33 L shell solution as shell 2, a mixed solution of 1.9 L core solution and 2 L shell solution as shell 3, a mixed solution of 0.95 L core solution and 2.67 L shell solution as shell 4, and 3.33 L pure shell solution as shell 5 were also added into CSTR one by one to encapsulated the core component and form multi-shell precursors {[(Ni0.8Co0.1Mn0.1)0.6]core[(Ni0.706Co0.147Mn0.147)0.089]shell1[(Ni0.612Co0.194Mn0.194)0.084]shell2[(Ni0.52Co 0.24Mn0.24)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 via co-precipitation 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. And the mixture of multi-shell precursors and Li2CO3 (Li/M=1.05) were calcined at 800 °C for 12 h under oxygen atmosphere to achieve the CGcathode LiNi0.7Co0.15Mn0.15O2. Materials characterization: A particle size analyzer, OMEC, LS-POP(6), was employed to measure particle size distribution. The structure, morphology, and chemical compositions were studied by X-ray diffractometry (XRD, Rigaku D/MAX-2500) and scanning electron microscope


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(SEM, JMS-6700F, JEOL), respectively. Surface valence is measured by X-ray photoelectron spectroscopy (XPS, Thermo Escalab250). Electron microprobe analysis (EMPA, Shimadzu EPMA-1600) was employed to analyse element content on the cross-section. For the analysis of differential scanning calorimetry (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 Ar-filled glovebox. After which, these delithiated cathodes were recovered from Al foil and enclosed into high-pressure crucible. Lastly the thermal data were measured by a NETZSCH 204F1 instrument at a rate of 5 °C min−1 from 50 to 300 °C. Electrochemical measurements: The detail processes of fabricating positive electrode can refer to the literature.36 After that, cathode disks with similar cathode loading of 2.5−3.0 mg cm−2 were punched out to assemble the CR2032 coin-type cells. 1M LiPF6 dissolved in EC and DMC solvent (3:7 by volume) was utilized as electrolyte. The median-potential of cathode is defined as the potential at which the discharge capacity reaches a half of 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 Li4Ti5O12 (LTO) anode and the CG-NCM cathode were cycled between 1.5 and 2.9 V at a rate 10C and 25 °C. ASSOCIATED CONTENT Supporting Information SEM, ICP-OES, XPS, XRD, and CVs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (X. Xu) *E-mail: [email protected] (H. Li) Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was financially supported by the NSF of Shandong Province (ZR2017BEM010, ZR2016JL015) and the NSFC (51672109). REFERENCES (1)

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Table of Contents Graphic


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Multishell Precursors Facilitated Synthesis of Concentration-Gradient

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