Tailoring Anisotropic Li-Ion Transport Tunnels on Orthogonally

Feb 23, 2017 - School of Physical and Electronic Science, Changsha University of Science and Technology, Changsha 410004, Hunan, People's Republic of ...
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Tailoring Anisotropic Li-Ion Transport Tunnels on Orthogonally Arranged Li-Rich Layered Oxide Nanoplates Toward HighPerformance Li-Ion Batteries Ming Xu,†,‡ Linfeng Fei,‡ Weibing Zhang,§ Tao Li,‡ Wei Lu,∥ Nian Zhang,⊥ Yanqing Lai,*,† Zhian Zhang,† Jing Fang,† Kai Zhang,† Jie Li,† and Haitao Huang*,‡ †

School of Metallurgy and Environment, Central South University, Changsha 410083, People’s Republic of China Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, People’s Republic of China § School of Physical and Electronic Science, Changsha University of Science and Technology, Changsha 410004, Hunan, People’s Republic of China ∥ University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Kowloon, Hong Kong, People’s Republic of China ⊥ Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: High-performance Li-rich layered oxide (LRLO) cathode material is appealing for next-generation Li-ion batteries owing to its high specific capacity (>300 mAh g−1). Despite intense studies in the past decade, the low initial Coulombic efficiency and unsatisfactory cycling stability of LRLO still remain as great challenges for its practical applications. Here, we report a rational design of the orthogonally arranged {010}-oriented LRLO nanoplates with built-in anisotropic Li+ ion transport tunnels. Such a novel structure enables fast Li+ ion intercalation and deintercalation kinetics and enhances structural stability of LRLO. Theoretical calculations and experimental characterizations demonstrate the successful synthesis of target cathode material that delivers an initial discharge capacity as high as 303 mAh g−1 with an initial Coulombic efficiency of 93%. After 200 cycles at 1.0 C rate, an excellent capacity retention of 92% can be attained. Our method reported here opens a door to the development of high-performance NiCoMn-based cathode materials for high-energy density Li-ion batteries. KEYWORDS: Orthogonal nanoplates, Li-rich layered oxide, spinel/rock-salt tunnels, electrode material, Li-ion batteries when charged above 4.5 V versus Li/Li+.14−20 However, extensive removal of Li+ ions leads to instability of the electrode structure, resulting in structural transformation. It has been shown that the surface reconstruction from a layered phase to a mixed spinel-like phase is the primary factor contributing to the modest cycle stability and intrinsic poor rate performance of LRLO.21−26 Because the structure of material, especially the surface structure, is a crucial factor that determines the rate of Li+ ion deintercalation/intercalation, the design of the structure and morphology of LRLO presents an effective route to enhance the electrochemical performance. A variety of strategies, such as hollow spheres, gradient composition, microrods, and so forth, have shown the potential for future development of LRLO.27−31 Recent reports have also evidenced that the

Li-ion batteries (LIBs) have been widely used as energy storage devices in consumer electronics and electrical vehicle market.1−3 Success in these fields depends on our ability to further increase their energy density to ensure longer cruising autonomy and lower cost for electrical vehicles.4 Many efforts to date have been devoted to the cathode side, which is considered to be the bottleneck for high energy and power densities.5−7 Over the past 20 years, scientists have switched their attention to high-energy and low-cost cathode materials due to limited energy densities of the current LIB materials, such as, LiCoO2, LiM2O4 (M = Ni, Mn), LiFePO4 and LiMO2 (M = Ni, Co, Mn).7−13 Currently, Li-rich layered oxides (LRLO) with the general formula xLi2MnO3·(1 − x)LiMO2 (M = Ni, Co, Mn, and so forth) have been the focus of intense research for high energy density LIBs as they represent a group of cathode materials with the potential to improve energy density and reduce cost for electrical vehicles.14 Several studies on LRLO have demonstrated that the Li2MnO3 component serves as the electrochemical active phase for Li extraction © 2017 American Chemical Society

Received: November 29, 2016 Revised: February 1, 2017 Published: February 23, 2017 1670

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Figure 1. (a) Density functional theory calculation of the surface adsorption energy of ethanol molecule on different terminated surfaces. (b) Ethanol-induced synthesis of OAN. It mainly involves the following three steps: (1) HMT decomposition to generate CO2 bubbles and initiate nucleation, (2) ethanol adsorption and crystal growth, which cause the formation of OAN structure, and (3) calcination to form mesoporous TM oxide precursor and final chemical lithiation to obtain target cathode material. SEM images of (c) NCM-0, (d) NCM-1, (e) NCM-2, (f) NCM-3, and (g) NCM-4 carbonate precursors.

surface structure is critical for Li+ ion transport and the rate capability of the LRLO cathode materials.32−37 In principle, LRLO in a hexagonal layered α-NaFeO2 structure has Li+ ion diffusion tunnels along either a- or b-axis (parallel to the c-plane or (001) plane). Therefore, the six equivalent side planes of the hexagonal unit cell, that is, (010), (11̅ 0), (10̅ 0), (010̅ ), (110̅ ), and (100) facets, indexed as {010}, are favorable active planes with an open structure for Li+ ions to intercalate into the bulk of LRLO.30,31,38,39 Many efforts have been devoted to the surface microstructure engineering to expose more {010} active planes.28,29,38,39 However, because the surface free energy of the {001} facet is lower than that of the {010} ones,28,29 it remains a great challenge to synthesize layered materials with highly exposed {010} planes. Moreover, the synthesized nanoplates with highly exposed {010} planes are prone to be stacked on the high energy surface, resulting in reduced surface area of the exposed {010} planes.29,38,39 Therefore, the design and synthesis of an antiagglomeration structure with highly exposed {010} planes are strongly desired for high-performance LRLO cathode materials. Motivated by the considerations above, instead of relying solely on {010}-faceted nanoplates, we propose a two-in-one strategy, that is, developing orthogonally arranged nanoplates (OAN) for LRLO that possesses both {010}-oriented planes and build-in spinel/rock-salt tunnels (ST) on the surface (the OAN-ST structure). To the best of our knowledge, the OANST structure is the first such example for LRLO cathode material to enable fast Li+ ion intercalation/deintercalation kinetics and at the same time to suppress the agglomeration of the {010}-oriented planes. As a result, the target material Li1.2Mn0.52Ni0.2Co0.08O2 with the OAN-ST structure is able to deliver an initial discharge capacity as high as 303 mAh g−1 and

an initial Coulombic efficiency of 93%. After 200 cycles at 1.0 C rate, an excellent capacity retention of 92% can be attainted. The properties of the cathode material and the underlying mechanism are studied using coin-cell-supported transmission electron microscopy (TEM), near-edge X-ray fine structure spectra (NEXAFS), Raman spectra and electrochemical impedance spectroscopy (EIS). Moreover, by comparison with typical LiNi0.8Co0.15Al0.05O2 (NCA), LiNi0.5Co0.2Mn0.3O2 (LMO523), and LiNi0.8Co0.1Mn0.1O2 (LMO811) cathode materials, Li1.2Mn0.52Ni0.2Co0.08O2 with the OAN-ST structure in an 18650-type cell configuration exhibits the highest energy density of 341 Wh kg−1. The concept of developing an OANST structure with kinetically favorable surfaces for Li+ ion intercalation/deintercalation will opens a door for comprehensive design of high-performance NiCoMn-based cathode materials in their commercial applications. Results and Discussion. To obtain the OAN-ST structure with {010}-oriented planes and at the same time to build the anisotropic Li+ ion transport tunnels on the surface, it is important to design an appropriate structure for the precursor, which can inhibit the agglomeration and/or stack of nanoplates. From thermodynamics, it is known that the growth of a certain crystallographic plane is closely related to its surface free energy.40,41 Previous reports show that solvent molecules with hydroxyl groups can be adsorbed on certain crystallographic planes, thereby altering the crystal growth thermodynamics.39−41 Herein, we choose transition metal (TM) carbonate as the precursor and find through first-principles calculations (Figure 1a; Figure S1 and calculation details in Supporting Information) that ethanol molecules show a strong tendency to be adsorbed on the (001) surface of TM carbonate. The stable binding geometry originates from the directional alignment of 1671

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Figure 2. (a) XRD patterns and (b) TG and dM/dT curves of the carbonate precursors. (c) BET of the TMO-2 precursor with OAN structure. (d) XRD pattern of the LRLO-E2 cathode material (inset: TM oxides obtained at 450 °C).

flower-like architecture. It is apparent that the appropriate amount of ethanol is a crucial factor to control the OAN structure. Too much ethanol will result in the growth of a large amount of nanoplates, crowded together to form a flower-like structure. EDX spectra, elemental mapping, and ICP characterizations (Table S2, Supporting Information) of the carbonate precursor indicate that the atomic ratio of Ni, Co, and Mn is almost independent of the amount of ethanol added and is consistent with the nominal composition (Figures S3−S6, Supporting Information). As shown in the XRD pattern (Figure 2a), NCM-2 can be indexed as rhombohedral phase (MnCO3, JCPDS#01-0981; NiCO3, JCPDS #78-0210; and CoCO3, JCPDS #01-1020), demonstrating the formation of carbonates. Thermal gravimetric (TG) (Figure 2b) characterization demonstrates the decomposition of carbonate precursor and the formation of TM oxide after heat treatment at 450 °C. The Brunauer−Emmett−Teller (BET) characterization (Figure 2c) also shows a high specific surface area of 137 m2 g−1 for the TMO-2 precursor with OAN structure. The high crystallinity and phase purity of the TMO-2 precursor and LRLO-E2 cathode material are distinctly confirmed by XRD measurement (Figure 2d). All of the diffraction peaks of LRLO-E2 can be indexed to the α-NaFeO2 type layered structure with space group of R3̅m without any impurity phase, except for the weak reflection peaks between 20° and 25°, which are characteristic of the superlattice structure (space group, C/2m).15,16,20 The difficulty in detecting spinel phase in XRD pattern may be due to the small amount and the locally nanosized distribution.47,48 However, it should be noted that the XRD pattern of LRLOE2 with the OAN-ST structure shows a broadened (003) peak with a large full width at half-maximum (fwhm, 0.0104 ± 0.004 nm−1), which is generally regarded to be associated with microstructural defects or heterostructures in the particles.47−50

ethanol molecules on the (001) surface of carbonate precursor that create a long chain structure as a result of hydrogen bonding among ethanol molecules. It is therefore expected that the addition of ethanol into the precursor reaction system will cause anisotropic growth of crystals, favoring the formation of OAN structure. The carbonate precursor with OAN structure can be obtained through the precipitation of TM salts, including nickel acetate (AcNi), cobalt acetate (AcCo), and manganese acetate (AcMn), under hydrothermal condition using HMT as precipitant (Figure 1b; with details in Experimental Section, Supporting Information). The HMT will decompose and generate a large amount of CO2 bubbles. TM ions can react with CO2 bubbles to produce precipitates, which will instantaneously nucleate and grow into particles. The addition of ethanol into this reaction solution causes a quite viscous environment, allowing slow nucleation and growth of the carbonate precursor. At the same time, the newly grown (001) surface will be covered by ethanol molecules, resulting in a stable OAN geometry in the carbonate precursor. Considering the important role played by ethanol in the formation of the OAN structure, a series of experiments were performed to find the optimum amount of ethanol added. SEM images of the carbonate precursors are shown in Figure 1c−g. Without adding ethanol into this reaction system, the NCM-0 precursor displays a hexahedron-shaped primary particle assembled microspheres (Figure 1c). A few nanoplates start to grow from the core of microspheres for the NCM-1 precursor (10 mL ethanol added, Figure 1d). Interestingly, the NCM-2 precursor (20 mL ethanol added, Figure 1e) presents an attractive OAN structure. However, after increasing the ethanol amount to 30 and 40 mL in the reaction system, the OAN geometry gradually disappears in the NCM-3 and NCM4 precursors (Figure 1f,g, respectively) and finally turns into a 1672

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Figure 3. (a) Schematic drawing of the formation of mesoporous TM oxides with oxygen vacancies left by the decomposition of carbonate precursor. (b) XPS depth-profiles of the O 1s peak of the mesoporous TM oxides. HRTEM images of TM oxides viewed at a direction (c) normal, and (d) parallel to the nanoplates (mesopores indicated by arrows). The insets are the corresponding bright-field images and fast Fourier transform (FFT) patterns. (e) Schematic illustration of OAN-ST structure in LRLO cathode material. (f) HRTEM image of LRLO-E2 with OAN-ST structure (insets: magnified image of a small area and the corresponding FFT pattern). (g) SAED pattern of LRLO-E2 along the zone axis of [010]H/[1̅10]C (subscripts H and C represent hexagonal layered and cubic spinel phases, respectively). The spots marked by black circles are shared by the L and S phases. The weak spot indicated by the green arrow can only be indexed as (111̅)S. (h) STEM and (i) HRTEM images of LRLO-E2 viewed normal to the nanoplate with mesopores indicated by arrows (inset: the corresponding TEM image).

It is interesting to note that the intensity ratio of I(110)/I(003) is 18.6%, higher than the standard value of 12.3% for randomly oriented powders (JCPDS card no. 87-1563), indicating the suppressed growth of the (001) plane. The Raman and NEXAFS spectra (Figure S7a−d, Supporting Information) of the as-synthesized LRLO-E2 cathode material all show the typical characteristics that belong to the layered phase of Li1.2Mn0.52Ni0.2Co0.08O2. As schematically shown in Figure 3a, the decomposition of carbonate precursor leaves a large amount of pores in TMO-2. Oxygen vacancies may be left in TMO-2 due to the evolution of CO2 gas.42−44 Depth-profiling X-ray photoelectron spectroscopy (XPS) (Figure 3b) shows the existence of oxygen vacancies throughout the bulk of mesoporous TMO-2 nanoplates. On the basis of the reported binding energies of O 1s spectrum,42,44−47 the band at 529.3 eV can be ascribed to lattice oxygen atoms (OMO, M = Ni, CO, Mn), whereas the band at 531.2 eV is intimately associated with the oxygen vacancies. The band at around 532.8 eV can be attributed to MOH species or H2O species. The mesoporous structure of the TMO-2 is clearly observed from the TEM and HRTEM images (Figure 3c,d) either from a direction normal or parallel to the nanoplate, demonstrating an interconnected porous structure. Similar to the mechanism proposed by Meng et al.,36 oxygen vacancies in pores will induce phase transformation during chemical lithiation for the LRLO-E2 cathode material. As schematically

shown in Figure 3e, this unique structure is quite favorable in the subsequent impregnation of LiOH to the formation of OAN-ST structure in the final product of LRLO, which is in contrast to the other samples (Figure S8, Supporting Information). HRTEM image viewed along a direction normal to the nanoplate of LRLO-E2 (Figure 3f) demonstrates a uniform distribution of porous tunnels. The sharp diffraction spots observed in Figure 3g (consistent with the simulation in Figure S9, Supporting Information) identify the existence of a spinel phase. The overlap of the diffraction spots of the spinel and layered phases (such as (105)L/(33-1)S and (003)L/ (111)S, where L and S refer to the layered and spinel phases, respectively) indicates an intimate lattice match between the two phases, which stabilize the structure. The built-in spinel/ rock-salt tunnels across the bulk of nanoplates of the OAN-ST structure can be further demonstrated by the STEM (Figure 3h) and HRTEM (Figure 3i) images viewed over a larger area of nanoplates. In view of the structural characteristic of TMO-2 precursor, it should be noted that the formation of the OAN-ST structure only occurs in pores within several nanometers through the OAN nanoplates. As presented in typical SEM (Figure S10, Supporting Information), the LRLO-E2 cathode material preserves the OAN structure from its TMO-2 precursor with little stacking of nanoplates. Figure S11 (Supporting Information) exhibits representative scanning TEM (STEM) 1673

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Figure 4. Electrochemical performance of LRLO-E2 with the OAN-ST structure and LRLO-E4 with stacked nanoplate structure: (a) initial charge/ discharge profiles at 0.1 C rate, (b) differential capacity versus potential plots during the initial cycle, (c) rate capability at different rates, (d) cycling performance at 0.2 C rate, (e) cycling performance at 1.0 C rate, and (f) discharge voltages upon cycling at 1.0 C rate. EIS plots of (g) fresh and (h) cycled electrodes.

profiles of each cathode material are in good agreement with their dQ/dV curves. The rate capability further highlights the advantages of this unique OAN-ST structure (Figure 4c). The LRLO-E2 exhibits the excellent rate performance at current rates from 0.1 to 5.0 C. Figure 4d,e shows the cycling performance of the LRLO-E2 at current rates of 0.2 and 1.0 C. In comparison with other samples (Figure S12, Supporting Information), the LRLO-E2 electrode shows a notable discharge capacity of around 264 mAh g−1 after 100 cycles at 0.2 C rate with excellent capacity retention of 92%. Significantly, a high capacity of 225 mAh g−1 is still retained for the LRLO-E2 after 200 cycles at 1.0 C rate, suggesting a highly stable electrode structure. Most importantly, the LRLO-E2 shows only about 0.38 V decay in discharge voltage (Figure 4f) over 200 cycles at 1.0 C rate, much smaller than that observed for the LRLO-E4 (about 0.95 V decay). The evolution of the discharge voltage profile upon cycling (Figure S13, Supporting Information) shows that in comparison with the LRLO-E4, the LRLO-E2 with the OAN-ST structure exhibits less voltage decay and superior discharge profile during the cycling test. Electrochemical impedance spectroscopy (EIS) studies on the cycled electrodes provide a measure of the charge-transfer resistance at electrode interface (Figure 4g,h). The corresponding resistance values of fresh and cycled electrodes, as well as the calculated Li+ ion diffusion coefficient (Table S4, Supporting Information) demonstrate enhanced surface kinetics of LRLO-E2 (as compared with LRLO-E4) before and after cycling. Overall, the OAN-ST structure has shown promise in the fast Li+ ion transport and the ability to accommodate volume change during repeated electrochemical energy storage cycles, leading to enhanced cycle life and suppressed voltage decay. The function of OAN-ST structure for enhancing electrochemical performance is quite different from surface modification by Li-ion/electron conductors, which mainly aim to facilitate deintercalation/intercalation kinetics.33,35 It is also in sharp contrast to the mesoporous coating that functions as a protecting shell to alleviate side reactions from the electrolytes or the interface treatment that relies on chemical reaction between the cathode and reactants to accelerate Li+ ion

bright-field (BF) and elemental mapping images of the LRLOE2 cathode material with the OAN structure, where Ni, Co, and Mn are found to distribute uniformly across the particle. The atomic percentages of Ni, Co, and Mn determined from the EDS spectrum are 25.2, 10.1, and 64.6%, respectively, which is in good agreement with the inductively coupled plasma (ICP) results (Table S2, Supporting Information) and the nominal composition of LRLO cathode material. To evaluate the advantages of the OAN-ST structure in the battery applications, LRLO-E0, LRLO-E1, LRLO-E2, LRLOE3, and LRLO-E4 were investigated as cathode materials for LIBs (Figure 4; Figure S12, Supporting Information). Figure 4a shows the initial charge/discharge profiles of the LRLO-E2 with the OAN-ST structure and LRLO-E4 with a stacked nanoplate structure for comparison. They exhibit similar curve shapes below 4.5 V and a plateau above 4.5 V, characteristic of the LRLO cathode material.15,16,21 The LRLO-E2 and LRLO-E4 can deliver an initial capacity of 303 and 280 mAh g−1, respectively, with the former displaying the highest initial discharge capacity at 0.1 C rate (265, 271, and 285 mAh g−1, for LRLO-E0, LRLO-E1, and LRLO-E3, respectively, Figure S12, Supporting Information). Remarkably, the LRLO-E2 exhibits the highest initial Coulombic efficiency of 93% with irreversible capacity of only 22 mAh g−1, indicating its extremely good electrochemical reversibility (higher than most of the Li-rich layered oxides reported, Table S3, Supporting Information). We emphasize that high Coulombic efficiency is critical for the operation of a practical battery. Early cycle Coulombic efficiencies are especially important because they account for most of the Li+ ions loss and electrolyte consumption during SEI formation.51−56 The remarkably high Coulombic efficiency results from two advantages of the OAN-ST structure. One is the exposed {010}-oriented planes of the OAN-ST structure that facilitate fast Li+ ion deintercalation/intercalation kinetics. The other is the built-in spinel/rock-salt tunnels on the surface of OAN structure that afford the anisotropic Li+ ion transport and allow the initial SEI formation without consuming too much Li+ ions. The corresponding differential capacity versus voltage (dQ/dV) curves are presented in Figure 4b. The potential plateaus and reproducibility of the charge/discharge 1674

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examined its structure characteristics on the LRLO-E2 after 100 cycles using XRD (Figure S14, Supporting Information) and SEM (Figure S15, Supporting Information). In comparison with LRLO-E4, our observation reveals that the OAN-ST structure of LRLO-E2 can be strongly held even after long-term cycling (Figure 5e). Furthermore, the cycled LRLO-SG cathode material prepared by a typical sol−gel method (Figures S16 and S17 and experimental details in Supporting Information) lends additional support to the OAN-ST protective mechanisms. In contrast to the fresh particles (Figure S17, Supporting Information), both the LRLO-E4 (with a stack of nanoplates) (Figure 5f) and the LRLO-SG particles (Figure 5g) clearly show a thick SEI layer on the surface of cathode particles, demonstrating the consumption of Li+ ions in the electrolyte and the agglomeration of Li+ ions on “blocked” surface during repeated deintercalation/intercalation processes. Both the electrode microstructure and the EIS data provide strong evidence for a stable electrode structure during cycling of the LRLO-E2 cathode material. Therefore, it is reasonable to conclude from the above results that the OAN-ST structure plays a protective role to the electrode by possessing a highly {010}-oriented OAN structure and uniformly distributed spinel/rock-salt tunnels that facilitate the Li+ ion transport and accommodate volume change during battery cycling. In order to further demonstrate the LRLO-E2 with the OAN-ST structure as a superior cathode material for practical LIB applications, a full cell with the LRLO-E2 cathode and a commercial Si/C anode was constructed (Figure S18, Supporting Information). For comparison, nickel-rich NCA, LMO523, and LMO811 were also used as cathodes in full cells, as shown in Figures S19−S21 (Supporting Information). For the full cell, relatively high charge and discharge capacities of 323 and 293 mAh g−1, respectively, were obtained at 0.1 C rate for the LRLO-E2 cathode, which showed an initial Coulombic efficiency of 91%. The reversible capacity retained at 234 mAh g−1 at 1.0 C rate after 100 cycles with capacity retention of 93%. As for those nickel-rich cathode materials prepared by typical coprecipitation method, discharge capacities of 207, 227, and 220 mAh g−1 with an initial Coulombic efficiency of 85, 82, and 83% were obtained at 0.1 C rate for the LMO523-Si/C, LMO811-Si/C, and NCA-Si/C full cells, respectively (see corresponding XRD, SEM, and electrochemical results in Figures S19−S21, Supporting Information). After 100 cycles at 1.0 C rate, the reversible capacity was dropped to 122, 173, and 130 mAh g−1, respectively (Figure S21, Supporting Information). The average working voltage of the LRLO-E2 cathode material at 0.1 C in a full cell is 3.53 V and able to deliver a specific energy density of 1016 Wh kg−1. By contrast, the specific energy densities achieved in LMO523, LMO811, and NCA cathode materials are only 746, 862, and 833 Wh kg−1, respectively. The mass fraction analysis of a commercial 18650 LIB is shown in Figure S22 (Supporting Information, similar analysis can be found in recent publications59,60), given a mass fraction of about 33 and 37% for the LRLO-E2 and other nickel-rich cathodes (including aluminum foil, PVDF, carbon black, and active material). The specific energy densities of the full cells (calculated based on the mass fraction) are 341, 276, 318, and 308 Wh kg−1 for the LRLO-E2-Si/C, LMO523-Si/C, LMO811-Si/C, and NCA-Si/C full cells, respectively. The above results show the great potential of the LRLO cathode material with the OAN-ST structure as a candidate for practical LIB applications.

Figure 5. Characterizations on LRLO-E2, LRLO-E4, and LRLO-SG cathode materials with 100% state of charge at 4.8 V (vs Li/Li+): TEM images of (a) LRLO-E2 with OAN-ST structure, (b) LRLO-E4 with stacked nanoplates, (c) LRLO-SG prepared by typical sol−gel method. (d) Schematic drawing of the structural stabilization mechanism in LRLO-E2 and structure collapse encountered in LRLO-E4 cathode materials. TEM images of cycled electrode: (e) LRLO-E2, (f) LRLOE4, and (g) LRLO-SG.

structure serves to reinforce the nanostructure by accommodating the volume change during cycling (as schematically shown in Figure 5d). In contrast, the LRLO-E4 particle with stacked nanoplates and the typical LRLO-SG particles show microcracks (Figure 5b,c) after severe volume change. Although Liu et al.58 reported that such cracks in the electrode particle could be partially healed by the corresponding volume expansion, unfortunately, a phase transformation from layered to spinel takes place through a nucleation and growth mechanism in a polycrystalline/amorphous matrix in the C2/m phase region after lattice fracture upon cycling.25 To substantiate the impressive characteristics of the OAN-ST structure, we 1675

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Nano Letters

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Conclusion. An orthogonally arranged {010}-oriented lithium-rich layered oxide nanoplates with built-in spinel/ rock-salt tunnels (OAN-ST) were developed that combine the merits of highly exposed {010} planes for enhanced redox reaction kinetics and the anisotropic Li+ ion transport tunnels. The OAN-ST structure also serves to reinforce the electrode by accommodating the volume change during cycling. The final cathode achieved is able to deliver an initial discharge capacity as high as 303 mAh g−1 and a Coulombic efficiency of 93%. After 200 cycles at 1.0 C rate, an excellent capacity retention of 92% can be attained. The full cell configuration with the OANST structured lithium-rich layered oxide as cathode produced a specific energy density of 1016 Wh kg−1 at 0.1 C rate and 341 Wh kg−1 in a 18650 LIB configuration, satisfying the energy density limit imposed by the range requirement for electronvolts. We believe this facile and inexpensive approach opens a door to the comprehensive design of high-performance Ni CoMn-based cathode materials for their commercial applications.

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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/acs.nanolett.6b04951. Additional information on method of theoretical calculations and experimental details; additional figures of microstructures, Raman, NEXAFS, BET, simulated electron diffraction pattern, and electrochemical characterizations; additional tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Linfeng Fei: 0000-0001-7329-0019 Weibing Zhang: 0000-0003-1306-4540 Haitao Huang: 0000-0002-3861-2702 Author Contributions

M.X. and L.F. contributed equally to this work and should be considered as cofirst authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support of the Teacher Research Fund of Central South University (2013JSJJ027). This project was supported by Grants from the Project of Innovation-driven Plan in Central South University (2015CXS018 and 2015CX001) and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. The work was also supported by Hong Kong Polytechnic University (1-ZVGH).

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