Toward Alleviating Voltage Decay by Na Substitution in Li-rich Mn

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Cite This: ACS Appl. Energy Mater. 2018, 1, 4065−4074

Toward Alleviating Voltage Decay by Sodium Substitution in Lithium-Rich Manganese-Based Oxide Cathodes Song Chen,†,‡ Zhuo Chen,*,† Min Xia,*,† Chuanbao Cao,*,† and Yunjun Luo† †

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Department of Materials Physics and Chemistry, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and System of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China S Supporting Information *

ABSTRACT: Lithium-rich manganese-based oxides (LMROs), as one of the most promising high-capacity cathodes, suffer from serious capacity fading and discharge voltage decay during repeated cycles. Here we have successfully enhanced cycle stability and rate capability of LMRO cathode material through introducing a certain amount of Na into LMRO microspheres. In particular, the discharge voltage decay per cycle significantly decreases from 4.40 to 1.60 mV. These enhancements may be attributed to the Na in Li layers, which can promote the kinetics of lithium ion diffusion and facilitate the electronic and ionic conductivity. More remarkably, Na dopant can effectively suppress the transformation from layered to spinel structure by serving as the fixed pillars in Li layers to inhibit the formation of three adjacent vacancies and Mn migration. In addition, full-cell investigations further show the Na-doped LMRO materials have great commercial value. Therefore, our findings may boost understanding in designing high-capacity and good stability cathode materials for LIBs. KEYWORDS: lithium-rich manganese-based oxides, doping, voltage decay, phase transformation, cycling stability, cathode

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INTRODUCTION

block the transformation from layered to spinel structure, then reducing voltage fade. So far, many efforts have been devoted to various doped LMROs, including Li-site doping, TM-site doping, and anion doping. Numerous studies have proven the effectiveness of TM-site cation doping, such as K+,13 Al3+,14,15 Ru3+,16 Ga3+,17 Cr3+,18 and Ti4+,19,20 and anion doping, such as F−,21 SiO44−,22 and BO33− and BO45−,23 for decreasing voltage decay. Wang et al.14 and Nayak et al.15 both substituted Al3+ for Ni2+ or Mn4+ in Li1.2Ni0.2Mn0.6O2, which exhibits significantly different levels of voltage decay per cycle. Yu et al.19 have reported that Ti4+ doping for Mn4+ in Li1.26Ni0.07Co0.07Mn0.6O2 shows 4.21 mV voltage decay per cycle, which is calculated from the 10th to the 130th discharge profiles. However, the rate performance

The layered lithium-rich manganese-based oxide cathodes, formulated as xLi2MnO3·(1−x)LMO2 (M = Ni, Co, and Mn, etc.), have aroused wide concern owing to their high reversible capacity (>250 mAh g−1), as well as low cost and high security.1−5 However, LMROs usually undergo large irreversible capacity loss in the first cycle, poor cycling stability, terrible rate performance, and continuous voltage decay, which restrict their commercial applications.6 In particular, the voltage decay is perhaps the largest obstacle because it leads to gradual energy density fading during cycling. It is considered to be strongly correlated to the undesirable electrochemical phase transformation from layered to spinel structure.7−10 Accompanying the extraction and insertion of lithium, some transition metal (TM) ions migrate from octahedral sites (TM layer) to tetrahedral sites (Li layer).11,12 Therefore, inhibiting the migration of TM ions by introducing foreign ions may © 2018 American Chemical Society

Received: May 9, 2018 Accepted: July 20, 2018 Published: July 20, 2018 4065

DOI: 10.1021/acsaem.8b00740 ACS Appl. Energy Mater. 2018, 1, 4065−4074

Article

ACS Applied Energy Materials

Figure 1. (a−c) SEM images and (d) EDS result of the hydroxide precursors (R = 1:3).

Na for Li has positive influences on inhibiting the layered-tospinel transformation and improving the electrochemical performance of LMRO materials. And, more notably, Kim’s33 first-principles calculation showed Li0.5MO2 layered materials have stronger driving forces and lower energy kinetic paths to convert to the spinel phase than Na0.5MO2. In addition, after Li are substituted with Na, the migration barriers of TM cations obviously increase owing to the larger ionic radius of Na+. At this point, substituting Na for Li in LMRO cathode materials may be worth exploring to realize a superior performance. However, to our best knowledge, these existing related studies mainly focus on investigating the impacts of Na substitution on capacity retention and rate performance of LMRO cathode materials, and there are few reports about influences of doping Na on voltage decay in LMROs. Herein, we have introduced a certain amount of Na into the Li layer of LMRO microspheres to investigate the distinct influences of Na doping on voltage fade and electrochemical performance of LMRO cathode materials. Compared to the undoped sample, the Li1.15Na0.05[Ni0.13Co0.13Mn0.54]O2 cathode achieves remarkably enhanced lithium-storage properties, cycle stability, and rate capability, and especially mitigates voltage decay during cycling (1.60 mV voltage decay per cycle calculated from the first to the 100th discharge profiles). Based on impedance spectra and diffusion kinetic analyses, remarkable improvement of LMRO materials in LIB can be ascribed to the effect of Na substitution, which can accelerate the lithium ion diffusion and promote the electronic and ionic conductivity. More importantly, from ex situ XRD and HRTEM, Na dopant can serve as the fixed pillars in Li layers to suppress the formation of three adjacent vacancies and Mn migration, thus significantly blocking the transformation from layered to spinel structure. Furthermore, full-cell investigations also show the Na-doped LMRO microspheres have huge

becomes worse than that of the undoped sample, due to lowering of the electronic conductivity caused by the electronically insulating nature of Ti4+. Li et al.21 have reported F− doping for O2− of Li1.2Ni0.13Co0.13Mn0.54O2. Though the voltage decay per cycle obviously decreases from 7.30 to 3.20 mV, the discharge capacity is only ∼125 mAh g−1 at 500 mA g−1. Compared to the undoped material, BO33− and BO45− codoping for O2− of Li1.2Ni0.13Co0.13Mn0.54O2 displays a 31% decreased voltage decay per cycle (2.92 mV calculated from the second to the 250th average discharge voltage), while the rate capability is not much improved (∼150 mAh g−1 reversible capacity at 400 mA g−1 ). 23 Although the aforementioned doping methods can relieve voltage fade to improve cyclic stability to a certain extent, the rate performance still remains to be much further enhanced in terms of LIB applications. Theoretical calculations indicate that the quite low energy in the high-voltage cycling stage induces the emergence of the spinel phase by migrating TM cations into the Li layer.24 The movement is realizable only when three adjacent sites in the Li layer all are vacant; that is, it is extremely sensitive to the local chemical environment of the targeted site.25 Therefore, substituting other cations for Li may be more helpful to suppress the migration of TM ions into the Li layer to reduce voltage decay and improve cycling performance. Additionally, Li-site doping with larger radius ions could enlarge the Li+ slab space to facilitate lithium ion diffusion between oxygen layers during cycling, thus leading to improving effectively the rate capability. Recently, it has been recognized that Na-substituted LMRO materials have been studied to enhance significantly cyclic stability and rate performance.26−29 Several groups30−32 have reported the introduction of Na into LMRO cathode materials by different methods, improving the kinetics of LMRO materials with good cyclic stability and rate capability to varying degrees. It is widely considered that substitution of 4066


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Figure 2. (a−c) XRD patterns. XPS spectra of (d) survey spectrum, (e) Li 1s, and (f) Na 1s for LMRO and Na-doped LMROs.

Figure 3. (a−c) SEM images. (d) TEM and (e) HRTEM images of Na0.05-LMRO. (f) HRTEM image of the circled region. The rhombohedral and monoclinic phases are marked as “R” and “M”, respectively. (g) Element mapping of Na, Ni, Co, and Mn and (h) EDS spectrum on the selected areas.

show that the surface of the microspheres is distinctly rough. The details about the impacts of water−ethanol volume ratios on the morphology are shown in Supporting Information (SI) Figure S1.The chemical composition of the precursors was determined by EDS (Figure 1d), which confirms the atomic ratio of Ni, Co, and Mn is close to 1:1:4, consistent with the theoretical ratio of elements. Moreover, all the XRD diffraction peaks of hydroxide precursors are in agreement with that of isostructural Ni1/6Co1/6Mn4/6CO3,34 being indexed to a hexagonal structure with a R3̅c space group (SI Figure S2). Subsequently, the aforementioned precursors were converted to Li1.2[Ni0.13Co0.13Mn0.54]O2. In order to enhance the

potential in the next-generation LIBs. Therefore, our work about the influence of Na on electrochemical performance of LMROs may shed some light on designing high-capacity cathode materials for LIBs.

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RESULTS AND DISCUSSION Based on the hydrothermal process given in Experimental Section, the well-developed microspherial hydroxide precursors are obtained. SEM images in Figure 1 indicate the synthesized precursors in an water−ethanol volume ratio (R) of 1:3 are composed of closely packed microspheres with an average diameter of about 1 μm. Panels b and c of Figure 1 4067


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Figure 4. (a) Cycling performance and (b) rate capability of all cathode materials. Galvanostatic charge/discharge profiles of the (c) Na0-LMRO, (d) Na0.02-LMRO, (e) Na0.05-LMRO, and (f) Na0.1-LMRO cathode materials.

wide-scan spectra of these materials indicate the presence of Li, Na, Ni, Co, Mn, and O elements (Figure 2d and SI Figure S3). The peak located at around 1071.5 eV is observed in all the Na-doped LMROs, which is attributed to Na 1s states. With the increasing amount of Na, the peak intensity gets stronger and stronger, which is in accordance with the XPS spectra of Na 1s shown in Figure 2f. From Figure 2e, the binding energy of Li slightly shifts to the low level, suggesting a change of chemical environment. After introducing Na+, the downward shift of Li binding energy may imply the weakening of Li−O bands, which can be conducive to facilitating Li ion transfer in Na-doped LMROs.37 The Ni 2p, Co 2p, and Mn 2p XPS spectra are shown in Figure S3a−c, respectively. The XPS peak positions of Ni 2p1/2 and Ni 2p3/2 are at around 854.5 and 872.8 eV, respectively, and they are identical for the samples before and after doping with Na, indicating the same oxidation state of Ni. The same phenomena are observed in the XPS spectra of Co 2p and Mn 2p. From the above XPS results, the introduction of Na+ seems to only affect the chemical environment of Li, whereas it has little influence on the other elements. SEM images in Figure 3a−c and SI Figure S4 show the calcined LMRO are composed of well-developed microspheres with an average size of approximately 1 μm, which basically reserve the microspherical architectures of the hydroxide precursors. However, the surfaces of the products are more rough than those of the precursors, which may be attributed to the formation of pores and void spaces during the calcined processes. By comparison, the cathode materials in the

electrochemical performance, we introduce the different contents of Na+ into Li1.2−xNax[Ni0.13Co0.13Mn0.54]O2 (x = 0, 0.02, 0.05, 0.1). All LMRO materials exhibit similar XRD patterns, as shown in Figure 2a. All peaks can be indexed to the O3 type layered α-NaFeO2 structure (space group R3̅ m) except for the additional weak superlattice peaks observed around 2θ = 20−25°, corresponding to the Li 2 MnO 3 component (space group, C2/m). The (006)/(102) and (018)/(110) peaks of all samples are distinctly split, revealing the formation of the typical hexagonal layered structure. For the Na-doped LMRO samples, no phase for any impurity is detected in the XRD profiles, indicating that Na+ may be introduced into the crystal lattice. The enlargements of the (003) and (104) peaks are shown in Figure 2b,c. The slight shift to the lower angle observed in the pictures means enlarging the Li+ slab, further suggesting the successful introduction of Na+ into the lattice.35 In addition, XRD data of all LMROs were analyzed by Rietveld refinement and the lattice parameters are shown in SI Table S1. With the increasing amount of Na, the cell volume of LMRO materials increases gradually. The ionic radius of Na+ (1.02 Å) is larger than those of Li+ (0.76 Å), Ni2+ (0.69 Å), Co3+ (0.54 Å), and Mn4+ (0.53 Å), thus leading to the lattice expansion after substituting for Li+. The enlargement in the Li+ slab space benefits the lithium ion diffusion between oxygen layers during the lithiation/delithiation process, thus hopefully improving the electrochemical property.36 To analyze the influence of Na+ on the other elements, XPS of all the samples were investigated and shown in Figure 2. The 4068


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Figure 5. (a) Discharge capacity above 3.5 V and (b) percentage of discharge capacity below 3.5 V for four LMRO samples during cycling. dQ/dV plots of (c) Na0-LMRO, (d) Na0.02-LMRO, (e) Na0.05-LMRO, and (f) Na0.1-LMRO cathode materials.

different doping amounts of Na+ present the similar size and morphology. Therefore, we choose Na0.05-LMRO as an example to demonstrate TEM and HRTEM images for a deeper understanding about the detailed structure and composition of the products. Figure 3d shows the typical bright-field TEM image for the solid Na0.05-LMRO spheres of about 1 μm, in accordance with SEM images. From the HRTEM images (Figure 3e,f), clear lattice fringes with interplanar spacing of 0.47, 0.24, and 0.20 nm are shown, which correspond to the (003), (101), and (104) planes of the LiNi1/3Co1/3Mn1/3O2 component (space group, R3̅m) or corresponding to the (001), (2̅01), and (202) planes of the Li2MnO3 component (space group, C2/m), respectively. In addition, the corresponding element mapping shows that Na, Ni, Co, and Mn elements are evenly distributed on the selected areas (Figure 3g). EDS spectrum further demonstrates the atomic ratios of Na, Ni, Co, and Mn are close to the theoretical ratios of elements in Li1.15Na0.05[Ni0.13Co0.13Mn0.54]O2 (Figure 3h). Electrochemical properties of electrode materials were investigated at 300 mA g−1 in the voltage range of 2.0−4.8 V. Cycling performance is shown in Figure 4a. From the picture, the LMRO without Na exhibits the initial discharge capacity of 236.6 mAh g−1. With the increasing cycles, the LMRO material shows fast capacity fading. After 100 cycles, the cathode only delivers the reversible capacity of only 85.6 mAh g−1, with a low capacity retention of 36.2%. However, for Na0.02-LMRO, Na0.05-LMRO, and Na0.1-LMRO materials,

the corresponding capacity retentions are 53.8%, 85.5%, and 83.7%, respectively. Figure 4b shows the rate capability of these LMRO electrode materials. The Na0.05-LMRO cathode delivers reversible charge capacities of 362.7, 243, 222.1, 208.6, and 190.4 mAh g−1 at 30, 100, 300, 500, and 1000 mA g−1, respectively. In addition, when the current density decreases to 30 mA g−1, the specific capacity of 246.8 mAh g−1 is retained. With the increasing current density, the charge capacities of Na0.02-LMRO cathode are 366.4, 245.7, 202.5, 145.3, and 128.7 mAh g−1, respectively, and those of Na0.1-LMRO cathode are 354.1, 222.6, 208.0, 180.0, and 158.7 mAh g−1, respectively. However, pure LMRO electrode without Na under the identical testing conditions only delivers 374.5, 210.2, 179.2, 134.4, and 97.8 mAh g−1, respectively. Na0.05LMRO and Na0.1-LMRO both show similar cycling stability; however, the excessive content of Na leads to a decrease of capacity. That is, the Na0.05-LMRO cathode exhibits the optimal performance. The detailed charge−discharge voltage profiles of the four LMRO electrodes are presented in Figure 4c−f. As shown, the initial charge curves of the four materials are composed of a sloping voltage region (4.5 V).38 The potential plateau is attributed to the loss of oxygen from the lattice, which is confirmed by the previous reports based on in situ XRD39 and differential electrochemical mass spectrometry (DEMS) testing techniques.40 Accompanied by the loss of oxygen, the Li2MnO3 4069


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Figure 6. Ex situ XRD patterns of (a) Na0-LMRO and (b) Na0.05-LMRO before and after different cycles.

Figure 7. HRTEM images and FFT patterns of different areas for (a) Na0-LMRO and (b) Na0.05-LMRO after cycling.

reduction peak corresponding to the reduction of Mn4+ to Mn3+ reduces from 3.44 to 3.00 V after 100 cycles (4.40 mV voltage decay per cycle). Compared with the undoped LMRO, the Na-substituted materials show obviously decreased discharge voltage fade. The average voltage decays per cycle of Na0.02-LMRO, Na0.05-LMRO, and Na0.1-LMRO are 2.80 mV (from 3.43 to 3.15 V), 1.60 mV (from 3.45 to 3.29 V), and 2.20 mV (from 3.45 to 3.23 V), respectively. To the best of our knowledge, the voltage decay of the Na0.05-LMRO electrode is superior to the great mass of other doped LMRO materials reported previously (SI Table S2). These results indicate the Na doping has a stabilizing influence on the layered structure leading to the better cycling property of Na-doped cathodes compared to the undoped counterpart, though the structure transition cannot be totally eliminated by Na substitution. The voltage decay marking the layered/spinel phase transformation, is further analyzed through comparing the LMRO and Nadoped LMRO during cycling using ex situ XRD and HRTEM. The ex situ XRD patterns of four samples before and after different cycles are shown in Figure 6 and SI Figure S6. It is clearly observed that the diffraction peaks from a short-ranged superstructure around 20−25° disappear due to the activation of the Li2MnO3 component.44 For the undoped LMRO material, the (003) peak is broadened, and the new peaks at ca. 18.7°, 31.8°, and 36.5° appear after cycling, which are considered to be related to the spinel structure (Figure 6a).27,30,36 After doping with a low amount of Na, the peak at about 18.8° still appears (SI Figure S5a). In addition, the weaker (018)/(110) splitting of the two Na0-LMRO and Na0.02-LMRO materials indicates the destruction of the layered structure.45 However, the (018)/(110) peaks for Na0.05-LMRO and Na0.1-LMRO samples are still distinctly split during cycling, and no impurity peak is obviously

phase is electrochemically activated and then leads to a high specific capacity in the following discharge process. However, the extraction of lithium ion and the evolution of oxygen directly lead to structural instability in subsequent cycles, such as the undesirable spinel formation in the layered host structure. The structural transformation from layered to spinel results in the gradual voltage decay and the appearance of a ∼3.0 V plateau as clearly observed in the LMRO electrode material (Figure 4c). However, Na-doped LMRO cathodes show obviously reduced voltage decay and enhanced electrochemical stability. Detailed comparisons about discharge capacity above 3.5 V and percentage of discharge capacity below 3.5 V are presented in Figure 5a,b. As shown, Na-doped samples exhibit significantly decreased capacity fade above 3.5 V, indicating the substitution of Na for Li may alleviate the conversion of layered to spinel structure.19 The corresponding dQ/dV profiles are illustrated in Figure 5c−f and SI Figure S5. From Figure S5, four LMRO electrodes show similar behavior about the extraction/insertion of lithium for the first cycle. The oxidation peaks appearing at about 4 and 4.6 V can be assigned to the reaction of TM ions (Co3+, Ni2+) and irreversible loss of oxygen, respectively.41 The peak intensity of ∼4.6 V decreases with the increasing amount of Na, suggesting the suppression of electrochemical removal of oxygen, which is in agreement with the result in Figure 4. The broad cathodic peak at 3.5−4.5 V corresponds to the redox reaction of Ni4+/Ni2+ and Co3+/ Co2+.42 The peak between 2.8 and 3.5 V is associated with the reduction of Mn4+ in the layered MnO2 component. As shown in Figure 5c, along with the cycling, the peak shifts to lower potential, which contributes to the continuous growth of spinel structure during the lithiation/delithiation process.21,43 Due to the serious transformation from layerd to spinel phase, the 4070


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Figure 8. Nyquist plots and the corresponding fitted parameters of (a, c) Na0-LMRO and (b, d) Na0.05-LMRO electrodes after different cycles at 300 mA g−1.

observed, implying the doped LMRO cathode with a certain amount of Na has a strong structural stability during repeated cycles (Figure 6b and SI Figure S6b). Figure 7 shows HRTEM images and the corresponding FFT patterns of different areas of Na0-LMRO and Na0.05-LMRO after 100 cycles. As shown in Figure 7a, though the layered structure of Na0-LMRO can be still observed, most regions are occupied by the spinel structure with high crystallinity, indicating the serious transformation from layered to spinel phase after cycling. The clear lattice fringes with interplanar spacings of 0.47 and 0.48 nm are shown, which correspond to the (003) planes of the LiNi1/3Co1/3Mn1/3O2 component and (111̅) planes of the LiMn2O4 phase, respectively. The structure conversion may induce the distortion and deformation of crystal lattice, even resulting in the formation of an amorphous structure in some regions. However, for the Na0.05-LMRO material, the layered/spinel phase transformation can be restrained significantly to some degree. The original layered structure is basically retained in most areas, although the spinel phase cannot be avoided completely in small regions. Lattice fringes are also apparently observed. No obvious amorphous area can be observed. More HRTEM images after cycling are shown in SI Figure S7. HRTEM results indicate that substituting Na for Li could alleviate effectively the layeredto-spinel phase transition of LMRO materials during repeated cycles. From the above, ex situ XRD and HRTEM associated with electrochemical analyses confirm that the layered/spinel phase transformation does exist in LMRO without Na material. In general, different structural evolutions can induce different kinetic processes during the repeated cycles. Therefore, electrochemical impedance spectroscopy (EIS) measurements were investigated to further understand the different structural stabilities of four electrodes, and the Nyquist plots of the corresponding fitted parameters are presented in Figure 8 and SI Figure S8. These data can be well-fitted with the equivalent circuit using Zsimpwin program (inset of Figure 8b). The

Nyquist spectra of these electrodes show similar characteristics with the overlapping semicircles in high-frequency and medium-frequency regions and the oblique line in the lowfrequency region. The typical semicircles can be ascribed to the SEI resistance (RSEI) and charge-transfer resistance (Rct), respectively, while the oblique line represents the Warburg impedance (W) relevant to the lithium ion diffusion.46−51 The x-intercept is internal resistance deriving from electrolyte and cell components (Re). From the Zsimpwin program, we could obtain each parameter value shown in the equivalent circuit. For convenience, we show the relevant parameters extracted from the electrochemical impedance spectra of all samples during cycling, as shown in SI Table S3. The variations of the corresponding fitted parameters are illustrated in Figure 8c,d (SI Figure S8c,d). By comparison, it is clearly observed that four samples show similar variation tendency for Re: Re almost has no change. As shown, the RSEI and Rct of the Na0.05LMRO material show no obvious increase after cycling. However, the undoped LMRO electrode displays the sharply increasing impedance, revealing a continuously structural evolution during cycling. Even more intuitively, the resistance increasing rate of the four samples before and after cycling is shown in SI Figure S9, further confirming that Na0.05-LMRO shows the lowest resistance changes for RSEI and Rct. The fittings DLi of the four cathodes according to equivalent circuits after cycling are shown in SI Figure S10 (see the detailed calculation equations in SI). Compared to the DLi of the other three electrodes, DLi of Na0.05-LMRO has the least change after cycling. The above results indicate that doping Na can relieve significantly the decomposition of electrolyte to form a more stable SEI film and promote the kinetics of lithium ion diffusion contributed to the enlargement in the Li+ slab space.30,33 To further explore its practical application, a full cell consisting of Na0.05-LMRO and the commercial graphite anode was assembled. The capacity of anode exceeds about 10% that of cathode. The specific capacity is estimated 4071


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Figure 9. Electrochemical characterization of full cells. (a) Galvanostatic charge−discharge profiles of graphite//Na0.05-LMRO. (b) Cycling performance at 50 mA g−1 for graphite//Na0-LMRO and graphite//Na0.05-LMRO. stoichiometric amounts of LiOH and NaOH. After homogeneously mixing, the mixture were preheated at 480 °C for 5h, and then calcined at 900 °C for 12h in air. During the synthetic processes, the doping amount of Na+ was controlled in different levels of x = 0, 0.02, 0.05, and 0.1, and the products are denoted as Na0-LMRO, Na0.02LMRO, Na0.05-LMRO and Na0.1-LMRO, respectively. Characterization. Powder X-ray diffraction (XRD) using PANalytical X-pert diffractometer with Cu Kα radiation was employed to identify the crystallography phase of the as-synthesized samples. The morphology and nanostructure were observed by field emission scanning electron microscopy (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, JSM-2100F), and highresolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20). Chemical compositions were characterized by energy dispersive spectrometer (EDS, Oxford INCA) and inductively coupled plasma atomic emission spectrometry (ICP-AES, ICAP6300). The valence states of Li, Ni, Co, Mn and Na were analyzed by X-ray photoelectron spectroscopy (XPS) on a PHI Quanteral II with a Al Ka radiation source. Electrochemical Measurements. Half-cell testing. The working electrodes were prepared by slurring Nax-LMRO, acetylene black, and polyvinylidene fluoride (PVDF) with the weight ratio of 80:10:10 in 1-methyl-2-pyrrolidinone (NMP). The above slurries were coated on an aluminum foil, and then dried in in a vacuum oven at 100 °C overnight. CR-2025 type coin cells were assembled using this cathode electrodes, lithium foil as the reference and counter electrodes, Celgard 2400 membranes as the separators, 1 M LiPF6 dissolved into the mixed solvent of equivoluminal ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte. The galvanostatic charge/ discharge measurements were performed between 2.0 to 4.8 V (vs Li+/Li) at various current densities (1C = 300 mA g−1) with the battery measurement system (LAND CT2001A). Cyclic voltammetry (CV) was carried out at a scan rate of 0.1 mV s−1 from 2.0 to 4.8 V using the electrochemical workstation (IM6e). Electrochemical impedance spectroscopic (EIS) was measured from 100 kHz to10mHz. Full-cell testing. The cathode and anode were the LMRO cathode material (Na0-LMRO, Na0.05-LMRO) and graphite, respectively. For graphite//LMRO full cell, the graphite anode was prepared by coating the slurry on a Cu foil with graphite, acetylene black, and PVDF in the weight ratio of 80:10:10 in NMP. The electrodes were 100 °C for 12 h. The galvanostatic charge/discharge measurements were performed at 50 mA g−1 from 1.5 to 4.8 V. The specific capacity was estimated on the basis of the weight of cathode.

according to the weight of cathode. The full cell was performed between 1.5 and 4.8 V. As shown in Figure 9a, graphite// Na0.05-LMRO full cell delivers the reversible discharge capacities of 221.1, 220.9, 209.4, and 178.1 mAh g−1 at first, second, 20th, and 50th cycles, respectively. However, the discharge capacities of graphite//Na0-LMRO under the identical testing conditions are 238.9, 235.4, 197.8, and 146 mAh g−1, respectively (SI Figure S11). After 50 cycles, the discharge capacity retentions for two full cells are 80.3% and 61.1%, respectively. The good cycling stability of graphite// Na0.05-LMRO electrode verifies the positive influence of Na dopant on the structure.

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CONCLUSIONS In summary, we have successfully incorporated a certain amount of Na into LMRO microspheres by a hydrothermal method. The Na-doped LMRO cathode exhibits remarkably improved electrochemical properties, which delivers high cycling performance (capacity retention of 85.5% after 100 cycles at 300 mA g−1, superior to the LMRO without Na), good rate capability (190.4 mAh g−1 at 1000 mA g−1), and low voltage decay per cycle (∼1.60 mV at 300 mA g−1 after 100 cycles). The enhanced cycling performance is likely to be ascribed to the introduction of sodium, which promotes the kinetics of lithium ion diffusion and facilitates the electronic and ionic conductivity. More remarkably, the doping of sodium might inhibit the formation of three adjacent vacancies and Mn migration, thus effectively mitigating the transformation from layered to spinel structure. Furthermore, when the cathode is paired with graphite anode, the full cell shows good cycling stability (capacity retention of 80.3% after 50 cycles). Therefore, our work makes it possible to design and commercialize the high-capacity and -stability candidate for the next-generation lithium ion battery cathodes.

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EXPERIMENTAL SECTION

Material Preparation. Synthesis of hydroxide precursors. The hydroxide precursors were prepared via a hydrothermal method. In a typical process, Ni(CH3COO)2·4H2O (0.4 g), Co(CH3COO)2·4H2O (0.4 g) and Mn(CH3COO)2·4H2O (0.4 g) were dissolved in the 40 mL mixed solution including 40−8 mL deionized water and 0−32 mL ethanol. After stirring for 10 min, 10 g urea was added to the above solution until it was transparent. Subsequently, the clear solution was transferred into the 50 mL Teflon-lined autoclave, and then heated at 150 °C for 24 h. After cooled to room temperature, the precipitates were washed with water and ethanol. Finally, the precursors were obtained after dried at 80 °C in vacuum overnight. Synthesis of Na-doped Li1.2‑xNax[Ni0.13Co0.13Mn0.54]O2 cathode materials. The above hydroxide precursors were mixed with the

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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/acsaem.8b00740. Supplementary SEM images and XRD patterns, XPS spectra, dQ/dV plots of LMRO samples, HRTEM images and FFT patterns, Nyquist plots and the 4072


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corresponding fitted parameters, resistance changes of four samples, variation curves of lithium ion diffusion coefficient, and galvanostatic charge/discharge profiles of graphite//Na0-LMRO electrode (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Z.C.) E-mail: [email protected]. *(M.X.) E-mail: [email protected]. *(C.C.) E-mail: [email protected] ORCID

Zhuo Chen: 0000-0002-0671-4974 Chuanbao Cao: 0000-0003-2830-4383 Yunjun Luo: 0000-0001-6927-2635 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472031). REFERENCES

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Toward Alleviating Voltage Decay by Na Substitution in Li-rich Mn

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