Benefits of Copper and Magnesium Co-substitution in Na0.5Mn0.6Ni0

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Benefits of Copper and Magnesium Co-substitution in Na0.5Mn0.6Ni0.4O2 as a Superior Cathode for Sodium-Ion Batteries Tao Chen, Weifang Liu, Fang Liu, Yi Luo, Yi Zhuo, Hang Hu, Jing Guo, Jun Yan, and Kaiyu Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01909 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Energy Materials

Benefits

of

Copper

and

Magnesium

Co-substitution

in

Na0.5Mn0.6Ni0.4O2 as a Superior Cathode for Sodium-Ion Batteries Tao Chen†, Weifang Liu†, Fang Liu†, Yi Luo†, Yi Zhuo†, Hang Hu†, Jing Guo†, Jun Yan*†, and Kaiyu Liu*† † Hunan

Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical

Engineering, Central South University, Changsha 410083, P.R. China E-mail: [email protected]* [email protected]* KEYWORDS: sodium-ion batteries, cathode, co-substitution, rate performance, transition metal oxide.

ABSTRACT: Transition metal oxides are considered to be one kind of the most promising cathode material for sodium ion batteries. Here, P2-type Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 cathode material was designed and synthesized by sol-gel method for the first time. The co-substitution of copper and magnesium in Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 inhibits the P2-O2 phase transition and enhance lattice spacing to reduce the resistance of sodium ion deintercalation and intercalation, which is beneficial to the improvement of electrochemical properties. A Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 electrode delivers an initial specific capacity of 126.1 mAh g-1 with a high average voltage of 3.6 V (2-4.6 V) and 96.7% capacity retention after 100 cycles at 0.1C. More importantly, the full cells using this cathode material and hard carbon as anode exhibit initial reversible specific capacity of 70.8 mAh g-1 with energy density of 226.56 Wh kg-1 and high capacity retention of 90.1% after 200 cycles at 0.5C. Therefore, 1 ACS Paragon Plus Environment

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Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 is able to be a cathode material with high operation voltage, good stability, high energy density and excellent rate performance for sodium ion batteries applications.

Keywords: sodium-ion batteries, cathode, co-substitution, rate performance, transition metal oxide. 1. Introduction lithium-ion batteries (LIBs) have attracted increasing attention as energy sources due to the urgent demand to develop a sustainable and highly efficient energy storage system.[1] However, the scarcity of lithium resources on earth has been keeping the price of the raw material for LIBs rising in recent years. Sodium ion batteries (SIBs) have been considered as the best substitutes because of abundant sodium resources and similar properties.[2] Compared with the lithium ion, the sodium ion has a larger ion radius, leading to a more slow diffusion kinetics of SIBs, which poses a challenge to the exploration of the suitable sodium cathode and anode materials.[3] It is generally believed that the performance of the cathode material (such as specific capacity, voltage and cyclicity) is the key factor to affect the energy density, safety and cycle life of a sodium ion battery. Accordingly, quite a large number of intercalation compounds have been researched constantly.[4-12] Among all of the available cathode materials, sodium transition metal oxides (NaxTMO2,TM represents for a transition metal) have attracted extensive attention due to their high energy density, high reversible capacity and high discharge voltage plateaus.[13-18] According to the coordination type of sodium ions and the stacking mode of oxygen, the most common structures are P2-type and O3-type, where Na ions occupy trigonal prismatic (P) and octahedral (O) lattice sites, respectively, while the number represents the least repeated units of oxygen.[19-22] Generally speaking, the O3 structure has a higher capacity because of its more sodium inlaid sites. Alternatively, the P2 phase structure keeps a wider layer spacing, which makes Na+ diffusion easier, and enables Na+ migrates from a trigonal prismatic vacancy to a nearby trigonal prismatic vacancy, showing a higher ionic conductivity.[21-25]

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Viewed from the reported study, the P2-type Na0.67Ni0.33Mn0.67O2 is regarded as a promising cathode material because of its high specific capacity, energy density and working voltage, which is mainly attributed to the high redox potential of Ni2+/Ni3+ and Ni3+/Ni4+ (both more than 3.0 V). However, the structure of Na0.67Ni0.33Mn0.67O2 is unstable during charge/discharge.[26-29] When charging voltage exceeds 4.2 V, the migration of oxide frame will lead to the P2-O2 phase transition in Na0.67Ni0.33Mn0.67O2 , resulting in a sharp decay in capacity and voltage after several cycles.[30-34] Substitutions or doping in MO2 layers of other metal elements are considered an effective method to suppress the irreversible P2-O2 phase transition.[24] In our previous work, we announced Cu-substituted P2-Na0.44Mn0.6Ni0.3Cu0.1O2 cathode delivers a reversible capacity of 149 mAhg-1 with a high energy density of 469 Whkg-1 at 0.1C current density. Also, we have noticed the capacity retention of P2Na0.44Mn0.6Ni0.3Cu0.1O2 is only 80% after 50 cycles at 0.1C, and the average voltage is only 3.1 V.[34] The data has further proved the unique advantages of this type of material and the promising future of the synthetic strategy, which impelled us continuously to explore this system.[34] In this work, we synthesized P2-type Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2, Na0.5Mn0.6Ni0.2Cu0.2O2, Na0.5Mn0.6Ni0.2Mg0.2O2 and Na0.5Mn0.6Ni0.4O2 (denoted hereafter as NMNCM, NMNC, NMNM and NMN, respectively) successfully by the sol-gel method. The NMNCM electrode delivers a better electrochemical property. We carried out a series of structural studies to demonstrate that the introduction of copper and magnesium can effectively inhibit the phase transition of P2-O2 and the lattice parameters are improved due to the larger ionic radius of Mg2+ and Cu2+. In order to evaluate the feasibility of NMNCM cathode in practical SIBs, we studied the performance of NMNCM full cells, with hard carbon as the anode material. All the investigations show the superiority of NMNCM with excellent cycling performance, high energy density and high operation voltage as a cathode material. 2. Experimental Section Synthesis of P2-Type NMNCM. The P2-type Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2) cathode materials were synthesized by a sol-gel method. The stoichiometric ratio of sodium carbonate (anhydrous), manganese (II) acetate tetrahydrate, nickel (II) acetate tetrahydrate, copper (II) acetate ACS Paragon Plus Environment

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monohydrate and magnesium (II) acetate tetrahydrate were dissolved into the 75 wt% citric acid solution and stirred for an hour to obtain a mixed solution. The resulting precursor complex was heated in a water bath at 80°C until a green gel was obtained. Then the gel was put into a drying oven for 24h at 100°C to yield a homogeneously powder. The powder was heated at 400°C for 10h in a muffle furnace. After cooling to room temperature, the powder was ground, pressed into pellet. Finally, the pellet was calcined at 900°C for 10h to generate the final target product. Characterization. The powder X-ray diffraction (XRD) patterns were characterized using Bruker Advance-D8 diffractometer with Cu (Kα) radiation. GSAS software was used to refine the unit cell parameters based on the Rietveld method. The content of metal element were confirmed by means of inductively-coupled plasma-atomic emission spectrometry (ICP-AES) analysis on a Perkin Eimer type instument (Optima5300DV). Scanning electron microscopy images (SEM) mesurements were carried out on a FEI Nova Nano SEM 230 equipment. JEOL-2100F electron microscope and FEI Tecnai F20 electron microscopewas were used to obtain Transmission electron microscopy (TEM) images, EDS maps and high-resolution transmission electron microscopy (HRTEM) images. X-ray photoelectron spectroscopy (XPS) analysis was collected on an ESCALAB MK II X-ray photoelectron spectrometer using Mg Ka radiation. Electrochemical test. Electrochemical researches were carried out in coin cells using a metal sodium plate as anode (half cell). 1.0 mol dm-3 NaClO4 dissolved in propylene carbonate (PC) was used as electrolyte solution. Electrode materials were composed of active material, acetylene black and polyvinylidene fluoride (PVDF) with mass ratio of 8 : 1 : 1. The electrode materials were uniformly pasted on Al foils and dried for 24h at 60°C. All the galvanostatic charge-discharge measurements were performed in the voltage range of 2.0-4.6 V on NEWARE battery test system. The coin cells were made in a glovebox filled with argon gas. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetric (CV) measurements were performed on a Autolab PG302N electrochemical workstation. For galvanostatic intermittent titration technique (GITT) measurement, the cells were charged and

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discharged at 17 mA g-1 for 30 min, followed by a rest for 1 hour. The chemical diffusion coefficients of Na ions could be calculated by the simplified equation: 2

Es

2

( )( )

4 mBvm D=  MBS

E

Where D is the chemical diffusion coefficient. mB is the mass of active materials. MB and vm are relative molecular mass and molar volume, respectively.  is the time for an applied galvanostatic current. S is the surface area of the electrode. Es is the quasi-equilibrium potential and E is the total transient change of cell voltage during a single titration. 3. Results and discussion The micromorphology of NMNCM particles was characterized by scanning electron microscopy images (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) in Figure 1a, b and Figure S1, respectively. The particle size is speculated to be 1-2µm and the morphology is irregular. The lattice spacing can be clearly observed, and the size is measured as 0.211nm that corresponds to (012) planes, which is agree with the XRD results. Energy dispersive spectrocopy (EDS) element maps in Figure 1c-g demonstrate that all the selected elements exhibit a homogeneous distribution over the whole particle. Selected area electron diffraction (SAED) pattern in Figure S2 indicates the structure of NMNCM is highly crystallized with the hexagonal layered lattice electron diffraction pattern.[18] Figure 1h presents the Rietveld-refined of the powder X-ray diffraction (XRD) patterns for NMNCM. Figure S3, S4 and Figure S5 show the refined XRD patterns of Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2). All the Bragg diffraction peaks of the four samples demonstrate a pure P2 phase with the space group P63/mmc (no.194), indicating that the substitutions of copper and magnesium do not influence the crystal structure.[35,36] The result of inductively-coupled plasma-atomic emission spectrometry (ICP-AES) analysis in Table S1 (Supporting Information) confirms the atomic ratio of Na:Mn:Ni:Cu:Mg as expected. The observed patterns of Rietveld refinement are consistant with fitting patterns. The parameters Rwp and Rp, which are the weighted profile and profile R-factors respectively, are below 10%, representing the high credibility of the Rietveld refinement. All the crystallographic 5 ACS Paragon Plus Environment


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data are shown in Table S2, Table S3, Table S4 and Table S5. The Rietveld-refined patterns suggest that both of the magnesium and copper ions are located in the prismatic 2a Wyckoff sites, occupying the sites of nickel. What’s more, two kinds of MO2 layer stacking sequences lead to two different positions of sodium ions (Naf and Nae) in each crystal cell, which are distributed in proportion of 1:1. The symmetric distribution of sodium ions improves the stability of the P2-type structure. The lattice parameters of the NMNCM sample are calculated to be a=b=2.8874(2) Å and c=11.1902(3) Å, which are larger than NMN (a=b=2.8867(4) Å and c=11.11565(3) Å) due to the larger ionic radius of Mg2+ and Cu2+ (Mn3+=0.645 Å, Mn4+=0.53 Å, Ni2+=0.69 Å, Cu2+=0.73 Å and Mg2+=0.72 Å).[37] Besides, when only Cu or Mg element is introduced into NMN samples, the increase of lattice parameters can also be observed. The improvement of interlayer spacing reduces the resistance when Na ions deintercalate and intercalate from crystal lattices. The structural model diagram of P2-type NMNCM is given in Figure 1h. Transition metals and oxygen ions are located at the MO2 (M=Mn, Ni, Cu and Mg) layer. In contrast, sodium ions with large radius (1.02 Å) are embedded in MO2 layers, sharing edges and faces with MO6 octahedra. The galvanostatic charge-discharge curves of NMNCM electrodes in Figure S6a show a high discharge specific capacity of 205.6 mAh g-1 with a high energy density of 575.6 Wh kg-1 in a voltage range of 1.5-4.5 V. Owing to the Jahn–Teller effects of Mn3+, which is formed below 2.0V, NMNCM exhibits a poor reversibility in 1.5-4.5 V (154.7 mAh g-1 retention after 24 cycles in Figure S6b).[30,31] The long-term cycling performances of the Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2) electrodes at 0.1 C are displayed in Figure 2a. The Cu-free and Mg-free NMN exhibits the poorest cycling stability due to the P2-O2 phase transition.[24,32,38] With the substitution of manganese and copper, obvious promotion of cycling performance can be found in NMNM and NMNC electrodes. Mg-substituted NMNM displays the lowest initial specific capacity of 118.5mAh g-1, which may due to the reduction of theoretical capacity by the inactive magnesium. The magnesium and copper co-substituted NMNCM electrode possesses the most excellent cycle stability, which has a initial specific discharge specific capacities of 126.1 mAh g-1 and a super high capacity retention of 96.7% after 100 cycles at 0.1C, ACS Paragon Plus Environment

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showing a high energy density of 453.96 Wh kg-1. The significant improvement of cycling performances of NMNCM electrode can be attributed to the inhibition of the irreversible P2-O2 phase transition and the improvement of lattice spacing by the co-substitution of Cu and Mg.[16, 39] Moreover, as shown in Figure 2b, NMNCM electrodes still remain a reversible specific capacity of 101.1 mAh g-1 and a capacity retention of 70.7% at a current density of 1A g-1. The rate properties of the Na0.5Mn0.6Ni0.4yCuy-xMgxO2

(0≤x≤y≤0.2) electrodes are shown in Figure S6c. All the electrodes were charged and

discharged from 0.1 C to 5 C and finally back to 0.1 C every five cycles. The NMNCM electrode shows excellent rate performance, which retains a reversible capacity of 98.8 mAh g-1 at 5C and a capacity retention of 95.5% (119.4 mAh g-1) after cycling at different currents. More noteworthy, even when only doped with Cu or Mg, the rate performance and cycle performance can be improved. Combined with the results of Rietveld-refined of the powder XRD patterns, the enhancement of the electrochemical properties of NMNCM is mostly due to the increase of lattice spacing, as a result of copper and magnesium co-substitution. For comparison, the electrochemical properties of various transition metal oxide cathode materials for SIBs in recent years were summarized in Table S6. To investigate the redox reaction of each charge and discharge plateaus, the cyclic voltammetric (CV) measurements were performed in Figure 2c. In the CV curve of NMNCM electrode, there are five oxidation peaks, but only four reduction peaks. This phenomenon occurs because the oxidation/reduction peaks of Cu2+/Cu3+ (3.93/3.73 V) and P2-O2 phase transition (4.29/3.89 V) overlap, which enhances the reduction peak area at 3.87 V significantly.[39] The reversible oxidation/reduction peaks at 3.70/3.52 V and 3.42/3.35 V are related to the redox reactions of Ni3+/Ni4+ and Ni2+/Ni3+, respectively.[34] The reversible peaks at 3.28/3.15 V are presumed to be the precipitation of CuO and NiO, which is discovered from the ex situ XRD in Figure 3. Besides, the reduction peaks of NMNC electrodes are totally higher than NMNM electrodes as shown in Figure S7. These minor peaks below 2.2V are attributed to the redox reactions of partial Mn3+/Mn4+. In contrast to the NMN electrode, the CV curve of NMNCM electrode shows a better symmetry. What’s more, the introduction of Cu makes the potentials of all reduction peaks in NMNCM improved. On the other hand, the CV curves illustrate Cu is active while Mg is inert in the whole charging and discharging process. The cycle performance of NMN is improved obviously via Mg substitution as shown in Figure 2a, while the discharge curves in Figure S7 show that the average voltage of reduction curve of Cu substitution is higher. Figure S6d ACS Paragon Plus Environment

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presents the galvanostatic discharge curves of the Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2) electrodes and Figure 2d shows the charge and discharge curves of NMNCM electrodes at 0.1C. NMN and NMNCM electrodes exhibit more obvious discharge plateaus than NMNM and NMNC electrodes. Four discharge plateaus can be observed at 4.06 V, 3.65 V, 3.40 V and 2.27 V in the NMNCM discharge curve. Due to the existence of high voltage plateaus, the average discharge voltage of NMNCM is up to 3.6 V. With the increase of cycle numbers, the structural degradation of NMNCM electrode is enhanced, resulting in the increase of capacity decay and the disappearance of the discharge plateaus. After placing the NMNCM sample in air for one month, it still keeps an initial specific capacity of 125.8mAhg-1 and capacity retention of 81.6%, indicating the NMNCM sample has the advantage of air stability. (Figure S6) The substitution of Cu and Mg into MO2 layer can enhance the surface structure as well, thus prevent the material from directly contacting with air to avoid oxidization.[40,41] Ex situ XRD patterns were characterized in Figure 3 to deeply reveal the structure evolution during the charging and discharging process. With the deintercalation of Na+, (002) and (004) peaks shifting to lower angle, indicate the reduction of lattice spacing. (Figure S8 ) As shown in Figure 3a, the peaks of CuO impurity are observed at 38.8° and 35.6° in the voltage range of 3.3-4.6 V. On the contrary, the peak of NiO impurity exists in 2-3.3V at 43.0°, which is caused by the reduction reaction of Ni2+/Ni3+. With the release of sodium ions, the valence of transition metal ions in MO2 layer increases. However, due to the relatively high redox potential of copper (Cu2+/Cu3+), a small amount of CuO which has not been able to react in time precipitates. When discharged below 3.3 V, Ni3+ has almost been converted to Ni2+. Because of the intercalation of Na+ can not keep up with the reaction rate, some NiO precipitates. Meanwhile, CuO returns to MO2 layer. There is 2.138 wt% CuO at 4.4 V and 1.070 wt% NiO at 2 V, which are calculated by the Rietveld method refinement as shown in Table S7. What’s more, the migration of these peaks is highly reversible when NMNCM electrodes are discharged back to 2.0 V and Na+ inserted back to the crystal lattice. The migration of other Bragg diffraction peaks has never been observed during the whole charging and discharging process, showing NMNCM electrodes maintain the P2 structure all the time. On the contrary, when NMN electrodes are charged up to 4.2 V, the disappearance of some diffraction peaks such as (100), (012) and (106) is discovered. (Figure 3b) With the deintercalated of sodium ions, the MO2 layer and oxygen layer glide to form O2 phase, which


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is consistent with previous research work[40]. It can be conclude that Cu and Mg co-substituted P2NMNCM can apparently inhibit the P2-O2 phase transition and make the whole charge-discharge process stay in single phase. Ex situ X-ray photoelectron spectroscopic (XPS) measurements were performed in Figure 4 to analyse the oxidation states of Mn, Ni, Cu and Mg during the Na+ deintercalation/intercalation processes. For the pristine NMNCM material, the location of 2p main peaks illustrates the existence of Mn4+, Ni2+, Cu2+ and Mg2+. When charged to 4.6 V, the binding energy of Ni 2p3/2 shifts to 855.6 eV and the binding energy of Cu 2p3/2 shifts to 933.7 eV, indicating that Ni2+ and Cu2+ is converted into Ni4+ and Cu3+, respectively.[40] While discharged to 2.0 V, a small amount of Mn4+ is reduced to Mn3+, leading to Mn 2p3/2 peak barely approach lower energy values by 0.4 eV. However, there is no shift on the binding energy of Mg1s, indicating the magnesium ions maintain a bivalent oxidation state and do not undergo redox reaction throughout the charge/discharge process between 2 and 4.6 V. In order to investigate the chemical kinetics behavior of Na+ in NMNCM, galvanostatic intermittent titration technique (GITT) twests were employed to estimate the diffusion coefficients of Na+. For a single titration process (Figure S9a and c), it is observed that there is a linear relationship between the voltage and the square root of the galvanostatic time (Figure S9b and d). The GITT curves and Na diffusivity versus state calculated D values are shown in Figure 5a and b. The variation of diffusion coefficients is similar during charging/discharging process. The D values are in the 10-10-10-9 cm2 s-1 order of magnitude. It is noticeable that the values at plateau of charge/discharge process are lower than that in the middle of each plateau, suggesting that the Na-intercalation/extraction reactions are diffusioncontrolled processes. While the D values at upper voltage plateau are obviously lower. Compared with the other cathodes for SIBs in Table S8, the calculated D values are evidently higher, which is considered to be the result of the increase of lattice parameters, so that the transportation channels of sodium ions increase and the resistances decreases. To evaluate the feasibility of NMNCM cathode in practical SIBs, the full cell performance was tested coupling NMNCM as the cathode material and hard carbon as the anode material, as shown in Figure 5c ACS Paragon Plus Environment

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and d. The full cells were cycled at a rate of 0.5C (based on the cathode) between 1.6 V and 4.5 V. The full cells deliver initial reversible specific capacity of 70.8 mAh g-1 and a high average operation voltage of 3.2 V. The average Coulombic efficiency is almost 100% during the whole cycling process, showing an excellent intercalation/deintercalation process of sodium ions between cathode and anode electrodes in this full system. Compared with the 1st cycle, though the discharge curve of the 100th cycle delivers some attenuation, the reversible efficiency is improved, which is due to the diffusion of sodium ion in the whole full cell system and the formation of the solid electrolyte interphase (SEI). The full cells exhibit high capacity retention of 90.1% after 200 cycles. Moreover, the energy density of this system is up to 226.56 Wh kg-1, which is higher than most of the recent published full cells. 4. Conclusions Significant performance improvement can be clearly observed in P2-type Na0.67Mn0.6Ni0.2Cu0.1Mg0.1O2 by the co-substitution of Cu and Mg. The Na0.67Mn0.6Ni0.2Cu0.1Mg0.1O2 cathode shows excellent capacity retention and high average operation voltage. The Rietveld-refined patterns and ex situ XRD results suggest the co-substitution of Cu and Mg increases the lattice spacing and inhibits the P2-O2 phase transition to enhance the stability of the structure, respectively. The improved structure can obviously enhance the migration ability of sodium ions. What’s more, Cu is better to enhance the average operation voltage and Mg is more beneficial to improve the cycling performance of NMNCM electrode. Besides, the full cell using this cathode material and hard carbon as anode exhibits excellent cycling performance and high energy density (226.56 Wh kg-1). All these results show that this new cathode material can be employed in future practical SIBs for large scale energy storage with high feasibility. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K.Y. Liu) *E-mail: [email protected] (J. Yan) ACS Paragon Plus Environment

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21471162) and Hunan Provincial Innovation Foundation for Postgraduate (No 502211822). REFERENCES [1] Thackeray, M. M.; Wolverton, C.; Isaacs, E. D. Electrical energy storage for transportation— approaching the limits of, and going beyond, lithium-ion batteries. Energy Environ. Sci. 2012, 5, 78547863. [2] Gogotsi, Y.; Simon, P. Materials science. True performance metrics in electrochemical energy storage. Science 2011, 334, 917-918. [3] Ellis, B. L.; Nazar, L. F. Sodium and sodium-ion energy storage batteries. Curr. Opin. Solid State Mat. Sci. 2012, 16, 168-177. [4] Berthelot, R.; Carlier, D.; Delmas, C. Electrochemical investigation of the P2–NaxCoO2 phase diagram. Nat. Mater. 2011, 10, 74-80. [5] Yuan, D.; Liang, X.; Wu, L.; Cao, Y.; Ai, X.; Feng, J.; Yang, H. A Honeycomb‐Layered Na3Ni2SbO6: A High-Rate and Cycle-Stable Cathode for Sodium-Ion Batteries. Adv. Mater. 2014, 26, 6301-6306. [6] Wu, C.; Jiang Y.; Kopold, P.; Van Aken, P. A.; Maier, J.; Yu, Y. Peapod-Like Carbon-Encapsulated Cobalt Chalcogenide Nanowires as Cycle-Stable and High-Rate Materials for Sodium-Ion Anodes. Adv. Mater. 2016, 28, 7276-7283.


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Figure 1. . a) TEM and b) HRTEM images of P2 NMNCM sample; EDS element maps of NMNCM, c) Na, d) Mn, e) Ni, f) Cu and g) Mg；h) Rietveld-refined pattern of the powder XRD data for NMNCM; XRD patterns of Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2). i) Structural model diagram of P2-type NMNCM.


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. Figure 2. Electrochemical performance of Na0.5Mn0.6Ni0.4-yCuy-xMgxO2 (0≤x≤y≤0.2) electrodes. a) Cycle performance of different samples at 0.1 C; b) the cycling performance of NMNCM electrode at a current density of 1 A g-1; c) CV curves at 0.2 mV s-1; d) galvanostatic charge-discharge curves of NMNCM electrode at 0.1C.


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Figure 3. Ex situ XRD patterns collected during charge/discharge process of the (a) NMNCM and (b) NMN electrodes between 2.0-4.6 V under a current rate of 0.1 C.

Figure 4. Ex situ XPS spectra of a) Mn 2p, b) Ni 2p, c) Cu 2p and d) Mg 1s in NMNCM samples.


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Figure 5. GITT curves and Na diffusivity versus state for NMNCM electrode material during a) charge and b) discharge processes; c) the charge-discharge curves and d) cycle performance of the full cell system at 0.5C.


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Abstract Graphic


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Benefits of Copper and Magnesium Co-substitution in Na0.5Mn0.6Ni0

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