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Triclinic Off-stoichiometric Na3.12Mn2.44(P2O7)2/C Cathode Materials for High Energy/Power Sodium-ion Batteries Huangxu Li, Zhian Zhang, Ming Xu, Weizhai Bao, Yanqing Lai, Kai Zhang, and Jie Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07577 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Triclinic Off-stoichiometric Na3.12Mn2.44(P2O7)2/C Cathode Materials for High Energy/Power Sodium-ion Batteries Huangxu Li, † Zhian Zhang, *,† Ming Xu, †,‡ Weizhai Bao, § Yanqing Lai, *,† Kai Zhang, † Jie Li † †

School of Metallurgy and Environment, Central South University, Changsha 410083, P. R.

China ‡

Department of Applied Physics, Hong Kong Polytechnic University, Kowloon, Hong Kong

§

Centre for Clean Energy Technology, University of Technology Sydney, Sydney, New South

Wales 2007, Australia * Email address: [email protected] and [email protected]

Abstract The application of sodium-ion batteries (SIBs) requires a suitable cathode material with low cost, non-toxic, high safety and high energy density, which is still a big challenge, thus basic research on exploring new types of materials is imperative. In this work, a manganic pyrophosphate and carbon compound Na3.12Mn2.44(P2O7)2/C has been synthesized through a feasible sol-gel method. Rietveld refinement reveals that the Na3.12Mn2.44(P2O7)2 adopts a triclinic structure (P−1 space group), which possesses spacious ion diffusion channels for facile sodium migration. The off-stoichiometric phase is able to offer more reversible Na+, delivering an enhanced reversible capacity of 114 mAh g-1 at 0.1C, and due to the strong “inductive effect” that (P2O7)4- groups imposing on Mn3+/Mn2+ redox couple, the Na3.12Mn2.44(P2O7)2/C presents high platforms above 3.6 V, contributing a remarkable energy density of 376 Wh kg-1, which is among the highest Fe-/Mn-based polyanion-type cathode materials. Furthermore, the offstoichiometric compound also presents satisfactory rate capability and long-cycle stability, with

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capacity retention of 75% after 500 cycles at 5C. Ex-situ XRD demonstrates a single phase reaction mechanism, and the density functional theory (DFT) calculations display two 1D sodium migration paths with low energy barriers in Na3.12Mn2.44(P2O7)2, which is vital for the facile sodium storage. We believe that this compound will be a competitive cathode material for large scale SIBs. Keywords: Na3.12Mn2.44(P2O7)2, off-stoichiometric, sodium-ion batteries, cathode, high energy, high power

1. Introduction Sodium-ion batteries (SIBs) have been experiencing rapid progress due to the merits of abundant sodium resources and similar electrochemical principles with lithium-ion batteries (LIBs).1-3 However, sodium has large atomic weight and ionic radius, leading to lower energy density, inferior sodium diffusion mobility and unsatisfactory structural stability of the electrode materials, which severely hinder the development of SIBs.

4-8

Thus, it is of great urgency to

explore advanced electrode materials with improved energy density, stable structure, low-cost and non-toxicity to promote the real application of SIBs. For cathode materials, concentration has been focused on polyanion-type compounds owing to their distinctive open framework, which is typically constructed by (XO4)n- (X = P, S, B, Si) tetrahedron anion units and MOx (M = Fe, Mn, Co, V) polyhedra units.

9, 10

The highly stable

X−O covalent bonds are strong enough to stabilize the lattice oxygen even at highly charged state, which is the key to ensure its high-level safety. The open framework also possesses many spacious interstices to accommodate large Na+, leading to lower volume change and less phase transition during sodiation and desodiation. In addition, compared with other types of materials, the inductive effect in polyanion-type materials leads to a higher redox potential, which benefits higher energy density.11,12 To data, various polyanion-type cathode materials such as NaFePO4,13 Na3V2(PO4)3,14-17 Na2FeSiO4,18,19 Na2CoSiO4,20 Na4Co3(PO4)2(P2O7),21 Na2+2xFe2-x(SO4)3,22 Na7V4(P2O7)4(PO4)23 etc. have been proposed as cathode materials for SIBs. Among them, the silicate normally limited by lower potential because of the weaker electronegativity of (SiO4)4group, and for sulfates, the thermal decomposition and sensitive surface remain problems for


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practical applications.24 By contrast, the pyrophosphate materials show high potential, moderate capacity and high safety. To date, Na2FeP2O7,25,26 Na2CoP2O7,27 Na2(VO)P2O7,28 NaVP2O7,29 Na7Fe4.5(P2O7)4

30

and Na7V3(P2O7)4

31

etc. were reported to show good sodium storage.

Nevertheless, most of them are hindered by either low energy density based on Fe2+/Fe3+ activation or the use of expensive and toxic V and Co elements. In this contest, manganese should be an attracting alternative due to its low cost, non-toxicity and high redox potential based on Mn3+/Mn2+ couple. In 2013, the Jung group found that Na2MnP2O7 could exhibit a reversible capacity about 90 mAh g-1 and achieve a high energy density around 300 Wh kg-1,32 which is higher than its iron counterpart, but still needs to be improved for applications. Recently, offstoichiometric sodium phase pyrophosphates were reported to demonstrate enhanced electrochemical performance due to their unique structures. Specifically, it is believed that the off-stoichiometric phase has more reversible sodium ions than the stoichiometric phase, enabling an improved energy density, and extra sodium ions tend to stabilize the framework upon deep charge/discharge process, thus offering better structural stability during long cycle.33-37 However, the advanced off-stoichiometric phase has never been introduced in Mn-based pyrophosphates to improve energy density. Research on the electrochemical performance and sodium storage reaction mechanism in off-stoichiometric phased Mn-based pyrophosphates is still unknown and imperative to understand. Herein, an off-stoichiometric phase manganic pyrophosphate Na3.12Mn2.44(P2O7)2/C (noted as NMP/C) material was fabricated through a feasible sol-gel approach. Structure information, kinetics and reaction mechanism of this NMP/C cathode material are systematically investigated to offer insight into this material. Based on Mn3+/Mn2+ redox, the NMP/C is able to achieve an impressive reversible capacity of 114 mAh g-1, outputting a high energy density of 376 Wh kg-1. In addition, the NMP/C displays decent power density and cycling stability, demonstrating promising prospects for large-scale applications of SIBs.

2.Experimental Section 2.1 Synthesis of the cathode materials



A typical sol-gel method was performed to synthesize the Na3.12Mn2.44(P2O7)2 and carbon compound materials. All the chemical reagents were used without purification. First of all, a certain amount of citric acid (C6H8O7, Aladdin, AR), which was adopted as both the carbon source and chelating agent, was dissolved into 60 deionized with constant stirring, and then stoichiometric amount of ammonium biphosphate (NH4H2PO4, Aladdin, AR), sodium acetate (CH3COONa, Aladdin, AR) and manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, Aladdin, AR) were dissolved into the citric acid solution, and kept constant stirring in the water bath at 90°C to form a gel. After that, the gel was further dried in a vacuum oven at 80°C to get porous aerogel precursor, which was annealed at 550°C for 9 h under an Ar atmosphere with a heating rate of 5°C·min-1 to obtain the final product. For comparison, a pure Na3.12Mn2.44(P2O7)2 (noted as NMP) material without the carbon matrix was synthesized by a traditional solid state method. Stoichiometric amount of NH4H2PO4, CH3COONa and Mn(CH3COO)2·4H2O were mixed in acetone by a ball-milling process at 400 rpm lasting 20 h. Finally, the powder-mixture were calcinated at 550°C for 9 hours in a tube furnace under argon atmosphere to obtain the NMP material 2.2 Materials characterization X-ray diffraction (XRD) patterns of the material was collected with a scan rate of 1° min-1 from 10° to 80° on RigakuMiniFlex 600. QUANTA 250 (FEI, US) field-emission scanning electron microscopy (FE-SEM) was employed to study morphology of the material and microstructure of material was observed by transmission electron microscopy (TEM, JEM-2100, Japan). The electron diffraction pattern was obtained through Fast Fourier Transform (FFT). Valence states of Mn was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Carbon content of the Na3.12Mn2.44(P2O7)2 material was measured by thermogravimetric (TG) test under an air condition. Brunauer–Emmett–Teller (BET) specific surface area of the materials were determined through N2 adsorption–desorption test based on a Micrometritics ASAP 2020 instrument (Norcross, GA, USA). Inductively coupled plasma optical emission spectroscopy (ICP, THERMO SCIENTIFIC, ICAP 7000) was used to study chemical composition of the samples. 2.3 Electrochemical measurements


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The material was assembled into CR2032 coin cells for electrochemical measurements. The cathode electrode consists of 70 wt% of active materials, 20 wt% of carbon black and 10 wt% of polyvinylidene fluoride (PVDF). The mass loading of active materials was around 1.4 mg cm-2. Sodium metal plate was used as the negative electrode. 1M NaClO4 dissolving in PC solution, with 5vol% addition of fluoroethylene carbonate (FEC) makes up the electrolyte. Galvanostatic charge/discharge, rate performance and long cycling were measured within a voltage window 1.5−4.5 V on land battery testing systems (CT2001A). Cyclic voltammetry (CV) tests were carried out with scanning speeds from 0.1 to 0.5 mV·s-1 in the range of 1.5−4.5 V on a PARSTAT 2273 Electrochemical workstation.

3. Results and Discussion A feasible sol-gel method was employed to synthesis and control the particle size of Na3.12Mn2.44(P2O7)2, as schematically illustrated in Figure 1. Citric acid was used as both chelating agent and carbon source. All chemical reagents can uniformly distribute in the solution to help forming a pure phase of the material. The gel forms as moisture evaporates. During the final calcination, crystalgrains of Na3.12Mn2.44(P2O7)2 gradually produce and the organic species in the gel pyrolyses into carbon matrix, which can limit the growth of Na3.12Mn2.44(P2O7)2 particles and prevent the small dimensional Na3.12Mn2.44(P2O7)2 nanograins from severe aggregations. Meanwhile, the decomposition of the organic species releases gases like carbon dioxide, leading to abundant pore structures in the composite, which should increase surface wettability of the material.

38

The calcination temperature of 550°C was determined by

thermogravimetric (TG) test (Figure S1). In order to confirm the phase and structure information of the NMP/C, Rietveld refinement was primarily employed (Figure 2a). The X-ray diffraction peaks can be indexed to a triclinic structure with space group of P−1. The fitting results are satisfactory, with low reliability-factors of Rwp = 8.96% and the cell parameters are: a = 6.531767 Å, b = 9.531739 Å, c = 11.089160 Å, α = 64.442°, β = 85.915°, γ = 73.419°, V = 595.868 Å3. The inset is the crystal structural illustration of Na3.12Mn2.44(P2O7)2 along a-axis. It shows that the triclinic-structured Na3.12Mn2.44(P2O7)2 is composed of MnO6 octahedra and PO4 tetrahedron, which combine into centrosymmetrical units of Mn2P4O22 and Mn2P4O20, and further establish a three-dimensional



skeleton by corner-sharing. This open framework has spacious interstices to accommodate sodium de-/insertion and keep structure stable.9,25 Besides, clear images of sodium-ion diffusion channels can be noticed along the directions of a, b, c axis, respectively (Figure 2b). Dimension of these channels are larger than the diameter of the Na+, allowing facile transport of Na+ in these channels.39 The XRD pattern for NMP can be seen from Figure S2, and no impurity was detected. XPS was performed to identify composition of the NMP/C and pure NMP materials (Figure 2c and Figure S3). The wide range spectrum proves that the samples consists of Na, Mn, P, O, C, and the atomic ratio of these elements were measured as Na: Mn: P = 3.132: 2.447: 4 by ICP (Table S1), indicating high purity of the as-synthesized materials. The narrow spectrum of Mn 2p has two components of Mn 2p1/2 (653.03 eV) and Mn 2p3/2 (641.08 eV), determining +2 valance state of manganese ions in Na3.12Mn2.44(P2O7)2.40,41 The signal located at around 446.0 eV is satellite, which appears in most transition-metal compounds and is derived from photoemission.36,42 Therefore, the high purity triclinic off-stoichiometric phase manganic pyrophosphates were successfully synthesized. Morphology and structure information of the materials were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The panoramic images of NMP/C show an irregular shape with abundant pores of the bulk material (Figure 3a and 3b). Low resolution of TEM image (Figure 3c) identifies the porous structure of the composite, and it is supposed to benefit electrolyte penetrating into the electrode and increase the contact area between electrolyte and electrode, enabling better utilization of the active materials. 43 Microstructure of the sample was further unclosed by high resolution TEM (HRTEM). Figure 3d demonstrates that the Na3.12Mn2.44(P2O7)2 nanograins are randomly embedded in a carbon matrix. The dimension of Na3.12Mn2.44(P2O7)2 particles are controlled under 25 nm and isolated by the carbon substrate, which can not only prevent particle aggregations, but also provides excellent electron pathways around the small-sized Na3.12Mn2.44(P2O7)2 particles. Since most polyaniontype materials suffer from poor electronic conductivity, this well designed structure are expected to shorten sodium-ion diffusion lengths and enhance electron transportation, thus greatly improving electrochemical performance of the Na3.12Mn2.44(P2O7)2. While for NMP, it shows much larger particle size due to severe aggregation (Figure S4). The carbon content of 7.4 wt% in NMP/C that confirmed by TG test (Figure 3e) is rational, since large carbon content can decease energy density when assembling full cells. Brunauer–Emmett–Teller (BET) specific


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surface area of the composite was measured to be 43.3 m2 g-1, and the inset demonstrates a mesoporous structure of the sample, which is in good accordance with the structure that SEM and TEM images presented. The energy dispersive spectrum (EDS) and the elemental mappings are displayed in Figure 3g, exhibiting the sample of homogenous distribution of C, Na, Mn, P, O elements. To investigate electrochemical performance of the as-produced materials, NMP/C and NMP were assembled into coin cells. Galvanostatic charge/discharge tests (Figure 4a) demonstrate that the NMP /C cathode can output a high initial capacity of 114 mAh g-1 at 0.1C (1C = 120 mAh g-1) in the voltage range of 1.5−4.5 V (vs Na+/Na). Theoretical capacity of this off-stoichiometric phase Na3.12Mn2.44(P2O7)2 can reach 118.1 mAh g-1 based on Mn3+/Mn2+ redox couple. Note that, the theoretical capacity of the stoichiometric Na2MnP2O7 is only 97.5 mAh g-1.32 The increased capacity is benefited from the off-stoichiometric phase with more reversible sodium ions (~2.31 Na-ion). Besides, the (P2O7)4- group has strong electronegativity. P–O covalent bonds drive the electron density away from the Mn center, thus weakening the covalence of the Mn–O bonds and leads to high redox plateau above 3.6 V.

12,39

As a result, the NMP/C achieves a satisfactory

-1

energy density over 376Wh kg . To the best of our knowledge, this is among the highest energy density reported on various Fe- and Mn-based polyanion compounds. Cyclic voltammetry (CV) tests (Figure 4b) were conducted to understand its reacting process. Three pairs of redox peaks can be identified, which located at 3.42/3.25, 3.81/3.63 and 4.00/3.80 V (vs Na+/Na), respectively, corresponding to different number of Na+ de-/intercalation. 32 In sharp contrast, the NMP shows inferior initial capacity delivering and low current response. The large particle size (Figure S4) and low electronic conductivity resulting in serious polarization should account for the results. Flexible power delivering capability of the materials were investigated under different C-rate. (Figure 4c). The NMP /C composite presents outstanding rate performance, with reversible capacity of 108.2, 104.3, 98.3, 92.1, 81.8 and 68.1 mAh g-1, at 0.1, 0.2, 0.5, 1, 2, and 5C respectively. Even under drastic current of 10C, it can still deliver 52.2 mAh g-1 of discharge capacity. And when the current turns back from 10C to 0.2C, the capacity regains to 102.2 mAh g-1, showing great reversibility. While for the NMP, it shows poor rate capability and become almost inactive when the C-rate goes to 2C. The differences between NMP/C and NMP indicate the vital impact of carbon materials and small particle size for electrochemical activity. The discharge curves at various current density of the NMP/C are depicted in Figure 4d. It shows that



their high voltage plateaus above 3.0 V are well maintained, which is key to high power density delivering. To show the prominent electrochemical property of the material vividly, energy density of the Na3.12Mn2.44(P2O7)2/C cathode against other advanced polyanion-type cathode materials that recently reported are presented in Figure 4e. 13,32,36,37,44-47 All these data were collected under 0.2C, and their energy density were calculated based on the weight of active materials in the cathode. It demonstrates that the energy density of Na3.12Mn2.44(P2O7)2 reaches ~330 Wh kg-1, apparently outperforming other Fe- and Mn-based polyanion-type cathodes including the sulfate, the phosphate and the silicate, whose energy density are normally less than 300 Wh kg-1. Besides, the Na3.12Mn2.44(P2O7)2/C cathode also exhibits great power delivering capability (Figure 4f). These improvements should be ascribed to the distinctive structure of the triclinic offstoichiometric phase, which offers improved capacity delivering and high redox potential. In addition to the requirements of high energy/power density, stable structure during cycling and the long lifespan under high rates are also necessary for practical applications. In Figure 5a, the NMP/C material displays negligible capacity decay after 50 cycles at 0.2C, in marked contrast to NMP, which only exhibits 44.5 mAh g-1 at 0.2C and falls quickly to 39.7 mAh g-1 after merely 20 cycles (Figure S5). The inset (Figure 5a) is the dQ/dV discharge curves of the 1st, 10th, 20th, 50th cycle. All these curves show 3 sharp peaks with the same location, and no new peaks appear, indicating the excellent stability of the electrode. 48 When the current increases to 1C, there is 84.3 mAh g-1 after 100 cycles, 91.6% of its initial capacity (Figure 5b). No clear irreversible phase changes have been detected from the dQ/dV patterns, which further proves the highly stable structure of the material. The triclinic off-stoichiometric structure with many roomy interstices to accommodate sodium-ion insertion/extraction is account for the enhanced stability and is also expected to show good long-cycle stability even under high rate. As shown in Figure 5c, the NMP/C has remarkable reversible capacity retention of 86% after 300 cycles at 2C. And even after 500 cycles under a current density as high as 5C, reversible capacity of the cathode still maintains 54.6 mAh g-1, 75% of the initial capacity. Note that the Coulombic efficiency under both 2C and 5C keep around 100%, demonstrating highly reversible Na-ion de-/insertion behavior. Therefore, the NMP/C material demonstrates overall advantages of low cost, non-


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toxicity, high energy/power density and cycling stability, which is desirable for practical applications of SIBs. To have a deeper insight of the sodiation and desodiation process, ex-situ XRD measurements were performed. The structural evolution of the Na3.12Mn2.44(P2O7)2 material during the first cycle was investigated, as shown in Figure 6a. The whole patterns remained unchanged, and there are no peaks disappearing or new peaks forming even in the magnified images, indicating a single phase transition mechanism during cycling. Tiny peaks’ ((011) reﬂection and (100) reﬂection) shifting was observed during desodiation, and returned to their original positions after sodiation, indicating reversible lattice breathing.37,

49

To see the details of sodium diffusion,

possible sodium ion diffusion pathways in Na3.12Mn2.44(P2O7)2 material are illustrated in Figure 6b-d. The energy barriers for each path were calculated based on density functional theory (DFT), as shown in Figure 6e. Both the pathway 1 (Na1 → Na3 → Na1, which approximately parallel to the b-axis ) and pathway 2 (Na2 → Na3 → Na2, which approximately parallel to the bc-plane) demonstrate low diffusion energy barrier of 3.05 eV and 2.42 eV, respectively, indicating a feasible sodium migration in the off-stoichiometric phase.50,51 While for the pathway 3, the result shows an “s-shaped” diffusion route with a relatively higher activation energy barriers at around 4.98 eV, thus the sodium migration is more likely to follow the former two 1 D pathways, that is coincident with its iron analogue. 37 These calculations provide a fundamental understanding for the energy storage in Na3.12Mn2.44(P2O7)2. To verify the facile sodium storage process, dynamics of the electrodes have also been investigated by electrochemical impedance spectroscopy (EIS). As shown in the Nyquist diagrams (Figure 7a), all spectra curves consist of a semicircle located in the high-medium frequency region and a straight line located in the low frequency region. Such patterns fit well with the equivalent circuit inset, where the Rs represents bulk resistance, and R[ct] is the charge transfer resistances. The simulation results (Table S2) demonstrate that the R[ct] for NMP/C and NMP are around 295 and 656 Ω, respectively, indicating enhanced electronic conductivity in the porous carbon modified material. The Na+ diffusion coefficient (DNa+) can be calculated, basing on following equation: 52

‫ܦ‬ே௔శ =

ோమ்మ

(1)

ଶ௡ర ி ర ఙೢ మ ஺మ ஼ మ



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Where the R, T, n, F, A and C is gas constant, absolute temperature, number of electron transfer per molecule, Faraday constant, surface area of the electrode and sodium ion concentration, respectively. σw represents the Warburg coefficient, which has a relationship with Z’ as follows:

ܼ ᇱ = ܴ௦ + ܴ௖௧ + ߪ௪ ߱ି଴.ହ

(2)

Figure 7b demonstrates a linear relationship between Z’ and ω-0.5, and the slope of fitting line of the NMP/C electrode are smaller than that of NMP, which further indicates faster sodium diffusivity of the NMP/C material. The apparent diffusion coefficient of sodium-ion during different sodiation/desodiation states was finally evaluated by CV (Figure 7c). The Randles Sevchik equation for semi-infinite diffusion of Na+ into NMP/C cathode was used for calculation. 53,54

య

భ

భ

‫ܫ‬௣ = 2.69 × 10ହ ݊మ ‫ܥܣ‬଴ ‫ܦ‬మ ‫ ݒ‬మ

(3)

where n represents the number of electrons transferred per species reaction, A represents the surface area of the electrode (0.785 cm-2), C0 represents the concentration of sodium ions (0.0086 mol cm-3), D represents the apparent sodium-ion diffusion coefficient, and ʋ represents the scanning rate, and Ip represents the peak current intensity. For an obvious distinction, the anodic peaks are labelled as a, b, c, while the cathodic peaks as a’, b’ and c’, respectively. The peak current has a linear relationship against square root of the scan rate (Figure 7d). Based on Equation (3), the apparent diffusion coefficients DNa+ can be calculated to be 1.67×10-12 (a), 9.98×10-12 (b), 1.41×10-11 (c), 1.84×10-12 (a’), 6.40×10-12 (b’) and 8.99×10-12 (c’). This fast sodium

diffusivity

is

key

for

the

outstanding

energy/power

delivering

of

the

Na3.12Mn2.44(P2O7)2/C cathode material. 55

4. Conclusion In summary, a triclinic off-stoichiometric phase manganic pyrophosphate material is successfully synthesized. The Na3.12Mn2.44(P2O7)2/C material can deliver a reversible capacity of


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114 mAh g-1 with 2.31 reversible sodium ions in the distinctive crystal structure, reaching a high energy density above 370 Wh kg-1. Among various Fe- and Mn-based polyanion cathode, the asprepared material demonstrates better high power outputting due to its high potential. In addition, the Na3.12Mn2.44(P2O7)2/C also exhibits enhanced structure stability and long lifespan, with 75% capacity retention after 500 cycles at 5C. The characteristic structure of the triclinic offstoichiometric phase, which has two 1D migration channels with low energy barriers, and the single phase transition mechanism with high apparent sodium diffusion coefficient around 10-11 ~10-12 S cm-2, is key for these advanced properties. This work provides a promising cathode candidate for the large-scale applications of SIBs, and offers a guidance to explore other types of off-stoichiometric manganic pyrophosphate materials.

ASSOCIATED CONTENT Supporting Information TG and DTG curves of the Na3.12Mn2.44(P2O7)2/C gel precursor; XRD pattern of the Na3.12Mn2.44(P2O7)2 sample synthesized by solid state method; XPS spectra and morphology of the NMP material; Cycling performance of the NMP cathode; Capacity retention of NMP/C and NMP at different C-rate; Morphology of the NMP/C electrode before/after cycling; ICP-OES analysis of the NMP/C and NMP cathode materials; Fitting values of the EIS plots of the NMP/C and NMP.

AUTHOR INFORMATION Corresponding Author * Email address: [email protected] and [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS



The authors thank the financial support of the National Natural Science Foundation of China (Grant No. 51674297) and the National Key Research and Development Program of China (2018YFB0104201).

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Figure

Figure 1. Scheme of the synthesis approach and structure of the NMP/C composite.



Figure 2. (a) Rietveld refinement of the NMP/C material (inset: crystal framework of Na3.12Mn2.44(P2O7)2 along the direction of a-axis. (b) Schematic representation of the Na3.12Mn2.44(P2O7)2 crystal structure with spacious channels along various directions for facile ion-diffusion. The Na (yellow), O (red), P (purple), and Mn (green) atoms are shown. (c) Wide range scanning XPS spectra of NMP/C and high-resolution spectra of Mn 2p.


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Figure 3. (a) Morphology of the NMP/C material. (a, b) SEM, (c) TEM and (d) HRTEM images of the NMP/C material. The inset shows the corresponding fast Fourier transform (FFT) patterns. White lines and arrows were used to mark the lattice plane, where the inter spacing is measured as 0.385 nm, corresponding to the (102) plane. (e) The TG curve tested under an air atmosphere. (f) N2 adsorption-desorption patterns with a inset of pore distribution. (g) STEM and EDS mapping images of the NMP/C.



Figure 4. Sodium storage performances of as-synthesized samples. (a) Galvanostatic charge/discharge curves at 0.1C and (b) the CV profiles of the first cycle tested under 0.1 mV s-1. (c) Rate performance of NMP/C and NMP and (d) corresponding charge/discharge profiles of the NMP/C material under different C-rate. (e) Comparison between the NMP/C and diverse polyanion-type positive electrode materials that recently reported in the aspects of average voltage, energy density, and specific capacity at 0.2C and (f) power density output under different current densities.


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Figure 5. (a, b) Cycling stability of the NMP/C tested under the 0.2C and 1C, respectively. The insets are the corresponding dQ/dV curves of different cycles. (c, d) The long cycling performance of NMP/C tested under 2C and 5C, respectively.



Figure 6. (a) Ex-situ XRD patterns of the NMP/C material collected from 5˚ to 35˚. (b, c, d) The schematic illustrations of sodium-ion diffusion pathways in Na3.12Mn2.44(P2O7)2. (e) The energy barriers for each pathway.


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Figure 7. Kinetics of the electrodes. (a) EIS plots of the NMP/C and NMP cathode after 10 cycles at 0.2C and (b) corresponding Z'- ω-0.5 patterns in the low frequency region. (c) The CV curves of NMP/C cathode under different scan rates and (d) the corresponding Ip – ʋ0.5 patterns in sodiation and desodiation processes.



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C Cathode

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