Iron Doping in Spinel NiMn2O4: Stabilization of the Mesoporous Cubic

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Iron Doping in Spinel NiMn2O4: Stabilization of the Mesoporous Cubic Phase and Kinetics Activation toward Highly Reversible Li+ Storage Yue Ma, Cheuk-Wai Tai, Reza Younesi, Torbjorn Gustafsson, Jim Yang Lee, and Kristina Edstrom Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03288 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Iron Doping in Spinel NiMn2O4: Stabilization of the Mesoporous Cubic Phase and Kinetics Activation toward Highly Reversible Li+ Storage Yue Ma*, Cheuk-Wai Tai, Reza Younesi, Torbjörn Gustafsson, Jim Yang Lee* and Kristina Edström* Dr. Y. Ma, Dr. R.Younesi, Dr. T. Gustafsson and Prof. K. Edström. Ångström Advanced Battery Centre (ÅABC), Department of Chemistry-Ångström Laboratory, Uppsala University, Box 538, SE-75121, Uppsala, Sweden. E-mail: [email protected]; [email protected] Dr. C. W. Tai. Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-10691, Stockholm, Sweden Prof. J. Y. Lee. Department of Chemical and Biomolecular Engineering, National University of Singapore, 119260, Singapore, Singapore E-mail: [email protected]

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 38

KEYWORDS: iron doping, nickel manganese oxide, hierarchical porosity, cation distribution, lithium-ion storage

ABSTRACT

Quaternary oxide structures with a three-dimensional macro/mesoporous network are synthesized via a facile nanocasting method followed by a calcination process. Structural engineering integrates multi-scale pores by using a hydrophilic membrane with tunable-porosity as the sacrificial template. Through tailoring the metal precursor ratio, the tetragonal sites of spinel oxide are preferentially occupied by iron, resulting in a stabilized mesoporous cubic phase. Crystal field theory together with compositional characterizations from EDS, XPS, Mössbauer and EELS direct our detailed analysis of the cation distribution in the spinel structures. Galvanostatic tests based on the best performing electrode exhibits a robust cycle life stable for 1200 cycles at a high current density of 1500 mA g−1. This good Li+ storage performance could be attributed to the mutually beneficial synergy of the optimal level of iron doping which improves the electrical conductivity and structural robustness; as well as the presence of extended, hierarchical macro/mesoporous network. Finally we demonstrate three feasible surface modification strategies for the oxide anodes toward better reversibility of Li+ storage.

Introduction

The demanding performance requirements for emerging applications, such as vehicle electrification and grid-scale energy storage have far surpassed the capability of conventional lithium-ion batteries (LIBs) technology. Therefore, alternative electrode chemistries are being


2

Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


investigated to enable the innovation of advanced LIBs with higher energy and power densities, strong safety features and long service life at a relative low cost.1-2 Among the potential anode candidates, transitional metal oxides can store a large quantity of Li+ (500-1000 mA h g-1) via a spatially varied conversion reaction.3 Manganese oxides, due to their natural abundance and low lithiation potential, particularly attract extensive interest.4-6 However, their poor electrical conductivities can seriously undermine the reversibility of the conversion reaction and consequently the battery cycle life.5 Research thus far has explored the nanoparticles with a reduced solid-state Li+ diffusion length,6-7 microsized secondary structures for improved material processability,8-10 and carbon compositing for enhanced electrode kinetics.11-12 Another possibility is to make use of ternary spinel manganese oxides (mMn2O4, m=Co, Ni, Zn), which have a significantly higher electrical conductivity and more versatile redox reactions than manganese oxides due to the presence of multiple aliovalent cations.13-16 The electrochemical properties of an electrode material hinge on its crystallinity, shape, phase purity and microstructure.17-20 Therefore considerable efforts have been invested in the synthesis of morphologically and compositionally well-defined structures. Nanostructures with interconnected macro/meso porosity have been developed as a model system for fast electrode kinetics: The macropores house the volume changes in the electroactive metal oxides during the conversion reaction, and also serve as reservoirs and thoroughfares for facile electrolyte percolation; while the mesopores provide the large interfacial contact between electrolyte and electrode to support a high Li+ flux across the solid/liquid interface.21 However, the scalable synthesis of hierarchical porous structures based on ternary (quaternary or multinary) oxides with deliberate control over phase purity, composition and interior pore structure has rarely been realized.22 This is mainly due to the high cost and limited choice of the current available


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 38

templates as well as the difficulty in chemical composition control. More importantly, the precursors must be compatible with the synthesis process to allow multiple metal cations to be incorporated according to the design, without relocation of mass, particle aggregation, and undesirable phase separation during an extended period of high-temperature reactions.23 Herein, we report a versatile synthesis route for the quaternary oxides FexNi1-xMn2O4 with the added possibility of tailoring the hierarchical porosity and engineering the cation distribution on preferential sites. By using amine-functionalized bromomethylated poly(2,6-dimethyl-1,4phenylene oxide) (BPPO) membrane with adjustable porosity as the sacrificial template, the interconnected macropores from the reverse-replication of the membrane pore structure can be incorporated in the as-synthesized quaternary oxide, and the mesopores were formed upon the gas release from the decomposition of the sacrificial polymer template. Iron was the deliberate dopant choice as it not only electrically integrated the electroactive manganese oxides, 24-25 but also stabilized the mesoporous cubic phase by its preferential occupancy in the tetrahedral sites of the cubic spinel structure. With the help of crystal field theory, we further rationalized the valence changes of metal cations due to iron doping and predicted their spatial distribution in the spinel oxides. When evaluated as an anode material for the LIBs, the quaternary oxide with hierarchical porosity demonstrated a very robust cycle life up to 1200 cycles at a high current density of 1500 mA g-1. Compositional investigations at various discharge-charge stages confirmed the presence of zero-valent iron, which electrically wires the manganese oxide during the conversion reaction with Li+. The methodology presented in this study opens up new possibilities of developing hierarchical porous multinary oxides for high performance Li+ storage. Moreover, three surface modification strategies to supress the irreversible capacity loss during the first cycle of oxide anodes were systematically investigated.


4

Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Results and Discussion

Scheme 1. Schematic figure of the preparation procedures for the Fe-doped NiMn2O4 by using a functionalized BPPO as the sacrificial template.

The preparation of Fe-doped NiMn2O4 with macro/mesopores is schematically illustrated in Scheme 1. The synthetic procedure begins with the amination of commercial BPPO (described by the equation in Process I) to graft the polymer surface with hydroxyl groups, followed by a modified Breath Figure (BF) method (Process II) described in a previous study.24 Briefly, water droplets formed during condensation at the toluene/air interface penetrate into the interior of the toluene solution of the aminated BPPO to form a micro-heterogeneous two-phase solution and the preparation is finalized by evaporation of toluene. By adjusting the volumetric ratio of the two phases, a functionalized BPPO polymer with tunable pore size range can be produced. As shown in Figure S1, if no toluene was added to dilute the polymer solution, a membrane with small pores ranging from ~100-200 nm was produced upon the BF method; when 20 mL or 40


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 38

mL additional toluene was added to dilute the polymer solution (details described in experimental section), pores ranging from ~500 nm-1 µm or 1-2 µm could be incorporated within the F-BPPO-20 and F-BPPO-40 membranes, respectively. The amine functionalization significantly increases the wettability of the polymer by aqueous solution, while the porosity tailoring through BF method enhances the surface area of the membrane accessible to the metal precursor solution and thus the product yield; in this regard, we chose F-BPPO-40 membrane with the largest macropores to template our metal oxide synthesis. When immersing the FBPPO-40 in the precursor solution with a predetermined cation ratio (Fe2+, Ni2+, Mn2+), the hydroxyl groups of the membrane actively anchor metal precursors via electrostatic attraction, forming a “metal hydroxide impregnated polymer” upon vacuum drying (Process III). Finally the ensuing calcination process converts the metal hydroxides into metal oxides accompanied with the oxidative decomposition of the membrane template (Process IV).


6

Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. FESEM images of Fe-doped NiMn2O4-2 at (a) low magnification and (b) high magnification; (c) TEM image of Fe-doped NiMn2O4-2; (d) HRTEM image of the selected area of a representative nanocrystal; (e) Nitrogen adsorption-desorption isotherms of Fe-doped NiMn2O4-2 and the corresponding pore size distribution (inset); (f) EDX element maps of Mn, Ni, Fe, and O for the selected area marked in (b); (g) TEM image of a localized region in Fedoped NiMn2O4-2 and its EFTEM element maps of (h) Mn, (i) Ni and (j) Fe; (k) the thickness map of the selected region shown in Figure 1g. The thickness variation is from 0.3 (blue)-0.8 (yellow) inelastic mean-free path. The dark-blue region at the upper right region is vacuum.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 38

Various spinel oxides were fabricated with increasing Fe/Ni molar ratio of 0, 0.2, 0.5 and 0.8 in the precursor solutions. These samples are hereafter referred to NiMn2O4-0，Fe-doped NiMn2O4-1, Fe-doped NiMn2O4-2 and Fe-doped NiMn2O4-3, respectively. The overall morphology of Fe-doped NiMn2O4-2, shown in the field-emission scanning electron microscopy (FESEM) images taken at different magnifications (Figure 1a and 1b), exhibits a 3D continuous structure with interconnected macropores inherited from the reverse replication of the F-BPPO40 membrane. A closer scrutiny of the oxide reveals the microstructure composed of nanocrystals (NCs) with ~ 10-20 nm in diameter, as shown in the transmission electron microscopy (TEM) images (Figure S2a and Figure 1c). Additionally, the uniformly distributed mesopores (some light-contrast regions marked by the white arrows in Figure 1c) are incorporated within the “solid” oxide. This low-scale mesoporosity originates from 1) lattice shrinkage due to the thermal dehydration of metal hydroxides; and 2) gas released during the thermal decomposition of the polymer template. High-resolution TEM (HRTEM) image of the selected region (white square in Figure 1c) of a representative nanocrystal reveals two sets of lattice fringes spaced by 0.257 nm and 0.213 nm (Figure 1d), which correspond to the (311) and (400) planes of cubic spinel NiMn2O4, respectively. The nitrogen adsorption and desorption isotherms of Fe-doped NiMn2O4-2 display a classical type IV isotherm with type H4 hysteresis with no adsorption limit at high p/p0 (Figure 1e), which purportedly suggests that the capillary condensation is operative. This isotherm type and hysteresis loop confirm the assemblage of mesopores in the interconnected macroporous structure.26 The broad pore size distribution ranging from ~ 30 Å to 100 Å (Figure 1e, inset) based on the Barrett-Joyner-Halenda (BJH) calculation of adsorption data agrees rather well with the TEM observation. The mesoporosity


8

Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contributes to a reasonably large Brunauer-Emmett-Teller (BET) surface of 65 m2 g−1. Figure 1f shows the energy dispersive X-ray (EDX) element maps of the selected area marked in Figure 1b: the overlapping Fe signal with Mn, Ni and O elemental maps suggests a uniform dispersion of Fe in the composite. In addition to macroscopic analysis, electron energy loss spectroscopy (EELS) was performed to investigate the local stoichiometry. In Figures 1h-k, energy-filtered TEM (EFTEM) images show the distribution of Mn, Ni, and Fe in a selected region of Fe-doped NiMn2O4-2, of which the thicknesses are quiet similar (see Figure 1k). It is apparent that Mn is uniformly distributed across the region and Fe is more scattered than Ni because of their concentration difference. It can be concluded that EDS and EFTEM results confirm the homogenous stoichiometry.

Scheme 2. Schematic view of the cubic spinel structure with octahedral (yellow) and tetrahedral units (red). Oxygen atoms are represented in purple. The tetrahedral (Ni) and octahedral (Mn) coordination is also represented.

NiMn2O4 spinel oxides with different iron doping amount were also fabricated. As summarized in Table S1, EDX analysis confirms the increasing molar ratios of Fe to Ni in the asfabricated oxides upon the increased dose of Fe precursor in the preparation: 0.23 for Fe-doped


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 38

NiMn2O4-1, 0.57 for Fe-doped NiMn2O4-2 and 0.89 for Fe-doped NiMn2O4-3, respectively. TEM images of NiMn2O4-0 without iron doping (Figure S2b and S2c) demonstrate the absence of mesopores among the nanocrystals. Besides, there exists one set of lattice fringes (marked by the red arrows in Figure S2c) corresponding to the (101) planes of the tetragonal spinel phase. The X-ray powder diffraction (XRD) pattern of NiMn2O4-0 recorded in Figure S3 shows diffraction peaks indexed to the two co-existing phases: 1) the cubic spinel (Fd3m) marked by black arrows and 2) the tetragonally distorted spinel (I41/amd) marked by blue arrows. It is thus speculated that the phase separation derived from lattice mismatch and the structural distortion results in the mass relocation, compromising the mesoporosity upon the crystallization of the spinel oxide. Upon the iron doping in the Fe-doped NiMn2O4-1, the packing of nanocrystalline building blocks became less compact (Figure S2d) compared to NiMn2O4-0. Accordingly, the diffraction peaks of Fe-doped NiMn2O4-1 (Figure S3), which correspond to the tetragonal phase, gradually weakened in accompany with more pronounced cubic phase. The continued increase in iron doping led to a phase pure Fe-doped NiMn2O4-2 where all the diffraction peaks can readily be indexed to the cubic spinel structure (ICDD 00-001-1110). This structure is generally isostructural to the cubic spinel Mn3O4 by forming tetrahedral MO4 (M=Ni or Fe) groups and octahedral MnO6 groups respectively, as shown in Scheme 2. This indicates that the Ni or Fe has been well integrated into the atomic structure without phase separation. On the other hand, the intense peaks of hematite (α-Fe2O3 ICCD 00-001-1053) appear in the XRD pattern of Fe-doped NiMn2O4-3 (Figure S3, marked by the magenta arrows), suggesting that the amount of iron doping likely exceeds the solubility limit in NiMn2O4 and thus phase separation occurred. This phenomenon echoes with the appearance of large particles indicated by the white arrows in Figure S2f, which adversely compromises the morphological uniformity. BET surface area for


10

Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiMn2O4-0, Fe-doped NiMn2O4-1 and Fe-doped NiMn2O4-3 was measured to be 22 m2 g-1, 46 m2 g-1 and 39 m2 g-1 respectively, suggesting their lower mesoporosity compared to Fe-doped NiMn2O4-2.

Figure 2. (a) Mn 2p, (b) Ni 2p core level XPS spectra of NiMn2O4-0, Fe-doped NiMn2O4-1 and Fe-doped NiMn2O4-2 electrodes; (c) Mössbauer spectrum of iron in the Fe-doped NiMn2O4-2 sample.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 38

Figure 2a exhibits the Mn 2p core level X-ray photoelectron spectroscopy (XPS) spectra of the as-developed spinel oxides. For the NiMn2O4-0, the well-defined peaks positioned at the binding energies (BE) of 641.0 eV and 653.3 eV originate from the spin-orbital splitting into Mn2p3/2 and Mn2p1/2 emissions, respectively. Direct analysis of the 2p region for 3d transition metal oxides (Mn or Ni for instance) by peak fitting procedures is not straightforward because of the overlapping of multiple final states and a serious broadening effect occurred in these strongly electron correlated systems.27 However, the satellite peak at 647.1 eV characteristic of divalent Mn, as indicated with the blue arrow in Figure 2a, gradually weakens with the increased amount of iron. Combined with the slight peak shift of Mn2p3/2 to higher BE upon the iron doping, it is suggested that higher oxidation states for the Mn species was introduced with the increased iron doping. Mn3s core-level XPS spectra shown in Figure S4a exhibits exchange splitting of the doublets: 5.8 eV for NiMn2O4-0 and 5.5 eV for Fe-doped NiMn2O4-2.28 The magnitude of splitting highly depends on the interaction strength of the 3s and 3d electrons and is proportional to 2S + 1, where S is the local spin of the 3d electrons in the ground state.29,30 In this regard, the reduced exchange splitting suggests the partial increase of the valence state of Mn2+ to Mn3+. In the Ni 2p spectra (Figure 2b), the BE difference between the main photoelectron peak and the ‘shake-up’ satellite (ΔEm,s) positively correlates to the intensity of coulombic attraction between the 2p and 3d electrons as well as the hybridization degree between ligand p and cation 3d levels.28 The reduction in ΔEm,s from 6.8 eV to 6.4 eV thus shows decreased number of unpaired electrons in the d orbitals, that is to say, the decrease of Ni valence upon the iron doping in the spinel structures.29 The gradual increase in the peak intensity for the Fe 2p spectra (Figure S4b) echoes with the increase of iron doping amount in the oxides. Moreover, the Fe 2p3/2 peak situated at ~ 711.2 eV and appearance of a broad satellite peak demonstrate the possibly


12

Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


existence of divalent or trivalent iron in the Fe-doped NiMn2O4-2. Mössbauer spectroscopy (MS) was also used for quantitative analysis of the chemical environment of Fe in the Fe-doped NiMn2O4-2 sample (Figure 2c). The results show the dominating presence of two sets of sharp and resolved sextets with hyperfine parameters (Table S2, supporting information), corresponding to the Fe atoms positioned at the tetrahedral sites (sextet 2, 86.6%) and those at the octahedral sites (sextet 1, 12.5%). This result is generally consistent to the predication from the crystal field theory of the dominating occupation of Fe to be in tetrahedral sites of the cubic spinel oxide. Additionally, the center shift (CS) values for both sextets are smaller than 0.5mm/s, indicating that divalent Fe cannot be observed in the sample within the detecting limit of the Mössbauer spectrum. Quantification analysis of the XPS spectra of Fe/Ni ratios in the spinel oxides are summarized in the Table S1. EELS survey spectrum of Fe-doped NiMn2O4-2 and high-resolution Mn, Ni, Fe and O edges obtained in a transmission electron microscope are shown in Figure S5 and S6. The Mn, Ni and Fe L2,3 edges are apparent because of their “white line” nature. The oxidation states of Mn, Ni and Fe can be determined by their near-edge features and the O K-edge, which are in consistent with the XPS results: As shown in Figure S6a, the small split at Mn L3 edge indicates the coexistence of Mn2+ and Mn3+.30-31 The Ni L2,3 edge does not show an apparent distortion like in Mn (Figure S6b). The core-loss feature in EELS spectrum can state that the dominating oxidation state of Ni is divalent.32 A slightly distorted profile of Fe L3 edge suggests the possible occurrence of 2+ and 3+ valence (Figure S6c).30-31 It is worthy to note that EELS information for the localized region and XPS/Mössbauer information from macroscopic region might have an observable difference.32 Unfortunately, the peak corresponding to transition from Ni 3d to O 2p cannot be determined at the first peak at O K


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

edge. Although the O K-edge has rich feature, most of them are coupled with the oxidation states of Mn and Fe. To elaborate on the aforementioned phase and valence variations upon the iron doping, we utilized the crystal field theory to calculate the thermodynamic preference (enthalpy-wise) of octahedral geometry for all the possibly involved metal-oxygen coordination. For a cation with specific electron configuration, the Octahedral Site Preference Energy (OSPE) - that is, the difference of crystal filed stabilization energies for a tetrahedral complex and the octahedral complex - was calculated to quantify the probability of occupancy in octahedral geometry. As summarized in Table S3, Ni2+ demonstrates a very strong preference for octahedral coordination due to the highest OSPE value (-38/45∆0), and it thus tends to migrate to the octahedral sites. This process is compensated by reverse migration of Mn3+ back to tetrahedral sites (Scheme 2).33 Since the divalent Mn2+ is more thermodynamically favourable situated in the four-fold coordination, the transformation of Mn3+ to Mn2+ occurs due to the Jahn-Teller lattice distortions.34 This probably explains the appearance of the distorted tetragonal symmetry of the spinel structure. When the Fe dopant is introduced to substitute Ni2+ in the spinel structure, both Fe2+ and Fe3+ prefer to occupy the tetragonal sites due to their lower OSPE values (-6/45∆0 for Fe2+ and 0∆0 for Fe3+) compared to Ni2+ (-38/45∆0) or Mn3+(-19/45∆0). Therefore, the substitution of Fe in the tetrahedral sites prevents the migration of Mn3+ and distortion of the cubic spinel phase, which is proofed by the aforementioned characterization results.


14

Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3 (a) Cyclic voltammograms of the Fe-doped NiMn2O4-2 electrode at a scan rate of 0.05 mV s-1; (b) the 1st, 2nd, 10th, 50th, 100th and 150th discharge-charge curves of Fe-doped NiMn2O4-2 electrode; (c) cycle stability of NiMn2O4-0, Fe-doped NiMn2O4-1 and Fe-doped NiMn2O4-2 electrodes measured at 200 mA g-1; (d) cycling stability of Fe-doped NiMn2O4-2 at different current densities; (e) long-term cycling performance of Fe-doped NiMn2O4-2 electrode at high current density of 1500 mA g-1.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

The interconnected macro-mesoporous network and compositional feature of the mixed cations in quaternary oxides render the Fe-doped NiMn2O4 electrode effective mixed-conducting properties for fast electron and Li+ transport. The electrochemical performance of the electrodes based on these oxides were thus examined and summarized in Figure 3. The representative cyclic voltammograms (CV) of the Fe-doped NiMn2O4-2 electrode during the first 5 cycles is shown in Figure 3a: during the initial cycle, three reduction peaks could be identified in the cathodic sweep. The broad peak centered at ∼1.0 V (I) associates with the initial reduction of Mn3+ to Mn2+ (described by Equation (1) as 2Li+ + NiMn2O4 + 2e−↔ xNiO + 2MnO + Li2O); the pronounced broad peak ranged from 0.76 V to 0.35 V (II) could be assigned to several overlapped electrochemical processes: 1) the irreversible decomposition of the electrolyte to form the solid-electrolyte interface (SEI); 2) and the reduction of Ni2+ (Fe2+/3+) to metallic Ni (Fe) nanoparticles. The third peak suited at 0.26 V (III) corresponds well with the further reduction of Mn2+ to Mn0 (described by Equation (2) as MnO + 2Li++ 2e−↔Li2O + Mn).35 During the following anodic sweep, the oxidation peaks at ∼1.29 V (IV) assigned to the reoxidation of Mn0 to Mn2+ while the broad peak ranges from 1.90-2.20 V (V) assigned to the overlapped oxidation process of Ni0 (Fe0) to their corresponding oxides.19 From the 2nd cycle onwards, two pairs of redox couples at 0.42/1.32 V and 1.13/1.98 V with stable peak positions and intensities echoes well from 2nd to 5th cycle, suggesting that the highly reversible conversion reaction starts to dominate the Li+ storage process. Figure 3b exhibits the discharge-charge curves of the Fe-doped NiMn2O4-2 electrode when it was galvanostatically cycled at a low current density of 200 mA g-1. The first discharge curve exhibits a typical multi-step discharge process: Three slope regions at ~1.0V, 0.75-0.5V and ~0.3V are generally in accordance with the cathodic peak I, II and III during the 1st CV cathodic


16

Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sweep. The discharge and charge capacity for the 1st cycle are 954 mA h g-1 and 638 mA h g-1, demonstrating a coulombic efficiency (CE) of 66.2%. This CE value increased to 94.8% during the 2nd cycle and maintained higher than 98.5 % after first 5 cycles. The good superimposability of the 2nd, 10th, 50th, 100th and 150th discharge-charge profiles implies the robust structural stability upon the repeated discharge-charge cycling. In sharp contrast, the cycling curves are quite different for the NiMn2O4-0 or Fe-doped NiMn2O4-1 electrode, where the cycle-to-cycle variations mirror the capacity fading under the galvanostatic conditions (Figure S7a and S7b). The cycling performance of various oxide materials with different amounts of Fe doping was shown in Figure 3c. Obviously, the Fe-doped NiMn2O4-2 electrode demonstrates the best capacity retention capability: in spite of the initial fading from 640 mA h g−1 to 597 mA h g−1 upon the first 20 cycles, the reversible capacity experiences a subsequent slight increase which finally stabilizes at 620 mA h g−1 after total 250 cycles. The capacity increase could be attributed to the reversible formation/dissolution of organic polymeric/gel-like layer by electrolyte decomposition, the gradual establishment of which brings about the extra capacity contribution through a so-called “pseudo-capacitance behavior”, which has been documented in similar nanosize metal oxide/Li cell systems.36-37 The electrode with insufficient iron doping (Fe-doped NiMn2O4-1) only maintains a moderate reversible capacity of 470 mA h g−1 after 186 cycles. This positive correlation between iron dopant amount and capacity retention capability is further validated in view of the even worse cyclability of NiMn2O4-0 electrode without iron doping: a discharge capacity of only 478 mA h g−1 can be achieved upon 100 cycles. On the other hand, the excessive iron dopant in the Fe-doped NiMn2O4-3 with a segregated hematite phase also delivers a rather poor capacity retention with 392 mA h g−1 retrievable after 60 repeated cycles (Figure S8). In this regard, the best cycling performance of Fe-doped NiMn2O4-2 among various


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

electrodes originates from the optimal level of iron doping that bestows the stabilized cubic structure and thus a percolating macro/mesopores network. Figure 3d exhibits the cycling stability of Fe-doped NiMn2O4-2 electrode at various current densities. Initially the electrode was cycled at 50 mA g-1 where the capacity stabilized to 653 mA h g-1 after 10 cycles. The current density was then increased stepwise to 500 mA g-1, 1000 mA g1

, 1500 mA g-1 and 2000 mA g-1 where stable capacities of 557 mA h g-1, 470 mA h g-1, 435 mA

h g-1 and 405 mA h g-1 were obtained after 15 cycles at each of these current densities. When the current density finally returned to its initial value of 50 mA g-1 after a total of 70 cycles, a capacity of 605 mA h g-1 was still recoverable and sustainable up to the 100th cycle without significant loss. The long-term cyclability of the Fe-doped NiMn2O4-2 electrode was also measured (Figure 3e): after pre-cycling at 500 mA g-1 for 15 cycles (for stabilization), the current density was increased to 1500 mA g-1 and a reversible capacity of 442 mA h g-1 was obtained; upon the continuous cycling until the 610th cycle, a capacity of ~ 410 mA h g-1 was still retrievable with 7 % capacity loss. After a rest period of 18 h, the electrode was re-cycled at the same current density of 1500 mA g-1 still delivering a remarkable cyclability with only 5 % capacity loss for another 600 cycles. We compared this result with those of similar Mn-based spinel oxide anodes in the literature, as shown in Table S4. Clearly, our Fe-doped NiMn2O4-2 electrode delivers a superior performance in terms of the robust cycle life, particularly when it was cycled at high current densities.


18

Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The 1st discharge-charge curve of the Fe-doped NiMn2O4-2 electrode using the LiClO4/ EC-DEC electrolyte system with circles marking the potential points of interest; (inset) Fe 2p core level spectra at the selected discharge and charge potentials.

EIS analysis measurements were also conducted to further verify the kinetics improvement of NiMn2O4 electrodes as a function of iron doping. The Nyquist plots of electrodes at opencircuit voltages in their native states (before cycling) and after cycling at 200 mA g-1 for 50 cycles are exhibited in Figure S9. Nyquist plots of all of the electrodes show the common features of a purely resistive response at the high frequency end, a semicircle in the high-tomiddle frequency region, and an inclined straight line in the low frequency region. The semicircular arc of the Fe-doped NiMn2O4-2 electrode was the smallest among the tested electrodes before cycling as shown in Figure S9a. EIS measurements taken after 50 cycles display the same trend (Figure S9b) of the smallest arc in the Nyquist plot of Fe-doped NiMn2O4-2. This is an indication of an overall smallest charge transfer resistance or, equivalently, a more facile charge transfer process at the electrode/electrolyte interface after iron doping. Additionally, the low-frequency inclined line represents the Warburg impedance (Zw),


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

associated with the solid-state diffusion of Li+ in the electrode materials. The Warburg-like response of Fe-doped NiMn2O4-2 electrode (Figure S9b) exhibits the largest slope after 50 cycles, which might suggest a higher Li+ mobility in the solid upon the Fe substitution of Ni in the spinel oxide.36, 38-39 We further investigated the oxidation states of Fe upon a full dischargecharge cycle by using our in-house XPS. The direct analysis of Fe valence during conversion reactions is rather challenging due to 1) the formation of the SEI which would block the Fe signal; 2) the F signal from the PVDF binder and LiPF6 salt in the electrolyte that interferes with the Fe 2p XPS peak.40 Therefore, we cycled the electrode in an analogous fluorine-free battery system (LiClO4/ EC : DEC (3:7) as the electrolyte and carboxymethyl cellulose (CMC) as the binder) to different potential stages and examined the iron valence. Due to the relative high lithiation potential of Fe2,3+ at ~ 1-0.8 V and delithiation of Fe0 at the potential of ~ 1.9-2.2 V, the electrochemically reduced zero-valent iron appears (BE positioned at 707.9 eV) when the electrode was discharged from the open-circuit voltage to 0.6 V (Figure 4, point 1), and the presence of Fe0 remains when the electrode was continued to be discharged to the fully lithiated state (point 2), and re-charged to 1.7 V (point 3). Considering the discharge plateau of manganese oxide at 0.3-0.5 V and charge plateau at 1.1-1.3 V, the zero-valent Fe particles could be retained during the oxidative formation of MnO. The electrical conductivity of iron is 1.0×107 S/m at room temperature (20 °C), which is more than one order of magnitude higher than that of manganese (6.9×105 S/m at 20 °C). In this sense, the choice of Fe dopant for manganese oxides is deliberate since the presence of zero-valent iron is expected to electrically integrate the manganese oxide during the reaction with Li+, providing a kinetic boost for the conversion reaction storage.


20

Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the zero-valent iron nanoparticles were believed to electrically integrate MnO particles to provide a kinetic boost for the conversion reaction.24

Figure 5 The postmortem SEM characterizations of (a) Fe-doped NiMn2O4-2 electrode after 250 cycles and (b) NiMn2O4-0 after 100 cycles at 500 mA g-1 and corresponding elemental maps of O, C, Mn, Fe, P, and F.

The critical influence of iron doping on the Li-storage reversibility is also reflected by the morphology variation of the electrodes observed post-cycling. After 250 cycles, the Fe-doped NiMn2O4-2 electrode (Figure 5a) exhibits distinguishable primary building blocks of nanocrystals with the well-preserved interparticle void space although the interconnected macroporous network collapsed; the visible Mn and Fe signals from EDS elemental mapping indicate that the as-formed SEI layer is conformally coated on the electrode but not thick enough to completely block the signal from the oxides. In sharp contrast, after only 100 cycles the NiMn2O4-0 electrode demonstrates an inhomogeneous polymer layer blocking most of the pores (Figure 5b). Additionally, the relative weaker Mn, Fe signals from the oxides and stronger P and


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

F signals originated from the species of the SEI layer (e.g. LiF, poly(ethylene oxide) (PEO)-type polymer and LiPF6 salt) further validate the presence of a thicker SEI layer on the electrode, the formation of which, of course, consumes the Li+ irreversibly and thus leads to a poor electrode cyclability.

Scheme 3 Schematic illustration of the structural features for Fe-doped NiMn2O4-2 electrode.

The good electrochemical performance of the Fe-doped NiMn2O4-2 electrode could be attributed to the unique structural and compositional features by integrating an optimal amount of iron doping and a connected network of macropores and mesopores that supports concurrent effective transport of electrons and ions, as schematically illustrated in Scheme 3: 1) the presence of Fe0 during the reduction/re-oxidation of manganese oxide renders a kinetic boost for the conversion reaction; 2) the iron dopant stabilizes the cubic spinel structure, in which the uniformly distributed mesopores enhance the interfacial contacts where a higher Li+ flux across


22

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the electrode/electrolyte interface is realized; 3) the three-dimensionally connected macropores serve as an electrolyte reservoir and render an effective electrolyte infiltration.

Figure 6 (a) TEM image and (b)HRTEM image of the RTIL-N-C-oxide composite;(c) the 1st discharge–charge curves of Fe-doped NiMn2O4-2 and RTIL-N-C-oxide composite; (d) Sum of irreversible capacity loss during the cycling of Fe-doped NiMn2O4-2 and RTIL-N-C-oxide composite. Furthermore, we provide several surface modification strategies to further optimize the CE for the spinel oxide anodes. The low CE of Fe-doped NiMn2O4-2 in the first cycle (i.e. 66.2%) originated from the interplay of partially irreversible electrochemical reduction of MnxOy to MnO as well as the consumption of Li+ during the SEI formation. Therefore, a room temperature ionic liquid (RTIL), 1-octyl-3-methylimidazolium chloride ([C8mim][Cl]) was utilized as the carbon source to apply a conformal N-doped carbon coating on the oxide surface. Due to its intrinsic mobility, the RTIL could easily diffuse into the porous structure interior and establish a connected 3D conductive carbon matrix after heat treatment. Additionally, the relatively long


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

carbon chain of [C8mim]+ could facilitate adhesion to and encapsulation of the oxide particles to establish a dense, continuous and conforming conductive network. Morphological examinations shown in Figure 6a and 6b indicate that [C8mim][Cl] was able to provide a conformal carbon encapsulation for the oxide with nano-thickness (~5-8 nm) upon pyrolysis. This capability could be attributed to the attractive electrostatic interaction between the negatively charged oxide surface (from the partial ionization of surface oxygen vacancy) and [C8mim]+ cations. The highresolution N 1s spectrum of the RTIL-N-C-oxide shows a bimodal distribution (Figure S10a, inset) which could be deconvoluted into two peaks with binding energies of 400.2 eV and 398.3 eV attributable to pyridinic (C=N) and pyrrolic (C–N) nitrogen respectively.25 The presence of nitrogen in multiple chemical states indicates that the ionic liquid carbon source [C8mim][Cl] had successfully transformed into N-doped carbon after pyrolysis. The presence of ~ 7 wt% RTIL derived N-doped carbon in the composite (Figure S10b) was expected to modify the electrochemistry at the electrode–electrolyte interface. The higher stability of the SEI layer on the RTIL-N-C-oxide composite could be illustrated by the charge– discharge curves in Figure 6c: The 1st cycle irreversible capacity loss for the RTIL-N-C-oxide is as low as 107 mA h g-1, which is a 66% reduction of that for the uncoated Fe-doped NiMn2O4 electrode (316 mA h g-1). Because of the effective surface chemistry modification, the 1st cycle CE value is significantly increased to 85% upon the RTIL-derived N-doped carbon encapsulation. This encouraging result, to the best of our knowledge, demonstrates the best CE value among all the Mn-based anode ever report. We further investigated this RTIL modification effect on the CE of Fe-doped NiMn2O4-2 upon the continued discharge-charge cycles. Figure S11 demonstrates the simultaneous realization of good capacity retention with high CE values for RTIL-N-C-oxide electrode. Figure 6d thus compares the sum of the irreversible capacity loss


24

Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(ICL) during the cycling of Fe-doped NiMn2O4-2 and RTIL-N-C-oxide electrodes. It is noted that the difference of the sum of ICL between two electrodes become more noticeable with the continued cycling. (209 mA h g-1 of the ICL difference for the initial cycle, 223 mA h g-1 for the first 5 cycles, 237 mA h g-1 for 10 cycles, 310 mA h g-1 for 50 cycles and 465 mA h g-1 for 100 cycles). This comparative analysis highlights the effectiveness of the N-doped carbon encapsulation in improving the reversibility of the oxide electrode. Besides, we also explored the influence of alternative alginate binder and electrolyte additive on improving the CE (details are described in Figure S12), demonstrating the feasible tailoring of the interfacial chemistry on the quaternary cubic spinel oxide towards higher reversibility. Conclusions In summary, using macroporous amine-functionalized BPPO membranes with tunable porosity as the sacrificial template, Fe-doped NiMn2O4 with hierarchically connected macropores and mesopores was successfully fabricated by a nanocasting method followed by controlled calcination. The macroporous network renders a facile electrolyte percolation; the void space from the macropores and mesopores collectively cushion the volume fluctuations upon the conversion reaction of the manganese oxide. The Fe dopant electrically integrates the manganese oxide during their conversion reaction with Li+; additionally, their preferentially occupancy in tetrahedral sites stabilizes the cubic spinel structures and thus the uniformly distributed mesopores for extensive interfacial electrode/electrolyte contact. Electrochemical measurements showed that the Fe-doped NiMn2O4-2 electrode could simultaneously realize a large reversible capacity of 620 mA h g-1 for 250 cycles at the current density of 200 mA g-1, good rate capability up to 2000 mA g-1 and robust cycle life even at very high current density (1200 cycles at 1500 mA g-1). This work also highlights the analysis of metal cation distribution


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

based on crystal field theory and several viable surface modification techniques, which could direct the synthetic procedures for other multinary spinel oxides with hierarchical structure for different application purposes. Experimental section Materials All chemicals were used as received without further purification. Iron (II) acetate (Fe(CO2CH3)2, 95%), nickel(II) acetate tetrahydrate (Ni(CO2CH3)2·4H2O, 98%), manganese(II) nitrate tetrahydrate (Mn(NO3)2·4H2O, 97%), Poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) powder,

diethanolamine ((HOCH2CH2)2NH, 99%), polyvinyl alcohol (MW=89000-98000,

99%), polyvinylidene diﬂuoride (PVDF) and N-methyl-pyrrolidone (NMP) were purchased from Sigma-Aldrich. Toluene (C7H8, >99.5%) from Merck were also used. Room temperature ionic liquid (RTIL), 1-Octyl-3-methylimidazolium chloride ([C8mim][Cl]) was synthesized by the method described in previous work.25 Ultrapure water (Millipore) with resistivity greater than 18.2 MΩ.cm was used as the primary solvent. Synthesis of Macro/Mesoporous NiMn2O4 with Different Iron Doping Ratios All reagents were used as received. The synthetic approach of functionalized BPPO membranes reported in the previous paper was modified in this project.22 Briefly, 200 mg BPPO membrane was dissolved in 10 mL toluene to react with 143.8 µL diethanolamine at 40 °C. Then the solution was diluted with toluene (0, 20 mL, or 40 mL) before it was transferred to a moist and well-ventilated atmosphere. After polymerization for 12h, the resulting functionalized BPPO


26

Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


membranes (F-BPPO-0, F-BPPO-20 and F-BPPO-40) were removed by soaking in ultrapure water and then dried at 80 °C for 4 h. 0.1g PVA was dissolved in 20 ml 1 M Mn(NO3)2.4H2O aqueous solution under constant magnetic stirring. 0.01 mmol Ni(CO2CH3)2·4H2O was then added to complete the preparation of the precursor solutions. F-BPPO-40 membranes were soaked in these solutions at 30 °C for 1h. The precursor-solution-impregnated membranes were then removed, rinsed with deionized water and dried in vacuum at 100 °C for 20 min. The precursor-impregnated polymer was heated in a muffle furnace from ambient temperature to 550 °C at 3 °C min-1 and maintained for 90 min. The calcinated products were designated as NiMn2O4-0. If the 0.002 mmol Fe(CO2CH3)2 was also dissolved in the precursor solution together with 0.01mmol Ni(CO2CH3)2·4H2O while keeping other experimental parameters identical as NiMn2O4-0, the as-calcinated product was designated as Fe-doped NiMn2O4-1. Similarly, Fe-doped NiMn2O4-2 was obtained when amount of Fe(CO2CH3)2 was increased to 0.005mmol; Fe-doped NiMn2O4-2 was obtained when 0.008 mmol Fe(CO2CH3)2 was used. Synthesis of RTIL-N-C-oxide composite 1g Fe-doped NiMn2O4-2 product was then dispersed in 2 ml acetone. 200 mL ([C8mim][Cl]) was added to the acetone suspension with stirring to form homogeneous slurry. Acetone in the slurry was then removed by a 4 h vacuum evaporation process, which also facilitated the infiltration of [C8mim][Cl] into the mesopores of the Fe-doped NiMn2O4-2. The powder after the vacuum evaporation process was heat-treated at 500 °C for 2 h in flowing N2 (at 200 standard cubic centimeters per minute, or sccm). The heat-treated product was identified as RTIL-N-Coxide.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

Materials characterization Scanning electron microscopy (SEM) was performed on a field-emission Zeiss 1550 instrument operated at 5-10 kV. Energy-dispersive X-ray (EDX) analysis was carried out during the FESEM session using a Horiba EMAX attachment analyzer. Transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images were taken on a JEOL JEM2100F microscope operating at 200 kV and equipped a Schottky field-emission gun, a Gatan Ultrascan 1000 CCD camera and a Gatan Imaging filter (GIF Tridiem 863) This microscope was also used to carry out electron energy loss spectroscopy (EELS) and energy-filtered TEM analysis. Gatan Microscopy Suite (GMS) was used to model the background in EELS spectra and to extract the core-loss information. For the TEM study, the samples were deposited on Cu supporting grid with holey carbon films. The EFTEM and EELS were performed in the freestanding regions of the samples, in order to avoid the contribution of carbon films. Powder X-ray diffraction patterns (XRD) of the spinel oxides were recorded on a Bruker D8 ADVANCE Diffractometer using Cu Kα radiation. BET measurements were carried out on a Micromeritics ASAP 2020 analyzer at 77 K equipped with the V3.04 E software as porosity analyzer. X-ray photoelectron spectroscopy (XPS) was conducted using a PHI Quantum 2000 spectrometer with monochromated Al-Kα radiation. The background under the XPS spectra was subtracted using the Tougaard-type function; the photo-electron peak positions and areas were obtained by a weighed least-square fitting of model curves (70% Gaussian, 30% Lorentzian) to the experimental data. The areas under the Fe2p, Ni2p and Mn2p spectra including the satellites

were used. The XPS quantitative analysis was carried out according to the equation = ∑ = / ∑ /

× 100% where CA is the relative concentration of element A in percent, nA is the


28

Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atomic concentration of element A, ASFA is the atomic sensitivity factor of element A and IA is the intensity (number of photoelectrons per second) of the characteristic photoelectron line of the element A. By using the atomic sensitivity factor, a relative error in the quantitative analysis can reach around 15%. Quantification analysis was conducted with the PHI MultiPak software (version 6). 57

Fe Mössbauer spectroscopy (MS) measurements were run in transmission geometry using

a spectrometer of constant acceleration type, with 1024 memory cells for storing the unfolded data. The source, 57CoRh, was always held at room temperature. The absorbers were prepared by mixing ∼8 mg of active material with a suitable amount of boron nitride, which was then spread evenly over the absorber disc (diameter 13 mm). The folded spectra, covering a velocity span of ± 10 mm/s or less, were least-squares ﬁtted with Lorenzian lines using the software Recoil. The center shift, CS, being the sum of the true isomer shift and the second-order Doppler shift, is given relative to metallic iron (α-Fe) at room temperature. The magnitude of the magnetic splitting, is given as the peak separation in the symmetrical sextet. The in-house XPS measurements were performed using a PHI5500 spectrometer operated using a monochromated Al-Kα radiation and an electron emission angle of 45°. Ar+-ion beam sputtering was performed for 20 min for each sample at pressure of 20×10-3 Pa with an emission current of 25 µA and beam energy of 4 kV. For the in-house XPS characterization of the Fedoped NiMn2O4-2 electrode at different potential stages, Ar+-ion beam sputtering was performed for 20 min for each sample at a pressure of 20×10-3 Pa with an emission current of 25 µA and beam energy of 4 kV.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

Specimens for post-mortem SEM analysis were treated as follows: the composite electrodes were cycled at a current density of 200 mA g-1 for certain cycles and then rested for 24 h. The anode materials were then disassembled from batteries in the glove box followed by washing with 1mL NMP. After vacuum drying at 120 °C for 1 h, the samples were mounted in the SEM holder in a re-circulating Ar glove box (MBRAUN labmaster 130), and then quickly transferred into the SEM, reducing the air exposure to less than 20 s. Electrochemical measurements Working electrodes were prepared by mixing electroactive materials with Super P and PVDF at a weight ratio of 8: 1: 1 and with a total weight loading of 1.5 ± 0.2 mg cm−2. A lithium metal foil was used as the counter electrode and LiPF6 solution (1 M) in an ethylene carbonate and diethyl carbonate mixture (3:7, v/v) was used as the electrolyte. Battery cell assembly was carried out in a re-circulating Ar glove box (MBRAUN labmaster 130) where the moisture and oxygen contents were below 1 ppm each. The test cells were discharged (Li+ insertion) and charged (Li+ extraction) galvanostatically at room temperature in the 0.1 V to 2.5 V voltage window at different current densities on a Digatron BTS-600 battery tester. A µAutolab Type III potententiostat/galvanostat with FRA2 frequency analyzer and Nova 1.5 software were used for electrochemical impedance spectroscopy (EIS). Impedance of the electrodes in the fully delithiated state was measured in the frequency range of 100 kHz to 0.1 Hz with a small perturbation of ±10 mV. For the XPS investigation of the Fe-doped NiMn2O4-2 electrode at different potential stages, a non-fluorine containing electrolyte system was employed by replacing the LiPF6 with 1M LiClO4 as the electrolyte salt and carboxymethyl cellulose (CMC) as the binder.


30

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgements This work has been funded by the Swedish Foundation for Strategic Research (SFF) within the project Road to Load, Swedish Research Council contract No 2012-4681 and The Swedish Energy Agency. We would like to acknowledge Tore Ericsson and Lennart Häggström for the assistance during the Mössbauer spectroscopy measurement. The Knut and Alice Wallenberg Foundation is acknowledged for an equipment grant for the electron microscopy facilities at Stockholm University. Supporting Information Available Supporting information contains detailed material about: (Figure S1) FESEM images of the BPPO membranes. (Figure S2) Electron microscope images of Fe-doped NiMn2O4. (Figure S3) XRD patterns. (Figure S4) XPS spectra for Ni 2p and Fe 2p. (Figure S5) EELS spectrum of Fedoped NiMn2O4-2. (Figure S6) The enlarged Mn, Ni, Fe and O L2,3 edges of EELS spectrum. (Figure S7) Discharge-charge curves. (Figure S8) Cycle stability of Fe-doped NiMn2O4-3 electrode. (Figure S9) Nyquist plots. (Figure S10) N 1s XPS spectrum and TGA curve of RTILN-C-oxide composite. (Figure S11) Cycling performance of RTIL-N-C-oxide. (Figure S12) Discharge–charge curves of electrode with alternative binder or electrolyte additive. (Table S1) EDX analysis of Fe: Ni molar ratios in oxides. (Table S2) Mössbauer hyperfine parameters for Fe-doped NiMn2O4-2. (Table S3) OSPE calculation for cations. (Table S4) Comparison of cycling performance with reference. (Table S5) Equivalent circuit parameters. This material is available free of charge via the Internet at http://pubs.acs.org.” Corresponding Author


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

Ångström Advanced Battery Centre (ÅABC), Department of Chemistry-Ångström Laboratory, Uppsala University, E-mail: [email protected]; References

1.

Tarascon, J. M.; Armand, M., Issues and challenges facing rechargeable lithium batteries.

Nature 2001, 414, 359-367. 2.

Armand, M.; Tarascon, J. M., Building better batteries. Nature 2008, 451, 652-657.

3.

Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M., Nano-sized transition-

metal oxides as negative-electrode materials for lithium-ion batteries. Nature 2000, 407, 496-499. 4.

Li, H.; Balaya, P.; Maier, J., Li-storage via heterogeneous reaction in selected binary

metal fluorides and oxides. J. Electrochem. Soc. 2004, 151, A1878-A1885. 5.

Gao, J.; Lowe, M. A.; Abrunaea, H. D., Spongelike Nanosized Mn3O4 as a High-Capacity

Anode Material for Rechargeable Lithium Batteries. Chem. Mater. 2011, 23, 3223-3227. 6.

Pasero, D.; Reeves, N.; West, A. R., Co-doped Mn3O4: a possible anode material for

lithium batteries. J. Power Sources 2005, 141, 156-158. 7.

Ma, Y.; Asfaw, H. D.; Edström, K., A General Method to Fabricate Free-Standing

Electrodes: Sulfonate Directed Synthesis and their Li+ Storage Properties. Chem. Mater. 2015, 27, 3957-3965. 8.

Yu, X. Q.; He, Y.; Sun, J. P.; Tang, K.; Li, H.; Chen, L. Q.; Huang, X. J., Nanocrystalline

MnO thin film anode for lithium ion batteries with low over potential. Electrochem. Commun. 2009, 11, 791-794.


32

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.

Fan, Q.; Whittingham, M. S., Electrospun Manganese Oxide Nanofibers as Anodes for

Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2007, 10, A48-A51. 10. Li, T.; Guo, C.; Sun, B.; Li, T.; Li, Y.; Hou, L.; Wei, Y., Well-shaped Mn3O4 tetragonal bipyramids with good performance for lithium ion batteries. J. Mater. Chem. A 2015, 3, 72487254. 11. Wang, C.; Yin, L.; Xiang, D.; Qi, Y., Uniform Carbon Layer Coated Mn3O4 Nanorod Anodes with Improved Reversible Capacity and Cyclic Stability for Lithium Ion Batteries. ACS Appl. Mater. Interfaces.2012, 4, 1636-1642. 12. Yue, H.; Li, F.; Yang, Z.; Li, X.; Lin, S.; He, D., Facile preparation of Mn3O4-coated carbon nanofibers on copper foam as a high-capacity and long-life anode for lithium-ion batteries. J. Mater. Chem. A 2014, 2, 17352-17358. 13. Li, J.; Xiong, S.; Li, X.; Qian, Y., A facile route to synthesize multiporous MnCo2O4 and CoMn2O4 spinel quasi-hollow spheres with improved lithium storage properties. Nanoscale 2013, 5, 2045-2054. 14. Kang, W.; Tang, Y.; Li, W.; Yang, X.; Xue, H.; Yang, Q.; Lee, C.-S., High interfacial storage capability of porous NiMn2O4/C hierarchical tremella-like nanostructures as the lithium ion battery anode. Nanoscale 2015, 7, 225-231. 15. Wang, L.; Liu, B.; Ran, S.; Wang, L.; Gao, L.; Qu, F.; Chen, D.; Shen, G., Facile synthesis and electrochemical properties of CoMn2O4 anodes for high capacity lithium-ion batteries. J. Mater. Chem. A 2013, 1, 2139-2143.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

16. Patrice, R.; Dupont, L.; Aldon, L.; Jumas, J. C.; Wang, E.; Tarascon, J. M., Structural and electrochemical properties of newly synthesized Fe-substituted MnO2 samples. Chem. Mater 2004, 16, 2772-2782. 17. Zhao, J.; Wang, F.; Su, P.; Li, M.; Chen, J.; Yang, Q.; Li, C., Spinel ZnMn2O4 nanoplate assemblies fabricated via "escape-by-crafty-scheme" strategy. J.

Mater. Chem.

2012, 22,

13328-13333. 18. Zhang, G.; Yu, L.; Wu, H. B.; Hoster, H. E.; Lou, X. W., Formation of ZnMn2O4 Ball-inBall Hollow Microspheres as a High-Performance Anode for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 4609-4613. 19. Yu, L.; Zhang, L.; Wu, H. B.; Zhang, G.; Lou, X. W., Controlled synthesis of hierarchical CoxMn3-xO4 array micro-/nanostructures with tunable morphology and composition as integrated electrodes for lithium-ion batteries. Energy. Environ. Sci. 2013, 6, 2664-2671. 20. Zhou, L.; Zhao, D.; Lou, X. W., Double-Shelled CoMn2O4 Hollow Microcubes as HighCapacity Anodes for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 745-748. 21. Hu, J.; Chen, M.; Fang, X.; Wu, L., Fabrication and application of inorganic hollow spheres. Chem. Soc. Rev. 2011, 40, 5472-5491. 22. Ma, Y.; Ji, G.; Lee, J. Y., Synthesis of mixed-conducting carbon coated porous [gamma]Fe2O3 microparticles and their properties for reversible lithium ion storage. J. Mater. Chem. 2011, 21, 13009-13014. 23. Yu, L.; Zhang, L.; Wu, H. B.; Lou, X. W., Formation of NixCo3−xS4 Hollow Nanoprisms with Enhanced Pseudocapacitive Properties. Angew. Chem. Int. Ed. 2014, 53, 3711-3714.


34

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24. Ma, Y.; Fang, C.; Ding, B.; Ji, G.; Lee, J. Y., Fe-Doped MnxOy with Hierarchical Porosity as a High-Performance Lithium-ion Battery Anode. Adv. Mater. 2013, 25, 4646-4652. 25. Ma, Y.; Zhang, C.; Ji, G.; Lee, J. Y., Nitrogen-doped carbon-encapsulation of Fe3O4 for increased reversibility in Li+ storage by the conversion reaction. J. Mater. Chem. 2012, 22, 7845-7850. 26. Sing, K. S. W., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). In Pure and Applied Chemistry, 1985, Vol. 57, 603. 27. Moses, A. W.; Flores, H. G. G.; Kim, J.-G.; Langell, M. A., Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2. Appl. Surf. Sci. 2007, 253, 4782-4791. 28. a. Cherkashinin, G.; Nikolowski, K.; Ehrenberg, H.; Jacke, S.; Dimesso, L.; Jaegermann, W., The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation. PCCP 2012, 14, 12321-12331. b. Fu, C.; Li, G.-S.; Luo, D.; Zheng, J.; Li, L. P.; J. Mater. Chem. A, 2014, 2, 1471-1483. 29. Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S., New interpretations of XPS spectra of nickel metal and oxides. Surf. Sci. 2006, 600, 1771-1779. 30. Garvie, L. A. J.; Craven, A. J.; Brydson, R., Use of electron-energy loss near-edge fine structure in the study of minerals. Am. Mineral. 1994, 79, 411-425. 31. Tan, H.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G., Oxidation state and chemical shift investigation in transition metal oxides by EELS. Ultramicroscopy 2012, 116, 24-33.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

32. Lin, F.; Nordlund, D.; Weng, T.-C.; Zhu, Y.; Ban, C.; Richards, R. M.; Xin, H. L., Phase evolution for conversion reaction electrodes in lithium-ion batteries. Nat Commun 2014, 5:3358, doi: 10.1038/ncomms4358. 33. Díez, A.; Schmidt, R.; Sagua, A. E.; Frechero, M. A.; Matesanz, E.; Leon, C.; Morán, E., Structure and physical properties of nickel manganite NiMn2O4 obtained from nickel permanganate precursor. J. Eur. Ceram. Soc. 2010, 30, 2617-2624. 34. Jadhav, R. N.; Mathad, S. N.; Puri, V., Studies on the properties of Ni0.6Cu0.4Mn2O4 NTC ceramic due to Fe doping. Ceram. Int 2012, 38, 5181-5188. 35. Qiu, Y.; Xu, G.-L.; Yan, K.; Sun, H.; Xiao, J.; Yang, S.; Sun, S.-G.; Jin, L.; Deng, H., Morphology-conserved

transformation:

synthesis

of

hierarchical

mesoporous nanostructures of Mn2O3 and the nanostructural effects on Li-ion insertion/deinsertion properties. J. Mater. Chem 2011, 21, 6346-6353. 36. Maier, J., Nanoionics: ion transport and electrochemical storage in confined systems. Nat Mater 2005, 4, 805-815. 37. Valvo, M.; Lindgren, F.; Lafont, U.; Björefors, F.; Edström, K., Towards more sustainable negative electrodes in Na-ion batteries via nanostructured iron oxide. J. Power Sources 2014, 245, 967-978. 38. Akila, R.; Jacob, K. T., Combined homo-hetero doping for enhancement of ionic conductivity. Solid State Ionics 1987, 25, 217-221.


36

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39. Rahman, M. M.; Wang, J.-Z.; Hassan, M. F.; Wexler, D.; Liu, H. K., Amorphous Carbon Coated High Grain Boundary Density Dual Phase Li4Ti5O12-TiO2: A Nanocomposite Anode Material for Li-Ion Batteries. Adv. Energy Mater. 2011, 1, 212-220. 40. Philippe, B.; Valvo, M.; Lindgren, F.; Rensmo, H.; Edström, K., Investigation of the Electrode/Electrolyte Interface of Fe2O3 Composite Electrodes: Li vs Na Batteries. Chem. Mater 2014, 26, 5028-5041.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

For Table of Contents Only


38

Iron Doping in Spinel NiMn2O4: Stabilization of the Mesoporous Cubic

Recommend Documents