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Stacking of Tailored Chalcogenide Nanosheets around MoO2-C Conductive Stakes Modulated by Hybrid POM#MOF Precursor Template: Composite Conversion-Insertion Cathodes for Rechargeable Mg-Li Dual-Salt Batteries Chenglong Wu, Jiulin Hu, Jing Tian, Fulu Chu, Zhenguo Yao, Yongjian Zheng, Dongguang Yin, and Chilin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Materials & Interfaces

Stacking of Tailored Chalcogenide Nanosheets around MoO2-C Conductive Stakes Modulated by Hybrid POMMOF Precursor Template: Composite Conversion-Insertion Cathodes for Rechargeable Mg-Li Dual-Salt Batteries

Chenglong Wu†,‡, Jiulin Hu‡, Jing Tian‡, Fulu Chu‡, Zhenguo Yao‡, Yongjian Zheng‡, Dongguang Yin †,* and Chilin Li ‡,*

†School

of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China

Email:[email protected] ‡State

Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Email: [email protected]

Keywords: Mg-Li dual-salt electrolyte, POM, MOF, NENU-5, chalcogenide cathodes, Mg-based batteries

Abstract: Mg anode has pronounced advantages in terms of high volumetric capacity, resource abundance and dendrite-free electrochemical plating, which make rechargeable Mg-based battery stand out as representative next-generation energy storage system utilized in the field of large-scale stationary electric-grid. However, sluggish Mg2+ diffusion in cathode lattices and facile passivation on Mg anode hinder the commercialization of Mg batteries. Exploring highly electroactive cathode prototype with hierarchical nanostructure and compatible electrolyte system with the capability of activating both anode and cathode is still a challenge. Here, we propose a POMMOF (NENU-5) core-shell architecture as hybrid precursor template to achieve the stacking of tailored chalcogenide nanosheets around MoO2-C conductive stakes, which can be employed as conversion-insertion cathodes (Cu1.96S-MoS2-MoO2 and Cu2Se-MoO2) for Mg-Li dual-salt batteries. Li-salt modulation further activates the capacity and rate performance at the cathode side by preferential Li-driven displacement reaction in Cu+ extrusible lattices. The heterogeneous conductive network and

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conformal dual-doped carbon coating enable a reversible capacity as high as 200 mAh/g with a Coulombic efficiency closing to 100 %. The composite cathode can endure a long-term cycling up to 400 cycles and a high current density up to 2 A/g. The diversity of MOF-based materials infused by functional molecules or clusters would enrich the nano-engineering of electrodes to meet the performance demand for future multivalent batteries.

Introduction Rechargeable multivalent batteries (e.g. Mg, Al, and Ca ones) are drawing more attentions owing to their higher safety than Li-ion batteries (LIBs) and Li-metal batteries (LMBs) in view of the potentially smoother anode evolution during cycling.1,2 Among them, Mg battery stands out, owing to the higher volumetric capacity (3833 mAh/cm3) of Mg anode than that of Li anode (2061 mAh/cm3), Mg resource abundance (1.94%) over Li (0.006%) in earth and dendrite-free plating and stripping for Mg anode.3,4 Since the theoretical reduction potential of Mg is merely 0.6 V higher than that of Li, the energy density of Mg battery is comparable to that of LIBs. However, the high charge density and polarity of Mg2+ often compromise its migration kinetics in cathode lattices and restrict the choice of Mg battery cathodes.5,6 Another concern comes from the incompatibility between the Mg anode and Cl-free electrolyte, which is prone to passivate the surface of Mg-metal or insulate the solid electrolyte interphase (SEI).7 So that exploring compatible electrolyte system and highly electroactive cathode prototype are crucial to develop rechargeable Mg batteries. Although great efforts have been made to explore the suitable Mg-storage prototypes, the well-defined hierarchical structures (or nanostructured composite cathodes) are still lacking. The Chevrel chalcogenides (Mo6X8, X=S, Se) and thiospinel Ti2S4 can serve as rare Mg-insertable cathode materials, which show acceptable Mg intercalation kinetics and cycling stability even without the assistance of Li-ion co-insertion from Mg-Li hybrid electrolyte.3,8 However their synthesis paths based on prior Cu+ extrusion from channels limit the availability of tailored nanostructures. Layered MoS2, TiS2 and VOPO4 were also reported to enable the reversibility of Mg insertion/extraction, benefiting from the expanded interlayer or molecule/cation filler to screen the electrostatic interaction with Mg2+ or MgCl+.9-12 Therefore it seems that insertion-type chalcogenides endowed with soft anion lattices show kinetic advantages over the oxide counterparts with


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potentially narrower interspace. However, their low reversible capacity (usually < 160 mAh/g) and moderate voltage (1.0-1.3 V vs. Mg2+/Mg) retard the practical application of Mg batteries dominantly driven by Mg-ions, especially under the protocol of high current density. For conversion-type chalcogenides, the successful samples for reversible Mg-storage are fewer. Cu-based chalcogenides (CuS and Cu2Se) appear to bear a reversible capacity slightly higher than 100 mAh/g however only under elevated temperature (50oC) or extremely low current density (5 mA/g).13,14 Extraction of conductive Cu and its wiring are favorable for the construction of Mg transportable interfaces or channels, which were also indicated from the cases of Chevrel and thiospinel phases pre-supported by Cu+.3,8 But such a kinetic performance is still not sufficient to achieve the theoretical capacity of Cu-based conversion cathodes. Furthermore, this problem is more serious under the participation of dissolvable polysulfide intermediates, which would migrate to and passivate the Mg anode, leading to a poorer reversibility.15 Mg-Li dual-salt system not only enables the kinetic activation at the cathode side owing to the preferential Li-insertion (from hybrid electrolyte reservoir) instead of slower Mg-insertion, but also the potential peel-off of passivation layer at the anode side.15-17 This Li-salt assisted strategy does not sacrifice the dominance of desired dendrite-free plating behavior of Mg anode in view of the earlier electrodeposition of Mg2+ over Li+ during charge.18 The simultaneous modification for both the electrodes in terms of rate and safety performance renders Mg-Li dual-salt batteries a promise as energy storage system in the field of smart-grid application.19 This dual-salt strategy is expected to enable a more effective activation on conversion-type chalcogenides, especially when combining with well-designed nano-morphology or hierarchical structures.20 Metal organic framework (MOF) is a good precursor for the synthesis of nanostructured chalcogenides by thermal sulfuration and selenylation due to its large specific surface area, rich hierarchical porosity and diversified building blocks.21,22 The infusion of targeted molecules or clusters with hetero-metal into the open cavities of MOFs would further enrich the preparation of heterogeneous chalcogenides with finer spatial distribution. Polyoxometalate (POM) clusters with a size as small as ~1 nm are allowed to enter into the voids of MOF smoothly with a strong interaction with the cavity walls.23,24 The immobility of POM nuclei during thermal process enables a controlled modulation on the growth of nanostructures of MOF and POM derivative products. In this work, we propose the infiltration of Keggin-type POM anion ([PMo12O40]3−) into Cu-based MOF HKUST-1



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([Cu3(BTC)2(H2O)x]n) to form NENU-5 ([Cu2(BTC)4/3(H2O)2]6[H3PMo12O40]) complex as POMMOF precursor template.25 The following sulfuration and selenylation lead to the growth of Cu1.96S and Cu2Se nanosheets from the MOF shells. The embedment of POM cores with interaction with the organic ligands of MOF is favorable for the formation of C-coated MoO2 pompons with different surface roughness and porosity depending on sulfuration or selenylation. The deeper sulfuration causes a phase segregation of MoS2 nano-ribbons from the pompon surface. These narrow MoS2 moieties still closely wrap the residual MoO2-C microspheres. In contrast, the selenylation of POM cores does not remarkably occur, although an evident etching effect on these MoO2-C pompons is observed from the formation of needle-like nanograins. These hierarchical nanostructures can promote the insertion-conversion reaction for Mg-Li dual-salt batteries. Their reversible capacities reach to 200 mAh/g at 0.1 A/g with high Coulombic efficiency (CE) closing to 100 %. They can also endure a long-term cycling up to 400 cycles and a high current density up to 2 A/g. The effects based on insufficient, excess or single-component sulfuration and selenylation are discussed.

Results and Discussion Figure 1 shows the scheme of synthetic procedures with corresponding photos of solution and powder. First of all, copper (II) acetate monohydrate, phosphomolybdic acid hydrate and L-glutamic acid were dissolved in deionized water. L-glutamic acid is used to adjust the size of NENU-5 deposit. The aqueous solution was mixed with the ethanol solution of 1,3,5-benzenetricarboxylic acid. The green precipitate of NENU-5 was collected by centrifugation and washed several times with ethanol, and then transferred into a vacuum furnace and dried at 70 °C for 24 hours.25 The NENU-5 powder was mixed with sulphur or selenium under a weight ratio of 10:1 by hand-milling for several times, and then the mixture was transferred into a quartz test tube, which was sealed under vacuum and put into a muffle furnace for pyrolysis at 600℃ (for sulfuration) or 700℃ (for selenylation) for 10 hours. The pyrolyzed products are correspondingly termed as S-NENU for sulfuration and Se-NENU for selenylation. X-ray diffraction (XRD) patterns of S-NENU and Se-NENU are shown in Figure 2a. S-NENU is composed by dominant tetragonal Cu1.96S (JCPDS No. 29-0578) and hexagonal MoS2 (JCPDS No.


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73-1508) phases, as well as a less amount of monoclinic MoO2 (JCPDS No.86-0135). The diffraction peaks located at 2θ = 32.65°, 39.1°, 45.35°, 45.4° and 53.75° are assigned to the (103), (104), (200), (114) and (212) planes of Cu1.96S respectively, and those at 2θ = 14.39° and 39.65° assigned to the (002) and (103) planes of MoS2 respectively. The pronounced (002) peak indicated an ordered stacking of S−Mo−S layers.26 The relatively weak diffraction peaks at 26.00°, 36.98° and 53.48° are respectively ascribed to the (011), (-211) and (-311) planes of MoO2, which is residual due to incomplete sulfuration. In contrast, Se-NENU consists of monoclinic Cu2Se (JCPDS No.27-1131) and monoclinic MoO2 (JCPDS No.86-0135), and their amounts are comparable from the intensity of XRD peaks. The diffraction peaks of (030), (221), (090), (012) and (402) for Cu2Se are located at 12.94°, 26.46°, 39.65°, 43.87° and 51.25° respectively. Unexpectedly, the Mo-based selenide (e.g. MoSe2) cannot be detected even under a higher selenylation temperature, therefore leaving a substantial residual of MoO2. These heterogeneous components are further confirmed by X-ray photoelectron spectra (XPS) in Figure 2b-g. For S-NENU, the Mo 3d spectra show the dominant Mo4+ signals at 230.0 eV for 3d5/2 and 233.1 eV for 3d3/2, which is attributed to the existence of MoS2 and MoO2 phases.27,28 A very weak Mo6+ signal at 236.1 eV for 3d3/2 is still detectable due to the residual of Mo6+-O from POM. The shoulder peak at 227.1 eV stems from the S 2s signal of MoS2.29 The sulfuration process is also confirmed by the appearance of Mo-S (or Cu-S) at 162.6 eV and S-S at 163.6 eV in S 2p.30,31 The S-S moieties are not sufficient and cannot be observed from XRD. C 1s shows the unusual C-P and C-O peaks at 285.5 and 286.5 eV respectively apart from C-C peak at 284.6 eV,27,32 indicating that P and O components in POM can successfully dope into the C layer (at least on the surface) obtained from the pyrolysis of organic ligand of MOF. The strong interaction between internal POM molecule and MOF shell ligand likely promotes such a heterogeneous doping. The substantial existence of P is also verified from the energy dispersive X-ray spectroscopy (EDX) element mapping as discussed later. In contrast, in Se-NENU the surface valence state of Mo is more complex, and it likely ranges from +6 to 0. The stronger reductibility of Se than S is responsible for the appearance of lower valence state of Mo (e.g. Mo2+ or Mo0 at 227 eV for Mo 3d5/2) especially at the near-surface region.33,34 The presence of Mo6+ is indicated from the Mo 3d3/2 peak at 237 eV, which is slightly higher than that for S-NENU due to the surface accumulation of more O-rich



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moieties for the former.35 The mixed valence state is beneficial to the construction of more conductive electrode network and kinetic improvement as shown later. Apart from MoO2, other MoOx stoichiometries cannot be found from the XRD pattern in view of the precipitation restriction of their bulk phases. The Se 3d peak is roughly located at 55 eV, stemming from the formation of Cu2Se.36 The C1s peak of Se-NENU displays the similar bonding situation (i.e. P and O co-doping into C) as that of S-NENU. Thermal sulfuration and selenylation cause a drastic morphology evolution from NENU-5 precursor to S-NENU and Se-NENU products as indicated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical NENU-5 particles are in octahedral shape with a grain size of about 1 μm (Figure S1), well agreeing with previous report.23 S-NENU mainly shows two kinds of morphology geometries, waxberry-like microspheres of 1-2 μm in size and leaf-like nanosheets of 40 mAh/g during the early 15 cycles. This capacity value is not bad for dominant Mg insertion at room temperature when combined with dense Mg foil, and it is even electrochemically activated up to 55 mAh/g after 10 cycles.9,10,13 However the capacity performance degrades gradually with the progress of deeper cycling and the capacity is decreased to 20 mAh/g after 40 cycles owing to the potential passivation of Mg anode and dissolution of active species (e.g. Cu+ and Sn2-). Se-NENU shows much lower reversible capacity than that of S-NENU, although its initial discharge capacity can reach to 60 mAh/g at 50 mAh/g with most the quasi-plateau capacity above 1 V (Figure S5c and d). The following capacity is below 20 mAh/g from the second cycle and further drops to less than 10 mAh/g after 15 cycles, indicating a minor capacity contribution from MoO2 even with loose nanostructure. The poor Mg-storage performance of MoO2 is also indicated from the pyrolyzed NENU-5 sample in inert atmosphere, which displays a reversible capacity as small as 6 mAh/g (Figure S6). This phenomenon indirectly demonstrates that the Mg-driven capacity in S-NENU mainly comes from the contribution of Cu1.96S and MoS2. Unexpectedly, Mg-ion is irreversibly trapped in Cu2-xSe lattices after the displacement reaction with Cu+.40 Mg-Li dual-salt electrolyte significantly promotes the capacity and rate performance of S-NENU and Se-NENU cathodes for Mg-based batteries. Although 1.0 M is not highly concentrated for LiCl salt, a high reversible capacity exceeding 200 mAh/g is achievable for both the chalcogenide samples (Figure 5), indicating a dominant Li-driven insertion-conversion behavior as previous reports on Li-Mg dual-salt hybrid batteries based on other insertion or conversion cathodes.20,41,42



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S-NENU shows a two-stage plateau curve for both the discharge and charge processes during the first cycle. The discharge plateaus locate at 0.8 and 0.3 V with larger capacity for the higher voltage plateau, whereas the charge plateaus lie at 1.2 and 1.6 V. Therefore the voltage polarization is still evident even for the Li-storage behavior (instead of Mg-storage) in cathode. With the progress of cycling, the two-stage plateaus are transited to one-stage plateau during the early cycles and then gradually become blurred. The sloped curves appear after 100 cycles and subsequently the similar curve profile is preserved for hundreds of cycles. Li-salt addition enables a long-term cycling performance for S-NENU with a highly stable capacity of 150 mAh/g at 100 mA/g for 200 cycles and of still 100 mAh/g after 375 cycles (Figure 5c). The reversible capacities are still close to 150 and 100 mAh/g at 200 and 500 mA/g respectively, and are recoverable to 150 mAh/g after undergoing high-rate charging of 2 A/g (Figure 5d). The evolution of reaction mechanism depending on the cycling number is possible as indicated from the evolution of voltage curves. The existences of dual cations (Li+ and Mg2+) in electrolyte and electroactive heterostructure (Cu1.96S and MoS2) complicate the reaction proceeding more or less, although the reversible cycling is mainly driven by kinetically favorable Li+ and the conversion reaction of Cu-based sulfides. The uplift and shortening of charge plateaus are likely associated with the deviation of Cu stoichiometry from pristine Cu1.96S during conversion reaction. The phase segregation and dissolution for Cu-S phases may be also responsible for the plateau evolution. These factors lead to the difficult overlap of reaction pathways and electrochemical curve profiles. Se-NENU displays a better kinetic performance than S-NENU under the architecture of Mg-Li dual-salt system. Se-NENU shows a multi-stage plateau behavior during the first discharge and charge processes and these plateaus are also gradually blurred during the following cycling (Figure 6a). The voltage polarization of Se-NENU is remarkably modified compared with S-NENU. The plateau-like regions are still observable in the voltage curves of Se-NENU even after 400 cycles, and the voltage gap between discharge and charge plateaus is as small as 0.25 V even after hundreds of cycles (Figure 6b). In contrast to S-NENU, in Se-NENU there is a capacity activation phenomenon from the 10th to 30th cycle with capacity increase from 150 to 200 mAh/g at 100 mAh/g. In the following 400 cycles, the capacity fading is quite slow and the capacity value is 150 mAh/g after 200 cycles and 100 mAh/g after 430 cycles (Figure 6c). The kinetic advantage of Se-NENU mainly lies in the rate performance, and the capacity is preserved at 175, 150, 125 and 75 mAh/g under 0.2, 0.5,


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1 and 2A/g (Figure 6d). The capacity can return to 200 mAh/g when the current density recovering to 100 mA/g. In Se-NENU, the Cu2Se sheets are not thin enough. Therefore some cycling steps are required to electrochemically attenuate these nanosheets and grind these conversion-type grains. The early cycling stages are accompanied with the gradual reinforcement of mass/charge transport (or electrolyte infiltration) across the thinner active species and richer phase boundaries, leading to the observation of abnormal capacity increase. The potential evolution of crystallinity and defect structure of conversion products may be also responsible for the accumulation of higher capacity. However the capacity activation and grain attenuation would increase the possibility of irreversible Mg trapping and redox site passivation, which likely leads to slightly worse reversibility degradation than S-NENU as observed in Figure 6(c). The better kinetics for Se-NENU is also reflected from the measurement of galvanostatic intermittent titration technique (GITT), which discloses a small voltage hysteresis of 0.2-0.4 V for most the lithiation stages (apart from the Li-deficient stages near the fully charged state) after 25 cycles (Figure 7). However the corresponding voltage hysteresis for S-NENU is close to or exceeds 0.5 V, and it is even larger when approaching to the cut-off charge voltage of 2 V. The open circuit voltage (OCV) plots show the similar evolution tendency as the nonequilibrium voltage curves. In OCV pattern, the featured plateau based on conversion reaction can be clearly distinguished from the sloped plots, which involve the contribution of insertion reaction. The pyrolyzed NENU-5 sample without additional sulfuration or selenization suffers from much lower reversible capacity of 50 mAh/g even under the assistance of Li-salt addition (Figure S7). Furthermore the discharge capacity mainly locates below 0.7 V and is released in a form of sloped curve rather than plateau. This result indicates that the capacity contribution of S-NENU and Se-NENU mainly comes from Cu1.96S, MoS2 and Cu2Se rather than MoO2. The practical capacity during the first discharge is 220 or 280 mAh/g for S-NENU or Se-NENU, which is close to the theoretical capacities of Cu1.96S, Cu2Se and MoS2 (i.e. 280, 260 and 334 mAh/g respectively) based on two-electron reaction. The irreversible trapping of Mg- or Li-ions during the early cycles does not influence the stabilization of capacity during the long-term cycling. The mobility of Cu+ in lattices is favorable for the fast electrochemical exchange between Cu+ and Li+, and is responsible for the achievement of high lithiation number. The conformal carbon coating can suppress the escape of extruded Cu moieties into electrolyte and therefore the degradation of reversible capacity. The 2D



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MoS2 does not cause the plateau capacity, and therefore its discharge capacity is mainly released in the lower voltage zone with sloped curve.42 We also attempted the effect of excess sulfuration or selenization on the electrochemical performance. When the sulphur content is increased to achieve a weight ratio of 5:1 for NENU-5: S, the reversible capacities are 60-70 and 160 mAh/g for Mg-storage in APC system and dominant Li-storage in APC-LiCl system respectively (Figure S8). The marginal increase of Mg-storage capacity should stem from the increased fraction of MoS2 in composite cathode compared with S-NENU prepared from a NENU-5-S ratio of 10:1. However the increase of MoS2 content does not promote the capacity performance of dual-salt batteries. Undesiredly, the curve profiles become even worse with quick dropping of discharge voltage in view of potential degradation of mixed conductive network or coarsing of nanostructures. After 30 cycles, the discharge capacity is mainly located below 0.5 V. When the selenium content is increased to achieve a weight ratio of 2:1 for NENU-5: Se, the magnesiation capacity in APC system is increased to 40 mAh/g and however the lithiation capacity in APC-LiCl system is on the contrary decreased to less than 100 mAh/g after merely 30 cycles, compared with Se-NENU sample (Figure S9). These results indicate that excess sulfuration or selenization likely compromises the conductive network of composite electrodes, which would particularly influence the lithiation kinetics. Moreover, it increases the risk of the generation of more dissoluble polysulfides or polyselenides, which would diffuse to and passivate the Mg anode and also influence the anode kinetics. Changing the precursor concentration (e.g. doubling the amount of copper(II) acetate monohydrate or phosphomolybdic acid hydrate) does not certainly guarantee a satisfactory POMMOF architecture as indicated from the degraded voltage profiles and capacities (Figure S10 and S11). The preservation of cycling stability of Cu-based cathode is a big challenge because of the facile electrochemical extrusion of Cu from lattices and its following dissolution in electrolyte. The dissolved Cu-ions would migrate to anode and corrode the anode (or be reduced by the anode) especially when the anode is Li metal, leading to the loss of active species and quick capacity fading. The electrochemical performance of Li-metal batteries based on S-NENU and Se-NENU cathodes in Figure S12 confirms the much worse capacity retention compared with the case using Mg anode, which appears to be a good solution to the cyclability of Cu-based cathodes because of better corrosion resistance of Mg. The electrochemical performance of sulfurated and selenylated Cu-MOF HKUST-1 (denoted as S-HKUST and Se-HKUST) without the infusion of POM molecules is shown


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in Figure S13 as a comparison. Without the assistance of pyrolyzed POM (i.e. metallic MoO2) as conductive stakes to reinforce the conductive network of electrode, the capacity retention of Cu-S and Cu-Se is still not satisfactory even using Mg as anode. Their capacities drop to 100 mAh/g or lower after merely 60 cycles. These control experiments further indicate the importance of injection of POM molecules and resulting modulation of electrode conductive network consisting of 2D (sulfides or selenides) and 3D (oxides) moieties with intimate contact. Apart from S-HKUST and Se-HKUST, the electrochemical performance of sulfurated and selenylated POM (denoted as S-POM and Se-POM) is also shown in Figure S14 to decouple the effects of different components in composite cathodes. S-HKUST and Se-HKUST display the distinct discharge plateaus around 1 V, which still exist even after capacity degradation. By contrast, S-POM and S-POM do not present plateau-like curves during discharge processes, and instead the sloped curves are observed with main capacity below 0.8 V. The better capacity retention of S-POM than Se-POM benefits from the more facile formation of MoS2 nanoribbons than MoSe2. These results indicate more similar curve profiles between S-NENU and S-HKUST as well as between Se-NENU and Se-HKUST, confirming that the dominant capacity release in S-NENU and Se-NENU is driven by Cu-based components rather than by Mo-based components. As shown in Figure S15, the Mg anode is very smooth before cycling except the strips caused during scraping the surface oxides before battery assembly. Figure 8 shows the SEM images and corresponding EDX mapping of cycled Mg anode to check the dissolution degree of Cu- and S-based components in electrolyte and their migration to anode side. After 40 cycles at 100 mA/g, the Mg surfaces are still roughly compact, and no dendrite can be observed. There are shallow pits left on the Mg surfaces for both the cases of S-NENU and Se-NENU. For the former, the contents of Cu and S migrating to Mg anode are as low as 0.12 and 0.11 mol% respectively, indicating a minor shuttle effect of Cu- and S-based species. For the case of Se-NENU, the content of dissolved Cu is expected to be lower, although the residual glass fibers bring some adhesive Cu-based solid electrode as indicated by the discrete signal of Cu around the separator residual. The lower solubility of Se-NENU is also reflected from the lower content of Se element (0.08 mol%). It is likely responsible for the smaller-sized pits at Mg surface and better rate performance for Se-NENU based cell. The preservation of plateau-like curves indicates the dominance of Li-driven conversion reaction in Cu2-yX (X = S, Se) phase at the cathode side.43,44 S-NENU is mainly composed of



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Cu1.96S, MoS2 and MoO2 with an element ratio of Cu:Mo:S:O roughly equal to 5:3:4:4.5, and Se-NENU mainly consists of Cu2Se and MoO2 with an element ratio of Cu:Mo equal to 5:3, from the EDX mapping analysis. Therefore the mole ratio of Cu1.96S, MoS2 and MoO2 is estimated to be 2.5: 0.75: 2.25 for S-NENU, and the mole ratio of Cu2Se and MoO2 is 2.5:3 for Se-NENU. The reversible capacities of S-NENU and Se-NENU are around 150 mAh/g, slightly higher than half of the theoretical values of Cu1.96S and Cu2Se. This result is in accordance with the fraction of Cu-based chalcogenides with dominant capacity contribution. The infusion of POM cores as molecule template is essential to modulate the sheet-like morphology of Cu2-yX and its thickness. The coating of MOF shells in turn serves as framework template to tailor the pompon-like morphology of MoO2 and its surface roughness. The homogeneous distribution of organic ligands in NENU-5 and their interaction both with Mo and Cu components enable a conformal coating of carbon on respective derivates after pyrolysis. MoO2 balls can act as conductive stakes to avoid the adherence (or aggregation) of surrounding Cu2-yX nanosheets with each other as well as to extend the electric contact with thin nanosheets. MoS2 nanoribbons also function as finer conductive wires between MoO2 and Cu1.96S apart from as active species. Such heterogeneous nanostructures promote the spread of conversion reaction not only in the bulk of Cu2-yX, but also between their grains. The firm carbon coating protects the extruded Cu from dissolution and therefore alleviates the loss of active species. These positive factors guarantee a satisfactory cycling performance of Cu2-yX based on Mg anode compared with naked counterparts.41 The shallow selenization in the case of Se-NENU is favorable for the exclusive formation of Cu2Se, which has been confirmed to possess much better Li-storage cyclability than CuSe2 and CuSe.44

Conclusion In summary, a series of nanostructured composite cathodes characterized by the stacking of tailored chalcogenide nanosheets around MoO2-C conductive stakes are proposed for Mg-Li dual-salt hybrid batteries. They are prepared by thermally sulfurating or selenizing a hybrid POMMOF (NENU-5) precursor template, which enables a desired modulation of nanosheet, nanoneedle and nanoribbon morphologies in pyrolyzed products in view of the two-sided interaction of organic ligands with center transitional metals (Mo and Cu) respectively in POM core and MOF shell. The dominate conversion reaction is accelerated in Cu2Se nanosheets, which are conformally coated by carbon


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layer to suppress the dissolution of extruded Cu+ into electrolyte. The heterogeneous conductive network and dual-doping in ultrathin carbon coating enable a fast interfacial transport and high reversible capacity of 200 mAh/g with a CE value closing to 100 %. The composite cathode can endure a long-term cycling up to 400 cycles and a high current density up to 2 A/g. The confinement of functional molecule or cluster into MOF open cavity is a good solution to heterogeneous nanoelectrodes for high-performance multivalent batteries.

Experimental Section Preparation of NENU-5 The reagents were obtained from commercial sources and used without further purification. The octahedral NENU-5 precursor was synthesized based on previous report.25 Typically, 1 mmol copper (II) acetate monohydrate (Aladdin Reagent Co.), 0.5 mmol L-glutamic acid (Aladdin Reagent Co.) and 0.3g phosphomolybdic acid hydrate (TCI Reagent Co.) were dissolved in deionized water of 40 ml and stirred for 20 min to get a green transparent solution. Then 0.67 mmol 1,3,5-benzenetricarboxylic acid (Aladdin Reagent Co.) was dissolved in 40 ml ethanol and was poured into the above solution under continuous stirring. A green precipitate appeared immediately. After further stirring for 14 h, the green precipitate was collected by centrifugation and washed for several times with ethanol, and then dried in an oven overnight at 70 °C for further use. The samples where the amount of precursor copper (II) acetate monohydrate or phosphomolybdic acid hydrate is doubled were also prepared as a comparison. Preparation of S-NENU or Se-NENU For the synthesis of S-NENU, 100 mg as-prepared NENU-5 was mixed with 10 mg sulphur powder (Aladdin Reagent Co.), and the mixture was hand-milled for 30 min. Then the mixture was transported to a quartz tube. After extracting the spare air, the quartz tube was sealed and put into a muffle furnace. S-NENU was synthesized by the thermal sulfuration of NENU-5 at 600 °C with a ramp rate of 2 °C/min. After cooling to room temperature, the black powder (denoted as S-NENU) was taken out from the quartz tube and milled for further use. The synthesis procedure of Se-NENU is similar to that of S-NENU, apart from the displacement of sulphur powder by selenium powder



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(Aladdin Reagent Co.). The selenization process was performed at a higher temperature of 700 °C. The samples with tuned ratio of S or Se were also prepared as a comparison. Sulfurated and selenylated single-components (either MOF or POM) were also prepared as control electrodes under the corresponding sulfuration and selenization conditions. The pyrolyzed products are correspondingly termed as S-HKUST for sulfuring Cu-MOF (HKUST-1) and Se-HKUST for selenylating Cu-MOF, as well as termed as S-POM for sulfuring POM and Se-POM for selenylating POM. Preparation of electrolyte solution A typical Mg electrolyte solution consists of 0.4 M all-phenyl complex (APC) in tetrahydrofuran (THF). 0.4 M APC electrolyte was prepared by dissolving 2 mmol AlCl3 (≥ 98%, TCI) in 3 mL THF under stirring overnight and then adding 2 mL PhMgCl (≥ 99%, 2.0 M in THF solution, Aladdin Reagent Co.) to the AlCl3/THF solution. The final solution was stirred overnight prior to use. The hybrid electrolyte solution consists of 1.0 M LiCl and 0.25 M APC in THF. The APC-LiCl electrolyte system was prepared by adding 4 mmol anhydrous LiCl (99.99%, Aladdin Reagent Co.) and 1mmol AlCl3 (≥98%, TCI) in 3 mL THF under stiring overnight and then adding 1 ml PhMgCl (≥99%, 2.0 M in THF solution, Aladdin Reagent Co.) to the above solution. The final solution was stirred overnight prior to use. A typical Li electrolyte was prepared by adding 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1. Physical characterization: The components and their crystallinity of S-NENU and Se-NENU were characterized by X-ray diffraction (XRD, Philips PW3710, 40 kV/30 mA) in a 2θ range of 10°~70° at a scanning rate of 2.0°/min using Cu Kα radiation. Scanning electron microscopy (SEM) images were obtained from FEI Magellan 400 equipped with energy dispersive X-ray spectrometer (EDX) in order to analyze the overview morphology and component distribution. The morphology, microstructure and phase assignment of S-NENU and Se-NENU were further analyzed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements (JEOL JSM-6700F, operated at 200 kV). X-ray photoelectron spectroscopy (XPS, ESCAlab-250) with an Al anode source was performed to detect the surface component, elemental valence and bonding situation of S-NENU and Se-NENU. Thermogravimetric analysis (TGA) was acquired by using a DSC 800 from PerkinElmer


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under O2 flowing, and the heating rate was 10 °C/min. Nitrogen adsorption/desorption isotherms and pore size distribution were measured on a Quantachrome Autosorb iQ gas sorption analyzer at -196 °C. To prepare the cycled Mg anodes for ex-situ SEM characterization, they were taken out from the 2032-type coin cells in the Ar-filled glove box, and then were washed three times by fresh THF to remove the residual electrolyte. The washed electrodes were dried in Ar filled glove box before measurement. Electrochemical characterization: Electrochemical experiments were tested based on an architecture of 2032-type coin cells which were assembled in an argon-filled glove box (

Stacking of Tailored Chalcogenide Nanosheets ... - ACS Publications

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