2D Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode

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Functional Inorganic Materials and Devices

2D Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode Material for Lithium-Ion Batteries Huibing Lu, Caihong Yang, Cunjun Li, Linjiang Wang, and Hai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00824 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

2D Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode Material for Lithium-Ion Batteries Huibing Lu, †,§ Caihong Yang, †,§ Cunjun Li, ‡ Linjiang Wang, ‡ and Hai Wang,*,†,§

†Ministry-Province

Jointly-Constructed Cultivation Base for State Key Laboratory of

Processing for Non-ferrous Metal and Featured Materials, Guangxi Zhuang Autonomous Region, Guilin 541004, China. ‡Key

Laboratory of New Processing Technology for Nonferrous Metals and Materials,

Ministry of Education, Guilin University of Technology, Guilin 541004, China. §Collaborative

Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and

Development of New Materials in Guangxi.

ABSTRACT

α-MoO3 has gained growing attention as anode material of lithium-ion batteries (LIBs) because it has high theoretical specific capacity of 1111 mAh g-1 and unique layer structure. However, the electrochemical reactions of MoO3 exhibit sluggish kinetics and structural instability caused by pulverization during charge and discharge. Herein, we report a new twodimensional (2D) Cr-doped MoO2.5(OH)0.5 (doped MoO2.5(OH)0.5) ultrathin nanosheets prepared by a facile hydrothermal process. The formation of the ultrathin nanosheets was

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clarified by a “doping-adsorption” model. Compared with doped MoO3, doped MoO2.5(OH)0.5 has larger expanded spacing of the {0l0} crystal planes for fast Li+ storage. The electrodes after cycling were investigated by ex situ TEM in combination with XPS analysis to reveal the reversible conversion reaction mechanism of doped MoO2.5(OH)0.5 nanosheets. Interestingly, for doped MoO2.5(OH)0.5 nanosheet electrodes, it was found that the as-formed intermediate LixMoO3 nanodots were well-dispersed in the mesoporous amorphous matrix and had an expanded (040) crystal plane after 10 cycles. These unique structural features increased the effective surface of intermediate products LixMoO3 to react with Li+ and shortened the diffusion length, and thus promoted the electrochemical reactions of doped MoO2.5(OH)0.5. Additionally, the presence of Cr also played a critical role in the reversible decomposition of Li2O and enhanced specific capacity. When employed as an anode in LIBs, doped MoO2.5(OH)0.5 nanosheets show superior reversible capacity (294 mAh g-1 at 10 A g-1 after 2000 cycles). Moreover, the reversible capacity after electrochemical activation, is quite stable throughout the cycling, thereby presenting a potential candidate anode material for LIBs.

KEYWORDS: MoO2.5(OH)0.5, nanosheets, energy storage, lithium-ion batteries, anode materials

1. INTRODUCTION Recently, MoO3, as conversion type electrodes of lithium-ion batteries (LIBs), had attracted tremendous interest because it has high theoretical specific capacity of 1111 mAh g1,

and unique layer structure.1-8 The layer structure of MoO3 can serve as a Li+ storage space

for fast Li+ diffusion and transportation. As most of transition-metal (TM) oxides, MoO3 also faced the same challenges, such as large-volume changes during phase evolution and resulting pulverization, which remarkably limited its practical application.9-11 Hence, to


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overcome this problem, various attempts have been made by morphologies regulations 1, 5, 7, 8, 12, 13

or the hybridization of MoO3 with carbon-based materials.4, 6, 14, 15 However, the results

are still not satisfactory, either due to complicated technology, poor cycling stability, or/and inadequate capacity improvement. The fact cannot be ignored that the poor electrical conductivity of most of TM compounds has limited their practical application as high-performance anodes for LIBs. Having this in mind, some metallic semiconductor nanocrystals with regulated shapes attracted our considerable interest.16, 17 Yet, this raises the question of whether we can find metallic Mobased compounds with a unique composition and structure. Recently, our group prepared some molybdenum-based materials with mixed valence states, for example, HxMoO3,18 MoO3-x19 and H4.5Mo5.25O18‧(H2O)1.36,20 and explored their structure-activity relationship. In the entire research process, we found that HxMoO3 nanobelts exhibited metallic characteristics and attractive physicochemical properties.18,

21

With inspiration from the

unique H insertion and mixed metal valence state, we attempt to utilize this strategy to improve the electrical conductivity of Mo-based compounds. MoO2.5(OH)0.5 with mixed valence state seems neglected when used as anodes. Why do we choose MoO2.5(OH)0.5? Apart from its mixed valence, there are still the following reasons: (1) the distance between two functionalized layered MoO6 octahedral layers of MoO2.5(OH)0.5, which is larger than that of MoO3 (Table S1); (2) the functional group of OH may be favourable for good conductivity.18, 21The earlier work on the molybdenum oxide hydroxide MoO2.5(OH)0.5 could retrospect to O. Glemser's research in 1947.22 As far as we know, previous synthesis methods for MoO2.5(OH)0.5 mainly included hydrothermal method,23 and heat treatment method,24 etc. We previously reported atomically 2D MoO2.5(OH)0.5 colloid.25 Yet, the applicability of 2D MoO2.5(OH)0.5 is still limited by drawbacks such as low yield and time consuming.



Generally speaking, the cycling performance and rate properties of TM oxides are strongly dependent on their composition, electronic structure, and microstructures. The heteroatom doping of anode materials was demonstrated to be an effective strategy in tuning the electronic structure and electrochemical properties.26-31 This can dramatically enhance their electrochemical performance. Among the various dopants, chromium (Cr) is considered as an interesting candidate, due to its small atomic size, low valance, and electron-donating ability.28,

32, 33

So far no report has been reported on the substitution and doping of

MoO2.5(OH)0.5. The comparison of the crystal structure of MoO3 and doped MoO2.5(OH)0.5 was presented in Figure 1. Moreover, from the point of view of micro nano-level structure, the electrode materials with nanosheets structure had short Li+ diffusion paths based on the equilibrium time equation τeq=l2 /D, where D is the diffusion coefficient, l is the diffusion path.34-36 Hence, to fully employ the structure advantages of MoO2.5(OH)0.5, it is possible to realize high-performance LIBs by heteroatom doping incorporation with MoO2.5(OH)0.5 nanosheets (Figure 2d). To the best of our knowledge, 2D doped MoO2.5(OH)0.5 nanosheet powders had not been reported yet, and their structure-activity relationship during chargedischarge process still lacked detailed investigation. Herein, we introduced a unique “doping-adsorption” strategy to prepare 2D Cr-doped MoO2.5(OH)0.5 nanosheets, and explored their possibilities as anode materials for the first time. When tested as anodes, the experimental results show that 2D Cr-doped MoO2.5(OH)0.5 nanosheets, as an ideal anode material, has played its advantages. The doped MoO2.5(OH)0.5 delivered high capacities of 294 mAh g-1 at 10 A g-1 even after 2000 cycles. Moreover, the reversible capacity after electrochemical activation is quite stable throughout the cycling. Furthermore, we can well understand the electrochemical mechanism of doped MoO2.5(OH)0.5 and its high lithium storage characteristics based on the its crystal structure change and microstructure evolution after cycling. Surprisingly, it was found that the as-


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formed intermediate LixMoO3 nanodots were well-dispersed in the mesoporous amorphous matrix and had an expanded (040) crystal plane after 10 cycles. These unique structural features increased the effective surface of intermediate products LixMoO3 to react with Li+ and shortened the diffusion path, and thus promoted the electrochemical reactions of doped MoO2.5(OH)0.5. Therefore, the 2D Cr-doped MoO2.5(OH)0.5 nanosheets are proposed as a promising anode candidate for LIBs.

Figure 1. The crystal structural comparisons of MoO3 and Cr-doped MoO2.5(OH)0.5 and their corresponding schematic representation of the typical layer structure viewed along different zone axis. The O sites are labelled with three different colors (blue and red and green). The shape of a single-unit cell is also presented.

2. EXPERIMENTAL SECTION 2.1 Preparation of Cr-doped Mo2.5(OH)0.5 nanosheets: 1 g of ammonium molybdate (NH4)6Mo7O24) was dissolved in 50 mL of deionized water. After continuously stirring for 30



min, 3 mL of HNO3 was then added into the beaker, followed by adding 2.5 g of sodium dodecyl sulfate (SDS), 12 mL of 1-Dodecanethiol into the solution, and stirred for another 30 min. Subsequently, 0.75 g of Cr(NO3)3·9H2O was added into above solution under intense stirring. The resulting suspension was then transferred to a 100 mL Teflon-lined autoclave, which reacted at 180 °C for 30 h. The as-prepared Cr-doped Mo2.5(OH)0.5 nanosheets powder was collected by filtration and fully washed by deionized water and ethanol for several times and dried in air. 2.2 Material characterizations: For the structure information of all samples, the crystal structure of the samples was characterized by XRD (PANanalytic X’Pert spectrometer); the morphologies of the samples were measured using FESEM (JEOL JSM6300), all samples for FESEM characterizations were prepared by dispersing the powders in the conductive carbon glue. The microstructure of all samples was further characterized by TEM and HRTEM (JEOL, JEM-2010F) at 200 kV, respectively. The information of pore structure of the samples was characterized by BET and pore size distribution curve (Quanta Chrome adsorption instrument). The chemical environment of the materials surface was characterized by XPS (ESCALAB MK II X-ray photoelectron spectrometer), and the composition information of organic matter or residue was characterized by FTIR (60-SXB IR spectrometer) and Raman (JY-HR800), respectively. The DSC and TG were carried out in STA449F3, Netzsch thermal analyser. To compare the optical band gaps of different electrode samples, UV-vis (Shimadzu) was used to characterize them between 200-800 nm. 2.3 Electrochemical evaluations: The as-prepared samples (active substance), conductive agent (acetylene black) and binder (polyvinylidene fluoride, PVDF) were mixed according to the weight ratio of 70:15:15, and then an appropriate amount of N-methyl pyrrolidone (NMP) solvent was added and stirred at room temperature for 24 h. The slurry was uniformly coated on copper foil with a diameter of 14 mm. It is then placed in a vacuum drying chamber for 12


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h and maintained at 90 ℃ to remove the NMP solvent from the electrode sheets. After lowering to room temperature, the electrode tablets (15MPa) are made by powder tablet press, and the active substance mass of each electrode tablet is about 2-3 mg. The electrochemical properties of the electrode materials prepared in this paper are tested by assembling CR2025type coin cells. The whole assembly process of the LIBs involved is carried out in a glove box in a sealed high purity argon atmosphere. The specific assembly process of CR2025-type coin cells is as follows: the as-prepared electrode sheet, separator, electrolyte, lithium metal sheet and gasket are placed in the center of the negative shell in turn. Then the positive shell is used to cover it, and the sealing machine is used to complete the sealing. The working electrode is the as-prepared electrode, the counter electrode is lithium sheets, and the separator is polypropylene porous membrane (Celgard 2400). The electrolyte used for LIBs is 1 M LiPF6 in ethylene carbonate and diethyl carbonate (volume ratio 1:1) mixed solution. The cycle performance and charge-discharge performance were completed using the Wuhan Blue Electric Test System (LAND-CT2001A) and tested in an incubator (25 ℃). The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested at room temperature on the assembled fresh battery or the battery after different cycles using the CHI 760E electrochemical workstation. Frequency range: 0.01 Hz-100 kHz.

3. RESULTS AND DISCUSSION Doped MoO2.5(OH)0.5 nanosheets were synthesized via a modified hydrothermal method.19 Owing to MoO3 crystal structural anisotropy, the growth rate is different in different directions. The {0l0} plane would preferentially grow based on lower surface energy model.30 Therefore, to suppress the growth of (010) plane, we introduced 1-Dodecanethiol as capping agents to reduce intrinsic surface energy of (010) crystal plane of MoO3. The H provided by 1-Dodecanethiol was expected to be chemisorbed to the surface of (010) crystal



plane of MoO3 (Figure 2b). It has been found that there was basically very little change in crystal structure by varying the process time, as shown in Figure S1a. One can see that the three samples can be indexed to the orthorhombic structure MoO2.5(OH)0.5 (PDF#74-1648). No obvious changes in the whole diffraction pattern or reflection positions are found. However, its crystallinity was gradually enhanced. Furthermore, we found that the diffraction positions of MoO2.5(OH)0.5 along (020), (040) and (060) planes shifted slightly to a low angle (10 h) and were finally stabilized at 30 h (Figure.S1b-d). For this peak at ca 10.4 ° (the dotted circle in Figure S1b), we found that the best match crystal phase may be Mo13O33(PDF#821930). This peak gradually disappears with the prolongation of hydrothermal time, and the reaction proceeds toward a more stable Cr-doped MoO2.5(OH)0.5 lattice direction. It implies that the Mo atoms in the MoO2.5(OH)0.5 lattice may be substituted by Cr atoms, since Cr (III) ions with a radius (0.075 nm) close to that of Mo5+ (0.075 nm) and Mo6+ (0.073 nm) are easy to enter MoO2.5(OH)0.5 lattice and to displace Mo5+ or Mo6+ ions. Furthermore, to understand whether Cr is doped into MoO2.5(OH)0.5 lattice, we compared the XRD refinement results of three samples obtained at different hydrothermal reaction times (Figure. S2). It was found that the lattice parameters of three different samples changed slightly relative to the standard refinement (Table S2), indicating that Cr is doped into MoO2.5(OH)0.5 lattice. The corresponding schematic diagram is shown in Figure. S2d. Additionally, from Figure 2c-e, one can observe that nanosheets’ surface become smoother as the reaction time prolongs. After further magnification of the SEM images of the 30 h-sample, we found that the thickness of the as-prepared doped MoO2.5(OH)0.5 sheets was ca 25 nm (Figure S3). The high-crystalline nanosheets is crucial for stabling MoO6 layer frame structure. Considering the layer spacing and crystallinity of doped MoO2.5(OH)0.5 (30 h), this sample was selected as an anode material in our work. To understand the role of organic matters and Cr source in synthesizing MoO2.5(OH)0.5 nanosheets, a controllable experiment was carried out. The


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obtained samples exhibited different morphologies and crystal phase (See Figure. S4 for the summary results). By comparison, we can draw a conclusion that the coexistence of organic thiol and the Cr source precursor leads to the formation of 2D doped MoO2.5(OH)0.5 nanosheets morphologies. This is also a new example of doping-induced morphological structure changes.26, 28, 29 On the basis of above experimental results, a “doping-adsorption” strategy was presented to elucidate the formation of 2D Cr-doped MoO2.5(OH)0.5 nanosheets, as shown in Figure 2a, b.

Figure 2. The transformation relationship from 1D MoO3 nanorods (a) to 2D Cr-doped MoO2.5(OH)0.5 nanosheets framework (b) by “doping-adsorption” strategy at different hydrothermal reaction times (t=10, 20 and 30 h) and their corresponding FESEM images (ce). To highlight the importance of the crystal structure of MoO2.5(OH)0.5 for LIBs applications, we selected doped MoO3 nanosheets as a comparison. The doped MoO3 nanosheets were obtained when doped MoO2.5(OH)0.5 nanosheets were calcined at 300 °C for 2 h. As shown in Figure 3, FESEM and TEM images of the doped MoO2.5(OH)0.5 (Figure 3a-c) and doped MoO3 (Figure 3d-f) samples were presented, respectively. It is found that the morphologies



of two samples are almost unchanged. Consider the morphologies’ effect on electrodes electrochemical properties, in our case, the effect factor of the morphologies is excluded since the electrodes have the same nanosheet morphologies. Noted that both XRD diffraction patterns are very similar and the difference is not clear from the Figure 3g. The layer spacing difference between the doped MoO2.5(OH)0.5 and doped MoO3 in the (020) planes is further verified in the Figure 3h. The lattice fringe was clearly observable in the HRTEM images of two samples, indicating their high crystallinity. The high-resolution TEM (HRTEM) images for doped MoO2.5(OH)0.5 and doped MoO3 nanosheets were presented in Figure 3c, f, respectively. The lattice spacing of approximately 0.36 and 0.34 nm corresponds to the two (040) planes of MoO2.5(OH)0.5 and MoO3, respectively, which are consistent with the XRD results (Figure 3g). Ultrathin nanosheets structure and surface defects of two samples are also clearly observed in Figure 3b, c and Figure 3e, f, respectively. The corresponding elemental mapping analysis clearly confirmed the successful incorporation of Cr into the MoO2.5(OH)0.5 crystal lattice, as shown in Figure S5. Noted that more than 20% of the carbon content is mainly from conductive carbon glue for better SEM observation because the samples we tested were dispersed in the surface carbon glue. A similar characterization of doped MoO3 also demonstrated that the successful doping of Cr into the MoO3 lattice (Figure S6). It is noteworthy that the presence of sulfur in Figure S5 originated from 1-Dodecanethiol in the reactants. Further analysis of sulphur had been explained in Figure S8 and Figure S9. It is worth emphasizing that our work is based on Cr doping. Elemental mapping is not at all a tool to confirm the presence of any element in a crystal lattice. It only gives an idea of the types of elements present in a sample. This means even if there may be any amorphous Cr-containing compounds nicely distributed on the Mo-based sample, it also will be observed in the elemental mapping as uniformly distributed Cr in the sample, and this does not necessarily indicate the doping of Cr in the crystal lattice of the Mo-based material. To prove


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the Cr doping in the crystal lattice, we carried out some appropriate characterization measures (See Figure. S7, S8 and a detailed discussion in Supplementary Note 1).

Figure 3. FESEM (a) and TEM images (b, c) of 2D Cr-doped MoO2.5(OH)0.5 nanosheets, FESEM (d) and TEM images (e, f) of 2D Cr-doped MoO3 nanosheets. (g) XRD patterns of doped MoO3 and doped MoO2.5(OH)0.5 nanosheets and enlargement section ranged from 2328° (h). Especially, the colour change of Mo-based compounds implied that the valence of Mo had changed and surface defects had been generated. This had been proved in our previous work.18, 19, 37 Judging from the dark blue powders, we can reasonably infer that the valence of Mo in the doped MoO2.5(OH)0.5 nanosheets was decreased, since it shows significant absorption in the visible region, and its band gap becomes narrower, compared with doped



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MoO3 nanosheets. Further UV-vis testing confirmed our hypothesis. As shown in Figure 4a, the doped MoO2.5(OH)0.5 nanosheets exhibited an obvious and intense absorption band in the visible region. A narrower band gap was also found (Figure 4b). These phenomena may be caused by the synergistic effects of oxygen vacancy,10, 38-40 the intervalence charge-transfer between Mo5+ and Mo4+,40, 41 and the interaction between Cr2p and O2p orbits electrons. The mixture valence of Mo5+ and Mo4+ were further verified in the XPS (Figure S9 and Supplementary Note 2). Surprisingly, Mo4+ existed in doped MoO2.5(OH)0.5 sample, while possible MoO2 and MoS2 crystal phases were not detected in XRD (Figure 3g). The importance of Mo4+ in improving batteries performance had been demonstrated by B. Dunn and his co-workers.40 It should be noted the band gap of as-prepared doped MoO2.5(OH)0.5 nanosheets was lower than that of other Mo-based compounds reported.19,

38, 39, 41-44

Combined with Raman spectroscopy (Figure S10c) and the contents of surface atomic percentage of O (Table S3), a large number of oxygen vacancies of doped MoO2.5(OH)0.5 nanosheets were further confirmed. Additionally, the surface crystallinity of doped MoO2.5(OH)0.5 nanosheets was also poor, compared with doped MoO3, which was observed in HRTEM (Figure 3e, f). To further reveal other structural features of two samples, we also performed TG-DSC and FTIR analysis, respectively (Supplementary Note 3). Generally, these unique structural features of 2D Cr-doped MoO2.5(OH)0.5 nanosheets make us expect their superior electrochemical performance.


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Figure 4. (a) The UV-vis diffuse reflectance spectra; (b) optical band-gap evaluation from the plots of (Ahγ)2 vs. hγ. The inset in (a) is the photographs of the doped MoO3 and doped MoO2.5(OH)0.5 nanosheets. The cyclic voltammogram (CV) measurements will provide more useful information on the electrochemical reaction kinetics of doped MoO2.5(OH)0.5 nanosheets. The CV profiles of doped MoO2.5(OH)0.5 nanosheets, bare MoO3, and doped MoO3 for the first few cycles at a scan rate of 0.1 mV s-1 were presented in Figure 5a-i, respectively. As can be seen in Figure 5a, b, four reduction peaks at 2.3, 1.65, 0.75, and 0.16 V appeared in doped MoO2.5(OH)0.5 in the first cathodic process. The broad peak at 2.3 V corresponds to the intercalation of Li+ into doped MoO2.5(OH)0.5, and the minor peak at 1.65 V may be ascribed to the decomposition of doped MoO2.5(OH)0.5 and the reduction of Mo5+ to Mo4+ (Figure 5b), whereas the intense peak 0.16 V corresponds to the reaction between the intermediate product LixMoO3 and Li+,2, 37, 45

and the formation of the solid electrolyte interface (SEI) (Figure 5a).3, 46 Additionally,

three new reduction peaks at 1.35, 0.42, and 0.08 V were found in the second cycle of doped MoO2.5(OH)0.5 nanosheets (Figure. 5a). Compared to pristine phase MoO3, the Cr-O-Mo nanoclusters formed by the doped MoO2.5(OH)0.5 phase after the second cycle contained



variable valence metals (Cr and Mo). Furthermore, we found that the doped MoO3 also existed 0.66 V and 0.07 V at the approximate peak position of 0.42 V and 0.08 V. To some extent, the contribution of Cr element of the specific capacity is reflected. Considering the rich valence states of Mo elemental, such as Mo6+, Mo5+ and Mo4+, accurate identification of both 0.42 V and 0.08 V peaks may require more advanced in-situ characterization tools. During the anodic scans, an obvious peak at 1.50 V is observed, which is related to the decomposition of Li2O (TMO+2Li++2e-⇋TM+Li2O, transition metal denoted as TM) (Figure 5c).9,

10, 28

Obviously, this oxidation peak disappeared in the CV curves of pristine MoO3

electrodes (Figure 5d-f) and doped MoO3 nanosheets (Figure 5g-i). Among the three electrodes, the oxidation peak intensity and reversible Li+ storage of the doped MoO2.5(OH)0.5 nanosheets electrode are clearly higher than those of the other two electrodes, revealing an interesting doped-dependent electrochemical behavior. As we know, the compound Li2O is electrochemical inertness, however, it can be activated during charge-discharge process, which will eventually affect the specific capacity of the electrodes. For doped MoO2.5(OH)0.5 nanosheets electrodes, however, the reduced metal nanoparticles (Mo or Cr) formed during Li+ intercalation may play a catalytic role to accelerate the reduction of Li2O.47 Therefore, the doped Cr in MoO2.5(OH)0.5 crystal lattice played an important role in prompting reversible reaction of Li2O. Furthermore, two oxidation-reduction pairs of 0.42-1.5 and 1.35-1.9 V are reversible from the second cycle. Based on the above analysis, it can be found that two main factors that affected the electrochemical performance of doped MoO2.5(OH)0.5. On one hand, the peak position and intensity at 1.35 V may correspond to lithium insertion into Li-Cr-O nanoclusters since pristine MoO3 did not show this peak except for the doped MoO3 and doped MoO2.5(OH)0.5.48 Interestingly, two new reduction peaks at 1.35 V and 1.43 V were activated from the second cycle for doped MoO2.5(OH)0.5 and doped MoO3, respectively. On the other hand, the


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reversibility of Li2O is obviously enhanced, which was expected to enhance the reversible specific capacity of doped MoO2.5(OH)0.5 nanosheets.

Figure 5. Cyclic voltammograms and its corresponding enlarged curves of doped MoO2.5(OH)0.5 (a-c), bare MoO3 (d-f) and doped MoO3 (g-i). Furthermore, we compared the voltage profiles of two electrode samples at 1 A g-1 from 0.01 to 3.0 V (Figure 6a). During the first discharge, it was observed that the apparent voltage of doped MoO2.5(OH)0.5 and doped MoO3 was stable at 2.3 V, and the oblique voltage below 0.5 V was stable, which is consistent with their CV results. Specially, the charge-discharge steps are divided into two regions: (i) higher than 1.5 V and (ii) lower than 1.5 V (vs Li+/Li).



The initial charge-discharge profile curves for doped MoO2.5(OH)0.5 (Figure S11a) would provide some extra information to help us understand their performance difference. For region (i), the potential plateau of 2.3 V corresponds to the Li+ insertion into the crystal lattice of doped MoO2.5(OH)0.5 nanosheets (MoO2.5(OH)0.5+xLi++xe-→LixMo O2.5(OH)0.5). The specific capacity provided is 127 mAh g-1. For region (ii), the conversion reaction between the LixMoO3 and Li+ corresponds to the formation amorphous Li2O of and Mo nanocrystals (Li2MoO3+4Li++4e-→3Li2O+Mo), leading to a broader potential plateau at 0.40.5 V and a high specific capacity is up to 1257 mAh g-1. As shown in Figure 6d, it can be seen that the first charge capacity of doped MoO2.5(OH)0.5 nanosheets electrode is ∼1084 mAh g-1 with ∼666 mAh g-1 capacity loss in the first cycle. The capacity loss is mainly attributed to several aspects: (1) the formation of Li2O; (2) the irreversible of Li inserted into the lattice of electrodes; (3) the formation of SEI film and the decomposition of electrolytes. Most of conversion type TM oxides exhibited those distinctive features above mentioned.9, 10, 48, 49

When the current density is 2 A g-1, the charge discharge profiles of the two electrodes

show a similar trend (Figure S12), and further analysis is not required here. Obviously, the specific capacities for doped MoO2.5(OH)0.5 nanosheet electrode are much higher than that of its counterpart at the same region. Furthermore, we found that doped MoO2.5(OH)0.5 nanosheets has first cycle Coulombic efficiency of 61%, which is higher than that of pristine MoO3 (53%) and doped MoO3(54%). From CV curves comparative analysis of two electrodes (Figure 5b, e, h and Figure 5c, f, i), the irreversible conversion capacity of Li2O was improved due to the introduction of Cr and the unique crystal structure of MoO2.5(OH)0.5. Moreover, in the second cycle, the Coulombic efficiency of doped MoO2.5(OH)0.5 increased rapidly to ∼87% and remained above 99% after 300 cycles, indicating superior electrochemical properties. Considering effects of specific surface area and pore size of electrodes on their electrochemical performance, we performed


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a BET test (Figure S13) and found that the specific surface area and pore volumes of doped MoO3 was slightly higher than that of doped MoO2.5(OH)0.5 nanosheets (Table S4), excluding the effects of the specific surface area and pore volume of the electrode materials. As shown in Figure 6b, the specific capacity retention and cyclic stability of the doped MoO2.5(OH)0.5 nanosheets were superior to doped MoO3 nanosheets at a current density of 2 A g-1, yielding the CE of 99.8%. For example, the capacity of the doped MoO2.5(OH)0.5 anode remains at ∼803 mAh g-1 after 600 cycles. Such a high specific capacity is relatively rare in previous Mo-based compound electrodes, while the doped MoO3 electrode only maintained ∼197 mAh g-1 after 600 cycles (Figure 6b). Furthermore, we found that the specific capacity of the doped MoO2.5(OH)0.5 electrodes gradually increased after the specific capacity went down and stabilized after an initial cycle of ca 125 cycles. For conversion type electrodes, essentially, the crystal structure and microstructure of the electrodes will be changed during charge-discharge, due to the phase transition. Initially, the specific capacity of electrodes decreased after initial few cycles, due to unstable electrodes and the large Li+ diffusion resistance within the electrodes; the specific surface area of active materials gradually increased caused by the pulverization of electrodes with an increase of cycle numbers, as a result, the specific capacity of electrodes tended to gradually increase. In our work, both doped MoO2.5(OH)0.5 and doped MoO3 are typical conversion reactions, the unique pore structure formed by the pulverization of doped MoO2.5(OH)0.5 nanosheets after cycling would benefit Li+ diffuse sufficiently and would lead to the continuous increase of its specific capacity compared with that of doped MoO3. While doped MoO3 was wrapped with the thick SEI film, indicating an aggregation state (Figure 8a, b). Although the capacity curve of doped MoO3 was stable, its specific capacity was relatively low. This is somewhat similar to electrochemical behaviours of commercial graphite electrode.



Additionally, the rate capability characteristics of two electrodes were evaluated at 0.1, 0.5, 1, and 2 A g-1, respectively, as shown in Figure 6c. Obviously, the doped MoO3 electrode showed poorer rate capability. For example, after cycling at 1 A g-1, the doped MoO3 delivered capacity was ∼465 mAh g-1, whereas the doped MoO2.5(OH)0.5 electrode maintained 692 mAh g-1 at 2 A g-1. The difference in battery performance may be due to electrodes degradation resulting from the large-volume change or agglomeration of nanoparticles during charge-discharge process. This will be further analysed in the mechanism discussion section. The Nyquist plots of doped MoO3 and doped MoO2.5(OH)0.5 are shown in Figure. 6e. The doped MoO2.5(OH)0.5 nanosheets exhibit a small semicircle at high frequencies, which indicates that the electron transfer resistance of the doped MoO2.5(OH)0.5 nanosheets was improved and the electrochemical reaction kinetics of the doped MoO2.5(OH)0.5 nanosheets had been enhanced. The excellent electrochemical properties of doped MoO2.5(OH)0.5 nanosheets stimulated us to further explore their cycling stability at higher rates. Even at the ultrahigh current rate of 10 A g-1, doped MoO2.5(OH)0.5 still delivered high capacities of 294 mAh g-1 even after 2000 cycles. Moreover, the reversible capacity after electrochemical activation is quite stable throughout the cycling. The results further confirmed that 2D Cr-doped MoO2.5(OH)0.5 nanosheet is a potential candidate for LIBs.


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Figure 6. The 1st, 2nd, 10th, 100th, 200th and 300th discharge-charge voltage profiles of (a) doped MoO2.5(OH)0.5 and (d) doped MoO3 nanosheets ; (b) cycling performances of doped MoO3 and doped MoO2.5(OH)0.5 nanosheets and coulombic efficiency of doped MoO2.5(OH)0.5 nanosheets at 2 A g-1; (c) rate performances of doped MoO3 and doped MoO2.5(OH)0.5 nanosheets; (e) the Nyquist plots of EIS of doped MoO3 and doped MoO2.5(OH)0.5 nanosheet electrodes; (f) cycling performance of doped MoO2.5(OH)0.5 nanosheets electrode as well as the corresponding CE. The chemical composition and the chemical states of surface elements for two electrodes after 10 cycles were studied by using XPS analysis, as shown in Figure 7. We note that the atomic percentage of F in doped MoO2.5(OH)0.5 was greater than that of doped MoO3 (Table S5), while LiF contents were reverse (Figure 7f). As we know, the LiF salt decomposed easily and formed HF, which would be detrimental for long cycle life of LIBs.4, 49, 50 The high-resolution core level spectrum of the Mo ion is shown in Figure 7a, and it consists of two obvious peaks at 235.6 and 232.5 eV corresponding to Mo 3d3/2 and Mo 3d5/2



states, respectively. The difference in binding energy between the two peaks (3.1 eV) corresponds to the Mo6+ state in the octahedral environment of MoO2.5(OH)0.5. A new peak at 230.3 eV can be ascribed to metal Mo.4, 51 Based on the relative strength of the peak of Mo, the content of Mo in the doped MoO3 cycle was higher than that of doped MoO2.5(OH)0.5. It was probably owing to the incomplete conversion of Mo to LixMoO3 during the charge process. This also explains why the electron transfer of nanocomposites in doped MoO3 was better than that of doped MoO2.5(OH)0.5 after cycling (Figure S14 and Supplementary Note 4). The oxides within the surface and SEI film of two electrodes after cycling is difficult to identify accurately. Therefore, the analysis of O 1s is fairly complicated in our case, because O element may be included in SO42-, CO32-, lattice oxygen, vacancy oxygen, and so on. It is difficult to quantify or qualitatively distinguish them only by XPS. However, the oxygen vacancies and conductivity of the electrode itself provided more space for Li+ storage and facilitated electrochemical reactions.19,

40, 52

Based on the elemental analysis of XPS

measurements, we found that the atomic percentage of O increased after 10 cycles, from 34.19% in the doped MoO2.5(OH)0.5 to 38.29% in the doped MoO3 (Table S5). To some extent, the effect of oxygen vacancies of doped MoO2.5(OH)0.5 on electrochemical performance improvement is reflected. Additionally, all the XPS peaks information of two electrodes after 10 cycles indicated similar features and little difference in intensity (Figure 7). The little difference in the XPS spectra of two samples prompted two unanswered questions: Is morphologies of electrodes after cycling similar? What is the reason for the performance difference of the two electrodes? The two issues would be further clarified in the next section.


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Figure 7. XPS spectra: (a) Mo 3d, (b) S 2p, (c) Cr 2p, (d) O 1s, (e) C 1s, and (f) F 1s for the doped MoO3 and doped MoO2.5(OH)0.5 nanosheet anodes after 10 cycles at 1 A g-1. The electrochemical process of Li+ and MoO3 was carried out, according to the following reaction equation:45, 53, 54 discharge MoO3 +2e- +2Li +   Li 2 MoO3

(1)

discharge

ˆˆ ˆˆ ˆ† Li 2 MoO3 +4e- +4Li + ‡ˆ ˆˆ charge ˆˆ Mo+3Li 2 O

(2)

As far as we know, there are relatively few studies on how to improve the chemical reaction process of these two reactions. How the microstructure of the pulverized electrodes after a few cycles affected their electrochemical reaction was also not very clear. To further gain insight into the structural features of electrode materials after cycling, we performed comparative research on microstructure-properties relationship of two electrode materials after 10 cycles.



Compared with doped MoO3, doped MoO2.5(OH)0.5 electrode after cycling exhibited a tightly connected porous structure without obvious aggregation occurred, as shown in Figure 8a-d. On the contrary, the doped MoO3 electrodes after cycling exhibited an aggregation structure (Figure 8a). Furthermore, TEM was used to investigate the microstructure changes of two electrodes after 10 cycles. Surprisingly, it was found that there are a number of small nanodots embedded in the mesoporous amorphous 2D matrix (Figure 8e, f). In this structure, the isolated dispersed nanoparticles are tightly fixed on the amorphous nanosheets, which can effectively avoid the volume expansion caused by Li+ insertion, ensure the transfer of electrons and Li+, and improve the electrochemical activity.47 Whereas there were almost no nanocrystals observed in the doped MoO3 (Figure 8c, d). The HRTEM images provided more detailed microstructure of the nanocomposites after cycling. As shown in Figure 8d, f, the nanodots have a lattice spacing of 0.34 and 0.41 nm, corresponding to the (040) planes of intermediate phase LixMoO3 of doped MoO3 and doped MoO2.5(OH)0.5, respectively. Obviously, the interplanar distance of the doped MoO2.5(OH)0.5 sample was significantly larger than that of the doped MoO3 sample. Based on the above discussion, the structure and properties relationship of the capacity improvement of doped MoO2.5(OH)0.5 nanosheet materials is shown schematically in Figure 8 I-III. The high specific capacity and excellent long-term stability of doped MoO2.5(OH)0.5 with synergistic structure and electronic modulation can be summarized in the following four aspects: (i) the sample after pulverization does not form agglomerates, which would increase the effective surface of the reaction between LixMoO3 and Li+ and shorten the diffusion path, thus promoting the electrochemical reaction. (Figure 8 I-III); (ii) the wide interplanar spacing of LixMoO3 facilitated the further Li+ diffusion; (iii) Cr incorporation in MoO2.5(OH)0.5 reduced the bandgap, thus enhanced its intrinsic conductivity and facilitated electron transportation; (iv) the open pore structure of pulverized nanoparticles can effectively adjust


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the strain caused by volume expansion during charge and discharge. In general, integrated advantages of doped MoO2.5(OH)0.5 anode materials were realized for superior electrochemical performance. t may open a window for improving the electrochemical properties of various electrode materials.

Figure 8. FESEM images of doped MoO3 with aggregation (a) and doped MoO2.5(OH)0.5 nanosheets without obvious aggregation (b) after 10 cycles; and their corresponding TEM images of aggregation of the pulverized doped MoO3 particles (c-d) and dispersion of the pulverized doped MoO2.5(OH)0.5 particles of (e-f). (I)-(III) The structure and properties relationship of the capacity improvement of doped MoO2.5(OH)0.5 nanosheet materials, compared with doped MoO3 during the lithiation-delithiation process. Line profile images



displaying measurements of ten atomic planes of the doped MoO3 (i) and doped MoO2.5(OH)0.5 (ii) taken from (d) and (f) as marked by the blue lines.


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4. CONCLUSIONS In summary, we provide the first report on the successful synthesis of 2D Cr-doped MoO2.5(OH)0.5 nanosheets via a unique “doping-adsorption” strategy. The doped MoO2.5(OH)0.5 exhibited a mixed ion valence state of Mo5+ and Mo4+, while the introduction of low-valence Cr elements led to more oxygen vacancies and defect-rich surface, which was supposed to be important factor for obtaining high specific capacity in LIBs. The role of Cr in the improvement of reversibility of conversion reaction of Li2O was also highlighted by comparing three anode materials, doped MoO3, doped MoO2.5(OH)0.5 and pristine MoO3. More importantly, as-formed intermediate LixMoO3 nanodots were found to be well-dispersed in the mesoporous amorphous matrix and to have a distinct (040) lattice expansion and open pore structure after cycling, which facilitated the Li+ diffusion and increased structural stability. As a result, the as-prepared doped MoO2.5(OH)0.5 achieved a high discharge specific capacity of ∼803 mAh g-1 at 2 A g-1 after 600 cycles, and superior long-term stability. The synthetic strategy of such doped MoO2.5(OH)0.5 nanosheets could be extended to other electrodes, thus providing new chances for the development of high-performance LIB electrode materials.

ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at DOI: Additional characterization results including XRD, SEM, TGA, XPS, FTIR, Raman, EIS, N2 sorption isotherms, pore-size distributions (PDF).

AUTHOR INFORMATION Corresponding Author


25


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* E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge financial support from National Natural Science Foundation of China

(No.

51462007,

41572034),

Guangxi

Natural

Science

Foundation

(No.

2018GXNSFAA138199), Guilin University of Technology and Guangxi Universities Key Laboratory of Non-Ferrous Metal Oxide Electronic Functional Materials and Devices.

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Xia,

W.;

Zhang,

Q.;

Xu,

F.;

Sun,

L.

New

Insights

into

Electrochemical

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2D Cr-Doped MoO2.5(OH)0.5 Nanosheets: A Promising Anode

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