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New Insights into Electrochemical Lithiation/Delithiation Mechanism of #-MoO3 Nanobelt by In Situ Transmission Electron Microscopy Weiwei Xia, Qiubo Zhang, Feng Xu, and Litao Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01671 • Publication Date (Web): 23 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016

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ACS Applied Materials & Interfaces

New Insights into Electrochemical Lithiation/Delithiation Mechanism of α-MoO3 Nanobelt by In Situ Transmission Electron Microscopy Weiwei Xia, †,# Qiubo Zhang, †,# Feng Xu, †,‡,* Litao Sun†,‡,* †

SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast

University, Nanjing 210096, China. ‡

Center for Advanced Materials and Manufacture, Joint Research Institute of Southeast

University and Monash University, Suzhou 215123, China

KEYWORDS: lithium ion batteries, in situ TEM, MoO3, electrochemical behavior, two-stage delithiation

ABSTRACT: The α-MoO3 nanobelt has great potential for application as anode of lithium ion batteries (LIBs) because of its high capacity and unique 1D layer structure. However, its fundmental electrochemical failure mechanism during first lithiation/delithiation process is still unclear completely. Here, we constructed an electrochemical setup with α-MoO3 nanobelt anode inside transmission electron microscope (TEM) to in situ observe the mircostructure evolution during cycles. Upon first lithiation, α-MoO3 nanobelt converted into numerous Mo nanograins within the Li2O matrix, with an obvious size expansion. Interestingly, α-MoO3 nanobelt was found to undergo a two-stage delithiation process. Mo 1

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nanograins were first transformed into crystalline Li1.66Mo0.66O2 along with the disappearance of Li2O and size shrink, followed by the conversion to amorphous Li2MoO3. This irreversible phase conversion should be responsible for the large capacity loss in first cycle. In addition, a fully reversile phase conversion between crystalline Mo and amorphous Li2MoO3 was revealed accompanying the formation and disapperance of the Li2O layer during the subsequent cycles. Our experiments provide direct evidence to deeply understand the distinctive electrochemical lithiation/delithiation behaviors of α-MoO3 nanobelt, shedding light onto the development of α-MoO3 anode for LIBs.

1.

INTRODUCTION

As a kind of promising rechargeable batteries, lithium ion batteries (LIBs) require new anode materials with high energy density, power density and good cyclability to meet the rapid development of electronic industry. Various metal oxides, such as SnO2,1-3 TiO2,4-5 Fe2O3,6 Al2O3,7 MoO3,8-11 Co3O412 and ZnO13-14, are possible anode candidates for LIBs due to their high lithium-taken abilities (>500 mAh g-1), low cost and environmental friendliness, compared with the commercial anode material graphite (372 mAh g-1). Among them, MoO3 has been studied for decades as a well-known lithium insertion compound with the high theoretical lithium storage capacity of 1117 mAh g-1.15 The orthorhombic MoO3 (α-MoO3) nanobelt, as the thermodynamically stable phase among various polymorphs, offers improved performance and thus be investigated as anode material for LIBs most widely.15-17 These improvements can be attributed to its unique 1D layer structure, which provides enhanced accommodation of the transformation strains caused by the lithium insertion/extraction, 2


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efficient 1D electron transport pathways and large contact area between electrode and electrolyte.15 However, α-MoO3 shows poor ionic and electronic conductivity, resulting in limited electrochemical activity.18 Moreover, the large volume expansion/constriction during lithiation/delithiation process can lead to anode pulverization and thus capacity loss. A variety of strategies have been adopted to overcome the restrictions, including the use of carbon-based (MWCNT, graphene) nanocomposites8-9, 17, 19 and the modification of MoO3 with MnO2/SnO2/In2O3.16, 20-22 In a well designed anode, these modifications can not only improve the lithium diffusion and the electronic conductivity, but also accommodate the volume change associated with the lithium insertion and extraction, achieving a high capacity retention and stable cycling performance.

Although the improved electrochemical performance has been acquired by the above mentioned methods, a few fundamental mechanisms concerning lithiation/delithiation processes still remain ambiguous due to the lack of direct evidence. It is well known that the α-MoO3 nanobelt anode suffers from a huge capacity fading in first charging/discharging cycle which can be attributed to the irreversible lithiation/delithiation process such as the irreversibility of the lithiated product, the formation of the solid electrolyte interphase (SEI) layer and the possible loss of electrical contact caused by the anode pulverization. Although the previous reports indicate that MoO3 can be transformed into Mo and Li2O after being lithiated completely,16, 18-19, 23 few attentions have been focused on the delithiation process. It still has no direct evidence to confirm the final delithiation product. Furthermore, no experiment data can provide the information on dynamic morphology and microstructure evolution of α-MoO3 nanobelt during cycles. That is to say, the nature of the irreversible 3



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electrochemical processes of α-MoO3 nanobelt anode and its influence on rapid capacity fading are still not understood completely.

Introducing in situ TEM technique to solve the above mentioned issues is an excellent option as it has the ability to monitor the dynamic processes of anode material during electrochemical reactions in real time.24-30 Here, an all solid nano-battery was constructed with an individual α-MoO3 nanobelt as anode inside TEM to visualize the microstructure, morphology and phase evolutions during lithiation/delithiation processes. The α-MoO3 nanobelt was transformed into many Mo nanograins embedded in the crystalline Li2O matrix during lithiation. Cracking and pulverization, which would lead to the loss of electrical contact between active material and current collector, was not observed in anode during lithiation process despite the obvious volume expansion. Meanwhile, we found that the anode cannot be converted to its original state, namely orthorhombic MoO3, but suffered a two-step phase change from Mo to crystalline Li1.66Mo0.66O2 and then transformed to amorphous Li2MoO3 during first delithiation process. This reversible phase conversion can be responsible for the huge capacity loss and low coulomb efficiency of MoO3 anode in first cycle. Our experiments provide direct observations for deeply understanding the electrochemical mechanisms of α-MoO3 nanobelt anode, which is beneficial for the successful design of advanced performance anode material.

2.

EXPERIMENTAL SECTION

2.1. Preparation of α-MoO3 nanobelts. The α-MoO3 nanobelts were synthesized according to the previous reports.17 Briefly, 2.0 g ammonium heptamolybdate tetrahydrate 4


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((NH4)6Mo7O24·4H2O) were dissolved into 15ml deionized water and formed a clear solution, in which the PH value was adjusted to about 1 by slowly adding a certain amount of HNO3 (5M). After that, the mixed solution was sealed in a Teflon-lined stainless autoclave at 180 °C for 30h. The obtained thick white colored precipitate of MoO3 were filtered and washed with distilled water, followed by drying in a hot air oven at 60 °C for 12h.

2.2. Characterization. The as prepared solid products were identified by X-ray diffraction (XRD) at room temperature using a diffractometer (XRD, ARL XTRA, Thermo Electron Co., USA) with Cu Ka radiation as the X-ray source. Scanning electron microscope (SEM) image were obtained on a microscope (SEM, JSM-7600F, JEOL, Japan) operated at 15kV to study the morphology of the samples. TEM images and selected area electron diffraction (SAED) were taken on a high resolution field emission transmission electron microscope (FEI Titan, 300KV).

2.3. In situ electrochemical experiments. A high resolution TEM (FEI Titan) with a fast responding charge-coupled device (CCD) camera was carried out for in situ observing the electrochemical behaviors of α-MoO3 nanobelt with the assistance of electron diffraction pattern (EDP), HRTEM imaging and EELS spectrum measurements. The α-MoO3 nanobelts, which were attached to Au rod by conductive silver colloid to ensure a good electrical contact, were used as anode. Metal lithium was adhered to a tungsten probe and regarded as counter electrode and lithium source. The natural oxide layer Li2O that generated in the holder loading process acted as the role of solid electrolyte which allows the transport of lithium ions. The building of nano-battery was accomplished in glove box that filled with argon gas

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as lithium is a kind of very active metal which can be oxidized easily. Afterwards the holder was transferred into TEM column immediately. The tungsten probe was driven by a piezo-positioner inside TEM to make the Li/Li2O and α-MoO3 nanobelt in contact with each other. A constant negative/positive potential was then applied to the α-MoO3 nanobelt anode with respect to lithium counter electrode to drive the transport of lithium ions through the solid-state electrolyte Li2O layer, thereby initiating the electrochemical lithiation/delithiation process.

3.

RESULTS AND DISCUSSION

The color photo image of the MoO3 smaples is shown in Figure 1 (a). Figure 1 (b) is the XRD spectrum of the as-prepared MoO3, in which all diffraction peaks can be perfectly indexed as the orthorhombic MoO3 (commonly as α-MoO3) phase with lattice parameters of a = 0.963 nm, b = 1.385 nm and c = 0.3696 nm (JCPDS no. 35-0609). As Figure 1 (c) shows, the α-MoO3 phase has a unique layer structure in which each layer is composed of two sublayers.31 The two sublayers are constituted by corners-sharing and edges-sharing octahedra [MoO6], resulting in a unique two-dimensional layer structure that is parallel to the (010) direction, and providing open channels for the transport of lithium ions and electrons.15 From the SEM image presented in Figure 1(d) and Figure S1, we can observe that the as-prepared α-MoO3 represent a belt-like morphology, with a width in the range of about 50-500 nm and a length of several micrometers. TEM image in Figure 1 (e) further confirms its nanobelts morphology, which is in good agreement with the SEM image. HRTEM image in the inset of Figure 1 (e) exhibits clear and ordered lattice fringes of 3.81 Å and 2.7 Å, in

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accordance with the (110) and (101) planes of a-MoO3 respectively (JCPDS no. 35-0609), indicating the good crystallinity of the α-MoO3 nanobelts. The electron diffraction pattern (EDP) of an individual α-MoO3 nanobelt is shown in Figure 1 (f), which reveals the single crystalline feature for the anode material.

The

electrochemical

experimental

setup

that

was

used

to

investigate

the

lithitaion/delithiation mechanism of α-MoO3 nanobelt inside TEM is illustrated in Figure 2 (a). Briefly, it contains α-MoO3 nanobelt anode, lithium counter electrode and solid electrolyte Li2O. The Li/Li2O layer was applied to a sharp tungsten tip that was attached to the piezo-driven biasing-probe which can make the α-MoO3 nanobelt and Li/Li2O in contact with each other by fine steps. A constant potential of -2 V was applied to the α-MoO3 nanobelt with respect to the lithium counter electrode to intiate the lithiation process. Movie S1 (Supporting Information) and Figure 2 (b1)–(b3) show the morphology evolution of an individual α-MoO3 nanobelt with the original width size of 74.5 nm in a low magnification, in which the red arrows denote the lithiation front. Obviously, lithiation begun from the side that in contact with the Li/Li2O layer and propagated towards to the other side, resulting in a gray contrast. The straight α-MoO3 nanobelt undergone both width swelling and elongation as a result of the lithium ions intercalations. The final width of the lithited anode was expanded to 98.9 nm, with an obvious width size expansion of 32%. We further claculated other five nanoblets, and their width expansions range from 26% to 34% with the average value of 29%, while the mean elongation is about 27%. However, we have no idea about the size expansion in the thickness direction as the image we observed during the in situ TEM experiment is the projection of the real object and the nanobelt-like MoO3 may show very different size 7



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expansion in width and thickness directions. Therefore, the specific volume expansion of the nanobelt-like MoO3 is almost impossible to be estimated from the experiments.

In order to further explore the microstructure change during lithiation, close view of the microstructure evolution of a segment of another MoO3 nanobelt was achieved and shown in Figure 2 (c1)–(c4) and Movie S2 (Supporting Information). The MoO3 nanobelt was just swelled at the initial stage of lithiation, and then coated with a crystalline layer which was increased as the lithiation process proceeded. The width expansion is about 34% depending on its size increase from 66.6nm to 89.7nm, similar to 32% for the above mentioned MoO3 nanobelt. The crystalline layer coated on the anode with a thickness of 3-5 nm was indified to be Li2O from the EDP of lithiated anode in Figure 2 (e). From the HRTEM image of the lithiated anode given in Figure 2 (d), lattice fringe is found about 2.66Å, in agreement well with the (111) plane of Li2O (JCPDS no.12-0254), also confirming the formation of crystalline Li2O layer. Furthermore, the lattice spacing marked with the white circles in Figure 2 (d) is measured to be 2.22 Å and can be assigned to the (110) plane of Mo (JCPDS no.01-1208), indicating the conversion of MoO3 to Mo nanograins. Therefore, we can conclude that MoO3 nanobelt has evolved from a single crystal to a belt with numerous Mo nanograins within the Li2O matrix after lithiation process. Figure 2 (e) shows the EDP of a lithited MoO3 nanobelt, in which the diffraction rings can be indexed as the mixture of Li2O and Mo, further confirming the nature of the lithiation product. Overall, the first lithiation reaction involves the reduction of MoO3 to Mo and the formation of crystalline Li2O, which can be expressed by the following equation:

8


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MoO3 + 6Li + + 6e −1 → Mo + 3Li 2O

Despite the obvious size expansion, no crackings and fractures were observed in the MoO3 anode during the lithiation process, which is benefical to the electrical contact between the anode material and current collector. Since the fracture can only be observed for Si nanoparticles over 150 nm in diameter,30 we further used MoO3 nanobelts with various sizes to explore the effect of width size on the microstructure of nanobelt during lithiation and no fractures were found. These results suggest that the MoO3 nanobelt exhibit promissing potential as anode material for LIBs.

During delithiation process, a constant positive potential of 3V was applied on the lithiated anode with respect to the lithium counter electrode. Figure 3 (a1)–(a3) show the electrochemical delithiation behaviors of a lithiated MoO3 nanobelt. Interestingly, we found that the delithiation process occurs in two stages. During the first stage (Figure 3 (a1)–(a2)), the anode shrunk in its width size from 208 nm to 176 nm and the coated Li2O layer was gradually disappeared as a result of the lithium ions extraction. However, the anode still exhibits the morphology with numerous nanograins, as the inset of enlarged anode in Figure 3 (a2) shows. As the delithiation process proceeds, the anode still shrunk which indicates that lithium ions were still extracted from the product after first stage delithiation under the drive of the potential. Notice that the nanograins were gradually vanished, resulting in a smooth surface (Figure 3 (a3)). Figure S2 (Supporting Information) also shows the similar delithiation behaviors of another sample, confirming that the unique two-stage delithiation process is ubiquitous with different samples. To understand the delithiation mechanism in depth, the

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dynamic process also has been given in Movie S3 (Supporting Information), which shows the dynamic microstructure evolution of anode during delithiation, including the disappearing of Li2O layer, the shrink of anode as well as the change and vanish of the nanograins. For further understanding the fundamental reaction mechanism of the lithiated MoO3 nanobelt during delithiation process, EDP patterns in different stages were achieved. Figure 3 (b) represents the EDP of anode in first stage (Figure 3 (a2)), in which the diffraction rings can be perfectly indexed as Li1.66Mo0.66O2 (JCPDS no.48-0003). The corresponding HRTEM image given in Figure 3 (c) shows lattice spacing of about 2.07 Å, in accordance with the (104) plane of Li1.66Mo0.66O2, further confirming the above conclusion. Since no Li2O phase was detected from the EDP and HRTEM, we believe that the Li2O was almost consumed up during the first stage. In the second stage, lithium ions were extracted from the anode continually under the drive of the potential, while the atomic ratio of Mo/O keeps a constant of 1:3, same to that in Li1.66Mo0.66O2, suggesting that the final delithiation product of MoO3 nanobelt anode is LixMoO3. The EDP (Figure 3 (d)) of the final anode after delithiation process shows diffused ring which reveals the amorphous delithiation product. EELS spectrum can be used to investigate the valence states of Mo as the M3,2 edges exhibit distinct valence-specific shapes.32 Here, the EELS spectrum of a fully delithiated anode (Figure 3 (e)) shows that the M3/M2 intensity ratio of Mo element is about 2.19, in agreement well with the valence state of 4+.32 Combined with the above analysis, we believe that the product is amorphous Li2MoO3. Although previous reports have shown that the reversible microstructure evolution of α-MoO3 during electrochemical lithiation/delithation process is between Mo and LixMoO3, no evidences have been used to confirm the specific product.15, 10


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33-34

Herein, we revealed that α-MoO3 nanobelt undergoes a unique two-stage delithiation

process and pointed out that the specific final product is amorphous Li2MoO3 by HRTEM, EDP and EELS techniques for the first time. The result is credible as EELS technique is an accurate tool to investigate the valence states of chemical elements. Based on above analysis, we hold opinion that the lithiated anode undergoes a two-stage phase change from Mo nanograins to Li1.66Mo0.66O2 nanograins followed by the transformation to amorphous Li2MoO3 during the first delithiation process, which can be expressed by the following equations:

Mo + 3 Li2O → 1.5 Li1.66 Mo 0.66O 2 + 3.5 Li+ + 3.5 e −1 1.5 Li1.66 Mo 0.66O 2 → Li 2 MoO 3 + 0.5 Li + + 0.5 e −1

Accordingly,

the

overall

reaction

during

delithiation

can

be

expressed

as Mo + 3 Li 2O → Li 2 MoO 3 + 4 Li + + 4 e −1 . This result is consistent with the previous conclusion that the amount of extraction lithium is about four in each cell during first delithiation process.23, 35 The delithiation process cannot convert the anode into its original state, resulting in an irreversible microstructure change and thus huge capacity loss during first cycle.

Previous studies indicate that the decomposition of Li2O is attributed to the electrocatalysis of the nanograins in the Li2O matrix. In the initial stage of the delithiation process, the small size effect of the Mo nanograins caused by the increased number of surface atoms can greatly enhance the electrochemical reactivity and make the conversion from Li2O to Li+ and O2-.10,36 Then Mo nanograins reacted with O2- and parts of Li+, thus forming Li1.66Mo0.66O2 nanograins 11



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during first stage. Meanwhile, about 3.5 Li+/Mo were extracted from the lithiated anode, accounting for the volume shrink during delithiation process. After the full conversion from Mo to Li1.66Mo0.66O2, Li+ were continually extracted from Li1.66Mo0.66O2 under the drive by the positive potential with the volume shrink, consequently resulting in amorphous Li2MoO3.

The microstructure evolution of a segment of α-MoO3 nanobelt during first two electrochemical lithiation/delithiation processes were investigated and displayed in Figure 4 to well understand the conversion mechanism during charging and discharging cycles. Figure 4 (a) and (a1) confirms the single crystalline feature of the pristine α-MoO3 nanobelt anode with a width of 56.1nm. In the following cycles, the constant potential of -2 V/3 V was applied to the α-MoO3 nanobelt to initiate the electrochemical lithiation/delithiation behaviors. After first lithiation, the width size of the anode was increased to 71.3nm with a size expansion of 27.1%, and a thin Li2O layer was visible on the surface of the lithiated nanobelt. This size expansion is a little smaller than the MoO3 nanobelts in Figure 2, which mainly attribute to the difference of local Li+ concentration. Meanwhile, the microstructure of the α-MoO3 nanobelt was evolved to numerous Mo nanograins in the Li2O matrix from a single crystal. The diffraction rings in the EDP of a lithiated anode (Figure 4 (b1)) further confirmed that the lithiation products are the mixture of Mo and Li2O. During first stage delithiation, the Li2O layer disappeared and the anode shrunk its size to 58.2 nm, as Figure 4 (c) shows. The EDP (Figure 4 (c1)) with diffraction rings confirmed the product of Li1.66Mo0.66O2. For the second delithiation stage, the anode further shrunk to 56.2 nm (Figure 4 (d)) and the final delithiation product are amorphous Li2MoO3 by the EDP in Figure 4 (d1). The anode after second lithiation process is displayed in Figure 4 (e), from that we can see 12


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the width size was expanded to 67.9 nm and Li2O layer was formed on the anode surface again. The size expansion is only about 20.8%, smaller than 27.1% during first lithiation process. This behavior may be mainly attributed to the fact that the formed Li2MoO3 after first cycle can accommodate less lithium ions than MoO3. EDP recorded from the second lithiated anode (Figure 4 (e1)) indicates that the products are Mo and Li2O, just like the first lithiation process. The second lithiation reaction can thus be expressed by the following equation: Li 2 MoO 3 + 4 Li + + 4e −1 →Mo + 3Li 2 O . Druing the second delithiation process, the Li2O layer disappeared again and the anode shrunk its width size to 57.1 nm after first delithiation stage and further decreased to 53.6 nm after fully delithiation, as shown in Figure 4 (f) and Figure 4 (g), respectively. The corresponding EDPs in Figure 4 (f1) and Figure 4 (g1) represent the similar feature with that in Figure 4 (c1) and Figure 4 (d1), suggesting that the electrochemical behavior during second delithiation process is the same as the first one. With the above discussions, we can conclude that the low coulomb efficiency and large capacity loss in first cycle is mainly due to the incompletely re-oxidized process of Mo to Li2MoO3 (Mo4+) rather than MoO3 (Mo6+). Meanwhile the better capacity retention and cycling performance during the subsequent cycles can be attributed to the relatively reversible phase and size change, as well as the improved conductivity of Li2MoO3 compared to α-MoO3 nanobelt.37

EELS performed in TEM is a well-established technique to quantificationally analyze the chemical at the nanoscale by calculating the intensity ratio from white-lines edges. Here, we used M2,3 edges to obtain the valence state information of Mo element in the first two lithiation/delithiation processes as it can keep a low experimental dispersion caused by the 13



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proximity between O-K and Mo-M2,3 edges and shows distinct valence-specific shapes.32 Figure 5 (a) shows the EELS spectrum of an initial α-MoO3 nanobelt in which the Mo-M3/M2 intensity ratio is about 1.95, corresponding to the valance state of 6+.32 The Mo-M3/M2 intensity ratio increased to 2.54 after first lithiation process (Figure 5 (b)), suggesting the formation of metal Mo. From the EELS spectrum of a delithiated anode in Figure 5 (c), we can see the Mo-M3/M2 intensity ratio decreased to 2.2, which agrees well with the valance state of 4+.32 The result implies the amount of the extraction lithium ions is only four for each cell, resulting in an irreversible phase conversion and thus large capacity fading in the first cycle, in agreement with the above conclusions. Figure 5 (d) shows that the Mo-M3/M2 intensity ratio was 2.5 after second lithiation process, similar to the first lithiation result, indicating that the second lithiation product is the same as the first lithiation one. In addition, from the EELS spectrum in Figure 5 (e), the Mo-M3/M2 intensity ratio after second delithiation process is calculated to 2.23, elucidating the 4+ valance state of Mo element, which is identical to the result after first cycle. The repeated valance state evolution of Mo elements revealed the reversible phase change between Mo0+ and Mo4+ during electrochemical lithiation/delithiation processes. These observations further confirmed the results that anode shows an irreversible microstructure evolution during first cycle but a reversible phase change between Mo and Li2MoO3 during subsequent cycles that from HRTEM images and EDP. Overall, the electrochemical reactions of α-MoO3 during lithiation and delithiation processees can be expressed as:

MoO3 + 6Li+ + 6e −1 → Mo + 3 Li2O (first lithiation)

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Mo + 3 Li2O ↔ Li2 MoO3 + 4Li+ + 4e −1

(subsequent delithiation/lithiation ) The theoretical lithium storage capacity of Li2MoO3 is calculated to be about 734 mAh g-1 according to its weight and the ability of accommodating lithium ions, which is much lower than MoO3 of 1117 mAh g-1. That is to say, the irreversible phase conversion during first electrochemical lithiation/delithiation cycle will lead to a theoretical capacity loss of 383 mAh g-1 after first cycle. In order to study the influence of irreversible phase conversion on capacity fading in first cycle, we checked other reports that used pure α-MoO3 as anode material for LIBs. The first cycle capacity loss was found to 760 mAh g-1 for ball-milled MoO3,35 370 mAh g-1 for porous MoO3 film,38 650 mAh g-1 for MoO3 nanoparticles,19 500 mAh g-1 for MoO3 nanorods,21 560 mAh g-1 for short-bar-like MoO3 particle,8 728 mAh g-1 for MoO3 nanobelt film,15 406 mAh g-1 for MoO3 nanobelt 17 and so on. The average capacity loss in first cycle can be thus calculated to about 568 mAh g-1 for pure MoO3. This result suggests that about 68% of the capacity loss of MoO3 in first cycle can be attributed to the irreversible phase conversion. Meanwhile, about 32% of the initial capacity loss is caused by other irreversible electrochemical behaviors such as the formation of SEI layer. The good capacity retention and cycling performance during subsequent cycles can be attributed to the reversible microstructure change between Mo and Li2MoO3 as well as the enhanced conductivity of Li2MoO3 than MoO3.

4.

CONCLUSIONS

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In summary, the electrochemical lithiation/delithiation behaviors of an individual α-MoO3 nanobelt was studied by in situ TEM tecnique for the first time. It was found that the α-MoO3 nanobelt was converted into many Mo nanograins within the Li2O matrix after first lithiation process. Interestingly, we found that the delithiation process, where the anode shrunk in volume, occurred in two stages. Mo nanograins were first transformed into crystalline Li1.66Mo0.66O2 nanograins which further converted into the amorphous Li2MoO3. The irreversible phase conversion from original MoO3 (1117 mAh g-1) to the final Li2MoO3 (734 mAh g-1) is suggested to be responsible for ~68% total capacity loss of MoO3 anode in first cycle, while the remaining ~32% capacity loss can be attributed to other irreversible electrochemical behaviors such as the formation of SEI layer. Our results also reveraled a reversile phase conversion between amorphous Li2MoO3 and crystalline Mo, accounting for the good capacity retention and improved cycling performance during the subsequent cycles. Overall, our findings provide direct evidence and new insights about the fundamental nature of electrochemical lithiation/delithiation behaviors of the α-MoO3 nanobelt, with the hope that it will assist the design of improved performance LIBs anode material.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx.

Additional SEM images of the as-prepared α-MoO3 nanobelt (PDF)

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Time-lapse morphology evolution of another α-MoO3 anode during first delithiation process (PDF)

The phases that involved in the cycles and the corresponding d-spacing (PDF)

Lithiation of the α-MoO3 nanobelt corresponding to Figure 2b (AVI)

Lithiation of the α-MoO3 nanobelt corresponding to Figure 2c (AVI)

Delithiation of the α-MoO3 nanobelt corresponding to Figure S2 (AVI)

AUTHOR INFORMATION

Corresponding Author *

Phone: +86 25 83792632. Fax: +86 25 83792939.

E-mail: [email protected] (F. Xu)；[email protected] (L.T. Sun).

Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program, Grant No. 2015CB352106), the National Natural Science Foundation of China (NSFC, Grant Nos. 61574034, 51372039, 91333118, 61274114, 11525415, 11327901, 11374332 and 51420105003), the Jiangsu Province Science and Technology Support Program (Grant No. 17



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(a)

(b)

(c)

(d)

(e)

(f)

Figure 1. (a) Photograph, (b) XRD spectrum, (c) the crystal structure of the as-prepared MoO3. (d) Scanning electron microscope (SEM) image and (e) TEM image confirm that the α-MoO3 is belt-like morphology. The inset in (d) is the HRTEM image of a MoO3 nanobelt, in which the clear and ordered lattice fringes suggest the good crystallinity. EDP in (f) indicates that the α-MoO3 nanobelt shows single crystalline feature.

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(b1)

(a)

(c1)

(c2)

(c3)

(b2)

(c4)

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(b3)

(d)

(e)

Figure 2. Experimental setup and the microstructure evolution of the MoO3 nanobelt during first lithiation. (a) Schematic illustration of the in situ TEM cell. (b1)–(b3) Typical morphology change during lithiation with both width swelling and axial elongation in a low magnification. The pristine MoO3 nanobelt was straight (b1) and bending deformation occurred during lithiation. The red arrows indicate the lithiation fronts. (c1)–(c4) Close view of the microstructure evolution of a segment of another MoO3 nanobelt during lithiation. The straight MoO3 nanobelt (c1) was just swelled (c2) at the beginning of lithiation, and then coated with Li2O layer (c3) which was increased as the lithiation process proceeded ((c1)–(c4)). The insets in (c2) and (c3) is the enlargement of the anode marked by the red boxes in (c2) and (c3), respectively. (d) HRTEM image and (e) EDP of a lithiated anode show that the lithiation product is the mixture of Mo and Li2O.

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(a1)

(b)

(a2)

(c)

(a3)

(d)

(e)

Figure 3. Microstructure evolution of the MoO3 nanobelt during first delithiation. (a1)–(a3) Close view of the microstructure evolution of a segment of lithiated MoO3 nanobelt during delithiation. (b) EDP (c) HRTEM image of the anode in (a2). (d) EDP (e) EELS spectrum of the fully delithiated anode in (a3). MoO3 nanobelt was found to undergo a two-stage delithiation process. Firstly, the coated Li2O layer gradually disappeared, and the size of the lithiated anode was shrunk as the delithiation process proceeded. In this stage, Mo nanograins were converted into crystalline Li1.66Mo0.66O2. Afterwards, Li1.66Mo0.66O2 vanished followed by the formation of amorphous Li2MoO3, accompanying with size shrunk of anode.

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(a)

(b)

(b1)

(e)

(e1)

Pristine

1st lithiation (c)

(a1)

1st delithiation (d) Intermediate

(c1)

2nd lithiation (f)

1st delithiation

(d1)

2nd delithiation (g) Intermediate

(f1)

2nddelithiation

(g1)

Figure 4. Microstructure evolution of MoO3 nanobelt in first two lithiation and delithiation processes. (a) The pristine MoO3 nanobelt with a size of 56.1 nm. (b) The first lithiated and 28


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(c, d) delithiated MoO3 nanobelt. (e) The second lithiated and (f, g) delithiated MoO3 nanobelt. (a1)–(g1) EDP recorded from the according MoO3 nanobelt anode in (a)–(g), revealing the phase conversion during first two cycles.

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(a)

(b)

(c)

(d)

(e)

Figure 5. EELS spectra of Mo/M3,2 edges in the first two electrochemical lithiation/delithiation cycles. (a) Original stage; (b,c) after the first (b)lithiation (c) delithiation process; (d,e) after the second (d)lithiation (e) delithiation process.

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TOC GRAPHIC In situ TEM observations uncover that α-MoO3 anode undergoes unique two-stage delithiation processes and irreversible microstructure change during first cycle. 52x18mm (300 x 300 DPI)


New Insights into Electrochemical Lithiation ... - ACS Publications

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