Delithiation Behaviors of

Feb 29, 2016 - Visualizing the Electrochemical Lithiation/Delithiation Behaviors of Black Phosphorus by in Situ Transmission Electron Microscopy. Weiw...
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Visualizing the Electrochemical Lithiation/Delithiation Behaviors of Black Phosphorus by in Situ Transmission Electron Microscopy Weiwei Xia,†,# Qiubo Zhang,†,# Feng Xu,*,†,‡ Hongyu Ma,§ Jing Chen,∥ Khan Qasim,∥ Binghui Ge,⊥ Chongyang Zhu,† and 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 § Research Center for Internet of Things, China University of Mining and Technology, Xuzhou 221008, China ∥ School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China ⊥ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Black phosphorus (BP) has drawn growing attention as the anode material for lithium-ion batteries (LIBs) because of its high theoretical lithium storage capacity. However, its electrochemical processes and fundamental failure mechanisms have not been completely understood due to the lack of direct evidence. Here, we report the direct visualization of the electrochemical lithiation/delithiation behavior of the BP anode in nano-LIBs using the in situ transmission electron microscopy technique. Upon lithiation, the BP anode is found to undergo obvious anisotropic size expansion and phase change from orthorhombic BP to amorphous LixPy compounds. Unexpectedly, the BP anode pulverizes suddenly during discharging, resulting in irreversibility of the lithiated product and thus poor electrochemical cycling performance. This finding discloses that the failure mechanism of the BP anode is mainly correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. The new mechanism insights would serve to provide viable solutions for eliminating rapid capacity fading that plagues the bulk BP LIBs. diffusion.21 These superior properties make BP a promising alternative as anode material for high performance LIBs. The LIBs with BP as anode material, however, show some limitations like the extremely low Coulombic efficiency (∼8%) and rapid capacity fading in the first charge/discharge cycle.19 In order to overcome the restrictions and improve the electrochemical cycling of the bulk BP anodes, great efforts have been devoted so far.16,19,20,22 Various carbon materials were incorporated into BP to form a robust P−C composite, improving the conductivity and ensuring good electrical connection between the active anode materials and current collectors.16,20 Although these methods have improved the electrochemical performance of the BP anodes, however, a few fundamental mechanisms concerning electrochemical processes still remain ambiguous due to the lack of direct evidence. Generally, the traditional methods, which can only provide the ex situ investigation of anode material, lack the ability to in situ

1. INTRODUCTION Improving the energy density, power density, and cycling performance of lithium-ion batteries (LIBs) are of great importance as the LIBs can be widely used as power sources for electronic devices and renewable power stations.1−3 At present, the commercial graphite anode material, with a moderate theoretical capacity of 372 mAh g−1 (LiC6), slows down the rapid development of high energy storage systems. Anode materials (such as metals,4,5 nonmetals,6−8 oxides,9−11 sulfides,12,13 phosphides,14 nitrides,15 etc.) that are believed to have high Li-uptake ability are thus sought for next-generation LIBs to meet the rapid development of the electronics industry. Recently, phosphorus has drawn increasing attention for LIBs because of its low atomic weight (Z = 15) and high theoretical capacity of 2596 mAh g−1 (Li3P).16 Typically, phosphorus has three mainly allotropes: white phosphorus (WP), red phosphorus (RP),17 and black phosphorus (BP). BP is the most thermodynamically stable form among them.16 With a special puckered double-layer structure,18,19 BP has high electronic conductivity (∼100 S/m)20 and large interlayer distance (0.54 nm) which serves to provide a faster lithium © XXXX American Chemical Society

Received: November 16, 2015 Revised: February 28, 2016

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Figure 1. (a) Schematic illustration of the in situ experimental setup which is used for directly observing the electrochemical behaviors of the BP sheet in the lithiation/delithiation process. (b) The corresponding TEM image of the nano-LIB constructed inside the TEM, in which white arrows indicate the possible diffusion pathway for lithium. (c) EDP of pristine orthorhombic BP, indicating the good crystallinity of the BP anode.

Figure 2. TEM images of the time-lapse size evolution of the BP anode along different directions during the first lithiation, showing obvious anisotropic size expansion.

2. EXPERIMENTAL SECTION Bulk BP was purchased from Smart Elements (product no. 003058). The constructions of a nanobattery, which would be used to study the electrochemical behavior of the BP anode during the first charging/discharging cycle inside TEM, are described in detail in the following. Anode material-irregular BP sheets were scraped from the bulk BP and attached to a Au rod by conductive silver colloid to ensure a good electrical contact. Metal lithium was adhered to tungsten and regarded as counter electrode and lithium source, while the natural oxide layer Li2O acted in the role of solid electrolyte which allows the transport of lithium ions. The building of the nanobattery was accomplished in a glovebox that filled with argon gas as lithium is a kind of very active metal which can be oxidized easily. Afterward, the holder was transferred into the TEM column immediately. The probe could be driven by the piezopositioner to make the Li/Li2O and BP sheet in contact with each other. During charging/discharging, a constant potential of −2 V/3 V was applied to the BP anode with respect to the lithium counter electrode to observe the electrochemical behaviors. The applied potential can drive the transport of lithium ions through the solid-state electrolyte Li2O layer, thereby ensuring the process of electrochemical reaction. Highresolution TEM (FEI Titan, 300 kV) with a fast responding

visualize the dynamic electrochemical lithiation/delithiation process that plagues the bulk BP LIBs. Recently, Cui et al. had done some in situ works focused on the sodiation behavior of BP,23 but no in situ observations of the electrochemical lithiation/delithiation process have been achieved so far. Thus, the nature of irreversible electrochemical processes and underlying mechanism behind the rapid capacity fading are still not understood completely. In this paper, we introduce the in situ transmission electron microscopy (TEM) technique to study the electrochemical lithiation/delithiation behaviors of the BP anode by constructing nano-LIBs. The rapid development of the in situ TEM technology in recent years makes it possible to realize real-time visualization of the microstructure evolution and phase change of electrode materials.13,24−29 Here, the in situ TEM technique is used to observe the volume expansion, phase change, and morphology evolution in the first lithiation/delithiation cycle of the BP anode. The failure mechanism that plagues the bulk BP anode is found to be mainly correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. The new mechanism insights would serve to seek viable solutions for improving the electrochemical cycling performance of the bulk BP LIBs. B

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concentration difference should be attributed to the lithium ions diffusion rate along different directions. Generally, the zigzag direction and armchair direction in layer structure BP are most studied.32 The migration barrier of Li diffusion on multilayer BP is about 0.05 eV along the zigzag direction, whereas, for the armchair direction, the barrier is about 0.2 eV.32 According to the following eq 1, diffusion constant D can be calculated as

charge-coupled device (CCD) camera was carried out for the real-time observation of morphology, size, and phase evolutions of the BP anode during the first lithiation/delithiation process with the assistance of electron diffraction pattern (EDP), HRTEM imaging, and EELS mapping measurements.

3. RESULTS AND DISCUSSION The schematic illustration of the electrochemical experimental setup which is used to in situ observe the electrochemical behaviors of the BP sheet in the lithiation/delithiation process is shown in Figure 1a. This electrochemical nano-LIBs device consists of a BP sheet anode that is attached to a Au rod, a metal lithium counter electrode adhered to the tungsten probe, and an all-solid electrolyte Li2O which allows the transport of lithium ions. The tungsten probe would be driven by the piezopositioner at fine steps to contact the BP anode. The lithiation/ delithiation process can be initiated by applying a constant negative/positive potential to the BP sheet anode with respect to the metal lithium counter electrode. The corresponding TEM image of a nano-LIB inside the TEM is shown in Figure 1b, where the white arrows denote the possible diffusion directions of lithium ions in BP. Figure 1c shows the EDP of a pristine BP sheet. The diffraction spots that can be indexed as the orthorhombic structure BP (JCPDS no. 74-1878) indicate the good crystallinity of the anode material, which is necessary to improve the electrochemical activity of BP LIBs. Figure 2 shows the time-resolved TEM images of the BP anode during the first lithiation process from Video V1 (Supporting Information). Lithium ions were inserted into the BP interlayer at the first step and then reacted with BP to form a new phase, resulting in lattice expansion and morphology evolution. The BP anode in Figure 2a−f was fully lithiated within 60 s as the anode never changed in size and morphology after 60 s even if the positive potential was applied for a longer time. The size was increased to 900 nm from 600 nm after lithiation and showed a size expansion of about 50% along the L1 direction, while the size expansion along the L2 direction was almost negligible. (The L1 and L2 directions are defined in Figure 2a.) The size expansion of BP exhibited obvious anisotropy during lithiation, increasing the difficulty to estimate volume expansion after lithiation because the size expansion in the thickness direction is unknown. As the previous studies report, crystalline Si also shows a huge anisotropic size expansion with significant swelling along the (110) direction, but negligible expansion along the (111) direction. The result can be attributed to that the invasion of the lithium ions is the slowest in the (111) direction but fastest in the (110) direction.30 Through in-depth in situ study, Liu et al. found that the anisotropic expansion is controlled by the orientation-dependent mobility/reaction rate at the phase boundary between the crystalline reactant and amorphous product.31 However, this mechanism is not suitable for explaining the anisotropic size expansion of the BP anode, as the intercalation mechanism of lithium ions in BP is very different from Si. BP has a unique layer structure and large interlayer distance (0.54 nm), providing an open channel for the lithium ions diffusion. As a result, lithium ions can be inserted into the BP interlayer along the channels during lithiation, rather than the lattice planes like Si. Meanwhile, no distinct phase boundary between the crystalline reactant and amorphous product as Si shows was observed during lithiation process. Therefore, we believe that the anisotropic size expansion in BP that is caused by the local lithium ions

⎛ −E ⎞ D ∼ exp⎜ a ⎟ ⎝ KBT ⎠

(1)

where Ea is the diffusion barrier and KB is the Boltzmann constant (1.38 × 10−23 J/K). T is the environment temperature (293 K in the experiment). On the basis of eq 1, the diffusion constant of lithium along the zigzag direction is about 300 times larger than that along the armchair direction. This means that the diffusion of lithium in the BP sheet along the armchair direction is almost forbidden, accounting for the obvious anisotropic size expansion. The size changes of the BP sheet in Figure 2a−f along L1 and L2 directions with charging time are plotted in Figure 3.

Figure 3. Size changes of the BP sheet corresponding to Figure 2 along L1 and L2 directions as a function of time during the first lithiation process.

The lithiation ratio of BP which can be reflected by the slope of the curve is not constant. Along the L1 direction, the lithiated ratio is about 13.2 nm/s during first 16 s and this value is lowered to 2.02 nm/s as the lithiation process proceeded. Slight size changes along the L2 direction during lithiation may be mainly caused by tilting of the BP sheet and measuring error. To verify the universality of the anisotropic size expansion, another BP sample was investigated, and its size evolution as the lithiation process proceeded is given in Figure S1 in the Supporting Information. As we can see from these TEM images, the anode size increased from 326 to 600 nm along the L2 direction, with a size expansion of 84%; meanwhile, the anode expanded its size from 1.4 to 1.62 μm along the L1 direction. This size expansion can be calculated to about 15.7%, showing an obvious anisotropic size expansion. Consequently, we believe that the anisotropic size expansion is a common issue of the BP anode in the lithiation process, although sometimes we cannot calculate the specific size of all samples because of the bend, tilt, and crimp behaviors. The “reaction front” which can be reflected by red dashed lines (i.e., the boundaries between the lithiated regions and the BP) during the lithiation process is visualized in Figure 4 and Video V2 (Supporting Information). Before lithiation, distinct C

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Figure 4. (a−d) The TEM images of the time-lapse morphology evolution during the first lithiation of the BP anode. Lithiation began from one side of the BP sheet, and the “reaction front” reflected by the red lines propagates toward the other side, resulting in a gray contrast and smooth surface due to the formation of new product. Enlarged TEM images of (e) pristine BP anode and (f) lithiated anode in the selected black boxes of (a) and (d), showing great morphology evolution from wrinkled structure to smooth surface.

Figure 5. Time-resolved TEM images from video frames show microstructure evolution of the BP sheet during the first cycle. (a) Pristine BP sheet; (b−e) charging process; (f−h) discharging process. The anode cracking and pulverization accompanying the huge size increase appeared in the delithiation process, greatly different from the commonly accepted understanding. (i) Typical magnified TEM image of the delithiated anode, showing obvious cracking.

D

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Figure 6. (a1) EELS mapping, (a2) EDP, and (a3) HRTEM image of the lithiated BP anode, which show that the product after lithiation is of an amorphous feature in most regions with gray contrast and a smooth surface. Meanwhile, the diffraction spots and clear and ordered lattice fringes that can be indexed to the orthorhombic BP are also observed only in “wrinkles” parts, suggesting the existence of unlithiated BP. (b1) EELS mapping, (b2) EDP, and (b3) HRTEM image of the BP anode after discharging. The predominant diffraction rings and lattice fringes are ascribed to the hexagonal Li3P, indicating the generation of Li3P during delithiation. Part of diffraction rings also can correspond to the orthorhombic BP which originates from the unlithiated BP during charging.

“wrinkles” can be observed on the surface of the BP anode (Figure 4a,e) due to the layer structure and nonuniformity of stress. After applying a negative potential to the BP anode, the “winkles” gradually disappeared and presented a gray contrast, indicating that lithiation had begun and propagated toward the other side. The lithiated area can be readily distinguished from the remaining BP area by “reaction front”, as shown in Figure 4b. The gray contrast in the lithiated area may be attributed to the formation of a new phase. Figure 4f is a representative magnified TEM image of the lithiated BP anode. It can be seen clearly that the lithiated anode surface became smoother compared with pristine BP (Figure 4e), even though some “wrinkles” still existed. According to the previous reports, corrugated graphene can be turned into flat because of the large stress induced by the lithium intercalation into the (0002) plane. The volume expansion of graphene is not only the result of real lattice expansion but also the influence of stress change.36 Here, as BP has a similar layer structure with graphite, we, therefore, surmise that the huge size expansion in the lithiation process is mainly caused by the lattice expansion and, moreover, also is slightly affected by the stress changes. The BP sheet here showed an inconspicuous size expansion, due to the fact that the BP sheet in the area marked with red dashed circles in Figure 4c,d was bent as the lithiation process proceeded owing to the limited distance between the Au anode and Li/ Li2O. It is interesting to note that cracking which appeared in most anode materials was absent for the BP anode during lithiation despite the huge size expansion. It is worth noting that the reaction front can be observed clearly in Figure 4 but is indistinct in Figure 2. This result may be caused by the different “intercalation direction” and the resulting diffusion ratio difference of lithium ions. In detail, if the “intercalation direction” is beneficial to the fast transport of lithium ions between the BP interlayer (for example, zigzag

direction), the generated stress can be released in time and, therefore, the inconspicuous lithiation front. On the contrary, if the intercalation of lithium ions is along the other direction (for example, armchair direction) that provides a low diffusion rate for lithium ions caused by the high barrier energy, which will lead to a huge stress concentration, and the anode would show an obvious lithiation front. Not all the sample can show an obvious lithiation front during lithiation because it is impossible to control the intercalation direction of lithium ions into BP inside TEM. Nevertheless, the main aim of proposing the lithiation front in this paper is to indicate the morphology evolution before and after lithiation. The morphology change from “wrinkles” to “smoother” and the resulting gray contrast after the lithiation process, which is reflected by the reaction front in Figure 4, can also be observed in Figure 2. Figure 5 and Video V3 (Supporting Information) show the microstructure evolution of the BP anode during the first charging/discharging. Obviously, similar to the above analysis, the BP anode also showed anisotropic size expansion during lithiation (Figure 5a−e). Notably, the sheet in Figure 5d,e seems to be smaller than that in Figure 5c, which is attributed to the tilting and crimping. For delithiation processes (Figure 5f−h), it is worth noting that cracking arose in the anode accompanied by a huge size increase in only 1 s after applying a constant positive potential to the BP anode relative to the lithium counter electrode(Figure 5f). As the delithiated process proceeded, the sheet size along the horizontal direction was further increased, and meanwhile, the crimping at the edge was stretched out. The size increase behavior during the delithiation process cannot be called “expansion” because it is caused by the anode cracking and pulverization, not the same as the volume expansion owing to the lattice expansion during the lithiation process. It can be calculated that the size of the BP anode expanded from 715 to 2166 nm along the horizontal direction E

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revealing that the lithium was not extracted from the lithiated anode. The black areas in the delithiated anode represent the existence of cracking, consistent with the observation in Figure 5i. Figure 6b2 shows the EDP of the BP anode after discharging. The diffraction rings can be perfectly indexed as the hexagonal Li3P (JCPDS no. 74-1160), indicating that the crystalline Li3P was produced by the transformation of amorphous LixPy during discharging. The HRTEM image (Figure 6b3) of the anode after discharging further confirmed the formation of Li3P. Notably, the diffraction rings in Figure 6b2 can also partly correspond to the orthorhombic BP (JCPDS no. 74-1878). Besides the unlithiated BP during charging, the appearance of BP after discharging may also originate from the product caused by the extraction of lithium from the lithiated anode. According to the above analysis, the BP anode suffers from a unique delithiation mechanism during discharging, which will be expressed by the following eqs 3 and 4.

with an overall expansion of about 77% after the whole cycle. In contrast, the size along the vertical direction was almost unchanged. These results indicated that the BP anode showed anisotropic size increase in the entire electrochemical process, which is greatly different from previous reports that almost all the anode materials have volume shrink during delithiation (such as Si,33 Ge,34 Sn,35 CoS2,12 Ce2O,36 and so on) as a result of the extraction of lithium ions. Although the volumes of these anode materials are not completely reversible, the shrinkage of the anode volume can guarantee the capacity retention and the reversibility of the lithiation/delithiation process to some extent. On the contrary, the BP sheet used here suffered from a huge size increase and cracking (Figure 5i) during discharging, which would lead to the loss of electrical contact between the active material and current collectors and, thereby, rapid capacity fading. Furthermore, it must be noted that the electrochemical behavior during the discharging process may be the greatest cause of the BP LIBs’ failure and more attention should be paid than before to get advanced BP LIBs. Although the anomalous size increase during the delithiation process is an unexpected result, it is not a special case in Figure 5. Actually, we have observed this phenomenon for all the samples. Another representative electrochemical lithiation/ delithiation process of the BP anode is given in Figure S2 in the Supporting Information. We also observed the similar size increase and anode pulverization during the delithiation process from TEM images, confirming that the distinct electrochemical delithiation behavior of BP is reproducible for different samples. Studying the phase change of the BP anode during the charging/discharging process is of great importance as the phase change is the basic cause of the size expansion, morphology change, and microstructure evolution. EELS mapping of the lithiated anode is shown in Figure 6a1, in which phosphorus and lithium distribute evenly on the anode surface. This means that lithium and phosphorus reacted with each other and formed a new product during lithiation. Figure 6a2 shows the EDP of the lithiated anode. The diffused rings denote that the amorphous phase was generated, while the diffraction spots indicate that crystalline parts still remained. In addition, the diffractions spots can be indexed as the (020), (111), (012), and (024) planes of orthorhombic BP, indicating that the BP sheet cannot be lithiated completely during charging. To better understand the lithitation product, a HRTEM image of the lithiated anode was further performed in Figure 6a3 and Figure S3 (Supporting Information). From that, clear lattice fringes of 0.256 and 0.337 nm were only observed in the “wrinkles” parts, corresponding to the (111) and (012) planes of BP, respectively (JCPDS no. 74-1878), which further confirmed the presence of BP. In contrast, most areas with gray contrast and a smooth surface exhibited an amorphous feature. Overall, this evidence confirmed that the crystalline parts in the lithiated anode are not generated during charging and the lithiation product is amorphous LixPy. However, we failed to detect the exact x and y due to the lack of structure information caused by the amorphous nature. Except for the unlithiated BP, the reaction between lithium ion and BP during the first lithiation process can be expressed as the following eq 2: x Li+ + x e−1 + y P → LixPy

LixPy → y Li3 P + (x − 3y)Li+ + (x − 3y)e−1

(3)

LixPy → y P + x Li+ + x e−1

(4)

Generally, the formation of BP caused by the extraction of lithium from the lithiated anode would lead to the lattice shrink and the resulting volume shrink. However, a huge size increase is found during discharging (Figure 5f−h). This means that the dominant delithiation mechanism of the BP anode is based on eq 3 rather than eq 4, and the phase change from amorphous LixPy to crystalline Li3P is the basic reason for the anode pulverization and size increase. Meanwhile, in accordance with the observation by Cui, the P−P bond cannot be rebuilt at the discharge stage,20 which further indicates that the delithiation behavior based on eq 4 is impossible to happen and the emerging BP after the first cycle is mainly from the unlithiated BP during charging. During the delithiation process, the lithiated anode cannot provide an effective transport pathway for the lithium extraction, leading to the local enrichment of lithium ions and the rapid formation of larger Li3P crystal grains which can be detected by the electron diffraction pattern. These behaviors will lead to the huge stress concentration and the resulting anode pulverization which cause the visualized size increase during the delithiation process. According to the above discussions, the BP sheet demonstrates a distinct electrochemistry reaction mechanism in the first cycle, which can be described as the following eq 5: charging

discharging

BP ⎯⎯⎯⎯⎯⎯⎯→ amorphous LixPy ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ hexagonal Li3P

(5)

In addition, we also found that the hexagonal Li3P was not transformed to BP under the continuous positive potential, which may be attributed to the loss of electrical contact after cracking, as well as the poor electronic conductivity of Li3P.20 In fact, we have discovered that the anode after discharging was easily destroyed by the electron accumulation from electron beam irradiation which cannot be conducted away in time, further confirming the above assumption. Accordingly, the irreversibility of the lithiated product would lead to a highly irreversible lithiation/delithiation process and extremely low first cycle Coulomb efficiency, as reported previously.6,19,20,22 Overall, we believe that the poor electronic conductivity of Li3P and loss of electrical contact after cracking are the basic reasons for the irreversible lithiation/delithiation process and rapid capacity fading. The failure mechanism of the BP anode is mainly correlated with the delithiation process rather than the

(2)

The product after discharging is also characterized in Figure 6b1−b3. As shown in EELS mapping, a large amount of lithium is found in the anode and presents a homogeneous distribution, F

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(3) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31−35. (4) Liang, W.; Hong, L.; Yang, H.; Fan, F.; Liu, Y.; Li, H.; Li, J.; Huang, J. Y.; Chen, L. Q.; Zhu, T.; Zhang, S. Nanovoid Formation and Annihilation in Gallium Nanodroplets under Lithiation-Delithiation Cycling. Nano Lett. 2013, 13, 5212−5217. (5) Li, X.; Dhanabalan, A.; Gu, L.; Wang, C. Three-Dimensional Porous Core-Shell Sn@Carbon Composite Anodes for High-Performance Lithium-Ion Battery Applications. Adv. Energy Mater. 2012, 2, 238−244. (6) Wang, C. M.; Li, X.; Wang, Z.; Xu, W.; Liu, J.; Gao, F.; Kovarik, L.; Zhang, J. G.; Howe, J.; Burton, D. J.; Liu, Z.; Xiao, X.; Thevuthasan, S.; Baer, D. R. In Situ TEM Investigation of Congruent Phase Transition and Structural Evolution of Nanostructured Silicon/Carbon Anode for Lithium Ion Batteries. Nano Lett. 2012, 12, 1624−1632. (7) Gu, M.; Li, Y.; Li, X. L.; Hu, S. Y.; Zhang, X. W.; Xu, W.; Thevuthasan, S.; Baer, D. R.; Zhang, J. G.; Liu, J.; Wang, C. M. In Situ TEM Study of Lithiation Behavior of Silicon Nanoparticles Attached to and Embedded in a Carbon Matrix. ACS Nano 2012, 6, 8439−8447. (8) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6, 1522−1531. (9) Su, Q. M.; Xie, D.; Zhang, J.; Du, G. H.; Xu, B. S. In Situ Transmission Electron Microscopy Observation of the Conversion Mechanism of Fe2O3/Graphene Anode During Lithiation Delithiation Processes. ACS Nano 2013, 7, 9115−9121. (10) Huang, J. Y.; Zhong, L.; Wang, C. M.; Sullivan, J. P.; Xu, W.; Zhang, L. Q.; Mao, S. X.; Hudak, N. S.; Liu, X. H.; Subramanian, A.; Fan, H. Y.; Qi, L. A.; Kushima, A.; Li, J. In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode. Science 2010, 330, 1515−1520. (11) Wang, D. L.; Yu, Y. C.; He, H.; Wang, J.; Zhou, W. D.; Abruna, H. D. Template-Free Synthesis of Hollow-Structured Co3O4 Nanoparticles as High-Performance Anodes for Lithium-Ion Batteries. ACS Nano 2015, 9, 1775−1781. (12) Su, Q. M.; Xie, J.; Zhang, J.; Zhong, Y. J.; Du, G. H.; Xu, B. S. In Situ Transmission Electron Microscopy Observation of Electrochemical Behavior of CoS2 in Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2014, 6, 3016−3022. (13) Wang, L.; Xu, Z.; Wang, W.; Bai, X. Atomic Mechanism of Dynamic Electrochemical Lithiation Processes of MoS2 Nanosheets. J. Am. Chem. Soc. 2014, 136, 6693−6697. (14) Alcántara, R.; Tirado, J. L.; Jumas, J. C.; Monconduit, L.; Olivier-Fourcade, J. Electrochemical Reaction of Lithium with CoP3. J. Power Sources 2002, 109, 308−312. (15) Shodai, T.; Okada, S.; Tobishima, S.-i.; Yamaki, J.-i. Study of Li3‑xMxN (M: Co, Ni or Cu) System for Use as Anode Material in Lithium Rechargeable Cells. Solid State Ionics 1996, 86−88, 785−789. (16) Park, C. M.; Sohn, H. J. Black Phosphorus and Its Composite for Lithium Rechargeable Batteries. Adv. Mater. 2007, 19, 2465−2468. (17) Li, W. J.; Chou, S. L.; Wang, J. Z.; Liu, H. K.; Dou, S. X. Simply Mixed Commercial Red Phosphorus and Carbon Nanotube Composite with Exceptionally Reversible Sodium-Ion Storage. Nano Lett. 2013, 13, 5480−5484. (18) Wang, Z. H.; Feng, P. X. L. Design of Black Phosphorus 2D Nanomechanical Resonators by Exploiting the Intrinsic Mechanical Anisotropy. 2D Mater. 2015, 2, 021001. (19) Stan, M. C.; Zamory, J. V.; Passerini, S.; Nilges, T.; Winter, M. Puzzling out the Origin of the Electrochemical Activity of Black P as a Negative Electrode Material for Lithium-Ion Batteries. J. Mater. Chem. A 2013, 1, 5293−5300. (20) Sun, J.; Zheng, G.; Lee, H. W.; Liu, N.; Wang, H.; Yao, H.; Yang, W.; Cui, Y. Formation of Stable Phosphorus-Carbon Bond for Enhanced Performance in Black Phosphorus Nanoparticle-Graphite Composite Battery Anodes. Nano Lett. 2014, 14, 4573−4580. (21) Yu, X.; Ushiyama, H.; Yamashita, K. Comparative Study of Sodium and Lithium Intercalation and Diffusion Mechanism in Black

lithiation one, which subverts the commonly accepted understanding. This finding will serve to provide viable solutions for eliminating rapid capacity fading that plagues the BP LIBs.

4. CONCLUSIONS In summary, we have constructed nano-LIBs with BP as anode material to in situ investigate their electrochemical lithiation/ delithiation behaviors inside TEM. Direct observations on the morphology and structure as well as phase evolution of the BP anode were achieved. Upon lithiation, orthorhombic BP was transferred into amorphous composite LixPy accompanied by a huge anisotropic size expansion. Moreover, the lithiated anode suffered from a huge size increase and cracking during delithiation. The appearance of cracking is supposed to be responsible for the loss of electrical contact and, thereby, a rapid capacity fading, while the generation of Li3P rather than BP after discharging is considered to account for the irreversible lithiation/delithiation process and poor cycling performance. Overall, our findings have disclosed that the failure mechanism of the BP anode is extremely correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. Our work will serve to provide viable solutions for eliminating rapid capacity fading that plagues the BP LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11218. TEM images (PDF) Supplementary video (AVI) Supplementary video (AVI) Supplementary video (AVI)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 83792632. Fax: +86 25 83792939. E-mail: [email protected]. Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.X. thanks Feng Wang and Wei Zhang from Brookhaven National Lab for their help in the in situ TEM technique. 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, 11374332, and 91333118), the Jiangsu Province Science and Technology Support Program (Grant Nos. BK20141118 and BK20151417), and the China Postdoctoral Science Foundation Funded Project (Grant Nos. 2014M550259 and 2015T80480).

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DOI: 10.1021/acs.jpcc.5b11218 J. Phys. Chem. C XXXX, XXX, XXX−XXX