In Situ TEM of Phosphorus-Dopant-Induced Nanopore Formation in

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In Situ TEM of Phosphorus Dopant-Induced Nanopore Formation in Delithiated Silicon Nanowires Jiakun Zhu, Mohan Guo, Yuemei Liu, Xiaobo Shi, Feifei Fan, Meng Gu, and Hui Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20436 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Applied Materials & Interfaces

In Situ TEM of Phosphorus Dopant-Induced Nanopore Formation in Delithiated Silicon Nanowires

Jiakun Zhu1, #, Mohan Guo2, #, Yuemei Liu1, Xiaobo Shi2, Feifei Fan3, 4, Meng Gu2*, Hui Yang1*

1Department

of Mechanics, Huazhong University of Science and Technology, Wuhan,

Hubei 430074, China 2Department

of Materials Science and Engineering, Southern University of Science and

Technology, No. 1088 Xueyuan Blvd, Shenzhen, Guangdong 518055, China 3Department of Mechanical Engineering, University of Nevada, Reno, Reno, NV 89557,

USA 4Nevada

Institute for Sustainability, University of Nevada, Reno, Reno, NV 89557,

USA

#Jiakun

Zhu and Mohan Guo contributed equally to this work.

*Corresponding author: [email protected] ; [email protected] ;

ABSTRACT: Through in situ transmission electron microscopy (TEM) observation, we report the behaviors of phosphorus (P) - doped silicon nanowires (SiNWs) during electrochemical lithiation/delithiation cycling. Upon lithiation, lithium (Li) insertion causes volume expansion and formation of crystalline Li15Si4 phase in the P-doped SiNWs. During delithiation, vacancies induced by Li extraction aggregate gradually, leading to the generation of nanopores. The as formed nanopores can get annihilated with Li reinsertion during the following electrochemical cycle. As demonstrated by our

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phase-field simulations, such first time observed reversible nanopore formation can be attributed to the promoted lithiation/delithiation rate by the P dopant in the SiNWs. Our phase-field simulations further reveal that the delithiation-induced nanoporous structures can be controlled by tuning the electrochemical reaction rate in the SiNWs. The findings of this study shed light on the rational design of high-power performance Si-based anodes.

KEYWORDS: Silicon nanowires, phosphorus dopant, lithiation/delithiation cycling, nanopore formation, phase-field modeling, Li-ion batteries

INTRODUCTION: Li-ion batteries (LIBs) appear to be tangible energy storage techniques for our daily life as they are indispensably used for portable electronics, electric transport, grid energy storage, and so on.1-4 Development of new electrode materials and structures with high energy density, long cycle life, good safety performance, and ultrafast charging/discharging rate is critical for LIBs.5-8 Among various anode materials, silicon (Si) has been recognized as one of the most attractive high capacity anode materials for LIBs because it possesses a capacity of 3579 mAh/g for Li15Si4 at room temperature, which is about ten times of conventional graphite anode (372 mAh/g).9-11 Meanwhile, Si has a low redox potential (0.5 V versus Li/Li+), and it is the second most abundant element in the earth’s crust as well as the plentiful of sources from the mature


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semiconductor industries.5, 8, 10-12 However, translating the Si-based materials into industry-scale usage as anodes for LIBs is not a straight forward process. On one hand, when bulk Si gets fully lithiated, it undergoes a volume expansion of ~300%.13-15 This issue is even severer when an electrode undergoes high charging/discharging rate, which is highly desired but often induces non-uniform Li distribution in the electrode. Such a huge volume change can always induce serious stress, causing fracture, pulverization, electrical disconnectivity, and destruction and regeneration of solid electrolyte interphase (SEI) layer on the fractured surfaces, leading to ultimate capacity loss of the battery during electrochemical cycling.13,

16-22

On the other hand, due to its intrinsic nature of

semiconductor, the poor electronic conductivity of Si can lead to low electrochemical reaction rate, which is inappropriate for high-power battery applications such as electric vehicles.23-24 These challenges greatly hinder the wide industrial application of Si-based materials as anodes. To alleviate the aforementioned problems and acquire highly reversible Si-based anodes, great efforts have been dedicated to seeking for solutions during the past few years. Synthesizing Si into nanostructures such as silicon nanowires (SiNWs),25-27 nanoparticles,14,

19, 28-29

and nanotubes30-31 is believed to be one of the effective

strategies to buffer volume change and meanwhile to facilitate high power capacity. The successful applications of the nanosized Si-based anode materials include the yolkshell-structured Si anode,32-34 mesoporous Si anode,35-36 hollow and porous hollow Si spheres,37-38 and core-shell structures.39-40 In addition, surface conductive coatings (i.e..



carbon coating, metallic coating) are usually used to enhance the conductivity and mechanical cyclability of Si-based anodes.41-44 However, the conductivity inside the Si structures cannot be changed by this approach, resulting in limited improvements of their high-power performance. Doping Si with other atoms,45 such as phosphorus (P),4648

boron (B),49-50 copper (Cu),51 and tin (Sn),52 is known to be another effective solution

to enhance the conductivity of Si. Among various dopants, P has attracted extensive attentions due to its high theoretical capacity, good electrical conductivity, and appropriately low redox potential.53 Previous studies have already demonstrated that P dopant can be used to enhance the electrical conductivity while ensuring better cyclic performance of Si nanostructured electrodes.46-48 For instance, Kim et al. reported that P-doped SiNWs exhibited an enhanced initial discharge capacity due to significant reduction of charge transfer resistance.47 Yan et al. demonstrated that the enhancement of electric conductivity caused by P dopant resulted in the improved cyclic performance and high capacity of Si nanorod.48 However, the microstructural evolution of P-doped Si electrodes during electrochemical cycling remains unrevealed so far due to the lack of direct evidence. Fortunately, the rapid development of the in situ transmission electron microscopy (TEM) technology in recent years makes it possible to real-timely image the electrochemical reactions and subsequent morphological changes in the Pdoped Si electrodes.27, 46, 54-55

RESULTS AND DISCUSSION: Using in situ TEM studies, we reveal in this work the lithiation/delithiation


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behaviors of P-doped SiNWs. The P-doped SiNWs used in this study are prepared by supercritical fluid-liquid-solid (SFLS) growth in toluence with trisilane and P, which has been widely used to fabricate SiNWs described in previous publications.56 The diameter and length of the P-doped SiNWs range from 45-70 nm and 2-10 m, respectively, and the P/Si ratio is 1.0 104 . The in situ TEM experiments are carried out based on an open-cell nanobattery configuration, which consists of a single P-doped SiNW as the working electrode, bulk Li as the counter electrode, and a native Li2O surface layer grown on the Li metal as the solid electrolyte, as schematically illustrated in Figure 1(a). In addition, the gold (Au) and tungsten (W) rods on each side of the nanobattery are working as current collectors. The assembled nanobattery is implanted in an aberration-corrected TEM/STEM at 300 kV, where the samples are tested on a Nanofactory Scanning Tunneling Microscopy (STM) holder. An overpotential -2 V is then applied via a potentiostat to drive the Li ions across the Li2O solid electrolyte for lithiation. During delithiation process, a reversed bias (+2 V) drives Li out of the NWs and back to the source. Such in situ TEM studies enable real-time imaging of electrochemical reactions in the P-doped SiNWs.46 As strong electron beam can cause lithiation retardation or even delithiation and initiate chemical lithiation by decomposing Li2O,54, 57-58 a very small electron beam dosage of ~400 e-/Å2‧s is adopted in this study to ensure that the in situ TEM observed lithiation/delithiation behaviors of the P-doped SiNWs are not sensitive to electron beam54, 58-59 and can reflect the reaction kinetics, structural change, and chemical evolution of the corresponding real battery electrochemistry.58



Figures 1(b)-(d) show the TEM snapshots of two P-doped SiNWs with initial diameters of ~61 nm at different lithiation stages. The corresponding video is available in the Supporting Information (Movie S1). As soon as the Li metal touched the SiNWs (Fig. 1(b)), Li quickly diffuses over a long distance along the SiNWs surface within a very short time before any appreciable radial lithiation occurred (see Movie S1 in Supporting Information), owing to the much higher Li diffusivity on the surface than that in the bulk of Si. The Li then flows radially into the SiNWs, leading to the volume expansion dominantly in the radial direction (Figures 1(c-d)), forming a structure consisting of a pristine core and a lithiated shell of amorphous LixSi (a-LixSi) in each NW, which is consistent with previous experimental observation.46 The scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images of a half way and full lithiated SiNW and the corresponding STEM-Electron Energy Loss Spectroscopy (EELS) elemental maps are also provided in Figure 2, which further demonstrate the lithiation-generated core-shell structures (Figure 2(a)). Similar core-shell structures have been widely observed in lithiated SiNWs and nanoparticles without any dopants.27, 34, 46 After full lithiation of the SiNW, uniform distribution of Li in the NW can be observed, as shown in Figure 2(b). Therefore, the P dopant makes no obvious difference on the first lithiation morphology of SiNWs comparing to that of dopant free SiNWs.


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Figure 1. Morphological evolution of two P-doped SiNWs under lithiation. (a) Schematics of the in situ nanobattery setup, consisting of a single P-doped SiNW as the working electrode, bulk Li as the counter electrode, and a native Li2O surface layer on the Li metal as the solid electrolyte. (b) Two pristine P-doped SiNWs with a diameter of ~61 nm. (c)-(d) TEM snapshots showing the uniform lithiation pattern in the radial direction of the NWs, forming core-shell structures.

Figure 2. STEM HAADF images and the corresponding EELS elemental maps of a Pdoped SiNW segment that undergoes: (a) Half way lithiation; (b) Full lithiation.

Previous TEM studies have already demonstrated that the high electrical



conductivity in germanium (Ge) NWs can easily lead to the formation of nanopores during delithiation,59 while similar delithiation-induced nanoporous structures are rarely observed in SiNWs due to the intrinsically low electrical conductivity in Si.60-61 However, with the enhancement of electrical conductivity by adding significant amounts of Sn into SiNWs, the formation of delithiation-induced nanopores was reported by Lu et al.52 As P dopant is believed to effectively promote the electrical conductivity and thus the lithiation/delithiation rates in Si, it is worthwhile to investigate whether the P dopant can cause the generation of a porous morphology in SiNWs undergoing delithiation or not. To see this, more focused TEM studies on the local regions of the lithiated P-doped SiNW are conducted herein. Figure 3 shows the local morphological evolutions of the lithiated P-doped SiNW during the following one delithiation/lithiation cycle, with the movies (Movies S2 and S3) provided in the Supporting Information. For each TEM snapshot, the corresponding electron diffraction pattern (EDP) is also measured. As shown in Figure 3(a), not only the P-doped SiNW but also the small Si fragments attached to the outer surface of the NW are fully lithiated. The diameter of the SiNW changes from ~61 nm in the pristine state to ~120 nm, corresponding to ~286% volume expansion, and the original Si phase is turned into crystalline Li15Si4 phase indicated by the diffraction pattern. The formation of the crystalline Li15Si4 phase is driven by the electronic similarity between the amorphous and crystalline Li15Si4 phase and the change from amorphous to crystalline is a congruent phase transformation process without long distance Li diffusion. Starting from the fully lithiated state (Figure


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3(a)), the SiNW is then delithiated by reversing the applied bias, which drives Li out of the SiNW and back to the Li source, leading to the shrinkage of the SiNW in the radial direction, as shown in Figure 3(b). Meanwhile, with the fast withdrawing of Li due to the promoted delithiation rate by P dopant, nanopores are nucleated inside the SiNW behind where delithiation takes place (delithiation front).59 With the delithiation front further sweeps to the center of the NW (Figure 3(c)), the delithiation-induced nanopores evolve into much larger sizes. In addition, the diameter of the SiNW is decreased to ~83 nm, which is much larger than the diameter of the pristine SiNW due to the generation of nanopores inside. Once the SiNW is fully delithiated, the applied bias is reversed back to lithiate the NW again. The reinsertion of Li finally leads to the annihilation of the nanopores, causing the increment of the diameter to ~124 nm and the re-crystallization of Li15Si4 phase, as shown in Figure 3(d). The detailed diffraction pattern index can be also obtained in Figure S1 in the Supporting Information through comparison to the standard diffraction patterns of Li2O and Li15Si4.



Figure 3. Morphological evolutions and the corresponding EDPs of a P-doped SiNW during delithiation/lithiation cycling. (a) After the P-doped SiNW with small Si fragments attached to the outer surface gets fully lithiated, the diameter of the SiNW increases from ~61 nm in the pristine state to ~120 nm, corresponding to ~286% volume expansion, and the original Si phase is turned into the crystalline Li15Si4 phase. (b) The lithiated SiNW is then delithiated, leading to the shrinkage of the SiNW in the radial direction. Meanwhile, nanopores are nucleated behind delithiation front inside the SiNW. (c) With the delithiation further proceeds to the final stage, the delithiationinduced pores evolve into much larger sizes. The diameter of the SiNW is decreased to ~83 nm, which is much larger than the diameter of the pristine SiNW due to the generation of nanopores inside. (d) The reinsertion of Li into the fully delithiated SiNW finally leads to the annihilation of the nanopores, causing the increment of the diameter to ~124 nm.

The similar nanopores formation and annihilation during delithiation/lithiation cycling are also observed in other portions of the SiNW, as demonstrated in Figure 4. After the P-doped SiNW gets fully delithiated at the end of the first lithiation/delithiation cycling (Figure 4(a)), the vacancies caused by Li extraction in the NW aggregate gradually, leading to the nucleation and growth of nanopores. Such formation of nanopores in the P-doped SiNW can be attributed to the improved electrical conductivity and thus delithiation rate caused by the P dopant than that in the dopant free SiNWs. In the second electrochemical cycling, the reinsertion of Li into the


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fully delithiated SiNW causes volume expansion of the SiNW in the radial direction, leading to the annihilation of the as formed nanopores, as indicated by Figure 4(b). It is worthwhile to note that the formation and annihilation of nanopores not only take place inside the SiNW, but also in the small Si fragments attached to the outer surface of the NW.

Figure 4. Nanopores formation and annihilation in the other portion of the P-doped SiNW during delithiation/lithiation cycling. (a) After fully delithiated, the vacancies caused by the extraction of Li aggregate gradually, leading to the nucleation and growth of nanopores. (b) With the reinsertion of Li, the as formed nanopores get annihilated.

To understand why nanopores are formed in the P-doped rather than dopant-free SiNWs in the delithiation process, a continuum phase-field model is developed herein to simulation the morphological evolution of the SiNW during delithiation.62-64 As demonstrated in our in situ experiment, with the extraction of Li from the SiNW, the generated vacancies aggregate gradually, leading to the formation of nanopores, which is similar to the formation of nanopores in GeNWs during delithiation. Such



phenomenon intrinsically involves the local dealloying of Li from LixSi phase (delithiation reaction), aggregation or annihilation of vacancies (rearrangement of active materials), and long-range transportation of Li from the delithiation reaction site to the surface of the SiNW and subsequently to the counter electrode.59 Our phase-field model in this study just focuses on the local delithiation reaction and the aggregation/ annihilation of vacancies that take place in the SiNW, and the long-range transportation of Li is ignored here by assuming that the Li is directly taken away from the reaction site during delithiation. In our model, a nonlinear phase-field equation is used to simulate the motion of reaction interface during delithiation reaction process,62-63 with a phase variable  adopted to distinguish the delithiated a-Si phase (   0 ) from the un-delithiated Li15Si4 phase (   1 ). The transition from   1 to   0 indicates the region where the electrochemical delithiation reaction takes place:

Li15Si 4  15Li   15e   4Si .

Moreover, to mimic the promoted delithiation reaction rate caused by the P dopant, a delithiation reaction rate constant L is introduced into the nonlinear phase-field equation. In addition, two diffusion equations are used to describe the spatial and temporal evolutions of Li and vacancies during delithiation, in which the delithiation reaction rate dependent Li extraction and the subsequently caused vacancy generation and aggregation/annihilation are considered. The normalized concentration of Li and vacancy are represented by cLi and cv , respectively. The aforementioned three coupling equations adopted in our phase-field model are implemented in the commercial COMSOL Multiphysics software, as detailed in the Supporting


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Information. With the variations of the delithiation reaction rate constant L that controls the rate of Li extraction, the evolutions of the phase parameter  , the normalized Li concentration cLi , and the normalized vacancy concentration cv in a small but representative segment of the SiNW are simulated, as shown in Figures 5-7. The corresponding movies, Movies S4-6, are provided in the Supporting Information. Color contours are used to represent the values of the phase parameter and concentration fields. From these figures, we can see that the simulated phase parameter  varies from

  1 in the un-delithiated region (red) to   0 in the delithiated region (bule) via a narrow delithiation reaction interface. As the long-range transportation of Li is ignored in the current study, Li is extracted directly from the SiNW in the delithiation reaction interface where 0    1 . Therefore, once the delithiation reaction interface sweeps from the outer surface (right hand side) toward the center (left hand side) of the SiNW, the Li concentration decreases from cLi  1 to cLi  0 . Meanwhile, the extraction of Li in the delithiation reaction interface causes the generation of vacancies in this region. With the reaction interface moves toward the center of the SiNW in the radial direction, the generated vacancies can either be annihilated by the local rearrangement of Si atoms (Figure 5) or gradually aggregate to form nanopores (Figures 6-7). Our simulation results also reveal that the introduced delithiation reaction rate constant

L

in the nonlinear phase-field equation can effectively control the

delithiation rate and determines whether nanopores will form in the SiNW or not. As shown in Figures 5-7, with the variation of L from 20 to 40 and 60, the time needed



to get one third (two third) of the sample delithiated decreases from 327 s to 220 s and 117 s (from 653 s to 440 s and 233 s). To fully delithiate the sample, the total time 980 s, 660 s, and 350 s is needed for each case, respectively. During delithiation process, extraction of Li can generate vacancies in the SiNW on one hand, while the local rearrangement of Si atoms, on the other hand, can annihilate the as formed vacancies. These two processes compete against each other. For the case with low delithiation rate ( L  20 ), the motion of delithiation reaction interface is slow enough that the local Si atoms have sufficient time to migrate around to annihilate the generated vacancies before they can aggregate and evolve into nanopores, as shown in Figure 5. However, with the increment of the delithiation rate (i.e., L  40, 60 in Figures 6-7), the Li extraction caused vacancies are generated in a much higher rate that the local rearrangement of Si atoms cannot effectively annihilate all of the formed vacancies. Therefore, the un-annihilated vacancies aggregate gradually, leading to the formation of nanopores in the SiNW. This can well explain why nanoporous structures can be observed in the P-doped rather than dopant free SiNWs during delithiation process. Moreover, the higher the delithiation rate, the denser the generated nanopores. When

L increases to 60, the simulated nanoporous morphology shown in Figure 7 is qualitatively consistent with that observed in the P-doped SiNWs in our in situ TEM experiment. It should be noted that, as a constant void mobility is assumed in the current study, the sizes of the obtained nanopores in different delithiation rate cases are quite similar. If the void mobility increases gradually, the size of the simulated nanopores raises accordingly, as shown in the Figure S2 in Supporting Information.


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Figure 5. The evolutions of the phase parameter 

(first row), normalized Li

concentration cLi (second row), and normalized vacancy concentration cLi (third row) at different delithiation stages with the normalized reaction rate constant L  20 . The phase parameter  varies from   1 in the un-delithiated region (red) to   0 in the delithiated region (blue) via a narrow delithiation reaction interface. With the sweeping of delithiation reaction interface from the outer surface (right hand side) toward the center (left hand side) of the SiNW, the Li concentration decreases from

cLi  1 to cLi  0 . The Li extraction generated vacancies in the delithiation reaction interface region are gradually annihilated by the local rearrangement of Si atoms.




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concentration cLi (second row), and normalized vacancy concentration cv (third row) at different delithiation stages with the normalized reaction rate constant L  40 . The phase parameter  varies from   1 in the un-delithiated region (red) to   0 in the delithiated region (blue) via a narrow delithiation reaction interface. With the sweeping of delithiation reaction interface from the outer surface (right hand side) toward the center (left hand side) of the SiNW, the Li concentration decreases from

cLi  1 to cLi  0 . The Li extraction generated vacancies in the delithiation reaction interface region aggregate gradually, leading to the formation of nanopores.


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concentration cLi (second row), and normalized vacancy concentration cv (third row) at different delithiation stages with the normalized reaction rate constant L  60 . The phase parameter  varies from   1 in the un-delithiated region (red) to   0 in the delithiated region (blue) via a narrow delithiation reaction interface. With the sweeping of delithiation reaction interface from the outer surface (right hand side) toward the center (left hand side) of the SiNW, the Li concentration decreases from

cLi  1 to cLi  0 . The Li extraction generated vacancies in the delithiation reaction interface region aggregate gradually, leading to the formation nanopores with a much higher density comparing to the case with L  40 .

In order to quantify the effect of delithiation reaction rate on the density of the as formed nanopores, we have calculated the porosities of the fully delithiated samples with different values of the normalized delithiation reaction rate constant L , as plotted out in Figure 8. When L  20 , as the delithiation rate is low enough that the vacancy annihilation is the dominant factor, all the Li extraction caused vacancies are



annihilated. Therefore, no nanopore is formed, leading to the porosity equals to 0. For the parameters adopted in our current simulation, L  20 can be regarded as the critical value above which nanopore will form. With the increment of L , the porosity increases gradually. When

L  60 , the calculated porosity reaches 0.25. It is

worthwhile to note that the porosity is calculated in the undeformed configuration, which should be larger than that obtained from the deformed configuration, as delithiation can lead to the shrinkage of the SiNWs. However, the obtained curve in Figure 8 still infers that different delithiation reaction rates can lead to varied levels of porosity in SiNWs after delithiation. Therefore, our modeling results suggest that structural porosity control can be achieved through tuning the reaction rate by means of, for example, different doping levels in the electrodes.

Figure 8. Variation of porosities of the fully delithiated SiNWs with different


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normalized delithiation reaction rate constants L . When L  20 , the delithiation rate is low enough that all the Li extraction caused vacancies are annihilated. Therefore, no nanopores are formed, leading to the porosity equals to 0. With the increment of the reaction constant L , the porosity increases gradually.

CONCLUSIONS: In summary, we have studied the electrochemical lithiation/delithiation behaviors of P-doped SiNWs with in situ TEM. Our results reveal that Li insertion during lithiation process causes volume expansion and the formation of crystalline Li15Si4 phase in the P-doped SiNWs. Upon delithiation, extraction of Li can generate vacancies in the SiNW on one hand, while the local rearrangement of Si atoms, on the other hand, can annihilate the as formed vacancies. These two processes compete against each other. With the enhancement of lithiation and delithiation rates by P dopant, vacancy generation is the dominant factor. Therefore, the vacancies induced by Li extraction in the P-doped SiNWs aggregate gradually, leading to the generation of nanopores. The as formed nanopores can get annihilated with Li reinsertion during the following electrochemical cycle. Such reversible formation of nanopores is observed in the Pdoped SiNWs for the first time. Moreover, a phase-field model is established in this study to better understand the nucleation and growth of the delithiation-induced nanopores in the P-doped SiNWs. By tightly coupling with in situ experiments, our phase-field simulations identify the promoted delithiation reaction rate by the P dopant in the SiNWs as the key parameter that leads to the formation of nanoporous structures. In addition, it is also revealed that the porosity of delithiation-induced nanoporous



structures can be controlled by tuning the electrochemical reaction rate in the SiNWs. The findings of this study shed light on the rational design of high-power performance Si-based anodes.

ASSOCIATED CONTENT: Supporting Information available: [ Description of phase-field simulation (PDF) In situ TEM movie of the first lithiation-induced morphological evolution in Pdoped SiNWs, Movie S1 (AVI) In situ TEM movie of the first delithiation-induced morphological evolution in P-doped SiNWs, Movie S2 (AVI) In situ TEM movie of the second lithiation-induced morphological evolution in P-doped SiNWs, Movie S3 (AVI) Phase-field simulation of a SiNW under delithiation with delithiation reaction rate constant L  20 , Movie S4 (AVI) Phase-field simulation of a SiNW under delithiation with delithiation reaction rate constant L  40 , Movie S5 (AVI) Phase-field simulation of a SiNW under delithiation with delithiation reaction rate constant L  60 , Movie S6 (AVI) ]

AUTHOR INFORMATION: Corresponding Authors *E-mail: [email protected] (M.G.). *E-mail: [email protected] (H.Y.).


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ORCID Meng Gu: 0000-0002-5126-9611 Hui Yang: 0000-0002-2628-4676

Author Contributions #

J.Z. and M.G. contributed equally to this work.

ACKNOWLEDGMENTS: H.Y. acknowledges the National 1000 Talents Program of China tenable at the Huazhong University of Science and Technology (HUST), China. M.G. acknowledges the support from the National Natural Science Foundation of China (No. 21802065). This work is also supported by the Pico Center at SUSTech that receives support from Presidential fund and Development and Reform Commission of Shenzhen Municipality. Part of the work is done at William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830.

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In Situ TEM of Phosphorus-Dopant-Induced Nanopore Formation in

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