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Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM
Author’s Accepted Manuscript Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM Yonghe Li, Junxia Lu, Xiaopeng Cheng, Huifeng Shi, Yuefei Zhang www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(18)30133-2 https://doi.org/10.1016/j.nanoen.2018.03.004 NANOEN2552
To appear in: Nano Energy Received date: 2 November 2017 Revised date: 2 March 2018 Accepted date: 2 March 2018 Cite this article as: Yonghe Li, Junxia Lu, Xiaopeng Cheng, Huifeng Shi and Yuefei Zhang, Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM Yonghe Lia, Junxia Lub*, Xiaopeng Chenga, Huifeng Shia, Yuefei Zhanga* a
Institute of Microstructure and Property of Advanced Materials, Beijing University
of Technology, Beijing 100124, People’s Republic of China b
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124,
People’s Republic of China [email protected] (J.Lu), [email protected](Y.Zhang) Corresponding Authors
Abstract Selenium (Se), a congener of Sulphur (S), is widely used as a cathode material for high energy-density lithium-ion batteries, named Li–Se batteries. It has been found that nanostructured Se confined in carbonaceous can lead to significantly improved rate capability and cyclic performance. However, the underlying mechanism of carbon coatings in view of surface/interface electro-chemo-mechanical effect at nanoscale remains poorly unexploited. Herein, equipped with in-situ transmission electron microscopy (TEM), we have investigated the type of lithium ions transportation, phase transformation, and coupling mechanical behavior of carbon conformably coated Se nanowire (NW) cathode reacted with Li. Intriguingly, We find a unique lithiation mechanism that the “leapfrog phase transformation” occurs at interface between carbon coating and Se NW cathode. The increasingly accumulated Li ions in leapfrog buckled region as a new platform would react with Se to form crystalline Li2Se from surface to interior. More importantly, this interfacial diffusion pathway of Li ions uniquely differs from the surface-coating directed Li transportation engineered where in Li ions initially diffuse into coatings and then react
with core materials of electrodes. Furthermore, we note a threshold diameter region of Se NWs with ~115–120 nm, above which the uniform carbon coating (~8.5 nm) shows remarkable crack and even delimitation after fully lithiation form. These observations provide reliable guidelines for the design of high-performance lithium-ion batteries by interface and surface engineering.
Graphic Abstract
We real time visualized the dynamical electrochemical lithiation – delithiation process in Se@carbon nanowires by in-situ transmission electron microscopy (TEM). The unique interfacial lithiation channel, leapfrog phase transformation, and size-dependent coating crack were jointly observed, which provides a new insight into the mechanism of Li-Se batteries systerm.
Keywords in-situ TEM; carbon coating; interfacial lithiation; crack; Li-Se batteries 1. Introduction The rapidly developing market for mobile electronics and plug-in hybrid electric vehicles (HEVs) has prompted the urgent need for high-energy-density batteries.[1,2] Based on alloy mechanism, selenium (Se), a congener of sulphur, has been introduced as a cathode material instead of sulphur for high energy-density lithium-ion batteries.[3-9] They preserve the merits of 1) significantly greater electrical conductivity (1 × 10-3 S m-1), which is approximately 20 orders of magnitude greater than that of sulphur (5×10-28 S m-1); [4,5] 2) high volumetric capacity (3253 Ah L-1), which is comparable to that of sulphur (3467 Ah L-1);[6,7] 3) a higher output voltage
(approximately 0.5 V higher than that of Li–S batteries) and a correspondingly higher energy density.[4] The above-mentioned merits suggest that the Li-Se battery could have higher utilization rate, better electrochemical activity, and faster electrochemical reaction with Li than Li-S battery. However, analogous to Li–S battery, many similar drawbacks still need to be solved, including the large volume expansion of Se during lithiation,[10] the safety problem caused by the formation of lithium dendrites, and the shuttle effect due to the solubility of lithium polyselenium (Li2Sex, 4≤x≤8).[11] In recent years, surface modification such as thin coating layers of metal oxide,[12,13] metal fluoride,[14] and carbonaceous materials [15-17] has been proven effective in improving the electrochemical performance of electrode materials in terms of altering the chemistry and mechanical behaviors. The surface layers applied in Se cathode not only act as a protective layer that can mitigate the unwanted and continuous side reactions and even fix the soluble lithium polyselenium at local, but also provide the mechanical confinement that buffers the volume change of the cathode during cycling. Among the materials of the effective layers, the carbonaceous materials, such as amorphous carbon, are excellent candidate coating materials due to their high conductivity, high mechanical strength, and easy coating process.[15] Recently, based on the merits derived from the carbon coating or confinement, Se-impregnated carbon composites synthesized by infusing Se into mesoporous carbon can deliver a reversible capacity of 480 mAh g-1 for 1000 charge–discharge cycles without considerable capacity loss, as reported by Luo et al.[18] Later, Yang et al. successfully confined Se in the form of cyclic Se8 molecules into ordered mesoporous carbon, which greatly improved Li–Se battery performance.[19] These exciting results further support the claim that the Se@carbon nanocomposites are a promising candidate as cathode material with high energy density in Li–Se battery. Despite large collections of research in carbon-coated Se cathodes focusing on materials processing and property characterization, it is urgent that the fundamental mechanisms remain to be unveiled by advanced real-time in situ methods to further improve the performance of Li–Se batteries. Recently, the pioneering Amine group[11,20] firstly used in-situ X-ray diffraction (XRD) and X-ray absorption near
edge structure (XANES) methods to demonstrate the phase change between pristine Se and lithiated Se structures and to determine the composition of the latter (Li2Sen (n ≥ 4), Li2Se2, and Li2Se coexist) during the discharge–charge cycles. These studies, however, do not provide specific information about the (de)lithiation behaviors on a local scale at surface/interface. Alternatively, the real-time observations by in-situ TEM have the unique merit of providing direct insights into dynamic compositional and microstructural evolutions during electrochemical reactions in electrodes.[21-24] In the very recent paper,[10] V. P. Dravid group firstly investigated the phase transformation and kinetics of pure Se nanotube cathode reacted with Li and Na. In this work, using in-situ TEM nanobattery set-up, we have studied Li ions transportation, phase transformation, and mechanical behaviors of the cathode of Se nanowire (NW) conformally coated by amorphous carbon. The interfacial diffusion pathway of Li ions and reversibility of phase transformation with t-Se and cubic Li2Se are both observed. With the initial lithiation process, we also observe a unique lithiation mechanism that the Li ions accumulated, reacted and expanded in leapfrog buckle places where preserve low interfacial adhesion. We further notice that the carbon shell could crack and even delamination under the ultrafast lithiation above the threshold core diameter with ~115–120 nm. These observations shed light on the deep understanding of the coating regulated the interfacial electrochemistry and mechanical behaviors, and provide the insights into the rational design of efficient and durable Se-based cathode materials. 2. Experimental section 2.1 Synthesis of Se@carbon nanowires All of the solvents and chemicals were of reagent quality and used without further purification. In a typical synthesis, 2 mmol SeO2, 5 mmol glucose and 0.2 g PVP were dissolved in a mixed solution of 12 mL deionized water and 18 mL ethylalcohol. After the reagents were fully dissolved, 5 mL ammonia was added dropwise to the solution, which was stirred for 10 min. The solution was transferred to a Teflon-lined
stainless steel autoclave (50 mL capacity) and heated at 160 °C in an electric oven for 20 h. After cooling, are dish-brown product was collected by centrifugation and washed thoroughly with water and ethanol before being dried at 60 °C for 3 h. 2.2 Structural characterization The crystal structure of the as-synthesized samples was characterized by XRD (Bruker, D8 Discover) using Cu Kα radiation (λ = 0.154 nm). The morphology and structure were obtained via transmission electron microscopy (TEM, FEI TECHNAI G20, operated at 200 kV) and scanning transmission electron microscopy (STEM) image with energy dispersive X-ray spectroscopy (EDXS) elemental mapping (Cs-corrected HRTEM, FEI G2, operated at 300 kV). In-situ electrical measurement experiments
were
performed
using
scanning
tunnelling
microscopy–TEM
(STM–TEM); the scanning tunnelling microscope (Nanofactory Instruments) was installed in a JEOL-2010F transmission electron microscope.
2.3 In-situ TEM Experiments The in-situ TEM observations were carried out using a Nanofactory TEM–STM holder inside the JEOL-2010F. A few Se@carbon NWs were attached to an Au rod with conductive Ag epoxy and served as the working electrode. Li metal was scratched with an electrochemically shaped tungsten (W) tip inside a glove box. The Au and W wires were mounted onto the TEM–STM holder. The assembly holder was loaded into the TEM chamber within a sealed plastic bag with an air exposure time of less than 5 s. By manipulating a piezo-driven stage on the TEM–STM holder with nanometre precision, the Li2O-covered Li-metal electrode was brought into contact with a single Se@carbon NW. After a reliable contact was made, a negative (-3 V) or positive voltage bias (+3 V) was applied to drive the lithiation or delithiation reaction, respectively.
3. Results and discussion The typical morphological and structural characterizations of Se@carbon NWs were shown in Fig. 1. The magnified TEM image and SAED pattern (Fig. 1a) show the carbon layer conformably coated single-crystalline Se NWs, evidenced by XRD pattern (PDF No. 06-0632) shown in Fig. S1. The closely magnified HRTEM in Fig. 1b features core-shell structure with coating thickness of ~6 nm. The chemical composition of as-prepared products shown in Figs. 1c and d was conducted by elemental mapping and electron energy loss spectroscopy (EELS) collection under STEM mode, which jointly verifies the existence and distribution of Se (core) and carbon (shell).
Fig. 1. Typical characterization of the as-prepared Se@carbon NW. (a) Magnified TEM image and (b) high-resolution (HR) TEM image of conformal coatings of amorphous carbon on Se NW.
(c) STEM mode EDS elemental mapping, and (d) corresponding EELS spectra, showing existence of elements of the carbon (shell) and selenium (core).
Fig. 2. Leapfrog phase transformation in a Se NW with a thin conformal carbon coating observed by in-situ TEM. (a) Schematic illustration of in situ TEM experimental setup of Li-Se@carbon
nano-battery. (b-f) TEM images showing the dynamic evolution of morphology of a Se@carbon NW during the first lithiation process.
Then, we investigate a single Se@carbon NW as cathode into a nano-battery setup and perform lithiaiton by in-situ TEM, as shown in Fig. 2. Fig. 2a is a schematic diagram of the assembled nanosized Li-Se@carbon open cell with a Se@carbon NW. The Se@carbon NWs mounted on an Au rod were used as the working electrode, and the bulk Li metal on a W tip was used as the counter electrode. The solid electrolyte consisted of naturally grown lithium oxide (Li2O) on Li metal. Lithiation occurred under an applied a bias voltage of -3 V between the Se@carbon NW and the Li counter electrode. The time-lapsed images in Figs. 2b-f and Video S1 show the dynamical morphological and structural evolution of a single Se@carbon NW during lithiation. Shortly after lithiation in Fig. 2b, we can clearly see the V-type lithiated front, differing substantially from the two-phase reaction induced sharply flat front in an Li–S battery (same main group) observed by Yushin et al.[25] The distinct V-type lithiation front pattern may stem from much fast surface diffusion than bulk diffusion (radial) in Se. If the surface diffusion is super-fast, it will lead to an H-shape lithiation morphology (an extreme case).[26] Observed from Fig.2b-f , the propagation speed of V-type lithiation front is calculated about 0.2-0.3 nm s-1. Importantly, the random buckled regions marked by “1”, “2”, “3” also appeared after the front (Figure 2b) From the closely view in Figure. 1, we did not find obvious delamination and defects between carbon sell and Se core at nanoscale. Beyond our capability of observation, there may be existence of certain interfacial atomic defects or weak bonds that as the lithium ions nucleated cites along the one-dimensional direction. The mass of lithium ions gradually accumulate at these defect cites and form the leapfrog buckled phenomena. In the following lithiation process shown in Figs. 2c-f, we observe that the lithiation front moves forward with lithiated LixSe grains left behind. And from the significantly expanding area of region marked in “1”, “2”, “3”, respectively, we confirm that these regions are becoming as the new platform for electrochemical reaction proceeding. The observed leapfrog phase transformation would accelerate the
lithiated kinetics by dividing the whole Se cathode into numerous random cites for electrochemical reaction.
Fig. 3. Selected images of sample evolution during the lithiation process of Se@carbon NW. (a-c) TEM images, (a’–c’) selected area electron diffraction (SAED) patterns, and (a’’–c’’) atomistic models. (a) and (c) correspond to the images of the pristine and lithiated products, respectively.
Fig. 3 shows three captured TEM images and corresponding SAED patterns of selected same place with different lithiation time. Initially, the pristine Se@carbon NW with a diameter of 182.9 nm shows the single-crystalline feature (Figs. 3a and a’). With lithiation process on, it should be noted that the image contrast of Se NW become weaker as the lithiation reaction proceeds in Fig. 3b. This indicates the transformation of the Se into lithium selenium (LixSe). We further observe that the V–type lithiation front emerged, and the corresponding SAED pattern in Fig. 3b’ verifies the direct appearance of cubic phase of polycrystalline Li2Se without an intermediate phase (e.g., Li2Sen (n ≥ 4), Li2Se2), which coincides with the recent in-situ reports during the solid-contact lithiation process.[10] After fully lithiation in Fig. 3c, we clearly observe the diameter of lithiated pattern increased to 249.5 nm,
with volume expansion of 186 %. The relative large expansion induced the carbon coating crack and even delimitation, especially in the buckled region. The SAED study in Fig. 3c’ reveals that light region in fully lithiated region corresponds to a polycrystalline Li2Se phase. The corresponding atomistic model (Figs. 3a’’-c’’) reveals the polycrystalline Li2Se grains formation during the Li-alloy process and leapfrog phase transformation region. Furthermore, the lithiation–delithiation process in Fig. S2 unveils the structural reversibility of single-phase reaction from t-Se to (back) cubic Li2Se, and mechanical flexibility accommodated by carbon shell during lithiation–delithiation cycle.
Fig. 4. Unusual Li ions diffusion pathway in single Se@carbon NW. (a) Magnified TEM images of pristine pattern and (b) partially lithiated pattern. (c-g) Close views of the morphology evolution of surface carbon layer in different lithiated states.
To further understand how this bulked surface occurs, the magnified TEM images are shown in Fig. 4. Fig. 4a shows the pristine defect-free Se NW uniformly coated by a thin carbon layer. The partially lithiated state (Fig. 4b) shows the formation of the two buckled regions induced by lithiation in Se@carbon NW marked by arrow, suggesting that the Li ions had passed by. However, from the close morphology evolution of surface coating in different lithiated state in Figs. 4c-g, the thickness (8.8 nm) of carbon layer in deep lithiated pattern (Fig. 4g) remained the same as that of pristine pattern in Fig. 4c. This suggests that the interface of coating and Se, rather than the carbon coating surface is the fastest and primary Li ions transport channel in the lithiation process. This interfacial diffusion pathway uniquely differs from the surface-coating directed Li transportation engineered [26-28] where in surface diffuse initially into surface coating and then react with core materials. The observed distinct Li ions transport pathway could be attributed to the lower intrinsic conductivity of amorphous carbon layer (<10-13 S m-1)[29] and high Li migration barrier (1.5-2.7 eV)[30], compared to higher intrinsic conductivity of Se material (10-3 S m-1). These large differences render the interface rather than amorphous carbon coating is the preferred pathway of Li transportation. Furthermore, the schematic illustration is shown in Fig. 5a. Owing to the fast interfacial lithiation, it is very easy to delaminate and buckle the carbon thin coating, forming the leapfrog bubbles as new Li ions platforms for the Li alloy reaction (2Li+ + Se → Li2Se) with minor expansion. With the bulk volume keep expanding, the carbon layer in buckled regions experience extra mechanical stress and are more feasible to be crack above a threshold diameter of Se nanowire. The unusual Li ion interfacial transportation observed here provides a new insight into the coating regulated lithiation form in LIBs.
Fig. 5. (a) Schematic illustration of the interfacial lithiation mechanism. (b) The dependence of the fracture occurrence of carbon shell during lithiation on the diameters (core), showing a threshold region within ~115–120 nm.
To shed light on the factors leading to the surfacial fracture induced by size effect, Fig. 5b summarizes the lithiation consequences for all the Se@carbon NWs we examined. With the similar thickness of carbon coating (~8.5 nm), it can be seen that carbon coating is prone to fracture and even delimitation when the Se NWs with larger diameter, while the smaller ones can be uniformly confined by non-cracked carbon coating during the lithiation. The marginal range of Se NWs thickness for coating cracked and non-crack cases indicates the existence of a threshold region of ~115–120 nm, which provides direct evidence of the mechanical robustness of smaller core NWs with carbon coatings, and further indicates the importance of size regulating damage control in LIBs. 4. Conclusion In summary, we visualized the electrochemical (de)lithiation behaviors of individual Se@carbon NW as cathode using in-situ TEM studies. The interfacial effects for the electrochemical and mechanical behaviors were systematically real-time unveiled. We observe a novel lithiation mechanism with leapfrog crystallization and phase transformation at interface between conformal carbon coating and Se cathode. We also find an interfacial diffusion pathway of Li ions
definitely differs from the surface-coating directed Li transportation engineered wherein Li ions initially diffuse into coatings and then react with core materials of electrodes. Finally, a threshold diameter region of Se NWs with ~115–120 nm is noticed, above which the carbon coating (~8.5 nm) shows remarkable crack or delimitation state after fully lithiation form. These findings provide a design guideline for advanced-type and durable surface coating regulated LIBs. Supporting information. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version. “Video S1. Leapfrog phase transformation during first lithiation process” Acknowlegement Y. Z. acknowledged the fundings from the NSFC (21676005), NSFC-DFG joint project (51761135129) and Beijing Natural Science Foundation (2172002), and Great Wall Scholarship Project. The authors also acknowledged Prof. Sulin Zhang from Penn State University for helpful discussion. Reference [1] M. Armand, J. M. Tarascon, Nature 451 (2008) 652–657. [2] Y. M. Chiang, Science 330 (2010) 1485–1486. [3] C. P. Yang, Y. X. Yin, Y. G. Guo, J. Phys. Chem. Lett. 6 (2015) 256−266. [4] A. Abouimrane, D. Dambournet, K. W. Chapman, P. J. Chupas, W. Weng, K. Amine, J. Am. Chem. Soc. 134 (2012) 4505–4508. [5] G. L. Xu, J. Z. Liu, R. Amine, Z. H. Chen, K. Amine, ACS Energy Lett. 2 (2017) 605−614. [6] Q. F. Cai, Y. Y. Li, L. Wang, Q. W. Li, J. Xu, B. Gao, X. M. Zhang, K. F. Huo, P. K. Chu, Nano Energy 6 (2017) 256–266. [7] J. T. Lee, H. Kim, M. Oschatz, D. C. Lee, F. X. Wu, H. T, Lin, B. Zdyrko, W. Cho, S. Kaskel, G. Yushin, Adv. Energy Mater. 5 (2015) 1400981–1400988. [8] K. Han, Z. Liu, J. M. Shen, Y. Y. Lin, F. Dai, H. Q. Ye, Adv. Funct. Mater. 25 (2015) 455–463.
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Yonghe Li received his Ph.D. degree (2018) under supervision of Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). His research interests focus on in situ Cs-TEM/STEM probing the electro-chemo-mechanics behaviors in high-performance Li/Na ion batteries and supercapacitors. Junxia Lu is an Assistant Professor of Beijing University of Technology. She received her Ph.D. degree (2010) in City University of Hong Kong. In the same year, she joined institute of laser engineering of Beijing University of Technology. She is interested in additive manufacturing, energy materials, corrosion science, and in-situ SEM experiments. Xiaopeng Cheng is s now a Ph.D. candidate supervised by Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). His research currently focus on atomic layer deposition (ALD)
modified electrode characterization.
materials,
and
corresponding
Cs-TEM/STEM
atomic
Huifeng Shi is now a Master’s candidate under Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). Her research currently focuses on in situ study of microstructure evolution of 3D structural microbattery by environmental scanning electron microscopy (ESEM). Yuefei Zhang received his B.S., M.S. from Taiyuan University of Technology in 1999 and 2002, respectively, and Ph.D. from Beijing University of Technology in 2008 supervised by Prof. Ze Zhang. From 2014-2015, he worked as visiting scholar in MIT cooperated with Prof. Ju Li. He is now a full professor in institute of microstructure and property of advanced materials, Beijing University of Technology. His research group interests are focus on the in situ TEM/SEM experiments to reveal the microscopic mechanisms that govern the performance of energy storage materials. Highlights 1. A unique lithiation mechanism that the “leapfrog phase transformation” occurs at interface of the core-shell structure. 2. The interfacial diffusion pathway of Li ions and reversibility of phase transformation are both observed. 3. A threshold diameter of Se NWs with~115–120 nm appears, above which the carbon coating (~8.5 nm) shows crack.
PII: DOI: Reference:
S2211-2855(18)30133-2 https://doi.org/10.1016/j.nanoen.2018.03.004 NANOEN2552
To appear in: Nano Energy Received date: 2 November 2017 Revised date: 2 March 2018 Accepted date: 2 March 2018 Cite this article as: Yonghe Li, Junxia Lu, Xiaopeng Cheng, Huifeng Shi and Yuefei Zhang, Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Interfacial lithiation induced leapfrog phase transformation in carbon coated Se cathode observed by in-situ TEM Yonghe Lia, Junxia Lub*, Xiaopeng Chenga, Huifeng Shia, Yuefei Zhanga* a
Institute of Microstructure and Property of Advanced Materials, Beijing University
of Technology, Beijing 100124, People’s Republic of China b
Institute of Laser Engineering, Beijing University of Technology, Beijing 100124,
People’s Republic of China [email protected] (J.Lu), [email protected](Y.Zhang) Corresponding Authors
Abstract Selenium (Se), a congener of Sulphur (S), is widely used as a cathode material for high energy-density lithium-ion batteries, named Li–Se batteries. It has been found that nanostructured Se confined in carbonaceous can lead to significantly improved rate capability and cyclic performance. However, the underlying mechanism of carbon coatings in view of surface/interface electro-chemo-mechanical effect at nanoscale remains poorly unexploited. Herein, equipped with in-situ transmission electron microscopy (TEM), we have investigated the type of lithium ions transportation, phase transformation, and coupling mechanical behavior of carbon conformably coated Se nanowire (NW) cathode reacted with Li. Intriguingly, We find a unique lithiation mechanism that the “leapfrog phase transformation” occurs at interface between carbon coating and Se NW cathode. The increasingly accumulated Li ions in leapfrog buckled region as a new platform would react with Se to form crystalline Li2Se from surface to interior. More importantly, this interfacial diffusion pathway of Li ions uniquely differs from the surface-coating directed Li transportation engineered where in Li ions initially diffuse into coatings and then react
with core materials of electrodes. Furthermore, we note a threshold diameter region of Se NWs with ~115–120 nm, above which the uniform carbon coating (~8.5 nm) shows remarkable crack and even delimitation after fully lithiation form. These observations provide reliable guidelines for the design of high-performance lithium-ion batteries by interface and surface engineering.
Graphic Abstract
We real time visualized the dynamical electrochemical lithiation – delithiation process in Se@carbon nanowires by in-situ transmission electron microscopy (TEM). The unique interfacial lithiation channel, leapfrog phase transformation, and size-dependent coating crack were jointly observed, which provides a new insight into the mechanism of Li-Se batteries systerm.
Keywords in-situ TEM; carbon coating; interfacial lithiation; crack; Li-Se batteries 1. Introduction The rapidly developing market for mobile electronics and plug-in hybrid electric vehicles (HEVs) has prompted the urgent need for high-energy-density batteries.[1,2] Based on alloy mechanism, selenium (Se), a congener of sulphur, has been introduced as a cathode material instead of sulphur for high energy-density lithium-ion batteries.[3-9] They preserve the merits of 1) significantly greater electrical conductivity (1 × 10-3 S m-1), which is approximately 20 orders of magnitude greater than that of sulphur (5×10-28 S m-1); [4,5] 2) high volumetric capacity (3253 Ah L-1), which is comparable to that of sulphur (3467 Ah L-1);[6,7] 3) a higher output voltage
(approximately 0.5 V higher than that of Li–S batteries) and a correspondingly higher energy density.[4] The above-mentioned merits suggest that the Li-Se battery could have higher utilization rate, better electrochemical activity, and faster electrochemical reaction with Li than Li-S battery. However, analogous to Li–S battery, many similar drawbacks still need to be solved, including the large volume expansion of Se during lithiation,[10] the safety problem caused by the formation of lithium dendrites, and the shuttle effect due to the solubility of lithium polyselenium (Li2Sex, 4≤x≤8).[11] In recent years, surface modification such as thin coating layers of metal oxide,[12,13] metal fluoride,[14] and carbonaceous materials [15-17] has been proven effective in improving the electrochemical performance of electrode materials in terms of altering the chemistry and mechanical behaviors. The surface layers applied in Se cathode not only act as a protective layer that can mitigate the unwanted and continuous side reactions and even fix the soluble lithium polyselenium at local, but also provide the mechanical confinement that buffers the volume change of the cathode during cycling. Among the materials of the effective layers, the carbonaceous materials, such as amorphous carbon, are excellent candidate coating materials due to their high conductivity, high mechanical strength, and easy coating process.[15] Recently, based on the merits derived from the carbon coating or confinement, Se-impregnated carbon composites synthesized by infusing Se into mesoporous carbon can deliver a reversible capacity of 480 mAh g-1 for 1000 charge–discharge cycles without considerable capacity loss, as reported by Luo et al.[18] Later, Yang et al. successfully confined Se in the form of cyclic Se8 molecules into ordered mesoporous carbon, which greatly improved Li–Se battery performance.[19] These exciting results further support the claim that the Se@carbon nanocomposites are a promising candidate as cathode material with high energy density in Li–Se battery. Despite large collections of research in carbon-coated Se cathodes focusing on materials processing and property characterization, it is urgent that the fundamental mechanisms remain to be unveiled by advanced real-time in situ methods to further improve the performance of Li–Se batteries. Recently, the pioneering Amine group[11,20] firstly used in-situ X-ray diffraction (XRD) and X-ray absorption near
edge structure (XANES) methods to demonstrate the phase change between pristine Se and lithiated Se structures and to determine the composition of the latter (Li2Sen (n ≥ 4), Li2Se2, and Li2Se coexist) during the discharge–charge cycles. These studies, however, do not provide specific information about the (de)lithiation behaviors on a local scale at surface/interface. Alternatively, the real-time observations by in-situ TEM have the unique merit of providing direct insights into dynamic compositional and microstructural evolutions during electrochemical reactions in electrodes.[21-24] In the very recent paper,[10] V. P. Dravid group firstly investigated the phase transformation and kinetics of pure Se nanotube cathode reacted with Li and Na. In this work, using in-situ TEM nanobattery set-up, we have studied Li ions transportation, phase transformation, and mechanical behaviors of the cathode of Se nanowire (NW) conformally coated by amorphous carbon. The interfacial diffusion pathway of Li ions and reversibility of phase transformation with t-Se and cubic Li2Se are both observed. With the initial lithiation process, we also observe a unique lithiation mechanism that the Li ions accumulated, reacted and expanded in leapfrog buckle places where preserve low interfacial adhesion. We further notice that the carbon shell could crack and even delamination under the ultrafast lithiation above the threshold core diameter with ~115–120 nm. These observations shed light on the deep understanding of the coating regulated the interfacial electrochemistry and mechanical behaviors, and provide the insights into the rational design of efficient and durable Se-based cathode materials. 2. Experimental section 2.1 Synthesis of Se@carbon nanowires All of the solvents and chemicals were of reagent quality and used without further purification. In a typical synthesis, 2 mmol SeO2, 5 mmol glucose and 0.2 g PVP were dissolved in a mixed solution of 12 mL deionized water and 18 mL ethylalcohol. After the reagents were fully dissolved, 5 mL ammonia was added dropwise to the solution, which was stirred for 10 min. The solution was transferred to a Teflon-lined
stainless steel autoclave (50 mL capacity) and heated at 160 °C in an electric oven for 20 h. After cooling, are dish-brown product was collected by centrifugation and washed thoroughly with water and ethanol before being dried at 60 °C for 3 h. 2.2 Structural characterization The crystal structure of the as-synthesized samples was characterized by XRD (Bruker, D8 Discover) using Cu Kα radiation (λ = 0.154 nm). The morphology and structure were obtained via transmission electron microscopy (TEM, FEI TECHNAI G20, operated at 200 kV) and scanning transmission electron microscopy (STEM) image with energy dispersive X-ray spectroscopy (EDXS) elemental mapping (Cs-corrected HRTEM, FEI G2, operated at 300 kV). In-situ electrical measurement experiments
were
performed
using
scanning
tunnelling
microscopy–TEM
(STM–TEM); the scanning tunnelling microscope (Nanofactory Instruments) was installed in a JEOL-2010F transmission electron microscope.
2.3 In-situ TEM Experiments The in-situ TEM observations were carried out using a Nanofactory TEM–STM holder inside the JEOL-2010F. A few Se@carbon NWs were attached to an Au rod with conductive Ag epoxy and served as the working electrode. Li metal was scratched with an electrochemically shaped tungsten (W) tip inside a glove box. The Au and W wires were mounted onto the TEM–STM holder. The assembly holder was loaded into the TEM chamber within a sealed plastic bag with an air exposure time of less than 5 s. By manipulating a piezo-driven stage on the TEM–STM holder with nanometre precision, the Li2O-covered Li-metal electrode was brought into contact with a single Se@carbon NW. After a reliable contact was made, a negative (-3 V) or positive voltage bias (+3 V) was applied to drive the lithiation or delithiation reaction, respectively.
3. Results and discussion The typical morphological and structural characterizations of Se@carbon NWs were shown in Fig. 1. The magnified TEM image and SAED pattern (Fig. 1a) show the carbon layer conformably coated single-crystalline Se NWs, evidenced by XRD pattern (PDF No. 06-0632) shown in Fig. S1. The closely magnified HRTEM in Fig. 1b features core-shell structure with coating thickness of ~6 nm. The chemical composition of as-prepared products shown in Figs. 1c and d was conducted by elemental mapping and electron energy loss spectroscopy (EELS) collection under STEM mode, which jointly verifies the existence and distribution of Se (core) and carbon (shell).
Fig. 1. Typical characterization of the as-prepared Se@carbon NW. (a) Magnified TEM image and (b) high-resolution (HR) TEM image of conformal coatings of amorphous carbon on Se NW.
(c) STEM mode EDS elemental mapping, and (d) corresponding EELS spectra, showing existence of elements of the carbon (shell) and selenium (core).
Fig. 2. Leapfrog phase transformation in a Se NW with a thin conformal carbon coating observed by in-situ TEM. (a) Schematic illustration of in situ TEM experimental setup of Li-Se@carbon
nano-battery. (b-f) TEM images showing the dynamic evolution of morphology of a Se@carbon NW during the first lithiation process.
Then, we investigate a single Se@carbon NW as cathode into a nano-battery setup and perform lithiaiton by in-situ TEM, as shown in Fig. 2. Fig. 2a is a schematic diagram of the assembled nanosized Li-Se@carbon open cell with a Se@carbon NW. The Se@carbon NWs mounted on an Au rod were used as the working electrode, and the bulk Li metal on a W tip was used as the counter electrode. The solid electrolyte consisted of naturally grown lithium oxide (Li2O) on Li metal. Lithiation occurred under an applied a bias voltage of -3 V between the Se@carbon NW and the Li counter electrode. The time-lapsed images in Figs. 2b-f and Video S1 show the dynamical morphological and structural evolution of a single Se@carbon NW during lithiation. Shortly after lithiation in Fig. 2b, we can clearly see the V-type lithiated front, differing substantially from the two-phase reaction induced sharply flat front in an Li–S battery (same main group) observed by Yushin et al.[25] The distinct V-type lithiation front pattern may stem from much fast surface diffusion than bulk diffusion (radial) in Se. If the surface diffusion is super-fast, it will lead to an H-shape lithiation morphology (an extreme case).[26] Observed from Fig.2b-f , the propagation speed of V-type lithiation front is calculated about 0.2-0.3 nm s-1. Importantly, the random buckled regions marked by “1”, “2”, “3” also appeared after the front (Figure 2b) From the closely view in Figure. 1, we did not find obvious delamination and defects between carbon sell and Se core at nanoscale. Beyond our capability of observation, there may be existence of certain interfacial atomic defects or weak bonds that as the lithium ions nucleated cites along the one-dimensional direction. The mass of lithium ions gradually accumulate at these defect cites and form the leapfrog buckled phenomena. In the following lithiation process shown in Figs. 2c-f, we observe that the lithiation front moves forward with lithiated LixSe grains left behind. And from the significantly expanding area of region marked in “1”, “2”, “3”, respectively, we confirm that these regions are becoming as the new platform for electrochemical reaction proceeding. The observed leapfrog phase transformation would accelerate the
lithiated kinetics by dividing the whole Se cathode into numerous random cites for electrochemical reaction.
Fig. 3. Selected images of sample evolution during the lithiation process of Se@carbon NW. (a-c) TEM images, (a’–c’) selected area electron diffraction (SAED) patterns, and (a’’–c’’) atomistic models. (a) and (c) correspond to the images of the pristine and lithiated products, respectively.
Fig. 3 shows three captured TEM images and corresponding SAED patterns of selected same place with different lithiation time. Initially, the pristine Se@carbon NW with a diameter of 182.9 nm shows the single-crystalline feature (Figs. 3a and a’). With lithiation process on, it should be noted that the image contrast of Se NW become weaker as the lithiation reaction proceeds in Fig. 3b. This indicates the transformation of the Se into lithium selenium (LixSe). We further observe that the V–type lithiation front emerged, and the corresponding SAED pattern in Fig. 3b’ verifies the direct appearance of cubic phase of polycrystalline Li2Se without an intermediate phase (e.g., Li2Sen (n ≥ 4), Li2Se2), which coincides with the recent in-situ reports during the solid-contact lithiation process.[10] After fully lithiation in Fig. 3c, we clearly observe the diameter of lithiated pattern increased to 249.5 nm,
with volume expansion of 186 %. The relative large expansion induced the carbon coating crack and even delimitation, especially in the buckled region. The SAED study in Fig. 3c’ reveals that light region in fully lithiated region corresponds to a polycrystalline Li2Se phase. The corresponding atomistic model (Figs. 3a’’-c’’) reveals the polycrystalline Li2Se grains formation during the Li-alloy process and leapfrog phase transformation region. Furthermore, the lithiation–delithiation process in Fig. S2 unveils the structural reversibility of single-phase reaction from t-Se to (back) cubic Li2Se, and mechanical flexibility accommodated by carbon shell during lithiation–delithiation cycle.
Fig. 4. Unusual Li ions diffusion pathway in single Se@carbon NW. (a) Magnified TEM images of pristine pattern and (b) partially lithiated pattern. (c-g) Close views of the morphology evolution of surface carbon layer in different lithiated states.
To further understand how this bulked surface occurs, the magnified TEM images are shown in Fig. 4. Fig. 4a shows the pristine defect-free Se NW uniformly coated by a thin carbon layer. The partially lithiated state (Fig. 4b) shows the formation of the two buckled regions induced by lithiation in Se@carbon NW marked by arrow, suggesting that the Li ions had passed by. However, from the close morphology evolution of surface coating in different lithiated state in Figs. 4c-g, the thickness (8.8 nm) of carbon layer in deep lithiated pattern (Fig. 4g) remained the same as that of pristine pattern in Fig. 4c. This suggests that the interface of coating and Se, rather than the carbon coating surface is the fastest and primary Li ions transport channel in the lithiation process. This interfacial diffusion pathway uniquely differs from the surface-coating directed Li transportation engineered [26-28] where in surface diffuse initially into surface coating and then react with core materials. The observed distinct Li ions transport pathway could be attributed to the lower intrinsic conductivity of amorphous carbon layer (<10-13 S m-1)[29] and high Li migration barrier (1.5-2.7 eV)[30], compared to higher intrinsic conductivity of Se material (10-3 S m-1). These large differences render the interface rather than amorphous carbon coating is the preferred pathway of Li transportation. Furthermore, the schematic illustration is shown in Fig. 5a. Owing to the fast interfacial lithiation, it is very easy to delaminate and buckle the carbon thin coating, forming the leapfrog bubbles as new Li ions platforms for the Li alloy reaction (2Li+ + Se → Li2Se) with minor expansion. With the bulk volume keep expanding, the carbon layer in buckled regions experience extra mechanical stress and are more feasible to be crack above a threshold diameter of Se nanowire. The unusual Li ion interfacial transportation observed here provides a new insight into the coating regulated lithiation form in LIBs.
Fig. 5. (a) Schematic illustration of the interfacial lithiation mechanism. (b) The dependence of the fracture occurrence of carbon shell during lithiation on the diameters (core), showing a threshold region within ~115–120 nm.
To shed light on the factors leading to the surfacial fracture induced by size effect, Fig. 5b summarizes the lithiation consequences for all the Se@carbon NWs we examined. With the similar thickness of carbon coating (~8.5 nm), it can be seen that carbon coating is prone to fracture and even delimitation when the Se NWs with larger diameter, while the smaller ones can be uniformly confined by non-cracked carbon coating during the lithiation. The marginal range of Se NWs thickness for coating cracked and non-crack cases indicates the existence of a threshold region of ~115–120 nm, which provides direct evidence of the mechanical robustness of smaller core NWs with carbon coatings, and further indicates the importance of size regulating damage control in LIBs. 4. Conclusion In summary, we visualized the electrochemical (de)lithiation behaviors of individual Se@carbon NW as cathode using in-situ TEM studies. The interfacial effects for the electrochemical and mechanical behaviors were systematically real-time unveiled. We observe a novel lithiation mechanism with leapfrog crystallization and phase transformation at interface between conformal carbon coating and Se cathode. We also find an interfacial diffusion pathway of Li ions
definitely differs from the surface-coating directed Li transportation engineered wherein Li ions initially diffuse into coatings and then react with core materials of electrodes. Finally, a threshold diameter region of Se NWs with ~115–120 nm is noticed, above which the carbon coating (~8.5 nm) shows remarkable crack or delimitation state after fully lithiation form. These findings provide a design guideline for advanced-type and durable surface coating regulated LIBs. Supporting information. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version. “Video S1. Leapfrog phase transformation during first lithiation process” Acknowlegement Y. Z. acknowledged the fundings from the NSFC (21676005), NSFC-DFG joint project (51761135129) and Beijing Natural Science Foundation (2172002), and Great Wall Scholarship Project. The authors also acknowledged Prof. Sulin Zhang from Penn State University for helpful discussion. Reference [1] M. Armand, J. M. Tarascon, Nature 451 (2008) 652–657. [2] Y. M. Chiang, Science 330 (2010) 1485–1486. [3] C. P. Yang, Y. X. Yin, Y. G. Guo, J. Phys. Chem. Lett. 6 (2015) 256−266. [4] A. Abouimrane, D. Dambournet, K. W. Chapman, P. J. Chupas, W. Weng, K. Amine, J. Am. Chem. Soc. 134 (2012) 4505–4508. [5] G. L. Xu, J. Z. Liu, R. Amine, Z. H. Chen, K. Amine, ACS Energy Lett. 2 (2017) 605−614. [6] Q. F. Cai, Y. Y. Li, L. Wang, Q. W. Li, J. Xu, B. Gao, X. M. Zhang, K. F. Huo, P. K. Chu, Nano Energy 6 (2017) 256–266. [7] J. T. Lee, H. Kim, M. Oschatz, D. C. Lee, F. X. Wu, H. T, Lin, B. Zdyrko, W. Cho, S. Kaskel, G. Yushin, Adv. Energy Mater. 5 (2015) 1400981–1400988. [8] K. Han, Z. Liu, J. M. Shen, Y. Y. Lin, F. Dai, H. Q. Ye, Adv. Funct. Mater. 25 (2015) 455–463.
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Yonghe Li received his Ph.D. degree (2018) under supervision of Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). His research interests focus on in situ Cs-TEM/STEM probing the electro-chemo-mechanics behaviors in high-performance Li/Na ion batteries and supercapacitors. Junxia Lu is an Assistant Professor of Beijing University of Technology. She received her Ph.D. degree (2010) in City University of Hong Kong. In the same year, she joined institute of laser engineering of Beijing University of Technology. She is interested in additive manufacturing, energy materials, corrosion science, and in-situ SEM experiments. Xiaopeng Cheng is s now a Ph.D. candidate supervised by Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). His research currently focus on atomic layer deposition (ALD)
modified electrode characterization.
materials,
and
corresponding
Cs-TEM/STEM
atomic
Huifeng Shi is now a Master’s candidate under Prof. Yuefei Zhang at institute of microstructure and property of advanced materials, Beijing University of Technology (BJUT). Her research currently focuses on in situ study of microstructure evolution of 3D structural microbattery by environmental scanning electron microscopy (ESEM). Yuefei Zhang received his B.S., M.S. from Taiyuan University of Technology in 1999 and 2002, respectively, and Ph.D. from Beijing University of Technology in 2008 supervised by Prof. Ze Zhang. From 2014-2015, he worked as visiting scholar in MIT cooperated with Prof. Ju Li. He is now a full professor in institute of microstructure and property of advanced materials, Beijing University of Technology. His research group interests are focus on the in situ TEM/SEM experiments to reveal the microscopic mechanisms that govern the performance of energy storage materials. Highlights 1. A unique lithiation mechanism that the “leapfrog phase transformation” occurs at interface of the core-shell structure. 2. The interfacial diffusion pathway of Li ions and reversibility of phase transformation are both observed. 3. A threshold diameter of Se NWs with~115–120 nm appears, above which the carbon coating (~8.5 nm) shows crack.