Origin of anomalous high-rate Na-ion electrochemistry in layered bismuth telluride anodes

Abstract

•Ultrafast Na-ion electrochemistry outperforming Li-ion and K-ion counterparts•In situ TEM of comparative lithiation, sodiation, and potassiation in layered Bi2Te3•Better sodiation kinetics induced by lower interfacial strain accommodation energy•Distinct reaction propagation dominated by chemo-mechanical stress concentration Sodium ranks the sixth most abundant element, making it promising for cost-effective Na-ion batteries to complement the dominant Li-ion technology. It is generally believed that Na-ion batteries can only deliver limited power due to the sluggish electrochemical reaction kinetics. Here, through a systematic comparison of Li+, Na+, and K+ reactions with van der Waals layered Bi2Te3, we reveal anomalous high-rate performance showing that sodium unexpectedly outperforms the lithium and potassium counterparts. A combination of electrochemical analysis, in situ transmission electron microscopy, first-principles calculation, and finite-element modeling is employed to elucidate the origin of this intriguing phenomenon, which provides insights into fundamental understanding of reaction mechanisms of lithiation, sodiation, and potassiation, and elucidates the reason for the superior Na storage capability. These findings open up new opportunities in making Na-ion batteries with not only low cost but also high-power performance. van der Waals layered metal chalcogenide Bi2Te3 has shown exceptional capacity and rate capability in alkali-ion batteries but the underlying reaction mechanism with Li+, Na+, and K+ remains undiscovered. It is unexpected that Na+ electrochemistry outperforms Li+ and K+ at high current densities. Here, in situ transmission electron microscopy is used to uncover nanoscale transformations during lithiation, sodiation, and potassiation, which follows two-step conversion and alloying reactions with Li+ and Na+, and three-step intercalation-conversion-alloying reactions with K+. Counterintuitively, sodiation exhibits the highest reaction kinetics, and its origin can be elucidated by first-principles and finite-element simulations in two aspects. The lower interfacial strain accommodation energy between Bi2Te3 and its Na-conversion products allows more facile sodiation phase transformation than Li- and K-ion reactions. The higher electrochemo-mechanical stress concentration at the concave-shaped sodiation reaction front facilitates continued Na-ion diffusion and reaction propagation. These fundamental insights are essential for fast-charging alkali-ion batteries. van der Waals layered metal chalcogenide Bi2Te3 has shown exceptional capacity and rate capability in alkali-ion batteries but the underlying reaction mechanism with Li+, Na+, and K+ remains undiscovered. It is unexpected that Na+ electrochemistry outperforms Li+ and K+ at high current densities. Here, in situ transmission electron microscopy is used to uncover nanoscale transformations during lithiation, sodiation, and potassiation, which follows two-step conversion and alloying reactions with Li+ and Na+, and three-step intercalation-conversion-alloying reactions with K+. Counterintuitively, sodiation exhibits the highest reaction kinetics, and its origin can be elucidated by first-principles and finite-element simulations in two aspects. The lower interfacial strain accommodation energy between Bi2Te3 and its Na-conversion products allows more facile sodiation phase transformation than Li- and K-ion reactions. The higher electrochemo-mechanical stress concentration at the concave-shaped sodiation reaction front facilitates continued Na-ion diffusion and reaction propagation. These fundamental insights are essential for fast-charging alkali-ion batteries. Lithium-ion batteries (LIBs) have become the cornerstone of energy sustainability. LIBs are an essential component of energy storage technology for applications in portable electronics, electric vehicles, drones, and utility-scale storage systems for wind and solar plants.1Tarascon J.M. Armand M. Issues and challenges facing rechargeable lithium batteries.Nature. 2001; 414: 359-367Crossref PubMed Scopus (15691) Google Scholar Such large-scale production of LIBs is rapidly depleting the precursors used for making electrode materials and driving the cost up. 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Rev. 2017; 117: 13123-13186Crossref PubMed Scopus (267) Google Scholar Among various metal chalcogenide candidates, Bi2Te3 stands out as the material of interest in this study for three major reasons: (1) despite limited reports in battery applications, Bi2Te3 turns out to possess one of the highest theoretical volumetric capacity of 3,093 mAh L−1, making it a promising anode that may outperform other 2D materials; (2) the facile synthesis of single-crystalline Bi2Te3 with well-defined planar hexagon morphology makes it a suitable model for quantitative in situ TEM studies; (3) Bi2Te3 is expected to be electrochemically active with respect to not only Li-ion but also Na- and K-ions, which is crucial for revealing its alkali-ion storage mechanisms and electrochemo-mechanical properties by comparing similarities and differences with different alkali-ion electrochemistry. Here, using van der Waals layered Bi2Te3 nanoplates as a well-defined model system, we have systematically evaluated the electrochemical performance in three distinct alkali (Li, Na, and K) ion batteries and elucidated their comparative ion storage mechanisms through the combination of in situ TEM experiments, first-principles calculations, and finite-element modeling. Noticeably, we have discovered the anomalous sodiation behavior and the associated unexpected high-rate capacity and further clarified that the origin is attributed to the unique kinetics and electrochemo-mechanical stability in Na reactions. This work provides practical implications in leveraging the fundamental understanding of alkali-ion storage mechanisms for the design of fast-charging anodes in beyond-lithium battery technologies. Bi2Te3 possesses a van der Waals-bonded layered structure with R3¯m space group. Separated by the van der Waals gap, the quintuple sheets consisting of covalently bonded Bi and Te atoms are arranged along the c axis to form a hexagonal unit cell, as shown in Figure 1A. As a result of distinct cleavage energies of different crystal planes, Bi2Te3 thermodynamically favors isotropic in-plane growth along a and b axes into a hexagonal shape. We have utilized the hydrothermal approach to synthesize single-crystalline Bi2Te3 nanoplates with uniform hexagonal morphology, with the addition of polyvinylpyrrolidone (PVP) to enhance better monodispersity and homogeneity in size and thickness.36Zhang Y. Hu L.P. Zhu T.J. Xie J. Zhao X.B. High yield Bi2Te3 single crystal nanosheets with uniform morphology via a solvothermal synthesis.Cryst. Growth Des. 2013; 13: 645-651Crossref Scopus (86) Google Scholar Surface coating is an effective method to suppress the volume change and stabilize the solid electrolyte interface (SEI) layer of electrode materials during the alkali-ion insertion/extraction process in batteries.37Akbari Garakani M. Abouali S. Cui J. Kim J.K. In situ TEM study of lithiation into a PPy coated α-MnO2/graphene foam freestanding electrode.Mater. Chem. Front. 2018; 2: 1481-1488Crossref Google Scholar Here, we coat a thin conductive layer of polypyrrole (PPy) via in situ polymerization of pyrrole monomer on the surface of Bi2Te3 nanoplates. Such surface coating does not affect the high purity and crystallinity of Bi2Te3 nanoplates as confirmed by X-ray diffraction (XRD) (Figure 1B), which identifies all characteristic peaks of Bi2Te3 without impurity. As illustrated in SEM and TEM images, the synthesized Bi2Te3/PPy maintains the hexagonal morphology with a uniform lateral size and ultrathin thickness (Figures 1C and 1D). A uniform PPy layer with a thickness of ∼6 nm can be clearly observed by high-resolution TEM (HRTEM) at the edge of Bi2Te3 nanoplate (Figure 1E). Furthermore, an atomically resolved high-angle annular dark-field (HAADF) scanning TEM (STEM) image (Figure 1F) verifies the (001) basal plane consisting of Bi and Te atoms consistent with the atomic model. We have tested the electrochemical performance of bare and PPy-coated Bi2Te3 in alkali-ion (Li, Na, and K) half-cells, and the results are shown in Figures 2 and S1. In the initial cycle, both Bi2Te3 and Bi2Te3/PPy delivered a similar reversible Li and Na storage capacity (422 mAh g−1 for LIBs and 406 mAh g−1 for NIBs at 0.1 A g−1), slightly higher than the theoretical capacity of Bi2Te3 (402 mAh g−1), whereas the K storage capacity is much lower (341 mAh g−1) even at a low current density of 0.05 A g−1. This indicates full Li and Na storage but incomplete K storage for Bi2Te3 anodes through a combination of conversion and alloying reactions. Despite the higher capacity from conversion and alloying reactions, the large volume change arising from multistep phase transformations may severely deteriorate the cyclic stability38Cui J. Xu Z.L. Yao S. Huang J. Huang J.Q. Abouali S. et al.Enhanced conversion reaction kinetics in low crystallinity SnO2/CNT anodes for Na-ion batteries.J. Mater. Chem. A. 2016; 4: 10964-10973Crossref Google Scholar and lead to a rapid capacity decay of bare Bi2Te3 anodes in all three cases (Figure S1A). It is interesting to note that the bare Bi2Te3 anode retains its stability after the first 30 cycles of capacity decay in NIBs and KIBs, possibly due to the buffering effect arising from the Na- or K-telluride matrix formed through the conversion reaction that endows stable and reversible alloying reaction between Bi and Na+/K+ ions.39Cui J. Yao S. Huang J.Q. Qin L. Chong W.G. Sadighi Z. et al.Sb-doped SnO2/graphene-CNT aerogels for high performance Li-ion and Na-ion battery anodes.Energy Storage Mater. 2017; 9: 85-95Crossref Scopus (67) Google Scholar The PPy coating can largely suppress the capacity fade and lead to the improved cyclic stability of Bi2Te3/PPy with 99%, 83%, and 71% capacity retention after 100 cycles for LIBs, NIBs, and KIBs, respectively. The largely improved cyclability is mainly attributed to the structural stability of Bi2Te3/PPy electrodes as confirmed by ex situ TEM characterization (Figure S2), in which the crystallinity of Bi2Te3/PPy is retained and no agglomeration is formed, despite the original hexagonal-shaped nanoplates being broken into smaller pieces. The rate performances of Bi2Te3/PPy in LIBs, NIBs, and KIBs are also tested (Figure 2B) and the corresponding charge/discharge voltage profiles (Figures 2C–2E) exhibit two major plateaus associated with conversion and alloying reactions, although the sluggish kinetics of K transport make them less prominent than the Li and Na counterparts. Despite the slight capacity loss due to the increasing polarization at high rates, the overall rate capability of PPy-coated Bi2Te3 is significantly better than that of uncoated Bi2Te3 (Figure S1B). It is worth noting that Bi2Te3/PPy in NIBs shows exceptional rate performance, especially at current densities higher than 1A g−1, and maintains the capacity of 231 mAh g−1 at the current density of 5 A g−1. This anomalous rate capability in Na electrochemistry has also previously been reported in layered metal chalcogenide electrodes,20Cui J. Yao S. Lu Z. Huang J.Q. Chong W.G. Ciucci F. et al.Revealing pseudocapacitive mechanisms of metal dichalcogenide SnS2/graphene-CNT aerogels for high-energy Na hybrid capacitors.Adv. Energy Mater. 2018; 81702488Crossref Scopus (113) Google Scholar,21Zhang F. Xia C. Zhu J. Ahmed B. Liang H. Velusamy D.B. et al.SnSe2 2D anodes for advanced sodium ion batteries.Adv. Energy Mater. 2016; 61601188Crossref Scopus (208) Google Scholar,24Zhang Q. Tan S. Mendes R.G. Sun Z. Chen Y. Kong X. et al.Extremely weak van der Waals coupling in vertical ReS2 nanowalls for high-current-density lithium-ion batteries.Adv. Mater. 2016; 28: 2616-2623Crossref PubMed Scopus (174) Google Scholar but the origin remains elusive. To further elucidate the difference in alkali-ion transport kinetics in Bi2Te3, we have conducted various electrochemical analyses. The differential capacity (dQ/dV) curves for Li, Na, and K electrochemical reactions are plotted and compared in Figure 2F. According to previous reports on other similar metal chalcogenide anodes,16Yao S. Cui J. Deng Y. Chong W.G. Wu J. Ihsan-Ul-Haq M. et al.Ultrathin Sb2S3 nanosheet anodes for exceptional pseudocapacitive contribution to multi-battery charge storage.Energy Storage Mater. 2019; 20: 36-45Crossref Scopus (42) Google Scholar,20Cui J. Yao S. Lu Z. Huang J.Q. Chong W.G. Ciucci F. et al.Revealing pseudocapacitive mechanisms of metal dichalcogenide SnS2/graphene-CNT aerogels for high-energy Na hybrid capacitors.Adv. Energy Mater. 2018; 81702488Crossref Scopus (113) Google Scholar,21Zhang F. Xia C. Zhu J. Ahmed B. Liang H. Velusamy D.B. et al.SnSe2 2D anodes for advanced sodium ion batteries.Adv. Energy Mater. 2016; 61601188Crossref Scopus (208) Google Scholar the peaks within 0–1 V are ascribed to alloying reactions, while the peaks higher than 1 V are mainly contributed by conversion reactions. The cathodic and anodic peaks of LIB and NIB curves are symmetric, implying fully reversible conversion and alloying reactions consistent with the charge/discharge tests. The dQ/dV curve for KIB shows cathodic peaks more pronounced than the corresponding anodic peaks, especially for the conversion reaction, indicating the difficulty in fully reversible K conversion reactions. The irreversible capacity loss in KIBs is attributed to the sluggish potassiation kinetics, which can be quantitatively evaluated by the diffusion coefficient of K+ ions in Bi2Te3. We measured the diffusion coefficients of Li+, Na+, and K+ ions in Bi2Te3 by galvanostatic intermittent titration technique (GITT).40Tang K. Yu X. Sun J. Li H. Huang X. Kinetic analysis on LiFePO4 thin films by CV, GITT, and EIS.Electrochim. Acta. 2011; 56: 4869-4875Crossref Scopus (326) Google Scholar The overpotentials of KIBs are much larger than those of LIBs and NIBs according to the measured voltage profiles (Figure S3), but their diffusion coefficients calculated from GITT tests are essentially similar to those during alloying reactions in LIBs and NIBs. On the contrary, diffusion coefficients of Na+ during the conversion reaction in NIBs are higher than those of LIBs and KIBs, which is consistent with the better rate performance of NIBs than the LIB and KIB counterparts (Figure 2B) and manifests the significant role of conversion kinetics in determining the rate capability. We conducted in situ TEM to unveil the similarity and difference in reaction mechanisms between lithiation, sodiation, and potassiation, and to specifically elucidate the origin of anomalous Na electrochemistry. It is expected that the electrochemical reaction between Bi2Te3 and alkali metals undergoes conversion and alloying processes with possibly intercalation at the beginning, although the conversion stage is predominant.41Ihsan-Ul-Haq M. Huang H. Wu J. Cui J. Yao S. Chong W.G. et al.Thin solid electrolyte interface on chemically bonded Sb2Te3/CNT composite anodes for high performance sodium ion full cells.Nano Energy. 2020; 71104613Crossref Scopus (27) Google Scholar In situ TEM based on the open-cell configuration has demonstrated its ability to offer unprecedented spatial resolution and ample chemical information that enables a comprehensive understanding of battery reaction mechanisms.42Cui J. Zheng H. He K. In situ TEM study on conversion-type electrodes for rechargeable ion batteries.Adv. Mater. 2020; 2000699https://doi.org/10.1002/adma.202000699Crossref Scopus (25) Google Scholar Here, we construct open cells for in situ TEM experiments that utilize monodispersed Bi2Te3 nanoplates as the working electrode and native oxide solid electrolyte covered alkali metals as the counter electrode to allow quantitative analysis of structure, morphology, and phase evolutions upon electrochemical discharge reactions.17Yao S. Cui J. Lu Z. Xu Z.L. Qin L. Huang J. et al.Unveiling the unique phase transformation behavior and sodiation kinetics of 1D van der Waals Sb2S3 anodes for sodium ion batteries.Adv. Energy Mater. 2017; 71602149Crossref Scopus (138) Google Scholar,20Cui J. Yao S. Lu Z. Huang J.Q. Chong W.G. Ciucci F. et al.Revealing pseudocapacitive mechanisms of metal dichalcogenide SnS2/graphene-CNT aerogels for high-energy Na hybrid capacitors.Adv. Energy Mater. 2018; 81702488Crossref Scopus (113) Google Scholar,43Kim S. Cui J. Dravid V.P. He K. Orientation-dependent intercalation channels for lithium and sodium in black phosphorus.Adv. Mater. 2019; 31: 1904623Crossref Scopus (23) Google Scholar The time-sequential TEM/STEM images of dynamic evolutions during in situ lithiation, sodiation, and potassiation are displayed in Figure 3 (raw images are shown in Figure S4 with the coloring algorithm shown in Figure S5), which are representatively selected from original videos (Videos S1, S2, and S3) recorded in real time. It is obvious that both lithiation and sodiation processes similarly undergo a conversion-dominant reaction pathway in which the pristine Bi2Te3 single crystals transform into ultrafine metallic Bi nanoparticles uniformly dispersed in the amorphous alkali-metal telluride (a-LixTe or a-NaxTe where x ≈ 2) matrix, forming a clear reaction front between pristine and conversion regions. The reaction front continues moving from the alkali-metal contact area toward the far end of Bi2Te3 nanoplate, along with the diffusion of alkali ions. The lithiation reaction front takes ∼50 s to complete the whole conversion reaction, leading to the corresponding propagation speed of ∼15.2 nm s−1 (Figure 3A). In contrast, the sodiation process shows unexpected better kinetics than lithiation. From an observation in the midst of sodiation (Figure 3B), it takes ∼25 s to finish the entire conversion reaction of the remaining nanoplate with a faster propagation speed of ∼28.2 nm s−1. The statistics of propagation speed from more in situ experiments are given in Figure S6. This higher sodiation speed is consistent with the better performance in rate testing (Figure 2B) and GITT measurement (Figure S3) for NIBs. It is also noted that the direct conversion phase separation without the preceding intercalation reaction is distinct from our previous observation on the lithiation of van der Waals layered SnSe2,44Kim S. Yao Z. Lim J.M. Hersam M.C. Wolverton C