FULL PAPER www.small-journal.com In Situ Formation of Hierarchical Bismuth Nanodots/ Graphene Nanoarchitectures for Ultrahigh-Rate and Durable Potassium-Ion Storage Yuanxin Zhao, Xiaochuan Ren, Zhenjiang Xing, Daming Zhu,* Weifeng Tian, Cairu Guan, Yong Yang, Wenming Qin, Juan Wang, Lili Zhang, Yaobo Huang, Wen Wen,* Xiaolong Li, and Renzhong Tai* urgent strategies worldwide.[1] Recharge- Metallic bismuth (Bi) has been widely explored as remarkable anode mate- able lithium-ion batteries have achieved rial in alkali-ion batteries due to its high gravimetric/volumetric capacity. great success during the last 40 years, while However, the huge volume expansion up to ≈406% from Bi to full potassia- they gradually display certain limitations tion phase K Bi, inducing the slow kinetics and poor cycling stability, hinders in further large-scale applications, such as 3 high cost, uneven geological distribution its implementation in potassium-ion batteries (PIBs). Here, facile strategy is and short supplies of lithium resources developed to synthesize hierarchical bismuth nanodots/graphene (BiND/G) (0.0017 wt%) around the world. As alternative composites with ultrahigh-rate and durable potassium ion storage derived energy storage sources, sodium ion batteries from an in situ spontaneous reduction of sodium bismuthate/graphene (SIBs) and potassium ion batteries (PIBs) composites. The in situ formed ultrafine BiND ≈( 3 nm) confined in graphene have recently attracted tremendous interest owing to their natural abundance, low cost layers can not only effectively accommodate the volume change during the and environmental friendliness. Despite alloying/dealloying process but can also provide high-speed channels for having a similar abundance with sodium ionic transport to the highly active BiND. The BiND/G electrode provides (sodium and potassium represent 2.36 and a superior rate capability of 200 mA h g−1 at 10 A g−1 and an impressive 2.09 wt% in the Earth’s crust, respectively), reversible capacity of 213 mA h g−1 at 5 A g−1 after 500 cycles with almost no potassium presents some specific advan- + capacity decay. An operando synchrotron radiation-based X-ray diffraction tages. K /K exhibit a lower standard redox potential of −2.93 V (vs E°) compared with reveals distinctively sharp multiphase transitions, suggesting its underlying that of Na+/Na (−2.71 V vs E°), implying a operation mechanisms and superiority in potassium ion storage application. higher working voltage and energy density of PIBs.[2] Moreover, potassium ions have much better conductivity and relatively lower 1. Introduction desolvation energy in organic solvents.[3] These merits of potas- sium make it a promising low-cost candidate for high-energy and With the tremendous demand for applications from mobile elec- power density energy storage applications. tronic devices, vehicles and stationary power stations, developing The progress of PIBs mainly follows the development of efficient energy storage technologies has become one of the most electrode materials, especially considering the large-size of Y. X. Zhao, Dr. X. C. Ren, Z. J. Xing, Dr. D. M. Zhu, Dr. W. M. Qin, Y. X. Zhao, C. R. Guan, Prof. Y. Yang Dr. J. Wang, Dr. L. L. Zhang, Prof. Y. B. Huang, Dr. W. Wen, School of Physical Science and Technology Prof. X. L. Li, Prof. R. Z. Tai ShanghaiTech University Shanghai Institute of Applied Physics Shanghai 201210, China Chinese Academy of Sciences Y. X. Zhao, Z. J. Xing, C. R. Guan Shanghai 201800, China University of Chinese Academy of Sciences E-mail: [email protected]; [email protected]; Beijing 100049, China [email protected] W. F. Tian Y. X. Zhao, Z. J. Xing, Dr. D. M. Zhu, Dr. W. M. Qin, Dr. J. Wang, College of Science Dr. L. L. Zhang, Prof. Y. B. Huang, Dr. W. Wen, Prof. X. L. Li, Prof. R. Z. Tai Henan University of Technology Shanghai Synchrotron Radiation Facility Zhengzhou 450001, China Shanghai Advanced Research Institute Chinese Academy of Sciences Shanghai 201204, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201905789. DOI: 10.1002/smll.201905789 Small 2019, 1905789 1905789 (1 of 9) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.small-journal.com Figure 1. Schematic diagram of the novel anode materials designation. a) 2D-SB laminated with graphene, b) in situ formed BiND/G, and c) KxBiy/ graphene nanocomposites. potassium ions, which draw great efforts to explore stable and These two completely different transition processes indicate high energy density electrode materials. Fortunately, signifi- that the underlying mechanism is strongly associated with the cant advances have been achieved in cathode materials, such unique structure and size of the anode material. Despite this [4] [5] [6] as Prussian blue, Prussian green, amorphous FePO4, great progress, research on the potassium storage properties [7] [8] FeSO4F, and Fe/Mn-based oxides, which exhibit competitive and mechanisms of nanosized Bi is generally in its early stage. energy densities with their counterparts in LIBs and SIBs. How- A facile synthesis of Bi nanoarchitecture and an in-depth explo- ever, as for anode materials, the achieving capacity and cycling ration of their related potassiation/depotassiation mechanisms performance as previously reported are far lower than those in are thus of particular scientific and technological significance. LIBs and SIBs. The former promising candidate for the anode Herein, we developed a novel bismuth nanodots/graphene of PIBs was predicted to be carbonaceous materials in regard (BiND/G) hierarchical composite in situ formed from 2D to their inexpensiveness and high electronic conductivity. sodium bismuthate (2D-SB) for ultrahigh-rate and durable Potassium ions could intercalate into graphite, similar to the potassium ion storage. Sodium bismuthate (SB) is a commer- intercalation behavior of lithium in LIBs,[9] but demonstrated cially available, inexpensive, nontoxic and potent inorganic low energy density because of the limited theoretical capacity oxidant. Due to its strong oxidation characteristics, SB can be of carbonaceous materials. Another series of PIB anodes can naturally reduced to metallic Bi under mild conditions (laser[17] alloy/dealloy with K (such as Sb,[10] Sn,[11] Bi,[12,13] and P[14]) or electron-beam irradiation[18]). Based on these characteristics and display much higher energy density. Among these mate- of SB, we have innovatively succeeded in obtaining 2D-SB by rials, bismuth (Bi) is an ideal choice due to its nontoxicity, large exfoliation and made it laminate with graphene, which is used lattice spacing and high theoretical capacity (385 mA h g−1). as a precursor to synthesize BiND/G (Figure 1a). The 2D-SB Recently, Zhang’s group achieved Bi microparticles as an anode can be easily in situ converted to BiND with isolated ultrafine for potassium storage that could deliver a stable capacity up to granules (≈3 nm) due to the 2D size effect. Spontaneous reduc- 392 mA h g−1 at 0.4 A g−1 after 100 cycles, corresponding to tion of 2D-SB/G to obtain BiND/G greatly simplified the fabri- an almost 97% capacity retention.[13] However, because K+ has cation process, providing a straightforward way to achieve the a much larger ionic radius (1.38 Å) than Na+ (0.97 Å) and Li+ ultrafine Bi nanodots/graphene composite (Figure 1b). The (0.76 Å), the potassiation and depotassiation processes invoke unique structure of BiND/G (shown in Figure 1b,c) efficiently huge volumetric changes and slow kinetics, resulting in fast accommodates the volume change because of the isolated capacity decay and poor rate capability. To address these issues, BiND and graphene and enables fast ionic and electronic trans- the design of Bi anode nanoarchitecture has been demon- port to the high-active BiND during the alloying/dealloying pro- strated as a key strategy that can suppress mechanical fracture. cess (Figure 1c). A robust solid electrolyte interface (SEI) film For instance, Lu et al.[15] developed unique ultrathin carbon can be formed, and better conductivity can be achieved benefit- film/carbon nanorods/Bi nanoparticle materials for a long cycle ting from the super hierarchical nanoarchitecture. As a result, life and exhibited a decent capacity of 121 mA h g−1 at a high BiND/G realizes high-rate capability (200 mA h g−1 at 10 A g−1) current density of 1 A g−1 with 75% capacity retention after and unprecedented cycling stability (213 mA h g−1 at 5 A g−1 700 cycles. Nevertheless, this anode suffered from a complex after 500 cycles with almost no capacity decay). Operando syn- fabrication process and limited electrochemical performance chrotron radiation-based X-ray diffraction (SR-XRD) reveals due to the heavy aggregation during cycling, restricting its fur- that the electrochemical mechanism of the BiND/G undergoes ther application. Exploring a simple and efficient method to sharp and stable phase transitions, which directly contributes to synthesize ultrafine Bi granules can be critical for pushing the its excellent electrochemical performance. Bi anode of potassium ion battery technology to the next level. What is more, the size of crystallite not only influences the electrochemical kinetics process but also may lead to different 2. Results and Discussion electrochemical mechanisms. Zhang’s group revealed that Bi microparticles suffer reversible, stepwise Bi–KBi2–K3Bi2–K3Bi Exfoliated 2D-SB has a well-maintained hexagonal structure transitions after the initial continuous surface potassiation.[13] similar to sodium bismuthate (Figure S1a, Supporting Informa- Guo’s work found that Bi nanoparticles undergo a solid-solu- tion), and their XRD patterns are shown in Figure S1b,c (Sup- tion reaction, followed by two typical two-phase reactions, cor- porting Information). Figure S1d,e (Supporting Information) [16] responding to Bi–Bi(K) and Bi(K)–K5Bi4–K3Bi, respectively.