Ni2P electrocatalysts decorated hollow carbon spheres as bi-functional mediator against shuttle effect and Li dendrite for Li-S batteries
Graphical Abstract
Well-dispersed Ni2P electrocatalysts implanted hollow carbon spheres (Ni2P-HCS) are designed as bi-functional mediators for advanced Li-S batteries. Ni2P-HCS exhibits robust chemisorption and high-efficient electrocatalysis to suppress the shuttle effect, and regulates the Li deposition behaviour to realize dendrite-free Li anode. Loading coefficient and catalytic effectiveness value are proposed to evaluate the intrinsic catalytic ability of electrocatalysts.
Introduction
Despite vast attractive superiority, such as high theoretical specific capacity (1675 mAh g−1 of sulfur) and low cost of active materials, [1], [2], [3] the commercialization of Li-S batteries is still plagued with several technical challenges associated with both S cathode and Li anode. [4], [5], [6], [7] At the S cathode side, the sluggish reaction kinetics of the multi-step conversion process, and the notorious shuttle effect of soluble lithium polysulfide intermediate (LiPSs) give rise to low active material utilization and rapid capacity degradation. [8], [9], [10] For the Li anode, severe Li dendrite growth ascribed to inhomogeneous distribution of Li ions (Li+) and surface current results in low Coulombic efficiency (CE) and short lifespan. [11], [12] Therefore, it is highly urgent to develop high-efficiency LiPSs immobilizer and Li dendrite inhibitor for high-performance Li-S batteries. The high-efficiency LiPSs immobilizer should have the following features: (1) excellent LiPSs capturing ability and catalytic activity toward Li-S redox reactions for suppressing the LiPSs shuttling behaviors; [13] (2) high electrical conductivity to ensure superior electron transport for accelerating the redox kinetics; [14] (3) abundant macro/mesoporous channels to allow penetration of electrolyte for fast Li+ migration. [15] A high-efficiency Li dendrite inhibitor should effectively regulate the Li+ flux and current distribution on electrode surface to restrain uneven nucleation and growth of Li. [16], [17].
Hollow porous carbon materials, with lightweight features and high electrical conductivity, have been widely studied as favorable S/Li hosts and interlayers for mitigating shuttle effect and regulating Li deposition. [18], [19], [20], [21] Besides, large surface area and high porosity of hollow carbon nanocages enlarge the electrolyte/material contact area and ensure rapid diffusion of Li+. [22], [23] However, nonpolar carbon renders weak surface affinity toward polar LiPSs, which leads to serious shuttle effect and poor capacity retention. [24], [25] In this regard, an emerging strategy is to implant polar mediator with electrocatalytic activity, such as transition metal oxides (TMOs), [26] sulfides (TMSs), [27], [28] into carbonaceous substrate to enable robust physiochemical interaction toward LiPSs and accelerated conversion process on polar surface. Moreover, when the carbon support decorated with lithiophilic polar materials (Co3O4, [29] SnO2[30]) serve as Li hosts, reduced Li nucleation barrier and suppressed dendrite growth can be realized. But the intrinsic poor electrical conductivity of TMOs/TMSs results in limited S utilization for Li-S redox reactions and increased voltage polarization during Li plating/stripping process. [31], [32] Transition metal phosphides (TMPs), with polar character [33] and metallic feature, [34], [35] provide strong chemical binding with LiPSs and fast electron transfer on reaction interface. Besides, excellent catalytic activity toward Li-S conversion process enables rapid redox reactions. [36], [37], [38] And lithiophilic TMPs can react with Li to produce mixed ion-electron conducting hosts composed of highly Li+ conducting lithium phosphide (Li3P, ~10−4 S cm−1) and corresponding metal, which ensures homogeneous Li+ flux and current distribution on electrode surface, and makes TMPs as promising high-efficiency Li dendrite inhibitor. [39], [40].
Herein, we proposed well-dispersed Ni2P decorated hollow carbon spheres (Ni2P-HCS) as high-efficiency bi-functional mediator to simultaneously inhibit shuttle effect and suppress Li dendrite for high-performance Li-S batteries. As interlayer, conductive Ni2P-HCS ensures excellent LiPSs anchoring capability, superior electrocatalytic effect on Li-S redox reactions and rapid charge/ion migration, which points to Ni2P-HCS as high-efficiency LiPSs immobilizer. The anchoring and electrocatalysis of LiPSs by Ni2P-HCS is investigated by in-situ X-ray diffraction and further verified by theoretical calculations. As a result, the Li-S coin cells with Ni2P-HCS interlayer deliver superb rate capacity of 642.7 mAh g−1 at 5 C and the pouch cells also present ultrahigh initial discharge capacity and favorable cycling stability. As Li host, the lithiophilic nature of Ni2P-HCS and as-formed mixed ion-electron conducting host (Li3P/Ni) are beneficial to homogenizing Li+ flux and surface electrical field for effectively suppressing dendritic growth. The well-designed Li-S full cells with high-efficiency bi-functional Ni2P-HCS exhibit excellent cycling stability over 500 cycles with fading rate of 0.05% per cycle at 1 C. Moreover, the Li-S full cells with high sulfur loading of 5.9 mg cm−2 and low E/S ratio (6.7 μL mg−1) realize high areal capacity of 6.67 mAh cm−2, suggesting the great potential of Ni2P-HCS for practical application in Li-S batteries. The loading coefficient (C/S, the ratio of areal loading catalyst to sulfur) was used to normalize the performance divergence with and without electrocatalyst. On this basis, catalytic effectiveness value (CEV) was proposed to evaluate the catalytic efficiency of diverse catalysts more intuitively.
Section snippets
Synthesis of Ni-MOF
Typically, 445 mg Ni(NO3)2.6 H2O, 160 mg trimesic acid, and 3 g polyvinylpyrrolidone (PVP) were fully dissolved in the 72 mL mixture solution (distilled water: methanol: DMF = 1:1:1, v/v/v) under vigorous stirring for 30 min (ambient temperature). The obtained uniform solution was transferred into 100 mL Teflon-lined autoclave and heated at 150 °C for 10 h. The light green Ni-MOF products were obtained by centrifuging, washing with distilled water three times, and drying at 80 °C.
Synthesis of Ni-HCS, Ni2P-HCS and HCS
The Ni-MOF
Results and discussion
Fig. 1a schematically illustrates the fabrication process of bi-functional Ni2P-HCS. The hollow Ni-based metal organic framework (Ni-MOF) precursor was synthesized by solvothermal reaction, which was further annealed under inert atmosphere for converting Ni ions to Ni nanocrystals (denoted as Ni-HCS). In the subsequent step, Ni-HCS was phosphorized to obtain the Ni2P-HCS. Notably, metalloid Ni2P-HCS possesses strong entrapment and catalytic conversion capability toward LiPSs. Moreover, the
Conclusions
In summary, the high-efficiency bi-functional Ni2P-HCS was developed for simultaneously alleviating shuttle effect and inhibiting Li dendrites in Li-S batteries. Experimental and computational results reveal the significant catalytic activity and lithiophilicity of Ni2P-HCS, empowering rapid LiPSs conversion and uniform Li+ deposition. When Ni2P-HCS is used as an interlayer, it affords robust LiPSs adsorption and promoted sulfur conversion kinetics owing to facile charge/mass transfer on the
CRediT authorship contribution statement
Yang Shan: Conceptualization, Methodology, Investigation, Writing – original draft, Xiao Ru: Writing – review & editing, Investigation, Conceptualization, Hu Tianzhao: Formal analysis, Investigation, Fan Xialu: Validation, Xu Ruogu: Investigation, Sun Zhenhua: Conceptualization, Methodology, Writing – review & editing, Supervision, Funding acquisition, Zhong Benhe: Methodology, Guo Xiaodong: Writing – review & editing, Supervision, Li Feng: Writing – review & editing, Supervision, Funding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51972313, 52020105010 and 51927803), National Key R&D Program of China (2016YFA0200100 and 2016YFB0100100), the Strategic Priority Research Program of Chinese Academy of Science (No. XDA22010602), Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. Y201942), Liaoning Revitalization Talents Program (No. XLYC1908015 and XLYC2007080) and DNL Cooperation Fund, CAS (DNL202019).
Author contributions
S.Y, R.X, Z.S.
Shan Yang received her B.S. degree from School of chemical engineering, Sichuan University, China, in 2018. She is currently a Ph. D student at School of chemical engineering, Sichuan University. Her research is focus on electrocatalysts for high-energy-density lithium-sulfur batteries.
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Shan Yang received her B.S. degree from School of chemical engineering, Sichuan University, China, in 2018. She is currently a Ph. D student at School of chemical engineering, Sichuan University. Her research is focus on electrocatalysts for high-energy-density lithium-sulfur batteries.
Ru Xiao received her B.S. (2019) in Materials Processing and Control from Northeastern University in China. She is currently a graduate student at the Institute of Metal Research, Chinese Academy of Sciences. Her research is focused on the design and application of novel nanocarbon materials and carbon-based composite materials for high-energy-density lithium-sulfur batteries.
Tianzhao Hu is a PhD candidate in School of Material Science and Engineering, Zhengzhou University. He is now united training under Prof. Feng Li’s supervision at Institute of Metal Research. His research mainly focuses on electric double-layer capacitor and hybrid capacitors.
Xialu Fan received her B.S. degree from chemistry, Beijing Normal University, China, in 2016. She is currently a Ph. D student at Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China. Her research is focus on lean electrolyte electrochemical performance of lithium-sulfur batteries.
Ruogu Xu received his B.S. degree from China University of Mining and Technology in 2019. He is currently a Ph.D. candidate at Institute of Metal Research, Chinese Academy of Science and at University of Science and Technology of China. His research interests are solid polymer electrolytes and solid-state batteries.
Zhenhua Sun is currently a professor at Institute of Metal Research, Chinese Academy of Sciences. He received his B.S. and Ph.D. degrees in inorganic chemistry from Jilin University in 2001 and 2006, respectively. Then he was a Postdoctoral Research Fellow in the Chinese University of Hong Kong from 2007 to 2009. His current research interests mainly focus on the synthesis and application of nano-carbon materials and carbon-based composite materials for electrochemical energy storage. He has published more than 90 papers in peer-reviewing journals including Nature Communications, Advanced Materials, etc. with 7200 citations (H-index 41).
Benhe Zhong obtained her bachelor's degree (1955) from Chengdu institute of technology （Chengdu university of science and technology in the past, integrated into sichuan university）. She is a distinguished professor of Sichuan university and a top scientist in the field of phosphorus chemical engineering. She has conducted studies on the basic and industrialization for clean processing and comprehensive utilization of phosphorus resources for about 60 years and gained series of major achievements. In recent years, her research interests involve the development of advanced energy materials and electrodes for lithium-ion batteries, sodium-ion batteris and lithium-sulphur batteries.
Xiaodong Guo is currently a professor in the School of Chemical Engineering of Sichuan University, visiting professor in university of Wollongong of Australia. He has published over 100 papers in Angew. Chem., Int. Ed., Adv. Mater., Adv. Energy. Mater and Adv. Sci. etc. by the first author/ Corresponding author.
Feng Li is a professor of the Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS). He received his Ph.D. in materials science at IMR, CAS in 2001 supervised by Prof. Hui-Ming Cheng. He mainly works on the carbon materials and energy materials. He has published more 300 papers on peer-reviewed journals, such as Nature Energy, Energy Storage Materials, Adv Mater, J Energy Chem, etc. with more 50000 citations and H-index about 95. He obtained the award of National Science Fund for Distinguished Young Scholars by National Foundation of Science China, and Highly Cited Researcher by Clarivate Analytics from 2016 to 2020.