Facile renewable synthesis of nitrogen/oxygen co-doped graphene-like carbon nanocages as general lithium-ion and potassium-ion batteries anode

Abstract

Environmentally-friendly carbon-based materials possess the potential applications as general anode for alkali-ion batteries. However, the existing carbon-based materials cannot satisfy the increasing demand for high energy density and need further active exploration. Herein, nitrogen/oxygen co-doped graphene-like carbon nanocages (NOGCN) is synthesized from biomass cytidine on hydro-soluble sodium chloride nanocrystals by a one-step method as a general lithium and potassium-ion batteries anode. All reactants are completely renewable and readily available. The nitrogen/oxygen-doping, large interlayer spacing and robust self-supporting nanocage architecture greatly favour electrolyte penetration and improve the kinetics for ion and electron transport, resulting in extraordinary electrochemical performance. The synthesized NOGCN electrodes exhibit a high lithiation storage capacity of 620 mA h g⁻¹ over 500 cycles at 500 mA g⁻¹, with continuously magnifying capacity. Moreover, the impressive reversible potassiation capacity (355 mA h g⁻¹ at 200 mA g⁻¹) and rate capability (114 mA h g⁻¹ at 1000 mA g⁻¹) were achieved despite the large-sized potassium ions. Kinetic analysis and density functional theory calculations elaborately illustrate the Li/K-absorption properties of the N/O-doped graphene-like structure, further demonstrating the chemical affinity and superiority in Li/K storage. This study provides a facile and completely renewable method to prepare promising general anode material for alkali-ion batteries.

Introduction

With the shortage of fossil fuels and growing influence of global climate-change issues, high energy density battery systems, such as alkali-ion batteries, are extensively regarded as feasible trends for renewable energy storage devices. Lithium-ion batteries (LIBs) possess high energy and power density and have been widely used as mature and reliable energy storage system in many application scenarios, especially in portable electronic devices and electric vehicles [1,2]. In view of large-scale stationary applications, the optimized type of alkali-ion batteries that use abundant potassium ionic shuttle as its operating principle has attracted significant attention [3,4]. Compared with other alkaline metals (ions), Li/Li⁺(−3.04 V vs. standard hydrogen electrode) and K/K⁺ (−2.93 V) exhibit the lowest redox potential in various kinds of nonaqueous electrolytes, implying higher output voltage plateau and energy density for LIBs and potassium-ion batteries (PIBs) system.

However, as a common commercial anode material for alkali-ion batteries, graphite can no longer satisfy the urgent need for energy density because of its low theoretical capacity (such as, 372 mA h g⁻¹ for LIBs and 278 mA h g⁻¹ for PIBs). Much as various designed materials have been proposed to use as anode material like silicon [5,6], germanium [7], alloy materials [8] and organic compounds [9], unstable electrochemical cycling performances seriously limit their practical applications. Owing to the long-cycling capacity and important role in the carbon-negative cycle, carbon-based materials are still considered as the most promising materials for anodes of alkali-ion batteries like LIBs/PIBs. Owing to the good ductility, carbon-based materials can maintain structural integrity after multiple charge/discharge cycles without electrode pulverization, and has the advantage of being suitable for alkali-ions with large size radius in theory. Recent research results further revealed that heteroatom-doping is an effective strategy for regulating the physical and chemical properties of carbon materials by improving electronic conductivity and creating more available active sites, with significantly enhanced alkali-ion storage [10]. For example, in a recent report by Wang et al., amorphous ordered mesoporous carbon (OMC) delivered a capacity of 218.8 mA h g⁻¹ at 200 mA g⁻¹ as anode for PIBs [11]. An enhanced rate capability has also been observed for instance, free-standing nitrogen-doped cup-stacked carbon nanotube (NCSCNT) was tested in PIBs with a capacity retaining 75 mA h g⁻¹ at 1000 mA g⁻¹ [12]. Moreover, Fan. et al. demonstrated that edge-thionic acid-functionalized GnPs (TAGnPs) displayed a rate capability of 80.6 mA h g⁻¹ at 5000 mA g⁻¹, and enabled stronger Li⁺ adsorption capability [13]. Unfortunately, the complex manufacturing process, high cost, and non-renewable raw material devalue their approach for mass production. Considering the unavoidable pollution of by-products and the carbon emission in process, these reports hardly constitute environmentally friendly methods. Furthermore, exploring general anode materials with decent electrochemical performances both in LIBs and PIBs is also a bottleneck for production application [14]. The general anode design needs to adapt to different alkali metal ion sizes and adsorption energy, and is apparently critical for decreasing the manufacturing costs and narrowing the gap between the world-wide LIB industry and PIB commercialization. It is still a long way to go to meet the universality and facile fabrication demands of high-performance anodes in LIBs and PIBs.

In this work, the nitrogen/oxygen co-doped graphene-like carbon nanocage (NOGCN) was synthesized through a one-step method from the biomass-derived sugar. The mixture of cytidine and sodium chloride was carbonized at low temperature (700 °C) to obtain NOGCNs. Cytidine can be regenerated from bio-waste [15] and sodium chloride nanocrystal templates can be completely reused after dissolution and recrystallization. All reactants and products in progress are non-toxic. NOGCNs exhibited high surface area, regular shapes and benign nitrogen/oxygen co-doping as expected. By virtue of the unique nanostructure and robust chemical bonds, NOGCNs exhibited promising capacity (620 mA h g⁻¹ at 500 mA g⁻¹ in LIBs 355 mA h g⁻¹ at 200 mA g⁻¹ in PIBs), excellent cyclic stability (492 mA h g⁻¹ at 1000 mA g⁻¹ after 1000 cycles in LIB, and 131 mA h g⁻¹ at 500 mA g⁻¹ after 300 cycles in PIB), impressive rate capability and rapid reaction kinetics. The density functional theory (DFT) calculations also indicate higher adsorption ability and extraordinary diffusivity achieved by abundant nitrogen/oxygen co-doping. Benefiting from the feasible material recycling and scalable process, our work provides a general high-performance anode design for LIBs and PIBs, and would be a cost-effective solution for advanced energy storage devices.