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Hierarchical NiCo2O4 Micro- and Nanostructures with Tunable Morphologies as Anode Materials for Lithium- and Sodium-Ion Batteries Fang Fu,† Jiadong Li,† Yuze Yao,† Xueping Qin,† Yubing Dou,† Haiyan Wang,†,‡ Jenkin Tsui,§ Kwong-Yu Chan,§ and Minhua Shao*,†,⊥ †
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, P.R. China ‡ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China § Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China ⊥ Energy Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P.R. China S Supporting Information *
ABSTRACT: NiCo2O4 microrods with open structures are successfully synthesized using a solvothermal method. Compared with those of dense microspheres, the onedimensional (1D) porous microrods show much higher capacities and stability for both Li- and Na-ion batteries due to the 1D open structure facilitating fast ion transport and buffering volumetric change during charge/discharge. This work demonstrates that the electrochemical performance of NiCo2O4 is highly dependent on morphologies of the active material.
KEYWORDS: binary metal oxide, solvothermal method, one-dimensional structure, porous hierarchical structure, batteries
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short cycling life.9 Many efforts are being made to overcome limitations of NiCo2O4 by surface modification and mixing with various carbons to form composites.10−13 Despite some encouraging progresses, the performance enhancement of NiCo2O4 is still unsatisfactory. It has been know that the performance of battery active materials is highly dependent on their structures.14 Secondary particles possessing a onedimensional (1D) structure have exhibited many advantages in energy storage applications.15 For LIBs and SIBs, 1D structure with confined dimension and small diameter benefits fast ion (Li+ and Na+) transport due to a short solid-state diffusion pathway for ions.16 Further, porous hierarchical structure in the secondary particle facilitates easy access of electrolyte into the active material, which effectively enhances the ion diffusion dynamics. Moreover, the porous hierarchical structure can provide additional space to buffer the volumetric expansion of active materials, resulting in an improved cyclability.17−21 In summary, a 1D porous hierarchical structure can provide adequate electrode/electrolyte contact, fast ion transport, and excellent structure stability.
INTRODUCTION Global warming and environmental problems have driven researchers to develop clean and green energy conversion and storage systems.1 Lithium-ion batteries (LIBs) have gained a huge success as power sources for portable devices. They have also been intensely explored to further enhance their performance while lowering the material cost for widespread use in electric vehicles, where arising sodium-ion batteries (SIBs) are an appealing alternative. Despite the fact that SIBs possess energy and power densities lower than those of LIBs due to the less negative redox potential of Na/Na+ over Li/Li+ and radius of Na+ larger than that of Li+, the abundant reserves and low cost of Na resources make them very attractive for electric vehicles and power grids.2,3 In general, the performance of batteries is mainly determined by electrode materials.4 Thus, developing a high-performance electrode material that possesses high capacity, rate capability, and cyclability is crucial for practical applications of LIBs and SIBs in electric vehicles. Binary metal oxide NiCo2O4 is an attractive anode material because of its low cost and high theoretical capacity5,6 In addition, the electrical conductivity and mechanical stability are higher than those of single-component metal oxides Co3O4 and NiO.7,8 The sluggish reaction kinetics and large volume change during charge/discharge result in poor rate performance and a © XXXX American Chemical Society
Received: February 14, 2017 Accepted: April 28, 2017 Published: April 28, 2017 A
DOI: 10.1021/acsami.7b02175 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a and b) SEM micrographs and (e) XRD pattern of rod-like Ni−Co precursors. (c and d) SEM micrographs and (f) XRD pattern of spherical-like Ni−Co precursors. X-ray diffraction (XRD, Philips PW-1830 X-ray diffractometer, Cu Kα radiation), and X-ray photoelectron spectroscopy (XPS, PHI5600). Electrochemical Tests. Electrochemical performances were examined in 2025 coin cells assembled inside the Ar-filled glovebox. Anodes were fabricated by pasting the mixture of as-prepared NiCo2O4 samples, carbon black (BP2000) (Cabot corporation, United States), and PVDF (HSV900, Arkema, France) in the weight ratio of 80:10:10 (8:1:1) onto a Cu foil. For the LIB fabrication, the metallic Li foil was used as the cathode and reference electrode, 1 M LiPF6 in ECDMC-DEC (1:1:1, v:v:v) as the electrolyte, and a porous polyethylene film as the separator. For the SIB fabrication, the metallic Na foil was served as the cathode and reference electrode, 1 M NaPF6 in FEC-PC (1:19, v:v) as the electrolyte, and glass fiber as the separator. The cells were cycled in the voltage range of 0.01−3.0 V at different current densities with a LAND-V34 battery-testing instrument (Wuhan, China). Cyclic voltammograms (CVs) and AC impedance (EIS) measurements were conducted on a CHI760 electrochemistry analyzer. The CVs were measured within 0.01−3.0 V at 0.01 mV s−1. EIS measurements were performed within 10 mHz to 100 kHz with an amplitude of 5.0 mV.
In this work, 1D porous NiCo2 O4 microrods were synthesized via a water-solvothermal method. The electrochemical performance was compared with that of microspheres synthesized by an isopropanol-solvothermal method. They exhibit different reversible capacity, rate capability, and lifetime when evaluated as anodes in LIBs and SIBs. The 1D porous NiCo2O4 microrods provided much better electrochemical performance owing to their unique morphology.
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EXPERIMENTAL SECTION
Preparation of 1D Porous NiCo2O4 Microrods. In a typical synthesis, 60 mL of Na2CO3 (2.49 g; 0.39 mol L−1) solution was added dropwise to an equal volume of an aqueous solution containing Co(CH3COO)2·4H2O (3.00 g; 0.20 mol L−1) and Ni(CH3COO)2· 4H2O (1.51 g; 0.10 mol L−1). The reaction solution was then heated in a Teflon autoclave at 180 °C for 5 h. After the solvothermal reaction, the pink precipitate was centrifuged, washed with water, dried at 100 °C overnight, and finally annealed at 400 °C for 5 h to generate NiCo2O4 microrods. Preparation of NiCo2O4 Microspheres. In a typical experiment, Co(NO3)2·6H2O (2.91 g; 0.125 mol L−1) and Ni(NO3)2·6H2O (1.45 g; 0.0625 mol L−1) were dissolved in 80 mL of isopropanol and stirred at 25 °C. After 0.5 h, the solution was sealed in an autoclave and maintained at 180 °C for 5 h. The brown precipitate was centrifuged and dried at 100 °C. The powder was annealed at 400 °C for 5 h to obtain NiCo2O4 microspheres. Characterization. Products were thoroughly characterized by scanning electron microscopy (SEM, JEOL-6300 and JEOL-6700F), energy dispersion X-ray (EDX, an EDX detector system attached to JEOL-6300), Brunauer−Emmett−Teller (BET, ASAP2020 analyzer),
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RESULTS AND DISCUSSION The morphology of Ni−Co precursors was investigated via SEM. Figures 1a and b display SEM images of rod-like precursor synthesized using the water-solvothermal approach. It is clear that the precursor consists of highly uniform microrods about 2 μm in diameter and 6 μm in length. Each microrod is composed of a smooth pyramid-shaped apex at both ends and apine bark-like rough surface in the middle. Nevertheless, the precursor synthesized using the isopropanol-solvothermal B
DOI: 10.1021/acsami.7b02175 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) SEM image of the rod-like Ni−Co precursor. (b, c, and g) Elemental mappings and EDX analysis of the precursor shown in panel a. (d) SEM image of the spherical-like Ni-Co precursor. (e, f, and h) Elemental mappings and EDX analysis of the precursor shown in panel d.
isotherm and the corresponding pore size distribution of the NiCo2O4 microrods. The BET specific surface area was calculated to be 3.0 m2 g−1. The sizes of the majority of the pores within the particles as analyzed by BJH equation for the desorption branch are below 20 nm. Such a highly open structure is desirable in batteries because it facilitates electrolyte penetration into the bulk material, increases the electrolyte/ active material contact area, and effectively accommodates the volume change. The formation of the porous structure during the calcination process can be attributed to release of CO2 during the thermal decomposition of Co−Ni−CO3 precursor: Co−Ni−CO3 → NiCo2O4+CO2. The NiCo2O4 prepared by the isopropanol-solvothermal method are hierarchical microspheres, constructed by densely packed nanoparticles with a small size of ∼10 nm (Figures 3c and d and S1b). Its BET area was calculated to be 3.8 m2 g−1. The pore size (
