Elsevier

Journal of Power Sources

Volume 292, 1 October 2015, Pages 23-30
Journal of Power Sources

High performance of Co-doped NiO nanoparticle anode material for rechargeable lithium ion batteries

https://doi.org/10.1016/j.jpowsour.2015.05.029Get rights and content

Highlights

  • Co-doped NiO sample was synthesized by low cost facile solvothermal method in polyol medium.

  • Co2+ ion substitutes Ni2+ and enhances the concentration of holes to improve p-type conductivity.

  • The electronic conductivity of NiO is largely improved through Co2+ doping.

  • Co doping significantly suppresses particles agglomeration, which offers a high surface area.

  • Greatly enhanced rate capability and excellent cycle retention of Co-doped NiO anode revealed.

Abstract

A comparative electrochemical study of undoped NiO and Co-doped NiO is performed in order to elucidate the effect of the Co distribution in the crystal lattice of NiO for energy storage applications. Both samples are synthesized using a facile solvothermal strategy and characterized systematically by X-ray diffraction, field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray photoelectron spectroscopy and charge/discharge measurements. It is found that doping process does not affect the phase structure of pristine NiO. However, it obviously significantly influences the morphology, suppresses the particles agglomeration and enhances the specific surface area of NiO. More importantly, the substitution of Co for Ni site enhances the p-type conductivity of NiO via the generation of holes. Consequently, the obtained Co-doped NiO anode displays superior Li-battery performance with a large reversible capacity, excellent cyclic performance, and good rate capability in comparison to undoped NiO.

Introduction

Over the past decades, great efforts have been dedicated to searching for alternative anode materials to replace commercial graphite anode of lithium ion batteries for improving their energy density and safety. Among the explored systems, 3d transition-metal oxides (MO) have received increasing research attention as the anodes of lithium-ion batteries, due to their higher theoretical specific capacity (700–1000 mAh g−1) than the commercial graphite anode (372 mAh g−1) and high energy density [1], [2]. Most transition metal oxide anodes (such as Co3O4, CuO, Fe3O4, SnO2 and NiO) exhibit the same lithium storage mechanism via the reversible conversion reaction viz., MOx + 2xLi+ + 2xe ↔ M + xLi2O [1], [2], [3]. In particular, nickel oxide (NiO) is considered as a promising anode for lithium-ion batteries owing to its high theoretical capacity (∼718 mAh g−1), higher density (6.81 g cm−3) than that of graphite (2.268 g cm−3), nontoxicity, environmental benignity, and low material cost [4]. Besides lithium ion batteries, NiO is also extensively used in fuel cells [5], solar cells [6], [7], and supercapacitors [8] applications. However, many transition metal oxide anodes suffer from poor cycling performance owing to their tendency to agglomerate during lithium ion insertion/extraction processes and the mechanical instabilities caused by the huge volume changes, resulting in increased diffusion lengths and electrical disconnection from the current collector [9], [10], [11]. In addition, the significant voltage hysteresis between charge and discharge is another major drawback of conversion reactions [12]. Therefore, it is believed that to fully use these conversion reactions in practical cells it is imperative to reduce this hysteresis which currently limits both the energy efficiency and the power capabilities of batteries using conversion reactions.

Several attempts have been made to overcome the above issues of NiO and especially to increase the electrical conductivity; the use of electrically conductive carbon and the preparation of nano-structured NiO (e.g. nanoplatelets, nanospheres, nanosheets, and nanoflakes) were the key points [11], [13], [14], [15], [16], [17], [18]. Carbon encapsulated nanocomposites facilitate the inter-particle connectivity and therefore tend to improve the overall electrical conductivity of the electrodes, whereas nano-structured electrodes with small particle-sizes and high surface areas improve the Li+ ion diffusion within the crystalline lattice of the NiO anodes. Recently, the research groups of J.P. Tu [11] and W. Pan [19] reported a novel strategy to improve the electrical conductivity of the NiO nanostructure. NiO is an intrinsic p-type semiconductor with a wide band gap of ∼3.6 eV [20], [21]. Hence, the electrical conductivity of NiO mainly depends on the concentration of the holes formed by oxygen vacancies in the material. W. Pan et al. reported that aliovalent Li ion doping increased the electrical conductivity of p-type NiO nanowires by 7 orders of magnitude [19]. It is believed that the Li+ ions occupied the Ni2+ sites and created excess holes in the NiO lattice (12O2(g)+Li2O2NiO2OOx+2LiNi+2h), which improved the electrical conductivity of NiO. On the other hand, J.P. Tu et al. reported that Co doping has a significant influence on the growth and electrochromic properties of NiO nanoflake arrays [11]. In addition, the Co doped NiO nanoflake arrays also show high rate capability, better capacity retention and enhanced electronic conductivity compared to pure NiO anodes for lithium ion battery applications [11], [22]. Thus, it is of particular importance to improve the electrical conductivity of NiO in order to enable its integration in semiconductor electronics. Therefore, it is important to note that preparing nano-scale p-type NiO electrodes in combination with metal ion doping would be a very effective and novel strategy to enhance the electrical conductivity of NiO nanoparticle electrodes for high power lithium ion batteries. However, the poor uniformity of the electrodes along with their complex fabrication procedures makes the resultant properties hard to control.

Our research group has focused mainly on the synthesis of nano-scale electrodes by the polyol-based approach. The polyol process has many advantages, since it allows for nanoscale synthesis at lower synthesis temperatures with short-term heat treatment, as well as it also acts as a stabilizer and inhibitor, which ultimately aids the formation of well-defined and uniformly dispersed nanocrystalline samples. In particular, the polyol-based solvothermal process is advantageous not only due to its simplicity, ease of operation and good reproducibility, but also because it enhances the solubility/reactivity of the precursors at high pressures and temperatures close to the boiling point of the polyol employed. To date, there is no report on a detailed studies of Co doped NiO nanoparticle electrodes synthesized by a facile chemical method for high power lithium ion batteries. Herein, we report an effective way to significantly improve the electrochemical performance of a Co-doped NiO anode prepared by a polyol-assisted solvothermal process followed by annealing at a moderate temperature of 500 °C for 5 h in an air atmosphere. As anticipated, the Co2+ partially substitutes Ni2+, resulting in an increase the concentration of holes and, therefore, improved p-type conductivity, which is useful to reduce the charge transfer resistance during the charge/discharge process. The superior electrochemical performance of the doped sample in comparison to the undoped sample clearly shows the Co doping effect, which increases the electrical conductivity, diffusion coefficient and kinetic properties during lithiation and delithiation. The obtained electrochemical data in the present work is better than the reported Co-doped NiO nanoflake arrays [11].

Section snippets

Material synthesis

Undoped NiO and Co-doped NiO nanoparticles were synthesized by the polyol-based solvothermal method. The detailed solvothermal synthesis procedure can be found in our previous paper [23]. Briefly, Nickel (II) nitrate hexahydrate [Ni(NO3)2.6H2O, 99.999%, Sigma–Aldrich, USA] was used as the starting precursor for the preparation of the undoped NiO anode, while cobalt (II) nitrate hexahydrate [Co(NO3)2·6H2O, >98%, Sigma–Aldrich, USA] was employed as a dopant precursor for the synthesis of the

Crystal structure and morphology

Fig. 1 shows the X-ray diffraction patterns of the undoped NiO and Co-doped NiO samples. All of the diffraction peaks for both samples are well indexed and exactly matched to the standard cubic NiO structure (JCPDS No. 001-1239) with a space group of Fm-3m (225) [4]. No obvious signals from possible impurities, such as Co, were observed in the pattern, implying that the Co doping did not change the original NiO structure. Therefore, it can be suggested that the Co ions are highly dispersed in

Conclusions

In summary, undoped and Co-doped NiO nanoparticle anodes were successfully synthesized in a polyol medium using a low-temperature solvothermal method, followed by annealing at a moderate temperature of 500 °C in air. The XRD pattern shows that pure cubic NiO is formed in the doped sample without any impurities of Co dopant, indicating that the Co was indeed uniformly distributed on and within the NiO sample. No significant variation in the primary particles size was observed in either sample,

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2014R1A2A1A10050821).

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    These authors contributed equally to this paper.

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