Elsevier

Electrochimica Acta

Volume 173, 10 August 2015, Pages 672-679
Electrochimica Acta

Understanding the electrochemical superiority of 0.6Li[Li1/3Mn2/3]O2-0.4Li[Ni1/3Co1/3Mn1/3]O2 nanofibers as cathode material for lithium ion batteries

https://doi.org/10.1016/j.electacta.2015.05.083Get rights and content

Highlights

  • Li-rich 0.6Li[Li1/3Mn2/3]O2-0.4Li[Ni1/3Co1/3Mn1/3]O2 nanofibers are successfully synthesized by the electrostatic spinning method.

  • The step-by-step CV tests revealed that nanofibers electrode presents higher discharge voltage platform and increased discharge energy density.

  • The enhanced electrochemical performance is attributed to the neat ion arrangement in crystal structure and the better electrochemical kinetics of nanofibers electrode.

Abstract

Solid solution cathode materials 0.6Li[Li1/3Mn2/3]O2-0.4Li[Ni1/3Co1/3Mn1/3]O2 with different morphologies were synthesized by electrospinning and coprecipitation method respectively. The field-emission scanning electron microscope images verified the successful formation of nanofibers for electrospinning and nanoparticles for coprecipitation and the nanofibers showed larger specific surface area according to the Brurauer Emmerr Teller procedure. The X-ray powder diffraction patterns and the corresponding lattice parameter refinements showed that both samples can be indexed to hexagonal α-NaFeO2 layered structure with space group of R-3m. And the material prepared by electrostatic spinning method has a tight atomic arrangement in the layer yet the different dimensions do not influence the intercalation and deintercalation of lithium ion through the interlayer. The discharge capacity of nanofibers electrode is 302.3 mAh g−1 at 0.05 C and the initial columbic efficiency is 76.2%, which are higher than 282.7 mAh g−1 and 68.2% of the nanoparticles electrode. The nanofibers electrode also presented better cycleability and rate capability, especially performed capacity of 126.6 mAh g−1 at 5 C, much higher than that of 109.4 mAh g−1 for nanoparticles electrode. The step-by-step cyclic voltammetry revealed that the nanofibers electrode performs higher discharge voltage platform and enhanced discharge energy density. The excellent electrochemical performance of nanofibers electrode is ascribed to the better conductivity and superior lithium ion diffusion ability according to the electrochemical impedance spectrum measurement.

Introduction

Increasingly serious energy and environmental problems exacerbate the demands for efficient and clear energy sources. Lithium ion batteries are currently the most widely used energy storage and conversion devices. In addition to the traditional advantages, the next generation of lithium ion batteries requires higher energy density and power density, longer life and less cost with electric vehicle and hybrid electric vehicle as the significant application background [1], [2], [3]. Lithium-rich oxides attracted much attention in recent years on account of the high theoretical discharge specific capacity, wide working voltage range, and good thermal stability [4], [5], [6], [7]. It can be considered as the solid solution of Li[Li1/3Mn2/3]O2 and traditional cathode material LiMO2 (M represents the transition metal ions), expressed by formula xLi[Li1/3Mn2/3]O2-(1-x)LiMO2. It also can be regarded as the oxide with layer structure in which the excess lithium occupy the position of transition metal layer, represented by Li1+xM1-xO2. Layered Li1.2[Ni0.13Co0.13Mn0.54]O2 and Li1.1[Ni0.23Co0.23Mn0.43]O2 cathodes [8], which belonged to the xLi[Li1/3Mn2/3]O2·(1-x)LiCo1/3Ni1/3Mn1/3O2 solid solution series, deliver 285 mAhg−1 and 250 mAhg−1 respectively at 0.05C rate and 2.0–4.8 V, which is almost twice as the common cathode materials such as LiCoO2, LiFePO4 and LiMn2O4. The extremely high capacity is much related to the 4.5 V plateau during the first charge process. Lu and Dahn [9] reported that oxygen is removed from the structure when the electrodes are charged to 4.5 V. Armstrong et al. [10] proposed that the ideal reactions can be represented byLi2MnO3MnO2+2Li+12O2+2e at the 4.5 V plateau. Kim et al. [11] have described the reaction at 4.5 V as a combination of electrochemical process and chemical process which can be represented by the equations 2Li2MnO34Li++2Mn4+O31.33+4eand2Mn4+O31.332MnO2+O2. The mechanism was also proved by Yabyychi et al. [6] who consider that the plateau is the complex result of the electrochemical reaction and the chemical reaction on the electrode surface. However, issues remain over the way of the practical working condition which hinders the further application such as the high irreversible capacity in the first cycle, the poor rate performance and cycling capability, which associated with the oxygen loss, structure instability at electrode/electrolyte interface, poor lithium ion diffusion ability and conductivity [12], [13], [14], [15]. Great efforts were made to overcome the aforementioned drawbacks such as acid treatment to suppress the oxygen loss [16], nanocrystallization to minish the lithium ion diffusion pathways [17], coating a surface layer to improve the conductivity [18], [19], [20], [21] and doping other elements to stabilize the structure [22]. Among these approaches, nanostructure electrode shows great potential to solve the kinetics problems of intercalation and deintercalation of lithium ion for its advantages of enhancing the diffusion ability and reducing the electrode polarization.

Electrostatic spinning technology is one of the most effective and flexible methods for the preparation of nanofibers, which drafts the polymer solution into fibers by high pressure electric field and static electric field [23]. Li1.2Ni0.17Co0.17Mn0.5O2 nanofibers with enhanced rate capability were synthesized by Ji et al. [24], however, the charge-discharge mechanism, detailed structure differences and the consequent electrochemical performance of nanofibers and nanoparticles materials need to be further investigated and understood for the development of solid solution cathode materials. In this paper, we propose to synthesis the 0.6Li[Li1/3Mn2/3]O2-0.4Li[Ni1/3Co1/3Mn1/3]O2 nanofibers by the electrostatic spinning technology. Meanwhile the nanoparticles material has also been prepared by the coprecipitation method so as to compare and interpret the morphology, structure and the electrochemical performance. The initial charge and discharge processes were thoroughly investigated by a step-by-step cyclic voltammetry method. Conductivity and the lithium ion diffusion at surface film area and material bulk were studied by electrochemical impedance spectroscopy at various voltages. Field-emission scanning electron microscope (SEM), X-ray powder diffraction (XRD), Inductively coupled plasma-atomic emission spectrometry (ICP-AES), Brurauer Emmerr Teller (BET) measurements, Differential capacity method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were adopted to elucidate the effects on structure and electrochemical performance of different dimensions of Li-rich cathode materials.

Section snippets

Preparation of 0.4Li[Li1/3Mn2/3]O2-0.6Li[Ni1/3Co1/3Mn1/3]O2 nanofibers and nanoparticles

Polyvinylpyrrolidone (PVP, MwE1300,000, Sinopharm Chemical Reagent Co., Ltd) of 5.96 g was dissolved in 40 mL ethanol under stirring for 6 h. Composition of analytical reagents lithium acetate, nickel acetate, cobalt acetate and manganese acetate (AR, Sinopharm Chemical Reagent Co., Ltd) with the cationic mole ratio Li:Ni:Co:Mn = 1.2:0.13:0.13:0.54 and the total mass of 5.96 g was dissolved in 40 mL deionized water. The ethanol solution and aqueous solution were then mixed and stirred for 12 h. The

Morphology and structure

The SEM images of precursors and final products prepared by electrospinning and coprecipitation method are shown in Fig. 1. The coprecipitated precursors present lamella in sharp and the final products are aggregation of particles sized approximately 200 nm as illustrated in Fig. 1(a) and (b). The electrospun precursors demonstrate fibers with diameters of about 200 nm and the corresponding final products are filaments with diameters of about 120 nm as described in Fig. 1(c) and (d). Smaller size

Conclusions

Li-rich solid solution cathode materials 0.6Li[Li1/3Mn2/3]O2-0.4Li[Ni1/3Co1/3Mn1/3]O2 with different morphologies of nanowire and nanoparticle were synthesized by electrospinning and coprecipitation method respectively. Their physical characteristics, electrochemical performances and charge-discharge mechanism were thoroughly studied. The lattice parameter refinements displayed that the material prepared by electrostatic spinning method has a closer atomic arrangement in the layer yet the

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (no.51172023), the National Basic Research Program of China (973 Program, 2013CB934002), the National High Technology Research and Development Program of China (863 Program, 2012AA110302), and the National Natural Science Foundation of China (no. 51372021).

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