Elsevier

Electrochimica Acta

Volume 296, 10 February 2019, Pages 1027-1034
Electrochimica Acta

Enhanced sodium-ion storage capability of P2/O3 biphase by Li-ion substitution into P2-type Na0.5Fe0.5Mn0.5O2 layered cathode

https://doi.org/10.1016/j.electacta.2018.11.160Get rights and content

Highlights

  • Facile sol-gel route is used to synthesis of layered cathode via Li-ion substitution.

  • Combination of P2 and O3 phases is confirmed using XRD refinement results.

  • P2+O3 Na0.5[Li0.10Fe0.45Mn0.45]O2 biphasic cathode exhibits good cycling performance.

Abstract

Integration of P2 and O3 phases in Na0.5Fe0.5Mn0.5O2 cathode via Li-ion substitution is proposed to enhance its electrochemical performance for sodium-ion battery applications. The formation of P2 and the combination of P2/O3 intergrowth were confirmed by X-ray diffraction refinement, high resolution transmission electron microscopy and X-ray photoelectron microscopy analyses. Various content of lithium was used to find optimum P2+O3 combinations. The optimized Li-ion substituted Na0.5(Li0.10Fe0.45Mn0.45)O2 showed a high initial discharge capacity of 146.2 mAh g−1 with improved cycling stability, whereas the pristine Na0.5Fe0.5Mn0.5O2 initially delivered a discharge capacity of 127.0 mAh g−1. In addition, the combination of P2+O3 increased its average voltage, which is important for achieving high energy density sodium-ion batteries. Overall, the prepared Na0.5(Li0.10Fe0.45Mn0.45)O2 electrode exhibited the improved cycling performance in terms of reversible capacity and rate capability compared to pristine Na0.5Fe0.5Mn0.5O2 electrode material.

Introduction

In these days, with a raising demand on energy worldwide because of the exhaustion of fossil fuels, the development of substitute energy conversion and storage systems is required for the advancement of mankind in various fields such as residential, industry, transportation, space exploration and military [1]. Over the past few decades, the development of lithium-ion batteries (LIBs) emerged rapidly due to urgent demand for developing sustainable energy storage systems for various electronic devices such as portable laptops, smartphones and so on; because of their high energy density and excellent cycle life [[2], [3], [4]]. However, to meet the essential demand of advanced power sources such as electric vehicles and large-scale energy storage systems, further development in the storage ability is required [5]. Among the various rechargeable battery systems, sodium-ion battery (SIB) is one of the promising candidates due to its low cost for large-capacity energy storage applications [[6], [7], [8], [9], [10]]. Numerous attempts have been made so far to develop more practical cathode materials for sodium-ion batteries. Sodium-based layered oxide NaxMO2 materials (M = 3d transition metal) have received much attention due to their reversible behaviour and large specific capacity. Such layered oxide materials can be classified into two types; P2 and O3, in which sodium layers are accommodated with the M layers in prismatic and octahedral structure, respectively [[11], [12], [13], [14], [15]]. The P2 phase sustains during sodium de-intercalation process due to its direct sodium ion diffusion and has a more open pathway. However, the structural changes are occurred by over-extraction of sodium ions at higher potential, which restricts the reversible charge and discharge cycles [[16], [17], [18], [19], [20], [21], [22], [23]]. In contrast, O3 phase acting as sodium ion accumulator can furnish sufficient amount of sodium ions during the electrochemical reaction. However, the intrusion of intermediate tetrahedral sites during sodium ion migration requires to overcome the high energy obstacle for O3 type structure, the ineluctable phase transition affecting the electrochemical reaction, and sluggish kinetics [16,[24], [25], [26]].

To solve these issues, utilizing the advantages of both phases can be a practical strategy to prepare the high-performance cathode materials for sodium-ion batteries. Recently, the combination of two phases in a single material by substituting cations has drawn considerable attention due to its excellent electrochemical performance as compared to the single structured electrode material [[27], [28], [29], [30], [31], [32], [33]]. For example, Guo et al. reported the biphasic Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+δ to deliver a high discharge capacity of 200 mAh g−1 at 0.1 C rate and achieved the stable cycling behaviour [28]. Li et al. reported that the P2 and O3 biphasic Na2/3Mn0.55Ni0.25Ti0.2-xLixO2 exhibited a discharge capacity of 158 mAh g−1 at 12 mA g−1, which was higher than Li-free cathode material (147 mAh g−1) [30]. More recently, Qi et al. reported the hybrid structure of Na0.78Ni0.2Fe0.38Mn0.42O2 could deliver a discharge capacity of 86 mAh g−1 with excellent high rate performance [32]. Kwon et al. reported the Na0.7[Mn1-xLix]O2+y had an excellent cycling stability than the pristine Na0.7MnO2+y, even though the pristine material showed a higher capacity [33]. However, very limited studies have been reported so far based on the combination of two phases in the cathode material of sodium-ion batteries. There are various types of synthesis techniques available for preparing the layered cathodes [34,35]. Among them, sol–gel technique provides many advantages, particularly, it is able to synthesize the solid-state material from the chemically homogeneous precursor. There is a possibility to produce complex inorganic and other nanomaterials even at shorter preparation times and lower preparation temperatures by snaring the randomness of the solution of chemicals and reagents [35,36]. In addition, sol–gel chemistry can control the morphology and size of the products. By considering all the above aspects, in this work, we report the combination of the P2 and O3 phases of lithium-ion substituted Na0.5[Lix(Fe0.5Mn0.5)1-x]O2 (LNFMO) as a promising cathode for sodium-ion battery applications by sol-gel technique, which delivers a specific capacity of 146.2 mAh g−1 with enhanced cyclic stability, average voltage, coulombic efficiency and rate capability compared to the single phase P2 structured Na0.5Fe0.5Mn0.5O2 (NFMO) material.

Section snippets

Materials synthesis

Layered LNFMO materials were synthesized by sol-gel technique, as schematically presented in Fig. 1. The starting precursor materials (acetates of Li, Na, Fe and Mn) and citric acid were mixed, and the mixture was kept with stirring at 80 °C for 12 h. Then, temperature was increased to 120 °C to form the xerogel. Finally, the obtained powder was heated to 400 °C for 5 h and followed by 900 °C for 12 h to obtain Na0.5[Lix(Fe0.5Mn0.5)1-x]O2. The same procedure was followed to prepare pristine

Results and discussion

Fig. S1 shows the XRD patterns of NFMO and LNFMO-n materials containing different amounts of lithium. Rietveld refinement of XRD data along with that of the pristine sample was investigated in order to determine the unit cell parameters, and the detailed XRD refinement results are summarized in the Supplementary Materials. The crystalline peaks for pristine NFMO can be indexed as a hexagonal lattice with the space group P63/mmc, which are corresponding to the characteristic peak of P2-type, as

Conclusions

Layered P2+O3 biphasic LNFMO cathode materials were successfully synthesized via lithium-ion substitution by a facile sol-gel approach, and their electrochemical properties were investigated for sodium-ion battery applications. Among various combinations of P2 and O3 biphase, the LNFMO-10 electrode exhibited the highest discharge capacity of 146.2 mAh g−1 with enhanced rate capability and good reversible behaviour compared to the pristine NFMO and other LNFMO-n materials. The improved capacity

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) funded by the Korea Government (2017R1C1B2012700) and the Basic Science Research Program of NRF, funded by the Ministry of Science, ICT and Future Planning (2016R1A4A1012224).

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