Elsevier

Journal of Power Sources

Volume 320, 15 July 2016, Pages 59-67
Journal of Power Sources

Atomic structural and electrochemical impact of Fe substitution on nano porous LiMnPO4

https://doi.org/10.1016/j.jpowsour.2016.04.061Get rights and content

Highlights

  • Fe substituted nanoporous LiMn1-xFexPO4 composite were synthesized by a sol-gel method.

  • This demonstrates the distinct influence of Fe content on electronic part on conductivity.

  • Mn is found to occupy the Li sites, which block the Li ion diffusion.

  • The antisite defect is overcome by iron substitution.

Abstract

The atomic structural and electrochemical properties of Fe substituted nano porous LiMn1-xFexPO4 (x = 0–0.8) composites are investigated and compared. X-ray scattering method is used for atomic structural investigation. Rietveld refinement shows that all Fe substituted composites have the same olivine structure (Pnma) with lithium occupying octahedral 4a sites, Fe2+ replacing Mn2+ at the octahedral 4c sites. The a, b, c parameters and cell volume decrease with the addition of Fe2+. When the nano porous LiMn1-xFexPO4 composites are evaluated as cathode materials in lithium cells at room temperature, x = 0.6, and 0.8 resulted in the best overall electrochemical performance, exhibiting stable cycling and high discharge capacities of 149 and 154 mA h g−1, respectively. The composites with above x = 0.4 show a fast lithium ions transfer with high electronic conductivity because Fe transition metal substitution reduce the partly occupation of Mn in the M1 (LiO6) sites and thereby Mn block the lithium ion diffusion pathway. We here firstly find the antisite defect in the high Mn content in porous LiMn1-xFexPO4 composites.

Introduction

Lithium metal phosphates (LiMPO4 where M=Fe, Mn, Co, Ni) are important cathode materials for lithium-ion batteries and have attracted the attention of many research groups. Beyond being environmentally friendly (non-toxic), having high theoretical capacities (∼170 mAh g−1), the strong covalent PO bonds drive the potential of the M3+/M2+ redox couple to values greater than 3.4 V vs Li+/Li. However, lithium metal phosphates are limited by poor electrical conductivities (10−9 S cm−1), which increase the resistance of the electrodes and decrease the charge-discharge capacities [1], [2], [3]. Several studies have been carried out to overcome the insulating nature as well as the low lithium diffusion in the structure: synthesizing small and phase-pure particles, in-situ coating the surface with a conductive material, for example carbon, dispersing metal particles, and solid-solution doping by cations have been successful approaches to achieve high and stable performance [4], [5], [6], [7], [8], [9], [10], [11], [12]. Techniques for improving electrical conductivity by carbon coating rely on initial addition of carbon, as such, or the pyrolysis of a carbon precursor during the synthesis [7], [13], [14], [15], [16], [17]. It is important that the amount of carbon is kept as low as possible, since the incorporation of low density carbon can adversely affect the density and volumetric energy density of LiMnPO4.

To improve the electrochemical properties of LiMPO4, many research groups have demonstrated different synthesis techniques, including the solid state method, sol-gel, mechanical activation, hydrothermal, co-precipitation, spray, microwave, and supercritical methods [7], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Our research group has focused on carbon coating and the synthesis method as means to improve the electrochemical properties of cathode materials [18], [19], [25], [26], [27]. In the family of olivine LiMPO4 (M=Mn, Fe, Co, Ni), LiMnPO4 is the most promising material for use as a positive cathode of high energy density. The Mn3+/Mn2+ redox couple operates at 4.1 V vs. Li/Li+, which is reasonably high as compared to the Fe3+/Fe2+ redox couple in LiFePO4. However, the Mn3+/Mn2+ redox couple has some serious drawbacks such as a much lower current durability, smaller effective energy density, and comparatively slower kinetics compared to LiFePO4, which limits its use. To utilize the inherent high capacity capability of LiMnPO4, some attempts have been made to synthesize mixed olivine phosphates with a general formula of LiMn1-xFexPO4. In this structure Fe and Mn coexist at the octahedral 4c sites, where the amounts of Fe can be manipulated to a proportion, which results in desirable material properties. LiMn1-xFexPO4 seems promising because its working voltage is not so high as to decompose the organic electrolyte, and not so low as to sacrifice energy density. The manipulation of x in LiMn1-xFexPO4 can be used to tailor its properties to a promising candidate material with various operating cell voltages of 3.4–4.1 V vs. Li+/Li. The amount of Fe in the LiMnxFe1-xPO4 composition is crucial, since a too high Fe amount renders decrease in energy density. The formation of LiMnxFe1-xPO4 by solid-solution method has been systematically studied by the groups of Padhi, Yamada and Li et al. [1], [28], [29], [30], [31], [32] who found that a comparatively high capacity was achieved with an average discharge voltage of 3.8 V and a Mn content of x = 0.5–0.85 [16], [33], [34]. Comparative X-ray and neutron diffraction investigations of the two solid-solution lines were performed as a function of Mn content to increase understanding of the electrochemical activity loss of Mn and the accuracy was almost the same on the atomic position [30]. The electrochemical behavior of cathode materials associated with its intrinsic defect and the cation antisite defect is most favorable [35]. LiMPO4 has an orthorhombic unit cell, where edge-sharing LiO6 (M1 site) octahedral forms chain along b, while corner-sharing FeO6 (M2 site) octahedral forms a zigzag pattern in the b-c plane [36]. However, transition metal (Mn and Fe) is placed in M1 site by the antisite defect and the occupied transition metal obstruct the lithium ion diffusion, resulting in poor electrochemical properties. The concentration of the defect on LiMPO4 is sensitive to synthesis condition and particle size [37], [38], [39].

In this work, nano porous LiMn1-xFexPO4 is synthesized by the sol-gel method, which was optimized to prepare highly porous materials. We focus on the atomic structural and electrochemical performance of the porous LiMn1-xFexPO4 with x = 0, 0.2, 0.4, 0.6 and 0.8 as a function of Fe content. The antisite defect is affected by high Mn concentration and the influence significantly appear in the range from x = 0 to x = 0.4. This indicates that Mn2+ ions are expected to sit on the Li sites up to x = 0.4. The atomic structural investigation is firstly performed in both nano porous LiMn1-xFexPO4 and sol-gel process. The approach is potentially useful to improve the electrochemical properties of porous metal-substituted cathode materials.

Section snippets

Experimental section

Li2CO3, FeC2O4·2H2O, Mn(COOCH3)2·4H2O and NH4H2PO4 (all chemicals of 99% purity from Aldrich) and citric acid (Shinyo Pure Chemicals, 99% purity) were used as the starting materials to synthesize porous LiMn1-xFexPO4 composites. These were dissolved in deionized water at room temperature and to the resulting solution was added citric acid solution. After homogenous mixing, the sol was dried by keeping it at 75 °C for 12 h during magnetic stirring. The gel state obtained was placed in a vacuum

Results and discussion

Fig. 1a shows XRD patterns of the series of synthesized nanoporous LiMn1-xFexPO4 (x = 0–0.8) samples. The crystal structure of all the samples exhibits a single phase olivine type orthorhombic Pnma structure. Peaks corresponding to significant amounts of impurities such as Fe3+or Mn3+ compounds are absent in the spectra and there is no evidence for crystalline carbon, or amorphous peaks present. This is undoubtedly due to the small amount and thin layer of carbon on LiMn1-xFexPO4. The amount of

Conclusions

Fe substituted nanoporous LiMn1-xFexPO4 composite were successfully synthesized by a sol-gel method. Substitution of iron in LiMnPO4 composite substantially changes the physicochemical and electrochemical properties. With an increasing amount of substituted Fe2+, the lattice parameters decrease linearly. The SEM analysis reveals formation of sponge-like porous particles in LiMn1-xFexPO4 composites, while composites with x = 0.2, 0.4, and 0.6 also has the large surface area and the high number

Acknowledgments

This work was supported by the Creativity and Innovation Project Fund (1,140009,01) of Ulsan National Institute of Science and Technology (UNIST) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2A16053515; NRF-2016R1A2A2A07005334).

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