• International Journal of Technology (IJTech)
  • Vol 13, No 1 (2022)

Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates

Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates

Title: Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates
Fitria Rahmawati, Dwi Aman Nur Romadhona, Desi Dyah Paramita, Witri Wahyu Lestari

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Cite this article as:
Rahmawati, F., Romadhona, D.A.N., Paramita, D.D.Lestari, W.W., 2022. Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates. International Journal of Technology. Volume 13(1), pp. 168-178

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Fitria Rahmawati Research Group of Solid-State Chemistry & Catalysis, Chemistry Department, Universitas Sebelas Maret, Jl. Ir. Sutami 36 A Kentingan, Surakarta, 57126, Indonesia
Dwi Aman Nur Romadhona Research Group of Solid-State Chemistry & Catalysis, Chemistry Department, Universitas Sebelas Maret, Jl. Ir. Sutami 36 A Kentingan, Surakarta, 57126, Indonesia
Desi Dyah Paramita Research Group of Solid-State Chemistry & Catalysis, Chemistry Department, Universitas Sebelas Maret, Jl. Ir. Sutami 36 A Kentingan, Surakarta, 57126, Indonesia
Witri Wahyu Lestari Research Group of Inorganic Materials, Chemistry Department, Universitas Sebelas Maret, Jl. Ir. Sutami 36 A Kentingan, Surakarta, 57126, Indonesia
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Abstract
Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates

In this research, a cyclic voltammetry (CV) method was applied to intercalate Na+ into an FePO4/Al substrate to produce NaFePO4/Al as a potential cathode material. The sodiation was conducted directly to FePO4 instead of applying delithiation to LiFePO4 followed by sodiation, as was done in previous research. CV was conducted within a potential window of 2.0–4.0 V using a scan rate of 0.05 mVs-1. The result was compared to LiFePO4/Al treated with a similar method. The various scan rate was then applied to understand its effect on the electrochemical activity recorded in the voltammogram and its impedance profile. The results show that the CV product of FePO4/Al (NFP(A)) was crystallized in an orthorhombic olivine NaFePO4, as a result of Le Bail refinement. Orthorhombic Na0.7FePO4, trigonal FePO4, and monoclinic FePO4 were presented as secondary phases. Meanwhile, the CV product of LiFePO4/Al (NFP(B)) was also crystallized in olivine NaFePO4 and possessed secondary phases similar to NFP(A) with an additional Fe2O3 phase. NFP(A) showed two significant peaks at 2.442 V and 3.534 V, confirming sodiation/de-sodiation and Fe3+/Fe2+ activity, respectively. Meanwhile, NFP(B) showed two peaks at 3.183 V and 3.04 V, corresponding to de-lithiation and sodiation, respectively. The Nyquist plots of both materials show a similar profile, with the impedance value of NFP(A) being lower than that of NFP(B). This confirms that the CV treatment of FePO4/Al is more facile than the treatment of the LiFePO4 layer, while also producing a cathode with higher electrical conductivity. Scan rate reduction to 0.04 mVs-1 produced a much lower impedance value, confirming higher electrical conductivity.

Electrochemical Na insertion; Electrical properties; Sodium ion-battery; Sodium iron phosphate

Introduction

    As important components in Lithium Ion-Batteries (LIBs), positive electrodes or cathodes have been studied for a few decades, with LiFePO4 (LFP) attracting the most attention due to its favorable kinetics in the lithium intercalation/de-intercalation process (Li et al., 2015; Sofyan et al., 2016; Yang et al., 2013), the ease of controlling its size and shape (Boesenberg et al., 2013), its low cost, its high thermal safety, its good reversible capacity, its long cycle ability, and the fact that it is environmentally friendly (Popovi?, 2011). However, sustainable lithium supply is now a concern because lithium is usually only mined from limited deposits of Li2CO3 and LiOH (Kim et al., 2019). Sodium-Ion Batteries (SIBs) might be a good alternative for LIBs due to their high natural abundance and uniform geographic distribution on earth (Li et al., 2015; Wang et al., 2016).

Iron-based phosphates have been investigated as SIB cathodes, such as in the molecular forms of Na2FeP2O7 (Kim et al., 2013), Na2FePO4F (Ellis et al., 2007), Na4Fe3(PO4)2(P2O7) (Kim et al., 2012), NaFePO4 (Kim et al., 2015), non-crystalline iron phosphate, FePO4 (Liu et al., 2012), composites of amorphous FePO4 nanospheres and carbon (Fang et al., 2014), and composites of FePO4 nanospheres and graphene (Fan et al., 2014). Even though amorphous FePO4 is easy to synthesize, olivine NaFePO4 is preferable, especially for recently studied aqueous SIBs that require electrode materials to have operating potentials within the electrochemical stability window of water, in which H2 and O2 evolution should not occur (Jeong et al., 2019). The potential window should be between -0.83 V and 1.23 V vs. SHE at 25oC and 1 atm (Castellan, 1983). Meanwhile, maricite NaFePO4 is not active electrochemically within 0–4.5 V due to cavities that trap Na ions and are not connected by pathways (Prosini et al., 2014). Further treatment to transform the maricite form into amorphous NaFePO4 provides electrochemical activity within 1.5–4.5 V (Xiong et al., 2019). However, the potential window is above the potential window of aqueous SIB, which is in the range of -0.5–0.85 V (vs. Ag/AgCl) in 1 M aqueous NaClO4 solution (Jeong et al., 2019).

NaFePO4 can be conventionally synthesized through solid-state reactions using FeC2O4.2H2O, Na2CO3, and NH4H2PO4 as precursors, followed by calcination at 600oC for 10 h under an argon gas atmosphere (Kim et al., 2015). The maricite structure dominates the product. However, within the maricite structure, the diffusion channels of sodium ion transport are blocked (Heubner et al., 2017). Meanwhile, direct chemical synthesis appears to be unsuccessful for olivine NaFePO4 preparation. Some researchers have produced olivine structures through ion-exchange methods using an organic-based electrochemical insertion into delithiated-FePO4 (Oh et al., 2012; Avdeev et al., 2013; Galceran et al., 2014). This method requires two steps: de-lithiation and sodium insertion into the delithiated-FePO4 in a new cell with sodium metal as the anode (Tang et al., 2016). The cyclic voltammetry profile of FePO4 with LiClO4 shows a single oxidation peak at 3.7 V and a single reduction peak at 2.85 V indicating de-lithiation/lithiation process of LiFePO4 (Hansen et al., 2016). Meanwhile, when the NaClO4 electrolyte was used, the peaks were at 3.65 V and 3.4 V, indicating de-lithiation of LiFePO4 and then Na+ insertion into the de-lithiated LiFePO4. The diffusion of Na+ into FePO4 is slower than that of Li+ (Heubner et al., 2016). This means that lithiation competes the Na+ diffusion easily.

Therefore, in this research, the Na+ intercalation was conducted directly to the FePO4 layer without passing through a de-lithiation step. The FePO4 was cast on an Al substrate to produce FePO4/Al which was then used as a working electrode. Cyclic voltammetry mode was applied within 2.0–4.0 V of potential windows with a scan rate of 0.05 mVs-1. A similar treatment was also conducted to LiFePO4/Al for comparison. In addition, the effect of varying the scan rate effect was checked to determine the electrical properties of the materials.

Conclusion

    Electrochemical sodiation was successfully applied in cyclic voltammetry mode to insert Na ions into the FePO4 layer directly. This finding offers a simpler electrochemical method to insert/extract Na+ directly as an alternative to the two-step de-lithiation and sodiation method. Impedance analysis even shows a similar semicircle trend, with a lower impedance value of NaFePO4 from FePO4, NFP(A), than the one prepared from the LiFePO4 layer, NFP(B). This research also found that by reducing the scan rate to as low as 0.04 mVs-1, the electrical conductivity increased for both NFP(A) and NFP (B).

Acknowledgement

    This work was carried out with the financial support of the Hibah Penelitian Dasar 2019, The Ministry of Research, Technology, and Higher Education, the Republic of Indonesia, contract number 719/UN27.21/PN/2019.

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