Nowadays, Lithium ion batteries (LIBs) can be found in every corner of our daily life. However, safety and cost of lithium ion batteries are the two major issues. Aqueous rechargeable lithium ion batteries are believed to be the promising candidates for LIBs due to nonflammable，low cost higher ionic conductivity of aqueous electrolyte [1]. To reduce the cost of aqueous lithium-ion cells, researchers focus on developing methods to synthesize electrode material in an energy efficient way.

It has been reported that TiP₂O₇ has a low intercalation /de-intercalation potential (2.6 V versus Li/Li⁺) [2], which makes it a promising anode material for aqueous lithium-ion batteries [3,4]. One report demonstrated a solid-state synthesis method annealing at 300^◦C for 2 hours, annealing at 800^◦C for 3 hours and cooling to room temperature for 4 hours [5]. The solid-state synthesis route is very common in industrial applications due to its simplicity. However, the long calcining duration and high temperature requirement increases the processing cost for the electrode materials.

One report applied microwave to synthesize C-NaTi₂(PO₄)_2, the negative electrode for aqueous sodium ion batteries [6]. Functional electrode materials can be synthesized in less than 20 minutes. In this work, a very fast microwave synthesis method was developed to produce C-TiP₂O₇ (Carbon and TiP₂O₇ Composite material). The precursor consists of (NH₄)₂HPO₄ and anatase TiO₂ in a molar ratio of 2:1, with 10wt%, 15wt% and 20wt%(the weight percent is relative to the precursor mixture) graphite added. After being ball milled together for 1 hour, the mixture was heated in a microwave oven (CEM Discover Proteomics System) for 10 minutes at 150W.

The SEM figure shows the surface of Graphite-TiP₂O₇ composite, and the particle size of Graphite-TiP₂O₇ composite is around 40nm (Figure 1a). In XRD test (Figure 1b), the pattern of product matches with the reference pattern (JCPDS #38-1468) at major peaks (511), (600), (630), (721), (660), (933). The products we get from this method are tested in cyclic voltammetry tests, which shows promising electrochemical performance (Figure 2a); after ten cycles, the specific capacity of 10% Graphite-TiP₂O₇ composite maintains 107.4 mAh/g. (Figure 2b).

Figure 1 a) SEM of Graphite-TiP₂O₇ composite. B) XRD patterns of Graphite-TiP₂O₇ composite and reference peaks.

Figure 2 a) Second cycles of cyclic voltammetry curves of 10% Graphite-TiP₂O₇ composite, 15% Graphite-TiP₂O₇ composite and 20% Graphite-TiP₂O₇ composite in 1 M Li₂SO₄ at scan rate 0.5 mV/s in range of −0.5 V and −1.6 V versus MSE. b) Discharge cycling performance of Graphite-TiP₂O₇ composite.

The total synthesis time is only 10 minutes for heating and 30 minutes for cooling, which is much less than for traditional solid state synthetic methods.

Reference:

1. Kim H, Hong J, Park K Y, et al. Aqueous Rechargeable Li and Na Ion Batteries[J]. Chemical Reviews, 2014, 114(23):11788.
2. Aravindan V, Reddy M V, Madhavi S, et al. Hybrid supercapacitor with nano-TiP₂O₇, as intercalation electrode[J]. Journal of Power Sources, 2011, 196(20):8850-8854.
3. Wang H, Huang K, Zeng Y, et al. Electrochemical properties of TiP₂O₇, and LiTi₂(PO₄)₃, as anode material for lithium ion battery with aqueous solution electrolyte[J]. Electrochemical Acta, 2007, 52(9):3280-3285.
4. Rai A K, Gim J, Song J, et al. Electrochemical and safety characteristics of TiP₂O₇ –graphene nanocomposite anode for rechargeable lithium-ion batteries[J]. Electrochemical Acta, 2012, 75(4):247-253.
5. Wu W, Shanbhag S, Wise A, et al. High Performance TiP₂O₇ Based Intercalation Negative Electrode for Aqueous Lithium-Ion Batteries via a Facile Synthetic Route[J]. Journal of the Electrochemical Society, 2015, 162(9): A1921-A1926.
6. Wu W, Mohamed A, Whitacre J F. Microwave Synthesized NaTi₂(PO₄)₃ as an Aqueous Sodium-Ion Negative Electrode[J]. Journal of the Electrochemical Society, 2013, 160(3): A497-A504.