New hydrogen titanium phosphate sulfate electrodes for Li-ion and Na-ion batteries

NASICON-type materials with general formula A_xM₂(PO₄)₃ (A = Li or Na, M = Ti, V, and Fe) are promising candidates for Li- and Na-ion batteries due to their open three-dimensional framework structure. Here we report the electrochemical properties of hydrogen titanium phosphate sulfate, H_0.4Ti₂(PO₄)_2.4(SO₄)_0.6 (HTPS), a new mixed polyanion material with NASICON structure. Micron-sized HTPS aggregates with crystallite grain size of ca. 23 nm are synthesized using a sol-gel synthesis in an acidic medium. The properties of the as-synthesized HTPS, ball-milled HTPS, and samples prepared as carbon composites using an in-situ glucose decomposition reaction are investigated. A capacity of 148 mAh g⁻¹ corresponding to insertion of 2 Li⁺ per formula unit is observed in the ball-milled HTPS over the potential window of 1.5–3.4 V vs. Li/Li⁺. Lithiation at ca. 2.8 and 2.5 V is determined to occur through filling of the M1 and M2 sites, respectively. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) are used characterize the HTPS before and after cycling. Evaluation of the HTPS in a Na-ion cell is also performed. A discharge capacity of 93 mAh g⁻¹ with sodiation at ca. 2.9 and 2.2 V vs. Na/Na⁺ is observed.