After the discovery of electrochemical Na⁺ insertion into non-graphitizable carbon[1], the hard carbon has been recognized as the most promising candidate of a negative electrode material for sodium ion batteries.  On another research front, various titanium oxides and titanates including TiO₂[2], spinel-type Li[Li_1/3Ti_5/3]O₄[3], NASICON-type NaTi₂(PO₄)₃[4] and Na₂Ti₃O₇[5] have also attracted much attention owing to its average redox potential where metal dendrite formation would not occur.  Among them, Na₂Ti₃O₇ delivers 177 mAh g^-1 at first deinsertion with approximately 50% in coulombic efficiency, and its insertion potential of 0.3 V possibly realizes a safer practical Na-ion battery system compared to hard carbon.  However, the specific capacity of titanium-based electrode is generally much lower than that of hard carbon, which causes consistent drawback to bring them into first choice.  Recently, as a new titanium-based negative electrode candidate, Doeff and her colleagues have reported lepidocrocite-type titanate with a primitive lattice (P-type) and proposed Na_0.8[Ti_1.73Li_0.27]O₂･nH₂O with a bilayer arrangement of water[6].  The material has a large theoretical capacity of 192 mAh g^-1 and delivers more than 140 mAh g^-1 of reversible capacity.  In this study, since there is a possibility that the presence of interlayer water disturbs insertion of Na⁺ ions, and P-type Na_0.8[Ti_1.73Li_0.27]O₂･nH₂O transforms into Na_0.8[Ti_1.73Li_0.27]O₂ with a C-centered lattice (C-type) as removal of interlayer water by drying, we synthesize C-type Na_0.8[Ti_1.73Li_0.27]O₂ to investigate the electrochemical performance and reaction mechanism for the first time.

  First, P-type Na_0.9[Ti_1.7Li_0.3]O₂･nH₂O was prepared by ionic-exchange from K_0.9[Ti_1.7Li_0.3]O₂ with 5 M NaCl solution, and then the obtained white powder was dried at 80 to 500 ºC.  The as-prepared materials were characterized by XRD, SEM, TG-DTA and their electrochemical property was measured in Na system.  The working electrodes, consisting of Na_0.9[Ti_1.7Li_0.3]O₂, acetylene black and binder with the mass ratio of 80:10:5, were prepared with N-methylprrolidone and the obtained slurry was casted on a Al foil current collector.  After drying the electrodes at 80 ºC in vacuum for overnight, 2023-type coin cells were assembled in a Ar-filled glove-box with 1 mol m^-3 NaPF₆ in PC against sodium metal.  The charge and discharge measurements were carried out at room temperature at a current density of 10 mA g^-1 in the voltage range between 2.0 and 0.1 V vs. Na/Na⁺.

  From XRD pattern of the synthesized sample, C-type Na_0.9[Ti_1.7Li_0.3]O₂ with lattice parameters of a = 3.7867(2), b = 14.4096(8) and c = 2.96935(19) Å was obtained.  As shown in Fig.1(a) and (b), C-type Na_0.9[Ti_1.7Li_0.3]O₂ shows first reversible capacity of 145 mAh g^-1 with nearly 80% in coulombic efficiency, and demonstrates excellent capacity retention at the 40^th cycle.  In addition, the reversible capacity even at 10C rate reached to 81% of that with C/17.5 (= 10 mA g^-1) (Fig. 2).  Detailed results of electrochemical performances and its charge and discharge mechanism will be discussed in the conference.

References:

[1] D. A. Stevens et al., J. Electrochem. Soc., 147(4), 1271 (2000).

[2] Y. Xu et al., Chem. Commun., 49, 8973 (2013).

[3] L. Zhao et al., Chin. Phys. B, 21, 028201 (2012).

[4] C. Delmas et al., Mater. Res. Bull., 22, 631 (1987).

[5] P. Senguttuvan et al., Chem. Mater., 23, 4109 (2011).

[6] M. Shirpour, J. Cabana and M. Doeff, Chem. Mater., 26, 2502 (2014).