Enhanced electrochemical behavior of Na0.66Li0.22Ti0.78O2/C layered P2-type composite anode material for Na-ion batteries

Within this paper, we present a comprehensive characterization of Na0.66Li0.22Ti0.78O2 - inexpensive, zero-strain anode material for Na-ion batteries in terms of the sodium-ion transport mechanism and electrochemical performance. Na0.66Li0.22Ti0.78O2 was synthesized via a citrate assisted sol-gel method for the first time, which resulted in a four times higher specific surface area as compared to the conventional synthesis method. EIS and DC polarization experiments showed that electrical conductivity in Na0.66Li0.22Ti0.78O2 is mainly ionic with bulk conductivity of 1.54.10(-4) S cm(-1) at room temperature and enthalpy of Na-ion migration equal to 0.37 eV. Structural changes during intercalation/deintercalation were investigated using operando X-ray diffraction and revealed that lattice parameters monotonically evolve, and P2 structure is maintained during the whole range of sodium insertion/extraction. GITT technique revealed that the chemical diffusion coefficient of sodium changes within two orders of magnitude between 4.8.10(-12) cm(2) s(-1) and 2.5.10(-10) cm(2) s(-1), and such changes were correlated with evolution of occupancy of sodium sites, and contraction of the interlayer gap. Electrochemical tests in both half and full cells show excellent performance of Na0.66Li0.22Ti0.78O2 in a wide range of current loads. The supremacy of sol-gel method fabricated materials is especially visible under high currents (10C), where 60% of theoretical capacity is preserved, which is two times higher than in the materials obtained via a standard high-temperature solid-state reaction. Full cells with a Na0.72Li0.24Mn0.76O2 cathode provided 2.88 V mid-point voltage and a discharge capacity of 132 mAh g(-1). Such results prove that Na0.66Li0.22Ti0.78O2 anodes may find their applications in both large-scale energy storage systems and high-power output devices.