Preparatiom And Characterization Of Li_xNa_yTi₆O₁₄（x+y=4） As Energy Storage Materials

In this paper, we adopted solid state method to prepare LixNayTi6O14(x+y=4), and explored their electrochemical performance and the properties of insertion and extraction of Li+. Then, the electrochemical performance of Li2Na2Ti6O14 through coating various carbons is studied. Detailed contents are as follows:Firstly, the results of XRD and TEM techniques show that there are two pure phase materials(Na2Li2Ti6O14 and Na2Ti3O7), and three mixed phase materials(Li4Ti6O14, Li3NaTi6O14 and LiNa3Ti6O14). Besides, the surface morphology change from square to round and return to square during the Na+ replaced by Li+ ions in LixNayTi6O14(x+y=4) subject. Meanwhile, Li4Ti6O14 possess highest original charge specific capacity(141.7 m Ah·g-1) and highest capacity retention about 97.9% after 50 cyclesSecondly, the diffusion coefficient of lithium ion for Na2Li2Ti6O14 is detected by cyclic voltammetry and electrochemical impendence spectra methods. The results of cyclic voltammetry method show that the insertion and extraction coefficient of Na2Li2Ti6O14 are 6.2254×10-12 and 4.8715×10-12cm2·s-1, respectively. Then, electrochemical impendence spectra method reveals that the Li+ diffusion coefficiet is about 10-12~10-14 cm2 s-1. In addition, Li2Na2Ti6O14 is not suitable for sodium ion batteries and its structure collapsed during discharge and charge cycles.Thirdly, the most suitable synthesized temperature is 800 ℃ and it obtains the smallest degree of polarization, highest specific capacity and capacity retention. Its structure change slightly between discharge to 1.0 V and 0.0 V. The extra capacity is due to the formation of SEI film on the surface of Na2Li2Ti6O14 particles. The results of ex-situ TEM and ex-situ XPS indicate the highly structure stability during insertion and extraction process.Lastly, Na2Li2Ti6O14 particles are coated by various carbons successfully. The XRD analysis reveals its structure keep stable after coating carbons. The SEM and TEM images show the Na2Li2Ti6O14 particles wrapped by CB, GN and CNT, thus accelerating their electrical conductivity. For comparion, CNT formed three dimensional conductive structure highly improves its electrochemical performance.