Synthesis parameter dependence of the electrochemical performance of solvothermally synthesized Li4Ti5O12

Qian Yang 0 1 2 Hailei Zhao 0 1 2 Jie Wang 0 1 2 Jing Wang 0 1 2 Chunmei Wang 0 1 2 Xinmei Hou 0 1 2 0 X. Hou School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing , Beijing 100083, China 1 H. Zhao Beijing Key Lab of Advanced Energy Materials, Beijing 100083, China 2 Q. Yang H. Zhao (&) J. Wang J. Wang C. Wang School of Materials Science and Engineering, University of Science and Technology Beijing , Beijing 100083, China Pure Li4Ti5O12 with high crystallinity was successfully synthesized by a solvothermal process. The effects of initial Li/Ti ratio and post-heating temperature on the phase evolution, particle morphology and electrochemical properties were systematically investigated. Excess lithium, compared to the theoretical value in Li4Ti5O12, was required to get pure Li4Ti5O12 due to the condensation reaction. Low Li/Ti ratio led to the appearance of secondary phase rutile TiO2, while high heattreatment temperature easily resulted in particle agglomeration of Li4Ti5O12 powder. The existence of rutile TiO2 decreased the specific capacity, and the particle agglomerate had a strong negative effect on the rate capability of electrode. The sample synthesized at the optimized condition exhibited a stable specific capacity of 150 mAh/g and a good rate performance. - Lithium-ion batteries have attracted much attention as important energy supply in portable electronic devices, hybrid electrical vehicles and electrical vehicles because of their high power and energy density [13]. At present, new electrode materials exhibiting excellent rate capability and high safety performance are urgently demanded to meet the requirement of electrical vehicles. The spinel lithium titanate Li4Ti5O12 is being considered as an ideal anode material in lithium-ion batteries due to its unique characteristics, including very flat charge/discharge voltage plateaus and a small structural change during charge/discharge processes. The zero-strain insertion characteristic provides material with an excellent cycling performance [4, 5], while that of the flat operating voltage at 1.55 V (versus Li?/Li) can avoid the deposition of dendritic metallic lithium, therefore a high operational safety can be expected [6, 7]. Despite the high Li deintercalation/intercalation potential, it can, in principle, be coupled with high-voltage cathodes such as LiNi0.4Mn1.6O4 to provide a cell with an operating voltage of approximately 3 V [8]. However, Li4Ti5O12 is an insulator, its rate capability is greatly limited by its inherently low lithium-ion diffusivity and electronic conductivity. Typical approaches to resolve this problem include employing nanoparticles to reduce the diffusion length of lithium ions, and increase the contact area between the electrode and the electrolyte [911], doping Li4Ti5O12 with aliovalent cation (Al3?, Ga3?, Co3?, Mg2?, Ta5?) [1214] in Li and Ti sites to produce mixed valence of Ti3?/Ti4?, and thus increase the electronic conductivity, and incorporating directly the conductive second phase (carbon, Ag and so on) [7, 15, 16]. Actually, the particle size and the crystalline ordering degree have strong impacts on the electrochemical properties of electrode. Small-sized active material can not only reduce the lithium-ion diffusion distance, but also increases the contact area with conductive reagent and electrolyte solution, thus can decrease the local current density and mitigate the electrode polarization. The high crystallinity is believed to be beneficial to the good cycling stability of electrode [9]. Compared to the doped materials, the pure material is easier to be synthesized and handled in practical operations. Many methods, including conventional solid-state reaction [1214], solgel method [6, 17, 18], solvothermal technique [1922], combustion synthesis [23], rheological phase reaction [11] and other synthesis routes, have been exploited to prepare Li4Ti5O12 materials. Among them, solvothermal technique with simple and flexible controls has spurred considerable interests. Although Li4Ti5O12 powders prepared by solvothermal method have been investigated extensively [1922], the work concerning the effect of the synthesis parameters on the electrochemical properties is very limited. Considering that the practical composition of the synthesized material via solvothermal route is usually different from the nominal composition, in this work, the effect of initial Li/Ti ratio in starting solution on the phase purity and the electrochemical properties was investigated. The influence of the post-heat-treatment temperature on the electrochemical performance of Li4Ti5O12 electrode was also addressed. The synthesized Li4Ti5O12 exhibited excellent rate capability and cycling performance, showing the solvothermal synthesis is a promising method to obtain high-performance Li4Ti5O12 anode material. The spherical precursors of Li4Ti5O12 powders were synthesized by solvothermal method using lithium acetate (LiAc, AR C99.0 %, Beijing Yili Fine Chemicals Co., Ltd.) and tetrabutyl titanate [Ti(O(CH2)3CH3)4, denoted as Ti(OR)4, AR C99.0 %, Beijing Jinlong Chemical Reagent Co., Ltd.) as Li and Ti cation sources, respectively. The molar ratios of the mixtures were fixed at different proportions (Li/Ti ratio = 0.81.4). Ti(OR)4 was dissolved in ethanol under magnetic stirring, and then LiAc was added into the mixtures with further stirring to obtain a homogeneous dispersion system. The concentration of Ti(OR)4 in ethanol was 1.4 9 10-4 mol/ml. The transparent solution was then transferred into a 100 ml teflon-lined stainless steel autoclave and kept at 180 C for 24 h. After cooling down to room temperature, a milky white precursor was prepared. The produced powder was washed and filtered with ethanol to eliminate the unreacted reagents and the partial organic compounds. The precipitate was dried at 80 C in air for 3 h. To obtain well-crystallized Li4Ti5O12, the precursor was calcined at 800 C for 2 h in air with a heating rate of 5 C/min. At last, the effect of heat-treatment temperature on the particle morphology and electrochemical properties was investigated. The precursor with the optimal Li/Ti ratio based on the above results was subjected to calcination at temperatures of 400, 600, and 800 C, respectively. Phase purity and crystallinity of the synthesized samples were identified by means of powder X-ray diffraction (XRD) performed on a Rigaku D/MAX-A diffractometer with Cu Ka radiation source (k = 1.54056 A ) in the range of 10 B 2h B 90 , while the morphology and size distribution of precursors and post-treated powders were observed on a LEO-1450 scanning electron microscope (SEM). The actual molar ratio of Li/Ti in the precursor was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES) (IRIS Intrepid II XSP). The thermal behavior of the precursor powders was examined by a thermogravimetrydifferential thermal analysis (TG DTA (...truncated)