Solvothermal Synthesis of Nasicon-Type LiTi2(PO4)3/C and Their Application As Electrode Material in Lithium-Ion Battery

With the rapid growth in the use of portable electronics and electric vehicles (EVs), high performance energy storage systems are in great demand, such as high energy density lithium ion batteries (LIBs) and high power density lithium-ion hybrid supercapacitors (Li-HECs). Although the energy storage and conversion mechanisms are different, there are “electrochemical similarities” of these two systems^{[1, 2]}. Common features are that the energy-providing processes take place at the phase boundary of the electrode/electrolyte interface and that electron and ion transport are separated.  NASICON-structured LiTi₂(PO₄)₃ is considered an outstanding electrode material for both LIBs and Li-HECs because of rapid Li⁺ diffusion in the bulk and on surface of LiTi₂(PO₄)₃^[3]. LiTi₂(PO₄)₃ can reversibly accommodate two additional lithium ions, operating on the Ti⁴⁺/Ti³⁺ at 2.48 V, via a two-phase mechanism that converts between LiTi₂(PO₄)₃ and Li₃Ti₂(PO₄)₃. In addition, this ceramic electrode material can not only well preserve its framework during Li⁺ extraction what leads to long cycle life, but also can be used as an electrode for aqueous electrolytes to improve safety performance^[4]. However, the vast majority of NASICON-type materials published in literature were synthesized by solid state process or sol-gel and high temperature post-calcination that resulted in polygonal or rounded particles and with relatively large particle size^[5-7].

Aiming to decrease the synthesis temperature and further improve the performance of LiTi₂(PO₄)₃/C by tailoring the particle size and morphology, we intended to prepare LiTi₂(PO₄)₃/C by solvothermal method. In our recent research, the “bare” LiTi₂(PO₄)₃ and LiTi₂(PO₄)₃/C were synthesized from an intermediate precursor of Li-Ti-P-O or Li-Ti-P-O-C via one-pot solvothermal method and subsequent heat treatment under 350-650 ^oC to obtain carbon coating. Stoichiometric amounts of LiOH·H₂O, Ti(OC₂H₅)₄, and NH₄H₂PO₄ were used as starting materials. Cetyltrimethylammonium bromide (CTAB) and oxalic acid (OA) are simultaneously used as surfactant and carbon source for the carbon coating on the surface of LiTi₂(PO₄)₃. Ammonia solution was used to adjust the pH value to a certain range. All the chemicals in this work are purchased from Sigma-Aldrich and used as received. The reaction took place in a reactor with the starting pressure at 7.8 bar and constant stirring for 3 days under 210 ^oC. Pressure and temperature changes are recorded. Additionally, LiTi₂(PO₄)₃/C was also prepared by using a Pechini-type polymerizable complex method for comparison. Crystalline phases, structures and morphology of the materials were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM) and Raman spectroscopy. The electrochemical experiments were performed using a coin cell incorporating metallic lithium foil as a counter electrode. The working electrode was prepared from a mixture of 90 wt% LiTi₂(PO₄)₃/C, 5 wt% carbon black as conductive additive, and 5 wt% Polyvinylidene difluoride dissolved in N-methylpyrrolidone as binder. This slurry was spread onto copper foil by tape casting and then dried at 100 ^oC for 12 h. Galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using a potentiostat from Biologic.

References

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