Revealing the reason for the unsuccessful fabrication of Li_(3)Zr_(2)Si_(2)PO_(12) by solid state reaction

NASICON type Li_(3)Zr_(2)Si_(2)PO_(12)can be synthesized via cation exchange method with Na_(3)Zr_(2)Si_(2)PO_(12)as precursor,which retains the skeleton structure and achieves an ionic conductivity higher than 3 mS cm^(-1)at room temperature.However,large-scale fabrication via cation exchange reaction seems unlikely considering the expensive precursors and complicated preparation process.Herein,the viability of solid-state reaction to prepare Li_(3)Zr_(2)Si_(2)PO_(12)is explored,which has important implication for its industrialization.The sintering was conducted using the raw materials of LiOH,SiO_(2),ZrO_(2)and NH_(4)H_(2)PO_(4)with the nominal stoichiometric ratio of Li_(3)Zr_(2)Si_(2)PO_(12).The results show that the final product is a Li_(3)PO_(4)·2ZrSiO_(4)composite with negligible Li+conductivity,other than the expected Li_(3)Zr_(2)Si_(2)PO_(12)with high Li+conductivity.Combined with thermodynamic calculations based on density functional theory(DFT),the competition between Li_(3)PO_(4)·2ZrSiO_(4)and Li_(3)Zr_(2)Si_(2)PO_(12)with NASICON phase is analyzed.It was found that the formation energy(AG)of Li_(3)PO_(4)·2ZrSiO_(4)is lower than that of Li_(3)Zr_(2)Si_(2)PO_(12).In addition,the decomposition of Li_(3)Zr_(2)Si_(2)PO_(12)with Li_(3)PO_(4)-2ZrSiO_(4)as products is a thermodynamically spontaneous reaction.The influences related to the coordination structures on the structural stability of NZSP are discussed as well.These results demonstrate that the fabrication of Li_(3)Zr_(2)Si_(2)PO_(12)through high-temperature sintering is difficult,and the development of a synthetic method with mild conditions is essential for the Li_(3)Zr_(2)Si_(2)PO_(12)preparation.