A series of lithium borate added electrolyte compounds, xLi(3)BO(3)-(1-x)Li3.5Si0.5P0.5O4 (0 <= x <= 0.2), are synthesized and characterized. This so-called LISICON electrolyte system is analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), solid state nuclear magnetic resonance (ss-NMR), electrochemical impedance spectra (EIS), and direct current (DC) polarization methods. From B-11 MAS NMR spectra, it is demonstrated that a small fraction of boron exists in the form of BO4, while its majority settles at grain boundaries in the form of BO3, indicating that lithium borate glasses play a role as sintering assistant. This prominently increases the relative density of samples, and is beneficial to the ionic conductivity. Further results show that the electrical conduction of lithium borate added LISICONs is dominated by Li+ ions, with a transference number of t(Li)(+) >= 0.996 and a corresponding ionic conductivity of about 6.5 x 10(-6) S cm(-1) at room temperature, almost two times that of pristine Li3.5Si0.5P0.5O4. (C) 2015 Elsevier B.V. All rights reserved.