Enhanced ionic conductivity of Li3.5Si0.5P0.5O4 with addition of lithium borate

• The addition of lithium borate to Li3.5Si0.5P0.5O4 prominently increases the ionic conductivity.
• 11B MAS NMR results demonstrate that the boron exists in the form of BO4 and BO3, and plays a role as sintering assistant.
• The synthesized electrolytes show prominence to another material with great potential for constructing solid state batteries.

A series of lithium borate added electrolyte compounds, xLi3BO3-(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 11B 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 tLi+ ≥ 0.996 and a corresponding ionic conductivity of about 6.5 × 10−6 S cm−1 at room temperature, almost two times that of pristine Li3.5Si0.5P0.5O4.

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