Electromechanical Failure of NASICON-Type Solid-State Electrolyte-Based All-Solid-State Li-Ion Batteries

Although inorganic all-solid-state Li-ion batteries (ASSLiBs) using stable oxide solid electrolytes are considered to be promising candidates for future energy storage devices, their relatively high cell impedance due to the restricted contact area and interfacial stability results in unsatisfactory electrochemical performance and fast capacity fading. The mechanism limiting performance and cycle life in such ASSLiBs still lack study and hence understanding. To overcome this bottleneck, we prepared a bulk ASSLiB, where a MnO2–CNT nanocomposite is used as a high-voltage anode to prevent reduction of the electrolyte by a lithium anode, high-voltage LiNi0.5Mn1.5O4 is used as a cathode, and ambient-air stable Li1.5Al0.5Ge1.5(PO4)3 (LAGP) is chosen as a solid-state electrolyte (SSE). This ASSLiB shows a maximum discharge capacity of 82 mAh g–1 at 0.15C and 23.8 °C. The electrochemical impedance study of the cell reveals a decrease in impedance after solid interface layer formation in cycle 1 followed by an increase after electrode lithiation/delithiation. Electrochemical evaluation and first-principles calculations were used to explore the decomposition of the LAGP after charge/discharge cycles. Decomposition of LAGP with the assistance of Li ions and free electrons from voids and grain boundaries leads mainly to the formation of Li4P2O7, Li3PO4, Ge5P6O25, AlPO4, and GeO. Finite element method simulations reveal that the volume expansion due to the formation of the decomposition products Li4P2O7, Li3PO4, and AlPO4 results in a maximum internal stress of 2.5–125 GPa for various Li excess ratios ranging from 0 to 6. This by far exceeds the failure stress of LAGP and results in crack formation and growth in the SSE on multiple cycling

Composite NASICON (Na₃Zr₂Si₂PO₁₂) Solid-State Electrolyte with Enhanced Na⁺ Ionic Conductivity: Effect of Liquid Phase Sintering

NASICON-type of solid-state electrolyte, Na3Zr2Si2PO12 (NZSP), is one of the potential solid-state electrolytes for all-solid-state Na battery and Na–air battery. However, in solid-state synthesis, high sintering temperature above 1200 °C and long duration are required, which led to loss of volatile materials and formation of impurities at the grain boundaries. This hampers the total ionic conductivity of NZSP to be in the range of 10–4 S cm–1. Herein, we have reduced both the sintering temperature and time of the NZSP electrolyte by sintering the NZSP powders with different amounts of Na2SiO3 additive, which provides the liquid phase for the sintering process. The addition of 5 wt % Na2SiO3 has shown the highest total ionic conductivity of 1.45 mS cm–1 at room temperature. A systematic study of the effect of Na2SiO3 on the microstructure and electrical properties of the NZSP electrolyte is conducted by the structural study with the help of morphological and chemical observations using X-ray diffraction (XRD), scanning electron microscopy, and using focused ion-beam-time of flight-secondary ion mass spectroscopy. The XRD results revealed that cations from Na2SiO3 diffused into the bulk change the stoichiometry of NZSP, leading to an enlarged bottleneck area and hence lowering activation energy in the bulk, which contributes to the increment of the bulk ion conductivity, as indicated by the electrochemical impedance spectroscopy result. In addition, higher density and better microstructure contribute to improved grain boundary conductivity. More importantly, this study has achieved a highly ionic conductive NZSP only by facile addition of Na2SiO3 into the NZSP powder prior to the sintering stage