Electrochemical Stability of Li10GeP2S12 and Li7La3Zr2O12 Solid Electrolytes

DOI: 10.1002/aenm.201501590 electrochemical stability window, and (3) chemical compatibility with the anode and cathode. In the past few years, major advances have been achieved in increasing the Li ionic conductivity of the solid electrolytes. The state-of-the-art solid electrolyte materials, such as Li-garnet Li7La3Zr2O12 (LLZO) and Li10GeP2S12 (LGPS) have achieved an ionic conductivity of 10−3 to 10−2 S cm−1,[1,2] which are comparable to commercial organic liquid electrolytes. The high ionic conductivity in solid electrolytes has ignited the research of all-solid-state Li-ion batteries. After achieving adequate Li ionic conductivity in the solid electrolyte materials, current research efforts turned to enhancing the electrochemical stability of the solid electrolytes and chemical compatibility between the solid electrolytes and electrodes, so that Li metal anode and high voltage cathode materials can achieve higher energy density in all-solid-state Li-ion batteries. To enable the highest voltage output of the solid-state battery by coupling a lithium metal anode with a high voltage cathode material, a very wide electrochemical stability window (0.0–5.0 V) is desired for an ideal solid electrolyte. The electrochemical stability window of solid electrolyte was typically obtained by applying the linear polarization on the Li/solid electrolyte/ inert metal (e.g., Pt) semiblocking electrode. Tested by this method, very wide electrochemical stability windows of 0.0 to 5.0 V were reported for both LGPS and LLZO.[2,3] However, the electrochemical performances of the bulk-type all-solid-state battery batteries assembled with these solid electrolytes[2,4] are far worse than the liquid-electrolyte based batteries even though the solid electrolyte has a comparable ionic conductivity to the liquid electrolyte. The high interfacial resistance is often blamed as the main limiting factor for the performance of the solid state battery.[5] The origin of the interfacial resistance, though still not fully understood, is often attributed to the poor physical interfacial contact, the formation of space charge layers,[6] and/or the formation of interphase layers due to the chemical reactions between the electrolyte and electrode.[7] Although a variety of interfacial processing techniques, such as dynamic pressing,[8] nanosizing,[9] cosintering,[10] screen printing,[11] surface coatings[12,13] have been attempted to engineer the interfaces between the electrodes and electrolytes, the performances of the solid-state battery are still much lower than the liquidelectrolyte based batteries. The limited electrochemical stability The electrochemical stability window of solid electrolyte is overestimated by the conventional experimental method using a Li/electrolyte/inert metal semiblocking electrode because of the limited contact area between solid electrolyte and inert metal. Since the battery is cycled in the overestimated stability window, the decomposition of the solid electrolyte at the interfaces occurs but has been ignored as a cause for high interfacial resistances in previous studies, limiting the performance improvement of the bulk-type solid-state battery despite the decades of research efforts. Thus, there is an urgent need to identify the intrinsic stability window of the solid electrolyte. The thermodynamic electrochemical stability window of solid electrolytes is calculated using first principles computation methods, and an experimental method is developed to measure the intrinsic electrochemical stability window of solid electrolytes using a Li/electrolyte/electrolyte-carbon cell. The most promising solid electrolytes, Li10GeP2S12 and cubic Li-garnet Li7La3Zr2O12, are chosen as the model materials for sulfide and oxide solid electrolytes, respectively. The results provide valuable insights to address the most challenging problems of the interfacial stability and resistance in high-performance solid-state batteries.