## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes
### Buffer Layers
#### 3.2.1 Buffer Layers at the Electrolyte/Cathode Interface  
Most solid electrolytes react with cathode materials and therefore they need to be protected by buffer layers in SSBs. The buffer layer at the solid electrolyte/cathode interface should provide a good ionic conduction path and also reduce the strain/stress. An ideal buffer layer at the electrolyte/cathode interface is required to have the following properties [293; 294]; (i) excellent Li\({}^{+}\) mobility, (ii) low interfacial stress variation by forming a conformal contact between the electrolyte and the cathode, (iii) a wide electrochemical window that can span the cathode's operating voltage and the electrochemical window of the electrolyte and (iv) low chemical reactivity with the electrolyte and the cathode. One of the most commonly used materials as a buffer layer is Li\({}_{3}\)BO\({}_{3}\)[295] which is a Li\({}^{+}\) conductor (\(2\times 10^{-6}\) S/cm at room temperature) with a low melting temperature (700 \({}^{\circ}\)C) that easily forms a conformal coating between the electrolyte and the electrode by annealing. Ohta et al. [238] employed Li\({}_{3}\)BO\({}_{3}\) as a buffer layer on LCO to overcome the poor contact between the electrolyte and the electrode. The authors claimed that the Li\({}_{3}\)BO\({}_{3}\) buffer layer could act as a bonding material. During the annealing process of Li\({}_{3}\)BO\({}_{3}\)-coated Nb-doped LLZO (LLZO-Nb), Li\({}_{3}\)BO\({}_{3}\) melts and forms a continuous contact that enhances the adhesion between the solid electrolyte and the cathode. Li\({}_{3}\)BO\({}_{3}\) was also used as a buffer layer by Park et al. [253] to provide sufficient interfacial contact between LCO and garnet solid electrolyte. Another method to decrease the interfacial resistance is to modification the cathode to form Li\({}^{+}\) conductors on the surface. Liu et al. [296] coated a Li-Ti-O precursor on the surface of NMC-523, which in turn formed a Li(Ti\({}_{0.1}\)Mn\({}_{0.9}\))\({}_{2}\)O\({}_{4}\) layer by sintering. In this work, Li(Ti\({}_{0.1}\)Mn\({}_{0.9}\))\({}_{2}\)O\({}_{4}\) buffer layer improved the interfacial contact between the cathode and Li\({}_{6.75}\)La\({}_{3}\)Zr\({}_{1.75}\)Ta\({}_{0.25}\)O\({}_{12}\) (LLZO-Ta) solid electrolyte, showing the reduced interfacial resistance from 12.5 k\(\Omega\) to 9.5 k\(\Omega\). Improved contact led to an enhanced initial specific discharge capacity to 123.3 mAh/g and increased capacity retention to 62% after 5 cycles. The authors also suggested that sputtering electrode or electrolyte materials as a buffer layer to form a cohesive interface are effective to enhance the contact of the interface. Lee et al. [297] prepared a buffer layer of common solid electrolyte material Li\({}_{5}\)La\({}_{3}\)Ta\({}_{2}\)O\({}_{12}\) between LCO and Li\({}_{5}\)La\({}_{3}\)Ta\({}_{2}\)O\({}_{12}\) by RF magnetron sputtering. According to this work, the Li\({}_{5}\)La\({}_{3}\)Ta\({}_{2}\)O\({}_{12}\) buffer layers reduced the contact resistance between LCO and Li\({}_{5}\)La\({}_{3}\)Ta\({}_{2}\)O\({}_{12}\) and increased the ion path between the cathode and the electrolyte. Bai et al. [298] designed a LCO buffer layer on a Li\({}_{7}\)(Al\({}_{0.1}\))La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) (LLZO-Al) electrolyte. The authors demonstrated that the LCO buffer layer possesses a large interfacial specific surface area and an excellent interfacial stability which reduces the interfacial resistance of NMC-523/LLZO-Al by 1279 \(\Omega\) compared to the unmodified interface. To suppress the formation of SCRs in SSBs with sulfide-based electrolytes, Ohta et al. [256] introduced a pure ionic conducting LTO buffer layer between electrolytes and electrodes. The authors claimed that the SCRs were less developed because of the similar chemical potential of LTO buffer layer and LCO and the poor electronic conduction between the Li\({}_{3.25}\)Ge\({}_{0.25}\)P\({}_{0.75}\)S\({}_{4}\) and LTO buffer layer.