## 3 Interfacial Phenomena between Solid-State Electrolytes and Electrodes
### Buffer Layers
#### 3.2.2 Buffer Layers at Solid-State Electrolyte/Anode  
As discussed earlier, Li metal is an ideal anode for SSBs owing to its high theoretical specific capacity, lowest negative electrochemical potential and low density. However, the high chemical reactivity between solid electrolytes and Li metals results in the formation of uncontrolled dendrites which leads to large volume change during cycling [86]. The lithium dendrite growth is known to cause the internal short circuits and the deterioration of SSBs. Han et al. [309] visualized the lithium dendrite growth in LLZO and Li\({}_{3}\)PS\({}_{4}\) solid electrolytes by time-resolved operando neutron depth profiling, revealing that lithium dendrites nucleate and grow directly inside LLZO and Li\({}_{3}\)PS\({}_{4}\). Wu et al. [310] showed that lithium dendrites can extend along the cracks and boundaries of solid electrolytes such as LLZO and can also lead to capacity fading and short-circuiting. To overcome these issues, an ideal buffer layer material on the electrolyte/anode is indispensable. The buffer layers should exhibit the following properties: (i) enhance the wettability between the lithium metal/electrolyte to stabilize the interface and (ii) suppress the growth of lithium dendrite. Han et al. [255] effectively addressed the large interfacial resistance due to poor wettability between the lithium metal and the garnet Li\({}_{7}\)La\({}_{2.75}\)Ca\({}_{0.25}\)Zr\({}_{1.75}\)Nb\({}_{0.25}\)O\({}_{12}\) electrolyte by using ALD deposited Al\({}_{2}\)O\({}_{3}\) buffer layers which can facilitate the molten Li metal coating of the garnet surface with no interfacial void space. The authors argued that the Al\({}_{2}\)O\({}_{3}\) buffer layer enhanced the wettability between lithium metal and the garnet electrolyte, which allowed effective Li\({}^{+}\) transport through the interface. According to the experimental results, a significant decrease of interfacial resistance from 1710 \(\Omega\) cm\({}^{2}\) down to 1 \(\Omega\) cm\({}^{2}\) was observed at room temperature due to the Al\({}_{2}\)O\({}_{3}\) buffer layer. Later, interfacial layers of ZnO proposed by the same group [88] increased the wettability and reduced the interfacial resistance between garnet electrolytes and the Li metal anode. Liu et al. [311] also demonstrated that Al\({}_{2}\)O\({}_{3}\) buffer layer on LATP (Li\({}_{1.3}\)Al\({}_{0.3}\)Ti\({}_{1.7}\)(PO\({}_{4}\))\({}_{3}\)) solid electrolyte could increase the wettability with the Li metal anode. The authors explained that the Al\({}_{2}\)O\({}_{3}\) buffer layer acts as the lithium transport pathway at the LATP/Li metal interface which largely reduces the interfacial  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Electrode**} & \multirow{2}{*}{**Electrolyte**} & \multirow{2}{*}{**Buffer Layer**} & \multirow{2}{*}{**Coating**} & \multicolumn{2}{c}{**Interface**} & \multirow{2}{*}{**Interface**} \\  & & & **Resistance w/** & & **Resistance** \\  & & & **Method** & **Buffer Layer** & **w/o Buffer** \\  & & & & **(\Omega\)-cm\({}^{2}\))** & **Layer (\(\Omega\)-cm\({}^{2}\))** & \\ \hline LCO & LLZO–Nb & Li\({}_{3}\)BO\({}_{3}\) & Screen & & 230 & & [238] \\  & & & printing & & & \\  & & & & Spray & 34.56 & 714.71 & [256] \\  & & & coating & & & \\  & & &  
resistance. Recently, Alexander et al. [312] proposed an effective buffer layer of LiNbO\({}_{3}\) on LLZO-Al (Li\({}_{6.28}\)Al\({}_{0.24}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\)) solid electrolyte, showing the drastically reduced interface resistance from 1078 \(\Omega\) cm\({}^{2}\) to 91 \(\Omega\) cm\({}^{2}\). Furthermore, the authors confirmed that the buffer layer could assist Li\({}^{+}\) conduction at the interface and provide good adhesion based on the stable voltage profile of LiNbO\({}_{3}\)-coated LLZO-Al/Li metal SSBs. Considerable progress was achieved to prevent lithium dendrite formation by introducing a buffer layer at the interface of solid electrolyte and Li metal anode.