## 5 Conclusions  
Replacing liquid electrolytes with solid counterparts allows SSBs to exhibit excellent safety, electrochemical stability and high energy density. Despite these advantages, further enhancement of the current SSBs is required to be used in practical applications. Improving the material properties of electrodes and electrolytes may accelerate the development of the next-generation energy storage systems. This review discussed key advances in battery materials and possible solutions to solve their issues. Surface modification, doping and nanostructuring have been successfully used to improve the electrode performance. For the solid electrolyte, many efforts have been devoted to improving the ionic conductivity by doping or altering the crystal structures. Indeed, several studies have improved the thermal and chemical stability of the solid electrolytes by replacing sensitive elements, such as transition metals. While we mainly focused on oxide-based solid electrolytes, LiRAP and sulfide-based solid electrolytes are becoming increasingly attractive owing to their outstanding ionic conductivity. However, the highly air sensitive nature of these electrolytes remains a substantial obstacle. The interfacial problems discussed in this review--insufficient mechanical contact between the electrode and the electrolyte, significant volume changes of electrodes and formation of SCRs and lithium metal dendrites--cause large interfacial resistances leading to the performance degradation of SSBs. Introducing thin films as buffer layers is a promising approach to overcome these interfacial issues. We discussed how to control the growth of thin films with various thin films deposition methods such as thermal evaporation, PLD, sputtering, CVD, ALD and sol-gel. Recent studies on the successful suppression of dendrites, SCRs and SEI layers by having thin-film buffer layers were also discussed systematically. Nanostructured thin films also play a key role in constructing high-performance SSTFBs, which can be implemented in wearable/flexible electronics and microelectronic devices. We summarized recent advances in thin-film electrodes and electrolytes and their impact on the SSTFB performances. While most thin films can be fabricated by different deposition methods, the film growth techniques may need to be carefully chosen depending on the material systems. Nevertheless, PLD has shown  
Figure 15: Conductivity summary of bulk [324] and thin-film LLZO deposited by PLD [324; 430; 431; 437], RF sputtering [59; 432], sol–gel [433; 434; 435] and CVD [57]. its unique advantage in controlling the physical properties of thin films, which directly influence the performance of SSTFBs. RF sputtering and sol-gel are also commonly used to synthesize thin-film electrodes for SSTFBs. To improve the performance of SSTFBs, a couple of different approaches such as applying epitaxial thin films, introducing the lattice strain and changing the crystallographic orientation of films have been utilized. However, the effect of changing the physical properties of thin films on the SSTFB performance is not yet fully understood as SSTFBs are still in their infancy. Apart from the topics covered in this review, we propose two promising approaches for the investigation to achieve further advances in SSBs and SSTFBs. Over the last few decades, the development of oxide thin films has led to many technological breakthroughs for energy and electronic devices. In particular, 2D planar heterostructures have been prevailingly investigated as they lead not only to improved functionalities but even to the occurrence of novel properties that do not belong to the bulk [438; 439]. For instance, the discovery of the formation of a conducting interface between two insulators [438], STO and LaAlO\({}_{3}\) (LAO), brought the breakthrough in the field of oxide electronics while also becoming the pole of attraction and inspiration for numerous studies [440; 441].