**3.5 Deposition technique for electrolyte materials in thin-film batteries**

A solid-state electrolyte is a solid ionic and electron-insulating material that promotes the movement of ions from the cathode to the anode during charging and the moving of ions from the anode to the cathode during discharging conditions. There are different types of electrolytes (glass, crystalline and ceramic) depending on the primary material makeup [53]. Several works have been done on solid-state electrolytes and more researchers keep working to get an electrolyte that will improve the thin-film battery characteristics such as reduced package size, increased safety and enhanced power and energy density [54]. Solid polymer electrolytes made of polyvinylpyrrolidone (PVP) with dopant p-amino benzoic acid (PABA) and poly pamino benzoic acid (PPA) were developed by Sangeetha et al. for improved dielectric property and ionic conductivity applications [55]. Solid-state electrolytes have been fabricated using pulsed laser deposition (PLD) technique [56, 57], sol-gel techniques [58], atomic layer deposition [59, 60], chemical solution deposition (CSD) [61], etc. for microbattery and other solid-state battery applications.

#### **3.6 Deposition technique for current collector materials**

Current collectors are made of copper and/or aluminum foils that collect electrons from the electrochemical reaction of the electrodes and external circuit and at the same time support the material layers of the cathode and the anode. For a material to function efficiently as a current collector, it must be cheap and available, very light material, a good electrical conductor and chemically stable and resistant to corrosion [62]. The importance of current collectors in the solid-state battery (SSB) industry has necessitated research focus on the materials for its fabrication and applications. Onestep galvanostatic electrodeposition of lithium in 3D porous copper-based nanoflake structures (3D Cu-NF@Cu foam) as anode current collector for high energy density lithium metal batteries was reported by Yuanyuan Xia et al. [63], electrode polymercarbon composite current collector foil for bipolar lithium-ion battery applications was reported by Fritsch et al. [64]. Also, Zhen Hou et al. designed a current collector using a nanostructured silver lipophilic layer on a copper foil through an electroless plating electrodeposition process for a stable lithium metal anode [65], fabrication of nickel-phosphorous-modified copper current collector using facile electroless plating electrodeposition method aimed at superior coulombic efficiency for microbattery applications [66], etc.
