Chemically fabricated LiFePO4 thin film electrode for transparent batteries and electrochromic devices

We report a new sol-gel approach of synthesis of LiFePO4 (LFP) thin film and its application as cathode materials for transparent Li-ion battery in half-cell configuration. LFP thin films were obtained from an alcoholic colloidal suspension of iron acetylacetonate (Fe(AcAc)3) and aqueous lithium dihydrogen phosphate (LiH2PO4) deposited on fluorine tin oxide (FTO) glass substrate, followed by heating at 450 °C under nitrogen gas for 1 h. X-ray diffraction (XRD) confirmed that the LFP films have an orthorhombic crystal system with space group Pnma (62). Scanning electron microscopy (SEM) shows spherical LFP nanoparticles aggregates homogenously deposited all over the surface of FTO substrate containing 3-D open pores. The electrochemical behaviors of thin film vs Li/Li+ cell were investigated by cyclic voltammetry and galvanostatic charge-discharge measurements. The cycle life was evaluated by running 1000 cycles of charge-discharge at a current density of 20 mA/cm2. The transmission spectra reveal 85-90% of transparency versus FTO as reference, which makes it a potential candidate as a complementary electrode in electrochromic devices (ECDs).