A "lithium-free" thin-film battery with an unexpected cathode layer

A metal/electrolyte/metal (M/E/M) device, with a lithium phosphorus oxynitride (LiPON) electrolyte layer sandwiched between a silver (Ag) thin film and a stainless steel (SS) substrate, was fabricated with a simple structural configuration of SS/LiPON/Ag. Cyclic voltammetric (CV) and charge/discharge measurements showed that the device was activated as a rechargeable lithium battery upon initial charging up to 3.7 V (vsLi/Li+). Electrochemical cycling between 4.0 and 0.5 V was demonstrated over 450 cycles at a current density of 30 mu A/cm(2). By using CV analyses, X-ray photoelectron spectroscopy (XPS), and ac impedance spectroscopy, an electrochemical mechanism of this battery is proposed. During initial charging of the as-prepared device of SS/LiPON/Ag at 3.7 V, the reduction of Li ion from LiPON takes place to form metallic lithium anode with in situ electroplating a lithium layer onto the SS substrate. Meanwhile the cathode current collector of metallic Ag was oxidized to Ag+ ion at the interface of Ag/LiPON, and then the Ag+ ions diffused into the LiPON electrolyte and displaced the moveable Li+ ions to create an unexpected cathode layer used as a cathode material. In this "lithium-free" thin-film battery, only lithium-ion electrolyte LiPON supplies lithium for creating both the Li anode and the Li-containing layer cathode during cycling. Both lithium anode and lithium-containing cathode films were absent prior to the initial charge, which greatly simplified the fabrication, packaging, and storage of all-solid-state, thin-film lithium batteries. (c) 2007 The Electrochemical Society.