Microstructural and Electrochemical Properties of rf-Sputtered LiFeO2 Thin Films

Lithium iron oxide (LiFeO2) thin films have been deposited by rf-magnetron sputtering technique and microstructural and electrochemical properties were studied. The films deposited at a substrate temperature 250°C with subsequent post annealing at 500°C for 4？h exhibited cubic rock-salt structure with Fm3m space group. The films exhibited well-defined oxidation and reduction peaks suggesting complete reversibility upon cycling. The as-deposited films exhibited an initial discharge capacity 15？ Ah/cm2· m, whereas the films post annealed at 500°C for 4？h in controlled oxygen environment exhibited 31？ Ah/cm2· m. 1. Introduction The substantial development of lithium ion batteries with high energy density and capacity originates from the identification of a good cathode host that can accommodate both Li+ ions and electrons with minimal structural changes [1, 2]. During charging, the lithium ions are deintercalated from the cathode host (e.g., LiCoO2) and inserted into the van der Waals gap between the anode layers (e.g., Li). Exactly, a reverse process occurs during discharge, involving the extraction of lithium from the van der Walls gap and intercalation into the cathode at their interstitial sites. The discharge/charge balance is maintained by reduction/oxidation of reversible Co3+ ions to Co4+ and maintaining the cathode structural stability, resulting in good reversibility for longer cycles. Hence, focus has been made mainly on oxides with transition metal ions, as many of them are believed to shift valence state easily [3]. In reality, oxygen ions participate strongly in accepting the electron, a fact which can be used to tailor the voltage of a cathode. Lithium transition metal oxides LiMO2 (M = Mn, Co, Ni, Fe, etc.) with the layered structure are commonly used as cathode materials for rechargeable lithium batteries [4–7]. Among various lithium transition metal oxides, LiCoO2 has been most widely used as a positive electrode material [8]. But, it has many disadvantages such as high toxicity, high cost, and low practical capacity [9]. Therefore alternative compounds based on other transition metals such as Fe, which are less expensive and nontoxic than Co, have to be developed. Various types of iron-containing systems including oxides and phosphates were investigated in recent years [10]. Among them, LiFeO2 has been paid more attention due to high abundance and nontoxicity of iron [11]. LiFeO2 has various crystalline structures such as -LiFeO2, -LiFeO2, -LiFeO2, layered LiFeO2, corrugated LiFeO2, and goethite type LiFeO2 [12]. The structure of the