Galvanostatic Intermittent Titration Technique (GITT) experiments were conducted to determine the lithium diffusion coefficient of LiNi0.8Co0.15Al0.05O2, used as the active material in a lithium-ion battery porous composite positive electrode. An electrochemical model, based on concentrated solution porous electrode theory, was developed to analyze the GITT experimental results and compare to the original GITT analytical theory. The GITT experimental studies on the oxide active material were conducted between 3.5 and 4.5V vs. lithium, with the maximum lithium diffusion coefficient value being 10−10 cm2 s−1 at 3.85V. The lithium diffusion coefficient values obtained from this study agree favorably with the values obtained from an earlier electrochemical impedance spectroscopy study.