Five kinds of nickel dioxide polymorphs, Li x NiO2 with x≈0, were prepared by treating LiNiO2 with sulfuric acid solutions, occasionally followed by low-temperature heating. Here we report their structure and properties as lithium insertion electrodes. Acid-treated Li0.10NiO2 with two layered phases turned into a single-layered compound at 160°C and then to a spinel-related compound at 170°C. Acid-treated Li0.04NiO2 contained a phase with a cadmium iodide structure, which was not observed with Li0.10NiO2. Heating this Li0.04NiO2 yielded a spinel-related compound on heating. The NiO distances in these compounds suggested that the nickel oxidation state was kept approximately +4. These ‘nickel dioxide’ polymorphs exhibited varied characteristics as lithium insertion electrodes. We discuss the electrode behavior together with the structural changes that occur during lithium insertion. We also examined the effect of the electrode drying condition on the cycling performance.