Coarse-grained force field for ion–water interactions
Forcefields for alkali ions in water have been developed for decades now, so the literature is rich with examples… and yet there still are significant developments (see this very new paper for only one such example).

If you want to have a forcefield for alkali ions that mix well with the TIP3P water model, you better look at what existing studies on such systems have done… this is an example, but you can find many others with searching through the literature.