Line intensity mapping (LIM) is an emerging observational technique to measure the large-scale structure of the Universe in three dimensions, traced by a redshifted emission line, without resolving individual objects. Future experiments promise to extend the observable volume beyond the redshift reach of traditional galaxy surveys, improving precision on the LCDM cosmological model and extensions to it. I will outline the science potential of mm-wave LIM experiments, highlighting the need for on-chip spectrometers to dramatically improve sensitivity over current instruments. I will introduce SPT-SLIM, a pathfinder for the South Pole Telescope that will demonstrate LIM using on-chip spectrometers. Finally I will discuss how this technology could power future LIM instruments with orders of magnitude more detectors and the sensitivity to constrain the expansion history at high redshift, primordial non-Gaussianity, and more.