Future missions to investigate the structure and evolution of the universe require highly efficient, low-temperature cryocoolers for low-noise detector systems. We propose to develop a highly efficient low-cost regenerator for regenerative cryocoolers with cooling temperatures in the range of 15 K and below. The proposed regenerator uses an innovative non-rare-earth material to achieve a volumetric specific heat of about 0.65 to 0.31 J/cm3-K at temperatures of 15 to 4.2 K. The large heat capacity will substantially reduce the thermal swing during periodic heat transfer and therefore improve the efficiency of low-temperature regenerative cryocoolers. The regenerator will be lightweight and easy to fabricate. In Phase I we will optimize the regenerator for a specific cooling application. We will use the resulting design and model to show that a regenerative cryocooler can achieve a very high efficiency. In Phase II we will build a prototype regenerator, measure its key performance parameters, and integrate it with an existing cryocooler to demonstrate its high thermal effectiveness.