Recycling of spent lithium ion batteries (LIBs) has received increasing attention in recent years, because of the increasing usage of LIBs in electronic products and the potential leakage of heavy metals to the soil when they are disposed to the landfills. Chemical precipitation has been widely applied in the recycling process of spent LIBs. However, most processes are developed based on trial and error, leading to the possibility of recovering the wrong product in the precipitation process or excess usage of chemicals. Solidliquid equilibrium (SLE) phase behavior governs the products to be recovered from the precipitation process and can be used to guide and optimize the process. Case studies on the recycling of LiFePO4 and LiCoxMn1-xO2 have been studied in this paper to demonstrate how the SLE phase behavior can be used to design the recovery process. Both case studies illustrate that pure metal salts can be recovered from the precipitation process with high recovery. The case studies also demonstrate how the SLE phase behavior helps to rationalize the separation process developed by previous researchers based on trial and error. The SLE phase behavior can be utilized to determine the optimal operating conditions such as the amount of precipitant to be added to the system. With the insights provided from the SLE phase behavior, new process alternatives can be generated. Process alternatives can be compared with the base case process to determine the optimal process for recycling metal salts from spent LIBs.