With the rise of electric vehicles (EVs) and thus lithium-ion batteries (LIBs), the number of end-of-life (EoL) LIBs after their first life in EVs is about to increase significantly. These end-of-first-life (EoFL) EV LIBs still have sufficient energy density for less-demanding second-life applications like stationary battery energy storage systems (BESSs) or mobile applications (e.g., forklifts, tools). Repurposing EoFL EV LIBs extends their lifespan, offering sustainability benefits and supporting several United Nations (UN) Sustainable Development Goals (SDGs). However, prevailing market entry barriers, such as high repurposing costs, little information on battery history and aging, or lacking performance indicators, hinder the widespread implementation of second-life applications. Thus, this study aims to identify preconditions for considering and selecting useful EoFL LIBs and to determine key performance indicators (KPIs) to minimize economic risks for a successful second-life market launch. KPIs were rated according to importance using a Likert scale, and reference values were introduced. A mixed-methods approach, using expert interviews, an exploratory workshop, and an online survey, was applied. Twelve important preconditions were identified, with the “availability of information on battery specification” and “compliance with standards and regulations” considered very important. In addition, 12 KPIs were derived, covering six economic, three environmental, and three technical and safety-related indicators. The KPIs “state of safety (SoS)” and “resource savings (Rsav)” were rated as highly important. Overall, the findings provide performance measurement guidance for repurposing companies, facilitating the market launch and adoption of second-life applications. Future research can build on these results and investigate variations among different battery types, ultimately promoting a circular economy.