To maximize circularity, remaining value in electric vehicle (EV) batteries can be retained through deploying different strategies such as reusing and repurposing to enable second life applications before they are recycled. Since EV battery ecosystems for the so-called battery second life are at early, emergence stages, they are characterized by uncertainties and high complexity. Despite previous analyses on the topic, collaboration in terms of roles, hierarchies, as well as the circular ecosystem dynamics remain unclear. We thus conduct a systematic literature review, applying a complex adaptive systems lens to map the literature concerning the three core dimensions of ecosystems: conceptual, structural, and temporal. Results point to the need to collaborate to enable circular ecosystems for EV battery second life, but also hint to high diversity of actors- over 40 types of actors potentially relevant for EV battery second life ecosystems - and various challenges for collaboration in such ecosystems.