Radical innovations can shift the global competitiveness of entire nations. While countries typically struggle to absorb knowledge about novel technologies quickly, in which knowledge tends to be spatially sticky, an important exception is the fast catch-up of the Korean Li-ion battery industry from Japan in the early 2000s. In this paper, we conduct an exploratory case study on this surprising success story. Focussing on patent co-inventions between Korea and Japan, we investigate their significance, as well as underlying types of co-inventions and types of transferred knowledge. To this end, we proceed in four steps: (1) a Poisson regression model; (2) social network analyses; (3) patent inventor tracking and (4) patent coding. Our results indicate that Korean–Japanese co-inventions hold significantly greater influence than other cross-country co-inventions, including with patents without cross-country collaboration. We find a pronounced knowledge-transfer intensity during the early 2000s and observe two types of co-inventions: organisation-level and inventor-level. Predominantly, we observe inventor-level co-inventions, i.e. Korean companies hiring experienced Japanese engineers, that proved important to transferring sticky knowledge. Moreover, while most patents target the design of core battery components, the share of manufacturing patents are—contrary to theoretical expectations—highest during the first half of the observation period. We also discuss our findings and draw implications for policy, industrial and academic players, including industry localisation policies, technology-inherent catch-up strategies and directions for future research.