Complex sentence: By June 2015, the plane had traversed Asia, and in July 2015, it completed the longest leg of its journey, from Japan to Hawaii, during which the aircraft's batteries sustained thermal damage that took months to repair.
Split into simpler sentences: By June 2015, the plane had traversed Asia, and in July 2015, it completed the longest leg of its journey, from Japan to Hawaii. During that leg, however, the aircraft's batteries sustained thermal damage that took months to repair.