Low-Power Wide-Area Networks (LPWAN) connect a large number of battery-powered wireless devices over long distances. Among them, Long Range Wide Area Networks (LoRaWAN) implement a Pure ALOHA medium access scheme to save device energy by minimizing the radio usage. However, frame collisions restrain the network scalability when the traffic load increases. In this context, synchronization is used to exploit the available bandwidth more efficiently by controlling the timing of frame transmissions and reducing the collision probability. Remarkably, the network throughput is increased at the cost of an extra energy demand due to the inherent overhead. In that, the entailed network scenario is still supposed to address low power applications. This paper timely presents LoRaSync, an energyefficient synchronization scheme designed for LoRa networks of any size. An accurate clock drift model was established based on measurements made on real cheap devices, and leveraged to support the design of LoRaSync. Our mechanism has been used to evaluate the same ALOHA-based random access but on a timeslotted basis, thus increasing the maximum achievable throughput compared to the legacy access. Throughput and energy efficiency models are established to evaluate the performances of a LoRaSync-operated network. These models are validated with a simulation environment mimicking large-scale deployments, and then used to determine the most energy efficient slot size for any traffic load. As a final proof of concept, LoRaSync has been implemented and tested on a LoRa testbed to demonstrate the feasibility of our solution on real hardware.