In this paper, we investigate the problem of optimal planning and deployment of multiple relays to support energy- efficient uplink transmissions of Internet of Things (IoT) devices. A novel approach is proposed to optimize the relay locations with the objective of minimizing the total energy consumption of the network. In addition, the uplink transmit power of IoT devices and the device-relay-channel association are jointly optimized to meet the QoS requirement of IoT devices. A mixed-integer linear programming (MILP) problem is formulated to obtain the optimal solution. We also design a low-complexity genetic algorithm to provide a sub-optimal solution to the problem.