We consider sensor networks where energy is a limited resource so that energy consumption must be minimized while satisfying given throughput requirements. Moreover, energy consumption must take into account both the transmission energy and the circuit processing energy for short-range communications. In this context, we analyze energy-efficient joint routing and link scheduling to achieve the optimal tradeoff between energy and delay. For networks composed of multiple clusters of nodes, we propose and analyze the cooperative multiple-input multiple-output (MIMO) approach where multiple sensor nodes in the same cluster cooperate in signal transmission and/or reception. We show that local information exchange within the cluster is not necessary for node cooperation based on Alamouti diversity codes if the transmissions are properly scheduled. We further show that the routing optimization problem based on cooperative MIMO can be solved by designing an equivalent single-input single-output (SISO) system, where each cluster is treated as a super node. For both SISO-based and MIMO-based cases, we derive the best energy-delay tradeoff curves and show that the cooperative MIMO approach dramatically improves the energy-delay performance.