Lithium metal, although attracting renewed interest for the next revolution in energy storage, continues to be challenged with the detrimental dendrite formation. Recent experimental reports have demonstrated the contrasting impact of thermal attributes on the electrodeposition morphology, showcasing the alleviation and/or aggravation of dendrite formation. Herein, we present a comprehensive discourse to discern the thermally activated physical mechanisms governing lithium electrodeposition morphology. We report that the synergistic effect of enhanced electrolyte transport and surface self-diffusion under a uniform thermal field (∼75 °C) enables adequate dendrite suppression, even at high reaction rates. However, in contrast to this, a localization of the thermal field substantially increases the exchange current density of the confined region, instigating the growth of needle dendrites. Based on our mesoscale analysis, we demarcate safety limits for such an event, beyond which dendrite growth is inevitably triggered. Therefore, though the operational strategy of elevating the cell temperature promises to resolve the challenge of stable electrodeposition, it comes along with the caveat. This fundamental study provides a detailed insight into underlying electrochemical-thermal complexations, critical to the performance and safety of metal-based rechargeable batteries.