Sustainable battery technologies are central to meeting the growing demand for electric vehicles (EV) and grid-scale energy storage systems, which are critical for accommodating the intermittent nature of renewable energy sources. Among the growing array of battery chemistries, sodium-ion batteries (NIBs) address many issues with lithium-ion battery technologies, including sustainability, safety, cost per energy density, and justice considerations related to critical mineral extraction and supply chains. Leveraging abundant and environmentally benign materials for electrodes, NIBs offer a competitive low-cost alternative for inexpensive EVs and largescale grid storage applications, which can be manufactured with existing fabrication facilities. The advancement of NIB technology for a broader range of applications requires anode materials with higher capacity and reversibility. Meeting these demands, hard carbon (HC) has emerged as the most promising anode material for commercial NIBs due to its high capacity, electrochemical stability, low cost, and the abundance of its precursors (e.g., biomass). However, a general mechanism for sodium storage in HC -critical for widescale commercialization -has been elusive because of the complexity and sensitivity of storage mechanisms to HC microstructure.