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Hard carbon (HC), a type of amorphous carbon materials, exhibits a multiscale microstructure comprising short-range ordered pseudo-graphene sheets and amorphous regions rich in defects and nanopores. Such structural complexity enables HC to achieve huge success in sodium-ion batteries (SIBs) with a plateau capacity in the low-potential regime extending the working voltage of full cells, similar to graphite in lithium-ion batteries (LIBs). However, this unique electrochemical signature is rarely observed in the application of HC in LIBs, due to the inherent differences in the HC microstructure and Li⁺ storage mechanism at the low-potential regime.  Herein, phenol-formaldehyde (PF) with controllable microstructures, a type of thermosetting resins, is employed as a model precursor for fabricating HCs, which is applicable to other thermo-setting resins and biomass precursors.^[1] The microstructures of the novolak-PF oligomers are carefully controlled. Material analyses reveal that low-cross-linking-density precursors tend to form pseudo-graphitic layers at early stage, generating abundant closed pores under suitable carbonization temperatures. A well correlation between Li-ion plateau capacity and closed pore volume along with in-situ XRD and Raman analyses confirms that the low-potential plateau results from Li⁺ filling into closed pores in these novolak-derived HCs. Consequently, the HC with optimal synthesis conditions achieves a reversible capacity of 550 mAh/g, including nearly 50% plateau capacity (255 mAh/g).^[2] This work provides a comprehensive understanding on the closed pore engineering and low-potential plateau origin of Li-ion storage, offering a promising route for microstructural engineering toward high-performance LIBs, which is applicable to Na-ion storage.