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Manganese dioxide (MnO₂) exists in a variety of polymorphs and crystallographic structures. The electrochemical performance of Li storage can vary depending on the polymorph and the morphology. In this study, we present a new approach to fabricate polymorph- and aspect-ratio-controlled α-MnO₂ nanorods. First, δ-MnO₂ nanoparticles were synthesized using a solution plasma process assisted by three types of sugars (sucrose, glucose, and fructose) as reducing promoters; this revealed different morphologies depending on the nucleation rate and reaction time from the molecular structure of the sugars. Based on the morphology of δ-MnO₂, the polymorphic-transformed three types of α-MnO₂ nanorods showed different aspect ratios (<i>c/a</i>), which highly affected the transport of Li ions. Among them, a relatively small aspect ratio (<i>c/a</i> = 5.1) and wide width of α-MnO₂-S nanorods (sucrose-assisted) induced facile Li-ion transport in the interior of the particles through an increased Li-ion pathway. Consequently, α-MnO₂-S exhibited superior battery performance with a high-rate capability of 673 mAh g⁻¹ at 2 A g⁻¹, and it delivered a high reversible capacity of 1169 mAh g⁻¹ at 0.5 A g⁻¹ after 200 cycles. Our findings demonstrated that polymorphs and crystallographic properties are crucial factors in the electrode design of high-performance Li-ion batteries.