Prof. Dr. Roberto Torresi

University of São Paulo

Plenary Title: “Sodium Rising: The Materials Science Behind the Next Battery Revolution”

Sodium-ion batteries are routinely presented as a cheaper version of lithium-ion technology. This lecture argues the opposite: sodium-ion is a distinct materials problem whose bottlenecks are structural, interfacial and kinetic, and therefore squarely within reach of the electrochemical toolbox this community already owns. Every advantage and every pathology of the technology follows from a small number of physical facts about the sodium ion — its size (1.02 Å against 0.76 Å for Li⁺), its standard potential (−2.71 V against −3.04 V vs SHE), its weak interaction with carbon and with solvents, and the absence of Na–Al alloying.
Each constant is developed into an explicit consequence. The larger ion demands more open host frameworks and tolerates compositions that lithium cannot host, at the price of larger and more anisotropic volume changes and of Na⁺/vacancy ordering. The 330-mV penalty is a permanent energy-density cost that no material discovery will recover, partly compensated by a wider anodic window in aqueous systems and by a milder driving force for electrolyte reduction. The lower de-solvation energy of Na⁺ reduces the activation barrier for interfacial charge transfer, giving better rate capability and low-temperature performance at comparable electrode design — one of the few places where sodium genuinely wins, and one that electrochemical impedance spectroscopy measures directly. The Na–Al phase diagram allows Al-current collectors at both electrodes and cells that can be discharged to 0 V for shipping and storage.
Cathodes are compared through a single trade-off triangle — energy density, stability against air, moisture and cycling, and cost and abundance — across three families: layered oxides (P2 versus O3, Na⁺/vacancy ordering and the high-entropy strategy), polyanionic frameworks (the inductive effect as a voltage design rule, and volume change per cycle as the cleanest predictor of cycle life) and Prussian blue analogues, where [Fe(CN)₆] vacancies and water are two symptoms of one synthesis problem, and where mechanochemical and controlled-precipitation routes become defect-engineering tools. The anode discussion starts from a negative result — Na⁺ does not intercalate into graphite in ordinary carbonate electrolytes — and centres on hard carbon, a material manufactured at scale and still incompletely understood. Electrolytes and interphases are then examined with methods familiar to this audience, including results from our own group. The lecture closes with cost, scale and sustainability, and with the position of the Ibero-American community, whose mineral base, energy matrix and established electrochemistry groups make it a legitimate actor rather than a spectator.
Sodium is not the cheaper lithium. It is the chemistry whose constraints happen to match a different market.
Acknowledgements: The author thanks FAPESP, CNPq and CAPES for financial support.

References
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