Altermagnets combine antiferromagnetic order with ferromagnetlike spin splitting, a duality that unlocks ultrafast spin-dependent responses. This unique property creates unprecedented opportunities for spin-current generation, overcoming the intrinsic limitations of conventional spin-transfer and spin-orbit torque approaches in magnetic memory technologies. Here, we establish a fundamental relationship between Fermi surface geometry and time-reversal-odd ($\mathcal{T}$-odd) spin currents in altermagnets through combined model analysis and first-principles calculations. We demonstrate that a $d$-wave altermagnet with a flat Fermi surface can achieve a theoretical upper limit of charge-to-spin conversion efficiency (CSE) of 100%. This mechanism is realized in the newly discovered room-temperature altermagnetic metal ${\mathrm{KV}}_2{\mathrm{Se}}_2\mathrm{O}$, which exhibits a CSE of $\sim 78\%$ at the charge neutrality point—nearly double that of ${\mathrm{RuO}}_2$, setting a new record for $\mathcal{T}$-odd CSE. Under electron doping, this efficiency further increases to $\sim 98\%$, approaching the theoretical limit. Our Letter advances the fundamental understanding of $\mathcal{T}$-odd spin currents via Fermi surface geometry engineering and provides key insights for developing next-generation altermagnet-based memory devices.

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• Electrical generation of spin carriers
• Fermi surface
• Spin current
• Altermagnets
• First-principles calculations