<p>The ever-increasing accumulation of spent lithium-ion batteries and nitrate-contaminated wastewater poses serious environmental challenges. Herein, we report a waste-to-value strategy that directly uses the acid leaching solution of spent LiCoO₂ battery cathodes as a precursor to electrodeposit Co(OH)₂-based electrodes for electrochemical nitrate reduction to ammonia. The effects of coexisting cations (Li⁺, Al³⁺, and Cu²⁺) are systematically investigated. An optimal Li⁺ molar ratio of 50%, corresponding to the native Li⁺/Co²⁺ ratio in the leachate, yields a faradaic efficiency of 86% and an ammonia yield rate of 0.27&#xa0;mmol&#xa0;cm⁻²&#xa0;h⁻¹. Compared with Na⁺ and K⁺, Li⁺ imparts superior activity due to specific structural modulation rather than solution conductivity. Trace Al³⁺ (~ 1%) induces severe agglomeration of the three-dimensional nanosheet-assembled spheres, lowering the yield rate, while a small amount of Cu²⁺ (~ 5%) promotes uniform dispersion, boosting the FE to 94% and the rate to 0.32&#xa0;mmol&#xa0;cm⁻²&#xa0;h⁻¹. When both impurities coexist at their actual levels (~ 1% Al³⁺ and ~ 3% Cu²⁺), the detrimental effect of Al³⁺ is counteracted by Cu²⁺, leading to a well-dispersed morphology. The electrode prepared directly from the authentic leachate achieves an outstanding FE of 96%, 100% ammonia selectivity, and a rate of 0.30&#xa0;mmol&#xa0;cm⁻²&#xa0;h⁻¹. This work provides a sustainable and cost-effective route for upcycling spent batteries into high-performance electrocatalysts for environmental nitrate remediation and ammonia synthesis.</p> Graphical Abstract <p></p>