Compared to aqueous electrolytes, fundamental understanding of the chemical and electrochemical processes occurring in non-aqueous electrolytes is far less developed. This is no different for Li-ion battery (LiB) electrolytes, where many questions regarding the solid-electrolyte interphase (SEI) on the anode side remain unanswered, including its chemical composition, the mechanism of formation, and its impact on LiB performance. Here, we present a detailed experimental and theoretical study of the electrochemistry of ethylene carbonate (EC) and its chemical relationship with trace amounts of water and HF across a vast range of electrode materials, from well-ordered single crystals to realistic graphite electrodes. We reveal the electrocatalytic nature of EC, HF, and water electroreductions at all interfaces, and unveil the catalytic role of water in EC electroreduction. Moreover, we show that these reactions are connected in a closed cycle by chemical reactions that take place either at the electrode/electrolyte interface or in the bulk of the electrolyte and demonstrate that the composition of the SEI depends predominantly on the balance between the (electro)chemistry of EC, water, and HF.