An efficient solid electrolyte interphase (SEI) on lithium (Li) metal anode is crucial to suppressing Li dendrite. Herein, a methoxide electrolyte additive, i.e., potassium methoxide, is revealed to induce preferential adsorption of anions on Li metal surface by a combination of theoretical and experimental studies. Cryogenic transmission electron microscope measurements and x-ray photoelectron spectroscopy depth profiles reveal that the as-formed inorganic-rich SEI possesses an amorphous and monolithic feature, which not only enables uniform diffusion of Li+ through SEI but also reduces diffusion barrier of Li+ along Li/SEI interface. Consequently, Li dendrite can be efficiently suppressed and the cycling lifetime of Li | Li symmetric cells can be prolonged to 3500 h at 1 mA cm-2 / 1 mAh cm-2. Besides, the LiFePO4 | Li full-cell batteries can be cycled stably over 400 cycles with an N/P ratio (i.e., the areal capacity ratio of negative to positive electrodes) of ~3, and a high LiFePO4 mass loading of ~20 mg cm-2.