The production of ammonia (NH3) in the industrial scale basically relies on the traditional technology of Haber-Bosch, which is operated under harsh conditions with high energy consumption and a huge number of greenhouse gas emissions. Electrochemical N-2 reduction reaction (NRR) is a promising route for artificial N-2-to-NH3 fixation with less energy consumption. However, an effective electrocatalyst, as a prerequisite of the NRR, is of significance. Here, we report that a spinel LiMn2O4 nanofiber acts as a noble-metal-free electrocatalyst for NH3 synthesis with excellent performance under ambient conditions. The electrocatalyst, which was tested in 0.1 M HCl, has an excellent Faradaic efficiency of 7.44% and a NH3 yield of 15.83 mu g h(-1) mg(cat.)(-1) at -0.50 V versus reversible hydrogen electrode. Moreover, it also possesses excellent electrochemical and structure stability.