Lithium is an element that has an electronic configuration of 2,1. So when forming bonds, it tries to release one of its electrons in the last shell to reach the octet state. Normally when atoms release electrons they get positive charge since the number of protons which bear positive charge, is higher than number of electrons which bear negative charge. So when reaching its octet state, lithium gains positive charge!
Hope this helps! Tbh am not good at explaining. If u have any doubts, ask me!