The lithium–sulfur (Li–S) battery is widely believed to be a promising candidate for grid energy storage and electric vehicles due to its high specific capacity. For performance improvement of Li–S batteries a full understanding of the actual reaction in the charge/discharge process is needed. Currently, a unified understanding of the role of lithium disulfide (Li2S2) in Li–S batteries has still not been reached. Here, we discover the most stable structure of Li2S2 by using first-principles unbiased structure searching calculations. The new compound exhibits intriguing structural features, including Li-centered tetrahedrons and S22− ions. Analysis of the thermodynamics and electrochemical activity indicates that Li2S2 acts as an intermediate product and spontaneously disproportionates into Li2S plus S in a Li–S battery. The finding of Li2S2 not only matched the lower plateau of the experimental discharge profile but also gave rise to a direct explanation of the experimental Raman peak at 514 and 174 cm−1 observed in the discharge process. Our work enables an opportunity for understanding the role of Li2S2 upon discharge in Li–S batteries.