What is the formula for lithium sulfide?

$L {i}_{2} S$
Lithium is a Group 1 metal .... it commonly forms a $L {i}^{+}$ ion... Sulfur is a Group 16 non-metal...upon reduction it commonly forms a ${S}^{2 -}$ ion...
${S}^{2 -} + 2 \times L {i}^{+} \equiv L {i}_{2} S$