nLab quotient chain complex

If $i : X^\bullet \hookrightarrow Y^\bullet$ is an inclusion of chain complexes in an additive category $A$, the quotient chain complex $Y^\bullet/X^\bullet$ is a cofibre of $i$ in the (discrete) category $Comp(A)$. Hence

$$Y^\bullet / X^\bullet \simeq Y^\bullet \oplus_{X^\bullet} 0.$$

(Recall that $Comp(A)$ admits pushouts.)