  We study the $B_c^+$ production in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$
TeV. In the quark-gluon plasma (QGP) produced in heavy-ion collisions, heavy
quarks make random motions with the energy loss. We employ the Langevin
equations to study the non-equilibrium distributions of heavy quarks and the
Instantaneous Coalescence Model (ICM) to study the hadronization process. Due
to abundant charm and bottom quarks in the QGP, their coalescence probability
is significantly enhanced compared with the situations in proton-proton
collisions. We find that the final production of $B_c^+$ is increased by the
coalescence process, which makes the nuclear modification factor ($R_{AA}$) of
$B_c^+$ larger than the unit. Our model explains the experimental data well at
semi-central and central collisions. The observation of $R_{AA}(B_c^+)>1$ is
regarded as an evident and strong signal of the existence of the deconfined
medium generated in heavy-ion collisions.
