  The negatively charged boron-vacancy center ($\mathrm{V}_{\mathrm{B}}^-$) in
hexagonal boron nitride (hBN) has recently emerged as a highly promising
quantum sensor. Compared to the nitrogen-vacancy (NV) center in diamond, the
change with temperature of the spin transition energy of
$\mathrm{V}_{\mathrm{B}}^-$ is more than an order of magnitude larger, making
it a potential nanoscale thermometer with superior sensitivity. However, the
underlying mechanism of the observed large temperature dependence remains an
open question. In this work, using isotopically purified
$\mathrm{h}{}^{10}\mathrm{B}{}^{15}\mathrm{N}$, we systematically characterize
the zero-field splitting, hyperfine interaction, and spin relaxation time of
$\mathrm{V}_{\mathrm{B}}^-$ from 10 to 350$~$K. We carry out first-principle
calculations of the $\mathrm{V}_{\mathrm{B}}^-$ spin-phonon interaction and
show that a second-order effect from finite-temperature phonon excitations is
responsible for the observed changes in experiments. By fitting our
experimental results to a physically motivated model, we extract the dominant
phonon mode which agrees well with our simulations. Finally, we investigate the
dynamic nuclear spin polarization process at cryogenic temperatures. Our
results provide key insights in $\mathrm{V}_{\mathrm{B}}^-$ centers and their
utilization as nanoscale thermometers and phonon sensors.
