  The multi-ton-scale liquid xenon detectors, with an excellent energy
resolution of a few keV, will be constructed to probe the dark-matter
particles. In this paper, we show that precision measurements of the low-energy
solar neutrinos via the elastic neutrino-electron scattering in this kind of
detectors are able to improve the present limits on neutrino lifetimes and
neutrino magnetic moments by about one order of magnitude. We carefully study
the impact of the unknown neutrino mass spectrum on the ultimate limits in the
case of non-radiative visible neutrino decays. In the case of invisible
neutrino decays, the lower bounds $\tau^{}_1/m^{}_1 \gtrsim 3 \times
10^{-2}~{\rm s}/{\rm eV}$ and $\tau^{}_2/m^{}_2 \gtrsim 8\times 10^{-3}~{\rm
s}/{\rm eV}$ at the $2\sigma$ level can be obtained for a total exposure of
$70~{\rm ton}\cdot {\rm year}$. Furthermore, a restrictive constraint on the
effective magnetic moment of neutrinos $\mu^{}_{\rm eff} \lesssim 2.6\times
10^{-12}~\mu^{}_{\rm B}$, with $\mu^{}_{\rm B}$ being the Bohr magneton, can be
achieved. This is among the best results that will be available in the
laboratory experiments in the near future.
