  Solid-state methods for cooling and heating promise a more sustainable
alternative to current compression cycles of greenhouse gases and inefficient
fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure
($p$) are especially encouraging in terms of large adiabatic temperature
changes ($|\Delta T| \sim 10$ K) and colossal isothermal entropy changes
($|\Delta S| \sim 100$ JK$^{-1}$kg$^{-1}$). However, BCE typically require
large pressure shifts due to irreversibility issues, and sizeable $|\Delta T|$
and $|\Delta S|$ seldom are realized in a same material. Here, we demonstrate
the existence of colossal and reversible BCE in LiCB$_{11}$H$_{12}$, a
well-known solid electrolyte, near its order-disorder phase transition at
$\approx 380$ K. Specifically, for $\Delta p \approx 0.23$ $(0.10)$ GPa we
measured $|\Delta S_{\rm rev}| = 280$ $(200)$ JK$^{-1}$kg$^{-1}$ and $|\Delta
T_{\rm rev}| = 32$ $(10)$ K, which individually rival with state-of-the-art
barocaloric shifts obtained under similar pressure conditions. Furthermore,
over a wide temperature range, pressure shifts of the order of $0.1$ GPa yield
huge reversible barocaloric strengths of $\approx 2$
JK$^{-1}$kg$^{-1}$MPa$^{-1}$. Molecular dynamics simulations were carried out
to quantify the role of lattice vibrations, molecular reorientations and ion
diffusion on the disclosed colossal BCE. Interestingly, lattice vibrations were
found to contribute the most to $|\Delta S|$ while the diffusion of lithium
ions, despite adding up only slightly to the accompanying entropy change, was
crucial in enabling the molecular order-disorder phase transition. Our work
expands the knowledge on plastic crystals and should motivate the investigation
of BCE in a variety of solid electrolytes displaying ion diffusion and
concomitant molecular orientational disorder.
