Thermoelectricity enables the conversion of waste heat into useful power, and it is becoming increasingly important for power recovery applications at high temperature, where thermoelectric performance is most efficient. Copper selenide has a simple crystal structure, and yet its thermal conductivity becomes comparable to that of a glass at temperatures above 800K, and this is very favourable for thermoelectric applications. The low thermal conductivity appears to be correlated with the onset of super-ionic diffusion of the copper ions above this temperature. We aim to test molecular dynamics simulations of the copper diffusion using quasi-elastic neutron scattering, and to relate this to changes in the lattice vibrations determined using inelastic neutron scattering, since these control the thermal conductivity.