  Thermal transistors that switch the thermal conductivity (\k{appa}) of the
active layers are attracting increasing attention as thermal management
devices. For electrochemical thermal transistors, several transition metal
oxides (TMOs) have been proposed as active layers. After electrochemical redox
treatment, the crystal structure of the TMO is modulated, which results in the
\k{appa} switching. However, the \k{appa} switching width is still small (< 4
W/mK). In this study, we demonstrate that LaNiOx-based solid-state
electrochemical thermal transistors have a \k{appa} switching width of 4.3
W/mK. Fully oxidised LaNiO3 (on state) has a \k{appa} of 6.0 W/mK due to the
large contribution of electron thermal conductivity (\k{appa}ele, 3.1 W/mK). In
contrast, reduced LaNiO2.72 (off state) has a \k{appa} of 1.7 W/mK because the
phonons are scattered by the oxygen vacancies. The LaNiOx-based electrochemical
thermal transistor exhibits excellent cyclability of \k{appa} and the
crystalline lattice of LaNiOx. This electrochemical thermal transistor may be a
promising platform for next-generation devices such as thermal displays.
