The LAX phases form an emerging class of orthorhombic and monoclinic compounds that share structural features with MAX phases, particularly their layered arrangement and the presence of A and X elements. In contrast to MAX phases, LAX structures incorporate late transition metals instead of early ones. In this work, we present a detailed first-principles study of the effect of temperature on quantum capacitance, electrical resistivity, and overall electronic behavior of the (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)AX (L = (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\); A = Al, Ga, Ge, In; X = C) family. We find that structural modification has a significant influence on both quantum capacitance and resistivity across the series. The results also indicate that these phases exhibit favorable electrical conductivity. Among them, (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)AlC shows particularly high conductivity. Moreover, variations at the A sites strongly influence the resistivity, underscoring the importance of atomic composition in controlling charge-transport behavior. Among the examined systems, the (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)AlC compound presents the highest residual resistivity ratio value, suggesting that increased disorder at the A layer enhances electron scattering and consequently elevates the resistivity. The LAX-phase compositions with A = Al, Ga, Ge, and In consistently function as effective negative electrodes, underscoring their suitability as a high-performance anode material and presenting a practical approach to adjusting their electrochemical response for targeted supercapacitor applications. Meanwhile, the quantum capacitance values for (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)GaC and (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)InC generally decrease as temperature increases. Collectively, these characteristics suggest that (Mn\(_{2/3}\)Ru\(_{1/3}\))\(_2\)AC compounds (A = Al, Ga, Ge, In) are strong candidates for applications such as protective coatings in electronic devices, current collectors or electrodes in high-temperature electrochemical systems and sensors, heat spreaders or integrated thermal-electrical management components, and functional materials for energy-storage technologies.
