Lithium-ammonia (Li-NH3) solutions are possible to be successfully made under the vacuum condition but there still remains a problem of undergoing stable and reliable decomposition in vacuum for high-efficiency thermoelectric power generation. This paper describes a new method for improving the thermoelectric conversion efficiency of Li-NH3 solutions in vacuum. The proposed method uses a ‘U’-shaped Pyrex vacuum tube for the preparation and decomposition of pure fluid Li-NH3 solutions. The tube is shaped so that a gas passageway (‘U’) connecting both legs of the ‘U’ helps to balance pressure inside both ends of the tube (due to NH3 gasification) during decomposition on the hot side. Thermoelectric experimental results show that solution reaction in the ‘U’-shaped tube proceeds more stably and efficiently than in the ‘U’-shaped tube, and consequently, thermoelectric conversion efficiency is improved. It is also proved that the proposed method can provide a reversible reaction, which can rotate between synthesis and decomposition in the tube, for deriving the long-time, high-efficiency thermoelectric power.