This paper reports firstly a medium-temperature ternary PCM of nitrate salt, containing 68 wt% KNO3, 18 wt% LiNO3, 14 wt% Ca(NO3)(2) in weight. To improve its thermal stability under high temperature and long term conditions, CsNO3 of quite low percentage (0-0.6 wt%) was attempted to add into the ternary nitrate salt to obtain quandary salts (KNO3-LiNO3-Ca(NO3)(2)-CsNO3). The thermal properties and stabilities of the nitrate salts without/with CsNO3 were then carefully measured and analyzed. The results show that a low percentage of CsNO3 does not influence the melting point of 113.0 degrees C, but reduces the fusion heat slightly by around 9.3%. The addition of CsNO3 essentially improves the thermal stability of salts at high temperature of 400 degrees C for long duration and the thermal cycling stability under (100-180 degrees C) for 800 cycles. Two composites with 5 wt% and 10 wt% expanded graphite (EG) combined with the nitrate salt, remarkably raise the thermal conductivity of PCM by more than 20 times and 30 times, respectively. The increase of EG particle with extremely high thermal conductivity not only enlarges the specific surface area but tends to build interconnected structures in the composite, thus enhances the effective thermal conductivity of the composite. The novel PCM we developed here shows suitable thermal properties and good thermal stability in long term that make the composite PCM potential in improving the heat performance of latent heat storage system in medium-temperature applications. (C) 2016 Elsevier B.V. All rights reserved.