## 1 Introduction  
Energy demand is projected to increase due to global economic growth and rising middle class populations. For example, the global energy demand rose by 2.1% according to the International Energy Agency (IEA) [1] in 2017, which is more than twice the previous year's rate. Meanwhile, fossil fuels still account for more than 70% of total consumption, leading to carbon emissions at a historical high of 32.5 gigatons in 2017. Therefore, the control of carbon emissions without reducing economic growth has been one of the world's greatest challenges [2]. Large scale utilization of renewable energy sources and enhancement of energy efficiency of current processes are regarded as two of the most promising approaches to addressing these challenges [3; 4]. One of the key issues that restricts the large scale deployment of renewable energy technologies is associated with intermittency and fluctuation which leads to a mismatch between energy supply and demand [5]. Energy storage offers an effective solution to the issue [6]. Energy storage could also enhance overall efficiency of energy networks [7]. There are many different energy storage technologies. This work is concerned with thermal energy storage (TES). Such a technology has a vital role to play in our current energy systems as heat is at the heart of the energy supply chain, linking primary energy and secondary energy sources [8]. In addition, TES has a relatively high technical maturity as well as the lowest costs [9]. It currently accounts for over 50% of the world's storage installations excluding pumped hydro [10]. Based on methods used to store heat, TES technologies can be broadly classified as either sensible heat storage, latent heat storage, and thermochemical energy storage. Our focus here is on latent heat storage, i.e., the use of phase change materials (PCM), particularly for high temperature applications (~700 \({}^{\circ}\)C). The work is motivated by industrial requirements, including concentrated solar power generation (e.g., tower technologies) and high temperature industrial waste heat recovery (e.g., glass, ironmaking, and steelmaking, etc.) [11; 12]. For applications over ~700 \({}^{\circ}\)C, as will be briefly reviewed below, there are a limited number of PCM choices. Inorganic salts are among the most popular candidates due to their high stability at high temperatures, high energy storage density, and low costs [13]. However, these PCMs often face the two issues of chemical incompatibility with constructive materials at high temperatures and relatively low thermal conductivity [8]. An effective way to minimize this incompatibility is through form-stabilization of PCM, namely the formation of a composite PCM. Such a composite PCM could confine the molten salt in the matrix structure and maintain shape stabilization during the phase transition process, thus reducing the direct contact and corrosion between the molten salt and containers. Additionally, this composite PCM could be further enhanced in terms of thermal conductivity through the addition of highly thermally conductive and chemically compatible additives. There have been lots of studies on inorganic salt-based form-stable composite PCMs with different phase change temperatures. Table 1 gives a summary of the recent literature published in this area. From Table 1, one can observe the following:  
* Most studies are on PCM use in low to medium temperature applications and eutectic nitrate salts are the most studied;
* Very few studies have been done on PCM use in high temperature applications (>500 \({}^{\circ}\)C);
* Carbon materials, particularly graphite, have been the main materials used to enhance PCM thermal conductivities, which are less likely to be applicable at high temperatures due to oxidation [5];
* Little work has been done on the thermal conductivity enhancement of salt-based composite PCMs for applications above ~500 \({}^{\circ}\)C;