*2.1. Materials*

### 2.1.1. Preparation of LSC

The LSC was prepared by using the sol-gel method [27,28], mixing a stoichiometric amount of lanthanum nitrate hexahydrate La(NO3)3·6H2O strontium nitrate Sr(NO3)2 with cobalt nitrate hexahydrate Co(NO3)2·6H2O in deionized water under constant stirring to ge<sup>t</sup> a clear solution. Citric acid (CA) was then added into the solution (CA and total metal ion in a 7:5 molar ratio), in which as a ligand to form a complex compound with the metal ion. Then, the pH value of the solution was adjusted to 9–10 by dropwise addition of aqueous ammonium hydroxide. The solution was slowly evaporated in a water bath at 70 ◦C for 10 h and the gel obtained was at the temperature of 150 ◦C overnight. Finally, the obtained powder was calcined at 900 ◦C for 6 h to obtain LSC nanoparticles.

### 2.1.2. Ball Milling of LSC

The prepared LSC powder was ball milled in a planetary ball mill, where 10 g of LSC powder and zirconia balls (mass ratio of zirconia balls to LSC powder of 20:1) were added to a ball mill jar, and then 200 mL of ethanol were added. The speed during ball milling was 300 rpm. The samples of ball milling of 20 and 40 h, respectively, were obtained for observation by scanning electron microscopy. Finally, the samples were dried at 50 ◦C to obtain the LSC nanoparticles after ball milling [29].

### 2.1.3. Preparation of the Nanocomposite

The matrix polymer was prepared by mixing 25% carbon black (CB), 45% low-density polyethylene (LDPE), and 30% ethylene-vinyl acetate copolymer (EVA) with an open mill at ◦C. Then, the LSC was mixed with the above matrix polymer in different mass percentages, as shown in Table 1. At last, the above materials were shaped by hot pressing by a vulcanizer.

**Table 1.** Sample notation and composition.

