## 1 Introduction  
Polypropylene (PP) is extensively used in many fields, including floor coverings, automobiles, building materials, electronics and electric materials, various home textiles, wall-coverings, and so on [1; 2; 3; 4; 5; 6]. However, relatively poor thermal stability and fire resistance of PP limit the conditions and range of its practical application [7; 8]. In particular, the damage to human life becomes serious because PP always produces CO during combustion [9; 10; 11; 12]. To reduce flammability and release of CO, the addition of flame retardants is an effective way [13; 14; 15; 16; 17]. Moreover, it is well known, to improve the strength and poor toughness of PP, the reinforcing and toughening agents are necessary additives [18]. Therefore, the addition of a small amount of flame retardant not only can increase the flame retardancy, smoke suppression, and thermal stability, but also can enhance the mechanical property of PP polymer, which is a desired goal in this work. In the past few decades, many researchers have been focusing on exploiting environmentally-friendly halogen-free fire retardants for PP materials. Inorganic fillers, such as magnesium hydroxide (MTH) and aluminum hydroxide (ATH), have been widely applied in flame-retardant polymeric materials [19]. These fillers could decrease the temperature of the burning materials by endothermic dehydration reaction and release water vapor into the gas phase to dilute the flame. However, the high loading of inorganic fillers (\(\approx\)60 mass%) is usually required to obtain a satisfactory flame retardancy of PP composites, which leads to the reduction in the processing characteristics and deteriorated mechanical properties of the materials [20,21]. The layered double hydroxides (LDH) materials as a result of their high aspect ratio, can improve mechanical properties of samples to a considerable level [22]. The LDH, also known as brucite-like layered materials, are a large class of anionic clays with a general formula of [M2\({}^{+}\)1\({}_{-}\)xM3\({}^{+}\)x(OH)2]\({}^{\times}\)[A\({}^{n-}\)]\({}_{X/n}\cdot\)mH\({}_{2}\)O, where M\({}^{2+}\) and M\({}^{3+}\) are divalent and trivalent metal cations, x = M\({}^{3+}\)/(M\({}^{2+}\)+M\({}^{3+}\)) molar ratio and A\({}^{n-}\) is an n-valent interlayer anion [23, 24, 25]. During combustion, when LDH loses the adsorbed water and interlayer water, combustible products are then diluted and cooled within the flame zone. Meanwhile, the generated char residue and mixed metal oxide can protect the bulk polymer from air exposure and suppress the release of smoke [26, 27, 28, 29]. Therefore, LDH has attracted considerable attention as a flame retardant and thermal stabilizer [30, 31, 32, 33, 34, 35, 36, 37, 38]. In particular, [PERSON] et al. confirmed that MgAl LDH and CoAl LDH can efficiently improve the flame retardancy of PP [39, 40]. In addition, the Ni-based LDH\({}^{\prime}\) catalysts are highly active and cost-effective in the partial oxidation and steam reforming of methane to produce H\({}_{2}\)[41, 42]. Furthermore, in our previous report, Ni-containing MgAl-LDH showed obviously higher flame retardant efficiency in comparison with MgAl-LDH in the ethylene vinyl acetate (EVA) matrix [32, 35]. However, the flame retardancy and smoke suppression of the Ni-containing MgAl LDH in the PP matrix have not been reported. Thus, by considering the advantages of Ni ions in catalysts and flame retardant in EVA, the Ni-containing MgAl LDH can also be expected to improve the comprehensive performance of PP/LDH composites. Since NiMgAl-LDH containing 0.2 mole Ni cations has never been reported, therefore, a PP composite with 1 wt %-20 wt % Ni\({}_{0.2}\)Mg\({}_{2.8}\)Al-LDH was prepared in the present work. X-ray diffraction (XRD) and morphology of Ni\({}_{0.2}\)Mg\({}_{2.8}\)Al-LDH and PP/LDH composite were characterized. Furthermore, the composite PP/LDH with comprehensive performance including enhanced flame retardancy, smoke suppression, thermal stability, and mechanical properties were obtained in this work.