# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 1 Introduction  
Currently, secondary batteries are receiving significant attention as core energy storage systems to realize a hyper-connected society in the 4 th industrial revolution. Especially, lithium secondary batteries are gaining a special attention as power sources due to their high energy and power densities. In terms of the production cost and performance of lithium secondary batteries, studies on the four major components, such as the positive electrode, negative electrode, separator, and electrolyte, are mainly being conducted. In particular, research studies on positive and negative active materials, which have the most significant influence on the capacity and cycle life, are being most actively carried out [1; 2; 3; 4; 5]. From the anode perspective, the goal is to obtain a next-generation high-capacity active material that exceeds the specific capacity of graphite (372 mAh g\({}^{-1}\)), which is currently in use [6]. For this purpose, various types of elements such as silicon, tin [7], sulfur [8; 9], and phosphorus [10] are being studied as promising candidates. They all have high specific capacities, but there are critical disadvantages; they have low cycling stability because the active materials are mechanically broken down as a result of a significant volume change during charging and discharging. They also have inferior high-rate characteristics due to their relatively low electronic conductivities. Various methods are being used to figure out these issues, including mixing silicon with carbon materials or manipulating the microstructure/morphology of silicon to enhance the cycling stability [11]. Some studies have focused on compounds such as oxides, fluorides, sulfides, and phosphides to improve the conductivity and cycle life [8; 12; 13; 14; 15; 16]. Among the next-generation negative active materials, transition metal phosphides (M-P, M = Ni [14], Fe [17], Co [18], Cu [19], etc.) are known to be combined with one phosphorus atom and three lithium atoms through a conversion mechanism (MP\({}_{y}\) + 3 yLi\(+3\)ye\({}^{-}\leftrightarrow\) yLi\({}_{3}\)P + M\({}^{0}\)), and these have much higher theoretical specific capacities than that of the graphite. In addition, the M-P bond can be broken relatively easily and can react with lithium, and the degree of polarization is approximately 0.4 V in a typical charge-discharge situation. These have an advantage because the reaction polarization is lower than those of metal oxides (\(\sim\)0.9 V), metal fluorides (\(\sim\)1.1 V), and metal sulfides (\(\sim\)0.7 V) [20]. They also react with lithium at a narrow range of low voltages, making them more suitable for configuring high-voltage batteries [21,22]. Nickel phosphide (Ni-P) has high utilization among the transition metal phosphides because it can form various stable phases [23]. In particular, NiP\({}_{2}\) is known to react reversibly with lithium as a lithium battery anode [21,22,24,25]. [PERSON] et al. reported that their NiP\({}_{2}\) anode had an initial specific capacity of 1250 mAh g\({}^{-1}\), which was close to the theoretical capacity, and maintained a capacity of 800 mAh g\({}^{-1}\) after ten cycles [21]. [PERSON] et al. also reported that a NiP\({}_{2}\) anode reacted reversibly with five lithium atoms [22]. However, Ni-P shows low coulombic efficiency due to significant capacity loss in the first cycle. In addition, there is still an issue with the deterioration of the cycling stability caused by a volume change, and its high-rate features are not as good as those of graphite [2,22,26,27]. To overcome this, particularly from the perspective of the nanostructure, studies related to the Ni-P anode active material are being widely conducted [10,14,15,21,25,27,28,29,30,31]. Most Ni-P crystalline materials are synthesized using the hot injection [28] and phosphorization methods [29].