# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 3 Results and Discussion
### Dependence of Electrochemical Properties on Phosphorus Content of Ni-P Thin Films  
Figure 3a-c show the charge and discharge curves for the first three cycles at a rate of 0.2 C (4.74, 6.38, and 8.84 mA cm\({}^{-2}\)), obtained from the P16, P21, and P28 thin films, respectively. Considering the phosphorus content in the thin film (Figure 2a-c), the theoretical specific capacities of P16, P21, and P28 were calculated to be 237, 319, and 442 mAh g\({}^{-1}\), respectively. However, the first charge capacities determined by the experiments were 404, 672, and 680 mAh g\({}^{-1}\), which were larger than the theoretical ones. These high initial charge capacities were probably due to the side reaction that formed a solid electrolyte interface [51]. After the first charge, a reversible charge-discharge response was observed. The initial discharge capacities of the three electrodes were 240, 300, and 342 mAh g\({}^{-1}\). Notably, the P28 specimen, which had the highest phosphorus content, showed approximately 77% of the theoretical capacity, which is comparable to that of commercial graphite. In addition, its volumetric capacity was estimated to be 2799 mAh cm\({}^{-3}\), which was about 3.3 times the theoretical value of graphite (850 mAh cm\({}^{-3}\)). For all the electrodes, voltage plateau regions (or inflection points) were observed around 0.65 and 1.5 V vs. Li\({}^{+}\)/Li on the charge curve, and around 1.3 and 2.0 V vs. Li\({}^{+}\)/Li on the discharge curve (see inset figure (dQ/dV curve)). It is known that the former was the result of lithiation and delithiation, which involve Li\({}_{3}\)P formation and decomposition reactions, and the latter was the result of the formation and decomposition reactions of an intermediate Ni\({}_{12}\)P\({}_{5}\) compound [30]. Figure 4 shows the coulombic efficiency and discharge capacity retention rate with the number of charge/discharge cycles at a 0.2 C rate. After about three cycles in which irreversible surface products were formed, the P21 and P28 electrodes showed coulombic efficiencies of approximately 95% or more and operated stably. After 20 cycles, the capacity retention rates were approximately 81% (241 mAh g\({}^{-1}\)) and 86% (293 mAh g\({}^{-1}\)) of the initial capacities, respectively. However, the P16 electrode, which had the lowest phosphorus content, deteriorated rapidly from the beginning of cycling. These results are contrary to the general expectation that the cycling stability would deteriorate as a result of the volume change during charge/discharge with the increase in phosphorus content in the Ni-P electrode. The reason for this interesting result could be understood by comparing the surface changes of the three electrodes after the cycles. Figure 5 shows the surface morphology of each electrode after 20 cycles. Electrodes P21 and P28 showed minor physical changes even after 20 cycles (Figure 5b,c), whereas the P16 electrode with low phosphorus content showed the partial detachment of active material, and several cracks were formed around the area where the detachment occurred (Figure 5a). This result implies that the relatively low cycling stability of the P16 electrode was caused by physical damage to the active materials. It was previously mentioned that a high reduction current density applied during the P16 electrode's fabrication could generate more hydrogen gas on the substrate surface, forming more voids between the substrate and thin film and reducing the adhesion between them (Figure 1). The adhesion test results confirmed that the P16 specimen, which had a low phosphorus content, had the worst adhesion (Figure 6). Thus, the electrode with a high P content had relatively severe stress changes in the thin film due to the volume change, but the high adhesion between the substrate and thin film prevented the thin film from being detached.