# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 3 Results and Discussion
### Effect of Surface Morphology  
The initial and cycling characteristics of the Ni-P electrodes explained in Section 3.2 show that relatively excellent characteristics could be obtained with a higher phosphorus content. However, the initial discharge capacity (342 mAh g\({}^{-1}\)) remained at 77% of the theoretical one (442 mAh g\({}^{-1}\)), indicating that the entire plated thin film was not effectively utilized for lithium storage. It is quite likely that such a low specific capacity might be caused by the low electrical conductivity and high reaction overvoltage of typical phosphide electrodes. Thus, this study tried to improve it by changing the surface structure. In particular, an attempt was made to increase the thin film utilization rate by modifying the surface to be more open-structured and have a large surface area. To change the surface morphology of the thin film, spherical micro-electrodeposits of Ni-P were created by electroplating. In a general Ni-P electrodeposition process, the Ni-P electro-deposit attains a spherical shape, because phosphorus and nickel are co-plated omni-directionally [52]. However, at this time, the plating of pure nickel, which has a planar growth feature, proceeds simultaneously. As a result, the overall electrodeposited layer is gradually changed from a spherical shape to a flat shape. On the other hand, it is known that when the plating solution contains NaCl (or when NaCl is generated by the reaction of the pH adjuster NaOH and NiCl\({}_{2}\cdot\)6H\({}_{2}\)O in the plating solution), nickel could grow anisotropically [53]. This means that the plating position of nickel is significantly limited by the presence of NaCl; thus, nickel hardly interferes with the spherical growth of the neighboring Ni-P. In this respect, instead of using ammonia hydroxide, which is generally used for pH control, NaOH was used to form spherical electrodeposits on the surface in this study (sample number 4 in Table 1). Except for the use of NaOH as a pH adjuster, other deposition conditions were basically determined by the trial and error method. In particular, as expected, it was experimentally confirmed that the density of spherical deposits increased with the concentration of P  
Figure 6: Bending test results for the qualitative analysis of the adhesion between the substrate and thin films: (**a**) schematic of bending test. Scanning electron micrographs of the nickel phosphide thin films after five repetitive bending cycles: (**b**) sample P16, (**c**) sample P21, and (**d**) sample P28. source, reduction current density, and deposition time. However, the film morphology was changed to be flat when they exceeded certain values, most likely due to the over-growth of the flat plating layer. From numerous experiments, the concentrations of pH adjuster and P source, current density, and deposition time given in Table 1 were finally selected for the well-defined spherical electrodeposits. As for the acidity, it was also experimentally proved that the lower the acidity was, the more spherical was the deposit we obtained in the pH range of 1.2-1.8. However, since lower acidity led to more vigorous H\({}_{2}\) gas evolution and then less uniform deposits, pH was set to 1.5 for the uniform thin film formation. Figure 7a,b show the surface and inclined views of a Ni-P thin film with the same phosphorus content (28 at%) as the P28 thin film partially covered with spherical electrodeposits (from now on referred to as P28_S). Unlike the P28 thin film with flat-surface morphology, P28_S had spherical electrodeposits with a diameter of approximately 150 nm formed on a thin and flat plating layer with a thickness of 50 nm. An image analysis showed that the density of the spherical electrodeposits was ca. 2.3 \(\times\) 10\({}^{9}\) cm\({}^{-2}\).