# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 4 Conclusions  
In this work, amorphous nickel phosphide (Ni-P) thin films with various compositions were fabricated using electroplating and tested as anode materials for secondary lithium batteries. From the experimental results, the following conclusions were drawn:  
1. Uniform, flat amorphous Ni-P thin films containing 16-28 at% phosphorus were successfully synthesized under different plating conditions. Interfacial cracks between the substrate and the thin film were not observed in high phosphorus content layers but lower phosphorus content resulted in severe interfacial cracks. This is most likely because more hydrogen gas was generated on the surface of the substrate; thus, more voids were formed between the substrate and thin film under the high reduction current applied when a thin film with a low phosphorus content was created;
2. As anodes for secondary lithium batteries, a higher phosphorus content for the thin film resulted in a higher specific capacity in general. In particular, when the phosphorus content was the highest (28 at%), the initial discharge capacity was 342 mAh g\({}^{-1}\), which was approximately 77% of the expected theoretical value. This is comparable to the theoretical specific capacity of commercially available graphite. Moreover, the capacity per volume was more than three times that of graphite;
3. The cycling stability of the thin film was improved as the phosphorus content increased. This was contrary to the general expectation that an increase in the phosphorus content would cause the cycling stability to deteriorate. This occurred because the electrode with high phosphorus content had high adhesion between the substrate and thin film, which prevented thin film separation, even with a significant stress change, whereas, because the electrode with low phosphorus content had poor adhesion between the substrate and thin film, severe separation and electrode deterioration occurred even with a relatively minor stress change;
4. To improve the specific capacity by increasing the utilization rate of the thin film, a Ni-P thin film containing fine spherical electrodeposits on the surface and high phosphorus content (28 at%) was fabricated. The initial specific discharge capacity was comparable to the theoretical specific capacity (445 mAh g\({}^{-1}\)). Even after 20 charge and discharge cycles, it showed a high specific capacity (341 mAh g\({}^{-1}\)), which was 1.2 times that of a flat electrode with the same composition. In addition, at the relatively high discharge rates of 0.5 and 1.0 C, the discharge capacity retention rate was 5-10%p higher than that of a flat electrode. Further work on the spherical electrodeposits with various densities, sizes, etc., would give a deeper understanding of their effect on the electrode characteristics for lithium battery anodes.  
Conceptualization, J.-Y.H. and H.-C.S.; Data curation, J.-Y.H.; Funding acquisition, H.-C.S.; Investigation, J.-Y.H.; Supervision, H.-C.S.; Writing--original draft, J.-Y.H.; Writing--review & editing, H.-C.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by National Research Foundation (NFR-2018R1A5A1025594) of Ministry of Science and ICT, and was supported by the Materials Technology Institute at Pusan National University.  
Not applicable.  
The data that support the findings of this study are available on request from the corresponding author.  
The authors declare no conflict of interest.