To understand the reaction mechanism, a series of characterization and control experiments are performed. First, the superficial morphologies, that cathode before reaction, and post-reaction with and without TPPA, are recorded by SEM (Figure ). Sn is a soft metal, and it presents a groove grain after sanding. Both cathodes after reaction with and without TPPA deposited substances on them which means a solid interphase successfully forms on each surface of them. The difference between them is that the cathode with TPPA shows an even and thin layer, while the non-TPPA one is attached with some irregular and agglomerate particles. At a scope of 50 and 10 μm, the solid interphase looks denser with TPPA. Some cracks can be observed, which may result from the volatilization of the solution after being taken from THF. It is noticed that parts of the layer will craze and shed when exposed to air. For the non-TPPA one, a functional membrane fails to form on the cathode. In the XPS pattern, it can be seen that the intensity of O is greatly strengthened, and Li, P, and N peaks appear (Figure ). The fine spectra of C 1s orbit emerges a new peak at 298.7 eV in post reaction sample which can be attributed to the C=O bond in polymers. This evidence suggests the formation of a SEI layer from the polymerization of TPPA combined with Li + . The Mg peak may be from trace Mg ions depositing on the cathode. A C=O bond peak, which belongs to 2-pipeidone, can be read in the IR spectrum at 1673 cm -1 , and its stretching frequency shifts to 1660 cm -1 when adding the LiBr (Figure ).