Role in Gravity
It was presumably a piece of collector's memorabilia brought up by a Chinese taikonaut only to be abandoned after the station was evacuated.
The Chinese coin was presumably destroyed upon re-entry. If not, the coin was buried or submerged in water after hitting the ground.
History and Description
Before the ancient Chinese got to their distinctive style of ancient coinage, they cast smaller versions of everyday objects like keys, swords and spades in bronze. Eventually in the Qin and western Han dynasties circa 202 B.C., the first circular coins were made with calligraphy around a square hole in the center. The purpose of the hole was so you could tie many coins into the shape of a dagger and hang it above your door to ward away bad spirits. These coins would be made like this for thousands of years until the fall of the last Qing emperor in 1912.
It is unknown what dynasty this coin was produced in because we see it only for a few seconds and it is small. However, judging by the wear on the surface, texture, thickness and the logic that the Chinese would have access to their own coins and wouldn't bring replicas into space, the ancient Chinese coin was probably a CGI replica of an authentic ancient coin. The coin was from later in ancient Chinese history, probably from the Song, Tang or Qing dynasties. It is mostlikely Qing judging by how many they made and the size and color of the coin.