I want to take a Raspberry Pi with me as wearable device so I have to attach a battery on it. The problem is that I fear the battery would explode some day thus destroy my body , just like Galaxy Note 7 for example. So what is the likelihood of explosion of the battery for Raspberry Pi on the market？
Lithium ion batteries have ruptured and blown up or caught fire. They are also small and light. In other words, they have a high energy density. Other things with high energy density include hydrogen, tnt and wood. Batteries need to be able to convert their energy efficiently and quickly into electricity. So wood is safe, but makes a lousy battery.
It's actually extremely difficult to make a cheap, light, safe battery that lasts a long time. If you want safety, you'll need to relax your constraints a bit and consider heavier, bulkier alternatives that may not last as long. Even alkaline batteries explode but they may be safer. If you do choose lithium ion, choose the smallest one that meets your need.