There is a Jisan brand 51000mAh high capacity Li-Polymer power bank selling online in some Malaysian e-commerce websites at pretty low price.
It claims to be manufactured in Korea, but when you search the Internet about "Jisan power bank", there is not much info other than the e-commerce webpages. The domain name "jisangroups.com" is even inactive. Most if not all the selling websites are based in Malaysia.
The power bank does have a premium metallic look and feel, but its packaging doesn't match the premium feeling. The English on the packaging (both the language and the text content) is not up to standard as well.
It provides 1 year limited warranty, with a Warranty Card inside, but there is no manufacturer detail nor contact, and the Warranty Card is not professionally designed and written. In fact, the warranty is provided by the dealer.
It says its protection circuit module is based on "Yeonhan Technology by Jisan".
All these signs make me suspect that this power bank is actually made in China by a manufacturer related to OEM factory called Yeon Han, and OEM to a Malaysian dealer with the Jisan brand, and possibly nothing to do with Korea.
This power bank is quite long (about 15cm), but not too heavy.
Given its slim design, I believe it uses the single cell 3.7v Li-Polymer flat cell as battery. This kind of battery is the same kind of Li-Po battery commonly used in hand phones and compact cameras nowadays, and is lighter and better design than the Li-Ion 18650 battery pack used in some other power banks.
However, looking at its size and weight, unless it uses a new kind of battery technology which can have extremely high energy density, I doubt its real capacity can be as high as 51000mAh. Its size and weight is more realistic for it to be around 15000mAh instead. I haven't tested on its real capacity yet, hopefully I am wrong and it really can discharge near to 51000mAh from fully charged state. Then it will be really value for money at its price.
Hopefully it has a good protection circuit module as it claimed. I do notice that its temperature doesn't change much during charging, which is a good sign.
After all, I believe this should be a good quality product, although seems to be overly done in marketing propaganda.