MacRumors did some digging and did, in fact, come across a Korean battery company called Orange Power, which has a patent application in Europe for a type of hollow battery
The Hollow Core battery page has been removed from the Orange Power site
Orange Power has a roadmap for their battery technology
The companies are said to be in partnership since the beginning of FY16, as one of the officials of Apple has been visiting the said South Korean battery developer. This company is expected to be solely responsible for its batteries. In this way, the tech giant is focused on securing its innovative battery technology reserved for its self-driving vehicles only.
The Korean company is said to be a small company of 20 employees with expertise in battery technologies. The company holds patented technologies reserved for hollow batteries. The batteries produced by this company are lithium ion battery with a hollow core and a thickness of two fingers and are expected to result in higher electrical output for the self-driving electric cars.
Apple is interested in this hollow-cored lithium ion battery as it provides a cell with more surface area to allow more efficient air flow to dissipate heat that is generated in duration of normal charge-discharge cycle. This would reduce the additional costs of installing a cooling device. The battery is easy to use as it can be engineered to make both parallel and series electrical circuits without the need of soldering or welding. It provides with the ease of replacing a single battery when it fails instead of replacing the entire bank
SOURCE- CNET, MacRumors, Orange Power, CNET