From Linkedin he finished in October 2022 so should be available for a near immediate or immediate start.
Core's deals with Yahua started in 2017 so they pre-dated Robert, although he is certain to have had contact with key Yahua individuals. On 9 August 2021 Core announced a $34m share issue to Ganfeng along with an off-take of 75ktpa. Ganfeng has subsequently sold most if not all its shares for a really big profit. You would have to guess that Robert Sills was a key part of the team arranging the Ganfeng deal. He will therefore have connections at this very important lithium industry player ($151b market cap). I'd guess that Ganfeng would be prepared to take a similar gamble with an equity Off-take + placement if they believed ESS could get into production. His appointment at ESS materially increases the chances of an off-take with either Yahua or Ganfeng.
It also increases the chances from zero to something just above zero in relation to a deal with Tesla. On 2 March 2022 Core announced deal with Tesla that was subject to a definitive execution agreement. This was supposed to be concluded by 27 August 2022, but the date to achieve this was extended. In October, Core announced that they were not confirming this deal. In October the Commercial Marketing Manager at Core stopped working there (see linked in for his end date at Core). Only those inside Core know the extent to which their Commercial Marketing Manager (Robert Sills) was involved in discussions with Tesla. He's got to have had some involvement. It's interesting his end date was the same month the Tesla deal was confirmed not to be going ahead and possibly indicates his recommendation was to conclude a deal.
His involvement with DSO is interesting and IMO favourable for ESS. He will have some very valuable insights, particularly in relation to the changing nature of enquiries, price and what specifications are important in the DSO area.
More speculatively, I presume he has a number of contacts beyond those with whom Core had off-take deals. Many of these could be useful to ESS.