When do you think we will have solide state batteries in smartphones?
The first generation of solid-state battery cells for smartphone use is already on the market, as I write this.
Solid State Batteries are Li ion and lithium polymer cells that are able to deliver higher capacity compared to conventional li-ion cells, but at a cost per charge: weight/capacity ratio and overall thickness tend to be lower due to lack of need for insulation layers or mechanical protection elements like canons (lead-acid) chemistries provide with high power outputs up to tens if not hundreds of Ampere hour rating levels being achievable from single 18650 sized units (which could then still be small). 

For your particular question though regarding future developments one might assume the possibility that other chemistry options would find their way into such applications - including smaller ones perhaps using even newer nano-particle materials, possibly combined with improved safety measures around heat management / energy conversion etc - potentially opening the door for yet smaller cells again with all those advantages in combination!
So my personal estimate right now does seem some years out but I'm happy to entertain updates in the meantime.