Hey ddzx, my B Sc. is in HPS not chemistry but, yeah, the surface of the graphite flakes (layers).
So...my grasp is that 'on the surface' means essentially the impurities are on the surface of the graphite flakes and/or what they call 'cold-welded' between adjacent layers of flakes: ie during the formation process of the flakes, mechanical and (light) layer-surface bonding are 'all' that trap the impurities 'in' the resource. Rather than 'intercalated' impurities, atomically embedded across/within layers (
intercalation: the reversible inclusion or insertion of a molecule or ion into compounds with layered structures)
Maybe the best (if simplistic!) way of differentiating might (?) be thus: if you stack a bunch of dirty plates in the sink after a dinner party, what you have is a series of impurities only lightly surface-bonded and 'mechanically' bonded, between the plate surfaces. You can clean them fairly easily via a process of 'milling', 'flotation' and maybe 'sizing' ie you separate and clean the stack with a good soak n' jostle n' light mechanical wipe n' sluice n' rinse. If your plates are super high-quality impervious grade china and whopping big fat ones at that (and there's only a few tiny scraps left anyway 'coz your better half is a really great cook), and if you're careful with your 'mechanical' washing up technique, deft and methodical and clever with water pressure etc...you probably won't even need warm water, much less detergent.
But if you clamp that dirty stack tightly together with a vice and stick it into the oven first, slow-baking the bastard at a lowish heat but for a very long time (not hot enough to burn off the impurities but long enough to melt 'em in), and if your plates are smallish and thin and a more porous, mass-produced ceramic (or whatevs cheap 'china' is made of), and if you're married to
@1973 (who word has it is a peerlessly expert whore in the bedroom, but in the kitchen, a...well, peerlessly expert whore...no offence '73) so there's stacks of uneaten gunk on each plate to start with...then you end up with what are more like 'intercalated' impurities. That is, to a more or less degree, impurities 'melded in' across and into and in the plates themselves, making a sort of layered stack-blend(ish) of plate and food scrap, with the structural 'line' between the two very much harder to isolate. So that to wash them, you'll have to re-heat it all to high temp again just to prise them apart, maybe soak them in strong acid for an hour, maybe re-heat them individually again, scrub them like buggery with steel wool, rinse, re-heat, fresh detergent, etc....and you'll probably end up losing bits of the plates' surface to the cleaning process along the way, maybe have to throw a few out completely.
OK, so not a perfect geo-chemical process analogy I am totes sure, ddzx...but that's how I understand it in general terms, without feeling the need to get too bogged down into the arcane chemistry. I know enough science to be very confident in the expertise of Frank Houllis and now Shailesh Upreti - who after yesterday's AGM I see as being this coming sector's presumptive 'King and Prince of the Battery Nerds' respectively, a powerhousing Industrial-Brainiac Double Act if there ever was one. But there's also oodles of more exhaustive (and non-Magnis) explanations from entirely unaligned experts around the traps, easily available on-line, including as circulated by rival plays. Maybe worth looking at the market leader's HC technical threads re: impurities, because they are going down a (necessarily) more intense processing route than we are, and there's an alternative case been built up over there by some seemingly qualified/knowledgeable posters as to why, in commercial terms, that isn't necessarily a battery marketplace deal maker/breaker.
Others here can be more authoritative on technical nuances than me too, obv.
@kingofhell is probably the best local go-to on this depth of FA. (Kingy, please step in if I've totally cocked that all up. (I'm a poet, not a lab rat, bra...)
Cheers, ddzx.
(20min delay)