It looks to me as though the root cause is Citroen's use of a cable with many small strands plus a crimp-on battery clamp. This arrangement relies on good connections from the inner lead surface of the crimped cylinder to the surfaces of the outer layer of strands to the next layer of strands to the next etc. Over time with moisture plus salt spray and and sulpher trioxide fumes and free oxygen (from battery charging), the copper strands become oxidised (or worse) giving rise to higher resistance with ever-accelerating oxidation.
I understand why Citroen do this: Finer strands in a cable reduces it's overall diameter, makes it more flexible and theoretically more reliable because a broken strand carries a much small percentage of the total current. Crimp connections are cheaper, fast, easy and require little skill to make reliably.
A well-made solder terminal requires more skill and time, but gives a guaranteed minimal-resistance connection with the interior of the connector filled with solder and the end of the PVC insulation melted into a seal preventing the ingress of corrosive chemicals into the core.
However, I very much doubt you will find battery cables with solder terminals or even someone to make them up.
In fact I don't think solder terminals are available anymore - ROHS elf 'n safety nonsense.
I think I'd better have a look at my battery cables when I've fitted the winter tyres tomorrow.
The only answer is make up your own with slightly over-size cable and SOLDER on crimp terminals.
