a suffix, appearing in words of Latin origin, denoting action or condition, used in Latin and in English to form nouns from stems of Latin adjectives (communion; union ), verbs (legion; opinion ), and especially past participles (allusion; creation; fusion; notion; torsion ).
Compare -cion, -xion.

< Latin -iōn- (stem of -iō) suffix forming nouns, especially on past participle stems; replacing Middle English -ioun < Anglo-French < Latin -iōn-

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World English Dictionary
suffix forming nouns
-ation Compare -tion indicating an action, process, or state: creation; objection
[from Latin -iōn-, -io]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
These same researchers have gotten viruses to do other creative tasks for them,
  including building a lithium-ion battery.
Tesla's vehicles use standard lithium-ion battery cells.
When lithium-ion batteries were invented, mobile phones became small enough to
  be slipped into a pocket.
It has developed lithium-ion batteries that are unusually cheap and easy to
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