I understand what you are saying as Hydroxide is also (not one or the other, both) used and some believe (mentioned in the below posting that it is also set to be a leader in it's own right, not discounting Carbonate.)
BUT.. i'll leave you with this section of a nice read only posted mid this year:
LITHIUM WILL BE A KEY COMPONENT OF BATTERIES FOR MANY YEARS
This increase in battery production will see a significant rise in demand for metals like cobalt, nickel and lithium. Lithium carbonate is currently the predominant form of the metal used by battery makers in China, where it is used in the LFP (lithium-iron-phosphate) batteries that are commonly used by Chinese vehicle manufacturers including BYD, FAW-Volkswagen, Geely and Great Wall Motors.
Lithium does not occur naturally in nature. Instead it is found in a variety of mineral salts which needs to be chemically processed to form the lithium compounds and chemicals required by industry. A number of lithium compounds are used by industry but it is lithium carbonate that is the most commonly used form of lithium, accounting for more than half of total demand. The lithium industry often expresses lithium production and trade in lithium carbonate equivalent (LCE) units.
China is already the world’s largest market for EVs, accounting for nearly half of global sales last year. This trend is expected to continue, supported by government policies driving the development and adoption of electric vehicles. This will require a significant increase in both lithium carbonate and increasingly lithium hydroxide, another lithium compound, which is the preferred basis for the next generation of lithium battery technologies.
Link to the full report: https://blogs.platts.com/2018/06/22/lithium-electric-vehicle-battery-technology/
