The formation of solid electrolyte interphase (SEI) and lithium ion intercalation are two crucial processes in lithium ion batteries. Given the complexity and challenges involved in investigating real batteries, a combination of model batteries comprising well-defined ultrathin graphite electrodes with surface-sensitive techniques can provide valuable information on these processes. Here, a comparative study of the performance of batteries using regular- and high-concentration electrolytes (RCE and HCE) is presented, aided by a myriad of correlative surface science techniques. It is found that the SEI formed in HCE possesses a thinner organic layer and more inorganic substances. Such an effective SEI protects the graphite electrode and facilitates the processes of Li+ intercalation/deintercalation, thereby improving the reversibility and cycling stability of the battery, superior to those with RCE. Moreover, the battery performance can be readily tuned by switching the electrolytes. The good cycling stability of the battery can still be maintained in RCE after the preformation of a robust SEI on the ultrathin graphite electrode in HCE. This work highlights the importance of electrolyte-modulated SEI formation to battery performance and provides powerful platforms to comprehensively study the SEI formation and ion intercalation processes.