The paper considers application of nanosized magnetite as an additive to the carbon matrix of a lithium-ion battery anode. We show that the use of 10–15 nm nanosized iron oxide Fe3O4 as an additive in the composition of the anode material Сarbon Black makes it possible to increase the capacity by almost 3 times: from approximately 250 mA·h·g–1 to 780 mA·h·g–1. Based on the results obtained, we propose a diagram of the sequence of intercalation–deintercalation processes of lithium ions into a carbon framework containing iron oxide molecules. The results obtained, confirmed by calculations and experimental studies, are fundamental for understanding the nature of the processes that affect the increase in the performance of batteries, as well as their applied nature for obtaining promising anode materials for lithium-ion batteries with increased electrical capacity.