Magnesium batteries are expected to be promising post lithium-ion batteries owing to several intriguing advantages, such as a relatively safe metal anode, low production cost, and high energy density. There are, however, still essential issues that hinder the development of practical magnesium batteries. In this chapter, we show that employing Mg ions with Li ions to construct Mg-Li dual-ion batteries is an effective approach to realize high performance rechargeable batteries with safe metal anodes. Beginning with a brief introduction of the concept, the fundamental electrochemistry is described for Daniell-type and rocking-chair-type Mg-Li dual-ion batteries. Furthermore, the co-intercalation behavior of Mg and Li ions is investigated theoretically and experimentally, which demonstrates that Mg intercalation can be facilitated by a concerted interaction between the preceding Li ions and following Mg ions. This provides a new perspective on solid-phase diffusion that is typically a rate-controlling process in battery systems.