High energy density, high capacity, high work voltage, low cost, highly safe rechargeable battery is the future development directions of storage battery technology. High-energy density Mg ion battery (MIB) is rechargeable battery with Mg or Mg alloy as anode. Mg ion battery is the most promising and important new type of green storage battery applicable in electric vehicle. The slow diffusion of Mg ion in the cathode material is one reason for the slow development of Mg ion battery. Thus, in this paper, we review five types of crystal structured Mg-transition metal complex oxides with one-dimensional tunnel structure, two-dimensional layer structure, three-dimensional spinel structure, three-dimensional NASICON structure, three-dimensional olivine structure, the preparation method and the electrahemical properties. Further, we also illustrate the diffusion performance of Mg ion in the solid cathode and the measures to improve the slowness diffusion. Finally, we point out the possible research directions of Mg-transition metal complex as cathode materials for Mg ion batteries in the future. Searching for high-energy density, high-capacity, high-voltage cathode materials and the compatible electrolyte is the key to realizing the third breakthrough of the Mg ion battery. We hope that this paper is favorable for understanding the cathode materials of Mg ion battery, promoting the development of Mg ion battery.

Contents
1 Introduction
2 One-dimensional tunnel structure
3 Two-dimensional layer structure
3.1 Mg-Ni-O
3.2 Mg-Ti-O
3.3 Mg-V-O
4 Three-dimensional spinel structure
4.1 Mg-Mn-O
4.2 Mg-Co-O
4.3 Mg-Fe-O
5 Three-dimensional post-spinel structure
6 Three-dimensional NASICON structure
7 Three-dimensional olivine structure
7.1 Mg-Fe-P-O
7.2 Mg-V-P-O-F
7.3 Mg-Mn-Si-O
7.4 Mg-Fe-Si-O
7.5 Mg-Co-Si-O
7.6 Mg-Ni-Si-O
8 Mg diffusion in the solid cathode
9 Outlook