Mg rechargeable batteries (MgRBs) stand out as a promising candidate beyond lithium ion battery technologies due to high volumetric energy density, resource abundance, and the dendrite-free deposition behavior of Mg, which ensures safe operation. Many of the advantages of MgRBs originate from the divalent nature and small ionic size of Mg ions; however, these properties also render the cation too polarizing to diffuse easily in most ion- intercalation materials. To overcome this challenge, we recently adopted an interlayer expansion approach via insertion of external pillars to transform two representative two-dimensional layered materials (MoS₂ and V₂O₅) into efficient Mg storage host materials.

To boost the Mg storage performance of MoS₂, we intercalated appropriate amount of poly(ethylene oxide (PEO) to expand MoS₂ structure through an exfoliation and restacking process. [1,2] The Mg solid state diffusivity and capacity increased accordingly with increasing amount of PEO. PEO_2L-MoS₂ (two layers of PEO in between MoS₂ layers) showed the best performance with Mg²⁺ diffusivity increase by two orders of magnitude and capacity increase of about 300% compared with bulk MoS₂. The flexible PEO serves as a structure pillar, which can increase the interlayer spacing but not block the diffusion of Mg²⁺ ions. In the case of V₂O₅, we turned to insert small water molecules in the layered structure to take advantage of the shielding effect of H₂O.[3] We prepared graphene decorated V₂O₅ aerogel nanowires, and demonstrated it to be an effective cathode material, with specific capacity of 330 mAh g^-1 at low rate, stable cyclability and good rate performance. The performance enhancement compared with bulk V₂O₅ is due to the increased space and charge shielding effect of crystal water in the aerogel to enhance the Mg²⁺ insertion and the porous structure of aerogel allowing easy access of electrolyte into the active material. Moreover, various interlayer spacing and crystal water amount were also studied and we found that the as-prepared sample with the highest amount of water and interlayer spacing showed the best performance.

The merit of the interlayer expansion lies at the enlarged space for enhanced Mg²⁺ ion mobility and weakened interaction between Mg²⁺ ions and the electrode frameworks. This approach could be further extended to a wide range of host materials for the storage of Mg²⁺ ion and other multivalent ions. 

1. Yanliang Liang, Hyun Deog Yoo, Yifei Li, Jing Shuai, Hector A. Calderon, Francisco Carlos Robles Hernandez, Lars C. Grabow, and Yan Yao, Nano Lett.2015, 15, 2194-2202.

2. Yifei Li, Yanliang Liang, Francisco C. Robles Hernandez, Hyun Deog Yoo, Qinyou An, Yan Yao, Nano Energy 2015, 15, 453-461.

3. Qinyou An, Yifei Li, Hyun Deog Yoo, Shuo Chen, Qiang Ru, Liqiang Mai, and Yan Yao, Nano Energy 2015, 18, 265-272.