The Proof‐of‐Concept of Anode‐Free Rechargeable Mg Batteries

Abstract The desperate pursuit of high gravimetric specific energy leads to the ignorance of volumetric energy density that is one of the basic requirements for batteries. Due to the high volumetric capacity, less‐prone formation of dendrite, and low reduction potential of Mg metal, rechargeable Mg batteries are considered with innately high volumetric energy density. Nevertheless, the substantial elevation in energy density is compromised by extremely excessive Mg metal anode. Herein, the proof‐of‐concept of anode‐free Mg2Mo6S8‐MgS/Cu batteries is proposed, in which MgS as the premagnesiation additive constantly decomposes to replenish Mg loss by electrolyte corrosion over cycling, while both Mg2Mo6S8 and MgS acts as the active material to reversibly provide high capacities. Besides, Mg2Mo6S8 shows superior catalytic activity on the decomposition of MgS and provides the strong affinity to polysulfides to restrain their dissolution. Consequently, the anode‐free Mg2Mo6S8‐MgS/Cu batteries deliver a high reversible capacity of 190 mAh g−1 with the capacity retention of 92% after 100 cycles, corresponding to the highly competitive energy density of 420 Wh L−1. The proposed anode‐free Mg battery here spotlights the great promise of Mg batteries in achieving high volumetric energy densities, which will significantly expedite the advances of Mg batteries in practice.


Material synthesis
Synthesis of Chevrel phase Mo6S8. Chevrel phase Mo6S8 is synthesized according to our previously reported method. 1 Specifically, MoS2, Cu, and Mo powders were ballmixing mixed and pressed into pellets with some iodine. After sealed in a swagelok stainless steel vessel, the mixture was gradually heated to 800 ºC and kept for 24 h under Ar to get Cu2Mo6S8. Mo6S8 powder was obtain by leaching out Cu from the asprepared Cu2Mo6S8 precursor in a 6 M HCl solution with oxygen bubbling.
Mg2Mo6S8 was obtained by immersing Mo6S8 into the 0.5 M di-nbutylmagnesium in heptane solution for 7 days and rinsing for 3 times using heptane, which is learned from the reported chemical lithiation process using n-butyllithium in heptane. [2][3][4] The duration of 7 days is applied to guarantee the complete chemical premagnesiation of Mo6S8 nanosheets in consideration of the slow magnesiation kinetics.
Mg2Mo6S8 and MgS were mixed at a weight ratio of 4:1 by ball-milling at 300 rpm for 2 h under Ar to form Mg2Mo6S8-MgS composite.

Electrochemical measurements
Cell assembly was carried out in an Ar-filled glovebox with O2 and H2O levels below 0.1 ppm. The Mg2Mo6S8-MgS composite cathode was prepared by pressing the asprepared composites, ketjen black, and PTFE at a weight ratio of 8:1:1 onto a stainlesssteel mesh. Anode-free Mg batteries were assembled using pouch cells with MACC as the electrolyte, Cu foil as the current collector of anode, and Whatman glass fibers as separators. The electrochemical test was conducted on a LAND-CT2001A battery test station (LAND Electronic Co.) with a voltage cutoff of 0.5−2.5 V at room temperature (RT). The specific capacities are calculated based on the mass of Mg2Mo6S8 only unless otherwise stated. Nyquist plots were recorded using Autolab PGSTAT302N (Metrohm, Switzerland) at a frequency range of 0.01−100 kHz. The capacity throughout the paper is calculated based on the mass of Mg2Mo6S8-MgS samples.

Material characterizations
The XRD patterns were measured using Cu Kα radiation on an X'Pert Pro MPD X-ray diffractometer from 10° to 80° (2θ). The morphologies of samples were investigated by scanning electron microscopy (Hitachi S-4800), transmission electron microscopy (JEM 2100Plus, JEOL Limited Corporation, Japan), and advanced spherical aberrationcorrected scanning transmission electron microscopy combined with low-angle annular dark field as well as annular bright field (JEM-ARM200CF, JEOL, Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS) spectra were recorded with a spectrometer having Mg/Al Kα radiation (ESCALAB 250 Xi, Thermo Fisher). All binding energies were corrected using the signal of carbon at 284.8 eV as an internal standard. For ex situ XPS measurements, pouch cells were disassembled in an argon-filled glovebox and the electrodes were washed in tetrahydrofuran (anhydrous, Alfa Aesar, 99.9%) for three times to remove the electrolyte, and then the dry samples were obtained and transferred to the machine with an argon-filled sealing tube as a transfer box. In this process, all samples were exposed to air within 3−4 s.

Theoretical calculations
The Vienna Ab Initio Package (VASP) was employed to perform all the density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew, Burke, and Enzerhof (PBE) formulation. [5][6][7] The projected augmented wave (PAW) potentials were applied to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. 8,9 Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 −4 eV. A geometry optimization was considered convergent when the force change was smaller than 0.04 eV/Å. Grimme's DFT-D3 methodology was used to describe the dispersion interactions. 10 The equilibrium lattice constants of unit cell were optimized when using a 2×2×1 Monkhorst-Pack k-point grid for Brillouin zone sampling. The Climbing Image-Nudged Elastic Band methods were employed to calculate the Mg ions migration barriers in the structures. Finally, the adsorption energies (Eads) were calculated as Eads = Ead/sub -Ead -Esub, where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively.