Kinetic- versus Diffusion-Driven Three-Dimensional Growth in Magnesium Metal Battery Anodes

Abstract

With twice the volumetric energy density as lithium, magnesium is a promising material for next-generation energy storage devices. Although magnesium rechargeable batteries were once believed to be even safer than lithium, the mechanisms governing multivalent metal deposition under practical operating conditions are still poorly understood. Through a comprehensive study of electrodeposition in coin cells over a wide range of current densities, we report a transition from charge-transfer-limited to diffusion-limited processes at timescales defined by the transport properties of the electrolyte. Our results explain controversial ideas such as 3D growth within the context of classical electrochemical theories and lay groundwork for future approaches to achieve stable electroplated multivalent metal electrodes.

Al2O3 Thin Films on Magnesium: Assessing the Impact of an Artificial Solid Electrolyte Interphase

“…Some work has also demonstrated that cycling Mg metal in this electrolyte can cause dendrite growth (Ding et al, 2018), although the deposits are not long and branch-like and have other formation characteristics that are different from dendrites. However, it is now more well-known that spherical Mg deposits can grow through separators and create soft-shorts in many different Mg electrolytes (Yoo et al, 2017;Merrill and Schaefer, 2019;Eaves-Rathert et al, 2020;Song et al, 2020). Results in the current work support the observation that soft-shorting is the origin of the overpotential decrease in Mg-Mg symmetric cells.…”

“…This high overpotential value is larger than some reported in the literature for Mg with this electrolyte (Tutusaus et al, 2017), and is consistent with the high overpotential region at the start of galvanostatic cycling of Mg foil. These high overpotentials are likely the true overpotential needed to deposit and strip Mg at the interface, while the small overpotential observed for Mg foil symmetric cells is due to soft-shorting of the cell recently observed by a few groups (Ding et al, 2018;Eaves-Rathert et al, 2020). Lower overpotentials were observed for a different trial with evaporated Mg thin films (Supplementary Figure S4), but in that case the overpotential does ultimately increase and reach the upper limit set for the measurement (4 V), the opposite of what would be expected for soft-shorting.…”

“…While the degradation layer is not completely passivating, it causes large voltage hysteresis issues in full-cell systems (Meng et al, 2019). However, more recent studies have suggested that this overpotential likely does not come from passivation, due to the observation of high overpotential for Mg plating after nucleation of the Mg deposits had already been observed (Eaves-Rathert et al, 2020). Some work has also demonstrated that cycling Mg metal in this electrolyte can cause dendrite growth (Ding et al, 2018), although the deposits are not long and branch-like and have other formation characteristics that are different from dendrites.…”

Recent Progress and Challenges in the Optimization of Electrode Materials for Rechargeable Magnesium Batteries

“…Moreover, the dendrite-free property (in specific conditions) and high coulombic efficiency in proper electrolytes make it more competitive. [27][28][29][30][31] However, the divalent Mg 2+ ions exhibit sluggish kinetics which originates from the strong polarizing nature and results in low or even no capacity in most cathode materials established for Li-storage. [32][33][34][35] Choosing Mo 6 S 8 as the cathode material is a breakthrough on the way to RMBs and it still surpasses many of cathodes even today, especially in cycling stability.…”

Practical energy densities, cost, and technical challenges for magnesium‐sulfur batteries

“…The low utilization of S due to low electrical conductivity of S and resultant MgS 35,36 The formation of Mg dendrites at high current densities 24,37‐41 The sluggish Mg 2+ transport; The Mg 2+ migration barrier calculated for MgS is ∼900 meV, suggesting that MgS can be electrochemically inactive and can significantly limit Mg transport in the composite electrode 42,43 …”