Dear Colleagues,

Lithium (Li) ion batteries have been used for a few decades as the main small- and medium-scale energy storage devices (electric vehicles), and as backup and smart-grid energy storage. However, sodium (Na), potassium (K) and multivalent (Mg, Ca, Al) ion batteries have emerged as candidates for medium- and large-scale stationary energy storage, mainly due to the abundance of Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺ ions in Nature, but also to the potential to surpass the energy density of lithium. Although many studies focus on lithium-ion batteries, only a few have been conducted in metal ion batteries beyond lithium.

One very important factor for the performance of metal-ion-based batteries and fast charge/discharge rates is the choice of organic electrolyte, since a good electrolyte can provide good ionic conductivity, high cycle life, and energy density, greatly reducing side reactions and electrochemical stability. Desirable properties for a good electrolyte include: (i) high polarity (dielectric constant ε > 15 is necessary for salt ion solvation and ion pairing limit), (ii) low viscosity to improve the cation mobility, (iii) chemical stability (to remain inert on the charged surfaces of the anode during cell operation), (iv) ability to remain liquid over a broad range of temperatures, and (v) safety (nontoxic and economical). Moreover, ionic liquids or ether-based electrolytes (polymer electrolytes) can be considered alternative electrolytes for high-performance rechargeable batteries. In addition, the research on anode materials and the passive layer between the electrolyte and anode (solid electrolyte interface, SEI) is also very limited for batteries beyond lithium. For instance, carbon-based materials (e.g., hard carbon, graphite, hollow carbon nanowires, carbon microspheres) represent a large category of anode materials, and are still considered the best, due to their natural abundance, low cost, and relatively good storage capacity. However, several other anode materials, such as nickel-titanium oxide, germanene nanosheets, phosphorus-based alloys etc., have recently been applied in novel rechargeable batteries.

There is therefore a need to develop research in this area in order to enable breakthrough development in beyond lithium-ion battery technology. This Special Issue of Batteries invites contributions addressing computational studies (DFT, molecular dynamics (ab-initio or classical), machine learning and theoretical models) in lithium-ion batteries and beyond, focusing on organic electrolyte phase, ionic liquids, ether-based electrolytes, anode materials, solid electrode interface, ion solvation, ion transport, transference number, and ionic conductivity.

Dr. Argyrios Karatrantos
Guest Editor

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• beyond lithium batteries
• electrolytes
• ionic liquids
• ether-based electrolytes
• polymer electrolytes
• anode materials
• ion transport
• solvation
• transference number
• conductivity
• solid electrolyte interface
• molecular dynamics
• DFT
• machine learning
• ab-initio

Title: Lithium metal under static and dynamic mechanical loading
Authors: Ed Darnbrough; David E J Armstrong
Affiliation: University of Oxford
Abstract: Macro-scale mechanical testing and Finite Element Analysis of lithium metal in compression is shown to suggest methods and parameters to produce thin lithium anodes. Consideration of engineering and geometrically corrected stress experiments shows that the increasing contact area dominates the stress increase observed during compression not strain hardening of lithium. Under static loading the lithium metal stress relaxes which means there is a speed of deformation (engineering strainrate limit of 6.4x10^{-5} s^{-1}) where there is no increase in stress during compression. Constant displacement tests show that the stress relaxation is dependent on the initial applied stress and the amount of athermal plastic work within in the material. Finite element analysis shows that barrelling during compression and the requirement for high applied stresses for compressing lithium with a small height to width ratio are friction and geometric effects, respectively. The outcomes of this work are then discussed in relation to the diminishing returns of stack pressure, the difficult of closing voids and potential methods for designing and producing sub-micron lithium anodes.