Electrochemically Stable Coating Materials for Li, Na, and Mg Metal Anodes in Durable High Energy Batteries

Abstract

Anodes made of Li, Na, or Mg metal present a rare opportunity to double the energy density of rechargeable batteries. However, these metals are highly reactive with many electrolytes and yield electronically conductive phases that allow continued electrochemical reduction of the electrolyte. This reactivity degrades cell performance over time and poses a safety risk. Surface coatings on metal anodes can limit reactivity with electrolytes and improve durability. In this paper, we screen the Open Quantum Materials Database (OQMD) to identify coatings that exhibit chemical equilibrium with the anode metals and are electronic insulators. We rank the coatings according to their electronic bandgap. We identify 92 coatings for Li anodes, 118 for Na anodes, and 97 for Mg anodes. Only two compounds that are commonly studied as Li solid electrolytes pass our screens: Li 3 N and Li 3 Furthermore, layered-layered materials with high Mn contents suffer from limit cycle life due to voltage and capacity fade.4,5 Substituting a new silicon-carbon composite anode for a conventional graphite anode offers only 20% improvement in cell energy density. 6 Metal anodes, which are comprised entirely or almost entirely of the mobile element in a battery, present a rare opportunity for major improvement in energy density. For example, in a lithium battery with a LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode, energy density can be doubled by substituting a lithium metal anode in place of a conventional graphite anode. 7 This doubling in energy density is attainable because metal anodes can eliminate host materials, polymeric binders, electrolytefilled pores, and even copper current collectors from the anode.8 Metal anodes comprised of pure elements also offer the lowest possible anode redox potential and therefore the highest possible cell voltage. In batteries where Na or Mg is the mobile element, anode host materials that operate by intercalation or conversion reactions exhibit particularly poor capacity, kinetics, and reversibility, and so Na or Mg metal anodes that eliminate these host materials are particularly attractive. 9,10A major challenge for the implementation of metal anodes is reactivity between the metals and electrolytes. Metal anodes must be electropositive to provide a sufficient cell voltage, but this electropositivity causes the metals to drive electrochemical reduction of electrolytes. Both liquid and solid electrolytes are often reactive at the anode surface.11,12 For graphite anodes in conventional lithium-ion batteries, reactivity can be mitigated by a passivation layer that forms in situ from the reaction products. 11,13 This passivation layer must be mechanically durable, electronically insulating to block electron transfer from the anode to the electrolyte, and chemically stable or metastable. For metal anodes, there is sparse evidence for passivation by in situ reactivity. Reactivity at the surface of metal anodes causes impedence growth that destroys cell performance, according to Luntz * Electrochemical Society Member....

Room‐Temperature Sodium–Sulfur Batteries and Beyond: Realizing Practical High Energy Systems through Anode, Cathode, and Electrolyte Engineering

“…In other words, precursors can either be physically or chemically attached to the Na metal surface, forming the artificial interphase. [ 113,114 ] From theoretical calculations of various 2D materials, [ 113 ] it was identified that the crystalline quality, defect patterns, increase in bond length, and proximity effect could favorably increase the ionic conductivity and surface diffusion properties. However, all materials were found to negatively impact the hardness or stiffness required to suppress dendrite growth.…”

“…developed an open quantum materials database (OQMD) to identify electronically insulating materials (i.e., transition metal compound films) that exhibit a stable equilibrium with the Na anode. [ 114 ] They identified 118 of such coatings (including binary, ternary, and quaternary compounds of oxides, nitrides, sulfides but no fluorides) that are both chemically stable with respect to Na metal and are electronically insulating. In spite of the theoretical ineffectiveness of graphene in stabilizing Na metal anodes, Wang et al.…”

Ionic Transport in Potential Coating Materials for Mg Batteries

“…Using first-principles calculations, we systematically assess the barriers and band gaps for Mg migration in a total of 27 candidate coating materials. In addition to the compositions listed above, we also considered materials that are stable against Mg metal 24,26 (potential anode coatings) and analogous chemistries that have been employed in Li-systems (e.g., Li-Nb oxides). [18][19][20][21][22] Although prior studies have demonstrated [27][28][29][30] that the lack of Mg (or multivalent) mobility in several structures relates to a combination of stronger electrostatic interactions of a 2+ charge with its surrounding anion environment (versus 1+ charge of monovalent ions) and strong coordination preferences, 17,25,[31][32][33][34] Mg mobility has not been rigorously quantified yet for potential coating chemistries.…”

Computational Design of Double-Layer Cathode Coatings in All-Solid-State Batteries

“…28 Automated computational screening of vast crystalline material databases enables efficient identification of additional coating candidates with high interfacial stability, thus greatly accelerating the discovery of candidate coating materials. 12,[29][30][31][32] In addition to the thermodynamic considerations, high Li-ion conductivity is also a key factor in achieving high rate capability. The identification of a single coating material that has all desirable properties, including high stability and high lithium-ion conductivity, has not yet been achieved.…”