Reductive Decomposition Kinetics and Thermodynamics That Govern the Design of Fluorinated Alkoxyaluminate/Borate Salts for Mg-Ion and Ca-Ion Batteries

Abstract

The rational design of electrolytes has been a longstanding challenge in chemistry and materials science. In this work, we demonstrate a computational rationale for improving the performance of weakly coordinating electrolytes in currently challenging multivalent-ion battery applications, based on enhanced thermodynamic and kinetic stability against reductive decomposition. A series of fluorinated alkoxyborate and alkoxyaluminate salts are systematically examined based on their reduction and oxidation potentials and, motivated by NMR spectroscopy, detailed reductive decomposition pathways involving the breaking of Al/B−O, C−O, or C−F bonds are obtained. Based on the decomposition kinetics, the hexafluoro-tert-isopropoxy (hfip) ligand for borates and the trifluoro-tert-butoxy (tftb) ligand for aluminates are identified as promising ligands for constructing the salt anions. This borate prediction corroborates previous experimental work on Mg[B(hfip) 4 ] 2 and Ca[B(hfip) 4 ] 2 , in which excellent electrochemical properties were reported. We find that steric factors govern the B−O bond-breaking decomposition kinetics while electronic factors are more important for aluminate salts. There is more charge transfer character in the aluminate transition states compared with borates for Al/B−O bond-breaking decomposition and thus electron-withdrawing ligands tend to stabilize the aluminate transition states. Such molecular-level understandings allow for better design principles for developing new electrolytes with improved stability and performance.

Comparative Studies on [B(HFIP)4]-Based Electrolytes with Mono- and Divalent Cations

“…As described before, such a solvation effect becomes prominent in the case of multivalent ion systems . The cathodic limits of [B­(HFIP) 4 ] − would indeed shift to the positive side by approximately +3 V upon ion-pair formation with Mg 2+ and Ca 2+ , and such contacted [B­(HFIP) 4 ] − anions are no longer stable even against moderately reductive magnesium. For the monovalent [B­(HFIP) 4 ] − -based electrolytes, inferior efficiency was confirmed for the Na-based electrolytes compared to the Li-based electrolytes.…”

“…Although the stabilization energies for Zn 2+ -glyme complex formation are unclear, the comparable ionic radii for the hexacoordinate states and the same valency of Mg 2+ and Zn 2+ support the formation of the SSIP-type solvate for Zn­[B­(HFIP) 4 ] 2 with G1 molecules. Moreover, recent molecular dynamic simulation studies have suggested the partial decomposition of [B­(HFIP) 4 ] − upon ion-pair formation with Mg 2+ . − Such structural instability during ion-pair formation further supports the formation of SSIP-type solvate structures for divalent salts. However, in the case of monovalent salts, the contribution of glyme coordination to complex structure formation is not very large.…”

“…The Coulombic efficiencies for metal deposition-dissolution reflect the reductive nature of each metal and the supposed solvation (or ion association) states. Basically, the reductive stability of the [B­(HFIP) 4 ] − is calculated to be approximately −4 V vs. SHE, with the value slightly depending on the calculation model used. ,, This value implies that [B­(HFIP) 4 ] − would potentially be stable against reductive metals including lithium. This favorable situation drastically changes upon ion-pair formation because the electric field of small metal ions strongly polarizes the electronic states of the interacting compounds.…”

“…The large crystal size of Mg deposits also suggests suppressed electrolyte decomposition and a favorable nucleus-growth mode during deposition owing to the mild polarization behavior. 62 43 The recent comprehensive work on Li metal battery electrolytes also emphasizes that diminishing electrolyte decomposition is essential to achieve high Coulombic efficiency for metal deposition/dissolution cycling. 4 In addition to the intrinsic stability of the electrolyte components against metal electrodes, the reaction products at the [electrode|electrolyte] interface, the so-called solid electrolyte interface (SEI), play a dominant role in electrochemical reactions.…”

“…The concept of WCAs has been proposed to improve the ionic conductivity and thermal stability of the electrolytes for lithium-based batteries since at least 1995. − Among the substantially large variety of WCAs, the quaternary borates and aluminates bearing fluorinated-alkoxide functional groups, such as tetrakis­(hexafluoro-iso-propoxyl)­borate ([B­(HFIP) 4 ] − ), tetrakis­(hexafluoro-iso-propoxyl)­aluminate ([Al­(HFIP) 4 ] − ), and tetrakis­(perfluoro-tert-butoxy)­aluminate ([Al­(PFTB) 4 ] − ), are promising owing to their excellent dissociativity and sufficient cathodic/anodic stability. The electrolytes based on these conducting salts indeed support reversible metal deposition/dissolution cycling with remarkable efficiencies and substantially contribute to paving the way for multivalent battery materializations. − Inspired by these success stories of WCA-based electrolytes in multivalent battery fields, the concept has recently been reimported in lithium and sodium battery chemistry, especially those utilizing metallic negative electrodes combined with high-performance positive electrodes. − Combined theoretical and experimental studies have also been conducted to understand the origin of such characteristics of WCAs, especially in magnesium battery electrolytes. ,− …”