Leveraging Cation Identity to Engineer Solid Electrolyte Interphases for Rechargeable Lithium Metal Anodes

Abstract

<p>Lithium metal anodes enable substantially higher energy density than current technologies for Li batteries. However, rechargeable Li metal anodes suffer from low Coulombic efficiency (loss of electrochemically active Li), leading to poor cycle life and safety. Engineering the electrolyte formulation to form a stable, well-functioning solid electrolyte interphase (SEI) is a promising approach to improving these performance figures of merit. While design rules have been established for selecting electrolyte solvents and salt anions to establish a more robust SEI, the impact of altering cation identity is not well understood. In this work, we demonstrate that alkali metal additives (here, K<sup>+</sup>) alter SEI composition and thickness. Through post-mortem elemental analyses, we show that K<sup>+</sup> ions do not directly participate in metal electrodeposition, but rather modify the chemical and electrochemical reactivity of the electrode-electrolyte interface. Through a combination of quantitative nuclear magnetic resonance (NMR) spectroscopic characterization and density functional theory (DFT) simulations, we show that decomposition of electrolyte solvent molecules, ethylene carbonate (EC) and dimethyl carbonate (DMC), at the lithium metal surface is suppressed in the presence of a K<sup>+</sup> additive. We attribute this to K<sup>+</sup> being a softer cation compared to Li<sup>+</sup>, leading to preferred pair formation between K<sup>+</sup> and the soft base carbonates, and thus increased salt-solvent coordination. Electrolyte cation engineering is an underexplored strategy to control the SEI, and we believe that the mechanistic understanding and insight developed in this work will spur further investigation of this promising approach.</p>

Reversible Switching of Battery Internal Resistance Using Iongate Separators

“…It is well known that nonuniform SEIs are extremely prone to future dendrite growth due to irregularities in surface resistance and Li + ion flux. [ 47 ] EDS elemental mapping for these anode areas (Figure S12, Supporting Information) show the iongate battery has a uniform distribution of C, O, F, and P—common components of SEI associated with LP30 electrolyte [ 48 ] —with almost no transition metal signal, while the normal battery has high concentrations of C, O, F, P, and additional signal from Ni, Mn, and Co in the thicker regions of the SEI. The elemental survey of the iongate battery anode detects at most 0.03 at% of any transition metal (Figure 4c) whereas the normal battery has ≈2 at% of the SEI composed of transition metals in a nearly exact ratio of 5:3:2 (Ni:Mn:Co) that agrees with the cathode chemistry (Figure 4f).…”

Potassium fluoride and carbonate lead to immediate cell failure in potassium-ion batteries

“…For example, KPF6 and KFSI are not reduced to KF, while their Li analogues readily decompose to form LiF. 45,55 In contrast, only KFSI decomposes to form small amounts of a partially potassiated KxF phase that does not decrease capacity retention to the same extent as KF. Likewise, KPF6 does not undergo a hydrolysis reaction with trace water in the electrolyte solvent like LiPF6, 56,57 limiting the amount of KF, HF, and fluorophosphates that are formed in KIBs.…”

“…The lack of organic decomposition is consistent with the lower reduction potential of carbonate solvents measured in K electrolytes, 44 as well as DFT calculations that show K + coordination with solvent molecules hinders charge transfer between the electrode surface and solvent, mitigating solvent reduction. 45 Solid-state NMR characterization of the SEI on hard carbon anodes after electrochemical cycling in KIBs. To probe changes in SEI composition due to FEC additives on the electrode surface, ex situ 1 H, 13 C, and 19 F SSNMR spectra were collected for HC electrodes after eleven galvanostatic cycles.…”

“…47 The 19 F resonance at 53 ppm is assigned to residual FSIanions and is observed in all KFSI-containing samples as electrodes are not washed prior to SSNMR analyses. Additionally, a small peak at -223 ppm is seen in the KFSI sample, and is assigned to the Torlon rotor cap, 45 which contains small amounts of F.…”