Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes

Abstract

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode−electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn 2+/4+ -based DRX oxyfluorides during initial cycling with a highvoltage charging cutoff (4.8 V vs Li/Li + ). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O 2 evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoridescavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of ex situ spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX−electrolyte interface and points toward potential routes of interfacial stabilization.

Dialing in the Voltage Window: Reconciling Interfacial Degradation and Performance Decay for Cation-Disordered Rocksalt Cathodes

“…Our initial experiments focused on a typical voltage window for DRX materials: 4.8-1.5 V vs Li/Li + . [6][7][8]23 As seen in Fig. 1, sizable quantities of CO 2 , and a small amount of O 2 that onsets at ∼4.6 V, are observed during charge over the course of cycling.…”

“…O 2 evolution, a process typically confined to the first charge, commonly occurs due to irreversible oxidation of DRX surface. [6][7][8] Furthermore, CO 2 evolution is typically observed due to decomposition of carbonate species within the cell at the DRX surface. These carbonates may be either native carbonate species on the DRX surface or the organic carbonate solvents used in the electrolyte.…”

“…[17][18][19][20] The organic carbonate solvents used in the electrolyte in this work are ethylene carbonate (EC) and ethyl methyl carbonate (EMC), both of which are prone to decomposition evolving CO 2 at potentials above ∼4.0 V vs Li/Li + . 6,7,21,22 To examine the nature and extent of the interfacial degradation that occurs during initial cycling of DRX materials, outgassing was measured using differential electrochemical mass spectrometry (DEMS). LMTO electrodes were cycled against Li metal counterelectrodes for four cycles in a set of different voltage windows while outgassing was measured.…”

“…In addition to simply depleting the electrolyte, this degradation initiates a cascade of deleterious processes driven by the reaction of electrolyte degradation products with other cell components. 7 While this degradation is known to lead to performance decay, there has yet to be a comprehensive study investigating the role of interfacial degradation in long-term performance decay for DRX materials. In fact, many of the studies that have looked at the cycling performance of DRX materials have employed electrolyte-flooded half-cells.…”

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes Enabled by a Graphite Conductive Additive

“…All samples also show a sloping CO 2 evolution profile which peaks at the end of the charge. Previous works have shown that the majority of CO 2 formed during the first cycle results from the decomposition of the native Li 2 CO 3 , and the subsequent CO 2 evolution is predominantly from electrolyte degradation. ,− Table shows the integrated CO 2 quantities for each sample. In the first cycle, LMTOF01/C65 and LMTOF01/Gr evolve more CO 2 compared to LMTOF01/KB.…”