Stabilizing Lithia-Based Cathodes through the In Situ Electrochemical Formation of an Inorganic MgF₂ Interfacial Coating

Abstract

Lithia (Li2O)-based cathodes are considered promising alternatives to commercial cathodes because of their high capacity owing to the anionic redox reaction. The capacity of most cathodes is based on the cationic redox reaction induced by heavy transition metals in the structure, whereas that of lithia-based cathodes is based on the anionic redox reaction related to oxygen, which is lighter in weight. However, charged lithia-based cathodes containing Li2O2 and superoxo species are highly reactive to electrolytes, which deteriorates the cathode’s electrochemical performance during cycling. Herein, a MgF2 coating is developed through an in situ electrochemical reaction to protect the vulnerable lithia-based cathode. Surface coating can effectively suppress undesirable parasitic reactions at the cathode/electrolyte interface. However, coating the surface of lithia-based cathodes is difficult because it generally requires a high-temperature treatment, causing unwanted side reactions between reactive lithia and the catalyst (or coating material). In contrast, coating via in situ electrochemical reactions can form a stable inorganic layer from dissolved salts in the electrolyte, which is suitable for lithia-based cathodes because no heating is required. Our MgF2 coating efficiently suppressed parasitic reactions between the cathode and electrolyte and increased the available capacity of the lithia-based cathode. Vinylene carbonate (VC) dissolved in the electrolyte also formed an interfacial layer on the cathode that mitigated parasitic reactions and enhanced the electrochemical performance of lithia-based cathode, which was consistent with previous results. However, the VC-based coating was thick and hindered the exchange of electrons and Li ions during cycling, whereas the MgF2 coating derived from a Mg salt was significantly thinner, thus facilitating electron and ion exchange. Therefore, the cell using a Mg salt showed electrochemical performance superior to those using a basic (conventional) electrolyte with and without a VC additive, confirming that the inorganic MgF2 coating effectively improves the electrochemical performance of lithia-based cathodes.

Interfacial Stabilization of Li2O-Based Cathodes by Malonic-Acid-Functionalized Fullerenes as a Superoxo-Radical Scavenger for Suppressing Parasitic Reactions

“…However, this layer is thinner than previously reported interfacial layers after cycling (50− 100 nm). 33,35 Previous studies generally used a highly reactive binder (PVDF) and salt (LiPF 6 ), whereas this experiment employed a PAN binder and LiTFSI salt, which are relatively stable and thus hindered the parasitic reactions and decreased the interfacial layer. However, the formation and growth of the interfacial layer clearly contributed to deteriorating the electrochemical performance during cycling.…”

“…An excessively deep charge depth results in the formation of highly reactive superoxo species (O 2 1– in LiO 2 ) and even the evolution of gaseous oxygen, which leads to a rapid capacity loss during cycling. , Moreover, the Li 2 O-derived superoxides on the cathode surface can easily be transformed into superoxo species through electron exchange with electrolytes or salts, which in turn activates parasitic (side) reactions that can form an unfavorable interfacial layer on the cathode surface. This layer disturbs the intercalation/deintercalation of lithium ions and limits the movement of electrons, both of which deteriorate the capacity and cycling performance of Li 2 O-based cathodes. − …”

“…In previous studies, − several approaches to decrease the parasitic reactions have been attempted. For example, the commonly used poly­(vinylidene fluoride) (PVDF) binder in the electrode and LiPF 6 salt in the electrolyte were replaced by polyacrylonitrile (PAN) binder and lithium bis­(trifluoro-methanesulfonyl)­imide (LiTFSI) salt, respectively, which efficiently alleviated the parasitic reactions between the Li 2 O-based cathode and electrolyte owing to the high stability of PAN and LiTFSI .…”

Conductive polymer polyaniline covering promotes the electrochemical properties of a nickel-rich quaternary cathode LiNi0.88Co0.06Mn0.03Al0.03O2

“…[11][12][13] However, the traditional covering materials, such as oxides, fluorides and phosphate compounds etc., are composed of plenty of inorganic nanoparticles, and by-products originating from the electrolyte could permeate and pass through the covering film, resulting in destruction of cathode structure after working for a long time. [14][15][16][17] In comparison with the inorganic covering materials, conductive polymers have demonstrated obvious superiority in terms of rapid electronic conductivity and prominent environmental stability, enabling them to effectively and enduringly maintain cathode structure stability and increase the rate performance of Ni-rich cathodes. 18,19 Among the many conductive polymers, polyaniline (PANI) is remarkable due to its favorable chemical and environmental stability, easy preparation and low cost.…”

Additive-Derived Surface Modification of Cathodes in All-Solid-State Batteries: The Effect of Lithium Difluorophosphate- and Lithium Difluoro(oxalato)borate-Derived Coating Layers

“…Coating the cathode with a material that exhibits high sulfide-electrolyte resistance minimizes parasitic (side) reactions between cathodes and sulfide-based solid electrolytes. This technique, commonly used in LIB systems, − stabilizes the cathode–sulfide electrolyte interface. However, the effectiveness of a coating varies significantly with the properties of the coating material and coating-layer morphology. − Effective cathode coating layers should meet several criteria.…”