Deciphering the Nature of an Overlooked Rate‐Limiting Interphase in High‐Voltage LiNi0.5_{0.5}Mn1.5_{1.5}O4_4 Cathodes: A Combined Electrochemical Impedance, Scanning Electron Microscopy and Secondary Ion Mass Spectrometry Study

High-voltage cathode active materials, such as LiNi$_{0.5}$Mn$_{1.5}$O$_4$ (LNMO), are of major interest for the development of high-energy lithium-ion batteries. However, it has been reported that composite cathodes based on high-voltage active materials suffer from high impedances and low rate capabilities. The origin of the high impedances has not yet been clarified. Here, we use a combination of electrochemical impedance spectroscopy (EIS), focused ion beam/scanning electron microscopy/energy-dispersive X-ray spectroscopy (FIB/SEM/EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for showing that in the case of LNMO-based cathodes, a major part of the cathode impedance is related to the formation of a passivating interphase on the Al current collector. Remarkably, the impedance of this interphase can be mitigated by the targeted formation of a distinct passivating interphase, namely on the surface of the LNMO particles. The interplay between these interphases is discussed

Deposition‐Type Lithium Metal All‐Solid‐State Batteries: About the Importance of Stack‐Pressure Control and the Benefits of Hot Pressing during Initial Cycling

Abstract The concept of using metallic lithium as anode material offers great potential for increasing the energy density of lithium‐ion batteries (LIBs). However, the ubiquitous formation of dendrites during lithium plating has prevented this so far. This is also true for all‐solid‐state batteries (ASSBs), for which dendrite formation has been observed particularly at high stack pressures and high current densities. An evolving and very promising strategy for suppressing dendrite formation is the usage of a thin carbon layer on the anode side, referred to as deposition‐type lithium metal anode. Here, it is shown that for ASSBs with carbon‐based deposition‐type anodes of this type, the discharge capacity can be significantly increased by either: i) active stack pressure control; or by ii) hot pressing of binder‐containing anode and separator. High discharge capacities exceeding 190 mAh g−1 are achieved at room temperature without using any expensive elements (thiophosphate‐based solid electrolyte, Ni‐rich NMC cathode active material). This observation points to the importance of ensuring good mechanical contact between the deposition‐type anode and the separator during cycling. While approach (i) is readily applicable for lab‐scale batteries, approach (ii) should be a promising option for future commercial ASSBs