Interfacial Manganese-Ion Dissolution in Cycled Li-Ion Cathodes Dependent on Electrolyte Chemistry—A Cryo-Electron Microscopy Study

In lithium-ion batteries, the pivot from scarce cobalt to earth-abundant elements for cathodes (composed of 0.3 Li2MnO3 and 0.7 LiMn0.5Ni0.5O2 w/w) has caused destabilization of the cathode interfaces during Li-ion cycling, losing manganese into the electrolyte. Electrolyte engineering is combating this interfacial destabilization using a combination of additives in the common Generation 2 (Gen2) electrolyte, 1.2M LiPF6 in EC:EMC (3:7 w/w). This study is focused on a mixture of additives targeting the interfaces on the cathode and anode. This work aims to understand how the combination of these additives has yielded higher stability cell performance as compared to the additives individually with the Gen2 electrolyte or to the Gen2 electrolyte alone.<br/>An electrolyte composed of additives together in the Gen2 electrolyte improved the energy and power density as well as the longevity of coin cell batteries cycled with the earth-abundant cathodes against graphite anodes. While electrolyte engineering is improving battery performance, the interfacial mechanism for this process is not understood. Here we used cryogenic electron microscopy (cryo-EM) sample preparation with air-free transfer and cryogenic scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) mapping to identify the nanoscale interfacial composition and the manganese oxidation state in cross-section at the surfaces of cycled cathode particles. This characterization allowed for the comparison of each individual additive with the Gen2 electrolyte to the additive combination with the Gen 2 electrolyte, each after 100 Li-ion charge and discharge cycles.<br/>Analysis from the different electrolytes has found Ni enrichment and Mn depletion at the cathode particle surface in certain samples. The manganese oxidation state at the cathode surface has a broader range of values, reaching down to a Mn mixed valency of 2+ and 3+ in the Gen2 electrolyte without additives. Since the Mn2+ is known to readily dissolve from the particles during cycling, this indicates that the combined cathode and anode additives are stabilizing the cathode particle surfaces to prevent Mn loss and impedance rise. This study reveals the differences between individual additives and the role that each additive played in concert to decrease interfacial impedance and prevent manganese ion dissolution. Cryo-EM methods were essential to obtaining this site-specific information from the beam-sensitive cycled cathode surfaces, without disturbing the formed cathode electrolyte interface or reduction of the transition metals in the cathode structure. The combination of electrolyte engineering with site-specific high-resolution cryogenic STEM/EELS characterization is yielding earth-abundant cathodes that operate with good capacity retention and a better understanding of the mechanism of this performance improvement.