Quantifying Mixed Redox and Parasitic Processes in Li-Rich Disordered Rocksalt Li-Ion Battery Cathodes

Abstract

Li-ion battery demand is expected to increase dramatically as the transportation and power generation sectors become electrified. To address the expense and scarcity of cobalt and nickel used in current layered cathodes, alternative transition metals must be explored. Disordered rocksalt (DRX) cathodes can be fully comprised of affordable, earth-abundant metals such as manganese and titanium and have shown high practical capacity but currently suffer from poor capacity retention and voltage fade [1]. Many researchers have attributed this to O redox, and the subsequent formation of deleterious reactive oxygen species, which can occur in DRX cathodes at states-of-charge as low as 40% [2,3]. Recently, researchers have increased the Mn content in DRX cathodes, with the intent of increasing Mn redox and suppressing O redox [4]. Furthermore, these Mn-rich DRX cathodes undergo a beneficial phase transformation upon electrochemical cycling, increasing capacity and almost eliminating voltage fade [4]. Yet, the redox contributions of Mn and O in these Mn-rich cathodes, especially after this phase transformation, are not understood.Here we decouple capacity contributions from Mn-redox, O-redox, and parasitic processes (e.g. electrolyte decomposition) for Mn-rich DRX using a combination of operando gas measurements and inductively coupled plasma – optimal emission spectrometry (ICP-OES). Mn-rich DRX cathodes were cycled to various states of charge in coin cells and the resulting Mn and O oxidation state were determined via titration mass spectrometry (TiMS) using oxalic acid and triflic acid, respectively. To account for lattice O2 loss, differential electrochemical mass spectrometry (DEMS) was used on select cells. To account for Mn dissolution during cycling, ICP-OES was used to quantify the amount of Mn remaining in the cathode as a function of cycle number. The results of this study provide insight into the various redox processes and interfacial degradation that occurs in DRX cathodes, thereby informing future design to improve high-voltage stability of these promising materials.[1]: Raphaële J. Clément et al 2020 Energy Environ. Sci. 2 345[2]: Tzu-Yang Huang et al 2023 Adv. Energy Mater. 13 2300241[3]: Roland Jung et al 2017 J. Electrochem. Soc. 164 A1361[4]: Zijian Cai et al 2023 Nat. Energy 1