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The cathode material in a lithium (Li) battery determines the system cost, energy density, and thermal stability. In anode-free batteries, the cathode also serves as the source of Li for electrodeposition, thus impacting the reversibility of plating and stripping. Here, we show that the reason LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes deliver lower Coulombic efficiencies than LiFePO4 (LFP) is the formation of tortuous Li deposits, acidic species in the electrolyte, and accumulation of “dead” Li0. Batteries containing an LFP cathode generate dense Li deposits that can be reversibly stripped, but Li is lost to the solid electrolyte interphase (SEI) and corrosion according to operando 7Li NMR, which seemingly “revives” dead Li0. X-ray photoelectron spectroscopy (XPS) and in situ 19F/1H NMR indicate that these differences arise because upper cutoff voltage alters electrolyte decomposition, where low-voltage LFP cells prevent anodic decomposition, ultimately mitigating the formation of protic species that proliferate upon charging NMC811. 

The role of the cathode–electrolyte interphase (CEI) on battery performance has been historically overlooked due to the anodic stability of carbonate-based electrolytes used in Li-ion batteries. Yet, over the past few decades, degradation in device lifetime has been attributed to cathode surface reactivity, ion transport at the cathode/electrolyte interface, and structural transformations that occur at the cathode surface. In this review, we highlight recent progress in analytical techniques that have facilitated these insights and elucidated not only the CEI composition but also the spatial distribution of electrolyte decomposition products in the CEI as well as cathode-driven reactions that occur during battery operation. With a deeper understanding of the CEI and the processes that lead to its formation, these advanced characterization tools can unlock routes to mitigate impedance rise, particle cracking, transition metal dissolution, and electrolyte consumption, ultimately enabling longer lasting, safer batteries. 

Abstract We present observations of the extremely luminous but ambiguous nuclear transient (ANT) ASASSN-17jz, spanning roughly 1200 days of the object’s evolution. ASASSN-17jz was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) in the galaxy SDSS J171955.84+414049.4 on UT 2017 July 27 at a redshift ofz= 0.1641. The transient peaked at an absoluteB-band magnitude ofM_B,peak= −22.81, corresponding to a bolometric luminosity ofL_bol,peak= 8.3 × 10⁴⁴erg s⁻¹, and exhibited late-time ultraviolet emission that was still ongoing in our latest observations. Integrating the full light curve gives a total emitted energy ofE_tot= (1.36 ±0.08) × 10⁵²erg, with (0.80 ± 0.02) × 10⁵²erg of this emitted within 200 days of peak light. This late-time ultraviolet emission is accompanied by increasing X-ray emission that becomes softer as it brightens. ASASSN-17jz exhibited a large number of spectral emission lines most commonly seen in active galactic nuclei (AGNs) with little evidence of evolution. It also showed transient Balmer features, which became fainter and broader over time, and are still being detected >1000 days after peak brightness. We consider various physical scenarios for the origin of the transient, including supernovae (SNe), tidal disruption events, AGN outbursts, and ANTs. We find that the most likely explanation is that ASASSN-17jz was a SN IIn occurring in or near the disk of an existing AGN, and that the late-time emission is caused by the AGN transitioning to a more active state.