Extending the charge cutoff voltage of cathode (e.g., LiCoO2) is a promising way to increase the energy density of Li-ion batteries, but critical challenges lie in the threats triggered by structural distortion and an unstable electrode/electrolyte interface. The general approach to enhance the stability of the cathode/electrolyte interface (CEI) consists of replacing the decomposition or sacrificing sources of carbonate solvents (e.g., EC) with concentrated or fluorinated electrolyte strategies. Herein, without following typical replacement strategies, we introduce a trace electrolyte additive and refine the dehydrogenation process of the original carbonate solvents, resulting in an enhanced CEI and long-term cycling stability of LiCoO2 up to 4.65 V. We demonstrate that cathode structure distortion, LiPF6 hydrolysis, and Co dissolution and shuttling have been simultaneously restrained. With the achievement of a long-life 250 and 270 Wh/kg pouch cells (assembled with a commercial graphite and SiO anodes), the refinement of the “old-school” electrolyte additive strategy opens up avenues toward the design of practical high-voltage full-cell systems.

Development of high-energy-density rechargeable battery systems not only needs advanced qualitative characterizations for mechanism exploration but also requires accurate quantification technology to quantitatively elucidate products and fairly assess numerous modification strategies. Herein, as a reliable quantification technology, titration mass spectroscopy (TMS) is developed to accurately quantify O-related anionic redox reactions (Li−O 2 battery and nickel-cobalt-manganese (NCM)/Li-rich cathodes), parasitic carbonate deposition and decomposition (derived from airexposure degradation and electrolyte oxidation), and dead Li 0 formation (Limetal battery and over-discharged graphite anode). TMS technology can harvest key information on products (e.g., quantification of oxidized lattice oxygen and solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI) components) and guide corresponding design strategy by enhancing understanding of the mechanism (e.g., clearly distinguish the catalytic target of highly oxidative Ni 4+ on the NCM cathode). Not limited as a rigid quantification tool for widely known products/mechanisms, TMS technology has been demonstrated as a powerful and versatile tool for the investigations of advanced batteries.

Cathode electrolyte interphase (CEI) layers derived from electrolyte oxidative decomposition can passivate the cathode surface and prevent its direct contact with electrolyte. The inorganics-dominated inner solid electrolyte layer (SEL) and organics-rich outer quasi-solid-electrolyte layer (qSEL) constitute the CEI layer, and both merge at the junction without a clear boundary, which assures the CEI layer with both ionic-conducting and electron-blocking properties. However, the typical "wash-then-test" pattern of characterizations aiming at the microstructure of CEI layers would dissolve the qSEL and even destroy the SEL, leading to an overanalysis of electrolyte decomposition pathway and misassignment of CEI architecture (e.g., component and morphology). In this study, we established a full-dimensional characterization paradigm (combining Fourier transform infrared, solution NMR, X-ray photoelectron spectroscopy, and mass spectrometry technologies) and reconstructed the original CEI layer model. Besides, the feasibility of this characterization paradigm has been verified in a wide operating voltage range on a typical LiNi x Mn y Co z O 2 cathode.