Beneficial Effects of Oxide-Based Additives on Li-and Mn-rich Cathode Active Materials

Abstract

Li- and Mn-rich layered oxides such as Li1.14(Ni0.26Co0.14Mn0.60)0.86O2 (LMR-NCM) are potential next-generation cathode active materials (CAMs) for lithium ion-batteries, promising an increased energy density at lower materials costs compared to state-of-the-art CAMs. However, their commercial viability is still inhibited by its strong gassing, poor cycling stability, and voltage fading, so various approaches such as post-treatments or additives are being investigated. Here, it is shown that the cycling performance of LMR-NCM//graphite coin-cells is drastically improved when assembled with 300°C dried glassfiber (GF) separators (“GF cells”) compared to cells with Celgard (CG) separators dried at 70°C (“CG cells”). The origin of this phenomenon is investigated by online electrochemical mass spectrometry, thermogravimetric analysis mass spectroscopy, water absorption, and X-ray photoelectron spectroscopy measurements. These reveal that the superior performance of the GF cells can be ascribed to the bulk water absorption capability of the 300°C dried glassfiber material as well as its ability to scavenge HF, whereby H2O and HF are produced by the (electro)chemical oxidation of the electrolyte and the decomposition of the LiPF6 salt. Similar performance enhancements can be observed for 300°C dried SiO2 nanoparticles added to the LMR NCM cathodes or for an HF/H+ scavenging electrolyte additive.

Depletion of Electrolyte Salt Upon Calendaric Aging of Lithium-Ion Batteries and its Effect on Cell Performance

“…Note that while here reaction 5 is used to describe the electrochemical electrolyte solvent oxidation (valid for CV conditions), the same results would be obtained when using reaction 4 (valid for OCV hold conditions). must (at least partially) suppress the reduction of HF or other protic species formed at the cathode; while we cannot present a definite answer, this suppression might be due to (i) the scavenging of HF, H + , and H 2 O by effective electrolyte additives, e.g., alkyl-silyl compounds, 83,84 (ii) the scavenging of HF, H + , and H 2 O by ceramic separator coatings (note that the large-format prismatic prototype cells contain a coated separator, while the single-layer pouch cells contain an uncoated polyolefine separator), [85][86][87] or (iii) a very good initial SEI which inhibits reduction of HF and H 2 O. 42 Finally, we want to put the findings of this study into context of existing literature.…”

Methods–Temperature-Dependent Gassing Analysis by On-Line Electrochemical Mass Spectrometry of Lithium-Ion Battery Cells with Commercial Electrolytes

“…The latter is required for the study of commercial cells or to investigate the effect of functionalized polyolefin separators. 30,31 When linear alkyl carbonates are required, the argon-filled headspace of the OEMS cell consists of a significant share of electrolyte vapor. The linear alkyl carbonate solvent molecules are composed of a large number of atoms (e.g., the molecular formula for EMC is C 4 H 8 O 3 ), so they have complex mass spectrometric fragmentation patterns, with strong mass signals at m/z (mass/charge) values that significantly contribute to the background signal on the relevant m/z mass signals related to hydrogen, ethylene, carbon monoxide, oxygen, and carbon dioxide.…”