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

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To achieve high energy densities at costs needed for automotive applications, Li- and Mn-rich NCMs (LMR-NCMs; Li1+x(NiaCobMnc)1-xO2, with a+b+c=1 and x»0.1-0.2), are promising cathode active material (CAM) candidates.[1] However, these materials still suffer from high gassing, particularly during formation, and detrimental voltage and capacity fading over their cycle life.[2–5] Different approaches are employed to fix these issues, such as the use of additives, novel synthesis routes, or post-treatments.[6–9] In this study, we investigated the effect of an oxide-based solid-additive for an LMR-NCM CAM. We have observed that with the use of a model oxide, not only HF, but also residual water could be scavenged from the electrolyte. Furthermore, we show that this effect is not limited to surface coatings, but it is possible to use a model oxide in a functionalized separator or mixed into the electrode slurry for the same beneficial effect.By conducting On-line Electrochemical Mass Spectrometry (OEMS) measurements, we analyzed the gas evolution of the LMR-NCM CAM Li1.14(Ni0.26Co0.14Mn0.60)0.86O2 during the first activation cycle. It is known from literature that not only O2 and CO2 are evolved in this process, but also HF and H2O, which in turn can be observed as POF3 in the gas-phase.[10,11] As seen in Figure 1, POF3 is evolved simultaneously with CO2 for LMR-NCM half-cells with a non-functionalized polypropylene (PP) based separator (blue line). Surprisingly, no POF3 is evolved, if an oxide-modified separator is used (orange line). This is evidence for the HF-scavenging effect of these oxides. Further electrochemical tests have shown that full-cells with an oxide-modified separator have a greatly increased cycling stability in comparison to cells with a non-functionalized separator. Using TGA-MS, XPS, and Karl-Fischer titration, we further elucidate the beneficial mechanism of other oxide-based additives.