The 1st cycle Coulombic efficiency (CE) of LiNi1/3Co1/3Mn1/3O2 (NCM) at 4.6 V vs. Li/Li(+) has been extensively investigated in NCM/Li half cells. It could be proven that the major part of the observed overall specific capacity loss (in total 36.3 mA h g(-1)) is reversible and induced by kinetic limitations, namely an impeded lithiation reaction during discharge. A measure facilitating the lithiation reaction, i.e. a constant potential (CP) step at the discharge cut-off potential, results in an increase in specific discharge capacity of 22.1 mA h g(-1). This capacity increase during the CP step could be proven as a relithiation process by Li(+) content determination in NCM via an ICP-OES measurement. In addition, a specific capacity loss of approx. 4.2 mA h g(-1) could be determined as an intrinsic reaction to the NCM cathode material at room temperature (RT). In total, less than 10.0 mA h g(-1) (=28% of the overall capacity loss) can be attributed to irreversible reactions, mainly to irreversible structural changes of NCM. Thus, the impact of parasitic reactions, such as oxidative electrolyte decomposition, on the irreversible capacity is negligible and could also be proven by on-line MS. As a consequence, the determination of the amount of extracted Li(+) ("Li(+) extraction ratio") so far has been incorrect and must be calculated by the charge capacity (=delithiation amount) divided by the theoretical capacity. In a NCM/graphite full cell the relithiation amount during the constant voltage (CV) step is smaller than in the half cell, due to irreversible Li(+) loss at graphite.
The tris(trimethylsilyl) phosphite (TMSPi) is considered as an ideal electrolyte additive for lithium ion batteries. In this work, its positive effect as well as its failure mechanism in a LiPF 6 containing electrolyte was studied by means of selected electrochemical, structural and analytical techniques. The LiNi 0.5 Co 0.2 Mn 0.3 O 2 /graphite cells with TMSPi as electrolyte additive were cycled between 2.8 and 4.6 V.Thanks to the compact cathode electrolyte interphase formed by the oxidative decomposition of TMSPi in a freshly prepared TMSPi containing electrolyte, both the discharge capacity and the cycling stability of cells were enhanced. However, our results also show that TMSPi actually reacts with LiPF 6 at room temperature. TMSPi is consumed by this spontaneous reaction after aging for certain time. In addition, a part of the fluorophosphates, generated from the hydrolysis of LiPF 6 , is bonded to one or two TMS groups, causing a decrease in the fluorophosphate content in the CEI film. Consequently, the cycling stability of the lithium ion cells with aged TMSPi containing electrolyte deteriorates. The obtained results offer important insights into the practical application of TMSPi, which means that TMSPi can only be used as an effective additive in a freshly prepared LiPF 6 containing electrolyte.
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