Abstract
The class of lithium ion battery (LIB) electrolytes most commonly used nowadays is under investigation for at least two decades. These electrolytes, consisting of organic carbonates and LiPF6, are known to undergo not only electrochemical reactions due to their limited electrochemical stability but also chemical reactions due to their sensitivity to hydrolysis. The interplay of chemical and electrochemical reactions as well as their interactions with cathodes, anodes and their surfaces are not yet fully clarified. To study these reactions different analytical techniques can be applied. This usually requires an extraction of the electrolyte from the battery involving a high risk of sample alterations due to volatile and highly reactive substances.
To overcome this issue a battery cell set-up for in situ NMR measurements of liquid electrolytes has been developed.1 It enables non-invasive investigations providing molecular information from the inside of an intact battery. The cylindrical battery cell consists of two electrodes in a heat-sealed polymer tube that is small enough to be placed in a standard 5 mm NMR glass tube (see Figure). To ensure gas tightness the NMR glass tubes are sealed with epoxy resin. All the common electrode materials including transition metal oxides, graphite as well as lithium metal can be applied as active electrode materials.
Electrochemical tests demonstrate a sufficient rate performance and a good cycling stability of the in situ cells. NMR measurements of the cells provide well-resolved spectra not distorted by the solid battery components. Furthermore, it can be shown that a loss equal or less than 0.1 % of the working electrode capacity by a one-electron reaction forming hydrogen or fluorine containing compounds is sufficient to reach the limit of detection (capacity: 1 to 6 mA h, 400 MHz NMR spectrometer). Hence, the sensitivity of this method is considered high enough to detect any significant electrochemical formation of soluble compounds exhibiting clear 1H or 19F NMR signals.
The results so far show that the in situ cells enable investigations on the passivating properties of the solid electrolyte interphase on the anode (SEI) by monitoring the formation of compounds electrochemically formed on the anode. Furthermore, they revealed temperature and potential dependent fluorination reactions of electrolyte solvents on the cathode surface. In addition, the measurements provide insight into reactions of the electrolyte that probably influence the SEI and are preventable by the cathode depending on the cathode material and processing. NMR spectroscopy also offers a method to investigate transition metal dissolution from the cathode active material since paramagnetic species, e.g., Mn2+, Ni2+ and Co2+ have a strong impact on the relaxation of nuclear spin magnetization.
1) S. Wiemers-Meyer, M. Winter and S. Nowak, Phys. Chem. Chem. Phys, 2017, 19, 4962-4966.
Figure 1