Feasibility of Chemically Modified Cellulose Nanofiber Membranes as Lithium-Ion Battery Separators.

Chemical modification of cellulose is beneficial to produce highly porous lithium-ion battery (LIB) separators, but introduction of high charge density adversely affects its electrochemical stability in a LiNi1/3Mn1/3Co1/3O2 (NMC)/graphite full cell. In this study, the influence of carboxylate functional groups in 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidized cellulose nanofibers (TOCNs) on the electrochemical performances of the LIB separator was investigated. X-ray photoelectron spectroscopy and in operando mass spectrometry measurements were used to elucidate the cause of failure of the batteries containing TOCN separators in the presence and absence of sodium counterions in the carboxylate groups and additives. For the TOCN separator with sodium carboxylate functional groups, it seems that Na deposition is the dominant reason for poor electrochemical stability of the cell thereof. The poor performance of the protonated TOCN separator, attributed to a high amount of gas evolution, is dramatically improved by adding 2 wt % of vinylene carbonate (VC) because of suppressed gas evolution. Unveiling the failure mechanism of the TOCN separators and successively implementing the strategies to improve performance, for example, removing Na, adding VC, and adjusting cycling rates, enable a remarkable cycling performance in the NMC/graphite full cell at ≈2 C (3 mA/cm2) of a fast discharging rate. Despite the aforementioned efforts and compromises required, an increased charge density of the TOCN is beneficial to acquire a mechanically stronger separator. In conclusion, the manufacturing process of cellulose nanofibers needs to be carefully adjusted to acquire a desired separator property. To the best of our knowledge, it is first reported to perform operando gas evolution measurements to systematically investigate the electrochemical stability of nanocellulose as an LIB separator material. The results elucidate not only the challenges for extensive applications of hygroscopic biomaterials for commercial LIBs but also the practical solutions to achieve high electrochemical stability of the materials.

Improving the Sensitivity of Solid-Contact Ion-Selective Electrodes by Using Coulometric Signal Transduction.

A fundamental limitation of potentiometric ion sensors is their relatively low sensitivity due to the logarithmic dependence between potential and activity. Here we address this issue by exploring a recently developed coulometric transduction method for solid-contact ion-selective electrodes (SCISEs). Spin-coated thin-layer ion-selective membranes are used to lower the membrane resistance and shorten the response time of the SCISEs. When using coulometric transduction, an optimized design of the K+-SCISE is able to detect a concentration change of 5 µM at a concentration level of 5 mM, corresponding to a 0.1% change in K+ activity. This indicates that SCISEs can provide extremely high sensitivity when employing coulometric transduction. Impedance measurements show that the coulometric transduction process for the K+-SCISE is limited by diffusion even for very thin ion-selective membranes. On the other hand, the H+-SCISE shows a low impedance and a fast coulometric response that is related to the high mobility of H+ in the H+-selective polymeric membrane as well as in the solid contact layer. The coulometric transduction method was used to detect small changes of pH in seawater and found to improve the sensitivity compared to classical potentiometry. The coulometric method was briefly tested also for determining activity changes of K+ in a serum sample.

Capacitive Model for Coulometric Readout of Ion-Selective Electrodes.

We present here a capacitive model for the coulometric signal transduction readout of solid-contact ion-selective membrane electrodes (SC-ISE) with a conducting polymer (CP) as an intermediate layer for the detection of anions. The capacitive model correlates well with experimental data obtained for chloride-selective SC-ISEs utilizing poly(3,4-ethylenedioxythiophene) (PEDOT) doped with chloride as the ion-to-electron transducer. Additionally, Prussian blue is used as a simple sodium capacitor to further demonstrate the role of the transduction layer. The influence of different thicknesses of PEDOT as a conducting polymer transducer, different thicknesses of the overlaying ion-selective membranes deposited by drop casting and spin coating, and different compositions of the chloride-selective membrane are explored. The responses are evaluated in terms of current-time, charge-time, and charge-chloride activity relationships. The utility of the sensor with coulometric readout is illustrated by the monitoring of very small concentration changes in solution.