Sodium-ion (Na-ion) batteries have received considerable attention in recent years as a cheaper, safer, more sustainable alternative to conventional lithium-ion (Li-ion) technology [1,2]. Battery cycle life is an important performance criterion with respect to the commercialisation of these. All batteries will experience, with time, some reduction in performance due to a variety of degradation mechanisms, some of which are linked to a rise in cell impedance [3]. Therefore, by carrying out a combination of constant-current, constant-voltage (CC/CV) cycling in combination with electrochemical impedance spectroscopy (EIS), it should be possible to identify the rate-limiting steps and problem interfaces inside a cell. Identifying such factors will make it possible to address them and therefore enhance overall battery performance.
We have applied electrochemical impedance spectroscopy (EIS) to layered oxide Na-ion cells in order to identify factors that limit cell performance. Figure 1 depicts the voltage profile for a typical cycle. These data were collected at charge and discharge rates of ~ C/10 and ~ C/5 respectively, using voltage limits of 4.2 V and 1.0 V. Figure 2 denotes a typical cycle life plot. A reversible capacity of 123 mAh/g (cathode) was retained after 100 cycles, indicating a charge retention of 98 %. Most of the capacity loss was recovered when subsequently cycled at lower rates, suggesting that this reduced capacity was due to increased cell impedance rather than an irreversible loss of Na.
EIS was performed on a range of different cell configurations, including a novel three-electrode pouch cell design. Spectra were recorded at varying states of health (SoH) and states of charge (SoC). Results were analysed by fitting ideal equivalent electrical circuits to impedance data, and using these to attribute resistances and capacitances for the different components present in the impedance response. Spectra were plotted using different complex formalisms in order to highlight different aspects of collected data, to the best of our knowledge this is an approach not previously reported in the analysis of batteries.
The cathode was found to be primarily responsible for resistive losses inside these layered oxide Na-ion cells, with the impedance associated with the positive electrode continuing to grow during cycling. Further characterisation and techniques to try and improve the performance of these sodium-ion batteries will be provided during the presentation.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)