A Fast Constant-Voltage Technique to Determine the Rating Performance with Application to Solid-State Batteries
Authors: Cele' Jacopo , Franger Sylvain , Lamy Yann , Oukassi Sami 
Published: September 2023

One of the main characterization necessary to compare different battery technologies is the extraction of the rating performance [1]. In Ragone or Ragone-like plot [2], [3] the evolution of the available capacity as a function of the discharge/charge current or power is plotted for different technologies in such a way to compare them not only at a particular discharge condition, but on the whole spectrum of available current density.
This comparison is of paramount interest because it may lead to unexpected results, as in the case of nanostructured electrode (e.g., Carbon nanofibers based electrode). That kind of electrode usually shows a poor capacity, compared to standard slurry electrode, at low current density, due to the intrinsic loss of active material linked to the 3D porous structure. Notwithstanding, when the comparison spans multiple decades of current density, the former technology will prove far better of the latter due to a superior electrolyte permeation and hence an almost zero loss of capacity with increasing current density[4].
Nowadays, the extraction of the rating performance is accomplished by discharging a cell at multiple current density and the final capacity value correspond to a single point on the rating plot. The corresponding test time is very high (>100h) even if lots of test channel are available. The test time will always be at least the time required to discharge a cell at the lowest probed current density.
In the present work, we propose different and faster methods to acquire the same information and compare them to the standard protocol both in terms of quality of results and total test time. Solid state Enfilm [5] has been considered as a test vehicle for this work
The first corresponds to start discharging a cell with the highest current under test and each time the voltage limit (e.g., 3V) is reached, to restart the discharge with a lower current, down to the minimum current under test. The resulting discharge profile while be composed by N-current step. This method limits the time at which the cell is discharge at the lowest current and strongly increases the amount of data: each test provides data for all probed current and not only one as in the case of the standard protocol.
The second method consists in discharging the cell at constant voltage. The voltage is fixed equal to the limiting voltage (i.e., 3V) and the current is registered. In this case, no previous knowledge of the maximum current is required and the number of points in the final plot are equal to the sampling of the measurement. The rating performance is directly available on the current versus capacity plot. The capacity available for each current density is obtained by integrating the discharge curve up to the desired current. This method can be considered the limit of the N-current step method for N approaching infinity.
Theoretical discussion and validity of application of the technique will be further discussed.

References

[1] H. Ji et al., “Ultrahigh power and energy density in partially ordered lithium-ion cathode materials,” Nat. Energy, vol. 5, no. 3, pp. 213–221, Mar. 2020, doi: 10.1038/s41560-020-0573-1.
[2] W. G. Pell and B. E. Conway, “Quantitative modeling of factors determining Ragone plots for batteries and electrochemical capacitors,” J. Power Sources, vol. 63, no. 2, pp. 255–266, Dec. 1996, doi: 10.1016/S0378-7753(96)02525-6.
[3] T. Christen and M. W. Carlen, “Theory of Ragone plots,” J. Power Sources, vol. 91, no. 2, pp. 210–216, Dec. 2000, doi: 10.1016/S0378-7753(00)00474-2.
[4] H. Zhang et al., “Electroplating lithium transition metal oxides,” Sci. Adv., vol. 3, no. 5, pp. 2–10, 2017, doi: 10.1126/sciadv.1602427.
[5] STMicroelectronics, “EnFilmTM - rechargeable solid state lithium thin film battery.” EFL700A39 datasheet, Jun. 2014.

Figure 1