Li-ion batteries (LIBs) offer particularly high performance among rechargeable batteries and are used in a variety of industrial domains. They were primarily used as a power supply for portable devices in the past. In recent years their applications have expanded to encompass stationary energy storage systems and electric vehicles (EVs), driving demand for lower-cost LIBs with even higher performance. Demand for LIBs for use in EVs is growing particularly rapidly as governments around the world fast-track measures to promote automobile electrification^1,2.

The quality of the Solid Electrolyte Interphase (SEI) on the negative electrode of Li-ion batteries has a strong impact on the cycle life so that clear determination of their properties is of essential importance to achieve progress in manufacturing processes³. Many kinds of measurements and tests are necessary at each state of the battery manufacturing process to assure the quality of each process. In addition, measurements and testing are essential in a variety of settings, during not only manufacturing, but also for battery research and finished-product inspections.

In this work, a new, cheap and easily-implementable methodology to estimate the electrically-insulating quality of the SEI in LIBs is proposed. First, a redox-mediator is added in the electrolyte after the SEI formation cycle, and the redox mediator leads to an internal self-discharge process that is inversely proportional to the electrically-insulating character of the SEI. Second, a few charge and discharge cycles are applied to the battery and the presence of the redox-mediator provokes a shuttle effect enables by the lack of electrically protecting character of the SEI which consumes charges decreasing the coulombic efficiencies, enhancing the sensibility to the SEI protecting nature.

The proposed methodology is probed on lithium iron phosphate batteries in pouch cell configuration with two redox mediators (ferrocene (FC) and methyl phthalimide (PHT)) by evaluating the influence of vinylene carbonate as electrolyte additive in the resulting SEI.

We believe that the findings based on the application of this mediator-enhanced coulometry can be used to accurately predict the cyclic behavior of LIBs under extended operating conditions, which is especially relevant for a better comprehension of future industrial needs for battery R&D in cell components and production fields.

Acknowledgment

The authors acknowledge the financial support by the NanoBat project. NanoBat has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement no. 861962

References

1. S. Manzetti and F. Mariasiu, Renewable & sustainable energy reviews, 51, 1004–1012 (2015).
2. E. Fan et al., Chemical Reviews (2020).
3. E. Ventosa, Current Opinion in Electrochemistry, 25, 100635 (2021) https://www.sciencedirect.com/science/article/pii/S2451910320301708.