Novel in-line quality control methodologies of the battery solid-electrolyte interface
As electric vehicles (EVs) are hitting the road in increasing numbers, batteries remain a key area of research. To understand and improve the processes of charging, discharging and ageing of EV batteries, component characterisation at the microscale under real conditions is required. The micro- and nano-scale impedance characteristics of batteries have not yet been investigated. In a global first, the EU-funded NanoBat project has filled this critical gap with gigahertz (GHz) radiofrequency nanotechnologies and microscale impedance measurement tools for lithium-ion batteries and beyond lithium-ion batteries. They will support battery production and Europe’s leadership position in this growing global market.
Zooming in on the solid electrolyte interphase
The electrode’s solid electrolyte interphase (SEI), created in a sophisticated process at the end of the battery cell production chain, accounts for a significant portion of EV battery production costs. This very thin layer – less than one thousandth of the diameter of a hair – is critical to the battery cell’s performance, reliability and durability and very often has abnormalities or defects that are difficult to detect. According to the NanoBat coordinator Ferry Kienberger of Keysight Technologies in Austria, “the NanoBat electrical measurement techniques and models focused on understanding and predicting the SEI layer’s reliability before the EV battery cell is built into a final battery module or pack.”
A broad-band frequency toolbox of high-resolution techniques under natural operating conditions
Regarding scanning probe microscopy, “we extended the operating frequency of the scanning microwave microscope to 20 GHz, increasing impedance resolution at the nanoscale by 100 times compared to low-frequency microscopes. The microscopes’ imaging time was decreased from about half an hour per frame to several minutes while maintaining lateral resolution,” Kienberger explains. “A newly calibrated electrochemical impedance spectroscopy (EIS) method that is 10 times more sensitive at frequencies of up to 100 kHz allowed researchers to make measurements on large EV battery cells (a pack of 396 cells) with impedance resolution of a few microohms,” adds Kienberger. The EIS system was metrologically calibrated and verified in a round-robin test with original equipment manufacturers (OEMs). Finally, the team developed a self-discharge measurement for cell quality control and a new high-voltage separator test. The former reduces cell testing time in EV battery production lines from 1 week to 20 minutes, which will reduce material waste and the costly disassembly of malfunctioning cell packs.
Satisfying discerning stakeholders, accelerating access
NanoBat demonstrated its nanoscience-based impedance models and metrological test methods in pilot-line manufacturing and OEM field tests. “Major players in the European EV battery manufacturing field (BMW and SAFT) are members of NanoBat’s stakeholder group and have collaborated on research and publications. They demand data accuracy, data reliability and high-level uncertainty analysis, all of which have been perfectly served by the NanoBat community,” notes Kienberger. Partner Pleione Energy (Greece) is now offering an EV battery pilot line service, and partner Poland introduced an open web platform for non-destructive materials characterisation, both integrating NanoBat outcomes. The Swiss Federal Institute of Metrology is offering the new NanoBat scanning probe microscopy method in their portfolio of services. The tremendous engagement on LinkedIn – up to 10 000 views per post – confirms that NanoBat has united diverse research units like universities, small enterprises and major European players in the electrification of cars and vehicles, supporting the European Green Deal and carbon neutrality strategies.
Keywords
NanoBat, EV, EV battery, battery cell, electric vehicle, SEI, solid electrolyte interphase, EIS, electrochemical impedance spectroscopy, scanning probe microscopy