By Gareth Hinds, NPL Fellow and Science Area Leader, Electrochemistry Group, National Physical Laboratory (NPL). https://www.npl.co.uk/
The boom in sales of electric vehicles (EVs) shows no signs of abating. According to the Society for Motor Manufacturers & Traders, 190,000 new EVs were sold in 2021 (11.6% of all new car sales). This is more than in the previous five years combined and comes against a backdrop of an automotive market that has stagnated due to the Covid pandemic and a global semiconductor shortage. There are now just under half a million EVs registered in the UK and that number looks set to continue its exponential increase.
We all know why this is happening. EVs are a critical component of the drive to decarbonise the transport sector. The UK government is committed to its ban on the sales of new petrol and diesel vehicles by 2030 in order to contribute to greenhouse gas emissions targets and improve air quality. Charging infrastructure is being rolled out, although the indications are that it may struggle to keep pace with the growing number of EVs on our roads.
The key component of an EV is the battery, which provides a highly efficient means of storing electrical charge to power the motor. The battery pack in an EV consists of hundreds (or sometimes thousands) of individual cells, which are arranged into sub-units called modules. Complex algorithms for the charge, discharge and cooling of each module are controlled by the battery management system (BMS) so that the performance and lifetime of each cell can be optimised.
Lithium-ion batteries are the technology of choice for automotive applications. Although they’ve been around since the early 1990s, it’s only in the last decade that their cost, performance and lifetime have improved sufficiently for large scale use in EVs. While a number of alternative battery chemistries are at various stages of research and development, none of these looks likely to displace lithium-ion in the short to medium term.
Battery testing is a vital element of cell, module and pack design, onboard diagnostics, lifetime prediction and end-of-life assessment, but as with many disruptive technologies the development of international standard test methods is lagging behind the rapid uptake of EVs. In particular, standard test methods for rapid and non-destructive assessment of performance, lifetime and safety are urgently required to underpin investor, regulator and end-user confidence in lithium-ion battery technology.
As anyone with a smartphone or laptop knows only too well, the amount of charge that can be stored in a lithium-ion battery decreases gradually over time in a process known as ‘capacity fade’. This is due to a range of material degradation mechanisms within the cell that combine to reduce the amount of lithium that can be stored in the electrodes. The state-of-health (SoH) of a lithium-ion cell is a key metric in the lifetime of the battery, which is usually defined as the capacity of the cell as a percentage of its original capacity.
Automotive manufacturers typically set a limit of 70-80% SoH for their EV battery packs; when this value is reached (typically after several thousand charge-discharge cycles) the battery pack is replaced to restore the full range of the EV. The used battery pack does not necessarily have to be discarded; it may still be useful for so-called ‘second life’ applications where the energy or power density requirements are less demanding, e.g. residential energy storage.
Testing has an important role to play in the cost-effective triage of EV batteries at the end of their first life. Conventionally, measurement of SoH is achieved by integration of current with time during slow discharge, which is expensive and prohibitively time-consuming, particularly as the number of used battery packs increases. Much research is focused on rapid and non-invasive techniques for SoH assessment, which can provide the basis for informed decisions on remanufacturing, repurposing or recycling. This is particularly important given the increasing scarcity of some of the critical raw materials, including cobalt, nickel and lithium.
Recent research at the National Physical Laboratory (NPL), in collaboration with partner laboratories across Europe in the LiBforSecUse project, has demonstrated the potential of electrochemical impedance spectroscopy (EIS) as a rapid means of assessing SoH at cell level. Key innovations include the development of low impedance standards for calibration of EIS instrumentation, tight control of experimental parameters such as cable length, terminal connection and measurement protocols, and establishment of the correlation between SoH and EIS data via well-defined and controlled life cycle testing.
As a result of the metrological infrastructure established in the LiBforSecUse project, the uncertainty in measurement of SoH has been reduced from around 10% to less than 1%. This enables Li-ion cells to be sorted rapidly and accurately according to their SoH at end of first life, significantly extending their performance and lifetime when assembled into a second life module/pack. The next steps will be to extend this measurement capability to module and pack level, making it available across a wider range of second-life applications, and to progress the test method to an international standard.
Standard test methods are also critical for efficient investment in materials R&D. The scientific literature is characterised by poor reproducibility due to a lack of standardisation of common laboratory test methods. The result of a measurement often depends to a large degree on the specific equipment used and the skill of the operator, meaning that test data from different organisations cannot be easily compared.
In collaboration with partners in the FutureCat project funded by the UK’s Faraday Institution, NPL is developing and maintaining a suite of standard protocols for fabrication, testing and post-mortem characterisation of Li-ion coin cells used in the development of next-generation cathode materials. Improving the quality and inter-comparability of data allows decisions to be made more rapidly on identification of the most promising new materials and ‘fast fail’ of others, accelerating the transition from the laboratory to real-world application.
In parallel with advances in battery testing standards, there is also a huge skills gap that needs to be addressed to support the seismic shift in automotive propulsion technology that will occur over the next decade. A new generation of electrical engineers and technicians capable of testing, diagnosing and repairing EV batteries need to be trained and deployed across the country in garages, MoT centres, test houses and research establishments. This will require significant investment in training courses, apprenticeships and accreditation by learned professional bodies.
A final important point is that increased UK engagement in the development of international standard test methods is urgently required to ensure the future competitiveness of UK companies in a growing global market. Without this, international standards will be imposed on the UK by the rest of the world, potentially hampering the commercial interests of UK companies. There is very strong representation from other European countries, the Far East and the USA on many international working groups through which future battery testing standards are being developed, but the UK is currently lagging behind in this area.
One potential reason for this is that the UK has a higher proportion of SMEs in the battery space compared to other countries and there is often a challenge around prioritising standardisation activities with limited resources. However, this may change going forward with the recent move to virtual meetings, meaning that committee and working group representation no longer requires international travel and lengthy in-person meetings. It is now possible to fit these activities much more efficiently and cost-effectively into an average working day via relatively short video conferences with a specific focus.
For further information on battery testing standards or to find out how to join the relevant BSI standards committee, please contact Nicola Young (BSI PEL/21 – Secondary cells and batteries) or Andreea Vieru (BSI PEL/69 – Electric vehicles).