1 Introduction

The application of lithium-ion batteries in daily life (alarm clocks, laptops, smartphones, etc.) and electric vehicles is becoming increasingly widespread, and it is a potential solution to reduce air pollution (the rapid transition from oil-based transportation to battery based clean transportation technology), widely commercialized due to its high energy density. However, under adverse loading conditions such as collisions or overheating caused by exceeding the design operating conditions of electric vehicles, lithium-ion batteries are prone to fire and explosion, which has prompted countries to establish strict safety standards (UL1642, UN38.3). However, the dynamics of battery failure behavior are not fully understood, and safety assessments need to be conducted to revise the standards.

Lithium iron phosphate batteries (LFPB) have higher thermal stability and collision safety compared to lithium cobalt oxide batteries (LCOB) and lithium manganese oxide batteries (LMOB or NMC), attracting the attention of electric vehicle manufacturers. Relevant research focuses on safety testing of batteries under mechanical and thermal abuse loads. Previous studies have included mechanical abuse testing (predicting failure loads under different quasi-static and dynamic loads), research on battery failure mechanisms (such as through “smart nail” testing, internal short-circuit ISC testing, nail induced ISC mechanism analysis, indentation testing of batteries of different capacities, etc.), research on the influence of battery shape and size on failure (such as the impact of cathode materials on the hazard level of 18650 batteries, simulation of thermal runaway behavior of batteries of different sizes, research on thermal runaway hazards of cylindrical LFPB), and research on the thermal behavior of batteries under charge and discharge conditions (temperature rise, hotspots, local degradation, temperature heterogeneity, cooling system research, etc.).

This article raises the issue of the applicability of different battery sizes to different types of electric vehicles, as battery thermal response is the main cause of failure and is size dependent (battery size is related to stored energy and energy release rate, and large-sized batteries have a higher risk of thermal runaway), while the size dependent failure behavior of cylindrical LFPB under mechanical abuse conditions has not been reported before. This article reveals the size dependence of LFPB under mechanical abuse conditions, analyzes the influence of its component size parameters on failure behavior, and studies its mechanical behavior under loading (layered and multi-component structure) by dissecting and analyzing four new LFPB sizes, considering the failure load and temperature rise of batteries with different diameters at the beginning of internal short circuit. This provides a basis for the development of battery safety standards and early failure detection systems.

2 Detailed description of the experimental process

2.1 Experimental Preparation and Sample Selection

Perform anatomical analysis on the all-new LFPB (18650, 22650, 26650, 32650) to study its internal structure and components. Subsequently, mechanical abuse testing was conducted on the new LFPB samples of various sizes to reduce experimental uncertainty. Calibration instruments were used in the laboratory environment to measure and ensure that thermocouples and other connections were intact during testing. Three sets of samples were tested on 18650 and 26650 batteries, while samples of other sizes were tested with the same configuration under similar loading conditions.

2.2 Anatomical analysis process

Discharge and disassembly preparation: Discharge the battery to zero charge (SOC), remove the non-conductive polymer coating on the outer shell, and then carefully cut the part of the outer shell near the positive and negative poles with a steel saw blade. The experiment follows standard safety protocols, and protective gloves and masks should be worn during operation, as disassembly may be dangerous due to short circuits, harmful electrolytes, or smoke release.

Component identification and extraction: After opening the battery, remove the positive terminal plate and safety valve component, take out the spiral wound core from the shell, and identify each component after disassembly, including the safety valve component, core (including electrode and diaphragm), shell, etc. It was also found that the 32650 battery has an additional central pin for exhaust.

2.3. Quasi static mechanical testing setup

Testing equipment and environment: Use DGBELL’s battery crush nail penetration test chamber for quasi-static lateral compression, longitudinal compression, and nail penetration testing. The test is conducted inside a transparent acrylic protective cover that protects the testing equipment and operators.

Test fixture and sample installation: Semi enclosed fixtures are used for nail testing of 18650 and 22650 batteries, while fully enclosed fixtures are used for nail testing of 26650 and 32650 batteries, with through holes for fixing nails.

Measurement and data recording: Compression and pinning tests are conducted at a loading speed of 5mm/min until the voltage drops to zero and the temperature begins to rise. The temperature is measured using a K-type thermocouple that complies with the IEC 584-2 standard and has an accuracy of ± 1.5 ℃. The thermocouple is attached to the positive electrode surface of the battery with Kapton tape (the positive electrode temperature is higher). The battery is connected to the voltage module and temperature module to record voltage and temperature data. The voltage and temperature modules have been calibrated to accurately record voltage, temperature, displacement, and load data.