Due to their performance advantages and cost reduction, Li-ion batteries have become an indispensable energy source in various strategic industries. As the demand for higher energy density increases, ensuring battery safety and longevity has become more crucial than ever. However, incidents involving battery fires and explosions, particularly in electric vehicles (EVs), still pose significant concerns for the electrification of transportation. Current commercial EV battery safety primarily relies on battery management systems (BMS) that can read battery voltage and current and with sensors placed out of the cells monitoring surface temperature and other parameters to estimate the battery’s health status. However, due to the lack of monitoring at multiple locations within each battery cell, some estimations may be spatially and temporally inaccurate, leading to higher error rates in large capacities and under abnormal conditions. This limitation could become more pronounced with the advent of higher energy density batteries, such as Li metal and Li-S batteries. This underscores the urgent need for novel technologies that can accurately monitor the internal states of batteries, providing more reliable data for enhanced safety and performance.

To overcome this limitation, we propose an innovative monitoring strategy by developing an integrated implantable mesh sensor system. This advanced sensor device features two temperature sensors, two pressure sensors, and three voltage sensors, enabling the detection of critical parameters for battery safety–internal temperature, pressure, and electrode potential within batteries–thereby enhancing the accuracy of battery monitoring systems. In this thesis, through conceptual rational design and systematic optimisation, this sensor system was successfully fabricated. The porosity (ranging from 97.8% to 77.8%) of the mesh structure was carefully examined to ensure minimal adverse impact on battery operations and performance. Moreover, a series of strategic modifications were introduced to improve the sensor device's reliability, sensitivity, and overall performance. First, a gold layer with a precisely controlled thickness of 150 nm was deposited onto the device as the temperature sensors, ensuring highly accurate and consistent temperature measurements across various operating conditions. The sensitivities of the outside sensor and the inside temperature of the device are calculated as 0.314% °C–1 and 0.295% °C–1, respectively. Furthermore, the device’s pressure-sensor response was significantly enhanced by spray-coating an optimised Ti3C2Tx/PVP composite. This composite material was meticulously fine-tuned to provide a highly responsive and precise pressure-sensing layer, capable of detecting a small pressure variation of 0.2 kPa, responding to the pressure within 40 ms with an average sensitivity of 0.0196 kPa–1. To ensure stability and accuracy in electrode potential measurements, the gold pad was pre-lithiated to form a LiAu alloy as a stable reference electrode. These approaches significantly improve the sensor device's overall performance.

With an optimised porosity of 83.27% and an ultra-thin design (~12 μm), the sensor device can be seamlessly inserted between the electrodes of a battery with minimal impact on performance. Separate sensor components in the device detect different parameters, ensuring minimal crosstalk between signals. It's multiparametric monitoring, non-invasive internal monitoring, and reduced crosstalk make this sensor device stand out compared to current sensing techniques, which typically achieve only one or two of these features.

The sensor devices were embedded into Li–S pouch cells between a cathode and separator to validate their superiorities in monitoring battery operations. These devices are capable of directly detecting internal temperature changes and can identify a temperature variation as small as 1 °C under low-current conditions, which surface-mounted thermocouples are unable to detect. In addition, the sensor devices can monitor the changes in individual electrode potential, providing a more accurate assessment of electrode health. Specifically, the monitored potential of the lithium metal anode remains stable, with fluctuations of less than 1%. More importantly, the sensor devices can detect internal pressure variations, enabling the identification of mechanical degradation within the batteries—an area where other online monitoring systems struggle. Their accuracy and comprehensive monitoring capabilities make these sensor devices highly promising for practical applications.