# Rapid Online Solid-State Battery Diagnostics with Optically Pumped Magnetometers
## 1 Introduction  
Lithium-ion rechargeable batteries (LIBs) currently are considered the most promising secondary battery technology to power mobile devices, electric transportation, and power tools. Conventional LIBs typically employ liquid or gel-based electrolytes. These are often volatile and flammable, which gives rise to safety concerns [1, 2]. Solid-state batteries incorporate ion conducting solid electrolytes instead, which avoid the issue of volatility and leakage. Among other things, solid electrolytes allow the use of electrode materials with high voltages (>5 V), because of their wider electrochemical window. Furthermore, the use of solid electrolytes could enable the use of lithium metal anodes, thereby providing a viable pathway for higher capacity devices, while maintaining the inherent safety of the battery [3]. Significant technological challenges need to be overcome to make solid-state-based LIBs a widespread reality, including, for example, the facilitation of fast ion transport and the establishment of good interfacial properties between electrodes and the electrolyte medium. All these factors highlight the need for the development of advanced diagnostic methodology that would allow proper device characterization at different stages of a battery's life. Currently usedtechniques which can give in situ/operando information include ultrasound diagnostics [4; 5], synchrotron-based scanning transmission X-ray microscopy [6], X-ray micro-diffraction [7], and Raman spectroscopy [8]. Manufacturers typically perform electrochemical testing and limited 2D X-ray scanning [9; 10]. X-ray tomography is another powerful (but costly) tool that is generally too slow for high-throughput use with commercial cells [9; 11]. Ultrasound diagnostics are useful for characterization of cells based on changes in density and mechanical properties [12; 13], but do not allow noncontact measurements, limiting their broad application. Rapid techniques that can be used nondestructively are typically limited to electrical impedance spectroscopy, and lately, ultrasound techniques. Recently, it was also demonstrated that magnetic susceptibility changes within cells could be measured nondestructively and without contact using an inside-out MRI (ioMRI) technique that used the \({}^{1}\)H nuclear spin resonance frequencies in water to measure the susceptibility-induced magnetic field changes surrounding a cell when placed in a strong magnetic field [14; 15]. It was further shown that the changes in the magnetic susceptibility could be tracked across the charge-discharge cycle, and that these changes followed the expected trends of the lithiation state of the cathode material. This approach therefore provided a single-point state-of-charge measurement and allowed for the identification of inhomogeneities or non-idealities of charge storage in electrochemical cells [14]. Extensions of this method have been developed to encompass the characterization of electrical current distributions (operando) [16], robust imaging protocols in the presence of strong background magnetic fields or distortions [17; 18], and a technique for mapping alternating magnetic fields produced by applied AC currents [19]. The latter approach is developed with the goal of providing a type of localized electrical impedance spectroscopy. It was further shown recently that ioMRI and an ultrafast inside-out NMR technique can be used to classify cells based on different types of defects [14; 20]. Superconducting-quantum-interference-device (SQUID)-based magnetic properties measurement systems (MPMS) were used to demonstrate the relation between battery charge state and susceptibility in the lab [21; 22; 23; 24]. An MPMS is usually used to measure the material susceptibility with high precision, and it contains two main parts: a SQUID magnetometer to measure the magnetic moment of the sample and a superconducting magnet to provide the background field.