Active electrode materials in lithium-ion batteries are porous. These materials are composed of a solid frame containing interconnected pores/channels, in which mass transport follows a tortuous pathway. Tortuosity is defined as the ratio of the average tortuous pathway to the projected straight path, and is a particularly important parameter indicating the diffusivity and conductivity properties of porous battery materials; so quantifying tortuosity is highly desirable to safeguard battery performance. Existing quantification methods are mostly based on impedance and polarisation measurements from specially-fabricated cells and numerical diffusion simulations using the microstructural tomographic images of porous materials. These methods, however, are generally cumbersome and barely applicable to in-production measurement. Here we report a novel alternative quantification method based on ultrasonic Biot waves in porous materials. This method utilises the fundamental fact that in a porous material with a rigid frame (which the electrode sheets are when immersed in the air), the slow Biot wave only travels in the liquid/gaseous phase, through the same tortuous pathway that the diffusion of ions and electrons happens in a finished battery. To achieve the quantification, we first develop a physical model relating the propagation of ultrasonic Biot waves to the tortuosity of porous battery materials. Based on this physical model, we then propose an inversion method to infer tortuosity from ultrasonic readings. The readings are acquired using air-coupled ultrasonic transducers in a non-destructive fashion, thus enabling real-time tortuosity quantification. This ultrasonic method measures the average tortuosity in a range of ~20 mm across the testing material, and can easily scan over a sheet to provide spatially resolved information. With these exciting features, the proposed method could offer a next-generation tortuosity quantification tool for quality control of porous battery material productions and a better understanding of battery performance.