Analytic Free-Energy Expression for the 2D-Ising Model and Perspectives for Battery Modeling

Although originally developed to describe the magnetic behavior of matter, the Ising model represents one of the most widely used physical models, with applications in almost all scientific areas. Even after 100 years, the model still poses challenges and is the subject of active research. In this work, we address the question of whether it is possible to describe the free energy A of a finite-size 2D-Ising model of arbitrary size, based on a couple of analytically solvable 1D-Ising chains. The presented novel approach is based on rigorous statistical-thermodynamic principles and involves modeling the free energy contribution of an added inter-chain bond $\Delta A_{\mathrm{bond}}(\beta ,N)$ as function of inverse temperature $\beta$ and lattice size N. The identified simple analytic expression for $\Delta A_{\mathrm{bond}}$ is fitted to exact results of a series of finite-size quadratic $N\times N$-systems and enables straightforward and instantaneous calculation of thermodynamic quantities of interest, such as free energy and heat capacity for systems of an arbitrary size. This approach is not only interesting from a fundamental perspective with respect to the possible transfer to a 3D-Ising model, but also from an application-driven viewpoint in the context of (Li-ion) batteries where it could be applied to describe intercalation mechanisms.