Magnetic Resonance and Magnetometry: Complimentary Tools for Probing Different Size Scales in Lithium-Ion Batteries

Abstract

Lithium-ion batteries (LiB) function because of interconnected chemical and physical reactions across a wide range of size scales—from the overlap of atomic orbitals to flexing of the “lattice” upon lithiation/delithiation to the size/morphology of the particles that make up an electrode film. The cathode electrode in a LiB is based on very high concentrations of transition metals like Fe, Co, and Ni with unique unpaired electron spin environments. Further complexity results from changes to the number of unpaired spins available via redox chemistry, and three-dimensional interactions between spin centers through the lattice. These longer range interactions include ferromagnetic/ferrimagnetic/antiferromagnetic ordering, super-exchange, and the presence of magnetic polarons. Thus, while LiB are commonly viewed first as electrochemical in nature, their magnetic nature is just as important to consider, and their performance and state of health should be interpreted in terms of magnetic changes in the material. We have previously observed fully constructed, commercial 18650 NCA, LCO, and LPO batteries have characteristic magnetic fields up to several hundred micro-Tesla, and this measured field changes in response to different SOH or SOC conditions of the cell. That such a strong magnetic field can be measured is both amazing and very surprising. In this review, we will explore LiB magnetic characterization across all size scales by reflecting on advances in SQUID magnetometry, NMR, EPR, and operando magnetometry. We make a first attempt at answering the question of why there is such a strong magnetic signal to measure on commercial LiB. Understanding the effect of a rich unpaired spin environment across size scales will undoubtedly lead to a better understanding of LiB function and may give insight to improved manufacturing approaches and longer use lifetimes. On the 80th anniversary of Zavoisky’s discovery of EPR, we consider the cathode materials of LiB a “symphony of unpaired electrons” and see that advances in EPR, NMR, and magnetometry are needed now more than ever to understand our technologically complex world.