Research Progress of Theoretical Studies on Polarons in Cathode Materials of Lithium-ion Batteries

Abstract

In addition to their extensive commercial application in electronic devices such as cell phones and laptops, lithium-ion batteries (LIBs) are most suitable to fulfill the energy storage requirements of electric vehicles because of their recognized safety, portability, and high energy density. Cathodes are the most important part of LIBs, and various cathode materials have been widely investigated over the past decades. Polaron formation has been attracting increasing attention in the research of cathode materials, as it limits electron conduction. In particular, polarons are responsible for low electronic conductivity in cathode materials like olivine phosphate. Polaron is a typical crystal defect caused by the integrated motion of lattice distortion and its trapping electrons. Research on the mechanism of polaron formation will provide theoretical guidance for the design of high-electronic-conductivity cathode materials and improvement of the electrochemical performance of LIBs. Theoretical calculation is a direct and important method to study polaron formation in a specific crystal material, because the presence of polarons and their formation mechanisms can be effectively verified through this method. In this article, we first introduce the basic physical concept of polarons and their dynamical model according to the Marcus and Emin-Holstein-Austin-Mott theories. A comparison of the general properties of large and small polarons, summarized in this chapter, reveals that small polaron formation more likely occurs in cathode materials. Moreover, the theoretical characterization, electrical impact, control and challenges of polarons are reviewed. Although a universal necessary and suitable condition for the theoretical characterization of polarons has not yet been found, we still propose three criteria that are proven to be feasible and practical for the theoretical identification of polarons when applied in combination. Experimental characterizations are also introduced briefly for reference, because the comparison with the experiment is suggested to be necessary and mandatory. The electrical impact caused by polarons results in low electronic conductivity, which has been broadly reported in layered, olivine, and spinel cathode materials. Doping can weaken the influence of polarons and, thus, significantly enhance the electronic conductivity, thereby becoming the most prevalent strategy for tuning polarons. Although theoretical calculations have been widely and effectively conducted in the study of polarons, some challenges may still be faced because of the intrinsic shortcomings of the traditional density functional theory, which need to be addressed. Finally, further research on polarons from the perspective of basic theory and practical applications is prospected.

Core–Shell Structured C@SiO2 Hollow Spheres Decorated with Nickel Nanoparticles as Anode Materials for Lithium‐Ion Batteries

“…Lithium-ion batteries (LIBs) with high energy density have been regarded as one of the prominent devices to meet the demand of portable electronic devices and electric vehicles. [1][2][3][4][5][6][7] the tensile hoop stress arising from the volume variation. And carbon not only greatly enhances electron transport but also boosts mechanical robustness.…”

Tuning a small electron polaron in FePO4 by P-site or O-site doping based on DFT+U and KMC simulation

“…2(b)) appears near the original Fermi level in all systems, which is a typical feature and a classic criterion to identify a small polaron according to previous studies. 17 The above calculated properties are considered as essential conditions for the current determination of polaron formation. 17…”

“…17 The above calculated properties are considered as essential conditions for the current determination of polaron formation. 17…”

“…In particular, polarons are deemed to account for the low electronic conductivity. 17 The barrier of free small polaron migration of FePO 4 was calculated to be ∼175 meV by Maxisch et al 18 with the GGA+ U calculation method. Ong et al 19 used hybrid density functional theory to investigate the small polaron migration in olivine MnPO 4 and FePO 4 , showing that the energy barrier of free electron polaron migration in the Mn olivine system (196 meV) is 63 meV higher than that in the Fe olivine system.…”

Realizing High-Performance Lithium Storage by Fabricating FeTiO3 Nanoparticle-Impregnated Multichannel Carbon Nanofibers with Promoted Reaction Kinetics

“…With the consumption of traditional fossil fuels and the resulting environmental problems, there is an increasing demand for renewable energy and corresponding sustainable energy storage technologies. Lithium-ion batteries (LIBs), as a predominant electrochemical energy storage technology, have achieved great commercial success in powering portable electronics and electric vehicles owing to their light weight, superior energy density, high power density, as well as long lifespan. − So far, studies on LIBs have been mainly focused on the development of electrode materials. − In terms of anodes, various carbon-based materials have been investigated. Nevertheless, the poor rate ability and comparatively low specific capacity seriously block their practical applications in LIBs. − Thus, exploring high-performance anode materials is crucial.…”