Interfacial Evolution and Accelerated Aging Mechanism for LiFePO4/Graphite Pouch Batteries Under Multi-Step Indirect Activation

Abstract

The dissolution of iron from the cathode and electrode/electrolyte interface (EEI) during long cycles significantly accelerates the aging process of LiFePO4 (LFP)/graphite batteries; there is a lack of systematic understanding of the spatial distribution of the EEI interface layer and the dissolve of Fe ions, especially in terms of the mechanism of the cathode–electrolyte interphase (CEI), solid electrolyte interphase (SEI), and iron dissolution. In this study, aged cells were subjected to continuous activation with constant current and multi-step segmented indirect activation (IA) and analyzed for capacity fade, impedance growth, and active Li+ mass loss at the EEI and nanoscale levels. The interaction between dissolved Fe2+ and the EEI in LFP/graphite pouch batteries was proposed and verified. The findings indicate that during IA process, the electric field facilitates the migration of solvated ions toward the electrodes, while simultaneously inhibiting the formation of organic species such as ROCO2Li. The SEI primarily consists of a mixture of organic and inorganic small molecules, forming a continuous and uniform film on the electrode surface. This study demonstrates that IA favors the formation of a uniform EEI and offers constructive insights for advancing accelerated lifetime prediction strategies in lithium-ion batteries.

1. Introduction

Lithium iron phosphate (LiFePO4, LFP) batteries have garnered significant attention in the field of lithium-ion batteries (LIBs) because of their high sustainability, superior thermal stability, extended cycle life, and cost-effectiveness [1–3]. Although LFP demonstrates exceptional structural stability and a high theoretical capacity of 170 mAh g−1, LFP/graphite batteries still suffer from capacity fading, impedance growth, metal dissolution, and material degradation over extended cycling. Additionally, direct contact between the electrode and electrolyte can induce unavoidable parasitic reactions, accompanied by ongoing surface structural reconstruction and the formation of passivation layers [4]. Concurrently, Fe ions irreversibly migrate into the Li layer in a highly attenuated state, resulting in cation mixing and phase transformation. These phenomena collectively compromise battery performance, increase polarization, and ultimately impede the design and scalability of LIBs. Therefore, performance degradation remains one of the foremost challenges to long-term operation, necessitating a comprehensive understanding of the mechanisms driving capacity loss and the development of effective mitigation strategies [5].

The electrolyte–electrode interphase (EEI) film and iron (Fe) dissolution are important incentives for accelerated aging in LFP batteries. Their interaction significantly impacts the battery cycle life, capacity fading, and safety performance, and the failure of the EEI is directly related to the aging of batteries [6]. Under ideal conditions, the EEI film gradually forms during the initial cycle and subsequently exists as a stable passivation layer throughout the entire service life. However, the formation of an EEI film involves a complex multistage electrochemical/chemical redox process, and the composition, structure, and function of EEI films also undergo dynamic changes during cycling. Concurrently, the formation of the EEI film consumes both the electrolyte and the active ions, potentially leading to losses in the accessible energy and/or power density of the battery [7].

The EEI, comprising both the cathode–electrolyte interphase (CEI) and the solid electrolyte interphase (SEI) at the anode, plays a pivotal role in determining the stability of the system, directly affecting electrolyte decomposition and the loss of active Fe [8, 9]. During prolonged lithiation/delithiation cycles, the organic outer layer of the CEI progressively thickens due to the oxidative decomposition of electrolyte components, such as ...