Correlation between lithium deposition on graphite electrode and the capacity loss for LiFePO₄/graphite cells

Highlights

• Lithium deposition on graphite electrode was quantitatively correlated with the capacity loss for a LiFePO₄/ graphite cell

• The growth of solid electrolyte interphase (SEI) is confirmed to be the most important cause for capacity decay.

• Cell voltage doesn’t considerably affect the stability of solid electrolyte interphase (SEI) on the graphite surface.

• Volume change of graphite particles is the important factor responsible for the damage and rearrangement of SEI film.

Abstract

Commercial 18650 LiFePO₄ (LFP)/graphite cells were subject to deep charge-discharge cycling and constant voltage storage tests. The capacity decay of the cells under charge-discharge cycling was correlated with lithium deposition on the graphite surface. The results show that lithium inventory loss is the main cause for the capacity loss and the majority of active lithium loss can be found on the graphite surface due to the growth of the solid electrolyte interphase (SEI). When the cells were held at different voltages for 60 days, negligible capacity loss was obtained compared to that underwent deep charge-discharge cycling, implying that cell voltage is not an important factor affecting the SEI stability and lithium consumption. The damage and repair of SEI film mainly results from the volume change of graphite particle due to lithium insertion and extraction.

Introduction

Lithium ion batteries (LIBs) have emerged as one of the most promising candidates for powering electric vehicles (EVs) due to our increasing demands on renewable resources that could replace fossil fuels [1]. In this respect, long-term cycling capability of LIBs (3000–5000 deep charge-discharge cycles) with 10–15 year calendar life is one of the major challenges. In recent years, extensive efforts have been made to fabricate and optimize electrode materials as these materials are always believed to be the key factors determining the battery performances [2], [3], [4], [5]. However, this is far from enough. As a matter of fact, various physical, chemical and electrochemical processes occurring in the interior of batteries also greatly affect the cell performance, especially in the process of long-term cycling and storage [6], [7], [8], [9], [10].

Although with a relatively lower energy density than 4 V-based LIBs, LiFePO₄ (LFP)/graphite cell has the advantages of high rate capability, high specific capacity, good safety, and low toxicity. Besides, the extremely small lattice change of LFP upon Li+ insertion/extraction endorses it with excellent long-term cycling stability. LFP/graphite cell is of great promise for the application in EVs [11], [12]. Nonetheless, the capacity fade of LFP/graphite cells remains the main problem hindering large-scale application. So far, different mechanisms regarding the capacity decay of LFP/graphite cells have been proposed, including structural damage of the electrode material, impedance rise, loss of active materials and lithium inventory loss. Among these factors, lithium inventory loss is generally considered as the most crucial factor responsible for the capacity decay of LFP/graphite cells. Dubarry et al. [13], [14], [15] attempted to identify the most important factor for the capacity loss and concluded that the loss of lithium inventory loss was the main cause of the capacity fade of a LFP/graphite cell. Strieble et al. [16], [17] discussed the aging mechanisms of a commercial LFP/graphite cell and showed that both the positive and negative electrodes well maintained their initial capacity, except a continuous cycleable lithium loss with increasing cycle number. Liu et al. [18], [19] confirmed that the loss of reversible lithium ion is responsible for most of the capacity loss for a LFP/graphite cell by using differential analysis of the discharge profiles. Deshpande et al. [20] attributed the irreversible capacity loss to the consumption of lithium in the formation and growth of solid electrolyte interphase (SEI) on newly exposed graphite surface upon cycling. We previously investigated the aging phenomenon of commercial 18650 LiFePO₄/graphite cells at different temperatures. The results revealed that most of the consumed lithium could be found on the graphite surface, illustrating that the majority of lithium loss was ascribed to the side reactions at the graphite/electrolyte interface [21].

SEI is a passivation film formed during the cell formation by the deposition of many kinds of Li salts on the negative or positive electrode surface through the decomposition of electrolyte components. It plays a crucial role in determining the electrochemical performances of the electrode. Generally, a good SEI is insoluble in the organic electrolyte, resistant to electron, and allows Li ion to migrate. More importantly, this film can prevent the intercalation of solvated Li ions and suppress further decomposition of the electrolyte components. SEI stability greatly affects the long-term cycling performance and storage property of LIBs. If the passivation layer is not very stable, its continuous growth and rearrangement will not only reduce charge-discharge efficiency in cycling, but also consume the limited active lithium ion within the cell. This will inevitably bring about capacity degradation of the cell due to the shortage of reversible lithium ion. It has been calculated that, if we want to get a Li-ion cell of 2500 cycle life, lithium consumption must be kept within 0.006% for each charge-discharge cycle. Although lithium consumption has been accepted to be the major factor for the capacity fade of LFP/graphite cells, up to today, the way of lithium consumption at the electrode/electrolyte interface and the main factors affecting lithium consumption remain unclear. There is no doubt that deep understanding of lithium consumption within the cell is a key scientific issue in battery technology. However, to the best of our knowledge, quantitative correlation between lithium deposition on the graphite electrode surface and the capacity loss for LFP/graphite cells under different conditions has rarely been reported [21]. The result will be helpful for deep understanding of the evolution of SEI film and the main factors affecting SEI stability.

In this study, commercial 18650 LFP/graphite cells were cycled at 1C rate until different capacity losses (5.7%, 9.2% and 15.8%) were attained. Correlation between the lithium deposition on the graphite electrode and the capacity loss was quantitatively investigated. Meanwhile, three other cells were held at different cell voltages (2 V, 3.45 V and 3.9 V, respectively) for 60 days. It is found that the cell voltage doesn’t considerably influence the SEI stability at the graphite/electrolyte interface. SEI growth and rearrangement is confirmed to be mainly caused by the volume change of graphite particles due to lithiation and delithiation in charge-discharge cycling.