Correlation between lithium deposition on graphite electrode and the capacity loss for LiFePO4/graphite cells
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.
Section snippets
Experimental
A group of LFP/graphite 18650 cells (about 40 g weight for each) was supplied by a commercial vendor. All the cells were formed at 0.05C rate for 3 electrochemical cycles between 2.0 and 3.9 V and the real capacity of each cell was obtained. After formation, one of which was used as the reference cell. It was then subject to complete discharge by holding at 2.0 V for 5 h to ensure all the reversible lithium ion return to the LFP positive electrode. Some of the formed cells were subject to
Results and discussion
Fig. 1 shows the cycling performance of three 18650 LFP/graphite cells with different capacity losses (5.7%, 9.2% and 15.8%). The initial discharge capacity at 1C rate was obtained to be 1.028 A h, 1.015 A h and 1.020 A h, respectively. With increasing cycle number, a good linear relationship between the cell capacity and cycle number is observed and the slope of the capacity-fading line seems very identical with each other. These results manifest good consistency between these cells. The small bumps
Conclusion
Long-term cycling behaviors of commercial LFP/graphite cells were investigated. Electrochemical and chemical analyses were conducted to quantitatively correlate lithium deposition on the graphite surface with the capacity loss of the cell. The results reveal that lithium inventory loss accounts for more than 80% of the capacity loss of the cell while more than 90% of the consumed lithium can be found on the graphite surface. The rapid capacity fade of the cells cycled between 2.0 and 3.9 V
Acknowledgements
This work was financially supported by the funding of National Natural Science Foundation of China (NSFC No. 21073129, 51272168 and 21473120).
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These authors contribute equally to this work.