Canrd Blog

Creating Avenues for New Research Development

“EES” Zhailing Dengyun and Kang Feiyu’s Team: Degradation of Intercalation Compounds and Evolution of Ether-Based Interfacial Phases in Graphite Anodes During Calendar Aging

In April 2024, the Shenzhen International Graduate School of Tsinghua University published an online paper in the journal Energy & Environmental Science (impact factor >32) titled “Degradation of sodium co-intercalation chemistry and ether-derived interphase on graphite anodes during calendar aging.” This study uses capacity loss and fluctuations in Coulombic efficiency during calendar aging as indicators to reveal the SEI (solid electrolyte interphase) evolution and degradation processes of intercalation products in graphite anodes at different states of charge.

Product Citation

Canrd, empowering your research journey to success! We are honored that Canrd product, “NaPF6,” contributed to the success of this research.

Research Background

Calendar aging is crucial for long-life batteries, referring to capacity loss caused by storage under open-circuit conditions. Extensive research on calendar aging has been conducted on commercial graphite anodes in lithium-ion batteries, concluding that capacity fade due to storage is caused by lithium inventory loss in carbonate electrolytes and the growth of the solid electrolyte interphase (SEI). Additionally, this calendar aging is highly dependent on the state of charge (SOC), with aging typically minimized at partial lithiation states.In sodium-ion batteries, the compatibility of graphite anodes with low-concentration ether electrolytes enables highly reversible co-intercalation involving the solvent, offering excellent rate capabilities due to fast ion diffusion and partial desolvation. However, high-rate performance is often accompanied by severe aging processes, and studies on the calendar life of graphite based on co-intercalation electrochemistry are almost non-existent. Therefore, understanding the calendar aging of graphite anodes in low-concentration ether electrolytes, especially regarding SOC dependence, remains a critical issue.

Capacity loss caused by calendar aging is inevitably related to the degradation of intercalation compounds in charged graphite anodes. In low-concentration ether systems, the ion de/intercalation process of graphite anodes involves significant volume changes (over 300%), which are detrimental to interfacial stability. Additionally, there is ongoing debate about the existence of SEI on graphite surfaces in low-concentration ether electrolytes, as conflicting results have been reported in the literature, such as the absence of SEI in high-resolution transmission electron microscopy (HRTEM) and its capture in atomic force microscopy (AFM). This has limited the study of graphite interfacial stability in ether systems.Today, whether it is SEI dissolution on hard carbon in carbonate-based electrolytes or continuous thickening of the interfacial layer on lithium anodes, the evolution of SEI during calendar aging is directly reflected in capacity or Coulombic efficiency loss. Even in highly efficient reported electrolytes, SEI formation after aging can still be observed with lithium metal growth. Therefore, calendar aging amplifies interphase evolution, making it an effective tool to investigate the presence of SEI on graphite and assess interfacial stability.

Research Question

Diagram 1: Schematic of intercalation compound degradation and associated SEI evolution on graphite at target SOCs (SOC100, a and SOC60, b) during calendar aging.

Figure 1. The impact of aging on the capacity and Coulombic efficiency (CE) of graphite anodes. (a) Voltage curves of graphite anodes stored at SOC60 and SOC100 after 300 hours of aging. (b) Capacity loss of graphite anodes after 300 hours of calendar aging at SOC100 and SOC60. (c) Changes in CE of graphite anodes due to calendar aging at target SOCs. (d-g) The effect of electrolyte volume (d) and operating temperature (f) on capacity loss after 300 hours of aging. Corresponding changes in CE at different electrolyte volumes (e) and operating temperatures (g). (h) The impact of high surface area graphite carbon on capacity loss in relation to storage time.

Figure 2. Phase transformation and volume contraction of graphite anodes during calendar aging. (a-b) Comparison of XRD patterns showing the phase transformation of aged graphite under SOC100 (a) and SOC60 (b) conditions. (c-f) Cross-sectional SEM images of graphite under SOC100 (c), SOC60 (d), aged SOC100 (e), and aged SOC60 (f) conditions. Insets show photographs of the corresponding graphite electrodes. (g-h) Thickness contraction of graphite after aging under SOC100 (g) and SOC60 (h) conditions.

Figure 3. Time-dependent changes in resistance and HRTEM images of SEI on graphite anodes during aging at target SOCs. (a-b) DRT analysis of RSEIR_{SEI} based on Nyquist plots of graphite stored at SOC100 (a) and SOC60 (b) for different durations. (c) Interfacial resistance values from EIS fitting for three separate cells. (d-g) Representative HRTEM images of SEI on graphite under SOC100 (d), SOC60 (e), aged SOC100 (f), and aged SOC60 (g) conditions.

Figure 4. Evolution of SEI components on graphite anodes at different SOCs during calendar aging. (a-d) XPS spectra of C1 (a, c) and F1 (b, d) on the graphite anode. (e-g) Comparison of HNMR spectra of unaged, aged, and post-aging cycled graphite anodes at SOC100 (e), SOC60 (f), and in the original electrolyte (g). The intensities are normalized to the HDO peak and graphite weight. (h) SEI components generated from electrolyte decomposition.

Research Summary

In this work, calendar aging of graphite anodes at target SOCs was quantified using capacity loss and CE changes as the primary indicators. Graphite anodes were stored for 300 hours to investigate the abnormal SOC-dependent aging in the DEGDME system. The results revealed that compared to fully sodiated SOC100, SOC60-aged graphite anodes exhibited more significant CE degradation and capacity loss. This aging behavior was identified as the influence of staged intercalation compound degradation on SEI evolution. During aging, the loss of dissolved Na led to the degradation of staged intercalation chemistry and volume contraction in charged graphite anodes, including the transformation of low-order t-GIC at SOC100 and high-order t-GIC at SOC60. At SOC100, SEI evolution was primarily characterized by growth, evidenced by the thickening of SEI on graphite and the increase of organic components such as alkoxides (CH3ONa and CH3OCH, CH2ONa). Surprisingly, under SOC60 conditions, the aged graphite anodes exhibited interface evolution where growth and destruction coexisted, leading to non-uniform thickness and subsequent SEI repair. SEI fracture was attributed to uneven phase transitions and substantial volume contraction caused by the formation of high-order t-GIC. Therefore, future research could focus on mitigating calendar aging by reducing volume changes and enhancing the reductive stability of electrolytes, particularly by inhibiting the formation of alkoxides.

Core Business

Company introduction

Welcome to Canrd Company (Canrd stands for “Creating Avenues for New Research Development”,website: www.canrd.com ). We specialized in:

  1. Electrochemical experiment consumables, including coin cell cases, cathodes, anodes, electrodes, electrolytes, and various other materials.
  2. Battery cell research and development outsourcing, offering Custom-made electrodes, dry cells, finished battery cells, and battery cell testing.
  3. Test line equipment, such as coin cell equipment lines, pouch cell test lines, cylindrical cell test lines, and more.

We have strong research and development capabilities.If you are interested, please feel free to contact us at any time.
Phone:19867737979
Email:contact@canrd.com
Wechat:contactcanrd

Leave a Reply

Your email address will not be published. Required fields are marked *