The role of carbon bond types on the formation of solid electrolyte interphase on graphite surfaces
Graphical abstract
Different carbon types of the SEI template organic salts contributing to different SEI stability on the natural graphite surface. By comparison, carbon double bond is more desirable for the in-situ development of a robust and flexible SEI layer and the cycling performance of the full cell with LFP cathode is significantly enhanced.
Introduction
Graphite is a dominant negative electrode material of the commercial lithium ion batteries (LIBs). However, the main drawbacks such as the very limited cycle-life and unsatisfactory rate performance greatly hinder the large application of LIBs into electric vehicles (EVs) and energy storage systems (ESSs). To meet the aggressive requirements of long cycle-life and high rate for large scale use, surface modification including surface coating and surface decoration is one of the important strategies for acquiring high performance graphite anode [[1], [2], [3], [4]]. The main functions of the interfacial decoration layer include inhibiting electrolyte decomposition, protecting the electrodes from corrosion, and accommodating electrode volume changes [[5], [6], [7]].
Solid electrolyte interphase (SEI) on graphite surface have been investigated for many years, especially in the advanced electrochemical systems based on lithium concept. The importance of the ultra-thin functional film on the active material surface can never be overemphasized. Generally, SEI is a naturally grown nano-layer on electrode surface via the decomposition of the electrolyte components. For graphite anodes, the composition, texture, conductivity and stability of SEI film play a crucial role on the overall electrochemical performances [[8], [9], [10], [11], [12]]. It must be pointed out that, during lithium insertion and extraction process, more than 10% volume change occurs for graphite particles. To ensure the SEI film is not damaged by the volume change, a good flexibility/ductility of the SEI film is highly required. However, the naturally grown SEI film consists of many inorganic lithium salts such as LiF and Li2CO3, which is always quite brittle. It is a big challenge for the film to stand up with large volume change of graphite particles [[13], [14], [15], [16]]. As the result, mechanical crack of SEI film inevitably brings about continuous growth and rearrangement of the SEI film and leads to constant lithium inventory loss of the cell. In commercial LIBs, it has been proved to be the root cause for the capacity-fading during deep charge-discharge cycles [[17], [18], [19]].
To circumvent the lithium consumption issue, it is very important to build up a robust and flexible SEI film, which is able to take up the volume change of graphite particles during lithium insertion and extraction. Although tremendous endeavour has been made with electrolyte optimizations such as introducing film-forming electrolyte additives, the approach is always less effective enhancing the SEI flexibility. Meanwhile, a considerable impedance rise is always observed due to the precipitation of excessive passivation species on graphite surface, particularly during long-term cycles [[20], [21], [22], [23]]. Physical surface coating or modification, for example, carbon layer coatings [24,25], inert metal oxide coatings with Al2O3 or ZrO2 [26], polymeric coatings [27,28] and conductive Ag or Ni metals [29], is another important approach to extent the life-span of graphite anode. As most of the reported physical coating materials are electrochemically inert, they do not participate in the formation of SEI layer. What is more, the mechanical mismatch between the graphite substrate and the coating layer is another serious problem during the cell operation [30]. By contrast, rare work has been carried out to enhance the mechanics of SEI film with electroactive SEI template materials. There is no doubt, the build-up of a flexible SEI layer to effectively accommodate the volume change of the graphite and thus reduce the lithium consumption is very crucial for developing LIBs of high reliability and durability. Previously, we proposed a novel route for constructing robust and elastic SEI on graphite surface by in-situ polymerization of sodium maleate template on graphite particles [31,32]. The unsaturated carbon bond in maleate turns into radical by accepting an electron and induces polymerization between the maleate monomers. The in-situ grown polymeric skeleton acts as the reinforcing grid for the SEI film, contributing to an increased strength and flexibility of the SEI film. The exquisite design and regulation of SEI film is able to take up the large volume change of graphite aroused from lithium insertion and extraction. However, the effects carbon bond types on the in-situ grown SEI film on graphite anode surface has never been investigated. There is no doubt, an in-depth study regarding the carbon types is of great significance for the selection of new SEI templates for developing high performance graphite anode.
In this work, sodium propionate, sodium acrylate and sodium propynate (the chemical structures for the three salts are shown in Fig. 1) are uniformly applied onto a natural graphite surface. The electrochemical performances of the graphite with single, double and triple carbon bond decoration are compared in both half cells and LiFePO4 based full cells. The results unequivocally show that, with similar chemical structure, the compound with carbon double bonds are superior to those containing the other two carbon bond types. As the result, life-span of the full cell combined with LiFePO4 cathode is significantly prolonged with the acrylate SEI template. It is concluded that constructing SEI film by in-situ polymerization between carbon double bonds is a new and effective approach for graphite electrode optimization.
Section snippets
Samples preparation
A natural graphite (NG, 10–20 μm in diameter) obtained from Beiterui New Energy Materials Co. Ltd. was adopted as the active material in this work. Sodium propionate (SB, single bond), sodium acrylate (DB, double bond) and sodium propiolate (TB, triple bond) (from Aladdin Bio-Chem Technology Co. Ltd) were used as the SEI templates on the graphite surface. As all the sodium salts are soluble in water, surface decoration of the NG was realized in the aqueous solutions. Specifically, with vigorous
Results and discussion
SEM and TEM images of the bare NG and the NG@SB (decorated with 3 wt% sodium propionate, single bond), NG@DB (decorated with 3 wt% sodium acrylate, double bond) and NG@TB (decorated with 3 wt% sodium propiolate, triple bond) are presented in Fig. 2. Compared to the bare graphite, all the decorated graphite particles show smooth and fuzzy surface, indicating the organic salts are dispersed onto the graphite particle surface. The corresponding TEM images clearly show a continuous and uniform
Conclusion
In summary, we investigated the effects of the carbon bond types on the formation of the SEI on graphite surface. Sodium propionate, sodium acrylate and sodium propynate were adopted as the SEI templates on the NG surface. It is found that carbon types have great impact on the electrochemical behavior of the graphite anode. For the organic salts containing different carbon types, double carbon bonds demonstrates the best film-forming properties and the NG@DB electrode exhibits higher rate
Acknowledgments
This work is sponsored by the National Natural Science Foundation of China (NSFC, contract no. 21875154 and 21473120) for the next generation of high performance lithium ion batteries.
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The authors contribute equally to this work.