Effect of Li2CO3-coating on the performance of natural graphite in Li-ion battery
Introduction
Graphite has been used as an anode material in state-of-the-art Li-ion batteries due to high capacity (339 mAh/g) and low potential (0.1–0.3 V vs. Li+/Li) of its lithium intercalation compound (LixC6, x=1). Reversible intercalation and deintercalation of Li+ ions with graphite are attributed to successful formation of a stable and protecting solid electrolyte interface (SEI) on the graphite surface, which is known to complete in initially few cycles [1], [2]. Previous studies have shown that formation of the SEI is greatly affected by electrolyte composition, morphology, and surface chemistry of graphite [3], [4]. For this reason, only a limited number of graphites have been found to be suitable for the anode of Li-ion batteries. To use natural graphite that is inexpensive and abundant, many researchers have currently focused on the surface modification of natural graphite [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Both chemical and physical approaches have been attempted to modify natural graphite. Chemically, the graphite surface can be modified by mild oxidization, which has been verified to deactivate its catalytic effect on the electrochemical reduction of electrolyte solvents [5], [6]. Physically, morphology and chemistry of graphite surface can be modified by coating a protecting layer such as oxides [7], [8], metals [9], [10], disordered carbons [11], [12], polymers [13], [14], or simply by mechanical milling [15]. It is believed that the coatings preferably covered the active locations such as catalytic sites and particle edges, which therefore reduce solvent reduction and increase cycling stability.
On the other hand, Li2CO3 has been used as an electrolyte additive to improve reversibility and cyclability of graphite in a Li-ion battery [16], [17]. It was reported that the presence of Li2CO3 in electrolytes favors the formation of more compact and conductive SEI. As a result, initial irreversibility and its related gas generation were significantly reduced. Because Li2CO3 is rarely soluble in organic electrolyte, Xue proposed dispersing Li2CO3 powder throughout the electrodes and separator of Li-ion battery in the procedure of making them [18]. This approach was a significant improvement in the storage performance at elevated temperature. In this work, we will use Li2CO3 aqueous solution to modify natural graphite and discuss the effect of Li2CO3-coating on the performance of graphite in Li-ion battery.
Section snippets
Experimental
The electrolyte used was a 1.0 m LiBF4 solution in a 1:1:3 (wt) mixture of propylene carbonate (PC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC). Water content of the electrolyte was determined to be 10–15 ppm by Karl–Fisher titration. Natural graphite (99%, 300 mesh, Alfa Aesar) was coated with Li2CO3 via the procedures described next. Graphite powder was added into a 0.5 wt% Li2CO3 aqueous solution and stirred for about 2 min to ensure that the powder was completely wetted.
Results and discussion
Previous studies have shown that using Li2CO3 as an electrolyte additive can help the formation of a more compact and conductive SEI, which further improves cycling stability of a Li-ion battery [16], [17]. This improvement is attributed to the participation of Li2CO3 into the SEI chemistry. However, such an improvement is limited since solubility of Li2CO3 in organic electrolytes is very low and the dissolved Li2CO3 cannot be fully utilized to form SEI. Therefore, in this work we use Li2CO3
Conclusions
Li2CO3-coating on graphite surface can effectively increase reversibility in the formation of SEI. It is speculated that the pre-coated Li2CO3 participates into components of the subsequently formed SEI. Due to very low solubility of Li2CO3 in organic electrolyte, the resulting SEI shows an improved stability at elevated temperatures. As a result, the Li2CO3-coated graphite shows lower self-delithiation in storage and better capacity retention in long-term cycling.
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