Optimize Electrolytes and Additives for Lithium-Ion Batteries Using C/SiO2 Synthesized from Rice Husk As an Anode Material
Authors: Vu Phat , Phan Thao Nhu , Nguyen Quynh Nhu , Yen Chuong Nguyen Kim , Tran Man V. , Le Phung 
Published: August 2024

Silicon-based anodes derived from agricultural waste, particularly rice husks, have recently been focused on replacing conventional graphite due to its abundance, low cost, fast charge ability, and remarkable theoretical capacity (~ 3600 mAh.g-1). SiO2 has several drawbacks, including low electrical conductivity and short cycling life associated with high-volume expansion (~ 300%) during the charge-discharge process. Hence, a hybrid composite C/SiO2 was an ideal solution to overcome these challenges.

To strengthen the electrochemical properties of LIBs, the compatibility between electrodes and electrolytes plays a crucial role. Lithium hexafluorophosphate (LiPF6) and Lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) are promising candidates for investigation due to their effectiveness cost. Regarding LiPF6, widely used in LIBs, offers high solubility and stability, while LiTFSI provides enhanced ionic mobility. However, LiTFSI is corrosive to aluminum current collectors, and LiPF6 unexpectedly dissociates to form HF acid and PF5, leading to cycling degradation. To mitigate these issues, additives such as Fluoroethylene-carbonate (FEC) and Vinylene Carbonate (VC) are introduced to enhance battery performance. This study employed FEC and VC, adding in the optimal electrolyte to stabilize the SEI layer, prevent lithium loss during cycling due to the instability of the interfacial layer, as well as to enhance high-rate performance. In detail, FEC could effectively prevent SEI degradation by enhancing the F-based component of this layer, stabilizing the SEI layers, and protecting electrodes. Simultaneously, VC offers a synergistic effect on the interphase modulation by reinforcing the inorganic/organic components. Thus, we investigate various electrolyte systems to evaluate half-cell performance of C/SiO2 synthesized from rice husk as the anode. Then, the optimal-chosen electrolytes were used to assemble full-cell LFP||C/SiO2 and investigated at high-rate performance.

According to the experimental sections, there are two main lithium-salts comprised of LiPF6 and LiTFSI, which dissolved in three solvent systems, including EC: DEC (1: 1-v/v); EC: DMC (1:1-v/v), and EC: DMC: DEC (1:1:1 – v/v/v). After optimizing the solvents, the system of EC: DEC (1: 1-v/v) shows the highest performance, which was chosen to investigate the impact of various ratios of FEC and VC additive. The 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC has the highest specific capacity compared to the LiPF6 samples, reaching the value of 220.0 mAh.g-1. Meanwhile, the 1M LiTFSI in EC: DEC (1:1-v/v) 2.5 wt% FEC + 2.5 wt% VC is the best electrolyte to offer a stable capacity of 223.4. mAh.g-1 after 50 cycles in comparison to LiTFSI samples. This phenomenon is plausible with the conductivity of 6.16 and 7.45 mS.cm-1, respectively. Furthermore, they undergo remarkable oxidation at 4.05 and 4.23 V at a current density of 0.002 mA, demonstrating they have a good stability potential window. Since there is no significant difference in half-cell performance, they were used to evaluate full-cell properties. According to battery performance, the full-cell LFP||C/SiO2 using LiPF6 + 5 wt% FEC shows that the initial capacity reaches 111.6 mAh.g-1 and is maintained 40% after 50 cycles with a stable CE above 96%. Full cell LiTFSI +2.5 wt% FEC + 2.5 wt% VC shows an initial discharge capacity of 159.9 mAh.g-1. However, this capacity value is significantly reduced to only 28.5 mAh.g-1 after 50 cycles and CE ~ 94%. Regarding rate capability results, full cell 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC illustrates better performance in all discharged rates than full cells using LiTFSI salt. Moreover, this full-cell shows a discharge capacity of ~ 50 mAh.g-1 at high-rate performance while full-cell using LiTFSI-based electrolyte displays a negligible capacity value. To explain, the total resistance of 25 Ohm is obtained for full cell LiPF6 + 5 wt%. FEC is three times lower than full-cell 1M LiTFSI + 2.5 wt% FEC + 2.5wt% VC. Furthermore, the HR-TEM ex-situ indicates that the stability of SEI layer formation is consistent in thickness but exhibits uneven distribution when using different electrolytes. Besides, the SEI components analyzed by XPS data indicate the presence of carbonate groups and LixPFyOz-based compounds due to the irreversible reactions on the SEI layer after the initial discharge cycle. This research brings some remarkable information to study the electrolyte compatibility for full-cell performance using C/SiO2 as an anode material.

Figure 1