Application of flame retardant localized high concentration electrolyte on silicon based anode
[Preface
Lithium-ion batteries are still constrained by problems such as insufficiently high energy density and poor safety performance. Silicon-based anode materials are regarded as one of the most promising alternatives to the existing commercially available graphitic carbon anode materials. However, silicon-based anode materials suffer from large volume changes during charging and discharging, thus hindering their commercialization, for which researchers have conducted numerous
modification studies and made significant progress. However, there are relatively few reports on the development of new electrolytes suitable for silicon-based anode materials. Dr. Ji-Guang Zhang of Pacific Northwest National Laboratory (PNNL) and others have reported a series of Localized High-Concentration Electrolytes (LHCE) since last year. Electrolytes (LHCEs) can effectively protect the lithium metal surface. Considering the commonality between silicon-based anode and lithium-metal anode protection, localized high-concentration electrolytes have also been used to improve the interfacial structure problem between silicon-based anode materials and electrolyte.
Recently, Dr. Wu Xu, Dr. Ji-Guang Zhang, and Dr. Haiping Jia at PNNL have modified a localized high-concentration electrolyte (1.2 M LiFSI/TEP-BTFE, named NFE-1 in this paper) previously reported by their group to have a flame-retardant effect by replacing a small portion of the electrolyte with a fluorinated ethylene carbonate (FEC). A new electrolyte (1.2 M LiFSI/(TEP-FEC)-BTFE, named NFE-2 in this paper) was modified by replacing a small portion of the flame retardant triethylphosphate (TEP) with fluorinated ethylene carbonate (FEC) to obtain a new electrolyte (1.2 M LiFSI/(TEP-FEC)-BTFE, named NFE-2 in this paper) suitable for silicon-carbon (Si/Gr) cathode, in which the dosage of FEC accounts for only 1.2 wt%. When it is used in Li||Si/Gr half-cells and Si/Gr||NMC full-cells, both of them show excellent cycling stability. The authors also found through mechanistic analysis that replacing TEP with a very small portion of FEC did not affect the solvation structure between LiFSI and TEP and FEC, and the generated SEI (solid electrolyte interface) and CEI (cathode electrolyte interface) membranes were both effective in protecting the corresponding The resulting SEI (solid electrolyte interface) and CEI (cathode electrolyte interface) films can effectively protect the corresponding anode and cathode surfaces, thus ensuring the electrochemical stability of the entire electrolyte for the battery. The paper entitled “High Performance Silicon Anodes Enabled by Nonflammable Localized High Concentration Electrolytes” was recently published in the prestigious international journal “Advanced Energy”. The paper was recently published in the prestigious international journal “Advanced Energy Materials”.
Introduction
In this paper, the authors applied flame retardant localized high concentration electrolyte (LHCE) to SiC anode (commercial BTR1000) for the first time, and further improved the formation and composition of SEI and CEI films by adding trace additive FEC (1.2 wt%). As shown in Fig. 1, the cycle life of Li||Si/Gr half-cells using conventional electrolyte depends on the amount of FEC added, and the half-cells can be stably cycled for 140 cycles when the amount of FEC reaches 10 wt%. In contrast, the Li||Si/Gr half-cell using NFE-2 localized high-concentration electrolyte containing 1.2 wt% FEC maintained 73.4% capacity and high CE (>99%) at 300 cycles. The LHCE electrolyte significantly improves the cycling performance of the half-cells compared to conventional electrolytes. In addition, the expansion of the electrodes in the lithium-embedded state was significantly suppressed.
The authors further prepared a full cell using Si/Gr as the negative electrode and NMC333 (face loading of 1.93 mAh/cm2) as the positive electrode. As seen in Fig. 2, the full cells exhibited significantly better cycling performance than the conventional electrolyte in both NFE-1 and NFE-2. Among them, up to 600 cycles can be stably cycled in NFE-2 containing the additive FEC. In addition, the full cell in NFE-2 also shows excellent high temperature performance, and NFE-2 also enables stable cycling of high loading electrodes for more than 100 cycles.
(b) Long cycle (a) and multiplication performance (b) of Si/Gr||NMC333 full cell in different electrolytes, tested at 25°C. (c-e) Full cell in different electrolytes, tested at 25°C. (f) Full cell in NFE-2 with high temperature. (c-e) Charge-discharge curves of the full cell in different electrolytes. (f) High-temperature test performance of full-cell Si/Gr||NMC333 in different electrolytes at 45 °C. (g) Cycling performance of full-cell Si/Gr||NMC333 with high loading in flame retardant LHCE (NFE-
2).
In addition, bis(2,2,2-trifluoroethyl) ether (BTFE) hardly dissolves lithium salts, so LHCE still retains the properties of the original high salt concentration electrolyte (HCE) (3.2 M LiFSI/TEP), but the salt concentration and viscosity of LHCE can be compared with those of conventional electrolytes.
(a) Nuclear magnetic resonance (NMR) spectra of 17O in different solvents and electrolytes. (b-c) Snapshots of localized highly concentrated electrolytes NFE-1 and NFE-2 in molecular dynamics (AIMD) simulations. (d) and (e) Radial distribution curves of Li-O (Li-OTEP, Li-OFEC, Li-OBTFE and Li-OFSi) in NFE-1 and NFE-2 obtained by AIMD simulation.
Component analysis of SEI on the negative surface and CEI on the positive surface of the whole cell after cycling in different electrolytes (Figs. 4 and 5) further explains the superiority of LHCEs. The content of LiF on the anode surface circulated in NFE-2 is higher than that in NFE-1 and conventional electrolytes.LiF can effectively mitigate the volume expansion effect of silicon to stabilize the silicon particles as well as the stability of silicon electrodes. The CEI film formed on the surface of the positive electrode circulating in LHCEs can effectively inhibit the Ni/Li mixing and thus improve the stability of the positive electrode during the cycling process.
(b-e) STEM-HAADF plots of NMC 333 electrode before cycling (b), full cell after 50 cycles of NMC 333 in E-control (c), NFE-1 (d) and NFE-2 (e). (f-i) Corresponding STEM-ABF plots.
Relaxation time (DRT) analysis maps based on cell impedance data, including contact impedance peaks, two SEI impedance peaks (RSEI), and two charge transfer impedance peaks (Rct). The DRT patterns of half-cell Li||Si/Gr and full-cell Si/Gr||NMC333 with different number of cycles in different electrolytes show a high degree of consistency, which suggests that the negative side of Si/Gr is the main source of impedance in the full-cell. There is no significant change in the RSEI and Rct for both full and half cells after cycling in NFE-2 for different numbers of times, which further suggests that the NFE-2 electrolyte contributes to the formation of high-quality membrane structures at the negative and positive electrode interfaces.
Relaxation time (DRT) analysis of Li||Si/Gr half-cells and Si/Gr||NMC333 full-cells after cycling different numbers of turns in different electrolytes. The relaxation time analysis is based on the impedance data of half-cell and full-cell.
[Summary
This study presents new ideas for the further development of electrolytes that are safe and stable for silicon anodes. The development of silicon-based negative electrodes not only relies on structural improvement, but the electrolyte also plays a crucial role in it. The development of electrolytes with flame retardant properties and in situ formation of stable interfacial structures can further improve the long cycle stability and safety of silicon anode, thus accelerating the development of high energy density batteries.
