The new ionic liquid electrolyte has a reversible capacity of 2200mAh/g.

Giovanni B. Appetecchi of the Italian National Institute of New Technologies, Energy and Environment tested the electrochemical properties of binderless Si and Ge nanowire (NW) anodes by properly combining PYR13TFSI and PYR13FSI IL with LiTFSI lithium salt to form an ionic liquid electrolyte. . The Si and Ge nanowire anodes exhibit excellent cycle performance in the ionic liquid electrolyte.

Rechargeable lithium batteries are the preferred energy storage system for many technologies due to their high weight and volumetric energy density. However, requirements for lithium ion battery (LIB) charging capacity and energy density, such as automobiles, renewable energy storage, smart grids, and consumer electronics, are increasing.

Silicon has a maximum specific capacity of 3579 mAh/g, and helium can provide a specific capacity of 1384 mAh/g, making it the most promising candidate for graphite replacement. Commercialized silicon-based and ruthenium-based anodes cause pulverization of active materials due to repeated expansion/contraction during lithiation/delithiation, separation from current collectors and formation of unstable solid electrolyte interface membranes (SEI membranes), resulting in electrolytes Constant consumption, low coulomb efficiency and poor cycle performance.

Ionic liquids (ILs) are salts composed of organic cations and inorganic/organic anions that are in a molten state at room temperature. Moreover, specific operating conditions can be matched by fine-tuning the physicochemical properties of the ILs (by simple modification of the IL structure and/or introduction of functional groups).

In addition, an appropriately combined IL mixture exhibits improved performance. In the past few years, ILs have been extensively studied as a safe electrolyte component to replace the volatile and harmful alkyl carbonates commonly used in commercial lithium ion batteries. In particular, based on N-alkyl-N-methylpyrrolidinium cation (PYR1A)+, bis(trifluoromethanesulfonyl)imide (TFSI)- and bis(fluorosulfonyl)imide (FSI) - Anionic ionic liquids show advantageous properties.

Giovanni B. Appetecchi of the Italian National Institute of New Technologies, Energy and Environment tested the electrochemical properties of binderless Si and Ge nanowire (NW) anodes by properly combining PYR13TFSI and PYR13FSI IL with LiTFSI lithium salt to form an ionic liquid electrolyte. . The Si and Ge nanowire anodes exhibit excellent cycle performance in the ionic liquid electrolyte.

Figure 1. SEM image of nanowire-Cu3Ge seed crystal 锗 nanowire (A) and Sn-seed Si nanowire (B)

Figure 2. CV plot of Li/Ge (A) and Li/Si (B) half cells in 0.1 LiTFSI-0.3 PYR13 TFSI-0.6 PYR13FSI + 5 wt% EC electrolyte, sweep rate: 0.1 mv/s.

The Si and Ge nanowire anodes exhibit excellent cycle performance in the ionic liquid electrolyte. At 0.1 C, the Ge and Si electrodes exhibited high reversible capacities of 1400 and 2200 mAh/g, respectively. These values ​​are very close to the previously reported performance of Ge NW and Si NW negative electrodes in an EC/DEC (1:1 V/V) standard electrolyte of 1 M LiPF6 supplemented with 3 wt% VC.

The Ge and Si electrodes have relatively low first coulombic efficiencies of 65% and 61%, respectively. However, it must be noted that the first coulomb efficiency of the binderless Si or Ge NW anode in conventional commercial electrolytes is reported to be between 60-80%, mainly due to the high level of such nanoscale electrodes. The specific surface area and the large interfacial contact area inherent between the electrolyte and the active material contribute to the formation of a large amount of SEI film. After several cycles, the Coulomb efficiency approached or exceeded 99%.

This indicates that a stable SEI film is formed in the quaternary electrolyte in the initial cycle, so that the active surface is inert to further decomposition of the electrolyte. When the current density reached 1 C, the Ge and Si electrodes remained at 69% and 54% of the capacity at 0.1 C.

The capacity of Si was observed to be higher in IL between 0.1 and 0.5 C compared to the conventional electrolyte. Above 1C, the performance of IL begins to lag behind commercial electrolytes. This may be due to the slower ion diffusion of the more viscous IL compared to the carbonate based solvent. It is worth noting that the more limited diffusion rate in IL becomes apparent only at relatively high current densities (≥1C).

Figure 3. Voltage capacity curves for Li/Ge (A) and Li/Si (B) half cells in 0.1 LiTFSI-0.3 PYR13 TFSI-0.6 PYR13FSI + 5 wt% EC electrolyte. The cycle was carried out at 0.1 C and 20 ° C in the test, while the first cycle was carried out at 0.02 C.

Figure 4. Voltage capacity curves of Li/Ge(A) and Li/Si(B) half cells at different current densities in 0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI +5wt% EC electrolyte.

Figure 5. Charge/discharge capacity (A), coulombic efficiency (B) and reversible capacity of Li/Ge and Li/Si half-cells in 0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI +5wt% EC electrolyte at different current densities Relationship with current density (C).

The Ge and Si NW electrodes retain a reversible capacity of more than 1000 mAh/g after 300 cycles of continuous current at 0.5C current density (100% depth of discharge), with a capacity retention of 86% (Ge) and 57% (Si). It was previously unachievable with any Si-based negative electrode tested in IL electrolytes. This long-term cycling stability is due to the electrolyte formulation promoting the conversion of Ge or Si nanowires into a porous network structure that can withstand extreme volume changes associated with lithiation/delithiation.

Figure 6. Voltage capacity curves of Li/Ge(A) and Li/Si(B) half cells at 0.5C and 20°C in 0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI +5wt% EC electrolyte, (c)Li Cycle performance of /Ge and Li/Si.

为了验证Ge和Si NWs在实际器件中的可行性，锗和硅电极与商业LCO正极极组成全电池，并在IL电解质进行了初步测试。锗和硅负极和LCO正极预先在Li/Ge，Li/Si和Li/LCO半电池中在20℃下以0.1C倍率循环（使用0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI +5wt％EC电解液）。

预氧化后，将半电池拆解，并将Ge，Si和LCO电极组合成全电池。对于Ge/LCO和Si/LCO全电池，在3.5-4.0 V范围内观察到明确的可重复平台，且具有超过1300（vs Ge）和1700（vs Si）mAh/g的相对稳定且高度可逆的容量。即使在最初的20个循环期间观察到容量的衰减，但循环100圈后仍保留1100（Ge/LCO）和1200（Si/LCO）mAh/g的高容量，证明了Ge和Si NW线在全电池中的可行性。

Figure 7. Voltage capacity curve of Ge/LCO and Si/LCO full cells at 20 °C in 0.1LiTFSI-0.3PYR13TFSI-0.6PYR13FSI +5wt% EC electrolyte, current density: 0.5C.