Ion‐Dipole Chemistry Drives Rapid Evolution of Li Ions Solvation Sheath in Low‐Temperature Li Batteries

Abstract

anode is approaching the theoretical capacity of 372 mAh g −1 and cannot satisfy the growing demand. Moreover, graphite anode undergoes severe capacity degradation and uncontrolled Li plating rather than Li intercalation below 0 °C. [2] Li metal anode, with high theoretical capacity of 3860 mAh g −1 and operational plating/ stripping at low temperatures, has been considered as the most promising candidate for high-energy-density batteries. Nevertheless, practical application of Li anode has been hampered by its uncontrollable dendrite electrodeposition, which lowers the Coulombic efficiency (CE) by reacting with electrolytes to form solid electrolyte interphase (SEI) and deteriorates the structure of Li metal batteries. [3] Unlike graphite anode with stable SEI during charging, the volume expansion of dendrite growth induces rupture of the fragile SEI film during plating/stripping. [4] The fresh exposed Li causes continuous electrolyte decomposition, together with forming thick SEI film. Such SEI has low ion conductivity and high concentration polarization, resulting in uneven Li + flux and uncontrollable dendrite growth. [5] As temperature decreases, the increased ion desolvation barrier further exacerbates the dendrite formation and short-circuiting of the battery. [6] During discharging, the Li + desolvation at cathode increases the polarization and reduces the discharging capacity, especially at low temperature. The ion-dipole chemistry is important for Li metal batteries, because it can directly impact the process of the Li + desolvation and the formation of a passivation layer, resulting in uniform Li deposition and high energy density.The formation of Li + solvation sheath is the competition between cation-anion, cation-dipole, and dipole-dipole interactions. [7] Because the charge density is localized on small Li + (0.09 nm), the Li + -dipole interaction is far stronger than the interaction between Li + and the anion. [8] Hence, most studies focus on reducing the ratio of solvent-to-anion in the Li + solvation sheath or weakening the interaction strength between the Li + and dipole for fast Li + desolvation and subsequent SEI formation. To enhance the participation of anions in the solvation sheath, lithium trifluoroacetate (LiTFA) coordinated with Li + by strong polar groups (CO) has been used to regulate the solvation structure and enable low Li + desolvation energy. [9] Combined with fluoroethylene carbonate (FEC), [10] Sluggish evolution of lithium ions' solvation sheath induces large chargetransfer barriers and high ion diffusion barriers through the passivation layer, resulting in undesirable lithium dendrite formation and capacity loss of lithium batteries, especially at low temperatures. Here, an ion-dipole strategy by regulating the fluorination degree of solvating agents is proposed to accelerate the evolution of the Li + solvation sheath. Ethylene carbonate (EC)-based fluorinated derivatives, fluoroethylene carbonate (FEC) and di-fluoro ethylene carbonate (DFEC) are used as the solvating a...

Ion‐Conductive Polytitanosiloxane Networks Enable a Robust Solid‐Electrolyte Interface for Long‐Cycling Lithium Metal Anodes

“…The electrochemical behavior of symmetric cells with different additives exhibits competitive stability in comparison with other carbonate electrolytes. [26,27,30,[45][46][47][48][49] To visualize the Li deposition process in situ, transparent Li|Li symmetric cells were assembled and monitored using an optical microscope under a high current density of 4.0 mA cm −2 . As shown in Figure 4a, obvious mossy-like protrusions were observed on the surface of the Li metal in the unmodified electrolyte within 5 min, which turned into dendritic Li deposits after plating for 10 min.…”

Hierarchical Sulfide‐Rich Modification Layer on SiO/C Anode for Low‐Temperature Li‐Ion Batteries

“…[3][4][5][6] In addition, the increased Li ions desolvation energy usually leads to formation of Li dendrites on the surface of graphite anode and thus the potential safety hazards. [7][8] Silicon-based materials have 10 times Li ions storage capability than graphite and low reactivity with electrolyte. [9][10] It shows a broad prospect as the next generation anode for LIBs.…”

Structural regulation chemistry of lithium ion solvation for lithium batteries

“…The high dielectric constant of EC makes it have a strong interaction with Li + , leading to a difficult de‐solvation process for Li + at low temperature. Through the fluorination modification of EC molecules, Wang et al 117 elaborated on the influence of polarity on the Li + de‐solvation process at low temperatures. The interaction strength and coordination ratio between EC, FEC, and difluoroethylene carbonate (DFEC) with Li + gradually decreased, as displayed in Figure 8G.…”

“…(H) The capacity of NCM811||Li batteries at 0.2 C from 20 to −30°C. (I) Schematic of the dynamic evolution of Li + solvation structure and different de‐solvated barriers 117 . Reproduced with permission: 2021, John Wiley & Sons 117 …”