Designing Temperature-Insensitive Solvated Electrolytes for Low-Temperature Lithium Metal Batteries.

Lithium metal batteries face problems from sluggish charge transfer at interfaces, as well as parasitic reactions between lithium metal anodes and electrolytes, due to the strong electronegativity of oxygen donor solvents. These factors constrain the reversibility and kinetics of lithium metal batteries at low temperatures. Here, a nonsolvating cosolvent is applied to weaken the electronegativity of donor oxygen in ether solvents, enabling the participation of anionic donors in the solvation structure of Li+. This strategy significantly accelerates the desolvation process of Li+ and reduces the side effects of solvents on interfacial transport and stability. The designed anion-aggregated electrolyte has a unique temperature-insensitive solvation structure and enables lithium metal anodes to achieve a high average Coulombic efficiency at room temperature and -20 °C. A high-loading LiFePO4||Li cell exhibited high reversibility with a 100% capacity retention after 150 cycles at room temperature, -20, and -40 °C. The practical 1 Ah-level LiFePO4||Li pouch-cell delivered 81% and 61% of the capacity at room temperature when charged and discharged at -20 and -40 °C, respectively. This strategy of constructing temperature-insensitive solvation by electronegativity regulation offers a novel approach for developing electrolytes of low-temperature batteries.

Enhancing Li+ Transport in NMC811||Graphite Lithium-Ion Batteries at Low Temperatures by Using Low-Polarity-Solvent Electrolytes.

LiNix Coy Mnz O2 (x+y+z=1)||graphite lithium-ion battery (LIB) chemistry promises practical applications. However, its low-temperature (≤ -20 °C) performance is poor because the increased resistance encountered by Li+ transport in and across the bulk electrolytes and the electrolyte/electrode interphases induces capacity loss and battery failures. Though tremendous efforts have been made, there is still no effective way to reduce the charge transfer resistance (Rct ) which dominates low-temperature LIBs performance. Herein, we propose a strategy of using low-polarity-solvent electrolytes which have weak interactions between the solvents and the Li+ to reduce Rct , achieving facile Li+ transport at sub-zero temperatures. The exemplary electrolyte enables LiNi0.8 Mn0.1 Co0.1 O2 ||graphite cells to deliver a capacity of ≈113 mAh g-1 (98 % full-cell capacity) at 25 °C and to remain 82 % of their room-temperature capacity at -20 °C without lithium plating at 1/3C. They also retain 84 % of their capacity at -30 °C and 78 % of their capacity at -40 °C and show stable cycling at 50 °C.

High-Energy Aqueous Sodium-Ion Batteries.

Water-in-salt electrolytes (WISE) have largely widened the electrochemical stability window (ESW) of aqueous electrolytes by formation of passivating solid electrolyte interphase (SEI) on anode and also absorption of the hydrophobic anion-rich double layer on cathode. However, the cathodic limiting potential of WISE is still too high for most high-capacity anodes in aqueous sodium-ion batteries (ASIBs), and the cost of WISE is also too high for practical application. Herein, a low-cost 19 m (m: mol kg-1 ) bi-salts WISE with a wide ESW of 2.8 V was designed, where the low-cost 17 m NaClO4 extends the anodic limiting potential to 4.4 V, while the fluorine-containing salt (2 m NaOTF) extends the cathodic limiting potential to 1.6 V by forming the NaF-Na2 O-NaOH SEI on anode. The 19 m NaClO4 -NaOTF-H2 O electrolyte enables a 1.75 V Na3 V2 (PO4 )3 ∥Na3 V2 (PO4 )3 full cell to deliver an appreciable energy density of 70 Wh kg-1 at 1 C with a capacity retention of 87.5 % after 100 cycles.

An Inorganic-Rich Solid Electrolyte Interphase for Advanced Lithium-Metal Batteries in Carbonate Electrolytes.

In carbonate electrolytes, the organic-inorganic solid electrolyte interphase (SEI) formed on the Li-metal anode surface is strongly bonded to Li and experiences the same volume change as Li, thus it undergoes continuous cracking/reformation during plating/stripping cycles. Here, an inorganic-rich SEI is designed on a Li-metal surface to reduce its bonding energy with Li metal by dissolving 4m concentrated LiNO3 in dimethyl sulfoxide (DMSO) as an additive for a fluoroethylene-carbonate (FEC)-based electrolyte. Due to the aggregate structure of NO3 - ions and their participation in the primary Li+ solvation sheath, abundant Li2 O, Li3 N, and LiNx Oy grains are formed in the resulting SEI, in addition to the uniform LiF distribution from the reduction of PF6 - ions. The weak bonding of the SEI (high interface energy) to Li can effectively promote Li diffusion along the SEI/Li interface and prevent Li dendrite penetration into the SEI. As a result, our designed carbonate electrolyte enables a Li anode to achieve a high Li plating/stripping Coulombic efficiency of 99.55 % (1 mA cm-2 , 1.0 mAh cm-2 ) and the electrolyte also enables a Li||LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) full cell (2.5 mAh cm-2 ) to retain 75 % of its initial capacity after 200 cycles with an outstanding CE of 99.83 %.

Solid-State Electrolyte Design for Lithium Dendrite Suppression.

All-solid-state Li metal batteries have attracted extensive attention due to their high safety and high energy density. However, Li dendrite growth in solid-state electrolytes (SSEs) still hinders their application. Current efforts mainly aim to reduce the interfacial resistance, neglecting the intrinsic dendrite-suppression capability of SSEs. Herein, the mechanism for the formation of Li dendrites is investigated, and Li-dendrite-free SSE criteria are reported. To achieve a high dendrite-suppression capability, SSEs should be thermodynamically stable with a high interface energy against Li, and they should have a low electronic conductivity and a high ionic conductivity. A cold-pressed Li3 N-LiF composite is used to validate the Li-dendrite-free design criteria, where the highly ionic conductive Li3 N reduces the Li plating/stripping overpotential, and LiF with high interface energy suppresses dendrites by enhancing the nucleation energy and suppressing the Li penetration into the SSEs. The Li3 N-LiF layer coating on Li3 PS4 SSE achieves a record-high critical current of >6 mA cm-2 even at a high capacity of 6.0 mAh cm-2 . The Coulombic efficiency also reaches a record 99% in 150 cycles. The Li3 N-LiF/Li3 PS4 SSE enables LiCoO2 cathodes to achieve 101.6 mAh g-1 for 50 cycles. The design principle opens a new opportunity to develop high-energy all-solid-state Li metal batteries.

A polymeric composite protective layer for stable Li metal anodes.

Lithium (Li) metal is a promising anode for high-performance secondary lithium batteries with high energy density due to its highest theoretical specific capacity and lowest electrochemical potential among anode materials. However, the dendritic growth and detrimental reactions with electrolyte during Li plating raise safety concerns and lead to premature failure. Herein, we report that a homogeneous nanocomposite protective layer, prepared by uniformly dispersing AlPO4 nanoparticles into the vinylidene fluoride-co-hexafluoropropylene matrix, can effectively prevent dendrite growth and lead to superior cycling performance due to synergistic influence of homogeneous Li plating and electronic insulation of polymeric layer. The results reveal that the protected Li anode is able to sustain repeated Li plating/stripping for > 750 cycles under a high current density of 3 mA cm-2 and a renders a practical specific capacity of 2 mAh cm-2. Moreover, full-cell Li-ion battery is constructed by using LiFePO4 and protected Li as a cathode and anode, respectively, rendering a stable capacity after 400 charge/discharge cycles. The current work presents a promising approach to stabilize Li metal anodes for next-generation Li secondary batteries.

Anion effects on the solvation structure and properties of imide lithium salt-based electrolytes.

The anion effect on Li+ solvation structure and consequent electrochemical and physical properties was studied on the basis of LiFSI-DMC (lithium bisfluorosulfonyl imide-dimethyl carbonate)- and LiTFSI-DMC (lithium bis(trifluoromethanesulfonyl imide)-dimethyl carbonate)-based dilute electrolytes, highly concentrated electrolytes, and localized concentrated electrolytes. With different anions, the electrolytes are different in possible solvation structures and charge distributions, leading to differences in terms of thermal properties, viscosity, ionic conductivity, electrochemical oxidation and reduction behaviors as well as LiNi0.6Mn0.2Co0.2|Li cell performances. The results indicate that the electronic structure of anions contributes greatly to the charge distribution of the Li+ solvation sheath, and consequently extends to the thermodynamics of the carbonate molecules, affecting reduction, oxidation reaction and products on the interface between electrolytes and electrodes. The comprehensive understanding of the solution structure and properties is necessary for the rational design of advanced electrolytes.