Ionic conductivity promotion of polymer membranes with oxygen-ion conducting nanowires for rechargeable lithium batteries

Manipulating the ionic conductivity and interfacial compatibility of polymer-in-dual-salt electrolytes enables extended-temperature quasi-solid metal batteries

Solid polymer electrolytes (SPEs) have shown great promise in the development of lithium-metal batteries (LMBs), but SPEs' interfacial instability and limited ionic conductivity still prevent their widespread applications. Herein, high-concentration hybrid dual-salt "polymer-in-salt" electrolytes (HDPEs) through formulation optimization were facilely prepared to simultaneously boost ionic conductivity, improve interfacial compatibility, and ensure a wide-temperature-range operation with high safety. An optimized electrolyte (HDPE-0.6) shows negligible corrosion to the aluminum current collector after manipulating the salt ratio of lithium bis(trifluoromethane)sulfonimide and lithium bis(oxalato)borate. In addition, HDPE-0.6 has excellent ionic conductivity (i.e., ∼0.536, ∼0.898, and ∼1.28 mS cm-1 at 0, 30, and 60 °C), approaching 1 mS cm-1 at room temperature. Furthermore, HDPE-0.6 exhibits a high lithium transference number of 0.6 and a high electrochemical oxidation stability potential of > 4.8 V vs. Li/Li+. Additionally, due to the formulation of high-concentration thermally stable lithium salts and the employment of flame-retardant trimethyl phosphate as the solvent, HDPE-0.6 has no safety issues. The resultant LiFePO4|HDPE-0.6|Li cell exhibits high discharge capacity, good rate capability, and excellent cycle stability at extended temperatures of 0, 30, and 60 °C. By coupling theoretical calculations and in-depth X-ray photoelectron spectroscopy, we attribute the excellent cycle stability to the formation of a stable interphase. Moreover, our formulation strategy is suitable for the Na3V2(PO4)3//Na battery when replacing the lithium salts with sodium salts (i.e., sodium bis(trifluoromethane)sulfonimide and sodium bis(oxalato)borate) to yield HDPE-0.6-Na, as demonstrated by excellent cycle stability (e.g., 98.6 % of capacity retention after 300 cycles). Our work demonstrates that the as-developed quasi-solid HDPEs are suitable for LMBs and sodium-metal batteries, and HDPEs can function normally in a wide temperature range.

Nonflammable, robust and flexible electrolytes enabled by phosphate coupled polymer-polymer for Li-metal batteries

Liquid organic electrolytes commonly employed in commercial Li-ion batteries suffer from safety issues such as flammability and explosions. Replacing liquid electrolytes with nonflammable electrolytes has become increasingly attractive in the development of safe, high-energy Li-metal batteries (LMBs). In this work, nonflammable, robust, and flexible composite polymer-polymer electrolytes (PPEs) were successfully fabricated by flame-retardant solution casting with polyimide (PI) and polyvinylidene fluoride (PVDF). The optimized nonflammable PPEs (e.g., PPE-50) demonstrate not only good mechanical properties (i.e., a high tensile strength of 29.6 MPa with an elongation at break of 87.2%), but also high Li salts dissolubility, the former of which ensures the suppression of Li dendrites, while the latter further improves the ionic conductivity (∼1.86 × 10^-4 S cm^-1 at 30 °C). The resulting symmetric cells (Li|PPE-50|Li) offer excellent Li stripping and plating stability for 1000 h at 0.5 mA cm^-2/0.25 mAh cm^-2 and 600 h at 2.0 mA cm^-2/1.0 mAh cm^-2. In addition, the LiFePO₄|PPE-50|Li half cells show high cycling performance (e.g., a reversible discharge capacity of 135.9 mAh g^-1 after 300 cycles at 1C) and rate capability (e.g., 117.2 mAh g^-1 at 4C). The PPE-50 is also compatible with a high-voltage cathode (e.g., LiNi_0.5Mn_0.3Co_0.2O₂), and the resulting batteries demonstrate long-term cycling stability with a high cut-off voltage of 4.5 V vs. Li/Li⁺. Because of the incorporation of a mechanically robust and thermally stable PI, a polar PVDF, and flame-retardant trimethyl phosphate (TMP) within PPEs, as well as the coordination between Li salts and TMP, and the interaction between Li salts and polymers (especially between Li bis(oxalato)borate) and PI, as well as the bis(oxalato)borate anion and PI), PPEs show great potential for practical and high-energy LMBs without safety concerns.

Well-Dispersed Garnet Crystallites for Applications in Solid-State Li-S Batteries

A uniform ceramic tape of well-dispersed garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ particles (LLZTO) with a high solid content of 70 wt % is prepared as an electrolyte in solid-state Li-S batteries. The use of appropriate dispersants is crucial for achieving fine dispersion and uniform distribution of LLZTO particles in the ceramic tape. This leads to improved surface flatness and mechanical strength of the ceramic tape. Moreover, the ionic conductivity increases remarkably at the same time from 10^-5 to 10^-4 to 10^-4 to 10^-3 S cm^-1, and the Li⁺ transport number doubles from 0.35 to 0.70. The Li-S battery constructed with the dispersed LLZTO electrolyte shows an adequate capacity of above 600 mA h g^-1 after 100 cycles at a discharge current of 84 mA g^-1, and it is capable of charging/discharging at a high current of 1672 mA g^-1. In comparison, the battery with a nondispersed LLZTO electrolyte functions only at the lower current of 84 mA g^-1 and fails to work after 25 cycles.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.