Influence of Chloride Ion Substitution on Lithium-Ion Conductivity and Electrochemical Stability in a Dual-Halogen Solid-State Electrolyte.

Li+ conducting halide solid-state electrolytes (SEs) are developing as an alternative to contemporary oxide and sulfide SEs for all-solid-state batteries (ASSBs) due to their high ionic conductivity, excellent chemical and electrochemical oxidation stability, and good deformability. However, the instability of halide SEs against the Li anode is still one of the key challenges that need to be addressed. Among halides, fluorides have shown a wider electrochemical stability window due to fluoride's high electronegativity and smaller ionic radius. However, the ionic conductivity of fluoride-based SEs is lower compared to other halide-based SEs. To achieve better interface stability with the Li anode, the presence of fluoride is not only advantageous for a wider potential window but also forms a stable passivation layer at the Li/SEs interface. Therefore, developing mixed halogen-based solid electrolytes, particularly fluorine and chlorine-based SEs are promising in ASSBs. Herein, we report dual halogen-based SEs, Li2ZrF6-xClx (0 ≤ x ≤ 2), synthesized via ball-milling. The X-ray diffraction results revealed that Li2ZrF6-xClx compounds crystallize in the trigonal phase (P3̅1m). Using impedance spectroscopy, an increase in Li+ conductivity with the increase in Cl content was observed for Li2ZrF6-xClx. Compared with x = 0, Li+ conductivity for the sample with x = 1 improved by ∼5 orders of magnitude. The Li+ conductivities for Li2ZrF5Cl1 at 25 and 100 °C are 5.5 × 10-7 and 2.1 × 10-5 S/cm, respectively. Moreover, Li2ZrF5Cl1 exhibits the widest electrochemical stability window and excellent Li interface stability. Our work indicates Li2ZrF6-xClx as an attractive material for optimization in the class of halide-based solid-state Li-ion conductors.

Horizons for Li-Ion Batteries Relevant to Electro-Mobility: High-Specific-Energy Cathodes and Chemically Active Separators.

Li-ion batteries (LIBs) today face the challenge of application in electrified vehicles (xEVs) which require increased energy density, improved abuse tolerance, prolonged life, and low cost. LIB technology can significantly advance through more realistic approaches such as: i) stable high-specific-energy cathodes based on Li1+ x Niy Coz Mnw O2 (NCM) compounds with either Ni-rich (x = 0, y → 1), or Li- and Mn-rich (0.1 < x < 0.2, w > 0.5) compositions, and ii) chemically active separators and binders that mitigate battery performance degradation. While the stability of such cathode materials during cell operation tends to decrease with increasing specific capacity, active material doping and coatings, together with carefully designed cell-formation protocols, can enable both high specific capacities and good long-term stability. It has also been shown that major LIB capacity fading mechanisms can be reduced by multifunctional separators and binders that trap transition metal ions and/or scavenge acid species. Here, recent progress on improving Ni-rich and Mn-rich NCM cathode materials is reviewed, as well as in the search for inexpensive, multifunctional, chemically active separators. A realistic overview regarding some of the most promising approaches to improving the performance of rechargeable batteries for xEV applications is also presented.