Fading Mechanisms and Voltage Hysteresis in FeF₂–NiF₂ Solid Solution Cathodes for Lithium and Lithium‐Ion Batteries

Abstract

The rapid development of ultrahigh-capacity alloying or conversion-type anodes in rechargeable lithium (Li)-ion batteries calls for matching cathodes for next-generation energy storage devices. The high volumetric and gravimetric capacities, low cost, and abundance of iron (Fe) make conversion-type iron fluoride (FeF 2 and FeF 3 )-based cathodes extremely promising candidates for high specific energy cells. Here, the substantial boost in the capacity of FeF 2 achieved with the addition of NiF 2 is reported. A systematic study of a series of FeF 2 -NiF 2 solid solution cathodes with precisely controlled morphology and composition reveals that the presence of Ni may undesirably accelerate capacity fading. Using a powerful combination of state-of-the-art analytical techniques in combination with the density functional theory calculations, fundamental mechanisms responsible for such a behavior are uncovered. The unique insights reported in this study highlight the importance of careful selection of metals and electrolytes for optimizing electrochemical properties of metal fluoride cathodes.

Cyclic Aminosilane‐Based Additive Ensuring Stable Electrode–Electrolyte Interfaces in Li‐Ion Batteries

“…The baseline electrolyte with a relatively high HF concentration formed an SEI with a large fraction of LiF and NiF 2 on the graphite surface (Figure 3e and Figure S7, Supporting Information) because LiF and NiF 2 can be formed by the reaction between HF and Li + (or Ni 2+ ). [ 15 ] In particular, NiF 2 is formed because the high reactivity of the electrolyte causes severe transition‐metal dissolution from the NCM cathode. [ 16 ] For electrolytes retrieved from NCM/graphite full cells before precycling, the Ni 2+ content of the electrolyte without TMS‐ON was 6 times higher than that of the 0.5 wt% TMS‐ON‐containing electrolyte (Figure S8, Supporting Information).…”

Uncharted Waters: Super-Concentrated Electrolytes

“…Thus, while the immediate local environment of cations (Li + or Na + ) still maintains the solvation structure of super-concentrated electrolytes, which is often responsible for the interphasial chemistries at electrode surfaces, the bulk properties (ion transport, viscosity, or wettability toward the electrodes and separators) were mainly defined by the average composition of the bulk electrolyte that still bear the nature of diluted regime. The simultaneous stabilization of the lithium metal and the high capacity and high voltage cathodes might have benefited from the highly fluorinated CEI formed by the partially fluorinated non-solvent and the defluorination of both LiFSI and LiPF 6, [39][40][41] while most of the ''oxidatively weak'' but highly Li + -solvating solvents were kept away due to the coulombic repulsion at the cathode surface. Because the strong salt aggregation is preserved locally in the electrolyte as a non-coordinated diluent is added, the preferential salt reduction that requires such aggregation is also preserved in the diluted regime.…”

3D Honeycomb Architecture Enables a High‐Rate and Long‐Life Iron (III) Fluoride–Lithium Battery

“…However, most of the previously reported metal fluoride particles did not yield uniform particles with a mean size of less than 100 nm and with controlled morphology . Hence, rate capability and cycle life were limited (only 100 cycles achievable in total, small active mass loading of ≈1.0 mg cm −2 or no indication of mass loadings) and these reports were far‐off from the standard of a practical application.…”

“…However, composite design for performance enhancement focused mainly on S‐based rather than on metal fluoride cathodes. At present, metal fluoride materials such as FeF 3 , FeF 2 , CuF 2 , and CoF 2 are facing severe limitations in terms of low reversibility, large voltage hysteresis, rather poor rate capability, detrimental active material dissolution, and poor cycle life . To mitigate these issues, ball milling, particle designs, doping chemistry, and electrolyte modifications have been reported with the target of improving transport and reaction kinetics, stabilizing reaction interface, and suppressing active material dissolution and the related shuttle effects .…”