Fe³⁺ Toxicity poses a significant threat to the performance and longevity of LiFePO₄ (LFP)∥Graphite batteries, a cornerstone of electric vehicles and grid energy storage. Celebrated for their low cost, high safety, and impressive reversible capacity, LFP batteries still grapple with cycle life and safety limitations. Among the key culprits is the crosstalk of iron (Fe) ions, which exhibit a stronger tendency to deposit on graphite anodes compared to other transition metals like Mn, Ni, and Co. While most research has focused on ions such as Mn²⁺, the distinct hazards of different Fe valence states—Fe²⁺ and Fe³⁺—remain underexplored. Recent breakthroughs reveal that Fe³⁺ Toxicity far exceeds that of Fe²⁺, disrupting the solid-electrolyte interphase (SEI), altering electrolyte decomposition pathways, and undermining battery performance.
The Hidden Danger of Fe³⁺ Toxicity: Electrochemical Performance Decline
The stark contrast between Fe²⁺ and Fe³⁺ in battery systems becomes evident in electrochemical tests. Initial Coulombic Efficiency (CE)—a critical metric for battery performance—follows a clear hierarchy: base group (no Fe ions) > Fe²⁺-added group > mixed Fe²⁺+Fe³⁺ group > Fe³⁺-added group. Fe³⁺ drastically reduces CE, indicating more irreversible reactions during the first charge-discharge cycle.
After 200 cycles, the Fe³⁺ group exhibits higher polarization (0.19V) than the base group (0.14V), meaning the battery requires more energy to overcome internal resistance. Electrochemical Impedance Spectroscopy (EIS) data reinforces Fe³⁺ Toxicity: both SEI resistance (RSEI) and charge transfer resistance (Rct) increase in the order of base group < Fe²⁺ group < Fe³⁺ group. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) further confirms the severity of Fe³⁺ Toxicity—Fe deposition on the anode reaches 1199.08 ppm in the Fe³⁺ group, nearly double the 593.75 ppm measured in the Fe²⁺ group. This excessive deposition directly contributes to capacity fade: after 300 cycles, the Fe³⁺ group retains only 44.08% of its capacity, compared to 62.50% for the Fe²⁺ group.
How Fe³⁺ Toxicity Degrades the SEI Layer
The SEI layer, a thin film forming on the anode surface, is vital for battery stability—it prevents unwanted electrolyte decomposition while enabling lithium-ion transport. Fe³⁺ Toxicity disrupts the SEI’s structure and composition far more severely than Fe²⁺.
Thickness is a key indicator of SEI health. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and cryogenic Transmission Electron Microscopy (TEM) show that the Fe³⁺ group’s SEI (9.12-17.55 nm) is significantly thicker than the Fe²⁺ group’s (7.20-13.76 nm) and the base group’s (3.15-7.64 nm). A thicker SEI increases resistance, hindering ion transport and accelerating performance decline.
Composition analysis reveals further damage from Fe³⁺ Toxicity. X-ray Photoelectron Spectroscopy (XPS) of the C1s spectrum shows that the Fe²⁺ group has a higher ROCO₂Li content (12.7%) than the base group (4.8%), while the Fe³⁺ group features a drastically higher C-O bond ratio (36.1% vs. 13.2% in the base group). Notably, the Fe³⁺ group’s C-Li signal—critical for Li-ion conduction—disappears entirely, whereas it remains at 4.5% in the Fe²⁺ group and 10.8% in the base group. In the F1s spectrum, Fe ions (especially Fe³⁺) reduce stable LiₓPᵧ content and increase LiₓPOFᵧ, a compound that weakens SEI stability.
Fe deposition morphology also differs: both Fe²⁺ and Fe³⁺ form organometallic complexes (not metallic Fe⁰) and stabilize as Fe²⁺ over time, but Fe³⁺ Toxicity leads to twice the deposition amount. Differential Scanning Calorimetry (DSC) tests confirm that SEI thermal stability follows the order base group > Fe²⁺ group > Fe³⁺ group, highlighting how Fe³⁺ Toxicity compromises battery safety.
Fe³⁺ Toxicity Alters Electrolyte Decomposition Pathways
Fe³⁺ Toxicity doesn’t just damage the SEI—it fundamentally changes how electrolytes break down, introducing harmful byproducts and accelerating battery degradation.
Fe²⁺ enhances conventional single-electron reduction of electrolytes (e.g., decomposition of DMC and FEC), increasing production of H₂, CH₄, and CO₂. In contrast, Fe³⁺ Toxicity drives a more destructive process: its high electronegativity attracts free radicals, and its high coordination number boosts radical combination probability, catalyzing two-electron reduction. This introduces new decomposition pathways, generating toxic gases like CO, C₂H₄, and C₂H₆.
Fe³⁺ Toxicity also modifies the decomposition of LiPF₆, a common electrolyte additive. Instead of forming stable LiF and PF₅, Fe³⁺ promotes the production of more LiₓPOFᵧ—further weakening SEI stability and exacerbating battery degradation. Gas chromatography (GC) tests confirm these differences, with the Fe³⁺-containing electrolyte showing distinct peaks for the new gaseous products.
Practical Implications: Fe³⁺ Toxicity in Real-World Batteries
In practical operating conditions, Fe ions dissolved from the LFP cathode exist primarily as Fe³⁺ in the electrolyte. UV-visible spectroscopy detects a characteristic peak at 226.1 nm in electrolytes from cycled batteries, with peak intensity increasing as cycles accumulate—confirming Fe³⁺ as the dominant and persistent form.
The long-term impact of Fe³⁺ Toxicity is devastating:
- Catalyzed electrolyte decomposition leads to gas accumulation, which degrades electrolyte wettability (ultrasonic imaging confirms severe wettability loss after 1500 cycles).
- Accelerated SEI overgrowth and increased LiₓPOFᵧ content drive continuous rises in RSEI and Rct, worsening polarization.
- Fe organometallic complex deposition damages SEI integrity, triggering ongoing capacity fade.
Mitigating Fe³⁺ Toxicity: A Promising Solution
To combat Fe³⁺ Toxicity, researchers propose preconstructing an artificial SEI—a strategy rooted in creating a robust barrier against Fe ion crosstalk. Formed in a base electrolyte without Fe ion interference, the artificial SEI features a uniform, dense structure composed of stable inorganic-organic composites (e.g., LiF and ROCO₂Li).
This artificial layer acts as a physical shield, inhibiting Fe³⁺ migration and deposition on the anode. By reducing direct contact between Fe³⁺ and the electrolyte, it suppresses Fe³⁺-catalyzed abnormal decomposition reactions, preventing SEI overgrowth and structural damage. This approach offers a viable path to mitigating Fe³⁺ Toxicity and extending LFP battery life.