Chemical Inertness Dominated Intrinsic Safety: Unraveling the “Dissolution–Catalysis–Runaway” Mechanism in Sodium-Ion Battery Cathode Materials

Abstract

While pursuing high energy density in sodium-ion battery cathodes, ensuring intrinsic safety remains challenging. This study establishes a complete evidence chain linking “intrinsic chemical stability-metal dissolution-electrolyte catalytic decomposition-thermal safety” using NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM), Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP), and NaCrO₂ (NCO) as models. We reveal that multivalent ions (Mn³⁺/Fe²⁺) in both NFM and NFPP trigger severe thermal runaway via a “dissolution-catalysis-runaway” cascade, despite their distinct structures. In contrast, NCO leverages the extreme chemical inertness of Cr³⁺ (unique d³ configuration and high Cr–O bond energy) to effectively sever this catalytic pathway, achieving counterintuitive high safety with minimal capacity sacrifice. This work elucidates that chemical inertness, rather than mere structural robustness, governs thermal safety, providing a new paradigm for designing intrinsically safe cathode materials.

The accelerating global energy transition has intensified concerns regarding the supply constraints of lithium-ion batteries, compelling the development of sustainable alternatives for grid-scale energy storage. , Sodium-ion batteries (SIBs), leveraging the abundance and low cost of sodium resources, have emerged as promising candidates. − However, their cathode materials face a critical performance tradeoff. Layered transition metal oxides such as NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) deliver high energy density yet suffer from severe thermal runaway risks, whereas polyanionic compounds like Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) exhibit structural stability but limited capacity. − Prevailing research predominantly attributes these safety failures to crystalline framework instability and attempts to mitigate risks by enhancing structural robustness. , However, this paradigm fundamentally overlooks the decisive role of intrinsic chemical inertness of transition metal (TM) ions in governing thermal runaway behaviors. − Multivalent ions (e.g., Mn³⁺, Fe^2+/3+) can dissolve from the cathode lattice under extreme electrochemical or thermal conditions, subsequently acting as homogeneous catalysts that lower the activation energy for electrolyte decomposition and initiate exothermic cascades. − Relying solely on macroscopic structural integrity therefore provides an incomplete safeguard, as it fails to suppress the microscopic chemical reactivity of dissolved metal species.

To address this critical gap, we propose a paradigm shift from structural stability to chemical inertness as the primary design criterion for intrinsically safe cathodes. This study selects NFM, NFPP, and NaCrO₂ (NCO) as representative model systems to establish, for the first time, a complete evidence chain linking intrinsic chemical stability-metal dissolution-electrolyte catalytic decomposition-device thermal safety. By leveraging the unique d³ electronic configuration and exceptionally high Cr–O bond energy of Cr³⁺ in NCO, we aim to investigate whether extreme chemical inertness can fundamentally disrupt the TM dissolution-catalysis pathway and suppress thermal runaway at its source. This work seeks to demonstrate that cathode safety is governed not merely by crystal structure robustness, but critically by the inherent chemical behavior of constituent transition metals. By quantifying the correlation between ionic inertness and thermal safety boundaries, this research provides essential theoretical guidance for transitioning from empirical structural engineering to rational, chemistry-driven design of high-safety SIB cathodes for large-scale applications.

Figure a–c shows the XRD patterns of the three cathode materials. Both NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) and NaCrO₂ (NCO) belong to the R3̅m space group and exhibit characteristic peaks of the sodium-rich O3-type layered structure. − The corresponding schematic structures appear in Figure d–f. NFM and NCO exhibit typical layered configurations with oxygen atoms in cubic close-packed arrays, forming alternating transition metal–oxygen slabs and sodium layers. Interlayer spacings calculated from the (003) peaks measure 5.36 Å for NFM (2θ ≈ 16.52°) and 5.32 Å for NCO (2θ ≈ 16.64°). In contrast, Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) exhibit an orthorhombic system featuring a robust three-dimensional network of corner-sharing FeO₆ octahedra, PO₄ tetrahedra, and P₂O₇ groups.

1.

Structural and morphological characterization. (a–c) XRD patterns of NFM, NFPP, and NCO. (d–f) Corresponding crystal structure schematics. (g–i) SEM images revealing distinct particle morphologies.

Distinct morphologies emerge from the different synthesis routes (Figure g–i). NFM exhibits a polycrystalline spherical morphology (D ₅₀ = 7.2 μm, BET = 0.75 m²/g). NCO forms blocky, single-crystal-like grains (D ₅₀ = 5.83 μm, BET = 0.58 m²/g). NFPP comprises irregularly shaped particles (D ₅₀ = 8.7 μm, BET = 9.32 m²/g). These structural and morphological disparities establish distinct frameworks for interfacial reactivity and ion transport.

Electrochemical testing employed coin-type half-cells. Figure a,b shows the 0.2 C charge–discharge profiles and dQ/dV curves (1 C = 100 mA g^–1). NFM delivers 135.1 mAh g^–1 between 2.0 and 4.0 V (median voltage 2.97 V), exceeding NFPP (100.5 mAh g^–1, 2.0–3.6 V, 2.95 V) and NCO (127.9 mAh g^–1, 2.0–3.6 V, 3.03 V). The higher capacity of NFM reflects its extended voltage window enabling multielectron redox. NFPP displays the flat voltage profile typical of polyanionic frameworks. NCO balances high voltage with substantial capacity. Rate capability testing spans 0.2 C to 40 C (Figure c). NFM and NFPP collapse at high rates, retaining merely 0.5% and 0.2% of capacity at 40 C. Neither recovers upon returning to 10 C, signaling irreversible structural damage. NCO retains 46.4% capacity at 40 C and recovers 87.9% at 10 C. The robust framework of NCO withstands the mechanical strain of high-rate cycling, unlike NFM and NFPP which undergo severe lattice distortion under polarization. The d³ electronic configuration of Cr³⁺ suppresses Jahn–Teller distortion, preserving structural integrity during rapid Na⁺ extraction. −

2.

Electrochemical performance and structural analysis. (a) GCD profiles and (b) dQ/dV curves at 0.2 C. (c) Rate capability. (d) Cycling performance at 1 C. (e–g) Schematics of d-orbital electron configurations. (h, i) In-situ XRD patterns of NFM and NCO.

Long-term cycling at 1 C (Figure d) shows NFPP and NCO retain 99.3% and 97.1% capacity after 150 cycles. NFM retains only 88.1%. This disparity reflects the d-orbital configurations of the transition metals. NFM (Figure e) contains Jahn–Teller (JT) active ions: high-spin Mn³⁺ (t ₂ g ³ eg ¹), low-spin Ni³⁺ (t ₂ g ⁶ eg ¹), and high-spin Fe⁴⁺. Their coexistence triggers cooperative Jahn–Teller distortion (CJTD), generating internal stress and particle cracking. NFPP and NCO suppress this degradation pathway. , Fe³⁺ in NFPP adopts high-spin d⁵ (t ₂ g ³ eg ², Figure f). Cr³⁺ in NCO adopts d³ (t ₂ g ³, Figure g). Both configurations lack orbital degeneracy. The suppressed JT activity preserves structural integrity during extended cycling.

In-situ XRD tracked structural evolution during cycling (Figure h). Charging NFM shifts the (003) and (006) peaks to lower angles (c-axis expansion) while shifting (101), (012), and (014) to higher angles (ab-plane contraction). This O3→P3 transition involves significant unit cell breathing. NCO undergoes the same phase transition but with markedly smaller peak displacements (Figure i), indicating suppressed volume strain. The minimal lattice breathing in NCO correlates with the JT-inactive d³ configuration of Cr³⁺, contrasting with the cooperative distortion in NFM.

Ah-level pouch cells (NFM || HC, 1 Ah; NFPP || HC, 660 mAh; NCO || HC, 1 Ah) enabled practical validation (Figure S1). At 1 C and room temperature­(Figure a), NFM delivers 118.3 mAh g^–1 between 1.5–4.0 V, exceeding NFPP (73.7 mAh g^–1) and NCO (96.2 mAh g^–1) limited to 3.6 V. The wider window of NFM accommodates higher capacity but compromises longevity: NFM retains only 83.64% capacity after 1000 cycles. NFPP and NCO achieve 97.84% and 92.37%, respectively (Figure b). Energy efficiency (EE, Figure c) unveils interfacial stability. NFM shows severe EE decay (91.52% → 81.15%), signaling rising polarization. NFPP (89.51%) and NCO (85.70%) maintain stable EE, indicating robust electrode–electrolyte interfaces. At 45 °C, all materials show slightly higher initial capacities due to faster kinetics (Figures d–f). After 1000 cycles, NCO retains 94.69% capacity, outperforming NFPP (86.51%) and NFM (83.12%). NFPP suffers accelerated fade at 45 °C relative to room temperature, likely reflecting accelerated Fe dissolution or interfacial degradation. NCO maintains exceptional structural integrity. EE trends mirror room-temperature behavior: NFM decays sharply (93.16% → 83.44%), while NFPP and NCO preserve high efficiencies.

3.

Performance of pouch cells: (a–c) room-temperature and (d–f) 45 °C electrochemical metrics.

Postcycling analysis reveals severe gassing in NFM cells but flat, intact NFPP and NCO cells (Figure S2), consistent with electrochemical data. The electrochemical deterioration of NFM stems from transition metal dissolution and catalytic side reactions. High temperature and voltage drive disproportionation of JT-active Mn³⁺ and Ni³⁺. Dissolved ions migrate to the anode, catalyzing electrolyte decomposition into CO₂ and C₂H₄. , Gas accumulation induces swelling and destabilizes the interface. Anisotropic lattice strain during cycling induces particle cracking, further raising impedance. NCO avoids this degradation despite its layered structure. The d³ configuration of Cr³⁺ suppresses JT distortion and metal dissolution, preserving cycling stability at 45 °C. NFPP utilizes a rigid polyanionic framework that minimizes volume strain and maintains structural reversibility. Its energy efficiency remains the most stable during long-term cycling.

ARC tests on fully charged pouch cells reveal distinct thermal profiles (Figure a). NCO requires 28.4 h to reach 200 °C and 28.9 h to reach maximum temperature (T _max), exceeding NFM (24.1 h, 24.3 h) and NFPP (23.6 h, 28.1 h). NCO delays thermal runaway with the highest trigger threshold. NFM shows the shortest incubation period and the steepest self-heating rate above 200 °C (Figure b). NFPP accelerates between 175 and 210 °C then moderates. NCO maintains stable, gradual heating without runaway signatures (Figures S3, S4). Figure c–e show the independent heating-rate curves. T ₁ (onset of self-heating at 0.02 °C min^–1) values are 96.9 °C (NFM), 112.3 °C (NFPP), and 95.1 °C (NCO). T ₂ (1 °C min^–1) values are 139.5 °C, 143.0 °C, and 141.9 °C. T ₃ (10 °C min^–1 threshold) values are 228.6 °C, 184.7 °C, and 239.2 °C. Thermal runaway initiation resistance: Characterized by T ₁. Here, NFPP performs best (T ₁ = 112.3 °C), followed by NFM (96.9 °C) and NCO (95.1 °C). The slightly lower T ₁ for NCO may reflect intrinsic surface chemistry, but this does not dictate the ultimate thermal fate of the cell. Thermal runaway propagation resistance: Characterized by T ₃ and the time to reach T _max. NCO shows the highest T ₃ (239.2 °C) and the longest time to T _max (28.9 h), indicating it most effectively suppresses the autocatalytic acceleration phase once self-heating begins.

4.

(a,b) ARC test results of pouch cells (4.0 V for NFM; 3.5 V for NFPP/NCO). (c–e) Self-heating rate analysis. (f) Aqua regia. (g) Excess NCO, NFM, and NFPP were separately immersed in aqua regia for 30 min. (h) ICP-MS analysis of transition-metal dissolution after aging charged cathodes in the actual electrolyte.

A chemical stability test simulated extreme thermal runaway conditions. Aqua regia (H⁺/NO₃ ^–/Cl^–) replicated the harsh environment from NaPF₆ hydrolysis [HF and PF₅ (strong Lewis acid)] (Figure f). NFM dissolves vigorously; NFPP partially; NCO minimally (Figure g). This dissolution severity (NFM > NFPP > NCO) matches the thermal runaway severity in ARC tests, confirming intrinsic chemical inertness as a key safety determinant. The ICP-MS results corroborate the transition metal dissolution trend in the actual electrolyte: NFM ≫ NFPP > NCO (Figure h). Despite dynamic phase transitions, NCO maintains exceptional chemical inertness via high Cr–O bond energy, minimizing dissolution. NFM suffers both acid sensitivity and JT-induced lattice instability. Dissolved ions catalyze electrolyte decomposition, accelerating gas generation and thermal runaway. NCO achieves intrinsic safety by suppressing metal dissolution and severing this catalytic pathway. −

TG-DSC isolated the cathode contributions to thermal runaway. Control experiments on pristine cathodes, Super P, hard carbon, PVDF, and their mixtures showed no significant exotherms (Figures S5, S6), confirming component stability in isolation. However, the electrolyte alone triggered pronounced exothermic behavior. TG curves (Figure a,b) show mass loss before 160 °C from EMC and PC codecomposition­(with cathode). The onset temperature for this decomposition follows the order: NCO > NFPP > NFM. DSC (Figure c) records exothermic peaks at 136.3 °C (NFM), 143.2 °C (NFPP), and 150.1 °C (NCO). NFM catalyzes decomposition at the lowest temperature. NCO exhibits the highest onset temperature, which corresponds to a prolonged incubation period observed in the ARC tests. Transition metal dissolution provides homogeneous catalysts that accelerate solvent breakdown. The low solubility of NCO deprives the system of these active sites, suppressing exotherms.

5.

Thermal analysis of cathode–electrolyte interactions. (a–c) TG/DTG/DSC for cathode-electrolyte mixtures. (d–f) TG/DTG/DSC for model compounds. (g) Schematic of the dissolution–catalysis–decomposition cascade mechanism.

Model compounds quantified specific ion effects (Figure d–f). Pure electrolyte (1 M NaPF₆ in EC: PC) loses ∼48.1% mass at 30–110 °C (EMC) and ∼38.5% at 110–153.9 °C (PC), leaving ∼13.4% NaPF₆ residue. Metal oxides uniformly lower decomposition temperatures. MnO₂ and NiO shift final peaks to 122.1 and 125.5 °C, well below the 153.4 °C of pure electrolyte. Fe₂O₃ exerts weaker catalysis at low temperature but induces additional mass loss above 240 °C, indicating NaPF₆ decomposition catalysis. The activity order is Mn⁴⁺ > Ni²⁺ > Fe³⁺ > Cr³⁺.

Integrating the above thermal analysis data with the intrinsic material properties, we propose a comprehensive “Dissolution-Catalysis-Runaway” cascade mechanism (Figure g). Initially, transition metal ions dissolve under thermal stress and form homogeneous catalysts. These ions lower activation energies for solvent and NaPF₆ dissociation (PF₆ ^– → PF₅ + F^–). The strong Lewis acid PF₅ triggers solvent chain reactions, releasing heat and gases (CO₂, C₂H₄) with concurrent HF generation. HF corrodes the cathode, accelerating further metal dissolution. This autocatalytic loop drives thermal runaway. NFM initiates this cascade readily. Jahn–Teller distortion lowers dissolution barriers for Mn and Ni. NCO blocks the pathway. The d³ configuration of Cr³⁺ eliminates JT activity. The elevated Cr–O bond energy suppresses dissolution at the source, severing the catalytic loop.

Figure a compares the three cathodes across seven metrics (Tables S1–S5). The symmetric d³ configuration and high Cr–O bond energy enable NCO to excel in thermal safety and chemical stability without sacrificing rate capability or energy density. Compared with NFPP, NCO achieves comparable thermal safety without accepting the energy density and rate capability penalties typically associated with polyanionic frameworks. This challenges the assumption that safety requires energy compromise. NFM suits portable electronics where energy density outweighs thermal management needs. NFPP fits stationary storage demanding longevity and low cost. NCO targets premium safety-critical applications including aviation, marine/submarine systems, military confined-space energy storage, and premium electric vehicles where thermal safety outweighs absolute cost minimization. (Figure b, Figure S7).

6.

(a) Multidimensional radar chart for performance evaluation. Note: The claim of “minimal sacrifice” in rate capability and energy density is benchmarked against NFPP, highlighting that NCO achieves comparable safety to NFPP without accepting the latter’s energy-density penalty. (b) Analysis of application scenarios based on the cost-performance-safety tradeoff.

This study maps intrinsic chemical stability to thermal safety in sodium-ion batteries, tracing the pathway from metal dissolution to electrolyte decomposition. Cathode safety stems not from crystal structure robustness, but from transition metal inertness. NFM and NFPP, despite structural differences, share susceptibility to multivalent ion dissolution (Mn³⁺/Fe²⁺), triggering dissolution–catalysis–runaway cascades. NaCrO₂ utilizes the d³ configuration and elevated Cr–O bond energy to suppress dissolution, severing the catalytic pathway to achieve intrinsic safety with minimal capacity penalty. This challenges the paradigm equating structural stability with safety, explaining why polyanionic frameworks fail to outperform layered oxides thermally. NaCrO₂ thus dominates safety-critical applications. Future design should prioritize transition metal inertness over structural frameworks, blocking thermal runaway at its origin.

Experimental Section

Including: materials; synthesis of NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM), Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP), and NaCrO₂ (NCO); materials characterization (XRD, SEM, BET, and ICP-MS); cathode electrode preparation; cell assembly and electrochemical measurements (coin cells and pouch cells); accelerated rate calorimetry (ARC) and thermogravimetric-differential scanning calorimetry (TG-DSC) tests; chemical stability evaluation (aqua regia immersion tests); cost analysis and application scenario selection. Details can be found in the Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.6c01259.

• Experimental details: synthesis procedures for NFM, NFPP, and NCO; materials characterization methods (XPS, SEM, DSC); electrochemical measurements for coin cells and pouch cells; ARC and TG-DSC test protocols; chemical stability evaluation (aqua regia immersion tests); cost analysis methods; Figure S1 (photographs of pouch cells and kilogram-scale cathode materials); Figure S2 (postcycling gas swelling photographs); Figure S3 (ARC test results with rigid fixture, self-heating rate analysis); Figure S4 (ARC test results with high-temperature tape fixation); Figure S5 (TG/DTG/DSC thermal analysis of NFM, NFPP, NCO, Super P, and PVDF); Figure S6 (TG/DTG/DSC analysis of cathode-SP, cathode-PVDF, and cathode-HC mixtures); Figure S7 (schematic of cathode material selection rationale); Table S1 (raw material price benchmarks); Table S2 (bill of materials and cost calculation for NFPP); Table S3 (bill of materials and cost calculation for NFM); Table S4 (bill of materials and cost calculation for NCO); Table S5 (comparative cost summary of NFM, NFPP, and NCO) (PDF)

The authors declare no competing financial interest.