NVPF Sodium-Ion Versus NMC and LFP Lithium-Ion Batteries in Thermal Runaway: Vent Gas Composition and Thermal Analysis

Abstract

In this study, cells with three different cell chemistries Na₃V₂(PO₄)₂F₃ (NVPF), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) and LiFePO₄ (LFP) are analyzed in exactly the same setup to compare the hazardous vent gases and their thermal behavior during thermal runaway (TR). Additionally, the influence of different triggers on the failure behavior of NVPF cells is elucidated. The innovative perspective is providing a direct comparison of the three cell chemistries, the influence of the trigger method on the vent gas composition and the thermal behavior. Of the three cell chemistries, LFP releases the least amount of vent gas at 0.02 mol/Ah (41% H₂, 27% CO₂, 8% CO), followed by NVPF at 0.05 mol/Ah (42% CO₂, 17% electrolyte solvent, 15% H₂ and 10% CO) and NMC at 0.07 mol/Ah (36% CO, 24% CO₂, 19% H₂). The maximum vent gas temperature increases from NVPF (265 °C) to LFP (446 °C) and NMC (1050 °C). As for the triggers, overcharge has the highest vent gas production of the NVPF cells at 0.07 mol/Ah. The results offer valuable insight into storage system design and expand the assessment of battery cells.

1. Introduction

Sodium-ion (Na-ion) batteries are becoming increasingly important in current chemical storage research. The reasons for this increase are the limited resources and the rising costs of rare metals of the lithium-ion (Li-ion) counterpart [1]. Furthermore, the demand for cost-effective energy storage solutions, particularly for transportation, continues to grow. Na-ion battery technology is widely recognized as being low-cost, resource-abundant and having a reduced environmental impact compared to Li-ion batteries [2]. Additionally, Na-ion batteries offer enhanced safety compared to state-of-the-art nickel-rich Li-ion systems [3,4].

Research on Na-ion batteries was conducted in the 1970s and 1980s but was disregarded due to the superior performance of Li-ion batteries [5]. The focus is back on Na-ion batteries, as they are considered a promising alternative to Li-ion with regard to energy storage systems. To achieve this, more effort is being put into improving the energy density by adapting the electrodes, foremost the cathode and the electrolyte [3,5,6]. Secondary to the focus on improving energy density is the safety of batteries, because early failure testing shows that this technology is safer compared to Li-ion batteries [2]. Nonetheless, as Li-ion batteries were improved, the safety aspects and particularly failure cases of NMC and LFP were extensively researched and published [7,8,9,10]. Na-ion systems do not show the same depth of research on failure cases [11,12,13,14]. Therefore, to expand the current understanding of possible hazards emanating from these systems, it is imperative to also maintain focus on safety evaluation.

New insights and risks of possible failures of the Na-ion battery need to be documented and updated. In this aspect, it is important that the same setup be used on Na-ion systems and Li-ion systems to ensure the quality of the comparison [15]. While the thermal behavior of Na-ion cells is intensively studied by various groups as the main characteristic of safety [16,17], the production of possibly hazardous gases also represents a safety issue, particularly depending on the use case. Some of the vented gases are toxic, easily flammable and, depending on the concentration, explosive [18]. It has been reported in literature that vent gas is mainly produced due to electrolyte decomposition, cathode and electrolyte reactions and solid electrolyte interface (SEI) decomposition [1,19,20]. The main components of Li-ion battery vent gases are carbon monoxide (CO), hydrogen (H₂), carbon dioxide (CO₂) and preserved electrolyte solvents, typically ethyl methyl carbonate (EMC), ethyl carbonate (EC) or dimethyl carbonate (DMC). Additionally, minor amounts of methane (CH₄), ethene (C₂H₄), butane (C₄H₁₀) and water (H₂O) are present in the mixture [10].

In this study, commercially available prismatic cells with three different cathode chemistries (NVPF, NMC and LFP) are triggered by two-sided overtemperature (OT-2 side) within the same setup to demonstrate the difference in the vent gas and thermal aspect. The uniformity of the experimental environment is crucial, as there are many factors influencing the results of TR experiments [10]. The OT-2 side trigger was chosen because this configuration enables homogeneous heating, which allows a more accurate determination of critical temperatures compared to the one sided overtemperature (OT-1 side) trigger. In order to evaluate the influence of the trigger method on the vent gas mixture, various trigger methods were applied to the prismatic NVPF cells. These methods include OT-1 side, OT-2 side, nail-penetration (Nail), overcharge (OC) and external short-circuit (Short) of the cell. The vent gas was analyzed by Fourier Transform Infrared spectroscopy (FTIR) and gas chromatography (GC).

The reported results are divided into two sections: first, the impact of cell chemistry on vent gas composition and thermal behavior is discussed. Secondly, the effect of different trigger methods on the same cell chemistry is investigated, highlighting their influence on vent gas composition and thermal behavior.

2. Materials and Methods

2.1. Investigated Cells

Three different prismatic hard-case cells with different cell chemistries (NVPF, NMC and LFP) were investigated in separate experiments. In addition to their distinct chemistries, the cells vary in dimensions and capacity, with further specifications provided in

Table 1. Specifications of tested cells.

Parameter	NVPF	NMC	LFP
Design	Prismatic hard case	Prismatic hard case	Prismatic hard case
Cathode	Na3V2(PO4)2F3	LiNi0.6Mn0.2Co0.2O2	LiFePO4
Anode	Hard carbon	Graphite	Graphite
Weight	650 g	960 g	1350 g
Gravimetric energy density	90 Wh/kg	235 Wh/kg	173 Wh/kg
Nominal voltage	3.7 V	3.7 V	3.2 V
Aging state	Fresh, unused	Fresh, unused	Fresh, unused
Start SOC	100%	100%	100%
Cell thickness	26.7 mm	28.0 mm	26.8 mm

. The solvent masses and exact electrolyte compositions were not disclosed to the authors. The cells presented in this study are used in electric vehicles, apart from the NVPF cell, which is used in stationary storage systems.

2.2. Conditions & Trigger

The reactor setup (Figure 1) and the analysis methods for the experiments are described in detail by Essl et al. [15] (pp. 6–10). In addition to the safety-relevant parameters listed in the publication by Essl et al., the parameter normalized vent gas (n_norm) is introduced in this article, referring to the total emitted vent gas (n_total) in relation to the cell capacity (Q_cell):

$${\mathrm{n}}_{\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m}}={\displaystyle \frac{{\mathrm{n}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}{{\mathrm{Q}}_{\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}}}$$

(1)

Deviations from the referenced setup include modifications to the gas transfer system connecting the reactor to the FTIR and GC. It consists of stainless-steel tubing, insulated with silicon foam and preheated to 170 °C prior to gas sampling.

The nail penetration setup was enhanced by incorporating a motor, enabling precise control of parameters such as penetration speed and depth, ensuring reliable and reproducible experimental conditions. In addition, the mechanical fixture was adapted: the NVPF cells were mechanically compressed to 2000 N (142.1 kPa), and the NMC and LFP cells to 3000 N (204.1 kPa and 129.6 kPa, respectively).

Prior to the experiments, the cells were cycled three times at room temperature and ambient pressure with charge/discharge rates of 0.55 C (NVPF), 0.66 C (NMC) and 0.33 C (LFP). The cells were brought to a final state of charge (SOC) of 100%, chosen to represent a worst-case condition for safety testing, as higher SOC levels are known to increase the likelihood and severity of TR [13,14,21]. These initial cycles serve to determine the actual capacity of each cell. At the beginning of the TR experiment, the reactor is kept at an ambient temperature and an N₂ atmosphere of 1.2 bar. The specifications for each trigger are shown in

Table 2. Parameter overview for the different trigger methods used in the experiments.

Triggers	OT-2 Side	OT-1 Side	OC	Nail	Short
Atmosphere	N2	N2	N2	N2	N2
Heat ramp/°C·min−1	2	2	-	-	-
SOC/%	100	100	>100	100	100
Current/A	-	-	+300	-	1000
Cell chemistry	NVPF, NMC, LFP	NVPF	NVPF	NVPF	NVPF

.

The TR is initiated by the trigger methods below.

2.2.1. Overtemperature

The large surface areas of the cell are homogeneously heated by external heater stripes, in combination with underlying copper distribution plates, which ensure effective heat spreading across the cell surface (one large surface area for the OT-1 side and both large surface areas for the OT-2 side). The applied heating protocol is designed to increase the cell surface temperature by ~2 °C min⁻¹.

2.2.2. Overcharge

The cell is charged from a SOC of 100% at a current of 300 A, using an open-ended cut off voltage, until TR occurs. The voltage of the cell immediately before TR is recorded.

2.2.3. Nail Penetration

A nail, driven by a motor, fully punctures the center of the large surface of the cell at a penetration speed of 20 mm/s. The nail used for penetration is made of 42CrMo4 and has a length of 60 mm, a diameter of 3 mm and a tip angle of 60°. The mechanical punctuation causes a rupture of the cell material and induces an internal short circuit.

2.2.4. Short Circuit

The positive and negative cell terminal are connected through a shunt of <1 mΩ, which is connected to a relay inside the reactor. By closing the relay, the two cell terminals are electrically connected, subjecting the cell to a short circuit.

3. Results and Discussion

3.1. Influence of Different Cathode Materials

The influence of the cell chemistry on vent gas composition and thermal behavior is researched using OT-2 side experiments. Detailed specifications of the cell materials—such as cathode, separator or electrolyte composition—were not available to the authors. Since multiple factors have an influence on the vent gas composition [10], the analysis presented is guided by prior experimental evidence with NMC cells, where energy density has been identified as the dominant parameter. Accordingly, the comparative assessment in this article focuses primarily on the cathode chemistry and energy density of the presented cells.

The NVPF cell has the lowest energy density of 89 Wh/kg, which is the reason why the cell shows little structural damage after the TR event, compared to the other two cell chemistries, apart from the burst plate. Additionally, it had the lowest recorded maximum cell surface temperature of 275 °C and maximum vent gas temperature of 265 °C (Figure 2), which is in good accordance with the literature [18]. The NVPF cell releases a total of 0.92 mol of vent gas comprised of small amounts of CO (10%) and H₂ (15%), as shown in Figure 2 and Figure 3. The amount of CO₂ reaches 42%, which is the largest share for the NVPF cell. Additionally, this cell exhibits the highest proportion of electrolyte solvent components in the vent gas, accounting for approximately 15%. A large portion of electrolyte solvent residue is consistent with the observations reported by Bhutia et al. [4]. These findings align with the low maximum temperatures observed during the experiment, as cathode degradation is known to accelerate at higher temperatures. Elevated thermal conditions promote increased oxygen release from the cathode material, which in turn facilitates the combustion of a greater proportion of the electrolyte solvent [22]. In this experiment, 135.36 ppm of HF was recorded. Bordes et al. [18] detected HF in their overtemperature experiment using the same cell chemistry, suggesting that approximately one-third of the fluorine from the electrolyte is converted to HF. Analyzing NMC cells in the same experimental setup, Essl et al. [10] suggests that the high reactivity of HF causes it to react with ejected particles and with components in the analysis pathway. Consequently, only smaller amounts of HF are recorded by the FTIR spectrometer in this setup than presumably produced by the failing cell.

The NMC cell has a gravimetric density of 230 Wh/kg [15], and the maximum cell surface temperatures reach 800 °C, as found by Yuan et al. [23] and Shen et al. [24]. The maximum vent gas temperature reaches 1000 °C, which is in accordance with the findings of Amano et al. [16]. The NMC cell releases a total of 4.21 mol of vent gas, with CO constituting the largest fraction, accounting for 36% of the vent gas composition. H₂ represents 19% of the total vent gas released. These gases pose safety and health risks, depending on the surrounding environmental conditions [25,26]. CO₂ comprises 24% of the vent gas composition. HF is not detected, likely due to its reaction within the reactor or the analysis pathway.

The LFP cell releases significantly less vent gas per Ah than the NMC cell, attributed to milder failure conditions and a more stable cathode [27]. The maximum cell surface temperature reaches 480 °C, while the vent gas temperature peaks at 446 °C, similar to the findings of Schöberl et al. [27] and Golubkov et al. [9]. The experiment results in a total vent gas release of 1.59 mol, with the major components being CO at 8%, CO₂ at 27% and H₂ at 41%. No HF is detected in this experiment.

When comparing the major vent gas components relative to battery capacity, the NMC cell exhibits the highest H₂ release at 13.2 mmol/Ah, followed by the LFP cell at 9.1 mmol/Ah, and the NVPF cell at 7.7 mmol/Ah. The LFP cell demonstrates the lowest emission of toxic CO at 1.8 mmol/Ah, whereas the NVPF and NMC cells release 5.1 mmol/Ah and 25.3 mmol/Ah, respectively. CO₂ emissions are observed across all three cell chemistries, with the LFP cell exhibiting the lowest release at 6 mmol/Ah, followed by the NMC cell at 16.8 mmol/Ah and the NVPF cell producing the highest amount at 21.5 mmol/Ah. Regarding the residual DMC electrolyte solvent, recorded values are 0.9 mmol/Ah for the LFP cell, 2.1 mmol/Ah for the NMC cell, and 7.7 mmol/Ah for the NVPF cell. A detailed list of gas components in the vent gas of the OT-2 side trigger is given in

Table 3. Amount of vent gas produced in mmol/Ah for NVPF, NMC and LFP cell chemistry. Each cell is triggered by the OT-2 side, with a heat ramp of 2 °C/min. The accuracy of the measurement is listed by Essl et al. [10] (p. 10).

Gases	NVPF /mmol/Ah	NMC /mmol/Ah	LFP /mmol/Ah
H2	7.7	13.2	9.1
CH4	1.0	5.2	1.1
CO	5.1	25.3	1.8
CO2	21.5	16.8	6.0
C2H4	3.1	3.5	0.7
C2H6	0.1	0.3	0.2
C2H2	0.0	0.3	0.0
C3H6	0.0	0.0	0.0
C3H8	0.0	0.0	0.0
H2O	0.4	2.1	1.1
DEC	0.0	0.0	0.0
DMC	7.7	2.1	0.9
EMC	0.0	0.0	0.0
C6H14	0.0	0.2	0.1
C4H10	2.6	1.3	0.9
HF	0.0	0.0	0.0

.

The lower and upper flammability limits (LFL, UFL) calculated using the improved Le Chatelier method [28] highlight the risk window in which a gas–air mixture is flammable or explosive if an ignition source is present. This window is between 7% and 28% for the vent gas mixture of the NVPF cell. The Li-ion cells have a calculated flammability window of 7% to 46% and 6% to 45% for the NMC and LFP chemistry, respectively. The LFL and UFL values of the individual gas components, as listed in Table S2, were utilized for the calculation. These results show that the risks posed by the vent gas mixture of the NVPF cells is reduced compared to those of the Li-ion cells, which is consistent with the findings of Baird et al. [29]. Even at low vent gas production rates, the explosion risk of the gas mixture can remain significant, as H₂ constitutes a major fraction of the composition [24,30,31], as shown by the risk window of the LFP cell. Even below the LFL, vent gases pose toxicity risks, foremost from CO. Regarding the thermal assessment, the NVPF cell exhibits the lowest cell surface temperatures, indicating a reduced risk of thermal propagation compared to the NMC cells.

3.2. Influence of TR Triggers

The most common TR triggers are applied to NVPF cells under identical experimental conditions to assess their impact on vent gas composition and thermal characteristics.

The overtemperature triggers (OT-2 side and OT-1 side) produce nearly identical amounts of vent gas, measuring 0.92 mol and 0.91 mol, respectively. The highest cell surface temperature is also in a similar range, at 305 °C for the OT-1 side, followed by the OT-2 side, at 275 °C. The vent gas temperatures spread more, with peaks at 265 °C and 199 °C for the OT-2 side and the OT-1 side trigger, respectively.

The overcharge trigger produces the most amount of vent gas, 1.23 mol, as shown in Figure 4. This can be explained by the additional energy input into the cell (7.6 V just before TR), as elucidated by Essl et al. [15]. It has been suggested that overcharge triggers induce increased side reactions, such as electrolyte reduction and SEI breakdown, leading to a higher release of vent gas [32]. Analyzing the vent gas composition (Figure 5), this trigger exhibits the highest recorded H₂ (23%) concentration, resulting from the side reactions, thereby increasing the explosion risk of the mixture. The maximum cell surface temperature reached 131 °C, and the peak vent gas temperature reached 259 °C.

The nail-triggered cell releases 0.76 mol of vent gas. It has a maximum cell surface temperature of 169 °C and a peak vent gas temperature of 162 °C. These temperature maxima remain significantly below the OT experiments because no external energy input was introduced via the mechanical triggering mechanism.

The lowest amount of vent gas is measured for the externally shorted-circuited cell, at 0.14 mol. The peak vent gas and cell surface temperatures are the lowest recorded, at 94 °C and 106 °C, respectively.

The milder TR conditions of the nail penetration and external short trigger, in comparison to the other triggers, results from lower or no additional energy input into the cell. As a consequence, not all decomposition reactions take place [17,33], resulting in lower vent gas production.

In the NVPF experiments, CO₂ was identified as the predominant component of the vent gas, comprising between 39% and 49% of the total vent gas composition. CO and H₂ were present in smaller proportions, ranging from 3% to 12% and from 2% to 23%, respectively. The total quantity of vent gas released is influenced by the magnitude of the energy input, and thus by the specific triggering method employed, as illustrated in Figure 4. Accordingly, the overcharge trigger resulted in the highest overall volume of vent gas released, along with the greatest relative concentrations of CO and H₂, compared to those observed for the other trigger types.

4. Conclusions

Different cell chemistries and different trigger methods have an influence on the failure behavior, thermal behavior and specifically the produced vent gases. The main results of the comparison of the NVPF, NMC and LFP cells triggered by the OT-2 side are listed below.

• The NVPF cell with an energy density of 90 Wh/kg experiences the least destructive TR: it releases 0.05 mol/Ah of vent gas with a maximum vent gas temperature of 265 °C. Compositionally CO₂, DMC, H₂, CO, butane, ethene, methane and O₂ are measured at >2%. The largest share is CO₂, at 42%. Smaller amounts of H₂ (15%) and CO (10%) were detected.
• The NMC cell has the highest energy density of 230 Wh/kg and showed the most destructive TR: the vent gas amounts to 0.07 mol/Ah and a maximum vent gas temperature of up to 1000 °C. Besides CO₂ (24%), two prominent vent gas components are CO (36%) and H₂ (19%).
• The LFP cell releases 0.02 mol/Ah, with an energy density of 173 Wh/kg. The vent gas contains H₂ as the major gas component at 41%, followed by CO₂ (27%) and a minor share of CO (8%). The vent gas temperature reaches a maximum of 446 °C.

Additionally, the trigger method influences the failure behavior and therefore changes the composition of the vent gas, as shown in the NVPF cells.

• Overcharging the cell results in the highest amount of vent gas produced, at 1.23 mol (0.07 mol/Ah). It contains the highest share of H₂ (23%) compared to the other triggers.
• The OT-1 and OT-2 side experiments resulted in 0.91 mol and 0.92 mol vent gas.
• The lowest amount of vent gas was produced by the externally short-circuited cell, at 0.14 mol.
• HF was solely recorded for the OT-2 side trigger (135.36 ppm); it is assumed that small amounts of HF were also produced for all other investigated cells and triggers, but it reacted with the ejected particles in the reactor and the analysis pathway.
• The highest vent gas temperature was recorded with the OT-2 side trigger (265 °C) and the lowest with the short trigger (94 °C). A maximum vent gas temperature of 259 °C was recorded for the OC trigger. The OT-1 side and nail-triggered cell reached maximum values of 199 °C and 162 °C.

Insights into vent gases, critical temperatures and their associated dangers provide a scientific basis for the development of safety guidelines for stationary energy storage systems. Identifying both the composition and concentration of vent gases emitted by different cell chemistries during cell failure enables a more accurate assessment of the risks posed to occupants, first responders and the surrounding environment. This information may directly inform requirements for ventilation design, setback distances and protective equipment specifications. Furthermore, the availability of such data facilitates evidence-based emergency response planning and risk communication, therefore strengthening public trust in small- and large-scale energy storage deployment.