Wen | Xiaoxiao
Photo: Xiaoxiao
Silicon (Si) is the most promising anode material for LIB and can replace the most advanced graphite anode because it provides a higher theoretical volume and weight capacity compared to graphite (759 mAh cm). In addition, silicon is the second most abundant element on Earth, making it economically attractive for battery applications.
On the other hand, silicon suffers from a huge volume expansion (≈300%) during (de-lithiation), due to the cracking and crushing of the particles of the active material, the loss of electronic contact, thus limiting the application of pure silicon in LIBs, resulting in significant capacity decay. In addition, continuous fracture and re-formation of the solid electrolyte interface (SEI) in each cycle results in significant loss of electrolyte and active lithium.
In order to overcome these challenges, great efforts have been attempted recently. Small amounts of Si (up to ≈3-8wt%; Often present in the form of SiO, x) embedding in graphite substrates with effective binders is a promising method for enhancing electrochemical performance. Alternative additive compounds, such as dioxocycloone derivatives, have been proposed for future LIBs, but only a few for silicon-based electrodes.
Nölle et al. have reported the use of non-fluorinated additive compounds. Raise NCM111 ||The Si full battery uses 5-methyl-1,3-dioxolane-2,4-dione (lacOCA), a compound with two functional SEI active moieties. This newly designed additive blend improves electrochemical performance in terms of capacity retention and C, by forming carbon monoxide and poly(lactic acid) as thin polymer layers on the surface of SEI.
【Results and Discussion】
1. Electrochemical behavior of electrolyte formulations containing HFPN derivatives
To investigate the role of HFPN derivatives in different electrolyte formulations, ||The silica X/C multi-layer bag battery was charged and discharged 700 times, and the electrochemical parameters were systematically analyzed. The specific capacity of all batteries containing different electrolyte formulations is normalized to the capacity of the first discharge cycle.
It can be seen that at the beginning of the experiment, all cells have a strong capacity decay. This capacity decay is expected to be mainly caused by the significant volume expansion of the silicon-based anode in NCM523Silica x/C battery that accelerates particle cracking, parasitic side reactions, and continuous consumption of alkaline electrolytes, electrolyte additives, and reactive lithium from NCM523.
Cells using STD electrolytes undergo continuous decay and reach a 70% healthy state (SOH70%) after only 170 cycles. Batteries that use only HFPN derivatives as electrolyte additives perform better than STD electrolyte batteries. Cells containing the electrolyte preparation STD-EtPFPN reached 70% of their SOH after 236 cycles, followed by STD-PhPFPN after 226 cycles and STD-HFPN after 217 cycles.
Cells containing STD-FEC as an electrolyte formulation extended SOH by 70% to 333 cycles and improved long-term performance by converting SOH by 70% compared to STD mixture without electrolyte additives 163 cycles. Cells containing dual additive electrolytes, namely STD-FEC-PhPFPN, STD-FEC-EtPFPN, and STD-FEC-HFPN, are capable of transferring SOHs by 70% with even higher cycles than STD-FEC electrolyte mixtures.
In a two-additive electrolyte formulation, similar capacity decay with a single-additive electrolyte mixture (STD-FEC) can be assumed. However, according to our electrochemical results, the two-additive electrolyte mixture has different fading processes. Thus, HFPN derivatives can observe a positive effect on single-additive electrolyte formulations STD-FEC.
The combination of HFPN derivatives with FEC may be responsible for the improved electrochemical performance in these dual-additive electrolyte formulations.
2. Effect of HFPN derivatives on gas evolution behavior and electrode membrane formation of NCM523 ||Silica X/C cells
Outgassing behavior of NCM523 ||Silica X investigated the gas inhibition properties of /C bag batteries using state-of-the-art electrolytes and cyclic phosphazene fluorinated compounds as additives. To evaluate the effect of fluorinated cyclic phosphazene compounds as additives on gassing behavior, NCM523 ||Silica x/C pouch battery (≈ 200 mAh battery capacity) is used with a practical electrolyte-to-battery capacity ratio of ≈4.5 g Ah.
The volume plot shows gas formation of different electrolyte compositions after SEI formation, after 720 h of storage at 60 °C, and after 700 charge/discharge cycles. Two-additive electrolyte mixtures show better gas reduction trends than single-additive electrolyte mixtures, especially for long-term performance.
The results of the dual-additive electrolyte after 700 cycles are particularly outstanding. The STD-FEC-EtPFPN mixture (22.0 mL) and STD-FEC-HFPN mixture (22.0 mL) were almost halved after 38 cycles compared to the STD-FEC formulation (700.0 mL). Outgassing results after storage at 60 °C showed a slight decrease in STD-FEC-EtPFPN mix and STD-FEC-HFPN mixture (0.25 mL) compared to STD-FEC mixture (0.21 mL).
It is known from the literature that phosphazene fluorinated compounds are commonly used as flame retardants and tend to have lower saturated vapor pressures than traditional electrolyte solvents (EC, DEC, DMC, EMC). Our results show that phosphazene fluoride compounds stabilize the electrolyte mixture STD-FEC when added as an additional additive and therefore appear to be effective in stabilizing conventional electrolyte solvents at high temperatures (60°C) and during long-term cycling.
Subsequently, this leads to a decrease in electrolyte decomposition, a decrease in parasitic side reactions, and a decrease in gas formation. Comparison of the two-additive electrolytes STD-FEC-EtPFPN (0.06 mL) and STD-FEC-HFPN (0.14 mL) with the single-additive electrolyte STD-FEC (0.07 mL) compared to storage experiments and gas formation after long-term cycling showed similar gassing results after SEI formation.
Based on these gas measurements, it can be assumed that FEC stabilizes the electrolyte during the initial phase of SEI formation. Therefore, gas measurements after SEI formation are very similar between STD-FEC and two dual-additive electrolyte mixtures. Comparing STD-FEC with gas measurements of a two-additive mixture after long-term performance and storage experiments shows that FEC as an additive cannot stabilize the electrolyte during long-term storage and long-term cycling.
In contrast, STD-FEC-HFPN mixtures and STD-FEC-EtPFPN mixtures can stabilize electrolytes more effectively during SEI formation and long-term cycling, as well as at high temperatures; Thus, FEC/EtPFPN and FEC/HFPN complement each other in double addition. Outgassing during SEI formation is attributed to electrolyte decomposition. Among EC/EMC-based electrolytes, CO, C2H4, and C2H6 are primarily detected as gases produced and can be mainly attributed to EC solvent decomposition.
Based on these results in the literature, it is concluded that the decomposition of EC occurs mainly during the initial charging process. Based on the gas measurements shown in the experiments, it can be assumed that electrolyte formulations containing FEC as additive compounds reduce EC/EMC decomposition during SEI formation.
To better understand electrolyte decomposition during SEI formation, the differential capacity (dQ/d V) of NCM523 vs. battery voltage/negative potential||Silica x plots up to 3.5 V for the first cycle of the /C bag cell.
3. Extopic surface study of SiO x/C anode surface
The surface of the negative electrode was studied by SEM and XPS to elucidate the difference in anode membrane formation between the STD-FEC and the STD-FEC-PhPFPN electrolyte mixture, which shows the greatest difference in the (∆V) plot and, therefore, enables better visualization of morphological changes.
In addition, XPS measurements were able to demonstrate the decomposition of PhPFPN on the negative electrode. In particular, silicon-containing anode materials have been reported to consume FEC intensively to form SEI rather than EC and other electrolytes. Thus, SiO surface x evaluated the /C composite electrode at the original and different stages of aging cells, elucidating the SEI formation capacity and SEI composition of the STD-FEC blend and the STD-FEC-PhPFPN blend.
SEM image of the negative electrode surface before and after SEI formation of two different electrolyte mixtures and after 200 cycles. In the SEM image, both SEI formation and particle cracking of both SiOx electrolyte mixtures can be clearly detected. The diagram of the original electrode shows the smooth surface x particles of SiO embedded in graphite (red area, EDX) with some graphite traces on the surface of SiO x particles.
Interestingly, different morphological features of the SEI structure of the two electrolyte formulations can be distinguished. The SEI structure of STD-FEC-PhPFPN mixtures is relatively smooth compared to the SEI structure of STD-FEC-PhPFPN mixtures.
4. Electrolyte analysis by gas chromatography-mass spectrometry
To analyze the decomposition/aging products of the electrolyte at the electrode-electrolyte interface, gas chromatography-mass spectrometry (GC-MS) measurements were performed. As mentioned above, it is assumed that HFPN derivatives decompose on the negative electrode surface together with other electrolyte substances (FEC, EC, EMC) to form SEI.
In addition, HFPN derivatives are able to stabilize the electrolyte to higher temperatures. To indirectly prove the hypothesis, the decomposition products after SEI formation and after 700 cycles were analyzed. STD-PhPFPN additive blends are used as model compounds for HFPN derivatives and related decomposition/aging products.
As circumstantial evidence, one can expect transesterification and oligomerization products resulting from EC decomposition in electrolytes during SEI formation. These transesterification and oligomerization products ('OHC) for EC/EMC systems are referred to in the literature as DMC, DEC, ethylmethyl-2,5-dioxahexanecarboxylate (EMDOHC), dimethyl-2,5-dioxanecarboxylate (DMDOHC), and diethyl-2,5-dioxanecarboxylate (DEDOHC).
These substances are associated with parasitic reactions, leading to deterioration of electrochemical performance and an increase in impedance growth. The decomposition of each electrolyte mixture can be observed after SEI formation. Compared to STD electrolyte mixtures, STD-FEC mixtures and STD-PhPFPNs significantly reduce the amount of DMC and DEC, and both mention "OHC".
These results are in good agreement with the observations discussed in the literature regarding anode film-forming additives and cycling-time transesterification and oligomerization product formation. In addition, a decrease in the production of lithium alcohol salts on the negative electrode surface can be confirmed, which is related to the formation of transesterification products such as DEC. Thus, reduced DEC formation means an indirect reduction of the appearance of Li-alkoxide. With GC-MS measurements, the breakdown products of PhPFPN after SEI formation can also be shown.
To better account for the long-term aging products of the STD-PhPFPN mixture, the electrolyte after 700 cycles was also studied by GC-MS. PhPFPN is completely consumed after 700 cycles. As a result, EC decomposition increases, resulting in an increase in transesterification products (e.g., DEC) and oligomeric products (e.g., 'OHC). In conclusion, we can assume that HFPN derivatives have similar effects on aging products such as FEC.
【Experimental part】
1. Pouch battery settings
Both sides of the electrode are coated, except for a small area on the side of the end of the foil. The bag cells were pre-dried (< 10−3 bar) in an oven at 80 °C under reduced pressure for 24 h in a drying chamber (dew point, -80 °C; <0.55ppm water). After the pre-drying process, cells are filled with 0.75 mL (≈ 0.90 g ±1%) electrolyte and hot-rolled for 87 sec at 5 °C at relative pressure using a vacuum sealer. Use a custom battery holder with repeatable pressure (200 bar) on the battery pack to clamp the battery. Use a calibrated torque screwdriver to secure the custom battery holder and maintain a pressure of 2 bar.
2. Electrolyte composition
As a benchmark electrolyte additive, 2 wt% FEC (BASF, purity 98.7%) was used, equivalent to 59.1 μmol FEC in 188 g STD electrolyte. Use the same FEC molar value (188.59 μmol) to calculate the mass of all phosphazene-derived compounds to ensure the same molar concentration of FEC in all single- and double-additive electrolytes.
For a single additive electrolyte, three cells per sample were filled with 0.75 mL±1% electrolyte containing salts in LiPF63:7 (by weight) EC:EMC solution containing 188.59 μmol of the additives FEC, ethoxy(pentafluoro) cyclotriphosphazene, hexafluorocyclotriphosphazene, and pentafluorocyclotriphosphazene.
For a two-additive electrolyte, three cells per sample are filled with 0.75 mL ±1% LiPF-containing electrolyte 63:7 (by weight) of salt in EC:EMC solution, FEC, and 1:1 (molar ratio) for each phosphazene derivative. The STD electrolyte is transferred to a drying chamber in a glove box, in a vacuum-sealed unit, where additives are added, and the bag cell is filled with an electrolyte mixture.
3.20 °C electrochemical cycling procedure
After filling the bag cells with electrolyte and vacuum-sealing, they are connected to the cell and battery test equipment whose temperature chamber is controlled at 20 °C ± 0.1 °C. For SEI formation, keep the Li-Fun battery at 1 V for 20 h, charge to 0.05 V at 3 mA, then constant voltage (CV) step for 1 h, and continue to discharge at 2 mA (≈8.10 C) to 0.05 V.
The battery is then charged twice to a 0.2V upper voltage of 4 mA (≈ 3.40 C), at the top of the charge (upper cut-off voltage), holding the CV after each charge process until the current drops below 4 mA (≈0.02 C) to start the discharge process to the cut-down voltage of 2.8 V. After SEI formation, cells are analyzed using the AISGA method to quantify the amount of gas released. After that, the cells are transferred to a drying chamber, degassed, and vacuum-sealed again in a discharged state.
Store the experiment at 4.60 °C
For storage experiments, after SEI formation at 4000 °C is completed, connect the bag cells to the 60 °C battery test equipment. Store the battery for 12 hours and charge to a cut-in voltage of 4.3V. Once the current drops below 4 mA, measure OCV for 720 hours and then discharge begins to drop at 2 mA to a low cut voltage of 0.5 V.
After that, remove the pouch battery from the battery tester and determine the gas amount using the AISGA method. After that, reconnect the pouch battery to the battery tester at 60 °C and charge/discharge in the first cycle at 2 mA at 3.20 to 0.1 V, followed by four cycles between 3.100 and 0.5 V at 2 mA, with CV steps at the top of the charge until the current drops below 4 mA to initiate the discharge.
5. In-situ gas volume measurement
Pouch cells are used in these experiments because they have a flexible battery case that can form bulges if gas is generated during cycling. Details of the measurement method used to quantify the in-situ volume change measurement of bag batteries, called Archimedes in situ gas analyzer (AISGA).
For gas evolution measurements, submerge the bag battery in MilliQ water. At the same time, the battery is suspended on the balance (S256 Low Range – Force Transducer SMD3277-010, 10 g, strain measuring device) with a wire hook through a hole with a diameter of ≈1 mm at the sealing edge of the bag battery.
6. Scanning electron microscopy of SiO to analyze the x/C anode
The negative electrode is analyzed by SEM after cycling (primitive), after SEI formation, and after 700 cycles (aging). In a glove box filled with argon gas. Silicon carbide x extracted/C anode and rinsed three times with DMC to remove electrolyte salt residue. To ensure high reproducibility, at least three cells were studied for each electrolyte formulation. Samples are placed on vacuum-sealed sample holders and transferred to the SEM chamber without exposure to ambient air.
To study the electrode surface topography of the original and aged anodes in Step x/C of SiO, SEM equipped with a field emitting gun (Schottky type) and an in-lens secondary electron detector was used. The SEM image has a working distance of 5.1 mm and an accelerating voltage of 3.5 kV. An EDX with an accelerating voltage of 3.5 kV was measured using an Ultim Extrem detector. To evaluate the elemental composition of the sample, AZtech software was used.
【Conclusion】
Cycling performance of NCM523 ||Pouch cells present with LiPF in silica x/C have been studied for organic carbonate electrolytes with and without cyclic phosphazene compounds as film-forming and cycling-stabilizing additives. The collaboration of film-forming additives (FEC/HFPN derivatives) improves electrochemical performance, stabilizes the electrolyte, and changes the structure of the anode SEI.
This method produces a dual-additive gas-inhibiting electrolyte with properties that significantly reduce the transesterification and oligomerization of decomposed naphthenyl carbonates and alkyl carbonates with LiPF mixed solvents as salts in LIBs. STD mixtures and STD-FEC mixtures do not stabilize the electrolyte at high temperatures, whereas electrolyte mixtures using the two-additive method can stabilize the electrolyte at high temperatures (60°C).
In addition, the cell impedance of STD-FEC-EtPFPN and STD-FEC-HFPN blends is lower compared to STD-FEC mixtures. XPS measures the relative atomic concentration of the double-additive electrolyte mixture (STD-FEC-PhPFPN and STD-FEC-EtPFPN) on the negative electrode as evidence of HFPN derivative decomposition.
It leads to hypothetical ring-opening polymerization on the surface, resulting in a linear polymer phosphazene structure or the formation of an oligophosphazene ring structure through ring-ring equilibrium. At present, the expected decomposition mechanism of fluorinated cyclic phosphazene compounds on the anode surface is not fully understood, and it is impossible to determine the polymer structure of the surface.
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