Detailed explanation of gas generation suppression scheme for lithium batteries based on electrolyte

In recent years, reports related to gas production in lithium-ion batteries have mainly focused on six types of gases, including H2, O2, olefins, alkanes, CO2, and CO. This article systematically discusses the generation mechanisms of these six types of gases during the use of lithium-ion batteries, as well as the relationship between the generation of these gases and changes in battery performance. Due to the fact that electrolyte is the main source of gas production in lithium batteries, and there have been numerous reviews and reports on improving battery stability and suppressing gas production through the modification of positive and negative electrode materials, this article proposes some corresponding suppression strategies from the perspective of electrolyte.
2 Gas production suppression strategies based on electrolyte perspective
The main source of gas production in batteries is the decomposition reaction of the electrolyte on the positive and negative electrode surfaces. Therefore, improving the stability of the electrolyte and building a stable SEI interface can effectively suppress gas production in batteries.
2.1 Improving electrolyte stability
Trace amounts of water, hydrofluoric acid, and reactive oxygen species in the electrolyte can reduce the chemical stability of the electrolyte and exacerbate gas production in the battery. In addition, EC, as one of the most easily decomposed solvents in the electrolyte, can greatly reduce the electrochemical stability of the electrolyte.
2.1.1 Use of Water Removing and Acid Suppressing Additives
The trace water that cannot be eradicated in the battery can not only undergo electrochemical decomposition on the electrode surface to produce gas, but also trigger the gradual hydrolysis of LiPF6 to produce a large amount of HF, which destroys the SEI interface and accelerates electrolyte decomposition [Figure 7 (a)]. In addition, water can directly attack ROCO2Li in the SEI interface to produce ROH, Li2CO3, and CO2 gases. The commonly used additives for water removal and acid suppression are silazane, siloxane, hypophosphite, and nitrogen-containing compounds [Figure 7 (b)].

The Si-N bond in silazane can react with water to inhibit the hydrolysis of LiPF6, such as trimethylsilyl imidazole and trimethylsilyl imidazolidinone. The Si-O bond in siloxanes can react with H2O to form - (Si-O) n -, or with HF to form Si-F bonds, such as dimethyldiphenyloxysilane. The P (III) in hypophosphite and the N atom in nitrogen-containing compounds, due to their lone pair electrons, can chelate hydrogen atoms in HF and H2O, such as N-acetylcaprolactam and cyclopentyl isocyanate. The combination of multiple functional groups has a stronger effect on water removal and acid inhibition.
2.1.2 Use of reactive oxygen species scavengers
Removing reactive oxygen species can prevent electrolyte oxidation and decomposition, reducing battery gas production. The lone pair electrons outside the P (III) nucleus in hypophosphite can react with O2 to convert into phosphate esters. Boron containing compounds can also capture oxygen in the electrolyte due to the electron deficient nature of B. The difference between these two types of adsorption is that the former can form P=O double bonds, which belongs to chemical adsorption [Figure 7 (c)], while the latter belongs to physical adsorption.
2.1.3 Reduce the content of cyclic carbonates in electrolyte solvents
The commonly used cyclic carbonate in electrolyte solvents is mainly EC, which is one of the most easily decomposed and gas producing solvents in electrolytes due to its strong reactivity with high nickel cathodes and poor oxidation resistance at high voltages [Figure 8 (a)]. It can undergo chemical/electrochemical oxidation at the positive electrode to produce CO2 and CO, as well as reduction at the negative electrode to produce C2H4, CH4, and CO. In addition, EC will decompose and produce a large amount of H2 under the joint action of positive and negative electrodes, so reducing the EC content in the electrolyte and even using the "EC free" system to increase electrolyte stability has gradually become a research hotspot.

Using efficient film-forming additives instead of EC in "EC free" electrolytes is an effective strategy. Wu et al. completely transferred the "builders" of SEI from cyclic carbonates to lithium salts by using three types of lithium salts in EMC solvents, resulting in a significant increase in temperature caused by thermal runaway of the battery [Figure 8 (b)]. Kang et al. further improved the stability of NCM811 positive electrode under high voltage by using EMD (EMC/DMC/DEC) - LiNO3 electrolyte [Figure 8 (c)].
2.1.4 Use of Fluorinated Solvents
Due to the fact that fluorine can lower the highest occupied molecular orbital (HOMO) energy level of molecules, fluorinated solvents have higher oxidation resistance, making them less prone to chemical or electrochemical oxidation decomposition and gas production. In addition, the strong electronegativity of fluorine element can reduce the solvation energy of the solvent system, making it easier for anions to enter the solvation shell of lithium ions and form LiF. The decomposition of fluorinated solvents themselves can also form SEI interfaces rich in LiF and other fluorine-containing substances at the battery interface, which can significantly enhance the chemical and mechanical stability of SEI and reduce gas production from electrolyte solvent decomposition. The commonly used fluorinated solvents in electrolytes currently include fluorinated carbonates, fluorinated carboxylic esters, fluorinated ethers, fluorinated sulfones, and fluorinated phosphines.
2.2 Building a stable electrode/electrolyte interface
The use of functional additives in electrolytes to construct a stable SEI can effectively reduce the decomposition and gas production of solvents at the positive and negative electrode interfaces. At present, the commonly used film-forming additives are mainly classified into six types according to their substance types: carbonate, sulfate, borate, phosphate/hypophosphite, nitrile, and lithium salt (Figure 9).

Vinyl carbonate (VC) and fluorinated vinyl carbonate (FEC) are two classic carbonate additives. The SEI film formed by VC is mainly composed of porous organic compounds with good toughness and ion mobility, while the SEI formed by FEC contains a large amount of LiF, thus having higher mechanical stability. The commonly used sulfate/sulfonate additives are mainly vinyl sulfate (DTD) and 1,3-propanesultone (PS), while SEI is mainly composed of organic compounds represented by ROSO3Li and inorganic compounds represented by Li2SO3. The commonly used boronic acid ester additives are triethyl borate (TEB) and tris (trimethylsilyl) boronic acid ester (TMSB), in which the central boron atom is in an electron deficient state, which can dissolve LiF in SEI and reduce battery interface impedance. Phosphate ester additives can combine with oxygen on the surface of the positive electrode to form P=O double bonds, which have better positive electrode protection and can inhibit the oxidation and decomposition of the electrolyte at the positive electrode to produce gas. The C ≡ N group in nitrile additives can chelate the surface of the positive electrode
The transition metal is used to inhibit its deposition on the negative electrode and avoid the occurrence of side reactions and gas production between the reduced metal and the electrolyte. There are many types of lithium salt additives, including lithium difluorophosphate (LiDFP), lithium tetrafluoroborate (LiBF4), lithium difluorosulfonylimide (LiFSI), lithium bis trifluoromethanesulfonimide (LiTFSI), lithium difluorooxalate borate (LiDFOB), and lithium difluorooxalate phosphate (LiDFOP), which have excellent positive electrode film-forming effects.
In addition to the six types of additives mentioned above, the types of film-forming additives involved in electrolytes include sulfones, ethers, and various heterocyclic compounds, which are used less frequently in commercial electrolytes and will not be described in detail.
Although traditional commercial additives have excellent film-forming properties, they also have certain shortcomings, thus requiring the development of new additives. PS is the most effective additive for suppressing battery gas production in electrolytes. Due to its carcinogenicity being regulated by the European Union, we have developed tetravinylsilane (TVS) as an alternative, which can participate in the formation of positive and negative electrode films to form silane polymers with high lithium ion conductivity, significantly inhibiting battery gas production and impedance growth during high-temperature storage [Figure 10 (a), (b)]. Triphenylphosphonic acid ester, as an excellent additive for water removal and acid suppression in commercial electrolytes, has a deteriorating effect on battery life. Using trifuranyl phosphonic acid ester (FuP) as a substitute can preserve the water removal and acid suppression function of the additive while preferentially improving the high-temperature performance of the battery in positive and negative electrode film formation compared to solvents [Figure 10 (c)].

On the other hand, a reasonable combination of additives can achieve a stable and low impedance SEI interface. By combining organic sulfate ester DTD with inorganic lithium salt LiDFP, Li2SO4 and ROSO2Li can be introduced into SEI containing LiF and LixPOyFz, which can suppress gas production during cycling and reduce impedance to improve fast charging performance [Figure 10 (d), (e)]. The combination of LiDFOP and dioxolane (DOL) can initiate DOL polymerization to form inorganic organic composite SEI rich in LiF and LixPOyFz [Figure 10 (f)].