Detailed description of high-pressure lithium-ion battery electrolyte additives and dry goods for application examples

The oxidative decomposition of ordinary lithium ion bath electrolyte at high voltage limits the development of high voltage lithium ion batteries. In order to solve this problem, it is necessary to design, synthesize a new type of high pressure resistant electrolyte or find a suitable electrolyte additive. However, in terms of economic benefits, it is more popular among researchers to develop suitable electrolyte additives to stabilize the electrode/electrolyte interface. In this paper, the research progress of electrolyte additives for high-voltage lithium-ion batteries is introduced, and it is divided into six parts according to the types of additives: boron-containing additives, organic phosphorus additives, carbonate additives, sulfur-containing additives ions, Liquid additives and other types of additives.

1, boron-containing additives

Boron-containing compounds are often used as additives in lithium-ion batteries with different positive materials. During the battery cycle, many boron-containing compounds form a protective film on the surface of the positive electrode to stabilize the interface between the electrodes and the electrolyte, thereby improving battery performance. . Considering this unique property of boron-containing compounds, many scholars have begun to apply it to high-voltage lithium-ion batteries to enhance the stability of the positive electrode interface.

Li et al. applied tris(trimethylalkane)borate (TMSB) to a high-voltage lithium-ion battery using Li[Li0.2Mn0.54Ni0.13Co0.13]O2 as a positive electrode material, and found that when there is 0.5% (mass fraction) In the presence of the TMSB additive, the capacity was maintained at 74% after cycling for 200 cycles (potential range 2-4.8 V, charge and discharge ratio 0.5 C), and the capacity was maintained at only 19% in the absence of additives.

In order to understand the mechanism of TMSB's modification on the surface of the positive electrode, ZUO added TMSB to the LiNi0.5Co0.2Mn0.3O2 graphite full cell, and analyzed the XPS and TEM of the positive electrode material respectively, and obtained the conclusion shown in the following figure: When no additives are present, as the number of cycles increases, a positive electrode electrolyte interface (CEI) film with LiF is formed on the surface of the positive electrode. This film is thicker and has higher impedance; after adding TMSB, it lacks electrons. The boron-containing compound increases the solubility of LiF on the surface of the positive electrode, and the formed SEI film is thinner and has lower impedance.

In addition to TMSB, boron-containing additives currently used in high-voltage lithium-ion batteries include lithium bis(oxalate)borate (LiBOB), lithium difluorooxalate borate (LiFOB), tetramethylborate (TMB), trimethyl borate. (TB) and trimethylcyclotrioxane, etc., these additives are preferentially oxidized during the recycling process than the electrolyte solvent, and the formed protective film covers the surface of the positive electrode. This protective film has good ionic conductivity. It can inhibit the oxidative decomposition of the electrolyte in the subsequent cycle and the destruction of the structure of the positive electrode material, stabilize the electrode/electrolyte interface, and finally improve the cycle stability of the high-voltage lithium ion battery.

2. Organic phosphorus additive

According to the relationship between the orbital energy and electrochemical stability of the front line: the higher the HOMO of the molecule, the more unstable the electrons in the orbit, the better the oxidizing property: the lower the LUMO of the molecule, the easier the electron is, and the better the reducing property.

Therefore, by calculating the frontier orbital energy of the additive molecule and the solvent molecule, the feasibility of the additive can be theoretically judged. SONG et al. used the Gaussian09 program to determine the tris(2,2,2-trifluoroethyl)phosphite (TFEP) at the B3LYP/6-311+ (3df, 2p) level by density functional theory (DFT). Triphenyl phosphite (TPP), tris(trimethylsilyl)phosphite (TMSP) and trimethyl phosphite (TMP) additives and solvent molecules are optimized to obtain the corresponding dominant conformation Frontline orbit analysis was performed. As can be seen in the figure below, the HOMO energy of these phosphite compounds is much higher than that of the solvent molecules, indicating that the phosphite compounds have higher oxidizability than the solvent molecules, and electrochemical oxidation is preferentially formed on the surface of the positive electrode to form SEI film coverage, on the surface of the positive electrode.

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In addition to phosphite additives, organophosphorus additives currently used include phosphate compounds. XIA et al. applied a triallyl phosphate (TAP) additive to a Li[Ni0.42Mn0.42Co0.16]O2(NMC442) graphite full cell and found that when there is TAP, the coulombic efficiency is significantly improved. , still has very high capacity retention. XPS results show that during the cycle, the allyl group may undergo cross-linking electropolymerization, and the obtained product covers the surface of the electrode to form a uniform SEI film.

3. Carbonate additives

Fluorinated fluorenyl (PFA) compounds have high electrochemical stability and are hydrophobic and oleophobic. When PFA is added to an organic solvent, the solvophobic PFA will agglomerate together to form a micelle. Due to this characteristic of PFA, ZHU et al. attempted to add a perfluoroalkyl group (TEM-EC, PFB-EC, PFH-EC, PFO-EC in the following figure) to ethylene carbonate in a high-pressure lithium-ion battery electrolyte. For the Li1.2Ni0.15Mn0.55Co0.1O2 graphite battery, when 0.5% (mass fraction) of PFO-EC is added, the performance of the battery during the long-term circulation is significantly improved, mainly because the additive forms a double during the cycle. The passivation film of the layer reduces both the degradation of the electrode surface and the oxidative decomposition of the electrolyte.

4, sulfur additives

In recent years, there have been many reports on the application of organic sulfonate as an additive to lithium ion batteries. PIRES added 1, 3-propane sultone (PS) to the electrolyte of high-voltage lithium-ion battery, which effectively inhibited the occurrence of side reactions on the electrode surface and the dissolution of metal ions. ZHENG et al. used dimethosulfonate (DMSM) as the electrolyte additive for high pressure LiNil/3Col/3Mn1/3O2 graphite battery. XPS, SEM and TEM analysis showed that the presence of MMDS has a good modification effect on the positive SEI film even the electrode/electrolyte interface impedance can also be significantly reduced under high pressure to improve the cycle stability of the positive electrode material. In addition, HUANG et al. studied the cycle performance of trifluoromethyl phenyl sulfide (PTS) additives in high-pressure lithium-ion batteries at room temperature and high temperature. Theoretical calculation data and experimental results show that PTS is preferentially oxidized than solvent molecules during the cycle, and the formed SEI film improves the cycle stability of the battery at high voltage. In addition, some thiophenes and their derivatives are also considered to be used as high-pressure lithium ion battery additives. When these additives are added, a polymer film is formed on the surface of the positive electrode to avoid oxidative decomposition of the electrolyte under high pressure.

5, ionic liquid additives

Ionic liquid is a low-temperature molten salt, which is widely used in lithium ion batteries because of its low vapor pressure, high electrical conductivity, non-flammability, thermal stability and high electrochemical stability.

At present, the reported literature mainly uses pure ionic liquids as ordinary lithium ion battery electrolytes. The research group of the Institute of Process Engineering of the Chinese Academy of Sciences, Li Fangfang, considered the unique physical and chemical properties of ionic liquids and tried to apply them as additives to high-pressure lithium. In the ion battery, four kinds of olefin-substituted imidazole bis(trifluoromethylsulfonyl)imide ionic liquids were respectively added to a 1.2 mol/L LiPF6/EC/EMC electrolyte, and the cycle performance test was performed. See below. The results show that the first charge and discharge efficiency is significantly improved, especially when adding 3% (mass fraction) of [AVlm][TFSI] ionic liquid, the discharge capacity and cycle performance of the battery are the best.

In addition, BAE et al. used bis(trifluoromethylsulfonyl)imide triethyl(2-methoxyethyl) quaternary phosphonium salt (TEMEP-TFSI) as an organic electrolyte additive, and found that TEMEP-TFSI can effectively improve Li/ The capacity retention of the LiMn1.5Ni0.5O4 half-cell also reduces the flammability of the electrolyte. The results of TEM and XPS show that the additive forms a stable protective film on the surface of LNMO, which effectively inhibits the decomposition of the electrolyte.

6, other types of additives

In addition to the types of additives mentioned above, CHEN et al. attempted to use silicone compounds as high-pressure lithium-ion battery additives when adding 0.5% (mass fraction) of allyloxytrimethylsilane (AMSL) to the electrolyte. The cycle performance and thermal stability of the battery are obviously improved; the results of SEM, XPS and FTIR analysis show that AMSL will form a protective film on the surface of the positive electrode: In addition, through the cycle performance and CV test of the graphite negative electrode, it is found that the discharge capacity will be added after adding the additive. A slight increase, compared with the CV curve without additives, the addition of AMSL will appear in the original reduction peak, a new reduction peak appears at a relatively high voltage, indicating that AMSL will be preferentially reduced, forming a stable SEI film coverage The surface of the graphite negative electrode inhibits the further reduction and decomposition of the electrolyte on the electrode surface and enhances the cycle stability. Since AMSL can simultaneously form an SEI film on the LiNi0.5Mn1.5O4 and the graphite negative electrode to stabilize the electrode interface, it is expected to become a kind of the ideal additive is used even further. Some benzene derivatives can also be used as high-pressure lithium ion battery additives. KANG et al. added 1,3,5-hydroxybenzene (THB) to carbonate electrolytes, which showed good thermal stability under high temperature and high pressure, sexual and electrochemical stability.

To sum up:

The traditional oxidative decomposition of organic carbonate electrolytes at high voltage and the dissolution of transition metal ions in positive electrode materials limit the capacity and application of high-voltage cathode materials. It is economical to develop high-pressure electrolyte additives to improve battery performance effective method. The high-pressure additives reported today generally oxidize preferentially over solvent molecules during the recycling process, forming a passivation film on the surface of the positive electrode, stabilizing the electrode/electrolyte interface, and finally realizing that the electrolyte can be stably present under high pressure.

From the current domestic and international research progress reported publicly, in the development of high-pressure electrolyte, the introduction of high-pressure additives generally can obtain 4.4-4.5V electrolyte. However, for cathode materials such as lithium-rich, lithium vanadium phosphate, and high-pressure nickel-manganese, since the chargeable voltage reaches 4.8V or more, it is necessary to develop an electrolyte that can withstand higher voltages in order to obtain higher energy density.

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