High - pressure lithium ion battery electrolyte additive details and application examples

The oxidation and decomposition of ordinary lithium ion cell electrolyte under high voltage limits the development of high voltage lithium ion battery. In order to solve this problem, it is necessary to design and synthesize new high voltage resistant electrolyte or find suitable electrolyte additives. However, from the perspective of economic benefits, the development of suitable electrolyte additives to stabilize the electrode/electrolyte interface is more favored by researchers. This paper introduces the research progress of electrolyte additives for high pressure lithium ion batteries, and divides them into six parts according to the types of additives: boron additives, organic phosphorus additives, carbonate additives, sulfur additives, ionic liquid additives and other types of additives.

1. Boron containing additives

Boron compounds are often used as additives in lithium ion batteries with different anode materials. During the battery cycle, many boron compounds will form a protective film on the anode surface to stabilize the interface between electrodes and electrolytes, thus improving the battery performance. Considering the unique properties of boron compounds, many scholars have begun to apply them to high voltage lithium ion batteries to enhance the interfacial stability of the anode.

Li et al. applied trimethylalane borase (TMSB) to the high-pressure lithium ion battery with Li[li0.2mn0.54 Ni0.13Co0.13]O2 as the positive electrode material, and found that when 0.5%(mass fraction) of TMSB additive was present, the capacity remained 74%(potential range 2-4.8v, charging and discharging ratio was 0.5c) after 200 cycles, while when there was no additive, the capacity remained only 19%.

To understand TMSB on the mechanism of action of the positive surface modification, ZUO, etc will be added to LiNi0.5 TMSB Co0.2 Mn0.3 O2 graphite whole cells, and the anode materials for the XPS and TEM analysis, have shown below conclusions: when there are no additives in, with the increase of cycling times, will gradually form a layer on the surface of the anode in the presence of LiF the positive electrolyte interface (CEI) membrane, the membrane thicker and higher impedance; When TMSB is added, the electron-deficient boron compounds will increase the solubility of the positive surface LiF, resulting in a thin SEI film with low impedance.

Today, in addition to TMSB applied to high pressure in the lithium ion battery boron additives include double oxalic acid boric acid lithium (LiBOB) and double lithium fluoride boric acid oxalate (LiFOB), tetramethyl boric acid ester (TMB), trimethyl borate (TB) and three methyl boron oxygen alkanes, etc., these additives in the process of circulation than electrolyte solvent by oxidation, form protective film to cover to the anode surface, this layer of protective film has good ionic conductivity, can inhibit the electrolyte in the subsequent cycle oxidation decomposition and destruction of the structure of the anode material, the stability of the electrode/electrolyte interface, And finally improve the high - pressure lithium - ion battery cycle stability.

2. Organic phosphorus additive

According to the relationship between the front-line orbital energy and electrochemical stability, the higher the HOMO of a molecule, the more unstable the electrons in the orbital, and the better the oxidability: the lower the LUMO of the molecule, the easier to get electrons, and the better the reductivity.

Therefore, the feasibility of additives can be judged theoretically by calculating the frontier orbital energy of additive molecules and solvent molecules. SONG using Gaussian09 procedures, such as using density functional theory (DFT) at B3LYP / 6-311 + (3 df, 2 p) level respectively to three (2,2,2 - trifluoro ethyl) phosphite (TFEP), three benzene heartland of phosphate (TPP), three (trimethyl silyl) phosphite ester (TMP) trimethyl phosphite (TMSP), and kind of additive and solvent molecules are optimized, take advantage of the corresponding conformation, and carries on the front line track analysis. As can be seen from the figure below, the HOMO energy of these phosphite compounds is much higher than that of solvent molecules, indicating that phosphite compounds have higher oxidability than solvent molecules, and electrochemical oxidation takes place on the surface of the positive electrode in priority, forming SEI film covering the surface of the positive electrode.

In addition to phosphite additives, currently used organic phosphorus additives also include phosphite compounds. XIA et al. applied TAP additive to Li[ni0.42mn0.42co0.16]O2(NMC442) graphene battery and found that TAP could significantly improve coulomb efficiency and maintain high capacity after long cycle. XPS results show that during the cycle, the allyl group may undergo crosslinked electropolymerization, and the resulting product covers the electrode surface, forming a uniform SEI film.

3, carbonate additives

Fluorine-anhui group (PFA) compounds have high electrochemical stability and hydrophobic and oil-phobic properties. When PFA is added to organic solvents, the hydrophobic PFA will agglomerate together to form micelles. Due to the characteristics of PFA, ZHU tried to perfluorinated alkyl (below in TEM - EC, the PFB - EC, PFH - EC, made - EC) instead of the ethylene carbonate are added to the high-pressure lithium ion battery electrolyte, for Li1.2 Ni0.15 Mn0.55 Co0.1 O2 graphite battery, when adding 0.5% (mass fraction) made - after EC, battery performance has improved significantly in the process of circulation for a long time, mainly because the additives in the process of loop forming double passivation membrane, at the same time reduce the oxidation degradation on the surface of the electrode and the electrolyte decomposition.

4, containing sulfur additives

In recent years, organosulfonate has been widely used as an additive in lithium ion batteries. 1, 3-propionic lactolide (PS) was added to the electrolyte of high pressure lithium ion batteries, effectively inhibiting the occurrence of electrode surface side reactions and the dissolution of metal ions. ZHENG et al. used DMSM as a LiNil/3Col/ 3mn1/3o2 graphite battery electrolyte additive. The results of XPS, SEM and TEM analysis showed that the presence of MMDS had a good modifying effect on the SEI film of the positive electrode, which could significantly reduce the interface impedance of the electrode/electrolyte and improve the cycling stability of the positive electrode material. In addition, HUANG et al. studied the cyclic properties of PTS additives at room temperature and high temperature of high pressure lithium ion batteries. The results of theoretical calculation and experimental analysis show that the PTS molecules are oxidized prior to the solvent molecules during the cycling process, and the SEI film formed improves the cycling stability of the battery under high voltage. In addition, some thiophene and its derivatives are also considered as high pressure lithium ion battery additives, when the addition of these additives, will form a polymer film on the surface of the cathode, to avoid the electrolyte in high pressure oxidation decomposition.

5. Ionic liquid additive

Ionic liquid is a kind of low-temperature molten salt, which is widely used in lithium ion batteries due to its advantages of low steam pressure, high conductivity, non-flammability, thermal stability and high electrochemical stability.

Reported from the literature at present is mainly the pure ionic liquids used as ordinary lithium ion battery electrolyte, institute of process engineering, Chinese academy of sciences li fly team considering the chemical and physical properties of ionic liquids is unique, try to apply as an additive to high pressure in the lithium ion battery, such as, respectively, to replace four olefin imidazole double (three fluorinated methyl sulfonyl) imide ionic liquid added to 1.2 mol/L LiPF6 / EC/EMC in the electrolyte, and carries on the cycle performance test, see below. The results show that the initial charge and discharge efficiency is significantly improved, especially when 3%(mass fraction) of [AVlm][TFSI] ionic liquid is added, the discharge capacity and cycle performance of the battery is the best.

Conclusion:

The continuous oxidation and decomposition of the conventional organic carbonate electrolyte at high voltage and the dissolution of the transition metal ions in the anode material limit the capacity and application of the high-voltage anode material. The development of high-voltage electrolyte additives is an economical and effective method to improve the battery performance. Currently reported high-pressure additives are generally oxidized prior to solvent molecules in the recycling process, forming a passivation film on the surface of the positive electrode, stabilizing the electrode/electrolyte interface, and finally realizing the stable existence of electrolyte under high pressure.

According to the research progress published at home and abroad, in the development of high-pressure electrolyte, the introduction of high-pressure additives can generally obtain 4.4-4.5v electrolyte. However, for lithium rich, lithium vanadium phosphate, high-voltage nickel-manganese and other anode materials, since the rechargeable voltage reaches 4.8v or even more than 5V, the electrolyte that can withstand higher voltage must be developed to obtain higher energy density.

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