极品斗地主

Electrolyte composition

- Dec 13, 2018-

1.1 organic solvent

 

The organic solvent is the main part of the electrolyte, and the performance of the electrolyte is closely related to the performance of the solvent。 Solvent oil commonly used in lithium ion battery electrolytes such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc。, generally not suitable for propylene carbonate (PC) ), ethylene glycol dimethyl ether (DME) and the like mainly used for lithium primary batteries。 PC is used in secondary batteries, and the compatibility with graphite anodes of lithium ion batteries is very poor。 During charging and discharging, PC decomposes on the surface of graphite anodes, and colleagues peel off the graphite layer, causing the cycle performance of the battery to decrease。 However, a stable SEI film can be established in the EC or EC+DMC composite electrolyte。 It is generally considered that a mixed solvent of EC and a chain carbonate is an excellent electrolyte of a lithium ion battery, such as EC+DMC, EC+DEC, and the like。 The same electrolyte lithium salt, such as LiPF6 or LiC104, PC+DME system always exhibits the worst charge and discharge performance (relative to EC+DEC, EC+DMC system) for mesophase carbon microsphere C-MVMB material。 But not absolutely, when the PC is used in related additives for lithium-ion batteries, it is beneficial to improve the low-temperature performance of the battery。

The organic solvent must be strictly controlled before use。 For example, the purity is required to be 99。9% or more, and the moisture content must be 10*10±6 or less。 There is a close relationship between the purity of the solvent and the stable voltage。 The oxidation potential of the organic solvent with the purity standard is about 5V。 The oxidation potential of the organic solvent is of great significance for studying the overcharge and safety of the battery。 Strict control of the moisture of organic solvents has a decisive influence on the preparation of qualified electrolytes。 Water below 10*l0?-6 can reduce the decomposition of LiPF6, slow down the decomposition of SEI film, and prevent gas rise。 The moisture content can be achieved by molecular sieve adsorption, atmospheric or vacuum distillation, and introduction of an inert gas。


1.2 electrolyte lithium salt

LiPF6 is the most commonly used electrolyte lithium salt and is the future direction of lithium salt development. As far as possible, LiCIO4, LiAsF6, etc. are also used as electrolytes in the laboratory. However, because the high temperature performance of the battery using LiC104 is not good, and LiC104 itself is easily exploded by impact, it is also a strong oxidant, which is not safe for use in batteries. Not suitable for industrial large-scale use of lithium-ion batteries,

LiPF6 is stable to the negative electrode, has large discharge capacity, high conductivity, small internal resistance, fast charge and discharge speed, but is extremely sensitive to moisture and HF acid, easy to react, and can only be operated in a dry atmosphere (such as gloves with an environmental moisture of less than 20x10). In the box), and not resistant to high temperature, decomposition reaction occurs at 80 °C-100 °C, and phosphorus pentafluoride and lithium fluoride are formed, which is difficult to purify. Therefore, when preparing the electrolyte, the self-decomposition and solvent heat caused by the dissolution of LiPF6 should be controlled. break down. The percentage of LiPF produced in China is generally up to standard, but the HF acid content is too high to be directly used to prepare the electrolyte and needs to be purified.


1.3 additives

There are many kinds of additives, and different lithium ion battery manufacturers have different requirements on the use and performance of the battery, and the focus of the selected additives is also different. In general, the additives used are mainly used in three ways:

(1) Adding anisole to the electrolyte to improve the performance of the SEI film

The addition of anisole to the lithium ion battery electrolyte can improve the cycle performance of the battery and reduce the irreversible capacity loss of the battery。 The anisole reacts with the desired product of the solvent to form LiOCH, which facilitates the formation of a highly stable and stable SEI film on the surface of the electrode, thereby improving the cycle performance of the battery。 The discharge platform of the battery can measure the energy that the battery can release above 3。6V, and to some extent reflect the large current discharge characteristics of the battery。 In practice, we have found that the addition of anisole to the electrolyte can extend the discharge platform of the battery and increase the discharge capacity of the battery。

(2) Adding metal oxide to reduce trace water and HF acid in the electrolyte

As mentioned earlier, lithium ion batteries are very strict with water and acid requirements in the electrolyte。 The carbodiimide compound can hydrolyze LiPF6 into an acid。 In addition, some metal oxides such as Al2O3, MgO, BaO, Li2CO3, CaCO3, etc。 are used to scavenge HF。 However, the acid removal rate is too slow relative to the hydrolysis of LiPF6, and it is difficult to filter out。 The total content of Li, P and F in the lithium battery electrolyte is 96。3%, and the sum of other major impurity elements such as Fe, K, Na, CI and A1 is 0。055%。


(3) Prevent overcharging and overdischarging

Traditional anti-overcharged through the internal protection circuit of the battery, it is now desired to add additives to the electrolyte, such as sodium imidazolium ring, biphenyls, carbazoles and other compounds, such compounds are in the research stage。


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