Several lithium salts are very soluble in carbonates and the resulting conductivities of around 10 mS/cm are adequate for batteries. Also, some carbonates clearly decompose at carbon anodes to form useful SEI layers. Are these all the primary reasons for using carbonates or am I missing some other crucial property? Maybe their price is lower compared to other solvents that would perform just as well or better?
Of course, price plays a role. Also, electrolyte should be liquid in the whole temperature range where the battery is intended to be used. For example, dimethyl carbonate melts at 2C, so it cannot be used alone, or the battery would freeze in winter.
As for SEI, it would be much better if electrolytes did not interact with electrodes at all. Unfortunately, they do, so formation of the good SEI layer is the best way to avoid further decomposition of the electrolyte.
Of course, price plays a role. Also, electrolyte should be liquid in the whole temperature range where the battery is intended to be used. For example, dimethyl carbonate melts at 2C, so it cannot be used alone, or the battery would freeze in winter.
As for SEI, it would be much better if electrolytes did not interact with electrodes at all. Unfortunately, they do, so formation of the good SEI layer is the best way to avoid further decomposition of the electrolyte.
Hello,
Alexandre is right. I think that this review should help you to answer all your questions about LIBs
Xu, K. Chem. Rev. 2004, 104 (10), 4303-4417
as alexander already said price is only one of many factors (and probably not the most important one). Electrolytes must meet a variety of requirements to be applied in batteries. Not only they must provide good ionic conductivity, while at the same time inhibiting electronic transfer in order to separate ionic from electronic flux, but also they must be stable in a wide range of potential and temperature towards the involved electrode materials. In order to satisfy the divers and sometimes contradictory demands, not a single component, but rather a mix of carefully matched solvents is used in practical applications.
Ethylene carbonate (EC) provides a Li+ ion-conducting but electronically- passivating layer on the negative electrodes such as carbon. Since, EC is solid at room temperature, it is mixed with propylene carbonate (PC) to yield a liquid electrolyte. PC alone does not form a good passivating layer on negative Li-based electrodes.
Both PC and EC are inexpensive and are produced by reacting CO2 with alkene epoxides. PC also has low toxicity and is used in cosmetic preparations.
Salt solubility (related to dielectric constant), ionic conductivity, viscosity, melting point, electrochemical stability window, safety, price, interactions with anode and cathode, etc. I think the main thing is that none of the cyclic or acyclic carbonates by themselves have all of the highest metrics for any of the above criteria. There has to be some trade-offs and optimization through solvent mixing (EC/DEC for example) or through use of additives. The same problem occurs with the choice of salt for electrolyte formulation - some are very good in one metric using specific anodes/cathodes but are terrible in others. LiPF6 is a perfect example - it is the middle of the ground in almost all metrics for a good electrolyte salt, yet it has significant thermal instability issues.
For EC not exist an alternative for carbon-based negative electrodes (at least I do not know which solvent can it conletely replace). The reason is a formation of SEI inte surface of lithium-carbon electrode. Bed side is EC high melting temperature. So really usefull electrolyte can be made by mixing of EC with linear carbonates (like DMC) and some esters (for example, some acetate or propionate).
LiPF6 is used in Li-ion batteries mainly because it assures the passivation of aluminum current collectors used for the positive electrode. Numerous attempts to replace LiPF6 with other salts (e.g. imides) failed because they do not passivate aluminum.
As mentioned earlier, price is the key as batteries are manufactured in large number. But the other reasons can be grouped in two major groups: (i) self performance such as ionic conductivity, stability in the working temp., etc. (ii) reaction with other battery components especially other solvents, SEI layer, electrodes, and formation of toxic gases when decompose
Moreover, it should be noted that cyclic has higher flash point than non-cyclic
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