Dear Lai Jiang,

Successful commercialization of LIBs is required, besides, a “successful” formula of electrolyte. It is known that the nature and composition of electrolyte play an important role in improvement of electrochemical parameters of LIBs: even small additives in electrolyte may considerably affect their electrochemical performance. Liquid electrolytes with organic carbonates as solvents (denoted as “organic electrolytes”) used at present in industrially manufactured LIBs are predominantly based on 1 M LiPF6 solutions in mixtures of cyclic alkyl carbonate: ethylene carbonate (EC) with one or two linear alkyl carbonates, such as DMC, DEC, and EMC, often with additives of VC or FEC, ES, or PS and others (the so called “functional electrolytes”). Notice that each LIBs producer uses his own formula of electrolyte, but it is usually not completely published due to commercial interests. The fact is that power density, cyclic life, and safety of LIBs largely depend on the conditions of formation on the surface of negative LIBs electrodes of a thin, stable layer of solid electrolyte interface (SEI; such a definition does not wholly exhaust the diversity of the properties of this layer, due to which it was suggested to denote is as a polyfunctional surface layer or insulating polyfunctional layer, IPL, in short) with a high Li-ion conductivity. In the literature, only the E1 value is usually presented in estimates of the cyclic behavior of negativemLIB electrodes basing solely on the only cause of loss in the charge capacity: IPL formation in the first cycle. However, IPL formed in the first cycle on the electrode surface is not ideal; it is to a certain degree characterized by a porous structure and presence of defects, and also partial dissolution. The nature of IPL components is potential–dependent. Physicochemical properties of IPL formed in the first and in the following cycles differ due to occurrence of secondary reactions and dissolution–deposition processes promoting accumulation of low-soluble products. In other words, irreversible losses in charge capacity Qirr(n) = Qch(n) – Qd(n) due to the binding of lithium ions into a product taking no part in the further electrode discharge process, are also accumulated in the further cycles after the first one (strictly speaking, IPL formation on the electrode surface is not the only cause of irreversible lithium loss, as its irreversible capture in the electrode material bulk is also possible, apart from its capturing by the surface). It is known that in order for IPL to be effective, it must to the maximum degree correspond to the following set of requirements:

—the layer formation process must be fast and the formed IPL must be characterized by high kinetic stability;

—the electron transport number must be close to zero (otherwise, electron tunneling can occur, which promotes continuous electroreduction of electrolyte);

—IPL must have high ionic conductivity, so that lithium ions could migrate fast under insertion into the bulk of the active electrode material and under insertion from it;

—the morphology and chemical composition of the insulating layer must be uniform to provide uniform current distribution;

—IPL must feature good adhesion to the electrode surface;

—the insulating layer must be characterized by high mechanical strength and be herewith flexible and elastic enough without destruction under expansion and contraction of the active electrode component in the processes of lithium insertion–extraction;

—IPL must contain insoluble and electrochemically stable reaction products.

The herewith applied electrolyte must be cheap, nontoxic, and operational in a wide range of temperatures and also it must consist of nonflammable and explosion–proof components.

Use of “functional electrolytes” promotes not only strengthening of spatial electrode organization in IPL formation and restructuring with enhancement of capacity retention, but also suppression of electroreduction of solvents at a decrease in the fraction of irreversible lithium accumulation.

As well known, IPL is a rather complex structure consisting of inorganic and organic components and its thickness may vary from several tenths of nanometer to lots of tens of nanometers. Different models of IPL structure were suggested by the groups of Peled, Aurbach, and Edstrцm, but they all agree in the fact that IPL represents a compact layer of inorganic products immediately adjacent to the electrode and the porous organic (organopolymer) layer formed on the side of electrolyte.

It is impossible to provide an accurate prediction of the exact route of IPL formation reactions due to their great diversity: one can only estimate their probabilities.

Hi Jiang,

The study of SEI is a very important and also difficult topic for LIBs. The research for SEI has been last for more than two decades, however, people even don't know whether it is proper to call it "solid-electrolyte interphase (interface)". I have attended "SIRBATT" Workshop this summer in Orlando, which is mainly focused on SEI. I recommend you to read some of their publications [https://www.liverpool.ac.uk/sirbatt/events/sirbattworkshoporlando/]. 

Back to your question, first of all, we don't say that SEI has HIGH ionic conductivity, it is just relatively high as compared to electronic conductivity. As for the composition, I believe no one can tell you what's the composition and the structure. The decomposition composites depends on your voltage window, electrolyte composition,  water and oxygen content, etc. For example, FEC is often used as additive of the electrolyte for Si based anode materials, and it will be reduced (consumed) before other solvent (such as EC, DMC, DEC), and the resulting SEI layer will be different with regular SEI layer.

In commercial LIBs, the electrolyte usually including 5-10 different additives to improve the electrochemical performance. You can refer to Dr. Jeff Dahn's papers for some electrolyte additive study.

Dear Lai Jiang,

Successful commercialization of LIBs is required, besides, a “successful” formula of electrolyte. It is known that the nature and composition of electrolyte play an important role in improvement of electrochemical parameters of LIBs: even small additives in electrolyte may considerably affect their electrochemical performance. Liquid electrolytes with organic carbonates as solvents (denoted as “organic electrolytes”) used at present in industrially manufactured LIBs are predominantly based on 1 M LiPF6 solutions in mixtures of cyclic alkyl carbonate: ethylene carbonate (EC) with one or two linear alkyl carbonates, such as DMC, DEC, and EMC, often with additives of VC or FEC, ES, or PS and others (the so called “functional electrolytes”). Notice that each LIBs producer uses his own formula of electrolyte, but it is usually not completely published due to commercial interests. The fact is that power density, cyclic life, and safety of LIBs largely depend on the conditions of formation on the surface of negative LIBs electrodes of a thin, stable layer of solid electrolyte interface (SEI; such a definition does not wholly exhaust the diversity of the properties of this layer, due to which it was suggested to denote is as a polyfunctional surface layer or insulating polyfunctional layer, IPL, in short) with a high Li-ion conductivity. In the literature, only the E1 value is usually presented in estimates of the cyclic behavior of negativemLIB electrodes basing solely on the only cause of loss in the charge capacity: IPL formation in the first cycle. However, IPL formed in the first cycle on the electrode surface is not ideal; it is to a certain degree characterized by a porous structure and presence of defects, and also partial dissolution. The nature of IPL components is potential–dependent. Physicochemical properties of IPL formed in the first and in the following cycles differ due to occurrence of secondary reactions and dissolution–deposition processes promoting accumulation of low-soluble products. In other words, irreversible losses in charge capacity Qirr(n) = Qch(n) – Qd(n) due to the binding of lithium ions into a product taking no part in the further electrode discharge process, are also accumulated in the further cycles after the first one (strictly speaking, IPL formation on the electrode surface is not the only cause of irreversible lithium loss, as its irreversible capture in the electrode material bulk is also possible, apart from its capturing by the surface). It is known that in order for IPL to be effective, it must to the maximum degree correspond to the following set of requirements:

—the layer formation process must be fast and the formed IPL must be characterized by high kinetic stability;

—the electron transport number must be close to zero (otherwise, electron tunneling can occur, which promotes continuous electroreduction of electrolyte);

—IPL must have high ionic conductivity, so that lithium ions could migrate fast under insertion into the bulk of the active electrode material and under insertion from it;

—the morphology and chemical composition of the insulating layer must be uniform to provide uniform current distribution;

—IPL must feature good adhesion to the electrode surface;

—the insulating layer must be characterized by high mechanical strength and be herewith flexible and elastic enough without destruction under expansion and contraction of the active electrode component in the processes of lithium insertion–extraction;

—IPL must contain insoluble and electrochemically stable reaction products.

The herewith applied electrolyte must be cheap, nontoxic, and operational in a wide range of temperatures and also it must consist of nonflammable and explosion–proof components.

Use of “functional electrolytes” promotes not only strengthening of spatial electrode organization in IPL formation and restructuring with enhancement of capacity retention, but also suppression of electroreduction of solvents at a decrease in the fraction of irreversible lithium accumulation.

As well known, IPL is a rather complex structure consisting of inorganic and organic components and its thickness may vary from several tenths of nanometer to lots of tens of nanometers. Different models of IPL structure were suggested by the groups of Peled, Aurbach, and Edstrцm, but they all agree in the fact that IPL represents a compact layer of inorganic products immediately adjacent to the electrode and the porous organic (organopolymer) layer formed on the side of electrolyte.

It is impossible to provide an accurate prediction of the exact route of IPL formation reactions due to their great diversity: one can only estimate their probabilities.

Dear Chunhui and Sergii, really thanks for your informative answer. The compositions of the SEI depend on various factros.

To be specifically. what are the possible electrolyte decompositions of EC/DMC (LP30 commercial battery grade electrolyte with LiPF6 as Li salt) on graphite and Si anode? Which reactions produce stable product that is favorable to form stable SEI( IPL as Sergii mentioned)?

In addition, during further cycling, some of the initial products of the decomposition of electrolyte will decompose, such as metastable carbonate. These will form stable lithiuim carbonate and lithium fluoride. So what are these reactions? 

First snapshot said that mechanism (I) form more gaseous products, is Li2CO3 abundant and less stable while mechanism (II) is more stable because less gaseous product.

But we can know that forming (CH2OCO2Li)2 will produce CH2=CH2 which is gaseous product. And (CH2OCO2Li)2 as a metastable carbonate will further decompose to from Li2CO3 ( (CH2OCO2Li)2→Li2CO3+CO2↑+C2H4↑+1/2 O2↑) 

, still will create gaseous product. This also agree with another snapshot I upload.

So which one is more resonable and why?

Article 093. A review on electrolyte additives for lithium-ion batteries

Dear Lai Jiang,

Let’s try to answer on your questions on the example of cycling Al-foil as active anode material in organic electrolytes of different formula compositions while the concentration and nature of the ionogenic salt remain unchanged: 1 M LiPF6 solution.

An electrolyte on the base of FEC with EMC as co-solvent and combination of VC and ES as additives appears to have the best combination of low impedance and high capacity retention. This is likely due to the formation of an IPL (SEI) which contains both a flexible polymer (poly(FEC)) and high lithium salts content with synergetic effect (Li2S2O4, Li2CO3, LiF). However, if the polymer content is too high, as observed for half-cells cycled with EC-based electrolyte containing additive of VC or FEC-based electrolyte without additives (do not shown on the figs), the IPL films are too thick and the half-cells resistance may dominate cycling performance. Also as very effective IPL (SEI)-forming additive I can recommend you fluoropropane sultone (J. Mater. Chem. A, 2013, 1, 11975, http://dx.doi.org/10.1039/c3ta12580g).

Sorry for mistake: designations for PC-containing electrolyte  without additives and with additives should be reversed.

Dear Sergii Kuksenko,

is there any Eis, also ?

Dear Ioannis Samaras,

What do you mean "Eis"?

Dear Sergii, by "Eis", we mean Electrochemical Impedance Spectroscopy (studies).

Note: See, please, a couple of related Fig. in some Refs (more are available) :

1. See Fig.4, 5  & 6  in https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120012917.pdf

2. See Fig.3 & 4 in http://www.cswang.umd.edu/publications/papers/30.pdf

Dear Ioannis,

I suppose that Qairr is more valuable parameter in this case than direct impedance data. It provides of the dynamics and depth of the occurring side reactions and is an indicator of the quality of IPL formed (including its periodic "restructuring") for n cycles. 

Thank you very much for paper from J. Electroanal. Chem., I've read it with great pleasure. To my regret, I can't to open the first ref. 

Best wishes,

Sergii

Dear Sergii, please try it now!

Dear Satish,

I am very grateful to you for the paper.

Good luck,

Sergii

Thank u Prof.

Dear Sergii,

by "Eis", we have a complementary  "view" of the resulting SEI layer. Also, the modeling parameters' evolution may offer a detailed and dynamic "view" of the SEI at any SOC in a cycle, as well as with cycle number.

Dear Ioannis,

I totally agree with you. However, the objective of this study was to show the possibility of spatial stabilization alloy-forming electrodes by establishing a "successful" formula of electrolyte for improvement of capacity retention and suppression of processes corresponding to irreversible capacity. Besides it was suggested to use Al-foil as a convenient material and the general approach as a methodological technique for accelerated composition of an acceptable electrolyte formula. Now that we know the benefits of FEC-EMC-VC-ES composition (see figs.), we can use a more subtle instrument, what is EIS.