Dear Raam,

the initial anode material has a very low conductivity, due to the natural thin film of the SiO2 on Si grains, as a SiO2/Si "egg model". So, only a percentage of the initial encapsulated Si grains-"eggs", appears as active (anode material). Then, Si grains expand in volume, upon (further) lithiation (as breaking the SiO2 thin film of the SiO2/Si eggs) and, finally, all the Li-Si alloys grains commit to the active (anode material). So, your procedure  "0.1C, 0.5C, 0.2C,1C, 2C and 3C"  follows a progressive "breaking eggs" model, in a clustered dominoes mode.

Dear Ramm

It could be that the anode structure of the Silicon or Silicon alloy has slightly changed to a composite structure of  Silicon or Silicon alloy with Silicon dioxide structure after the first rate capacity test, especially if the first test was conducted in aerated (open) environment, not in a O2 free cell    This could trigger some inactive materials in the  structure of Silicon or Silicon alloys to be active again in the present of Silicon dioxide structure.   It is well known that the Si has high resistivity in a comparison to Si2O, refer to the following:

file:///C:/Users/hp/AppData/Local/Microsoft/Windows/INetCache/IE/V1TFF6J9/generalpropertiesSi62002.pdf

Dear Raam,

the initial anode material has a very low conductivity, due to the natural thin film of the SiO2 on Si grains, as a SiO2/Si "egg model". So, only a percentage of the initial encapsulated Si grains-"eggs", appears as active (anode material). Then, Si grains expand in volume, upon (further) lithiation (as breaking the SiO2 thin film of the SiO2/Si eggs) and, finally, all the Li-Si alloys grains commit to the active (anode material). So, your procedure  "0.1C, 0.5C, 0.2C,1C, 2C and 3C"  follows a progressive "breaking eggs" model, in a clustered dominoes mode.

It might be assumed that the binder hinders access of the electrolyte to the surface of silicon particles until pores appear in the course of several cycles. This could be eliminated by an additional decrease in the amount of the binder or in the electrode thickness. The decrease in the amount of binder introduced into the electrode is also an important factor for raising the specific capacity of the electrode. However, the content

of the binder can only be diminished while it continues to perform its main function, i.e., maintains a contact between active particles and provides adhesion of the electrode layer to the current collector in alternating changes in particle sizes in cycling, which may adversely affect the retention of the reversible electrode capacity.

I agree with the previous comments of Dr. Samaras and Kuksenko. Increase in the charge capacity might be due to the easy access of electrolyte ions to the active materials surface after the first cycle. Initially electrolyte ions are not able to reach all areas of the active elctrode material which shows less capacity. With further cycling electrolyte contacts the whole area (including micro and mesopores in case of porous material) increasing the charge capacity. 

The surface area of the active materials has increased  after the first rate capacity test of the Si or Si alloys due to the surface oxidation of Si structure to  SiO2 microporous structure.  

Dear Raam, can you please inform about how many cycles carried out at each C rate, additionally can you please provide the literature which you were mentioning. Ideally, the first cycle exhibits the highest capacity. Moreover, the after various C rates causes pulverization and you cell might have reached in complex nanoparticles of Si due to which higher oxidation is taking place (remember this could be dangerous) may deliver a unconventionally sudden rise in capacity.  Please do read more about solid electrolyte interphase layer and its breaking due to volumetric expansion and contraction of Si.

The Answer of Ioannis Samaras is quite intuitive and it is true that there is always SiO2 film covering Si grains . However, I have a question which might be helpful for my understanding of these alloying materials.  

During the first few cycles, i expect the reduction of SiO2 to silicon  (During Lithiation). So after prolonged cycles, the presence of SiO2 is unlikely, unless you are making SiO2 during the oxidation process which i have never observed if we are delithiating upto 1.5 V vs Li. 

So in my opinion, during first few cycles, the utilisation of all active material is not possible due to diffusion barriers.(If you are using nano particles, you might get complete utilisation). Once you make your material porous enough, the reaction become easier and you get maximum utilisation of your active material, consequently increasing the capacity. (I have no proof of that, its just a speculation)

Thank you everyone. The answers were very useful and informative. @ Arthur Braun, I will surely acknowledge the people for the advises.:)

@Edmund Samuel, Five cycles were made at each C rate. In some of the literatures, I could see in the plot, it is slightly higher than the inital C-rate but it was not discussed. If I remember, the exact literature, I will send it.

@Umair Gulzar I am using nano Si particles dispersed in carbon matrix

@Raam Kasinathan, if at each C-rate the cycles were 5, then please check differential capacity it could be possible the formation of SEI layer again. Due to pulverization apart from agglomeration, the particles become smaller which could contribute enhancement of the capacity. Additionally, from differential capacity, you may be able to understand the probable formation of LixSiy during your retention cycles. Please see the link below, if you are getting much higher value after retention (in comparison to first value) then differential capacity analysis, as well as its post-cycle composition, could help you much. Else if the retention value is just near to first value then it should not be a much difficult for analyzing the reason for retention capacity.

http://pubs.acs.org/doi/pdfplus/10.1021/acsami.6b11500

Hi, Raam,

I can say you what I would do at your place: I would perform the inverse order of capacity with brand NEW battery material (decreasing C), and after that the first order  (increasing C) with the SAME materials. Cheers

Stefano

---------------------------------------------

Stefano di Stasio, PhD

Electronics Telecommunications Eng. Uni FEDERICO II NAPOLI I

Specialist in Optics, Uni FIRENZE

PhD in Thermo-mechanical Eng. Uni FEDERICO II NAPOLI

Master in Digital Signal Processing, Politecnico di MILANO

Lab CNR IM of Nanostructures, light scattering and X-ray techniques, Head

Italian National Research Council

tel. 081 7177 122 (room); 081 7177 171 (lab);  mobile:  347 0525117

n° ORCID profile:  000-0001-6479-8996

e:mail:  [email protected]

Hello Mr. Raam

Can you suggest the best way to disperse Si particles in carbon matrix? any preference for carbon matrix( graphite, CNT, graphene?

you need before the average pore size of carbon matrix and particle dimension

Hello Raam,

For your study, other than Rate capabililty test, do you have other electrochemical tests, such as CV and EIS. If you have the CV, please perform the CV test again for your high rate cycled cells at the same conditions. Check whether you have other peaks that not belong to your active material. At the same time, perform the cycle life test at contant C rate, see whether in the same cycle range, you have a capacity increase or not? If yes, probably the capacity increase is not because of the high rate training. If no, then high rate test could be one of the reason for the capacity increase.  As advised by Dr. Ioannis Samaras, the conductivity, the active Si percentage in your electrode may increase after cycling, but I recommend you first see whether your capacity in the initial cycles are reasonable or not, as compared to theoretical capacity. If your first cycle capacity is much less than theoretical capacity, I would personally believe your electrode is not well cast. The reason is that: at beginning, not all your silicon in the electrode is active, then in the later cycles, due to the volume change, more silicon particles are exposed to the electrolyte or be connected into the conductive path ways, which will lead to the capacity increase. Usually, for these type of electrodes, your cell repeat-ability will not be good, or you will have a large capacity derivation from cell to cell. 

I would prefer to know you casting composition and the post treatment, such as whether you have good electronic conductive path, what's the thickness, loading level and the density of your electrode. If you just cast your slurry and dried without calendering, you may not have good electric paths. However, during cycling, in the coin cell, you will have some type of pressure in it, and the volume change may connect some inactive silicon to contribute in the later cycles.