Irreversible capacity loss

SEl surface films form both on the anode and the cathode and this means that a certain amount of electrolyte is permanently consumed. The irreversible process of SEl formation immobilizes a certain amount of lithium ions within the insoluble salt that constitutes the SEl. Since most LiBs are built as cathode-limited, in an attempt to avoid the lithium metal deposition on the carbonaceous anode at the end of charging, the consumption of the limited lithium ion source during the initial cycles leads to some permanent capacity loss of the cell. Thus, cell energy density and the corresponding cost are compromised. The extent of this irreversible capacity loss depends on the anode-electrolyte-cathode combination chosen. 

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In order to improve the electrochemical performance with respect to lower irreversible capacity losses, several attempts have been made to modify the carbon surface. Here the work of Peled s [38, 130-132] and Takamura s groups [133-138] deserves mention. A more detailed discussion can be found Chapter III, Sec. 6. 

This difference is the irreversible capacity loss (<2jr). Dahn and co-workers [71] were the first to correlate <21R with the capacity required for the formation of the SE1. They found that <2ir is proportional to the specific surface area of the carbon electrode and, assuming the fonnation of an Li2C03 film, calculated an SEI thickness of 45 5 A on the carbon particles, consistent with the barrier thickness needed to prevent electron tunneling [1,2]. They concluded [71] that when all the available surface area is coated with a film of the decomposition products, further decomposition ceases. 

The electrochemical characteristics of these three materials are summarized in Table 1. The reversible capacity, the irreversible capacity loss and capacity below a certain voltage (0.5V in this case) were identified to be the key important electrochemical parameters at the time. 

Material Charge capacity at Discharge capacity (2V vs l.i/l.i+), mAh/g Irreversible capacity loss, %... 

The electrochemical galvanostatic charge-discharge curves at different rates are presented by Figure 6. The data is also summarized in Table 6. In our testing, the irreversible capacity loss of SLC1015 was seen to... 

At the electrochemical performance level, these novel natural graphite-based materials surpass mesophase carbon s characteristics as related to cell/battery safety performance, low irreversible capacity loss, and good rate capability even at high current densities. 

Figure 10. Irreversible capacity losses (A) and reversible capacity at 10-th cycle (B) for two specific current values of modified Graphite-type materials annealed at different temperatures.

At 50°C, a marked improvement was seen over the baseline cell, both in terms of more stable cycling, a higher rate capability, and less first cycle irreversible capacity loss. Reasons for the improvement in performance appear to be related to a lowering of the electrode material impedance and a smaller first cycle irreversible capacity by suppression of PC solvent cointercalation due to a de-solvation catalyzed by Cu metal. 

The contribution by Rouzaud et al. teaches to apply a modified version of high resolution Transmission Electron Microscopy (TEM) as an efficient technique of quantitative investigation of the mechanism of irreversible capacity loss in various carbon candidates for application in lithium-ion batteries. The authors introduce the Corridor model , which is interesting and is likely to stimulate active discussion within the lithium-ion battery community. Besides carbon fibers coated with polycarbon (a candidate anode material for lithium-ion technology), authors study carbon aerogels, a known material for supercapacitor application. Besides the capability to form an efficient double electric layer in these aerogels, authors... 

Progressive irreversible capacity loss is sometimes confused witb the reversible memory effect processes. The former is a quite different... 

Passivating surface films are formed, and thus Li intercalation is highly reversible. The initial irreversible capacity loss due to the surface reactions is about 10-30% of the reversible capacity. This behavior is typical of EC-based solutions and solutions containing C02 (e.g., MF/C02/LiAsF6). 

The surface films formed are not sufficiently passivating, and their formation involves partial exfoliation of the graphite. Thick and resistive surface films are formed. The irreversible capacity loss is pronounced, and the electrode cannot reach all the Li-graphite intercalation stages (MF, DMC, and ether solutions). 

Figure 27 shows a typical chronopotentiogram of the first lithiation-delithiation cycle of a petroleum coke electrode [357], It demonstrates the irreversible capacity loss due to the carbon s surface reactions (plateau around 1 V versus Li/Li+), the sloping potential profile, and the fact that the maximal reversible capacity is less than that of graphite. However, its structural disorder makes this electrode... 

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