Irreversible capacity

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. 

Zhu et al. [94] reported the synthesis of Sn02 semiconductor nanoparticles by ultrasonic irradiation of an aqueous solution of SnCLj and azodicarbonamide under ambient air. They found that the sonochemically synthesized Sn02 nanoparticles improved remarkably the performance of Li ion batteries such that there was about threefold increase (from 300 to 800 mAh/g) in the reversible capacity in the first lithiation to delithiation cycles. Similarly the irreversible capacity also increased by about 70% (from 800 to 1400 mA h/g). Wang et al. [95] reported the synthesis of positively charged tin porphyrin adsorbed onto the surface of silica and used as photochemically active templates to synthesise platinum and palladium shell and... 

The Li-Ion system was developed to eliminate problems of lithium metal deposition. On charge, lithium metal electrodes deposit moss-like or dendrite-like metallic lithium on the surface of the metal anode. Once such metallic lithium is deposited, the battery is vulnerable to internal shorting, which may cause dangerous thermal run away. The use of carbonaceous material as the anode active material can completely prevent such dangerous phenomenon. Carbon materials can intercalate lithium into their structure (up to LiCe). The intercalation reaction is very reversible and the intercalated carbons have a potential about 50mV from the lithium metal potential. As a result, no lithium metal is found in the Li-Ion cell. The electrochemical reactions at the surface insert the lithium atoms formed at the electrode surface directly into the carbon anode matrix (Li insertion). There is no lithium metal, only lithium ions in the cell (this is the reason why Li-Ion batteries are named). Therefore, carbonaceous material is the key material for Li-Ion batteries. Carbonaceous anode materials are the key to their ever-increasing capacity. No other proposed anode material has proven to perform as well. The carbon materials have demonstrated lower initial irreversible capacities, higher cycle-ability and faster mobility of Li in the solid phase. 

Acetylene/carbon black is also quite effective but has an initial irreversible capacity that cannot be ignored. The amount of irreversible loss for acetylene black component ranges up to 20%. The particle size of conductive additives is recommended to be less than 5 microns. The addition is very effective to improve to improve 1) cycle life, 2) high power capability, and 3) the initial charge efficiency (reduce the initial irreversible... 

Figure 6. Integrated irreversible capacities ofLiCn in 1 MLiBr in S02/acetonitrile (AN) and 0.5 MLiClC>4 in PC as electrolytes. Carbon Highly graphitic carbon fiber PI 00 (Amoco), i = 50 pA mg 1, cut-off 0-2 V vs. Li/Li+ [4].

Figure 10. Integrated irreversible capacities of LiC in y-butyrolactone based electrolytes without (full symbols) and with (open symbols) C02 as electrolyte additive using various electrolyte salts LiCl04 (top, left), LiBF4 (top, right), LiPF6 (bottom, left), LiN(S02CF3)2 (bottom, right). Carbon Lonza KS44 synthetic graphite, i = 10 pA mg 1, cut-off 0-1.5 V vs. Li/Li+ [12],...

Figure 1. Comparison of the reversible and irreversible capacities of commercial graphites.

Commercial and non-commercial carbons were tested for their applicability as anode of lithium-ion battery. It was found that Superior Graphite Co s materials are characterized both by high reversible capacities and low irreversible capacities and thus can be regarded as good candidates for practical full cells. Cylindrical AA-size Li-ion cells manufactured using laboratory techniques on the basis of SL-20 anode had initial capacities over 500 mAh (volumetric energy density ca. 240 Wh/dm3). Boron-doped carbon... 

Irreversible Capacity vs. Surface Area and Particle Size... 

Figure 5. Irreversible capacity vs. particle size (a) and vs. specific surface area (b) of graphite electrodes comprising of synthetic flakes in EC-PC and EC-DMC solutions, as indicated.

In the case of EC-DMC solutions, since the surface species are deposited quickly and form very compact passivating films, the passivation of the active mass is obtained before products such as ethylene gas have the chance to be accumulated in crevices and an internal pressure to grow. Indeed, in these solutions the irreversible capacity depends inversely on the size of the particles (as expected). 

Figure 6. Potential vs. capacity curves obtainedfrom cycling tests of synthetic graphite flakes in EC-PC/LiCIO4 solutions in different discharge rates. Notice that as the discharge rate decreases - the irreversible capacity decreases accordingly.

The irreversible capacity results from formation of a surface-electrolyte interface (SEI) layer, and is believed to be caused by decomposition of the electrolyte on the surface of active material during few first charge cycles [3-5]. The values of irreversible capacity and the SEI are functions of the type of active material and the electrolyte. Also, the safety issue, which is believed to be associated with stability of SEI, has been identified as a major parameter in the equation [6-7]. The contribution of the negative electrode to the thermal runaway is believed to be related to the nature and also to the surface area of the active material [8-9]. 

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. 

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