Reversible capacity

For practical applications the important factor is the reversible capacity A H/M) which is defined as the plateau width. Reversible capacity can be considerably less than the maximum capacity (H/ M) max [ 59]. A classic example is vanadium hydride. At 3 5 3 K vanadium forms the so-called monohydride (x 1) at hydrogen pressure 

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Fig. 2. The master graph of reversible capacity for lithium plotted versus heat treatment temperature for a variety of carbon samples. The three regions of commereial relevance are marked. Solid symbols are data for soft carbons, open symbols are data for hard carbons.

Lithium insertion in microporous hard carbons (region 3 in Fig. 2) is described in section 6. High capacity hard carbons can be made from many precursors, such as coal, wood, sugar, and different types of resins. Hard carbons made from resole and novolac resins at temperatures near 1000°C have a reversible capacity of about 550 mAh/g, show little hyteresis and have a large low voltage plateau on both discharge and charge. The analysis of powder X-ray diffraction. 

Figure 7 shows voltage profiles, for the second cycle of most of the graphitic carbon samples listed in Table 1. The curves have been sequentially offset by 0.1 V for clarity. Most striking is a reduction of the maximum reversible capacity, or Q ,, (<2 =372-x ,3,), as P increases. 

The cell made from BrlOOO appears most promising. Its reversible capacity is about 540 mAh/g and it has a long low voltage plateau. Similar results were found for the second cycles of samples made from Ar and Cr resins, except that the capacities were smaller. 

Additional samples were prepared from the three resins and were heated at temperatures between 940° and 1100°, under different inert gas flow rate and with different heating rates. The samples have different microporosities and show different capacities for lithium insertion. The results for all the carbons prepared from resins are shown in Fig. 32, which shows the reversible capacity plotted as a function of R. The reversible capacity for Li insertion increases as R decreases. This result is consistent with the result reported in reference 12,... 

Fig. 32. Reversible capacity of microporous carbon prepared from phenolic resins heated between 940 to 1 I00°C plotted as a function of the X-ray ratio R. R is a parameter which is empirically correlated to the fraction of single-layer graphene sheets in the samples.

The physicochemical properties of carbon are highly dependent on its surface structure and chemical composition [66—68], The type and content of surface species, particle shape and size, pore-size distribution, BET surface area and pore-opening are of critical importance in the use of carbons as anode material. These properties have a major influence on (9IR, reversible capacity <2R, and the rate capability and safety of the battery. The surface chemical composition depends on the raw materials (carbon precursors), the production process, and the history of the carbon. Surface groups containing H, O, S, N, P, halogens, and other elements have been identified on carbon blacks [66, 67]. There is also ash on the surface of carbon and this typically contains Ca, Si, Fe, Al, and V. Ash and acidic oxides enhance the adsorption of the more polar compounds and electrolytes [66]. 

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... 

Electrochemical tests in half-cells allow the preliminary assessment of the WUT carbon as well as of the impact of grinding on the electrochemical performance. The data from the chart on the Figure 5 indicate that the material has high reversible capacity (similar to the capacities of the commercial graphites described earlier). 

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... 

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. 

Knowing that for any pure natural graphite the capacity at low rate should be very close to the theoretical capacity, it did not become a surprise when the reversible capacity value of LBG1025 reached 365 mAh/g at C/20 rate it was still as high as 349.5mAh/g at C/5 rate. 

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

As to the intercalation capacity, its relation to the carbon crystalline structure is ambiguous. The reversible capacity is reported to go through a minimum at d,M2 = 0.343 nm. The Lc dependence of capacity is reported to be of a similar shape (a minimum near Lc 5 nm, an increase when... 

Lc 1 nm or Lc>20 nm). Highly graphitic materials (<7002 within 0.335 -0.338 nm and Lc>30 nm) show some increase of the capacity when the lowest limit of d002 is achieved and Lc increases. The specific reversible capacity vs crystallite size or vs treatment temperature diagrams are often published. They look as smooth curves with a minimum within medium temperatures or medium crystallite sizes [7,11,17]. 

The source carbon materials show a significant electrochemical activity for lithium intercalation though the reversible capacity is relatively low and tends to reduce with cycling. For the thermally expanded graphite... 

Thus, the experiments carried out suggest that using the theoretical concept proposed by Figure 2, it is possible to create negative electrodes that would feature reversible capacity exceeding 400 mA-h/g. 

Both carbon materials were tested for their initial electrochemical performance in the 2-electrode electrochemical cells with Li metal as a counter electrode. Our findings have shown that with both types of carbon materials, achieving near theoretical reversible capacity upon Li+ deintercalation was possible. Thus, in a typical half cell environment (a CR2016 type coin cell with graphite and Li metal electrodes, a 1M LiPF6,... 

The comparative 2016 coin cell data of the uncoated vs. Si-coated spheroidized natural graphite precursor vs Li metal counter electrode (Figure 5) reveals a remarkable effect of metal addition onto the reversible capacity of anode. Thus, the starting material s reversible capacity increased from approximately 350 mAh/g to over 510 mAh/g (data taken at 0.8V vs Li/Li+), which is 1.37 times higher than the theoretical value of the reversible capacity for graphite. 

As it is seen from the data of Figure 8, all modified materials have poor cycling performance their reversible capacities fade faster than the one of initial non-modified material, and become lower after the first 8-10 charge-discharge cycles. Thus, we can conclude that no positive effect is achieved by means of modification of the Carbon-Type material with bimetal tri-nuclear complex of Co(III)-Ni(II). 

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.

The reversible capacity of the modified G500 material is seen to be constantly higher than that of the initial graphite while the performance of other modified samples is worse. 

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