Factores que influyen en las dificultades de la comprensión lectora

Lithium-ion batteries are comprised of cells that employ lithium intercalation com- pounds as the positive and negative materials [38]. As a battery is cycled, lithium ions (Li+_{) exchange between the positive and negative electrodes. They are also re-} ferred to as rocking chair batteries as the lithium ions rock back and forth between the positive and negative electrodes as the cell is charged and discharged [2]. The positive electrode material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material with a tunneled structure, such as lithium manganese oxide (LiMn2O4), on a current collector of aluminum foil. The negative electrode material is typically a graphitic carbon, also a layered material, on a copper current collector.

In the charge/discharge process, lithium ions are inserted or extracted from in- terstitial space between atomic layers within the active materials. The first batteries marketed, and the majority of those currently available, utilize LiCoO2 as the pos- itive electrode material. Lithium cobalt oxide offers good electrical performance, is easily prepared, has good safety properties, and is relatively insensitive to process variation and moisture. More recently lower cost or higher performance materials, such as LiMn2O4 or lithium nickel cobalt oxide (LiNi1−xCoxO2), have been intro- duced, permitting development of cells and batteries with improved performance [37, 18]. The batteries that were first commercialised employed cells with coke neg- ative electrode materials [39]. As improved graphites became available, the interest

2.2 Li-ion cell structure and chemistry 15

has shifted to graphitic carbons as negative electrode materials as they offer higher specific capacity with improved cycle life and rate capability [35, 36].

Intercalation processes

Li-ion batteries typically consist of two electrodes, an anode and a cathode with a separator between them to prevent shorting [40]. The cell is filled with electrolyte. Figure 2.3 illustrates a typical Li-ion cell sandwich consisting of a graphite anode and a LiCoO2 cathode. The electrodes consist of active materials bound together

Figure 2.3: Schematic of a typical Li-ion cell.(Source: [1])

with an electronically insulating binder and conductive additives. Each electrode is pasted onto current collectors. During charge, Li is removed from the cathode (or positive electrode), transferred through the separator via the electrolyte and is inserted into the anode [41]. The reverse occurs on discharge. The difference in voltage of the cathode and anode is the cell voltage. The amount of Li that is stored in each of these materials is related to the capacity (often given in mAh/g). The product of the voltage and the capacity is the energy. How quickly the Li is transferred from one electrode to the other (or how quickly the energy is removed) is related to the power [38].

Figure 2.4 shows the typical steady-state charge (constant current) of the anode and cathode of a Li-ion cell with a graphite anode and a LiCoO2 cathode in an organic electrolyte consisting of a Li salt (lithium hexafluoro phosphate, LiPF6) in a solvent (e.g., ethylene carbonate and diethyl carbonate) [37]. This is the battery

Figure 2.4: Steady state charge curve of a Li-ion cell.(Source: [1])

used in laptops and cell phones [28]. The voltage of each electrode is represented with respect to a Li-metal reference electrode. As the Li is removed from the cathode, its potential increases, while the potential of the anode decreases with insertion of Li. The process of Li moving in and out of the electrodes is referred to as intercalation/deintercalation. The voltage of the battery is the difference in voltage of the cathode and the anode, which increases as charge proceeds. The abscissa represents how much Li is stored in the cell, while the ordinate shows at what voltage the Li is inserted/removed from the materials. In order to increase the energy of the battery three avenues can be pursued, namely:

• Increase the voltage of the cathode

2.2 Li-ion cell structure and chemistry 17 • Increase the capacity of the cell

However, the thermodynamics of electrochemical reactions other than the inter- calation of Li (referred to as side reactions) limits these quantities [41]. The three side reactions worth mentioning in this figure are the oxidation of the solvent that occurs above 4.2 V, Li-metal deposition that occurs below 0 V, and solvent reduction that occurs below 1 V . These reactions not only limit the energy of the cell, but they are also implicated in the life and safety problems associated with Li-ion bat- teries. Fortunately, it has been observed that Li can intercalate into many different anode and cathode materials. At present, three classes of cathodes, four classes of anodes, and four classes of electrolytes are being considered for use in Li-ion cells. Depending on the combination of the anode, cathode, and electrolyte, one can have a completely new battery with changes to the energy, power, life, safety character- istics and low temperature performance [39, 34]. These classes are illustrated in Figure 2.5 for the three components of the battery.

Figure 2.5: Schematic of a Li-ion cell with the various anode, cathode, and elec- trolytes that are presently being considered. Changing the combination results in changes to the energy, power, safety, life, and cost.(Source: [1])