These batteries have represented a turning point in the field of power sources for a variety of applications. This is the result of such strong points as:
In contrast, some weak points also need to be mentioned:
A Li-ion cell is based on two electrodes able to insert Liþ in their structure. The term insertion includes both bi- and tri-dimensional structures. In the case of bi-dimensional (layered) structures, the term intercalation is preferentially used. At present, most commercial Li-ion cells have carbon as a negative, LiCoO2 as a positive, and an organic liquid or polymeric electrolyte. However, after many years of predominance of the C/LiCoO2 couple, new electrode materials have emerged, especially as substitutes for the positive (see later). Carbons capable of Liþ intercalation can be roughly classified as graphitic and non-graphitic. Pure graphite is crystalline while non-graphitic carbons contain more or less extended amorphous areas. Both have been utilized as negative electrodes in Li-ion cells.
In pure graphite, up to 1 Liþ can be intercalated per 6C atoms, that is the limiting composition is LiC6.
During the first charge, electrolyte decomposition and formation of a solid electrolyte interface (SEI) on C occur. Such a process is necessary as the behaviour of the C electrode depends on the characteristics of this layer. Electrolyte decomposition starts at 0.8V vs Li/Liþ and SEI formation continues down to 0.2 V; at this potential Liþ intercalation begins. When the voltage approaches 0V, the charge is stopped to avoid Li plating on C. Because of the SEI formation, the first-charge capacity exceeds the first-discharge capacity. The difference (irreversible capacity) should be minimized to reduce excess of the Liþ source (the positive electrode) in a real battery.
LiCoO2, the most common positive electrode, has a layered (hexagonal) structure from which Li can be de-intercalated upon charge and re-intercalated upon discharge.
The maximum practical delithiation for LiCoO2 is 60% (160 Ah/kg). Beyond this value, structural changes limit reversibility of the above reaction. More Liþ can be deintercalated from LiNiO2, so that a capacity of about 200Ah/kg may be obtained. However, this material is difficult to synthesize, has a remarkable capacity loss on cycling, and a limited thermal stability. Much better performance is obtained by partly substituting, in LiCoO2, Co with Ni and another metal (e.g. Al or Mn).
In recent years, new positive electrodes have gained attention, especially in view of building larger and more powerful Li-ion batteries. A positive electrode, wherein part of Co is replaced by Ni and Al (NCA), allows extending the specific energy up to 240 Wh/kg and the energy density to 630 Wh/L. If Mg-doped Li-Mn spinel is used, the power capability is increased: these Li-ion cells can be used in power tools and are considered as power sources for HEVs. Another excellent positive is nanosized-doped LiFePO4, capable of very high power outputs. An entirely new system, where both electrodes have been changed with respect to the traditional C/LiCoO2 couple, is Nexelion by Sony, which is based on Sn-Co-C as a negative and LiCoxNiyMnzO2þLiCoO2 as a positive. The new negative electrode uses nanoparticles that allow minimizing changes in particle shape during charge/ discharge, as was instead the case of other Sn-based anodes. It can accept considerably more lithium in its structure upon charge with respect to a conventional C anode. The Nexelion cell can be recharged to 90% capacity in 30 min at the 2C rate.
Li-ion cells can use both liquid and polymeric electrolytes. Among the prerequisites of organic liquid electrolytes to be used in these cells, two are of particular importance due to the specific nature of the electrode materials. First, the electrolyte has to grant a stable and efficient SEI on graphite, thus limiting self-discharge (Liþ deintercalation) while allowing fast reversible Liþ transport. Second, the electrochemical window of the electrolyte has to range from 0V to at least 4.3V vs Li/Liþ. Electrolytes commonly used in Li-ion batteries are based on LiPF6 and a binary solvent mixture, EC-DMC or EC-DEC (EC, ethylene carbonate; DMC, dimethyl carbonate; DEC, diethyl carbonate).
These solutions are stable up to 60C, and can be used down to 20/30C. Temperatures above 60C are problematic for Li-ion cells operation: while at the positive electrode parasitic reactions with the electrolyte are fastened, the SEI on the C surface becomes unstable. It can reform, but irreversible losses have been observed. Additives to improve the SEI are vinylene carbonate, used by SAFT and Sanyo, and methyl cinnamate, used by NEC.
In a few applications, working temperature below 20/30C may be reached. In these cases, the conductivity of the binary solutions becomes too low, and ternary or quaternary mixtures of carbonates must be used, for example LiPF6 in EC-DMC-DEC or LiPF6 in EC-DMC-DEC-EMC (EMC, ethyl methyl carbonate).
One of the greatest concerns with liquid electrolytes is their flammability, which becomes a source of risk in case of cell venting. Additives that may lower the flammability (fire retardants) include trimethyl and triethyl phosphate, and other P-containing organic compounds.
The electrolyte is supported on a microporous separator (polyethylene or polypropylene). Commercial cells are mainly available in cylindrical or prismatic form factors. Ribbons of electrodes and separators are wound together.
The negative electrode is supported on a thin Cu foil, while the positive electrode is supported on a thin Al foil. The case normally contacts the positive electrode and is made of stainless steel. However, the last generation of prismatic cells uses an Al case to take advantage of its lower weight.
In 1999, batteries with polymeric electrolytes have been commercialized. They can offer some advantages over the conventional ones, for example no leakage, flexibility and very thin form factors. The use of the so-called gel polymer electrolytes has resulted in commercial products for consumer applications with performance characteristics comparable to those of liquid-electrolyte cells. A gel polymer electrolyte (GPE) is formed by immobilizing a liquid electrolyte in a polymeric matrix. An example is provided by the electrolyte used in Sony's cells: a solution of LiPF6 in PC-EC is added to a copolymer of poly(vinylidene fluoride) (PVDF) and poly(hexafluoropropylene) (PHFP).. The conductivity of this gel electrolyte is comparable to those of liquid electrolytes.
The evolution of Li-ion batteries has been remarkable in these years, with increasing capacity, energy, reliability and safety. For the popular 18 650 cell size (diameter: 18 mm, height: 65 mm), the progress in capacity and energy density since 1993. . The use of Li-ion batteries in portable (not only consumer) electronics is well known. These batteries are also increasingly used in industrial applications.
Applications only thought possible for aqueous batteries are now at hand for Li-ion systems too. In aerospace applications, where price is not the main issue, these batteries are already used, for example in GEO and LEO satellites and in rovers for planetary missions, such as the Mars Exploration Rover. Forthcoming applications include: traction (advanced vehicles are available), uninterruptible power supplies (UPS) and energy storage systems. Relatively smaller batteries are used in cleaners, motor-assisted bicycles, power tools, etc.
Cellular phones account for the largest share, followed by notebooks. The growing use in power tools is noteworthy: until 2004, only Ni-Cd and Ni-MH were used in this segment, but the new Li-ion chemistries have changed the scenario.
Finally, the long-term forecast for Li-ion batteries are to be used in portable applications and HEVs. The demand for these batteries features an almost linear growth till the year 2016, with cellular phones and notebook computers representing major shares. The power tool and HEV segments will become increasingly relevant. For the latter, this would be more evident if the battery value, not the number of battery sold, were taken into account.
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