ABSTRACT
Lithium phosphates (LiMPO4) with an olivine structure have attracted extensive interests as potential cathode materials for lithium-ion batteries. In the crystalline structure, phosphorous occupies tetrahedral sites, while the transition metal (M) occupies octahedral sites and lithium forms one-dimensional chains along the [010] direction [51]. The most widely studied phosphate is LiFePO4. Owing to its high power density and long cycle life, LiFePO4 has a great potential in manufacturing lithium-ion batteries to power the next-generation EVs and HEVs. However, the electronic conductivity of LiFePO4 is relatively low (ca. 10-9 S cm-1 for the pure LiFePO4). In order to overcome this shortcoming, many methods have been used, among which the introduction of hierarchical 3D mixed conducting networks is one of the best choices in enhancing the power and rate performance of LiFePO4 [52], and this work will be further discussed in Section 6.5.3. 6.4 NANOSTRUCTURED ELECTRODE MATERIALSAlthough lithium-ion batteries have achieved great commercial success, further application of these batteries is now limited by their performance (e.g., charge/discharge rate) due to the fact that most lithium-ion batteries are based on micrometer-sized electrode materials, thus poor in their kinetics, lithium-ion intercalation capacities, and structural stability; limits will always exist if no improvement is made on the intrinsic diffusivity of Li ions in the solid state (ca. 10-8 cm2 s-1) or other material properties [8]. Therefore, in order to meet the demands of next-generation HEVs and clean energy storage, people may still face challenges in developing new electrode materials with high power density (viz. high rates), high energy density, longer cycle life, and improved safety [14]. As a result, people naturally turn to nanomaterials, for the sake of size effects, ultrahigh specific surface areas and other attractive features they possess. For example, reduced dimensions of nanomaterials may accelerate
the intercalation/deintercalation rates of Li ions, hence enabling the high-power characteristic of the battery. However, using nanomaterials does not necessarily mean taking a panacea. Before using nanomaterials, it is extremely important to understand effects, both positive and negative, of nanomaterials on the performance of lithium-ion batteries. 6.4.1 Advantages of NanomaterialsGenerally, the greatest obstacle to the use of lithium-ion batteries in HEVs or EVs lies in its relatively low power density, which results from kinetic problems in solid-state electrode materials, i.e., the slow Li+ and e-diffusion rate. And the mean diffusion (or storage) time, τeq, can be expressed by the diffusion coefficient, D, and the diffusion length, L, as shown in Eq. 6.3: Clearly, the value of τeq can be reduced through two approaches: One is to increase the value of D, and this can be achieved through doping foreign atoms; the other approach is to reduce the value of L, and this is what nanomaterials are for [53]. Although the first method can improve the mixed conduction, only limited rate-performance enhancement can be observed, and sometimes this method may even bring about unstable crystal structures [14]. However, through nanostructuring, the crystal structure of electrode material remains the same, while the diffusion distance of Li ions can be greatly shortened. Thus, τeq can be reduced dramatically. In this way, fast insertion/extraction of lithium ions in the electrode material can be realized on the condition that no deterioration will come to the structure or performance of the material. Moreover, it has been reported that electrode materials inactive toward Li insertion may become active when ‘‘going nano.” For example, low Li diffusion rate of rutile TiO2 along the ab-plane (Dab ~ 10-15 cm2 s-1) often makes Li insertion into rutile extremely hard. However, nano-sized rutile TiO2 (10-40 nm) is able to
eq 2 L D
τ = (6.3)
reversibly accommodate Li up to Li0.5TiO2 (168 mAh g-1) at 1-3 V vs. Li+/Li with excellent capacity retention on cycling compared with its micrometer-sized counterparts [54]. The difference in rutile TiO2 is mainly related to the drastic decrease in the diffusion time τeq, which is evidently caused by the shortening of Li diffusion path. This is the same for β-MnO2, which, when made mesoporous, will allow reversible lithium intercalation without destruction of the rutile structure [55]. In addition, with a large surface area, there will be a higher contact area between the material and the electrolyte. Hence, more lithium ions can be quickly absorbed onto and stored in the fine particles; meanwhile, the specific current density of active material can be significantly reduced, which will then enable a high lithium-ion flux across the interface. As a result, material performance such as capacity and high rate performance (or high power) can be improved. It can also be concluded from the above example that nano-sized rutile TiO2 with a specific surface area of ca. 110 m2 g-1 also exhibits an excellent high rate performance (100 mAh g-1 at 10 C and 70 mAh g-1at 30 C, where 1 C = 336 mA g-1) [54]. Another advantage brought about by nanomaterials is the enhanced structural stability. When material has a particle radius rp larger than the critical nucleation radius rc for that phase, structural transition to thermodynamically undesirable structures may take place. Therefore, by using nanoparticles with rp < rc, it is possible to eliminate such transitions. For example, layered LiMnO2 suffers from serious structural instability during the Li insertion/extraction process, which is responsible for its cycling capacity fade. In order to overcome such difficulties, materials with nanocrystalline structures are introduced, for a much higher Li-intercalation capacity than convention materials resulting from an easily accommodated lattice stress due to Jahn-Teller distortion [56]. In nanoparticles, it is widely accepted that the smaller the particles are, the more atoms these particles will have at the surface. Since the charge accommodation occurs mainly at or very near the surface, the need for diffusion of Li+ in the solid phase of nanomaterials may be reduced, which will then account for an enhancement in the charge and discharge rate of the electrode as well as a reduction
in the volumetric change and lattice stress caused by repeated Li insertion and expulsion.Besides, using nanomaterials will enable some lithium-storage mechanisms available for mass storage. One such new mechanism is the so-called “conversion” mechanism [32], which is first found in transition metal oxides, then in fluorides, sulfides, and nitrides [33, 57, 58], and the mechanisms can described by Eq. 6.4: where X = O, S, F, or N and M = Fe, Co, Ni, Cr, Mn, Cu, and so on. As can be seen, the mechanism is mainly related to the reversible in situ formation and decomposition of LiyX upon Li uptake and release, and the reversible capacities in these systems are usually in the range of 400-1100 mAh g-1. It is reported that electrodes made of CoO nanoparticles can achieve a specific capacity of 700 mAh g-1 with almost 100% capacity retention for up to 100 charge/discharge cycles [32], In addition to conversion mechanism, other mechanisms such as interfacial Li storage [59] and nanopore Li storage [24] have also been proposed, and more research work will follow this direction in the future.Nanostructured electrode materials also possess other merits, such as the change of electrode potential (or the thermodynamics of the reaction) [60], and the more extensive range of solid-solution-existing composition [61]; these merits, along with other above-mentioned advantages, provide infinite possibilities to nanomaterials. 6.4.2 Disadvantages of NanomaterialsFor nanomaterials, the excess surface free energy should be taken into consideration for the chemical potential, as shown in Eq. 6.5: where 2(γ/r)V gives the excess surface free energy, γ is the effective surface tension, V is the partial molar volume, and r is the effective grain radius.
