The success of Li-ion batteries is based on their high volumetric and gravimetric energy-density available for storage. This has enabled the realization of small portable devices like mobile telephones and laptops. However, important demands also include material and production costs, safety, cycle life (number of charge/ discharge cycles), and high (dis)charge rates. Addressing all these demands makes it very challenging to find better electrode and electrolyte materials to improve Li-ion batteries [2, 4]. To gain more insight into the challenges in battery research it is useful to express the performance-related demands in more specific material properties of the electrodes and electrolytes. With respect to the general demands of cost and safety we merely state that batteries require abundant and cheap materials that are intrinsically stable during battery operation. For more extensive literature we refer the reader to some excellent reviews [2, 4—6].

The gravimetric and volumetric energy density of a battery is CVOCP, where C is the specific or volumetric capacity (mAhg-1 or mAgcm-3), respectively, and VOCP is the open circuit, or equilibrium potential of the battery. Large gravimetric energy — density is required for automotive applications, and large volumetric energy den­sities are essential in mobile electronic equipment. It is important to realize that the specific Li capacity of the electrode materials is not necessarily the only decisive factor, and that the actual capacity of an electrode also depends significantly on the electrode morphology [7—9]. Large gravimetric energy-density requires dense electrodes and hence low porosities. In this context nano-sizing of electrode materials, aimed at higher storage capacities and higher rates, typically carries the disadvantage of low tap densities (powder packing) leading to less dense electrodes and compromising both volumetric and gravimetric energy-density.

The power density is the product of the specific current and the voltage that the battery can deliver. The battery voltage is defined by the difference in potential between the electrodes and the current via the internal resistance of the battery. High power densities, allowing for fast (dis)charge, require low internal resistance of the battery. The various charge-transport phenomena inside a battery contribute to this internal resistance, including the electronic conductivity through the elec­trodes, the ionic conductivity through the electrode and electrolyte, and finally the charge transfer through the interface between the electrode and electrolyte. Gen­erally, the Li ion and electronic conductivity through the electrodes are assumed to be rate limiting in Li-ion batteries. Note that because the electrode is porous the ionic conductivity of the electrodes includes both the transport of ions through the solid-state electrode as well as through the electrolyte dispersed in the electrode pores. The solid-state transport of ions through electrodes depends on the specific electrode host material and is typically orders of magnitude slower than in liquid electrolytes. The overall Li-ion conductivity within the electrodes also depends strongly on the electrode morphology, such as characterized by the porosity [10] and the interconnectivity of the pores [11, 12].

High energy and power density are conflicting demands, as evidenced by the impact of electrode morphology and electrode thickness on these. High porosities, such as in nanostructured electrodes, generally lead to fast ionic transport throughout the electrodes responsible for high (dis)charge rates, and hence high power densities. However, the downside of large porosities is the larger volume required to store the same amount of energy, resulting in smaller volumetric energy- density. Because in most cases either electronic or ionic conductivity through the electrodes is rate limiting [7—9], thin electrodes can be charged faster than thick electrodes. Hence, the power density of a battery can be improved by the use of thinner electrodes. However, building a battery from thin electrodes leads to a smaller amount of active electrode material per gram of battery because of the relatively larger amount of current collector, electrolyte, and packing materials.

The cycle life of batteries is determined by a combination of the chemical and physical processes that occur during (dis)charging in the electrolyte and electrodes of a battery. Generally, electrodes undergo structural changes upon Li insertion and extraction. Large volumetric changes upon Li insertion and extraction lead to mechanical failure of the electrode. The result is generally that part of the active material is electronically disconnected from the electrode and therefore inactive. Consequently, small structural changes, such as occurring in Li4Ti5O12 spinel (where the change in the unit-cell volume, AVunit_ceU, & 0.1 %) [13], and moderate structural changes, such as occurring in LiFePO4 (AVunit_ceu & 6.5 %) [14], contribute to batteries with long cycle-life. The other extreme is the alloying reaction of Li with silicon up to the Li4.4Si composition resulting in a volume expansion >300 %, which leads to mechanical failure of the electrodes containing relatively large silicon particles within only a few charge/discharge cycles [15]. Nevertheless, the very high capacity associated with this type of alloying reaction has motivated chemists and material scientists to develop smart strategies to maintain the mechanical coherence of these electrodes [15—17].

The second factor that is important for the cycle life, and also to safety, is the thermodynamic stability of the electrolyte with respect to the positive and the negative electrodes (Fig. 7.2). The electrodes have electrochemical potentials (anode) and ^C (cathode) equal to their Fermi energies sF. If ^A is above the lowest unoccupied molecular orbital (LUMO) of the electrolyte, the anode will reduce the electrolyte. This reduction decomposes the electrolyte unless a passivating layer, generally referred to as the solid electrolyte interface (SEI) layer, is formed. The SEI layer often forms in the first few cycles and can act to electronically insulate the anode from the electrolyte, preventing further reduction. The SEI growth tends to stop after a few cycles, resulting in stable battery performance. Similarly, if ^C is below the highest occupied molecular orbital (HOMO) of the electrolyte, the cathode will oxidize the electrolyte unless a passivating SEI layer is formed, electronically insulating and preventing further oxidation. Therefore, not only should the

electrolyte provide a wide stability window in terms of voltage, but the gap between its LUMO and HOMO must be larger than the difference between the chemical potentials of the electrodes. That is, the electrolyte voltage window should be positioned such that LUMO > and HOMO < fic. The large open-cell voltage (VCeld of Li-ion batteries (e. g. 3.6 V) that is responsible for their high energy and power densities requires electrolyte stabilities that exceed that of water (Elumo-Ehomo ^ 1.3 V) or water-containing electrolytes, leading to the application of non-aqueous electrolytes. Such stability demands do not apply if an electroni­cally-insulating layer is formed upon electrolyte reduction or oxidation which can passivate further reactions. An example of passivation of SEI layers is the graphite or carbon-based anodes that work at about 0.5 V below the stability of typical carbonate electrolyte solutions, and hence 0.5 V above the LUMO of the electrolyte. The first few cycles with these anodes and carbonate-based electrolyte solutions results in the reduction of the electrolyte which leads to the formation of a stable SEI layer that prevents further reduction of the electrolyte. Importantly, the SEI layer does not grow significantly in subsequent cycles. Therefore, strategic combinations of elec­trodes, electrolytes, and SEI layers can result in better-performing batteries.

Improving Li-ion batteries with respect to energy and power density, manu­facturing costs, safety, and cycle life is clearly a formidable challenge. A common starting point is to lower costs and use environmentally-benign materials for both the electrodes and the electrolyte, noting that the cathode can account for as much as 40 % of the cost of the battery. The electrolyte should have high Li-ion conductivity over a practical ambient-temperature range and a large stability (potential) window allowing use of large differences in electrode potentials. The electrodes should work within the stability window of the electrolyte and allow fast (dis)charge of a large reversible capacity. This not only depends on the electrode material and composition but also on the electronic and ionic ‘wiring’ of the electrodes determined by the electrode morphology. The development of new electrode and electrolyte materials and the improvement of existing materials requires a fundamental understanding of the Li insertion and transport mechanisms that involve both structural and kinetic phenomena. Probing these is a tremendous challenge given the complexity of the system and the difficulty of probing a light element such as Li, particularly under in situ conditions.