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Lithium Metal Batteries

1.1 History

Li metal has been widely regarded as the ideal anode material due to its ultrahigh theoretical specific capacity (3860?mAh?g-1) and very low redox potential (~3.040?V vs. standard hydrogen electrode) [1,2]. The first visible Li metal secondary battery was operated by Stanley Whittingham at Exxon in the 1970s. In the late 1980s, Li metal batteries based on MoS2 cathode and excess Li were commercialized and sold to the market, which could be cycled hundreds of times. But due to safety accidents, including fires caused by Li dendrites, all the cells were recalled [3]. Since then, safety concerns about Li metal anodes still prevail. Although NEC and Mitsui realized an ultra-long cycling stability of Li metal cells over 500?000 cycles, the safety issue is not solved. Later, Sony Company developed a safe Li-ion battery based on a carbonaceous anode instead of Li metal, which can overcome the safety concerns related to Li dendrites, and these kinds of cells were commercialized and are in use until today [4]. As a result, the studies on Li metal anodes were halted.

Now, with the emergence of vehicle devices and hybrid grids, the state-of-the-art Li ion batteries based on graphite anodes are reaching their theoretical capacity, which cannot fulfill the increasing demand for high energy density [5]. Therefore, the application of Li metal as anode attracts the researchers. Over the four decades of research on Li metal anodes, great achievements have been made in improving the energy density and cycling stability of Li metal batteries [6]. Among them, various cathode alternatives have been reported, such as intercalation cathodes [7], sulfur/selenium/tellurium composites [8-10], oxygen/air electrodes [11], iodine/bromine flow cathode [12], and lithium-free transition metal oxides [13]. These batteries have their own advantages, but also exhibit their intrinsic problems, which hamper practical applications of Li metal batteries. But these battery systems all have common problems related to Li metal [14]. First, Li metal can react with the electrolyte to form a solid electrolyte interphase (SEI) layer on the surface, which will consume the electrolyte and increase the internal resistance. Second, Li dendrites will grow and cause safety concerns. Third, large volume change of Li metal can break the SEI and lead to further reaction between Li metal and the electrolyte, resulting in the failure of cells. All these disadvantages need to be addressed before the commercialization of Li metal batteries.

1.2 Types

In this chapter, we summarize several different types of Li metal batteries, such as lithium-oxygen batteries, lithium-sulfur batteries, lithium-selenium/tellurium batteries, lithium-iodine/bromine batteries, and lithium-free transition metal oxide Li batteries. In each section, we first introduce the working mechanisms, then the design strategies for cathode structures, the modification of electrolyte, and the protection methods for Li metal anodes.

1.2.1 Lithium-Oxygen Batteries

With the continuous consumption of nonrenewable fossil fuels and new requirements for carbon neutrality, green energy systems have been paid much attention. Among them, Li-O2 batteries using O2 as cathode and Li metal as anode have the highest energy density (~3500?Wh?kg-1). The cathode, O2, can be inexhaustibly extracted from open air and show the lightweight property to reduce the whole weight of Li-O2 cells. Unlike cathodes in Li-ion batteries, that consist of high-cost nickel or/and cobalt metal, the abundance and cheap O2 cathodes can dramatically reduce the cost of Li-O2 batteries. Moreover, the Li anode exhibits high specific capacity and lowest electrochemical potential. Although Li-O2 batteries have so many advantages over Li-ion batteries, their practical applications are hampered by the sluggish oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) and random deposition of discharge products (Li2O2), leading to high overpotentials, low capacity, and unstable cycling performance [15]. In this section, the working mechanism of Li-O2 batteries is discussed first and then the reasonable design for cathode and anode protection is summarized. Finally, problems are summarized and future directions are provided.

1.2.1.1 Working Mechanism of Li-O2 Batteries

As shown in Figure 1.1a, a Li-O2 battery consists of three parts: Li metal anode, electrolyte, and porous cathode [16]. Unlike other energy storage systems, the Li-O2 battery is a semi-open system, which can allow O2 penetrate into the cathode side to participate in electrochemical reactions. During discharge, the Li anode will be oxidized to generate Li+, and the oxygen is reduced on the cathode side, which can react with Li ions to form Li2O2, as described by Eq. (1.1):

Figure 1.1 (a) Schematic operation proposed for rechargeable aprotic Li-O2 batteries.

Source: Reproduced with permission Kwak et al. [16].

(b) Schematic of the Li2O2 growth pathways.

Source: Reproduced with permission Lyu et al. [17]. Reproduced with permission of (2017) Royal Society of Chemistry.

It looks very simple according to the reaction equation, but this direct reaction seems unlikely to occur electrochemically because the two-electron process requires much higher entropic barriers than that of a one-electron process. Actually, the discharging process is much more complicated than the equation [18], which is shown in Figure 1.1b. O2 is first adsorbed on the active sites of cathode materials, donated as , and then will get one electron and reacts with Li+ to form (Eq. (1.2)):

After that, the have three possible reaction pathways to finally transfer to Li2O2, as described by the following equation:

The forms through electrochemical reduction (Eq. (1.3)) or disproportionation on the surface (Eq. (1.4)), both of which are referred to as the surface growth model. It has been reported that the electrochemical reduction pathway can be easily realized due to the low kinetic barrier and free energy [19]. The adsorbed may also diffuse into the solution by modulating the electrolyte, which can be converted into Li2O2 via disproportionation in solution (Eq. (1.5)), which is regarded as the solution growth model.

The most expected is the four-electron transfer process, i.e. the O2 is fully reduced to lithium oxide (Li2O) because of its higher electron transfer number. However, this reaction pathway is kinetically impossible because Li2O is not observed by any spectroscopic technique at a cutoff voltage of above 2.0?V. Moreover, during the charging process, the Li2O could not convert into O2 [20]. On the cathode side, carbonaceous materials as matrixes can be decomposed on high voltage or react with Li2O2 to form Li2CO3 [21]. These solid side products will accumulate on the cathode surface, leading to blockage of electron and O2 channels and the failure of battery. Therefore, cathode materials with suitable pore and relatively high pore volume are desirable. The superoxide as discharge product can react with the electrolyte and form side products on the cathode surface. In addition, due to the intrinsic sluggish ORR/OER process, the rate capability of Li-O2 batteries is poor and catalysts should be applied on the cathode side [22,23]. In view of semi-open property of Li-O2 batteries, Li metal anodes are unstable with O2 and H2O, leading to gas generation and corrosion of Li anode. Thus, long-term stable Li-O2 batteries will not be realized until proper strategies are made for cathode structure design, electrolyte modification, and Li anode protection. From this discussion, we conclude that even Li-O2 batteries can provide high energy density, the challenges still exist on the way to make them applicable.

1.2.1.2 Cathode Design of Li-O2 Batteries

The structures and morphologies of cathode materials in Li-O2 batteries directly affect the ORR and OER activities, thus determining the energy density and coulombic efficiency of Li-O2 batteries. The commonly used cathodes are based on carbon materials due to their high conductivity, high surface area, low cost, lightweight property, and various and controllable structures [24]. The ideal cathode structure should have high chemical/electrochemical stability in the reversible cycling process; high surface area with mesoscale pores for large Li2O2 storage, suitable porosity to control the size of Li2O2 product; and high electrical conductivity to reduce the overpotential caused by the insulator and insoluble Li2O2 product.

For the cathode design, at the beginning, researchers largely focused on preparing nanosized materials with high conductivity and high surface area. For example, Zhang's group reported a porous air electrode with...