Towards high-energy and durable lithium-ion batteries via atomic layer deposition: elegantly atomic-scale material design and surface modification

At present, we are mostly relying on fossil fuels (including coal, oil, and natural gas) for energy supplies. As finite energy sources, they are quickly depleting as well as causing environmental issues [2]. In this context, seeking new forms of energy becomes imperative. Of all possible candidates, solar radiation and wind [3, 4] become the primary ones lauded because of their renewability, cleanness, and abundance. The two are exclusively converted into electricity for use. Given their intermittent operations determined by the alternating daytime/nighttime and varying weather conditions, they are therefore unable to generate and supply electricity smoothly. To this end, electrical energy storage (EES) [4] becomes particularly critical for widely implementing solar and wind energy. Batteries are among the most successful electrochemical devices, enabling EES in the form of chemical energy. Lithium-ion batteries (LIBs), commercialized in 1991 [5], dominate consumer electronics due to their superior performance versus their competitors. To fill the foreseeable gaps left by the depletion of fossil fuels, we are expecting LIBs to power transportation and support smart grids in the future. However, the state-of-the-art LIBs are still undesirable in energy density, rate capability, durability, safety, and cost [6]. In this situation, next-generation LIBs are undergoing intensive investigation, and various research routes are being sought.

Atomic layer deposition (ALD) [7], delivered in the 1970 s [8] and traditionally used as a thin-film technique in the semiconductor industry, first emerged in nanotechnology at the very beginning of the 21st century and had immediately attracted great interest due to its unique capabilities of nanostructuring materials [9] at the atomic level. The exceptional characteristics of ALDs were further identified in controlling material crystallinity [10] and fine-tuning compositions of complex materials [11]. These characteristics well distinguished ALD from other methods for novel nanostructured materials and made it possible to use them for many important applications. The first practice of ALD in LIBs appeared in 2000, when V₂O₅ nanofilms [12] were electrochemically investigated as LIB cathodes. Thereafter, no further work was reported until 2007, when ALD TiN [13] was coated on Li₄Ti₅O₁₂ to tailor the interface between the anode and electrolyte for improved battery performance. More extensive studies on ALD for battery applications were exposed beginning in 2010, and all the emerging applications were thoroughly reviewed by Meng et al [14] in 2012, and featured battery components' design and interface tailoring. Since then, many more research attempts have been conducted using ALD. At this point, Liu and Sun timely presented a new review [1] in which updated advances were summarized, mainly including two aspects (as illustrated in figure 1): (i) design of nanostructured battery components; and (ii) interface tailoring between electrode and electrolyte. In either way, there are many bright spots worthy of being highlighted.

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In the first aspect, as clearly documented in the review by Liu and Sun, ALD is capable of developing various battery components (i.e., anodes, electrolytes, and cathodes). In battery applications, ALD could either coat powder-based materials for nanocomposites as electrodes in bulk-type batteries or directly deposit each component as nanofilms on two-dimensional (2D) or into three-dimensional (3D) substrates for microbatteries. While the former is very attractive for nanostructuring battery components, the latter shows great potential for commercialization. In terms of material classes, there are more species studied since Meng's review [14]. Specifically in the case of anodes, Liu and Sun noted that, besides binary oxides, ALD demonstrated its fine-tuning capability in preparing ternary materials (e.g., Li₄Ti₅O₁₂ [11]). Furthermore, ALD sulfides [15, 16] were first reported as new electrodes. Compared to anodes, ALD made more progress on cathodes. Besides the V₂O₅ formerly reported, Liu and Sun thoroughly tracked new successes by ALD in synthesizing other complex cathodes including FePO₄ [17], LiCoO₂ [18], Li_xMn₂O₄ [19], and LiFePO₄ [20]. In addition, ALD also has explored more inorganic solid-state electrolytes and new species include Li₃N [21], LiNbO₃ [22], LiTaO₃ [23]. All these new achievements by ALD have greatly expanded its capabilities from synthesizing binary to ternary or more complex materials, featuring its fine-tunability on material compositions at the atomic level.

Another distinguished aspect well described by Liu and Sun focused on various ALD nanofilms coated on different electrodes for enhanced battery performance. Besides the ones (Al₂O₃, TiO₂, and TiN) reviewed by Meng et al [14] ZrO₂ [24] is one new coating material studied for anodes. In comparison, more coating films were investigated for cathodes. Of them, it is worth noting that two lithium-containing solid-state electrolytes (LiAlO_x [25, 26] and Li_5.1TaO_z [27]) were first used to surface-modify cathodes for better performance. The beneficial effects of all the coating films lie in the formation of a protective layer of artificial solid electrolyte interphase (SEI) on different electrodes, leading to a reduction in side reactions, an improvement in mechanical properties, and enhancements in electrical/ionic conductivity. ALD coatings [26, 28] were also tried as a way to sustain the structural stability of cathodes and keep battery voltage from fading. To date, however, studies have revealed that ALD ultrathin films were very effective in capacity retention but were undesirable for alleviating voltage decay.

Apparently, Liu and Sun have prepared a detailed summary of the latest advances of ALD in LIBs. All the research mentioned in the review have recognized ALDs as a novel and viable technique for high-energy and durable LIBs. However, there is still a need to explore ALD in terms of electric vehicles and smart grids. For instance, inorganic solid-state electrolytes represent an important route to secure battery safety, but to date, the electrolytes created by ALD were too low in their ionic conductivities to replace state-of-the-art liquid electrolytes. Furthermore, the most recent studies demonstrated that ALD is also an effective technique for other advanced battery technologies such as lithium-sulfur batteries [29], lithium-air batteries [30], and sodium-ion batteries [31]. With an increased awareness of the capabilities of ALDs to function as better batteries, as the review tried to deliver, it is reasonable to believe that ALD is likely to bring us more technical breakthroughs in future battery studies.

Acknowledgments