While all major components are essential in LIBs, the cathode material has been limiting a shift to the next stage of Li-ion battery technology. As a key player of cathode active materials, intercalation compounds have been most widely used due to their unique ability to contain Li-ions over large concentration intervals.32,33Good intercalation compounds feature with one, two or three dimensional channels through Li-ions can migrate moderately. Lithium intercalation into the host material lead to a change in the electronic properties of transition metal and anion framework. As lithium is stored in intercalation compound, the corresponding valence electron is supplied through external circuit to balance the valence state of host and alters the nature of bonds between transition metal. Thus, the host structure with layered structure (O3 type) generally exhibits phase transition upon (dis)charging to decrease its thermodynamic potential (‘O’ refers the sites of alkali ion; in this case Li in octahedral site, the following number ‘3’ represents the number of transition metal slab along the c-axis) (Figure1-14).34
Recently, three types of intercalation compound are being intensively studied, and categorized as the followings : 1) layered lithium transition metal oxide,35,362) spinel-type lithium transition metal oxide37,38and 3) olivine LiMPO4(M=Fe, Mn, etc.) (Figure 1-15).39-41Each type of cathode material has its intrinsic pros and cons. For example, Li-rich layered cathode materials provide exceptionally high capacity (> 250mAh g─1) and energy density (~ 1,050 Wh kg─1), but their structural stability is inferior compared to other types. On the other hand, while spinel-type oxide materials have a superior rate capability, its usable specific capacity limits to only ~100 mAh g─1and exhibit poor cycle life especially at high temperature. Lastly, the olivines have been known as cathode materials with cheap, safe and have excellent cycle life and rate capability, but it has limited gravimetric (~ 590 Wh kg─1) and volumetric (2000 Wh l─1) energy densities because of their low density (~3.4 kg l─1) and low average voltage (~3.5 V vs. Li/Li+).
LiCoO2 and LiNiO2, O3 type layered transition metal oxides, were the first to be investigated as cathode materials.42-44 In particular, LiCoO2 (rhombohedral R-3m group) is one of the first commercialized cathode materials in LIBs, and still has been widely applied in portable devices.
LiCoO2 can deliver a high gravimetric (~ 650 Wh kg─1) and volumetric (~ 3,000 Wh kg─1) energy density due to its high operation voltage (~3.9V vs. Li/Li+) and true density (~5.05 g cc─1) (Figure 1- 16). However, in practice, only half on the Li is available for operation are reversibly cycled. Moreover, high cost of cobalt encourages the researchers to seek for other layered cathode material with Co-lean and Ni-rich composition.
To outperform LiCoO2 in terms of both performance and cost, LiNixMnyAlzCo1-x-y-zO2 cathode material has been intensively studied, in which TM layers are occupied by TM ions and Al. Among various TM compositions, LiNi1/3Mn1/3Co1/3O2 (NCM111), LiNi0.5Mn0.5O2, LiNi0.8Co0.15Al0.05O2 (NCA),
and LiNi0.8Co0.1Mn0.1O2(NCM811) have been most intensively studied because the synergetic effects of each transition metal in this class of materials in terms of specific capacity, safety and rate capability are greatly improved.45-49In particular, NCM811 cathode material exhibits a high gravimetric (~ 740 Wh kg─1) and specific capacities (~200 mAh g─1) (Figure 1-17). Furthermore, as the amount of Ni replaced Co increases, the price of Ni-rich cathode materials significantly decreases. Nowadays, Ni- rich materials have been highly regarded as a next generation cathode for electric vehicles and smart grid for now. While most of the traditional layered materials are stoichiometric materials, there have been numerous attempts to achieve very high capacities and energy densities from Li-excess layered materials. 50,51Li-excess cathode materials designed as solid-solution compounds (or nano-composite compounds) between Li(Li1/3Mn2/3)O2and Li(Ni/Mn/Co)O2can demonstrate more than ~250 mAh g─1 at an average voltage of ~3.6 V, delivering a high energy density (~ 900 Wh kg─1) (Figure 1-18).
Nevertheless, excess oxygen evolution and voltage fading issues associated dramatic crystal structure transformation needs to be improved for practical applications.52
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Figure 1-14. Schematic diagram of phase transition of LiCoO2 during de-intercalation from (a) O3- type layered cathode and (b) O2-type layered cathode. Blue octahedrons represent CoO6, red balls indicate oxide ions, and yellow balls represent lithium ions.34 Reproduced from Ref. 34 with permission from Royal Society of Chemistry. Copyright 2012.
Figure 1-15. Illustration of crystal structures and electrochemical reactions of various cathodes. (a) layered lithium transition metal oxide, (b) spinel lithium transition metal oxide, and (c) olivine LiMPO4
(M =Fe, Mn, etc.).53 Reproduced from Ref. 53 with permission from Royal Society of Chemistry.
Copyright 2016.
Figure 1-16. Voltage profiles of bare- and Al2O3-coated LiCoO2between 2.75 and 4.4 V at the rate of 0.1 C (1C ≈ 140mAh g─1).54Reprinted with permission from Parket al. Chem. Mater. 2000, 12, 12, 3788–3791. Copyright 2000 American Chemical Society.
Figure 1-17. Voltage profiles of the NCM (LiNi0.8Co0.1Mn0.1O2) and NS-NCM (nano-structured stabilizer-incorporated NCM) at 25 oC, where the operating voltage ranged from 3.0 to 4.3 V (charge and discharge C-rate: 0.5 and 1C, 1C ≈ 200mAh g─1).55Reproduced from Ref. 55 with permission from Royal Society of Chemistry. Copyright 2018.
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Figure 1-18. Voltage profiles of Li1.2Ni0.2Mn0.6O2 (LNMO) where the operating voltage ranged from 2.0 to 4.6 V with a charge and discharge C-rate of 0.1 for 3 cycles and 0.3C for subsequent 97 cycles (1C ≈ 250mAh g─1).56 Reprinted with permission from Gu et al. ACS Nano 2013, 7, 1, 760–767.
Copyright 2013 American Chemical Society.