State-of-the-art cathode materials for Lithium-ion batteries
i) 1st generation cathode: LCO
The first cathode material developed for Li-ion batteries is LiCoO2 (abbreviated as LCO). It has a layered (hexagonal) structure with space group R-3M. The structure consisting of Li-O-Co-O-Li layers along the c axis is shown in Figure 1. Both Co and Li occupy alternating octahedral sites coordinated by oxygen. The oxidation states are +1, +3, and -2 for lithium (Li), cobalt (Co) and oxygen (O), respectively. Only the octahedra with cobalt centeral atoms are shown.
Figure 1. Structure of LiCoO2.
During charge (oxidation direction of Eq.1), lithium ions are removed from the structure, and for every mole of lithium ion removed, an equivalent of cobalt is oxidized from +3 to +4, for charge neutrality.
A comparison of the theoretical/practical lithium storage capacity as well as other parameters of state-of-the-art cathode materials for lithium-ion batteries is presented in Table 1.
Table 1. Overview of state-of-the-art cathode materials for lithium-ion batteries
(V vs. Li/Li+)
|Tm: Transition metal|
ii) 2nd generation cathode material: NCM family
The second generation of cathode materials began with LiNiuCoyMnzO2 (NCM or NMC). It has the same layered (hexagonal) structure with space group R-3M as LCO (similar to Figure 1, but with Mn and Ni on some Co atomic positions). Instead of only cobalt in LCO, there is partial substitution of Co(+3) with Ni(+2) and Mn(+4) in NCM, such that u + y + z = 1. During charge (oxidation direction of Eq.2), lithium ions are removed from the structure, and for every mole of lithium ion removed, an equivalent of cobalt and nickel are oxidized from +2+ to +3 and from +3+ to +4, respectively, for charge neutrality. Mn(+4) remains electrochemically inactive.
Based on the u, y and z values, NCM can be classified as 111, 523, 622, and 811. By varying the composition as such, each type of NCM presents some characteristic advantage over the other, as showcased in Table 1 and Figure 2. By increasing the nickel content, the material is usually referred to as nickel-rich NCM.
Unfortunately, though NCM8111 delivers better lithium storage capacity, it has the challenge of capacity fading as a result of structural instability. The structural instability causes oxygen evolution, hence the material is not yet applied in commercial cells.
iii) 3rd generation cathode material: manganese spinel family
The third generation of cathode materials began with LiMn2O4 (LMO). It has a spinel (cubic) structure with space group Fd-3M. As shown in Figure 3, MnO6 octahedra are connected to each other through edge-sharing, whilst lithium is in tetrahedral sites. The oxidation states are +1, +3/+4, and -2 for lithium (Li), manganese (Mn) and oxygen (O), respectively. As per the formula, there are two distinct manganese atoms in LMO, one is in the oxidation of +3 while the other is in +4. During charge, lithium ions are removed from the structure, and for every mole of lithium ion removed, an equivalent of Mn3+ is oxidized to Mn4+.
Figure 3. Structure of LiMn2O4
LMO received a lot of attention as Mn is much cheaper and less toxic than Co or Ni. However, Mn+3 causes Jahn-Teller distortions which lead to crystal structural instability and capacity fading. Mn dissolution also occurs in the battery electrolyte which affects performance. As a result, the Ni-subsituted version, LiNi0.5Mn1.5O4 emerged. In this modified version, called LNMO, Mn4+ stabilizes the structure and is inactive, whilst Ni2+ is the active cation. However, the high operating voltage of 4.7 v vs. Li/Li+ does not favour its use with most electrolytes.
iv) 4th generation cathode material: Nickel-rich NCA
The fourth generation cathode material is LiNi0.8Co0.15Al0.05O2 (NCA). It has the same layered (hexagonal) structure with space group R-3M as LCO (similar to Figure 1), but with more Ni, and a bit of Al on some Co atomic positions.
v) 5th generation cathode material: LFP
The fifth generation cathode material is LiFePO4 (LFP). It has the olivine-type structure with space group Pnma in which Li+ and Fe2+ occupy octahedral sites, while P is located in tetrahedral sites in a slightly distorted hexagonal close-packed (HCP) oxygen array, as shown in Figure 4. During charge (oxidation direction), lithium ions are removed from the structure, and for every mole of lithium ion removed, an equivalent of Fe2+ is oxidized to Fe3+, for charge neutrality.