Intro to Cathode Materials

In most lithium-ion batteries both the cathode and the anode are intercalation hosts that lithium-ions can reversibly move in and out of, with lithium-ions and electrons moving towards the negative electrode (“anode”) during charge, and towards the positive electrode (“cathode”) during discharge, as shown in Figure 1.

The most common lithium-ion chemistry, typically a layered oxide cathode and a graphite anode, has an electrochemical cell reaction displayed in Equation 1 (where MO₂ = transition metal oxide, and forward reaction is discharge, full cell reaction). The cathode half reaction for the same cell is shown in Equation 2 (the forward reaction is again discharge and x is the extent of discharge).

Li_xC₆+Li_1-XMO₂⇌ C₆+LiMO₂       [1]

Li_(y+x)MO₂⇌ Li_yMO₂+xLi⁺ +xe^-       [2]

Lithium-ion cathode material LiCoO₂ (LCO) was the first commercially available layered transition metal oxide cathode material. LCO was discovered by John Goodenough in 1980, commercialized by SONY, and is still used in a large majority of lithium-ion batteries today. LCO’s low stability at high states of charge (SOC) combined with the high cost of cobalt has led to a partial substitution to other transition metals, like Ni and Mn, resulting in LiNiO₂ (LNO), LiNi_0.8Co_0.15Al_0.05O₂ (NCA), and LiNi_xMn_yCo_1−x−yO₂ (NMC) cathode materials. NMC having equal amounts of nickel, manganese, and cobalt gives the formula: LiNi_1/3Mn_1/3Co_1/3O₂ and is referred to as NMC111. The most commonly used lithium-ion battery cathode materials are displayed in Table 1. 

Table 1: Commonly used lithium-ion cathode materials

In addition to transition metal oxides, other lithium-ion intercalation cathode materials include polyanion compounds and metal chalcogenides. LiFePO₄ (LFP) is a popular polyanion olivine cathode material due to its low toxicity and thermal stability. Its low electronic and ionic conductivities were found to be improved by decreasing particle size and adding a carbon coating. The resulting nano-LFP is known to have better high-rate capacities than other cathode materials, but has a lower working voltage than commonly used layered metal oxides resulting in a lower energy density. 

Tradeoffs of different cathode materials

Compared to layered oxide materials (LCO, NMC, NCA), cathodes with a spinel or olivine structure (LMO and LFP respectively) are lower in cost, more environmentally safe, more thermally stable, and possess higher-rate capabilities. Layered oxide cathodes have higher capacities than spinel, but have lower thermal stability, lower rate capabilities, and are more expensive (due mostly to cobalt and nickel content).

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Not an electrochemistry resource this time!

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