Battery Cathode

Battery cathodes include the following types:

LiC0O₂

lithium cobalt oxide (LiCoO₂) was first reported by Goodenough group in the 1980s and commercialized by Sony in 1991, due to its high volumetric energy density and excellent reliability. LiCoO₂ was a dominating cathode material for LIBs, particularly for portable electronics applications; however, it has challenges in EV market, due to the following reasons: 

• high materials cost. It has high cobalt content which is the most expensive battery material. 
• low specific capacity (e.g. 150 mAh/g).
• Chemical instability of Li_1-x CoO₂ for x > 0.5.

LiNiO₂

The research of LiNiO2 is actually several years earlier than that of LiCoO2. Compared with LiCoO2, it has several advantages:

• High capacity. At charge cut-off voltage of 4.3 V (vs. Li/Li+), state-of-the-art discharge capacities of LiNiO2 and LiCoO2 are approximately 250 and 150 mAh/g, respectively.
• Oxygen stability. The band structure of LiNiO2 is presented in the following figure. The little orbital overlap between the Ni3+/4+:3d band and O2-:2p band enables higher degree of reversible charge and discharge in LiNiO2 than that of LiCoO2, without the release of oxygen from the structure. 

LiNiO2 family has numerous intrinsic issues: 

• Phase instability and susceptible to off-stoichiometry
• Li/Ni mixing. The reasons are: (1) Li⁺ and Ni²⁺ has similar ion radii and they can occupy each other’s position; (2) when Li/Ni mixing is present, the Ni2+ ions in the Li layer can form strong 180^o Ni²⁺ -O^2- -Ni³⁺ inter-planar superexchange structure; (3) Li⁺ in the TM layer relieves the magnetic frustration in a 2D triangular lattice [Ni2+/Ni3+ magnetic frustration (both have net spin)–>Li+/Ni2+ or Li+/Ni3+ (Li+ has no net spin)]
• Instability of Ni³⁺
• low Coulombic efficiency
• thermal instability
• multiple phase transitions during cycling

LiNi_xMn_yCo_zO₂

To understand and improve the structural and thermal stability of LiNiO2 by partially replacing Ni atoms with dopants such as Mg, Mn, Cr, Co, Fe, Al, Ga, Zr, Ti, Sb, and W. Among them, Co and Mn doping were studied the most. 

The benefit of Mn doping

Partial doping with Mn is desired because:

• Mn is cost effective 
• Mn⁴⁺(0.65A) has similar ion radii as Ni³⁺ (0.6A)

Benefits of Mn doping

Mn doping improved capacity retention, due to improved structural stability, compared with LiNiO2. Suppression of the phase transition from H2 to H3 (at about 4.2V), as shown in the comparison of dQ/dV data between Mn-doped LiNiO₂ and original LiNiO₂. However, higher Mn content in layered LiNi_1-xMn_xO₂leads to reduced power capacity. 

Benefits of Co doping

Co addition has been shown to improve the structural stability and electrochemical property, as Ni³⁺/⁴⁺ and Co³⁺/⁴⁺ redox pairs exhibit similar redox behavior at high voltages. The advantages of Co doping include:

• Decrease Li/Ni mixing
• Suppress multiphase transitions such as M+H2 and H2+H3 (above figure)
• Electron conductivity can be improved because of the overlap of O 2p orbital with that of Co³⁺/⁴⁺redox pairs.

Disadvantage of Co doping

Co is the most expensive Lithium battery materials. Extensive work have been done to replace the Co while keeping the battery performances same.

LFP (LiFePO4)

The Advantages of lithium iron phosphate or LFP

safety

Disadvanatge

The main downside of LFP is low energy density and therefore driving range

Recently, olivine LiFePO4 (LFP) has been employed as the cathode material in Tesla’s short-range Model 3, which could be one of the breakthroughs in terms of price per energy (US$80 kWh-1) (ref)

Crystalline structure

LNMO

Battery performances Comparison Among Cathode technologies

Chemistry Vs Application

Major Cathode Materials suppliers

More Vendors

LCO	NCA	NMC	LFP	LNMO
Nichia		Nichia	Nichia
Targray	Targray	Targray	Targray	Targray
		POSCO
		ECOPRO

 Problems in Cathode and solutions

 High cost of raw materials  

Battery cost parity with internal combustion engine is US$100 kWh-1.

Volumetric energy in (Wh/L ) based on used material volume and gravimetric energy in (Wh/kg ) of the 10 considered cell chemistries plus the Panasonic NCA Use Case (ref)

Total materiall costs of all 10 considered cell chemistries plus Panasonic NCA Use Case differentiated in combined CAM cost, anode cost, and secondary material costs

Solutions

Cobalt free cathode

The function of Cobalt is to suppress cation (Li/Ni) mixing and enable high rate capability. Manthiram group developed Cobalt free NMA, NMCAM cathode which have similar function as NCA and NMC (ref).

NMC/electrolyte contact

Surface modification of NMC. LZO-coated NMC was prepared by the sol–gel method.

Example: the LZO coating solution was prepared from anhydrous 2-propanol, lithium methoxide (10% lithium methoxide in a methanol solution) and zirconium(iv) tetrapropoxide in a 200:20:1 mole ratio. NMC was dispersed in the above solution and stirred for 1 h; subsequently, the propanol was evaporated under vacuum at 50 °C in a water bath, with continuous sonication to prevent the aggregation of the NMC particles. After filtration, the precursor was heated at 300 °C for 1 h in the presence of air to obtain the LZO-coated NMC (Ref). 

Cathode composite making

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^[1] https://doi.org/10.1002/adma.202002718

^[1] Energies 2019, 12, 504; doi:10.3390/en12030504

^[1] http://dx.doi.org/10.1021/acs.accounts.9b00033

^[2] https://doi.org/10.1016/j.ensm.2020.09.020