Among the various components involved in a lithium-ion cell, cathodes (the positive or oxidizing electrodes) currently limit the energy density and dominate the battery cost. Today’s common cobalt (Co) and manganese (Mn) based cathodes were developed to overcome safety concerns with Li-metal anodes. This FAQ begins with a brief look at the longer-term trajectory of Li-ion cathode developments, dives into cathode structures and chemistries currently being used, and closes by reviewing efforts to develop Co-free, and Li-free cathodes, including the use of disordered rocksalts, needed to support long-range electric vehicles, electric aircraft, and other advanced applications.
Today’s Li-based cathode materials are a relatively new development. From the 1960s to the early 1980s, various Li-free cathodes were paired with metallic Li anodes to produce rechargeable batteries. However, these Li-metal-based rechargeable batteries were inherently unsafe due to Li dendrites piercing the separator, shorting the battery, and causing fires. Today’s Li-ions were developed to eliminate elemental Li by moving the active material from the anode to the cathode, resulting in a much safer structure. The anodes in Li-ions are made from materials such as graphite, silicon, and silicon oxides that can act as hosts for Li-ions delivered by the cathode (a discussion of Li-ion anodes can be found in “Li-ion Parts 3 – Anodes”)
Li-ion batteries are based on the intercalation and deintercalation of Li ions. The cathode is the source of the ions inserted into the lattice structure of the anode during charging (intercalation) and extracted from the anode during discharge (deintercalation). A range of cathode materials has been developed that deliver specific levels of performance, including Li2MnO3, LiFePO4, Li2MO3-LiM’O2, Li(NiMnCo)1/3O2 (also called NMC) (Figure 1). The two most common are LiCoO2 and LiMn2O4.
Figure 1: Initially, various Li-free cathodes were paired with metallic Li anodes to produce rechargeable batteries; the development of Li-ion batteries required the invention of various Li compounds to provide the Li ions for intercalation/deintercalation in a graphite anode. (Image: Joule)
A Li-ion cathode is built on a thin aluminum foil current collector that holds the frame of the cathode coated with a combination of active material, conductive additive, and binder. The active material is the source of the Li-ions. The additive increases the conductivity of the cathode, and the binder helps maintain the cathode’s structure and maintain good contact with the aluminum foil.
Different cathode materials contain varying amounts of Li. The higher the Li content, the larger the battery capacity. The battery voltage is determined by the potential difference between the cathode and the anode. Anode materials have a more limited range of potentials, making the cathode the key element determining the battery’s voltage, energy, and power densities.
Co-based cathodes have high specific heat capacity, high volumetric capacity, high discharge voltage, good cycling capability, and low self-discharge. The Co-material forms a pseudo tetrahedral structure that supports two-dimensional Li-ion diffusion. It suffers from the drawbacks of low thermal stability and relatively high cost. Mn-based cathodes with a cubic lattice structure can support three-dimensional Li-ion diffusion. These Mn cathodes are less expensive than Co cathodes, but the Mn tends to dissolve into the electrolyte during cycling, resulting in poor stability.
Lithium Cobalt Oxide (LiCoO2), LCO, was the first layered transition metal oxide cathode. It has been the most commercially successful cathode and the majority of commercial Li-ion batteries today still use LCO. LCO has a theoretical specific capacity of 275 mAh/g, a theoretical volumetric capacity of 1365 mAh/cm, low self-discharge, high discharge voltage, and good cycling performance.
Other cathode materials have been developed for specific performance demands. For example, lithium iron phosphate (LiFePO4) is a polyanion oxide with low cost, is safe, and has high cycle durability, making it useful in electric vehicles. However, a carbon conductive agent is required to overcome its low electrical conductivity, increasing the cost of these Li-ions.
Lithium manganate (LiMn2O4) cathodes have a spinel structure (cubic close-packed oxides with eight tetrahedral and four octahedral sites per formula unit) that can support high discharge rates, making it suitable for high-rate applications. The spinel LiNi0.5Mn1.5O4 (LNMO) has a high operating voltage (about 4.7 V), is low cost, with high ionic conductivity, and good thermal stability. However, LNMO also suffers from rapid capacity decay and poor cycle life, especially at high temperatures, limiting its commercial viability.
The performance of today’s Li-ion cathode materials is thought to be reaching its limit. Existing material structures such as layered transition metals oxides, olivines (orthorhombic crystals), and spinels have different performance tradeoffs. For example, layered oxides such as LiCoO2 are toxic, expensive to produce, have limited potential windows, and limited capacity. Li-rich layered oxides, such as Li1+xM1−xO2, where M is a mixture of transition metals (Ni, Mn, and Co), are promising since they can have high discharge voltages over 4.5 V and deliver high specific capacities. However, they experience large voltage decay during cycling and high irreversible capacity loss at the first cycles, limiting their utility. Olivine materials such as LiFePO4 have higher stability than layered oxides at elevated temperatures due to their high thermal and structural stability. But the low ionic conductivity of olivines is an obstacle to their use for high-energy Li-ion batteries.
Layered and spinel oxides offer good electronic conductivity, while the polyanion oxides such as lithium iron phosphate have poor electronic conductivity. To overcome their poor conductivity, polyanion oxide cathodes require the particles to be small and coated with conductive carbon, increasing the processing cost and quality control challenges to maintain consistent performance. Spinel and layered oxides have close-packed structures with high densities, while polyanion oxides have lower densities. The need to coat the particles results in lower volumetric energy densities, making polyanion oxides less suited for use in a range of applications from portable electronic devices to electric vehicles, where layered oxide cathodes excel.
Polyanion oxides have performance advantages making them suited for grid storage of electricity from renewable sources like solar and wind:
- higher thermal stability than the layered and spinel oxide cathodes.
- polyanion cathodes with optimally small particles coated with carbon can sustain high charge-discharge rates, despite a lower volumetric energy density.
- polyanion cathodes can be produced using abundant transition metals like Fe, unlike the layered and spinel oxides, offering sustainability advantages.
Regardless of the chemical or material structure, the use of Li ions limits the maximum energy density of these batteries to about 300Wh/kg. To get to the higher energy densities needed for long-range EVs, electric aircraft, and other applications, new approaches such as Co-free and Li-free cathodes are required.
Cobalt-free cathodes
Conventional cathodes rely on large amounts of Co, which is not an abundant element and is expensive. Disordered rocksalt (DRX) cathode materials have emerged as a possible solution to the scarcity and cost challenges of using Co. A DRX consists of interpenetrating face-centered lattices of positive cations and negative anions randomly distributed within specific sublattices (Figure 2A). The random structure of DRXs contrasts with the layered structures of conventional cathode materials like LiCoO2, wherein Li and Co cations form alternating layers within the cation sublattice. DRX materials are not new but have small energy storage capacities resulting from sluggish Li diffusion within the disordered structure. More recently, it has been found that using inexpensive and abundant metals such as Mn, Fe, Fe, and V, in the DRX structure can overcome the capacity limitation of DRXs, and at the same time, substantially improve the sustainability and reduce the cost of Li-ions.
Figure 2: DRX Li-ion cathode structure (A) with assessments of sustainability (B) and estimated associated cost-effectiveness (C). (Image: Joule)
Referring to Figure 2 (above):
(A) DRX crystal structure with cations (Li, TM = transition metal, such as Co) and anions (such as O, F, S) forming an interpenetrating face-centered lattice.
(B) Comparison of natural abundance (orange bars) versus price (blue bars) of selected TM elements found in DRXs.
(C) Comparison of cathode costs per kilowatt-hour for eight materials: L-LCO, layered LiCoO2; L-NMC111, layered LiNi1/3Mn1/3Co1/3O2; L-NCA, layered LiNi0.8Co0.15Al0.05O2; L-NMC811, layered LiNi0.8Mn0.1Co0.1O2; S-LMNO, spinel LiNi0.5Mn1.5O4; S-LMO, spinel LiMn2O4; O-LFP, olivine LiFePO4, D-LMTO, DRX, Li1.2Ti0.4Mn0.4O2.
DRXs are also being developed for Li-ion anodes that support fast charging, higher cell voltages, and longer cycle lives (see “Li-ion Parts 3 – Anodes”).
Li-Free cathodes
Developments in solid-state electrolytes that can eliminate concerns with dendrites forming and shorting Li batteries are causing a resurging interest in Li-metal anodes as replacements for Li-ion cathodes (trends in solid-state electrolytes are discussed in “Li-ion Parts 5 – Electrolytes”). The development of Li-metal anodes will have the potential to produce energy densities of 1,000 to 1,600 Wh/kg and 1,500 to 2,200 Wh/L at the cell level, 3X to 7X higher than today’s Li-ions.
Figure 3: Li-free cathodes are being developed for high capacity and high energy density batteries. (Image: Joule)
Combined with Li/C anodes, choices for low-cost, environmentally benign, and high-energy-density Li-free cathode materials include S, FeF3, CuF2, FeS2, and MnO2. The development of solid-state electrolytes is only one piece of the puzzle that needs to be solved to enable the commercial development of rechargeable batteries with Li-free cathodes. Challenges include high voltage hysteresis, large volume changes in the electrodes that lead to mechanical stresses, parasitic reactions with the electrolytes, and capacity fading.
Summary
The cathode is the most important component in today’s Li-ions. Cathodes limit the performance and drive the cost of these batteries. Li-ion cathode technology has matured, and there are numerous choices with various tradeoffs between power/energy densities, operating temperature capabilities, self-discharge rates, cycle lives, cost, etc. To continue moving Li-battery performance forward, new solutions are needed for cathodes. In the near-term, Co-free cathodes could significantly reduce the cost and improve the sustainability of Li-ion batteries. In the longer term, the development of advanced electrolytes that enable the use of Li-free cathodes will allow the development of Li-metal batteries with far superior energy/power densities.
References
A reflection on lithium-ion battery cathode chemistry, Nature Communications
Cathode Materials for Lithium-ion Batteries, NEI Corp.
Li-free Cathode Materials for High Energy Density Lithium Batteries, Joule
Recent advances in the design of cathode materials for Li-ion batteries, RSC Advances
Toward high-energy Mn-based disordered-rocksalt Li-ion cathodes, Joule
nice notes on Li ion batteries