NMC: 

LiNixCoyMnzO₂ (generally abbreviated as NCM or NMC) is a layered lithium transition-metal oxide that serves as one of the most important cathode materials in lithium-ion batteries. In the chemical formula, x + y + z = 1, indicating that the total molar fraction of nickel (Ni), cobalt (Co), and manganese (Mn) equals unity. Structurally, NCM crystallizes in a hexagonal α-NaFeO₂–type structure with the space group R-3m, which is characterized by a well-ordered layered arrangement of cations. In this lattice, Li⁺ ions occupy the 3a octahedral sites, the transition-metal ions (Ni, Co, Mn) occupy the 3b octahedral sites, and oxygen anions reside at the 6c sites. This ordered stacking of –O–TM–O–Li–O–TM–O– layers along the c-axis provides two-dimensional diffusion pathways for lithium ions, which is essential for reversible electrochemical intercalation and de-intercalation.

NCM compositions are commonly categorized according to the molar ratio of Ni:Co:Mn, since this ratio strongly influences electrochemical performance, thermal stability, and structural robustness. For instance, NCM111 (1:1:1) offers good structural stability and moderate capacity, while NCM523 and NCM622 provide higher specific capacities due to increased nickel content. NCM811, with approximately 80% nickel, delivers very high gravimetric energy density but suffers from pronounced structural and interfacial instability. Nickel primarily contributes to capacity through the Ni²⁺/Ni⁴⁺ redox couple, cobalt enhances electronic conductivity and structural ordering, and manganese improves thermal and structural stability due to the presence of electrochemically inactive Mn⁴⁺.

During electrochemical charging, lithium ions are extracted from the Li layers, and the material undergoes a sequence of reversible and partially irreversible phase transitions. Initially, the structure exists in a hexagonal H1 phase. As lithium de-intercalation proceeds, it transitions through a monoclinic (M) distortion, followed by a second hexagonal H2 phase, and eventually reaches a highly delithiated H3 phase at high voltages. The H2 → H3 transition is particularly critical because it involves a sudden contraction of the c-lattice parameter and anisotropic volume change. This abrupt lattice shrinkage generates substantial internal mechanical stress, which contributes to particle fracture and micro-crack formation.

A major contributor to long-term capacity fading in NCM materials is irreversible structural degradation associated with lithium loss and oxygen instability. At high states of charge, especially above ~4.2 V vs. Li/Li⁺, the lattice oxygen becomes thermodynamically unstable. Simultaneously, Ni⁴⁺ is partially reduced to Ni²⁺, and oxygen evolution reactions (OER-like processes) may occur at the surface. The released oxygen can react with the electrolyte, producing gaseous by-products such as CO₂ and CO, which accelerate interfacial degradation. This oxygen loss leads to the formation of surface reconstruction layers, typically transforming the original layered structure into spinel-like or rock-salt phases (Fm-3m). These reconstructed layers possess poor lithium-ion conductivity and increase charge-transfer resistance and impedance growth.

Another critical degradation pathway is intergranular and intragranular micro-cracking induced by anisotropic lattice strain during repeated cycling. The volumetric mismatch between the H2 and H3 phases generates tensile stress within secondary particles, leading to crack initiation at grain boundaries. In Ni-rich compositions, these cracks can propagate to the particle surface, enabling electrolyte infiltration deep into the particle interior. This internal exposure enhances parasitic side reactions, transition-metal dissolution, and loss of electrical contact, all of which accelerate capacity decay and impedance rise.

Additionally, cation mixing (Li⁺/Ni²⁺ disorder) is a significant structural issue in NCM systems, particularly those with high nickel content. Because the ionic radius of Ni²⁺ (≈0.69 Å) is close to that of Li⁺ (≈0.76 Å) in octahedral coordination, nickel ions can migrate into lithium layers during synthesis or deep delithiation. This disorder blocks the two-dimensional lithium diffusion channels, decreases lithium-ion mobility, and reduces the degree of reversible lithium intercalation. Over extended cycling, cation mixing contributes to kinetic limitations, voltage hysteresis, and progressive loss of specific capacity.

In summary, NCM cathode materials achieve high energy density through nickel-driven redox activity within a layered R-3m framework, yet they are intrinsically susceptible to phase transitions, oxygen release, surface reconstruction, micro-cracking, impedance growth, and Li/Ni cation disorder. These interconnected degradation mechanisms become increasingly severe as nickel content rises, making compositional optimization, surface coating, bulk doping, and electrolyte engineering essential strategies for enhancing structural stability and cycle life.

LFP:

LiFePO₄ (LFP) is an olivine-type lithium iron phosphate cathode material that crystallizes in the orthorhombic Pmnb space group. Its crystal structure is composed of two fundamental building units: FeO₆ octahedra and PO₄ tetrahedra. The FeO₆ octahedra share oxygen corners with neighboring polyhedra, forming a rigid three-dimensional framework, while the PO₄ tetrahedra act as highly stable polyanionic groups that enhance structural and thermal stability. Within this framework, Li⁺ ions occupy octahedral sites and migrate through well-defined zigzag channels, which are crystallographically aligned along a specific axis of the lattice.

Unlike layered cathode materials such as LiCoO₂ (LCO) or NCM, lithium-ion transport in LFP occurs through one-dimensional (1D) diffusion tunnels. During electrochemical delithiation, lithium ions are extracted from the structure, and LFP is transformed into FePO₄, which retains a crystal structure closely related to the parent phase. This transformation is often described as a two-phase reaction between LiFePO₄ and FePO₄, accompanied by the oxidation of Fe²⁺ to Fe³⁺. Although this structural similarity supports good reversibility, the 1D nature of lithium diffusion makes LFP particularly sensitive to structural defects that block lithium-ion pathways.

Extensive studies have shown that the degradation of LFP cathodes is primarily associated with the formation of lithium vacancy defects and antisite defects. Lithium vacancies arise from the irreversible extraction of Li⁺ ions during repeated cycling or high-voltage operation. The loss of lithium destabilizes the local charge balance, leading to the oxidation of Fe²⁺ to Fe³⁺ and promoting the migration of iron ions into lithium sites, thereby forming Fe-on-Li antisite defects. These antisite defects are especially detrimental because iron ions residing in lithium channels act as strong physical and electrostatic barriers to lithium-ion diffusion.

The presence of Fe³⁺ in lithium sites further aggravates structural irreversibility. Due to the strong electrostatic repulsion associated with Fe³⁺ ions, their migration back to the original Fe sites during subsequent lithiation becomes energetically unfavorable. As a result, antisite defects tend to accumulate over prolonged cycling, progressively blocking the 1D lithium diffusion channels and causing a decline in rate capability and reversible capacity.

In addition to bulk defects, surface degradation also contributes significantly to LFP performance deterioration. During cycling, an amorphous surface layer can form on LFP particles, consisting of phases such as FePO₄, FeO, Fe₃O₄, and Li₃PO₄. This reconstructed surface layer exhibits poor lithium-ion conductivity and acts as a kinetic barrier, hindering lithium transport across the electrode–electrolyte interface. The problem is further exacerbated by the inherently low electronic conductivity of LFP, which necessitates the use of conductive carbon coatings.

Considering the strictly one-dimensional lithium diffusion pathways in the LFP crystal, effective regeneration or direct recovery of degraded LFP materials must address antisite defects before re-lithiation and recrystallization. If these defects are not eliminated, lithium-ion transport remains obstructed even after lithium replenishment. Moreover, the conductive carbon coating that is typically applied to LFP particles can crack or delaminate during long-term cycling, reducing electronic conductivity and requiring supplementary carbon recoating to restore electrode performance.

Because Fe³⁺ ions experience strong electrostatic repulsion, their migration within the lattice requires a high activation energy, making defect healing kinetically difficult. Therefore, strategies that involve the reduction of Fe³⁺ to Fe²⁺, either through electrochemical reduction or the use of chemical reductants, are considered promising. Reducing Fe³⁺ lowers the migration energy barrier, facilitates iron-ion relocation back to their original lattice sites, and thereby enables the elimination of antisite defects. Such approaches are critical for improving lithium diffusion kinetics, restoring structural integrity, and extending the cycle life of LFP cathode materials.

Ref: Wu, J., Zheng, M., Liu, T., Wang, Y., Liu, Y., Nai, J., Zhang, L., Zhang, S. and Tao, X., 2023. Direct recovery: A sustainable recycling technology for spent lithium-ion battery. Energy Storage Materials, 54, pp.120-134.