High-nickel layered oxide cathodes, such as LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.8Co0.15Al0.05O2 (NCA), are critical for advancing lithium-ion batteries due to their high energy density. However, these materials suffer from intrinsic challenges, including poor electronic conductivity, structural instability during cycling, and cation mixing. To mitigate these issues, dopants such as magnesium (Mg), zirconium (Zr), and titanium (Ti) are introduced into the cathode lattice. These dopants modify the atomic and electronic structure, enhancing performance through distinct mechanisms.

**Magnesium (Mg) Doping**
Mg is commonly used as a dopant due to its ability to stabilize the crystal structure and improve electronic conductivity. When Mg substitutes for nickel (Ni) in the transition metal layer, it acts as a pillar to suppress detrimental phase transitions. The Mg2+ ion has a similar ionic radius to Ni2+ (0.72 Å vs. 0.69 Å), allowing for minimal lattice distortion. However, Mg2+ is electrochemically inactive, meaning it does not participate in redox reactions. This inactivity helps maintain structural integrity during charge and discharge by reducing lattice strain.

At the atomic level, Mg doping mitigates cation mixing—a phenomenon where Ni2+ migrates into Li+ sites, obstructing Li+ diffusion. By occupying Ni sites, Mg reduces the likelihood of Ni2+ displacement due to its stronger bonding with oxygen. Additionally, Mg doping introduces hole carriers into the oxygen lattice, enhancing electronic conductivity. This occurs because Mg2+ has a lower valence than Ni3+, creating oxygen vacancies or holes that facilitate electron hopping between transition metal ions.

**Zirconium (Zr) Doping**
Zr is another effective dopant, primarily known for improving structural stability and Li+ diffusion kinetics. Zr4+ has a larger ionic radius (0.84 Å) compared to Ni2+, which expands the interlayer spacing in the cathode material. This expansion reduces the energy barrier for Li+ movement, enhancing rate capability. Zr doping also forms a robust Zr-O bond, which strengthens the lattice framework and inhibits oxygen loss at high voltages.

The presence of Zr4+ in the transition metal layer suppresses harmful phase transitions from the layered structure to rock-salt or spinel phases during cycling. This suppression occurs because Zr4+ stabilizes the oxygen sublattice, preventing collapse during Li+ extraction. Furthermore, Zr segregates to grain boundaries, forming a passivation layer that reduces side reactions with the electrolyte. This segregation also minimizes microcrack formation, a common issue in high-nickel cathodes due to anisotropic volume changes.

**Titanium (Ti) Doping**
Ti doping is employed to enhance both structural and thermal stability. Ti4+ has an ionic radius (0.745 Å) close to Ni2+, enabling substitution without significant lattice distortion. Ti doping reduces cation mixing by preferentially occupying Ni sites and forming strong Ti-O bonds, which anchor the lattice. Unlike Mg, Ti4+ can participate in charge compensation mechanisms, indirectly stabilizing Ni oxidation states.

At high voltages, Ti doping mitigates oxygen evolution by strengthening the metal-oxygen bonds. This stabilization is critical because oxygen loss leads to capacity fade and safety risks. Ti also modifies the electronic structure by introducing defect states that improve electronic conductivity. These states arise from the hybridization of Ti 3d and O 2p orbitals, creating additional pathways for electron transport.

**Comparative Analysis of Dopant Effects**
The effectiveness of dopants depends on their ionic radius, valence state, and bonding characteristics. Below is a comparison of key properties:

| Dopant | Ionic Radius (Å) | Valence State | Primary Effect |
|--------|------------------|--------------|----------------|
| Mg | 0.72 | +2 | Structural pillar, reduces cation mixing |
| Zr | 0.84 | +4 | Expands interlayer spacing, stabilizes oxygen |
| Ti | 0.745 | +4 | Strengthens metal-oxygen bonds, enhances thermal stability |

**Synergistic Dopant Combinations**
In some cases, dual doping is employed to leverage complementary benefits. For example, Mg-Ti co-doping combines Mg’s structural stabilization with Ti’s electronic effects. The Mg2+ ions prevent cation mixing, while Ti4+ enhances charge transfer kinetics. Similarly, Zr-Mg co-doping improves both Li+ diffusion and mechanical stability. These combinations are tailored to address specific degradation modes in high-nickel cathodes.

**Challenges and Limitations**
While doping improves performance, excessive dopant concentrations can degrade capacity. Dopants like Mg and Zr are electrochemically inactive, diluting the active material. Optimal doping levels are typically below 2-5% to balance stability and energy density. Additionally, inhomogeneous dopant distribution can create localized strain, necessitating precise synthesis techniques.

**Conclusion**
The strategic use of dopants such as Mg, Zr, and Ti addresses critical limitations in high-nickel cathodes. Mg enhances structural integrity and electronic conductivity, Zr improves Li+ diffusion and oxygen stability, and Ti strengthens thermal and electrochemical resilience. These atomic-level modifications enable high-energy-density batteries with prolonged cycle life and safety, paving the way for next-generation energy storage.