Advanced Powder Materials: IEST SPFT Enable B/Zr Co‑Doping for Lattice Regulation and Internal Stress Relief in Ultra‑High‑Nickel Single‑Crystal Cathodes

1. Degradation of Ultra-High-Nickel Single-Crystal Cathodes

Ultra-high-nickel NCM cathodes offer high specific capacity and reduced cobalt dependence, but their structural stability becomes increasingly difficult to maintain as the nickel content and upper cutoff voltage increase. During charge and discharge, unstable Ni⁴⁺ can promote irreversible phase transitions and interfacial side reactions. The nickel-rich composition also undergoes anisotropic lattice and volume changes that generate internal stress during deep cycling.

The source study identifies a coupled degradation sequence rather than a single isolated failure mode:

• Ni-rich layered lattices undergo non-uniform structural evolution at high states of charge.
• Internal stress accumulates as the lattice contracts or distorts.
• Intragranular cracks can initiate and propagate inside otherwise single-crystalline particles.
• Cracking exposes fresh surfaces to the electrolyte and accelerates side reactions.
• Particle pulverization, phase transformation, impedance growth, and electrochemical-mechanical performance loss can then reinforce one another.

Single-crystal architecture reduces grain-boundary-driven fracture compared with conventional polycrystalline particles, but it does not eliminate intragranular cracking. The remaining problem is therefore a coupled lattice–interface–mechanics problem.

2. B/Zr Co-Doping for Lattice-Bulk Regulation

The study proposes a lattice-bulk regulation strategy instead of treating doping as a uniform bulk substitution problem. The two dopants are assigned complementary structural functions:

• B incorporation: B is distributed throughout the particle interior, where it reinforces the lattice framework and helps relieve internal strain.
• Zr enrichment: Because of its limited solubility in the bulk lattice, Zr is preferentially enriched near the particle surface, where it suppresses Li/Ni cation mixing and helps stabilize the interface during high-cutoff-voltage cycling.
• Combined effect: The spatially differentiated roles of B and Zr coordinate bulk lattice stabilization with surface protection, improving structural integrity, Li⁺ transport, and interfacial stability.

This complementary distribution allows the two dopants to address different but interconnected stages of cathode degradation.

2.1 Spatial Distribution and Local Structure

SCNCM@0.5%Zr/B was synthesized through a co-precipitation route. TOF-SIMS depth profiling showed that the B signal remained relatively stable with increasing sputtering depth, indicating distribution through the particle interior. By contrast, Zr showed a stronger signal near the surface, indicating preferential surface enrichment.

HRTEM and HAADF-STEM further revealed that the layered structure remained intact within approximately 10 nm of the particle surface along the [100] zone axis. The observed (101) lattice spacing changed from 0.242 nm to 0.482 nm (this figure should be verified against the published paper — see note below), while the (003) lattice spacing increased slightly from 0.472 to 0.477 nm after B/Zr modification, indicating a change in the local interlayer environment.

These results support the complementary roles of bulk B incorporation and near-surface Zr enrichment in stabilizing different regions of the cathode particle.

Figure 1. TEM, HRTEM, TOF-SIMS, and HAADF-STEM characterization of SCNCM and SCNCM@0.5%Zr/B before cycling.

2.2 Electrochemical Performance and Li⁺ Transport Kinetics

The lattice modification is not only mechanical. By changing the local crystal environment, B/Zr co-doping also affects lithium-ion transport. The study reports that the calculated Li⁺ migration barrier decreases from 0.60 eV in the original SCNCM to 0.40 eV after B/Zr co-doping.

The source attributes the improved transport behavior to the combined structural effects of the two dopants. The altered lattice environment provides more favorable conditions for Li⁺ migration, while the preserved layered structure near the particle surface helps maintain a continuous diffusion pathway.

For high-nickel single-crystal cathodes, this relationship is important because mechanical stabilization and transport kinetics can interact. A structural modification that suppresses cracking but significantly impedes lithium transport would introduce a different bottleneck. The study instead targets both properties through coordinated lattice regulation.

Figure 2. Comparison of electrochemical properties in pristine and B/Zr-modified SCNCM pouch-type full cells at 25°C/50°C.

2.3 DFT Insights into Lattice Stabilization and Li⁺ Migration

DFT calculations indicated that B/Zr co-doping produced controlled local lattice distortion and a lower formation energy than Zr-only doping, resulting in a more energetically favorable modified structure.

The source describes four interconnected effects: B occupies tetrahedral interstitial sites within the oxygen framework and helps mitigate internal lattice strain, whereas Zr preferentially accumulates near the surface and suppresses Li/Ni cation mixing. Charge-density and density-of-states analyses further indicate stronger local bonding interactions and modified electronic states near the Fermi level.

B/Zr co-doping also produced a more favorable Li⁺ migration network and reduced the calculated migration barrier from 0.60 eV in pristine SCNCM to 0.40 eV in the co-doped structure. These calculations explain how the complementary distributions of B and Zr improve lattice stability and lithium-ion transport.

Figure 3. DFT analysis of lattice distortion, formation energy, charge redistribution, electronic structure, and Li⁺ migration in pristine and B/Zr-modified SCNCM.

3. Stress Evolution and Particle-Level Mechanical Stability

3.1 Lattice Strain and Phase Evolution during Cycling

3.2 Electrochemical Performance

The prepared cathodes were paired with conventional graphite negative electrodes to assemble pouch full cells. The electrochemical evaluation used a 2.75–4.2 V voltage window and included testing at both 25°C and 50°C.

The source reports favorable electrochemical performance for SCNCM@0.5%Zr/B under both temperature conditions. The published paper further reports that a 1.8 Ah pouch cell based on the optimized material delivered 164.2 mAh g⁻¹ after 1,500 cycles, corresponding to 82.3% capacity retention. This long-cycle result is important because the paper does not present lattice modification as a short-term electrochemical optimization; it connects the structural strategy with extended full-cell durability.

At the material level, the electrochemical results need to be interpreted together with structural and mechanical measurements. Capacity retention alone cannot identify whether degradation originates from lattice collapse, interfacial side reactions, lithium transport limitations, or particle fracture.

3.3 Single-Particle Mechanical Testing with IEST SPFT2000

The IEST SPFT2000 was used in the study to evaluate the mechanical response of cathode particles recovered after 500 and 1,000 cycles. During testing, an individual particle is compressed under controlled displacement while the applied force and particle deformation are recorded. At both cycle counts, the modified SCNCM@0.5%Zr/B particles exhibited higher crushing strength than the pristine SCNCM particles. Unlike microscopy, which primarily reveals the presence and morphology of cracks, single-particle compression directly quantifies the residual load-bearing capability of electrochemically aged particles.

From a battery-engineering perspective, this creates a useful particle-to-cell linkage:

• Crystal engineering: B/Zr co-doping changes lattice structure and local strain.
• Particle mechanics: SPFT2000 measures how the aged particle responds to controlled compression.
• Electrode robustness: Higher particle strength can reduce crack-driven degradation and help maintain particle integrity under electrode processing and cycling stresses.
• Cell durability: Particle-level integrity contributes to maintaining electrochemical contact and limiting exposure of fresh surfaces to the electrolyte.

IEST SPFT2000: Particle-Level Mechanical Characterization

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