Dual-Element-Modified Single-Crystal LiNi0.6Co0.2Mn0.2O2 as a Highly Stable Cathode for Lithium-Ion Batteries

Mechanically and Chemically Co-Robust Ni-Rich Cathodes with Ultrahigh Capacity and Prolonged Cycle Life

Ni-rich layered oxide (NRLO) materials are considered highly promising cathode for lithium-ion batteries. However, their practical application is limited by capacity loss and interface instability caused by chemical and mechanical failure during cycling. Doping has been identified as a direct and effective method to address these challenges. However, mechanistic understanding of doping enhanced electrochemical performance is still unclear. In this study, the introduction of high-valent Nb ions was employed to achieve mechanical-chemical coupling regulation, thereby concurrently improving the capacity and cycle life of NRLO. First, Nb5+ doping was conducted to refine secondary grains, achieving a "grain refinement" effect similar to that in ceramics and alloys, while further stabilizing the grain boundaries. The intergrain fusion structure of NCM811-0.5Nb effectively dissipates lattice strain under highly delithiated state, suppresses oxygen loss, and prevents cracks that lead to fracture during cycling. Moreover, Nb doping stabilizes the monoclinic phase during phase transitions and promotes the formation of highly stable spinel twin boundaries after cycling. This effectively reduces the Li diffusion barrier, leading to improved reversible specific capacity and rate capability. Lastly, the strong Nb─O bonding restrains oxygen release and transition metal/Li antisite mixing, thus mitigate rock-salt phase formation. This study demonstrates a comprehensive understanding of the concurrent capacity and stability enhancement mechanisms attributed to Nb-doping and highlights the significant potential of the synergistic regulation of mechanical and chemical coupling in improving the capacity and lifespan of NRLOs by Nb-doping.

Effect of TiO2 Coating on Structure and Electrochemical Performance of LiNi0.6Co0.2Mn0.2O2 Cathode Material for Lithium-Ion Batteries

High-nickel ternary LiNi0.6Co0.2Mn0.2O2 (NCM622) is a promising cathode material for lithium-ion batteries due to its high discharge-specific capacity and energy density. However, problems of NCM622 materials, such as unstable surface structure, lithium-nickel co-segregation, and intergranular cracking, led to a decrease in the cycling performance of the material and an inability to fully utilize high specific capacity. Surface coating was the primary approach to address these problems. The effect of TiO2 coating prepared by the sol-gel method on the performance of LiNi0.6Co0.2Mn0.2O2 was studied, mainly including the morphology, cell structure, and electrochemical properties. LiNi0.6Co0.2Mn0.2O2 was coated by TiO2 with a thickness of about 5 nm. Compared with the pristine NCM622 electrode, the electrochemical performance of the TiO2-coated NCM622 electrodes is improved. Among all TiO2-coated NCM622, the NCM622 cathode with TiO2 coating content of 0.5% demonstrates the highest capacity retention of 89.3% and a discharge capacity of 163.9 mAh g-1, in contrast to 80.9% and145 mAh g-1 for the pristine NCM622 electrode, after 100 cycles at 0.3 C between 3 and 4.3 V. The cycle life of the 5 wt% TiO2-coated NCM622 electrode is significantly improved at a high cutoff voltage of 4.6 V. The significantly enhanced cycling performance of TiO2-coated NCM622 materials could be attributed to the TiO2 coating layer that could block the contact between the material surface and the electrolyte, reducing the interface side reaction and inhibiting the transition metal dissolution. At the same time, the coating layer maintained the stability of layered structures, thus reducing the polarization phenomenon of the electrode and alleviating the irreversible capacity loss in the cycle process.

A Novel 4-Fluorophenyl Isocyanate Additive Constructing Solid Cathodes-Electrolyte Interface for High-Performance Lithium-Ion Batteries

Building a stable cathode-electrolyte interface (CEI) is crucial for achieving high-performance layered metal oxide cathode materials LiNixCoyMn1-x-yO2 (NCM). In this work, a novel 4-fluorobenzene isocyanate (4-FBC) electrolyte additive that contains isocyanate and benzene ring functional groups is proposed, which can form robust and homogeneous N-rich and benzene ring skeleton CEI film on the cathode surface, leading to significant improvement in the electrochemical performance of lithium-ion batteries. Taking LiNi0.5Co0.2Mn0.3O2 (NCM523) as an example, the NCM523/SiO@Graphite pouch full cells with electrolytes containing a mass fraction of 1% 4-FBC additives demonstrate improved capacity retention after 200 cycles, retaining capacity retention rates of 81.3%, which is much higher than that of 39.1% without additive. The improvement can be ascribed to the mitigation of electrolyte decomposition and inhibition of transition metal ions the dissolution from the cathode material due to the stable CEI film. Moreover, the electrochemical performance enhancement can also be achieved in high voltage and Ni-rich cathode materials, indicating the universality and effectiveness of this strategy for the practical applications of high energy density lithium-ion batteries.

Interfacial Robustness and Improved Kinetics of Single-Crystal Ni-Rich Co-Free Cathodes Enabled by Surface Crystal-Facet Modulation

The elimination of Co from Ni-rich layered cathodes is critical to reduce the production cost and increase the energy density for sustainable development. Herein, a delicate strategy of crystal-facet modulation is designed and explored, which is achieved by simultaneous Al/W-doping into the precursors, while the surface role of the crystal-facet is intensively revealed. Unlike traditional studies on crystal structure growth along a certain direction, this work modulates the crystal-facet at the nanoscale based on the effect of W-doping dynamic migration with surface energy, successfully constructing the core-shell (003)/(104) facet surface. Compared to the (003) plane, the induced (104) facet at the surface can provide more space for Li+-activity, enabling lower interfacial polarization and higher Li+-transport rate. Additionally, bulk Al-doping is beneficial for enhancing the Li+-diffusion from the exterior surface to the interior lattice. With improved interfacial stability and restrained surface erosion, the product exhibits superior capacity retention and boosted rate performance under the elevated temperature.

Optimized In Situ Doping Strategy Stabling Single-Crystal Ultrahigh-Nickel Layered Cathode Materials

Single-crystal Ni-rich cathodes offer promising prospects in mitigating intergranular microcracks and side reaction issues commonly encountered in conventional polycrystalline cathodes. However, the utilization of micrometer-sized single-crystal particles has raised concerns about sluggish Li+ diffusion kinetics and unfavorable structural degradation, particularly in high Ni content cathodes. Herein, we present an innovative in situ doping strategy to regulate the dominant growth of characteristic planes in the single-crystal precursor, leading to enhanced mechanical properties and effectively tackling the challenges posed by ultrahigh-nickel layered cathodes. Compared with the traditional dry-doping method, our in situ doping approach possesses a more homogeneous and consistent modifying effect from the inside out, ensuring the uniform distribution of doping ions with large radius (Nb, Zr, W, etc). This mitigates the generally unsatisfactory substitution effect, thereby minimizing undesirable coating layers induced by different solubilities during the calcination process. Additionally, the uniformly dispersed ions from this in situ doping are beneficial for alleviating the two-phase coexistence of H2/H3 and optimizing the Li+ concentration gradient during cycling, thus inhibiting the formation of intragranular cracks and interfacial deterioration. Consequently, the in situ doped cathodes demonstrate exceptional cycle retention and rate performance under various harsh testing conditions. Our optimized in situ doping strategy not only expands the application prospects of elemental doping but also offers a promising research direction for developing high-energy-density single-crystal cathodes with extended lifetime.

A uniform and high-voltage stable LiTMPO4 coating layer enabled high performance LiNi0.8Co0.15Mn0.05O2 towards boosting lithium storage

LiTMPO₄ materials, such as LiNiPO₄, can maintain structural stability and Li⁺ transport activity up to 4.8 V, showing great potential to stabilize layered nickel-rich cathodes at high voltage. But achieving a uniform LiTMPO₄ coating layer remains a great challenge. Herein, an ultrathin and uniform LiTMPO₄ layer (mainly LiNiPO₄) is successfully coated on the surface of LiNi_0.8Co_0.15Mn_0.05O₂ (NMC@LTMP) via utilizing the surface chelation of phytic acid with NMC precursors and a subsequent high-temperature in situ reaction. The reconstructed surface and interface could act as stable paths for Li⁺ transport and efficient barriers against electrolyte corrosion. Thus, harmful side reactions like solid electrolyte interphase overgrowth, irreversible phase transformation, and metal dissolution are inhibited simultaneously. Impressively, the optimized NMC@LTMP2 cathode exhibits remarkably improved capacity, as high as 215 mA h g^-1 at 2.8-4.5 V, with capacity retention of 87.21% after 200 cycles and outstanding rate capability of 140 mA h g^-1 at 10C, significantly better than a pristine cathode. Furthermore, a pouch cell assembled with an NMC@LTMP2 cathode and graphite anode also exhibits robust capacity retention of 82.42% after 100 cycles. These results provide useful insights towards enabling the application of NMC cathodes via developing facile modification methods.

Enabling high energy lithium metal batteries via single-crystal Ni-rich cathode material co-doping strategy

High-capacity Ni-rich layered oxides are promising cathode materials for secondary lithium-based battery systems. However, their structural instability detrimentally affects the battery performance during cell cycling. Here, we report an Al/Zr co-doped single-crystalline LiNi_0.88Co_0.09Mn_0.03O₂ (SNCM) cathode material to circumvent the instability issue. We found that soluble Al ions are adequately incorporated in the SNCM lattice while the less soluble Zr ions are prone to aggregate in the outer SNCM surface layer. The synergistic effect of Al/Zr co-doping in SNCM lattice improve the Li-ion mobility, relief the internal strain, and suppress the Li/Ni cation mixing upon cycling at high cut-off voltage. These features improve the cathode rate capability and structural stabilization during prolonged cell cycling. In particular, the Zr-rich surface enables the formation of stable cathode-electrolyte interphase, which prevent SNCM from unwanted reactions with the non-aqueous fluorinated liquid electrolyte solution and avoid Ni dissolution. To prove the practical application of the Al/Zr co-doped SNCM, we assembled a 10.8 Ah pouch cell (using a 100 μm thick Li metal anode) capable of delivering initial specific energy of 504.5 Wh kg^-1 at 0.1 C and 25 °C.

Interfacial Reviving of the Degraded LiNi0.8 Co0.1 Mn0.1 O2 Cathode by LiPO3 Repair Strategy

Nickel-rich cathode materials, owing to their high energy density and low cost, are considered to be one of the cathodes with the most potential in next-generation lithium-ion batteries. Unfortunately, this kind of cathode with highly active surface is easy to react with H₂ O and CO₂ when exposed to ambient air, resulting in the formation of lithium impurities and interfacial phase transition as well as deterioration of the electrochemical properties. In this work, the evolution mechanism of the structure and interface of LiNi_0.8 Co_0.1 Mn_0.1 O₂ during air-exposure is systematically investigated. Furthermore, a facile reviving strategy is proposed to restore the degraded LiNi_0.8 Co_0.1 Mn_0.1 O₂ by using LiPO₃ as the repair agent. The lithium impurities on the surface of the degraded sample can transform into the repair/coating layer, and part of the rock salt phase on the subsurface can revive to layered phase after repair heat treatment. As a result, the optimized cathode delivers an initial discharge capacity of 198.3 mAh g^-1 at 0.1C and a capacity retention of 85.5% after 50 cycles. Although slightly lower than the bare sample (201 mAh g^-1 and 88%), they are obviously higher than the exposed samples (166.5 mAh g^-1 and 40.4%). The regenerated electrochemical properties should be attributed to the multifunctional repair layer that can efficiently reduce the surface lithium impurities, prevent the corrosion of electrolyte, and improve the interfacial Li⁺ diffusion kinetics. This work can effectively reduce the waste of the degraded Ni-rich ternary materials and realize the transformation of "waste" into wealth.

Reduction of Surface Residual Lithium Compounds for Single-Crystal LiNi0.6Mn0.2Co0.2O2 via Al2O3 Atomic Layer Deposition and Post-Annealing

Surface residual lithium compounds of Ni-rich cathodes are tremendous obstacles to electrochemical performance due to blocking ion/electron transfer and arousing surface instability. Herein, ultrathin and uniform Al2O3 coating via atomic layer deposition (ALD) coupled with the post-annealing process is reported to reduce residual lithium compounds on single-crystal LiNi0.6Mn0.2Co0.2O2 (NCM622). Surface composition characterizations indicate that LiOH is obviously reduced after Al2O3 growth on NCM622. Subsequent post-annealing treatment causes the consumption of Li2CO3 along with the diffusion of Al atoms into the surface layer of NCM622. The NCM622 modified by Al2O3 coating and post-annealing exhibits excellent cycling stability, the capacity retention of which reaches 92.2% after 300 cycles at 1 C, much higher than that of pristine NCM622 (34.8%). Reduced residual lithium compounds on NCM622 can greatly decrease the formation of LiF and the degree of Li+/Ni2+ cation mixing after discharge–charge cycling, which is the key to the improvement of cycling stability.