Influence of Single-Crystalline Morphology on High-Nickel Layered Oxide Cathodes for Lithium-Ion Batteries
Authors: Mesnier Alex , Manthiram Arumugam 
Published: September 2023

Single-crystal, high-nickel layered oxides have seen increasing interest due to their potential to address the surface reactivity and mechanical resilience issues inherent to these cathode compositions. Great strides have been made in methods for synthesizing single-crystal layered oxides, but much is still unknown about the impact of single-crystalline morphology on the electrochemical behavior of these materials.
Here, we report a comparative study on the electrochemical operation of single- and polycrystalline LiNiO2, respectively termed SCLNO and PCLNO. SCLNO is synthesized with a molten-salt-assisted method, allowing the crystals to form at the same temperature as used for solid-state synthesis of PCLNO. As a result, the cathodes possess similar lattice structures with pure hexagonal layered phase and comparable levels of cation mixing (~1.5%), enabling a robust comparison of single-crystalline and polycrystalline morphology with minimal confounding variables. Charge/discharge performance, rate capability, and cycling stability reveal insights on the behavior single-crystal layered oxides and the performance tradeoffs they elicit. Particular attention is directed towards Li+ diffusion and phase evolution dynamics within the two morphologies, as characterized by in-situ and ex-situ X-ray diffraction, differential capacity analysis, and galvanostatic intermittent titration technique (GITT).
As has been reported in other cathode systems,1 single-crystal LNO (SCLNO) delivers a lower initial capacity of about 200 mA h g-1, compared to nearly 230 mA h g-1 for polycrystalline LNO (PCLNO), most of which is lost in the low-voltage discharge region known to be particularly susceptible to kinetic hindrance.2 Indeed, GITT indicates a lower effective Li+ diffusivity in SCLNO. Interestingly, the single-crystal cathode exceeds the polycrystalline cathode in rate capability; SCLNO delivers 78% capacity at 10C discharge rate (158 mA h g-1) while PCLNO achieves only 25% capacity (57 mA h g-1). Analysis of the charge/discharge profiles of the materials reveals the H2-H3 transition is still present in SCLNO at high discharge rates, while it disappears completely in PCLNO. Despite the conflicting nature of the observed lower diffusivity, yet improved rate capability, further analysis suggests that the elimination of grain boundaries in single-crystalline cathodes may be responsible for both phenomena. Details of the experimental techniques, and results from the electrochemical analysis will be presented during the meeting. The findings from this study will contribute to understanding single-crystalline layered oxides and aid the design of higher-performance cathode materials.

References

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