Jin-Myoung Lim

Layered-type, nickel-rich lithium nickel manganese cobalt oxides (NMCs) are well-established cathode materials for high-performance lithium-ion batteries. These materials exhibit stable cycling within mild voltage windows (up to 4.3 V vs. Li/Li⁺) but can be charged to higher potentials (4.8 V vs. Li/Li⁺) to access additional capacity. However, high-voltage operation is generally avoided due to severe interfacial and chemomechanical degradation, such as electrolyte decomposition reactions and non-uniform buildup of chemomechanical strains that can result in particle fracture and compromised cyclability. In this work, we find that a conformal graphene coating enables significant enhancements in the cycle life and coulombic efficiency of NMC cathodes during high voltage electrochemical cycling. Postmortem surface chemical analysis reveals that graphene-coated NMC electrodes exhibit reduced spectral intensities corresponding to electrolyte decomposition products, suggesting that the interfacial graphene layer limits parasitic electrode-electrolyte interactions. Furthermore, global and local structural analysis shows that the graphene coating mitigates mechanical degradation, which was evidenced by reduced microstrain, particle fracture, and electrochemical creep. Based on these observations, we propose a mechanistic relationship between the spatial uniformity of lithium flux and primary particle-level mechanical degradation, and show that a conformal graphene coating is well-suited to address these issues associated with high voltage chemomechanical degradation. Overall, these results delineate a pathway for rationally mitigating high-voltage chemomechanical degradation of nickel-rich cathodes that can be applied to existing and emerging classes of battery materials. https://doi.org/10.1021/acsaem.1c01995

In this study, we present improved power characteristics and suppressed phase transition by incorporating elemental doping into a P2-type cathode of sodium ion batteries. A Cu-doped Fe–Mn based P2-type Na_0.67Cu_0.125Fe_0.375Mn_0.5O₂ cathode was designed based on the calculations of the electronic structure and then examined experimentally. Using first principles, we introduced instrinsic p-type conductivity by elemental doping with Cu. Introduction of Cu generated electron holes above the Fermi level in the electronic structure, which is typical of p-type semiconductors. Charge analyses suggested that the hole generation was driven primarily by the greater reduced characteristics of Cu as compared with those of Fe and Mn. In addition, introduction of Cu retaining high reduced property also suppressed phase transition from the P2 to Z phase by Fe migration to empty Na layers mainly. Electrochemical experiments revealed improved power characteristics upon the introduction of p-type conductivity. This could be attributed to the increase in the electronic conductivity by hole generation in the valence band. This study suggests that the introduction of p-type conductivity could be a rational tactic for the development of promising cathode materials for high performance sodium ion batteries.

To achieve the high energy densities demanded by emerging technologies, lithium battery electrodes need to approach the volumetric and specific capacity limits of their electrochemically active constituents, which requires minimization of the inactive components of the electrode. However, a reduction in the percentage of inactive conductive additives limits charge transport within the battery electrode, which results in compromised electrochemical performance. Here, an electrode design that achieves efficient electron and lithium‐ion transport kinetics at exceptionally low conductive additive levels and industrially relevant active material areal loadings is introduced. Using a scalable Pickering emulsion approach, Ni‐rich LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode powders are conformally coated using only 0.5 wt% of solution‐processed graphene, resulting in an electrical conductivity that is comparable to 5 wt% carbon black. Moreover, the conformal graphene coating mitigates degradation at the cathode surface, thus providing improved electrochemical cycle life. The morphology of the electrodes also facilitates rapid lithium‐ion transport kinetics, which provides superlative rate capability. Overall, this electrode design concurrently approaches theoretical volumetric and specific capacity limits without tradeoffs in cycle life, rate capability, or active material areal loading.

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (Na_xMnO₂·yH₂O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries.

A double-hierarchical sulfur host has been synthesized in which hierarchical carbon spheres, constructed from building blocks of hollow carbon nanobubbles used for loading sulfur, are sealed by a polar MoS₂ coating that is composed of ultrathin nanosheets (MoS₂@HCS composite).

A synergistic approach for advanced cathode materials is proposed. Sodium manganese oxide with a layered-tunnel hybrid structure was designed, synthesized, and subsequently investigated. The layered-tunnel hybrid structure provides fast Na ion diffusivity and high structural stability thanks to the tunnel phase, enabling high rate capability and greatly improved cycling stability compared to that of the pure P2 layered phase while retaining the high specific capacity of the P2 layered phase. The hybrid structure provided a decent discharge capacity of 133.4 mAh g^–1 even at 8 C, which exceeds the reported best rate capability for Mn-based cathodes. It also displayed an impressive cycling stability, maintaining 83.3 mAh g^–1 after 700 cycles at 10 C. Theoretical calculation and the potentiostatic intermittent titration technique (PITT) demonstrated that this hybrid structure helps enhance Na ion diffusivity during charge and discharge, attaining, as a result, an unprecendented electrochemical performance.

A fundamental understanding of anomalous redox mechanisms in hexacyanometallate compounds, compared with conventional NaMO₂ systems (M: transition metals), is presented based on first-principles calculations and experimental validations. From theoretical calculations, we identified low-spin and high-spin states of Fe ions coordinated by the cyanide group (–CN) with the same oxidation state (Fe²⁺) in Na₂Fe₂(CN)₆. Considering the site dependency of d electronic spin configurations based on the crystal field theory (CFT) of transition metals (TMs), we calculated the thermodynamic mixing energy using Na₂Fe₂(CN)₆ and Na₂Mn₂(CN)₆ for obtaining a thermodynamically stable phase of Na₂FeMn(CN)₆. The phase stabilities of Na₂Fe_2−xMn_x(CN)₆ among many atomic configurations and lattice parameters originating from octahedral structures (i.e., Fe(CN)₆ and Mn(NC)₆) are highly dependent on the electronic structures of TMs with spin states. From partial density of states (PDOS) and spatial electron distributions, it was observed that Fe²⁺ in the low-spin state (t⁶ _2g) and Mn²⁺ in the high-spin states (t³ _2g and e² _g) in the stable phase lead to higher redox potentials (∼3.55 V vs. Na/Na⁺) with the removal of Na⁺ as compared to that of Na₂Fe₂(CN)₆. In addition, lattice parameters from x = 0 to x = 1 in Na₂Fe_2−xMn_x(CN)₆ are increased due to the larger ionic radius of Mn²⁺ in the high-spin states. On the other hand, Fe²⁺ in the high-spin states (t⁴ _2g and e² _g) and Mn²⁺ in the low-spin state (t⁵ _2g) in the most unstable phase of Na₂FeMn(CN)₆ would have lower redox potentials. Based on the fundamental correlation between redox potentials and CFT with spin configurations of TMs, we suggest a material design concept for intercalation compounds with higher energy densities for rechargeable battery systems.