Spectroscopy and Imaging

Cobalt-free lithium nickel oxide (LNO) has garnered significant interest as the end member of high-nickel layered oxide cathodes for next-generation batteries. However, its practical performance notably underperforms expectations because of the structural degradation and unstable interfacial chemistry with electrolytes during cycling. Here, we report that a durable cathode-electrolyte interface (CEI), enriched by in situ formed sulfides and borides, can inhibit LNO structural degradation and suppress Ni ion dissolution. With the CEI protection, the stability of LNO can be remarkably extended, and batteries demonstrate a capacity retention rate of 84% (30 °C) and 79% (50 °C) after 200 cycles at 1C, respectively. These results demonstrate that enriching CEI with sulfur-containing species can effectively stabilize the interfacial chemistry of LNO, particularly at an elevated temperature of 50 °C. This finding provides valuable perspectives on designing electrolytes for cobalt-free LNO and other high-Ni cathodes toward the development of next-generation high-energy-density lithium-ion batteries.

Read more at ACS Energy Letters:
https://pubs.acs.org/doi/10.1021/acsenergylett.4c00652

Lithium-rich layered oxides (LLOs) have gained significant attention due totheir high capacity of over 250 mAh g⁻¹, which originates from the chargecompensation of oxygen anions activated under high voltage. However, thecharge compensation of oxygen anions is prone to over-oxidation, leading toserious irreversible oxygen release, surface-interface reactions, and structuralevolution. These detriments make LLOs undergo fast voltage decay andcapacity fading, which have hindered their practical applications for manyyears. Herein, this work develops a multifunctional co-doping strategy andconstructs W─O bonds with strong bonding interaction and covalence, lowbond energy Li─S bonds with non-binding electrons near the Fermi level, andcontinuous and homogeneous surface spinel-like layer induced by W/Sco-doping. Their synergistic effect significantly mitigates the irreversibleoxygen release and surface-interface reactions and improves structuralstability of Li-rich layered cathodes. Thus, the designed and prepared Co-freeLi-rich layered cathode (Li_1.232Mn_0.574Ni_0.191W_0.003O_1.995S_0.005) deliverssuperior voltage and capacity stability. Its capacity retention after 400 cycles isas large as 86%, and its voltage decay rate from the 10^th to the 400^th cycle isonly 0.626 mV cycle⁻¹.

Read more at Advanced Functional Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202404044

The irreversible deterioration of electrochemical performance in Ni-rich cathode materials, attributed to crack propagation and undesired side reactions, poses a critical barrier to the further development of high-energy power batteries for electrical vehicles (EVs). Herein, a concentration gradient strategy is proposed for synthesizing a Ni-rich cathode with enhanced mechanical and electrochemical stability to address the issues related to the irreversible structural deterioration. Notably, the concentration gradient structure contributes to superior mechanical strength in secondary particles due to the radially orientated primary particles resulted from Mn composition grading, which effectively alleviate the internal strain caused by structural changes and fatigue destruction during successive cycling. Moreover, the Mn-rich surface minimizes the parasitic side reactions at the electrode–electrolyte interface. Benefitting from the above, the concentration gradient sample can deliver ≈180.1 mA h g⁻¹ at 1 C and retain 96.2% of its initial discharge capacity after 100 cycles. This work demonstrates that the concentration gradient structure can simultaneously improve the mechanical and chemical stabilities of Ni-rich cathode and offers a feasible way for designing stable lithium-ion batteries with high energy density

Read more at Advanced Functional Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202400956

Ordinary metals contain electron liquids within well-defined ‘Fermi’ surfaces at which the electrons behave as if they were non-interacting. In the absence of transitions to entirely new phases such as insulators or superconductors, interactions between electrons induce scattering that is quadratic in the deviation of the binding energy from the Fermi level. A long-standing puzzle is that certain materials do not fit this ‘Fermi liquid’ description. A common feature is strong interactions between electrons relative to their kinetic energies. One route to this regime is special lattices to reduce the electron kinetic energies. Twisted bilayer graphene is an example, and trihexagonal tiling lattices (triangular ‘kagome’), with all corner sites removed on a 2 × 2 superlattice, can also host narrow electron bands for which interaction effects would be enhanced. Here we describe spectroscopy revealing non-Fermi-liquid behaviour for the ferromagnetic kagome metal Fe₃Sn₂. We discover three C₃-symmetric electron pockets at the Brillouin zone centre, two of which are expected from density functional theory. The third and most sharply defined band emerges at low temperatures and binding energies by means of fractionalization of one of the other two, most likely on the account of enhanced electron–electron interactions owing to a flat band predicted to lie just above the Fermi level. Our discovery opens the topic of how such many-body physics involving flat bands could differ depending on whether they arise from lattice geometry or from strongly localized atomic orbitals.

Read more at Nature:
https://www.nature.com/articles/s41586-024-07085-w

Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention, yet their origin remains a topic of debate. The discovery of ScV₆Sn₆, a bilayer kagome metal featuring an intriguing $\sqrt{3}\times <!--\; \times \; -->\sqrt{3}\times <!--\; \times \; -->3$  CDW order, offers a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering and density functional theory to investigate the electronic structure and phonon modes of ScV₆Sn₆. We identify topologically nontrivial surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS aligning with the in-plane component of the CDW vector near the $\stackrel{¯<!--\; ¯\; -->}{K}$ point. Additionally, Raman measurements indicate a strong electron-phonon coupling, as evidenced by a two-phonon mode and new emergent modes. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV₆Sn₆.

Read more at Nature Communications:
https://www.nature.com/articles/s41467-024-45859-y

Electronic nematicity that spontaneously breaks rotational symmetry is a generic phenomenon in correlated quantum systems including high-temperature superconductors and the AV₃Sb₅ (A can be K, Rb or Cs) family of kagome superconductors. However, the underlying mechanism of nematicity in these systems is hard to identify because of its entanglement with other ordered phases. Recently, a family of titanium-based kagome superconductors ATi₃Bi₅ have been synthesized, where electronic nematicity occurs in the absence of charge order. It provides a platform to study nematicity in its pure form, as well as its interplay with orbital degrees of freedom. Here we reveal the band topology and orbital characters of the multiorbital RbTi₃Bi₅. We use polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to identify the coexistence of flat bands, type-II Dirac nodal lines and non-trivial topology in this compound. Our study demonstrates the change in orbital character along the Fermi surface contributed by the kagome bands, implying a strong intrinsic interorbital coupling in the Ti-based kagome metals. Furthermore, doping-dependent measurements uncover the orbital-selective features in the kagome bands, which can be explained by d–p hybridization. Hence, interorbital coupling together with d–p hybridization is probably the origin of electronic nematicity in ATi₃Bi₅.

Read more at Nature Physics:
https://www.nature.com/articles/s41567-023-02215-z