Designing light-element materials with large effective spin-orbit coupling.

Spin-orbit coupling (SOC), which is the core of many condensed-matter phenomena such as nontrivial band gap and magnetocrystalline anisotropy, is generally considered appreciable only in heavy elements. This is detrimental to the synthesis and application of functional materials. Therefore, amplifying the SOC effect in light elements is crucial. Herein, focusing on 3d and 4d systems, we demonstrate that the interplay between crystal symmetry and electron correlation can significantly enhance the SOC effect in certain partially occupied orbital multiplets through the self-consistently reinforced orbital polarization as a pivot. Thereafter, we provide design principles and comprehensive databases, where we list all the Wyckoff positions and site symmetries in all two-dimensional (2D) and three-dimensional crystals that could have enhanced SOC effect. Additionally, we predict nine material candidates from our selected 2D material pool as high-temperature quantum anomalous Hall insulators with large nontrivial band gaps of hundreds of meV. Our study provides an efficient and straightforward way for predicting promising SOC-active materials, relieving the use of heavy elements for next-generation spin-orbitronic materials and devices.

Charge transport mechanisms in potassium superoxide.

Rechargeable metal-air batteries based on superoxide discharge products are attractive due to the facile one-electron redox process of O2/O2-. Recently, a K-O2 battery has been reported that showed a significantly lower discharge/charge potential gap than the Li-O2 battery systems. Here, we perform first-principles calculations on potassium superoxide (KO2) to unravel the charge transport mechanism in this discharge product. The concentration and mobility of intrinsic carriers are calculated. The results show that hole polarons and negatively charged potassium ion vacancies are the main charge carriers. The conductivity associated with polaron hopping (2 × 10-12 S cm-1) is 8 orders of magnitude higher than that of Li2O2, and the ionic conductivity has a comparable value (1 × 10-13 S cm-1). Our calculation results can rationalize the experimental findings and provide a theoretical basis for the understanding of superoxide discharge products in metal-air batteries.

A hybrid superconcentrated electrolyte enables 2.5 V carbon-based supercapacitors.

A hybrid solvent in salt electrolyte was developed by hybridizing aqueous and organic solvents in concentrated lithium bis(fluorosulfonyl)imide (LiFSI) salts, such an electrolyte provides an unprecedented electrochemical window of 5.35 V, which is even comparable to traditional organic electrolytes, and enables a super-stable carbon-based symmetric supercapacitor with a long life of 10 000 cycles at an operating voltage of 2.5 V.

Discovery of space aromaticity in transition-metal monoxide crystal Nb3O3 enabled by octahedral Nb6 structural units.

An octahedral Nb6 structural unit with space aromaticity is identified for the first time in a transition-metal monoxide crystal Nb3O3 by ab initio calculations. The strong Nb-Nb metallic bonding facilitates the formation of stable octahedral Nb6 structural units and the release of delocalization energy. Moreover, the Nb atoms in continuously connected Nb6 structural units share their electrons with each other in a continuous space of framework, so that the electrons are uniformly distributed. The newly discovered aromaticity in the octahedral Nb6 structural units extends the range of aromatic compounds and broadens our vision in structural chemistry.

Electrochemical Nitrogen Reduction Reaction Performance of Single-Boron Catalysts Tuned by MXene Substrates.

A boron (B) center, which has an electronic structure mimicking the filled and empty d orbitals in transition metals, can effectively activate the triple bond in N2 so as to catalyze the nitrogen reduction reaction (NRR). Here, by means of density functional theory, we have systematically investigated the catalytic performance of a single B atom decorated on two-dimensional transition metal carbides (MXenes). The B-doped Mo2CO2 and W2CO2 MXenes exhibit outstanding catalytic activity and selectivity with limiting potentials of -0.20 and -0.24 V, respectively. Importantly, we have found that, although a high tendency of B-to-adsorbate electron donation can promote the hydrogenation of *N2 to *N2H, it would also severely hamper the *NH2 to *NH3 conversion due to the strong B-N bonding. Such an electron-donation effect can be reasonably tuned by the transition metal in the MXene substrate, which enables us to achieve optimized catalytic performance with a certain moderate degree of electron donation.

An Ordered Ni6 -Ring Superstructure Enables a Highly Stable Sodium Oxide Cathode.

Sodium-based layered oxides are among the leading cathode candidates for sodium-ion batteries, toward potential grid energy storage, having large specific capacity, good ionic conductivity, and feasible synthesis. Despite their excellent prospects, the performance of layered intercalation materials is affected by both a phase transition induced by the gliding of the transition metal slabs and air-exposure degradation within the Na layers. Here, this problem is significantly mitigated by selecting two ions with very different MO bond energies to construct a highly ordered Ni6 -ring superstructure within the transition metal layers in a model compound (NaNi2/3 Sb1/3 O2 ). By virtue of substitution of 1/3 nickel with antimony in NaNiO2 , the existence of these ordered Ni6 -rings with super-exchange interaction to form a symmetric atomic configuration and degenerate electronic orbital in layered oxides can not only largely enhance their air stability and thermal stability, but also increase the redox potential and simplify the phase-transition process during battery cycling. The findings reveal that the ordered Ni6 -ring superstructure is beneficial for constructing highly stable layered cathodes and calls for new paradigms for better design of layered materials.

Discovering unusual structures from exception using big data and machine learning techniques.

Recently, machine learning (ML) has become a widely used technique in materials science study. Most work focuses on predicting the rule and overall trend by building a machine learning model. However, new insights are often learnt from exceptions against the overall trend. In this work, we demonstrate that how unusual structures are discovered from exceptions when machine learning is used to get the relationship between atomic and electronic structures based on big data from high-throughput calculation database. For example, after training an ML model for the relationship between atomic and electronic structures of crystals, we find AgO2F, an unusual structure with both Ag3+ and O22-, from structures whose band gap deviates much from the prediction made by our model. A further investigation on this structure might shed light into the research on anionic redox in transition metal oxides of Li-ion batteries.

Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li(NixMnyCoz)O2.

Ni/Li exchange (disordering) usually happens in layered Li(NixMnyCoz)O2 (NMC) materials and affects the performance of the material in lithium-ion batteries. Most of previous studies attributed this phenomenon to the similar size of Ni2+ and Li+, which implies that Ni2+ should be more favorable than Ni3+ to be located at Li 3b sites in the Li slab. However, this theory cannot explain why in Ni-rich NMC materials where most Ni cations are Ni3+, Ni/Li exchange happens even more frequently. Using extensive ab initio calculations combined with experiments, here we report that a superexchange interaction between transition metals plays a dominating role in tuning the Ni/Li disordering in NMC materials. Under this scheme, we also propose a new charge compensation mechanism that describes that after Ni3+/Li exchange the nearest Co3+ transforms to Co4+ in Ni-rich NMC materials. On the basis of this theory, the existence of Co4+ in the initial Ni-rich NMC samples was predicted for the first time, which was further confirmed by our synchrotron-based soft X-ray absorption spectroscopy.

Tuning of Thermal Stability in Layered Li(NixMnyCoz)O2.

Understanding and further designing new layered Li(NixMnyCoz)O2 (NMC) (x + y + z = 1) materials with optimized thermal stability is important to rechargeable Li batteries (LIBs) for electrical vehicles (EV). Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)3-O-Li3-x'): each O atom bonds with three transition metals (TM) from the TM-layer and three to zero Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. The synergistic effect between Li vacancies and raised valence state of Ni during delithiation process can aggravate instability of oxygen, and oxygen coordinated with more nickel (especially with high valence state) in LSCU becomes more unstable at a fixed delithiation state. The Ni/Li mixing would decrease the thermal stability of the "Ni═Mn" group NMC materials but benefit the thermal stability of "Ni-rich" group, because the Ni in the Li layer would form 180° Ni-O-Ni super exchange chains in "Ni-rich" NMC materials. Mn and Co doping can tune the initial valence state of Ni, local coordination environment of oxygen, and the Ni/Li disorder, thus to tune the thermal stability directly.

Pre-Lithiation of Li(Ni1-x-yMnxCoy)O2 Materials Enabling Enhancement of Performance for Li-Ion Battery.

Transition metal oxide materials Li(NixMnyCoz)O2 (NMCxyz) based on layered structure are potential cathode candidates for automotive Li-ion batteries because of their high specific capacities and operating potentials. However, the actual usable capacity, cycling stability, and first-cycle Coulombic efficiency remain far from practical. Previously, we reported a combined strategy consisting of depolarization with embedded carbon nanotube (CNT) and activation through pre-lithiation of the NMC host, which significantly improved the reversible capacity and cycling stability of NMC532-based material. In the present work we attempt to understand how pre-lithiation leads to these improvements on an atomic level with experimental investigation and ab initio calculations. By lithiating a series of NMC materials with varying chemical compositions prepared via a conventional approach, we identified the Ni in the NMC lattice as the component responsible for accommodating a double-layered Li structure. Specifically, much better improvements in the cycling stability and capacity can be achieved with the NMC lattices populated with Ni(3+) than those populated with only Ni(2+). Using the XRD we also found that the emergence of a double-layer Li structure is not only reversible during the pre-lithiation and the following delithiation, but also stable against elevated temperatures up to 320 °C. These new findings regarding the mechanism of pre-lithiation as well as how it affects the reversibility and stability of NMC-based cathode materials prepared by the conventional slurry approach will promote the possibility of their application in the future battery industry.

Prelithiation Activates Li(Ni0.5Mn0.3Co0.2)O2 for High Capacity and Excellent Cycling Stability.

Transition metal oxide materials Li(NixMnyCoz)O2 (NMC) based on layered structures are expected to replace LiFePO4 in automotive Li-ion batteries because of their higher specific capacity and operating potential. However, the actual usable capacity is much lower than the promised theoretical value [Uchaker, E.; Cao, G. Nano Today 2014, 9, 499-524; Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359-367], in addition to the often poor cycling performance and the first-cycle Coulombic efficiency, for which Mn(II)-dissolution, its immobilization in solid electrolyte interface (SEI), oxidation of electrolytes by Ni, and other parasitic process thereat have been held responsible [Zhan, C., et al. Nat. Commun. 2013, 4, 2437; Wang, L., et al. J. Solid State Electrochem. 2009, 13, 1157-1164; Lin, F., et al. Nat. Commun. 2014, 5, 4529]. Previously, we reported a composite Li(Ni0.5Mn0.3Co0.2)O2 (NMC532) depolarized by the embedded carbon nanotube (CNT) and achieved capacity close to the theoretical limit [Wu, Z., et al. Nano. Lett. 2014, 14, 4700-4706]; unfortunately, this high capacity failed to be maintained in long-term cycling due to the degrading contacts between the active ingredient and CNT network. On the basis of that NMC532/CNT composite, the present work proposes a unique "prelithiation process", which brought the cathode to low potentials before regular cycling and led to an interphase that is normally formed only on anode surfaces. The complete coverage of cathode surface by this ∼40 nm thick interphase effectively prevented Mn(II) dissolution and minimized the side reactions of Ni, Co, and Mn at the NMC interface during the subsequent cycling process. More importantly, such a "prelithiation" process activated a structure containing two Li layers near the surface of NMC532 particles, as verified by XRD and first principle calculation. Hence, a new cathode material of both high capacity with depolarized structure and excellent cycling performance was generated. This new structure can be incorporated in essentially all the NMC-based layered cathode materials, providing us with an effective tool to tailor-design future new cathode materials for lithium batteries.