Intrinsic enhancement of the rate capability and suppression of the phase transition via p-type doping in Fe-Mn based P2-type cathodes used for sodium ion batteries.

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 Na0.67Cu0.125Fe0.375Mn0.5O2 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.

Phase-Inversion Polymer Composite Separators Based on Hexagonal Boron Nitride Nanosheets for High-Temperature Lithium-Ion Batteries.

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Triggered reversible phase transformation between layered and spinel structure in manganese-based layered compounds.

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 (NaxMnO2·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.

Cu4SnS4-Rich Nanomaterials for Thin-Film Lithium Batteries with Enhanced Conversion Reaction.

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu4SnS4-rich composite of nanoparticles and nanotubes is synthesized and applied for thin and flexible Li-metal batteries. Unlike the Cu2SnS3-rich electrode, the Cu4SnS4-rich electrode cycles stably with an enhanced conversion capacity of ∼416 mAh g-1 (∼52% of total capacity) after 200 cycles. The lithiation/delithiation mechanisms of Cu-Sn-S electrodes and the voltage ranges of conversion and alloying reactions are informed by in situ X-ray diffraction tests. The conversion process of three Cu-Sn-S compounds is compared by density functional theory (DFT) calculations based on three algorithms, elucidating the enhanced conversion stability and superior diffusion kinetics of Cu4SnS4 electrodes. The reaction pathway of Cu-Sn-S electrodes and the root cause for the unstable capacity are revealed by in situ/ex situ characterizations, DFT calculations, and various electrochemical tests. This work provides insight into developing energy materials and power devices based on multiple lithiation mechanisms.

High-Modulus Hexagonal Boron Nitride Nanoplatelet Gel Electrolytes for Solid-State Rechargeable Lithium-Ion Batteries.

Solid-state electrolytes based on ionic liquids and a gelling matrix are promising for rechargeable lithium-ion batteries due to their safety under diverse operating conditions, favorable electrochemical and thermal properties, and wide processing compatibility. However, gel electrolytes also suffer from low mechanical moduli, which imply poor structural integrity and thus an enhanced probability of electrical shorting, particularly under conditions that are favorable for lithium dendrite growth. Here, we realize high-modulus, ion-conductive gel electrolytes based on imidazolium ionic liquids and exfoliated hexagonal boron nitride (hBN) nanoplatelets. Compared to conventional bulk hBN microparticles, exfoliated hBN nanoplatelets improve the mechanical properties of gel electrolytes by 2 orders of magnitude (shear storage modulus ∼5 MPa), while retaining high ionic conductivity at room temperature (>1 mS cm-1). Moreover, exfoliated hBN nanoplatelets are compatible with high-voltage cathodes (>5 V vs Li/Li+) and impart exceptional thermal stability that allows high-rate operation of solid-state rechargeable lithium-ion batteries at temperatures up to 175 °C.

Atomic-Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe2 single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe2 → Lix SnSe2 → Li2 Se + Sn → Li2 Se + Li17 Sn4 ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe2 nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe2 with potential implications for the broader class of 2D metal chalcogenides.

A highly efficient double-hierarchical sulfur host for advanced lithium-sulfur batteries.

Li-S batteries have attracted enormous interest due to their potentially high energy density, non-toxicity and the low cost of sulfur. The main challenges of sulfur cathodes are the short cycling life and limited power density caused by the low conductivity of sulfur and dissolution of Li polysulfides. Here we design a new double-hierarchical sulfur host to address these problems. Hierarchical carbon spheres (HCSs), constructed from building blocks of hollow carbon nanobubbles for loading sulfur, are sealed within a thin, polar MoS2 coating composed of ultrathin nanosheets. Experimental and theoretical studies show a strong absorption of the MoS2 nanoshell to polysulfides, and the excellent stability of the MoS2 nanosheets after the adsorption of polysulfides. Moreover, MoS2 promotes the electrochemical redox kinetics in Li-S batteries. Benefiting from the unique hierarchical, hollow and compositional characteristics of the host, the S/MoS2@HCS composite electrode shows a high capacity of 1048 mA h g-1 at 0.2C, good rate capacity and long cycling life with a slow capacity decay of 0.06% per cycle.

Self-Assembled Cu-Sn-S Nanotubes with High (De)Lithiation Performance.

Through a gelation-solvothermal method without heteroadditives, Cu-Sn-S composites self-assemble to form nanotubes, sub-nanotubes, and nanoparticles. The nanotubes with a Cu3-4SnS4 core and Cu2SnS3 shell can tolerate long cycles of expansion/contraction upon lithiation/delithiation, retaining a charge capacity of 774 mAh g-1 after 200 cycles with a high initial Coulombic efficiency of 82.5%. The importance of the Cu component for mitigation of the volume expansion and structural evolution upon lithiation is informed by density functional theory calculations. The self-generated template and calculated results can inspire the design of analogous Cu-M-S (M = metal) nanotubes for lithium batteries or other energy storage systems.

Mn-Based Cathode with Synergetic Layered-Tunnel Hybrid Structures and Their Enhanced Electrochemical Performance in Sodium Ion Batteries.

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.

Hexacyanometallates for sodium-ion batteries: insights into higher redox potentials using d electronic spin configurations.

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 Na2Fe2-xMnx(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 (t) and Mn2+ in the high-spin states (t and e) 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 Na2Fe2-xMnx(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 (t and e) and Mn2+ in the low-spin state (t) 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.

Intrinsic Origins of Crack Generation in Ni-rich LiNi0.8Co0.1Mn0.1O2 Layered Oxide Cathode Material.

Ni-rich LiNi0.8Co0.1Mn0.1O2 layered oxide cathodes have been highlighted for large-scale energy applications due to their high energy density. Although its specific capacity is enhanced at higher voltages as Ni ratio increases, its structural degradation due to phase transformations and lattice distortions during cycling becomes severe. For these reasons, we focused on the origins of crack generation from phase transformations and structural distortions in Ni-rich LiNi0.8Co0.1Mn0.1O2 using multiscale approaches, from first-principles to meso-scale phase-field model. Atomic-scale structure analysis demonstrated that opposite changes in the lattice parameters are observed until the inverse Li content x = 0.75; then, structure collapses due to complete extraction of Li from between transition metal layers. Combined-phase investigations represent the highest phase barrier and steepest chemical potential after x = 0.75, leading to phase transformations to highly Li-deficient phases with an inactive character. Abrupt phase transformations with heterogeneous structural collapse after x = 0.81 (~220 mAh g-1) were identified in the nanodomain. Further, meso-scale strain distributions show around 5% of anisotropic contraction with lower critical energy release rates, which cause not only micro-crack generations of secondary particles on the interfaces between the contracted primary particles, but also mechanical instability of primary particles from heterogeneous strain changes.

Design of Surface Doping for Mitigating Transition Metal Dissolution in LiNi0.5 Mn1.5 O4 Nanoparticles.

In lithium-ion batteries (LIBs) comprising spinel cathode materials, the dissolution of transition metals (TMs) in the cathodes causes severe cyclic degradation. We investigate the origin and mechanism of surface TM dissolution in high-voltage spinel oxide (LiNi0.5 Mn1.5 O4 ) nanoparticles to find a practical method for its mitigation. Atomic structures of the LiNi0.5 Mn1.5 O4 surfaces are developed, and the electronic structures are investigated by first-principles calculations. The results indicate that titanium is a promising dopant for forming a more stable surface structure by reinforcing metal-oxygen bonds in LiNi0.5 Mn1.5 O4 . Experimentally synthesized LiNi0.5 Mn1.5 O4 with titanium surface doping exhibits improved electrochemical performance by suppressing undesirable TM dissolution during cycles. The theoretical prediction and experimental validation presented here suggest a viable method to suppress TM dissolution in LiNi0.5 Mn1.5 O4 .

Phase Separation and d Electronic Orbitals on Cyclic Degradation in Li-Mn-O Compounds: First-Principles Multiscale Modeling and Experimental Observations.

A combined study involving experiments and multiscale computational approaches is conducted to propose a theoretical solution for the suppression of the Jahn-Teller distortion which causes severe cyclic degradation. As-synthesized pristine and Al-doped Mn spinel compounds are the focus to understand the mechanism of the cyclic degradation in terms of the Jahn-Teller distortion, and the electrochemical performance of the Al-doped sample shows enhanced cyclic performance compared with that of the pristine one. Considering the electronic structures of the two systems using first-principles calculations, the pristine spinel suffers entirely from the Jahn-Teller distortion by Mn(3+), indicating an anisotropic electronic structure, but the Al-doped spinel exhibits an isotropic electronic structure, which means the suppressed Jahn-Teller distortion. A multiscale phase field model in nanodomain shows that the phase separation of the pristine spinel occurs to inactive Li0Mn2O4 (i.e., fully delithiated) gradually during cycles. In contrast, the Al-doped spinel does not show phase separation to an inactive phase. This explains why the Al-doped spinel maintains the capacity of the first charge during the subsequent cycles. On the basis of the mechanistic understanding of the origins and mechanism of the suppression of the Jahn-Teller distortion, fundamental insight for making tremendous cuts in the cyclic degradation could be provided for the Li-Mn-O compounds of Li-ion batteries.

Underlying mechanisms of the synergistic role of Li2MnO3 and LiNi1/3Co1/3Mn1/3O2 in high-Mn, Li-rich oxides.

For large-scale energy storage applications requiring high energy density, the development of Li-rich oxides with enhanced cyclic stabilities during high-voltage operations and large specific capacities is required. In this regard, high-Mn, Li-rich oxides (HMLOs; xLi2MnO3 (1 - x)LiNi1/3Co1/3Mn1/3O2 at x > 0.5) warrant an in-depth study because of their good cyclic performance at high operating voltages and potentially large specific capacities. Here, to understand the synergistic effects and enhanced cyclic stability of HMLOs, mechanically blended HMLO (m-HMLO) and chemically bonded HMLO (c-HMLO) were prepared and investigated. c-HMLO exhibits relatively high reaction voltages, large specific capacities, and enhanced cyclic stabilities (∼99%) at a high operating voltage (∼4.8 V vs. Li/Li(+)) compared with m-HMLO. First-principles calculations with electronic structure analysis were performed using an atomic model developed by Rietveld refinement using as-synthesised c-HMLO. The redox mechanisms of Ni, Co, and Mn ions were determined via the partial density of states of the ground states predicted using the cluster expansion method, which elucidates that LiNi1/3Co1/3Mn1/3O2 stabilises the transition metal (TM) layer of Li2MnO3 and separates Li delithiation potentials in Li2MnO3 in the HMLO. Kinetic analyses including electronic structures revealed that the interlayer migration of TMs from the TM layer to the Li layer depends on the crystal field stabilisation. Thus, TMs with reduced character in the tetrahedral sites than the octahedral sites owing to the effects of crystal field stabilisation, such as Ni ions, in HMLOs would face a higher interlayer migration barrier, impeding phase transformation into spinel phases. Furthermore, Cu ions could constitute a doping source for HMLOs to improve the material's cyclic stability through this mechanism. These characteristics may be widely applied to explain experimental phenomena and improve the properties of cathode materials for Li-ion batteries.

Understanding of Surface Redox Behaviors of Li2MnO3 in Li-Ion Batteries: First-Principles Prediction and Experimental Validation.

Critical degradation mechanism of many cathode materials for Li-ion batteries is closely related to phase transformations at the surface/interface. Li2MnO3 in x Li2MnO3 ⋅(1-x) LiMO2 (M=Ni, Co, Mn) provides high capacity, but the Li2MnO3 phase is known to degrade during cycling through phase transformation and O2 evolution. To resolve such degradation problems, it is critical to develop a fundamental understanding of the underlying mechanism. Using first-principles calculations, we identified the surface delithiation potential (<4.5 V vs. Li/Li(+) ) of Li2MnO3, which is significantly lower than the bulk redox potential. A lower Mn oxidation state at the surface would reduce the delithiation potential compared with the fully oxidized Mn(4+) in the bulk. As a result, the delithiation would be initiated from the surface, which induces a phase transformation of Li2MnO3 into a spinel-like structure from the surface. These theoretical findings have been confirmed by experimental analyses. Based on these detailed mechanistic understanding, it would be possible to develop rational approaches to modify and coat the surface to suppress degradation mechanisms.