In Situ Phase-Transformation Forming a Bicontinuous (Ni, Co, Ti, and Al)-Oxide/Li2CO3 Interpenetrating Network Electrolyte for Solid Oxide Fuel Cells.

Designing heterogeneous electrolytes with superior interface charge transfer is promising for low-temperature solid oxide fuel cells (LT-SOFCs). However, a rational construction with optimal interfaces to maximize ionic conduction remains a challenge. Here an in situ phase-transformation strategy is demonstrated to prepare a highly conductive heterogeneous electrolyte. A pristine LiNiO2-TiO2 nanocomposite precursor undergoes chemical reactions and phase-transformation upon heating and feeding H2, destroying the original phases, and forming new species, including an amorphous Li2CO3 scaffold within a (Ni, Co, Al, and Ti)-oxide (NCAT) matrix. It creates an intertwining and continuous network inside the electrolyte with plentiful interfaces. The in situ formed NCAT/Li2CO3 heterogeneous electrolyte displays superior ionic conductivity and impressive fuel cell performance. This work emphasizes the potential of rational heterogeneous structure design and interface engineering for LT-SOFC electrolyte through an in situ phase-transform approach. The generated interfaces enhance ion transport, presenting an opportunity for further optimizing electrolyte candidates, and lowering the operating temperatures of SOFCs.

Atomic-Scale Cryo-TEM Studies of the Electrochemistry of Redox Mediator in Li-O2 Batteries.

Rechargeable aprotic lithium (Li)-oxygen battery (LOB) is a potential next-generation energy storage technology because of its high theoretical specific energy. However, the role of redox mediator on the oxide electrochemistry remains unclear. This is partly due to the intrinsic complexity of the battery chemistry and the lack of in-depth studies of oxygen electrodes at the atomic level by reliable techniques. Herein, cryo-transmission electron microscopy (cryo-TEM) is used to study how the redox mediator LiI affects the oxygen electrochemistry in LOBs. It is revealed that with or without LiI in the electrolyte, the discharge products are plate-like LiOH or toroidal Li2 O2 , respectively. The I2 assists the decomposition of LiOH via the formation of LiIO3 in the charge process. In addition, a LiI protective layer is formed on the Li anode surface by the shuttle of I3 - , which inhibits the parasitic Li/electrolyte reaction and improves the cycle performance of the LOBs. The LOBs returned to 2e- oxygen reduction reaction (ORR) to produce Li2 O2 after the LiI in the electrolyte is consumed. This work provides new insight on the role of redox mediator on the complex electrochemistry in LOBs which may aid the design LOBs for practical applications.

Grain Boundary Electronic Insulation for High-Performance All-Solid-State Lithium Batteries.

Sulfide electrolytes with high ionic conductivities are one of the most highly sought for all-solid-state lithium batteries (ASSLBs). However, the non-negligible electronic conductivities of sulfide electrolytes (≈10-8  S cm-1 ) lead to electron smooth transport through the sulfide electrolyte pellets, resulting in Li dendrite directly depositing at the grain boundaries (GBs) and serious self-discharge. Here, a grain-boundary electronic insulation (GBEI) strategy is proposed to block electron transport across the GBs, enabling Li-Li symmetric cells with 30 times longer cycling life and Li-LiCoO2 full cells with three times lower self-discharging rate than pristine sulfide electrolytes. The Li-LiCoO2 ASSLBs deliver high capacity retention of 80 % at 650 cycles and stable cycling performance for over 2600 cycles at 0.5 mA cm-2 . The innovation of the GBEI strategy provides a new direction to pursue high-performance ASSLBs via tailoring the electronic conductivity.

An All-Solid-State Battery Based on Sulfide and PEO Composite Electrolyte.

Replacing liquid electrolytes with solid polymer electrolytes (SPEs) is considered as a vital approach to developing sulfur (S)-based cathodes. However, the polysulfides shuttle and the growth of lithium (Li) dendrites are still the major challenges in polyethylene oxide (PEO)-based electrolyte. Here, an all-solid-state Li metal battery with flexible PEO-Li10 Si0.3 PS6.7 Cl1.8 (LSPSCl)-C-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) composite cathode (FCC) and PEO-LSPSCl-LiTFSI composite electrolyte (S-CPE) is designed. The initial capacity of the Li|S-CPE|FCC battery is 414 mAh g-1 with 97.8% capacity retention after 100 cycles at 0.1 A g-1 . Moreover, the battery displays remarkable capacity retention of 80% after 500 cycles at 0.4 A g-1 . Cryo-transmission electron microscopy (Cryo-TEM) reveals rich large-sized Li2 CO3 particles at the Li/PEO interface blocking the Li+ transport, but the layer with rich Li2 O nanocrystals, amorphous LiF and Li2 S at the Li/S-CPE interface suppresses the growth of lithium dendrite and stabilizes the interface. In situ optical microscopy demonstrates that the excellent cyclic stability of FCC is ascribed to the reversible shuttle of P-S-P species, resulting from the movement of ether backbone in PEO. This study provides strategies to mitigate the polysulfide shuttle effect and Li dendrite formation in designing high energy density solid-state Li-S-based batteries.

A Coral-Like FeP@NC Anode with Increasing Cycle Capacity for Sodium-Ion and Lithium-Ion Batteries Induced by Particle Refinement.

We present a coral-like FeP composite with FeP nanoparticles anchored and dispersed on a nitrogen-doped 3D carbon framework (FeP@NC). Due to the highly continuous N-doped carbon framework and a spring-buffering graphitized carbon layer around the FeP nanoparticle, a sodium-ion battery with the FeP@NC composite exhibits an ultra-stable cycling performance at 10 A g-1 with a capacity retention of 82.0 % in 10 000 cycles. Also, particle refinement leads to a capacity increase during cycling. The FeP nanoparticles go through a refining-recombination process during the first cycle and present a global refining trend after dozens of cycles, which results in a gradually increase in graphitization degree and interface magnetization, and further provides more active sites for Na+ storage and contributes to a rising capacity with cycling. The capacity ascending phenomenon can also extend to lithium-ion batteries.

Deciphering the Lithium-Ion Conduction Mechanism of LiH in Solid-Electrolyte Interphase.

Lithium hydride (LiH) has been widely recognized as the critical component of the solid-electrolyte interphase (SEI) in Li batteries. Although the formation mechanism and structural model of LiH in SEI have been extensively reported, the role in electro-performance of LiH in SEI is still ambiguous and has proven challenging to explored due to the complicated structure SEI and the lack of advanced in situ experimental technology. In this study, the isotopic exchange experiments combined with isotopic tracer experiments is applied to solidly illustrate the superior conductivity and Li+ conduction behavior of the LiH in natural SEI. Importantly, in situ transmission electron microscopy analysis is utilized to visualize the self-electrochemical decomposition of LiH, which is significantly distinctive from LiF and Li2O. The critical experimental evidence discovered by the work demonstrates ion transport behaviors of key components in the SEI, which is imperative for designing novel SEI and augurs a new area in optimizing the performance of lithium batteries.

Unveiling the Nature of Ultrastable Potassium Storage in Bi0.48Sb1.52Se3 Composite.

The conversion and alloying-type anodes for potassium-ion batteries (PIBs) have drawn attention. However, it is still a challenge to relieve the huge volume expansion/electrode pulverization. Herein, we synthesized a composite material comprising Bi0.48Sb1.52Se3 nanoparticles uniformly dispersed in carbon nanofibers (Bi0.48Sb1.52Se3@C). Benefiting from the synergistic effects of the high electronic conductivity of Bi0.48Sb1.52Se3 and the mechanical confinement of the carbon fiber that buffers the large chemomechanical stress, the Bi0.48Sb1.52Se3@C//K half cells deliver a high reversible capacity (491.4 mAh g-1, 100 cycles at 100 mA g-1) and an extraordinary cyclability (80% capacity retention, 1000 cycles at 1000 mA g-1). Furthermore, the Bi0.48Sb1.52Se3@C-based PIB full cells achieve a high energy density of 230 Wh kg-1. In situ transmission electron microscopy (TEM) reveals an intercalation, conversion, and alloying three-step reaction mechanism and a reversible amorphous transient phase. More impressively, the nanofiber electrode can almost return to its original diameter after the potassiation and depotassiation reaction, indicating a highly reversible volume change process, which is distinct from the other conversion type electrodes. This work reveals the stable potassium storage mechanisms of Bi0.48Sb1.52Se3@C composite material, which provides an effective strategy to enable conversion/alloying-type anodes for high performance PIBs for energy storage applications.

Revealing the Electrochemistry of Solid-State Li-SeS2 Battery via In-Situ Transmission Electron Microscopy.

Sex Sy is considered as a promising cathode material as it can deliver higher energy density than selenium (Se) and offer improved conductivity and enhanced reaction kinetics compared with S. However, the electrochemistry of the Li-SeS2 all-solid-state battery (ASSB) has not been well understood to date. Herein the electrochemistry of Li-SeS2 battery was revealed by in-situ transmission electron microscopy. The charge products were phase-separated Se and S, rather than the widely believed SeS2 . Among the various Sex Sy cathodes, SeS2 achieved the best electrochemical performance. The Li-SeS2 ASSB delivered a high reversible capacity of 1052 mAh g-1 at 1 A g-1 over 350 cycles, and a high areal capacity of 4 mAh cm-2 was also achieved with a high cathode mass loading of 7.6 mg cm-2 . These results represent the best performance achieved to date in the Li-SeS2 ASSB and brings us one step closer toward its practical applications.

Cryo-EM Revealing the Origin of Excessive Capacity of the Se Cathode in Sulfide-Based All-Solid-State Li-Se Batteries.

Selenium (Se), whose electronic conductivity is nearly 25 orders higher than that of sulfur (S) and whose theoretical volumetric capacity is 3254 mAh cm-3, is considered as a potential alternative to S to overcome the poor electronic conductivity issue of the S cathode in the lithium (Li)-S battery. However, the study of the Li-Se battery, particularly a Li-Se all-solid-state battery (ASSB), is still in its infancy. Herein, we report the performance of Li-Se ASSBs at both room temperature (RT) and high temperature (HT, 50 °C), using a Li10Si0.3PS6.9Cl1.8 (LSPSCl) solid-state electrolyte and Li-In anode. With a Se loading of 7.6 mg cm-2, the Li-Se battery displayed a record high reversible capacity of 6.8 mAh cm-2 after 50 cycles at HT, which exceeds the theoretical areal capacity of 5.2 mAh cm-2 for Se. Moreover, the RT Li-Se ASSB delivered an initial areal capacity of about 2 mAh cm-2 at a current density of 1 A g-1 for 1200 cycles with a capacity retention of 67%. Cryo-electron microscopy revealed that the excessive capacity of Se at HT can be attributed to the formation of a previously unknown S5Se4 phase during charging, which participated reversibly in a subsequent redox reaction. The formation of the S5Se4 phase originated from the reaction of Se with S, which was generated by the decomposition of LSPSCl at HT. These results unlock the electrochemistry of a Li-Se ASSB, suggesting that a Li-Se ASSB is a viable alternative to a Li-S battery for energy storage applications.

Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification.

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF2 and polymer-derived carbon (FeF2 @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF2 embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF2 @PDC enables a reversible capacity of 500 mAh g-1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF2 @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g-1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe3 O4 layers on the surface of FeF2 prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF2 electrochemistry, and a strategy to radically improve the electrochemical performance of FeF2 cathode for lithium-ion battery applications.

Novel K2Ti8O17 Anode via Na+/Al3+ Co-Intercalation Mechanism for Rechargeable Aqueous Al-Ion Battery with Superior Rate Capability.

A promising aqueous aluminum ion battery (AIB) was assembled using a novel layered K2Ti8O17 anode against an activated carbon coated on a Ti mesh cathode in an AlCl3-based aqueous electrolyte. The intercalation/deintercalation mechanism endowed the layered K2Ti8O17 as a promising anode for rechargeable aqueous AIBs. NaAc was introduced into the AlCl3 aqueous electrolyte to enhance the cycling stability of the assembled aqueous AIB. The as-designed AIB displayed a high discharge voltage near 1.6 V, and a discharge capacity of up to 189.6 mAh g-1. The assembled AIB lit up a commercial light-emitting diode (LED) lasting more than one hour. Inductively coupled plasma-optical emission spectroscopy (ICP-OES), high-resolution transmission electron microscopy (HRTEM), and X-ray absorption near-edge spectroscopy (XANES) were employed to investigate the intercalation/deintercalation mechanism of Na+/Al3+ ions in the aqueous AIB. The results indicated that the layered structure facilitated the intercalation/deintercalation of Na+/Al3+ ions, thus providing a high-rate performance of the K2Ti8O17 anode. The diffusion-controlled electrochemical characteristics and the reduction of Ti4+ species during the discharge process illustrated the intercalation/deintercalation mechanism of the K2Ti8O17 anode. This study provides not only insight into the charge-discharge mechanism of the K2Ti8O17 anode but also a novel strategy to design rechargeable aqueous AIBs.

Ultrastable Anode/Electrolyte Interface in Solid-State Lithium-Metal Batteries Using LiCux Nanowire Network Host.

High interfacial resistance and uncontrollable lithium (Li) dendrite are major challenges in solid-state Li-metal batteries (SSLMBs), as they lead to premature short-circuiting and failure of SSLMBs. Here, we report the synthesis of a composite anode comprising a three-dimensional LiCux nanowire network host infiltrated with Li (Li* anode) with low interfacial impedance and superior electrochemical performance. The Li* anode is fabricated by dissolving Cu foil into molten Li followed by solidification. The Li* anode exhibits good wettability with Li6.4La3Zr1.4Ta0.6O12 (LLZTO) and high mechanical strength, rendering low Li*/LLZTO interfacial impedance, homogeneous deposition of Li, and suppression of Li dendrites. Consequently, the Li* anode-based symmetric cells and full cells with LiNi0.88Co0.1Al0.02O2 (NCA), LiFePO4 (LFP), and FeF2 cathodes deliver remarkable electrochemical performance. Specifically, the Li*/LLZTO/Li* symmetrical cell achieves a remarkably long cycle lifetime of 10 000 h with 0.1 mA·cm-2; the Li*/LLZTO/NCA full cell maintains capacity retention of 73.4% after 500 cycles at 0.5C; and all-solid-state Li*/LLZTO/FeF2 full cell achieves a reversible capacity of 147 mAh·g-1 after 500 cycles at 100 mA·g-1. This work demonstrates potential design tactics for an ultrastable Li*/garnet interface to enable high-performance SSLMBs.

Phase-pure pentlandite Ni4.3Co4.7S8 binary sulfide as an efficient bifunctional electrocatalyst for oxygen evolution and hydrogen evolution.

Developing an efficient non-noble bifunctional electrocatalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in the same electrolyte is significant for lowering the cost of electrochemical water splitting. Herein, a phase-pure pentlandite Ni4.3Co4.7S8 bifunctional electrocatalyst was synthesized via a hydrothermal process using a commercial nickel foam as the nickel source. The active metallic nickel source and the chelating agent ethylenediamine play important roles in the formation of phase-pure pentlandite Ni4.3Co4.7S8 binary sulfide. Physicochemical characterizations, electrochemical measurements and density functional theory (DFT) computations illustrate that the material has an exposed high-indexed (022) surface with a biomimetic hydrogenase-like structure, and that the pentlandite phase has metallic characteristics, with next-nearest neighbor metal-metal bonds, as well as there being a high overlap of density of state (DOS) at the Fermi-level due to the synergistic effect between Ni and Co ions. In addition, there is an elevation of the d-state center (from -2.84 to -1.52 eV) with high occupation of the anti-bonding eg (dx2-y2 and dz2) d-orbitals. These properties endow the Ni4.3Co4.7S8 bifunctional electrocatalyst with higher catalytic activity for OER than RuO2, with comparative activity for HER to commercial Pt/C and with a low over-potential for all water splitting in an alkaline electrolyte. The studies here provide a novel strategy to synthesise phase-pure pentlandite nickel cobalt binary sulfides and boost their applications in electrochemical water splitting.