The Raw Mixed Conducting Interphase Affords Effective Prelithiation in Working Batteries.

Contact prelithiation is an important strategy to compensate the initial capacity loss of lithium-ion batteries. However, the dead Li generated from inadequate Li conversion reduces the cycling stability of rechargeable batteries. Herein a mono-solvent dimethyl carbonate (DMC) electrolyte was employed in contact prelithiation for the first time. We discover that the low-organic-content raw electrolyte interphase (REI) induced by this electrolyte on Li source and anode is a mixed ion/electron conductor. Therefore, electron channels can be maintained even when the Li source has been completely wrapped by the DMC-derived REI. As a result, an outstanding Li source utilization of 92.8 % and a negligible dead Li yield can be realized. This strategy renders batteries with a very high initial Coulombic efficiency (90.0 %) and an excellent capacity retention (94.9 %) over 210 cycles, highlighting the significance of REI for effective contact prelithiation.

Tuning P2-Structured Cathode Material by Na-Site Mg Substitution for Na-Ion Batteries.

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na-Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as "pillars" to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create "Na-O-Mg" and "Mg-O-Mg" configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.

Ordered mesoporous nanofibers mimicking vascular bundles for lithium metal batteries.

Hierarchical self-assembly with long-range order above centimeters widely exists in nature. Mimicking similar structures to promote reaction kinetics of electrochemical energy devices is of immense interest, yet remains challenging. Here, we report a bottom-up self-assembly approach to constructing ordered mesoporous nanofibers with a structure resembling vascular bundles via electrospinning. The synthesis involves self-assembling polystyrene (PS) homopolymer, amphiphilic diblock copolymer, and precursors into supramolecular micelles. Elongational dynamics of viscoelastic micelle solution together with fast solvent evaporation during electrospinning cause simultaneous close packing and uniaxial stretching of micelles, consequently producing polymer nanofibers consisting of oriented micelles. The method is versatile for the fabrication of large-scale ordered mesoporous nanofibers with adjustable pore diameter and various compositions such as carbon, SiO2, TiO2 and WO3. The aligned longitudinal mesopores connected side-by-side by tiny pores offer highly exposed active sites and expedite electron/ion transport. The assembled electrodes deliver outstanding performance for lithium metal batteries.

Multifunctional α-MoO3 Nanobelt Interlayer with the Capacity Compensation Effect for High-Energy Lithium-Sulfur Batteries.

Spatial hindrance of lithium polysulfide (LiPS) diffusion by inserting a barrier interlayer has been deemed as an effective strategy to restrict the shuttle effect in lithium-sulfur batteries (LSBs). However, the extra interlayer without reversible capacity production inevitably reduces the actual energy density of the battery. Herein, a freestanding α-MoO3 nanobelt interlayer with the decoration of TiN nanoparticles and carbon nanotubes (denoted as MCT) is established. To investigate the capacity compensation effect of the MCT during cell operations, X-ray absorption near-edge spectrometry is conducted. It is revealed that MoO3 can sustain a reversible Li intercalation/deintercalation in a voltage range of 1.8-2.8 V, providing 180 mAh g-1 of extra capacity for compensating sulfur cathode. In addition, the adsorption of the lithiated α-MoO3 toward LiPSs is further evaluated. By matching a high-loading sulfur cathode (3.0 mg cm-2), a superior capacity of 713.3 mAh g-1 can be retained after 100 cycles under the MCT assistance.

Unblocked Electron Channels Enable Efficient Contact Prelithiation for Lithium-Ion Batteries.

Contact prelithiation is strongly considered for compensating the initial capacity loss of lithium-ion batteries, exhibiting great potential for ultralong cycle life of working batteries and the application of large-scale energy-storage systems. However, the utilization of the sacrificial Li source for contact prelithiation is low (<65%). Herein the fundamental mechanism of contact prelithiation is described from the perspective of the Li source/anode interfaces by regulating the initial contact state, and a clear illustration of the pathogeny for capacity attenuation is successfully delivered. Specifically, creating plentiful electron channels is an access to making contact prelithiation with a higher Li utilization, as the mitigated local current density that reduces the etching of Li dissolution and SEI extension on electron channels. A vacuum thermal evaporation for depositing the Li film enables the contact interface to possess an adequate electron channel construction, rendering a Li utilization of 91.0%, and the dead Li yield is significantly reduced in a working battery.

Ethylene-Carbonate-Free Electrolytes for Rechargeable Li-Ion Pouch Cells at Sub-Freezing Temperatures.

Sub-freezing temperature presents a significant challenge to the survival of current Li-ion batteries (LIBs) as it leads to low capacity retention and poor cell rechargeability. The electrolyte in commercial LIBs relies too heavily on ethylene carbonate (EC) to produce a stable solid electrolyte interphase (SEI) on graphite (Gr) anodes, but its high melting point (36.4 °C) severely restricts ion transport below 0 °C, causing energy loss and Li plating. Here, a class of EC-free electrolytes that exhibits remarkable low-temperature performance without compromising cell lifespan is reported. It is found that at sub-zero temperatures, EC forms highly resistive SEI that seriously impedes electrode kinetics, whereas EC-free electrolytes create a highly stable, low-impedance SEI through anion decomposition, which boosts capacity retention and eliminates Li plating during charging. Pouch-type LiCoO2 (LCO)|Gr cells with EC-free electrolytes sustain 900 cycles at 25 °C with 1 C charge/discharge, and LiNi0.85 Co0.10 Al0.05 O2 (NCA)|Gr cells last 300 cycles at -15 °C with 0.3 C charge, both among the best-performing in the literature under comparable conditions. Even at -50 °C, the NCA|Gr cell with EC-free electrolytes still delivers 76% of its room-temperature capacity, outperforming EC-based electrolytes.

Graphite prelithiation by solid electrochemical corrosion of lithium metal with a superficial mosaic structure.

A transformative concept of solid electrochemical corrosion has been put forward, in which solid-state electrolyte LiPON has been applied to replace the liquid one to prelithiate graphite with Li-metal. Thus, high prelithiation efficiency and low polarization of the treated anode can be obtained, with a unique mosaic structure left at the surface.

Boosting Reversibility of Mn-Based Tunnel-Structured Cathode Materials for Sodium-Ion Batteries by Magnesium Substitution.

Electrochemical irreversibility and sluggish mobility of Na+ in the cathode materials result in poor cycle stability and rate capability for sodium-ion batteries. Herein, a new strategy of introducing Mg ions into the hinging sites of Mn-based tunnel-structured cathode material is designed. Highly reversible electrochemical reaction and phase transition in this cathode are realized. The resulted Na0.44Mn0.95Mg0.05O2 with Mg2+ in the hinging Mn-O5 square pyramidal exhibits promising cycle stability and rate capability. At a current density of 2 C, 67% of the initial discharge capacity is retained after 800 cycles (70% at 20 C), much improved than the undoped Na0.44MnO2. The improvement is attribute to the enhanced Na+ diffusion kinetics and the lowered desodiation energy after Mg doping. Highly reversible charge compensation and structure evolution are proved by synchrotron-based X-ray techniques. Differential charge density and atom population analysis of the average electron number of Mn indicate that Na0.44Mn0.95Mg0.05O2 is more electron-abundant in Mn 3d orbits near the Fermi level than that in Na0.44MnO2, leading to higher redox participation of Mn ions.

Whole-Voltage-Range Oxygen Redox in P2-Layered Cathode Materials for Sodium-Ion Batteries.

Oxygen-redox of layer-structured metal-oxide cathodes has drawn great attention as an effective approach to break through the bottleneck of their capacity limit. However, reversible oxygen-redox can only be obtained in the high-voltage region (usually over 3.5 V) in current metal-oxide cathodes. Here, we realize reversible oxygen-redox in a wide voltage range of 1.5-4.5 V in a P2-layered Na0.7 Mg0.2 [Fe0.2 Mn0.6 □0.2 ]O2 cathode material, where intrinsic vacancies are located in transition-metal (TM) sites and Mg-ions are located in Na sites. Mg-ions in the Na layer serve as "pillars" to stabilize the layered structure during electrochemical cycling, especially in the high-voltage region. Intrinsic vacancies in the TM layer create the local configurations of "□-O-□", "Na-O-□" and "Mg-O-□" to trigger oxygen-redox in the whole voltage range of charge-discharge. Time-resolved techniques demonstrate that the P2 phase is well maintained in a wide potential window range of 1.5-4.5 V even at 10 C. It is revealed that charge compensation from Mn- and O-ions contributes to the whole voltage range of 1.5-4.5 V, while the redox of Fe-ions only contributes to the high-voltage region of 3.0-4.5 V. The orphaned electrons in the nonbonding 2p orbitals of O that point toward TM-vacancy sites are responsible for reversible oxygen-redox, and Mg-ions in Na sites suppress oxygen release effectively.

Freestanding Double-Layer MoO3/CNT@S Membrane: A Promising Flexible Cathode for Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have been regarded as a promising candidate of secondary batteries to satisfy the enormous demand for electric vehicles and energy storage applications. However, Li-S batteries still suffer from severe capacity fading due to the shuttle effect of lithium polysulfides. Here, we develop a freestanding double-layer MoO3/carbon nanotube@S (FMC@S) membrane by hydrothermal and suction filtration strategy, without polymer binder and current collector substrate. FMC@S contains a polysulfide blocking layer and an active material layer. Except for S content, the two layers have the same components and are integrated together, so there is no distinct interface between the two layers, which can facilitate ion and electron transport. As a result, the FMC@S cathode delivers promising capacity retention and rate capability. The hierarchical integrated design provides a new strategy to develop high-performance flexible cathodes for Li-S batteries.

Improving the Electrochemical Performance and Structural Stability of the LiNi0.8Co0.15Al0.05O2 Cathode Material at High-Voltage Charging through Ti Substitution.

LiNi0.8Co0.15Al0.05O2 (NCA) has been proven to be a good cathode material for lithium-ion batteries (LIBs), especially in electric vehicle applications. However, further elevating energy density of NCA is very challenging. Increasing the charging voltage of NCA is an effective method, but its structural instability remains a problem. In this work, we revealed that titanium substitution could improve cycle stability of NCA under high cutoff voltage significantly. Titanium ions with a relatively larger ion radius could modify the oxygen lattice and change the local coordination environment of NCA, leading to decreased cation migration, better kinetic and thermodynamic properties, and improved structural stability. As a result, the Ti-substituted NCA cathode exhibits impressive reversible capacity (198 mA h g-1 at 0.1C) with considerable cycle stability under a cutoff voltage up to 4.7 V. It is also revealed that Ti could suppress oxygen release in the high-voltage region, benefitting cycle and thermal stabilities. This work provides valuable insight into the design of high-voltage layered cathode materials for high-energy-density LIBs.

Synthesis of complementary hierarchical structured Si/C composites with high Si content for lithium-ion batteries.

Si/C composites are considered as the most promising anode materials for next-generation lithium-ion batteries (LIBs) due to their high specific capacity and low cost. However, the commercialized Si/C composites cannot maintain a Si content over 10 wt% for sustaining an acceptable cycle life. To achieve long-term cycle stability for Si/C composites with high Si content is still very challenging. Here, we report a rationally designed double-morphology Si/graphene (DMSiG) composite with a high Si content of 78 wt%, and prove its feasibility as a high performance anode material for LIBs. DMSiG composes of Si quantum-dot decorated graphene and mesoporous Si spheres with a complementary hierarchical structure. The graphene framework enhances the electronic conductivity, alleviates the aggregation of mesoporous Si spheres and provides space and flexibility to buffer the volume change during cycling. Mesoporous Si spheres contribute to a large reversible capacity and support the hierarchical architecture of DMSiG. The Si quantum-dots help to build firm connections between graphene and mesoporous Si spheres to avoid their separation during cycling. Coupling these features together, the DMSiG anode delivers a high reversible capacity of 1318 mA h g-1 at a current density of 500 mA g-1 and 684 mA h g-1 at 2000 mA g-1.