The Unusual Conductivity of Na+ in PEO-Based Statistical Copolymer Solid Electrolytes: When Less Means More

A High-Rate and Long-Life Sodium Metal Battery Based on a NaB3H8 ⋅ xNH3@NaB3H8 Composite Solid-State Electrolyte

All-solid-state sodium metal batteries are promising for large-scale energy storage applications owing to their intrinsic safety and cost-effectiveness. However, they generally suffer from sodium dendrite growth or rapid capacity fading, especially at high rates, mainly due to poor wettability, sluggish ionic transport, or low interfacial stability of the solid electrolytes. Herein, we report a novel composite, NaB3H8 ⋅ xNH3@NaB3H8 (x<1), as a new class of solid electrolyte for high-rate batteries. NaB3H8 ⋅ xNH3@NaB3H8 is obtained from the sticky NaB3H8 ⋅ NH3 after removal of NH3 partially at room temperature. It delivers an ionic conductivity of 0.84 mS cm-1 at 25 °C and reaches 20.64 mS cm-1 at 45 °C after an order-disorder phase transformation. It also reveals a good capability of dendrite suppression and remarkable stability against sodium metal. These performances enable the all-solid-state Na//TiS2 battery with a high capacity of 232.4 mAh g-1 (97.2 % of theoretical capacity) and long-term cycling stability at 1 C. Notably, this battery shows superior long-life cycling stability even at 5 and 10 C, which has been rarely reported in all-solid-state sodium metal batteries. This work opens a new group of solid electrolytes, contributing to fast-charging or high-power-density sodium metal batteries.

Polycarbonate-Based Solid-Polymer Electrolytes for Solid-State Sodium Batteries

Solid-polymer electrolytes comprised of polypropylene carbonate (PPC) and varied sodium bis(fluorosulfonyl)imide (NaFSI) salt concentrations are investigated for implementation as a conductive solid polymer electrolyte into solid-state cathode composites utilizing a sodium-layered oxide active material. The ionic conductivity generally increases with NaFSI salt content, reaching ≈1 mS cm-1 at 80 °C at the highest salt concentration (PPC:NaFSI = 0.5:1). Through an all-in-one slurry casting method, Na2/3 Ni1/3 Mn2/3 O2 cathode composites are fabricated in which the dispersed PPC electrolyte acts as the primary binder. Enabled by a bilayer polymer electrolyte system, cycling performance with the PPC cathode electrolyte is optimized with respect to salt concentration and anode material. The best cyclability is achieved with a moderate salt concentration electrolyte (PPC:NaFSI = 5:1), showcasing an initial capacity of 83 mA h g-1 with a remarkable 80% capacity retention after 150 cycles at C/5 rate and 60 °C. The superior performance of the lower salt concentration electrolyte is attributed to better electrochemical stability, as confirmed by linear sweep voltammetry and online electrochemical mass spectrometry measurements. These results underscore the potential of carbonate-based polymer electrolytes and the importance of balancing electrolyte conductivity and stability in cell design.

Enabling high rate capability and stability all-solid-state batteries via cationic surfactant modification of composite electrolyte

The garnet-type solid electrolyte Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was modified with a cationic surfactant Cetyltrimethylammonium Bromide (CTAB) to improve the dispersion of LLZTO inorganic particles in Poly (ethylene oxide) (PEO) electrolyte (PEO-LLZTO@CTAB) by a liquid phase casting method. During fabrication, the cationic modifier CTAB is uniformly adsorbed on the surface of LLZTO particles, which can effectively reduce their surface energy and lead to a thin CTAB surface coating layer. This fabricated PEO-LLZTO@CTAB can avoid the aggregation of LLZTO particles in the composite solid-state electrolyte (CSSE) and improve the interfacial contact at the PEO/LLZTO interface, thus reducing the overall resistance of PEO-LLZTO@CTAB/Li half-cell and inhibiting the dendrite growth during cycling. The all-solid-state batteries (ASSBs) with LiFePO4 (LFP) as the cathode, PEO-LLZTO@CTAB as the electrolyte and Li as the anode exhibit a high initial discharge capacity of 146.6 mAh-g-1, excellent rate performance and high-capacity retention of 91.0 % after 300 cycles at 0.2 C multiplier and 60 °C within the voltage range of 2.7-4.0 V.

Durable and Adjustable Interfacial Engineering of Polymeric Electrolytes for Both Stable Ni-Rich Cathodes and High-Energy Metal Anodes

Achieving stable cycling of high-voltage solid-state lithium metal batteries is crucial for next-generation rechargeable batteries with high energy density and high safety. However, the complicated interface problems in both cathode/anode electrodes preclude their practical applications hitherto. Herein, to simultaneously solve such interfacial limitations and obtain sufficient Li⁺ conductivity in the electrolyte, an ultrathin and adjustable interface is developed at the cathode side through a convenient surface in situ polymerization (SIP), achieving a durable high-voltage tolerance and Li-dendrite inhibition. The integrated interfacial engineering fabricates a homogeneous solid electrolyte with optimized interfacial interactions that contributes to tame the interfacial compatibility between LiNi_x Co_y Mn_z O₂ and polymeric electrolyte accompanied by anticorrosion of aluminum current collector. Further, the SIP enables a uniform adjustment of solid electrolyte composition by dissolving additives such as Na⁺ and K⁺ salts, which presents prominent cyclability in symmetric Li cells (>300 cycles at 5 mA cm^-2 ). The assembled LiNi_0.8 Co_0.1 Mn_0.1 O₂ (4.3 V)||Li batteries show excellent cycle life with high Coulombic efficiencies (>99%). This SIP strategy is also investigated and verified in sodium metal batteries. It opens a new frontier for solid electrolytes toward high-voltage and high-energy metal battery technologies.

Giant Thermopower of Hydrogen Ion Enhanced by a Strong Hydrogen Bond System

Ionic thermoelectric materials based on organic polymers are of great significance for low-grade heat harvesting and self-powered wearable temperature sensing. Here, we demonstrate a poly(vinyl alcohol) (PVA) hydrogel that relies on the differential transport of H⁺ in PVA hydrogels with different degrees of crystallization. After the inorganic acid is infiltrated into the physically cross-linked PVA hydrogel, the ionic conductor exhibits a huge ionic thermopower of 38.20 mV K^-1, which is more than twice the highest value reported for hydrogen ion transport thermoelectric materials. We attribute the enhanced thermally generated voltage to the movement of H⁺ in the strong hydrogen bond system of PVA hydrogels and the restrictive effect of the strong hydrogen bond system on anions. This ionic thermoelectric hydrogel opens up a new way for thermoelectric conversion devices using H⁺ as an energy carrier.