Zhenyou Li

Rechargeable calcium batteries are promising for sustainable energy storage but lack cathode materials that reversibly intercalate Ca ²⁺ at practical rates. While considerable efforts have focused on improving ionic conductivity, this work investigates how electronic conductivity governs Ca ²⁺ storage performance by employing bilayer δ‑V ₂ O ₅ ·nH ₂ O (VOH) as a model cathode. Electronic conductivity is first enhanced by pre‐intercalating 1‑butyl‑1‑methylpyrrolidinium cations (Pyr ⁺ ) into the molecular layers. These cations attract the electron cloud of lattice oxygen, increasing the density of states at the Fermi level and shortening the electron transport distance between adjacent oxygen atoms, thereby resulting in higher electronic conductivity. Further improvements are achieved by coating the active particles with conductive polyaniline (PANI) and incorporating carbon nanotubes (CNTs) into the cathode formulation. Electrochemical measurement reveals a linear correlation between storage capacity and electronic conductivity. The resulting P‑PyrV@CNT cathode delivers a high capacity of 320 mAh g ⁻¹ (eight‑fold increase over pristine VOH) and excellent cycling stability over 2000 cycles (80% retention). Structural and elemental analysis confirms reversible Ca ²⁺ intercalation into the cathode bulk, accompanied by a coupled electron‐ion motion. Overall, these findings identify electronic conductivity as a rate‐limiting factor for Ca ²⁺ intercalation in layered oxides, offering new design principles for high‐performance cathodes for Ca batteries.

The practical implementation of calcium metal anodes in rechargeable batteries is persistently hindered by surface passivation, which impedes Ca ²⁺ transport and induces inhomogeneous Ca deposition. Herein, we demonstrate that introducing a tailored trace amount of water (∼139 ppm) fundamentally modifies the interfacial chemistry in a state‐of‐the‐art Ca[B(hfip) ₄ ] ₂ /DME electrolyte. Mechanistic investigations reveal that instead of triggering deleterious bulk side reactions, the trace water stably enters Ca ²⁺ solvation sheath, directing a controlled electrochemical hydrolysis pathway during cycling. This process generates a thin, uniform bilayer solid electrolyte interphase (SEI) comprising an outer hybrid organic‐inorganic layer and an inner inorganic layer rich in CaH ₂ , CaO, and CaF ₂ , while concurrent H ₂ evolution effectively eliminates the passivating native oxide layer. Consequently, this tailored SEI enables highly stable Ca plating/stripping for over 400 h at 0.2 mA cm ⁻² and over 300 h at 1 mA cm ⁻² . Furthermore, the high anodic stability of the electrolyte enables reliable operation of various high‐voltage cathodes within a wide voltage window (up to 4.5 V) for up to 70 cycles. These results highlight the critical role of a stable SEI for Ca metal anodes, but also illustrate how subtle electrolyte modification can profoundly regulate the interfacial chemistry toward high‐performance multivalent batteries.

The practical implementation of calcium metal anodes in rechargeable batteries is persistently hindered by surface passivation, which impedes Ca ²⁺ transport and induces inhomogeneous Ca deposition. Herein, we demonstrate that introducing a tailored trace amount of water (∼139 ppm) fundamentally modifies the interfacial chemistry in a state‐of‐the‐art Ca[B(hfip) ₄ ] ₂ /DME electrolyte. Mechanistic investigations reveal that instead of triggering deleterious bulk side reactions, the trace water stably enters Ca ²⁺ solvation sheath, directing a controlled electrochemical hydrolysis pathway during cycling. This process generates a thin, uniform bilayer solid electrolyte interphase (SEI) comprising an outer hybrid organic‐inorganic layer and an inner inorganic layer rich in CaH ₂ , CaO, and CaF ₂ , while concurrent H ₂ evolution effectively eliminates the passivating native oxide layer. Consequently, this tailored SEI enables highly stable Ca plating/stripping for over 400 h at 0.2 mA cm ⁻² and over 300 h at 1 mA cm ⁻² . Furthermore, the high anodic stability of the electrolyte enables reliable operation of various high‐voltage cathodes within a wide voltage window (up to 4.5 V) for up to 70 cycles. These results highlight the critical role of a stable SEI for Ca metal anodes, but also illustrate how subtle electrolyte modification can profoundly regulate the interfacial chemistry toward high‐performance multivalent batteries.

Titanium disulfide (TiS ₂ ) is a promising layered cathode for sodium‐ion batteries (SIBs), but suffers from rapid capacity fading, which was often ascribed to multi‐step phase transitions. Herein, we demonstrated that solvent co‑intercalation that typically occurs at low Na ⁺ content plays a more decisive role in driving structural degradation by promoting lattice distortion and triggering layer delamination. By replacing the conventional solvent propylene carbonate with ethyl methyl carbonate (EMC), which is a linear carbonate with a lower dielectric constant and larger kinetic diameter, solvent uptake is significantly suppressed. Further improvement is achieved by introducing fluoroethylene carbonate (FEC) and increasing NaFSI concentration to 3 m in an EMC/FEC (9:1 v/v) blend. This formulation enables the formation of a uniform, thin cathode‐electrolyte interphase, which is robust and rich in NaF that facilitates desolvation of Na ⁺ and further limits solvent co‐intercalation. Consequently, the TiS ₂ cathode delivers exceptional long‐term cycling stability (88.9% capacity retention after 1200 cycles at 2000 mA g ⁻¹ ) and superior rate capability (75% capacity retention from 20 to 2000 mA g ⁻¹ ). Since solvent co‐intercalation is a common issue in layered hosts during Na ⁺ intercalation, the insights and electrolyte design strategy presented here provide a generalizable approach to developing high‐performance, durable cathodes for SIBs.

Rechargeable calcium batteries (RCBs) are a promising sustainable energy storage technology with high theoretical energy density. However, their development is hindered by the lack of suitable cathodes that enable facile and reversible Ca ²⁺ storage. This study investigates the Ca ²⁺ storage mechanism in tunneled α‐K _0.03 MnO ₂ cathodes, revealing distinct electrochemical behaviors in two cell configurations using either activated carbon (AC) anodes or Ca metal anodes. While Ca ²⁺ insertion/extraction dominates the charge storage process in Ca metal anode systems, negligible Ca ²⁺ insertion occurs with AC anodes. Despite that, detailed mechanistic investigations of the Ca metal anode systems indicate sluggish Ca ²⁺ diffusion within the α‐K _0.03 MnO ₂ framework, leading to irreversible cation trapping. Moreover, progressive Ca ²⁺ accumulation causes deep calciation that triggers irreversible phase transition into CaMn ₂ O ₄ and eventually complete structural degradation. To address these issues, a composite cathode combining ultrasmall, low‐crystallinity MnO ₂ nanoparticles with graphene oxide (u‐MnO ₂ @GO) is developed, demonstrating improved Ca ²⁺ diffusion kinetics, enhanced cycling stability over 60 cycles, and superior rate capability up to 50 mA g ⁻¹ . This work provides critical insights into Ca ²⁺ storage mechanisms in oxide cathodes and offers effective strategies for designing high‐performance cathodes for RCBs.

The Patrónite vanadium tetrasulfide (VS₄) follows a simultaneous cationic and anionic redox (SCAR) mechanism in magnesium batteries, which renders a delocalized electronic structure for fast kinetics and meanwhile enables multielectron reactions for delivering high capacity. In contrast to most research that focuses on the hydrothermal route resulting in VS₄ nanoparticles, herein a more industrially relevant synthesis route is targeted, utilizing a solid‐state approach leading to micron‐sized VS₄. The obtained 2 × 10 μm VS₄ needles show good water/air stability, allowing an environmentally friendly coatings procedure using polyvinylpyrrolidon binder in isopropanol. Electrochemical investigation shows that the micron‐sized VS₄ cathode delivers a maximum capacity of 420 mAh g⁻¹ in a tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)₄]₂)‐based electrolyte, however suffers from fast capacity fading and overcharging. Targeting these issues, a concentrated Mg[B(hfip)₄]₂ electrolyte in bis(2‐methoxyethyl)ether is applied, which enables stable cycling for 300 cycles at the expense of a reduced capacity. Multimodal characterizations confirm a reversible SCAR mechanism of VS₄ during cycling. However, pulverization of the electrode and fragmentation of the VS₄ backbones are evident, leading to the leakage of active material into electrolyte and causing capacity decay. To avoid active material loss, crystal engineering or surface protection strategies should be developed toward practical Mg batteries.

Achieving multicationic redox reactions with low lattice strain accumulation upon repeated sodiation/desodiation processes is pivotal for developing high‐energy and long‐durability Na superionic conductor (NASICON)‐type cathodes but still a formidable task. Herein, a novel NASICON‐structured high‐entropy Na_3.4VMn_0.2Fe_0.2Al_0.3Cr_0.3(PO₄)₃ (HE‐NVMFACP) cathode is delicately devised, where the entropy‐driving stepwise Fe²⁺/Fe³⁺, V³⁺/V⁴⁺/V⁵⁺, Mn²⁺/Mn³⁺/Mn⁴⁺, and Cr³⁺/Cr⁴⁺ redox couples not only trigger the multielectron transfer chemistry, but also alleviate the lattice strain accumulation. Consequently, the HE‐NVMFACP cathode exhibits a high reversible capacity of 151.3 mAh g⁻¹ with an admirable energy density of 520.5 Wh kg⁻¹, and an impressive “quasi‐zero strain” behavior (1.33% cell volume change) during the whole charge‐discharge process. Meanwhile, an excellent rate capability of 90.4 mAh g⁻¹ at 50 C and an ultralong cycling life of 91.9% capacity retention after 5000 cycles are demonstrated. Advanced in situ/ex situ techniques reveal the complete solid‐solution behavior and highly reversible stepwise multicationic redox mechanism of HE‐NVMFACP upon Na⁺ uptake/release, and the rapid electrode process kinetics of HE‐NVMFACP are confirmed by systematic electrochemical measurements and theoretical computations. Furthermore, the HE‐NVMFACP||hard carbon Na‐ion full batteries are assembled in both coin and pouch configurations, manifesting bright application prospects. The present high‐entropy strategy provides invaluable perspectives for designing high‐energy and long‐life NASICON‐cathodes for sodium‐ion batteries.