Owing to high energy density and economic viability, rechargeable Mg batteries are considered alternatives to lithium ion batteries. However besides the chevrel phase, none of the conventional inorganic cathode materials demonstrate reversible intercalation/deintercalation of Mg+2 ions in an anhydrous electrolyte system. The lack of high voltage and high capacity cathode frustrates the realization of Mg batteries. Previous studies indicate that vanadium pentoxide (V2O5) has the potential to reversibly insert/extract Mg ions. However, many attempts to utilize V2O5 demonstrate limited electrochemical response, due to hindered Mg ion mobility in solid. Here, monodispersed spherical V2O5 with a hierarchical architecture is rationally designed, through a facile and scalable approach. The V2O5 spheres exhibit initial discharge capacity of 225 mA h g−1 which stabilizes at ≈190 mA h g−1 at 10 mA g−1, much higher than previous reports. The V2O5 spheres exhibit specific discharge capacity of 55 mA h g−1 at moderate current rate (50 mA g−1) with negligible fading after 50 cycles (≈5%) and 100 cycle (≈13%), while it retains ≈95% columbic efficiency after 100 cycles demonstrating excellent stability during Mg+2 ion intercalation/deintercalation. Most interestingly, exact phase and morphology are completely retained even after repeated Mg+2 ion intercalation/deintercalation at different current rates, demonstrating pronounced electrochemical activity in an anhydrous magnesium electrolyte.

Owing to its high energy density, LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NMC811) is a cathode material of prime interest for electric vehicle battery manufacturers. However, NMC811 suffers from several irreversible parasitic reactions that lead to severe capacity fading and impedance buildup during prolonged cycling. Thin surface protection films coated on the cathode material mitigate degradative chemomechanical reactions at the electrode−electrolyte interphase, which helps to increase cycling stability. However, these coatings may impede the diffusion of lithium ions, and therefore, limit the performance of the cathode material at a high C-rate. Herein, we report on the synthesis of zirconium phosphate (Zr x PO y ) and lithium-containing zirconium phosphate (Li x Zr y PO z ) coatings as artificial cathode−electrolyte interphases (ACEIs) on NMC811 using the atomic layer deposition technique. Upon prolonged cycling, the Zr x PO y -and Li x Zr y PO zcoated NMC811 samples show 36.4 and 49.4% enhanced capacity retention, respectively, compared with the uncoated NMC811. Moreover, the addition of Li ions to the Li x Zr y PO z coating enhances the rate performance and initial discharge capacity in comparison to the Zr x PO y -coated and uncoated samples. Using online electrochemical mass spectroscopy, we show that the coated ACEIs largely suppress the degradative parasitic side reactions observed with the uncoated NMC811 sample. Our study demonstrates that providing extra lithium to the ACEI layer improves the cycling stability of the NMC811 cathode material without sacrificing its rate capability performance. KEYWORDS: LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), metal phosphate, atomic layer deposition (ALD), surface passivation, suppressed parasitic reactions, high rate performance

MXenes are a large class of 2D materials that consist of few‐atoms‐thick layers of transition metal carbides, nitrides, or carbonitrides. The surface functionalization of MXenes has immense implications for their physical, chemical, and electronic properties. However, solution‐phase surface functionalization often leads to structural degradation of the MXene electrodes. Here, a non‐conventional, single‐step atomic surface reduction (ASR) technique is adopted for the surface functionalization of MXene (Ti3C2Tx) in an atomic layer deposition reactor using trimethyl aluminum as a volatile reducing precursor. The chemical nature of the modified surface is characterized by X‐ray photoelectron spectroscopy and nuclear magnetic resonance techniques. The electrochemical properties of the surface‐modified MXene are evaluated in acidic and neutral aqueous electrolyte solutions, as well as in conventional Li‐ion and Na‐ion organic electrolytes. A considerable improvement in electrochemical performance is obtained for the treated electrodes in all the examined electrolyte solutions, expressed in superior rate capability and cycling stability compared to those of the non‐treated MXene films. This improved electrochemical performance is attributed to the increased interlayer spacing and modified surface terminations after the ASR process.

To mitigate the associated challenges of instability and capacity improvement in Na3V2(PO4)2F3 (NVPF), rationally designed uniformly distributed hollow spherical NVPF and coating the surface of NVPF with ultrathin (≈2 nm) amorphous TiO2 by atomic layer deposition is demonstrated. The coating facilitates higher mobility of the ion through the cathode electrolyte interphase (CEI) and enables higher capacity during cycling. The TiO2@NVPF exhibit discharge capacity of >120 mAhg−1, even at 1C rates, and show lower irreversible capacity in the first cycle. Further, nearly 100% capacity retention after rate performance in high current densities and 99.9% coulombic efficiency after prolonged cycling in high current density is reported. The improved performance in TiO2@NVPF is ascribed to the passivation behavior of TiO2 coating which protects the surface of NVPF from volume expansion, significantly less formation of carbonates, and decomposition of electrolyte, which is also validated through post cycling analysis. The study shows the importance of ultrathin surface protection artificial CEI for advanced sodium‐ion battery cathodes. The protection layer is diminishing parasitic reaction, which eventually enhances the Na ion participation in reaction and stabilizes the cathode structure.

High‐Ni‐rich layered oxides [e.g., LiNixCoyMnzO2; x > 0.5, x + y + z = 1] are considered one of the most promising cathodes for high‐energy‐density lithium‐ion batteries (LIB). However, extreme electrode–electrolyte reactions, several interfacial issues, and structural instability restrict their practical applicability. Here, a shortened unconventional atomic surface reduction (ASR) technique is demonstrated on the cathode surface as a derivative of the conventional atomic layer deposition (ALD) process, which brings superior cell performances. The atomic surface reaction (reduction process) between diethyl‐zinc (as a single precursor) and Ni‐rich NMC cathode [LiNi0.8Co0.1Mn0.1O2; NCM811] material is carried out using the ALD reactor at different temperatures. The temperature dependency of the process through advanced spectroscopy and microscopy studies is demonstrated and it is shown that thin surface film is formed at 100 °C, whereas at 200 °C a gradual atomic diffusion of Zn ions from the surface to the near‐surface regions is taking place. This unique near‐surface penetration of Zn ions significantly improves the electrochemical performance of the NCM811 cathode. This approach paves the way for utilizing vapor phase deposition processes to achieve both surface coatings and near‐surface doping in a single reactor to stabilize high‐energy cathode materials.