Se-Hee Lee

Ni rich LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) is one of the most promising cathode materials for high energy all solid-state batteries. However, its poor ionic and electronic conductivity along with high interfacial instability with thiophosphate-based solid electrolytes result in severe capacity loss with cycling. In this paper, a polyacrylonitrile (PAN) coating is applied on the surface of NMC811 followed by a simple heat treatment in air. The PAN coating layer not only helps to improve the electronic conductivity of the cathode composite but also prevents the direct contact of NMC811 active material with the solid electrolyte, reducing the decomposition of the electrolyte and formation of passivating layers with low ionic conductivity. Compared to pristine NMC811, the electrochemical properties of PAN-NMC composites are significantly improved. When cycled at 0.1 C rate, PAN-NMC composite shows an initial capacity of 192.16 mAh g ⁻¹ and capacity retention of 68.5% over 100 cycles compared to the lower capacity of 173.06 mAh g ⁻¹ and retention 58.68% for the pristine NMC811. These results indicate that the PAN coating greatly improves the conductivity and cycling performance of NMC811.

High nickel layered oxide LiNi _0.8 Mn _0.1 Co _0.1 O ₂ (NMC811) was coated with a nanometer layer coating of a lithium ion conducting solid electrolyte, lithium phosphorus oxynitride (LiPON) by using RF-magnetron sputtering. The cells with LiPON coated NMC811 exhibit much improved cycling performance compared to the cells with pristine NMC811 with 64.1% and 42.6% capacity retention respectively over 100 cycles in an all solid-state battery. The LiPON layer provides interfacial stability at high voltages, suppresses the growth of impedance with cycling and improves the rate capability. Thicker coatings show a negative impact on the performance owing to the increase in electronic resistance with increasing thickness of the LiPON layer. The dQ/dV analysis, electrochemical impedance spectroscopy (EIS), and the overpotential study during galvanostatic cycling were conducted to elucidate the improvement of the cycling stability and enhancement of Li ⁺ transport through LiPON layers surrounding NMC811.

A slurry-coated sheet-style Si-based anode is developed for use in all-solid-state Li-ion batteries. Inexpensive, mixed-conducting polyacrylonitrile (PAN) is utilized as both binder and conductive additive, enabling Si-rich electrodes (70 wt%) to attain large reversible capacities ∼1,500 mAh g ⁻¹ (Si) at 1 C rates (>3 mA cm ⁻² ). Cross sectional analysis of a discharged all-solid-state half-cell indicates that the Si-PAN anode achieves the largest volumetric specific capacity ever reported for a Li-ion electrode (>1500 mAh cm ⁻³ ). Ex situ Raman spectroscopic studies reveal that the anodes’ reversibility is attributed to the preservation of small tetrahedrally coordinated clusters within their Si nanoparticles upon discharge. We therefore argue that careful lithiation limitations should be implemented in all Si-based Li-ion anodes to preserve this local tetrahedral atomic structure. The slurry-coated sheet-style Si-PAN anodes have been successfully cycled in all-solid-state full-cells against high-voltage nickel-rich NMC 811 composite cathodes to encourage future commercialization of similar sheet-style all-solid-state designs.

In this study, the ionic conductivity of 77.5Li₂S·22.5P₂S₅ and 77.5Li₂S·(22.5–x)P₂S₅·xP₂O₅ glassy solid-state electrolytes (SSEs) and the relationship of conductivity and lithium deposition behavior in a symmetric lithium metal cell in terms of P₂O₅ concentration have been investigated. The mechanical milling method has been used to prepare the glassy electrolytes. By adding only a small amount (0.25 mol %) of P₂O₅, both the ionic conductivity and the longest cycling performance improve with no signs of short circuiting up to 54 cycles. All symmetric lithium metal cells using 77.5Li₂S·(22.5–x)P₂S₅·xP₂O₅ (mol %) electrolytes with P₂O₅ addition of x < 9 display longer cycling performance compared to cells with pure Li₂S·P₂S₅ SSEs (x = 0), indicating the suppression of dendrite growth with the addition of P₂O₅. The reduced dendritic lithium growth is attributed to higher ionic conductivity and better compaction of the electrolyte pellets with P₂O₅ addition. The better compaction is confirmed by examination of the pellet density of each SSE composition. The trends established by this study will help inform the optimal performance of 77.5Li₂S·(22.5–x)P₂S₅·xP₂O₅ glassy solid-state electrolytes (SSEs) in solid-state battery applications.

Ionic liquid (IL) electrolytes offer a number of advantages over conventional carbonate-based electrolytes but commonly suffer from poor performance at high charge and discharge rates due to their relatively low ionic conductivity and high viscosity. While increasing the lithium salt content of an IL electrolyte beyond optimal levels exacerbates both of those characteristics, this work demonstrates the surprising kinetic capabilities of highly concentrated IL electrolytes. Results presented herein show that NMC-811 half-cells cycled in PYR₁₃FSI with a high-concentration of LiFSI exhibit superior rate performance as compared to lower-concentration IL solutions. Furthermore, extended cycling shows that higher LiFSI concentrations promote enhanced stability during long-term cycling, significantly outperforming the capabilities of a conventional organic electrolyte. Transference number calculations, differential capacity analyses, and electrochemical impedance spectroscopy were used to illuminate the underlying mechanisms contributing to the performance improvements at high concentrations, ultimately revealing the significance of LiFSI molarity in the formation of a robust and conductive solid-electrolyte interphase layer capable of promoting rapid lithium-ion transport as well as stable long-term cycling. The results of this study highlight the unique capability of IL electrolytes to enable successful implementation of a challenging high-energy electrode material.

As an alternative to conventional carbonate-based electrolytes, ionic liquid (IL) electrolytes exhibit several advantageous characteristics including non-flammability, negligible volatility, high electrochemical and thermal stability, and the ability to form robust yet conductive passivation layers on a number of high-energy electrode materials. However, IL electrolytes traditionally suffer from poor rate performance owing to their high viscosity and low ionic conductivity as compared to traditional electrolyte solutions. The following study will assess the kinetic capabilities of an IL electrolyte paired with a high-energy, nickel-rich layered cathode. The effects of temperature, electrode mass loading, and electrode composition are investigated in order to form an understanding of the kinetically limiting factors. Cycling tests performed at elevated temperatures not only show that IL electrolytes enable stable high-temperature operation but that elevated temperatures facilitate substantial improvements in rate capabilities. Analysis by electrochemical impedance spectroscopy ultimately illuminates a critical correlation between electrode wettability and rate performance. The trends established by this study will to help inform the optimal performance of IL electrolytes in high-rate applications.

In order to meet the critical energy-storage challenges of the future, a next-generation lithium-ion battery will need to achieve a higher energy density and longer cycle life. While increasing the nickel content in layered LiMO₂ (M = Ni, Mn, Co) significantly improves the capacity of the material, nickel-rich cathodes cycled in conventional organic electrolytes commonly suffer from crystallographic phase transformation and the growth of a resistive interfacial layer, both of which result in voltage fade and capacity degradation during cycling. However, pairing a nickel-rich cathode with an appropriate ionic liquid (IL) electrolyte enables exceptional cycling stability and energy retention. This work demonstrates how a pyrrolidinium-based IL electrolyte not only allows for cycling to higher voltages but shows a 95% energy retention and average discharge capacity of 189 mAh g⁻¹ over 150 cycles between 3 and 4.5 V vs. Li/Li⁺ with a nickel-rich layered cathode. Based on electrochemical and crystallographic analyses, the exceptional performance of the cells cycled in IL is attributed to the stability of the electrode-electrolyte interfacial layer formed by the IL which protects the active material and suppresses the structural degradation commonly observed in nickel-rich cathodes.

This study demonstrates the application of Si/C composite fibers as anode materials for all-solid-state lithium-ion batteries. Using polyacrylonitrile as the carbon precursor, Si/C fibers were prepared through electrospinning and subsequent heat-treating processes. To investigate the correlation between fiber diameter and electrochemical performance, we prepared three electrodes (A, B, C), containing Si/C fibers with ∼2 μm, ∼1 μm and ∼0.1 μm diameters, respectively. Our results revealed that although the composition of all three electrodes was nearly the same, the Si/C fiber based electrodes exhibited better capacity retention when their fiber diameters were smaller. Normalized to the total mass of electrode composite, the solid-state half-cell prepared with the smallest diameter (∼0.1 μm) Si/C fibers achieved a reversible specific capacity of ∼700 mAh g⁻¹ (normalized to electrode mass) over 70 cycles. We believe that this report can serve as an informative approach toward the utilization of electrospun Si/C fibers as anode materials for all-solid-state lithium-ion batteries.

All-solid-state batteries utilizing ceramic electrolytes have several advantages compared to the conventional batteries using liquid electrolytes, such as enhanced safety and high temperature operation. Additionally, the compressive pressure required for operation of all-solid-state batteries has been shown to suppress capacity decay which originates from the expansion of high capacity electrode materials such as silicon (Si). Si is an attractive anode active material due to its high theoretical capacity and natural abundance. However, the volume expansion (400%) during lithiation and the following mechanical pulverization and active material isolation leads to fast capacity decay hindering more widespread commercialization of this material. Recently we reported several articles utilizing Sn as an electrochemically active and mixed conductive matrix to host Si. The highly reversible Si-Sn hybrid anode, which was prepared by simple powder mixing, could maintain 700 mAh/g electrode specific capacity for 50 cycles, and the utilization of both Sn and Si was confirmed through electrochemical analysis and XRD. The ductile nature of Sn was pointed out as one of the important factors as it can adapt well to the elevated pressure originating from volume expansion of the active materials during lithiation and thus act conformally on Si leading to greater reversibility. Especially the fact that Sn is lithiated prior to Si, indicated that Sn would expand first, preventing extensive expansion and the following mechanical degradation of Si particles inside the electrode matrix. In this study, we present a more advanced method to prepare Si-Sn hybrid electrodes, using sputtering technique. Multilayered electrodes were prepared by sequentially sputtering Sn and Si, which eventually turned out to result in a more homogeneous mixing between the two active materials during cycling, therefore leading to Sn more efficiently stabilizing the electrochemical performance of Si. The morphology of the electrode during cycling was monitored by ex-situ FIB-TEM analyses, which showed cracking of the individual layers and mixing of Sn and Si. Sn was first sputtered as a single layer anode, and the cycling test showed a voltage profile with clear plateau regions corresponding to the four lithiation phases of Sn, and very good cycling stability showing specific charge and discharge capacities of ~850 mAh /g (at 55^th cycle). On the other hand, Si sputtered as a single layer did not perform well as an anode in terms of cycling stability. The Si-sputtered anode showed fast capacity decay with 33% of irreversible capacity loss in the first cycle, and at the 50^th cycle the charging capacity was only 18% of the initial charging capacity. When Sn and Si are sputtered sequentially in a layered fashion, the cycling stability significantly improved. Especially, the sputtered Sn layer between the stainless steel substrate and the Si layer appeared to be critical for stable cycling. The Sn-Si-sputtered anode showed much improved cycling stability compared to the Si-only anode, and outperformed the Si-Sn-anode (sputtered Si first, then Sn) as well. The Sn-Si-sputtered anode showed 0.956 mAh total charging capacity at the 40^th cycle, which corresponds to 2926 mAh/g of Si specific capacity, assuming that the Sn layer reached maximum capacity each cycle. The Si-Sn-sputtered anode on the other hand, reached 0.607 mAh total charging capacity at the 58^th cycle, which corresponds to 1453 mAh/g Si specific capacity. The thin layer of Sn (≈0.32 µm thickness, by calculation with mass and density) which is located between the Si layer (≈0.76 µm) and stainless-steel substrate is expected to act as a mixed conductive layer which improves and retains the interface between Si and the substrate during cycling. The Sn-Si-Sn trilayer-sputtered anode performed with excellent cycling stability, maintaining 1.44 mAh total charging capacity at the 51^st cycle, which corresponds to 3860 mAh/g Si specific capacity. Compared to the Sn-Si bilayer anode, the trilayer anode shows less irreversible capacity loss in the first cycle. This effect was attributed to the top Sn layer which is expected to prevent loss of contact due to Si pulverization which happens during the first lithiation cycle.