Phosphate-Based Glass Electrolytes in Solid-State Lithium-Ion Batteries: Overcoming Development Challenges

Electrolyte additives for bulk-type all-oxide solid-state lithium-ion batteries

The development of solid-state batteries (SSBs) is crucial for advancing next-generation battery technology. Compared to conventional lithium-ion batteries using organic liquid electrolytes, SSBs offer higher energy density, improved safety, and wide operating temperature range. Nonetheless, constructing a practical SSB with lithium-metal anode (LMA) and optimal inorganic solid electrolyte (SE) presents significant challenges. In this presentation, we introduce our findings addressing the two primary obstacles impeding the remarkable advancement of SSBs.The electrochemical instability at the interface between LMA and SE is the first persistent issue to overcome. Solid materials constituting SSBs exhibit distinct physical and chemical properties. When these materials, in the form of electrodes or electrolytes, are assembled into an electrochemical cell configuration, they establish solid–solid interfaces within the energy storage system. Achieving electrochemically stable SSBs requires non-reactive interfaces between LMA and SE with conformal contact. Previous studies have predominantly focused on developing interface modification methods to enhance the long-term cycling stability of SSBs. However, such approaches often offer phenomenological solutions without a thorough understanding of the interface dynamics. Additionally, the role of charges within an electric field in the operation of lithium-ion batteries is often overlooked. The LMA-SE interface of SSBs experiences an in situ electric field during the charging process, where the SEs exhibit semiconducting properties distinct from liquid electrolytes. Consequently, identifying the governing mechanism of interface stabilization cannot rely solely on conventional electrochemical techniques or a materials science perspective alone.Interface modeling by tailoring energy band alignment is a fundamental approach to achieving the cycling stability of SSBs. In this study, we used Li1.3Al0.3Ti1.7(PO4)3 (LATP), a representative NASICON-type SE, alongside a Li-metal foil. Although LATP possesses excellent electrochemical stability with a high anodic limit (4.31 V) and small decomposition energy (- 65 meV per atom at 5 V), its direct contact with LMA leads to the formation of a reactive interface having mixed ionic-electronic conducting interphases (MCI). The presence of MCI expedites the reduction of LATP’s metallic constituents by enabling electron transfer across the interface. Given the heightened risk of interfacial instability in the electrochemical cell, controlling interfacial reactions is imperative for constructing SSBs. Herein, we deposited titanium (45 nm) onto the surface of LATP to stabilize the inherently reactive LMA-LATP interface. The pure transition metal interlayer transformed into an ion-conducting crystal structure through electrochemical interactions with Li-metal. This titanium compound self-induced interlayer (TSI) served as a stable Li-ion conductor and electron buffer between LMA and LATP. Lithium symmetric cells having TSI maintained constant overpotential over 1000 cycles and exhibited significantly reduced impedance.Another critical obstacle to realizing practical SSBs is the intrinsic structural instability of SEs. This physical instability shortens the lifespan of SSBs and severely diminishes their electrochemical performance at the typical ambient temperatures of electric vehicle power batteries (25-50 °C). Over the past decade, various solid-state Li-ion conductors have been developed such as Li7La3Zr2O12 (garnet-type oxide SE) and Li6PS5Cl (argyrodite-type sulfide SE). By associating lattice dynamics and ionic transport in these materials, previous studies could improve the ionic conductivity of SE through major substitution with large cations like tantalum or partial substitution with large anions. Recently, antiperovskite-type SEs with the anion-centered unit of Li3AB, where A represents hydroxyl and B represents halogen groups, have attracted attention. The representative composition Li2OHCl is renowned for its facile fabrication process through solid-state reaction, high ionic conductivity (10-3 S cm-1), and excellent compatibility with LMA. Still, significant limitations exist in its use for constructing practical SSBs, as is common with most other types of SEs.We found that substituting trace amounts of aliovalent cations to modify atomic interactions in SEs can significantly enhance the electrochemical reliability of SSBs. This strategy effectively resolves the intrinsically unstable structure of SEs and simultaneously enhances the LMA-SE interfaces. As a proof-of-concept, we substituted lithium in Li2OHCl with metal elements that do not experience crystal field stabilization energy. The resulting modified electrolytes led to prolonged lifespan and superior rate performance in lithium symmetric cells at room temperature. The cells with the modified SEs exhibited a remarkable critical current density exceeding 10.5 mA cm-2 and maintained a stable voltage profile for 3000 hours at 10.0 mA cm-2, whereas the pristine cells achieved a relatively low critical current density of 2.0 mA cm-2 and a short lifespan of 1000 hours at 1.0 mA cm-2. These findings underscore a strong correlation between the physical properties of inorganic materials and the electrochemical stability of SSBs. Finally, we anticipate that the revealing results will contribute to the advancement of high-power SSBs employing solid electrolytes and metallic electrodes.

Reversible thixotropic gel electrolytes for safer and shape-versatile lithium-ion batteries

Solid polymer electrolytes for bipolar lithium ion batteries requiring electrochemical stability of 4.5 V vs. Li/Li+ are presented. Thus, imidazolium-containing poly(ionic liquid) (PIL) networks were prepared by crosslinking UV-photopolymerization in an in situ approach (i.e., to allow preparation directly on the electrodes used). The crosslinks in the network improve the mechanical stability of the samples, as indicated by the free-standing nature of the materials and temperature-dependent rheology measurements. The averaged mesh size calculated from rheologoical measurements varied between 1.66 nm with 10 mol% crosslinker and 4.35 nm without crosslinker. The chemical structure of the ionic liquid (IL) monomers in the network was varied to achieve the highest possible ionic conductivity. The systematic variation in three series with a number of new IL monomers offers a direct comparison of samples obtained under comparable conditions. The ionic conductivity of generation II and III PIL networks was improved by three orders of magnitude, to the range of 7.1 × 10−6 S·cm−1 at 20 °C and 2.3 × 10−4 S·cm−1 at 80 °C, compared to known poly(vinylimidazolium·TFSI) materials (generation I). The transition from linear homopolymers to networks reduces the ionic conductivity by about one order of magnitude, but allows free-standing films instead of sticky materials. The PIL networks have a much higher voltage stability than PEO with the same amount and type of conducting salt, lithium bis(trifluoromethane sulfonyl)imide (LiTFSI). GII-PIL networks are electrochemically stable up to a potential of 4.7 V vs. Li/Li+, which is crucial for a potential application as a solid electrolyte. Cycling (cyclovoltammetry and lithium plating-stripping) experiments revealed that it is possible to conduct lithium ions through the GII-polymer networks at low currents. We concluded that the synthesized PIL networks represent suitable candidates for solid-state electrolytes in lithium ion batteries or solid-state batteries.

All-solid-state lithium-ion batteries (ASSLIBs) have been considered suitable alternatives to commercial lithium-ion batteries (LIBs) in the aspect of safety issues that come from the use of inflammable solid electrolytes to replace organic flammable electrolytes. Nevertheless, there apparently remains much room for a better understanding of their properties and behaviors in order to upgrade their performance to reach the practical application level. Ni-rich layered oxides, LiNi1-x-yCoxMnyO2 (NCM), are promising cathodes for high-energy ASSLIBs because of their high capacities and redox potentials, and low material cost when compared with conventional LiCoO2. However, certain challenges associated with their use in ASSLIBs must be addressed for their effective use and industrialization. In particular, the structural integrity of the NCM electrodes during the long-term charge/discharge process suffers from the formation of an intergranular and/or intragranular cracking behavior. These microcracks are initiated at the grain boundaries from the anisotropic chemical strain of the randomly oriented polycrystal (PC) primary cathode particles. Solid evidence behind the electrochem-mechanical breakdown of an NCM cathode with liquid electrolytes has been well discussed and reported in the literature. However, the breakdown in ASSLIBs may not obey as same as in the liquid-electrolyte LIBs and display exclusive mechanics-oriented material damage in the presence of rigid solid-state electrolytes (SSEs). The primary objective of this research is to gain a deeper understanding of various cracking modes in ASSLIBs, which we believe is instrumental to improving the lifespan and cell performance as a whole. ForASSLIBs, stacking pressures greater than 100 MPa under battery operation are currently commonly used. The use of high stacking pressure increases manufacturing and usage difficulties and overall cost. Lowering the battery-operation stacking pressure, for example to the level of the liquid-electrolyte batteries, will certainly facilitate the wider applications of ASSLIBs. In this context, composite cathodes consisting of a LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode and brittle Li3InCl6 (LIC) SSE have been assessed for ASSLIB applications under low operation stacking pressure (coin-cell configuration; ~2.0 MPa). The study reveals the effects of material and structural characteristics of the composite cathode, including the NCM particle size and size distribution, active-material spatial distribution, particle crystallinity (polycrystal or single-crystal), and electrochemical operation parameters (voltage range, current rates) on the cycle stability of the ASSLIBs as a result of the electrochem-mechanical behaviors of the oxide cathode. Meanwhile, a novel solvent-based synthesis process of LIC for achieving high-density cathodes is introduced.

Since the electrochemical potential of lithium metal was systematically elaborated and measured in the early 19th century, lithium-ion batteries with liquid organic electrolyte have been a key energy storage device and successfully commercialized at the end of the 20th century. Although lithium-ion battery technology has progressed enormously in recent years, it still suffers from two core issues, intrinsic safety hazard and low energy density. Within approaches to address the core challenges, the development of all-solid-state lithium-ion batteries (ASSLBs) based on halide solid-state electrolytes (SSEs) has displayed potential for application in stationary energy storage devices and may eventually become an essential component of a future smart grid. In this Review, we categorize and summarize the current research status of halide SSEs based on different halogen anions from the perspective of halogen chemistry, upon which we summarize the different synthetic routes of halide SSEs possessing high room-temperature ionic conductivity, and compare in detail the performance of halide SSEs based on different halogen anions in terms of ionic conductivity, activation energy, electronic conductivity, interfacial contact stability, and electrochemical window and summarize the corresponding optimization strategies for each of the above-mentioned electrochemical indicators. Finally, we provide an outlook on the unresolved challenges and future opportunities of ASSLBs.

Instead of commercial lithium-ion batteries (LIBs) using organic liquid electrolytes, all-solid-state lithium-ion batteries (ASSBs) employing solid electrolytes (SEs) are promising for applications in high-energy–density power applications and electric vehicles due to their potential for improving safety and achieving high capacity. Although remarkable progress in SEs has been achieved and has resulted in high ionic conductivity, which now reaches values comparable to those of liquid electrolytes, the typical use of a slurry process for the fabrication of conventional ASSBs inevitably causes harmful reactions between sulfide materials and polar solvents. Here, we studied the efficient infiltration process of SE slurry into conventional composite LIB electrodes (NCM622) for achieving high-energy-density ASSBs via a scalable solution-based fabrication process. Two methods are proposed to ensure that SE materials are evenly distributed and sufficiently infiltrated into the porous structures of LIB electrodes. The LPSCl SE solutions were effectively infiltrated into the electrodes at higher processing temperatures and the temperature was subsequently optimized at above the boiling point of the ethanol solvent due to the dynamic motion of SE molecules via a convective flow during solvent vaporization. Moreover, the porous LIB composite electrodes with a mixture of active materials of different particle sizes formed and filled capillary pores resulting in a high electrode density. The LPSCl SE-infiltrated NCM622 electrodes that used this strategy could remarkably improve the initial discharge capacity of ASSBs to as high as 177 mAh/g. These ASSBs also showed excellent performance even at high loading values (about 17 mg/cm2), making them competitive with LIBs using conventional liquid electrolytes.

Interfacial self-healing polymer electrolytes for Long-Cycle silicon anodes in High-Performance solid-state lithium batteries

Halide solid electrolytes have been considered as the most promising candidates for practical high-voltage all-solid-state lithium-ion batteries (ASSLIBs) due to their moderate ionic conductivity and good interfacial compatibility with oxide cathode materials. Aliovalent ion doping is an effective strategy to increase the ionic conductivity of halide electrolytes. However, the effects of ion doping on the electrochemical stability window of halide electrolytes and carbon additive on electrochemical performance are still unclear by far. Herein, a series of Zr-doped Li3-xEr1-xZrxCl6 halide solid electrolytes (SEs) are synthesized through a mechanochemical method and the effects of Zr substitution on the ionic conductivity and electrochemical stability window are systematically investigated. Zr doping can increase the ionic conductivity, whereas it narrows the electrochemical stability window of the Li3ErCl6 electrolyte simultaneously. The optimized Li2.6Er0.6Zr0.4Cl6 electrolyte exhibits both a high ionic conductivity of 1.13 mS cm-1 and a high oxidation voltage of 4.21 V. Furthermore, carbon additives are demonstrated to be beneficial for achieving high discharge capacity and better cycling stability and rate performance for halide-based ASSLIBs, which are completely different from the case of sulfide electrolytes. ASSLIBs with uncoated LiCoO2 cathode and carbon additives exhibit a high discharge capacity of 147.5 mAh g-1 and superior cycling stability with a capacity retention of 77% after 500 cycles. This work provides an in-depth understanding of the influence of ion doping and carbon additives on halide solid electrolytes and feasible strategies to realize high-energy-density ASSLIBs.

Enabling fast-charging capability for all-solid-state lithium-ion batteries

Due to the hidden benefits such as the possibilities to make energy density high, safety improved, and lifespan extended, solid-state batteries are a focal point in battery technology research. Current liquid electrolyte-based lithium-ion batteries, despite their maturity, have inherent issues like safety risks and limited energy density. While the advancement in solid-state battery technology can potentially overcome certain challenges, it also encounters its own issues including high contact interface impedance and ion transmission efficiency. For solid-state batteries to supplant traditional lithium-ion batteries in the future, these challenges must be addressed. The research provides an in-depth analysis of the three major types of solid-state electrolytes currently prevalent in the market, including oxide, sulfide, and polymer electrolytes. This is especially true for the electrochemical analysis of batteries after the electrolyte has been used. Each has its unique properties, synthesis methods, and limitations. Oxide electrolytes excel in high-temperature ionic conductivity but underperform at room temperature. Moreover, the synthesis of oxide solid electrolytes requires fine control over temperature and atmosphere, thus demanding high technical expertise. The sulfide category shows promise in room temperature conductivity, albeit with stability issues. Polymer electrolytes, however, are flexible and processable but have generally lower ionic conductivity. To enable large-scale applications, future research needs to focus on improving these electrolytes’ performance and developing cost-effective and efficient synthesis methods. The importance of research into solid-state lithium-ion batteries is tied to their capacity to transform the energy storage sector.

The adoption of solid electrolytes in rechargeable batteries has bestowed the energy storage devices with advantages such as high energy density, improved life span and most importantly improved safety. Changing from the flammable liquid organic electrolytes to the non-flammable solid inorganic and polymeric electrolytes brings about a drastic change to the overall set-up of the battery pack, allowing for the fabrication of compact batteries suitable for application in small electronic devices and wearable technologies. However, the electrolyte phase change also brings about changes in the electrolyte–electrode dynamics which impact the overall performance of the battery. Therefore, an advancement of the electrolyte with regard to safety and performance issues should be accompanied by an analogous development of electrode materials that will be complementary to the optimized operation of the electrolyte. Hence in this chapter we discuss the prospective electrode material that can be applied in cathode active material (CAM) in all-solid-state batteries (ASSBs). We take a special look at the CAMs that have been developed for use in all-solid-state lithium-ion batteries (ASSLIBs) and to a lesser extent the lithium metal batteries (ASSLBs) as lithium technology batteries have dominated the commercial application of rechargeable batteries. Particular attention is given to materials such as the lithiated transition metal compounds that are comprised of layered and spinel oxides (LixMyOz, M = Mn, Co, Ni, etc.), and the three-dimensional polyanionic compounds (LixMyXOz, X = P, S, Si, Mo, W, etc.) and fluorinated polyanionic compounds (LixMyXOzF). Detailed discussions are given of the structural identities of such materials and the different preparation techniques/methods that have been used so far to synthesize materials of different sizes and morphologies, and how these aspects affect the electrochemical performance of the electrode material. We also give some examples of reported literature that employed the discussed cathode active materials in ASSLIBs and ASSLBs, as well as how drawbacks associated with these materials have been dealt with in order to exploit the full functionality of the electrode materials.KeywordsSolid-state batteryLithium-ion batteryCathode materialTransition metal compoundsCapacity

Recent development of ionic liquid-based electrolytes in lithium-ion batteries

In solid-state lithium-ion batteries, composite polymer-ceramic electrolytes (CPEs) of poly(ethylene oxide) (PEO) and nanosized Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles combine polymer-like flexibility and processability with significantly enhanced ionic conductivity and electrochemical stability from the ceramic filler, forming a promising electrolyte system. However, the poor interfacial compatibility between LLZTO and PEO leads to inhomogeneous filler distribution during electrolyte preparation, compromising both interfacial stability and discharge capacity in solid-state batteries. To address this, we developed an effective modification approach involving PEG-functionalized LLZTO through ultrasonic-assisted solution processing, followed by composite electrolyte fabrication with PEO. This method significantly improves LLZTO dispersion homogeneity within the polymer matrix. The optimized CPEs demonstrate superior electrochemical performance, including high ionic conductivity (∼1.0 × 10–4 S cm–1 at 25 °C), exceptional electrochemical stability (up to 5.24 V vs Li+/Li), and excellent interfacial compatibility with lithium metal anodes. Consequently, the assembled Li//CPE//LiFePO4 all-solid-state batteries deliver a high initial discharge capacity of 147.1 mAh g–1 and outstanding cycling stability with 99.0% capacity retention after 100 cycles at 0.5C (55 °C).

Building a C-P bond to unlock the reversible and fast lithium storage performance of black phosphorus in all-solid-state lithium-ion batteries