Solid State Rechargeable Battery
Introduction
Lithium -ion batteries (LIBs) have been considered as the most promising energy storage devices due to their relatively high energy density [1,2]. LIBs have many uses in various applications such as portable electronics, electric and hybrid electric vehicles, stationary energy storage systems, and others. Compared with LIBs, Na-ion batteries have attracted much attention in recent years as a substitute for LIBs in power storage applications due to their low cost and abundant sodium resources [3]. Aluminum-ion batteries are promising alternatives due to their low cost, nontoxic nature, abundant soil, and three-electron redox couples that offer competitive storage capacity compared to single-electron Li-ion storage [4,5]. Conventional lithium-ion batteries containing organic liquid electrolytes suffer from safety issues and low energy density. However, solid-state lithium batteries are expected to use metal lithium anodes due to the use of non- flammable solid electrolytes, which can significantly increase energy density. The introduction of metal anodes makes solid-state batteries (SSBs) promising as next-generation batteries with high energy density. Especially metallic lithium has high theoretical specific capacity (3860mAhg-1), low density (0.53gcm-3) and the lowest electrochemical potential (~3.04Vvs. standard hydrogen electrode (SHE)). Compared with liquid electrolyte-based Li-ion batteries, SSBs are considered safer, have longer life cycles, higher energy density, and lower packaging requirements [6–12]. As such, SSBs have received considerable attention over the past few decades. In this brief review, we briefly introduce the progress and existing challenges of solid-state lithium, sodium, and aluminum batteries, and also propose several possible research directions to circumvent these challenges.
Fundamentals of Solid Electrolytes in Solid-state Batteries
The Theory of Ion Conduction in Solid Electrolytes
For inorganic solid electrolytes, ionic conduction follows the Arrhenius formula (Equation 1):
where σi is the ionic conductivity, A is the pre-exponential factor, T is the absolute temperature (in Kelvin), Ea is the activation energy, and k is the Boltzmann constant. To calculate the mobility of carriers, use Equation 2:
In the formula, q is the carrier charge, D is the metal ion diffusion coefficient, k is the Boltzmann constant, and T is the absolute temperature [13]. The movement of a single particle jumping from an occupied position to an adjacent unoccupied position with equivalent energy can be described by random walk theory [14]. The relationship between conductivity and diffusion coefficient follows the Nernst -Einstein equation (Equation 3):
where Nc is the number of mobile ions. Ionic conductivity is proportional to Nc and d, and for polymer electrolytes, ionic conductivity usually follows the Arrhenius or Vogel-Tammann-Fulcher (VTF) equation, or both [15]. In general, the VTF behavior seems to be more suitable for solid polymer electrolytes, as shown in the formula 4 described
In the formula, B is the pseudo-activation energy of conductivity, and T0 is the reference temperature, which is generally 10-50K lower than the glass transition temperature (Tg ). The ion motion behavior is related to the long-range motion of the polymer segments. Effective medium theory describes the conductivity of composite electrolyte materials consisting of a conducting phase and an insulating phase.
Structure and Electrochemical Process of Solid-State Lithium Battery
Figure 1 schematically shows the structure of an all-solid-state battery. The battery consists of a positive electrode, a solid electrolyte (Mn+ ionic conductor, M=Li, Na, Al), a negative electrode, and a current collector. The solid electrolyte acts as both an ion conductor and a separator in SSBs . Electrodes are attached to both sides of the electrolyte. Solid-state batteries require less packaging and thus may reduce manufacturing costs. Mn+ ions detached from the negative electrode during discharge are transported to the positive electrode through the solid electrolyte, while electrons pass through an external circuit that powers the device. During charge/discharge, the potential for reactions and strain formation in the electrodes may lead to interfacial delamination. This phenomenon is not conducive to the cycle stability of the battery. In general, intimate interfacial contact between electrodes and electrolytes can be formed using surface modification techniques such as ball milling, PLD coating, and softening glass methods .
Figure 1 : Schematic diagram of a solid-state battery based on Mn+ ion conduction
Research Progress of Solid State Lithium Batteries
Solid lithium electrolytes (fast lithium- ion conductors) are an essential component of rechargeable solid-state lithium batteries. In general, solid electrolytes should have high Si , negligible Se , wide voltage window, chemical compatibility with electrodes, and low cost. Li3N, LiPON2, Li2S-based glass, NASCON type oxide Li1+xAlTix (PO2-x) (LATP), Garnet-type Li7 La3 Zr2O12 (LLZO), perovskite Li0.05-3xLa0.5+xTiO3, anti-perovskite Li3OCl0.5 Br0.5 and polymer electrolytes and other electrolytes because of their good Lithium-ion conductivity has been studied [6]. Inorganic electrolytes are usually too hard and brittle for flexible battery applications. However, the mechanical strength of polymer electrolytes is not satisfactory. In order to solve the problems faced by inorganic solids and polymer electrolytes, a hybrid electrolyte composed of a polymer matrix and an inorganic filler material that combines the advantages of both has been developed .
related institutions prepared a garnet membrane (HSE) composed of Li7La3Zr2O12 particles and poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) polymer matrix for high-performance Solid-state lithium battery [8]. The HSE membrane exhibits high ionic conductivity, a wide electrochemical window of ~5.3Vvs. Li+ /Li, and excellent flexibility (Fig. 2A–C). A solid-state lithium battery with this HSE film, Li metal anode, and LiFePO4 cathode exhibits an initial reversible discharge capacity of 120 mAhg-1 at a current density of 0.5 C at room temperature. After 180 cycles, the battery maintained a capacity retention of 92.5% at 0.5C (Fig. 3C).
Figure 2. A) Temperature dependence of Li + ion conductivity of HSE membranes with 20 mL liquid electrolyte and HSE membrane soaked with 20 mL liquid electrolyte; B) Comparison of linear sweep voltammograms between pure PVDF-HFP electrolyte and HSE; C) The photo of the HSE membrane in the bent state; D) the photo of the LED screen displaying the letters of BINN powered by the flexible battery with this hybrid electrolyte.
Figure 3. A) The first charge-discharge curves of solid-state lithium batteries at different current densities; B) The rate performance of solid-state lithium batteries in the range of 3.0-3.8Vvs.Li+/Li potential tested at 25°C ; (C) Tested at 0.5C rate Cyclability performance; D) Voltage curves of lithium plating/stripping cycles in symmetric Li|HSE|Li batteries at 0.05, 0.1, and 0.2 mA cm-2 , respectively [8] .
In addition to developing solid electrolytes with high ionic conductivity, improving the cyclability of metal batteries also requires the stabilization of metal anodes. Related institutions also reported an organic-inorganic composite membrane (CPM) composed of PVDF-HFP and LLZO particles to protect lithium anodes [17]. The CPM-modified lithium symmetric battery has no obvious voltage hysteresis within 500 h at a voltage of 2 mAcm−2. Furthermore, the CPM-modified Li|LFP cell can be stably operated at 1C for 800 cycles and maintains an average Coulombic efficiency as high as ~99.95%. In addition, it was also found that a composite electrolyte membrane composed of polyacrylonitrile (PAN)-Li6.5La3Zr1.5Ta0.5 O12 (LLZTO) matrix and double salts LiClO4 and Mg (ClO4)2 can improve the cycle of lithium batteries Stability [18]. Magnesium salt is beneficial to promote the decomposition of LiPF6 in the electrolyte to generate fluoride ions. As a result, a stable magnesium fluoride protective layer is formed on the surface of the lithium negative electrode, which can effectively inhibit the growth of lithium dendrites and increase the cycle life of the battery.
In order to improve the rate performance of solid-state lithium batteries, further research is needed to use ion-conducting polymer matrix as a binder to prepare positive electrodes or to coat ion-conducting materials on positive electrode particles. In addition, to improve the cyclability of solid-state lithium batteries, a stable lithium metal anode is also necessary.
Research Progress of Solid State Sodium Batteries
Solid-state sodium batteries have the advantages of high energy density, high safety, and being an abundant sodium resource. The solid electrolytes studied include b-alumina (Na2O·11Al2O3), Na3Zr2Si2PO12, Na3P1-xAsxS4 (0≤x≤0.5) Na3PSe4. 94Na3PS4-6Na4S3S4, NaS4S4, 50Na2S-50P2S5, 60Na2S-40GeS2, 50Na2S-50SiS2 etc. [19]. However, the low conductivity of solid electrolytes and the high interfacial resistance between the electrolyte and electrodes are two major challenges for the practical application of solid-state sodium batteries. In order to solve the problem of low conductivity of Na3Zr2 Si2PO12(NZSP) solid electrolyte at room temperature, we prepared Ca2+ doped Na3Zr2 Si2PO12 with NaSICON structure, which has higher ionic conductivity at room temperature, reaching 1.67X10-3 Scm-1 [10]. Neutron powder diffraction experiments (NPD) reveal the anisotropic thermal displacement of Na atoms and a more rigid framework structure, which is favorable for Na diffusion, by substituting Ca2+ for Zr in NZSP.
The researchers developed a robust, Ca-doped NZSP-type monolithic structure to address the poor interfacial contact between electrodes and electrolyte. The monolithic solid-state battery with Na metal anode and Na3V2(PO4)3 cathode maintained a capacity of 94.9 mAhg−1 at 1 °C after 450 cycles (Fig. 4C). In addition, it also represents. It exhibits high-rate capability and excellent cyclability. This unique monolithic electrolyte architecture design provides a promising approach to realize high-performance solid-state sodium batteries.
The current findings suggest that scaling up the process of monolithic cells is required as a key technology in future research. In addition, stabilizing metallic sodium anodes is a key technology to improve the cyclability of solid-state sodium batteries.
Figure 4. A) Schematic diagram of the overall NZSP solid electrolyte; B) Schematic diagram of the full SSB; (c~e) Electrochemical performance of solid-state sodium batteries
Research Progress of Solid Aluminum Batteries
Aluminum (Al) has many advantages: it is abundant , light in weight, and has three electrons per Al atom, allowing the realization of a theoretical specific capacity of 2980mAhg-1 and a volumetric capacity of 8046AhL-1 [4,5] . However, the development of aluminum batteries is hampered by solid electrolytes with high electrical conductivity. Through high-temperature NPD experiments and atomic-resolution scanning transmission electron microscopy (STEM) analysis, we determined the diffusion mechanism of Al in (Al0.2Zr0.8)20/19Nb (PO4 )3, and the Al shift indicated that Al3+ Ions diffuse through the structure through the vacancy mechanism. Figure 5A shows the Z-contrast (Z: atomic number) atomic resolution High Angle Annular Dark Field (HAADF) image of (Al0.2Zr0.8)20/19Nb (PO4)3 taken along the [010] band axis. Yellow arrows indicate Al3+ ion columns, identified in {102} crystal planes. Figure 5B shows the intensity profile of the Al3+ ion column along the dashed line ab in Figure 5A. The variation of the intensity of Al3+ sites indicate the random distribution of Al3+ and vacancies, which facilitates the transport of Al3+ in ion channels. Furthermore, we report for the first time a VO nanorod/rGO (reduced graphene oxide).
A rechargeable solid-state battery assembled with dense (Al0.2Zr0.8)20/19Nb (PO4)3 particles as the electrolyte and Al as the negative electrode. The addition of a small amount of molten salt electrolyte composed of NaCl (99.99%) and AlCl (99.9%) (molar ratio 1:1.63) improved the diffusion of Al ions at the cathode/electrolyte interface. As shown in Figure 5F, at 120°C, the first discharge specific capacity of the battery reached 7.5mAhg-1, while the charge specific capacity was 6.5mAhg-1. When the temperature rises to 150°C, the capacity value of the battery is ~10mAhg-1.
Figure 5. STEM images of (Al0.2Zr0.8)20/19Nb (PO4)3; C) Schematic diagram of the Swagelok cell for electrochemical tests; D) Preparation process of Al anode; E) (Temperature dependence of Al3+ ionic conductivity of Al0.2Zr0.8)20/19 Nb (PO4)3; F) Solid-state V2O5 nanorod/rGO|Al battery at 2 mAg-1 at 120 The first discharge-charge curves tested at ℃ and 150℃ [11].
Solid-state aluminum batteries urgently need solid electrolytes with high ionic conductivity, which requires further research. Effective interfacial contact between the electrolyte and the electrodes is also a necessary condition for realizing high-performance batteries.
Scientists have previously studied solid-state batteries with other ion-conducting electrolytes. Magnesium (Mg) batteries present another promising alternative to overcome the problems of poor safety and low energy density faced by LIBs. However, the development of magnesium batteries has been hampered by the poor mobility of magnesium ions in solids. Through ab initio calculations and experimental characterization, Ceder et al. reported for the first time the rapid conduction of magnesium ions in spinel MgSc2Se4, which can be integrated with magnesium magnesium cathodes, such as spinel MgTi2S4 and rutile Mo6S8 in solid state Magnesium battery [20]. Their theoretical calculations also predict that other chalcogenide spinels may have higher Mg mobility.
Conclusion and Outlook
To sum up, solid-state batteries have received considerable attention in recent years due to safety concerns. Although the scientific community has made great progress in solid-state lithium batteries over the past few decades, the low ionic conductivity of solid electrolytes and poor interfacial contact between the electrolyte and electrodes are two major challenges that researchers still face . Solid-state Na batteries and Al batteries are emerging technologies because of their advantages in low cost and high volume-energy density, respectively, compared with Li batteries. The review authors provide an overview of the progress of solid-state lithium, sodium, and aluminum batteries developed in our laboratory. For solid-state batteries, solid electrolytes are a key component.
The development of solid electrolytes with high ionic conductivity has become an urgent need. Achieving and maintaining a good contact between solid electrodes and solid electrolytes is crucial to reduce interfacial resistance. Utilizing soft polymer interlayers, small amounts of gel or liquid electrolytes have been shown to be feasible approaches to improve ion transport in interfaces. In addition, researchers can employ other strategies to improve cathode performance. For example, scientists can use ion-conducting polymer matrix as a binder to prepare positive electrodes or coat ion-conducting materials on positive electrode particles, such as LiNbO3 and Li1+xAlxTi2-x(PO4)3 and other materials.
Furthermore, metal anode protection has been shown to be crucial to achieve long-term stability of solid-state batteries. Especially solid electrolyte interphase (SEI) film-forming additives and artificial SEI provide feasible strategies to suppress Li dendrites and improve the long-term stability of Li batteries. Combining experimental and theoretical calculation methods can reveal the interfacial evolution during charge-discharge cycling, thereby enhancing the performance of solid-state batteries. The commercialization of solid-state batteries will take some time.
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