A solid-state electrolyte (SSE) is a solid ionic conductor and electron-insulating material and it is the characteristic component of the solid-state battery. It is useful for applications in electrical energy storage (EES) in substitution of the liquid electrolytes found in particular in lithium-ion battery. [1] [2] The main advantages are the absolute safety, no issues of leakages of toxic organic solvents, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability. [3] This makes possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte thanks to the property of lithium dendrite suppression in the presence of a solid-state electrolyte membrane. The use of a high capacity anode and low reduction potential, like lithium with a specific capacity of 3860 mAh g−1 and a reduction potential of -3.04 V vs SHE, in substitution of the traditional low capacity graphite, which exhibits a theoretical capacity of 372 mAh g−1 in its fully lithiated state of LiC6, [4] is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. [5] Moreover, this allows the reach of gravimetric and volumetric energy densities, high enough to achieve 500 miles per single charge in an electric vehicle. [6] Despite the promising advantages, there are still many limitations that are hindering the transition of SSEs from academia research to large-scale production, depending mainly on the poor ionic conductivity compared to that of liquid counterparts. However, many car OEMs (Toyota, BMW, Honda, Hyundai) expect to integrate these systems into viable devices and to commercialize solid-state battery-based electric vehicles by 2025. [7] [8]
The first inorganic solid-state electrolytes were discovered by Michael Faraday in the nineteenth century, these being silver sulfide (Ag2S) and lead(II) fluoride (PbF2). [9] The first polymeric material able to conduct ions at the solid-state was PEO, discovered in the 1970s by V. Wright. The importance of the discovery was recognized in the early 1980s. [10] [11]
However, unresolved fundamental issues remain in order to fully understand the behavior of all-solid batteries, especially in the area of electrochemical interfaces. [12] In recent years the needs of safety and performance improvements with respect to the state-of-the-art Li-ion chemistry are making solid-state batteries very appealing and are now considered an encouraging technology to satisfy the need for long range battery electric vehicles of the near future.
In March 2020, the Samsung Advanced Institute of Technology (SAIT) published research on an all-solid-state battery (ASSB) using an argyrodite-based solid-state electrolyte with a demonstrated energy density of 900 Wh L−1 and a stable cyclability of more than 1000 cycles, reaching for the first time a value close to the 1000 Wh L−1. [13]
For Solid State Batteries (SSBs) / Solid Electrolytes (SEs) to become a major market challenger it must meet some key performance measurements. [14] [15] [16] The major criteria that an SSB/SE should have are: [12] [17]
It is hard for one material to fulfill all the above criteria, hence a number of other approaches can be used for example a hybrid electrolyte system which combines the advantages of inorganic and polymer electrolytes.
SSEs have the same role of a traditional liquid electrolyte and they are classified into all-solid-state electrolyte and quasi-solid-state electrolyte (QSSE). All-solid-state electrolytes are furthermore divided into inorganic solid electrolyte (ISE), solid polymer electrolyte (SPE) and composite polymer electrolyte (CPE). On the other hand, a QSSE, also called gel polymer electrolyte (GPE), is a freestanding membrane that contains a certain amount of liquid component immobilized inside the solid matrix. In general the nomenclatures SPE and GPE are used interchangeably but they have a substantially different ionic conduction mechanism: SPEs conducts ions through the interaction with the substitutional groups of the polymer chains, while GPEs conducts ions mainly in the solvent or plasticizer. [23]
All-solid-state electrolytes are divided into inorganic solid electrolyte (ISE), solid polymer electrolyte (SPE) and composite polymer electrolyte (CPE). They are solid at room temperature and the ionic movement occurs at the solid-state. Their main advantage is the complete removal of any liquid component aimed to a greatly enhanced safety of the overall device. The main limitation is the ionic conductivity that tends to be much lower compared to a liquid counterpart. [24]
Inorganic solid electrolyte (ISE) are a particular type of all-solid-state electrolyte that is constituted by an inorganic material in the crystalline or glassy state, that conducts ions by diffusion through the lattice. [25] The main advantages of this class of solid-state electrolyte are the high ionic conductivity (of the order of a few mS cm−2 at room-temperature), high modulus (of the order of GPa) and high transfer number compared to other classes of SSEs. [26] They are generally brittle and with this comes a low compatibility and stability towards the electrode, with a rapidly increasing interfacial resistance and a complicated scale-up from academic to industry. [27] They can be oxides, sulfides or phosphates-based and the crystalline structures include LISICON (lithium superionic conductor) (e.g. LGPS, LiSiPS, LiPS), argyrodite-like (e.g. Li6PS5X, X = Cl, Br, I), [28] garnets (LLZO), [29] NASICON (sodium superionic conductor) (e.g. LTP, LATP, LAGP), [30] lithium nitrides (e.g. Li3N), [31] lithium hydrides (LiBH4), [32] lithium phosphidotrielates [33] and phoshidotetrelates, [34] perovskites (e.g. lithium lanthanum titanate, "LLTO"), [35] lithium halides (LYC, LYB)., [36] RbAg4I5. [37] [38] Some ISEs can be glass ceramics assuming an amorphous state instead of a regular crystalline structure. Popular examples are lithium phosphorus oxynitride (LIPON) [39] and the lithium thiophosphates (Li2S–P2S5). [40]
Solid polymer electrolyte (SPE) are defined as a solvent-free salt solution in a polymer host material that conducts ions through the polymer chains. Compared to ISEs, SPEs are much easier to process, generally by solution casting, making them greatly compatible with large-scale manufacturing processes. Moreover, they possess higher elasticity and plasticity giving stability at the interface, flexibility and improved resistance to volume changes during operation. [23] A good dissolution of Li salts, low glass transition temperature (Tg), electrochemical compatibility with most common electrode materials, a low degree of crystallinity, mechanical stability, low temperature sensitivity are all characteristics for the ideal SPE candidate. [41] In general though the ionic conductivity is lower than the ISEs and their rate capability is restricted, limiting fast charging. [42] PEO-based SPE is the first solid-state polymer in which ionic conductivity was demonstrated both through inter and intra molecular through ion hopping, thanks to the segmental motion of the polymeric chains [43] because of the great ion complexation capability of the ether groups, but they suffer from the low room-temperature ionic conductivity (10−5 S cm−1) [44] due to the high degree of crystallinity. The main alternatives to polyether-based SPEs are polycarbonates, [45] polyesters, [46] polynitriles (e.g. PAN), [47] polyalcohols (e.g. PVA), [48] polyamines (e.g. PEI), [49] polysiloxane (e.g. PDMS) [50] [51] and fluoropolymers (e.g. PVDF, PVDF-HFP). [52] Bio-polymers like lignin, [53] chitosan [54] and cellulose [55] are also gaining a lot of interest as standalone SPEs or blended with other polymers, on one side for their environmentally friendliness and on the other for their high complexation capability on the salts. Furthermore, different strategies are considered to increase the ionic conductivity of SPEs and the amorphous-to-crystalline ratio. [56]
With the introduction of particles as fillers inside the polymer solution, a composite polymer electrolyte (CPE) is obtained, the particles can be inert to the Li+ conduction (Al2O3, TiO2, SiO2, MgO, zeolite, montmorillonite, ...), [57] [58] [59] with the sole purpose of reducing the crystallinity, or active (LLTO, LLZO, LATP...) [60] [61] if ISE's particles are dispersed and depending on the polymer/inorganic ratio the nomenclature ceramic-in-polymer and polymer-in-ceramic is often used. [62] Copolymerization, [63] crosslinking, [64] interpenetration, [65] and blending [66] may also be used as polymer/polymer coordination to tune the properties of the SPEs and achieve better performances, introducing in the polymeric chains polar groups like ethers, carbonyls or nitriles drastically improve the dissolution of the lithium salts.
Quasi solid-state electrolytes (QSSEs) are a wide class of composite compounds consisting of a liquid electrolyte and a solid matrix. This liquid electrolyte serves as a percolating pathway of ion conduction while the solid matrix adds mechanical stability to the material as a whole. As the name suggests, QSSEs can have a range of mechanical properties from strong solid-like materials to those in a paste form. [67] [68] [69] QSSEs can be subdivided into a number of categories including gel polymer electrolytes (GPEs), Ionogel electrolytes, [70] and gel electrolytes (also known as "soggy sand" electrolytes). The most common QSSE, GPEs have a substantially different ionic conduction mechanism than SPEs, which conduct ions through the interaction with the substitutional groups of the polymer chains. Meanwhile, GPEs conduct ions mainly in the solvent, which acts as plasticizer. [71] The solvent acts to increase the ionic conductivity of the electrolyte as well as soften the electrolyte for improved interfacial contact. The matrix of GPEs consist of a polymer network swollen in a solvent that contains the active ions (e.g., Li+, Na+, Mg2+, etc.). This allows for the composite to contain both the mechanical properties of solids and the high transport properties of liquids. A number of polymer hosts have been used in GPEs, including PEO, PAN, PMMA, PVDF-HFP, etc. The polymers are synthesized with increased porosity to incorporate solvents such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC). [72] [73] [74] Low molecular weight poly(ethylene glycol) (PEG) or other ethers or aprotic organic solvents with high dielectric constant like dimethylsulfoxide (DMSO) can also be mixed the SPE matrix. [75] [76] UV and thermal cross-linking are useful ways to polymerize in-situ the GPE directly in contact with the electrodes for a perfectly adherent interface. [77] Values of ionic conductivity on the order of 1 mS cm−1 can be easily achieved with GPEs, as demonstrate the numerous research articles published. [78]
Emerging subclasses of QSSEs use matrix materials and solvents. Ionogels, for example use ionic liquids as a solvent that has improved safety including non-flammability and stability at high temperatures. [70] [79] Matrix materials in ionogels can vary from polymer materials [80] to inorganic nano-materials. [68] These matrix materials (as with all QSSEs) provide mechanical stability with a storage moduli up to 1 MPa or higher. Meanwhile, these materials can provide ionic conductivities on the order of 1 mS cm−1 without using flammable solvents. However, gel electrolytes (i.e. "soggy sand" electrolytes) can achieve liquid-like ionic conductivities (~ 10 mS cm−1) while being in the solid state. Matrix materials such as SiO2 nanoparticles are typically paired with low viscosity solvents (e.g., ethylene carbonate (EC)) to create a gel, whose properties can be modified based on the matrix loading. [81] Matrix content ranging from 10 to 40 wt% can shift the mechanical properties of the electrolyte from a soft paste into a hard gel. [67] However, a tradeoff between mechanical strength and ionic conductivity as one goes up with changing matrix content the other suffers. [82] Despite this, matrix content in these materials can have added benefits including enhanced lithium transference number due to functionalized matrix materials. [83] These new classes of QSSEs are an active area of research to develop the optimal combination of matrix and solvent. [67] [81]
The versatility and properties of the solid-state electrolyte widen the possible applications towards high energy density and cheaper battery chemistries that are otherwise prevented by the current state-of-the-art of Li-ion batteries. Indeed, by introducing a SSE in the battery architecture there's the possibility to use metallic lithium as anode material, with the possibility to achieve a high energy density battery thanks to its high specific capacity of 3860 mAh g−1. [84] The use of a lithium metal anode(LMA) is prevented in a liquid electrolyte above all because of the dendritic growth of a pure Li electrode that easily cause short circuits after few cycles; other related issues are volume expansions, solid-electrolyte-interface (SEI) reactivity and 'dead' lithium. [85] The usage of a SSE guarantees a homogeneous contact with the metallic lithium electrode and possess the mechanical properties to impede the uncontrolled deposition of Li+ ions during the charging phase. At the same time, a SSE finds very promising application in lithium-sulfur batteries solving the key issue of the polysulfide "shuttle" effect by blocking the dissolution of polysulfide species in the electrolyte that rapidly causes a reduction of capacity. [86]
An electrolyte is a medium containing ions that are electrically conductive through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases, dissolved in a polar solvent like water. Upon dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.
A lithium-ion or Li-ion battery is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. In comparison with other commercial rechargeable batteries, Li-ion batteries are characterized by higher specific energy, higher energy density, higher energy efficiency, a longer cycle life, and a longer calendar life. Also noteworthy is a dramatic improvement in lithium-ion battery properties after their market introduction in 1991: over the following 30 years, their volumetric energy density increased threefold while their cost dropped tenfold.
A lithium polymer battery, or more correctly, lithium-ion polymer battery, is a rechargeable battery of lithium-ion technology using a polymer electrolyte instead of a liquid electrolyte. Highly conductive semisolid (gel) polymers form this electrolyte. These batteries provide higher specific energy than other lithium battery types. They are used in applications where weight is critical, such as mobile devices, radio-controlled aircraft, and some electric vehicles.
Molten-salt batteries are a class of battery that uses molten salts as an electrolyte and offers both a high energy density and a high power density. Traditional non-rechargeable thermal batteries can be stored in their solid state at room temperature for long periods of time before being activated by heating. Rechargeable liquid-metal batteries are used for industrial power backup, special electric vehiclesand for grid energy storage, to balance out intermittent renewable power sources such as solar panels and wind turbines.
In materials science, fast ion conductors are solid conductors with highly mobile ions. These materials are important in the area of solid state ionics, and are also known as solid electrolytes and superionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily in solid oxide fuel cells. As solid electrolytes they allow the movement of ions without the need for a liquid or soft membrane separating the electrodes. The phenomenon relies on the hopping of ions through an otherwise rigid crystal structure.
The electrochemical window (EW) of a substance is the electrode electric potential range between which the substance is neither oxidized nor reduced. The EW is one of the most important characteristics to be identified for solvents and electrolytes used in electrochemical applications. The EW is a term that is commonly used to indicate the potential range and the potential difference. It is calculated by subtracting the reduction potential from the oxidation potential.
An Ion gel is a composite material consisting of an ionic liquid immobilized by an inorganic or a polymer matrix. The material has the quality of maintaining high ionic conductivity while in the solid state. To create an ion gel, the solid matrix is mixed or synthesized in-situ with an ionic liquid. A common practice is to utilize a block copolymer which is polymerized in solution with an ionic liquid so that a self-assembled nanostructure is generated where the ions are selectively soluble. Ion gels can also be made using non-copolymer polymers such as cellulose, oxides such as silicon dioxide or refractory materials such as boron nitride.
Solid-state ionics is the study of ionic-electronic mixed conductor and fully ionic conductors and their uses. Some materials that fall into this category include inorganic crystalline and polycrystalline solids, ceramics, glasses, polymers, and composites. Solid-state ionic devices, such as solid oxide fuel cells, can be much more reliable and long-lasting, especially under harsh conditions, than comparable devices with fluid electrolytes.
A solid-state battery is an electrical battery that uses a solid electrolyte for ionic conductions between the electrodes, instead of the liquid or gel polymer electrolytes found in conventional batteries. Solid-state batteries theoretically offer much higher energy density than the typical lithium-ion or lithium polymer batteries.
The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.
LISICON is an acronym for LIthiumSuper Ionic CONductor, which refers to a family of solids with the chemical formula Li2+2xZn1−xGeO4.
A potassium-ion battery or K-ion battery is a type of battery and analogue to lithium-ion batteries, using potassium ions for charge transfer instead of lithium ions.
A separator is a permeable membrane placed between a battery's anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell.
Aluminium-ion batteries are a class of rechargeable battery in which aluminium ions serve as charge carriers. Aluminium can exchange three electrons per ion. This means that insertion of one Al3+ is equivalent to three Li+ ions. Thus, since the ionic radii of Al3+ (0.54 Å) and Li+ (0.76 Å) are similar, significantly higher numbers of electrons and Al3+ ions can be accepted by cathodes with little damage. Al has 50 times (23.5 megawatt-hours m-3) the energy density of Li and is even higher than coal.
Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas of research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and reducing cost.
NASICON is an acronym for sodium (Na) super ionic conductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. NASICON compounds have high ionic conductivities, on the order of 10−3 S/cm, which rival those of liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.
Calcium (ion) batteries are energy storage and delivery technologies (i.e., electro–chemical energy storage) that employ calcium ions (cations), Ca2+, as the active charge carrier. Calcium (ion) batteries remain an active area of research, with studies and work persisting in the discovery and development of electrodes and electrolytes that enable stable, long-term battery operation. Calcium batteries are rapidly emerging as a recognized alternative to Li-ion technology due to their similar performance, significantly greater abundance, and lower cost.
A polymer electrolyte is a polymer matrix capable of ion conduction. Much like other types of electrolyte—liquid and solid-state—polymer electrolytes aid in movement of charge between the anode and cathode of a cell. The use of polymers as an electrolyte was first demonstrated using dye-sensitized solar cells. The field has expanded since and is now primarily focused on the development of polymer electrolytes with applications in batteries, fuel cells, and membranes.
Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li
1+xAl
xGe
2-x(PO
4)
3. LAGP belongs to the NASICON family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10–5 - 10–4 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like Li
10GeP
2S
12 (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of Li
1.5Al
0.5Ge
1.5(PO
4)
3, which is also the typically used material in battery applications.
Fluoride batteries are rechargeable battery technology based on the shuttle of fluoride, the anion of fluorine, as ionic charge carriers.