All‐solid‐state Li‐ion batteries with commercially available electrolytes: A feasibility review

Abstract

The all‐solid‐state battery (ASSB) concept promises increases in energy density and safety; consequently recent research has focused on optimizing each component of an ideal fully solid battery. However, by doing so, one can also lose oversight of how significantly the individual components impact key parameters. Although this review presents a variety of materials, the included studies limit electrolyte‐separator choices to those that are either fully commercial or whose ingredients are readily available; their thicknesses are predefined by the manufacturer or the studies in which they are included. However, we nevertheless discuss both electrode materials. Apart from typical materials, the list of anode materials includes energy‐dense candidates, such as lithium metal, or anode‐free approaches that are already used in Li‐ion batteries. The cathode composition of an ASSB contains a fraction of the solid electrolyte, in addition to the active material and binders/plasticizers, to improve ionic conductivity. Apart from the general screening of reported composites, promising composite cathodes together with constant‐thickness separators and metallic lithium anodes are the basis for studying theoretically achievable gravimetric energy densities. The results suggest that procurable oxide electrolytes in the forms of thick pellets (>300 μm) are unable to surpass the performance of already commercially available Li‐ion batteries. All‐solid‐state cells are already capable of exceeding the performance of current batteries with energy densities of 250 Wh kg⁻¹ by pairing composite cathodes with high mass loadings and using separators that are less than 150 μm thick, with even thinner electrolytes (20 μm) delivering more than 350 Wh kg⁻¹.

INTRODUCTION

While lower battery prices¹ and renewable energy costs² have led to the affordable large-scale grid storage of electrical energy, the mobile electric sector still struggles to compete with internal combustion engines in terms of power and energy density. The personal vehicle market prioritizes the implications of these limitations, as public acceptance is heavily influenced by vehicle comfort that are mainly ascribable to the ranges³ and charging times⁴ of electric vehicles. Even though state-of-the-art and even more upcoming Li-ion batteries attempt to overcome these concerns,^5,6 the all-solid-state battery (ASSB) concept may provide possible improvements, especially in terms of energy density^7–9 and safety owing to the use of supposedly nonflammable solid electrolytes. Janek and Zeier surveyed the prospects of solid-state batteries in detail a few years ago,¹⁰ including their remaining problems, and recently complemented their evaluation with new trends, including composite cathodes with highly conducting solid electrolytes. Moreover, these researchers advocated diversifying the approaches and materials used as well as focusing on the main issues that hinder their broad application.¹¹ Kim et al. discussed more specifically how these challenges manifest themselves in oxide and sulfide electrolytes and presented possible routes for overcoming them.¹² They concluded that three points are essential: (1) the cost-effective and scalable production of highly conductive solid electrolytes that are both dense and thin (20 μm), (2) coupling solid electrolytes with suitable electrode materials, including the active material, binder, and additives, and (3) enabling current densities above 0.6 mA cm⁻² by addressing (i) lithium-dendrite formation, (ii) interfacial electrochemical stability, and (iii) physical contact that worsens over time. In contrast, Randau et al. presented a benchmarking study that explored how various reported cells of a specific ASSB type compare in terms of selected performance parameters; for example, specific energy with respect to cell thickness and cycling stability.¹³ A comparison of lithium-metal ASSBs with various electrolyte–cathode combinations links the maximum allowed cell resistance to reachable specific energies. Commonly, these reviews not only broadly screen available materials but also specify the desired parameters of all individual battery components. While providing such minimum requirements provides guidance when optimizing each battery component, ultimately leading to an ideal ASSB, it can also introduce vagueness in terms of the impact of achieving only a few target parameters or even just one.

In this review, we not only list commercially available or at least state-of-the-art materials for solid electrolyte separators but also consider theoretically reachable energy densities at available or already reported thicknesses. Consequently, isolating the electrodes represents the remaining challenge. Although various options have been presented for anodes, close attention is paid to lithium metal. Cathodes are covered more comprehensively, as their composite nature leads to problems associated with material compatibility. Furthermore, the maximum energy density is determined by the solid–electrolyte fraction. Finally, the reports assess the extent to which theoretical energy densities can arise through various pairings.

Additionally, we present measures that mitigate interfacial issues, including coatings between electrolytes and electrodes, as well as safety aspects and the upscaling potentials of the respective solid electrolytes.

MATERIALS

Separator solid electrolytes

Solid(-state) electrolytes (SEs/SSEs) are solid-ion conductors that can be classified into three categories: inorganic, polymer, and composite, with some oxides, sulfides (both inorganic), and composites listed in Table 1. Oxide electrolytes are typically rigid and have high voltage-stability windows, good ionic conductivities, and are highly stable in air.³⁵

TABLE 1 Examples of various electrolytes, including their chemical compositions, as well as reported conductivities, bending strengths or moduli, and potential windows.

Family	Electrolyte group	Chemical composition	Conductivity in S/cm @RT, up to	aBending strength in MPa or bmodulus in GPa	Potential Window (V vs. Li/Li+)	Ref.
Oxide	NASICON-type	$\mathrm{L}{\mathrm{i}}_{1+x}\mathrm{A}{\mathrm{l}}_x\mathrm{T}{\mathrm{i}}_{2-x}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_3$ (LATP)	$1.9\times {10}^{-3}$	140 MPaa	2.65–4.6	14,15
$\mathrm{L}{\mathrm{i}}_{1+x}\mathrm{A}{l}_x\mathrm{G}{\mathrm{e}}_{2-x}{\left(\mathrm{P}{\mathrm{O}}_4\right)}_3$ (LAGP)	$6.2\times {10}^{-3}$	100 MPaa	1.85–4.9	15–17
${\mathrm{Li}}_2\mathrm{O}-{\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{Si}{\mathrm{O}}_2-{\mathrm{P}}_2{\mathrm{O}}_5-\mathrm{Ti}{\mathrm{O}}_2-\mathrm{Ge}{\mathrm{O}}_2$ (LICGC)	$1\times {10}^{-4}$	140 MPaa	–	18
Lithium-based perovskite	$\mathrm{L}{\mathrm{i}}_{3x}\mathrm{L}{\mathrm{a}}_{\left(2/3-\mathit{x\gamma }\left(1/3\right)-2x\right)}\mathrm{Ti}{\mathrm{O}}_3$ (LLTO)	$1\times {10}^{-3}$	79 MPaa	2.1–3.68	19,20
Garnet	$\mathrm{L}{\mathrm{i}}_7\mathrm{L}{\mathrm{a}}_3\mathrm{Z}{\mathrm{r}}_2{\mathrm{O}}_{12}$ (LLZO)	$5\times {10}^{-4}$	–	0.05–3.61	21,22
$\mathrm{L}{\mathrm{i}}_{6.65}\mathrm{G}{\mathrm{a}}_{0.15}\mathrm{L}{\mathrm{a}}_3\mathrm{Z}{\mathrm{r}}_{1.9}\mathrm{S}{\mathrm{c}}_{0.1}{\mathrm{O}}_{12}$ (Sc:LLZO)	$1.8\times {10}^{-3}$	–	1.65–3.7	23,24
$\mathrm{L}{\mathrm{i}}_{6.4}\mathrm{L}{\mathrm{a}}_3\mathrm{Z}{\mathrm{r}}_{1.4}{\mathrm{Ta}}_{0.6}{\mathrm{O}}_{12}$ (Ta:LLZO, LLZTO)	$1\times {10}^{-3}$	120 MPaa	–	24,25
	$\mathrm{L}{\mathrm{i}}_3\mathrm{P}{\mathrm{O}}_4+{\mathrm{N}}_2$ (LiPON)	$1\times {10}^{-6}$	80 GPab	0.0–4.3	26
Sulfide	$\mathrm{L}{\mathrm{i}}_{10}\mathrm{Ge}{\mathrm{P}}_2{\mathrm{S}}_{12}$ (LGPS)	$1.2\times {10}^{-2}$	–	1.19–2.38	20,27,28
${\mathrm{Li}}_{10}\mathrm{Ge}{\mathrm{P}}_2{\mathrm{S}}_{12-x}{\mathrm{O}}_x$	$1\times {10}^{-2}$	–	0.0–10.0	29
${\mathrm{Li}}_6\mathrm{P}{\mathrm{S}}_5\mathrm{Cl}$ (LPSCl, powder)	$3\times {10}^{-3}$	–	1.11–2.19	30
${\mathrm{Li}}_6\mathrm{P}{\mathrm{S}}_5\mathrm{Cl}$ (LPSCl, pellet)	$2\times {10}^{-3}$	42.1 GPab	–	20,31,32
$\mathrm{L}{\mathrm{i}}_{6.6}\mathrm{S}{\mathrm{i}}_{0.6}\mathrm{S}{\mathrm{b}}_{0.4}{\mathrm{S}}_5\mathrm{I}$	$2.4\times {10}^{-2}$	–	–	33
Composite	PEO + 10 wt% LLZTO	$2\times {10}^{-4}$	–	0.0–4.0	34

Generally, sulfides exhibit the highest ionic conductivities and deformable mechanical properties; they can be further classified into thio-LISICONs, which are derived from the Li₃PO₄ solid electrolyte by exchanging oxygen for sulfur, argyrodite Li₆PS₅X (where X = Br, Cl, I), LGPS (Li₁₀GeP₂S₁₂), and LGPS-type (Li_11−xM_2−xP_1+xS₁₂, where M = Ge, Sn, Si).^36,37 In comparison, halide electrolytes have the general Li_3+mM_1+n X₆ formula where M represents one or multiple metallic elements and X represents one or multiple halogen elements. Halides are generally less ionically conductive than sulfides; however, they are more electrochemically stable, especially when combined with oxide cathodes. Moreover, they are more compatible with lithium anodes than sulfides.³⁸ Composite materials combine flexibility and adaptability to volume change with the superior ionic conductivities of inorganic materials, which enables the electrolyte properties to be fine-tuned as desired.^39,40 In general, all SEs still suffer from low ionic conductivities when compared to liquid electrolytes, with only some sulfide electrolytes delivering comparable values of 10⁻² S cm^–1 or higher.

Electrochemical stability

Figure 1 summarizes the electrochemical windows of various SSEs, which shows that most SSE/electrode interfaces are thermodynamically unstable, with solid–electrolyte interphases (SEIs) formed at anode sides.⁴¹ While such interfaces may extend the electrochemical stability window, they frequently hinder ion transport owing to their poor ionic conductivities and mixed electronic–ionic conductivity properties. Consequently, this inefficiency fails to inhibit further reactions between the electrode and SSE, resulting in significant self-discharging and polarization. Modifying the interface can enhance SSE/anode-material compatibility. Chen et al. comprehensively reviewed this topic and examined anode/SSE compatibility in detail.⁴² To sustain low interfacial impedance across multiple cycles, an SSE must be thermodynamically stable with respect to the anode. This condition is satisfied when the anode potential is within the electrochemical window of the SSE; otherwise the SSE is reduced by the anode, which indicates that the anode material and the SSE are inherently incompatible.⁴² The oxygen-substituted LGPS-type solid electrolyte reported by Sun et al. (${\mathrm{Li}}_{10}\mathrm{Ge}{\mathrm{P}}_2{\mathrm{S}}_{12-x}{\mathrm{O}}_x$) deserves special attention; it not only features conductivities of around 10⁻² S cm^–1, but is also electrochemically stable between −0.5 and 10 V (vs. Li/Li⁺).²⁹

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An SSE must also be thermodynamically stable with respect to the cathode to sustain a low interfacial impedance across multiple cycles. This condition is satisfied when the cathode potential is within the electrochemical window of the SSE. Otherwise, the SSE is oxidized at the cathode.⁴² In a similar manner to the anode side, Figure 1 shows that most SSE/cathode interfaces are thermodynamically unstable, leading to the formation of cathode–electrolyte interphases (CEIs) at the cathode sides.⁴¹ Furthermore, inherent cathode-material/SSE incompatibility leads to the formation of a space-charge layer at the SSE/cathode interface.⁴²

Mechanical properties

If mechanical properties are reported, they are often not determined under standardized conditions, which makes it difficult to provide fair comparisons. Of course, this is possibly due to the different flexural modes associated with the various electrolyte groups. Oxide electrolytes are stiff and brittle; hence, their maximum bending strengths prior to breakage is of interest (~10²–10³ MPa). On the other hand, the flexible nature of a sulfide electrolyte is better assessed by its elastic (Young's) modulus (~10² GPa) (Table 1). Ideally, a proper comparison should feature only members of the same electrolyte group and use the same testing procedure.

Safety aspects

Several safety aspects associated with solid electrolytes must be considered. Firstly, the nonflammability of a solid electrolyte becomes nuanced when closely inspected.⁴³ Oxide electrolytes are incombustible owing to their ceramic characteristics; however, some electrolytes tend to yield at elevated temperatures when combined with lithium anodes.⁴⁴ The same cannot be said for sulfur- or polymer-containing electrolytes. Compared with the state-of-the-art liquid electrolyte (1 M LiPF₆ in 1:1 (v:v) EC:DMC), sulfide electrolytes do not produce flammable products during combustion; however, heating them still produces a flame, as does grinding them with Li_0.5CoO₂ (as the active cathode material) and lithium. The latter combination simulates the thermal runaway of a full cell containing two electrode materials and Li₆PS₅Cl, which exhibits a more violent reaction than that associated with the combustion of the sulfide electrolyte alone, indicative of a thermal safety risk in the event of a short circuit. However, not all sulfide electrolytes show this heightened thermal risk; for example, Li₃PS₄ does not exhibit such behavior.³⁰ Moreover, the sulfide-electrolyte/cathode-material combination determines the thermal-runaway risk. In addition to LCO, Li₆PS₅Cl reacts violently with the Li_1–xNi_0.8Co_0.1Mn_0.1O₂ nickel-rich cathode but is stable in the presence of LiFePO₄.⁴⁵

Because polymers are considered for composite-electrolyte and composite-cathode (see next section) applications, their compatibilities with other materials are highly important in relation to thermal runaway. Polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) are considered nonflammable, whereas polyethylene oxide (PEGMA) and polyethylene glycol diacrylate (PEGMA) are flammable.⁴⁶ In addition to identifying flame retardants that thermally stabilize polymers,⁴⁷ correct pairings within composites are crucial.^44,48

Moreover, most solid electrolytes are unstable in air owing to reactions with O₂, H₂O, or CO₂, which provides manufacturing hurdles because dry rooms and, in extreme cases, inert atmospheres, are indispensable.⁴⁹ Fabricating sulfide electrolytes is also risky; not only do electrolytes need to be protected from the aerobic atmosphere, but personnel also need to be shielded from potentially hazardous H₂S. Although H₂S concentrations are manageable in dry rooms for some production steps, others must be performed in an inert atmosphere (Ar, N); hence, odor development is unavoidable, and locally high concentrations must be registered using personal gas detectors.^43,50

Upscaling aspects

Upscaling all-solid-state-battery production and achieving desired component thicknesses requires advancements in both materials and manufacturing techniques.³³ Traditional thick pellets are not only unsuitable for large-scale production but are also undesirable in terms of energy density (see Section 3).^51,52 In this regard, considering conductivity and mechanical strength as parameters that need to be optimized is useful; here, a solid electrolyte that is either too thin or too thick favors either conductivity or mechanical strength.

In particular, oxide electrolytes, such as lithium garnets (LLZO), perovskites (LLTO), and NASICON-type (LATP) structures are typically produced in pellet form using high-temperature sintering, which is energy-intensive and hence a cost factor.⁵³ Emerging methods, such as ultrafast high-temperature sintering, wet-chemical deposition, and advanced film-fabrication techniques, including pulsed-laser deposition (PLD) and aerosol-deposition methods (ADMs) have been explored with the aim of creating thin, uniform solid electrolyte (SE) layers. These methods aim to ensure mechanical stability and ionic conductivity while maintaining uniform thickness across large areas. However, thin-film deposition methods are rarely scaled owing to their inherently low cost-effectiveness.⁵² Optimizing thin oxide-electrolyte production involves developing high-throughput processes and precise coating methods to achieve the desired form factor and performance. Moreover, manufacturing hurdles not only include the electrolyte membrane itself but also the further processing of the as-then-prepared thin sheet. Techniques that combine them with cathode materials, such as co-sintering at high temperatures, present challenges ascribable to chemical incompatibilities.⁵⁴

Compared with oxide electrolytes, sulfides are malleable, which is a good feature for any production process that involves cold pressing. Composite cathodes that include sulfide electrolytes (as opposed to oxide electrolytes) can therefore omit the sintering step and save energy. The inherent flexibility of sulfide electrolytes provide another way of quickly achieving large-scale production by imitating the manufacturing processes of established lithium-ion batteries (LIBs). Treating the separator as just another layer that needs to be cast enables one to use existing machines, thereby accelerate upscaling.^37,51 Also, the production of sulfide electrolyte base materials needs to be evaluated based on their potentials to be prepared on large scales. As an example, one might examine the liquid-phase synthesis method, which is fast, inexpensive, and has the potential to be highly scalable. To that end, precursors that are less expensive than Li₂S are crucial, while environmental issues also need to be considered, which is mainly due to the use of essential organic solvents.⁵⁵

Although sulfides are easier to work with than oxide electrolytes, from a mechanical perspective, composites are even more machinable. The polymer within a composite electrolyte functions as the matrix, which not only engenders the electrolyte itself with flexibility, but also improves electrolyte/electrode contact.⁴⁰

In addition to good machinability, new techniques with promising upscaling potentials have emerged. The manufacturing process itself is typically also adaptable using a flexible material; for example, it can be designed such that 3D-printed batteries are possible.^52,56 Recently, Sakuú announced a battery that not only showcased more than 80% of its initial capacity after 1000 cycles but was also supposedly fully dry-printed.⁵⁷ Therefore, solid electrolytes can conceivably be manufactured at scale without the use of toxic solvents.

While all the materials presented above have garnered attention as possible solid–electrolyte candidates, here, we only compare four of them (see Figure 2), which were mainly chosen on the basis of their availabilities, ionic conductivities, and demonstrated thicknesses.

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We first discuss commercially available LICGC™ AG-01 reported by Ohara¹⁸ to have an ionic conductivity of 1 × 10⁻⁴ S cm^–1, which is stable toward air and water. Circular and square samples are available in thicknesses of 150 and 20 μm, respectively. In terms of mechanical stability, the sample exhibited a bending strength of 140 MPa. Although its air stability is favorable for research and low-scale production, the low ionic conductivity of this solid–electrolyte candidate renders it unsuitable for use in commercial applications. It has also been shown to be unstable when in direct contact with Li metal.⁵⁹

We next discuss LLZTO (Li_6.5La₃Zr_1.5Ta_0.5O₁₂), which has attracted attention owing to its projected ionic conductivity of more than 1 × 10⁻³ S cm^–1.²⁵ Unfortunately, this material is sensitive to oxidation and water⁶⁰ and is lithiophobic,⁶¹ which is why an interlayer is required. Samples can be polished down to approximately 200–300 μm on the laboratory scale, but only 700-μm-thick pellets are commercially available.²⁵ The pellets exhibit flexural strengths of approximately 120 MPa. Thus, while it is mechanically strong, thinner samples are required to deliver the energy densities required for automotive applications (see Section 3).

PEO-LLZTO, which is a hybrid electrolyte composed of polyethylene oxide (PEO) mixed with LLZTO, is the third example. The ionic conductivity of this hybrid electrolyte reportedly depends on the weight percentage of LLZTO (10 or 40 wt%) and lies between 2 × 10⁻⁴ and 4 × 10⁻⁴ S cm^–1, with thicknesses usually below 100 μm.^62–65 This hybrid electrolyte is reported to exhibit good cycling stability, is inexpensive, and offers more applications possibilities owing to its flexibility that is ascribable to its large PEO content. Unfortunately, the mechanical strength of this hybrid is insufficient for use in large-scale applications, such as automotive batteries.⁶⁵ Because LLZTO is used, contact with air or water is detrimental, however, both these limitations can be overcome using additional polymers or other materials as additives by combining their strengths.⁶⁶

We finally discuss the LPSCl sulfide electrolyte, which can deliver ionic conductivities of up to 2 × 10⁻³ S cm^–1 for its pellets depending on the synthesis method.³¹ However, owing to its brittle nature,⁶⁷ LPSCl requires a support framework, such as PVDF⁶⁸ or HNBR,⁶⁹ to fabricate thinner, flexible membranes, which decreases the ionic conductivity by up to 70%. The electrolyte is also poorly stable toward lithium⁷⁰ and requires buffer layers as a consequence.⁷¹ The high ionic conductivity of this electrolyte facilitates its use in future applications that require fast charging.

In the next section, we discuss in more detail the use of solid electrolytes as ingredients in composite cathodes to boost ionic conductivity, as well as compatibility issues.

Cathodes

As mentioned previously, significant improvements have been made in relation to the ionic conductivities of bulk solid electrolytes (SE) and lithium-metal-anode/SE interfaces. In a similar manner to conventional Li-ion batteries, the most commonly used cathode active materials are layered or spinel lithium transition-metal oxides (Li_xM_yO_z, M = Co, Ni, Mn, etc.) and olivine polyanionic compounds (LiMPO₄, M = Fe, Mn, Ni, Co, etc.).⁷² In particular, NCM (LiNi_xCo_yMn_zO₂) and LCO (LiCoO₂) are commonly used as high-capacity cathode active materials (CAM) for LIBs and ASSBs, respectively. However, compared with conventional Li-ion batteries, the cathode structure and SE/cathode interface require several changes to enable effective charge transport between the active material and the SE separator.⁶⁹ Owing to the semiconducting nature of most cathode materials, conducting additives are required to enable electron transport; consequently, cathodes for conventional LIBs consist of active-material particles, conducting additives, and binder materials, which form a porous structure that is infiltrated by the liquid electrolyte. However, pathways for ion transport must be artificially created in an ASSB by preparing composite electrodes that contain the SE.^73,74 The mixing ratio and method are decisive for providing optimal electrode designs that enable sufficient electronic and ionic conductivities, as expressed by the tortuosity of the electrode.⁷⁵ At the same time, the proportion of active material should be as high as possible to maximize the specific capacity and energy density of the cathode.

Various reviews are dedicated to preparation strategies for ASSB cathodes with the aim of improving interfacial ionic conductivity and stability.^{69,72,74–76} However, the areal capacities reported in most studies are insufficient for use in practical applications. In 2021, a break-even analysis by Kravchyk et al. found that ASSBs with Li-garnet solid electrolytes require areal capacities of at least 3.5 mAh cm⁻² to deliver similar or even higher energy densities than conventional LIBs.⁷⁷ The thin electrodes (<1 mAh cm⁻²) commonly employed in laboratory cells cannot fulfill this capacity requirement even for very thin (20–50-μm-thick) solid electrolytes. Therefore, this section focuses on strategies that enable composite cathodes for solid-state batteries with high areal capacities while concurrently providing high active-material utilization. Table 2 lists various composite cathodes that reportedly exceed the minimum areal capacity requirement. Notably, many of these electrodes are prepared using techniques in which the SE separator is deposited directly on top of the composite cathode, or vice versa. Although this approach leads to improved interfacial charge transfer, it may not be compatible with commercial battery-production processes; nevertheless, the underlying strategies are still worth discussing and can, in many cases, be modified to produce freestanding cathodes.

TABLE 2 High areal capacity composite cathodes and their compositions, SEs used as ion-conducting additives, specific capacities with mass loadings, and areal capacities.

Cathode composition	Solid electrolyte (SE)	Specific capacity	Areal capacity	Ref.	#
Composite (polymer/oxide) and oxide electrolytes
LFP:CB:SE, 8:1:1	90% PEO + 10% LLZTO	118 mAh g−1 @ 15.2 mg cm−2	1.79 mAh cm−2	Bi et al.58	a
NCM(811):SE, 6:4 (vol.)	PEGMA + MePrPyl TFSI + LiTFSI	199 mAh g−1 @ 83.9 mg cm−2	16.7 mAh cm−2	Huang et al.78	b
NCM(811):CB:SE, 8:1:N.A.	LAGP + PEG-BATGIC	154 mAh g−1 @ 13 mg cm−2	2.0 mAh cm−2	Zhang et al.79	c
NCM(622):CB:PVDF:LLZTO:LiClO4, 80:10:7:2:1	PDOL	140 mAh g−1 @ 21 mg cm−2	2.94 mAh cm−2	Zhang et al.80	d
LFP:ITO:SE, 87.7:8.8:3.5	PEO:LiTFSI: nano-SiO2	157 mAh g−1 @ 95 mg cm−2	15 mAh cm−2	Chen et al.81	e
Sulfide electrolytes
LAZO@LNO:SE:CNF, ~65:30:5	LPSCl	180 mAh g−1 @ 25 mg cm−2	4.5 mAh cm−2	Wang et al.82	f
LZO@LCO:LPSCl:VGCF, 60:35:5	LPSCl (nano soft carbon and LLZTO interface layer)	120 mAh g−1 @ 111.1 mg cm−2	15 mAh cm−2	Yang et al.83	g
NCM(811):SE:PTFE, 70:28:2	LPSCl	180 mAh g−1 @ 71.4 mg cm−2	13 mAh cm−2	Liang et al.84	h
NCM(830611):SE:CB, 70:27.5:2.5	LPSCl	205 mAh g−1 @ 43 mg cm−2	8.7 mAh cm−2	Doerrer et al.85	i
HfO2@NMC(851005):SE:CB, 69:30:1	LPSCl	200 mAh g−1 @ 10.5 mg cm−2	2.1 mAh cm−2	Kitsche et al.86	j
NCM(701515):SE:CB:BR, 70:27.5:1:1.5	LPSClBr	165 mAh g−1 @ 8 mg cm−2	1.3 mAh cm−2	Kwon et al.87	k
Halide electrolytes
NMC90:SE, 8:2	LIC	107 mAh g−1 @ 72.6 mg cm−2	9.8 mAh cm−2	Ma et al.88	l
LCO:SE, 8:2	Li2In1/3Sc1/3Cl4	138 mAh g−1 @ 52.4 mg cm−2	7.2 mAh cm−2	Zhou et al.89	m
NMC85:SE, 9:1	Li2In1/3Sc1/3Cl4	192 mAh g−1 @ 21.6 mg cm−2	4.1 mAh cm−2	Zhou et al.89	n
LCO:SE, 85:15	LIC	131.7 mAh g−1 @ 48.7 mg cm−2	6 mAh cm−2	Wang et al.90	o
NMC(830611):SE:CNF, 65:30:5	Li3InCl6	210 mAh g−1 @ ~14 mg cm−2	3 mAh cm−2	Gao et al.91	p

For a better overview, Figure 3 shows the areal capacities of state-of-the-art composite cathodes against their respective mass loadings. The dotted line provides guidance for the eye and corresponds to a specific capacity of about 157 mAh g⁻¹.

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Composite (polymer/oxide) and oxide electrolytes

Bi et al. combined the high conductivities of polymeric electrolytes with the mechanical stiffnesses of garnet-type oxide electrolytes to develop a composite cathode containing a 90%-PEO/10%-LLZTO SE. The LiFePO₄-based electrode displayed specific capacities of approximately 155, 142, and 118 mAh g⁻¹ at LFP mass loadings of about 6.4, 9.9, and 15.2 mg cm⁻², respectively. These researchers also demonstrated that these composite electrodes outperformed a conventional electrode coupled with a freestanding electrolyte, which delivered only 118 and 30 mAh g⁻¹ at 6 and 10.2 mg cm⁻², respectively.⁹² Chen et al. followed a similar approach by combining PEG, LiN(CF₃SO₂)₂, and a nano-SiO₂ electrolyte with an electrolyte-infused LiFePO₄/ITO pellet composite cathode. The resulting improved ionic and electronic properties led to a specific capacity of 157 mAh g⁻¹ at a LiFePO₄ mass loading of 95 mg cm⁻², which corresponds to an exceptionally high areal capacity of 15 mAh cm⁻².⁸¹ Similarly, Huang et al. reported a hybrid cathode composed of NMC(811)-rich pillars surrounded by a [LiTFSI + PEGMA + MePrPyl TFSI] polymer composite electrolyte with a volumetric CAM-to-SE ratio of 6:4. A specific capacity of 199 mAh g⁻¹ at an NMC811 mass loading of 83.9 mg cm⁻² was achieved using a novel directional freezing and polymerization process by enabling low-tortuosity ion-transport pathways. The associated areal capacity of 16.7 mAh cm⁻² is among the highest reported values for comparable systems and well above the previously established threshold of 3.5 mAh cm⁻².⁷⁸

Sulfide electrolytes

Solid sulfide electrolytes exhibit exceptionally high ionic conductivities and favorable mechanical properties, making them promising candidates for all-solid-state batteries. However, pairing them with high-voltage cathodes is difficult because sulfide electrolytes suffer from narrow electrochemical stability windows, leading to their oxidation. Moreover, continuous operation results in interfacial debonding and rapid capacity loss.⁹⁰ Preventing interfacial reactions is paramount for overcoming cathode/sulfide-electrolyte compatibility issues, which is commonly accomplished by introducing interfacial buffer layers with high ionic and low electronic conductivities between the cathode and solid electrolyte.^93,94

Wang et al. combined an LNO composite cathode with an LPSCl electrolyte using atomic layer deposition to introduce an ultrathin LAZO buffering layer. This coating reduces contact loss and side reactions, thereby enabling high-voltage operation. The resulting full cell features a specific capacity of 180 mAh g⁻¹ at an LNO mass loading of 25 mg cm⁻², which corresponds to an areal capacity of 4.5 mAh cm⁻².⁸²

Active-material utilization can be improved by introducing conducting additives. Kitsche et al. also followed an atomic-layer-deposition strategy and reported an NMC(851005), LPSCl, and carbon black composite cathode with a high specific capacity of 200 mAh g⁻¹ and an areal capacity of 2.1 mAh cm⁻². The applied HfO₂ coating layer enabled high-voltage operation, further highlighting the effectiveness of the buffer layer.⁸⁶

Based on a similar concept, Yang et al. fabricated a soft-carbon cubic-phase nano Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) interface layer that connected a LiZrO₂-coated (LZO-coated) LiCoO₂ (LCO)/LPSCl/VGCF composite cathode to an LPSCl electrolyte. The resulting assembly exhibited improved and uniform Li⁺ migration at the interfacial layer, which suppressed Li-dendrite formation. The enhanced electrochemical performance was accompanied by a specific capacity of 120 mAh g⁻¹ at an LCO mass loading of 111 mg cm⁻² and an exceptionally high areal capacity of 15 mAh cm⁻².⁸³

Doerrer et al. used a different approach to increase cathode/sulfide-electrolyte interfacial stability that involved a single-crystal particulate NMC(830611), LPSCl, and carbon-black (70:27.5:2.5) composite cathode with superior surface interactions and interfacial contact compared to polycrystalline NMC systems; this cathode delivered one of the highest specific capacities reported to date (205 mAh g⁻¹) at an NMC mass loading of 43 mg cm⁻², which translates into an areal capacity of 8.7 mAh cm⁻².⁸⁵

Halide electrolytes

To avoid stability issues associated with the use of sulfide electrolytes, Wang et al. used Li₃InCl₆ in combination with a Li₃InCl₆-coated LiCoO₂ composite cathode. The high ionic conductivities of halide electrolytes and their stabilities toward high-voltage cathodes enabled them to deliver a specific capacity of 131.7 mAh g⁻¹ at an LCO mass loading of 48.7 mg cm⁻², which corresponds to an areal capacity of 6 mAh cm⁻².⁹⁰

Ma et al. achieved an even higher areal capacity of 9.8 mAh cm⁻² with the same electrolyte. Freeze-drying resulted in small Li₃InCl₆ particles, which improved the charge-transmission capabilities of NCM90 and the solid–electrolyte–composite cathode, thereby demonstrating that high-areal-capacity systems are achievable without complicated coatings or cathode compositions.⁶¹

For the study discussed below, we selected composite cathodes with the highest specific and areal capacities, which are indicated in bold in Table 2. A more detailed overview of each design is shown in Figure 4. The colored regions of each column represent the mass portions of each component of the composite cathode, whereas the blue line shows the areal capacity achieved by the respective design. Minimizing the portion of the cathode composed of inactive materials, such as conducting additives or binders, is beneficial for creating a high-energy-densities full battery. However, an optimized CAM/other-component ratio facilitates faster and more homogeneous ion conduction, thereby improving active material utilization. Generally, a high areal capacity at a relatively low mass loading is desirable for maximizing the specific energy of the cell. Thus, composite cathodes (b), (e), (h), and (i) appear to provide the optimal properties required for competitive ASSBs.

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Anodes

The anode within an ASSB is a critical component because it significantly influences key performance parameters, such as energy density, safety, lifespan, and fast-charging ability.⁹⁵ Current research efforts are directed toward developing novel materials and interfaces that address existing challenges and unlock the complete potential of ASSB anode materials.

Lithium metal is an important anode material for an ASSB because it has the highest theoretical capacity and lowest potential among known options.^96–98 Nevertheless, lithium-metal anodes face numerous challenges that must be addressed, which highlights the fact that ASSBs with lithium-metal anodes are far from transitioning from laboratory development to commercial applications.^99,100 Hence, in addition to lithium metal, alternative anode materials, such as insertion anodes (e.g., graphite anodes), alloy anodes (e.g., Si-based anodes), and conversion anodes (e.g., metal hydrides) have been investigated for use in ASSBs.⁹⁹ Although their theoretical capacities may not match that of lithium metal, they offer distinct advantages in terms of mitigating lithium dendrites and enhancing battery safety.^99,100 Liu et al. provide a comprehensive overview of alternative anode materials for use in all-solid-state lithium-ion batteries in their review.⁹⁹

Lithium metal

Owing to its unique characteristics, lithium metal is a prime candidate for use as an ASSB anode material. With a theoretical capacity of 3860 mAh g⁻¹ and the lowest potential (−3.04 V vs. the standard hydrogen electrode) among known anode materials, it is the optimal anode material for SSB use.^96,98 Nevertheless, the substantial volume expansion experienced by lithium during charging, which averages at around 5 μm per 1 mAh cm⁻² of plated Li,¹⁰¹ is a primary obstacle that needs to be addressed. This expansion creates elevated pressure levels, which may result in the lithium metal slowly creeping through the pores of the electrolyte.¹⁰² Such dendrite formation during cycling can lead to short circuits that ultimately result in a shorter battery lifespan.¹⁰³ In addition, lithium-metal anodes face other challenges, including capacity decay, increasing overpotential, and potential safety risks.⁹⁹ Hence, numerous approaches have been proposed to enhance the performance of lithium-metal anodes, including designing artificial solid–electrolyte interfaces, regulating the lithium-ion deposition path, and increasing the buffer layer at the electrode–electrolyte interface.^104–106 Zhang et al. provide a thorough summary of recent strategies aimed at improving Li-metal anodes.¹⁰⁴

Graphite

Graphite has emerged as the predominant anode material for commercial secondary lithium-ion batteries (LIBs) owing to its numerous advantages that include abundant raw-material resources, cost-effective production methods, structural stability, and environmental friendliness.^107,108 In contrast to lithium-metal anodes, graphite anodes are typically exceptionally cycling stable in ASSBs with minimal volume changes.^109,110 Nonetheless, graphite anodes are not without certain limitations, including a comparatively modest theoretical charge capacity (372 mAh g⁻¹¹¹¹) and thermodynamic instability when used with certain solid electrolytes, such as sulfide-based electrolytes.¹¹² In addition, in a similar manner to a lithium-metal anode, the deposition of lithium on the surface of a graphite anode can result in the formation of Li dendrites during charging.¹¹² Recent research efforts have been directed toward overcoming these limitations by reducing the size of the graphite particles to the nanometer scale and exploring the use of graphite-based composites.⁹⁹

Silicon-based anodes

Silicon-based anodes have been extensively researched owing to their high specific capacities (3580 mAh g⁻¹ for Li₁₅Si₄), low redox potentials (0.35 V vs. Li⁺/Li), the high abundance of silicon, and affordability.⁹⁹ As an alloyed anode, silicon exists in a range of alloy phases, including LiSi, Li₁₂Si₇, Li₁₅Si₄, and Li₂₂Si₅.⁹⁹ Unlike liquid lithium-ion batteries, the high stability of the solid electrolyte prevents the formation of a solid–electrolyte interphase (SEI) layer between the silicon anode and the electrolyte. Notably, sulfide solid electrolytes and silicon anodes are thermodynamically stable and highly compatible.⁹⁹ Moreover, the lower operating potential of silicon reduces concerns regarding lithium-dendrite formation, and unlike lithium metal, silicon is stable under ambient conditions, negating the need for a protective atmosphere; hence, making it well-suited for mass production.⁹⁹ However, silicon-based ASSB anodes face several challenges, particularly the substantial effects of volume expansion and their inherently low conductivities.¹¹³ These limitations have been promisingly addressed in recent studies. Decreasing particle size to the nanometer scale is an effective strategy for enhancing the surface contact area and mitigating volume expansion in a Si-based anode during charging/discharging cycles.^99,114 Silicon–carbon composite materials and silicon alloy materials significantly reduce the volume expansion experienced by silicon and its electronic resistance, thereby significantly enhancing the rate capability of the silicon-based anode.^99,114

Conversion anodes

Conversion electrodes that employ the conversion reaction also exhibit promising performance as ASSB anode materials owing to their low potentials and high capacities. The electrochemical mechanism of a conversion electrode is based on the following reaction: ${\mathrm{M}}_a{\mathrm{X}}_b+\left(\mathit{bn}\right){\mathrm{Li}}^{+}+\left(\mathit{bn}\right){\mathrm{e}}^{-}\leftrightarrow a\mathrm{M}+b{\mathrm{Li}}_n\mathrm{X}.$ In this context, M represents a metal or a blend of metals, X represents negatively charged nitrogen, oxygen, sulfur, halogen, or phosphorus ions, and n indicates the oxidation state of X. Conversion electrodes composed of metal oxides, hydrides, nitrides, sulfides, phosphides, fluorides, and other materials have been extensively studied for use in lithium-ion battery applications.

Nevertheless, electrode degradation is a major issue for conversion-electrode materials. Electrodes that rely on the conversion reaction chemically interact with lithium to form new phases with uncontrolled spatial distributions. Therefore, insulating compounds with low conductivities and high overpotentials are formed, which compromise electrochemical reversibility. Furthermore, challenges, such as poor kinetics, limited cycling life, and low reversibility, hinder the potential commercialization of conversion electrodes. Recently, progress toward addressing these issues has been made. Again, reducing the particle size to the nanometer scale has proven to be an effective strategy. Moreover, efforts have focused on modifying the surfaces of conversion electrodes and forming electrolyte-additives layers on the electrodes.

Metal-hydride-based conversion electrodes have demonstrated significant promise as potential ASSB anode candidates owing to their low potentials, minimal polarizations, high specific volumetric and gravimetric capacities, and cost-effectiveness. An ASSB with a metal-hydride-based anode requires an ideal solid electrolyte with excellent conductivity for both lithium and hydrogen ions.⁹⁹ Solid electrolytes that meet these criteria include LiBH₄ and Li₂S–P₂S₅. Among metal hydrides, MgH₂ stands out owing to its high theoretical specific capacity of 2038 mA h g⁻¹, an operating potential of 0.5 V (compared to Li/Li⁺), cost-effectiveness, and natural abundance in nature, and has become a key research focus as an ASSB anode.⁹⁹ However, despite its numerous advantages, SSBs that use MgH₂ exhibit poor charge/discharge reversibilities.

Anode-free ASSBs

An anode-free ASSB is a lithium-metal battery in which a lithium anode is formed during the first charging cycle. Specifically, lithium ions from the cathode are reversibly plated onto a bare current collector as lithium metal during the initial charging cycle.^111,115 This setup offers several advantages over current lithium-based batteries, such as higher volumetric and gravimetric energy densities, better cell safety, simpler manufacturing methods, and enhanced lifespans.^111,115 However, anode-free SSBs still encounter significant challenges, including rapid capacity degradation,¹¹¹ whose primary causes include low coulombic efficiency and a gradual increase in the overpotential of the lithium-metal anode.¹¹¹ Furthermore, the substantial volume changes within an anode-free ASSB caused by lithium plating and stripping severely affect cycling stability and shorten the lifespan of the battery cell.¹¹⁵ Additionally, anode-free ASSBs are subjected to non-uniform lithium-ion fluxes during lithium plating.¹¹¹ Current research is directed toward resolving these issues by developing SEs with high ionic conductivities, forming passivated layers that are kinetically inert and stable against lithium metal at the interface, and broadly modifying the electrolyte–anode interface. Huang et al. presented a comprehensive overview of recent strategies aimed at enhancing anode-free ASSBs.¹¹¹

In recent years, various anode materials have been investigated for use in ASSBs. The main research areas are discussed above. The structure and size of the active material, use of a binder, selection of a suitable solid electrolyte, and the manufacturing process significantly affect the electrochemical properties of the anode.⁹⁹

Graphite, lithium-metal, and anode-free ASSBs were selected for further discussion. Graphite was selected because it is the predominant anode material used in commercial secondary LIBs today and serves as a lower benchmark limit. Li metal has exceptional characteristics; hence it is the prime candidate for use as an ASSB anode material. With a theoretical capacity of 3860 mAh g⁻¹ and the lowest potential (−3.04 V vs. the standard hydrogen electrode) among known anode materials, it theoretically provides the optimal ASSB-anode-material solution. Anode-free ASSBs were selected because they show promising future potential. Although still in their early stages, anode-free ASSBs offer several advantages over current lithium-based batteries, including even higher volumetric and gravimetric energy densities than those of ASSBs with lithium-metal anodes.

Economic aspects

Estimating the costs associated with anode materials, such as graphite, silicon-based anodes, and conversion electrodes, presents considerable challenges, particularly when compared against the anode-production costs of current LIBs. Nevertheless, research efforts have focused on estimating the potential costs associated with producing ASSBs with lithium-metal anodes, as well as anode-free ASSBs. Heubner et al. provide preliminary cost assumptions in their study.¹¹⁶

The high reactivity of lithium components with ambient air necessitates stringent dry-room conditions for ASSBs with lithium-metal anodes, resulting in significant acquisition, operating, and maintenance costs.¹¹⁶ Additionally, fabricating sufficiently thin lithium foil with acceptable purities and microstructures over large areas is extremely challenging and expensive, potentially leading to lithium foil prices in the 300–400 USD kg⁻¹ range, or even up to 1000 USD kg⁻¹ for films with advantageous thicknesses (≪100 μm). However, currently, there are no available models or practical experiences that enable costs to be reliably compared on an industrial scale. Consequently, whether or not these costs are competitive with those associated with currently used LIB anode materials remains uncertain.¹¹⁶

Anode-free ASSBs offer substantial processing-cost advantages by eliminating active-material-related anode-manufacturing steps that contribute 12%–18% to production costs during LIB manufacturing.¹¹⁷ Additionally, the removal of the anode coating reduces material consumption and simplifies cell production, further lowering costs. Moreover, the process conditions are significantly less demanding because lithium metal is absent during assembly, thereby considerably reducing acquisition, operating, and maintenance costs. However, no models or practical experiences currently exist that facilitate industrial-scale costs to be reliably compared.¹¹⁶

Interfacial issues

Although reasonably high SSE ionic conductivities have been reported, the internal resistances of the corresponding ASSBs remain large, which is mainly ascribable to the interfacial charge-transfer resistance associated with Li ions. Dendrite formation and significant anodic volume expansion have been identified as the primary issues on the anode side. In particular, Si-based anode materials experience substantial volume expansions during charging and discharging cycles, resulting in fracturing and delamination.⁹⁹ In the case of a lithium-metal anode, volume expansion leads to higher pressure levels that can result in the lithium metal gradually infiltrating electrolyte pores.¹⁰² Furthermore, the SSE must be thermodynamically stable with respect to the anode to maintain low interfacial impedance over multiple cycles. This stability is achieved when the anode potential falls within the electrochemical window of the SSE; the SSE is reduced by the anode if this condition is not met, indicative of an inherent incompatibility between the anode material and the SSE.⁴² Recently, Ma et al. comprehensively reviewed this topic and discussed the compatibility of the anode/electrolyte interface in detail.¹¹⁰

The cathode/electrolyte interface mainly suffers from interfacial instability and poor physical contact that arise from poor mechanical stability and charge-layer formation owing to chemical and electrochemical instability, resulting in the oxidation and decomposition of the SE. Challenges associated with these issues and their solutions are discussed thoroughly in the review articles of Jiang et al. and Lou et al.^118,119 Promising solutions involve the previously discussed introduction of protective coatings and buffer layers that enable the use of high-voltage cathodes without compromising stability. Furthermore, electrode composition and processing, structural design, and active-material morphology play essential roles. Although not as severe in cathodes as in anodes, volume expansion can harm mechanical stability, especially at low stack pressures, necessitating the use of low-volume-change CAMs and buffer layers.¹²⁰ The interested reader is referred to several dedicated reviews.^121–123

ENERGY DENSITIES OF ASSBs WITH STATE-OF-THE-ART MATERIALS

The gravimetric energy densities of hypothetical full cells were estimated using various previously introduced cathode designs to determine ASSB feasibility based on commercially available solid electrolytes. A simple cell design with copper and aluminum current collectors and a Li-metal anode was considered in these calculations. The gravimetric energy density, E_spec, of the battery is defined as the ratio of the total energy, E, to its total mass, M, and the total energy of the battery can be calculated as the product of the total capacity, Q, and cell voltage, V. However, because total values are usually not readily available, areal densities are preferred in these calculations; consequently, the areal capacity, q_A, and the areal mass density of each component, m_A,x, were used. For simplicity, the battery was assumed to be mass-balanced with equal anodic and cathodic areal capacities. Owing to the very low weight of the Li-metal anode (usually <5% of the cell mass), the anode mass has relatively little influence. Although anode-free cells can achieve higher energy densities, this difference is negligible. Consequently, the following equation can be used to calculate gravimetric energy density: $E_{\mathit{\text{spec}}}=\frac{q_A·V}{m_A}=\frac{q_A·V}{m_{A,\mathit{\text{Cath}}}+m_{A,\mathit{\text{Anode}}}+m_{A,\mathit{\text{Separ}}}+m_{A,\mathit{CC}}},$ with the cathodic areal capacities listed in Table 2. The areal mass densities are defined as follows:

• m_A,Cath: Value taken from Table 2 divided by the active-material ratio in the cathode.
• m_A,Anode: Calculated as $\frac{q_A}{q_{\mathit{\text{spec}},\mathit{Li}}}$ with the specific capacity of lithium, $q_{\mathit{\text{spec}},\mathit{Li}}=3.86$ mAh mg⁻¹.
• m_A,Separ: Calculated as $\rho ·d$, using the density, $\rho$, and the thickness, d, of the solid electrolyte separator ($\rho =3.05$ g cm⁻³ for LICGC™,¹⁸ $\rho =5.26$ g cm⁻³ for LLZTO,¹²⁴ $\rho =1.13$ g cm⁻³ for PEO,⁹¹ $\rho =1.64$ g cm⁻³ for LPSCl¹²⁵).
• m_A,CC: Current collector materials: $\rho =8.9$ mg cm⁻² for 10 μm copper foil and $\rho =2.7$ mg cm⁻² for 10 μm aluminum foil.¹²⁶

Respective cell voltages of 3.4 V (LFP), 3.75 V (NCM) and 4.0 V (LCO) were used.¹²⁷

The resulting gravimetric energy densities of the various cathodes are plotted against CAM mass loadings in Figure 5. Only reported cathodes with areal capacities and mass loadings that lead to energy densities comparable to those of commercial LIBs are included in the graph. The colors of the lines and symbols correspond to the material classes of the separator and cathode additive, respectively. Therefore, a matching line and symbol color implies that the same or at least similar electrolytes are used for both parts of the cell. It should be noted that different materials can be combined; however, interactions that impair stability or charge transport cannot be excluded. The horizontal dotted lines indicate two thresholds: 250 Wh kg⁻¹ for state-of-the-art conventional LIBs, and 360 Wh kg⁻¹ for the already commercialized quasi-ASSB by WeLion currently used in NIO electric vehicles.¹²⁸

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Thin and low-density solid electrolytes are clearly key to surpassing the aforementioned thresholds. For example, cells with 700- or 300-μm-thick LLZTO electrolytes cannot deliver energy densities above ~200 Wh kg⁻¹, and a significantly thinner (~30 μm) electrolyte is required to exceed 360 Wh kg⁻¹, which may prove challenging considering the brittleness of LLZTO. Adding low-density and more-flexible PEO dramatically increases the maximum energy density to ~375 Wh kg⁻¹, even for thicker (50–100 μm) electrolytes. However, ultra-thin 20 μm LICGC™ delivers the highest specific energy among the commercially available options. Owing to their comparably low densities, LPSCl-based cells deliver even higher energy densities at similar thicknesses, which seems feasible as 35–120-μm-thick electrolyte films have already been successfully prepared in the past.^129–131

The highest energy density was achieved using the design reported by Doerrer et al.,⁸⁵ who used monocrystalline NCM to deliver an exceptional specific capacity of 205 mAh g⁻¹, which corresponds to almost 100% active-material utilization. This strategy facilitates the use of relatively low cell mass while maintaining high areal capacity. The specific energy is expected to exceed 400 Wh kg⁻¹ when combined with 20 μm LPSCl as a compatible electrolyte separator. However, it should be noted that the use of monocrystalline NCM is associated with higher material costs because it is synthesized in a more complex manner than commonly employed polycrystalline materials. A comparison of approaches that use polycrystalline CAMs reveals that mass loadings between ~70 and ~100 mg cm⁻² appear to yield optimal performance, with values close to 400 Wh kg⁻¹ also achievable within these limits. Yang et al. reported⁸³ that higher mass loadings lead to lower material utilization owing to the long ion-conduction pathways within the electrodes; consequently, the additional inactive mass effectively decreases the gravimetric energy density of the full battery.

Oxide-based ASSBs deliver the best performance by combining a commercially available 20 μm LICGC™ glass electrolyte with the cathode preparation strategies reported by Huang et al.⁷⁸ and Chen et al.,¹² who both suggested combining polymers with inorganic oxide materials, thereby rendering this approach as a promising option for achieving energy densities close to 400 Wh kg⁻¹. Similar results can be achieved by potentially improving the compatibility between the composite cathode and the solid electrolyte using the same separator composition (LLZTO + PEO).

To put these results into perspective, the energy densities of ASSBs used in electric vehicles can be estimated by considering the average ranges of vehicles powered by state-of-the-art LIBs. According to the International Energy Agency, BEVs have an average driving range of 350 km with a single charge, which corresponds to ~54% of the median range of gasoline-powered cars.¹³² Assuming an average energy density of 250 Wh kg⁻¹ for current-generation automotive batteries, ranges of 490–560 km should be feasible with ASSBs based on the results presented above. Therefore, delivering energy densities of between 450 and 500 Wh kg⁻¹ is considered to be a future goal based on these estimations, as it would enable ranges similar to those of conventional combustion-engine-based vehicles.

Limitations arising from the use of conventional Li-ion batteries, particularly in electric vehicles, have prompted the industry to investigate high-energy-density ASSBs. In this regard, QuantumScape and ONE follow cost-effective, anode-free approaches to yield energy densities similar to those suggested in this study.^7,8 Optimization that affords lower prices is another key factor for commercial applications; hence, the slightly lower gravimetric energy densities can be rationalized in terms of more cost-effective production procedures. CATL announced a “condensed” semi-solid-state battery with an exceptional energy density of 500 Wh kg⁻¹, which surpasses the values discussed in this review.⁹ The undisclosed chemistry, however, includes non-solid components, most likely liquid interfacial layers that improve ionic-conduction pathways in the cathode. In addition to achieving competitive energy densities, other factors, such as cycling stability, safety, and production costs, play important roles in evaluating ASSB viability. High capacity retention during frequent battery charging and discharging is a key factor that determines battery performance. The reported cycling lives of batteries that employ cathode designs used to evaluate potential energy densities are shown in Figure 6. With more than 98% capacity retention after 4000 cycles at a high cycling rate of 25 C, the cathode proposed by Yang et al. demonstrated the best cycling stability. The same design maintained 94% of its initial capacity after 350 cycles at 1 C.⁸³ Doerrer et al. reported a capacity retention of 96% after 50 cycles at 0.5 mA cm⁻² for their high-energy-density single-crystal cathode.⁸⁵ Similarly, Huang et al. and Wang et al. carried out relatively short stability testing for 200 cycles at 0.5 and 0.1 C, respectively, whereas Chen et al. did not report any cycling-stability data.⁸¹ Therefore, longer cycling is required to determine how economically competitive these batteries are.

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Furthermore, evaluating production expenses and costs linked to each cathode and battery design is of utmost importance when assessing the possibility of realistically implementing them in the market. While exactly quantifying material and processing costs on an industrial scale is nearly impossible, it appears likely that manufacturing methods compatible with already existing machinery used to produce electrodes for conventional LIBs are the most economically viable. Although the design proposed by Wang et al. (entry b in Table 2) enables high energy densities, the directional polymer-freezing technique is significantly more intricate than established slurry-casting and calendaring methods used to produce LIBs.⁹⁰ Other approaches that lead to similar energy densities, such as those reported by Chen et al. (entry e in Table 2), Liang et al. (entry h in Table 2), and Doerrer et al. (entry i in Table 2), rely on much simpler powder-pressing methods.^81,84,85 The design reported by Chen et al. (entry e in Table 2) appears to be the most cost-efficient among those that use high-energy-density cathodes as it uses less expensive LFP instead of NCM or LCO and does not rely on additional coatings. In general, composite-cathode preparation for ASSBs is expected to be costlier than the cathode manufacturing processes used for conventional LIBs because it involves a high-temperature sintering step in most cases. The anode and electrolyte must be considered when assessing the production costs of a full ASSB. Although lithium prices are subject to large fluctuations, they affect ASSBs and LIBs equally. Consequently, the anode does not significantly affect the economic competitiveness of solid-state batteries that use Li-metal or anode-free configurations. The costs associated with the SE itself are difficult to estimate, as they depend highly on the production scale and method.

Schnell et al. performed bottom-up calculations that focused on sulfide- and oxide-based electrolytes to assess the competitiveness of ASSBs with conventional LIBs.¹³³ To that end, they not only addressed the developmental forecasts for ASSBs but also accounted for progress in LIB production. In addition to omitting the electrolyte filling and simplifying the formation procedure, they identified bipolar stacking (in a similar manner to fuel cell applications) as a potential way of decreasing costs per battery module compared with the classical parallel stacking configurations used in LIBs. Using this approach in combination with an NMC811 cathode and a lithium-metal anode led to predicted manufacturing costs between 132 and 86 USD kWh⁻¹, which is expected to break even with a LIB that uses a Si/C anode if the price of the LPS sulfide separator can be reduced to about 50 USD kg⁻¹. ASSBs using LLZ oxide separators only facilitate a 267 to 123 USD kWh⁻¹ reduction, which is not expected to be price competitive with Si/C-anode-based LIBS, even if the LLZ price were to be reduced to 10 USD kg⁻¹. The Nissan Motor Corporation aims to reduce the price of ASSBs for electric vehicles even further, to 75 USD kWh⁻¹ by 2028 and has projected projects even lower prices of 65 USD kWh⁻¹ thereafter, which would make electric vehicles as cost-effective as their gasoline-powered counterparts.¹³⁴ However, these predictions only serve as rough estimates because they depend heavily on the assumed technological learning rate. Currently, the manufacturing costs of ASSBs are significantly higher than those of LIBs, and efforts to upscale production to the industrial level are required in order to establish realistic values. Adopting current industrial methods for producing LIBs is paramount for cost-effective upscaling by making use of existing infrastructure wherever possible. However, major changes to conventional LIB-production are necessary; hence, concurrently developing the product and process is expected to yield the best results for ASSB upscaling.^133,135 To assess scalability, Zhang et al. constructed three 1-Ah ASSBs using Ohara LICGC™ powder together with PEO-LiTFSI as the polymer matrix. ASSB cells were composed of a composite electrolyte, NMC622 cathodes, and MCMB (MesoCarbon MicroBead) graphite anodes. In doing so, these researchers demonstrated the scalable production of practical ASSBs that exhibit consistent electrochemical performance with confirmed safety.¹³⁶

CONCLUSION

In this review, we discussed commercially available state-of-the-art materials for solid electrolyte separators and calculated the potential energy densities of ASSBs with respect to currently feasible material thicknesses. This approach separately calculates potential energy densities for selecting and combining different electrode materials. Although various options have been presented for the anode, we focused on promising materials, such as lithium metal and the anode-free ASSB concept. Cathodes are more thoroughly examined in this review, which addressed compatibility issues arising from their composite nature. Together, our findings revealed the highest possible theoretical energy densities for different material combinations. Additionally, we discussed interfacial measures, such as coatings between electrolytes and electrodes, as well as safety aspects and upscaling potentials.

Among the separators discussed in this review, LPSCl exhibited the highest ionic conductivity. However, manufacturing a thin membrane requires a support framework for mechanical stability and lithium compatibility, which can significantly reduce the conductivity, depending on the material used. LLZTO separators also exhibit relatively high ionic conductivities; however, they require interlayer materials to ensure good contact with lithium and to protect against the atmosphere. Furthermore, commercial LLZTO electrolytes must also be thin. The PEO-LLZTO hybrid electrolyte is highly stable with lithium and can easily be modified by incorporating additional materials in order to optimize other parameters. Ohara LICGC, which has the advantages of being commercially available and highly stable toward air and water, still has low ionic conductivity as its major drawback.

Cathodes paired with composite polymer-oxide electrolytes and sulfide electrolytes that use interfacial layers exhibit the highest areal capacities, with LPSCl, PEO, and PEGMA notable examples of such electrolytes. Halide electrolytes also exhibit relatively high areal capacities; they also enable simpler systems devoid of complicated coatings and cathode compositions owing to their high-voltage stabilities. Furthermore, composite cathodes containing the respective solid electrolytes are crucial for achieving high areal capacities regardless of the type of solid electrolyte separator used. NMC, LCO, and LFP are CAMs that can be used to fabricate high-areal-capacity composite cathodes.

Various anode materials for use in ASSBs have been investigated in recent years. Lithium metal is an essential anode material because it exhibits the highest theoretical capacity and lowest potential among known options. However, lithium-metal anodes still face many challenges that must be overcome, which highlights the fact that ASSBs with lithium-metal anodes are far from transitioning from laboratory development to commercial applications. Therefore, aside from lithium metal, alternative anode materials, such as insertion anodes (e.g., graphite anodes), alloy anodes (e.g., Si-based anodes), and conversion anodes (e.g., metal hydrides), have been investigated for ASSB use.⁹⁹ Although their theoretical capacities do not match those of lithium metal, they offer distinct benefits in mitigating lithium dendrites and enhancing battery safety. The anode-free-ASSB concept has shown significant potential for the development of next-generation ASSBs, despite being in its early stages.

Overall, our selection of various material combinations and subsequent calculations show that ASSBs with exceptional energy densities that far exceed 250 Wh kg⁻¹ appear feasible using market-available thin solid-state electrolytes and state-of-the-art design strategies for high-capacity composite cathodes. In conclusion, our feasibility study showed that ASSBs with gravimetric specific energies of approximately 350 and 400 Wh kg⁻¹ can be realistically produced based on the current state of research and technology.

ACKNOWLEDGMENTS

We thank the German Research Foundation (DFG) under Germany's excellence strategy—EXC 2089/1-390776260 (“e-conversion”) 3.4 V (LFP). Open Access funding enabled and organized by Projekt DEAL.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

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