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Recent Research Progress on All-Solid-State Mg Batteries

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07 October 2023

Posted:

10 October 2023

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Abstract
Current Li battery technology employs graphite anode and flammable organic liquid electrolytes. Thus, current Li battery is always facing the problems of low energy density and safety. Additionally, sustainable supply of Li due to scarce abundance of Li sources is another problem. All-solid-state Mg battery is expected to solve the problems owing to non-flammable solid-state electrolytes, high capacity/safety of divalent Mg metal anode and high abundance of Mg sources. Therefore, solid-state electrolytes and all-solid-state Mg battery have been researched intensively last two decades. However, realization of all-solid-state Mg battery is still far. In this article, we review recent research progress on all-solid-state Mg battery so that researchers can pursue recent research trend of all-solid-state Mg battery. At first, solid-state electrolyte research is described briefly in the categories of inorganic, organic and inorganic/organic composite electrolytes. After that, the recent research progress of all-solid-state Mg battery is summarized and analyzed. To help readers, we tabulate electrode materials, experimental conditions and performances of all-solid-state Mg battery so that the readers can find necessary information at a glance. In the last, challenges to realize the all-solid-state Mg batteries are visited.
Keywords: 
magnesium battery
;  
solid electrolyte
;  
ceramic electrolyte
;  
polymer electrolyte
;  
all-solid-state Mg battery
Subject: 
Chemistry and Materials Science  -   Electrochemistry

1. Introduction

Since Li-ion battery (LIB) was commercialized in 1991, its application into portable electronic devices such as laptop computers and mobile phones has been widely achieved which has affected our daily life significantly [1,2]. As the most successful battery technology, LIBs possess several advantages including high energy density, no memory effect, good capacity retention, etc., overcoming last generation lead-acid and nickel hydrogen batteries [3,4]. Current LIBs rely on the intercalation mechanism. The energy density of LIBs has reached 240 Wh kg-1 and 670 Wh L-1 at the cell level due to the innovation and development of materials and cell design in these two decades [5,6]. However, the inherent limitation in the theoretical capacity of current graphite-based anodes makes LIBs be almost impossible to meet the increasing demand for energy density [7]. Li metal anode is an ideal anode material due to its ultimate high theoretical capacity (Table 1) which can improve the energy density of the batteries. Typically, Li-LMO cells (LMO means Li transition metal oxides) have revealed high energy density of ~ 440 Wh kg-1 [8]. However, dendric growth of Li metal and scarce abundance of Li source have hindered commercialization of the Li metal anode.
Solid-state electrolytes which are solid-state ion conductor can suppress the dendrite growth due to their high mechanical strength [9]. In addition, their inflammable nature and wide electrochemical window can improve the safety and energy density of LIBs. Therefore, solid-state electrolytes and all-solid-state Li batteries have been researched intensively especially in the last decade [10]. Regarding the low abundance of Li, it is considered that high abundance elements such as Na, K, Mg, Ca, Zn and Al are employed as a charge carrier instead of Li. Particularly, metal anodes of multivalent ions (Mg2+, Ca2+, Zn2+, Al3+) possess higher volumetric capacities than monovalent Li metal anode. Table 1 summarizes representative properties of metal anodes. Mg has high abundance in the earth crust, high volumetric capacity and relatively low redox potential. Mg metal was believed to not form dendrite. Although the Mg dendrite formation was found in 2017 [11], less prone to dendrite formation of Mg metal comparing to Li and other metals is verified in both experimental and theoretical studies [12,13], making Mg metal be an ideal anode material. Therefore, research on all-solid-state Mg battery, Mg2+-ion conductive solid-state electrolytes have been intensively carried out recently[14,15].
Solid-state electrolytes are categorized into three groups, organic, inorganic and organic-inorganic composite [16]. Typically, organic solid-state electrolytes are flexible and easy to process for large-scale production. Contrary, inorganic solid-state electrolytes commonly possess high ionic conductivity, high transference number, wide electrochemical window. The organic-inorganic solid-state electrolytes are proposed and developed to address the defects of organic and inorganic solid-state electrolytes which can combine the advantages of the flexibility and easy processing of organic solid-state electrolytes and high conductivity of inorganic solid-state electrolytes [17]. Some good review articles focusing on solid-state electrolytes have been published recently [18,19], while research on all-solid-state Mg batteries has yet to be reviewed.
Therefore, in this review article, we focus on recent, especially there five years (2018 ~), research on all-solid-state Mg battery. At first, solid-state electrolyte research is described briefly in the categories of inorganic, organic and inorganic-organic composite electrolytes. After that, the recent research progress of all-solid-state Mg battery is summarized and analyzed. To help readers, we tabulate electrode materials, experimental conditions and performances of all-solid-state Mg battery so that the readers can find necessary information at a glance. In the last, challenges to realize the all-solid-state Mg batteries are visited.

2. Solid electrolytes for Mg battery

2.1. Inorganic electrolyte

2.1.1. Oxides

Na+ super ion conductor (NASICON), Na1+xZr2P3-xSixO12 is well-known for permitting fast migration of Na+ ion due to the well-ordered three-dimensional network structure [20,21]. By the successful application of NASICON structure to Li+ ion conductive ceramic electrolyte such as LiZr2(PO4)3 [22,23], Li1+xAlxTi2-x(PO4)3 [24,25], and Li1+xAlxGe2-x(PO4)3 [26,27], it has been highly interested in developing NASICON-type multivalent ion conductors [28,29].
The first report on NASICON-type Mg2+ ion conductors was MgZr4(PO4)6 (MZP) in 1987 [30]. The ionic conductivity was 2.9 × 10-5 and 6.1 × 10-3 S cm-1 at 400 and 800 oC, respectively. MZP was assigned as the rhombohedral structure at first, but later it was ascribed to the monoclinic structure, which is like β-Fe2SO4. Nakayama et. al. simulated Mg2+ migration energy of 0.63 and 0.71 eV in the rhombohedral and monoclinic structures, respectively [31]. Thus, heteroatom doping into Zr4+ site to stabilize the rhombohedral structure has been carried out [32,33]. Contrary, another strategy, introduction of Hf4+ into Mg2+ site has also been attempted [34]. However, all results exhibited too low Mg2+ ion conductivity, ~ 10-5 S cm-1 even at 500 oC. Those cannot be applied for all-solid-state Mg battery operated at ambient temperature. It is noted that some promising results were reported by Mohamed’s group. They reported σ = 3.97 × 10-4 S cm-1 at room temperature in Mg1.05Zn0.4Al0.3Zr1.3(PO4)3. However, this material demonstrated almost the same conductivity, 5.82 × 10-4 S cm-1 at 500 oC [35]. The extremely low activation energy of 0.039 eV was about one order of magnitude lower than that of typical Li+ ion conductive ceramics. They also estimated Mg2+ ion transference number by the Bruce method to 0.84 [33]. This implies 16 % of electric charge is carried by anion, i.e. oxygen ion. In addition, they measured electrochemical window of Mg0.5Si2(PO4)3 to 3.21 V, however, as shown in Figure 1, it seems decomposition of electrolyte commences below 2 V [36]. These data are quite weird. We need to re-check their results.
In other research, although magnesium phosphate (Mg2.4P2O5.4) [37], magnesium silicate (Mg0.6Al1.2Si1.8O6) [38], magnesium tangstate (MgHf(WO4)3) [39] are researched, still a significant improvement of conductivity is needed. Indeed, all-solid-state Mg batteries using oxide-based solid electrolyte have not been reported yet.

2.1.2. Chalcogenides

Although chalcogenide-based Li+ ion conductive ceramics like sulfides have succeeded greatly [40], chalcogenide-based Mg2+ ion conductive ceramics were not researched intensively. Only one paper was published about MgS-P2S5 system in 2014 [41]. However, since Canepa et. al. reported trinary spinel chalcogenides with high Mg2+ ion mobility in 2017 [42], research on the chalcogenide-based Mg2+ ion conductive ceramics has been activated. They predicted the least migration energy of Mg2+ ion appeared in MgY2S4, MgY2Se4 and MgSc2Se4 and the values were 360, 361 and 375 meV, respectively (Figure 2). However, only MgSc2Se4 has been successfully synthesized so far. The Mg2+ ion conductivity of MgSc2Se4 was estimated to ~ 1 × 10-4 S cm-1, comparable to Li+ ion conductive ceramic electrolytes (garnet-type and NASICON-type) [43]. Unfortunately, electronic conductivity was also relatively high about 4 × 10-8 S cm-1. Thus, Fichtner et. al. synthesized Se-excess, Ti4+, Ce4+-doped MgSc2Se4 to reduce the electronic conductivity, but the electronic conductivity was not drastically lowered [44]. Based on this, they used MgSc2Se4 as a cathode material for Mg battery using liquid electrolyte. Kundo et. al. studied electronic conduction mechanism of MgSc2Se4 and found electronic conductive layer was formed on the surface of particles during ball-milling process [45]. In fact, the electronic conductivity was reduced by avoiding ball-milling process. In addition, the ionic and electronic conductivities of MgSc2Se4 were largely influenced by sintering process, particularly cooling process. Indeed, the field-assisted synthesis could sinter MgSc2Se4 in very short time, leading to low electronic conductivity of ≈ 10-11 S cm-1 [46]. Lowering the electronic conductivity of MgSc2Se4 is a critical issue to apply for all-solid-state Mg batteries. Advanced sintering techniques which can provide rapid heating/cooling rates and short heat treatment, such as spark plasma sintering (SPS) [23], flash sintering [47], microwave sintering [48] and ultrafast high temperature sintering [49] should be applied for sintering MgSc2Se4.
Electrochemical window of MgSc2Se4 has not been studied yet, but Au/MgSc2Se4/Au cell was stable during 3 V of applied voltage [46]. Therefore, MgSc2Se4 would possess reasonable electrochemical window for all-solid-state Mg battery application.
The development of MgSc2Se4 has two directions, electrolytes and electrodes. The electronic conductivity of MgSc2Se4 must be lowered for electrolytes while it is maintained for electrodes. The all-solid-state Mg battery composed of MgSc2Se4-based electrode and MgSc2Se4-based electrolyte should have an intimate electrode/electrolyte interface due to similar chemical composition and structure, resulting in high and stable performances.
Properties of oxide- and chalcogenide-based electrolytes are summarized in Table 2.

2.1.3. Hydrides

In 2012, Matsuo et. al. reported possible Mg conduction in Mg(BH4)2 based on FPMD simulation [61]. Later, Higashi et. al. experimentally proved Mg ion conduction of Mg(BH4)2 and Mg(BH4)(NH2) [62]. Since then, Mg(BH4)2-based electrolytes have been researched the most intensively among the inorganic solid electrolytes. Also, all-solid-state Mg battery with inorganic electrolytes has been fabricated by using only Mg(BH4)2-based electrolytes (Chapter 3.1). Although the ionic conductivity of Mg(BH4)(NH2) is 1 × 10-6 S cm-1 at 150 oC [62], it is influenced by the synthetic parameter, for example, the ionic conductivity of 3 × 10-6 S cm-1 at 100 oC was obtained in glass-ceramic like Mg(NH4)(NH2) [63]. As other strategies, modification of BH4 ligands [64,65], partial oxidation [66], compositing with ceramic oxides such as MgO, YSZ, TiO2 and Al2O3 [67-70] have been attempted and all achieved improvement of Mg ion conductivity to 10-5 ~ 10-6 S cm-1 at ambient temperature which is slightly lower than those of Li and Na ion conductive inorganic solid electrolytes [10]. Additionally, they exhibited stable Mg plating/stripping behavior (Figure 3). However, the modification narrowed the electrochemical window to about 1.2 ~ 1.4 V. This restricts the choice of cathode materials and decreases the energy density of all-solid-state Mg batteries. Although the moderate ionic conductivity and stability against Mg metal anode of Mg(BH4)(NH2)-based inorganic solid electrolytes are desirable, improvement of anodic stability must be considered.
Table 3. Properties of hydride-based solid electrolytes.
Table 3. Properties of hydride-based solid electrolytes.
Electrolyte σtotal
(S cm-1)		 Temperature (oC) Activation energy (eV) Electrochemical window (V) Ref
Mg(BH4)2 1 × 10-9 150 - - [62]
Mg(BH4)(NH2) 1 × 10-6 150 - 3
Mg(en)1(BH4)2 5 × 10-8 30 1.6 1.2 [64]
6 × 10-5 70
Mg(BH4)(NH2) glass ceramics 3 × 10-6 100 1.3 - [71]
Mg(BH4)2 1.6NH3-75 wt.% MgO 1.2 × 10-5 RT 1.12 1.2 [69]
Oxidized Mg(BH4)2 7.89 × 10-6 RT - - [66]
Mg(BH4)2 1.5THF-75 wt.% MgO 9.8 × 10-7 30 1.4 1.2 [70]
1.7 × 10-4 70
Mg(BH4)2(NH3BH3)2 1.3 × 10-5 30 1.47 1.2 [65]
Mg3(BH4)4(NH2)2 4.1 × 10-5 100 0.84 1.48 [72]
Amorphous Mg(BH4)2 2NH3 5 × 10-4 75 1.99 1.4 [73]
Mg(BH4)2 1.5NH3-60wt.% YSZ 3 × 10-4 50 - 1.3 [67]
Mg(BH4)2 1.5NH3-60wt.% TiO2 1.12 × 10-3 50 0.87 -
Mg(BH4)2 1.6NH3-67 wt.% Al2O3 2.5 × 10-5 22 0.56 1.2 [68]

2.1.4. MOF (Metal-organic framework)

MOFs are crystalline solids composed of metal ions coordinated by multifunctional organic molecules with a three-dimensional porous structure. The composition and structure of MOFs can be easily adjusted via the rational selection of the metal ion and organic molecules [74]. Due to the porous structure, diffusivities of guest ions in the pores would be similar to those in molten salt state [75]. Therefore, MOFs have been studied as ionic conductors. To introduce guest ions, pores of MOFs are filled with liquid electrolytes. Thus, MOF-based solid electrolytes would be categorized into liquid-solid composite electrolyte.
Comparing to MOF-based Li+ ion conductive electrolyte, the study for Mg2+ ion conductor is still few, only 8 papers have been published so far. The conductivity of MOF-based Mg ion conductive electrolytes ranges from 10-4 ~ 10-6 S cm-1. Since MOF-based solid electrolytes contain liquid electrolytes, the transference number of Mg2+ ion should be studied as well as their electrochemical window. Only three papers reported those, 0.25 ~ 0.49 of transference number and about 3 V vs. Mg/Mg2+ of oxidative stability [76-78]. Contrary, stable Mg plating/stripping behavior was observed in 4 papers (Figure 4) [76-79]. Therefore, Mg metal anode can be applied for the MOF-based electrolytes. Typically, MOF-based Mg ion conductive electrolytes contain around 45 ~ 55 wt.% of solvent which is comparable to gel-polymer electrolytes [80], however, their conductivities and mechanical properties are not inversely proportional. Hassen et. al. reduced the liquid content in MOFs to around 20 wt.% and reported ionic conductivity was not primarily affected [76]. Also, the same group found conductivity enhancement by treatment of MOF at 150 oC for 24 h due probably to removal coordinated water.
The MOF-based electrolytes are likely to be stable for Mg metal anode, relatively high anodic stability, ~ 3 V and high conductivity which makes them a good candidate for all-solid-state Mg batteries. However, there are still a lot of unknowns. Particularly, the correlation of pore structure and composition of liquid electrolyte (salt, solvent, and salt concentration) with chemical/electrochemical properties must be clarified.
Table 4. Properties of MOF-based solid electrolytes.
Table 4. Properties of MOF-based solid electrolytes.
MOF Liquid electrolyte σtotal
(S cm-1)		 Temperature (oC) Activation energy (eV) Ref
Mg2(dobpdc) Mg(TFSI)2/triglyme 1.3 × 10-4 RT 0.11 ~ 0.19 [80]
Mg(OPhCF3)2+Mg(TFSI)2/triglyme 2.5 × 10-4 RT
MIT-20 MgBr2/PC 8.8 × 10-7 RT 0.37 [81]
Cu4(ttpm)2∙0.6CuCl2 MgCl2/THF 1.2 × 10-5 RT 0.32 [82]
MgBr2/THF 1.3 × 10-4 RT 0.24
MOF-74 Mg(TFSI)2/MgCl2/DME 3.17 × 10-6 RT 0.53 [79]
Mgbp3dc α-Mg3(HCOO)6/DMF 3.8 × 10-5 RT 0.669 [76]
UiO-66 Mg(TFSI)2/[EMIM][TFSI] 5.8 × 10-5 RT 0.67 [77]
MOF-177 Mg(TFSI)2/diglyme 1.6 × 10-5 RT 0.33 [78]
MIL-101 Mg(TFSI)2 + MeCN vapor 1.9 × 10-3 25 0.18 [83]

2.2. Organic electrolyte

Organic electrolytes, that is, polymer electrolytes, are composed of polymer hosts and Mg salts (solid polymer electrolytes, SPEs). In some cases, fillers and plasticizers are added to improve the properties. SPEs have been reported the most, while SPEs with plasticizers are most widely employed for all-solid-state Mg batteries. Herein, recent research on organic electrolytes is briefly summarized. Recently, novel organic electrolytes, i.e., organic crystal electrolytes, are developed. Organic crystal electrolytes are introduced at the end of this section. For Mg2+ ion conductive electrolytes, only inorganic fillers are employed. Therefore, filler-added polymer electrolytes are reviewed in the next section “2.3 Organic-inorganic composite electrolytes”.

2.2.1. Solid polymer electrolytes

SPEs are composed of host polymers and Mg salts. The Lewis-base moieties of host polymers allow the dissociation of Mg salts, resulting in emerging Mg2+ ion conduction. Accordingly, the host polymers contain atoms with lone-pair electrons such as oxygen, fluorine, and nitrogen atoms. Figure 5 depicts the structures of various host polymers.
Common host polymers such as PEO have been used in Mg2+-ion conductive SPEs [84,85]. Different from Li+ ion conductive SPEs, water-soluble polymers like PVP and PVA are also used [86-89]. Such polymers allow using water as a solvent, facilitating preparation processes and reducing production costs. The ionic conductivities of SPEs using the water-soluble polymer hosts are comparable (10-5 ~ 10-6 S cm-1) to the conventional polymer hosts. However, other properties like the electrochemical window are seldom studied. Studies on the other properties must be carried out to clarify the applicability of the water-soluble polymer hosts.
Natural polymers are also used as host polymers for Mg2+ ion conductive SPEs [94-109]. Natural polymers are attractive in terms of environmental friendliness and resource abundance. Natural polymer-based SPEs exhibit better conductivity, around one order of magnitude higher than synthetic polymer-based SPEs. Notably, SPEs composed of potato starch and Gellan gum reveal high ionic conductivity of ~10-2 S cm-1 [97] which is comparable to Li10GeP2S12 and even liquid electrolytes [110]. In addition, these SPEs possess high flexibility and are promising for all-solid-state Mg batteries. However, their application to all-solid-state Mg batteries has yet to be reported. The environmental friendliness of natural polymer means the natural polymers will decompose naturally in long-term. Thus, the long-term stability of natural polymer-based SPEs must be tested.
To improve the properties of SPEs, a polymer blend, that is, a mixture of two host polymers, is also studied [111-121]. By the blending, the ionic conductivity increases by one order of magnitude, 10-3 ~ 10-4 S cm-1, which is applicable to all-solid-state Mg batteries. Recently, the blend of natural and synthetic polymers emerges as a new research trend in SPEs [122-125]. For example, in the blend of methyl cellulose (MC) and PVA, the hydrogen bond forms between MC and PVA, stabilizing the polymer blend [124]. In addition, rich-oxygen atoms in MC facilitate the dissociation of Mg salts. As a result, the ionic conductivity increased to ~ 10-4 S cm-1 which is applicable to all-solid-state Mg batteries [126].
Regarding the Mg salts, in addition to commonly used metal salts in LIBs like TFSI, ClO4-salts, more cost-effective MgSO4, Mg(NO3)2, MgCl2, etc. are used. Thus, water-soluble polymers such as PVA and PVP are used for these salts. The ionic conductivity was not influenced by the Mg salts. Aziz et. al. added LiFSI into PEC-Mg(TFSI)2 SPE [92]. The ionic conductivity was improved by one order of magnitude by the addition. Also, the Li-contained SPE demonstrated stable Mg stripping and plating. The usage of mixing salt is a new concept for the SPEs. In this system, contribution of Li-ion conduction must be considered to estimate the ionic conductivity. However, characterization techniques to extract only Mg2+ ion conduction has not been developed yet. Therefore, characterization of the system should be paid extra consideration.
Many types of polymers and Mg salts are studied for Mg2+-ion conductive SPEs. Most studies focus on the ionic conductivity, however, other properties such as electrochemical window, transference number and compatibility with electrodes are also important for application of all-solid-state batteries. Thus, studies on SPEs should be performed more comprehensively.
Table 5. Properties of SPEs.
Table 5. Properties of SPEs.
Polymer Mg salt σtotal
(S cm-1)		 Temp.(oC) Ea (eV) Window
(V vs. Mg/Mg2+)		 Transference
number		 Ref
PEO Mg(TFSI)2 1.8 × 10-6 0 0.68 - - [84]
1.6 × 10-4 50
PEO Mg(ClO4)2 1.42 × 10-6 RT - - - [85]
PVP MgCl2 1.42 × 10-5 RT - - - [86]
PVP MgSO4 1.05 × 10-5 RT - - - [87]
PVA MgSO4 1 × 10-9 27 0.37 - - [88]
PVA MgCl2 5 × 10-7 35 - - - [89]
Polysaccharide Mg(ClO4)2 5.66 × 10-4 RT 0.09 3.93 0.43 [90]
PEC Mg(ClO4)2 5.2 × 10-5 90 - - - [91]
PEC Mg(TFSI)2 2.3 × 10-6 80 - 2.0 - [92]
PAGE Mg(TFSI)2 4.1 × 10-4 90 - - - [93]
Potato starch MgCl2 3.2 × 10-2 RT 0.002 4.6 - [94]
Sodium alginate Mg(NO3)2 4.58 × 10-3 RT - 3.5 0.31 [95]
MC Mg(NO3)2 1.02 × 10-4 RT - 3.23 - [96]
Gellan gum Mg(ClO4)2 1.06 × 10-2 RT - 2.86 0.33 [97]
Natural rubber Mg(Tf)2 4.9 × 10-3 30 - 2.5 - [98]
I-Carrangeenan Mg(NO3)2 6.1 × 10-4 30 0.17 - - [99]
Agarose Mg(NO3)2 1.48 × 10-5 RT 0.044 3.57 - [100]
CA Mg(NO3)2 9.19 × 10-4 RT - 3.65 0.35 [101]
K-Carrageenan MgCl2 4.76 × 10-3 30 - 1.94 0.26 [102]
Chitosan Mg(Tf)2 9.58 × 10-5 RT 0.36 - - [103]
K-Carrageenan Mg(NO3)2 7.05 × 10-4 RT - 4.42 0.32 [104]
Methyl cellulose Mg(CH3COO)2 2.6 × 10-5 RT - 3.47 - [105]
I-carrageenan Mg(ClO4)2 2.18 × 10-3 RT 0.05 - 0.313 [106]
Chitosan MgCl2 4.6 × 10-4 - - - - [107]
Pectin Mg(NO3)2 7.7 × 10-4 RT - 3.8 0.29 [108]
Pectin MgCl2 1.14 × 10-3 RT - 2.05 0.301 [109]
PEO-PVDF MgTFSI 1.2 × 10-5 25 - - - [111]
PEO/PVDF-HFP MgBr2 3.9 × 10-4 RT 0.26 1.86 - [112]
PVA-PAN Mg(ClO4)2 2.94 × 10-4 RT 0.21 3.65 0.27 [113]
PVDF-HFP+
PVAc		 Mg(ClO4)2 1.60 × 10-5 30 0.33 3.5 - [114]
PVP-PVA Mg(NO3)2 3.8 × 10-5 30 0.475 - - [115]
PVA-PAN MgCl2 1.01 × 10-3 RT 0.07 3.66 - [116]
Poly(VdCl-co-AN-co-MMA) Mg(NO3)2 1.6 × 10-4 RT 0.19 3.2 0.36 [117]
PEO/PO Mg(TFSI)2 1.5 × 10-5 30 - - - [118]
PCL-PTMC Mg(TFSI)2 2.52 × 10-8 25 - - - [119]
PVA-PAN Mg(NO3)2 1.71 × 10-3 RT 0.36 3.4 0.30 [120]
PVDF-HFP+PVAc Mg(ClO4)2 3.85 × 10-5 30 3.37 3.68 - [121]
CS+MC MgCl2 2.75 × 10-3 30 - 3.86 - [122]
Corn silk+PVA MgCl2 1.28 × 10-3 RT - 2.11 0.32 [123]
Methyl cellulose-PVA Mg(NO3)2 3.25 × 10-4 27 - 2.62 - [124]
PEO-Starch MgBr2 7.8 × 10-9 RT - - - [125]

2.2.2. Polymer electrolytes with plasticizers (Gel-polymer electrolytes)

In general, plasticizers are used to soften a material, to increase its plasticity and to decrease its viscosity. In polymer electrolytes, plasticizers lower Tg (glass-transition temperature) and activate segmental motion of polymer chains, enhancing ionic conductivity. Studies on all-solid-state Mg batteries have been done using GPEs the most.
As Li+-ion conductive polymer electrolytes, low molecular weight solvents and ionic liquids have been used as a plasticizer (Figure 6). Because plasticizers soften SPEs, optimum amount of plasticizers must be found to balance ionic conductivity and mechanical properties of GPEs. The optimum amount of plasticizers varies significantly by the plasticizers used. In the GPE using PYR14TFSI ionic liquid, the highest ionic conductivity was obtained at 10 wt.% of plasticizer [127]. Contrarily, the plasticizer amount of 200 wt.% was reported in TEGDME system [128]. In such high plasticizer content, it is questioned whether the ionic conduction is mainly caused by the segmental motion of polymer or dissolved Mg salt in the liquid part.
In GPEs, synthetic polymers are mostly studied. Their conductivity ranges from 10-4 to 10-3 S cm-1 which can be applied for all-solid-state batteries. Gupta et.al. reported high ionic conductivity of 2 × 10-2 S cm-1 in [PVdF-HFP(30wt.%)-EMIMBr(70 wt.%)](30 wt.%)-[PC-Mg(ClO4)2 (0.3 M)] (70 wt.%)system [129]. However, cell data was not reported although the high ionic conductivity is promising. Mixed plasticizers like EC-SN[130] and EC-DEC[131] also have been studied, but they did not improve properties comparing with single plasticizers significantly.
The inorganic magnesium aluminum chloride complex (MgCl2-AlCl3, MACC) has been studied in liquid electrolytes [132]. By the addition of AlCl3, dissociation of MgCl2 is promoted, increasing solubility of MgCl2 and Mg2+-ion concentration. As a result, Mg stripping/plating over potential can be drastically decreased [133]. Wang et. al. applied this concept to GPEs for the first time [134]. The ionic conductivity of their PVDF-HFP-based GPEs containing MgCl2-AlCl3 salt and TEGDME plasticizer was 4.7 × 10-4 S cm-1. Although this value was comparable to other GPEs, the reversibility of Mg stripping/plating was drastically improved. In polymer electrolyte research, an effect of Mg salt has not been studied intensively. Their results clearly show the importance of Mg salt on performance of all-solid-state Mg batteries.
As another important study, single-ion conductive polymer electrolyte is reported by Schaefer et. al. [135] In the single-ion conductive polymer electrolyte, anion part of Mg salt is polymerized with host polymers. Thus, mobility of anion is zero, in other words, cation transference number is 1. Therefore, undesired side reactions caused by anions can be avoided completely. The authors prepared P(PEGDMA)-P(TFSI) [poly (ethylenglycol) dimethacrylate- poly styrensulfonyl (trifluoromethylsulfonyl)] network (Figure 7). In this structure, TFSI moiety is involved in the polymer chain, resulting in immobilization of anion. This type of polymer generally shows low ionic conductivity due to low segmental motion of polymer chain. Thus, DMSO plasticizer was added and the Mg2+-ion conductivity was increased to 8.8 × 10-4 S cm-1. It is noted that the high conductivity was achieved by only Mg2+-ion transportation. Some other studies reported higher conductivities, however, the comductivities contain anion transportation. Therefore, the high Mg2+-ion conductivity of single-ion conductive polymer is very attractive. Unfortunately, cell data was not reported.
In GPEs, mixed salt like MACC and the single-ion conductive polymer are studied. Such studies have been carried out only in GPEs. Their superior properties are promising to apply for all-solid-state Mg batteries. This concept should be investigated intensively and also used in other types of polymer electrolytes such as organic-inorganic composite electrolytes.
Table 6. Properties of GPEs.
Table 6. Properties of GPEs.
Polymer Mg salt Plasticizer σtotal
(S cm-1)		 Temp. (oC) Ea (eV) Window
(V vs. Mg/Mg2+)		 Transference
number		 Ref
P(PEGDMA)-P(STFSI) DMSO 8.8 × 10-4 30 - 1.5 (1.0) [135]
PVDF Mg(SO3CF3)2 TEGDME 4.6 × 10-4 55 0.62 1.0 0.74 [136]
PEO Mg(Tf)2 PYR14TFSI 3.7 × 10-4 RT - - 0.40 [127]
PEO Mg(Tf)2 EMIM-BF4 9.4 × 10-5 RT 0.26 4.0 0.22 [137]
PVdC-co-AN Mg(TFSI)2 EC+SN 1.9 × 10-6 RT 0.04 3.8 0.59 [130]
PVdC-co-AN Mg(TFSI)2 SN 1.6 × 10-6 RT 0.09 3.2 - [138]
PVDF-HFP Mg(ClO4)2 EDiMIMBF4 8.4 × 10-3 RT 0.33 - - [139]
PVDF-HFP Mg(ClO4)2 EMIMBr,PC 2.0 × 10-2 RT 0.02 - - [129]
PEC Mg(TFSI)2 TEGDME 5.2 × 10-6 80 - - - [140]
PECH-OH MgCl2 TEGDME 6.2 × 10-5 30 0.25 3.2 0.79 [128]
PVDF-HFP Mg(Tf)2 SN+EMITf 4 × 10-3 26 0.104 4.1 - [141] Bull. Mater. Sci. 41 2018 147
Poly(VdCl-co-An-co-MMA) MgCl2 SN 1.4 × 10-3 RT 0.26 3.3 0.31 [142]
c-PTHF Mg(TFSI)2 TEGDME 4.5 × 10-5 30 - - - [143]
CS Mg(CH3COO)2 glycerol 1.1 × 10-4 RT - - - [144]
k-carrageenan Mg(NO3)2 EC 7.3 × 10-3 30 - 4.59 0.39 [145]
PVDF-HFP/PVAc Mg(ClO4)2 EMITF 9.1 × 10-4 30 0.28 3.59 - [146]
Hydroxy propyl Mg(TFSI)2 TEGDME 1.73 × 10-3 25 - - - [147]
PVDF-HFP Mg(Tf)2 EC-DEC 2.4 × 10-4 70 - 5.0 0.42 [131]
PVDF Mg(ClO4)2 PC 1.5 × 10-3 RT - 5.0 0.47 [148]
PVDF-HFP Mg(ClO4)2 TEGDME 9.8 × 10-4 RT - 4.6 - [149]
PTHF MgBOR 2.0 × 10-3 25 - 2.57 0.3 [150]
PVA Mg(Tf)2 EMITf 1.2 × 10-3 RT - - - [151]
PVDF-HFP Mg(ClO4)2 PC 1.6 × 10-3 RT - 5.5 - [152]
PAN Mg(ClO4)2 PC 3.3 × 10-3 30 0.1 4.6 0.6 [153]
PVDF-HFP MgCl2-AlCl3 TEGDME 4.7 × 10-4 25 - 3.1 - [134]
PEO Mg(Tf)2 PC-DEC 3.0 × 10-5 RT 0.14 3.5 0.32 [154]
CS:Dextran Mg(CH3COO)2 Glycerol 1.2 × 10-6 RT - 1.5 - [155]

2.2.3. Organic crystal electrolytes

Recently, Moriya’s group reported a new type of organic electrolyte, organic crystal electrolytes. They are composed of organic molecules and Mg salts and possess ion conduction paths in the crystal lattice (Figure 8). The paths are precisely controlled by organic molecules and Mg salts. Only two Mg ion conductive organic crystal electrolytes have been reported so far [156,157]. In both cases, room temperature conductivities were higher or comparable to Mg(BH4)2-based inorganic electrolytes and the cation transference number was higher than that of polymer-based electrolytes. These properties would be improved by adjusting the ion conduction paths. Unfortunately, cell data using the organic crystal electrolytes are not available at the moment. There is still a lot of room to develop organic crystal electrolytes.
Table 6. Properties of organic crystals.
Table 6. Properties of organic crystals.
Crystal σtotal
(S cm-1)		 Temp.
(oC)		 Ea (eV) Transference number Ref
Mg(TFSA)2(CPME)2 2 × 10-7 30 0.72 0.74 [156]
[N1122][Mg(η2-TFSA)2(μ2-η1-η1-TFSA)] 2.5 × 10-6 40 1.21 0.46 [157]

2.3. Organic-inorganic composite electrolytes

2.3.1. Solid polymer electrolytes with fillers

In the Li+ ion conductive polymer electrolytes, various types of fillers such as organic/inorganic fillers and active/passive (Li+ ion conductive/non- Li+ ion conductive) fillers are researched. Contrarily, in the Mg2+ ion conductive polymer electrolytes, only inorganic passive fillers, especially metal oxides, are studied. In addition, only nano-particle morphology, not nano-wire, nano-sheet, etc. is employed. Among the studies, the highest conductivity was obtained in filler contents of 3 ~7 wt.% regardless of the fillers. An interesting study was carried out by Jayanthi et. al. in which ferroelectric material, BaTiO3, was added to PVDF-HFP/MgTf polymer electrolyte as a filler [158]. The presence of ferroelectric domains in the polymer electrolyte facilitates salt dissociation and helps amorphization of the polymer, resulting in enhancement of ionic conductivity. Similar study was reported in Na+ ion conductive polymer electrolyte [159]. In this case, K0.5Na0.5NbO3 (KNN) was used as a ferroelectric filler and it decreased ionic conductivity of polymer electrolyte while stability against Na metal anode was improved. As a result, better performance of all-solid-state Na battery was obtained. This is a good result that the performance of electrolytes is determined by not only conductivity but also interface properties between electrodes and electrolytes. Thus, electrolyte study must include construction and evaluation of all-solid-state battery. Unfortunately, studies on only solid electrolytes have been reported much more than all-solid-state batteries.
Table 7. Properties of SPEs with fillers.
Table 7. Properties of SPEs with fillers.
Polymer Mg salt Filler σtotal
(S cm-1)		 Temp. (oC) Ea (eV) Window
(V vs. Mg/Mg2+)		 Transference number Ref
PVA/PVP MgCl2 CuS 4.3 × 10-6 RT - - - [160]
MC MgCl2 ZnO 1.2 × 10-4 RT - - - [161]
PVDF Mg(NO3)2 MgO 1.0 × 10-4 RT 0.32 - - [162]
PEG Mg(CH3COO)2 CeO2 3.4 × 10-6 RT - - - [163]
PMMA Mg(Tf)2 TiO2 1.8 × 10-6 RT - - - [164]
CS Mg(NO3)2 MnO2 1.2 × 10-3 30 - 1.7 - [165]
PVDF-HFP Mg(Tf)2 BaTiO3 4.1 × 10-4 RT - - - [158]
PEO Mg(Tf)2 MgO 1.6 × 10-4 25 0.14 - - [166]
PVDF Mg(NO3)2 Al2O3 9.5 × 10-6 RT - - - [167]
PVDF Mg(NO3)2 ZnO 5.2 × 10-5 RT 0.29 - - [168]
PEO MgCl2 B2O3 7.2 × 10-6 25 - - - [169]
PVDF-HFP/PVAc Mg(ClO4)2 MgTiO3 5.8 × 10-3 30 0.25 4.0 0.34 [170]
CS MgCl2 V2O5 1.4 × 10-3 RT - 1.7 - [171]
PVDF-HFP MgClO4 ZrO2 6.6 × 10-2 30 - - - [172]
PVDF-HFP MgCl2 ZnO 1.3 × 10-5 RT - - - [173]

2.3.2. Solid polymer electrolytes including plasticizers and fillers

In this system, polymer electrolytes contain both inorganic fillers and plasticizers. As mentioned, low molecular weight solvents and ionic liquids are employed as a plasticizer. Contrarily, metal oxides are used as a filler. Aziz et. al. used Ni metal nanoparticles as a filler [174]. Metal fillers have not been applied for Mg2+-ion conductive polymer electrolytes except this paper. To clarify effect of metal filler, more research is needed. Sharma et. al. added EC-PC and MgAl2O4 into PVDF-HFP/Mg(Tf)2 polymer electrolyte [177]. This system revealed high transference number of 0.66 which is one of the highest transference number in Mg2+-ion conductive polymer electrolytes.
The ionic conductivity of filler/plasticizer-containing polymer electrolytes ranges 10-5 to 10-3 S cm-1. These values are comparable to other types of polymer electrolytes. Thus, benefits of usage of both plasticizers and fillers cannot be emphasized. Currently, individual effect of plasticizers and fillers on properties of polymer electrolytes has yet to be fully understood. Thus, the individual effect of plasticizers and fillers must be clarified at first. Then, more complicated system, i.e. polymer electrolytes containing both plasticizers and fillers, should be developed based on the individual effect.
Table 8. Properties of SPEs with plasticizers and fillers.
Table 8. Properties of SPEs with plasticizers and fillers.
Polymer Mg salt Plasticizer Filler σtotal
(S cm-1)		 Temp. (oC) Ea (eV) Window (V vs. Mg/Mg2+) Transference number Ref
CS/MC Mg(CH3COO)2 Glycerol Ni 1.0 × 10-4 RT - 2.48 - [174]]
PEO Mg(ClO4)2 EMIMFSI SiO2 5.4 × 10-4 RT 0.36 4.0 - [175]
CS Mg(CH3COO)2 Glycerol Ni 1.1 × 10-5 RT - 2.4 - [176]
PVDF-HFP Mg(Tf)2 EC-PC MgAl2O4 4.0 × 10-3 RT - - 0.66 [177]
PVDF-HFP Mg(ClO4)2 PTR14RFSI TiO2 1.6 × 10-4 30 0.13 - 0.23 [178]
PVDF-HFP Mf(TFSI)2 TEGDME SiO2 8.3 × 10-4 RT - - - [179]
PTHF Mg(BH4)2-LiBH4 diglyme TiO2 4.2 × 10-4 40 0.003 - 0.5 [180]

3. All-solid-state Mg battery

3.1. Inorganic electrolyte

Research on all-solid-state Mg battery using oxide-based electrolytes is not reported. The Mg2+-ion conductivity of the oxide electrolytes is 10-6 ~ 10-7 S cm-1 at room temperature. This is too low to support ion conduction in all-solid-state batteries operated at room temperature. Thus, improvement of room temperature ionic conductivity to at least 10-4 S cm-1 level is needed at first. In the case of MgSc2Se4-related materials, although they possess high Mg2+-ionic conductivity, their electronic conductivities are also high. Thus, these materials are not studied for electrolytes for all-solid-state batteries.
All-solid-state Mg batteries using inorganic electrolytes are reported only in borohydride electrolytes, i.e. Mg(BH4)2-related materials. While these materials demonstrate good Mg stripping/plating behaviors [62,64,65,67,69], only two papers are reported with respect to full cell configurations. In both cases, TiS2 is used as a cathode and electrochemical tests were carried out above room temperature to obtain reasonable capacity. The obtained discharge capacity was about 100 mAh/g although the C rate was low. Also, capacity decay occurs rapidly. TiS2 cathode exhibits large (> 100 mAh/g) and stable (> 100 cycles) capacities at room temperature at reasonable C rate (1 ~ 2 C) in liquid electrolytes [132]. Thus, the low performance of all-solid-state battery would be caused low conductivity of solid electrolyte and high impedance/low stability of cathode/electrolyte interface.
As mentioned, improvement of room temperature ionic conductivity is needed to realize all-solid-state Mg batteries using inorganic electrolytes. Additionally, various cathode materials and properties of electrode/electrolyte interface must be tested and characterized, respectively. In summary, all-solid-state Mg batteries using inorganic electrolytes are still far from realization.

3.2. MOF

In the all-solid-state Mg batteries using MOF-based electrolytes, since MOFs are used to support liquid electrolytes, they can be said “Quasi-solid electrolytes”. Although stable Mg stripping/plating is observed, only one paper reported in the full cell configuration. The full cell using PTCDA (Perylenetetracarboxilic dianhydride) cathode demonstrates small discharge capacity of 36 mAh/g. The PTCDA cathode is employed for Na and K batteries [181, 182] and reveals a good performance. However, it has not been applied for Mg batteries even including liquid electrolytes. Therefore, the poor performance of all-solid-state Mg batteries using MOF-based quasi-solid electrolyte is attributed to some factors such as cathode itself, cathode/electrolyte interface, electrolyte itself and so on. Usage of common cathode materials like MoS6 can reduce the factors, facilitating the evaluation of all-solid-state Mg batteries. Consequently, the common cathode materials should be used for the MOF-based quasi-solid electrolytes at this moment.
Since stable Mg stripping/plating was achieved at room temperature in MOF-based solid electrolytes, they would be promising for all-solid-state Mg battery applications. Therefore, studies on cathode side must be carried out intensively.

3.3. Organic electrolyte

Although studies on SPEs (without fillers and plasticizers) are reported by many groups, SPEs are not applied for all-solid-state Mg batteries. Some groups studied primary Mg batteries using SPEs [90,95,99,106,108,109,113,120,122,124,145]. Since this review article focuses on rechargeable all-solid-state Mg batteries, the studies on primary batteries are not introduced here. The main reason for lack of research on SPEs for rechargeable all-solid-state Mg batteries is their relatively low ionic conductivity. The conductivity of most SPEs ranges 10-5 ~ 10-7 S cm-1 at RT. Improvement of the conductivity to 10-3 ~ 10-4 S cm-1 at least is needed to achieve reasonable performance of all-solid-state Mg batteries. Potato starch [94] and Gellun gum [97]-based SPEs exhibit extremely high ionic conductivity of ~10-2 S cm-1. These are 2 orders of magnitude higher than other SPEs. Close investigation for these SPEs, such as reproducibility and characterization procedures, should be performed since such very high conductivity of these SPEs cannot be accepted easily.

3.3.1. Gel polymer electrolyte

Gel polymer electrolytes (GPEs) composed of post polymers, Mg salts and plasticizers possess higher conductivity, 10-3 ~ 10-4 S cm-1, than SPEs. Additionally, their high flexibility makes battery construction easy. Thus, all-solid-state Mg batteries using GPEs are reported the most. The all-solid-state Mg batteries using GPEs can be operated at room temperature [128, 134, 141, 150]. However, in all cases, initial and steady-state capacities are low. Because stable Mg stripping/plating is observed in GPEs, cathode itself and cathode/electrolyte interface would be the reason for the low performance. Detail characterization and post-mortem analysis of the interface and cathode should be carried out.
Ge et. al. fabricates pouch cell-type all-solid-state Mg batteries for the first time [128]. Only this study reports the performance of pouch cell-type all-solid-state Mg battery with GPE. The pouch cell can reduce a weight of battery cases, resulting in high energy density. Additionally, the authors performed safety tests, such as cutting the pouch cell and flammability tests of GPE and pouch cell. The study is meaningful in verifying the possible application of pouch cell configuration for all-solid-state Mg batteries, although improvement of performance of the pouch cells is needed.
Sheha et. al. studies dual polymer/liquid electrolyte (Figure 9) [136]. The electrolyte is composed of two layers, liquid electrolyte and GPE. Both electrolytes are separated by a glass fiber membrane. The liquid electrolyte (APC, all phenyl complex) and GPE are faced on cathode and anode (Mg metal) sides, respectively. In the all-solid-state battery, a poor contact between porous electrode and solid electrolyte increases impedance of the battery and causes low performance. At the moment, effective solution of the contact issue has yet to be found. Their concept would be helpful in solving the contact issue. Therefore, the dual electrolyte configuration is likely to be applied for first generation all-solid-state Mg batteries. The authors report high initial capacity using BaTiO3 cathode, but the capacity is rapidly decayed within 15 cycles. The reason for low cyclability is unclear since cyclability of BaTiO3 cathode has not been tested in liquid electrolytes. The dual electrolytes should be studied using common cathode materials at first. For safety, usage of the flammable liquid electrolytes must be minimized. Influence of the liquid electrolyte on safety of the batteries, formation of CEI (cathode-electrolyte interphase) and properties of new interface, i.e. GPE/liquid electrolyte should be studied for successful application of the dual electrolyte system (In fact, the battery is not pure all-solid-state Mg batteries since the batteries contain a small amount of liquid electrolyte).
In the GPE research, Mo6S8 which is the most commonly used cathode material, is applied for the all-solid-state battery [128, 150]. This facilitates evaluation of the all-solid-state batteries since performance of cathode is studied in liquid electrolytes for long time. Mo6S8 cathode provides high capacity (130 mAh g-1 at 0.1 C and 98 mAh g-1 at 0.5 C) in liquid electrolytes [183, 184]. Contrarily, the all-solid-state batteries using GPEs demonstrate about 70 mAh g-1 (68 mAh g-1 at 0.1 C [150] and 73 mAh g-1 at 0.3 C [128]) even though same cathode materials are used. Because ionic conductivity of electrolytes would not largely influence battery performance at such low C rate, high impedance at cathode-electrolyte interface could be a reason for the low performance. Common electrode materials should be used for all-solid-state batteries more intensively. It is interesting to note that better battery performance was obtained in the PECH-OH-based GPE comparing to the PTHF-based GPE. As shown in Table 6, the PTHF-based GPE revealed about 2 order of magnitude higher Mg2+-ion conductivity than the PECH-OH-based GPE. This is a good example that battery performance is determined by not only properties of electrolytes. GPE is the most promising for all-solid-state Mg batteries at this moment. The ionic conductivity of GPEs is comparable to Li+-ion conductive polymer electrolytes. Thus, compatibility with electrodes and properties of electrode/electrolyte interface would determine the performance of all-solid-state Mg batteries. Many studies on GPEs are reported, while their application to all-solid-state Mg batteries is seldom researched. Such study is strongly required, not only simply study on properties of GPEs, to realize all-solid-state Mg batteries.

3.4. Organic-inorganic composite electrolytes

Only 3 papers have been reported in terms of all-solid-state Mg batteries using the organic-inorganic composite electrolytes recently [170,179,180]. Comparing with GPEs, all-solid-state Mg batteries with the composite electrolytes demonstrate better initial performance and cyclability. Particularly, when SPEs contain both fillers and plasticizers, very stable cyclability was achieved although only 2 papers were reported. The compatibility of polymer electrolytes with electrodes, especially cathodes, is likely to be improved by adding fillers. Wang et al. successfully prepared the pouch cell-type all-solid-state Mg batteries [180]. The cell demonstrated excellent performance. However, the solid electrolyte contains two salts, Mg(BH4)2 and LiBH4. The ratio of Mg/Li is 0.1/1.5. Thus, it is unclear influence of Li intercalation on observed capacity.
Despite fewer examples, fillers would improve the stability of all-solid-state Mg batteries. Application of the organic-inorganic composite electrolytes for all-solid-state Mg batteries must be studied more intensively. Although pouch cell-type all-solid-state battery was reported, common coin cell configuration and cathode materials should be adopted at this moment because the application of organic-inorganic composite electrolytes for all-solid-state Mg battery is still infant stage.
Table 9. Properties of all-solid-state Mg batteries.
Table 9. Properties of all-solid-state Mg batteries.
Electrolyte Cathode Initial capacity Capacity retention Temp. (oC) Note Ref
Borohydrides
Mg(BH4)(NH2) Pt - - - Mg plating on Pt [62]
Mg(BH4)(NH3BH3)2 Mo - - - Mg plating on Mo [65]
0.4Mg(BH4)2•NH3-0.6Mg(BH4)2•2NH3@MgO Mg - - 60 Stable Mg stripping/plating more than 100 cycles at 0.25 mA cm-2 [69]
Mg(en)1(BH4)2 Pt - - 60 Stable Mg stripping/plating in 20 cycles at 10 mV s-1 [64]
Mg(BH4)2 1.5NH3-YSZ Mg - - 60 Stable Mg stripping/plating in 300 cycles at 0.1 mA cm-2 [67]
Mg(BH4)∙2NH3 TiS2 141 mAh/g at 0.05C 31 % at 25th cycle 75 111 mAh/g at 0.2C,
72 mAh/g at 0.5C		 [73]
Mg(BH4)2∙1.5THF-MgO TiS2 94 mAh/g at C/50 32 % at 5th cycle 55 SS current collector was oxidized [70]
MOF
Mg(TFSI)2/MgCl2/DME
in MOF-74		 Mg - - RT Stable Mg stripping/plating in 100 cycles at 0.05 mA cm-2 [79]
Mgbp3dc in α-Mg3(HCOO)6/DMF Mg - - RT Stable Mg stripping/plating in 8 cycles at 0.1 μA cm-2 [76]
Mg(TFSI)2/[EMIM][TFSI] in UiO-66 PTCDA 36 mAh/g at 1 mA/g 61 % at 3rd cycle 60 Stable Mg stripping/plating more than 200 cycles at 3.14 μA cm-2 [77]
GPE
PVDF-TEGDME-Mg(Tf)2 BaTiO3 557 mAh/g at 20 mA/g 12 % at 15th cycle 55
  • Full cell configuration: Mg/SE/APC/Cathode
  • Stable Mg stripping/ plating more than 25 cycles at 0.02 mA cm-2
[136]
PECH-OH-MgCl2-TEGDME Mo6S8 73 mAh/g at 0.3 C 84 % at 100th cycle 30 Pouch cell data [128]
PVDF-HFP-Mg(Tf)2- SN+EMITf MnO2 40 mAh/g at 38 μA cm-2 12.5 % at 8th cycle RT [141]
PTHF-MgBOR Mo6S8 68 mAh/g at 0.1 C 74 % at 100th cycle 25 Stable Mg stripping/plating more than 1000 cycles at 0.1 mA cm-2 [150]
PVDF/HFP-MgCl2/AlCl3-TEGDME MoS2 121 mAh/g at 40 mA/g 58 % at 1700th cycle 25 Stable Mg stripping/plating more than 400 cycles at 1.0 mA cm-2 [134]
Filler
PVDF-HFP/PVAc-Mg(ClO4)2-MgTiO3 Mo6S8 120 mAh/g at 0.5 C 87 % at 30th cycle RT [170]
SPE+filler+plasticizer
PVDF-HFP-Mg(TFSI)2-SiO2 TiO2 129 mAh/g at 50 mA/g 99 % at 100th cycle RT Stable Mg stripping/plating more than 100 cycles at 0.2 mA cm-2 [179]
PTHF- Mg(BH4)2/LiBH4-Diglyme-
TiO2 		TiS2 225 mAh/g at 0.5 C 98 % at 70th cycle 22 Stable Mg stripping/plating more than 100 cycles at 0.1 mA cm-2 [180]

4. Challenges

The all-solid-state Mg battery is a good option to replace LIBs due to high safety, energy density and resource abundance. Thanks to many efforts of researchers, technologies for the all-solid-state Mg battery have progressed significantly. Despite the significant progress, further research is still required to realize the all-solid-state Mg battery. Herein, challenges are summarized.
(1)
Inorganic electrolytes
The Mg2+-ion conductivity of current inorganic electrolytes are 10-6 ~ 10-7 S cm-1 at room temperature in ceramic electrolytes (except weird results [33, 35, 59]). Even hydride-based inorganic electrolytes, the conductivity is ~10-5 S cm-1. For all-solid-state Mg batteries, required conductivity is > ~10-4 S cm-1 at room temperature to ensure room temperature operation. Thus, improvement of ionic conductivity must be investigated in inorganic electrolytes. Some groups have studied Mg2+-ion conductivity of ceramic electrolytes intensively, however, effective solution to enhance the conductivity is yet to be found at this moment. Thus, thin film ceramic electrolytes would be effective to compensate the low conductivity. Currently, only one paper is reported with respect to thin film ceramic electrolytes [58]. The thin film ceramic electrolytes should be studied more intensively. In the hydride-based electrolytes, addition of inorganic fillers is likely to improve the ionic conductivity effectively [67,68]. The effect of fillers should be researched.
(2)
Study on Mg salts
Comparing to the inorganic electrolytes, organic polymer electrolytes are more promising in application of all-solid-state Mg battery at this moment. Various polymer hosts have been studied. On the other hand, systematic study on Mg salts is not carried out. Fichtner et. al. reported corrosion of battery case and current collectors by Cl-contained salt [150]. Although various types of Mg salts such as MgSO4, MgCl2, Mg(ClO4)2, Mg(TFSI)2, Mg(NO3)2, Mg(Tf)2, have been employed so far, suitable Mg salt for the all-solid-state Mg battery is still under investigation. Therefore, effect of Mg salts on properties of SPEs (and GPEs) and performance of all-solid-state Mg batteries should be studies systematically.
(3)
Mechanical properties of solid electrolytes
High flexibility is one of the benefits of SPEs and GPEs which facilitates construction of the all-solid-state Mg battery. However, quantitative evaluation of the flexibility, i.e. mechanical properties has not been carried out yet. The mechanical properties of solid electrolytes influence cell pressure, contact with electrodes, suppression of Mg dendrite formation, etc. significantly. Consequently, both electrochemical and mechanical properties of SPEs and GPEs must be characterized precisely.
(4)
Construction of all-solid-state Mg battery
As mentioned in previous section, most research focus on solid electrolyte and only some papers try to fabricate all-solid-state Mg batteries. Compatibility of solid electrolytes with all-solid-state Mg battery cannot be evaluated by not only ionic conductivity, electrochemical window, transference number, etc. Therefore, solid electrolytes must be evaluated in all-solid-state batteries in addition to the above mentioned properties. Particularly, properties of electrode/solid electrolyte interface which largely affect performance of all-solid-state Mg batteries can be characterized only in the all-solid-state battery configuration.
(5)
Cathode materials
Some groups employ novel cathode materials for the all-solid-state Mg batteries. In this case, performance of the cathode material is unknown. Thus, it is impossible to clarify the reason for poor performance of the all-solid-state Mg battery, i.e. cathode itself or other components. Common cathode materials such as Mo6S8 and TiS2 which are well characterized in liquid electrolytes should be used to characterize and evaluate the all-solid-state Mg batteries. After that, development of cathode materials for all-solid-state batteries would be carried out.
All-solid-state Mg battery would replace current LIBs owing to high safety, energy density and resource abundance although the research is still infant stage. To realize the all-solid-state Mg battery, above mentioned studies must be carried out intensively. Additionally, research on Mg battery using liquid electrolytes should be referred especially for the cathode selection.

Author Contributions

J.P. and G.J.; analysis, writing—original draft preparation, K.Z. and M.K.; writing—review and editing, M.K.; supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Linear sweep voltammogram of Mg0.5Si2(PO4)3. Reproduced with permission [36]. Copyright 2016, Elsevier.
Figure 1. Linear sweep voltammogram of Mg0.5Si2(PO4)3. Reproduced with permission [36]. Copyright 2016, Elsevier.
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Figure 2. Computed Mg2+ ion migration barrier in trinary spinel chalcogenides. Reproduced with permission [42]. Copyright 2017, Nature Communications.
Figure 2. Computed Mg2+ ion migration barrier in trinary spinel chalcogenides. Reproduced with permission [42]. Copyright 2017, Nature Communications.
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Figure 3. Galvanostatic cycling of a symmetric Mg/Mg(BH4)2 1.6NH3-MgO cell at 60 oC with a constant current density of 0.25 mA cm-2. Reproduced with permission [69]. Copyright 2020, American Chemical Society.
Figure 3. Galvanostatic cycling of a symmetric Mg/Mg(BH4)2 1.6NH3-MgO cell at 60 oC with a constant current density of 0.25 mA cm-2. Reproduced with permission [69]. Copyright 2020, American Chemical Society.
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Figure 4. Mg plating/stripping test of UiO-66-Mg(TFSI)2/[EMIM][TFSI] electrolyte at current density of (a) 1.57 μA cm-2 and (b) 3.14 μA cm-2 at 60 oC. Reproduced with permission [77]. Copyright 2022, Wiley-VCH GmbH.
Figure 4. Mg plating/stripping test of UiO-66-Mg(TFSI)2/[EMIM][TFSI] electrolyte at current density of (a) 1.57 μA cm-2 and (b) 3.14 μA cm-2 at 60 oC. Reproduced with permission [77]. Copyright 2022, Wiley-VCH GmbH.
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Figure 5. Structures of various host polymers used in Mg2+ ion conductive polymer electrolytes.
Figure 5. Structures of various host polymers used in Mg2+ ion conductive polymer electrolytes.
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Figure 6. Structures of various plasticizers used in Mg2+ ion conductive polymer electrolytes.
Figure 6. Structures of various plasticizers used in Mg2+ ion conductive polymer electrolytes.
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Figure 7. Structures of magnesiated P(PEGDMA)-P(STFSI) network.
Figure 7. Structures of magnesiated P(PEGDMA)-P(STFSI) network.
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Figure 8. Packing view with ball and stick model of Mg(TFSA)2(CPME)2. Mg-ions are emphasized as a large sphere. (A) along the a-axis. (B) along the b-axis, and (c) along the c-axis (Mg: green, C: gray, N: pale blue, O: red, F: pale green, S: dark yellow. H atoms are omitted for clarity). Reproduced with permission [156]. Copyright 2021, Frontiers.
Figure 8. Packing view with ball and stick model of Mg(TFSA)2(CPME)2. Mg-ions are emphasized as a large sphere. (A) along the a-axis. (B) along the b-axis, and (c) along the c-axis (Mg: green, C: gray, N: pale blue, O: red, F: pale green, S: dark yellow. H atoms are omitted for clarity). Reproduced with permission [156]. Copyright 2021, Frontiers.
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Figure 9. Configuration of Mg battery using the dual GPE/liquid electrolyte (Mg/GPE/APC/BaTiO3 cathode). Reproduced with permission [136]. Copyright 2020, American Chemical Society.
Figure 9. Configuration of Mg battery using the dual GPE/liquid electrolyte (Mg/GPE/APC/BaTiO3 cathode). Reproduced with permission [136]. Copyright 2020, American Chemical Society.
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Table 1. Properties of various metal anodes.
Table 1. Properties of various metal anodes.
Li Na K Mg Ca Zn Al
Standard redox potential
(E vs. SHE)		 -3.04 -2.71 -2.93 -2.37 -2.87 -0.76 -1.66
Volumetric capacity (mAh/cm3) 2062 1128 591 3883 2073 5851 8046
Specific capacity (mAh/g) 3861 1166 685 2205 1337 820 2980
Abundance (%) 0.002 2.7 2.4 2.08 5 0.008 8.2
Ionic radius (Å) 0.76 1.02 1.38 0.72 1.00 0.74 0.535
Relative atomic mass 6.94 22.98 39.1 24.31 40.08 65.39 26.98
Mass to charge 6.94 22.98 39.1 12.16 20.04 32.7 8.99
Table 2. Properties of various oxide- and chalcogenide-based solid electrolytes.
Table 2. Properties of various oxide- and chalcogenide-based solid electrolytes.
Electrolyte σtotal
(S cm-1)		 Temperature (oC) Activation energy (eV) Electrochemical window (V) Ref
Oxides
MgZr4(PO4)6 2.9 × 10-5 400 0.868 - [30]
6.1 × 10-3 800
Mg0.5Zr2(PO4)3 1.1 × 10-6 30 0.0977 ~ 2.5 [50]
7.1 × 10-5 500
MgZr4(PO4)6 7.23 × 10-3 725 0.84 - [51]
MgZr4(PO4)6+Zr2O(PO4)2 6.9 × 10-3 800 1.41 - [52]
Mg0.7(Zr0.85Nb0.15)4(PO4)6 5.71 × 10-3 800 0.95 - [32]
Mg1.4Zr4P6O24.4+0.4Zr2O(PO4)2 6.89 × 10-3 800 1.41 -
Mg1.1(Zr0.85Nb0.15)4P6O24+0.4Zr2O(PO4)2 9.53 × 10-3 800 1.28 -
Mg1.1Zr3.4Nb0.6P6O24.4 + Zr2O(PO4)2 9.53 × 10-3 800 1.26 - [53]
Mg0.9Zr1.2Fe0.8(PO4)3 1.25 × 10-5 RT 0.14 - [33]
7.2 × 10-5 500
Mg0.5Ce0.2Zr1.8(PO4)3 3.8 × 10-7 200 0.307 - [54]
Mg1.05Zn0.4Al0.3Zr1.3(PO4)3 3.97 × 10-4 RT 0.039 - [35]
5.82 × 10-4 500
Mg0.35(Zr0.85Nb0.15)2(PO4)3 1.1 × 10-6 350 1.18 - [55]
Mg0.5ZrSn(PO4)3 2.47 × 10-5 500 0.79 - [56]
Mg0.7Zr3.4Nb0.6(PO4)6 7.7 × 10-4 600 0.954 - [57]
3.7 × 10-3 750
Mg0.6Zr1.8Fe0.2(PO4)3 thin film 1.8 × 10-7 25 0.141 < 175oC

0.511 > 175oC		 - [58]
2.3 × 10-6 200
Mg0.625Si1.75Al0.25(PO4)3 1.54 × 10-4 RT - 2.51 [59]
Mg0.5Si2(PO4)3 1.83 × 10-5 - ~ 3.21 [36]
Mg0.105Hf0.95Nb(PO4)3 1.2 × 10-4 600 0.639 - [34]
Mg2.4P2O5.4 ALD 1.6 × 10-7 500 1.37 - [37]
Mg0.6Al1.2Si1.8O 6 2.3 × 10-6 500 1.32 - [38]
MgSO4-Mg(NO3)2-MgO 2.2 × 10-6 RT 0.17 - [60]
MgHf(WO4)3 2.5 × 10-4 600 0.835 - [39]
Chalcogenides
80(0.6MgS 0.4P2S5) 20MgI2 2.1 × 10-7 200 - - [41]
MgSc2Se4 9.2 × 10-5 RT - - [44]
MgSc2Se4 ~1 × 10-4 25 0.38 [42]
MgSc2Se4 8 × 10-5 RT - - [45]
MgSc2Se4 1.78 × 10-5 RT - - [46]
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