1. Introduction

2. Solid electrolytes for Mg battery

2.1. Inorganic electrolyte

2.1.1. Oxides

Na⁺ super ion conductor (NASICON), Na_1+xZr₂P_3-xSi_xO₁₂ 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], Li_1+xAl_xTi_2-x(PO₄)₃ [24,25], and Li_1+xAl_xGe_2-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 Mg_1.05Zn_0.4Al_0.3Zr_1.3(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 Mg_0.5Si₂(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 (Mg_2.4P₂O_5.4) [37], magnesium silicate (Mg_0.6Al_1.2Si_1.8O₆) [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 T_g (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, K_0.5Na_0.5NbO₃ (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

3.4. 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

(1)

Inorganic electrolytes

(2)

Study on Mg salts

(3)

Mechanical properties of solid electrolytes

(4)

Construction of all-solid-state Mg battery

(5)

Cathode materials

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