The Effect of Anion-sublattice Structure on the Displacement Reaction in Copper Sulfide Cathodes of Rechargeable Magnesium Batteries

Abstract

Copper sulfides (Cu 2¹x S, where x denotes position defects) with different structures of the S 2¹ sublattice, such as cubic digenite and hexagonal chalcocite, have been investigated as cathode materials for magnesium (Mg) batteries. Rechargeable performance is only achieved using a cubic digenite cathode; however, using hexagonal chalcocite cathode yields poor rechargeable performance. The face-centered cubic S 2¹ sublattice in cubic digenite, which enables Cu + and Mg 2+ ions to be displaced from each other without causing the lattice to collapse, could play a key role in improving the performance of rechargeable cathodes.Keywords: Rechargeable magnesium battery | Anion-sublattice structure | Copper sulfide cathodeCurrently, a rechargeable lithium (Li)-ion battery is one of the key systems for energy storage.1,2 However, the growing demand for reduced cost and safety in energy storage for largescale applications has motivated the development of secondary batteries using non-Li cations. For this purpose, a magnesium (Mg) battery is one of the promising alternatives owing to its relatively high stability of magnesium metal upon air exposure and high terrestrial abundance. Despite their promising characteristics, the development of Mg batteries has been slow. In fact, although relevant materials have been extensively researched, only a limited number of transition metal compounds have been found as suitable materials for the insertion-type cathode of rechargeable Mg batteries. 312 One of the reasons for this is that Mg 2+ ions travel through the electrodes too slowly. This is due to the bivalent nature of Mg 2+ ions and their highly stable coordination with hard Lewis bases, as per the hard and soft Lewis acids and bases (HSAB) theory. 13 A new type of cathode materials should be developed for Mg batteries other than the insertion-type one.One promising approach is to research materials that enable the displacement reaction, which is a type of conversion reaction found in the study of Li-ion batteries.14 In this reaction, the insertion of guest cations induces the displacement and extrusion of transition metal cations from the original lattice sites, while maintaining the sublattice of anions. In this situation, the energy barrier caused by lattice strains should be suppressed such that the exchange of cations proceeds more smoothly in the reaction process than in most conversion reactions that require a reconstruction of the anion sublattice. Recently, based on the displacement reaction, we reported that β-Cu 2 Se is suitable as a cathode material for rechargeable Mg batteries. 15 In the discharge process for this material, Mg 2+ ions diffuse to the face-centered cubic (fcc) sublattice composed of Se 2¹ anions (i.e. a Se 2¹ sublattice) to induce a position displacement of Cu + ions that are reduced to Cu 0 (i.e., Cu metal) at the outside of the lattice, leading to the formation of MgSe (+2Cu 0 ). Importantly, the Se 2¹ sublattice in MgSe also has an fcc structure. This result suggested that a key factor for suc...

A High‐Rate Rechargeable Mg Battery Based on AgCl Conversion Cathode with Fast Solid‐State Mg2+ Diffusion Kinetics

“…The reversibility of conversion reaction could be well‐explained by hard and soft acids and bases theory. Since X 2 n − anion is bonded with bivalent Mg 2+ ion (hard acid), using soft X 2 n − anions (such as S 2− and Se 2− ) is expected to weaken the Mg—X bonds, and thus could improve the reversibility of conversion reaction . However, selection of M element is also of great significance.…”

“…A soft acid M 2 m + cation (such as Cu + ) would have a relatively stronger interaction with X n − anion and assist the dissociation of discharge product (Mg n X m ). For example, Cu 2 S and Cu 2 Se, which are combinations of soft‐acid cation (Cu + ) and soft‐base anion (S 2− and Se 2− ), show superior Mg‐storage performances as conversion cathodes . Therefore, a promising conversion cathode could be a combination of soft anions and soft cations.…”

“…Since X 2nÀ anion is bonded with bivalent Mg 2þ ion (hard acid), using soft X 2nÀ anions (such as S 2À and Se 2À ) is expected to weaken the Mg-X bonds, and thus could improve the reversibility of conversion reaction. [9,10,[24][25][26][27][28][29] However, selection of M element is also of great significance. A soft acid M 2mþ cation (such as Cu þ ) would have a relatively stronger interaction with X nÀ anion and assist the dissociation of discharge product (Mg n X m ).…”

“…For example, Cu 2 S and Cu 2 Se, which are combinations of soft-acid cation (Cu þ ) and soft-base anion (S 2À and Se 2À ), show superior Mg-storage performances as conversion cathodes. [26][27][28][29][30][31] Therefore, a promising conversion cathode could be a combination of soft anions and soft cations. With the aforementioned knowledge in mind, we think Ag þ should be another perfect soft-acid cation candidate similar to Cu þ , considering the large ion radius of Ag þ and high conductivity of Ag-containing materials.…”

Alloy Anode Materials for Rechargeable Mg Ion Batteries

“…In parallel to the development of Mg electrolytes, [ 26 ] substantial efforts have also been put into discovering new cathode materials capable of improved voltage and specific capacity over the last two decades, since the operating voltage (≈1.1 V) and gravimetric capacity (128 mAh g −1 ) of Mo 6 S 8 cathode used in the first Mg battery prototype are inadequate. [ 7,27 ] As a consequence, a remarkable number of new materials other than Chevrel phase (Mo 6 S 8 , [ 28 ] Mo 6 Se 8 , [ 29 ] Mo 6 S 6 Se 2 , [ 30 ] Cu x Mo 6 S 8 , [ 31 ] ) have been introduced and evaluated as potential Mg cathodes through experimental and theoretical explorations, such as Prussian blue analogs (Ni/Fe, [ 32 ] Cu/Fe, [ 33 ] V/Fe, [ 34 ] Na 0.69 Fe 2 (CN) 6 [ 35 ] ), multifarious metal oxides (V 2 O 5 , [ 36 ] V 2 O 5 –P 2 O 5 , [ 37 ] VO x , [ 38 ] VO 2 , [ 39 ] V 2 MoO 8 , [ 40 ] Mo 2.5+ y VO 9+ δ , [ 41 ] Co 3 O 4 , [ 42 ] MnO 2 , [ 43 ] Mn 3 O 4 , [ 44 ] MoO 3 , [ 45 ] RuO 2 , [ 46 ] TiO 2 [ 47 ] ), sulfides (MoS 2 , [ 48 ] TiS 2 , [ 49 ] TiS 3 , [ 50 ] Ti 2 S 4 , [ 51 ] VS 2 , [ 52 ] VS 4 , [ 53 ] WS 2 , [ 54 ] CoS, [ 55 ] Cu 2‐ x S, [ 56 ] CuS, [ 57 ] α‐MoS x [ 58 ] ), selenides (Ni 0.75 Fe 0.25 Se 2 , [ 59 ] NiSe 2 , [ 59 ] WSe 2 , [ 60 ] TiSe 2 , [ 61 ] VSe 2 , [ 62 ] NbSe 2 , [ 63 ] CuSe 2 , [ 64 ] ) or borides (MoB 2 , TiB 2 , ZrB 2 ), [ 10 ] organic compounds (DMBQ, [ 65 ] 14 PAQ, [ 66 ] P(NDI2OD‐T2), [ 67 ] MCNB, [ 68 ] PHVCl, […”