Operation Mechanism in Hybrid Mg–Li Batteries with TiNb₂O₇ Allowing Stable High-Rate Cycling

Abstract

We studied the structural evolution and cycling behavior of TiNb2O7 (TNO) as cathode in a non-aqueous hybrid dual-salt Mg-Li battery. A very high fraction of a pseudocapacitive contribution to the overall specific capacity makes the material suitable for ultrafast operation in a hybrid battery, comprised of a Mg-metal anode, and a dual-salt APC-LiCl electrolyte with Li and Mg cations. Theoretical calculations show that Li-intercalation is predominant over Mg-intercalation into the TNO in a dual-salt electrolyte with Mg 2+ and Li + , while experimentally up to 20% Mg-co-intercalation was observed after battery discharge.In hybrid Mg-Li batteries, TNO shows capacities which are about 40 mAh g -1 lower than in single-ion Libatteries at current densities up to 1.2 A g -1 . This is likely due to a partial Mg co-intercalation, or/and location of Li-cations on alternative crystallographic sites in the TNO structure in comparison to Liintercalation process in Li-batteries. Generally, hybrid Mg-Li cells show a markedly superior applicability for a very prolonged operation (above 1000 cycles) with 100% Coulombic efficiency and a capacity retention higher than 95% in comparison to conventional Li-batteries with TNO after being cycled either under a low (7.75 mA g -1 ) or high (1.55 A g -1 ) current density.The better long-term behavior of the hybrid Mg-Li batteries with TNO is especially pronounced at 60 °C. The reasons for this are an appropriate cathode electrolyte interface containing MgCl2-species as well as a superior performance of the Mg-anode in APC-LiCl electrolytes with a dendrite-free, fast Mg deposition/stripping. This stable interface stands in contrast to the anode electrolyte interface in Li-batteries with a Li-anode in conventional carbonate-containing electrolytes, which is prone to dendrite formation, thus leading to a battery shortcut.

Mg-Li Hybrid Batteries: The Combination of Fast Kinetics and Reduced Overpotential

“…These capacitive storage values are very close to that observed in Li batteries, which indicates the Mg-Li hybrid batteries can achieve fast charging technology comparable to Li batteries. Subsequently, Maletti et al further investigated the structural evolution and cycling behavior of TNO electrodes in Mg-Li hybrid batteries [47]. Some initial Li insertion into TNO to expand the interlayers can facilitate Mg 2+ coinsertion start-ing from the composition Li 0.2 TiNb 2 O 7 .…”

“…As known, a major advantage of Mg-Li hybrid batteries is the adoption of Mg metal anode with smooth deposition morphology. Especially at a rate up to 20 C (Figure 5(a)), compared with Li metal batteries, although the Mg anode suffers a higher overpotential in APC-LiCl electrolyte, it can prevent the local concentration of current and the growth of dendrites [47]. In addition to the characteristic advantages of Mg anode, Li + will also have magical effects on the Mg anode side in the Mg-Li hybrid system, including tailoring SEI and reducing the overpotential.…”

Magnesium/Lithium Hybrid Batteries Based on SnS 2 -MoS 2 with Reversible Conversion Reactions

“…Specifically, the reaction kinetics at the cathode is highly improved as the reactions are dominated by Li + . In addition, the advantages of using metal Mg as the anode are maintained because Mg 2+ are deposited at the anode side with reversibility [14,[17][18][19]. The dual-salt electrolyte acts as the Li-ion reservoir to ensure the reaction on the cathode side during cycling.…”

“…Despite that, an anodic stability limit of 3.0 V (vs. Mg/Mg 2+ ) has been reported for the Mg electrolyte all-phenyl-complex (APC) [20]. This limit decreases to 2.5 V (vs. Mg/Mg 2+ ) when LiCl or LiBF 4 is added [14,15,[17][18][19]. Therefore, the redox potential of the chosen cathode must be within the electrochemical stability window of the complex electrolyte and remain chemically inert to electrolyte components to avoid any parasitic reactions.…”

“…In light of the success in the commercialisation of LIBs, many efforts have been devoted to the intercalation-type cathode materials, such as Mo 6 S 8 , Li 4 Ti 5 O 12 , VS 2 , and TiO 2 , shown in Table S1 [14,[17][18][19][24][25][26][27][28][29][30]. However, the development of these cathode materials is hindered by their low specific and energy density.…”