Researchers from Helmholtz Institute Ulm and Cheongju University have developed a new lithium-metal battery that claims to offer an extremely high energy density of 560 watt-hours per kilogram with remarkably good stability.

Despite lithium-metal batteries having a higher-capacity energy storage solution than current Li-ion batteries, their stability poses a challenge because the electrode materials react with common electrolyte systems.

Nickel is a promising material for cathodes owing to its high specific capacity. However, the high nickel content introduces new challenges, such as poor cycling and thermal instability. Also, due to the similar ionic radius of Ni²⁺and Li⁺, nickel ions can easily migrate from the transition metal layer into neighboring lithium vacancies which is detrimental for lithium-ion diffusion.

Despite these drawbacks of Ni-rich cathodes, substantial efforts have been devoted toward designing materials with the ideal balance of high specific capacity and high safety characteristics required by the market. To enhance the structural stability of the cathodes, they are lattice doped into the transition metal layer of magnesium, calcium, aluminum, and titanium to prevent Ni2+ migration into the lithium layer and strengthen the metal-oxygen bonding to restrain oxygen release and improve the thermal stability.

Another approach to mitigate the performance decay of such cathode materials is applying a protective surface coating. This prevents direct contact between the cathode active materials and the electrolyte, reduces the negative impact of the attack by highly reactive hydrogen fluoride or some other components, and, thus, alleviates the parasitic reactions stimulated by the presence of Ni⁴⁺.

The researchers went ahead with the second method, using a combination of low-cobalt, nickel-rich layered cathode (NCM88) for cathode and a non-volatile, non-flammable ionic liquid electrolyte with two anions (ILE), bis(fluorosulfonyl) imide (FSI), and bis(trifluoromethanesulfonyl)imide (TFSI), for the electrolyte.

ILE was substituted for the commonly used, commercially available organic electrolyte (LP30), in which particle cracks occur on the cathode. LP30 reacts within these cracks and destroys the structure. In addition, a thick, moss-like lithium-containing layer forms on the cathode.

With the electrolyte ILE, the cathode exhibits remarkable electrochemical performance, achieving an initial specific capacity of 214 mAh g−1 and outstanding capacity retention of 88% over 1,000 cycles. More importantly, this electrolyte enables an average Coulombic efficiency of 99.94%.

Mercom had reported that collaborative research between the University of Liverpool, United Kingdom, and National Tsing Hua University (NTHU), Taiwan, had revealed a new charge storage mechanism that can allow rechargeability within calcium-air batteries.

Earlier this year, researchers from the Massachusetts Institute of Technology and other organizations found a novel electrolyte that could allow lithium-ion batteries to store about 420 watt-hours per kilogram. Such batteries can now typically store about 260 watt-hours per kilogram of energy.