Chinese researchers have developed a new strategy to improve the capacity of manganese-based semi-solid battery systems.Using the new method, scientists at Shanghai University reported a 409 percent boost in battery capacity over the reported manganese-sulfate (MnSO₄)- based electrolyte benchmark when they switched to a Manganese dioxide (MnO₂) semi-solid slurry electrolyte.The significant improvement in capacity was observed even when the systems were operated at higher current densities.These modifications could make them a suitable candidate for large-scale energy storage systems, researchers said.Bottlenecks of traditional Mn-battery systemsConventional manganese-based systems have been a preferred choice for battery materials as manganese is environmentally safe, low-cost, and is abundantly available.But the traditional redox-based batteries have limited solubility in electrolyte and restrict the amount of energy they can store.The alternative solid MnO₂ could overcome the challenge as it increases the active-material content. However, these semi-solid flow batteries typically cannot charge or discharge as quickly as desired because the chemical reactions occur slowly. This makes them unsuitable for operations at high charge-discharge rates,To overcome the two limitations of the conventional Mn-based systems, the team, instead of relying solely on dissolved manganese salts, added solid MnO₂ into the electrolyte. This resulted in a slight change in the electrochemical reaction pathway, the study reported.”During discharge, solid MnO₂ is electrochemically converted into soluble Mn²⁺, allowing the battery to access more active material without being constrained by the initial solubility limit,” the researcher said.Mn solids enhance capacityThis innovative way of increasing the amount of active manganese helped reduce the common-ion effect that impacts sulfuric acid electrolytes, limiting their solubility.The approach drastically improved capacity: a slurry mix with 4 Molar (M) MnO₂ achieved a soluble manganese concentration of 3.76 M. According to the study, this increased the battery’s storage capacity to 156.2 ampere-hours per liter (Ah L⁻¹), a 409 percent improvement over the MnSO₄-based electrolyte used as a benchmark.Performance of these new battery systems at higher current densities was also enhanced. The battery storage capacity was retained above 151 Ah L⁻¹ at the same slurry concentration when a current density of 30 milliamperes per square centimeter (mA cm⁻²) was applied, the study reported. These retained energy efficiencies exceeded 71 percent at the said current density.The researchers said these operating current conditions are 30 times those previously reported for MnO₂-based semi-solid redox flow batteries, showing that high active-material loading does not necessarily come at the expense of reaction rate.The researchers also discovered why the battery performs better over time.During charging, the MnO₂ inside the battery changes into a more reactive form that speeds up the chemical reactions needed to store and release energy. This shift allows the battery to operate efficiently at higher charging and discharging rates while maintaining a high storage capacity.In their lab tests, a battery using a 1 M MnO₂ slurry completed 100 charge-discharge cycles with about 99 percent Coulombic efficiency and 76 percent energy efficiency. However, the researchers said further work is needed to reduce capacity loss caused by manganese crossover and particle buildup.The findings were published in Nano-Micro letters.