Promoting solution phase discharge in Li-O₂ batteries containing weakly solvating electrolyte solutions

On discharge, the Li-O₂ battery can form a Li₂O₂ film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li₂O₂ particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the LiO 2 intermediate involved in the formation of Li₂O₂. However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO₂ or Li₂O₂. Here we demonstrate that introduction of the additive 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li₂O₂ in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li₂O₂ on the cathode surface, which would otherwise lead to Li₂O₂ film growth and premature cell death. It also halves the overpotential during discharge, increases the capacity 80-to 100-fold and enables rates >1 mA cm areal^-2 for cathodes with capacities of >4 mAh cm areal^-2. The DBBQ additive operates by a new mechanism that avoids the reactive LiO₂ intermediate in solution.