posted on 2022-01-11, 17:06authored byYoko Hase, Takeshi Uyama, Kiho Nishioka, Juntaro Seki, Kota Morimoto, Nobuhiro Ogihara, Yoshiharu Mukouyama, Shuji Nakanishi
The
large overpotential of nonaqueous Li–O2 batteries
when charging causes low round-trip efficiency and decomposition of
the electrode materials and electrolyte. The origins of this overpotential
have been enthusiastically explored to date; however, a full understanding
has not yet been reached because of the complexity of multistep reaction
mechanisms. Here, we applied structural and electrochemical analysis
techniques to investigate the reaction step that results in the increase
of the overpotential when charging. Rietveld refinement of ex situ powder X-ray diffraction showed that a Li-deficient
phase of Li2O2, Li2–xO2, formed when discharging and was present over
the course of charging. The galvanostatic intermittent titration technique
revealed that the rate-determining process in the first step of charging
was a solid–solution type of delithiation. The chemical diffusion
coefficient of Li+ ions in Li2–xO2, DLi, decreases as
the cell voltage increases, which in turn leads to a decrease in the
oxidation rate of Li2–xO2. Under galvanostatic conditions, the deceleration of oxidation induces
further increase of the cell voltage; therefore, an intrinsic mechanism
of positive feedback to increase the cell voltage occurs in the first
step. The results demonstrate that the continuity of the first step
can be extended by the suppression of changes in any of the elements
of the positive feedback loop, i.e., the oxidation rate, cell voltage,
or DLi.