For convenience, below is a summary of the primary degradation / aging mechanisms of the anode and cathode in Li-ion cells. Notice in particular how high SOC plays a role in the majority of the degradation processes.
To expand on my remarks in post #5, here is some further data on the degradation caused by float-charging and storing at high SOC and high temperatures. Below is a graph from this 2014 study of calendar life degradation (no cycling) for Sanyo UR18650E cells stored at various SOCs at 50°C. As the study shows, you get only 107 days lifetime storing at 100% SOC, but 2.7x that (288 days) storing at 95% vs. 100% SOC, and 14x that (4 years) storing at 0% SOC.
Ecker et al. said:
During calendar aging tests cells were stored at different SOC and temperatures. Fig. 6 shows capacity fade and resistance increase for cells stored at 50°C at different SOC. As expected, cells stored at lower SOC exhibit a longer lifetime. Especially cells stored at 100% SOC show a much faster degradation compared to other SOC. These cells only reach a lifetime of 107 days at 50°C until capacity reaches 80% of the initial value (linear interpolation). At this time the resistance increased by a factor of 1.3. For comparison, cells stored at 95% SOC reach 288 days until end of life (linear interpolation), whereas a cell stored at 0% SOC has an extrapolated lifetime of about 4 years (linear extrapolation of the last three measured data points)