Distribution of relaxation times investigation of $$\hbox {Co}^{3+}$$ Co 3 + doping lithium-rich cathode material $$\hbox {Li}[\hbox {Li}_{0.2} \hbox {Ni}_{0.1} \hbox {Mn}_{0.5} \hbox {Co}_{0.2}]\hbox {O}_{2}$$ Li [ Li 0.2 Ni 0.1 Mn 0.5 Co 0.2 ] O 2