Ni, Co Hydroxide Modified by Partial Substitution of OH^– with Cl^– for Boosting Ultra-Fast Redox Kinetics up to 500 mV s⁻¹ in Supercapacitors

Metal oxides/hydroxides have attracted great attention as battery-type electrodes for supercapacitors due to their high theoretical capacitance. However, their poor electrical conductivity and ion transport kinetic severely limit their rate performance, hindering the practical application for high power devices. Herein, a simple co-precipitation method to synthesize partial substitution of OH^– with Cl^– in Co, Ni layered hydroxide in saturated NaCl solution is reported. The Cl^– substitution significantly increases the interlayer H₂O content from 5.3% to 10.2% by changing the polarity of the hydroxide layers, leading to ultrafast ion transport through the Grotthuss behavior. Moreover, the Cl^– substitution also changes the electron configuration of Ni and Co, enhancing the electrical conductivity and redox activity. Thanks to the robust electrochemical kinetics, the Ni_0.85Co_0.15(OH)_1.77Cl_0.23 shows well-maintained redox peaks at an ultrahigh scan rate up to 500 mV s^–1 and an outstanding rate performance (1076.3 C g^–1 at 1 A g^–1 and 333.6 C g^–1 at 100 A g^–1), which are among the best in the previously reported Ni(OH)₂ based electrodes. Therefore, this simple anion substitution strategy opens up a new concept for achieving ultrahigh rate battery-type electrodes.