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Reusing valuable cathode materials from end-of-life (EOL) Li-ion batteries can help decrease dependence on mining of raw materials for producing cathodes, while preventing commodity prices from rising. This study employed chemically delithiated cathodes that are analogous to spent cathodes but free of impurities to fundamentally elucidate the effectiveness of cathode regeneration. Two lithium cobalt oxides (LCOs) at different degrees of delithiation were synthesized via chemical delithiation. Their material and electrochemical characteristics were systematically compared before and after hydrothermal-based cathode regeneration. The material and electrochemical characteristics were further evaluated and compared with those of pristine LCO. Both LCOs, at high and low states of health (SOH), recovered their reversible capacity and cycle performance comparable to those of pristine LCO. However, the high-rate performance (2C) of the regenerated LCOs was not comparable to that of pristine LCO. The slight increase in cell resistance of the regenerated LCOs was attributed to their lower high-rate performance, which was identified as a key challenge of cathode regeneration. Our study provides valuable insights into the effectiveness of cathode regeneration by elucidating the process underlying regeneration of disordered Li-deficient LCOs at different levels of SOH.