The development of new energy has promoted the wide application of electric vehicles, but it has also led to a large number of spent lithium-ion batteries (LIBs). Microwave-assisted pyrolysis (MAP) of electrode materials from spent LIBs and biomass has been proven to be an effective method for recovering valuable metals. However, the microwave absorption of these electrode materials, the pyrolysis characteristics of biomass, and their product compositions have not been clearly identified. Herein, the feasibility of several electrode materials from spent LIBs (graphite, LiFePO₄, and oxide cathodes) as microwave absorbers in MAP was evaluated by vector net analysis. Furthermore, the MAP characteristics of biomass (cellulose, hemicellulose, lignin, and peanut shell) and these electrode materials were investigated, and relevant mechanisms were proposed through product analysis. The results showed that all electrode materials exhibited medium or strong microwave absorption (tanδε > 0.02) at 2.45 GHz, indicating that they are suitable microwave absorbers for MAP. These spent electrode materials significantly increased the mass loss and yield of gaseous products in the MAP of the above biomass, promoting H₂ production and CH₄ decomposition. The oxide cathodes could facilitate the dehydration and aromatization of cellulose and hemicellulose, while LiFePO₄ and graphite might increase the aromatic content in the liquid products due to their strong reducibility for biomass deoxygenation. Moreover, the characterizations of solid products revealed the phase transitions of spent electrode materials before and after MAP with biomass. This study could provide an experimental and theoretical basis for the application of spent electrode materials from lithium-ion batteries in the microwave-assisted pyrolysis of biomass.