The functionalization of cross-linked chitosan (Chito) by an original sulfonating process allows synthesizing a highly efficient sorbent (Sulfo-C) for Li(I) recovery from acidic solution (at pH ≈2). FTIR and XPS analyses show the main contributions of amine and sulfonic groups in the binding mechanism. Maximum sorption capacity reaches 20 mmol Li g−1, and the sorption isotherms are fitted by the Langmuir Dual Site equation; the process being spontaneous, exothermic and enthalpy-driven. The sorption remains highly efficient in 1 M NaCl solution, while the uptake remains strongly selective in the presence of equimolar concentrations of alkali-earth and heavy elements (Fe > Zn > Mg > Ca > Ni > Al cations). This selectivity (SCLi/Metal varying between 14 and 36) is controlled by the pH (optimum close to 2.2) and slightly increases (up to 16–69) with adding oxalic acid (1% w/w). The equilibrium is achieved, under selected experimental conditions, within 30–90 min of contact (depending on temperature). Uptake kinetics are fitted by the pseudo-first order rate equation, though the contribution of resistance to intraparticle diffusion cannot be neglected. Bound metal can be completely desorbed using hydrochloric acid solution (0.2 M); the kinetics of desorption is fast (within 20 to 30 min). Both sorption and desorption performances are remarkably stable for 18 cycles (loss in sorption less than 10.4% at the 18th cycle). The functionalized sorbent is highly efficient for the recovery of lithium from a spent Li-ion battery (mobile cell battery). The sorbent shows high selectivity for Li(I) against Co(II), Mn(II), Ni(II), Fe(III) and Al(III) (SCLi/Metal: 26–85); much lower against Cd(II) (i.e., 2.3).