The fire extinguishment of a mixture of lithium-ion cathode and electrolyte is investigated based on the use of flame retardants. Four boehmite-based flame-retardants are embedded in the LiFePO4 cathode including 0.95μm AlOOH-L, 0.35μm AlOOH-S, and their individual microcapsules. The microcapsules are denoted as en-AlOOH-L and en-AlOOH-S. Here, the shell material is the chemically stable poly(urea-formaldehyde). The rheology of the cathode slurries improves as flame-retardants are added. This is especially significant for cases using microencapsulated boehmites, en-AlOOH-L and en-AlOOH-S. At 5–15wt%, the smaller AlOOH-S and en-AlOOH-S exhibit much better fire-extinguishing efficiency values—40-50% versus 30-40% for the larger AlOOH-L and en-AlOOH-L. This is because the smaller powder can be easier to form a dense barrier against further decomposition of the combustible materials. Among the four retardants, en-AlOOH-S is the best. It shows good fire retardancy in the cathode/electrolyte mixture without significant electrochemical sacrifice of the cathode.