Several factors need to be taken into consideration. First of all, what type of electrochemical technique are you employing? If it is cyclic voltammetry, then it is not a good choice; try using square wave stripping voltammetry instead. It is unclear which type of salt you are using for preparing lithium-ion solutions. In terms of material fabrication, try to add some sheet-like materials, such as graphitic carbon nitride, graphene oxide, or something like CNTs.

I'm sorry but as must ask, what type of electrochemical mechanism do you think is going to make you able to detect the Li ion? The intercalition or de-intercalation of Li ion in a material that already has Li+ in its composition? Is not that easy, furthermore the compounds LiFePO4 and LiMnO4 both decompose slowly in water, making your sensor unreliable. If we go even deper in the analysis some other cation such a Na+ and K+ are able the intercale (to some extent) in the structure. Making also the sensor to be non-specific. This idea need further discution, and further understanding of the mechanism behind Li-materials. As someone that works in both Li ion cathode materials and also electroanalysis I am open to guide and give more information on the matter.. but adding any other conductive material won't really help you

Electrochemical sensing of lithium ions in water can be effectively achieved using lithium ion selective electrodes (ISEs) that offer precise, rapid, and reliable measurements with minimal sample preparation. Alternative sensitive methods like surface-enhanced laser-induced breakdown spectroscopy (SENLIBS) also provide fast and accurate lithium quantification in various water sources using affordable substrates like aluminum.