Potassium sodium niobate (KNN) is a well known piezoelectric material and a developing candidate for replacing lead based high performance piezoceramics due to limitations of hazardous materials in electronic devices. Lithium substitution in KNN structure leads a crystal transition from orthorhombic to tetragonal symmetry at room temperature and enhances ferroelectric properties. In this study, lithium substituted KNN piezoceramics with high relative densities were prepared by spark plasma sintering (SPS). Densification, crystal structure, microstructure and ferroelectric behavior of the samples were investigated. It was observed that lithium niobate based secondary phases exist in sintered KNN ceramics. These secondary phases showed significant effects on ferroelectric properties. Maximum (P_m) and remnant (P_r) polarizations were determined as 27 and 20 µC/cm², respectively for pure KNN at the electric field of 20 kV/cm. When the electric field was increased to 30 kV/cm, P_m did not change significantly, but remained below P_r. For Li substituted samples, P_r decreased with increasing Li content.