Forming a solid solution phase with careful selection of end members and their compositions yields the opportunity to create unexpected or superior properties beyond averaging of their end member properties. To form a multi-component solid solution phase, several factors are generally considered, such as crystallographic parameters, chemical valance, and radius of cations or anions for minimum formation energy and a wide range of composition tuning. Recently, the beneficial combination of two Li⁺-storage mechanisms in a single compound, referred to as hybrid anode materials, has been noticed to complement the shortcomings of two different reaction mechanisms. A new strategy to extend the concept of the present hybrid anode material is to find out the material systems to form the complete solid solution whose end members have the different types of reaction mechanisms with Li⁺, which can additionally control the ratio of two different materials.

In this talk, the rational design of solid solution phases and their synthesis strategies are introduced to improve the electrochemical properties of secondary battery applications. First, ternary solid solution phases were introduced as anode materials for lithium-ion batteries (LIBs) by cation exchange in binary metal phosphides to compensate for the drawbacks of end members. We suggest a new concept to tailor the electrochemical performance using the solid solutions of Mn-Fe-P and Mn-V-P, which are composed with alloying type MnP, conversion type FeP and insertion type VP. Second, P-rich tetraphosphide compounds were introduced as high capacity of anode materials for next generation sodium-ion batteries (SIBs). The cation-exchanged Mn_1-xV_xP₄ (x = 0.25) phase was introduced as an anode material for Na-ion batteries to obtain the higher reaction kinetics of the high capacity of MnP₄ anode. In addition, the anion-exchanged NiP_2-xS_x anodes were proposed to minimize the undischarged capacity in SIB applications by upshifting the P redox potential.