Chromium Tetraphosphide (CrP4) as a Promising Anode Material for Lithium-Ion and Sodium-Ion Batteries

Lithium-ion batteries (LIBs) are the most widely used power source in portable devices, electrical vehicles (EVs), and energy storage system (ESS). Graphite or silicon anode materials are commercialized in LIBs, but high price and limited distribution of lithium resources cannot meet the increasing demand for LIBs. Besides, sodium-ion batteries (SIBs) have been spotlighted as an alternative to LIBs due to the natural abundance of sodium resources and similar energy storage mechanisms to that of LIBs. However, the commercialized graphite (C) is electrochemically less active for SIB application due to thermodynamically unstable state of the ternary intercalation compounds and Si materials, which is the highest specific capacity anode for LIB application (~ 4200 mA h g^-1), have limitation for sodium ion storage in room temperature because intermetallic phase of NaSi_x is inactive to sodium ions at temperature up to 60 ^oC. Therefore, the new type of anode materials is necessary to be developed which operates electrochemically active in Li and Na cells with high energy density.<br/>Recently, numerous studies have been conducted on metal oxides and sulfides, such as TiO₂ and MoS₂ anodes, but such materials have limitations to meet the requirement for high-energy density anodes due to their low reversible capacity and high reaction potential. Transition metal phosphides (MP_x), especially P-rich phases (x &gt; 2), have been attracted much interest owing to their high reversible capacity (&gt; 1500 mA h g^-1) & low react-potential and similar reaction mechanisms in both Li and Na cells. In particular, the MP₄ (M = V, Cr, Mn, Fe, Mo, W, Zn) type materials can deliver the highest specific capacity as transition metal phosphides with the highest number of P component.<br/>In this study, we first synthesized CrP₄ phase (C2/c, monoclinic) via high energy mechanical milling (HEMM) process without high-temperature process. The CrP₄ phase was first introduced as an anode for LIBs and SIBs and their electrochemical performance and reaction mechanisms were investigated. Based on the XRD analysis, the CrP₄ electrode showed the conversion reaction in both LIB and SIB application. The CrP₄ electrode delivered discharge/charge capacities of 1776 / 1540 mA h g^-1 at the current density of 100 mA g^-1 in Li cell and 933 / 736 mA h g^-1 at the current density of 50 mA g^-1 in Na cell. P-rich MP_x electrode inevitably showed the rapid capacity fading during Li/Na ion uptake/release due to the low electronic conductivity (~ 10^-14 S cm^-1) and large volume expansion (~ 490 %) of P element. To enhance the cyclabilty and high-rate capability of the CrP₄ electrode, CrP₄/graphene nanocomposite was fabricated using commercial graphene nanosheet by HEMM. As-fabricated CrP₄/graphene electrode exhibited the enhanced long-term cyclabilty and high-rate capability for both Li and Na cells, which delivered the specific capacity of 700 mA h g^-1 after 100^th cycle at the current density of 1000 mA g^-1 in LIBs and 300 mA h g^-1 after 200^th cycle at the current density of 500 mA g^-1. Highly conducting graphene nanosheets improved the electrical conductivity of the whole electrode and alleviated the aggregation of adjacent CrP₄ active materials. The simple and large scalable synthesis process and its superior Li/Na ion storage performance of the CrP₄/graphene nanocomposite electrodes make it a promising high-energy density anode materials for both LIB and SIB application.